43 research outputs found
An Improved Estimate of the Mass of Dust in Cassiopeia A
Recent observations of sub-millimeter continuum emission toward supernova
remnants (SNR) have raised the question of whether such emission is caused by
dust within the SNR and thus produced by the supernova itself or along the
line-of-sight. The importance of the present work is to establish evidence for
the production of large amounts of dust in supernovae. The best tests can be
made for young supernovae in our galaxy. Cassiopeia A is the best candidate for
a measurement.Comment: 7 pages, 3 figures, accepted by A&A for publicatio
A Search for Molecular Gas in GHz Peaked Spectrum Radio Sources
We present searches for molecular gas (CO, OH, CS, and Ammonia) in six GHz
Peaked Spectrum (GPS) radio sources. We do not detect gas in any source and
place upper limits on the mass of molecular gas which are generally in the
range 1E9 to a few times 1E10 solar masses. These limits are consistent with
the following interpretations: (1) GPS sources do not require very dense gas in
their hosts, and (2) The GPS sources are unlikely to be confined by dense gas
and will evolve to become larger radio sources
Deuterated Ammonia in Galactic Protostellar Cores
We report on a survey of \nh2d towards protostellar cores in low-mass star
formation and quiescent regions in the Galaxy. Twenty-three out of thirty-two
observed sources have significant (\gsim 5\sigma) \nh2d emission.
Ion-molecule chemistry, which preferentially enhances deuterium in molecules
above its cosmological value of \scnot{1.6}{-5} sufficiently explains these
abundances. NH2D/NH3 ratios towards Class 0 sources yields information about
the ``fossil remnants'' from the era prior to the onset of core collapse and
star formation. We compare our observations with predictions of gas-phase
chemical networks.Comment: 16 Pages, 7 Figures, Accepted to Ap.J., to appear in the June 20,
2001 editio
The Detection of New Methanol Masers in the 5_{-1}-4_0E Line
Fifty-one objects in the 5_{-1}-4_0E methanol line at 84.5 GHz were detected
during a survey of Class I maser sources. Narrow maser features were found in
17 of these. Broad quasi-thermal lines were detected towards other sources. One
of the objects with narrow features, the young bipolar outflow L 1157 was also
observed in the 8_0-7_1A+ line at 95.2 GHz; a narrow line was detected at this
frequency. Analysis showed that the broad lines are usually inverted. The
quasi-thermal profiles imply that the line opacities are not larger than
several units. These results confirm the plausibility of models in which
compact Class I masers appear in extended sources as a result of an appropriate
velocity field.
Measurements of linear polarization at 84.5 GHz in 13 sources were made. No
polarization was found except a tentative detection of a weak polarization in M
8E
HI Narrow Self-Absorption in Dark Clouds: Correlations with Molecular Gas and Implications for Cloud Evolution and Star Formation
We present the results of a comparative study of HI narrow self-absorption
(HINSA), OH, 13CO, and C18O in five dark clouds. The HINSA follows the
distribution of the emission of the carbon monoxide isotopologues, and has a
characteristic size close to that of 13CO. This confirms that the HINSA is
produced by cold HI which is well mixed with molecular gas in well-shielded
regions. The ratio of the atomic hydrogen density to total proton density for
these sources is 5 to 27 x 10^{-4}. Using cloud temperatures and the density of
HI, we set an upper limit to the cosmic ray ionization rate of 10^{-16} s^{-1}.
Comparison of observed and modeled fractional HI abundances indicates ages for
these clouds to be 10^{6.5} to 10^{7} yr. The low values of the HI density we
have determined make it certain that the time scale for evolution from an
atomic to an almost entirely molecular phase, must be a minimum of several
million years. This clearly sets a lower limit to the overall time scale for
star formation and the lifetime of molecular clouds
Low-frequency radio absorption in Cassiopeia A
Cassiopeia A is one of the best-studied supernova remnants. Its shocked
ejecta emits brightly in radio and X-rays. Its unshocked ejecta can be studied
through infrared emission, the radio-active decay of Ti, and low
frequency free-free absorption due to cold gas internal to the shell. Free-free
absorption is affected by the mass, geometry, temperature, and ionisation
conditions in the absorbing gas. Observations at the lowest radio frequencies
constrain a combination of these properties. We use LOFAR LBA observations at
30-77 MHz and L-band VLA observations to compare -matched images with a
common resolution of 17". We simultaneously fit, per pixel, for the emission
measure and the ratio of the emission from the unabsorbed front of the shell
versus the absorbed back of the shell. We explore the effects that low
temperatures and a high degree of clumping can have on the derived physical
properties, such as mass and density. We also compile published radio flux
measurements, fit for the absorption processes that occur in the radio band,
and consider how they affect the secular decline of the source. We find a mass
in the unshocked ejecta of for an assumed gas
temperature of K. This estimate is reduced for colder gas temperatures
and if the ejecta are clumped. We measure the reverse shock to have a radius of
" 6". We also find that a decrease in the amount of mass in the
unshocked ejecta (as more and more material meets the reverse shock and heats
up) cannot account for the observed low frequency behaviour of the secular
decline rate. To reconcile our low frequency absorption measurements with
models that predict little mass in the unshocked ejecta we need the ejecta to
be very clumped, or the temperature in the cold gas to be low ( K).
Both conditions can jointly contribute to the high absorption.Comment: Accepted for publication in A&A v2: including the DOI, language edit
Radio Astrochemistry
This paper contains a short overview of the chemistry of interstellar clouds based on radio astronomical measurements.
First, an account of the properties of the Interstellar Medium (ISM) is followed by a historical review up to the time of
the discovery of complex molecules. This is followed by a list of molecules and a short collection of
molecular abundances, followed by an account of possible chemistries in molecular clouds, and
finally, examples of typical galactic sources.