P. R. Jewell

Interstellar Glycolaldehyde: The First Sugar

Interstellar glycolaldehyde (CH OHCHO) has been detected in emission toward the Galactic center source 2 Sagittarius B2(N) by means of millimeter-wave rotational transitions. Glycolaldehyde is an important biomarker since it is structurally the simplest member of the monosaccharide sugars that heretofore have gone undetected in interstellar clouds. There is no consensus as to how any such large complex molecules are formed in the interstellar clouds. It may be that the typical environment of dense interstellar clouds is favorable to glycolaldehyde synthesis by means of the polymerization of formaldehyde (H CO) molecules either on grain surfaces or in the 2 gas phase. Alternatively, we speculate that glycolaldehyde and other complex molecules may undergo assembly from functional molecular groups on grain surfaces. Utilizing common chemical precursors, a chance process could account for the high degree of isomerism observed in complex interstellar molecules (e.g., methyl formate, acetic acid, and glycolaldehyde). This work suggests that the phenomenon of isomerism be investigated further as a means of potentially constraining interstellar chemistry routes for those individual sources where the condition of good source-beam coupling can be achieved.

A Rigorous Attempt to Verify Interstellar Glycine

In 2003, Kuan and coworkers reported the detection of interstellar glycine (NH2CH2COOH) based on observations of 27 lines in 19 different spectral bands in one or more of the sources Sgr B2(N-LMH), Orion KL, and W51 e1/e2. They supported their detection report with rotational temperature diagrams for all three sources. In this paper we present essential criteria that can be used in a straightforward analysis technique to confirm the identity of an interstellar asymmetric rotor such as glycine. We use new laboratory measurements of glycine as a basis for applying this analysis technique, both to our previously unpublished 12 m telescope data and to the previously published Swedish-ESO Submillimetre Telescope (SEST) data of Nummelin and colleagues. We conclude that key lines necessary for an interstellar glycine identification have not yet been found. We identify some common molecular candidates that should be examined further as more likely carriers of several of the lines reported as glycine. Finally, we illustrate that a rotational temperature diagram used without the support of correct spectroscopic assignments is not a reliable tool for the identification of interstellar molecules.

Green Bank Telescope Detection of New Interstellar Aldehydes: Propenal and Propanal

The new interstellar molecules propenal (CH2CHCHO) and propanal (CH3CH2CHO) have been detected largely in absorption toward the star-forming region Sagittarius B2(N) by means of rotational transitions observed with the 100 m Green Bank Telescope (GBT) operating in the range from 18 GHz (λ ~ 1.7 cm) to 26 GHz (λ ~ 1.2 cm). The GBT was also used to observe the previously reported interstellar aldehyde propynal (HC2CHO) in Sagittarius B2(N), which is a known source of large molecules presumably formed on interstellar grains. The presence of these three interstellar aldehydes toward Sagittarius B2(N) strongly suggests that simple hydrogen addition on interstellar grains accounts for successively larger molecular species: from propynal to propenal and from propenal to propanal. Energy sources within Sagittarius B2(N) likely permit the hydrogen addition reactions on grain surfaces to proceed. This work demonstrates that successive hydrogen addition is probably an important chemistry route in the formation of a number of complex interstellar molecules. We also searched for but did not detect the three-carbon sugar glyceraldehyde (CH2OHCHOHCHO).

Detection of Acetamide (CH3CONH2): The Largest Interstellar Molecule with a Peptide Bond

Acetamide (CH3CONH2) has been detected in emission and absorption toward the star-forming region Sagittarius B2(N) with the 100 m Green Bank Telescope (GBT) by means of four A-species and four E-species rotational transitions. All transitions have energy levels less than 10 K. The Sgr B2(N) cloud is known to have a cold halo with clumps of gas at several different velocities. Absorption features are largely characterized by local standard of rest (LSR) velocities that are typical of the two star-forming cores with systemic LSR velocities of +64 and +82 km s-1. Continuum sources embedded within the star-forming cores give rise to the absorption from the molecular gas halo surrounding the cores. Emission features are seen at an approximate intermediate LSR velocity of +73 km s-1 that characterizes the widespread molecular halo that has a spatial scale of a few arcminutes. Two low-energy transitions of formamide (HCONH 2) were also observed with the GBT toward Sagittarius B2(N) since formamide is the potential parent molecule of acetamide; both molecules are the only interstellar species with an NH2 group bound to a CO group, the so-called peptide bond, that provides the linkage for the polymerization of amino acids. While the acetamide transitions observed appear to be confined to the cold (~8 K) halo region, only the 101-0 00 transition of formamide appears to be exclusively from the cold halo; the 312-313 transition of formamide is apparently contaminated with emission from the two hot cores. The relative abundance ratio of acetamide to formamide is estimated to be in the range of ~0.1 to ~0.5 in the cold halo. The exothermic neutral-radical reaction of formamide with methylene (CH2) may account for the synthesis of interstellar acetamide in the presence of shock phenomenon in this star-forming region.

Interstellar Antifreeze: Ethylene glycol

Interstellar ethylene glycol ( ) has been detected in emission toward the Galactic center source HOCH CH OH 22 Sagittarius B2(N-LMH) by means of several millimeter-wave rotational torsional transitions of its lowest energy conformer. The types and kinds of molecules found to date in interstellar clouds suggest a chemistry that favors aldehydes and their corresponding reduced alcohols—e.g., formaldehyde ( )/methanol ( ), acetal- 2 any such large complex molecules are formed in the interstellar clouds, atomic hydrogen (H) and carbon monoxide (CO) could form formaldehyde on grain surfaces, but such surface chemistry beyond that point is uncertain. However, laboratory experiments have shown that the gas-phase reaction of atomic hydrogen (H) and solid-phase CO at 10-20 K can produce formaldehyde and methanol and that alcohols and other complex molecules can be synthesized from cometary ice analogs when subject to ionizing radiation at 15 K. Thus, the presence of aldehyde/ reduced alcohol pairs in interstellar clouds implies that such molecules are a product of a low-temperature chemistry on grain surfaces or in grain ice mantles. This work suggests that aldehydes and their corresponding reduced alcohols provide unique observational constraints on the formation of complex interstellar molecules. Subject headings: ISM: abundances — ISM: clouds — ISM: individual (Sagittarius B2(N-LMH)) — ISM: molecules — radio lines: ISM

The Spatial Scale of Glycolaldehyde in the Galactic Center

We previously reported the spectral detection of the first interstellar sugar, which is known as glycolaldehyde (CH2OHCHO), by observing six separate millimeter-wave rotational transitions with the NRAO 12 m telescope while pointed toward the Sagittarius B2 North hot core source known as the Large Molecule Heimat (LMH) source. In the present BIMA array work, we have spatially mapped Sgr B2 using the 808–717 transition of glycolaldehyde at 82.4 GHz. We find that glycolaldehyde has a spatial scale of ≥60 unlike its isomers methyl formate and acetic acid, which are concentrated in the LMH source that has a spatial scale of ≤5. We estimate that the relative abundance ratios of (acetic acid) : (glycolaldehyde) : (methyl formate) are ∼1 : 0.5 : 26 within the LMH source. It is likely that the conditions of the LMH source favor the chemically reactive nature of glycolaldehyde over its isomers and other large molecules such as dimethyl ether. The ensuing chemistry leads to glycolaldehyde destruction in the LMH source and glycolaldehyde survival outside of the LMH source in extended cloud extremities. This scenario is supported by comparison of line widths, which shows that glycolaldehyde possesses a factor of 2–3 greater line width than those of other complex molecules that are confined largely to the LMH source. Subject headings: ISM: abundances — ISM: clouds — ISM: individual (Sagittarius B2) — ISM: molecules — radio lines: ISM

Green Bank Telescope Observations of Interstellar Glycolaldehyde: Low-Temperature Sugar

Interstellar glycolaldehyde (CH2OHCHO) has been detected with the 100 m Green Bank Telescope (GBT) toward the star-forming region Sagittarius B2(N) by means of the 110-101, 211-202, 312-303, and 413-404 rotational transitions at 13.48, 15.18, 17.98, and 22.14 GHz, respectively. An analysis of these four high signal-to-noise ratio rotational transitions yields a glycolaldehyde state temperature of ~8 K. Previously reported emission-line detections of glycolaldehyde with the NRAO 12 m telescope at millimeter wavelengths (71-103 GHz) are characterized by a state temperature of ~50 K. By comparison, the GBT detections are surprisingly strong and are seen in emission at 13.48 GHz, emission and absorption at 15.18 GHz, and absorption at 17.98 and 22.14 GHz. We attribute the strong absorption observed by the GBT at the higher frequencies to the correspondingly smaller GBT beams coupling better to the continuum source(s) in Sagittarius B2(N). A possible model for the two-temperature regions of glycolaldehyde is discussed.

Confirmation of Interstellar Acetone

We present new observations of interstellar acetone [(CH3)2CO] 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 NT = 2.9(3) × 1016 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.

Interstellar isomers : The importance of bonding energy differences

We present strong detections of methyl cyanide (CH3CN), vinyl cyanide (CH2CHCN), ethyl cyanide (CH3CH2CN), and cyanodiacetylene (HC4CN) molecules with the Green Bank Telescope (GBT) toward the Sgr B2(N) molecular cloud. Attempts to detect the corresponding isocyanide isomers were only successful in the case of methyl isocyanide (CH3NC) for its JK = 10-00 transition, which is the first interstellar report of this line. To determine the spatial distribution of CH3NC, we used archival Berkeley-Illinois-Maryland Association (BIMA) array data for the JK = 4K-3K (K = 0-3) transitions, but no emission was detected. From ab initio calculations, the bonding energy difference between the cyanide and isocyanide molecules is >8500 cm-1 (>12,000 K). Thus, cyanides are the more stable isomers and would likely be formed more preferentially over their isocyanide counterparts. That we detect CH3NC emission with a single antenna (Gaussian beam size ΩB = 1723 arcsec2) but not with an interferometer (ΩB = 192 arcsec2) strongly suggests that CH3NC has a widespread spatial distribution toward the Sgr B2(N) region. Other investigators have shown that CH3CN is present both in the LMH hot core of Sgr B2(N) and in the surrounding medium, while we have shown that CH3NC appears to be deficient in the LMH hot core. Thus, large-scale, nonthermal processes in the surrounding medium may account for the conversion of CH3CN to CH3NC, while the LMH hot core, which is dominated by thermal processes, does not produce a significant amount of CH3NC. Ice analog experiments by other investigators have shown that radiation bombardment of CH3CN can produce CH3NC, thus supporting our observations. We conclude that isomers separated by such large bonding energy differences are distributed in different interstellar environments, making the evaluation of column density ratios between such isomers irrelevant unless it can be independently shown that these species are cospatial.

Cyclopropenone (c-H2C3O): A New Interstellar Ring Molecule

The three-carbon keto ring cyclopropenone (c-H2C 3O) has been detected largely in absorption with the 100 m Green Bank Telescope (GBT) toward the star-forming region Sagittarius B2(N) by means of a number of rotational transitions between energy levels that have energies less than 10 K. Previous negative results from searches for interstellar c-H2C3O by other investigators attempting to detect rotational transitions that have energy levels ~10 K or greater indicate no significant hot core component. Thus, we conclude that only the low-energy levels of c-H2C3O are populated because the molecule state temperature is low, suggesting that c-H2C3O resides in a star-forming core halo region that has a widespread arcminute spatial scale. Toward Sagittarius B2(N), the GBT was also used to observe the previously reported, spatially ubiquitous, three-carbon ring cyclopropenylidene (c-C3H2 ), which has a divalent carbon that makes it highly reactive in the laboratory. The presence of both c-C3H2 and c-H2C3O toward Sagittarius B2(N) suggests that gas-phase oxygen addition may account for the synthesis of c-H 2C3O from c-C3H2. We also searched for but did not detect the three-carbon sugar glyceraldehyde (CH2OHCHOHCHO).