THE ASTROPHYSICAL JOURNAL, 486 : L55–L58, 1997 September 1 © 1997. The American Astronomical Society. All rights reserved. Printed in U.S. A.
THE PROPER MOTIONS OF THE WARM MOLECULAR HYDROGEN GAS IN HERBIG-HARO 1 ALBERTO NORIEGA-CRESPO1 Infrared Processing and Analysis Center, California Institute of Technology and Jet Propulsion Laboratory, Pasadena, CA 91125;
[email protected]
PETER M. GARNAVICH1 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138;
[email protected] AND
SALVADOR CURIEL, ALEJANDRO C. RAGA,
AND
SANDRA AYALA
Instituto de Astronomı´a, UNAM, 04510, Me´xico D.F., Me´xico;
[email protected],
[email protected],
[email protected] Received 1997 May 9; accepted 1997 June 18
ABSTRACT We have measured the proper motions of three molecular hydrogen (v 5 1– 0 2.121 mm) knots in the Herbig-Haro 1 object using a 4.4 yr baseline (1992–1997). The HH 1F knot, which probably corresponds to the 129 tip of the working surface of HH 1, has a proper motion of 0.19 H 0.10 (arcsec yr21 ) and a position angle of 315225 deg. This motion is comparable to that determined in the atomic emission lines of Ha and [S II] ll 6717y31, and confirms that the warm molecular H2 gas is partaking of the same motion as the atomicyionic gas. Subject headings: ISM: individual (HH 1) — ISM: jets and outflows — ISM: molecules — stars: formation 1997) and to determine whether or not the warm molecular H2 gas has the same dynamical properties as the atomicyionic gas. There are many difficulties in attempting this, since early infrared arrays were small (typically 128 3 128 pixels with 10"7 pixel21 ) and have only been available for a few years. The small detector size results in a field of view (FOV) comparable to the size of the object, with few stars for comparison, a problem that is compounded by the short time base of the available observations. Nevertheless, the success in determining the proper motions of Herbig-Haro objects using optical CCD images in HH 1–2 (Raga et al. 1990; Reipurth et al. 1993; Eislo ¨ffel et al. 1994), HH 34 (Heathcote & Reipurth 1992; Eislo ¨ffel & Mundt 1992; Devine 1997), HH 46y47 (Eislo ¨ffel & Mundt 1994), HH 110 (Reipurth, Raga, & Heathcote 1996), and HH 111 (Reipurth, Raga & Heathcote 1992) has encouraged us to try a similar approach with the near-infrared images. The HH 1 system is particularly suitable for this project, since the optical condensations have proper motions as large as 0"18 yr21 (1380 km s21 at the distance of Orion), and the expectation is that the warm molecular hydrogen gas could be moving at comparable velocities.
1. INTRODUCTION
The measurement of the proper motions of the HerbigHaro (HH) 1 and 2 objects provided a new perspective of the nature of HH objects and their relationship to young stellar objects (YSOs) and bipolar outflows. The cornerstone study by Herbig & Jones (1981) of HH 1–2, based on photographic plates, showed that HH 1 and 2 were moving in opposite directions and at large velocities. A few years later Pravdo et al. (1985), using VLA radio continuum observations, identified an embedded object between HH 1 and 2 as the driving source. Thus, the HH 1–2 system became a bipolar outflow powered by a YSO named VLA 1, with about 50 LJ (Harvey et al. 1986) and with knots moving relative to it at velocities as large as 380 km s21. Several detailed optical studies of the proper motions, based on CCD images, of HH 1–2 have since then been carried out (Raga, Barnes, & Mateo 1990; Reipurth et al. 1993; Eislo ¨ffel, Mundt, & Bo ¨hm 1994), which show the presence of other outflows in the region and a more complex dynamics. The radial velocities of HH 1 (e.g., Solf et al. 1991) and HH 2 (e.g., Bo ¨hm & Solf 1992) are small for most condensations, with values of uVLSRu # 30 km s21. The large tangential velocities in conjunction with the low radial velocities indicate that the entire outflow lies essentially on the plane of the sky. This conclusion is further supported by the study of the scattered light ahead of HH 1F (Noriega-Crespo, Calvet, & Bo ¨hm 1991; Henney 1994), one of the brightest optical condensations. In this study we carry out a first attempt to measure the proper motions of some of the HH 1 knots in the nearinfrared, specifically in the v 5 1– 0 S(1) 2.121 mm line of molecular hydrogen. The emission at 2 mm traces the warm molecular H2 gas with an excitation temperature of 2000 –3000 K (see, e.g., Gredel 1994). The motivation is to explore the possibility that the molecular hydrogen gas in stellar jets is moving at velocities of $100 km s21 (e.g., Garnavich et al.
2. THE ANALYSIS
The data consist of images from three epochs, which cover a period of 4.42 yr (see Table 1). We selected the first image (from 1992 October) as our reference image, with a pixel scale of 0"72 per pixel. The individual data frames were reduced in the usual way, i.e., dark- and sky-subtracted and then flatfielded, and finally combined into a single image using the common stars. The shifts of the knots between different epochs were determined by placing the comparison images (1995, 1997) in the reference frame (at the same scale) of the 1992 image, using the three common stars and the IRAF tasks GEOMAP and GEOTRAN. These tasks take into account translations, rotations, and magnifications, but they do not correct systematic errors that could appear from optical distortions in the camera. We do not expect this to be a problem for any of the
1 Guest Observer, Kitt Peak National Observatory, NOAO, which is operated by AURA, Inc., under cooperative agreement with the National Science Foundation.
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TABLE 1 LOG
OF THE INFRARED
Date
IR Array
1992 October 5. . . . . . . . . . . . 1995 November 30 . . . . . . . . 1997 February 3. . . . . . . . . . .
HgCdTe 128 3 128 InSb 256 3 256d NICMOS 3 256 3 256e
DATA Scale (arcsec pixel21 )
Telescope
0.72 0.20 0.85
APO 1.8 mb,c KPNO 2.1 mb SPMO 2.2 mb
a
a
GRIM II Camera. Filter: 1% 2.12 mm. Filter: 4% 2.22 mm. d COB Camera. e CAMILA Camera (Cruz-Gonza´lez, Salas, & Ruiz 1996). b c
infrared cameras used. The centroids of the well-defined and bright condensations (at the scale of the reference frame) can be measured to 0.1– 0.2 pixel. For fainter or more complex morphology knots, the centroids have an uncertainty of 0.3– 0.4, too large to make them useful for measuring the proper motion. This was the case for HH 1–5, the VLA jet, and HH 144 in the 1997 image. The Cryogenic Optical Bench (COB) 1995 image was used to measure only the HH 1–7 knot (see below) because of the small pixels and resulting small FOV. We adopted a 0.1 pixel uncertainty in the X and Y positions, which translates into a formal error for the shifts between two epochs of 0.45 pixel. The superposition of the 1992 and 1997 images is displayed in Figure 1. The gray scale corresponds to the 1992 epoch and the contours to the 1997 (both intensity scales are linear). Notice the shift of HH 1F condensation toward the northwest. Because of the large pixels (0"72 in the reference image) and the complex structure of most of the condensations, only three knots were finally used. The shifts between epochs were measured in two ways; first, by comparing the centroids obtained from a two-dimensional Gaussian fit, and second, by using a cross-correlation method between small sections around the selected knots. This last method has been successfully applied to obtain of the proper motions of the optical condensations in HH 30 (Lope´z et al. 1996) and HH 110 jets (Reipurth, Raga, & Heathcote 1996). In Table 2 we present the offsets derived from the cross-correlation method, although the results from the two-dimensional Gaussian fit are very similar. For comparison, we have included the proper motions of the optical condensations obtained by Eislo ¨ffel et al. (1994) using primarily [S II] images. The COB images from 1995 were taken to study the VLA 1 jet in H2 at a high spatial resolution. The image of the jet (see Fig. 2) indeed shows how well the jet is collimated in H2 and displays in more detail some of the knots, particularly the HH 1–7 knot, which resembles a bow shock. The smaller pixels and FOV of the COB, however, place the reference stars right at the edge of the image. Because of this, we were unable to use them to measure the proper motions of most of the knots. The only exception is the HH 1–7 knot, which despite this problem clearly shifts its position along the jet flow when the 1992 and 1995 combined images are compared. The difference between the centroids over a period of 3.15 yr gives a proper motion of 132 0.21 H 0.10 (arcsec yr21 ) with a position angle P.A. 5 324237 deg. This value is within the limits of that obtained using the 4.42 yr period, although a bit larger, and is included for completeness in Table 2.
3. DISCUSSION
The proper motions of the three condensations measured in the HH 1 object in the warm molecular gas are of the order of those obtained in the atomic jet gas. At a distance of 440 pc for Orion, this corresponds to flow velocities in H2 of about 150 – 400 km s21. If the molecular hydrogen gas is excited by shocks, there is then an upper limit of 30 –50 km s21 before it becomes dissociated. From this, we conclude that direct shock acceleration is not driving the H2 flow. This situation is not very different to what is observed in the atomic jet gas, where flow velocities are as high as 400 km s21 but the shocks generating the optical emission seldom reach velocities higher than 100 km s21 (Raga, Bo ¨hm, & Canto ´ 1996). This discrepancy between the proper motion velocities and the observed excitation of the optical spectrum has been interpreted in terms of a jet from a variable velocity source, in which relatively weak (but fast-moving) working surfaces can be generated in the interaction region between material ejected at different velocities. A similar process seems to be taking place in HH 1 with respect to the warm molecular gas. The case for H2, however, is a bit more intriguing since in addition to shocks, there are several other ways in which molecular gas can be excited (Hartigan et al. 1996). An important difference between the atomicyionic and H2 emission is that the first one arises from the recombination region behind shocks. Emission from H2 behind dissociating shocks is unlikely, since the reformation of molecules cannot be a major source of H2, as implied by the small H2 column densities measured in HH flows (see, e.g., Gredel 1994; Gredel, Reipurth, & Heathcote 1992). It has been suggested that the H2 emission from HH 1F arises from the Mach disk, in regions where oblique (weak) shocks are enough to excite the H2 but not to destroy it (Davis, Eislo ¨ffel, & Ray 1994), which is possible as long as the Mach disk moves at about 380 km s21. The Hubble Space Telescope images of the HH 1 working surface (Hester et al. 1994) show that the very tip (within 20) resembles a bow shock, so some H2 emission could arise from its wings as well. We have previously pointed out that turbulent entrainment is probably as important as shocks as a source of H2 emission in HH jets (see, e.g., Raga 1995; Garnavich et al. 1997). The entrainment would occur in a mixing layer where the atomic jet gas meets the molecular environment. What makes this possibility appealing is that in turbulent mixing layers the H2 is collisionally excited (as in shocks), emits in small column densities, and has flow velocities approaching the velocity of the jet beam (Taylor & Raga 1995). The large flow velocity implied by the proper motions of the warm molecular H2 gas in HH 1 is not an unique
No. 1, 1997
HERBIG-HARO 1 WARM MOLECULAR HYDROGEN GAS
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FIG. 1.—Superposition of two near-infrared images centered on the v 5 1– 0 2.121 mm H2 emission line of the HH 1 object. The gray-scale image corresponds to 1992 October and the contours to 1997 February. Both intensity scales are linear, and the field is 1"5. Notice the shift toward the northwest of the HH 1F knot. The three stars common to the two epochs are marked (1–3), as well as some of the brighter knots (the near-infrared nomenclature comes from Davis et al. 1994). The boxes show schematically the regions where the cross correlation was performed.
211 the collimated SiO (dense molecular gas) reaches velocities of up to 30 km s21 (Chandler & Richer 1997). The possibility of measuring the proper motion of the warm molecular gas provides a new insight on the kinematics of
case, since signatures of molecular gas at high velocities are found in other objects. For instance, in HH 111 the CO flow can reach velocities of about 500 km s21, and there are “CO bullets” moving at about 240 km s21 (Cernicharo & Reipurth 1996), and in HH
TABLE 2 PROPER MOTIONS
Knot HH HH HH HH a b
1F . . . . . . . . . . . . . . . . . . . . 1–4. . . . . . . . . . . . . . . . . . . . 1–7. . . . . . . . . . . . . . . . . . . . 1–7b . . . . . . . . . . . . . . . . . . .
OF THE
HH 1 KNOTS
IN
H2
Opticala Vtan (arcsec yr21 )
P.A. (deg)
H2 2.12 mm Vtan (arcsec yr21 )
P.A. (deg)
0.178 H 0.014 0.085 H 0.017 … …
317.5 H 4.2 319.2 H 12.6 … …
0.19 H 0.10 0.07 H 0.10 0.16 H 0.09 0.21 H 0.10
315129 225 329112 247 304136 230 324132 237
From Eislo ¨ffel, Mundt, & Bo ¨hm 1994. From COB over a period of 3.15 yr.
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FIG. 2.—Gray-scale narrowband image of the VLA 1 jet in the HH 1–2 system centered on the v 5 1– 0 2.121 mm H2 emission line taken with the COB system. The FOV is 500 and is oriented as in Fig. 1.
stellar jets and an alternative method to study the dynamics of embedded flows invisible at optical wavelengths but detectable in the near-infrared. The data for this project were obtained at three different observatories over a period of nearly 5 years; we want to thank
the staff of each place for their help. We thank Karl-Heinz Bo ¨hm and Steve Heathcote (the referee) for their insightful comments. The research of A. N.-C. is supported by the NASA Long Term Astrophysics program through a contract with the Jet Propulsion Laboratory (JPL) of the California Institute of Technology.
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