11,955 research outputs found

### Highly ionized atoms in cooling gas

The ionization of low density gas cooling from a high temperature was calculated. The evolution during the cooling is assumed to be isochoric, isobaric, or a combination of these cases. The calculations are used to predict the column densities and ultraviolet line luminosities of highly ionized atoms in cooling gas. In a model for cooling of a hot galactic corona, it is shown that the observed value of N(N V) can be produced in the cooling gas, while the predicted value of N(Si IV) falls short of the observed value by a factor of about 5. The same model predicts fluxes of ultraviolet emission lines that are a factor of 10 lower than the claimed detections of Feldman, Brune, and Henry. Predictions are made for ultraviolet lines in cooling flows in early-type galaxies and clusters of galaxies. It is shown that the column densities of interest vary over a fairly narrow range, while the emission line luminosities are simply proportional to the mass inflow rate

### Velocity dispersions in galaxies: 1: The SO galaxy NGC 7332

A Coude spectrum of the SO galaxy NGC 7332 with 0.9 A resolution from 4186 to 4364 A was obtained with the SEC vidicon television camera and the Hale telescope. Comparisons with spectra of G and K giant stars, numerically broadened for various Maxwellian velocity distributions, give a dispersion velocity in the line of sight of 160 + or - 20 km/sec with the best fit at G8III. The dispersion appears to be constant within + or - 35 km/sec out to 1.4 kpc (H = 100 km/sec/mpc). After correction for projection, the rotation curve has a slope of 0.16 km/sec/pc at the center and a velocity of 130 km/sec at 1.4 kpc where it is still increasing. For an estimated effective radius of 3.5 kpc enclosing half the light, the virial theorem gives a mass of 1.4 x 10 to the 11th power solar masses if the mass-to-light ratio is constant throughout the galaxy. The photographic luminosity is 8.3 x 10 to the 9th power solar luminosities so that the M/L ratio is 17

### Wind-Interaction Models for the Early Afterglows of Gamma-Ray Bursts: The Case of GRB 021004

Wind-interaction models for gamma-ray burst afterglows predict that the
optical emission from the reverse shock drops below that from the forward shock
within 100s of seconds of the burst. The typical frequency $\nu_m$ of the
synchrotron emission from the forward shock passes through the optical band
typically on a timescale of minutes to hours. Before the passage of $\nu_m$,
the optical flux evolves as $t^{-1/4}$ and after the passage, the decay
steepens to $t^{-(3p-2)/4}$, where $p$ is the exponent for the assumed
power-law energy distribution of nonthermal electrons and is typically $\sim
2$. The steepening in the slope of temporal decay should be readily
identifiable in the early afterglow light curves. We propose that such a
steepening was observed in the R-band light curve of GRB 021004 around day 0.1.
Available data at several radio frequencies are consistent with this
interpretation, as are the X-ray observations around day~1. The early evolution
of GRB 021004 contrasts with that of GRB 990123, which can be described by
emission from interaction with a constant density medium.Comment: 16 pages, 1 figure, submitted to ApJ

### Optical Signatures of Circumstellar Interaction in Type IIP Supernovae

We propose new diagnostics for circumstellar interaction in Type IIP
supernovae by the detection of high velocity (HV) absorption features in Halpha
and He I 10830 A lines during the photospheric stage. To demonstrate the
method, we compute the ionization and excitation of H and He in supernova
ejecta taking into account time-dependent effects and X-ray irradiation. We
find that the interaction with a typical red supergiant wind should result in
the enhanced excitation of the outer layers of unshocked ejecta and the
emergence of corresponding HV absorption, i.e. a depression in the blue
absorption wing of Halpha and a pronounced absorption of He I 10830 A at a
radial velocity of about -10,000 km/s. We identify HV absorption in Halpha and
He I 10830 A lines of SN 1999em and in Halpha of SN 2004dj as being due to this
effect. The derived mass loss rate is close to 10^{-6} Msun/yr for both
supernovae, assuming a wind velocity 10 km/s. We argue that, in addition to the
HV absorption formed in the unshocked ejecta, spectra of SN 2004dj and SN
1999em show a HV notch feature that is formed in the cool dense shell (CDS)
modified by the Rayleigh-Taylor instability. The CDS results from both shock
breakout and radiative cooling of gas that has passed through the reverse shock
wave. The notch becomes dominant in the HV absorption during the late
photospheric phase, ~60 d. The wind density deduced from the velocity of the
CDS is consistent with the wind density found from the HV absorption produced
by unshocked ejecta.Comment: 38 pages, 12 figures, ApJ, in pres

### Use of Optimization to Develop a Correlation Model for Predicting Residual NAPL Saturation

Predicting the residual saturation of a trapped non-aqueous phase liquid contaminant is critical to estimating the region of contamination, the design of remediation strategies, and risk assessment. Models were developed to predict residual NAPL saturation utilizing optimization and non-linear error functions, consequently allowing for a broader mathematical approach to model development. The input parameters evaluated represent soil and fluid properties: the uniformity coefficient (Cu), the coefficient of gradation (Cc), the capillary number (Nc), the bond number (Nb) and the total trapping number (Nt). Overall, the model that performed best was based on a second-order equation with the independent variables Cu and Nt1 using the sum of the squares of the errors. The nonlinear error function based on a derivative of Marquardt’s Percent Standard Deviation performed best for three other cases

### Instabilities and Clumping in Type Ia Supernova Remnants

We present two-dimensional high-resolution hydrodynamical simulations in
spherical polar coordinates of a Type Ia supernova interacting with a constant
density interstellar medium. The ejecta are assumed to be freely expanding with
an exponential density profile. The interaction gives rise to a double-shocked
structure susceptible to hydrodynamic instabilities. The Rayleigh-Taylor
instability initially grows, but the Kelvin-Helmholtz instability takes over,
producing vortex rings. The nonlinear instability initially evolves toward
longer wavelengths and eventually fades away when the reverse shock front is in
the flatter part of the supernova density distribution. Based on observations
of X-ray knots and the protrusion in the southeast outlin of Tycho's supernova
remnant, we include clumping in the ejecta. The clump interaction with the
reverse shock induces Rayleigh-Taylor and Kelvin-Helmholtz instabilities on the
clump surface that facilitate fragmentation. In order to survive crushing and
to have a bulging effect on the forward shock, the clump's initial density
ratio to the surrounding ejecta must be at least 100 for the conditions in
Tycho's remnant. The 56Ni bubble effect may be important for the development of
clumpiness in the ejecta. The observed presence of an Fe clump would then
require a non-radioactive origin for this Fe, possibly 54Fe. The large radial
distance of the X-ray emitting Si and S ejecta from the remnant center
indicates that they were initially in clumps.Comment: 27 pages, 4 postscript figures, 5 GIF figures submitted to
Astrophysical Journa

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