The Astrophysical Journal, 576:255–263, 2002 September 1 # 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A.
FORMIC ACID IN ORION KL FROM 1 MILLIMETER OBSERVATIONS WITH THE BERKELEY-ILLINOIS-MARYLAND ASSOCIATION ARRAY Sheng-Yuan Liu Astronomy Department, California Institute of Technology, Pasadena CA 91125;
[email protected]
and J. M. Girart,1 A. Remijan, and L. E. Snyder Astronomy Department, University of Illinois, Urbana, IL 61801;
[email protected],
[email protected],
[email protected] Received 2002 January 8; accepted 2002 May 8
ABSTRACT We present Berkeley-Maryland-Illlinois Association array observations of formic acid (HCOOH) at 1 mm toward the Orion KL region. Near the compact ridge, HCOOH emission is spatially resolved; its partial shell morphology is different from that of other complex O-bearing molecules such as methyl formate and dimethyl ether. This unique distribution suggests that HCOOH is located in a layer that delineates the interaction region between the outflow and the ambient quiescent gas. HCOOH is also detected toward the hot core. For both cases, ejection of grain mantles is likely to be responsible for the observed HCOOH. Subject headings: astrochemistry — ISM: abundances — ISM: individual (Orion Kleinmann-Low) — ISM: molecules observations of selected HCOOH transitions with higher J quantum numbers. These lines will presumably have stronger line intensities because of their higher line strengths. Furthermore, since these transitions lie at higher frequencies, the obtainable spatial resolution is naturally enhanced when the same aperture is used. The Orion KL region is the nearest (500 pc) source among the three where Liu et al. (2001) detected HCOOH emission. It is currently undergoing massive star formation, as indicated by the large bulk bolometric luminosity and the clustering of infrared sources identified in various IR bands (e.g., Wynn-Williams et al. 1984; see Genzel & Stutzki 1989 for a review). The source IRc2 (2000 = 5h35m14 9513, 2000 = 5 220 29>78) has been conventionally considered as the primary heating source in this region (Downes et al. 1981), although recent observations have put this into question (Dougados et al. 1993; Gezari, Beckman, & Werner 1998). Gezari et al. (1998), for example, suggested that the ionizing star of the nearby radio continuum source I (2000 = 5h35m14 9505, 2000 = 5 220 30>45) is a more significant luminosity source. The complexity of the Orion KL region is revealed by molecular thermal and maser spectral line observations. Several cloud components, including the hot core, the plateau, the compact ridge, and the extended ridge, have been identified, each of which exhibits different physical and chemical characteristics (e.g., Blake et al. 1987). At approximately 200 south of source I, the hot core source is a dense and warm clump of molecular gas. Wright, Plambeck, & Wilner (1996) found a gas kinetic temperature of 200–300 K in the hot core at a resolution of 300 –600 . Given the proximity, the hot core gas may be closely associated with the radio source I or the IRc2 source and represents remnant material from the formation of stars in this region. The component with the broadest emission line width from outflow/shock tracers (such as CO, SO, SO2, SiO, and H2) is generally identified as the plateau source. Detailed studies show that these species actually trace outflows at different velocities and scales (Plambeck et al. 1982; Wright et al. 1983, 1996; Masson et al. 1987; Stolovy et al. 1998). Because
1. INTRODUCTION
Formic acid (HCOOH) is the simplest organic acid and the first identified acid in the interstellar medium. Presently, the published astronomical HCOOH data are mainly obtained from single-element telescope spectral line surveys (Woods et al. 1983; Sutton et al. 1985; Blake et al. 1986; Turner 1991; Ziurys & McGonagle 1993; Schilke et al. 1997). The identified HCOOH lines in these surveys, however, are mostly very weak, and there is no detailed information regarding the HCOOH spatial distribution. Since HCOOH shares common structural elements with biologically important species such as acetic acid (CH3COOH) and glycine (NH2CH2COOH), the study of HCOOH should aid future searches for and observations of these species. Recently, Liu, Mehringer, & Snyder (2001) carried out the first set of interferometric HCOOH observations toward galactic hot molecular core (HMC) sources with the Berkeley-Illinois-Maryland Association (BIMA) array.2 The purpose of those observations was to detect and investigate the distribution of HCOOH gas in hot cores. Liu et al. (2001) successfully mapped HCOOH emission in three regions: Orion KL, Sgr B2, and W51. Based on the location of HCOOH emission, a grain-surface origin for HCOOH was suggested. In this picture for the HMC chemistry, enhanced abundances of complex and highly saturated molecules are attributed to grain surface chemistry and mantle evaporation processes (Miao et al. 1995; Mehringer & Snyder 1996; Snyder 1997; see van Dishoeck & Blake 1998 for a review). The above conclusion is, however, partially limited by the fact that the observed HCOOH features are still weak, and its emission appears to be either spatially unresolved or barely resolved. To overcome this, we carried out follow-up
1 Current address: Departament d’Astronomia i Meteorologia, Universitat de Barcelona, Catalonia, Spain. 2 Operated by the University of California, Berkeley, the University of Illinois, and the University of Maryland with support from the National Science Foundation.
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radio source I is coincident with the center of the strong SiO maser emission, it is suggested to be the most likely driving source of the outflow activities (Wright et al. 1995). The compact ridge, located 1000 southwest of IRc2, is another dense clump cooler than the hot core. The large abundances of highly saturated O-rich species in this region were suggested as a result of the outflows interacting with the quiescent ambient molecular material (Blake et al. 1987). The extended ridge is an extended, cold region that is oriented northeast-southwest. It is dominated by emission from simple C-rich species that represent the ambient quiescent gas (Blake et al. 1987). In this paper, we present new observational results for HCOOH emission toward Orion KL in the 1 mm band. With improved resolution, the HCOOH emission features are resolved both spectroscopically and spatially. The HCOOH emission appears to be located both near the compact ridge and in the hot core.
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November and December. Typical system temperatures were between 800 and 1300 K for the 225 GHz observations and between 900 and 1500 K for the 262 GHz observations. The FWHM primary beam was 0
