Notes on Crockett et al. (2015), "HEXOS toward Orion KL II. Chemical Implications"

I read the following paper recently:

http://adsabs.harvard.edu/abs/2015ApJ...806..239C

also with my advisor as second-author.

 
Abstract here:

We present chemical implications arising from spectral models fit to the Herschel/HIFI spectral survey toward the Orion Kleinmann-Low nebula (Orion KL). We focus our discussion on the eight complex organics detected within the HIFI survey utilizing a novel technique to identify those molecules emitting in the hottest gas. In particular, we find the complex nitrogen bearing species CH₃CN, C₂H₃CN, C₂H₅CN, and NH₂CHO systematically trace hotter gas than the oxygen bearing organics CH₃OH, C₂H₅OH, CH₃OCH₃, and CH₃OCHO, which do not contain nitrogen. If these complex species form predominantly on grain surfaces, this may indicate N-bearing organics are more difficult to remove from grain surfaces than O-bearing species. Another possibility is that hot (T_kin ∼ 300 K) gas phase chemistry naturally produces higher complex cyanide abundances while suppressing the formation of O-bearing complex organics. We compare our derived rotation temperatures and molecular abundances to chemical models, which include gas-phase and grain surface pathways. Abundances for a majority of the detected complex organics can be reproduced over timescales ≳10⁵ years, with several species being underpredicted by less than 3σ. Derived rotation temperatures for most organics, furthermore, agree reasonably well with the predicted temperatures at peak abundance. We also find that sulfur bearing molecules that also contain oxygen (i.e., SO, SO₂, and OCS) tend to probe the hottest gas toward Orion KL, indicating the formation pathways for these species are most efficient at high temperatures.

One central question that I found relevant is here:
"Though the chemistry in [hot cores] remains poorly understood, theoretical studies argue that the evaporation of icy grain mantles, precipitated by the formation of protostars, plays a key role in the production of many complex molecules. However, the extent to which these species form on grain surfaces as opposed to in the gas phase remains unknown."

 Basically, this is a more in-depth analysis of the results of the previous paper. There's an interesting bit of technical trivia in the analysis that's worth highlighting:

In this study, we use the molecular fits as templates for the data, analyzing the emission from the models rather than the data. Examining the HIFI scan in this way has two advantages. First, we are able to easily separate emission from different spatial/velocity components. And second, we do not have to be concerned with line blends because we examine the model emission on a per molecule basis.

This figure is worth showing, too:




The hot core has more N-bearing organics, while the compact ridge has more O-bearing organics.

Paper I has LTE models for the T_rot and total column density Ntot ("from which abundances are computed") of each species.

Here, the analysis allows for temperature gradients.

The conclusions of this paper:

Complex N-bearing organics, i.e. cyanides and NH2CHO, probe hotter environments than complex species which contain no nitrogen.

"The inclusion of up to thousands of lines per molecule in our analysis thus places these results on a strong statistical footing."

Crucially, the following two items indicate that the interpretation of these observational results hinge on future observational work:

item 1.

"Gerin et al. 1992 reports a CH2DCN/CH3CN ratio >= 0.005 toward the Orion KL hot core, commensurate with D/H ratios measured for H2O (Neill 2013b) and CH3OH (Neill 2013a), suggesting at least some methyl cyanide forms via grain surface chemistry at low temperatures."

"Another possibility is that hot gas phase chemistry may be producing the highly excited cyanides in the hot core. [Models are discussed.] In this scenario, cyanides naturally trace hotter material because they form efficiently in the gas phase at higher temperatures."

"If gas phase formation routes are active for N-bearing organics in the hottest gas, we would expect gradients in the D/H ratios of complex N-bearing species like methyl cyanide."

I asked myself (when reading) "Has anyone tried to constrain gradients in D/H for both CH3CN and CH3OH toward the same source? Can NOEMA do this? Is ALMA needed?"

item 2.

"If both O- and N-bearing complex organics form predominantly on grain surfaces, cyanides along with NH2CHO may be more difficult to remove from grain surfaces than O-bearing species. Within a hot core, we might then expect oxygen bearing organics to be released during an earlier, presumably cooler epoch and/or further from the central protostar. Measured excitation temperatures should thus be higher toward the same location, and spatial temperature variations larger and clumpier for N-bearing organics relative to O-bearing species because the former traces hotter material along the line of sight."... "Our results indicate the need for excitation temperature maps derived from both O- and N-bearing organics at sub-arcsecond resolution to see if differences exist. Such observations will surely be attainable with ALMA."

These are very clear follow-up experiments, and I'll look into whether they've been carried out - the results seem central to my work.