59 research outputs found
Physical properties of CO-dark molecular gas traced by C
Neither HI nor CO emission can reveal a significant quantity of so-called
dark gas in the interstellar medium (ISM). It is considered that CO-dark
molecular gas (DMG), the molecular gas with no or weak CO emission, dominates
dark gas. We identified 36 DMG clouds with C emission (data from Galactic
Observations of Terahertz C+ (GOT C+) project) and HINSA features. Based on
uncertainty analysis, optical depth of HI of 1 is a reasonable
value for most clouds. With the assumption of , these clouds
were characterized by excitation temperatures in a range of 20 K to 92 K with a
median value of 55 K and volume densities in the range of
cm to cm with a median value of
cm. The fraction of DMG column density in the cloud ()
decreases with increasing excitation temperature following an empirical
relation +1.0. The relation
between and total hydrogen column density is given by
=. The values of in the
clouds of low extinction group ( mag) are consistent with the
results of the time-dependent, chemical evolutionary model at the age of ~ 10
Myr. Our empirical relation cannot be explained by the chemical evolutionary
model for clouds in the high extinction group ( mag). Compared to
clouds in the low extinction group ( mag), clouds in the high
extinction group ( mag) have comparable volume densities but
excitation temperatures that are 1.5 times lower. Moreover, CO abundances in
clouds of the high extinction group ( mag) are
times smaller than the canonical value in the Milky Way. #[Full version of
abstract is shown in the text.]#Comment: Accepted for publishing in Astronomy & Astrophysics. 13 pages, 8
figure
Dust-Gas Scaling Relations and OH Abundance in the Galactic ISM
Observations of interstellar dust are often used as a proxy for total gas
column density . By comparing thermal dust data
(Release 1.2) and new dust reddening maps from Pan-STARRS 1 and 2MASS (Green et
al. 2018), with accurate (opacity-corrected) HI column densities and
newly-published OH data from the Arecibo Millennium survey and 21-SPONGE, we
confirm linear correlations between dust optical depth , reddening
and the total proton column density in the range
(130)10cm, along sightlines with no molecular gas
detections in emission. We derive an / ratio of
(9.41.6)10cmmag for purely atomic sightlines
at 5, which is 60 higher than the canonical value of
Bohlin et al. (1978). We report a 40 increase in opacity
=/, when moving from the low column
density (510cm) to moderate column
density (510cm) regime, and suggest that
this rise is due to the evolution of dust grains in the atomic ISM. Failure to
account for HI opacity can cause an additional apparent rise in ,
of the order of a further 20. We estimate molecular hydrogen column
densities from our derived linear relations, and hence
derive the OH/H abundance ratio of 110
for all molecular sightlines. Our results show no evidence of systematic trends
in OH abundance with in the range
(0.110)10cm. This suggests
that OH may be used as a reliable proxy for H in this range, which includes
sightlines with both CO-dark and CO-bright gas.Comment: The revised manuscript is accepted for publication in The
Astrophysical Journa
Abundance ratios of OH/CO and HCO+/CO as probes of the cosmic ray ionization rate in diffuse clouds
The cosmic-ray ionization rate (CRIR, ) is one of the key parameters
controlling the formation and destruction of various molecules in molecular
clouds. However, the current most commonly used CRIR tracers, such as H,
OH, and HO, are hard to detect and require the presence of
background massive stars for absorption measurements. In this work, we propose
an alternative method to infer the CRIR in diffuse clouds using the abundance
ratios of OH/CO and HCO/CO. We have analyzed the response of chemical
abundances of CO, OH, and HCO on various environmental parameters of the
interstellar medium in diffuse clouds and found that their abundances are
proportional to . Our analytic expressions give an excellent
calculation of the abundance of OH for 10 s,
which are potentially useful for modelling chemistry in hydrodynamical
simulations. The abundances of OH and HCO were found to monotonically
decrease with increasing density, while the CO abundance shows the opposite
trend. With high-sensitivity absorption transitions of both CO (1--0) and
(2--1) lines from ALMA, we have derived the H number densities () toward 4 line-of-sights (LOSs); assuming a kinetic temperature of
, we find a range of
(0.140.03--1.20.1)10 cm}. By comparing the
observed and modelled HCO/CO ratios, we find that in our diffuse
gas sample is in the { range of 10 10 s. This is 2 times higher
than the average value measured at higher extinction, supporting an attenuation
of CRs as suggested by theoretical models.Comment: 22 pages, 9 figures, accepted by Ap
Dependence of Chemical Abundance on the Cosmic Ray Ionization Rate in IC 348
Ions (e.g., H, HO) have been used extensively to quantify the
cosmic-ray ionization rate (CRIR) in diffuse sightlines. However, measurements
of CRIR in low-to-intermediate density gas environments are rare, especially
when background stars are absent. In this work, we combine molecular line
observations of CO, OH, CH, and HCO in the star-forming cloud IC~348, and
chemical models to constrain the value of CRIR and study the response of the
chemical abundances distribution. The cloud boundary is found to have an
of approximately 4 mag. From the interior to the exterior of the
cloud, the observed CO line intensities drop by an order of magnitude.
The calculated average abundance of CO (assuming C/C = 65)
is (1.20.9) 10, which increases by a factor of 6 from the
interior to the outside regions. The average abundance of CH (3.30.7
10) is in good agreement with previous findings in diffuse and
translucent clouds ( 5 mag). However, we did not find a decline
in CH abundance in regions of high extinction (8 mag) as
previously reported in Taurus. By comparing the observed molecular abundances
and chemical models, we find a decreasing trend of CRIR as
increases. The inferred CRIR of = (4.71.5)
10 s at low is consistent with H measurements
toward two nearby massive stars.Comment: 21 pages, 11 figures. Submitted to Ap
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