66,499 research outputs found
HST/FOS Eclipse Observations of the Nova-like Cataclysmic Variable UX Ursae Majoris
[abridged abstract]
We present and analyze Hubble Space Telescope observations of the eclipsing
nova-like cataclysmic variable UX UMa obtained with the Faint Object
Spectrograph. Two eclipses each were observed with the G160L grating (covering
the ultraviolet waveband) in August of 1994 and with the PRISM (covering the
near-ultraviolet to near-infrared) in November of the same year. The system was
50% brighter in November than in August, which, if due to a change in the
accretion rate, indicates a fairly substantial increase in Mdot_acc by >~ 50%.
Model disk spectra constructed as ensembles of stellar atmospheres provide
poor descriptions of the observed post-eclipse spectra, despite the fact that
UX UMa's light should be dominated by the disk at this time. Suitably scaled
single temperature model stellar atmospheres with T_eff = 12,500-14,500 K
actually provide a better match to both the ultraviolet and optical
post-eclipse spectra. Evidently, great care must be taken in attempts to derive
accretion rates from comparisons of disk models to observations.
One way to reconcile disk models with the observed post-eclipse spectra is to
postulate the presence of a significant amount of optically thin material in
the system. Such an optically thin component might be associated with the
transition region (``chromosphere'') between the disk photosphere and the fast
wind from the system, whose presence has been suggested by Knigge & Drew
(1997).Comment: 35 pages, including 12 figures; to appear in the ApJ (Vol. 499
A self consistent chemically stratified atmosphere model for the roAp star 10 Aquilae
Context: Chemically peculiar A type (Ap) stars are a subgroup of the CP2
stars which exhibit anomalous overabundances of numerous elements, e.g. Fe, Cr,
Sr and rare earth elements. The pulsating subgroup of the Ap stars, the roAp
stars, present ideal laboratories to observe and model pulsational signatures
as well as the interplay of the pulsations with strong magnetic fields and
vertical abundance gradients. Aims: Based on high resolution spectroscopic
observations and observed stellar energy distributions we construct a self
consistent model atmosphere, that accounts for modulations of the
temperature-pressure structure caused by vertical abundance gradients, for the
roAp star 10 Aquilae (HD 176232). We demonstrate that such an analysis can be
used to determine precisely the fundamental atmospheric parameters required for
pulsation modelling. Methods: Average abundances were derived for 56 species.
For Mg, Si, Ca, Cr, Fe, Co, Sr, Pr, and Nd vertical stratification profiles
were empirically derived using the ddafit minimization routine together with
the magnetic spectrum synthesis code synthmag. Model atmospheres were computed
with the LLModels code which accounts for the individual abundances and
stratification of chemical elements. Results: For the final model atmosphere
Teff=7550 K and log g=3.8 were adopted. While Mg, Si, Co and Cr exhibit steep
abundance gradients Ca, Fe and Sr showed much wider abundance gradients between
log tau_5000=-1.5 and 0.5. Elements Mg and Co were found to be the least
stratified, while Ca and Sr showed strong depth variations in abundance of up
to ~ 6 dex.Comment: 9 pages, 15 figure
Cold fronts and multi-temperature structures in the core of Abell 2052
The physics of the coolest phases in the hot Intra-Cluster Medium (ICM) of
clusters of galaxies is yet to be fully unveiled. X-ray cavities blown by the
central Active Galactic Nucleus (AGN) contain enough energy to heat the
surrounding gas and stop cooling, but locally blobs or filaments of gas appear
to be able to cool to low temperatures of 10^4 K. In X-rays, however, gas with
temperatures lower than 0.5 keV is not observed. Using a deep XMM-Newton
observation of the cluster of galaxies Abell 2052, we derive 2D maps of the
temperature, entropy, and iron abundance in the core region. About 130 kpc
South-West of the central galaxy, we discover a discontinuity in the surface
brightness of the hot gas which is consistent with a cold front. Interestingly,
the iron abundance jumps from ~0.75 to ~0.5 across the front. In a smaller
region to the North-West of the central galaxy we find a relatively high
contribution of cool 0.5 keV gas, but no X-ray emitting gas is detected below
that temperature. However, the region appears to be associated with much cooler
H-alpha filaments in the optical waveband. The elliptical shape of the cold
front in the SW of the cluster suggests that the front is caused by sloshing of
the hot gas in the clusters gravitational potential. This effect is probably an
important mechanism to transport metals from the core region to the outer parts
of the cluster. The smooth temperature profile across the sharp jump in the
metalicity indicates the presence of heat conduction and the lack of mixing
across the discontinuity. The cool blob of gas NW of the central galaxy was
probably pushed away from the core and squeezed by the adjacent bubble, where
it can cool efficiently and relatively undisturbed by the AGN. Shock induced
mixing between the two phases may cause the 0.5 keV gas to cool non-radiatively
and explain our non-detection of gas below 0.5 keV.Comment: 11 pages, 9 figures, A&A, in pres
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