Assuming that the glycine lines are optically thin and LTE populated, the Tmb upper limit is directly proportional to the column density of glycine through the relation (Combes et al. 1996):
where Z is the partition function, is the line strength, µ is the dipole moment, is the upper level energy, T is the gas temperature and Tbg is the background temperature.
The three lines of conformer I in the 3 mm band all have similar upper level energies ( cm-1) and line strengths (). We therefore averaged the three lines together to increase the detection sensitivity. No line appears at a level of 7 mK km s-1. We performed the same analysis for the six lines in the 1.3 mm band (level energies cm-1, line strengths ) and derived an upper limit of 6 mK km s-1. Using the molecule parameter values quoted by Combes et al. (1996), we obtained the curve of the upper limit of the glycine column density as a function of the gas temperature, shown in Fig. 2. Depending on the gas temperature, or in other words on where the glycine is supposed to originate, the upper limit of the glycine column density (averaged on the beam) varies from 0.7 to 1.5 cm-2 for temperature varying from 20 K to 100 K. Using the density and temperature structure of the envelope surrounding IRAS16293 (Ceccarelli et al. 2000a), we can convert the upper limit of the glycine column density into an upper limit of the glycine abundance. In the cold envelope, the glycine abundance is therefore . The limit of the abundance in the hot core is much higher, because of the dilution factor ( using the 216 GHz lines): .
As already mentioned, glycine is among the simplest amino acids. A few theoretical studies have recently been performed to assess the possibility of glycine formation in space. There are two classes of such models for large molecule formation: the first describes the formation of large molecules via reactions occurring in the gas phase (e.g. Millar et al.1997), while a second class considers molecule formation on the grain surfaces (e.g. Tielens 1983). Glycine formation in the gas phase was recently studied by Chakrabarti & Chakrabarti (2000). In this model, the authors computed the glycine abundance in the envelopes surrounding low mass protostars and found that an abundance of a few 10-10 can be reached in the envelope at about 2000 AU (" in our source). The upper limit of the glycine abundance which we derive from our observations rules out such high values of glycine abundance in the cold region of the envelope. Indeed this is not surprising: in their chemical model, Chakrabarti & Chakrabarti (2000) used the UMIST database (Millar et al. 1997) where they have added several new species and reactions. In particular, the have assumed that glycine is formed from successive neutral-neutral reactions with rate constants taken to be cm3 s-1 (temperature independent). Such rapid reactions are unusual for neutral species and always involve radicals (open-shell species). In the case of glycine, since only stable (closed-shell) species are involved in its formation, the rate constants used by Chakrabarti & Chakrabarti (2000) are probably largely overestimated.
As previously stated, it is possible that glycine (as well as other large molecules) forms on grain surfaces due to multiple reactions on the grains which act as catalysts. Recently Bakes et al. (1999) reported a study of glycine formation on grain surfaces although exact estimates of abundances are not yet available. Charnley (1999) and Ehrenfreund & Charnley (2000) suggested that amino acids can form by combination of gas-grain chemistry involving evaporation of alcohols, amino alcohols and formic acid and exothermic reactions which lead to the synthesis of glycine and other amino acids in hot cores. Another possibility is the synthesis of glycine as a result of UV or X-ray irradiation of the grain dirty mantles. An experiment where ice composed of water with small amounts of formaldehyde and methanol (similar to that found around protostars) was illuminated by UV radiation resulted in the synthesis of quinones (Bernstein et al. 1999). Whatever the actual route of formation, if formed on grain mantles glycine would be released into the gas phase near the center, when the dust temperature exceeds 100 K (the ice sublimation temperature) in the hot core region. Of course, the survival of glycine in the harsh environment around protostars, where X-rays and UV photons are copiously emitted, may be difficult (Ehrenfreund, Bernstein, Dworkin, Sandford, Allamandola, in preparation). Our observations show that glycine is less abundant than molecular hydrogen in the hot core region. Further searches at higher spatial resolutions could help to set a lower limit on the glycine abundance.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 2000