110 research outputs found
Mass Loss From Planetary Nebulae in Elliptical Galaxies
Early-type galaxies possess a dilute hot (2-10E6 K) gas that is probably the
thermalized ejecta of the mass loss from evolving stars. We investigate the
processes by which the mass loss from orbiting stars interacts with the
stationary hot gas for the case of the mass ejected in a planetary nebula
event. Numerical hydrodynamic simulations show that at first, the ejecta
expands nearly symmetrically, with an upstream bow shock in the hot ambient
gas. At later times, the flow past the ejecta creates fluid instabilities that
cause about half of the ejecta to separate and the other half to flow more
slowly downstream in a narrow wake. When radiative cooling is included, most of
the material in the wake (>80%) remains below 1E5 K while the separated ejecta
is hotter (1E5-1E6 K). The separated ejecta is still less than one-quarter the
temperature of the ambient medium and the only way it will reach the
temperature of the ambient medium is through turbulent mixing (after the
material has left the grid). These calculations suggest that a significant
fraction of the planetary nebula ejecta may not become part of the hot ambient
material. This is in contrast to our previous calculations for continuous mass
loss from giant stars in which most of the mass loss became hot gas. We
speculate that detectable OVI emission may be produced, but more sophisticated
calculations will be required to determine the emission spectrum and to better
define the fraction of cooled material.Comment: 34 pages with 20 figures. Higher quality figures are in the ApJ
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The optical counterpart of an Ultra-luminous X-Ray Source in NGC 5204
Ultra-luminous X-Ray sources are extra-nuclear point sources in external
galaxies with -- erg/s and are among the most poorly
understood X-ray sources. To help understand their nature, we are trying to
identify their optical counterparts by combining images from the Hubble Space
Telescope and the Chandra Observatory. Here we report upon the optical
counterpart for the ULX in NGC 5204, which has average X-ray luminosity of
erg/s and has varied by a factor of 50% over the last 10
years. A unique optical counterpart to this ULX is found by carefully comparing
the Chandra ACIS images and HST WFPC2 and ACS/HRC images. The spectral energy
distribution and the HST/STIS FUV spectrum of this object show that it is a B0
Ib supergiant star with peculiarities, including the 1240 N V emission
line that is uncommon in B stellar spectra but has been predicted for X-ray
illuminated accretion disks and seen in some X-ray binaries. Study of its FUV
spectrum leads to a binary model for this ULX in which the B0 Ib supergiant is
overflowing its Roche Lobe and accreting onto the compact primary, probably a
black hole. This picture predicts an orbital period of days for
different black hole mass, which can be tested by future observations
A Scattered Light Echo around SN 1993J in M81
A light echo around SN 1993J was observed 8.2 years after explosion by a HST
WFPC2 observation, adding to the small family of supernovae with light echoes.
The light echo was formed by supernova light scattered from a dust sheet, which
lies 220 parsecs away from the supernova, 50 parsecs thick along the line of
sight, as inferred from radius and width of the light echo. The dust inferred
from the light echo surface brightness is 1000 times denser than the intercloud
dust. The graphite to silicate fraction can not be determined by our BVI
photometric measurements, however, a pure graphite model can be excluded based
on comparison with the data. With future observations, it will be possible to
measure the expansion rate of the light echo, from which an independent
distance to M81 can be obtained.Comment: 10 pages, 6 figures, in AASTeX format, submitted to ApJ Part
Optical studies of the ultraluminous X-ray source NGC1313 X-2
NGC1313 X-2 was among the first ultraluminous X-ray sources discovered, and
has been a frequent target of X-ray and optical observations. Using the HST/ACS
multi-band observations, this source is identified with a unique counterpart
within an error circle of 0\farcs2. The counterpart is a blue star on the
edge of a young cluster of years amid a dominant old stellar
population. Its spectral energy distribution is consistent with that for a
Z=0.004 star with 8.5 about years old, or for an O7 V
star at solar metallicity. The counterpart exhibited significant variability of
mag between two F555W observations separated by
three months, reminiscent of the ellipsoidal variability due to the orbital
motion of this ULX binary.Comment: 21 pages, 7 figures, scheduled for the ApJ June 10, 2007, v662n 1
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A Two Hour Quasi-Period in an Ultra-luminous X-Ray source in NGC628
Quasi-periodic oscillations and X-ray spectroscopy are powerful probes of
black hole masses and accretion disks, and here we apply these diagnostics to
an ultraluminous X-ray source (ULX) in the spiral galaxy NGC628 (M74). This
object was observed four times over two years with the Chandra X-ray
Observatory and XMM-Newton, with three long observations showing dramatic
variability, distinguished by a series of outbursts with a quasi-period (QPO)
of 4,000-7,000 seconds. This is unique behavior among both ULXs and Galactic
X-ray binaries due to the combination of its burst-like peaks and deep troughs,
its long quasi-periods, its high variation amplitudes of %, and its
substantial variability between observations. The X-ray spectra is fitted by an
absorbed accretion disk plus a power-law component, suggesting the ULX was in a
spectral state analogous to the Low Hard state or the Very High state of
Galactic black hole X-ray binaries. A black hole mass of -- is estimated from the -- scaling relation found in the
Galactic X-ray binaries and active galactic nuclei.Comment: 12 pages, 3 figures. accepted for publication in ApJ Lette
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