288 research outputs found
The analysis of spectra of novae taken near maximum
A project to analyze ultraviolet spectra of novae obtained at or near maximum optical light is presented. These spectra are characterized by a relatively cool continuum with superimposed permitted emission lines from ions such as Fe II, Mg II, and Si II. Spectra obtained late in the outburst show only emission lines from highly ionized species and in many cases these are forbidden lines. The ultraviolet data will be used with calculations of spherical, expanding, stellar atmospheres for novae to determine elemental abundances by spectral line synthesis. This method is extremely sensitive to the abundances and completely independent of the nebular analyses usually used to obtain novae abundances
The Salpeter plasma correction for solar fusion reactions
We review five different derivations that demonstrate that the Salpeter
formula for the plasma corrections to fusion rates is valid at the center of
the sun with insignificant errors (~ percent). We point out errors in several
recent papers that have obtained a variety of answers, some even with the wrong
sign or the wrong functional dependence.Comment: Related information at http://www.sns.ias.edu/~jn
The Brightest Black Holes
I suggest that there are two classes of ultraluminous X-ray sources (ULXs),
corresponding to super-Eddington mass inflow in two situations: (a)
thermal-timescale mass transfer in high-mass X-ray binaries, and (b)
long-lasting transient outbursts in low-mass X-ray binaries. These two classes
are exemplified by SS433 and microquasars like GRS 1915+105 respectively. The
observed ULX population is a varying mixture of the two, depending on the star
formation history of the host galaxy. ULXs in galaxies with vigorous star
formation (such as the Antennae) are generally SS433--like, while ULXs in
elliptical galaxies must be of the microquasar type. The latter probably have
significantly anisotropic radiation patterns. They should also be variable, but
demonstrating this may require observations over decades. The close analogy
between models of X-ray binaries and active galactic nuclei (AGN) suggests that
there should exist an apparently super-Eddington class of the latter, which may
be the ultrasoft AGN, and a set of X-ray binaries with Doppler--boosted X-ray
emission. These are presumably a subset of the ULXs, but remain as yet
unidentified.Comment: 4 pages, no figures; accepted for MNRAS Letter
The porous atmosphere of eta Carinae
We analyze the wind generated by the great 20 year long super-Eddington
outburst of eta-Carinae. We show that using classical stellar atmospheres and
winds theory, it is impossible to construct a consistent wind model in which a
sufficiently small amount of mass, like the one observed, is shed. One expects
the super-Eddington luminosity to drive a thick wind with a mass loss rate
substantially higher than the observed one. The easiest way to resolve the
inconsistency is if we alleviate the implicit notion that atmospheres are
homogeneous. An inhomogeneous atmosphere, or "porous", allows more radiation to
escape while exerting a smaller average force. Consequently, such an atmosphere
yields a considerably lower mass loss rate for the same total luminosity.
Moreover, all the applications of the Eddington Luminosity as a strict
luminosity limit should be revised, or at least reanalyzed carefully.Comment: 10 pages, aastex, 3 eps figures, To appear in ApJ Let
Non-LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei
We present self-consistent models of the vertical structure and emergent
spectrum of AGN accretion disks. The central object is assumed to be a
supermassive Kerr black hole. We demonstrate that NLTE effects and the effects
of a self-consistent vertical structure of a disk play a very important role in
determining the emergent radiation, and therefore should be taken into account.
In particular, NLTE models exhibit a largely diminished H I Lyman discontinuity
when compared to LTE models, and the He II discontinuity appears strongly in
emission for NLTE models. Consequently, the number of ionizing photons in the
He II Lyman continuum predicted by NLTE disk models is by 1 - 2 orders of
magnitude higher than that following from the black-body approximation. This
prediction has important implications for ionization models of AGN broad line
regions, and for models of the intergalactic radiation field and the ionization
of helium in the intergalactic medium.Comment: 11 pages; 2 postscript figures; LaTeX, AASPP4 macro; to appear in the
Astrophysical Journal (Letters
Observations and simulations of recurrent novae: U Sco and V394 CrA
Observations and analysis of the Aug. 1987 outburst of the recurrent nova V394 CrA are presented. This nova is extremely fast and its outburst characteristics closely resemble those of the recurrent nova U Sco. Hydrodynamic simulations of the outbursts of recurrent novae were performed. Results as applied to the outbursts of V394 CrA and U Sco are summarized
The heat capacity and derived thermophysical properties of the high TC superconductor YBa2Cu3O7âÎŽ from 5.3 to 350 K
The heat capacity of the perovskite highâTC superconductor YBa2Cu3O7âÎŽ was measured from 5.3 to 350 K in an adiabatic calorimetric cryostat. A break in the heatâcapacity curve, associated with the critical temperature for superconductivity was observed between 90.09 and 92.59 K. The transition temperature was identified as 91.44 K, and ÎCp,m was calculated to be 0.559R at that temperature. The lattice heat capacity was evaluated by means of the recently developed Komada/Westrum phonon distribution model and the apparent characteristic temperature ÎKW was calculated to be 107.7 K. The excess electronic heat capacity for the superconducting phase was evaluated and the energy gap was identified as 234. RâK. Excess contribution, resulting from magnetic impurities, was noted below 20 K. Thermodynamic properties at selected temperatures are presented.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71226/2/JCPSA6-92-11-6794-1.pd
Optically thick clumps: not the solution to the Wolf-Rayet wind momentum problem?
The hot star wind momentum problem η = Mν∞/(L/c)» much greater than 1 is revisited, and it is shown that the conventional belief, that it can be solved by a combination of clumping of the wind and multiple scattering of photons, is not self-consistent for optically thick clumps. Clumping does reduce the mass loss rate M, and hence the momentum supply, required to generate a specified radio emission measure epsilon, while multiple scattering increases the delivery of momentum from a specified stellar luminosity L. However, in the case of thick clumps, when combined the two effects act in opposition rather than in unison since clumping reduces multiple scattering. From basic geometric considerations, it is shown that this reduction in momentum delivery by clumping more than offsets the reduction in momentum required, for a specified ε. Thus the ratio of momentum deliverable to momentum required is maximal for a smooth wind and the momentum problem remains for the thick clump case. In the case of thin clumps, all of the benefit of clumping in reducing η lies in reducing M for a given ε so that extremely small filling factors f ≈ 10-4 are needed. It is also shown that clumping affects the inference of M from radio ε not only by changing the emission measure per unit mass but also by changing the radio optical depth unity radius Rrad, and hence the observed wind volume, at radio wavelengths. In fact, for free-free opacity proportional to αn2, contrary to intuition, Rrad increases with increasing clumpiness
Ultraviolet Emission Line Ratios of Cataclysmic Variables
We present a statistical analysis of the ultraviolet emission lines of
cataclysmic variables (CVs) based on ultraviolet spectra of 20
sources extracted from the International Ultraviolet Explorer Uniform Low
Dispersion Archive. These spectra are used to measure the emission line fluxes
of N V, Si IV, C IV, and He II and to construct diagnostic flux ratio diagrams.
We investigate the flux ratio parameter space populated by individual CVs and
by various CV subclasses (e.g., AM Her stars, DQ Her stars, dwarf novae,
nova-like variables). For most systems, these ratios are clustered within a
range of decade for log Si IV/C IV and log He II/C IV
and decades for log N V/C IV . These
ratios are compared to photoionization and collisional ionization models to
constrain the excitation mechanism and the physical conditions of the
line-emitting gas. We find that the collisional models do the poorest job of
reproducing the data. The photoionization models reproduce the Si IV/C IV line
ratios for some shapes of the ionizing spectrum, but the predicted N V/C IV
line ratios are simultaneously too low by typically decades. Worse,
for no parameters are any of the models able to reproduce the observed He II/C
IV line ratios; this ratio is far too small in the collisional and scattering
models and too large by typically decades in the photoionization
models.Comment: LaTeX format, uses aaspp4.sty, 28 pages, 11 Postscript figures,
accepted for publication in The Astrophysical Journal 10/16/9
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