JCMT/SCUBA SUBMILLIMETER WAVELENGTH IMAGING OF THE INTEGRAL-SHAPED FILAMENT IN ORION Doug Johnstone. and John Bally

Transcription

1 The Astrophysical Journal, 510:L49 L53, 1999 January The American Astronomical Society. All rights reserved. Printed in U.S.A. JCMT/SCUBA SUBMILLIMETER WAVELENGTH IMAGING OF THE INTEGRAL-SHAPED FILAMENT IN ORION Doug Johnstone Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, Ontario M5S 3H8, Canada; johnstone@cita.utoronto.ca and John Bally Center for Astrophysics and Space Astronomy, Campus Box 389, and Department of Astrophysical, Planetary, and Atmospheric Sciences, Campus Box 391, University of Colorado, Boulder, CO 80309; bally@nebula.colorado.edu Received 1998 August 24; accepted 1998 October 27; published 1998 November 30 ABSTRACT We present the first high dynamic range and sensitivity images of the submillimeter wavelength continuum emission at 450 and 850 mm of the integral-shaped filament in the northern portion of the Orion A cloud, which contains the nearest site of ongoing high-mass star formation. The images trace the morphology and spectral index of optically thin emission from interstellar dust, and they constrain the grain temperature and emissivity. The images reveal a remarkable chain of compact sources embedded in a narrow (! pc), high column density filament that extends over the 50 (7 pc) length of the map, with faint extended structure surrounding it. While many compact sources contain extremely young protostars, others may be pre collapse phase cloud cores. The brightest region, associated with OMC-1, contains a remarkable group of dust filaments that radiate radially away from this high-luminosity core and that coincide with the filaments of NH 3 emission. The spectral index is uniform between 450 and 850 mm, except for the ridge sources, the photoheated H ii region edges including the Orion bar, and the location of molecular hydrogen shocks. Subject headings: H ii regions ISM: individual (Orion Nebula) stars: formation 1. INTRODUCTION The integral-shaped filament (ISF; see Bally et al. 1987) in the northern portion of the Orion A molecular cloud is one of the most active sites of star formation near the Sun. It is associated with the Orion Nebula and the Trapezium Cluster of 700 young stars (Hillenbrand 1997; Hillenbrand & Hartmann 1998), and it contains the OMC-1 cloud core immediately behind the Nebula and two other extensively studied star-forming cores, OMC-2 and OMC-3, located about 15 and 25 to the north (Castets & Langer 1995). The ISF has spawned several thousand young stars in the past few million years and contains dozens (possibly hundreds) of embedded young stellar objects (Chini et al. 1997) that power dozens of molecular outflows, Herbig-Haro objects (Reipurth, Bally, & Devine 1997), and molecular hydrogen emitting shocks and jets (Yu, Bally, & Devine 1997). In this Letter, we present high dynamic range maps of the ISF at 450 and 850 mm and discuss the morphology, the spectral index distribution, and the grain properties in this region. 2. OBSERVATIONS We obtained 7.5 resolution, 450 mm and 14 resolution, 850 mm maps of the entire ISF with the 15 m James Clerk Maxwell Telescope (JCMT) located near the summit of Mauna Kea, Hawaii, in 1998 February with the focal-plane instrument SCUBA (Submillimeter Common-User Bolometer Array), which uses 91 bolometers at 450 mm and 37 bolometers at 850 mm to obtain images at both wavelengths simultaneously (Holland et al. 1998). Data were obtained with the new scanmapping technique in 10 # 10 patches using three chopper throws (20, 30, and 65 ) in right ascension and in declination in order to provide six independent chopped difference maps at each wavelength that sample different spatial frequency components of the intensity distribution. The standard reduction package was used to flat-field, remove bad pixels, calibrate, L49 and make images. Images with a specific chop throw and angle were combined to form six 50 # 10 chopped difference images that were Fourier-transformed and combined in Fourier space. An inverse transform produced the final images at each wavelength (cf. Emerson 1995 and Jenness, Lightfoot, & Holland 1998). Low-level and low spatial frequency negative residuals were removed by fitting and subtracting a third-order polynomial two-dimensional baseline function from the final image. Structures with scales larger than the largest chopper throw (65 ) are missing from the map. Frequent observations of HL Tau using the same chop throws and data reduction procedures were used for the calibration. The calibration uncertainty and noise levels are 0% and 0.04 Jy beam at 850 mm and 30% and 0.3 Jy beam at 450 mm, respectively. The calibration uncertainty is dominated by atmospheric transmission fluctuations. 3. MORPHOLOGY The observed 50 # 10 region (Fig. 1) covers most of the ISF mapped in CO by Bally et al. (1987), in C O by Dutrey et al. (1991), in CS by Tatematsu et al. (1993), and in NH 3 by Cesaroni & Wilson (1994). The ISF mass, derived from the molecular data, is about 10 4 M,, with an average density of 4 3 about n(h 2) 10 cm. The submillimeter data confirm the remarkable filamentary structure of the ISF. The main ridge of emission consists of a chain of compact sources confined to a narrow (!1 or 0.2 pc) ridge extending north-south for over 50 (7 pc). Fainter filaments and clumps extend orthogonal to the ridge for several arcminutes. The average intensity distribution orthogonal to the spine of the ISF can be fitted by a broken power law (see Fig. a 2) of the form S 850(R) R, where 0.75! a! 1 from within a beam radius at the center of the ridge to a projected distance Re and 3! a! 5 beyond Re. The ridge appears to have a distinct edge at R e, where the intensity profile steepens significantly.

2 L50 INTEGRAL-SHAPED FILAMENT IN ORION Vol. 510 Fig. 1. The 850 mm emission and mm spectral index distribution in the ISF at the northern end of the Orion A molecular cloud. The Orion Nebula is located directly in front of the main concentration of emission from the OMC-1 core at the center of the image. Note the filaments extending radially away from this core. Left panel: the 850 mm image showing the observed flux from 0.1 to 2 Jy beam with a linear transfer function. Middle panel: the 850 mm image showing the observed flux from 100 mjy beam to 20 Jy beam with a logarithmic transfer function. Right panel: the mm spectral index (g) in the range of 2 6 with a linear transfer function. Higher resolution and color images are available via the first author. The surface brightness of a self-gravitating isothermal cylinder observed in an optically thin tracer should be constant 3 to a core radius r0 beyond which it declines as R (Ostriker 1964). Assuming a central molecular hydrogen density of cm and a central velocity dispersion of 1 km s, the core radius at a distance of 460 pc is expected to be at r0 25. Although our observations show a break at a larger radius, the intensity profile is clearly not flat inside r 0, and no parameters produce a good fit to the self-gravitating isothermal cylinder model. However, recent models of self-gravitating cylinders that contain ordered magnetic fields and external pressure (Fiege & Pudritz 1998) predict broken power-law intensity profiles similar to what we observe. Our data are best fitted by these models if the ISF contains a predominantly toroidal magnetic field. The dozens of class 0 sources (Chini et al. 1997) and outflows (Yu et al. 1997) indicate that self-gravitating, star-forming cores have condensed from the ridge. After condensation from a cylinder but prior to the onset of star formation, such cores are expected to be isothermal spheres with an r radial surface brightness profile. The superposition of such cores along the spine of the ISF can also produce the observed shallow ( R ) power-law index near the center of the ridge, provided that the spacing between each core is approximately the ridge width. Our data provide some evidence for two length scales, one with a spacing of about 9 ( 1.3 pc, close to the spacing noted by Dutrey et al. 1991) that corresponds to the spacing of OMC- 1, OMC-2, OMC-3, and OMC-4 (see below) and the other with a spacing of about 150 ( 0.3 pc) that corresponds to the spacing of the sources identified by Chini et al. (1997) and to the width of the ridge. The most prominent region in the ISF is OMC-1, which is located directly behind the Orion Nebula. Two deeply embedded subcondensations, separated by 90, correspond to the 10 5 L Orion-KL region containing IRc2 (Menten & Reid 1995), and the 10 4 L Orion S source (Schmid-Brugk et al. 1990)., Wiseman & Ho (1996, 1998) mapped the OMC-1 region in NH 3 (2, 2) and (1, 1) with the VLA at 8 resolution, revealing filamentary structure emanating from the Orion-KL core (cf. Murata et al. 1990; Martín-Pintado et al. 1990; Rodríguez-

3 No. 1, 1999 JOHNSTONE & BALLY L51 Fig. 2. A series of crosscuts showing the 850 mm flux density per beam (roughly proportional to the dust column density) plotted as a function of the projected distance from the center of the filament. Note the broken power law, which consists of a shallow (index ) region inside followed by a steep portion (index 4) at larger distances. The dot-dashed line shows the Ostriker (1964) self-gravitating isothermal cylinder model evaluated for the mean conditions in the ISF. The dotted line shows a power law with an index of. Franco et al. 1992). The submillimeter images show the same filamentary structures and bright knots and demonstrate that at least a dozen dusty filaments radiate away from the OMC-1 core for at least 2 5 ( pc). The filaments delineate more than a half-dozen V-shaped cavities that open away from the OMC-1 core. The narrow ends of the cavities converge on the elongated core of OMC-1, extending from Orion-KL to Orion S and not just on Orion-KL. The submillimeter surface brightness implies H 2 masses in the range of 1 10 M, for the filaments, in rough agreement with mass estimates from molecular emission line studies. These filaments may be the walls of cavities excavated from the molecular cloud by now extinct outflows powered by stars formed from the OMC-1 core. These cavities are much larger than the currently active outflows traced by the H 2 fingers emerging from near IRc2, the high-velocity CO associated with the Orion-KL, and the Orion S outflow. Although there is pervasive, moderate-velocity ( 10 km s ) CO and HCO emis- sion throughout the region, the angular resolution of the available maps is insufficient to make a direct association of this gas with either the dust cavities or the filaments. The maximum velocity and spatial extents of the cavities imply that, if they 4 are old outflow cavities, they must be older than 5 # 10 yr (more than 10 times the age of the Orion-KL outflow and the H 2 fingers) and that these outflows must have displaced tens of solar masses of matter. The Orion bar is over 5 (0.65 pc) in extent, and at 850 mm, it breaks up into two parallel ridges (noted at 1300 mm by Mezger, Wink, & Zylka 1990). The bar is an edge-brightened, photodissociating region (PDR) at the rim of the Orion Nebula, separating the H ii region from the molecular cloud. The second ridge, visible at longer wavelengths, is illuminated by free-free emission within the photoionization front on the inside of the PDR. A bipolar cavity is centered on M43, which is a small H ii region that is powered by the B0.5 star NU Orionis 6 northeast of OMC-1. The cavity is bounded by a wall of submillimeter emission that is probably associated with the hot dust in the PDR and the swept-up shell at the edge of the expanding H ii region. The pinched waist of the cavity is located around the massive star and implies that higher density material has impeded the expansion of the nebula in this direction, indicating either that this star formed from a nearly edge-on sheet or that, until recently, the star was surrounded by a nearly edge-on disk. Each lobe extends about 3.5 (0.5 pc) from the nebula center. The 450 and 850 mm maps of the ridge that contain OMC- 2 and OMC-3 look similar to the 1300 mm map (Chini et al. 1997). However, these higher sensitivity maps reveal fainter off-ridge emission that consists of wisps, filaments, and knots spreading out from the ISF. Some of these filaments may also correspond to the walls of known outflow cavities. The spine of the ridge breaks up into numerous compact sources, some of which coincide with known young stellar objects (YSOs) (the brightest sources were identified by Chini et al. 1997) that drive H 2 jets (Yu et al. 1997) and CO outflows. However, it is not possible to determine from our data which of the fainter sources contain YSOs, are self-gravitating clumps, or are just gravitationally unbound concentrations of dust. As noted by Chini et al. (1997), the ridge of dust emission is offset to the east of the peak of the CO, C O, and CS emission between OMC-2 and OMC-3 by about 1 2. The bright molecular emission peak that is observed west of the submillimeter peaks MMS 8 and MMS 9, although clearly visible in the 850 and 450 mm maps, remains conspicuously faint. This dense region may be cold and possibly in a precollapse phase. Two bright regions of shock-excited H 2 emission that lie between MMS 8 and MMS 9 and this molecular peak (H 2 knots 4 and 5 in the G and H flows of Yu et al. 1997) coincide with unresolved 850 mm knots that are very faint at 450 mm, resulting in a very shallow spectral index. Our 850 mm flux may be contaminated by shock-excited lines of SO, SO 2, and CO J 3 2 that lie within the passband of the SCUBA 850 mm filter. A second, unresolved shallow spectral index knot that is coincident with an H 2 region lies just north of FIR 3/4 (H 2 knot 65). The bright V-shaped concentration of knots about 15 south of OMC-1 (first noted in molecular line maps by Dutrey et al and observed at 1.3 mm at the IRAM 30 m by R. Chini 1997, private communication) form a fourth major concentration of submillimeter emission in the ISF. We will refer to this region as OMC-4. The morphology of this region is unlike either OMC-1 or the northern OMC-2/3 ridge. Two straight filaments beaded by knots of emission suggest a much more ordered structure than is observed along the northern ridge. The peak emission in OMC-4 is much fainter than OMC-2/3 (1.5 Jy beam at 850 mm vs. about 7.5 Jy beam ), implying much lower column densities and masses or much colder dust. The OMC-4 region appears less luminous and turbulent and shows no evidence of outflow activity. It may still be in a precollapse phase of protostar evolution. 4. SPECTRAL INDEX MAP The 850 and 450 mm maps are of sufficient sensitivity and dynamic range (about 5000:1) to construct a high-fidelity spec-

4 L52 INTEGRAL-SHAPED FILAMENT IN ORION Vol. 510 Fig. 3. A gray-scale plot of the density of points binned by a spectral index and logarithm of 850 mm flux density per beam. A plot of the mean spectral index as a function of 850 mm flux, along with j error estimates, is overlaid. The symbols denote individual peaks of emission: knots in OMC- 2/3 (asterisks), knots in OMC-1 (crosses), and knots in OMC-4 (diamonds). tral index map (right panel of Fig. 1). The 450 mm map was convolved with a Gaussian function so as to convert the 7.5 resolution of this map to the 14 resolution of the 850 mm data. g The spectral index g, defined such that Sn n between 450 and 850 mm, is determined at each position in the image. The largest uncertainty in the resulting spectral index map arises from 450 mm calibration errors. A second uncertainty is produced by the linear baseline offsets introduced by the image reconstruction algorithm, which effects the spectral index measurement at low flux levels. The spectral indices span the range of 2! g! 4.5 and reflect temperature and/or dust emissivity variations within the ridge. For optically thin dust emission at temperature T d, the observed flux at each frequency should be proportional to the Planck b function B n(t d) and the optical depth t n, where b reflects the change in dust emissivity with frequency or, equivalently, the change in optical depth. For sufficiently warm dust, Td 50 K, the emission at submillimeter wavelengths falls on 2 the Raleigh-Jeans tail of the Planck function where Bn n. Observations and theoretical models of dust emissivity imply 1! b! 2 with b 2 in the general interstellar medium and b 1.5 in dense cores. For optically thick dust, Sn B n(t d). Thus, for dust temperatures higher than Td 10 K, the spectral index should vary within the range 1! g! 4, which is the observed range within our map. The spectral index at each point within the map was binned according to the underlying 850 mm flux, and the mean spectral index within each flux bin was computed (Fig. 3). Despite the wide range in individual values, the mean spectral index as a function of 850 mm flux is remarkably constant, with g for S 850 mm! 10 Jy. The sharp drop-off in spectral index at S850 mm 1 10 Jy is due entirely to the Orion-KL source, which is known to have an anomalously low spectral index (see also Goldsmith, Bergin, & Lis 1997). A single-temperature, single dust emissivity fit to the mean spectral index along the ridge requires Td 30 K and b 2. The peak emission within the entire map arises from the bright northern clump in OMC-1. At 850 mm, the peak flux is 167 Jy in a 14 beam, while at 450 mm, the peak flux is 490 Jy in a 7.5 beam or 731 Jy in a beam convolved to match the 850 mm observations. Serabyn & Weisstein (1995) produced an empirical fit to the line emission, mostly SO and SO 2, that almost uniformly fills 4 2 both the submillimeter bandpasses; Fn 3.9 # 10 nghz Jy. Thus, spectral line contamination may contribute 47 Jy at 850 mm (28%) and 179 Jy at 450 mm (24%) to the SCUBA fluxes. Moreover, Groesbeck (1994) finds that half the flux from 830 to 910 mm is produced by lines. The brightest peaks in OMC- 2/3 also have anomalously low spectral indices. Figure 3 shows the spectral indices of the 19 brightest peaks (stars). Eighteen sources clump around g , while MMS 6 has g 2.3 similar to that of Orion-KL ( g 2.2). Although much fainter, seven sources in OMC-4 (the diamonds in Fig. 3) have an average g Similar spectral indices for compact sources have been found in NGC 2024 by Visser et al. (1998). Although structure in the submillimeter spectral index is dominated by dust emissivity changes, deviations from the Raleigh-Jeans law at low temperatures also decrease g. Thus, it is difficult to separate temperature and emissivity changes within the map. Either cold dust ( Td 20 K) or a low grain emissivity ( b 1.5) may be responsible for the low spectral indices of the compact sources. A comparison with 1300 mm data (cf. Chini et al. 1997) might resolve the ambiguity since the longer wavelength spectral index is determined mainly by the grain emissivity, while the shorter wavelength spectral index is sensitive to the grain temperature. However, contamination by the surrounding ridge makes separation of the compact sources from the background impossible. Visser et al. (1998) use 3 mm interferometric measurements to probe the cause of the low spectral indices of NGC 2024 compact sources, and they conclude that the grain emissivity is low, with b 1.5. Future observations with the submillimeter array are required to constrain better the roles of low-grain emissivity or temperature for the compact sources. We wish to thank the SCUBA team at the JCMT for their support during both the observations and the data analysis. The JCMT is operated by the Joint Astronomy Centre on behalf of PPARC of the UK, the Netherlands OSR, and NRC Canada. We also gratefully acknowledge the assistance of B. Reipurth, R. Chini, J. Fiege, R. Pudritz, and the referee P. Ho. This research has been funded by an NSERC Canada Postdoctoral Fellowship awarded to Doug Johnstone. REFERENCES Bally, J., Langer, W. D., Stark, A. A., & Wilson, R. W. 1987, ApJ, 313, L45 Castets, A., & Langer, W. D. 1995, A&A, 294, 835 Cesaroni, R., & Wilson, T. L. 1994, A&A, 281, 209 Chini, R., Reipurth, B., Ward-Thompson, D., Bally, J., Nyman, L.-Å., Sievers, A., & Billawala, Y. 1997, ApJ, 474, L135 Dutrey, A., Duvert, G., Castets, A., Langer, W. D., Bally, J., & Wilson, R. W. 1991, A&A, 247, L9 Emerson, D. T. 1995, in ASP Conf. Ser. 75, Multifeed Systems for Radio Telescopes, ed. D. T. Emerson & J. M. Payne (San Francisco: ASP), 309 Fiege, J., & Pudritz, R. 1998, MNRAS, in press

5 No. 1, 1999 JOHNSTONE & BALLY L53 Goldsmith, P. F., Bergin, E. A., & Lis, D. C. 1997, ApJ, 491, 615 Groesbeck, T. D. 1994, Ph.D. thesis, Caltech Hillenbrand, L. A. 1997, AJ, 113, 1733 Hillenbrand, L. A., & Hartmann, L. W. 1998, ApJ, 492, 540 Holland, W. S., Cunningham, C. R., Gear, W. K., Jenness, T., Laidlaw, K., Lightfoot, J. F., & Robson, E. I. 1998, Proc. SPIE, in press Jenness, T., Lightfoot, J. F., & Holland, W. S. 1998, Proc. SPIE, in press Martín-Pintado, J., Rodrígues-Franco, A., & Bachiller, R. 1990, ApJ, ApJ, 357, L48 Menten, K. M., & Reid, M. J. 1995, ApJ, 445, 157 Mezger, P. G., Wink, J. E., & Zylka, R. 1990, A&A, 228, 95 Murata, Y., Kawabe, R., Ishiguro, M., Morita, K., Kasuga, T., Takano, T., & Hasegawa, T. 1990, ApJ, 359, 125 Ostriker, J. 1964, ApJ, 140, 1056 Reipurth, B., Bally, J., & Devine, D. 1997, AJ, 114, 2708 Rodríguez-Franco, A., Martín-Pintado, J., Gómez-Gonzáles, J., & Planesas, P. 1992, A&A, 264, 592 Schmid-Brugk, J., Güsten, R., Mauersberger, R., Schultz, A., & Wilson, T. L. 1990, ApJ, 362, L25 Serabyn, E., & Weisstein, E. W. 1995, ApJ, 451, 238 Tatematsu, K., et al. 1993, ApJ, 404, 643 Visser, A. E., Richer, J. S., Chandler, C. J., & Padman, R. 1998, MNRAS, in press Wiseman, J. J., & Ho, P. T. P. 1996, Nature, 382, , ApJ, 502, 676 Yu, K. C., Bally, J., & Devine, D. 1997, ApJ, 485, L45