1,098 research outputs found
The Evolution of Galaxies and Their Environment
The Third Teton Summer School on Astrophysics discussed the formation of galaxies, star formation in galaxies, galaxies and quasars at high red shift, and the intergalactic and intercluster medium and cooling flows. Observation and theoretical research on these topics was presented at the meeting and summaries of the contributed papers are included in this volume
The Propagation and Survival of Interstellar Grains
In this paper we discuss the propagation of dust through the interstellar
medium (ISM), and describe the destructive effects of stellar winds, jets, and
supernova shock waves on interstellar dust. We review the probability that
grains formed in stellar outflows or supernovae survive processing in and
propagation through the ISM, and incorporate themselves relatively unprocessed
into meteoritic bodies in the solar system. We show that very large (radii >= 5
micron) and very small grains (radii <= 100 Angstrom) with sizes similar to the
pre-solar SiC and diamond grains extracted from meteorites, can survive the
passage through 100\kms shock waves relatively unscathed. High velocity (>= 250
km/s) shocks destroy dust efficiently. However, a small (~10%) fraction of the
stardust never encountered such fast shocks before incorporation into the solar
system. All grains should therefore retain traces of their passage through
interstellar shocks during their propagation through the ISM. The grain
surfaces should show evidence of processing due to sputtering and pitting due
to small grain cratering collisions on the micron-sized grains. This conclusion
seems to be in conflict with the evidence from the large grains recovered from
meteorites which seem to show little interstellar processing.Comment: 19 pages, 5 figures (.eps), LaTeX, to appear in "Astrophysical
Implications of the Laboratory Study of Presolar Materials" AIP Conference
Proceedings, 1997 T.J. Bernatowicz and E. Zinner (eds.
Physical conditions in photodissociation regions: Application to galactic nuclei
Infrared and sub-millimeter observations are used in a simple procedure to determine average physical properties of the neutral interstellar medium in Galactic photodissociation regions as well as in ensembles of clouds which exist in the nuclei of luminous infrared galaxies. The relevant observations include the Infrared Astronomy Satellite (IRAS) infrared continuum measurements, infrared spectroscopy of the fine-structure lines of SiII 35 microns, OI 63 microns, and CII 158 microns, and the 2.6 mm CO (J=1-0) rotational transition. The diagnostic capabilities of the OI 145 microns line is also addressed. Researchers attribute these emission lines as well as the continuum to the atomic/molecular photodissociation region on the surfaces of molecular clouds which are illuminated by strong ultraviolet fields. They use the theoretical photodissociation region models of Tielens and Hollenbach (1985, Ap. J., 291, 722) to construct simple diagrams which utilize line ratios and line to continuum ratios to determine the average gas density n, the average incident far-ultraviolet flux G sub o, and the temperature of the atomic gas T
Spitzer spectral line mapping of supernova remnants: I. Basic data and principal component analysis
We report the results of spectroscopic mapping observations carried out
toward small (1 x 1 arcmin) regions within the supernova remnants W44, W28,
IC443, and 3C391 using the Infrared Spectrograph of the Spitzer Space
Telescope. These observations, covering the 5.2 - 37 micron spectral region,
have led to the detection of a total of 15 fine structure transitions of Ne+,
Ne++, Si+, P+, S, S++, Cl+, Fe+, and Fe++; the S(0) - S(7) pure rotational
lines of molecular hydrogen; and the R(3) and R(4) transitions of hydrogen
deuteride. In addition to these 25 spectral lines, the 6.2, 7.7, 8.6, 11.3 and
12.6 micron PAH emission bands were also observed. Most of the detected line
transitions have proven strong enough to map in several sources, providing a
comprehensive picture of the relative distribution of the various line
emissions observable in the Spitzer/IRS bandpass. A principal component
analysis of the spectral line maps reveals that the observed emission lines
fall into five distinct groups, each of which may exhibit a distinct spatial
distribution: (1) lines of S and H2 (J > 2); (2) the H2 S(0) line; (3) lines of
ions with appearance potentials less than 13.6 eV; (4) lines of ions with
appearance potentials greater than 13.6 eV, not including S++; (5) lines of
S++. Lines of group (1) likely originate in molecular material subject to a
slow, nondissociative shock that is driven by the overpressure within the
supernova remnant, and lines in groups (3) - (5) are associated primarily with
dissociative shock fronts with a range of (larger) shock velocities. The H2
S(0) line shows a low-density diffuse emission component, and - in some sources
- a shock-excited component.Comment: 43 pages, including 21 figures. Accepted for publication in Ap
Moment equations for chemical reactions on interstellar dust grains
While most chemical reactions in the interstellar medium take place in the
gas phase, those occurring on the surfaces of dust grains play an essential
role. Chemical models based on rate equations including both gas phase and
grain surface reactions have been used in order to simulate the formation of
chemical complexity in interstellar clouds. For reactions in the gas phase and
on large grains, rate equations, which are highly efficient to simulate, are an
ideal tool. However, for small grains under low flux, the typical number of
atoms or molecules of certain reactive species on a grain may go down to order
one or less. In this case the discrete nature of the opulations of reactive
species as well as the fluctuations become dominant, thus the mean-field
approximation on which the rate equations are based does not apply. Recently, a
master equation approach, that provides a good description of chemical
reactions on interstellar dust grains, was proposed. Here we present a related
approach based on moment equations that can be obtained from the master
equation. These equations describe the time evolution of the moments of the
distribution of the population of the various chemical species on the grain. An
advantage of this approach is the fact that the production rates of molecular
species are expressed directly in terms of these moments. Here we use the
moment equations to calculate the rate of molecular hydrogen formation on small
grains. It is shown that the moment equation approach is efficient in this case
in which only a single reactive specie is involved. The set of equations for
the case of two species is presented and the difficulties in implementing this
approach for complex reaction networks involving multiple species are
discussed.Comment: 12 pages, submitted for publication in A&
Effective grain surface area in the formation of molecular hydrogen in interstellar clouds
In the interstellar clouds, molecular hydrogens are formed from atomic
hydrogen on grain surfaces. An atomic hydrogen hops around till it finds
another one with which it combines. This necessarily implies that the average
recombination time, or equivalently, the effective grain surface area depends
on the relative numbers of atomic hydrogen influx rate and the number of sites
on the grain. Our aim is to discover this dependency. We perform a numerical
simulation to study the recombination of hydrogen on grain surfaces in a
variety of cloud conditions. We use a square lattice (with a periodic boundary
condition) of various sizes on two types of grains, namely, amorphous carbon
and olivine. We find that the steady state results of our simulation match very
well with those obtained from a simpler analytical consideration provided the
`effective' grain surface area is written as , where, is
the actual physical grain area and is a function of the flux of atomic
hydrogen which is determined from our simulation. We carry out the simulation
for various astrophysically relevant accretion rates. For high accretion rates,
small grains tend to become partly saturated with and and the
subsequent accretion will be partly inhibited. For very low accretion rates,
the number of sites to be swept before a molecular hydrogen can form is too
large compared to the actual number of sites on the grain, implying that
is greater than unity.Comment: 8 pages, 5 figures in eps forma
Summer School on Interstellar Processes: Abstracts of contributed papers
The Summer School on Interstellar Processes was held to discuss the current understanding of the interstellar medium and to analyze the basic physical processes underlying interstellar phenomena. Extended abstracts of the contributed papers given at the meeting are presented. Many of the papers concerned the local structure and kinematics of the interstellar medium and focused on such objects as star formation regions, molecular clouds, HII regions, reflection nebulae, planetary nebulae, supernova remnants, and shock waves. Other papers studied the galactic-scale structure of the interstellar medium either in the Milky Way or other galaxies. Some emphasis was given to observations of interstellar grains an
Photoionization of Galactic Halo Gas by Old Supernova Remnants
We present new calculations on the contribution from cooling hot gas to the
photoionization of warm ionized gas in the Galaxy. We show that hot gas in
cooling supernova remnants (SNRs) is an important source of photoionization,
particularly for gas in the halo. We find that in many regions at high latitude
this source is adequate to account for the observed ionization so there is no
need to find ways to transport stellar photons from the disk. The flux from
cooling SNRs sets a floor on the ionization along any line of sight. Our model
flux is also shown to be consistent with the diffuse soft X-ray background and
with soft X-ray observations of external galaxies.
We consider the ionization of the clouds observed towards the halo star HD
93521, for which there are no O stars close to the line of sight. We show that
the observed ionization can be explained successfully by our model EUV/soft
X-ray flux from cooling hot gas. In particular, we can match the H alpha
intensity, the S++/S+ ratio, and the C+* column. From observations of the
ratios of columns of C+* and either S+ or H0, we are able to estimate the
thermal pressure in the clouds. The slow clouds require high (~10^4 cm^-3 K)
thermal pressures to match the N(C+*)/N(S+) ratio. Additional heating sources
are required for the slow clouds to maintain their ~7000 K temperatures at
these pressures, as found by Reynolds, Hausen & Tufte (1999).Comment: AASTeX 5.01; 34 pages, 2 figures; submitted to Astrophysical Journa
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