Abstract. A small filamentary cloud in Norma hosts a number of young low-mass ... Graham & Frogel (1985) suggested a value of about 700 pc, noting the con-.
Handbook of Star Forming Regions Vol. II c 2008 Astronomical Society of the Pacific, Bo Reipurth, ed.
Low Mass Star Formation in the Norma Cloud Bo Reipurth Institute for Astronomy, University of Hawaii 640 N. Aohoku Place, Hilo, HI 96720, USA Markus Nielbock Max-Planck-Institut f¨ur Astronomie, K¨onigstuhl 17, D-69118 Heidelberg, Germany Abstract. A small filamentary cloud in Norma hosts a number of young low-mass stars in various stages of evolution, from visible Hα emission stars to embedded sources detected only in the sub-millimeter regime. The best known source is V346 Nor, an FU Orionis star that brightened in the early 1980s. The morphology of the cloud complex and an apparent age gradient along the cloud suggests that star formation in this region was triggered by an external event.
1.
Overview
The Norma Cloud is a filamentary dark cloud at the approximate position of α(J2000) = 16h 34m , δ(J2000) = −45◦ 00′ or in Galactic coordinates l = 338.6, b = +1.9. It was first listed as two distinct dark clouds Sandqvist 187 and 188 (Sa 187/188) (Sandqvist 1977). In the recent catalog of Dobashi et al. (2005) it is listed as TGU 2076. On optical images, the Norma cloud is recognized as a dark filament ranging over 40′ in right ascension and 15′ in declination, with a characteristic “head-tail” structure. The actual areal coverage amounts to approximately 150 square-arcminutes. The Norma Cloud is a small and dense condensation with an average visual extinction of AV = 7.6 (Nielbock & Chini 2005) inside the much thinner so called Norma Dark Cloud which extends over 40◦ between the constellations of Scorpius and Centaurus. Several extinction measurements have demonstrated that the Norma Dark Cloud is located somewhere between our local and the next inner Galactic spiral arm (Ramberg 1966; Schnur 1970; Haug & Bredow 1977; Waldhausen & Marraco 1993). Nevertheless, the distance to the Norma Cloud is still quite uncertain. A first estimate of 200 pc was given by Cohen et al. (1984), who speculated that the cloud is related to the Lupus complex. Graham & Frogel (1985) suggested a value of about 700 pc, noting the considerable uncertainty of this estimate. Reipurth (1985) adopted a distance of 300 pc, and Gregorio Hetem et al. (1988) favored a nearby distance of only 150 pc. Most recently, Nielbock & Chini (2005) measured the extinction towards two stars, located at distances of approximately 390 pc and 490 pc, and found a substantial increase in extinction towards the latter, suggesting that parts of the cloud could be present already at 490 pc. All these determinations are little more than educated guesses, and for the present purposes we adopt the 700 pc distance of Graham & Frogel (1985) for no better reason than it is the most commonly employed. 1
2
Figure 1. Image of the Norma Cloud retrieved from the second edition of the Digitized Sky Survey (DSS2 red). All objects mentioned in the text are marked in the picture. The region studied by Moreira et al. (2000) is indicated by a rectangular box. The coordinates are given in R.A. and Dec. for epoch J2000.
2.
Cloud Structure
Alvarez et al. (1986) studied the global properties of the Norma Cloud by means of CO (J=1-0) mapping at a low resolution of 8.′ 8 (HPBW). Assuming a distance of 700 pc, the authors estimated the total mass of the cloud to be 500 M⊙ with a column density of N (H2 ) = 1.4 × 1021 cm−2 . Using continuum observations at 1.2 mm (250 GHz) with a beam size of 24′′ (HPBW), Nielbock & Chini (2005) found a cloud mass of 340 M⊙ and a column density of N (H2 ) = 1.4 × 1022 cm−2 , also using a 700 pc distance. For the most part, this discrepancy can be explained with the different spatial resolutions. Fig. 1 displays an image of the “Digitized Sky Survey” (DSS2 red) showing that the Norma Cloud consists of a filamentary tail in the East (Sa 188) and a more confused head in the West (Sa 187). The perturbed structure is probably caused by molecular outflows from the young stellar population. Most observations are concentrated on the western part. The wealth of sources discovered around the Herbig-Haro objects HH 56 and HH 57 is described in Sect. 3. 3.
Herbig-Haro Flows and their Energy Sources
The best known source in the Norma clouds is the FU Orionis object V346 Nor, discovered by Graham (1983). It is located in the western head of the cloud complex, see Figure 1, and is a bright near- and far-infrared (IRAS 16289–4449) source (Elias 1980; Reipurth & Wamsteker 1983; Cohen et al. 1984, 1985; Cohen & Schwartz 1987; Prusti et al. 1993). Near-infrared photometry is summarized by Molinari, Liseau, & Lorenzetti (1993) and Reipurth et al. (1997). At the assumed distance of 700 pc, V346 Nor has a luminosity of ∼135 L⊙ (Prusti et al. 1993). The FU Ori nature of V346 Nor was further documented in detailed studies by Graham & Frogel (1985) and Reipurth
3
Figure 2. The region around HH 56 and 57 seen in a CCD image obtained through a [SII] filter. North is up and east is left. From Reipurth et al. (1997).
(1985). A weak stellar absorption spectrum was seen by Cohen et al. (1986). The star is partly embedded and surrounded by a reflection nebula (Graham & Frogel 1985; Reipurth 1985; Scarrott et al. 1987). Circumstellar dust has been detected at sub-mm wavelengths by Weintraub et al. (1991), Reipurth et al. (1993) and Sandell & Weintraub (2001). This has been confirmed by mid-infrared photometry and spectroscopy by Sch¨utz et al. (2005), who also detected the silicate absorption band at 9.7 µm. The 3 µm water ice band was detected by Graham & Chen (1991), and further studied by Graham (1998). Mm observations are presented by Alvarez et al. (1986) and Nielbock & Chini (2005), and a molecular outflow has been mapped by Evans et al. (1994) and Reipurth et al. (1997). SiO emission has been detected by Codella et al. (1999). V346 Nor is associated with a compact Herbig-Haro object HH 57, discovered by Schwartz (1977) and studied spectroscopically by Schwartz & Dopita (1980) (see Figure 2) as well as Eisl¨offel et al. (2000), who interpreted their results in terms of a planar J-type shock. CCD images of the region are shown by Graham & Frogel (1985), Reipurth (1985), and Reipurth et al. (1997). A proper motion determination was attempted by Schwartz et al. (1984). As seen in Figure 2, a second Herbig-Haro object, HH 56, is found close to V346 Nor and HH 57. HH 56, discovered by Schwartz (1977), is a bright bow shock in a bipolar collimated outflow (Reipurth et al. 1997). Optical spectra are presented by Schwartz & Dopita (1980), Wilking et al. (1990) detected HH 56 in molecular hydrogen emission, and Gredel (1994) obtained infrared spectra. Schwartz, Jones, & Sirk (1984) attempted a proper motion determination. A little reflection nebula, Re 13, is illuminated by the likely energy source of HH 56 (Reipurth 1981), which is detected in the infrared (Prusti et al. 1993), at submm wavelengths (Sandell & Weintraub 2001), and at 1.3 mm Reipurth et al. (1993). Spectra of Re 13 show Hα emission (Alvarez et al. 1986; Cohen, Dopita, & Schwartz 1986). A small HH knot, HH 56N and a distant bow shock to the south, HH 56S, forms a well defined flow axis through Re 13
4 (Reipurth et al. 1997). A sub-millimeter map of the source region has been presented by Sandell & Weintraub (2001), who find that there is another object, named Re 13 SM, offset from Re 13, which appears to be a very young embedded source (Figure 3). Mm observations are presented by Alvarez et al. (1986) and Nielbock & Chini (2005), and a molecular outflow has been mapped by Evans et al. (1994) and Reipurth et al. (1997). SiO emission has been detected by Codella, Bachiller, & Reipurth (1999).
Figure 3. Deconvolved 850 µm image of the region around V346 Nor and Re 13 with 14 arcsec resolution. From Sandell & Weintraub (2001).
Table 1. List of the known Herbig-Haro objects in the Norma Cloud sorted by their declination. Name HH 56 N HH 56 HH 57 HH 56 S
4.
R.A. (J2000) h
m
s
16 32 30.8 16h 32m 29.s 3 16h 32m 31.s 9 16h 32m 18.s 6
Dec. (J2000) ◦
′
′′
–44 54 40 –44◦ 54′ 57′′ –44◦ 55′ 37′′ –44◦ 57′ 41′′
Comments knot bow shock knot bow shock
Other Star Formation
Nielbock & Chini (2005) have carried out a 1.2 mm survey of the entire Norma cloud complex. Figure 4 shows their map, scaled to match the optical image in Figure 1. They detected six cold sources, MMS 1 - 6, some coincident with sources known from other wavelength studies, and listed in Table 2. Moreira et al. (2000) found a second site of active star formation around IRAS 16295–4452 by CO and CS molecular spectroscopy. The area covered is indicated by the frame in Figs. 1 and 4. They detected a molecular core of 35 M⊙ , which
5
Figure 4. Image of the Norma Cloud obtained with SIMBA/SEST at a wavelength of 1.2 mm. The millimeter map is scaled to the DDS2 image in Fig. 1. The beam size is indicated in the lower left corner. From Nielbock & Chini (2005).
divides into three distinct millimeter continuum sources MMS 3, 4, and 5 (Nielbock & Chini 2005). MMS 4 coincides with the IRAS 16295–4452 and is probably responsible for a bipolar outflow found in the region. The emission line stars Sz 136 and Sz 137 (Schwartz 1977) are located in a ridge further to the north. Two IRAS point sources (IRAS 16305–4455, IRAS 16311–4457) are located inside the filamentary cloud tail to the East (Sa 188). IRAS 16305–4455 was detected by IRAS in the 25 µm and 60 µm filter bands only. The MSX survey detected this source in the A band (8.28 µm). Nielbock & Chini (2005) could establish a poor estimate of the 1.2 mm flux density only. Altogether, this source exhibits an observed luminosity of L = 4.2 L⊙ and a total gas mass of Mgas = 0.1 M⊙ . IRAS 16311–4457, also known as ESO Hα 284, is a weak IRAS 25 µm point source. It was detected by MSX in the A and C band (8.28 µm and 12.13 µm, respectively). Reipurth et al. (1997) found a CO outflow centered on the source. Combining all available data, Nielbock & Chini (2005) derived a luminosity of L = 9.4 L⊙ and a total gas mass of Mgas = 0.8 M⊙ . Nielbock & Chini (2005) report on a new bright millimeter source, MMS 6, located at the easternmost tip of the cloud tail (see Figure 4). The source is not detected by IRAS, but roughly coincides with a mid-infrared absorption feature in the MSX-A data equivalent to an extinction of at least AV = 145 mag. This value corresponds to a column density of Ngas = 2.7 × 1024 cm−2 . It appears that MMS 6 could be a cold Class 0 source. Judging from the apparent age spread of the known YSOs detected in the Norma Cloud so far, with more evolved stars in the cloud head to the West and the youngest sources in the tail, Nielbock & Chini (2005) speculated that the cloud might have undergone a process as proposed by Hartmann (2002) and Burkert & Hartmann (2004) explaining the formation of filamentary dark clouds by the gravitational collapse of finite sheets. On the other hand, Carlqvist & Gahm (1992) suggested Sandqvist 187
6 Table 2. List of all known young stellar sources in the Norma Cloud sorted by their declination. The first column quotes their common name, while the the last column, apart from general comments about the sources, also lists alternative names used in the literature. Name
R.A. (J2000)
Dec. (J2000) Comments and alternative names
Re 13 SM V346 Nor Re 13 Sz 137 Sz 136 MMS 3 MMS 4
16h 32m 27.s 8 16h 32m 32.s 1 16h 32m 27.s 2 16h 33m 12.s 1 16h 33m 04.s 5 16h 33m 04.s 8 16h 33m 07.s 9
-44◦ 55′ 30′′ -44◦ 55′ 31′′ -44◦ 55′ 36′′ -44◦ 56′ 14′′ -44◦ 57′ 17′′ -44◦ 58′ 19′′ -44◦ 58′ 34′′
MMS 5 IRAS 16306–4455 MMS 6 IRAS 16311–4457
16h 33m 07.s 2 16h 34m 15.s 6 16h 35m 36.s 8 16h 34m 43.s 1
-44◦ 59′ 24′′ -45◦ 01′ 27′′ -45◦ 02′ 48′′ -45◦ 03′ 39′′
MMS 1 MMS 2, FU Ori star, drives HH 57 emission line star, drives HH 56 emission line star emission line star 12 CS peak in Moreira et al. (2000) 13 CO peak in Moreira et al. (2000), IRAS 16295–4452 12 CO peak in Moreira et al. (2000) MIR absorption at 8.28 µm (MSX) ESO Hα 284
and 188 as possible examples of filamentary clouds, where plasma effects might have supported their formation. As Reipurth et al. (1997) pointed out, the initial impulse to form stars could have been triggered by an external event, possibly a supernova located to the W-N-W.
Acknowledgements. We thank Richard D. Schwartz for his careful refereeing of this chapter. We have made use of NASA’s Astrophysics Data System Bibliographic Services, the SIMBAD database operated at CDS, Strasbourg, France, and the Digitized Sky Survey. MN acknowledges the support by the Deutsche Forschungsgemeinschaft, DFG project number SFB 591/B2. BR was supported in part by the NASA Astrobiology Institute under Cooperative Agreement No. NNA04CC08A and by the NSF through grants AST-0507784 and AST-0407005. References Alvarez, H., Bronfman, L., Cohen, R., Garay, G., Graham, J., & Thaddeus, P. 1986, ApJ, 300, 756 Burkert, A. & Hartmann L. 2004, ApJ, 616, 288 Carlqvist, P. & Gahm, G. F. 1992, IEEE Transactions on Plasma Science, 20, 867 Codella, C., Bachiller, R., & Reipurth, B. 1999, A&A, 343, 585 Cohen, M. & Schwartz, R. D. 1987, ApJ, 316, 311 Cohen, M., Harvey, P .M., & Schwartz, R .D. 1985, ApJ, 296, 633 Cohen, M., Dopita, M. A., & Schwartz, R .D. 1986, ApJ, 302, L55 Cohen, M., Schwartz, R. D., Harvey, P. M., & Wilking, B. A. 1984, ApJ, 281, 250 Dobashi, K., Uehara, H., Kandori, R., Sakurai, T., Kaiden, M., Umemoto, T., & Sato, F. 2005, PASJ, 57, No. SP1, S1 Eisl¨offel, J.. Smith, M. D., & Davis, C. J. 2000, A&A, 359, 1147
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