J/A+A/622/A26 Molecular ion abundances in diffuse ISM (Gerin+, 2019)
Molecular ion abundances in the diffuse ISM : CF+, HCO+, HOC+,
and C3H+.
Gerin M., Liszt H., Neufeld D., Godard B., Sonnentrucker P., Pety J.,
Roueff E.
<Astron. Astrophys. 622, A26 (2019)>
=2019A&A...622A..26G 2019A&A...622A..26G (SIMBAD/NED BibCode)
ADC_Keywords: Interferometry; Interstellar medium; Diffuse clouds; H II regions;
Spectra, millimetric/submm
Keywords: ISM: general - ISM: clouds - ISM: molecules - radio lines: ISM
Abstract:
The transition between atomic and molecular hydrogen is associated
with important changes in the structure of interstellar clouds, and
marks the beginning of interstellar chemistry. Most molecular ions are
rapidly formed (in ion-molecule reactions) and destroyed (by
dissociative recombination) in the diffuse ISM. Because of the
relatively simple networks controlling their abundances, molecular
ions are usually good probes of the underlying physical conditions
including for instance the fraction of gas in molecular form or the
fractional ionization.
In this paper we focus on three possible probes of the molecular
hydrogen column density, HCO+, HOC+, and CF+.
We presented high sensitivity ALMA absorption data toward a sample of
compact HII regions and bright QSOs with prominent foreground
absorption, in the ground state transitions of the molecular ions
HCO+, HOC+, and CF+ and the neutral species HCN and HNC, and
from the excited state transitions of C3H+(4-3) and 13CS(2-1).
These data are compared with Herschel absorption spectra of the ground
state transition of HF and p-H2O. We show that the HCO+, HOC+,
and CF+ column densities are well correlated with each other. HCO+
and HOC+ are tightly correlated with p-H2O, while they exhibit a
different correlation pattern with HF depending on whether the
absorbing matter is located in the Galactic disk or in the central
molecular zone. We report new detections of C3H+ confirming that
this ion is ubiquitous in the diffuse matter, with an abundance
relative to H2 of ∼7x10-11.
We confirm that the CF+ abundance is lower than predicted by simple
chemical models and propose that the rate of the main formation
reaction is lower by a factor of about 3 than usually assumed. In the
absence of CH or HF data, we recommend to use the ground state
transitions of HCO+, CCH, and HOC+ to trace diffuse molecular
hydrogen, with mean abundances relative to H2 of 3x10-9, 4x10-8
and 4x10-11 respectively, leading to sensitivity
N(H2)/∫τdv of 4x1020, 1.5x1021, and 6x1022cm-2/km/s,
respectively.
Description:
The observations have been obtained with ALMA during its cycle 2. The
bandpass calibrators were J1427-4206, J1717-3342, J1924-2914, and the
phase calibrator was J1752-2956. As flux calibrator the solar system
objects Titan, Venus, and Ceres were used. The data were calibrated
and deconvolved using CASA and the resulting data-cubes were then
exported to GILDAS for further analysis. The individual files are
labelled according to the source and molecule observed.
Each file is labelled with the source name and the molecule.
C3H for C3H, CFP for CF+, HCN for HCN, HNC for HNC, 13CS for
13CS, C3HP for C3H+, HCOP for HCO+, HOCP for HOC+,
It has a brief header containing ancillary information, followed by
the continuum divided spectrum, listed as LSRK velocity in the first
column in km/s and the variable (line/continuum-1) in the second
column.
The first line provides the format of the coordinates, and the units.
TC is to be interpreted as the continuum flux in Jy
Frequency is the doppler-shifted rest frequency of the line
observed in the first channel.
For example the first lines of the file SGRSHCOP.TXT containing the
spectrum of HCO+ toward the southern position near SgrB2-S are as
follows:
RA:hh.mmssss; Dec:dd.mmsss; L,B: decimal deg.; TC: Jy; Frequency: MHz
SOURCE = 'SgrS-HCO^+ '
RA = 17.472046404
DEC = -28.23448380
L = 0.6562967426
B = -0.042985450
TC = 0.6156601138
Frequency = 89135.437743
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
sources.dat 144 60 List of spectra
sp/* . 60 Individual spectra
--------------------------------------------------------------------------------
Byte-by-byte Description of file: sources.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 15 A15 --- Source Source designation
17- 18 I2 h RAh Right ascension (J2000)
20- 21 I2 min RAm Right ascension (J2000)
23- 30 F8.5 s RAs Right ascension (J2000)
32 A1 --- DE- Declination sign (J2000)
33- 34 I2 deg DEd Declination (J2000)
36- 37 I2 arcmin DEm Declination (J2000)
39- 45 F7.4 arcsec DEs Declination (J2000)
47- 60 F14.10 deg GLON Galactic longitude
62- 73 F12.9 deg GLAT Galactic longitude
75- 87 F13.11 Jy TC Continuum flux
88-100 F13.6 MHz Freq Doppler-shifted rest Frequency of the line
observed in the first channel
102-115 A14 --- FileName Name of the file with spectrum
in subdirectory sp
117-136 A20 --- Name Name of the QSO or HII region
137-144 A8 --- Mol Observed molecule
--------------------------------------------------------------------------------
Byte-by-byte Description of file (#): sp/*
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 19 E19.10 km/s VLSR LSRK velocity
20- 34 E15.12 --- I/Ic-1 Intensity, normalized to continuum, minus 1
(I/Ic-1)
--------------------------------------------------------------------------------
Acknowledgements:
Maryvonne Gerin, maryvonne.gerin(at)lra.ens.fr
(End) Patricia Vannier [CDS] 05-Nov-2018