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Ethylene Glycol in Comet C/1995 O1 (Hale-Bopp)

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Ethylene Glycol in Comet C/1995 O1 (Hale-Bopp) A&A 418, L35–L38 (2004) Astronomy DOI: 10.1051/0004-6361:20040116 & c ESO 2004 Astrophysics Ethylene glycol in comet C/1995 O1 (Hale-Bopp) J. Crovisier1,D.Bockel´ee-Morvan1 ,N.Biver1,P.Colom1, D. Despois2, and D. C. Lis3 1 Observatoire de Paris, 92195 Meudon, France 2 Observatoire de Bordeaux, BP 89, 33270 Floirac, France 3 California Institute of Technology, MS 320-47, Pasadena, CA 91125, USA Letter to the Editor Received 23 January 2004 / Accepted 18 March 2004 Abstract. We report the detection of ethylene glycol (HOCH2CH2OH) in comet C/1995 O1 (Hale-Bopp) from the analysis of archival radio spectra. Its production rate is ≈0.25% that of water, making it one of the most abundant organic molecules in cometary ices. This detection strengthens the similarity between interstellar and cometary material. Key words. astrobiology – astrochemistry – comets: general – comets: individuals: C/1995 O1 (Hale-Bopp) – radio lines: solar system – solar system: formation 1. Introduction survey of the comet at radio wavelengths was undertaken with the Institut de Radioastronomie Millim´etrique (IRAM) Investigations of the composition of comets provide clues to 30-m telescope and Plateau de Bure Interferometer (PdBI, used the physical conditions and chemical processes which occurred here in the single dish mode), and at the Caltech Submillimeter in the primitive Solar Nebula. They could also help in under- Observatory (CSO) 10-m telescope. Observations are de- standing the origin of life on Earth, because comets might have scribed in detail by Bockel´ee-Morvan et al. (2000) and played a crucial role in the first stages of the Solar System Crovisier et al. (2004). evolution by seeding the early Earth with prebiotic molecules. At the time the observations were carried out, no line fre- Decisive progress was made in the last decade with the iden- quency predictions were available for ethylene glycol. Soon tification of many cometary molecules, especially from radio after line predictions were published by Christen & M¨uller spectroscopy (Bockel´ee-Morvan et al. 2000, 2004). The most (2003) and incorporated in the Cologne Database for Molecular complex species identified were acetaldehyde and methyl for- Spectroscopy (M¨uller et al. 2001; http://www.cdms.de), we mate. However, the presence of more complex, still unidenti- were able to assign to this molecule several of the unidentified fied species was indicated in the exploration of comet Halley, lines in our survey as well as several previously unnoticed fea- from mass spectroscopy and from evidence of an organic re- tures (Table 1, Figs. 1 and 2). Theory predicts the existence of fractory component in cometary grains (Mitchell et al. 1987; several (10) conformers (Christen et al. 2001). The conformer Fomenkova 1999). detected in the comet and in the interstellar medium is the one We report here the detection of ethylene glycol in of lowest energy, aGg. comet C/1995 O1 (Hale-Bopp) from an analysis of archival Altogether, eight lines of ethylene glycol were detected spectra. Ethylene glycol HOCH CH OH, also known as 2 2 with the IRAM 30-m telescope with a signal-to-noise ratio 1,2-ethanediol, is commonly used as antifreeze in coolant flu- ranging from 3 to 7. The PdBI observed the 230.578 GHz line ids for car engines. It was recently discovered in the interstellar independently on two days (Fig. 2), with a S/N of 10 on the medium by observing four of its rotational lines at millimetre second day. Two more lines were observed at the CSO with wavelengths (Hollis et al. 2002). a S/N of 3 and 6. All the observed spectral features coincide, within observational uncertainties, with the ethylene glycol line 2. Observations and analysis frequencies, and their widths are similar to other cometary lines (although most observations were made with low spectral res- C/1995 O1 (Hale-Bopp) was an exceptionally highly pro- olution). The relative line intensities are reasonably consistent ductive comet, with a water production rate reaching with model predictions (see below). No strong line is predicted, 1031 molec. s−1 at perihelion, in early April 1997. Its appari- within the spectral range of our survey, that does not appear in tion offered the opportunity to investigate the chemical com- the spectra. Therefore, the identification is secure. position of a comet with unprecedented sensitivity. A spectral The inner, collisional coma sampled here is close to thermal Send offprint requests to:J.Crovisier equilibrium. A convenient way to analyze the observations of e-mail: [email protected] several molecular rotational lines is then the rotation diagram Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20040116 L36 J. Crovisier et al.: Ethylene glycol in comet Hale-Bopp Table 1. Lines of ethylene glycol observed in comet Hale-Bopp. a b −1 Transition Frequency Eu Telescope Date r ∆ Int. Tbdv [mK km s ] [GHz] [cm−1]dd/mm/yy [AU] [AU] [min] Modelc Observed 121,12,0–111,11,1 106.9074 25.3 IRAM 30-m 03/04/97 0.91 1.37 145 70 75 ± 16 157,9,0–147,8,1 147.1318 57.7 IRAM 30-m 08/04/97 0.92 1.42 130 72 181 ± 26 157,8,0–147,7,1 147.1324 57.7 92 168,9,0–158,8,1 157.2863 68.3 IRAM 30-m 05/04/97 0.92 1.39 85 94 239 ± 58 168,8,0–158,7,1 157.2864 68.3 74 172,15,0–162,14,1 168.3863 54.9 IRAM 30-m 03/04/97 0.91 1.37 205 198 194 ± 63 251,25,0–241,24,1 226.6433 102.7 IRAM 30-m 11/04/97 0.93 1.45 65 222 440 ± 76 250,25,0–240,24,1 226.6435 102.7 284 Letter to the Editor 215,16,1–205,15,0 227.3158 88.5 IRAM 30-m 05/04/97 0.92 1.39 310 178 135 ± 22 232,21,0–223,20,0 227.5035 96.2 54 72 ± 22 224,18,0–214,17,1 227.5871 94.0 214 114 ± 22 243,22,0–233,21,1 230.5771 104.0 IRAM PdBI 6, 11/03/97 1.00 1.40 34 116 192 ± 18 224,19,1–214,18,0 230.5783 92.3 98 2510,16,1–2410,15,0 263.3025 144.4 CSO 08/04/97 0.92 1.42 44 52 53 ± 16 2510,15,1–2410,14,0 263.3026 144.4 40 291,29,0–281,28,1 263.3921 136.4 CSO 08/04/97 0.92 1.42 44 62 121 ± 21 290,29,0–280,28,1 263.3921 136.4 82 a The energy levels are noted here JKa,Kb,v,wherev is the quantum number associated with OH tunneling motions (Christen & M¨uller 2003). b Integration time (on comet + comparison field). c Line area predicted by our LTE model (Crovisier et al. 2004) for a production rate of ethylene glycol of 0.002 relative to that of H2O(which is 0.8 to 1.1 × 1031 molec. s−1). A uniform rotational temperature of 110 K and a spherical coma expanding with a velocity of 1.04 km s−1 are assumed (Biver et al. 1999). β is set to 2 × 10−5 s−1 at 1 AU. method (Goldsmith & Langer 1999). The plot of log (Nu/gu) lower temperature (Trot = 65 ± 8 K) hard to reconcile with the (where Nu is the column density of the upper state and gu its temperature retrieved from other molecules. statistical weight) for individual lines as a function of the up- If we use Trot = 110 K determined from other molecules, per state energy Eu should be distributed along a straight line the weighted average of the line area ratios [observed]/[model related to the rotational temperature Trot and the total column prediction] from Table 1 leads to a production rate of ethylene density. In the present case, this method is more difficult to ap- glycol 0.17 ± 0.05% relative to water (the error representing ply because the lines were not observed simultaneously and the dispersion between the individual lines). Together with our may therefore be affected by short-term variations of cometary rotation diagram analysis for small β values, this provides a activity. Moreover, the observed line intensities must be cor- robust determination of the abundance of 0.25 ± 0.12%. rected for the differences in the instrumental field of view. The rotation diagram for the glycol lines observed with the IRAM 30-m telescope (i.e., restricted to the 3−11 April period) 3. Discussion is shown in Fig. 3. Column densities have been normalized to the same field of view, assuming a parent molecule spatial dis- The detection of ethylene glycol completes our inventory −5 −1 tribution with a photodestruction rate β = 2 × 10 s at 1 AU of “CHO” molecules in comet Hale-Bopp (Table 2). This (i.e., comparable to those of other CHO-containing molecules molecule of 10 atoms is the most complex species ever iden- such as methanol or ethanol). The data are reasonably well fit- tified in a comet by means of spectroscopy. With an abun- ted by a straight line. The linear regression corresponds to a dance of 0.25% in number (or ≈1% in mass) relative to wa- ± rotational temperature of 77 11 K (which is somewhat lower ter, it is one of the most abundant CHO molecules in comet than the ≈110 K temperature derived at the same time from ob- Hale-Bopp.
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