THE ASTROPHYSICAL JOURNAL, 480 : L71–L74, 1997 May 1 q 1997. 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, 1002 W. Green Street, Urbana, IL 61801;
[email protected],
[email protected],
[email protected] AND
FRANK J. LOVAS Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899 Received 1996 July 17; accepted 1997 February 7
ABSTRACT We have detected acetic acid (CH3COOH) in the Sgr B2 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 ,72 A blend near 90.2 GHz. The * corresponding line from the E symmetry species was sought but may be blended with a line from another species. Interstellar CH3COOH was confirmed using the OVRO array, with which we detected the 9 ,9– 8 ,8 E blend near * * 100.9 GHz. The corresponding line from the A symmetry species was sought but was found to be blended with the 7 1–7 0 E line of CH3SH. Our CH3COOH 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 B2[LMH]) — ISM: molecules — radio lines: ISM fied as the major source of complex species in Sgr B2 (Snyder et al. 1994; Miao et al. 1995) using the Berkeley-IllinoisMaryland Association (BIMA) Array.3 These workers exploited the ability of interferometric arrays to image a relatively large field of view (in this case 120) with high spatial resolution (100) to identify Sgr B2(LMH) as the main source of complex molecules in Sgr B2. Using this technique, the position of Sgr B2(LMH) has been determined to within 20. It is of critical importance to realize that Sgr B2(OH), which has been well explored as the pointing center of several spectral line surveys and as a target of the CH3COOH search of Wootten et al. (1992), is located over 1#5 from Sgr B2(LMH). The NRAO 12 m telescope, which was used in the Wootten et al. search, has a half-power radius of 10#5 in the 3 mm band. Thus, Wootten et al. would not have been able to detect weak CH3COOH lines from Sgr B2(LMH). We were motivated to search Sgr B2(LMH) for CH3COOH 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 CH3COOH search. We used the BIMA and the Caltech Owens Valley Radio Observatory4 (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 CH3COOH would come from a very compact region, as has been the case for all other complex species so far observed in Sgr B2 with the BIMA array (Miao et al. 1995). In addition, a recent full-synthesis imaging study of ethyl cyanide (CH3CH2CN) in Sgr B2 using the BIMA array and the NRAO 12 m telescope shows that the only source of
1. INTRODUCTION
There has been considerable interest in searching for interstellar acetic acid (CH3COOH), because in the laboratory a bimolecular synthesis of glycine (NH2CH2COOH), the simplest biologically important amino acid, occurs when acetic acid combines with NH 12 . Consequently, an interstellar CH3COOH source may also contain glycine. In addition, CH3COOH 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 (HCOOCH3 ), has the C–O–C backbone and is easily detectable in many hot-core sources such as OMC-1 (e.g., Turner 1989), G34.3 1 0.2 (Mehringer & Snyder 1996), and the Large Molecule Heimat (LMH) source in Sgr B2 (Snyder, Kuan, & Miao 1994; Miao et al. 1995). Snyder (1997) has recounted the history of unsuccessful radio searches for interstellar CH3COOH. The most recent such search was conducted by Wootten et al. (1992), who searched for both centimeter- and millimeter-wavelength transitions in OMC-1, W51 Main, and Sgr B2(OH). Wootten et al. placed upper limits on the CH3COOH/HCOOCH3 abundance ratio of 0.001 in OMC-1, 0.002 in Sgr B2(OH), and 0.01 in W51 Main. These small ratios indicate that HCOOCH3 is much easier to form than CH3COOH under interstellar conditions. Sgr B2(LMH) is arguably the best source in the Galaxy for seeking complex, saturated organic molecules. It has an extremely high H2 column density of ?1025 cm22 (Lis et al. 1993; Kuan, Mehringer, & Snyder 1996), so species with very low abundances may still be detected. It was only recently identi-
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 93-14079.
1 Current address: California Institute of Technology, Downs Laboratory of Physics, MC 320-47, Pasadena, CA 91125. 2 The subscript * is defined to mean K or K p 2 5 0 or 1.
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MEHRINGER ET AL. TABLE 1 PARAMETERS
OF
CH3COOH LINES SOUGHT
Transitiona 8*,8–7*,7 8*,8–7 ,7 * 9*,9–8 ,8 * 9*,9–8 ,8 *
E ................. A ................. E ................. A .................
IN
SGR B2(LMH)
nb (MHz)
Eu (K)
S m 2x (D2 )
90203.35 (0.05) 90246.26 (0.05) 100855.02 100897.83
20.3 20.3 25.1 25.1
43.2 43.2 49.0 49.0
a Each of the * lines consists of two a-type and two b-type degenerate transitions. For example, the 90203 MHz line consists of the 8 0,8–7 0,7 E and 8 1,8–7 1,7 E a-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 m 2x value is the sum of all four transitions in each group. b Values in parentheses are the 1 s uncertainties in the measured rest frequencies.
CH3CH2CN emission is the compact Sgr B2(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 CH3COOH lines in Sgr B2(LMH). This study marks the first time that a molecule has been detected and confirmed in the ISM using interferometric arrays. 2. OBSERVATIONS
The initial search was carried out with the BIMA array. We searched for the 8 * ,8–7 ,7 A and E lines of CH3COOH, with * rest frequencies of 90246.26 H 0.05 MHz and 90203.35 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 273 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 B2 is bright enough that self-calibration of the antenna-based phases was 22 possible. During imaging, the data were weighted by T sys to optimize the noise level. After the initial CH3COOH 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 CH3COOH with calculated rest frequencies of 100897.83 MHz and 100855.02 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 CH3COOH lines sought in this study. Listed are the transition quantum numbers, the rest frequency (n), the energy of the upper level (Eu ), and the product of the line strength and the relevant component of the dipole moment (S m 2x ). The spectra of these lines are presented in Figure 1. We have detected the 90246 MHz and 100855 MHz CH3COOH lines at the 4 s level.
Vol. 480
Furthermore, the positions of these two features are coincident to within the uncertainties, strengthening the identification. These positions are listed in Table 2, along with the peak line intensities (I0 ). Also in this table is listed the peak position of the blend of HCOOCH3 lines near 90228 GHz (see below). Contour plots of emission from the two CH3COOH lines overlaid on a gray-scale figure of the 8.4 GHz continuum are shown in Figure 2. These two lines have similar quantum mechanical parameters (see Table 1), and hence we expect the line intensities to be comparable. From Table 2 it can be seen that this is indeed the case; both lines have peak intensities of 10.2 Jy beam21. Emission from the other two CH3COOH lines is apparently blended with lines from other molecules. While the 90203 MHz line peaks at the expected velocity, it is about 2 times too intense to be solely emission from CH3COOH, assuming LTE holds (the quantum mechanical parameters for this line are practically identical to those for the 90246 MHz line, so the intensities of these two lines should be equal). The 100898 MHz line of CH3COOH is masked by the 7 1–7 0 E line of CH3SH (Fig. 1d). CH3COOH column densities can be calculated from the 90246 MHz and 100855 MHz line data. For this calculation, we use equation (1) of Miao et al. (1995) and the parameters of the 100855 MHz line with W 5 2 Jy beam21 km s21 (0.2 Jy beam21 spectral peak 3 a typical FWHM of 10 km s21 ), Tr 5 200 K, and OVRO beam dimensions ( u a 3 u b 5 11"5 3 4"4). Because CH3COOH is an asymmetric rotor, its (e.g., partition function is well approximated by Qr 5 CT 3/2 r Townes & Schowlow 1955), where C 5 14.1K 23/2. The derived CH3COOH column density is NCH3COOH 5 7.3 3 1015 cm22. For an H2 column density range of NH2 5 (1– 8) 3 1025 cm22 (Lis et al. 1993; Kuan et al. 1996), the CH3COOH fractional abundance is XCH3COOH 5 (0.9 –7) 3 10210. Another relevant molecule whose column density we can compare to NCH3COOH is the isomer of CH3COOH, HCOOCH3. Mehringer & Snyder (1996) used the measurements of Miao et al. (1995) and a value of Tr 5 200 K to calculate NHCOOCH3 5 1 3 1017 cm22 in Sgr B2(LMH). This value is in good agreement with the value NHCOOCH3 5 2 3 1017 cm22 derived by Kuan & Snyder (1996) from another transition, taking Tr 5 200 K. In the present data, two strong HCOOCH3 lines, the 8 0,8–7 0,7 A and E lines near 90.23 GHz, were included. Based on the intensities of these lines and assuming Tr 5 200 K, we calculate NHCOOCH3 5 1 3 1017 cm22. Therefore, if the CH3COOH and HCOOCH3 emissions peak at the same location, the relative CH3COOH/HCOOCH3 abundance ratio is (4 –7) 3 1022. However, inspection of Table 2 shows that the emission peaks from these species are separated by about 30; thus, in the region where CH3COOH peaks, this ratio is a lower limit, and in the region where HCOOCH3 peaks, this ratio is an upper limit. High-sensitivity observations with spatial resolutions of 20 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 CH3COOH/HCOOCH3 abundance ratio is only weakly dependent on the assumed value of Tr(NCH3COOH/NHCOOCH3 F exp [Eu,CH3COOH 2 Eu,HCOOCH3/kTr] 5 exp [5.0 K/Tr]). Our data indicate that the CH3COOH and HCOOCH3 emission peaks, both of which are pointlike in our beams, are
No. 1, 1997
DETECTION OF INTERSTELLAR ACETIC ACID
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FIG. 1.—Spectra of the CH3COOH lines sought in this study. The abscissa is the rest frequency scaled for a source at vLSR 5 164 km s21, 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 s21 ). They were taken at a single 20 square pixel at a(J2000) 5 17h47m19!9, d(J2000) 5 2288229190. The angular resolution for these data is 10"8 3 7"1 (P.A. 5 198), 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 beam21. While the flag for the 15NNH1 line in (b) is shown for a velocity of 164 km s21, this line occurs at the higher velocity of 1168 km s21. Both 15NNH1 and HCOOCH3 emission peak at the same position, but the difference of 14 km s21 in the velocities of these species indicates that they do not occupy the same volume. Other molecules, such as CH3CH2CN and CH2CHCN, which are also formed primarily via grain-surface reactions, have velocities similar to that of HCOOCH3 (Miao et al. 1995), in the 63– 64 km s21 range. In contrast, species which are formed primarily in the gas phase such as 15NNH1, HNO, CCS, and HC13CCN (Kuan & Snyder 1994) are observed in this same region to have velocities of 68 –70 km s21, 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 s21 ). They were taken at a single 10 square pixel at a(J2000) 5 17h47m19!9, d(J2000) 5 2288229200. The angular resolution for these data is 11.50 3 4.40 (P.A. 5 2198), and the rms noise in a line-free channel is 0.05 Jy beam21. The line at the low-frequency edge of the spectrum in (d) is most likely due to the 22,0–31,3 line of SO2, which has a rest frequency of 100,878.113 MHz.
separated by about 30 (0.1 pc; see Table 2). 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 CH3COOH and HCOOCH3 lines observed in this study are at roughly the same energy above the ground state. Thus, in one region, the formation of CH3COOH is more likely relative to HCOOCH3 than in the other. Wlodarczak & Demaison (1988) have outlined a possible mechanism for the synthesis of CH3COOH. In this model,
CH3COOH is formed principally in the gas phase (Huntress & Mitchell 1979). Based on this formation scheme, Wlodarczak & Demaison (1988) predicted a CH3COOH/HCOOCH3 number ratio in Sgr B2 of 0.1. Because Wootten et al. (1992) did not detect CH3COOH in Sgr B2(OH), they argued that the above mechanism could only produce a CH3COOH/ HCOOCH3 number ratio of less than 0.001. However, our results for the CH3COOH emission peak are consistent with the original prediction of Wlodarczak & Demaison. On the other hand, toward the HCOOCH3 peak, which is about 30 from the CH3COOH peak, it is likely that the original prediction fails. The basic question remains of what the major CH3COOH
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MEHRINGER ET AL. TABLE 2 PARAMETERS
OF THE
CH3COOH
AND
Species
n (MHz)
CH3COOH . . . . . . . . . . . . . . . . . . . .
90246.26 100855.02
HCOOCH3 . . . . . . . . . . . . . . . . . . . .
90229.61
HCOOCH3 EMISSION PEAKS
a(J2000)a,b
IN
SGR B2(LMH)
d(J2000)a,c (Jy beam21 )
17 47 19.92 (0.07) 17 47 19.89 (0.05)
228 22 19.5 (1.3) 228 22 20.3 (1.3)
17 47 19.847 (0.008)
228 22 16.80 (0.15)
I0a 0.21(0.05) 0.19(0.05) ...
Values in parentheses are the 1 s uncertainties. Right ascension in units of hours, minutes, and seconds. c Declination in units of degrees, arcminutes, and arcseconds. a
b
formation process is. While we cannot rule out the gas-phase formation process of Huntress & Mitchell (1979), we do note that the CH3COOH emission peak lies very close to the emission peaks of several other complex species, such as HCOOCH3, CH3CH2CN, and CH2CHCN (Miao et al. 1995). Furthermore, the velocity of the CH3COOH 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 CH3COOH. In contrast, species formed primarily in the gas phase, such as 15NNH1, HNO, CCS, and HC13CCN (Kuan & Snyder 1994) are observed in this same region to have velocities of 68 –70 km s21, thus indicating that there is a clear separation between gas and grain-surface species. Future chemical models should incorporate CH3COOH formation paths in order to try to reproduce the observed abundance of this species in Sgr B2(LMH). 4. SUMMARY
We have discovered CH3COOH in the Sgr B2(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 CH3COOH column density is 7 3 1015 cm22. Its fractional abundance relative to H2 and to its isomer, HCOOCH3, are (0.9–7) 3 10210 and (4–7) 3 1022, respectively. 2. The CH3COOH emission peak is offset from the HCOOCH3 emission peak by about 30 (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 CH3COOH remains an unanswered question. While a gas-phase series of reactions cannot be ruled out, the close relationship of the CH3COOH 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 CH3COOH. 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 93-20239.
REFERENCES Huntress, W. T., Jr., & Mitchell, G. F. 1979, ApJ, 231, 456 Kuan, Y.-J., Mehringer, D. M., & Snyder, L. E. 1996, ApJ, 459, 619 Kuan, Y.-J., & Snyder, L. E. 1994, ApJS, 94, 651 ———. 1996, ApJ, 470, 981 Lis, D. C., Goldsmith, P. F., Carlstrom, J. E., & Scoville, N. Z. 1993, ApJ, 402, 238 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, 26, 906 Millar, T. J., Olofsson, H., Hjalmarson, Å., & Brown, R. D. 1988, A&A, 205, L5
Snyder, L. E. 1997, Origins, Life and Evolution Biosphere, 27, in press Snyder, L. E., Kuan, Y.-J., & Miao, Y. 1994, in The Structure and Content of Molecular Clouds, 25 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., 102, 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, 192, 313 Wootten, A., Wlodarczak, G., Mangum, J. G., Combes, F., Encrenaz, P. J., & Gerin, M. 1992, A&A, 257, 740
FIG. 2.—Contour images of CH3COOH emission overlaid on a gray-scale image of the 8.4 GHz continuum of Sgr B2N from Mehringer, Palmer, & Goss (1997). Contour levels in both panels are at 20.12, 0.12, 0.15, 0.18, and 0.21 Jy beam21 and the numbers on the gray-scale wedge are in units of mJy beam21. (a) Emission from the 90246 MHz transition in a 1.8 km s21 wide channel with a resolution of 10"9 3 7"2 (P.A. 5 198). (b) Emission from the 100855 MHz transition in a 1.5 km s21 wide channel with a resolution of 11"2 3 4"3 (P.A. 5 2198). MEHRINGER et al. (see 480, L72)
PLATE L10
