39 research outputs found
Large-Scale Spectroscopic Mapping of the Ophiuchi Molecular Cloud Complex I. The CH to NH Ratio as a Signpost of Cloud Characteristics
We present 2.5-square-degree CH N=1-0 and NH J=1-0 maps of the
Ophiuchi molecular cloud complex. These are the first large-scale maps
of the Ophiuchi molecular cloud complex with these two tracers. The
CH emission is spatially more extended than the NH emission. One
faint NH clump Oph-M and one CH ring Oph-RingSW are identified
for the first time. The observed CH to NH abundance ratio
([CH]/[NH]) varies between 5 and 110. We modeled the CH
and NH abundances with 1-D chemical models which show a clear decline
of [CH]/[NH] with chemical age. Such an evolutionary trend is
little affected by temperatures when they are below 40 K. At high density
(n 10 cm), however, the time it takes for the abundance
ratio to drop at least one order of magnitude becomes less than the dynamical
time (e.g., turbulence crossing time 10 years). The observed
[CH]/[NH] difference between L1688 and L1689 can be explained by
L1688 having chemically younger gas in relatively less dense regions. The
observed [CH]/[NH] values are the results of time evolution,
accelerated at higher densities. For the relative low density regions in L1688
where only CH emission was detected, the gas should be chemically younger.Comment: Accepted by ApJ, 45 pages, 10 figure
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
Evidence of Dark Contents in the Center of NGC 6517
Millisecond pulsars can serve as effective probes to investigate the presence
of Intermediate-mass Black Holes (IMBHs) within Galactic globular clusters
(GCs). Based on the standard structure models for GCs, we conduct simulations
to analyze the distributions of pulsar accelerations within the central region
of NGC 6517. By comparing the measured accelerations of pulsars obtained from
their period derivatives to the simulated distribution profiles, we
demonstrate that a central excess of dark mass is required to account for the
measured accelerations. Our analysis, which relies on existing pulsar timing
observations, is currently unable to differentiate between two possible
scenarios: an IMBH precisely situated at the core of the cluster with mass
, or a central concentration of stellar
mass dark remnants with a comparable total mass. However, with additional
acceleration measurements from a few more pulsars in the cluster, it will be
possible to differentiate the source of the nonluminous matter.Comment: 6 pages, 3 figures, 1 table. Accepted for publication in MNRA