135 research outputs found
Dust in regions of massive star formation
It is suggested that protostars increase mass by accreting the surrounding gas and dust. Grains are destroyed as they near the central protostar creating a dust shell or cocoon. Radiation pressure acting on the grains can halt the inflow of material thereby limiting the amount of mass accumulated by the protostar. General constraints were considered on the initial dust-to-gas ratio and mass accretion rates that permit inflow. These results were constrained further by constructing a numerical model, including radiative deceleration on grains and grain destruction processes. Also the constraints on dust properties were investigated which allow the formation of massive stars. The obtained results seem to suggest that massive star formation requires rather extreme preconditioning of the grain and gas environment
The chemistry of fluorine-bearing molecules in diffuse and dense interstellar gas clouds
We present a theoretical investigation of the chemistry of fluorine-bearing
molecules in diffuse and dense interstellar gas clouds. The chemistry of
interstellar fluorine is qualitatively different from that of any other
element, because - unlike the neutral atoms of any other element found in
diffuse or dense molecular clouds - atomic fluorine undergoes an exothermic
reaction with molecular hydrogen. Over a wide range of conditions attained
within interstellar gas clouds, the product of that reaction - hydrogen
fluoride - is predicted to be the dominant gas-phase reservoir of interstellar
fluorine nuclei. Our model predicts HF column densities ~ 1.E+13 cm-2 in dark
clouds and column densities as large as 1.E-11 cm-2 in diffuse interstellar gas
clouds with total visual extinctions as small as 0.1 mag. Such diffuse clouds
will be detectable by means of absorption line spectroscopy of the J = 1 - 0
transition at 243.2 micron using the Stratospheric Observatory for Infrared
Astronomy (SOFIA) and the Herschel Space Observatory (HSO). The CF+ ion is
predicted to be the second most abundant fluorine-bearing molecule, with
typical column densities a factor ~ 100 below those of HF; with its lowest two
rotational transitions in the millimeter-wave spectral region, CF+ may be
detectable from ground-based observatories. HF absorption in quasar spectra is
a potential probe of molecular gas at high redshift, providing a possible
bridge between the UV/optical observations capable of probing H2 in low column
density systems and the radio/millimeter-wavelength observations that probe
intervening molecular clouds of high extinction and large molecular fraction;
at redshifts beyond ~ 0.3, HF is potentially detectable from ground-based
submillimeter observatories in several atmospheric transmission windows.Comment: 34 pages, including 11 figures (10 color), accepted for publication
in Ap
Infrared emission from ultracompact H II regions
Models of circumstellar dust shells around ultracompact (UC) H II regions were constructed that accurately fit the observed IR flux distributions. The models assume spherically symmetric dust shells illuminated by stars whose bolometric luminosity is inferred from the integrated FIR flux densities. Assuming ionization by a single zero age main sequence (ZAMS) star, the relations of Panagia were used to infer the stellar radius and effective temperature for a given luminosity. The grain mixture in the dust shell consists of bare graphite and silicate grains with the optical properties of Draine and Lee and the size distribution of Mathis et al. The computer code of Wolfire et al was used to solve the radiative transfer equations through a spherical dust shell. The model provides monochromatic luminosities, dust temperatures, and opacities through the shell. Aside from the stellar and dust properties, the only other input parameters to the model are the distance to the shell, the form of its density distribution, and its outer radius. Predictions of the model are compared with observations of a typical UC H II region and the run of dust temperature with radius and the optical depth with frequency are discussed
The PhotoDissociation Region Toolbox: Software and Models for Astrophysical Analysis
The PhotoDissociation Region Toolbox provides comprehensive, easy-to-use,
public software tools and models that enable an understanding of the
interaction of the light of young, luminous, massive stars with the gas and
dust in the Milky Way and in other galaxies. It consists of an open-source
Python toolkit and photodissociation region models for analysis of infrared and
millimeter/submillimeter line and continuum observations obtained by
ground-based and sub-orbital telescopes, and astrophysics space missions.
Photodissociation regions (PDRs) include all of the neutral gas in the ISM
where far-ultraviolet photons dominate the chemistry and/or heating. In regions
of massive star formation, PDRs are created at the boundaries between the H II
regions and neutral molecular cloud, as photons with energies 6 eV 13.6 eV photodissociate molecules and photoionize metals. The gas is heated
by photo-electrons from small grains and large molecules and cools mostly
through far-infrared fine-structure lines like [O I] and [C II]. The models are
created from state-of-the art PDR codes that includes molecular freeze-out;
recent collision, chemical, and photo rates; new chemical pathways, such as for
oxygen chemistry; and allow for both clumpy and uniform media. The models
predict the emergent intensities of many spectral lines and FIR continuum. The
tools find the best-fit models to the observations and provide insights into
the physical conditions and chemical makeup of the gas and dust. The PDR
Toolbox enables novel analysis of data from telescopes such as ISO, Spitzer,
Herschel, STO, SOFIA, SWAS, APEX, ALMA, and JWST.Comment: 22 pages, 10 figures, includes code listing
Star Formation in Atomic Gas
Observations of nearby galaxies have firmly established, over a broad range
of galactic environments and metallicities, that star formation occurs
exclusively in the molecular phase of the interstellar medium (ISM).
Theoretical models show that this association results from the correlation
between chemical phase, shielding, and temperature. Interstellar gas converts
from atomic to molecular only in regions that are well shielded from
interstellar ultraviolet (UV) photons, and since UV photons are also the
dominant source of interstellar heating, only in these shielded regions does
the gas become cold enough to be subject to Jeans instability. However, while
the equilibrium temperature and chemical state of interstellar gas are
well-correlated, the time scale required to reach chemical equilibrium is much
longer than that required to reach thermal equilibrium, and both timescales are
metallicity-dependent. Here I show that the difference in time scales implies
that, at metallicities below a few percent of the Solar value, well-shielded
gas will reach low temperatures and proceed to star formation before the bulk
of it is able to convert from atomic to molecular. As a result, at extremely
low metallicities, star formation will occur in a cold atomic phase of the ISM
rather than a molecular phase. I calculate the observable consequences of this
result for star formation in low metallicity galaxies, and I discuss how some
current numerical models for H2-regulated star-formation may need to be
modified.Comment: 9 pages, 8 figures, ApJ in press; very minor changes from previous
versio
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