552 research outputs found
Star Formation and Gas Dynamics in Galactic Disks: Physical Processes and Numerical Models
Star formation depends on the available gaseous "fuel" as well as galactic
environment, with higher specific star formation rates where gas is
predominantly molecular and where stellar (and dark matter) densities are
higher. The partition of gas into different thermal components must itself
depend on the star formation rate, since a steady state distribution requires a
balance between heating (largely from stellar UV for the atomic component) and
cooling. In this presentation, I discuss a simple thermal and dynamical
equilibrium model for the star formation rate in disk galaxies, where the basic
inputs are the total surface density of gas and the volume density of stars and
dark matter, averaged over ~kpc scales. Galactic environment is important
because the vertical gravity of the stars and dark matter compress gas toward
the midplane, helping to establish the pressure, and hence the cooling rate. In
equilibrium, the star formation rate must evolve until the gas heating rate is
high enough to balance this cooling rate and maintain the pressure imposed by
the local gravitational field. In addition to discussing the formulation of
this equilibrium model, I review the current status of numerical simulations of
multiphase disks, focusing on measurements of quantities that characterize the
mean properties of the diffuse ISM. Based on simulations, turbulence levels in
the diffuse ISM appear relatively insensitive to local disk conditions and
energetic driving rates, consistent with observations. It remains to be
determined, both from observations and simulations, how mass exchange processes
control the ratio of cold-to-warm gas in the atomic ISM.Comment: 8 pages, 1 figure; to appear in "IAU Symposium 270: Computational
Star formation", Eds. J. Alves, B. Elmegreen, J. Girart, V. Trimbl
Spatial Variations in Galactic H I Structure on AU-Scales Toward 3C 147 Observed with the Very Long Baseline Array
This paper reports dual-epoch, Very Long Baseline Array observations of H I
absorption toward 3C 147. One of these epochs (2005) represents new
observations while one (1998) represents the reprocessing of previous
observations to obtain higher signal-to-noise results. Significant H I opacity
and column density variations, both spatially and temporally, are observed with
typical variations at the level of \Delta\tau ~ 0.20 and in some cases as large
as \Delta\tau ~ 0.70, corresponding to column density fluctuations of order 5 x
10^{19} cm^{-2} for an assumed 50 K spin temperature. The typical angular scale
is 15 mas; while the distance to the absorbing gas is highly uncertain, the
equivalent linear scale is likely to be about 10 AU. Approximately 10% of the
face of the source is covered by these opacity variations, probably implying a
volume filling factor for the small-scale absorbing gas of no more than about
1%. Comparing our results with earlier results toward 3C 138 (Brogan et al.),
we find numerous similarities, and we conclude that small-scale absorbing gas
is a ubiquitous phenomenon, albeit with a low probability of intercept on any
given line of sight. Further, we compare the volumes sampled by the line of
sight through the Galaxy between our two epochs and conclude that, on the basis
of the motion of the Sun alone, these two volumes are likely to be
substantially different. In order to place more significant constraints on the
various models for the origin of these small-scale structures, more frequent
sampling is required in any future observations.Comment: 16 pages with 10 figures in 24 files; AASTeX format; accepted by A
Very small grains in the Milky Way and external galaxies
These studies of the infrared colors of reflection nebulae, HL HI clouds, HII regions and external galaxies have shown the following results. Different classes of objects locate in different regions on the R vs F sub v (60)/F sub v (100) diagram. This is determined both by differences in dust properties and by differences in the illuminating radiation field. For example, HL clouds and reflection nebulae almost have the same behavior since both are in the diffuse Interstellar Medium (ISM) and can be expected to have similar grain populations; the small difference in their infrared colors can be explained by the difference of the illuminating radiation field. On the other hand, the dramatic difference of R=vF sub v (12)/F sub v (far IR) between HII region and diffuse ISM may be due to the destruction of the Very Small Grain (VSG) component in the HII regions, although radiation transfer effects may play a part as well. The ratio R=vF sub v (12)/F sub v (far IR) is approximately constant in normal spiral galaxies. This implies that the mass ratio b=Mass (VSG)/Mass (dust) does not vary greatly from one galaxy to another
Parsec-scale magnetic fields in Arp 220
We present the first very-long-baseline interferometry (VLBI) detections of
Zeeman splitting in another galaxy. We used Arecibo Observatory, the Green Bank
Telescope, and the Very Long Baseline Array to perform dual-polarization
observations of OH maser lines in the merging galaxy Arp 220. We measured
magnetic fields of 1-5 mG associated with three roughly parsec-sized
clouds in the nuclear regions of Arp 220. Our measured magnetic fields have
comparable strengths and the same direction as features at the same velocity
identified in previous Zeeman observations with Arecibo alone. The agreement
between single dish and VLBI results provides critical validation of previous
Zeeman splitting observations of OH megamasers that used a single large dish.
The measured magnetic field strengths indicate that magnetic energy densities
are comparable to gravitational energy in OH maser clouds. We also compare our
total intensity results to previously published VLBI observations of OH
megamasers in Arp 220. We find evidence for changes in both structure and
amplitude of the OH maser lines that are most easily explained by variability
intrinsic to the masing region, rather than variability produced by
interstellar scintillation. Our results demonstrate the potential for using
high-sensitivity VLBI to study magnetic fields on small spatial scales in
extragalactic systems.Comment: 9 pages, accepted to MNRA
Recent X-Ray Observations of SN1986J with ASCA and ROSAT
We present ASCA and ROSAT observations of SN 1986J covering the period 1991
August to 1996 January. From observations with the ROSAT HRI and PSPC, we find
that the 0.5-2.5 keV flux decreased proportional to during this
period; the ASCA data are consistent with this result and extend it to the 2-10
keV band. ASCA spectra from 1994 January and 1996 January are consistent with
thermal emission from a solar metallicity plasma at an equilibrium temperature
kT = 5-7.5 keV, somewhat hotter than that observed from other X-ray supernovae.
These spectra also show a clear Fe K emission line at 6.7 keV with FWHM <
20,000 km/s (90% confidence). This limit on the line width is consistent with
the reverse shock model of Chevalier & Fransson (1994), but does not rule out
the clumpy wind model of Chugai (1993).Comment: 20 pages, 9 postscript figures, latex, uses aastex4.0, submitted to
The Astrophysical Journa
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