371 research outputs found
HCO, c-C3H and CF+ : three new molecules in diffuse, translucent and "spiral-arm'' clouds
%methods {We used the EMIR receiver and FTS spectrometer at the IRAM 30m to
construct absorption spectra toward bright extra-galactic background sources at
195 kHz spectral resolution ( 0.6 \kms). We used the IRAM Plateau de
Bure interferometer to synthesize absorption spectra of \hthcop\ and HCO toward
the galactic HII region W49.} %results {HCO, \cc3h\ and CF\p\ were detected
toward the blazars \bll\ and 3C111 having \EBV\ = 0.32 and 1.65 mag. HCO was
observed in absorption from ``spiral-arm'' clouds in the galactic plane
occulting W49. The complement of detectable molecular species in the 85 - 110
GHz absorption spectrum of diffuse/translucent gas is now fully determined at
rms noise level at \EBV\ = 0.32 mag (\AV\ = 1 mag)
and /\EBV\ mag overall.} %conclusions {As
with OH, \hcop\ and \cch, the relative abundance of \cc3h\ varies little
between diffuse and dense molecular gas, with N(\cc3h)/N({\it o-c}-\c3h2)
0.1. We find N(CF\p)/N(HCO\p) , N(CF\p)/N(\cch)
0.005-0.01 and because N(CF\p) increases with \EBV\ and with the
column densities of other molecules we infer that fluorine remains in the gas
phase as HF well beyond \AV\ = 1 mag. We find N(HCO)/N(HCO\p) = 16
toward \bll, 3C111 and the 40 km/s spiral arm cloud toward W49, implying X(HCO)
, about 10 times higher than in dark clouds. The behaviour of
HCO is consistent with previous suggestions that it forms from C\p\ and \HH,
even when \AV\ is well above 1 mag. The survey can be used to place useful
upper limits on some species, for instance N(\hhco)/N(\HH CS) 32 toward
3C111, compared to 7 toward TMC-1, confirming the possibility of a gas phase
formation route to \hhco.}Comment: A\%A in pres
The Shape of Cas A
Based on optical, IR and X-ray studies of Cas A, we propose a geometry for
the remnant based on a "jet-induced" scenario with significant systematic
departures from axial symmetry. In this model, the main jet axis is oriented in
the direction of strong blue-shifted motion at an angle of 110 - 120 degrees
East of North and about 40 - 50 degrees to the East of the line of sight.
Normal to this axis would be an expanding torus as predicted by jet-induced
models. In the proposed geometry, iron-peak elements in the main jet-like flow
could appear "beyond" the portions of the remnant rich in silicon by projection
effects, not the effect of mixing. In the context of the proposed geometry, the
displacement of the compact object from the kinematic center of the remnant at
a position angle of ~169 degrees can be accommodated if the motion of the
compact object is near to, but slightly off from, the direction of the main
"jet" axis by of order 30 degrees. In this model, the classical NE "jet," the
SW "counter-jet" and other protrusions, particularly the "hole" in the North,
are non-asymmetric flows approximately in the equatorial plane, e.g., out
through the perimeter of the expanding torus, rather than being associated with
the main jet. We explore the spoke-like flow in the equatorial plane in terms
of Rayleigh-Taylor, Richtmyer-Meshkov and Kelvin-Helmholz instabilities and
illustrate these instabilities with a jet-induced simulation.Comment: 25 pages, 4 figures. Accepted for publication in the Astrophysical
Journa
Carbon isotope fractionation and depletion in TMC1
12C/13C isotopologue abundance anomalies have long been predicted for
gas-phase chemistry in molecules other than CO and have recently been observed
in the Taurus molecular cloud in several species hosting more than one carbon
atom, i.e. CCH, CCS, CCCS and HCN. Here we work to ascertain whether these
isotopologic anomalies actually result from the predicted depletion of the 13C+
ion in an oxygen-rich optically-shielded dense gas, or from some other more
particular mechanism or mechanisms. We observed 3mm emission from
carbon, sulfur and nitrogen-bearing isotopologues of HNC, CS and \HH CS at
three positions in Taurus(TMC1, L1527 and the ammonia peak) using the ARO 12m
telescope. We saw no evidence of 12C/13C anomalies in our observations.
Although the pool of C+ is likely to be depleted in 13C 13C is not depleted in
the general pool of carbon outside CO, which probably exists mostly in the form
of C^0. The observed isotopologic abundance anomalies are peculiar to those
species in which they are found.Comment: Accepted for publication in The Astrophysical Journal (mail journal
Reanalysis of Copernicus Measurements on Interstellar Carbon Monoxide
We used archival data acquired with the Copernicus satellite to reexamine CO
column densities because self-consistent oscillator strengths are now
available. Our focus is on lines of sight containing modest amounts of
molecular species. Our resulting column densities are small enough that
self-shielding from photodissociation is not occurring in the clouds probed by
the observations. While our sample shows that the column densities of CO and H2
are related, no correspondence with the CH column density is evident. The case
for the CH+ column density is less clear. Recent chemical models for these
sight lines suggest that CH is mainly a by-product of CH+ synthesis in low
density gas. The models are most successful in reproducing the amounts of CO in
the densest sight lines. Thus, much of the CO absorption must arise from denser
clumps along the line of sight to account for the trend with H2.Comment: 19 pages, 6 figures. Accepted for publication in Ap
A High-Resolution Survey of HI Absorption toward the Central 200 pc of the Galactic Center
We present an HI absorption survey of the central 250 pc of the Galaxy. Very
Large Array (VLA) observations were made at 21 cm in the DnC and CnB
configurations and have a resolution of ~15"(0.6 pc at the Galactic Center (GC)
distance) and a velocity resolution of ~2.5 km/s. This study provides HI data
with high spatial resolution, comparable with the many high resolution
observations which have been made of GC sources over the past ten years. Here
we present an overview of the HI absorption toward ~40 well-known continuum
sources and a detailed comparison of the ionized, atomic and molecular
components of the interstellar medium for the Sgr B, Radio Arc and Sgr C
regions. In these well-known regions, the atomic gas appears to be closely
correlated in both velocity and distribution to the ionized and molecular gas,
indicating that it resides in photo-dissociation regions related to the HII
regions in the GC. Toward the majority of the radio continuum sources, HI
absorption by the 3-kpc arm is detected, constraining these sources to lie
beyond a 5 kpc distance in the Galaxy.Comment: 59 pages, including 41 figures; accepted for publication in
Astrophysical Journal Supplement Series in December 201
Recent star formation in the inner Galactic Bulge seen by ISOGAL. I - Classification of bright mid-IR sources in a test field
Context: The stellar populations in the central region of the Galaxy are
poorly known because of the high visual extinction and very great source
density in this direction.
Aims: To use recent infrared surveys for studying the dusty stellar objects
in this region.
Methods: We analyse the content of a 20x20 arcmin^2 field centred at
(l,b)=(-0.27,-0.06) observed at 7 and 15 microns as part of the ISOGAL survey.
These ISO observations are more than an order of magnitude better in
sensitivity and spatial resolution than the IRAS observations. The sources are
cross-associated with other catalogues to identify various types of objects. We
then derive criteria to distinguish young objects from post-main sequence
stars.
Results: We find that a sample of about 50 young stellar objects and
ultra-compact HII regions emerges, out of a population of evolved AGB stars. We
demonstrate that the sources colours and spatial extents, as they appear in the
ISOGAL catalogue, possibly complemented with MSX photometry at 21 microns, can
be used to determine whether the ISOGAL sources brighter than 300 mJy at 15
microns (or [15] < 4.5 mag) are young objects or late-type evolved stars.Comment: 15 pages, 12 figures. Accepted for publication in Astronomy and
Astrophysic
Molecular absorption lines toward star-forming regions : a comparative study of HCO+, HNC, HCN, and CN
Aims. The comparative study of several molecular species at the origin of the
gas phase chemistry in the diffuse interstellar medium (ISM) is a key input in
unraveling the coupled chemical and dynamical evolution of the ISM. Methods.
The lowest rotational lines of HCO+, HCN, HNC, and CN were observed at the
IRAM-30m telescope in absorption against the \lambda 3 mm and \lambda 1.3 mm
continuum emission of massive star-forming regions in the Galactic plane. The
absorption lines probe the gas over kiloparsecs along these lines of sight. The
excitation temperatures of HCO+ are inferred from the comparison of the
absorptions in the two lowest transitions. The spectra of all molecular species
on the same line of sight are decomposed into Gaussian velocity components.
Most appear in all the spectra of a given line of sight. For each component, we
derived the central opacity, the velocity dispersion, and computed the
molecular column density. We compared our results to the predictions of
UV-dominated chemical models of photodissociation regions (PDR models) and to
those of non-equilibrium models in which the chemistry is driven by the
dissipation of turbulent energy (TDR models). Results. The molecular column
densities of all the velocity components span up to two orders of magnitude.
Those of CN, HCN, and HNC are linearly correlated with each other with mean
ratios N(HCN)/N(HNC) = 4.8 1.3 and N(CN)/N(HNC) = 34 12, and more
loosely correlated with those of HCO+, N(HNC)/N(HCO+) = 0.5 0.3,
N(HCN)/N(HCO+) = 1.9 0.9, and N(CN)/N(HCO+) = 18 9. These ratios
are similar to those inferred from observations of high Galactic latitude lines
of sight, suggesting that the gas sampled by absorption lines in the Galactic
plane has the same chemical properties as that in the Solar neighbourhood. The
FWHM of the Gaussian velocity components span the range 0.3 to 3 km s-1 and
those of the HCO+ lines are found to be 30% broader than those of CN-bearing
molecules. The PDR models fail to reproduce simultaneously the observed
abundances of the CN-bearing species and HCO+, even for high-density material
(100 cm-3 < nH < 104 cm-3). The TDR models, in turn, are able to reproduce the
observed abundances and abundance ratios of all the analysed molecules for the
moderate gas densities (30 cm-3 < nH < 200 cm-3) and the turbulent energy
observed in the diffuse interstellar medium. Conclusions. Intermittent
turbulent dissipation appears to be a promising driver of the gas phase
chemistry of the diffuse and translucent gas throughout the Galaxy. The details
of the dissipation mechanisms still need to be investigated
Dense Ionized and Neutral Gas Surrounding Sgr A*
We present high resolution H41a hydrogen recombination line observations of
the 1.2' (3 pc) region surrounding Sgr A* at 92 GHz using the OVRO Millimeter
Array with an angular resolution of 7" x 3" and velocity resolution of 13 km/s.
New observations of H31a, H35a, H41a, and H44a lines were obtained using the
NRAO 12-m telescope, and their relative line strengths are interpreted in terms
of various emission mechanisms. These are the most extensive and most sensitive
observations of recombination line to date. Observations of HCO+ (1 - 0)
transition at 89 GHz are also obtained simultaneously with a 40% improved
angular resolution and 4-15 times improved sensitivity over previous
observations, and the distribution and kinematics of the dense molecular gas in
the circumnuclear disk (CND) are mapped and compared with those of the ionized
gas. The line brightness ratios of the hydrogen recombination lines are
consistent with purely spontaneous emission from 7000 K gas with n_e = 20,000
cm near LTE condition. A virial analysis suggests that the most
prominent molecular gas clumps in the CND have mean densities of 10^7 cm^{-3},
sufficient to withstand the tidal shear in the Galactic Center region.
Therefore, these clumps may survive over several dynamical times, and the CND
may be a dynamically stable structure. We estimate a total gas mass of 3 x 10^5
solar mass for the CND. \Comment: 34 pages including 11 figures (4 jpgs), Latex, uses aastex. The full
pdf format file including high resolution figures is available at
http://www.astro.umass.edu/~myun/papers/SgrA.pdf . To appear in the 20
November 2004 (V616) issue of the Astrophysical Journa
Constraining the variation of fundamental constants using 18cm OH lines
We describe a new technique to estimate variations in the fundamental
constants using 18cm OH absorption lines. This has the advantage that all lines
arise in the same species, allowing a clean comparison between the measured
redshifts. In conjunction with one additional transition (for example, an
HCO line), it is possible to simultaneously measure changes in ,
and . At present, only the 1665 MHz and 1667 MHz lines
have been detected at cosmological distances; we use these line redshifts in
conjunction with those of HI 21cm and mm-wave molecular absorption in a
gravitational lens at to constrain changes in the above three
parameters over the redshift range . While the constraints
are relatively weak ( 1 part in ), this is the first
simultaneous constraint on the variation of all three parameters. We also
demonstrate that either one (or more) of , and must vary with
cosmological time or there must be systematic velocity offsets between the OH,
HCO and HI absorbing clouds.Comment: 5 pages, no figures. Accepted for publication in Phys. Rev. Let
The Dynamics of Molecular Material Within 15 pcs of the Galactic Center
We report the results of a 5-field mosaic of the central 15pc of the Galaxy
in the (1,1) and (2,2) lines of NH3. Two narrow filaments or streamers are seen
running parallel to the Galactic plane. The southern streamer appears to carry
gas directly toward the nuclear region from the 20 km/s cloud. The eastern
streamer, which we will denote the molecular ridge, appears to be the denser
part of the 50 km/s cloud which lies immediately east of the Sgr A East complex
and extends in the south towards the 20 km/s cloud. This ridge of gas carries
the kinematical signatures of interactions with Sgr A East as well as a SNR
which lies south of the Galactic center. The bulk motion of the gas, the
enhanced line widths, and the heating of the molecular material all suggest an
active evolutionary phase for the gas immediately adjacent to the nucleus.Comment: 11 pages, 13 figures, to appear in The Astrophysical Journa
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