J/A+A/587/A130 HCO and CH3O in prestellar cores (Bacmann+, 2016)
The origin of gas-phase HCO and CH3O radicals in prestellar cores.
Bacmann A., Faure A.
<Astron. Astrophys. 587, A130 (2016)>
=2016A&A...587A.130B 2016A&A...587A.130B (SIMBAD/NED BibCode)
ADC_Keywords: Interstellar medium ; Spectroscopy
Keywords: astrochemistry - ISM: molecules - ISM: abundances
Abstract:
The recent unexpected detection of terrestrial complex organic
molecules in the cold (∼10K) gas has cast doubts on the commonly
accepted formation mechanisms of these species. Standard gas-phase
mechanisms are inefficient and tend to underproduce these molecules,
and many of the key reactions involved are unconstrained.
Grain-surface mechanisms, which were presented as a viable
alternative, suffer from the fact that they rely on grain surface
diffusion of heavy radicals, which is not possible thermally at very
low temperatures.
One of the simplest terrestrial complex organic molecules, methanol is
believed to form on cold grain surfaces following from successive H
atom additions on CO. Unlike heavier species, H atoms are very mobile
on grain surfaces even at 10K. Intermediate species involved in grain
surface methanol formation by CO hydrogenation are the radicals HCO
and CH3O, as well as the stable species formaldehyde H2CO. These
radicals are thought to be precursors of complex organic molecules on
grain surfaces.
We present new observations of the HCO and CH3O radicals in a sample
of prestellar cores and carry out an analysis of the abundances of the
species HCO, H2CO, CH3O, and CH3OH, which represent the various
stages of grain- surface hydrogenation of CO to CH3OH.
The abundance ratios between the various intermediate species in the
hydrogenation reaction of CO on grains are similar in all sources of
our sample, HCO:H2CO:CH3O:CH3OH∼10:100:1:100. We argue that
these ratios may not be representative of the primordial abundances on
the grains but, rather, suggest that the radicals HCO and CH3O are
gas-phase products of the precursors H2CO and CH3OH, respectively.
Various gas-phase pathways are considered, including neutral-neutral
and ion-molecule reactions, and simple estimates of HCO and CH3O
abundances are compared to the observations. Critical reaction rate
constants, branching ratios, and intermediate species are finally
identified.
Description:
Line parameters for the 8 prestellar cores observed, in each of the
molecules HCO, CH3O and CH3OH. For each molecule and each object,
the frequency of the observed transition, the rms in the spectrum, the
peak main beam temperature, the linewidth, the integrated line
intensity and the error on the integrated line intensity are given.
The peak main beam temperature, and linewidth and the integrated
intensity were derived by Gaussian fitting of the spectra. The
corresponding reduced (folded and baseline-subtracted) spectrum is
given in antenna temperature units.
Objects:
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RA (ICRS) DE Designation(s)
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04 18 06. +27 37 L1495AS = LDN 1495
05 04 09.7 +32 43 09 L1512 = LDN 1512
04 55 18.8 +30 38 04 L1517B = LDN 1517B
04 29 21.0 +26 14 19 L1521E = LDN 1521E
16 34 42.1 -24 36 11 L1689B = LDN 1689B
16 30 50.7 -23 41 03 L1709A = LDN 1709A
18 16 44. -08 18 54 L429 = LDN 429
04 32 44.6 +24 23 13 TMC2 = TMC-2
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File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 52 8 Observed sources and their properties
table3.dat 78 32 Line parameters for HCO transitions
table4.dat 81 96 Line parameters for CH3O transitions
table5.dat 81 148 Line parameters for CH3OH transitions
sp/* . 252 Spectra for all sources and all transitions
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Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 A7 --- Source Source name
10- 11 I2 h RAh Right ascension (J2000)
13- 14 I2 min RAm Right ascension (J2000)
16- 20 F5.2 s RAs Right ascension (J2000)
22 A1 --- DE- Declination sign (J2000)
23- 24 I2 deg DEd Declination (J2000)
26- 27 I2 arcmin DEm Declination (J2000)
29- 32 F4.1 arcsec DEs Declination (J2000)
34- 37 E4.1 cm-2 nc(H2) H2 column density
39- 47 A9 --- Region Region
49- 52 A4 --- r_nc(H2) References for nc(H2) (1)
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Note (1): References as follows:
1 = Roy et al. (2014A&A...562A.138R 2014A&A...562A.138R)
2 = Brady Ford & Shirley (2011ApJ...728..144F 2011ApJ...728..144F)
3 = Tafalla et al. (2004A&A...416..191T 2004A&A...416..191T)
4 = Tafalla & Santiago (2004A&A...414L..53T 2004A&A...414L..53T)
5 = Bacmann et al. (2002A&A...389L...6B 2002A&A...389L...6B)
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Byte-by-byte Description of file: table3.dat table5.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 A7 --- Source Source name
11- 19 F9.2 MHz Freq Frequency of the transition
21- 25 F5.1 mK rms rms noise in spectrum
27- 30 I4 mK Tmb ?=- Line peak intensity, Tmb
33- 36 F4.2 km/s dv ?=- Linewidth
39 A1 --- l_Int Limit flag on intensity
40- 44 F5.3 K.km/s Int Integrated intensity
49- 53 F5.3 K.km/s e_Int ?=- Error on integrated intensity
55- 81 A27 --- FileName Name of corresponding spectrum file in
subdirectory sp (1)
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Note (1): in a few cases, two or three transitions are on the same spectrum.
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Byte-by-byte Description of file: table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 A7 --- Source Source name
11- 19 F9.2 MHz Freq Frequency of the transition
21- 25 F5.1 mK rms rms noise in spectrum
27- 30 I4 mK Tmb ?=- Line peak intensity, Tmb
33- 36 F4.2 km/s dv ?=- Linewidth
39 A1 --- l_Int Limit flag on intensity
40- 46 E7.1 K.km/s Int Integrated intensity
49- 55 E7.1 K.km/s e_Int ?=- Error on integrated intensity
57- 81 A25 --- FileName Name of corresponding spectrum file in
subdirectory sp (1)
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Note (1): in a few cases, two transitions are on the same spectrum.
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Byte-by-byte Description of file: sp/*
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 13 F13.4 MHz Freq Frequency
15- 29 E15.4 K Int Intensity in Ta* unit
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Acknowledgements:
Aurore Bacmann, aurore.bacmann(at)obs.ujf-grenoble.fr
(End) Patricia Vannier [CDS] 07-Jan-2016