228 research outputs found
Constraints on changes in fundamental constants from a cosmologically distant OH absorber/emitter
We have detected the four 18cm OH lines from the gravitational
lens toward PMN J0134-0931. The 1612 and 1720 MHz lines are in conjugate
absorption and emission, providing a laboratory to test the evolution of
fundamental constants over a large lookback time. We compare the HI and OH main
line absorption redshifts of the different components in the
absorber and the lens toward B0218+357 to place stringent
constraints on changes in . We obtain
,
consistent with no evolution over the redshift range . The
measurements have a sensitivity of or to fractional
changes in and over a period of Gyr, half the age of
the Universe. These are among the most sensitive current constraints on changes
in .Comment: 4 pages, 3 figures. Final version, with minor changes to match the
version in print in Phys. Rev. Let
Beyond the pseudo-time-dependent approach: chemical models of dense core precursors
Context: Chemical models of dense cloud cores often utilize the so-called
pseudo-time-dependent approximation, in which the physical conditions are held
fixed and uniform as the chemistry occurs. In this approximation, the initial
abundances chosen, which are totally atomic in nature except for molecular
hydrogen, are artificial. A more detailed approach to the chemistry of dense
cold cores should include the physical evolution during their early stages of
formation. Aims: Our major goal is to investigate the initial synthesis of
molecular ices and gas-phase molecules as cold molecular gas begins to form
behind a shock in the diffuse interstellar medium. The abundances calculated as
the conditions evolve can then be utilized as reasonable initial conditions for
a theory of the chemistry of dense cores. Methods: Hydrodynamic shock-wave
simulations of the early stages of cold core formation are used to determine
the time-dependent physical conditions for a gas-grain chemical network. We
follow the cold post-shock molecular evolution of ices and gas-phase molecules
for a range of visual extinction up to AV ~ 3, which increases with time. At
higher extinction, self-gravity becomes important. Results: As the newly
condensed gas enters its cool post-shock phase, a large amount of CO is
produced in the gas. As the CO forms, water ice is produced on grains, while
accretion of CO produces CO ice. The production of CO2 ice from CO occurs via
several surface mechanisms, while the production of CH4 ice is slowed by
gas-phase conversion of C into CO.Comment: 9 pages, 3 figures, 2 table
CO emission and variable CH and CH+ absorption towards HD34078: evidence for a nascent bow shock ?
The runaway star HD34078, initially selected to investigate small scale
structure in a foreground diffuse cloud has been shown to be surrounded by
highly excited H2. We first search for an association between the foreground
cloud and HD34078. Second, we extend previous investigations of temporal
absorption line variations (CH, CH+, H2) in order to better characterize them.
We have mapped the CO(2-1) emission at 12 arcsec resolution around HD34078's
position, using the 30 m IRAM antenna. The follow-up of CH and CH+ absorption
lines has been extended over 5 more years. In parallel, CH absorption towards
the reddened star Zeta Per have been monitored to check the homogeneity of our
measurements. Three more FUSE spectra have been obtained to search for N(H2)
variations. CO observations show a pronounced maximum near HD34078's position,
clearly indicating that the star and diffuse cloud are associated. The optical
spectra confirm the reality of strong, rapid and correlated CH and CH+
fluctuations. On the other hand, N(H2, J=0) has varied by less than 5 % over 4
years. We also discard N(CH) variations towards Zeta Per at scales less than 20
AU. Observational constraints from this work and from 24 micron dust emission
appear to be consistent with H2 excitation but inconsistent with steady-state
bow shock models and rather suggest that the shell of compressed gas
surrounding HD34078, is seen at an early stage of the interaction. The CH and
CH+ time variations as well as their large abundances are likely due to
chemical structure in the shocked gas layer located at the stellar wind/ambient
cloud interface. Finally, the lack of variations for both N(H2, J=0) towards
HD34078 and N(CH) towards Zeta Per suggests that quiescent molecular gas is not
subject to pronounced small-scale structure.Comment: 19 pages, 15 figures, accepted for publication in A&
Phase Calibration Sources in the Northern Sky at Galactic Latitudes \mid b \mid < 2.5\arcdeg
The Jodrell Bank - VLA Astrometric Survey (JVAS) of flat-spectrum sources
yielded a catalog of 2118 compact radio sources in the northern sky. Those
sources are being used as phase calibrators for many synthesis arrays. JVAS
suffers from a zone of avoidance because Galactic confusion prevented selection
of flat-spectrum sources at \mid b \mid < 2.5\arcdeg. This confusion problem
was overcome by selecting variable GB6 sources in the JVAS zone of avoidance. A
subset of these sources were observed at 8.5 GHz with the NRAO VLA in the JVAS
style, leading to 29 compact radio sources in the zone of avoidance whose
positions have been measured to an rms accuracy of about 10~mas or less. This
extension of the JVAS lists to lower Galactic latitudes (1) improves the
prospects for VLBA phase-referencing of Galactic targets and (2) strengthens
the lists' usefulness for studies of the Galactic interstellar medium.Comment: with 2 figures, to appear in the Astronomical Journa
Infalling Gas Towards the Galactic Center
VLA maps of ammonia emission were made for the Galactic Center region. The
NH3(1,1) and NH3(2,2) transitions were observed in three 2' x 2' fields
covering Sgr A* and the region 3' immediately south of it. In the central 3
parsecs surrounding Sgr A* we find emission which appears to be associated with
the circumnuclear disk (CND), both morphologically and kinematically. This
central emission is connected to a long, narrow 2 pc x 10 pc streamer of clumpy
molecular gas located towards the south, which appears to be carrying gas from
the nearby 20 km/s giant molecular cloud (GMC) to the circumnuclear region. We
find a velocity gradient along the streamer, with progressively higher
velocities as the gas approaches Sgr A*. The streamer stops at the location of
the CND, where the line width of the NH3 emission increases dramatically. This
may be the kinematic signature of accretion onto the CND. The ratio of the
NH3(2,2)/NH3(1,1) emission indicates that the gas is heated at the northern tip
of the streamer, located inside the eastern edge of the CND. The morphology,
kinematics and temperature gradients of the gas all indicate that the southern
streamer is located at the Galactic Center and is interacting with the
circumnuclear region.Comment: 11 pages, 10 figures, accepted by The Astrophysical Journal (figure 1
contours have been corrected
Models of turbulent dissipation regions in the diffuse interstellar medium
Supersonic turbulence is a large reservoir of suprathermal energy in the
interstellar medium. Its dissipation, because it is intermittent in space and
time, can deeply modify the chemistry of the gas. We further explore a hybrid
method to compute the chemical and thermal evolution of a magnetized
dissipative structure, under the energetic constraints provided by the observed
properties of turbulence in the cold neutral medium. For the first time, we
model a random line of sight by taking into account the relative duration of
the bursts with respect to the thermal and chemical relaxation timescales of
the gas. The key parameter is the turbulent rate of strain "a" due to the
ambient turbulence. With the gas density, it controls the size of the
dissipative structures, therefore the strength of the burst. For a large range
of rates of strain and densities, the models of turbulent dissipation regions
(TDR) reproduce the CH+ column densities observed in the diffuse medium and
their correlation with highly excited H2. They do so without producing an
excess of CH. As a natural consequence, they reproduce the abundance ratios of
HCO+/OH and HCO+/H2O, and their dynamic range of about one order of magnitude
observed in diffuse gas. Large C2H and CO abundances, also related to those of
HCO+, are another outcome of the TDR models that compare well with observed
values. The abundances and column densities computed for CN, HCN and HNC are
one order of magnitude above PDR model predictions, although still
significantly smaller than observed values
Obscuration of the Parsec Scale Jets in the Compact Symmetric Object 1946+708
We present results of VLA and VLBA observations of the 1.420 GHz neutral
hydrogen absorption associated with the Compact Symmetric Object 1946+708
(z=0.101). We find significant structure in the gas on parsec scales. The peak
column density in the HI (N_HI~2.2x10^23 cm^-2 (T_s/8000K)) occurs toward the
center of activity of the source, as does the highest velocity dispersion
(FWHM~350 \kms). In addition, we find that the continuum spectra of the various
radio components associated with these jets strongly indicate free-free
absorption. This effect is particularly pronounced toward the core and inner
components of the receding jet, suggesting the presence of a screen local to
the source, perhaps part of an obscuring torus.Comment: revised version, some text added, 1 figure changed; accepted to
Astrophysical Journal, 22 page LaTeX document includes 8 postscript figure
The relation between column densities of interstellar OH and CH molecules
We present a new, close relation between column densities of OH and CH
molecules based on 16 translucent sightlines (six of them new) and confirm the
theoretical oscillator strengths of the OH A--X transitions at 3078 and 3082
\AA (0.00105, 0.000648) and CH B--X transitions at 3886 and 3890 \AA, (0.00320,
0.00210), respectively. We also report no difference between observed and
previously modelled abundances of the OH molecule.Comment: 4 pages, 0 figures, accepted for publication in MNRA
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