Interferometric Observations of Large Biologically Interesting Interstellar and Cometary Molecules

Home , Cyanoacetylene, Cyanopolyyne, Methyl isocyanide, Stellar chemistry

PCA ETR:PERSPECTIVE FEATURE: SPECIAL

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 interstellar 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 understanding 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 observations 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. more extended interstellar cloud (see, for with single-element telescopes in torsion- Croix, U.S. Virgin Islands; the Plateau example, refs. 23, 29, and 30). This core ally excited methanol (20) and ammonia de Bure Interferometer array in France; is smaller than the Oort Cloud, but its (21). Initially, the primary research inter- the Giant Meterwave Radio Telescope mass may be several thousand MJ (31). est in hot molecular cores focused more (GMRT) near Pune, India; the e- This hot molecular core is named the on excitation conditions than on the MERLIN Array headquartered at Jodrell Large Molecule Heimat source, or Sgr chemistry of interstellar molecule forma- Bank in Great Britain; the Nobeyama B2(N-LMH), because of the extraordinar- tion. However, there were strong hints Millimeter Array (NMA) in Japan; and ily high column densities of the large, that the chemistry might be very interest- the Australian Compact Array (ACTA) in partly hydrogen-saturated molecules (32). ing in compact regions associated with Narrabri, Australia. A new array, the Examples are vinyl cyanide (CH2CHCN), molecular sources. For example, the mo- Combined Array for Research in Milli- ethyl cyanide (CH3CH2CN), methyl lecular cloud region around Sgr B2 con- meter Astronomy (CARMA) is nearing formate (HCOOCH3), dimethyl ether sistently has been the best source of the completion on a new high site near ((CH3)2O), acetone ((CH3)2CO), and ace- most complex molecules detected in Bishop, CA. This array is the result of the tic acid (CH3COOH). It is very important the Galaxy. Goldsmith et al. (22) used the merger of the BIMA and OVRO arrays. to note that these molecules cannot be Nobeyama Radio Observatory 45-m with Currently under construction are the Ata- formed efficiently by the mechanisms of 15Љ angular resolution to study the non- cama Large Millimeter Array (ALMA) in cool gas ion-molecule chemistry. One uniform density distribution, or clumps, in northern Chile (26) and the Allen Tele- may suppose that large, partly hydrogen- the central 2Ј of the Sgr B2 complex. scope Array (ATA) at Hat Creek, CA saturated molecules could be formed by They observed HC3N, SO, OCS, and (27). The Square Kilometre Array (SKA) condensation reactions in which molecular HNCO and found differences in the spa- is in the planning stage (28). ions and neutrals react to form larger tial distributions that they attributed to From the time it started operation as a products that contain more hydrogen at- variations in the chemical abundances of 3-antenna array in 1985 until it ceased oms. However, detailed gas-phase calcula- the various clumps. Vogel et al. (23) used operation as a 10-antenna array in June, tions by Leung et al. (33), Bettens and the Very Large Array to observe NH3 2004, to become part of the future Herbst (34), and Herbst (personal com- with synthesized beams of Ϸ5Љϫ3Љ and CARMA project, the BIMA Array of- munication) show that only carbon clus- found two hot, compact clumps in the fered several advantages for research on ters or hydrocarbons with one and two direction of Sgr B2(M) and Sgr B2(N). interstellar chemistry. High angular reso- hydrogen atoms prevail when the product Recognition of Sgr B2(N) as an outstand- lution over a large field of view and spa- complexity reaches the level of three or ing source of large interstellar species tial filtering of extended emission allowed four carbon atoms. Therefore, condensa- started to emerge from the results of the the study of compact objects such as hot tion chemistry can generate large mole- submillimeter bandscans of Sgr B2(M) molecular cores. Simultaneous measure- cules, but it will not produce large, partly and Sgr B2(N) by Sutton et al. (24). They ment of upper and lower sideband spec- hydrogen-saturated molecules. Conse- found that Sgr B2(N) had considerably tral windows doubled the amount of quently, without a quantitative molecular higher column densities of complex available spectral line data. Excellent re- formation theory, observations have led chemical species such as dimethyl ether ceiver and correlator systems allowed the the way in this area of interstellar chemis- ((CH3)2O), often by a factor of Ϸ5. The detection of weak signals. State-of-the art try. Indeed, from an observational per- high spatial resolution of their submillime- data archiving eased the normally painful spective, the chemistry of hot molecular ter wavelength observations made them process of combining interferometric cores is distinctly different from the con- more sensitive to emission from compact, array data sets. Fairly flexible scheduling ventional ion-molecule chemistry found in dense cores. These embedded hot molecu- allowed promising observational results to extended cool clouds. In support of this

12244 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0601750103 Snyder Downloaded by guest on October 2, 2021 this information, hot core chemistry models will lack the predictive power that is routinely achieved by models of ion-molecule chemistry. Many of the large, partly hydrogen- saturated molecular species observed in hot molecular cores are also of biological interest. The chemical mechanisms that generate these biomolecules could be transferred to newly formed planets during a bombardment phase by the grains in comets, asteroids, and meteorites, so there is a potential connection between prebiotic organic chemistry and the chemistry of the interstellar medium. An immediate goal in the study of biomolecules is the detection of glycine (NH2CH2COOH), the simplest biologically important amino acid. Although the existence of glycine is Fig. 1. Beam-averaged column densities ͗NT͘ determined from Sgr B2(N-LMH) interferometric measurements: acetic acid (CH3COOH); formic acid (HCOOH); acetone ((CH3)2CO); ethyl cyanide (CH3CH2CN); and yet to be confirmed in the interstellar me- methyl formate (HCOOCH3). [Reproduced with permission from Snyder et al. (43) (Copyright 2002, American dium (41), an interesting prediction can Astronomical Society).] be made based on observations with interferometric arrays. Two important biomolecules, formic acid (HCOOH) and acetic view, the BIMA Array surveys of Sgr tion) used a combined gas–surface and acid (CH3COOH), share the C–C–O B2(N-LMH) and Orion show that the gas-phase process to model hot core backbone with glycine, and the latter is emission from the cyanopolyyne HC3Nis chemistry because it was found that just one NH2 group away from glycine. extended into the cooler outer regions of standard models that use methanol and Data from the BIMA Array show that dense clouds (35, 36). This distribution gas-phase chemistry at Ϸ100 K have millimeter wavelength transitions of for- would be expected for species formed pri- severe failings. Their new model pro- mic acid, acetic acid, methyl formate marily from cool gas ion-molecule reac- duces large abundances of HCOOCH3, (HCOOCH3), acetone ((CH3)2CO), di- tions, which exclude both endothermic HCOOH, and (CH3)2O during the hot methyl ether ((CH3)2O), ethyl cyanide reactions and exothermic reactions that protostellar switch-on phase. An impor- (CH3CH2CN), and methyl cyanide require activation energy to overcome a tant feature of this model is that it re- (CH3CN) are concentrated around hot potential barrier (see, for example, ref. quires large, gas-phase molecular ions ϩ ϩ molecular cores in sources such as Sgr 37). In contrast, the spatial distribution of such as CH3OH2 ,CH3OH2OCH2 , B2(N-LMH), W51 e1͞e2, and G34.3ϩ0.2 ϩ ϩ ϩ the partly hydrogen-saturated molecule H2COH ,H2COHOCH ,CH , and ϩ 2 3 (see, for example, refs. 42 and 43). Beam- CH2CHCN, which has only two more H HC(OH)OCH3 to produce methyl for- averaged column densities have been de- atoms, is observed to be compact. This mate. However, a major obstacle to obser- termined with the BIMA Array using picture is consistent with a formation vational confirmation of this type of hot comparable angular resolution for several chemistry that occurs directly on the man- core chemistry model is that there are few large molecular species that have compact tles of dust grains in hot molecular cores laboratory measurements of large molecu- distributions around the hot molecular (T Ͼ 150 K) or in the heated enriched gas lar ions. In addition, only a small number core in Sgr B2(N-LMH) and are shown in of large molecules that originated on the of interstellar ions has been identified: ϫ 16 Ϫ2 ϩ Fig. 1: 0.6(1) 10 cm for acetic acid; ϩ ϩ ϩ ϩ ϩ 16 Ϫ2 dust grains before being driven off by, for CH ,CO ,SO , HCO ,HN2 , HOC , 1.1(3) ϫ 10 cm for formic acid; ϩ ϩ ϩ ϩ 16 Ϫ2 example, radiation from a hot molecular HCS , HCNH , HOCO ,H3O , 2.9(3) ϫ 10 cm for acetone; 4.6(1) ϫ ϩ ϩ ϩ 16 Ϫ2 core. This rich, active chemistry is concen- H2COH ,HC3NH , and H3 . Despite 10 cm for ethyl cyanide; and trated in a compact hot core geometry this small number, interstellar molecular 11.2(10) ϫ 1016 cmϪ2 for methyl formate. that enhances detections of molecular ro- ions have had a long history of association The synthesized beams were very similar tation spectra by telescopes that have high with astronomical observations. Further- for these remarkably high column densi- spatial resolution, such as interferometric more, recent spectral line surveys suggest ties: 11Љϫ4Љ for acetic acid; 12.5Љϫ5.4Љ arrays. that the number of unidentified interstel- for acetone; and 14Љϫ4Љ for formic acid, How close are we to acquiring a quanti- lar species is at least as great as the iden- ethyl cyanide, and methyl formate (43). If tative molecular formation theory for tified species (35, 36). Molecular ions the chemistry of glycine depends on acetic large, partly hydrogen-saturated interstel- could be very important candidates for acid as a precursor, it appears from Fig. 1 lar molecules? A discussion of the chemis- the carriers of many of these unidentified that the column density of glycine would try of organic molecules in hot molecular spectral lines because ionization by cosmic be no higher than Ϸ1016 cmϪ2 in Sgr cores has been given elsewhere (38), so rays, stellar radiation, and accelerated B2(N-LMH). However, these column den- only the most pressing observational con- electrons produce them abundantly in as- sities may be underestimated in certain cerns will be outlined here. It is known tronomical plasmas and, once formed, instances. First, these values are depen- that a large variety of molecular species they may have long lifetimes in the rela- dent on the coupling between the source can be formed on ice mantles in the labo- tively low-density interstellar clouds (14). and the telescope beam because they were ratory: alcohols, quinones, esters, and It is critical that the molecular ions central determined from spatially unresolved ar- even amino acids (39, 40). At this point in to hot core chemistry be identified be- ray measurements. As an example, when time, however, the production of large, cause there are clear cases where labora- ethyl cyanide was measured toward Sgr partly hydrogen-saturated interstellar mol- tory frequency assignments followed by B2(N-LMH) with the BIMA Array using ecules is not well understood. Recently, array observations could define the pre- subarcsecond resolution (31), the esti- Garrod and Herbst (personal communica- dominant formation mechanisms. Without mated mean column density was found to

Snyder PNAS ͉ August 15, 2006 ͉ vol. 103 ͉ no. 33 ͉ 12245 Downloaded by guest on October 2, 2021 be Ϸ 5times higher than that found by noise ratio. It also appears in the spectra density to the column densities of other using 14Љϫ4Љ spatial resolution (44). Sec- of glycolaldehdye (50), ethylene glycol large molecular species. In a typical acetic ond, an assumption of low optical depth (51), and vinyl alcohol (52). Thus, these acid source, the column density of acetic may cause an underestimate. Third, the three large molecules have velocity com- acid appears to be Ϸ15–100 times below gas excitation temperature may be higher ponents associated with the Sgr B2(N-h) the abundance of methyl formate, Ϸ2–10 for the hot core of Sgr B2(N-LMH) than core, even though they have extended times below the abundance of formic acid, the 170–200 K that is typically used. emission. On the other hand, the glycolal- and Ϸ2–10 times below the abundance of Finally, often there is some fraction of dehyde isomers acetic acid and methyl ethyl cyanide. In those sources where ace- extended flux that is resolved out by formate are concentrated in Sgr B2(N- tic acid was not detected, such as interferometric measurements. LMH). This example demonstrates the W3(OH) and W3(H2O), these limits were A problem with associating biomol- importance of employing higher spatial not reached. Toward the Orion hot core ecules exclusively with hot molecular resolution to analyze the complex chemis- and compact ridge, these detection limits cores first appeared with observations of try of hot molecular cores. were reached, but acetic acid was not de- the acetic acid isomer glycolaldehyde Interferometric searches have been con- tected. However, the W3 sources and toward the hot molecular core in Sgr ducted for new sources of acetic acid and Orion sources may differ from the other B2(N-LMH). BIMA Array observations other large molecules that are outside the sources. For example, the survey observa- show that, unlike acetic acid, the glycolal- galactic center region because it is impor- tions reveal no detection of acetic acid in dehyde emission is extended, and not tant to learn how well the astrochemistry any hot molecular core that shows a confined to the LMH (45). Another alde- found in and near the hot molecular cores chemical differentiation of the O-bearing hyde, acetaldehyde (CH3CHO), was im- Sgr B2(N-LMH) and Sgr B2(N-h) really and N-bearing species, such as Orion, aged with the Giant Meterwave Radio represents the astrochemistry of hot mo- where the hot core and compact ridge Telescope (46) and found to be spatially lecular cores in the disk of the Galaxy. To have been characterized as N-rich and extended and not concentrated in Sgr date, 18 galactic sources outside of Sgr O-rich molecular sources, respectively B2(N-LMH). Similarly, the Green Bank B2(N-LMH) have been surveyed for in- (57). The observations clearly show this Telescope (GBT) detections of the terstellar acetic acid with the BIMA Array chemical differentiation toward both two new interstellar aldehydes, pro- (42, 53–55). As a result, two other hot Orion and W3(OH) (42). Therefore, it is penal (CH2CHCHO), and propanal molecular core sources of acetic acid have possible that the formation of acetic acid (CH3CH2CHO), as well as ethylene glycol been found: W51e2 and G34.3ϩ0.2. Ace- may depend strongly on a mix of nitrogen (HOCH2CH2OH), the sugar alcohol of tic acid in W51e2 has a column density and oxygen chemistry, but these results glycolaldehyde, show that they are ex- comparable to that in Sgr B2(N-LMH). are very preliminary. tended (47). To study the kinematics of Acetic acid in G34.3ϩ0.2 has a column Isomers present another set of interest- Sgr B2(N-LMH) and its surrounding re- density range of (0.77–1.64) ϫ 1015 cmϪ2 ing problems. Remijan et al. (58) used the gion, ethyl cyanide observations were and a relative (acetic acid)͞(methyl for- Green Bank Telescope to observe the made at 7-mm wavelength with the Very mate) column density ratio of Ϸ3.3 ϫ cyanide series methyl cyanide (CH3CN), Ϫ2 Large Array (48). The naturally weighted 10 , which is comparable to the ratio vinyl cyanide (CH2CHCN), ethyl cyanide synthesized beam was 1.5Љϫ1.4Љ, so beam found toward Sgr B2(N-LMH) and (CH3CH2CN), and cyanoacetylene Ϫ2 dilution was minimal. At this resolution, W51e2 [(3–6) ϫ 10 ] (53). In addition, (HC3N) toward Sgr B2(N). They searched two emission sources of ethyl cyanide ap- Cazaux et al. (56) used the Institute de for the corresponding isocyanide isomers, peared. The bulk of the ethyl cyanide Radioastronomie Millime´trique (IRAM) but were successful only in detecting emission resides in the Sgr B2(N-LMH) 30-m telescope to detect several partly methyl isocyanide (CH3NC). BIMA Array hot core source, but Ϸ5Љ north of Sgr hydrogen-saturated molecules in IRAS archival data searches for methyl isocya- B2(N-LMH) there is a secondary ethyl 16293-2422. Their detections may include nide were not successful, which suggests cyanide emission source. It is coincident acetic acid, but at a column density below that the methyl isocyanide distribution with the quasithermal methanol emission the BIMA Array upper limit (42). Even may be extended whereas the methyl cya- core ‘‘h’’ reported by Mehringer and Men- though the available acetic acid survey nide is more concentrated in Sgr B2(N). ten (49) and thus can be labeled Sgr sample is too small to draw statistically Furthermore, ab initio calculations show B2(N-h). Extended ethyl cyanide absorp- significant conclusions, the current data that the binding energy differences be- tion can be seen east of Sgr B2(N-LMH) suggest that a detectable amount of acetic tween cyanides and isocyanides are and is apparently absorbing against the acid is present in regions that follow cer- Ͼ12,000 K, which makes cyanides the continuum emission. The molecular emis- tain constraints (42, 54, 55). First, the more stable isomer. Consequently, it has sion associated with Sgr B2(N-LMH) has three main acetic acid sources are within been suggested that large-scale nonther- a velocity centered near 64 km⅐sϪ1 with 7 kiloparsecs (1 parsec ϭ 3.09 ϫ 1016 m) mal processes in the extended region may respect to the local standard of rest of the Galactic center, but more extensive account for the conversion of methyl cya- (LSR). This is the dominant LSR emis- observations will be necessary to deter- nide to methyl isocyanide (58). As a re- sion velocity observed at 3-mm wave- mine how important galactocentric dis- sult, isomers separated by large binding length with the BIMA Array for vinyl tances are as a constraint. On the other energy differences may be distributed in cyanide, ethyl cyanide, methyl formate, hand, it is apparent that mass has an in- different interstellar environments, and methanol, acetone, formic acid, and acetic fluence on whether acetic acid is detect- hence may not be cospatial. A more com- acid. By treating Sgr B2(N-LMH) as a able. The acetic acid detections that have plex situation is found in the case of the rotating molecular disk, Hollis et al. (48) been made with the BIMA Array are in isomers glycolaldehdye, methyl formate, calculated that its mass is Ϸ 2,600 MJ. star-forming regions with total masses and acetic acid in Sgr B2(N-LMH). Note This is close to the mass estimated by Ͼ200 MJ. As these sources are observed the high column density of methyl for- Vogel et al. (23). Sgr B2(N-h) has lower with next-generation arrays that have mate in Fig. 1. It is Ϸ18 times higher than mass but a very interesting chemistry. The higher spatial resolution and greater sensi- that of its isomer acetic acid. Mehringer et peak Sgr B2(N-h) LSR velocity is cen- tivity than current arrays, they may be al. (59) found that these two isomers have tered near 72 km⅐sϪ1, which is a velocity resolved into multiple sources, each emission peaks separated by Ϸ 3Љ (0.1 also seen in the 3-mm wavelength ethyl smaller than the original. Remijan et al. parsec). Because the observed transitions cyanide data (31) with a lower signal-to- (42) compared the acetic acid column are relatively low-lying and roughly at the

12246 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0601750103 Snyder Downloaded by guest on October 2, 2021 same excitation energy, excitation differ- 2422. They suggest that hot molecular col was identified in comet Hale–Bopp ences were ruled out. Hence, the reason cores found in low-mass regions are some- (67). Methyl formate may provide a that these two isomers are not cospatial what colder (Ϸ100 K) and much smaller connection to acetic acid and even to must be the result of chemical differences. (Ͻ20 astronomical units, AU; 1 AU ϭ glycine in comets. BIMA Array observa- High-sensitivity observations with spatial 1.5 ϫ 1011 m) than hot molecular cores tions of Hale–Bopp missed the sensitiv- resolutions of 2Љ or better are needed to found in high-mass regions. Because there ity required to image cometary methyl determine accurate column density ratios are some differences in physical condi- formate by at least a factor of 3. This in these two closely spaced regions. The tions, the chemistry may also exhibit dif- situation will change as observations third isomer, glycolaldehyde, has a spatial ferences that should be possible to directly with new interferometric arrays bring a scaleϾ 60Љ, whereas methyl formate and observe with the next generation of inter- new era in the imaging of large co- acetic acid are largely confined to a ferometric arrays. metary biomolecules. They will have an region Ϸ5Љ in diameter (45). The line appropriately wide field of view to ac- widths of glycolaldehyde are 2–3 times Comets: Potential Carriers of Prebiotic count for the usual uncertainties in the wider than those of other large, partly Organic Chemistry ephemerides. Multiple baselines will hydrogen-saturated molecules confined Grains in cometary nuclei may have car- make them sensitive to many cometary to the hot molecular core, which is ried prebiotic organic chemistry from source sizes, and near wavelengths of 1 consistent with a formation chemistry the interstellar clouds to the early Earth mm, they will have a gain over the that favors glycolaldehyde outside the (see, for example, ref. 38). Arrays can BIMA Array of at least an order of core. These extended distributions may sample the gaseous component of the magnitude in sensitivity. be the result of chemistry due in part grain chemistry in cometary nuclei, but to shock heating of grains (46, 47). yet from a spectroscopic standpoint, we Conclusions As telescope sensitivity and spatial reso- have little quantitative information The radio discoveries of interstellar lution have improved, it has been found about the chemical composition of co- polyatomic molecules in the 1960s broke that the typical high-mass source is re- metary grains (64). Theory links the the diatomic barrier and led to new solved into multiple sources of lower composition of comets to that of the research directions in the study of inter- mass. Indeed, L. Looney, J. Tobin, and B. interstellar medium, but in contrast to stellar chemistry. One direction is the Fields (personal communication) have the more than 135 identified gas-phase interferometric study of large, partly argued that our Sun was born in a cluster species in the interstellar medium and in hydrogen-saturated molecules in regions that contained at least one massive star, circumstellar shells, only about half as of star formation. BIMA Array obser- which exploded near the still-forming Sun many have been identified in comets. vations have shown that those mol- and seeded the early solar nebula with Millimeter wavelength interferometric ecules associated with hot molecular radioactive ejecta. Their conclusion that spectroscopy is very well suited for iden- cores in high-mass sources such as Sgr our Sun may have been born in a massive tifying parent molecules because in com- B2(N-LMH) have unexpectedly high star-forming region is consistent with the ets three special problems arise: the column densities. Their formation is as- evidence that the majority of stars in the observed lines are variable from day to sumed to be driven by a unique hot Galaxy formed in clusters (60). This ob- day; the peak emission may be displaced core chemistry, not only because they servation suggests that many low-mass from the nucleus; and often at the time are associated with hot dusty core re- stars, such as our Sun, may be formed in of the observations of new comets the gions, but also because ion-molecule regions containing high-mass objects. ephemeris is not well enough known to formation mechanisms cannot generate Consequently, it can be argued that the locate the nucleus more accurately than their observed abundances. Perhaps the chemistry of hot molecular cores in high- Ϸ2Ј. Therefore, interferometric studies most important large, partly hydrogen- mass sources should be studied to deter- of large, partly hydrogen-saturated mol- saturated biomolecule identified in hot mine its relevance to the chemistry of the ecules of biological interest in hot mo- cores is acetic acid, because of its struc- solar nebula, but there are many gaps in lecular cores have been extended to tural proximity to glycine. Acetic acid our knowledge about the evolutionary comets. A goal is the detection and has not been reported in comets, but an progression from a hot molecular core to mapping of large biomolecules, which isomer, methyl formate, has been found. a primordial solar nebula. Evidence that may be formed along chemical pathways Furthermore, methanol, an important the solar nebula and its prebiotic chemis- similar to those for hot molecular core product of grain chemistry, has been try pass through the hot molecular core species. So far, there are a few clues for mapped interferometrically in three phase has been problematical because of what we should expect to find. For ex- comets. Therefore, if no more data were the dearth of observational evidence for ample, methanol is a chemically impor- available, a simple scenario could be the existence of a hot core phase in low- tant grain mantle cometary molecule hypothesized for enriching the prebiotic mass star formation regions. Until re- that has been successfully imaged with organic chemistry on rocky planets such cently, hot molecular cores were found the BIMA Array in comets Hale–Bopp as the early Earth. The Sun was born in only in regions of massive star formation, (C͞1995 O1), LINEAR (C͞2002 T7), a cluster that had a chemistry that gen- even though many young stellar regions and NEAT (C͞2001 Q4) (65). Bockele´e- erated large partly hydrogen-saturated have physical conditions that are sugges- Morvan et al. (66) conducted an exten- biomolecules such as acetic acid and tive of hot molecular cores (61). Some sive study of comet Hale–Bopp with the amino acids. Hence, planets passed low-mass young stellar regions even have California Institute of Technology Sub- through a seeding flux of grain chemis- grain mantle chemistry products such as millimeter Observatory, the Institute de try in space debris carried by asteroids, formaldehyde and methanol (62, 63). But Radioastronomie Millime´trique 30-m, comets, and meteorites during a Great observations that appear to provide the and the Plateau de Bure array, all in Bombardment phase. Thus planetary missing evidence have been reported by autocorrelation mode. Their detection seeding chemistry would start out to be Cazaux et al. (56), who used the Institute of methyl formate provided the first very similar to hot core chemistry. A de Radioastronomie Millime´trique 30-m clear link between the chemistry of major obstacle, however, is that current telescope to indirectly document the exis- large partly hydrogen-saturated mole- models for hot core chemistry are in a tence of a hot molecular core in the low- cules in comets and that in high-mass very early stage of development. As a mass young stellar object IRAS 16293- hot molecular cores. Later, ethylene gly- result, differentiation in isomers has

Snyder PNAS ͉ August 15, 2006 ͉ vol. 103 ͉ no. 33 ͉ 12247 Downloaded by guest on October 2, 2021 been observed, but is not well under- abundances around hot molecular cores the formation chemistry of large, partly stood. The extended source structure for by searching for both emission and ab- hydrogen-saturated species and its rele- some large, partly hydrogen-saturated sorption spectra and establishing definitive vance to prebiotic planetary chemistry species such as certain sugars, alde- temperature distributions by means of could come from direct observations of hydes, and alcohols may require non- sensitive, high spatial resolution observa- the chemistry of the debris clouds around thermal formation mechanisms such as tions. If molecules that trace shocks (e.g., newly formed planetary systems. This shock heating of grains. HCOϩ) can be observed in the extended information would be important for In principle, major advances in under- gas, the shock boundaries and tempera- providing a chemical road map for the standing the formation chemistry of large, ture distributions will allow the correct composition of cometary nuclei, which partly hydrogen-saturated species can astrochemistry to be modeled. Armed may have carried prebiotic organic come from observations with the next with correct model predictions, it will be chemistry from the interstellar clouds generation of sensitive, high-resolution possible to conduct meaningful searches to the early Earth. arrays. The sensitivity of the BIMA Array for compact distributions of new large limited observations of absorption spectra biomolecules, such as ethyl methyl ether, Useful preprints were provided by B. D. to mostly small, abundant molecules such diethyl ether, ethyl formate, butyl cyanide, Fields, E. Herbst, J. M. Hollis, L. W. Looney, as CO, SiO, HCN, HNC, HCOϩ, and and propyl cyanide. Success would lead to and T. Oka. Helpful information was contrib- uted by R. M. Crutcher, D. N. Friedel, A. J. C2H toward the hot molecular core in Sgr the large data set required for meaningful Ϸ Љ Kemball, P. R. Jewell, F. J. Lovas, A. J. B2(N-LMH) with 10 resolution (35, comparisons of the chemistries of hot mo- Remijan, E. C. Sutton, B. E. Turner, and 36). With the next generation of arrays, it lecular cores found in giant molecular T. L. Wilson. Support was received from the will be possible to directly observe the clouds, which are distributed from the Laboratory for Astronomical Imaging at the chemical behavior of compact distribu- galactic center to the edge of the galactic University of Illinois and from National Sci- tions of large biomolecules with enhanced disk. Real progress toward understanding ence Foundation Grant AST 02-28953.

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