J/A+A/647/A42 Detected CH2 spectra (Jacob+, 2021)
Hunting for the elusive methylene radical.
Jacob A.M., Menten K.M., Gong Y., Bergman P., Tiwari M., Bruenken S.,
Olofsson A.O.H.
<Astron. Astrophys. 647, A42 (2021)>
=2021A&A...647A..42J 2021A&A...647A..42J (SIMBAD/NED BibCode)
ADC_Keywords: Molecular clouds ; Spectroscopy
Keywords: ISM: abundances - radiative transfer - ISM: molecules - ISM: clouds -
ISM: lines and bands - methods: observational
Abstract:
The NKaKc=404-313 transitions of ortho-CH2 between 68 and 71GHz
were first detected toward the Orion-KL and W51 Main star-forming
regions. Given their high upper level energies (225K) above the
ground state, they were naturally thought to arise in dense, hot
molecular cores near newly formed stars. However, this has not been
confirmed by further observations of these lines and their origin has
remained unclear. Quite in general, only a paucity of observational
data exist for CH2 and, while astrochemically an important compound,
its actual occurrence in astronomical sources is poorly constrained.
In this work, we aim to investigate the nature of the elusive CH2
emission and address its association with hot cores and examine
alternative possibilities for its origin. Owing to its importance in
carbon chemistry, we also extend the search for CH2 lines by observing
an assortment of regions guided by the hypothesis that the observed
CH2 emission likely arises from the hot gas environment of
photodissociation regions (PDRs).
We carried out observations using, first, the Kitt Peak 12m telescope
to verify the original detection of CH2 toward different positions in
the central region of Orion Molecular Cloud 1. These were followed-up
by deep integrations using the higher angular resolution of the Onsala
20 m telescope. We have also searched for the NKaKc=212-303
transitions of para-CH2 between 440-445GHz toward the Orion giant
molecular cloud complex using the APEX 12m telescope. We also
obtained auxiliary data for carbon recombination lines with the
Effelsberg 100m telescope and employed archival far infrared data.
Our and other recent observations of the Orion region reported here,
rule out an association with hot and dense gas. We find that the
distribution of the CH2 emission follows closely that of the
[CII] 158 um emission while CH2 is undetected toward the hot core
itself. The observations suggest that its extended emission rather
arises from hot but dilute layers of PDRs, but not from the denser
parts of such regions, in particular the Orion Bar. This hypothesis
was corroborated by comparisons of the observed CH2 line profiles
with those of carbon radio recombination lines (CRRLs), well known PDR
tracers. In addition, we report the detection of the 70GHz fine-, and
hyperfine structure components of ortho-CH2 toward the W51 E, W51 M,
W51 N, W49 N, W43, W75 N, DR21, and S140 star-forming regions, and
three of the NKaKc=404-313 fine- and hyperfine structure
transitions between 68-71GHz toward W3 IRS5. While we have no
information on CH2's spatial distribution in these regions, except
for W51, we again see a correspondence of the profiles of CH2 lines
with those of CRRLs. We see stronger CH2 emission toward the
extended HII region W51 M than toward the much more massive and
denser W51 E and N regions, strongly supporting an origin of CH2 in
extended dilute gas. We also report the non-detection of the
212-303 transitions of para-CH2 toward Orion. Furthermore, using
a non-LTE radiative transfer analysis, we can constrain the gas
temperatures and H2 density to (163±26)K and
(3.4±0.3)x103cm-3, respectively, for the 68-71GHz ortho-CH2
transitions toward W3 IRS5, for which we have the highest quality data
set. This analysis confirms our hypothesis that CH2 originates in
warm and dilute PDR layers. Our analysis suggests that for the
excitation conditions under the physical conditions that prevail in
such an environment, these lines are masering, with weak level
inversion. The resulting amplification of the lines' spontaneous
emission greatly aides their detection.
Description:
The observations of Orion-KL were carried out in 2005
October-November (project id: 5029) and between 2006 January and
April using the 3mm receiver of the KP 12m telescope. In 2019 April
and May we observed the NKaKc=404-313 transitions of o-CH2
using the 4 mm receiver on the Onsala 20m telescope (project
id: O2018b-07) which was followed up with further observations in 2020
January and February. The detected CH2 spectra can be found here.
Objects:
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RA (ICRS) DE Designation(s)
------------------------------------------------------------------
05 35 14.16 -05 22 21.5 Orion KL = NAME Orion-KL
05 35 12.9 -05 24 10 Orion S = NAME Orion S
05 35 22.30 -05 24 33.0 Orion Bar = NAME Orion Bright Bar
05 35 19.8 -05 25 10 Orion Bar 2 = [VPT2002] Orion Bar 2
02 25 40.54 +62 05 51.4 W3 IRS 5 = NAME W3 IRS 5
19 23 41.9 +14 30 36 W51E = NAME W 51 IRS 1
19 23 42. +14 30 W51M = W 51m
19 23 40.0 +14 30 51 W51N = NAME W 51 North
19 10 13.2 +09 06 12 W49N = NAME W 49A North
18 47 32 -01 56 30 W43 = SNR G030.8-00.0
20 39 01.6 +42 19 38 DR21 = GRS G081.70 +00.50
20 38 35.9 +42 37 22 W75N = NAME Cyg X FIR 33
22 19 07.8 +63 17 07 Sh 2-140 = LBN 106.60+05.16
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 159 22 List of fits spectra
fits/* . 22 Individual fits spectra
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 22 I3 --- Nx Number of pixels along X-axis
24- 46 A23 "datime" Obs.date Observation date
48- 59 E12.6 Hz bFreq Lower value of frequency interval
61- 71 E11.6 Hz BFreq Upper value of frequency interval
73- 80 F8.1 Hz dFreq Frequency resolution
82- 86 F5.1 km/s Vlsr ? Source LSR velocity
88- 91 F4.1 km/s e_Vlsr ? rms uncertainty on Vlsr
93 I1 Kibyte size Size of FITS file
95-118 A24 --- FileName Name of FITS file, in subdirectory fits
120-159 A40 --- Title Title of the FITS file
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Acknowledgements:
Arshia Maria Jaocb, ajacob(at)mpifr-bonn.mpg.de
(End) Patricia Vannier [CDS] 25-Dec-2020