J/A+A/675/A69 First detection of CD toward IRAS 16293-2422 (Jacob+, 2023)
First detection of deuterated methylidyne (CD) in the interstellar medium.
Jacob A.M., Menten K.M., Wyrowski F., Sipila O.
<Astron. Astrophys. 675, A69 (2023)>
=2023A&A...675A..69J 2023A&A...675A..69J (SIMBAD/NED BibCode)
ADC_Keywords: Molecular clouds ; Infrared sources ; Spectroscopy
Keywords: astrochemistry - ISM: molecules -
ISM: individual objects: IRAS 16293-2422 - line: identification -
molecular data
Abstract:
While the abundance of elemental deuterium is relatively low
(D/H ∼ a few x 10-5), orders of magnitude higher D/H abundance
ratios have been found for many interstellar molecules, enhanced by
deuterium fractionation. In cold molecular clouds (T<20K) deuterium
fractionation is driven by the H2D+ ion, whereas at higher
temperatures (T>20-30K) gas-phase deuteration is controlled by
reactions with CH2D+ and C2HD+. While the role of H2D+ in
driving cold interstellar deuterium chemistry is well understood,
thanks to observational constraints from direct measurements of
H2D+, deuteration stemming from CH2D+ is far less understood,
as a result of the absence of direct observational constraints of its
key ions. Therefore, making use of chemical surrogates is imperative
in order to explore deuterium chemistry at intermediate temperatures.
Formed at an early stage of ion-molecule chemistry directly from the
dissociative recombination of CH3+ (CH2D+), CH (CD) is an
ideal tracer for investigating deuterium substitution initiated by
reactions with CH2D+. This paper reports the first detection of CD
in the interstellar medium (ISM), carried out using the APEX 12m
telescope towards the widely studied low-mass protostellar system
IRAS 16293-2422. Observed in absorption toward the envelope of the
central protostar, the D/H ratio derived from the column densities of
CD and CH is found to be 0.016±0.003. This is an order of magnitude
lower than the values found for %that derived using other small
molecules like C2H and H2CO observed in emission but whose
formation, which is similar to that of CH, is also initiated via
pathways involving warm deuterium chemistry. Gas-phase chemical models
reproducing the CD/CH abundance ratio suggest that it reflects 'warm
deuterium chemistry' (which ensues in moderately warm conditions of
the ISM) and illustrates the potential use of the CD/CH ratio in
constraining the gas temperatures of the envelope gas clouds it
probes.
Description:
Observations of the N=1, J=3/2-1/2 hyperfine structure splitting
transitions of CD near 440GHz were performed in June 2021 and April
and June 2022 using the high-frequency module of the new version of
the First Light APEX Submillimetre Heterodyne Receiver, nFLASH460, on
the APEX 12m telescope under the project id:M9530C_107. We report the
detect towards IRAS 16293-2422 as well as the non-detection towards
star-forming regions SgrB2M, G34.26+0.15 and W49N. In addition to the
results of the CD observations described above, we use complementary
Herschel/HIFI archival data of CH at 532/526 GHz published in
Bottinelli et al. (2014MNRAS.441.1964B 2014MNRAS.441.1964B). Furthermore toward W49N we
also report the non-detection of the N=2-1, J=3/2-1/2
hyperfine structure splitting lines of CD near 1325GHz using the
4GREAT instrument on SOFIA under project id: 83_0701.
Objects:
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RA (2000) DE Designation(s)
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16 32 22.56 -24 28 31.8 IRAS 16293-2422 = JCMTSE J163222.8-242837
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File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 165 2 List of fits spectra
fits/* . 2 Individual fits spectra
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Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
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- 78 I6 Hz dFreq Frequency resolution
80- 82 F3.1 km/s vlsr Source LSR velocity
84- 87 F4.2 K Tc Main-beam brightness continuum temperature
at 440GHz
89- 94 A6 --- Parity Parity of the N=1, J=3/2-1/2 transitions of CD
96 I1 Kibyte size Size of FITS file
98-121 A24 --- FileName Name of FITS file, in subdirectory fits
123-165 A43 --- Title Title of the FITS file
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Acknowledgements:
Arshia Maria Jaocb, ajacob51(at)jhu.edu
References:
Bottinelli et al., 2014MNRAS.441.1964B 2014MNRAS.441.1964B,
CH in absorption in IRAS 16293-242
Jacob et al., 2020A&A...640A.125J 2020A&A...640A.125J,
First detection of 13CH in the interstellar medium
(End) Patricia Vannier [CDS] 12-May-2023