Experiments involving the irradiation of organic, mixed ices with UV photons or charged particles indicate that glycine and related molecules may be formed through photo-chemical processes, and glycine was recently identified in cometary samples returned to Earth from comet Wild 2; this is encouraging, as interstellar and cometary ices are similar in many respects, and cometary ices are thought to be (at least) partially composed of ice material that was originally formed in the interstellar medium.
In spite of the lack of an interstellar detection of glycine, molecules of similar structure and complexity — such as acetic acid (CH3COOH) and amino acetonitrile (NH2CH2CN) — have been firmly identified. This led me to investigate both whether glycine may indeed be formed within or upon dust-grain ices in star-forming cores known as hot cores, and whether it would be detectable following likely sublimation into the gas phase.
Glycine chemistry simulations
Building on my previous hot-core chemical models, I constructed a new reaction set for glycine and related molecules, based around the addition of radicals within the dust-grain ice mantles. I included four mechanisms for glycine formation:
- H + NHCH2COOH → NH2CH2COOH
- NH2 + CH2COOH → NH2CH2COOH
- NH2CH2 + COOH → NH2CH2COOH
- NH2CH2CO + OH → NH2CH2COOH
Glycine abundance (shown in black) is seen to peak at around 200 K, as a result of its sublimation from the grains. A medium time-scale model run is shown here; the peak gas-phase fractional abundance of glycine ranges from ~8 x 10-11 for the shortest warm-up timescale to 8 x 10-9 for the longest.
Glycine is found to form in/upon the ice mantles in the broad temperature range of ~40 — 120 K. Each of reactions (2) — (4) play a significant role in formation, with the precise influence dependent on time-scale. A plausible gas-phase mechanism for glycine formation in hot cores, involving the reaction of protonated hydroxylamine (NH2OH2+) and acetic acid (CH3COOH) was also tested; its influence was found to be minimal compared to formation within/upon the ice mantles, producing a fractional abundance of only ~10-12.
The peak abundances calculated for glycine are quite high; however, the high temperature at which it is predicted to sublime from the grains (~200 K, which is dependent on my binding energy estimate for glycine on water ice) indicates that it would be released in a much more compact region than most hot-core molecules.
To determine whether glycine would be detectable in a hot core, I constructed a spectroscopic modelling code, written in IDL. The code takes an observationally-determined temperature/density/radius profile for a hot core and maps all the chemical abundances onto this framework according to their temperature dependences. This produces a spherically-symmetric three-dimensional grid of absolute abundances of all species at all points in the source. Combining this with the temperature structure allows the emission and absorption characteristics of each molecule to be calculated, using published line lists (CDMS, JPL, Splatalogue), so that the overall emission and absorption coefficients are known at all points (assuming local thermodynamic equilibrium). Radiative transfer is then calculated along lines of sight through the hot core, in frequency bins covering a range of mm/sub-mm wavelengths.
This process produces a data cube (X x Y x freq.) that may be convolved with a telescope beam to determine the strength of line emission at any chosen frequency, allowing simulated spectra to be constructed that may be directly compared with observed spectra. The method naturally takes account of beam-dilution effects and line blending for all molecules that are present in both the line lists and the chemical model.
The southern-hemisphere hot-core source NGC 6334 IRS 1 was chosen for simulation, due to its proximity (~1.7 kpc) and its strong, fairly narrow (~5 km/s), molecular rotational emission lines. The observational temperature/density profile of van der Tak (2000) was used, and a fixed line width of 5 km/s was chosen for all positions in the model. Radiative transfer calculations were conducted in the approximate 218 — 250 GHz range. The results were convolved with telescope beams of size appropriate to the James Clerk Maxwell telescope (12m), the IRAM 30m telescope, and ALMA.
Similar line convolution for emission from methanol (CH3OH) and methyl formate (HCOOCH3), both typical hot core molecules, produced integrated line intensities for those species that fell within ~50 % of the values observed by Bisschop et al. (2007) for the simulated source (NGC 6334 IRS 1). This shows that the underlying spectroscopic model is plausible, and in this case very accurate.
Searching for glycine
Based on these combined chemical/physical and spectroscopic simulations, it appears that glycine may exist, and be detectable, in some hot cores.
It is essential that future searches for glycine choose nearby hot-core sources whose putative glycine emission region may be resolved with ALMA. Effective searches will also require a careful analysis of which lines may be both strong and unblended with lines from other molecules (using methods outlined here). The use of ALMA will reduce the amount of line blending to some degree as larger-scale emission from more typical hot-core molecules may be resolved out, using an appropriate array configuration.
For more information please see the publication, Garrod (2013).