DETECTION AND CONFIRMATION OF INTERSTELLAR ACETIC ACID

Transcription

1 THE ASTROPHYSICAL JOURNAL, 480 : L71 L74, 1997 May The American Astronomical Society. All rights reserved. Printed in U.S.A. DETECTION AND CONFIRMATION OF INTERSTELLAR ACETIC ACID DAVID M. MEHRINGER, 1 LEWIS E. SNYDER, AND YANTI MIAO Department of Astronomy, University of Illinois, 100 W. Green Street, Urbana, IL 61801; dmehring@socrates.caltech.edu, snyder@astro.uiuc.edu, yanti@astro.uiuc.edu AND FRANK J. LOVAS Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 0899 Received 1996 July 17; accepted 1997 February 7 ABSTRACT We have detected acetic acid (CH 3 COOH) in the Sgr B Large Molecule Heimat source using the Berkeley-Illinois-Maryland Association (BIMA) Array and the Caltech Owens Valley Radio Observatory (OVRO) Millimeter Array. With the BIMA array, we initially detected the 8 *,8 7 *,7 A blend near 90. GHz. The corresponding line from the E symmetry species was sought but may be blended with a line from another species. Interstellar CH 3 COOH was confirmed using the OVRO array, with which we detected the 9 *,9 8 *,8 E blend near GHz. The corresponding line from the A symmetry species was sought but was found to be blended with the E line of CH 3 SH. Our CH 3 COOH observations represent the first detection and confirmation of an interstellar molecule using interferometric arrays; all past detections and confirmations of new molecules have been made on the basis of single-element telescope observations. Subject headings: ISM: abundances ISM: clouds ISM: individual (Sagittarius B[LMH]) ISM: molecules radio lines: ISM 1 Current address: California Institute of Technology, Downs Laboratory of Physics, MC 30-47, Pasadena, CA The subscript * is defined to mean K p or K 0or1. L71 1. INTRODUCTION There has been considerable interest in searching for interstellar acetic acid (CH 3 COOH), because in the laboratory a bimolecular synthesis of glycine (NH CH COOH), the simplest biologically important amino acid, occurs when acetic acid combines with NH. Consequently, an interstellar CH 3 COOH source may also contain glycine. In addition, CH 3 COOH is important for astrochemical studies because it contains the elusive C C O backbone; interstellar molecules with this structure appear to have less potential for formation than their counterparts with C O C backbone structure (Millar et al. 1988). For example, an isomer of acetic acid, methyl formate (HCOOCH 3 ), has the C O C backbone and is easily detectable in many hot-core sources such as OMC-1 (e.g., Turner 1989), G (Mehringer & Snyder 1996), and the Large Molecule Heimat (LMH) source in Sgr B (Snyder, Kuan, & Miao 1994; Miao et al. 1995). Snyder (1997) has recounted the history of unsuccessful radio searches for interstellar CH 3 COOH. The most recent such search was conducted by Wootten et al. (199), who searched for both centimeter- and millimeter-wavelength transitions in OMC-1, W51 Main, and Sgr B(OH). Wootten et al. placed upper limits on the CH 3 COOH/HCOOCH 3 abundance ratio of in OMC-1, 0.00 in Sgr B(OH), and 0.01 in W51 Main. These small ratios indicate that HCOOCH 3 is much easier to form than CH 3 COOH under interstellar conditions. Sgr B(LMH) is arguably the best source in the Galaxy for seeking complex, saturated organic molecules. It has an extremely high H column density of?10 5 cm (Lis et al. 1993; Kuan, Mehringer, & Snyder 1996), so species with very low abundances may still be detected. It was only recently identified as the major source of complex species in Sgr B (Snyder et al. 1994; Miao et al. 1995) using the Berkeley-Illinois- Maryland Association (BIMA) Array. 3 These workers exploited the ability of interferometric arrays to image a relatively large field of view (in this case 1 ) with high spatial resolution (10 ) to identify Sgr B(LMH) as the main source of complex molecules in Sgr B. Using this technique, the position of Sgr B(LMH) has been determined to within. It is of critical importance to realize that Sgr B(OH), which has been well explored as the pointing center of several spectral line surveys and as a target of the CH 3 COOH search of Wootten et al. (199), is located over 1#5 from Sgr B(LMH). The NRAO 1 m telescope, which was used in the Wootten et al. search, has a half-power radius of 10#5 inthe 3 mm band. Thus, Wootten et al. would not have been able to detect weak CH 3 COOH lines from Sgr B(LMH). We were motivated to search Sgr B(LMH) for CH 3 COOH because emission lines that may be due to interstellar glycine have been detected there (Miao et al. 1994; Snyder 1997), but this source had never been targeted for a CH 3 COOH search. We used the BIMA and the Caltech Owens Valley Radio Observatory 4 (OVRO) millimeter arrays in our search. We opted to use arrays rather than single-element telescopes because an array is insensitive to large-scale emission. We expected that emission from CH 3 COOH would come from a very compact region, as has been the case for all other complex species so far observed in Sgr B with the BIMA array (Miao et al. 1995). In addition, a recent full-synthesis imaging study of ethyl cyanide (CH 3 CH CN) in Sgr B using the BIMA array and the NRAO 1 m telescope shows that the only source of 3 Operated by the University of California at Berkeley, the University of Illinois, and the University of Maryland, with support from the National Science Foundation. 4 Observations with the Owens Valley Radio Observatory Millimeter-Wave Array are supported by NSF grant AST

2 L7 MEHRINGER ET AL. Vol. 480 TABLE 1 PARAMETERS OF CH 3 COOH LINES SOUGHT IN SGR B(LMH) Transition a b (MHz) E u (K) S x (D ) 8 *,8 7 *,7 E (0.05) *,8 7 *,7 A (0.05) *,9 8 *,8 E *,9 8 *,8 A a Each of the * lines consists of two a-type and two b-type degenerate transitions. For example, the 9003 MHz line consists of the 8 0,8 7 0,7 E and 8 1,8 7 1,7 Ea-type transitions and the 8 0,8 7 1,7 E and 8 1,8 7 0,7 E b-type transitions. The listed S x value is the sum of all four transitions in each group. b Values in parentheses are the 1 uncertainties in the measured rest frequencies. CH 3 CH CN emission is the compact Sgr B(LMH) source (Miao & Snyder 1997). Because arrays are not sensitive to emission smoothly distributed on relatively large spatial scales, our observations are immune to some extent from confusion with other lines from simpler, more common molecules. Such confusion plagues single-element telescope searches for weak lines. We have detected emission from at least two unblended CH 3 COOH lines in Sgr B(LMH). This study marks the first time that a molecule has been detected and confirmed in the ISM using interferometric arrays.. OBSERVATIONS The initial search was carried out with the BIMA array. We searched for the 8 *,8 7 *,7 A and E lines of CH 3 COOH, with rest frequencies of H 0.05 MHz and H 0.05 MHz, respectively. These rest frequencies were measured at the National Institute of Standards and Technology with a spectrometer configuration described previously by Suenram & Lovas (1980). The unresolved extragalactic source NRAO 530 was observed to calibrate the antenna-based complex gains, and 3C 73 was used to calibrate the bandpass responses. The data were edited, calibrated, concatenated, and imaged using the MIRIAD software package of the BIMA consortium. The continuum emission from Sgr B is bright enough that self-calibration of the antenna-based phases was possible. During imaging, the data were weighted by T sys to optimize the noise level. After the initial CH 3 COOH line detection using the BIMA array, the OVRO array was used to confirm the detection of this new interstellar molecule. We searched for the 9 *,9 8 *,8 A and E lines of CH 3 COOH with calculated rest frequencies of MHz and MHz, respectively (Wlodarczak & Demaison 1988). The calibrators were the same as those from the BIMA array observations. The data were calibrated using the MMA software package of Caltech. The calibrated u-v data were ported to the AIPS software package of NRAO, where they were imaged and self-calibrated. 3. RESULTS AND DISCUSSION Table 1 lists the quantum mechanical parameters of the CH 3 COOH lines sought in this study. Listed are the transition quantum numbers, the rest frequency ( ), the energy of the upper level (E u ), and the product of the line strength and the relevant component of the dipole moment (S x ). The spectra of these lines are presented in Figure 1. We have detected the 9046 MHz and MHz CH 3 COOH lines at the 4 level. Furthermore, the positions of these two features are coincident to within the uncertainties, strengthening the identification. These positions are listed in Table, along with the peak line intensities (I 0 ). Also in this table is listed the peak position of the blend of HCOOCH 3 lines near 908 GHz (see below). Contour plots of emission from the two CH 3 COOH lines overlaid on a gray-scale figure of the 8.4 GHz continuum are shown in Figure. These two lines have similar quantum mechanical parameters (see Table 1), and hence we expect the line intensities to be comparable. From Table it can be seen that this is indeed the case; both lines have peak intensities of 10. Jy beam 1. Emission from the other two CH 3 COOH lines is apparently blended with lines from other molecules. While the 9003 MHz line peaks at the expected velocity, it is about times too intense to be solely emission from CH 3 COOH, assuming LTE holds (the quantum mechanical parameters for this line are practically identical to those for the 9046 MHz line, so the intensities of these two lines should be equal). The MHz line of CH 3 COOH is masked by the E line of CH 3 SH (Fig. 1d). CH 3 COOH column densities can be calculated from the 9046 MHz and MHz line data. For this calculation, we use equation (1) of Miao et al. (1995) and the parameters of the MHz line with W Jy beam 1 km s 1 (0. Jy beam 1 spectral peak a typical FWHM of 10 km s 1 ), T r 00 K, and OVRO beam dimensions ( a b 11"5 4"4). Because CH 3 COOH is an asymmetric rotor, its partition function is well approximated by Q r CT 3/ r (e.g., Townes & Schowlow 1955), where C 14.1K 3/. The derived CH 3 COOH column density is N CH3 COOH cm. For an H column density range of N H (1 8) 10 5 cm (Lis et al. 1993; Kuan et al. 1996), the CH 3 COOH fractional abundance is X CH3 COOH (0.9 7) Another relevant molecule whose column density we can compare to N CH3 COOH is the isomer of CH 3 COOH, HCOOCH 3. Mehringer & Snyder (1996) used the measurements of Miao et al. (1995) and a value of T r 00 K to calculate N HCOOCH cm in Sgr B(LMH). This value is in good agreement with the value N HCOOCH cm derived by Kuan & Snyder (1996) from another transition, taking T r 00 K. In the present data, two strong HCOOCH 3 lines, the 8 0,8 7 0,7 A and E lines near 90.3 GHz, were included. Based on the intensities of these lines and assuming T r 00 K, we calculate N HCOOCH cm. Therefore, if the CH 3 COOH and HCOOCH 3 emissions peak at the same location, the relative CH 3 COOH/HCOOCH 3 abundance ratio is (4 7) 10. However, inspection of Table shows that the emission peaks from these species are separated by about 3 ; thus, in the region where CH 3 COOH peaks, this ratio is a lower limit, and in the region where HCOOCH 3 peaks, this ratio is an upper limit. High-sensitivity observations with spatial resolutions of or better are necessary to determine accurate ratios in these two closely spaced regions. Finally, because both these molecules are asymmetric rotors, the temperature dependence of their partition functions is the same. Because we are using transitions with comparable energies for the calculation of column densities, the CH 3 COOH/HCOOCH 3 abundance ratio is only weakly dependent on the assumed value of T r (N CH3 COOH/N HCOOCH3 F exp [E u,ch3 COOH E u,hcooch3 /kt r ] exp [5.0 K/T r ]). Our data indicate that the CH 3 COOH and HCOOCH 3 emission peaks, both of which are pointlike in our beams, are

3 No. 1, 1997 DETECTION OF INTERSTELLAR ACETIC ACID L73 FIG. 1. Spectra of the CH 3 COOH lines sought in this study. The abscissa is the rest frequency scaled for a source at v LSR 64 km s 1, and lines are labeled for this velocity. The BIMA array spectra in (a) and (b) have spectral resolutions of 0.54 MHz (1.8 km s 1 ). They were taken at a single square pixel at (J000) 17 h 47 m 19!9, (J000) The angular resolution for these data is 10"8 7"1 (P.A. 19 ), and the rms noise in a line-free channel (shown by the vertical bars near the lower left corner of the spectra) is 0.05 Jy beam 1. While the flag for the 15 NNH line in (b) is shown for a velocity of 64 km s 1, this line occurs at the higher velocity of 1 68 km s 1. Both 15 NNH and HCOOCH 3 emission peak at the same position, but the difference of 14 kms 1 in the velocities of these species indicates that they do not occupy the same volume. Other molecules, such as CH 3 CH CN and CH CHCN, which are also formed primarily via grain-surface reactions, have velocities similar to that of HCOOCH 3 (Miao et al. 1995), in the km s 1 range. In contrast, species which are formed primarily in the gas phase such as 15 NNH, HNO, CCS, and HC 13 CCN (Kuan & Snyder 1994) are observed in this same region to have velocities of km s 1, thus indicating that there is a clear separation between gas and grain-surface species. The OVRO array spectra in (c) and (d) have spectral resolutions of 0.50 MHz (1.5 km s 1 ). They were taken at a single 1 square pixel at (J000) 17 h 47 m 19!9, (J000) 8 0. The angular resolution for these data is (P.A. 19 ), and the rms noise in a line-free channel is 0.05 Jy beam 1. The line at the low-frequency edge of the spectrum in (d) is most likely due to the,0 3 1,3 line of SO, which has a rest frequency of 100, MHz. separated by about 3 (0.1 pc; see Table ). This separation suggests that there is a significant difference in the chemical processes occurring in these two regions. Thus, these results indicate there can be substantial differences in the chemistries of molecular cores on scales of only 0.1 pc. We can rule out differences in excitation as being the cause of the offset of the emission peaks because all the CH 3 COOH and HCOOCH 3 lines observed in this study are at roughly the same energy above the ground state. Thus, in one region, the formation of CH 3 COOH is more likely relative to HCOOCH 3 than in the other. Wlodarczak & Demaison (1988) have outlined a possible mechanism for the synthesis of CH 3 COOH. In this model, CH 3 COOH is formed principally in the gas phase (Huntress & Mitchell 1979). Based on this formation scheme, Wlodarczak & Demaison (1988) predicted a CH 3 COOH/HCOOCH 3 number ratio in Sgr B of 0.1. Because Wootten et al. (199) did not detect CH 3 COOH in Sgr B(OH), they argued that the above mechanism could only produce a CH 3 COOH/ HCOOCH 3 number ratio of less than However, our results for the CH 3 COOH emission peak are consistent with the original prediction of Wlodarczak & Demaison. On the other hand, toward the HCOOCH 3 peak, which is about 3 from the CH 3 COOH peak, it is likely that the original prediction fails. The basic question remains of what the major CH 3 COOH

4 L74 MEHRINGER ET AL. TABLE PARAMETERS OF THE CH 3 COOH AND HCOOCH 3 EMISSION PEAKS IN SGR B(LMH) Species (MHz) (J000) a,b (Jy beam 1 ) (J000) a,c I 0 a CH 3 COOH (0.07) (1.3) 0.1(0.05) (0.05) (1.3) 0.19(0.05) HCOOCH (0.008) (0.15)... a Values in parentheses are the 1 uncertainties. b Right ascension in units of hours, minutes, and seconds. c Declination in units of degrees, arcminutes, and arcseconds. formation process is. While we cannot rule out the gas-phase formation process of Huntress & Mitchell (1979), we do note that the CH 3 COOH emission peak lies very close to the emission peaks of several other complex species, such as HCOOCH 3,CH 3 CH CN, and CH CHCN (Miao et al. 1995). Furthermore, the velocity of the CH 3 COOH lines are also similar to the velocities of these other complex species, strongly suggesting that they are all nearly cospatial. The other species appear to be formed as a result of grain-surface chemistry, and so we think it is likely that grain-surface chemistry also plays an important role in the formation of CH 3 COOH. In contrast, species formed primarily in the gas phase, such as 15 NNH, HNO, CCS, and HC 13 CCN (Kuan & Snyder 1994) are observed in this same region to have velocities of km s 1, thus indicating that there is a clear separation between gas and grain-surface species. Future chemical models should incorporate CH 3 COOH formation paths in order to try to reproduce the observed abundance of this species in Sgr B(LMH). 4. SUMMARY We have discovered CH 3 COOH in the Sgr B(LMH) source using the BIMA and OVRO millimeter arrays. Our observations represent the first detection and confirmation of a molecule in the ISM using interferometric arrays. The main results of our study are the following: 1. The CH 3 COOH column density is cm. Its fractional abundance relative to H and to its isomer, HCOOCH 3, are (0.9 7) and (4 7) 10, respectively.. The CH 3 COOH emission peak is offset from the HCOOCH 3 emission peak by about 3 (0.1 pc). Because both positions were determined from relatively low-lying transitions, excitation differences can be ruled out as the cause of the offset. Thus, the offset must be the result of chemical differences between the two regions. 3. The formation mechanism of CH 3 COOH remains an unanswered question. While a gas-phase series of reactions cannot be ruled out, the close relationship of the CH 3 COOH emission peak with emission peaks of other complex species that are formed primarily as the result of grain-surface chemistry suggests that grain-surface chemistry also plays an intimate role in the formation of CH 3 COOH. Future chemical models must address this problem. We thank H. R. Dickel, J. R. Dickel, and J. R. Forster for help with acquiring the BIMA array data and M. Fleming and J. Wirth for their after-hours antenna repair work. We thank C. D. Wilson and S. L. Scott for help with acquiring the OVRO array data. We thank an anonymous referee for helpful comments. We acknowledge support from the Laboratory for Astronomical Imaging at the University of Illinois and NSF grant AST Huntress, W. T., Jr., & Mitchell, G. F. 1979, ApJ, 31, 456 Kuan, Y.-J., Mehringer, D. M., & Snyder, L. E. 1996, ApJ, 459, 619 Kuan, Y.-J., & Snyder, L. E. 1994, ApJS, 94, , ApJ, 470, 981 Lis, D. C., Goldsmith, P. F., Carlstrom, J. E., & Scoville, N. Z. 1993, ApJ, 40, 38 Mehringer, D. M., Palmer, P., & Goss, W. M. 1997, in preparation Mehringer, D. M., & Snyder, L. E. 1996, ApJ, 471, 897 Miao, Y., Mehringer, D. M., Kuan, Y.-J., & Snyder, L. E. 1995, ApJ, 445, L59 Miao, Y., & Snyder, L. E. 1997, ApJ, 480, L67 Miao, Y., Snyder, L. E., Kuan, Y.-J., & Lovas, F. J. 1994, BAAS, 6, 906 Millar, T. J., Olofsson, H., Hjalmarson, Å., & Brown, R. D. 1988, A&A, 05, L5 REFERENCES Snyder, L. E. 1997, Origins, Life and Evolution Biosphere, 7, in press Snyder, L. E., Kuan, Y.-J., & Miao, Y. 1994, in The Structure and Content of Molecular Clouds, 5 Years of Molecular Radio Astronomy, ed. T. L. Wilson & K. J. Johnston (Berlin: Springer), 187 Suenram, R. D., & Lovas, F. J. 1980, J. Am. Chem. Soc., 10, 7180 Townes, C. H., & Schowlow, A. L. 1955, Microwave Spectroscopy (New York: McGraw-Hill) Turner, B. E. 1989, ApJS, 70, 539 Wlodarczak, G., & Demaison, J. 1988, A&A, 19, 313 Wootten, A., Wlodarczak, G., Mangum, J. G., Combes, F., Encrenaz, P. J., & Gerin, M. 199, A&A, 57, 740

5 FIG.. Contour images of CH 3 COOH emission overlaid on a gray-scale image of the 8.4 GHz continuum of Sgr BN from Mehringer, Palmer, & Goss (1997). Contour levels in both panels are at 0.1, 0.1, 0.15, 0.18, and 0.1 Jy beam 1 and the numbers on the gray-scale wedge are in units of mjy beam 1.(a) Emission from the 9046 MHz transition in a 1.8 km s 1 wide channel with a resolution of 10"9 7" (P.A. 19 ). (b) Emission from the MHz transition in a 1.5 km s 1 wide channel with a resolution of 11" 4"3 (P.A. 19 ). MEHRINGER et al. (see 480, L7) PLATE L10