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Interferometric Observations of Large Biologically Interesting Interstellar and Cometary Molecules

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Interferometric Observations of Large Biologically Interesting Interstellar and Cometary Molecules SPECIAL FEATURE: PERSPECTIVE Interferometric observations of large biologically interesting interstellar and cometary molecules Lewis E. Snyder* Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL 61801 Edited by William Klemperer, Harvard University, Cambridge, MA, and approved May 26, 2006 (received for review March 3, 2006) Interferometric observations of high-mass regions in interstellar molecular clouds have revealed hot molecular cores that have sub- stantial column densities of large, partly hydrogen-saturated molecules. Many of these molecules are of interest to biology and thus are labeled ‘‘biomolecules.’’ Because the clouds containing these molecules provide the material for star formation, they may provide insight into presolar nebular chemistry, and the biomolecules may provide information about the potential of the associated inter- stellar chemistry for seeding newly formed planets with prebiotic organic chemistry. In this overview, events are outlined that led to the current interferometric array observations. Clues that connect this interstellar hot core chemistry to the solar system can be found in the cometary detection of methyl formate and the interferometric maps of cometary methanol. Major obstacles to under- standing hot core chemistry remain because chemical models are not well developed and interferometric observations have not been very sensitive. Differentiation in the molecular isomers glycolaldehdye, methyl formate, and acetic acid has been observed, but not explained. The extended source structure for certain sugars, aldehydes, and alcohols may require nonthermal formation mechanisms such as shock heating of grains. Major advances in understanding the formation chemistry of hot core species can come from obser- vations with the next generation of sensitive, high-resolution arrays. biomolecules ͉ comets ͉ chemistry ne of the most exciting moti- the predominant astrochemical theories of both the laboratory measurements of vating factors for studying the day indicated that only diatomic mole- Woods et al. (9) and the astronomical ob- large, biologically interesting cules could exist in the interstellar clouds. servations of Snyder et al. (10). Thus the interstellar molecules is the Efforts to find new molecules either were X-ogen line is the J ϭ 1–0 transition of Opossibility of learning more about how concentrated on unlikely and͞or difficult- ϩ HCO , which turned out to be the first prebiotic organic chemistry started on to-detect diatomic species or were put on detected rotational spectrum of a molecu- early Earth. Earth formed Ϸ4.5 billion hold because telescope referees consid- lar ion. In the meantime, Watson (11) and years ago (BYA), not long after the Sun, ered the proposed molecular search candi- Herbst and Klemperer (12) published ion- and it may have been hot, dry, and sterile. dates to be too speculative to schedule. molecule chemistry models that became As discussed by Bernstein et al. (1), space This situation started to change dramati- the basis for the quantitative treatment of debris bombarded Earth, and some of it cally in late 1968 and early 1969 when the formation of interstellar molecules in had enough energy to knock off material Cheung et al. (3, 4) used the 6-m tele- gas-phase reactions. An important predic- for the formation of the Moon. This scope at Hat Creek Observatory to detect tion was that HCOϩ is the most abundant ‘‘Great Bombardment’’ ceased Ϸ4.0 BYA, interstellar ammonia (NH3) and then in- polyatomic ion in dense interstellar and from that point in time it appears terstellar water vapor (H2O). These detec- clouds, primarily produced by CO reacting ϩ ϩ that Earth could safely support life. It is tions exposed the incorrectness of the with H3 . The detection of interstellar H3 , generally agreed that the best established ‘‘diatomic molecule’’ barrier and allowed the most abundant ion in large diffuse evidence for the oldest life on Earth is others to gain time to conduct molecular clouds and hence in interstellar space, was contained in Australian and South Afri- searches on many other radio telescopes. reported in 1996 by Geballe and Oka (ref. can microfossils from 3.5 BYA, but For example, in 1969, Snyder, Buhl, Zuck- 13; also see ref. 14). isotopic carbon samples that may have erman, and Palmer were able to use the During the next 30 years, observational belonged to living organisms 3.9 BYA National Radio Astronomy Observatory interstellar chemistry branched out in sev- have been found in rocks from Green- (NRAO) 140-foot telescope to detect in- eral directions. As an example of one di- land. In either case, the relatively short terstellar formaldehyde (H2CO), which rection, quantitative tests of ion-molecule time interval suggests that terrestrial life marked the start of the study of interstel- chemistry models were very successful in may have required help from extraterres- lar organic chemistry (5). After that, the certain types of cool dust clouds, such as trial chemistry. rate of new molecular detections was so TMC-1. These objects were preferred be- The first known interstellar molecules high that about 50 more molecules had cause they could be modeled both physi- ϩ were the diatomic species CH, CH , and been discovered in interstellar space by cally and chemically (see, for example, ref. CN. They were identified over 70 years the late 1970s. Currently, more than 135 15 and references therein). The observa- ago by the absorption features they super- molecules have been detected in interstel- tions usually were made with single- imposed on the optical spectra of hot lar and circumstellar environments in our element telescopes because sensitivity was stars. W. S. Adams, A. E. Douglas, T. galaxy (6). more important than high spatial resolu- Dunham, G. Herzberg, A. McKellar, L. In 1970, Buhl and Snyder (7) reported tion. It was determined that ion-molecule Rosenfeld, and P. Swings were among the the detection of a strong unidentified and atom-radical reactions can produce pioneers who made these interstellar iden- emission line in five galactic sources. This tifications in the late 1930s and early line was labeled ‘‘X-ogen’’ simply because 1940s. In 1963, the radio detection of OH it was unidentified. Shortly thereafter, Conflict of interest statement: No conflicts declared. was reported at 18-cm wavelength by Klemperer (8) argued that the best candi- This paper was submitted directly (Track II) to the PNAS Weinreb et al. (2). But by 1968, research date for the carrier of the X-ogen line was office. ϩ directed toward finding other interstellar HCO . Five years later, Klemperer’s pro- *E-mail: [email protected]. molecules was in a stagnant state because posed identification was confirmed by © 2006 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0601750103 PNAS ͉ August 15, 2006 ͉ vol. 103 ͉ no. 33 ͉ 12243–12248 Downloaded by guest on October 2, 2021 molecules up to intermediate structural lar cores in turn allowed future molecular be explored further on a timely basis. As a complexity with low to moderate hydro- discoveries with millimeter wavelength result of these advantages, the BIMA Ar- gen saturation. Examples include CO, interferometric arrays. ray became the most active in the world ϩ ϩ C2H, C2S, HCO ,N2H , cyclic C3H, lin- for conducting interferometric observa- ϩ ear C3H, H2CO, HNCO, HOCO ,NH3, The Importance of Interferometry for tions of large, partly hydrogen-saturated C4H, cyclic C3H2, linear C3H2,CH2NH, the Study of Large Interstellar Molecules interstellar molecules in compact regions HC3N, HCOOH, H2C2O, C4H2,C5H, with Complex Structures of star formation and molecular emission CH3CN, CH3OH, C6H, CH3C2H, and Interferometric arrays permit mapping of from comets. CH3CHO. However, ion-molecule chemis- radio sources with much higher angular Typically, observational star formation try in cool dust clouds fails to produce resolution than allowed by single-element programs consist of either studies of re- interstellar molecules that are at the next telescopes (see, for example, ref. 25). The gions that form high-mass stars (Ͼ10 solar level of complexity in large, partly hydro- ability to kinematically resolve individual masses, MJ), or studies of regions that gen-saturated species (i.e., the structure cores thereby greatly lessens the spectral form low-mass stars (Ͻ10 MJ). Many contains three or more H atoms with no line confusion encountered with single- BIMA Array observational studies of in- carbon–carbon multiple bonds), such as element telescopes. Numerous arrays have terstellar chemistry at millimeter wave- CH3CH2OH, CH3CH2CN, (CH3)2O, and been used to study interstellar and circum- lengths have been directed toward hot CH3OCHO (see, for example, ref. 16). stellar molecules, molecular masers, and molecular cores in high-mass regions. Another important direction in the interstellar chemistry. The list includes the There are good reasons for this emphasis. study of interstellar chemistry started with Berkeley–Illinois–Maryland Association First, a more complex chemistry has been the realization that many interstellar mo- (BIMA) Array formerly at Hat Creek, detected in high-mass regions than in low- lecular clouds were not uniform, but CA; the OVRO Millimeter Array for- mass regions. Of the more than 135 gas- clumpy (see, for example, ref. 17). High- merly in Owens Valley, CA; the Submilli- phase interstellar molecules that have temperature compact regions called hot meter Array (SMA) on Mauna Kea in been found, about half were initially de- molecular cores are dense (n Ͼ 106 Hawaii; the National Radio Astronomy tected in Sgr B2. Interferometric studies cmϪ3), hot (T Ͼ 150 K), and among the Observatory (NRAO) Very Large Array have shown that the emission from most most visible markers of the earliest phase (VLA) near Socorro, NM; the NRAO large molecules reaches a peak toward Sgr of star formation (18, 19). Some of the Very Long Baseline Array (VLBA) ex- B2(N) in a hot molecular core within the first molecular studies were conducted tending from Mauna Kea, HI, to St.
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