Low Mass Star Formation in the Lynds 1641 Molecular Cloud

Handbook of Star Forming Regions ASP Conference Series, Vol. ..., 2008 Bo Reipurth, ed.

Low Mass Star Formation in the Lynds 1641 Molecular Cloud L.E. Allen Harvard-Smithsonian Center for Astrophysics 60 Garden Street, Cambridge, MA 02138, USA C.J. Davis Joint Astronomy Centre, 660 North A’oh¯ok¯u Place, University Park, Hilo, Hawaii 96720, USA Abstract. The Lynds 1641 cloud makes up the bulk of Orion A south of the Orion Nebula Cluster. Although it contains no rich clusters comparable to the ONC, it is forming stars in numerous dense molecular cores found primarily along a ridge of gas that extends the length of the cloud (approximately 2.5 degrees). Optical, X-ray, and infrared surveys have detected hundreds of young, predominantly low-mass stars. Most of the protostars are clustered in small groups or aggregates of N=5–40 members, but the more evolved T-Tauri stars appear to be located both in and around these aggregates. L 1641 has thus become a case study for the relative importance of “distributed” vs. “clustered” star formation. Recent results from the Spitzer Space Telescope survey of L 1641 confirms the existence of a significant distributed population, in which 44% of the young stars are in low surface density regions of fewer than 10 stars/pc2 . L 1641 is extremely rich in molecular outflows and HH–objects, containing as many as 85 molecular outflows and dozens of HH–shocks. Some of the HH flows are quite spectacular and at least a few comprise pc-scale systems, most notably the HH 303/310 outflow, whose largest lobe extends 6.3 pc from the driving star.

1. Introduction As the nearest giant molecular cloud (GMC) complex, the Orion region offers a unique opportunity to study the star–forming history of a GMC. Its modest distance of ∼450 pc means that with small–aperture telescopes we are able to observe star formation over the full range of stellar masses, from the most massive O stars (∼40 M⊙ ) to late M stars at and beyond the hydrogen burning mass limit (∼0.08 M⊙ ). Lynds 1641 (Figure 1) is one of the several ∼ 104 M⊙ molecular clouds making up the Orion GMC complex. As originally defined by Lynds (1962; 1965), L 1641 is centered at (RA,Dec) 05h 37.4m , -06d 58m (J2000) and has an area of 6.3 deg2 . It lies just south of the L1640 cloud where the ONC is located, and forms a northwest to southeast oriented ridge which extends some 2.5 degrees, covering 300-400 pc2 (at d∼450 pc). At the very southern end of this ridge, Lynds (1962; 1965) named two additional clouds, L1647 (western leg) and L1648 (eastern leg). Together, these clouds make up the Orion A GMC. Since the two southern clouds are rarely distinguished from L 1641 in the literature, it is practical to refer to the entire cloud structure south of L1640 (or south of δ = −06d 10m ; J2000) as L 1641, and that is what we shall do in this contribution (though see Carballo & Sahu 1994). It is worth noting that the clouds known as OMC4 and OMC5 (Johnstone & Bally 2006), located to the north of L 1641 1

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Figure 1. The Northern part of L 1641 molecular cloud is seen in this image composed of an Hα image combined with R, G, B, broadband filter images. North is up and east is left. Courtesy Johannes Schedler.

3 between δ2000 = 05h 30m and δ2000 = 06h 00m , are excluded from this review, as they are considered separately from L 1641 in most of the literature. Unlike its immediate neighbor to the north, L 1641 is not forming massive stars. Its most massive known member is the B4 V Herbig Ae/Be star HD 38023 (α2000 :5:42:21, δ2000 :−8:08; Racine 1968); thus it contains no rich clusters, but it does contain several small groups or aggregates of young stars, as well as relatively isolated young stars distributed throughout the cloud. Perhaps because of its proximity to and difference from the ONC region, L 1641 has been thoroughly surveyed across the electromagnetic spectrum. 2. Distance to L 1641 Typically, the distance to the L 1641 cloud is assumed to be the same as the distance to the ONC, however Wilson et al. (2005) present some evidence that there is a distance gradient from the northern to southern end of the Orion A GMC of approximately −100 pc. The distance estimates to various pieces of Orion A are highly uncertain, mainly because they are based on photometric or parallax measurements of very few stars. The oft-used distance of 450 pc is consistent with most estimates. For a more detailed discussion of distance determinations to Orion A, the reader is referred to the chapter by Muench et al. in this volume. 3. Cloud Structure: Radio, Millimeter and Submillimeter mapping It is difficult to discuss the structure of the L 1641 cloud without including the rest of Orion A, since most global physical properties (such as molecular mass) are presented in the literature for the entire GMC, rather than for the individual clouds. Menon (1958) made the first complete 21 cm survey of the entire Orion region, deriving a total mass in HI of ∼105 M⊙ , though no separate value was given for the individual molecular clouds. The observed HI velocity field was interpreted as that of a large shell, roughly 70 pc in radius and centered on the Orion Nebula, expanding at a velocity of ∼ 10 km s−1 . Gordon (1970) mapped the Orion GMC in HI at higher resolution and concluded that the observed velocity structure was consistent with rotation of the gas, rather than expansion. Kutner et al. (1977) produced the first large scale map in 12 CO, covering 28 square degrees. They estimated the mass of the Orion A cloud to be about 105 M⊙ , though much of that is concentrated north of L 1641, in the ONC region. This mass was confirmed by Maddalena et al. (1986), whose 12 CO map of the Orion and Monoceros clouds covered 850 square degrees. Because 13 CO is optically thin in most directions and therefore a better probe of total column density than the more abundant 12 CO, Bally et al. (1987) mapped Orion A in 13 CO (Figure 2). They calculated a mass in molecular gas in their 8 square degree map of 5×104 M⊙ , about 25% of that mass contained in the elongated integral-sign filament at the northern end of the cloud that includes NGC 1977, the ONC and, just south of the ONC, NGC 1999. The Bally et al. 13 CO map showed L 1641 to be dominated by two clumpy filaments of gas. The main filament contains about 30% of the emission from 13 CO south of δ = −6, is on average 20 arcmin wide and more than 3 degrees long, and extends from the northwest to the southeast, parallel to the Galactic equator (Figure 2). A second smaller filament runs nearly north and south and connects to the larger ridge

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Figure 2. The Orion A cloud in 13 CO. The molecular gas is concentrated in a relatively narrow filament, starting at the northern end with the well-known integral-sign filament and the rich Orion Nebula cluster. South of the ONC are several small aggregates, including L 1641 North, V380 Ori, L 1641 South, and the HBC 498 group. In this and subsequent figures, integrated intensity of 13 CO is shown in contours of 6, 12, 18 and 24 K-km s−1 . The grayscale level of 13 CO emission in this image varies from 6-24 K-km s−1 . Coordinates are Equatorial, epoch 2000. (From Bally et al. 1987).

5 at α = 5h 38m 26s , δ = −6d 23m 21s (J2000). Numerous condensations of molecular gas are found along these filaments, with typical masses of a few tens to ∼ 100 M⊙ . According to Bally et al., interclump gas produces about 25% of the 13 CO emission from the cloud. Sakamoto et al. (1997) carried out higher-resolution observations (HPBW=15”) in the 12 CO(J = 1 − 0) line along the minor axis of the L 1641 cloud, covering a rectangular area bounded by 5h 33m 34s to 5h 44m 29s in α2000 and from −6h 16m 21s to −6h 43m 47s in δ2000 . They also observed a strip at δ2000 = −6h 43m from α2000 = 5h 33m 56s to 5h 36m 56s in the 12 CO(J = 1 − 0) and (J = 2 − 1) lines simultaneously. Similar to Bally et al. (1987), Sakamoto et al. (1997) identify two distinct clouds components, “clumps” with high brightness temperature (∼ 25 K) and small line width (∼ 1.5 km s−1 ) and extended molecular gas with low brightness temperature (∼ 2.5 K) and broad line width (∼ 2.5 km s−1 ). Molecular gas density extracted from the multi-line data was ≥ 3 × 103 cm−3 in the clumps and an order of magnitude less in the extended gas. There is an overall velocity gradient ranging from VLSR = +3 km s−1 at the southern end of L 1641 to VLSR = +14 km s−1 at the northern end, as shown in Figure 3. The origin of this gradient is unknown. One theory, posited by Kutner et al. (1977) and Maddalena et al. (1986), is that it is due to rotation about an axis perpendicular to the Galactic plane and in the opposite direction of Galactic rotation. Heyer et al. (1992) argue that the sharp velocity shifts (of ∼1 km s−1 ) observed between adjacent spectra are not consistent with the continuous motion expected from rotation. An alternative explanation was put forth by Bally et al. (1987), who suggested the gradient could be due to large–scale expansion of the cloud, driven by the stellar winds of the Ori OB1 association. Wilson et al. (2005) argue that the coherence of the gradient over such a large distance (∼ 100 pc) is unlikely to result from purely internal processes. They suggest that the gradient could have been formed by the passage through the cloud of a large shell similar to those driven by the Ori OB association. Uchida et al. (1991) argued that some of the observed motions in the gas are evidence of a helical structure, which they attribute to the structure of the magnetic field. In their model, this rotating helical filament in L 1641 acts as an angular momentum drain on the Orion KL region, allowing it to collapse. Broad and multiple velocity component line profiles are seen in L 1641, indicating that along many lines of sight, several different features exist at velocities differing by a few km s−1 (Bally et al. 1987). In contrast, the northern filament of Orion A, where the ONC is located, mostly single-velocity line components are seen. Bally et al. argue that L 1641 is therefore less dynamically relaxed than the northern part of the GMC. Hartmann & Burkert (2007) argued that the entire Orion A cloud is undergoing a large–scale gravitational collapse. They modeled the cloud as an elliptical rotating sheet with a smooth surface density gradient along its major axis. Qualitatively, their model result resembles the morphology of Orion A, showing a V–shaped structure which narrows to an integral–shaped filament at one end and contains a density enhancement at roughly the position of the ONC (Figure 4). Millimeter surveys for dense cores in L 1641 have been made using various molecular lines. The first was made by Few & Booth (1979), who mapped most of the cloud in the 111 –110 transition of formaldehyde (λ =6 cm), finding two large concentrations correlated with dust extinction. Takaba et al. (1986) reported on two concentrations of gas traced by several molecules, including HCN and HCO+ . Molecular surveys of IRAS sources were conducted by Harju et al. (1991) in NH3 (1,1) and by Chen, Fukui,

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& Yang (1992) in 13 CO and HCO+ . Chen, Fukui, & Iwata (1993) studied the coincidence of cores (as traced by 13 CO) with IRAS point sources. Their sample of 40 IRAS sources contained eight CO outflow sources; of these, six were associated with 13 CO cores. Conversely, only five of the 32 non–outflow sources had retained their cores, suggesting that dense cores are dissipated by high–velocity outflows in the early stages of star formation. A less biased search for dense cores was conducted in the CS (1-0) line and reported by Tatematsu et al. (1993) (Figure 5). Submillimeter line emission from their 125 high density cores was mapped by Wilson et al. (1999), who measured the J = 3 − 2 and J = 2 − 1 lines of 12 CO and C18 O, and the J = 3 − 2 line of 13 CO, as well as the (J, K) = (1, 1) and (2, 2) transitions of NH . Another survey at 3 submillimeter wavelengths was conducted by Ikeda et al. (1999), in the 3 P1 –3 P0 fine structure line of atomic carbon (CI, 492 GHz) and the J = 3 − 2 transition of CO (346 GHz). More recently, Ikeda, Sunada & Kitamura (2007) mapped the northern part of L 1641 in H13 CO+ (J = 1 − 0), with a spatial resolution of 25′′ .

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Figure 3. Longitude–velocity map of Orion A from Wilson et al. (2005), integrated over −22◦ < b < −17◦ . The ONC is at b∼209.3 deg, V380 is at b∼210.7 deg, HBC498 is at b∼212.5 deg.

Figure 4. A comparison of the 13 CO emission map from Bally et al. (1987) with the final state of the cloud collapse model by Hartmann & Burkert (2007). (From Hartmann & Burkert 2007).

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Figure 5.

CS cores from the survey of Tatematsu et al. (1993).

Submillimeter continuum observations probe cloud structure on a smaller scale, by tracing optically thin emission from cold dust. Johnstone & Bally (2006) presented 850 µm and 450 µm maps of L 1641 made with SCUBA. The dense condensations, or ”clumps” of dust identified at 850 µm are mainly concentrated along narrow filaments, running along or parallel to the long axis of the molecular cloud (Figure 6). The similar morphologies of the dust continuum cores traced by 850 µm emission and the dense molecular cores traced by H13 CO+ (J = 1 − 0) was pointed out by Ikeda, Sunada & Kitamura (2007), and is illustrated in Figure 7, which shows the small region around the L 1641-N cluster. 4. Young Stellar Objects in L 1641 Numerous surveys have been undertaken to find protostars and pre-main sequence (PMS) stars in L 1641. Here we review the various means by which these objects are identified, and the papers in which these methods were employed. 4.1. Hα emission line surveys A traditional means of searching for pre–main sequence stars has been through slitless Hα emission line surveys, a method which has its origins in the pioneering work of

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Figure 6. 850 µm clumps identified by Johnstone & Bally (2006). The circle size represents the size of the clump. From Johnstone & Bally (2006).

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Figure 7. Comparison of Ikeda, Sunada & Kitamura (2007) H13 CO+ (J = 1 − 0) and the 850 µm clumps of Johnstone & Bally (2006) of the L 1641-N region. From Ikeda, Sunada & Kitamura (2007).

Haro and collaborators in the late 1940’s and early 1950’s (Haro 1950, 1953; Haro, Iriarte & Chavira 1953). This technique is generally sensitive to Hα equivalent widths ˚ and provides a lower limit to the number of PMS stars since large larger than 3–5A, fractions of cloud populations may remain embedded in molecular material for 1 Myr or more, and be obscured at optical wavelengths. In addition, PMS stars can vary in their Hα emission, by as much as a factor of two or three. Nevertheless, objective prism surveys have been effective in identifying large numbers of PMS stars. Parsamian & Chavira (1982) built on Haro’s early efforts, surveying a 5 deg x 5 deg region centered on the Orion Nebula. Their catalog of 534 Hα emission line stars approximately doubled Haro’s original list of 255. Wouterloot & Brand (1992) surveyed one square degree in L 1641 and detected 112 Hα emission stars, including ”weak” emitters. The most spatially comprehensive study in Orion A was the Kiso Hα survey, which covered an area of 300 deg2 to a limiting magnitude of V = 17.5 (Nakano et al. 1995; Kogure et al. 1989; Wiramihardja et al. 1989, 1991, 1993). A deeper survey was conducted by Pettersson et al. (2008), using objective prism films obtained with the ESO Schmidt telescope. Centered at 5h 35m , −5d 25m and covering a 5 x 5 degree area, the ESO survey increased the number of Hα–emitting stars known in the surveyed region by a factor of ∼6. In Figure 8 all stars from the ESO survey with strong Hα emission (3-5 on Pettersson et al.’s scale), are plotted on contours of 13 CO integrated intensity (Bally et al. 1987). Not surprisingly, the density of Hα–emitting stars is highest near the ONC region. More notably, many of the Hα sources in L 1641 appear to be unassociated with any known cluster.

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Figure 8. Hα emission–line stars from the survey of Pettersson et al. (2008). Only strong (strength 3-5) Hα emitters are plotted. The dashed line shows the southern limit of their survey.

4.2. X-ray emission surveys Unlike Hα emission line searches, X–ray surveys are sensitive to the detection of both “classical′′ T–Tauri stars and “weak–line′′ T–Tauri stars. Results from 710 deg2 of the ROSAT All–Sky Survey (RASS), centered on but exceeding the area of the Orion molecular clouds, are reported by Sterzik et al. (1995) and in Alcala et al. (1996). RASS detected over 2000 X–ray sources, ∼1/2 of which were interpreted to be 107 108 yr old pre–main sequence stars. These stars may be associated with the Orion clouds but could also be young stars somewhat in the foreground. More sensitive, but spatially less extensive X–ray studies of the on–cloud regions were conducted by Strom et al. (1990), in L 1641 using data from the Einstein Observatory. Four overlapping IPC frames covering an area ∼2.5 deg2 and centered on V380 Ori contained 65 X–ray sources. Comparison with Hα emission line surveys led Strom et al. to conclude that the ratio of all PMS stars (as detected via X–rays) to those detectable by their Hα emission is ∼3:1, yielding a density of young stars in L 1641 of ∼32 deg−2 .

11 Recently, Wolk et al. (2008) surveyed seven fields in L 1641 using the XMM Xray Observatory. Guided by the spatial distribution of infrared excess sources detected in a Spitzer 3.6 - 25 µm imaging survey (Section 4.4), they selected regions shown in Figure 9. At the time of writing, six of the seven fields had been obtained and the data were being processed.

Figure 9. XM M − N ewton surveys in L 1641. The Spitzer 4.5 µm image is shown in grayscale. Small blue dots indicate the Class II sources identified by the Spitzer data. The large red circles show fields previously observed by XMM - Newton. Large cyan circles and rectangles show the fields recently surveyed by Wolk et al. Small red points show the X-ray sources as of September 2007.

4.3. Near–Infrared imaging Near–infrared imaging surveys have added much to our knowledge of the young embedded stellar populations in L 1641. Prior to the Two Micron All Sky Survey (2MASS), the near-IR surveys were mostly small in angular extent, and were centered on known sites of star formation, e.g. molecular outflows or IRAS point sources. Chen et al. (1993) presented multi–band imaging (BVRI and HK′ nbLM) of three IRAS sources (05338−0624, 05339−0626, and 0536−0702) associated with CO molecular outflows. They found stellar density enhancements around all three sources, and

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determined that at least 60% of the near–IR sources near the IRAS sources are pre–main sequence stars. In addition, they identified candidates for the near–IR counterpoints of the IRAS sources. Hodapp and Deane (1993) imaged a 30′ x 10′ region centered on L 1641 North at K′ , following with K–band spectra of selected stars as well as I–band polarimetry of stars near the North cluster and K′ polarimetry of stars near the L 1641 North molecular outflow. Based on a comparison of their observed K ′ luminosity functions with theoretical models and on their HR diagram (for 12 stars), they derived an age for the North cluster of ∼0.5 Myr. Strom, Strom & Merrill (1993) surveyed 0.77 deg2 (49 pc2 for d=480 pc), to limiting magnitudes (5σ) of 16.8, 15.8, and 14.7 mag at JHK. They identified a population of ∼1500 candidate pre–main sequence stars spread throughout the cloud in a low–density, “distributed” population, as well as a cluster (∼150 stars) which they call L1641-South, and seven small aggregates comprised of 10–50 stars and having enhanced surface densities over the distributed population, by factors ranging from 4 to 10. Comparing extinction–corrected J–band luminosity functions with theoretical models, they derived an age for the aggregates of ∼1 Myr, and for the distributed population of 5–7 Myr. Chen & Tokunaga (1994) surveyed 59 IRAS sources associated with dense molecular gas in L 1641, and found that ∼1/4 of them are associated with small groupings of bright near–infrared sources. They showed that the surface densities of these groupings were significantly higher than that of background fields. With the availability of the Two Micron All Sky Survey (2MASS), it became possible to study the young stellar populations across the entire cloud. Carpenter (2000) used 2MASS data to investigate the spatial distribution of young stars in Orion A and three other molecular clouds. Making background–subtracted surface density maps, he identified compact clusters and young stars in a low–density population distributed over the molecular cloud. Carpenter, Hillenbrand & Skrutskie (2001) used time-series photometry obtained with the southern 2MASS telescope to identify and examine the near-infrared variability properties of a 0.84 x 6 degree region, centered near the Trapezium. Their sample region encompassed the northernmost part of L 1641, but misses most of the cloud. In Figure 10, sources they identified as variables are plotted on CO contours. Of the more than 1235 near-infrared variables identified in this study, 233 were found to be periodic, and 22 were identified as eclipsing binary candidates. For declinations south of −06 degrees where L 1641 is located, 213 near-IR variables were identified. Among those, 42 were found to be periodic variables, and 4 were reported to be eclipsing binary candidates. Near–infrared polarimetric imaging of 33 YSO was performed by Casali (1995), who found that the polarization values are correlated with the infrared colors of the sources. Point sources were found to have a strong wavelength dependence, whereas extended sources had a flatter dependence on wavelength, as expected for scattering from small grains. Finally, Orion A and B are currently being targeted by the UKIRT Infrared Deep Sky Survey. The Galactic Clusters sub-survey will image 314 deg2 in JHK to a depth of K=18.7 with sub-arcsecond resolution (Lawrence et al. 2007). These data will clearly provide a powerful tool for studies of the PMS population.

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Figure 10. Near-infrared variables in and near L 1641, as determined from an analysis of multi-epoch 2MASS data. Plotted are stars of variablility index 0.55 and higher, as described in Carpenter, Hillenbrand & Skrutskie (2001). Dashed lines show the boundaries of the variability survey.

4.4. Mid-to-Far-Infrared imaging Strom et al. (1989) investigated 123 sources identified in the IRAS Point Source Catalog and located within the boundaries of the L 1641 molecular cloud. They compiled existing and new photometry in the U BV RIJHKL bands as well as the IRAS 12–100 µm fluxes. Of the 123 IRAS sources examined, 107 appeared to be associated with stars, 93 of which appear to be members of a young population located near the surface of the molecular cloud or embedded within it. These sources are plotted in Figure 11. Chen, Tokunaga & Fukui (1993) analyzed co–added IRAS data and identified 224 sources (98 point and 126 extended), 122 on the cloud, and 102 outside the cloud. Of the 123 sources in the IRAS PSC presented by Strom et al. (1989), 106 met their selection criteria. Based on an analysis of IRAS color temperature and flux ratios, and on correspondence with 13 CO column density, Chen, Tokunaga & Fukui (1993) concluded that the sources inside L 1641 (mostly point sources) are embedded YSO, while the sources outside the cloud (mostly extended objects) are probably infrared cirrus heated by the interstellar radiation field.

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Figure 11.

IRAS sources from Strom et al. (1989).

ISOCAM broadband and CVF observations of L 1641–North were made by Ali & Noriega-Crespo (2004). They detected a total of 34 sources in the 7.65 x 8.40 arcminute region covered by the broadband filters LW2 (6.7 µm) and LW3 (14 µm). In addition they obtained low-resolution spectra (R = λ/δλ ∼ 40) for the seven brightest sources within the central 3.2 x 3.2 arcminute portion of the field. One notable conclusion from Ali & Noriega-Crespo (2004) was that their source number 10, located at α2000 = 05h 36m 24.6s , δ2000 = −06d 22m 41s has brightened by ∼200 mJy between the epochs of the IRAS and ISO observations, from the point source detection limit of 0.3 Jy for IRAS to 481 mJy, measured by the ISO CVF at 12 µm. More recently, Megeath et al. (2008) surveyed L 1641 (along with most of the Orion clouds) with Spitzer using IRAC and MIPS. Using the Spitzer data in combination with 2MASS JHK, they identified 135 protostars and 632 probable Class II stars (Figure 12). At the time of writing, analysis of the Spitzer data is still in progress (Megeath et al. 2008), however the mosaicked images from IRAC and MIPS provide a glimpse of the rich scientific harvest to come. Figure 13 shows the entirety of L 1641 in a combined IRAC+MIPS false color image. Regions around L 1641–North and V380 (upper right), L 1641C (center) and L 1641–South plus HBC 498 (lower left), are outlined in white and shown (expanded) in Figures 14, 15, and 16.

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Figure 12. Stars with infrared excess,as determined from Spitzer/IRAC data at 3.5-8.0 µm, or by a combination of 2MASS H and K bands and Spitzer 3.5, 4.5µm data. From data presented in Megeath et al. (2008).

4.5. Radio Continuum Morgan, Snell, & Strom (1990) surveyed L 1641 in the radio continuum at 6 and 21 cm, identifying 67 objects. Four sources were identified as low-luminosity YSO and have no optical counterparts. Three sources were identified as HH or “HH-like” objects.

5. Clustering in L 1641 While L 1641 lacks any rich cluster comparable to the ONC, it does contain several distinguishable groups or aggregates of young stars (Gomez & Lada, 1998). The most prominent of these are the L 1641 North cluster and the group around HBC 498 (aka the Cohen–Kuhi group). Strom, Strom & Merrill (1993) identified six other aggregates containing five or more young stars. All but one of these (L 1641 South) contain at least one IRAS source.

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Figure 13. False color image (red=24 µm, green=5.8 µm, blue=3.6 µm) from Spitzer of L 1641. Expanded views of outlined regions are shown in Section 5. From data presented in Megeath et al. (2008).

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Figure 14. North (upper) and V380 (lower) regions are shown in this expanded view of the box in upper right of Fig. 13. These two aggregates are rich in molecular outflows, with aproximately a dozen between them (Section 6), and in embedded sources (Section 5). In this false color image, red=24 µm, green=5.8 µm, and blue=3.6 µm. From data presented in Megeath et al. (2008).

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Figure 15. L 1641-C, corresponding to the center boxed region in Fig. 13. L 1641C takes its name from the prominent molecular outflow in that region. In this false color image, red=24 µm, green=5.8 µm, and blue=3.6 µm. From data presented in Megeath et al. (2008).

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Figure 16. Prominent regions in the southern end of the cloud (lower box in Fig. 13), from left to right: L 1641-South, the DL Ori group, and the HBC 498 group. In this false color image, red=24 µm, green=5.8 µm, and blue=3.6 µm. From data presented in Megeath et al. (2008).

5.1. L 1641 North L 1641 North contains several IRAS sources and approximately 40 young stars, as evidenced by their near-IR excess emission and optical emission lines. This group takes its name from the molecular outflow associated with IRAS 05338-0624 and discovered by Fukui et al. (1986), whose initial CO (J=1-0) observations were followed with higher–resolution maps in CO, HCO+ and HCN to study the distribution of dense gas (Fukui et al. 1988). Wilking, Blackwell & Mundy (1990) and Morgan & Bally (1991) presented additional CO observations. Detailed information on the L 1641 N outflow and associated HH objects, is presented in Section 6.5. The group has been extensively observed in the near-IR. Chen et al. (1993) imaged a 4 x 4 arcminute region centered on the L 1641-N outflow in the B, V, R, I, H, and K ′ photometric bands, and sub-regions in nbL and M . They identified 34 infrared sources, of which most were found to be young stars based on their spectral energy distributions. A larger scale near-IR survey (JHK) was conducted by Strom, Strom & Merrill (1993), who imaged a 26 x 22 arcminute area centered about an arcminute south of the Chen et al.’s center. Their analysis, based largely on the J-band luminosity function, concluded that L 1641 North contains ∼43 young members concentrated in an ”aggregate” and having an average surface density of 124 stars pc−2 , surrounded by a lower–density ”distributed” population of young stars having an average surface density of 24 stars pc−2 . Hodapp & Deane (1993) used K ′ and I-band polarization maps to identify sources with localized reflection nebulosity and to map the projected magnetic field, finding evidence for flattened gas and dust features oriented orthogonally to the direction of the magnetic field. They also present K-band spectra of 26 stars, and based on their median age of ∼ 3 × 105 yr (as indicated by their position in the H-R diagram), argue for a relatively short history (106 yr) of star formation in the region. Additional spectra were acquired in the R-band by Allen (1996), who placed ∼100 stars in the H-R diagram (including those of Hodapp & Deane 1993). Figure 17 shows the H-R diagram for L 1641 North, along with three other clustered regions in the cloud.

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Figure 17. H-R diagrams for four regions in L 1641, from the R-band spectroscopic sample of Allen (1996). Filled symbols represent stars with near-infrared excess; open symbols those without. Triangles in L 1641 North are from the Kband spectroscopy of Hodapp & Deane (1993). The group labelled ”C–K” is the HBC 498 group (aka ”Cohen–Kuhi” group).

Observations at longer wavelengths have targeted the outflow sources. The IRAS source 05338-0624 was detected in the radio continuum at 2.7 mm (Wilking et al. 1989), but not at 6 cm (Morgan, Snell & Strom 1990). Xiang & Turner (1995) discovered an H2 O maser associated with this source. Anglada et al. (1997) mapped a region around this source in NH3 and discovered two peaks; one associated with IRAS 05338-0624 and the other near IRAS 05339-0626, three arcminutes to the south. 5.2. V380 Ori group The group associated with V380 Ori is perhaps best known for the spectacular outflow HH 1/2 and its driving source VLA 1 (see Section 6.1). V380 Ori itself (IRAS 05339−0644) is a Herbig Ae/Be intermediate–mass emission–line star (see Section 8.1), of spectral type A0 or A1 (Allen 1995). It drives the outflow associated with the HH 35 knots (see Table 4). Also of note in this small aggregate are the H2 O maser source 05338−0647 (Pravdo et al. 1985), IRAS 05339−0646 (KMS 31; Strom et al. (1989)), and HH 3. Disk masses were measured by Reipurth et al. (1993) for three objects in this group: 4.3 M⊙ for VLA 1 and 0.44 M⊙ for V380 Ori. They obtained no detection on the HH 3 pointing.

21 5.3. HBC 498 group This partially embedded group was first described by Cohen & Kuhi (1979), who ˚ and near-infrared photometric performed an optical spectrophotometric (∆λ = 7A) (JHK) study of the five stars associated with reflection nebulosities. Due to its proximity to DL Ori (05h 41m 25.35s , −08d 05m 54.9s ), this is often referred to as ”the DL Ori group” and Cohen and Kuhi (1979) designated the sources ”DL Ori/G1” through ”DL Ori/G5”. With the exception of DL Ori/G2, all five objects in the Cohen & Kuhi study are in the Herbig–Bell Catalogue of emission–line stars, including the eponymous HBC 498, which has been associated with both the IRAS point source 05390-0807 (Strom, Margulis, & Strom 1989) and with the X-ray RASS source RXJ0540.8-0806 (Alcala et al. 1996; 2000). Another IRAS source, 05384-0808 (KMS 85), was detected by Strom et al. (1989b) at H and K. Their coordinates for the near-IR source do not match any of those of the DL Ori/G1-G5 group. Table 1 gives coordinates and, where possible, corresponding source names from other surveys for the objects in the original C-K survey. Table 1.

Sources in the HBC 498 Group

C-K designation

HBC designation

RA (J2000)

Dec (J2000)

GGD7 designation

Refs

DL Ori/G1 DL Ori/G2 DL Ori/G3 DL Ori/G4 DL Ori/G5 ——-

HBC 498 —HBC 497 HBC 495 HBC 496 ——-

5:40:48.04 5:40:51.39 5:40:47.01 5:40:46.15 5:40:46.76 5:40:46.11 5:40:48.62

−8:05:57.9 −8:02:53.4 −8:07:31.8 −8:05:23.7 −8:04:54.8 −8:07:30.7 −8:06:57.9

IRS3 IRS1 IRS4 IRS2 —IRS5 —-

1–6 1,2,4 1–4 1–4 1–3 4 5

References: (1) Cohen & Kuhi (1979), (2) Chavarria-K et al. (2005), (3) Herbig & Bell (1988) (4) Carballo et al. (1988), (5) Strom et al. (1989), (6) Alcala et al. (1996)

5.4. L 1641 South cluster Reported by Strom, Strom & Merrill (1993), this relatively populous (N∼130) cluster surrounds the Herbig Ae/Be B4 V star HD 38023 (Racine 1968). There is a small amount of nebulosity (vdB 55) associated with this star (van den Bergh 1966). Based on its H-R diagram (Figure 17), this group appears to be slightly older than the other clusters discussed here. A somewhat older age for L 1641-South is consistent with its appearance, showing no evidence for deeply embedded sources. Just south of this group is the molecular outflow L 1641-S2, listed in Table 5. 6. Prominent Herbig-Haro and H2 Flows in L 1641 L 1641 is home to an abundance of well-known Herbig-Haro (HH) objects and H2 jets. We list some of the more spectacular and best-studied flows in Table 2, and describe them in detail below (a complete list of HH objects is given in Table 3). The flows in

L 1641

22

the OMC 2/3 region, north of δ = −05d 30m ; J2000, are discussed in the chapter by Peterson & Megeath. Table 2.

Prominent Herbig-Haro and H2 outflows in L 1641

HH or H2 designation∗

Driving source

HH 1/2 HH 34/86/88 HH 38/43/64 HH 83/84 HH 303-310 HH 403/404 SMZ 59 SMZ 72 SMZ 76 DFS 123

VLA-1 HH 34-IRS HH 43MMS1 HH 83-IRS L 1641-N L 1641-N V380 Ori-NE Haro 4-255FIR L 1641 S3 IRS IRS 2035

RA (2000)

Dec (2000)

Source ( L⊙ )

Spec. Class

Jet Len. (pc)

Refs

5:36:22.8 5:35:29.8 5:37:57.5 5:33:32.6 5:36:18.1 5:36:18.1 5:36:36.1 5:39:19.6 5:39:55.1 5:37:17.1

−6:46:06 −6:26:58 −7:07:00 −6:29:45 −6:22:10 −6:22:10 −6:38:53 −7:26:19 −7:30:27 −6:49:49

50 45

0 I 0 II

5.9 3.0 1.4 1.5 12.8 10.6 0.3 0.16 2.7 1.0

1,2,3 4,5 6,7 8,9 10,11,7 10,11,7 12 13 7 14

10

13 70

0/I 0 0 0/I

∗ SMZ

and DFS refer to the H2 flow nomenclature of Stanke, McCauchrean & Zinnecker (2002) and Davis et al. (2008), respectively. References: (1) Bally et al. (2002), (2) Eislöffel, Mundt & Böhm (1994), (3) Ogura (1995) (4) Reipurth et al. (2002), (5) Devine et al. (1997), (6) Eislöffel & Mundt (1997), (7) Stanke, McCauchrean & Zinnecker (2000), (8) Reipurth, Bally & Devine (1997b), (9) Reipurth et al. (2000b), (10) Reipurth, Devine & Bally (1998), (11) Mader et al. (1999), (12) Davis et al. (2000b), (13) Stanke, McCauchrean & Zinnecker (2002), (14) Davis et al. (2008). 6.1. HH 1/2 Due to their brightness, HH 1 and HH 2 (Figure 18) have been the subject of detailed studies at optical (Mundt, Brugel & Bührke 1987; Solf, Böhm, & Raga 1988; Böhm & Solf 1992; Eislöffel, Mundt & Böhm 1994; Riera et al. 2005), infrared (Davis et al. 1994; Noriega-Crespo & Garnavich 1994; Gredel 1996; Davis, Smith & Eislöffel 2000a; Eislöffel, Smith & Davis 2000) and millimeter (Chernin & Masson 1995; Correia et al. 1997; Torrelles et al. 1994; Moro-Mart´ın et al. 1999) wavelengths. Highspatial-resolution imaging with HST detail the complex morphology and excitation structure, along the jet that points towards HH 1, as well as in the cluster of clumpy bows that comprise HH 1 and HH 2 (Ray et al. 1996; Hester, Stapelfeldt & Scowen 1998; Bally et al. 2002; Reipurth et al. 2000a,b). Spectroscopy of the HH objects indicates that the flow lies close to the plane of the sky (Choe et al. 1985; Raga & Noriega-Crespo 1998; Solf et al. 1991), while proper motion studies show that HH 1 and 2 are moving away from their source at speeds of 200-400 km s−1 (Herbig & Jones 1981; Eislöffel, Mundt & Böhm 1994; Bally et al. 2002). However, HH 1 and HH 2 (separated by 0.34 pc) may only be inner working surfaces within a much larger flow, the full extent of which is traced by the huge bow shocks HH 401 and HH 402, which Ogura (1995) identify in large-scale optical images. The embedded source of HH 1/2, VLA 1 (Lbol ∼ 50 L⊙ ) is undetected at optical and near-IR wavelengths. First observed at centimeter wavelengths by Pravdo et al.

23

HH 1

HH 2

Figure 18. Optical HST images of HH 1 and its jet (top), and HH 2 (bottom), with Hα and [S II] emission colored turquoise and red, respectively (courtesy of John Bally).

24

L 1641

(1985), the source, jet and HH 1/2 objects have since been studied at 6 cm and 3.6 cm by Rodr´ıguez et al. (2000). A neighboring radio source, VLA 2, drives the westerly HH 144/145 outflow (Reipurth et al. 1993a). Cernicharo et al. (2000) also present ISO observations of the source region. The collimated knotty jet from VLA 1 (Strom et al. 1985) is very much an archetypal flow, and consequently has been analysed spectroscopically by a number of groups (Choe et al. 1985; Solf et al. 1991; Noriega-Crespo et al. 1997; Nisini et al. 2005). Nisini et al. present complete flux-calibrated spectra covering the spectral range from 0.6–2.5 µm. Different zones of excitation are resolved along the jet. The mass flux of atomic and molecular jet components are also distinguished, the mass of the former (∼2.2×10−7 M⊙ yr−1 ) being two orders of magnitude higher than the latter. The HH bow shocks have also been used to test shock models (Hartmann & Raymond 1984; Hartigan, Raymond & Hartmann 1987; Raga et al. 1988a). HH 1/2 have been observed at ultraviolet wavelengths (Böhm et al. 1987; Raymond, Hartigan & Hartman 1988; Raymond, Blair & Long 1997); HH 2 is also a source of X-rays (Pravdo et al. 2001), implying shocked regions with temperatures exceeding 106 K. Far-IR observations of HH 1/2 reveal pure-rotational H2 emission from gas at ∼600 K, as well as [C II] 158 µm emission from a PDR associated with the HH shocks (Molinari & Noriega-Crespo 2002). Giannini et al. (2001) also report rotational emission from CO, H2 O and OH, which they attribute to non-dissociative shock fronts, while Lefloch et al. (2003) observe ionised neon in HH 2. Finally, the influence of HH 2 on the ambient gas directly downstream has been studied at a number of wavelengths (Davis, Dent & Bell Burnell 1990; Torrelles et al. 1992; Dent et al. 2003; Girart et al. 2005; Lefloch et al. 2005; Viti, Girart & Hatchell 2006).

6.2. HH 34 HH 34 comprises a collimated knotty jet and bow-shock, with a faint bow HH 34N in the counter-flow direction (Reipurth et al. 1986; Raga & Mateo 1988b; Bührke, Mundt & Ray 1988, see Figure 19). HH 34 may in fact be part of a parsec-scale flow that includes HH 173/86/87/88 to the south and HH 126/85/40/33 to the north (Bally & Devine 1994; Reipurth et al. 1997a; Eislöffel & Mundt 1997; Devine et al. 1997), although Davis et al. (2008) have recently identified a protostar coincident with HH 40, which may account for at least the HH 33/40 complex (Figure 20). Masciadri et al. (2002) have recently modelled the dynamics of such a giant flow. HH 34 is driven by a low mass Class I object (45 L⊙ ) which may be a binary (Cohen & Schwartz 1987; Reipurth et al. 2002). HH 34-IRS coincides with a cold, dense core (Reipurth et al. 1993b; Dent et al. 1998) and a very compact reflection nebula (Reipurth et al. 2000b). The source has also been detected with the VLA (Rodr´ıguez & Reipurth 1996). The jet itself has been imaged in the optical by many groups (e.g. Reipurth et al. 1986; Mundt, Brugel & Bührke 1987; Raga, Mundt & Ray 1991; Reipurth et al. 2002); proper motions are reported by Heathcote & Reipurth (1992) and Eislöffel & Mundt (1992). HST observations are presented by Ray et al. (1996) and Reipurth et al. (2002). The jet is rather faint in H2 2.12 µm emission (Stanke, McCauchrean & Zinnecker 2002; Davis et al. 2008), although it is much brighter in [FeII] 1.644 µm emission (Reipurth et al. 2000b; Stanke, McCauchrean & Zinnecker 2002; Davis et al.

25

Figure 19. An optical image showing the knotty jet that drives the southerly HH 34 bow shock. The bow in the counterflow, HH 34-N, is evident to the north. NTT data courtesy of Bo Reipurth.

26

L 1641

2003). Optical spectroscopy of the jet is presented by Heathcote & Reipurth (1992); similar data are analysed in detail by Bacciotti & Eislöffel (1999). Infrared line emission from the southern, blue-shifted jet has been traced to within a few 10s of AU of the driving source (Davis et al. 2001, 2003; Takami et al. 2006). At near-IR wavelengths, [FeII] and H2 emission lines trace a fast, high-excitation, collimated jet and a slow, low-excitation, molecular wind, respectively. Further downwind, Beck et al. (2007) present optical IFU spectra of the flow, while Podio et al. (2006) combine optical and infrared long-slit spectra to probe density and temperature stratification behind the shock fronts in the jet (see also Bacciotti & Eislöffel 1999). In comparison to optical lines, Podio et al. find that the [FeII] H-band forbidden emission lines derive from regions of higher electron density though lower electron temperature. They trace an even denser component in calcium lines. HH 34 itself is very much a textbook example of a jet working surface, exhibiting both a bow shock and Mach disk (Reipurth & Heathcote 1992; Morse et al. 1992). The bow likely sweeps up and entrains ambient gas to form the weak molecular outflow mapped in CO by Chernin & Masson (1995). 6.3. HH 38/43/64 CCD images of HH 38/43 were first presented by Strom et al. (1986) and later by Eislöffel & Mundt (1997); HH 64 was found by Reipurth & Graham (1988). HH 38 and HH 43 were initially thought to be driven by an embedded IRAS source, HH 43-IRS (Cohen & Schwartz 1983; Cohen, Harvey & Schwartz 1985), which was later resolved into a double star (Gredel 1994; Moneti & Reipurth 1995). However, deep infrared images now show that this is not the case. The HH 43 flow is instead driven by a more deeply embedded Class 0/1.3 mm source, HH 43-MMS1 (Stanke, McCauchrean & Zinnecker 2000), that is situated midway between HH 43 and HH 64 and is associated with an ammonia core (Anglada et al. 1989). Even so, HH 43-IRS may also drive a small flow that runs parallel with HH 38/43/64 (Stanke, McCauchrean & Zinnecker 2002; Davis et al. 2008). Optical spectra of HH 43 have been presented by Böhm & Solf (1990), BeckWinchatz, Böhm & Noriega-Crespo (1994) and Schwartz & Greene (1999); UV observations are discussed by Böhm, Scott & Solf (1991) and Moro-Mart´ın et al. (1996). HH 38 and particularly HH 43 have also been the subject of spectroscopic studies at infrared wavelengths. Gredel (1994), Giannini et al. (2002) and Caratti o Garatti et al. (2006) present 1-2.5 µm spectroscopy, while in a series of papers Schwartz et al. have studied the kinematics of the H2 emission from these objects (Schwartz et al. 1995; Schwartz & Greene 1999, 2003). Schwartz et al. find that this flow lies very close to the plane of the sky. This would explain why searches have so far failed to detect high-velocity CO emission from this striking jet (e.g. Edwards & Snell 1984; Morgan et al. 1991) 6.4. HH 83/84 HH 83 is a collimated jet associated with a bright, conical reflection nebula. The jet probably drives the distant HH 84 bow shock to form a parsec-scale flow (Reipurth 1989; Mundt, Ray & Raga 1991; Ogura & Walsh 1991; Reipurth, Bally & Devine 1997b; Reipurth et al. 2000b). The central source, a 10 L⊙ Class II young stellar object, has been studied at infrared (Reipurth 1989; Moneti & Reipurth 1995) and centimeter (Rodr´ıguez & Reipurth

27

30"

DFS123

SMZ68 SMZ58 SMZ67

HH43 HH34−IRS

HH33/40

DFS122 SMZ42 V380 Ori NE

HH65

SMZ76 (L1641−S3)

Figure 20. H2 2.122 µm thumb-nails of some of the molecular outflows in L 1641 (Davis et al. 2008). Contours of 1300 µm dust continuum emission are superimposed (from Stanke & Williams 2007); the circles mark the positions of Spitzer– identified protostars (from Megeath et al. 2008).

28

L 1641

1998) wavelengths. Additional photometry is presented by Reipurth et al. (1993b) and Dent et al. (1998). Nakano et al. (1994) report the presence of a rotating 7000 AU, 0.4 M⊙ circumstellar disk orientated perpendicular to the jet axis. Early optical spectra were discussed by Ogura & Walsh (1991); a more recent analysis is given by Hovhannessian et al. (2004). A detailed optical/near-IR spectroscopic analysis of the HH 83 jet is given by Podio et al. (2006). Bally, Castets & Duvert (1994b) identify only a very slow, poorly collimated molecular flow from HH 83-IRS, and the jet is undetected in H2 emission (Reipurth et al. 2000b; Davis et al. 2008). Both factors are consistent with HH 83 being in a later stage of evolution.

6.5. HH 303-310 and HH 403/404 (L 1641-N) The L 1641-N core, centered on IRAS 05338-0624, contains an embedded cluster of about 20-25 stars (Strom, Margulis, & Strom 1989; Chen et al. 1993; Hodapp & Deane 1993, see also Figure 21). A north-south bipolar outflow centered on the cluster was first mapped in CO by Fukui et al. (1988) and Wilking et al. (1990). This flow is probably associated with the parsec-scale system of HH objects, HH 61/62/303/310 (Reipurth, Devine & Bally 1998; Mader et al. 1999). HH 303 is a chain of knots stretching 1′ -2′ northward from L 1641-N. The flow continues north to include bows of increasing size; HH 306-309. HH 310 is a cluster of bow-shaped objects which may represent the northern end of this giant flow. The counter-lobe of HH 303-310 is prominent in H2 2.122 µm emission (Stanke, McCauchrean & Zinnecker 2000, 2002; Davis et al. 2008), although HH 61 and 62 (discovered by Reipurth & Graham 1988) may mark the southern terminus of the flow. At 6.3 pc, the northern lobe of the HH 303/310 outflow is arguably the longest known flow lobe from a low mass young star. Its dynamic age of ∼ 104 yrs is a sizable fraction of the expected lifetime of its protostellar source. If one includes the southern lobe - HH 60/61 and SMZ 49 in the nomenclature of Stanke et al. - the total flow length measures almost 13 pc. The large, knotty bow shocks HH 403/404 (Mader et al. 1999) to the northeast of L 1641-N and the faint knots that comprise HH 127 (Reipurth 1985; Ogura & Walsh 1991) to the southwest probably delineate a second giant flow from L 1641-N. The bright knots HH 301 and 302 are part of a much shorter flow from L 1641-N. Additional flows in the vicinity of the IR cluster include the collimated 0.6 pc long jet HH 305, as well as HH objects HH 296-299, HH 304 and HH 316 (Reipurth, Devine & Bally 1998; Mader et al. 1999). Stanke et al. (2000,2002) have mapped this region in H2 emission. They identify four outflows (SMZ 50-54) emanating from the L 1641-N core. G˚alfalk & Olofsson (2007) use proper motions and Spitzer observations to identify the emdedded sources of these flows (see also Davis et al. 2008), and to establish the direction of propagation of each feature (Figure 21). They associate the easterly, blue-shifted jet (51 in Stanke et al. 2002) from source 172 with HH 301 and HH 302. Notably, source 172 may also have brightened considerably over the last decade (G˚alfalk & Olofsson 2007). The centre of L 1641-N harbours seven cm radio sources (Anglada et al. 1998) and at least four dominant mid-IR sources (Ali & Noriega-Crespo 2004; G˚alfalk & Olofsson 2007); the 24µm sources labelled 115 and 116 in Figure 21 are associated with millimeter sources MM 1 and MM 3. The latter likely drives the giant north-south flow (HH 303-310 and H2 jet SMZ 49)

29

Figure 21. Colour Ks /H2 /I-band near-IR image of L 1641-N, with arrows indicating the proper motions of H2 emission features (′ x′ denotes no detected motion). Spitzer 24 µm contours are over-plotted revealing the most embedded sources. See G˚alfalk & Olofsson (2007) for details.

30

L 1641

Stanke & Williams (2007) have used 1.3 mm single-dish and interferometric CO 2-1 maps to further disentangled the protostars and outflows in L 1641-N (see also McMullin et al. 1994). They identify four possible CO outflows: a giant north-south bipolar flow associated with HH 303-310 (the flow first mapped by Fukui et al. 1988), a northeast-southwest flow associated with the dominant millimeter source MM 1, and a small bipolar flow driven by star 145 in Figure 21 (seen also in H2 emission; Davis et al. 2008). The fourth flow is associated with the east-west HH 301/302 flow from L 1641N-172; Stanke & Williams suggest that the red lobe of this outflow has been deflected in a southerly direction, although this distinctive CO lobe has no optical nor near-IR counterpart. 6.6. V380 Ori NE This bipolar H2 flow is situated ∼ 5′ to the northeast of V380 Ori in NGC 1999 (Figure 20). The lobes of the flow are clearly defined in (sub)millimeter CO maps (Morgan et al. 1991; Davis et al. 2000b). The flow is driven by an embedded low-mass “Class I′′ protostar (Zavagno et al. 1997). Bright, curving arcs of H2 emission are observed in both outflow lobes (Davis et al. 2000b; Stanke, McCauchrean & Zinnecker 2002). These molecular shock fronts are closely tied to peaks in the bipolar CO outflow. On morphological grounds, they are also indicative of precession. Finally, V380 Ori NE is one of a select number of bipolar flows that exhibit sub-millimeter SiO emission (Gibb et al. 2004), which is consistent with the extreme youth and embedded nature of this source. 6.7. Haro 4-255 FIR, HH 469/470 The region includes two orthogonal flows emanating from a reflection nebula close to an Hα emission-line star, Haro 4-255. Haro 4-255 itself drives a compact HH jet and knotty HH object, HH 470, that is seen also as a faint H2 bow shock in the nearIR (Stanke, McCauchrean & Zinnecker 2002). A neighboring far-IR source, Haro 4255 FIR (L = 13 L⊙ ; Evans, Levreault & Harvey 1986) powers the prominent HH/H2 jet and bow shock HH 469/SMZ 72 which extends towards the east-northeast (Davis & Eislöffel 1995; Aspin & Reipurth 2000; Stanke, McCauchrean & Zinnecker 2002). Stanke, McCauchrean & Zinnecker (2002) discovered distant H2 knots which may be associated with SMZ 72; the total length of the flow may be as large as 2 pc. Highvelocity molecular line emission has been mapped by Levreault (1988) and Morgan & Bally (1991). The FIR source also coincides with an NH3 core (Anglada et al. 1989), and is only 16′′ north of a VLA 3.6 cm continuum peak (Anglada et al. 1992, 1998). 6.8. L 1641-S3/SMZ 76 The wide-field H2 images of Stanke et al. (2000,2002) reveal a huge, curving, molecular flow in the southern regions of L 1641 which they label SMZ 76. HH 65 represents just a small part of SMZ 76 (Reipurth & Graham 1988). Although located close to the Re 50 and Re 50 N reflection nebulae and the luminous, FU Ori-type IRAS source 05380-0728 (Reipurth & Bally 1986; Strom et al. 1993), these are probably unrelated to the outflow. Instead, SMZ 76 seems to be driven by a second IRAS source, 05375-0731, situated ∼9′ west-southwest of Re 50 N (Stanke, McCauchrean & Zinnecker 2000). A possible counter-lobe has recently been identified by Davis et al. (2008). Far-IR and submillimeter observations of IRAS 05375-0731 are presented by Zavagno et al. (1997); Wouterloot et al. (1989) also find an NH3 core associated with this

31 source. Infrared images of the source region are presented by Chen & Tokunaga (1994), where the source is labelled L 1641-S3 IRS. Stanke, McCauchrean & Zinnecker (2000) obtained 1.3 mm continuum observation of L 1641-S3 IRS and infer a luminosity of ∼70 L⊙ for this Class 0 type object. Maps of high-velocity CO are presented by Reipurth & Bally (1986) and Morgan & Bally (1991). The high-resolution CO maps of Lee et al. (2002), which are centred on Re 50 and Re 50N, are shown here in Figure 22. 6.9. HH 131 HH 131 is a perculiar, helical object that lies ∼14′ to the west of L 1641-S. Discovered by Ogura & Walsh (1991), it is somewhat unusual in that it has no associated molecular cloud, but rather may be associated with a cloud of atomic gas. There is no obvious driving source in the IRAS Point Source catalogue, and HH 131 lies beyond the bounds of most surveys of PMS stars in Orion. HH 131 is also associated with large bow shocks, the morphologies of which suggest an origin from within the L 1641 molecular ridge (Wang et al. 2005). 6.10. HH 999 HH 999 are bright Hα bow shocks associated with a remarkable molecular outflow traced in H2 1-0S(1) and CO J=1-0 line emission by Yun et al. (2001). Situated southeast of L1641, this 0.4 pc long bipolar outflow is driven by the low-mass Class I source IRAS 06047-1117 (HH 999-IRS). K-band observations of the central source and reflection nebula have recently been published by Connelley, Reipurth & Tokunaga (2007); Integral Field Spectroscopy of HH 999-IRS are described by Davis et al. (2009). Table 3.: Herbig-Haro Objects in L 1641 (south of δ = −05d 30m ; J2000)∗ HH

RA (2000)

Dec (2000)

Outflow

H2

Note

1 2 3 33 34 35 36 38 40 43 61 62 64 65 83 84 85 86 87 88 89 127

5 36 20.4 5 36 25.5 5 36 11.7 5 35 17.9 5 35 29.9 5 36 22.5 5 36 46.6 5 38 21.8 5 35 20.7 5 38 10.8 5 36 12.8 5 36 12.9 5 37 47.8 5 40 18.9 5 33 31.6 5 34 11.4 5 35 23.9 5 35 40.3 5 35 43.2 5 35 44.0 5 37 47.7 5 35 50.0

−6 45 08 −6 47 16 −6 43 04 −6 17 42 −6 27 01 −6 41 52 −6 44 14 −7 11 38 −6 18 23 −7 09 23 −7 07 02 −7 11 02 −7 05 30 −7 25 05 −6 29 38 −6 34 00 −6 19 47 −6 36 13 −6 37 30 −6 37 55 −6 46 05 −7 00 17

HH 1/2 HH 1/2 HH 1/2? HH 33/40 HH 34 V380 Ori DFS 121 HH 38/43 HH 33/40 HH 38/43

Y Y Y Y Y Y Y Y Y Y N N Y Y N N Y Y Y Y Y N

Very bright HH/H2 bow shock Very bright cluster of HH/H2 knots Bright, compact HH/H2 knot Bright, compact HH/H2 bow shock Bright bow shocks driven by collimated jet Group of small HH/H2 knots Clumpy HH/H2 bow shock Bright bow-shaped group of HH/H2 knots Bright, elongated HH/H2 knot Bright cluster of HH/H2 knots Diffuse HH bow shock Small cluster of HH knots Part of parsec-scale HH 38/43 flow HH knot in large, curving H2 flow Reflection neb., jet and bow shock Bow shock Chain of knots; part of giant flow Bright HH/H2 knots; part of giant flow Bright HH/H2 bows; part of giant flow Bright HH/H2 knots; part of giant flow Bipolar HH/H2 jet and reflection neb. Small HH knots; counterlobe of HH 403/404

HH 38/43 L1641-S3 HH 83/84 HH 83/84 HH 34 HH 34 HH 34 HH 34 HH 89 L 1641-N

L 1641

32 HH

RA (2000)

Dec (2000)

Outflow

H2

Note

130 5 36 45.9 −6 49 56 V380 Ori N HH Bow shock 134 5 35 36.3 −6 30 18 N Small cluster of HH knots near HH 34 144 5 36 21.2 −6 46 07 HH 144 Y Faint knots west of the HH 1 jet 145 5 36 14.3 −6 46 22 N Faint group of HH knots (with HH 144?) 146 5 36 22.6 −6 48 22 N Faint HH knot SW of HH 2 147 5 36 22.8 −6 44 58 SMZ 63 Y Group of compact HH/H2 knots 148 5 36 23.3 −6 43 15 V380 Ori Y Three HH knots near V380 Ori 173 5 35 34.8 −6 33 01 HH 34 N Faint HH object in giant HH 34/86/88 flow 183 5 38 18.1 −7 02 26 V883 Ori N Single HH object near bright star 222 5 35 41.9 −6 23 03 N Large arcs or streamers near V571 Ori 292 5 36 59.8 −6 33 26 N Small bipolar HH flow 296 5 36 11.7 −6 14 23 N Faint group of HH knots near L 1641-N 297 5 36 15.7 −6 16 33 N Single faint HH knot near L 1641-N 298 5 36 21.7 −6 21 51 L 1641-N N Chain of three HH knots in L 1641-N 299 5 36 09.6 −6 20 00 SMZ 48 Y HH/H2 jet or filament near L 1641-N 301 5 36 39.0 −6 21 17 L 1641-N Y HH/H2 bow in same flow as HH 302 302 5 36 48.3 −6 20 38 L 1641-N Y HH/H2 knot in same flow as HH 301 303 5 36 19.1 −6 19 35 L 1641-N Y Collimated chain of HH knots 304 5 36 37.1 −6 14 55 L 1641-N Y Elongated chain of HH/H2 knots 305 5 36 23.4 −6 15 44 N Chain of six HH knots 306 5 36 08.1 −6 08 46 L 1641-N N Group of HH bows; part of giant N-S flow 307 5 36 07.7 −6 04 15 L 1641-N N Group of HH bows; part of giant N-S flow 308 5 36 01.1 −6 00 25 L 1641-N N Large, faint HH bow; part of giant N-S flow 309 5 36 03.1 −5 49 47 L 1641-N N Cluster of HH bows; part of giant N-S flow 310 5 35 49.4 −5 36 05 L 1641-N N Sweeping HH bows; N end of giant N-S flow 316 5 35 54.7 −6 04 54 N Curved, collimated jet near HH 307 322 5 35 13.2 −6 19 52 HH 33/40 N HH knot SW of HH 33/40 323 5 35 12.9 −6 17 42 HH 33/40 N Diffuse HH object W of HH 33/40 324 5 35 40.4 −6 18 34 SMZ 52 Y Group of HH/H2 knots east of HH 34 325 5 35 50.7 −6 29 19 N Faint knots east of HH 134 326 5 35 49.3 −6 32 04 N Very faint HH knots NE of HH 86-88 327 5 35 50.7 −6 35 29 N Small jet/filament NE of HH 86-88 330 5 35 42.0 −5 04 43 SMZ 8 Y Optical/H2 bow shock 401 5 35 12.0 −6 29 38 HH 1/2 N Huge, sweeping bow shock 402 5 37 17.6 −7 00 40 HH 1/2 N Huge, sweeping bow shock 403 5 37 03.6 −5 52 54 L 1641-N N Long, fragmented bow 404 5 37 15.8 −5 44 04 L 1641-N N Large, knotty HH bow shock 405 5 37 24.8 −5 43 36 Y Small HH/H2 jet pointing at HH 406 406 5 37 38.8 −5 39 22 N HH bow shock associated with HH 405? 407 5 35 17.3 −5 59 39 L 1641-N N Large, clumpy bow shock 449 5 38 42.8 −7 12 44 Haro4-249 N Faint chain of HH knots 469 5 39 19.0 −7 26 27 Haro4-255FIR Y HH/H2 jet and bow shock 470 5 39 28.8 −7 28 16 Haro4-255 Y HH/H2 bow shock ∗ Catalogues of H2 flows in L 1641 (SMZ and DFS sources) are given by Stanke, McCauchrean & Zinnecker (2002) and Davis et al. (2008).

33

Figure 22. From Lee et al. (2002). CO emission in the vicinity of Re 50 and Re 50N, superposed on a 2.12 m image (without continuum subtraction) provided by Stanke et al. (2000). The dotted line outlines the observed region. The beam size is 14”.5 x 10”.4. (a) The CO emission integrated from −6 to 12 km s−1 . The solid lines connected to VLA 1 indicates the possible outflow axis of the CO emission in this region. The contours start at 13 Jy beam−1 km s−1 , with a step size of 6.5 Jy beam−1 km s−1 ; (b), (c), and (d) each show a channel map averaged over 1.5 km s−1 with the center velocity indicated at the upper left-hand corner in each panel. The contours start at 0.5 Jy beam−1 , with a step size of 1.0 Jy beam−1 . The noise level is 0.3 Jy beam−1 .

34

L 1641

7. Surveying Molecular Outflows in L 1641 Although optical surveys in Hα and [SII] reveal many – if not most – of the jets and outflows in low mass star-forming regions, perhaps a truly complete survey requires observations at longer wavelengths, where extinction is less of a problem. It is therefore not surprising that Orion A has been the subject of a large number of such studies. Although wide-field imaging of the OMC 2/OMC 3 region was given by Yu et al. (1997) and Yu et al. (2000), and many of the well-known HH objects have been observed with small-scale (though often high resolution) infrared imagers (see references above), the first truly comprehensive H2 survey of outflow activity in Orion A – north and south of M 42 and throughout much of L 1641 – was given by Stanke, McCauchrean & Zinnecker (2002), building on their earlier imaging of parsec-scale flows in L 1641-N, L 1641-S3 and HH 38/43/64 (Stanke, McCauchrean & Zinnecker 2000). More recently, Davis et al. (2008) have obtained homogeneous maps of a 8 square degree field, comparing their images with the (sub)mm surveys of Nutter & Ward-Thompson (2007) and Stanke & Williams (2007), and the catalogue of protostars extracted from Spitzer observations by Megeath et al. (2008). A handful of new flows are found in regions not covered by Stanke et al. (see e.g. Figure 20). Most notable, perhaps, is the spectacular H2 jet and bow shocks DFS 123 which has no optical HH counterpart. The flow has not been mapped in (sub)millimeter molecular lines, though the source is identified in the Spitzer observations of Megeath et al. The molecular gas that constitutes L 1641 (and Orion A) has been mapped at modest spatial resolution in various transitions and isotopes of CO (Wilson et al. 1970; Kutner et al. 1977; Maddalena et al. 1986; Bally et al. 1987; Sakamoto et al. 1997; Nagahama et al. 1998; Ikeda et al. 1999; Wilson et al. 2005). Williams, Plambeck & Heyer (2003) present 10′′ resolution CO 1-0 maps of the high-velocity gas in OMC 2/OMC 3, and are able to associate may of the CO flows in the region with H2 jets and bow shocks, as well as the embedded far-infrared/millimeter sources from Chini et al. (1997) and Yu et al. (1997). A similar survey of outflows around L 1641-N is presented by Stanke & Williams (2007). For lower-resolution CO maps of OMC 2/3 see also Yu et al. (2000) and Aso et al. (2000). A catalogue of CO outflows is given by Wu et al. (2004). In Table 4 we list the flows found in L 1641.

8. Individual Objects of Interest 8.1. V380 Ori V380 Ori is a Herbig AeBe star, it lies in a dense globule and illuminates a bright optical reflection nebula (Figure 23). It is associated with a “loop′′ of HH emission and is probably powering HH 35 (Reipurth 1989; Corcoran & Ray 1994). Molecular outflows have also been observed in the vicinity of V380 Ori (Levreault 1988; Morgan et al. 1991). V380 Ori is a binary of separation 0.154′′ with both components qualifying as intermediate-mass stars (Leinert et al. 1997; Smith et al. 2005; Baines et al. 2006). The binary is apparently surrounded by a geometrically thin, optically thick accretion disk (Hillenbrand et al. 1992; Shevchenko 1999); Hillenbrand et al. propose a high mass accretion rate of 5 × 10−6 M⊙ y−1 . Optical spectra suggest a combined type

35 Table 4. Molecular outflows in L 1641 (south of δ = −05d 30m ; J2000), from the catalogue of Wu et al. (2004). Associated HH objects or H2 flows are given in the last two columns (SMZ and DFS H2 sources are listed in Stanke et al. 2002 and Davis et al. 2008, respectively). Num

Name

RA (2000)

Dec (2000)

072 073 083 085 086 087 088 089 090 091 092 093 094 101 102 105 106 107 114

Ori A-W HH 83 HH 34 L 1641-N CS-star (HH 1) 05339-0647 VLA 3 VLA 2 HH 1/2 VLA 1 05339-0646 V380 Ori V380 Ori NE V380 Ori S L 1641-C Haro 4-255 L 1641-S3 L 1641-S L 1641-S4 L 1641-S2

5 32 42 5 33 32 5 35 30 5 36 19 5 36 21 5 36 23 5 36 23 5 36 23 5 36 23 5 36 24 5 36 26 5 36 36 5 36 26 5 38 45 5 39 22 5 39 56 5 40 28 5 40 49 5 42 47

−5 35 48 −6 29 44 −6 26 56 −6 22 13 −6 45 35 −6 44 57 −6 45 22 −6 44 58 −6 46 07 −6 44 45 −6 42 38 −6 39 12 −6 54 12 −7 01 03 −7 26 45 −7 30 26 −7 27 28 −8 06 51 −8 17 05

HH

H2 DFS 131

HH 83 HH 34 HH 303/310 HH 147? HH 147? HH 147? HH 1/2 HH 147? HH 35

HH 469/470 HH 65 HH 65?

SMZ 55/56 SMZ 49 SMZ 63 ? ? SMZ 64 ? SMZ 60 SMZ 59 SMZ 66 SMZ 72/73 SMZ 76 SMZ 76?

of late B (Strom et al. 1972; Rossi et al. 1999). Complete UV, optical and near-IR spectroscopy and photometry are presented by Rossi et al. (1999); they confirm the presence of a strong IR excess from circumstellar dust with strong permitted emission lines superimposed. In the 3µm spectroscopy of Acke & van den Ancker (2006) the Pfund series is visible, though PAH and nanodiamond features are not detected. Carmona et al. (2005) also failed to detect CO fundamental ro-vibrational emission at 4.7µm. V380 Ori is, however, a strong X-ray source (Zinnecker & Preibisch 1994; Stelzer et al. 2006), further indication that it drives a dense, high-velocity wind. 8.2. V883 Ori (IC 430, Haro 13a) Discovered on photographic plates in the late 19th Century, V883 Ori is a luminous (Lbol ∼ 230 L⊙ ) though embedded IR source with a faint optical counterpart that illuminates the bright reflection nebula IC430 (Haro 1953; Allen et al. 1975). Based on optical spectroscopy, Strom et al. (1993) were the first to argue that it is an FUor type object. Reipurth & Aspin (1997) present near-IR spectra which exhibit pronounced CO absorption bands at 2.3µm, consistent with disk accretion, while Greene, Aspin, & Reipurth (2008) have since shown with high-resolution data that V883 Ori is spectroscopically identical to FU Ori. Strom et al. (1993) also note that V883 Ori has a flat SED, as is often the case with FUors.

L 1641

36

Figure 23.

HST WFPC2 image of V380 Ori and associated nebula NGC 1999.

Molinari, Liseau, & Lorenzetti (1993) have collated near- and mid-IR photometry over a 25-year period. More recently, the source has been detected in dust continuum emission at 1.3mm (Reipurth et al. 1993b) and at 850 and 450µm (Dent et al. 1998; Sandell et al. 2001). Dent et al. also present C18 O spectral line data, although a molecular outflow has not been found toward V883 Ori (Morgan et al. 1991). Ground-based N-band spectroscopy and photometry were obtained by Scheitz et al. (2005) who detect broad silicate absorption. V883 Ori is also believed to be the driving source of HH 183 (Strom et al. 1986) 8.3. Re 50 and Re 50N Re 50 is a bright, optical reflection nebula that appeared 40-50 years ago (Reipurth et al. 1986; Scarrott & Wolstencroft 1988; Casali 1991; Colomé et al. 1996); Re 50N is an infrared nebula situated ∼2′ to the north (Scarrott & Wolstencroft 1988; Casali 1991; Colomé et al. 1996; Stanke, McCauchrean & Zinnecker 2000). Re 50N is associated with one of the most luminous IRAS sources in L 1641 (IRAS 05380-0728; Lbol ∼ 230 L⊙ ; Zavagno et al. 1997). Re 50N also coincides with a 6 cm VLA source (Morgan, Snell & Strom 1990; Anglada, Rodr´ıguez, & Torrelles 1996) and is more closely aligned with the IRAS position than its optical cousin, although both features appear to be associated with a north-south bipolar molecular outflow (Fukui et al.

37 1986; Morgan et al. 1991; Lee et al. 2002). This bipolar flow is possibly associated with HH 65, although L 1641S is a rather complex region (Figure 22; see the discussion in Stanke et al. 2000). Hα profiles observed toward the reflection nebula associated with Re 50 exhibit P Cygni profiles, suggesting that the source that illuminates this nebula is an FUor (Strom et al. 1993). However, near-IR spectra of Re 50N are essentially featureless and do not include CO bandhead absorption as is expected for FUors (Reipurth & Aspin 1997). M-band spectroscopy reveals broad CO absorption at 4µm toward Re 50N (Pontoppidan et al. 2003) while ISO mid-IR spectra exhibit deep 10µm silicate absorption (Quanz et al. 2007). Both suggest the presence of a disk or flattened envelope is seen edge-on. Acknowledgements We are grateful to Thomas Stanke for his help in navigating the outflows in L 1641, Lynne Hillenbrand for summarizing some of the literature, Tom Megeath for providing Spitzer data prior to publication, Rob Gutermuth for preparing the Spitzer images shown here, Steve Rodney for technical assistance with the manuscript, and Karen Strom for generating much interest in L 1641. References Acke, B. & van den Ancker, M. E. 2006, A&A, 457, 171 Alcala, J.M.,Terranegra, L., Wichmann, R., Chavarria-K., C., Krautter, J., et al. 1996, A&AS, 119, 7A Ali, B. & Noriega-Crespo, A. 2004, ApJ, 613, 374 Allen, L. E. 1996, Ph.D. Thesis, University of Massachusetts, (http://www.astro.umass.edu/theses/allen/thesis.html) Allen D.A., Strom, K.M., Grasdalen, G.L., Strom, S.E., & Merrill, K.M. 1975, MNRAS, 173, 47P Anglada, G., Rodr´ıguez, L.F., Cantó, J., Estalella, R., & Torrelles, J.M. 1992, ApJ, 395, 494 Anglada, G., Rodr´ıguez, L.F., Torrelles, J.M., Estalella, R., Ho, P.T.P., Cantó, J., López, R. & Verdes-Montenegro, L. 1989, ApJ, 341, 208 Anglada, G., Villuendas, E., Estalella, R., Beltrán, M.T., Rodr´ıguez, L.F., Torrelles, J.M., & Curiel, S. 1998, AJ, 116, 2953 Anglada, G., Rodr´ıguez, L. F., & Torrelles, J. M. 1996, ApJ, 473, L123 Aspin C. & Reipurth, B. 2000, MNRAS, 311, 522 Aso, Y., Tatematsu, K., Sekimotot, Y., Nakano, T., Umemoto, T., Koyama, K., & Yamamoto, S. 2000, ApJS, 131, 465 Bacciotti, F. & Eislöffel, J. 1999, A&A, 342, 717 Baines, D., Oudmaijer, R. D., Porter, J. M., Pozzo, M., 2006, MNRAS, 367, 737 Bally, J., Langer, W.D., Stark, A.A., & Wilson, R.W. 1987, ApJ, 312, L45 Bally, J., Castets, A., & Duvert, G. 1994b, ApJ, 423, 310 Bally, J. & Devine, D. 1994, ApJ, 428, L65 Bally, J., Heathcote, S., Reipurth, B., Morse, J., Hartigan, P., & Schwartz, R. 2002, AJ, 123, 2627 Beck, T.L., Riera, A., Raga, A.C., & Reipurth, B. 2007, AJ, 133, 1221 Beck-Winchatz, B., Böhm, K.H., & Noriega-Crespo, A. 1994, PASP, 106, 1271 Böhm, K.-H., Bührke, T., Raga, A.C., Brugel, E.W., Witt, A.N., & Mundt, R. 1987, ApJ, 316, 349 Böhm, K.H., Scott, D.M., & Solf, J. 1991, ApJ, 371, 248 Böhm, K.H. & Solf, J. 1990, ApJ, 348, 297 Böhm, K.-H. & Solf, J. 1992, AJ, 104, 1193 Bührke, T., Mundt, R., & Ray, T.P. 1988, A&A, 200, 99

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