3,916 research outputs found
Mergers, cooling flows, and evaporation
Mergers (the capture of cold gas, especially) can have a profound influence on the hot coronal gas of early-type galaxies and clusters, potentially inducing symptoms hitherto attributed to a cooling flow, if thermal conduction is operative in the coronal plasma. Heat can be conducted from the hot phase into the cold phase, simultaneously ionizing the cold gas to make optical filaments, while locally cooling the coronal gas to mimic a cooling-flow. If there is heat conduction, though, there is no standard cooling-flow since radiative losses are balanced by conduction and not mass deposition. Amongst the strongest observational support for the existence of cooling-flows is the presence of intermediate temperature gas with x-ray emission-line strengths in agreement with cooling-flow models. Here, x-ray line strengths are calculated for this alternative model, in which mergers are responsible for the observed optical and x-ray properties. Since gas around 10(exp 4) K is thermally stable, the cold cloud need not necessarily evaporate and hydrostatic solutions exist. Good agreement with the x-ray data is obtained. The relative strengths of intermediate temperature x-ray emission lines are in significantly better agreement with a simple conduction model than with published cooling-flow models. The good agreement of the conduction model with optical, infrared and x-ray data indicates that significantly more theoretical effort into this type of solution would be profitable
Comments on "The long-period Galactic Cepheid RS Puppis. I. A geometric distance from its light echoes"
The luminous Galactic Cepheid RS Puppis is unique in being surrounded by a
dust nebula illuminated by the variable light of the Cepheid. In a recent paper
in this journal, Kervella et al. (2008) report a very precise geometric
distance to RS Pup, based on measured phase lags of the light variations of
individual knots in the reflection nebula. In this commentary, we examine the
validity of the distance measurement, as well as the reality of the spatial
structure of the nebula determined by Feast (2008) based upon the phase lags of
the knots. {Kervella et al. assumed that the illuminated dust knots lie, on
average, in the plane of the sky (otherwise it is not possible to derive a
geometric distance from direct imaging of light echoes). We consider the
biasing introduced by the high efficiency of forward scattering. We conclude
that most of the knots are in fact likely to lie in front of the plane of the
sky, thus invalidating the Kervella et al. result. We also show that the flat
equatorial disk structure determined by Feast is unlikely; instead, the
morphology of the nebula is more probably bipolar, with a significant tilt of
its axis with respect to the plane of the sky. Although the Kervella et al.
distance result is invalidated, we show that high-resolution polarimetric
imaging has the potential to yield a valid geometric distance to this important
Cepheid.Comment: 10 pages, 5 figures, 1 table; accepted by Astronomy & Astrophysic
Line strength variations in beta Cephei
The line strength variations of the resonance line of C IV (1550A, 2s 2S - 2P) observed by OAO-2 were confirmed by IUE observations. In addition, the NV resonance line (1204A, 2s 2S - 2P), the Si III line (1206A, 3p 1P-1D, multiplet 11) and the Si IV resonance line (1395A, 3s 2S - 2P) all vary in line strength essentially in phase with the C IV variation. The (preliminary) period of the variation is 6.02/12.04 days
Hydrodynamic models for novae with ejecta rich in oxygen, neon and magnesium
The characteristics of a new class of novae are identified and explained. This class consists of those objects that have been observed to eject material rich in oxygen, neon, magnesium, and aluminum at high velocities. We propose that for this class of novae the outburst is occurring not on a carbon-oxygen white dwarf but on an oxygen-neon-magnesium white dwarf which has evolved from a star which had a main sequence mass of approx. 8 solar masses to approx. 12 solar masses. An outburst was simulated by evolving 1.25 solar mass white dwarfs accreting hydrogen rich material at various rates. The effective enrichment of the envelope by ONeMg material from the core is simulated by enhancing oxygen in the accreted layers. The resulting evolutionary sequences can eject the entire accreted envelope plus core material at high velocities. They can also become super-Eddington at maximum bolometric luminosity. The expected frequency of such events (approx. 1/4) is in good agreement with the observed numbers of these novae
Numerical modelling of the classical nova outburst
A mechanism is described that promises to explain how nova outbursts take place on white dwarf of 1 solar mass or less and for accretion rates of 4 x 10 to the -10 solar mass/yr or greater
Studies of hydrodynamic events in stellar evolution. 3: Ejection of planetary nebulae
The dynamic behavior of the H-rich envelope (0.101 solar mass) of an evolved star (1.1 solar mass) as the luminosity rises to 19000 solar luminosity during the second ascent of the red giant branch. For luminosities in the range 3100 L 19000 solar luminosity the H-rich envelope pulsates like a long-period variable (LPV) with periods of the order of a year. As L reaches 19000 solar luminosity, the entire H-rich envelope is ejected as a shell with speeds of a few 10 km/s. The ejection occurs on a timescale of a few LPV pulsation periods. This ejection is associated with the formation of a planetary nebula. The computations are based on an implicit hydrodynamic computer code. T- and RHO-dependent opacities and excitation and ionization energies are included. As the H-rich envelope is accelerated off the stellar core, the gap between envelope and core is approximated by a vacuum, filled with radiation. Across the vacuum, the luminosity is conserved and the anisotropy of the radiation is considered as well as the solid angle subtended by the remnant star at the inner surface of the H-rich envelope. Spherical symmetry and the diffusion approximation are assumed
A Search for Candidate Light Echoes: Photometry of Supernova Environments
Supernova (SN) light echoes could be a powerful tool for determining
distances to galaxies geometrically, Sparks 1994. In this paper we present CCD
photometry of the environments of 64 historical supernovae, the first results
of a program designed to search for light echoes from these SNe. We commonly
find patches of optical emission at, or close to, the sites of the supernovae.
The color distribution of these patches is broad, and generally consistent with
stellar population colors, possibly with some reddening. However there are in
addition patches with both unusually red and unusually blue colors. We expect
light echoes to be blue, and while none of the objects are quite as blue in V-R
as the known light echo of SN1991T, there are features that are unusually blue
and we identify these as candidate light echoes for follow-on observations.Comment: 13 pages, Latex, 5 Postscript Tables, 42 Postscript figures, accepted
for publication in the A&AS. Figures 1 through 36 are available at the web
address: http://www.stsci.edu/~boffi
The Effects of Changes in Reaction Rates on Simulations of Nova Explosions
Classical novae participate in the cycle of Galactic chemical evolution in
which grains and metal enriched gas in their ejecta, supplementing those of
supernovae, AGB stars, and Wolf-Rayet stars, are a source of heavy elements for
the ISM. Once in the diffuse gas, this material is mixed with the existing
gases and then incorporated into young stars and planetary systems during star
formation. Infrared observations have confirmed the presence of carbon, SiC,
hydrocarbons, and oxygen-rich silicate grains in nova ejecta, suggesting that
some fraction of the pre-solar grains identified in meteoritic material come
from novae. The mean mass returned by a nova outburst to the ISM probably
exceeds ~2 x 10^{-4} Solar Masses. Using the observed nova rate of 35 per year
in our Galaxy, it follows that novae introduce more than ~7 x 10^{-3} Solar
Masses per year of processed matter into the ISM. Novae are expected to be the
major source of 15N and 17O in the Galaxy and to contribute to the abundances
of other isotopes in this atomic mass range. Here, we report on how changes in
the nuclear reaction rates affect the properties of the outburst and alter the
predictions of the contributions of novae to Galactic chemical evolution. We
also discuss the necessity of including the pep reaction in studies of
thermonuclear runaways in material accreted onto white dwarfs.Comment: 9 pages, 2 figures, as it appeared in the Proceedings of the Tours
2006 Symposium on Nuclear Physic
- …