5,422 research outputs found
Resolving distance ambiguities towards 6.7 GHz methanol masers
Distances to most star forming regions are determined using kinematics,
through the assumption that the observed radial velocity arises from the motion
of the source with respect to the Sun resulting from the differential rotation
of Galaxy. The primary challenge associated with the application of this
technique in the inner Galaxy is the kinematic distance ambiguity. In this
work, we aim to resolve the kinematic distance ambiguity towards a sample of
6.7 GHz methanol masers, which are signposts of the early stages of massive
star formation. We measured 21 cm HI absorption spectra using the Very Large
Array in C and CnB configurations. A comparison of the maximum velocity of HI
absorption with the source velocity and tangent point velocity was used to
resolve the kinematic distance ambiguity. We resolved the distance ambiguity
towards 41 sources. Distance determinations that are in conflict with previous
measurements are discussed. The NE2001 spiral arm model is broadly consistent
with the locations of the star forming complexes. We find that the use of
vertical scale height arguments to resolve the distance ambiguity can lead to
erroneous classifications for a significant fraction of sources.Comment: Accepted by Astronomy & Astrophysic
A Herschel/HIFI Legacy Survey of HF and H2O in the Galaxy: Probing Diffuse Molecular Cloud Chemistry
We combine Herschel observations of a total of 12 sources to construct the
most uniform survey of HF and H2O in our Galactic disk. Both molecules are
detected in absorption along all sight lines. The high spectral resolution of
the Heterodyne Instrument for the Far-Infrared (HIFI) allows us to compare the
HF and H2O distributions in 47 diffuse cloud components sampling the disk. We
find that the HF and H2O velocity distributions follow each other almost
perfectly and establish that HF and H2O probe the same gas-phase volume. Our
observations corroborate theoretical predictions that HF is a sensitive tracer
of H2 in diffuse clouds, down to molecular fractions of only a few percent.
Using HF to trace H2 in our sample, we find that the N(H2O)-to-N(HF) ratio
shows a narrow distribution with a median value of 1.51. Our results further
suggest that H2O might be used as a tracer of H2 -within a factor 2.5- in the
diffuse interstellar medium. We show that the measured factor of ~2.5 variation
around the median is driven by true local variations in the H2O abundance
relative to H2 throughout the disk. The latter variability allows us to test
our theoretical understanding of the chemistry of oxygen-bearing molecules in
the diffuse gas. We show that both gas-phase and grain-surface chemistry are
required to reproduce our H2O observations. This survey thus confirms that
grain surface reactions can play a significant role in the chemistry occurring
in the diffuse interstellar medium n_H < 1000 cm^-3.Comment: 53 pages; 12 figures, accepted for publication in ApJ main journa
A Herschel [C II] Galactic plane survey II: CO-dark H2 in clouds
ABRIDGED: Context: HI and CO large scale surveys of the Milky Way trace the
diffuse atomic clouds and the dense shielded regions of molecular hydrogen
clouds. However, until recently, we have not had spectrally resolved C+ surveys
to characterize the photon dominated interstellar medium, including, the H2 gas
without C, the CO-dark H2, in a large sample of clouds. Aims: To use a sparse
Galactic plane survey of the 1.9 THz [C II] spectral line from the Herschel
Open Time Key Programme, Galactic Observations of Terahertz C+ (GOT C+), to
characterize the H2 gas without CO in a statistically significant sample of
clouds. Methods: We identify individual clouds in the inner Galaxy by fitting
[CII] and CO isotopologue spectra along each line of sight. We combine these
with HI spectra, along with excitation models and cloud models of C+, to
determine the column densities and fractional mass of CO-dark H2 clouds.
Results: We identify 1804 narrow velocity [CII] interstellar cloud components
in different categories. About 840 are diffuse molecular clouds with no CO, 510
are transition clouds containing [CII] and 12CO, but no 13CO, and the remainder
are dense molecular clouds containing 13CO emission. The CO-dark H2 clouds are
concentrated between Galactic radii 3.5 to 7.5 kpc and the column density of
the CO-dark H2 layer varies significantly from cloud-to-cloud with an average
9X10^(20) cm-2. These clouds contain a significant fraction of CO-dark H2 mass,
varying from ~75% for diffuse molecular clouds to ~20% for dense molecular
clouds. Conclusions: We find a significant fraction of the warm molecular ISM
gas is invisible in HI and CO, but is detected in [CII]. The fraction of
CO-dark H2 is greatest in the diffuse clouds and decreases with increasing
total column density, and is lowest in the massive clouds.Comment: 21 pages, 19 figures, accepted for publication in A&A (2014
Ionized gas at the edge of the Central Molecular Zone
To determine the properties of the ionized gas at the edge of the CMZ near
Sgr E we observed a small portion of the edge of the CMZ near Sgr E with
spectrally resolved [C II] 158 micron and [N II] 205 micron fine structure
lines at six positions with the GREAT instrument on SOFIA and in [C II] using
Herschel HIFI on-the-fly strip maps. We use the [N II] spectra along with a
radiative transfer model to calculate the electron density of the gas and the
[C II] maps to illuminate the morphology of the ionized gas and model the
column density of CO-dark H2. We detect two [C II] and [N II] velocity
components, one along the line of sight to a CO molecular cloud at -207 km/s
associated with Sgr E and the other at -174 km/s outside the edge of another CO
cloud. From the [N II] emission we find that the average electron density is in
the range of about 5 to 25 cm{-3} for these features. This electron density is
much higher than that of the warm ionized medium in the disk. The column
density of the CO-dark H layer in the -207 km/s cloud is about 1-2X10{21}
cm{-2} in agreement with theoretical models. The CMZ extends further out in
Galactic radius by 7 to 14 pc in ionized gas than it does in molecular gas
traced by CO. The edge of the CMZ likely contains dense hot ionized gas
surrounding the neutral molecular material. The high fractional abundance of N+
and high electron density require an intense EUV field with a photon flux of
order 1e6 to 1e7 photons cm{-2} s{-1}, and/or efficient proton charge exchange
with nitrogen, at temperatures of order 1e4 K, and/or a large flux of X-rays.
Sgr E is a region of massive star formation which are a potential sources of
the EUV radiation that can ionize the gas. In addition X-ray sources and the
diffuse X-ray emission in the CMZ are candidates for ionizing nitrogen.Comment: 12 pages, 9 figure
Millimeter Wave Localization: Slow Light and Enhanced Absorption
We exploit millimeter wave technology to measure the reflection and
transmission response of random dielectric media. Our samples are easily
constructed from random stacks of identical, sub-wavelength quartz and Teflon
wafers. The measurement allows us to observe the characteristic transmission
resonances associated with localization. We show that these resonances give
rise to enhanced attenuation even though the attenuation of homogeneous quartz
and Teflon is quite low. We provide experimental evidence of disorder-induced
slow light and superluminal group velocities, which, in contrast to photonic
crystals, are not associated with any periodicity in the system. Furthermore,
we observe localization even though the sample is only about four times the
localization length, interpreting our data in terms of an effective cavity
model. An algorithm for the retrieval of the internal parameters of random
samples (localization length and average absorption rate) from the external
measurements of the reflection and transmission coefficients is presented and
applied to a particular random sample. The retrieved value of the absorption is
in agreement with the directly measured value within the accuracy of the
experiment.Comment: revised and expande
The Arecibo Methanol Maser Galactic Plane Survey - II: Statistical and Multi-wavelength Counterpart Analysis
We present an analysis of the properties of the 6.7 GHz methanol maser sample
detected in the Arecibo Methanol Maser Galactic Plane Survey. The distribution
of the masers in the Galaxy, and statistics of their multi-wavelength
counterparts is consistent with the hypothesis of 6.7 GHz maser emission being
associated with massive young stellar objects. Using the detection statistics
of our survey, we estimate the minimum number of methanol masers in the Galaxy
to be 1275. The l-v diagram of the sample shows the tangent point of the
Carina-Sagittarius spiral arm to be around 49.6 degrees, and suggests
occurrence of massive star formation along the extension of the Crux-Scutum
arm. A Gaussian component analysis of the maser spectra shows the mean
line-width to be 0.38 km/s which is more than a factor of two larger than what
has been reported in the literature. We also find no evidence that faint
methanol masers have different properties than those of their bright
counterparts.Comment: Accepted by ApJ; Revised footnote number 3 on page 8 based on private
communicatio
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