283 research outputs found
The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
In this series of papers we study the structure of the atomic to molecular
transition in the giant atomic-molecular complexes that are the repositories of
most molecular gas in galaxies, with the ultimate goal of attaining a better
understanding of what determines galaxies' molecular content. Here we derive an
approximate analytic solution for the structure of a photodissociation region
(PDR) in a cloud of finite size that is bathed in an external dissociating
radiation field. Our solution extends previous work, which with few exceptions
has been restricted to a one-dimensional treatment of the radiation field. We
show that our analytic results compare favorably to exact numerical
calculations in the one-dimensional limit. However, our more general geometry
provides a more realistic representation than a semi-infinite slab for
atomic-molecular complexes exposed to the interstellar radiation field,
particularly in environments such as low-metallicity dwarf galaxies where the
curvature and finite size of the atomic envelope cannot be neglected. For
clouds that are at least 20% molecular we obtain analytic expressions for the
molecular fraction in terms of properties of the gas and radiation field that
are accurate to tens of percent, while for clouds of lower molecular content we
obtain upper limits. As a side benefit, our analysis helps clarify when
self-shielding is the dominant process in H_2 formation, and under what
circumstances shielding by dust makes a significant contribution.Comment: 19 pages, 11 figures, emulateapj style, accepted to ApJ. Discussion
slightly changed from previous version, and some new analytic approximations
added. Underlying results unchange
Get Me Out of Here
My thesis paper Get Me Out of Here discusses paintings I made and presented while preparing to finish the Hunter MFA program in spring 2017
The Atomic-to-Molecular Transition in Galaxies. III. A New Method for Determining the Molecular Content of Primordial and Dusty Clouds
Understanding the molecular content of galaxies is a critical problem in star
formation and galactic evolution. Here we present a new method, based on a
Stromgren-type analysis, to calculate the amount of HI that surrounds a
molecular cloud irradiated by an isotropic radiation field. We consider both
planar and spherical clouds, and H_2 formation either in the gas phase or
catalyzed by dust grains. Under the assumption that the transition from atomic
to molecular gas is sharp, our method gives the solution without any reference
to the photodissociation cross section. We test our results for the planar case
against those of a PDR code, and find typical accuracies of about 10%. Our
results are also consistent with the scaling relations found in Paper I of this
series, but they apply to a wider range of physical conditions. We present
simple, accurate analytic fits to our results that are suitable for comparison
to observations and to implementation in numerical and semi-analytic models.Comment: 14 pages, 5 figures, accepted to Ap
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
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
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
The Spitzer Space Telescope First-Look Survey: Neutral Hydrogen Emission
The Spitzer Space Telescope (formerly SIRTF) extragalactic First-Look Survey
covered about 5 square degrees centered on J2000 17:18 +59:30 in order to
characterize the infrared sky with high sensitivity. We used the 100-m Green
Bank Telescope to image the 21cm Galactic HI emission over a 3x3 degree field
covering this position with an effective angular resolution of 9.8 arcmin and a
velocity resolution of 0.62 km/s. In the central square degree of the image the
average column density is N(HI) = 2.5 x 10^{20} cm-2 with an rms fluctuation of
0.3 x 10^{20}. The Galactic HI in this region has a very interesting structure.
There is a high-velocity cloud, several intermediate-velocity clouds (one of
which is probably part of the Draco nebula), and narrow-line low velocity
filaments. The HI emission shows a strong and detailed correlation with dust.
Except for the high-velocity cloud, all features in the HI map have
counterparts in an E(B-V) map derived from infrared data. Relatively high
E(B-V)/N(HI) ratios in some directions suggest the presence of molecular gas.
The best diagnostic of such regions is the peak HI line brightness temperature,
not the total N(HI): directions where Tb > 12 K have E(B-V)/N(HI) significantly
above the average value. The data corrected for stray radiation have been
released via the Web.Comment: Accepted for publication in the Astronomical Journal, April 2005. 25
pages includes 11 figures. The data and higher resolution figures are
available from http::/www.cv.nrao.edu/fls_gb
Physical-chemical modeling of the low-mass protostar IRAS 16293-2422
We present detailed gas-phase chemical models for the envelope of the
low-mass star-forming region IRAS 16293-2422. By considering both time- and
space-dependent chemistry, these models are used to study both the physical
structure proposed by Schoier et al. (2002), as well as the chemical evolution
of this region. A new feature of our study is the use of a detailed,
self-consistent radiative transfer model to translate the model abundances into
line strengths and compare them directly with observations of a total of 76
transitions for 18 chemical species, and their isotopes. The model can
reproduce many of the line strengths observed within 50%. The best fit is for
times in the range of 3e3 - 3e4 yrs, and requires only minor modifications to
our model for the high-mass star-forming region AFGL 2591. The ionization rate
for the source may be higher than previously expected -- either due to an
enhanced cosmic-ray ionization rate, or, more probably, to the presence of
X-ray induced ionization from the center. A significant fraction of the CO is
found to desorb in the temperature range of 15-40 K; below this temperature,
\~90% or more of the CO is frozen out. The inability of the model to explain
the HCS+, C2H, and OCS abundances suggests the importance of further laboratory
studies of basic reaction rates. Finally, predictions of the abundances and
spatial distributions of other species which could be observed by future
facilities (e.g., Herschel-HIFI, SOFIA, millimeter arrays) are provided.Comment: 15 pages, 11 Figures, accepted for publication by A&
G28.17+0.05: An unusual giant HI cloud in the inner Galaxy
New 21 cm HI observations have revealed a giant HI cloud in the Galactic
plane that has unusual properties. It is quite well defined, about 150 pc in
diameter at a distance of 5 kpc, and contains as much as 100,000 Solar Masses
of atomic hydrogen. The outer parts of the cloud appear in HI emission above
the HI background, while the central regions show HI self-absorption. Models
which reproduce the observations have a core with a temperature <40 K and an
outer envelope as much as an order of magnitude hotter. The cold core is
elongated along the Galactic plane, whereas the overall outline of the cloud is
approximately spherical. The warm and cold parts of the HI cloud have a
similar, and relatively large, line width of approximately 7 km/s. The cloud
core is a source of weak, anomalously-excited 1720 MHz OH emission, also with a
relatively large line width, which delineates the region of HI self-absorption
but is slightly blue-shifted in velocity. The intensity of the 1720 MHz OH
emission is correlated with N(H) derived from models of the cold core. There is
12CO emission associated with the cloud core. Most of the cloud mass is in
molecules, and the total mass is > 200,000 Solar Masses. In the cold core the
HI mass fraction may be 10 percent. The cloud has only a few sites of current
star formation. There may be about 100 more objects like this in the inner
Galaxy; every line of sight through the Galactic plane within 50 degrees of the
Galactic center probably intersects at least one. We suggest that G28.17+0.05
is a cloud being observed as it enters a spiral arm and that it is in the
transition from the atomic to the molecular state.Comment: 35 pages, inludes 12 figure
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