CONFIRMATION OF INTERSTELLAR ACETONE Lewis E. Snyder, F. J. Lovas, 1 David M. Mehringer, 2 Nina Yanti Miao, 3 and Yi-Jehng Kuan 4. J. M.

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1 The Astrophysical Journal, 578: , 2002 October 10 # The American Astronomical Society. All rights reserved. Printed in U.S.A. CONFIRMATION OF INTERSTELLAR ACETONE Lewis E. Snyder, F. J. Lovas, 1 David M. Mehringer, 2 Nina Yanti Miao, 3 and Yi-Jehng Kuan 4 Department of Astronomy, 1002 West Green Street, University of Illinois, Urbana IL 61801; snyder@astro.uiuc.edu J. M. Hollis Earth and Space Data Computing Division, Code 930, NASA Goddard Space Flight Center, Greenbelt, MD and P. R. Jewell National Radio Astronomy Observatory, P.O. Box 2, Green Bank, WV Received 2002 April 30; accepted 2002 June 12 ABSTRACT We present new observations of interstellar acetone [(CH 3 ) 2 CO] from both the NRAO 12 m and the BIMA array. We report NRAO 12 m detections of 13 new acetone emission features that can be assigned to 20 acetone transitions. These assignments are based on the measured and calculated frequencies in 2002 of Groner and coworkers, and they confirm the interstellar acetone identification in 1987 by Combes and coworkers. In addition, our BIMA array observations show that acetone emission is concentrated in the vicinity of the hot molecular core Sgr B2 (N-LMH). The beam-averaged column density for acetone is hn T i¼2:9ð3þ10 16 cm 2. This value is consistent with the 1990 conclusions of Herbst, Giles, & Smith that the observed acetone abundance is too high to be explained by gas-phase synthesis reactions. Subject headings: ISM: abundances ISM: clouds ISM: individual (Sagittarius B2(N-LMH)) ISM: molecules radio lines: ISM 1. INTRODUCTION The detection of interstellar acetone [(CH 3 ) 2 CO] was reported 15 years ago by Combes et al. (1987). Acetone is one of the most important molecules in organic chemistry, and it was the first 10 atom molecule reported in the interstellar medium. The observations of Combes et al. (1987) were taken in the direction of Sgr B2 (OH) ( ¼ 17 h 44 m 11 s, ¼ [B1950.0]) with the IRAM 30 m, the National Radio Astronomy Observatory (NRAO) 12 m, and the NRAO 43 m telescopes. Detections of the blended J ¼ 8 7 acetone quadruplets at 82.9 GHz and J ¼ at GHz were reported. These detections were based on the acetone rest frequencies of Vacherand et al. (1986), which were determined from both laboratory measurements and frequency fitting. Several interesting developments followed the interstellar acetone detections. First, because the reported acetone detection spectra were weak and blended, several review articles written shortly after the detection listed interstellar acetone as an unconfirmed interstellar identification (see, e.g., Irvine & Knacke 1989; Turner 1989a). Next, Herbst, Giles, & Smith (1990) showed that the observed interstellar acetone abundances were too high to be explained by radiative association between acetaldehyde (HCOCH 3 ) and CH + 3, followed by dissociative recombination. As a result, it appears that interstellar acetone formation cannot be explained by gas-phase ion-molecule synthetic reactions. Finally, Herbst et al. (1990) suggested that the Sgr B2 acetone may be located in a region similar 1 Current address at Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD Current address at National Center for Supercomputing Applications, University of Illinois, Urbana, IL Current address at Bellsoft Technologies, 80 Nandan Lu, Academy of Sciences Building, Suite 1401, Shanghai , China. 4 Current address at Department of Earth Sciences, National Taiwan Normal University, and Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box , 106 Taipei, Taiwan, Republic of China. 245 to the compact ridge source in Orion, where oxygen containing organic molecules has abundances too high to be explained by the formation models of the time. Consequently, we concluded that interstellar acetone warranted further observational study. We decided to (1) confirm the acetone detection by detecting new lines, (2) determine the precise location of the strongest source of the acetone emission in the Sgr B2 region, and (3) use this new information to determine the best available values for the acetone column densities and relative abundances. To accomplish these goals, we used the 12 m radio telescope of the NRAO 5 and the BIMA array. 6 Our preliminary results were reported by Snyder et al. (1997). In the interim, Groner et al. (2002) measured and fitted new rest frequencies for acetone, and we have applied these to our data. Our final results are reported in this paper. 2. OBSERVATIONS 2.1. Rest Frequencies Groner et al. (2002) used over 1000 measured laboratory spectra to predict the frequencies and intensities through 600 GHz of over 10,000 rotational-torsional transitions of acetone in its vibrational-torsional ground state. We have used several of these frequencies to identify new interstellar acetone transitions in the 3 mm range. Following the convention of Lovas (1992), the parentheses immediately following a calculated or measured rest-frequency value in this paper contain a 2 estimate of the uncertainty in the least significant digit. All other listed uncertainties are 1. 5 The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. 6 Operated by the University of California, Berkeley, the University of Illinois, and the University of Maryland, with support from the National Science Foundation.

2 246 SNYDER ET AL. Vol. 578 For astronomical identifications, it is important to note that the spin weights derived by Myers & Wilson (1960) for the A 1 A 1 : EE : EA 1 : A 1 E symmetry states are 6 : 16 : 4 : 2 for the K a K c ¼ ee; oo (symmetric) rotational states and 10 : 16 : 4 : 6 for the K a K c ¼ eo; oe (antisymmetric) states, respectively. For convenience, we have eliminated the subscript 1 of the A label. Thus, 8 0,8 7 1,7 AE is an ee, oo transition in the A 1 E symmetry state and 8 2,7 7 1,6 AA is an eo, oe transition in the A 1 A 1 symmetry state. Spin weighting is discussed in the Appendix An Overview of the Observations As stated in x 1, our first goal was to confirm the interstellar acetone detection report of Combes et al. (1987) by finding new acetone lines. The best way to accomplish this was to locate the source of the strongest acetone emission in the Sgr B2 region and search for new acetone lines in that direction. In previous work with the BIMA array, an interesting result had emerged: array maps of methyl formate (HCOOCH 3 ), vinyl cyanide (CH 2 CHCN), ethyl cyanide (CH 3 CH 2 CN), and acetic acid (CH 3 COOH) showed that the emission from these large molecules has an intensity peak in Sgr B2 (N) at ¼ 17 h 44 m 1092, ¼ (B1950.0), which is the hot molecular core region known as the Large Molecule Heimat source, Sgr B2 (N-LMH) (Snyder, Kuan, & Miao 1994; Mehringer et al. 1997). Combes et al. (1987) would not have detected acetone emission from Sgr B2 (N) because it is 1<3 north of Sgr B2 (OH), outside of both the 12 m and 30 m beams. The best defined group of acetone lines detected by Combes et al. (1987) was the cluster of J ¼ 8 7 transitions between 82,908 and 82,924 MHz; this is where we made our initial NRAO 12 m observations. We found that the acetone 8 7 transitions reached an intensity peak in the direction of Sgr B2 (N). We continued the 12 m acetone search observations of Sgr B2 (N) at frequencies of 81,836, 91,648, and 92,740 MHz. In all, we were able to identify 13 new emission features from the 12 m data, which can be assigned to 20 new interstellar acetone lines. Next, we used the BIMA array to determine that the exact location of the intensity peak of the 8 7 acetone emission from Sgr B2 (N) is indeed Sgr B2 (N-LMH), just as it is for methyl formate, vinyl cyanide, ethyl cyanide, and acetic acid. Detailed discussions of the NRAO 12 m and BIMA array observations are given in the next two sections. We did not attempt to detect acetone emission from Sgr B2 (OH) because both the earlier observations of Kuan & Snyder (1994, 1996) and Kuan, Mehringer, & Snyder (1996), as well as our own 12 m measurements, suggested that any Sgr B2 (OH) acetone emission would be below our sensitivity limits. For example, Kuan & Snyder (1994) and Kuan et al. (1996) showed that both Sgr B2 (N) and Sgr B2 (M) have significant 3 mm dust continuum emission, but Sgr B2 (OH) was not detected down to an rms noise level of 0.1 Jy beam 1. No methyl formate was detected toward Sgr B2 (OH) by Kuan & Snyder (1996) Fig. 1. NRAO 12 m spectra in the frequency range 81,686 81,986 MHz, centered at 81,836 MHz. This range includes the EE, AA, AE, and EA symmetrystate frequencies of the 7 1,6 6 2,5 and 7 2,6 6 1,5 acetone [(CH 3 ) 2 CO] transitions listed in Table 1. The ordinate is T R, and the abscissa is rest frequency calculated with respect to V LSR ¼ 64:0kms 1. The rms noise level is K, as indicated by the small vertical bar on the left.

3 No. 1, 2002 CONFIRMATION OF INTERSTELLAR ACETONE 247 at an rms noise level of 0.19 Jy beam 1. However, as the sensitivity of millimeter-wavelength arrays improves, a future array search for acetone in Sgr B2 (OH) should be attempted NRAO 12 m Observations Single-element observations of (CH 3 ) 2 CO were made during 1995 March with the NRAO 12 m radio telescope. The NRAO 3 mm single-sideband SIS receiver had a sideband rejection 25 db and an effective system temperature (referenced to above the atmosphere and including rear and forward spillover efficiencies) of 300 K. Chopper calibration corrected for atmospheric extinction, and telescope losses and the resultant data are on the T R temperature scale (Kutner & Ulich 1981). Data were taken while position switching 30 0 in azimuth. The conventional beam efficiency, which includes all antenna losses outside the main beam, is 58%. The FWHM is at 82.9 GHz. The spectrometer is the NRAO Hybrid Spectrometer; two 256 channel filter banks were used as a backup. The Hybrid Spectrometer was operated with two polarization intermediate frequencies (IFs) in parallel, with 300 MHz bandwidth and 768 Hanning-smoothed channels per IF, giving an effective spectral resolution of MHz. The filter banks were operated with two IFs in parallel, with 1000 khz resolution in one bank and 500 khz in the other. The NRAO 12 m data were reduced using the NRAO data reduction package UNI- POPS. The NRAO hybrid correlator data are shown in Figures 1 4; these are all autocorrelation spectra. Figure 1 shows the observed transitions in the direction of Sgr B2 (N) in the frequency range 81,686 81,986 MHz, with center ¼ 81; 836 MHz at V LSR ¼ 64:0 km s 1. This range includes the EE, AA, AE, and EA symmetry-state frequencies of the 7 1,6 6 2,5 and 7 2,6 6 1,5 acetone transitions. Table 1 lists the molecular parameters for each identified line in Figure 1: rest frequency (2 uncertainty), molecular identification, quantum numbers of each identified transition, upper energy level for each transition E upper (K), line strength S ij, corrected antenna temperature T R (K), and FWHM line width DV (km s 1 ). In this frequency range, the line width uncertainty is 2.9 km s 1, and the conversion from T R to Janskys is 32.4(32) Jy K 1. Unidentified lines are listed with the standard U label. The acetone identifications in Figure 1 are new detections based on frequencies from Groner et al. (2002). Both the 7 1,6 6 2,5 AE, EA and the 7 2,6 6 1,5 AE, EA transitions are doubly degenerate. Thus, the eight new J ¼ 7 6 acetone lines in the 81,836 MHz band are assigned to six emission features. In addition to acetone, lines of formamide (HCONH 2 ), ethyl cyanide (CH 3 CH 2 CN), cyanoacetylene (HC 3 N), and methylenimine (H 2 CNH) are identified. U is almost frequency coincident with the 2 3=2 J ¼ 59=2 57/2 f transition of 1, 3, 5 hexatriynyl (C 6 H) at 81,778.1(4) MHz (Lovas 1992), but the e-component at 81,801.1(4) MHz is missing. Furthermore, C 6 H has not previously been detected in Sgr B2 sources. The reality of U needs to be confirmed. It may be due to a false absorption feature generated by position switching into weak emission from an extended HC 3 Ncloud,orit TABLE 1 Sgr B2 (N) Spectra between 81,686 and 81,986 MHz (Fig. 1) (MHz) Species Transition E upper (K) S ij T R (K) DV (km s 1 ) 81, (2) a... HCONH 2 4 1,4 3 1, , U , U , (29) b... CH 3 CH 2 CN 18 1, , , U ,777 c... U , (16) d... (CH 3 ) 2 CO 7 1,6 6 2,5 AE e 81, (16) d... (CH 3 ) 2 CO 7 1,6 6 2,5 EA e 81, (14) d... (CH 3 ) 2 CO 7 2,6 6 1,5 EA e 81, (16) d... (CH 3 ) 2 CO 7 2,6 6 1,5 AE e 81, (12) d... (CH 3 ) 2 CO 7 1,6 6 2,5 EE e 81, (12) d... (CH 3 ) 2 CO 7 2,6 6 1,5 EE e 81, (18) d... (CH 3 ) 2 CO 7 1,6 6 2,5 AA e 81, U , (18) d... (CH 3 ) 2 CO 7 2,6 6 1,5 AA , U ,881.5 a... HC 3 N ,906 f... U ,935.0 g... HC 3 N l,1f 1f , U , U e 81,980.1 h... H 2 CNH 11 2,9 11 2, a Previous line detection listed by Lovas b New ethyl cyanide line detection identified from Lovas c Almost frequency coincident with the hexatriynyl radical C 6 H (see text). d New interstellar acetone line detection identified from Groner et al e Blended line; approximate DV was used. f Possibly false absorption feature or c-hc 13 CCH (see text). g New cyanoacetylene line detection identified from Lafferty & Lovas h New methylamine line detection identified from Kirchhoff, Johnson, & Lovas 1973.

4 248 SNYDER ET AL. Vol. 578 may be due to the 3 1,2 3 0,3 transition of c-hc 13 CCH in absorption at 81, (50) MHz (Pickett et al. 1998). Figure 2 shows the 12 m spectral data obtained in the search that was conducted for the acetone intensity peak in Sgr B2. The frequency range was 82,755 83,055 MHz with center ¼ 82; 905 MHz at V LSR ¼ 64:0 km s 1. As previewed in x 2.2, this range includes the EE, AA, AE, and EA symmetry-state frequencies of the and acetone transitions, which were the best-defined acetone lines detected by Combes et al. (1987). Table 2 follows the format of Table 1 and lists the parameters for the spectral lines shown in Figure 2. In this frequency range, the line width uncertainty is 2.8 km s 1, and the conversion from T R to Janskys is 32.4(32) Jy K 1. The bottom spectrum was observed in the direction of Sgr B2 (OH), where the observations of Combes et al. (1987) were taken. The middle spectrum was observed in the direction of Sgr B2 (M) ( ¼ 17 h 44 m 1096, ¼ [B1950.0]), which is north of Sgr B2 (OH). The top spectrum was observed in the direction of Sgr B2 (N), which is another north of Sgr B2 (M). The Sgr B2 (N) acetone spectra are clearly stronger than either the Sgr B2 (M) or Sgr B2 (OH) spectra. Fig. 2. NRAO 12 m spectra from 82,755 to 83,055 MHz, centered at 82,905 MHz. This range includes the EE, AA, AE, and EA symmetry-state frequencies of the 8 0,8 7 1,7 and 8 1,8 7 0,7 acetone [(CH 3 ) 2 CO] transitions listed in Table 2. The abbreviation 8 *,8 7 *,7 is used for blended 8 0,8 7 1,7 and 8 1,8 7 0,7 transitions. The ordinate is T R, and the abscissa is rest frequency calculated with respect to V LSR ¼ 64:0 kms 1. The rms noise levels are K for Sgr B2 (OH), K for Sgr B2 (M), and K for Sgr B2 (N), as indicated by the small vertical bars on the left.

5 No. 1, 2002 CONFIRMATION OF INTERSTELLAR ACETONE 249 TABLE 2 Sgr B2 (N) Spectra between 82,755 and 83,055 MHz (Fig. 2) (MHz) Species Transition E upper (K) S ij T R (K) DV (km s 1 ) 82, (44) a... HC 3 N l,1f 1f b 82,776.77(34) c SO , , b 82, (10) d... HCOCH ,9 10 0,10 A , U b 82, U b 82, (14) e,f... CH 3 C 13 CH b 82, (101) e,f... CH 3 OH 22 5, ,19 A b 82, (15) e,f... CH 3 C 13 CH b 82, (16) e,f... CH 3 C 13 CH b 82, (20) e... (CH 3 ) 2 CO 8 0,8 7 1,7 AE b 82, (20) e... (CH 3 ) 2 CO 8 1,8 7 0,7 AE b 82, (18) e... (CH 3 ) 2 CO 8 0,8 7 1,7 EA b 82, (18) e... (CH 3 ) 2 CO 8 1,8 7 0,7 EA b 82, (14) e... (CH 3 ) 2 CO 8 0,8 7 1,7 EE b 82, (14) e... (CH 3 ) 2 CO 8 1,8 7 0,7 EE b 82, (22) e... (CH 3 ) 2 CO 8 0,8 7 1,7 AA b 82, (22) e... (CH 3 ) 2 CO 8 1,8 7 0,7 AA b 82, (10) e,g... SO , , , (7) e,h... c-c 3 H 2 3 1,2 3 0, , U , (19) i... CH 3 CH 2 CN 10 2,9 10 1, b 83, (45) e,g SO 2 8 1,7 8 0, j 14 b a New cyanoacetylene line detection identified from Lafferty & Lovas b Blended line; approximate DV was used. c Possible weak sulfur dioxide ( 33 SO 2 ) line blended with HC 3 N; rest frequency from Lovas d New acetaldehyde line detection identified from Kleiner, Lovas, & Godefroid e Line previously reported in Sgr B2 (OH) by Combes et al f Blended methyl acetylene and methanol transitions. CH 3 C 13 CH frequencies from new fits. CH 3 OH frequency from Xu & Lovas 1997, where S ij includes dipole moment squared (Debye 2 ). g Rest frequency from Lovas h Rest frequency from Vrtilek, Gottlieb, & Thaddeus i New ethyl cyanide line detection identified from Lovas j In Sgr B2 (M), T R ðkþ ¼0:18 and DV ¼ 18 km s 1. However, because of the low spatial resolution, only a general location for the acetone emission peak can be determined from these data. For the precise source location, the BIMA array observations described in x 2.4 were utilized. Other species identified in this frequency band include cyanoacetylene (HC 3 N) blended with sulfur dioxide ( 33 SO 2 ), acetaldehyde (HCOCH 3 ), methanol (CH 3 OH) blended with methylacetylene (CH 3 C 13 CH), SO 2, cyclopropenylidene (c-c 3 H 2 ), ethyl cyanide (CH 3 CH 2 CN), and 34 SO 2. Figure 3 shows the observed transitions in the direction of Sgr B2 (N) in the frequency range 91,498 91,798 MHz with center ¼ 91; 648 MHz at V LSR ¼ 64:0 km s 1. This range includes the EE, AA, AE, and EA symmetry-state frequencies of the 8 1,7 7 2,6 and 8 2,7 7 1,6 acetone transitions. Table 3 follows the format of Table 1 and lists the molecular parameters for the lines shown in Figure 3. In this frequency range, the line width uncertainty is 2.6 km s 1, and the conversion from T R to Janskys is 33.0(33) Jy K 1. The acetone identifications in Figure 3 are new detections based on frequencies from Groner et al. (2002). The AE and EA symmetry 8 1,7 7 2,6 and 8 2,7 7 1,6 transitions are blended, but the EE transitions are partially resolved, as are the AA. Thus, the eight new J ¼ 8 7 acetone lines in the 91,648 MHz band are assigned to five emission features. However, the spin weights show that the AA transitions should be only 38% (for oo, ee) and 63% (for eo, oe) of the strengths of their corresponding EE transitions. In fact, Figure 3 and Table 3 show that the AA transitions are 4.5 times stronger. This suggests that the AA transitions have either nonthermal excitation or an underlying unidentified line. Other identifications in Figure 3 include the previously detected blended lines of ethyl cyanide, sulfur dioxide, and cyanoacetylene listed by Lovas (1992), a new line of gauche-ethanol (g-ch 3 CH 2 OH) identified from Pickett et al. (1998), and two new blended lines of methyl formate (HCOOCH 3 ) identified from Oesterling et al. (1999). The unidentified lines U91.636, U91.654, U91.662, and U that were detected by Turner (1989b) in Sgr B2 (OH) and listed by Lovas (1992) are possibly the acetone lines between 91,634.6 and 91,662.0 MHz. Figure 4 shows the observed transitions in the direction of Sgr B2 (N) in the frequency range 92,590 92,890 MHz with center ¼ 92; 740 MHz at V LSR ¼ 64:0 km s 1. This range includes the EE, AA, AE, and EA symmetry-state frequencies of the 9 0,9 8 1,8 and 9 1,9 8 0,8 acetone transitions. Table 4 follows the format of Table 1 and lists the molecular parameters for the lines shown in Figure 4. In this frequency range, the line width uncertainty is 2.5 km s 1, and the conversion from T R to Janskys is 33.1(33) Jy K 1. Based on the frequencies of Groner et al. (2002), the four acetone AE and EA symmetry 9 0,9 8 1,8 and 9 1,9 8 0,8 transitions in Figure 4 are blended with U92.724, but the EE and AA transitions are clearly detected. Therefore, these four EE and AA acetone lines have been assigned to two emission features. A

6 250 SNYDER ET AL. Vol. 578 Fig. 3. NRAO 12 m spectra from 91,498 to 91,798 MHz, centered at 91,648 MHz. This range includes the EE, AA, AE, and EA symmetry-state frequencies of the 8 1,7 7 2,6 and 8 2,7 7 1,6 acetone [(CH 3 ) 2 CO] transitions listed in Table 3. The abbreviation 8 *,7 7 *,6 is used for blended 8 1,7 7 2,6 and 8 2,7 7 1,6 transitions. The gauche ethanol line is indicated by g-ch 3 CH 2 OH. The ordinate is T R, and the abscissa is rest frequency calculated with respect to V LSR ¼ 64:0kms 1. The rms noise level is K, as indicated by the small vertical bar on the left. new line of vinyl cyanide (CH 2 CHCN) has been identified, but the remaining lines in Figure 4 are unidentified BIMA Array Observations The six-element BIMA millimeter-wavelength array was used in its B and C configurations in 1995 March and May to observe the blended 8 0,8 7 1,7 EE and 8 1,8 7 0,7 EE transitions of acetone at 82,916.5 MHz. At this frequency, the BIMA array primary beam has a FWHM of 2<4. Each of the two observing sessions had a duration of 6 hr. The unresolved extragalactic source (J2000.0) was observed to calibrate the complex gains, and 3C 273 was observed to calibrate the bandpass responses. Data were edited, calibrated, and imaged using the Multichannel Image Reconstruction, Image Analysis, and Display (MIRIAD) software package of the BIMA consortium. During imaging, the data were weighted by Tsys 2 to optimize the noise level. The angular resolution in the final images is 12>5 5>4 (P:A: ¼ 4 ) with a pointing uncertainty of 1 00 rms. The spectral window containing the acetone lines was located in the lower sideband and had a total bandwidth of 50 MHz (180 km s 1 ). This band was divided into 128 channels for a spectral resolution of 0.39 MHz (1.4 km s 1 ). To improve the signal-to-noise ratio, the data were averaged into bins that were two channels wide, reducing the spectral resolution to 0.78 MHz (2.8 km s 1 ). The typical rms noise in a 2.8 km s 1 wide channel image is 140 mjy beam 1 (0.37 K). Figure 5 shows the BIMA array contour map of the blended 8 0,8 7 1,7 EE and 8 1,8 7 0,7 EE acetone transitions at 82,916.5 MHz. Within the pointing uncertainty, the map peaks in the direction of Sgr B2 (N-LMH), the hot molecular core located in Sgr B2 (N) (see x 2.2). The ultracompact H ii regions K 1,K 2, and K 3 are each marked (Gaume et al. 1995). In addition, at least 24 H 2 O masers are clustered in this region (Reid et al. 1988). Subarcsecond images of ethyl cyanide and vinyl cyanide in the direction of Sgr B2 (N- LMH) show that these species peak very near K 3 (Liu & Snyder 1999). Figure 6 shows the cross-correlation spectrum taken with the BIMA array at the peak of the Sgr B2 (N-LMH) acetone contour map of Figure 5.

7 No. 1, 2002 CONFIRMATION OF INTERSTELLAR ACETONE 251 TABLE 3 Sgr B2 (N) Spectra between 91,498 and 91,798 MHz (Fig. 3) (MHz) Species Transition E upper (K) S ij T R (K) DV (km s 1 ) 91, (13) a... CH 3 CH 2 CN 10 1,9 9 1, b 16 c 91, (4) a,d... SO , , b 16 c 91, (43) a... HC 3 N l, b 16 c 91, (49) a... HC 3 N l,2e 2e b 16 c 91, (44) a... HC 3 N l,2f 2f b 16 c 91, U , (18) e... (CH 3 ) 2 CO 8 1,7 7 2,6 AE b 16 c 91, (16) e... (CH 3 ) 2 CO 8 1,7 7 2,6 EA b 16 c 91, (18) e... (CH 3 ) 2 CO 8 2,7 7 1,6 AE b 16 c 91, (16) e... (CH 3 ) 2 CO 8 2,7 7 1,6 EA b 16 c 91, (14) e... (CH 3 ) 2 CO 8 1,7 7 2,6 EE b 16 c 91, (14) e... (CH 3 ) 2 CO 8 2,7 7 1,6 EE b 16 c 91, (20) e... (CH 3 ) 2 CO 8 1,7 7 2,6 AA b 16 f 91, (20) e... (CH 3 ) 2 CO 8 2,7 7 1,6 AA b 16 f 91, U b 16 c 91, U , (100) g... g-ch 3 CH 2 OH 11 0,11 ( 1 ) 11 1,11 ( 0 ) b 16 c U , (25) h... HCOOCH 3 8 1,8 7 0,7 E b 16 c 91, (29) h... HCOOCH 3 8 1,8 7 0,7 A b 16 c a Previous line detection listed by Lovas b Blended line; approximate intensity was used. c Blended line; approximate DV was used. d Frequency from new laboratory data fit. e New interstellar acetone line detection from Groner et al f Possibly blended with an unidentified line; approximate DV was used. g New gauche ethanol line identification from Pickett et al h New methyl formate line identification from Oesterling et al RESULTS AND DISCUSSION The autocorrelation spectra from the NRAO 12 m telescope record the total flux density. Hence, the NRAO 12 m is best suited for observing spectral lines from sources that fill the half power beamwidth. Because of the relatively large beamwidth, our NRAO 12 m data are not suitable for determining either the size of a compact emission source or its precise location to better than As a consequence, the spectra in Figures 1 4 detect the molecular emission from the Sgr B2 (N) molecular cloud, but the spatial resolution is not high enough to determine whether a particular species, such as acetone, has an emission peak in the small source Sgr B2 (N-LMH). From Figure 2, we note that both the CH 3 CH 2 CN line and the blend of the CH 3 OH and CH 3 C 13 CH lines peak in Sgr B2 (N), but the SO 2 and c- C 3 H 2 lines are stronger in Sgr B2 (M). This suggests that both SO 2 and c-c 3 H 2 come from an extended source of emission, which would be consistent with formation via gasphase ion-molecule reactions. On the other hand, from full synthesis mapping, the CH 3 CH 2 CN emission is known to be confined to a small region of 0:15 0:28 pc in the direction of Sgr B2 (N-LMH) (Miao & Snyder 1997). Because the original acetone detection by Combes et al. (1987) was in the direction of Sgr B2 (OH) and our array observations were in the direction of Sgr B2 (N-LMH), it is informative to examine our data for evidence of extended acetone emission. The NRAO 12 m data (Fig. 2) show that the acetone emission is weak from Sgr B2 (OH) through Sgr B2 (M), but stronger toward Sgr B2 (N). On the other hand, the BIMA array data (Fig. 5) show emission only in the direction of Sgr B2 (N-LMH). The flux contained in the velocityintegrated blend of the 8 0,8 7 1,7 and 8 1,8 7 0,7 EE transitions of acetone toward Sgr B2 (N-LMH) can be calculated from the NRAO 12 m spectra (Fig. 2) and from the BIMA array spectra (Fig. 6). By applying the conversion of 32.4 Jy K 1 (x 2.3) to the NRAO 12 m spectra, we find that the BIMA array spectra contain only 32% of the flux in the NRAO 12 m spectra. This suggests that the acetone emission has a strong, compact component toward Sgr B2 (N-LMH) (measured mainly by the BIMA array) and a low-intensity component (measured mainly by the NRAO 12 m) that is extended toward Sgr B2 (M) (and also Sgr B2 [OH]) on scales larger than those to which the array is sensitive. The BIMA array data can be used to calculate the acetone column density measured toward the compact or hot core component, Sgr B2 (N-LMH). The spectrum of the blended 8 0,8 7 1,7 EE and 8 1,8 7 0,7 EE acetone transitions in Figure 6 can be used to find the beam-averaged column density hn T i in the optically thin limit. From equation (A2), hn T i ¼ 1:02 W EE qe E u =T a b g EE Sl cm 2 ; where W EE, the total integrated intensity, and other terms are defined in the Appendix. Here, g EE ¼ g EE ðee; ooþ ¼ g EE ðoe; eoþ ¼16, W EE ¼ 17:3 Jy beam 1 km s 1 for a FWHM line width of 14 km s 1, E u ¼ 18:7 K, a b ¼ 12>5 5>4, Sl 2 ¼ 63:4 D 2, ¼ 82:9165 GHz, and q ¼ 261:67T 3=2 (Groner et al. 2002). For T ¼ 170ð13Þ K (Pei, Liu, & Snyder 2000), hn T i¼2:9ð3þ10 16 cm 2. For an H 2 column density range of N H2 ¼ ð1 8Þ10 25 cm 2 (Lis et al. 1993; Kuan et al. 1996), the acetone frac- ð1þ

8 Fig. 4. NRAO 12 m spectra from 92,590 to 92,890 MHz, centered at 92,740 MHz. This range includes the EE, AA, AE, and EA symmetry-state frequencies of the 9 0,9 8 1,8 and 9 1,9 8 0,8 acetone [(CH 3 ) 2 CO] transitions listed in Table 4. The abbreviation 9 *,9 8 *,8 is used for blended 9 0,9 8 1,8 and 9 1,9 8 0,8 transitions. The ordinate is T R, and the abscissa is rest frequency calculated with respect to V LSR ¼ 64:0kms 1. The rms noise level is K, as indicated by the small vertical bar on the left. TABLE 4 Sgr B2 (N) Spectra between 92,590 and 92,890 MHz (Fig. 4) (MHz) Species Transition E upper (K) S ij T R (K) DV (km s 1 ) 92, (12) a... CH 2 CHCN 10 1,10 9 1,9 11 = , U b 92, U b 92, U b 92, U b 92, U c 13 d 92, (22)... (CH 3 ) 2 CO 9 0,9 8 1,8 AE c 13 d 92, (22)... (CH 3 ) 2 CO 9 1,9 8 0,8 AE c 13 d 92, (20)... (CH 3 ) 2 CO 9 0,9 8 1,8 EA c 13 d 92, (20)... (CH 3 ) 2 CO 9 1,9 8 0,8 EA c 13 d 92, (16) e... (CH 3 ) 2 CO 9 0,9 8 1,8 EE c 10 d 92, (16) e... (CH 3 ) 2 CO 9 1,9 8 0,8 EE c 10 d 92, (24) e... (CH 3 ) 2 CO 9 0,9 8 1,8 AA c 10 d 92, (24) e... (CH 3 ) 2 CO 9 1,9 8 0,8 AA c 10 d 92, U , U , U , U , U b 92, U b U a New interstellar vinyl cyanide line detection identified from new laboratory data fits. b Blended line; can t determine DV. c Blended line; approximate intensity was used. d Blended line; approximate DV was used. e New interstellar acetone line detection from Groner et al

9 CONFIRMATION OF INTERSTELLAR ACETONE 253 Fig. 6. BIMA array cross-correlation spectrum taken at the peak of the Sgr B2 (N-LMH) acetone contour map in Fig. 5. At 64 km s 1, the center frequency is 82,916.5 MHz, which is the rest frequency of the blended 8 0,8 7 1,7 EE and 8 1,8 7 0,7 EE acetone transitions. The ordinate is flux density per beam I in Jy beam 1 ; the abscissa is V LSR calculated with respect to the center frequency. The velocity resolution is 2.8 km s 1. The rms noise per channel is 0.14 Jy beam 1, as indicated by the small vertical bar on the left. Fig. 5. BIMA array contour map of the blended 8 0,8 7 1,7 EE and 8 1,8 7 0,7 EE acetone transitions at 82,916.5 MHz in the direction of Sgr B2 (N-LMH). The synthesized beam is 12>5 5>4 (P:A: ¼ 4 ), and the contour levels are at 33%, 47%, 60%, 73%, and 87% of the peak intensity of 1.5 Jy beam 1. The cross marks the position of the 3 mm continuum peak. K 1, K 2, and K 3 are the locations of ultracompact H ii regions (Gaume et al. 1995). tional abundance range is X ðch3 Þ 2 CO ¼ð4 30Þ Acetone is surprisingly abundant in Sgr B2 (N-LMH). We note that beam-averaged column densities have now been determined with the BIMA array using comparable angular resolution for several other large molecular species in Sgr B2 (N-LMH). Remijan et al. (2002) found 6:1ð6Þ10 15 cm 2 for acetic acid (CH 3 COOH), and Liu, Mehringer, & Snyder (2001) found 11:0ð27Þ10 15 cm 2 for formic acid (HCOOH), 46:3ð14Þ10 15 cm 2 for ethyl cyanide (CH 3 CH 2 CN), and 11:2ð10Þ10 16 cm 2 for methyl formate (HCOOCH 3 ). The synthesized beams were very similar: for acetic acid and for formic acid, ethyl cyanide, and methyl formate. Figure 7 shows a graphic comparison of these beam-averaged column densities. For the reasons outlined here, these column density estimates may be underestimated in certain instances (see also Liu & Snyder 1999). First, these values are dependent on the coupling between the source and the telescope beam because they are determined from spatially unresolved array measurements. As an example, when ethyl cyanide was measured toward Sgr B2 (N-LMH) by Liu & Snyder (1999) with the BIMA array with subarcsecond resolution, the estimated mean column density was found to be 5 times higher than that found by Liu et al. (2001) using spatial resolution. Second, the assumption of low optical depth may cause an underestimate. Third, the gas excitation temperature may be higher for the hot core of Sgr B2 (N- LMH) than the K that is typically used. Fourth, there is often some fraction of extended flux that is resolved out by interferometric measurements. The NRAO 12 m data can be used to estimate the acetone column density measured toward the low-density extended component in Sgr B2 (N). Following the procedure used in equation (1), equation (A3) can be applied to the spectrum of the blended 8 0,8 7 1,7 EE and 8 1,8 7 0,7 EE acetone transitions in Figure 2 to find the beam-averaged acetone column density hn T i in the optically thin limit. From Figure 2 and Table 2, W EE ¼ 1:68 K km s 1. The effective rotational temperature for the acetone emission in the NRAO 12 m beam is not known, so for this estimate we consider a range of temperatures from 8 to 170 K (Ikeda et al. 2001; Pei et al. 2000). These temperatures give an estimate for hn T i between 2: and 1: cm 2. Combes et al. (1987) found an acetone column density of cm 2 for Sgr B2 (OH) using a rotational temperature of 20 K. Presumably, this acetone would be part of a low-density component extending from Sgr B2 (N) through Sgr B2 (M) to Sgr B2 (OH). We mentioned in x 1 that Herbst et al. (1990) showed that interstellar acetone formation cannot be explained by known gas-phase ion-molecule reactions because the observed acetone abundance is too high. Their conclusions were predicated on the acetone column density of cm 2, the abundance relative to H 2 of , and the

10 254 SNYDER ET AL. Vol. 578 observational results from our array measurements of Sgr B2 (N-LMH) appear to be consistent with the conclusions of Herbst et al. (1990): the observed acetone abundance is too high to be explained by known gas-phase synthesis reactions. Fig. 7. Beam-averaged column densities determined from Sgr B2 (N-LMH) interferometric measurements: acetic acid (CH 3 COOH), formic acid (HCOOH), acetone [(CH 3 ) 2 CO], ethyl cyanide (CH 3 CH 2 CN), and methyl formate (HCOOCH 3 ). Synthesized beams were for acetic acid and for formic acid, ethyl cyanide, and methyl formate. abundance relative to acetaldehyde of 0.3 found by Combes et al. (1987) in the direction of Sgr B2 (OH). For Sgr B2 (N-LMH), our values are 600 times higher for the acetone column density and 8 60 times higher for the acetone abundance relative to H 2. Herbst et al. (1990) calculated that the abundance ratio of acetaldehyde to acetone ([HCOCH 3 ]/[(CH 3 ) 2 CO]) should be if both are formed by gas-phase synthesis. We can estimate this ratio for Sgr B2 (N-LMH) to see how it compares with Sgr B2 (OH), even though there are no published interferometric measurements of acetaldehyde in Sgr B2 (N-LMH) made with angular resolution comparable to our BIMA array measurements of acetone. Ikeda et al. (2001) have reported Sgr B2 (N) observations of acetaldehyde that were made using the 45 m radio telescope of the Nobeyama Radio Observatory (NRO) with a beamwidth. Without knowledge of the beam filling, it is not possible to precisely compare the NRO 45 m acetaldehyde measurements of Sgr B2 (N) with the BIMA array acetone measurements of Sgr B2 (N-LMH). However, since acetaldehyde shares some common structural elements with formic acid and methyl formate, we can make an estimate by scaling the NRO formic acid and methyl formate column densities of Ikeda et al. (2001) against the interferometrically determined values for Sgr B2 (N-LMH) discussed previously (Fig. 7). The scale factors are 43 for methyl formate and 155 for formic acid. Hence, the estimated acetaldehyde column density in Sgr B2 (N-LMH) measured by an array with angular resolution comparable to that of our acetone measurements would be ð1:7 6:2Þ10 16 cm 2 ; this gives a column density ratio of ½HCOCH 3 Š=½ðCH 3 Þ 2 COŠ 0: Thus, the 4. SUMMARY The NRAO 12 m telescope was used to confirm the detection of the blended 8 0,8 7 1,7 EE and 8 1,8 7 0,7 EE transitions of interstellar acetone originally reported toward Sgr B2 (OH) by Combes et al. (1987). These transitions were observed toward Sgr B2 (OH), Sgr B2 (M), and Sgr B2 (N). The acetone emission toward Sgr B2 (N) was found to be about twice as strong as that toward Sgr B2 (OH). In addition, the following new transitions were identified toward Sgr B2 (N) using the assignments of Groner et al. (2002): 7 1,6 6 2,5 and 7 2,6 6 1,5 AA, EE, AE, and EA (Fig. 1); 8 1,7 7 2,6 and 8 2,7 7 1,6 AA, EE, AE, and EA (Fig. 3); and 9 1,9 8 0,8 and 9 0,9 8 1,8 AA and EE (Fig. 4). The assignment of these 20 new acetone lines verifies the identification of interstellar acetone. The BIMA array was used to map the blended 8 0,8 7 1,7 EE and 8 1,8 7 0,7 EE acetone transitions. The peak emission was found to be toward Sgr B2 (N-LMH), a hot molecular core located in Sgr B2 (N). This is consistent with the NRAO 12 m results. The lower bound estimated for the beam-averaged column density is 600 times higher than that found for Sgr B2 (OH) by Combes et al. (1987). We make this column density comparison with two caveats based on assumptions stated by Combes et al. (1987). First, the 20 K temperature estimate for Sgr B2 (OH) is 8.5 times lower than the 170 K found for Sgr B2 (N-LMH) by Pei et al. (2000). Second, it was assumed that the Sgr B2 (OH) acetone source is uniformly extended so that there is similar source coupling by both the IRAM 30 m and NRAO 12 m beams. If it should turn out that the Sgr B2 (OH) acetone emission is coming from either a hotter source or a more compact source, then the column density would increase. In any case, the high acetone column density and high abundance of acetone relative to acetaldehyde are consistent with the conclusions of Herbst et al. (1990) that the observed acetone abundance is too high to be explained by known gas-phase synthesis reactions. We thank P. Groner and M. C. H. Wright for helpful comments. An anonymous referee made useful suggestions that improved the text. Y.-J. K. acknowledges support from grants NSC M and NSC M J. M. H. received support from NASA RTOP We acknowledge support from the Laboratory for Astronomical Imaging at the University of Illinois, and NSF AST APPENDIX SPIN WEIGHTING AND THE ACETONE COLUMN DENSITY The spin weighting (introduced in x 2.1) and rotational symmetry must be taken into account in the calculation of acetone column density. Spin weighting has been examined for dimethyl ether [(CH 3 ) 2 O] by Lovas, Lutz, & Dreizler (1979) and for acetone by Groner et al. (2002). Because dimethyl ether and acetone have the same point-group symmetry (C 2V ) and similar internal rotation, the same arguments apply to both. The acetone spin weights for the AA : EE : EA : AE symmetry states are

11 No. 1, 2002 CONFIRMATION OF INTERSTELLAR ACETONE : 16 : 4 : 2 for K a K c ¼ ee; oo and 10 : 16 : 4 : 6 for K a K c ¼ eo; oe. Following Groner et al. (2002), the effective acetone partition function q can be written as q ¼ Q TðABCÞ g T ; ða1þ where Q T (ABC) is the standard rotational partition function, g T is the total spin weight ð2i þ 1Þ 6, and is the symmetry number. For acetone, ¼ 2 because of C 2V symmetry, and I ¼ 1 2. The strongest acetone transitions observed by the BIMA array were the blended 8 0,8 7 1,7 EE and 8 1,8 7 0,7 EE transitions (Fig. 5, Table 2). The first transition has K a K c ¼ ee; oo (symmetric) rotational states, and the second has K a K c ¼ oe; eo (antisymmetric) states. Normally, the integrated intensity W T ¼ R I dv can be measured directly. In this case, the blending makes the direct measurement of each EE component impossible. However, it is possible to measure the sum of the two integrated intensities, W EE ¼ W EE ðee; ooþþw EE ðoe; eoþ. Then the expression for the beam-averaged column density hn T i given by Miao et al. (1995) can be adapted to the blended 8 0,8 7 1,7 EE and 8 1,8 7 0,7 EE transitions of acetone. For the BIMA array cross-correlation observations, hn T i ¼ 2:04 W EE ðee; ooþ 2 a b g EE ðee; ooþ þ W EEðoe; eoþ QT ðabcþg T e E u=t g EE ðoe; eoþ Sl cm 2 : ða2þ In equation (A2), W EE is in Jy beam 1 km s 1, E u is the upper level rotational energy in K, h a and h b are the FWHM synthesized beam dimensions in arcseconds, S is the line strength, l 2 is the square of the dipole moment in Debye 2, and is the frequency in GHz. A helpful simplification is that for acetone, g EE ðee; ooþ ¼g EE ðoe; eoþ ¼16, g T ¼ 64, and q-values have been calculated for several different temperatures by Groner et al. (2002). The acetone transitions in this paper are all b-type, so only the b-component of the dipole moment [2.93(3) D] is needed (Peter & Dreizler 1965; Groner et al. 2002). For the NRAO 12 m autocorrelation observations, the expression for the beam-averaged column density hn T i given by Snyder et al. (2001) can also be adapted to the blended 8 0,8 7 1,7 EE and 8 1,8 7 0,7 EE transitions of acetone: hn T i ¼ 1:67 W EE ðee; ooþ 2 g EE ðee; ooþ þ W EEðoe; eoþ QT ðabcþg T e E u=t g EE ðoe; eoþ Sl cm 2 : ða3þ The terms in equation (A3) are defined the same as for equation (A2), except that the integrated intensity is now R T R dv in K km s 1. Combes, F., Gerin, M., Wootten, A., Wlodarczak, G., Clausset, F., & Encrenaz, P. J. 1987, A&A, 180, L13 Gaume, R. A., Claussen, M. J., De Pree, C. G., Goss, W. M., & Mehringer, D. M. 1995, ApJ, 449, 663 Groner, P., Albert, S., Herbst, E., DeLucia, F. C., Lovas, F. J., Drouin, B. J., & Pearson, J. C. 2002, ApJS, 142, 145 Herbst, E., Giles, K., & Smith, D. 1990, ApJ, 358, 468 Ikeda, M., Ohishi, M., Nummelin, A., Dickens, J. E., Bergman, P., Hjalmarson, A., & Irvine, W. M. 2001, ApJ, 560, 792 Irvine, W. M., & Knacke, R. 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