173 research outputs found
Main-sequence stellar eruption model for V838 Mon
We propose that the energy source of the outburst of V838 Mon and similar
objects is an accretion event, i.e., gravitational energy rather than
thermonuclear runaway. We show that the merger of two main sequence stars, of
masses 1.5 Mo and 0.1-0.5 Mo can account for the luminosity, large radius, and
low effective temperture of V838 Mon and similar objects. Subsequent cooling
and gravitational contraction lead such objects to move along the Hayashi
limit, as observed. By varying the masses and types of the merging stars, and
by considering slowly expanding, rather than hydrostatic, envelopes, this model
can account for a large range in luminosities and radii of such outburst
events.Comment: 9 page
Simultaneous Spectroscopic and Photometric Observations of Binary Asteroids
We present results of visible wavelengths spectroscopic measurements (0.45 to
0.72 microns) of two binary asteroids, obtained with the 1-m telescope at the
Wise Observatory on January 2008. The asteroids (90) Antiope and (1509)
Esclangona were observed to search for spectroscopic variations correlated with
their rotation while presenting different regions of their surface to the
viewer. Simultaneous photometric observations were performed with the Wise
Observatory's 0.46-m telescope, to investigate the rotational phase behavior
and possible eclipse events. (90) Antiope displayed an eclipse event during our
observations. We could not measure any slope change of the spectroscopic albedo
within the error range of 3%, except for a steady decrease in the total light
flux while the eclipse took place. We conclude that the surface compositions of
the two components do not differ dramatically, implying a common origin and
history. (1509) Esclangona did not show an eclipse, but rather a unique
lightcurve with three peaks and a wide and flat minimum, repeating with a
period of 3.2524 hours. Careful measurements of the spectral albedo slopes
reveal a color variation of 7 to 10 percent on the surface of (1509)
Esclangona, which correlates with a specific region in the photometric
lightcurve. This result suggests that the different features on the lightcurve
are at least partially produced by color variations and could perhaps be
explained by the existence of an exposed fresh surface on (1509) Esclangona.Comment: 21 pages, 14 figures, 1 table, accepted for publication in
Meteoritics & Planetary Science (MAPS
A Thousand and One Nova Outbursts
Multicycle nova evolution models have been calculated over the past twenty
years, the number being limited by numerical constraints. Here we present a
long-term evolution code that enables a continuous calculation through an
unlimited number of nova cycles for an unlimited evolution time, even up to (or
exceeding) a Hubble time. Starting with two sets of the three independent nova
parameters -- the white dwarf mass, the temperature of its isothermal core, and
the rate of mass transfer on to it -- we have followed the evolution of two
models, with initial masses of 1 and 0.65 solar masses, accretion rates
(constant throughout each calculation) of 1e-11 and 1e-9 solar-masses/yr, and
relatively high initial temperatures (as they are likely to be at the onset of
the outburst phase), through over 1000 and over 3000 cycles, respectively. The
results show that although on the short-term consecutive outbursts are almost
identical, on the long-term scale the characteristics change. This is mainly
due to the changing core temperature, which decreases very similarly to that of
a cooling white dwarf for a time, but at a slower rate thereafter. As the white
dwarf's mass continually decreases, since both models lose more mass than they
accrete, the central pressure decreases accordingly. The outbursts on the
massive white dwarf change gradually from fast to moderately fast, and the
other characteristics (velocity, abundance ratios, isotopic ratios) change,
too. Very slowly, a steady state is reached, where all characteristics, both in
quiescence and in outburst, remain almost constant. For the less massive white
dwarf accreting at a high rate, outbursts are similar throughout the evolution.Comment: To be published in MNRA
A fully 3-dimensional thermal model of a comet nucleus
A 3-D numerical model of comet nuclei is presented. An implicit numerical
scheme was developed for the thermal evolution of a spherical nucleus composed
of a mixture of ice and dust. The model was tested against analytical
solutions, simplified numerical solutions, and 1-D thermal evolution codes. The
3-D code was applied to comet 67P/Churyumov-Gerasimenko; surface temperature
maps and the internal thermal structure was obtained as function of depth,
longitude and hour angle. The effect of the spin axis tilt on the surface
temperature distribution was studied in detail. It was found that for small
tilt angles, relatively low temperatures may prevail on near-pole areas,
despite lateral heat conduction. A high-resolution run for a comet model of
67P/Churyumov-Gerasimenko with low tilt angle, allowing for crystallization of
amorphous ice, showed that the amorphous/crystalline ice boundary varies
significantly with depth as a function of cometary latitude.Comment: 19 pages, 10 figure
Long-term dynamics of Methone, Anthe and Pallene
We numerically investigate the long-term dynamics of the Saturn's small
satellites Methone (S/2004 S1), Anthe (S/2007 S4) and Pallene (S/2004 S2). In
our numerical integrations, these satellites are disturbed by non-spherical
shape of Saturn and the six nearest regular satellites. The stability of the
small bodies is studied here by analyzing long-term evolution of their orbital
elements.
We show that long-term evolution of Pallene is dictated by a quasi secular
resonance involving the ascending nodes () and longitudes of
pericentric distances () of Mimas (subscript 1) and Pallene (subscript
2), which critical argument is . Long-term
orbital evolution of Methone and Anthe are probably chaotic since: i) their
orbits randomly cross the orbit of Mimas in time scales of thousands years);
ii) numerical simulations involving both small satellites are strongly affected
by small changes in the initial conditions.Comment: 9 pages; 4 figures. Submitted to Proceedings IAU Symposium No. S263,
200
An Extended Grid of Nova Models: II. The Parameter Space of Nova Outbursts
This paper is a sequel to an earlier paper devoted to multiple, multicycle
nova evolution models (Prialnik & Kovetz 1995, first paper of the series),
which showed that the different characteristics of nova outbursts can be
reproduced by varying the values of three basic and independent parameters: the
white dwarf mass-M_{WD}, the temperature of its isothermal core-T_{WD} and the
mass transfer rate-Mdot. Apart from being the largest computational classical
novae parameter-space survey, we show here that the parameter space is
constrained by several analytical considerations and find its limiting
surfaces. Consequently, we extend the grid of multicycle nova evolution models
presented in Paper I almost to its limits, adding multicycle nova outburst
calculations for a considerable number of new parameter combinations. In
particular, the extended parameter pace that produces nova eruptions includes
low mass transfer rates down to 5e-13 Msun/yr, and more models for low T_{WD}.
Resulting characteristics of these runs are added to the former parameter
combination results, to provide a full grid spanning the entire parameter space
for Carbon-Oxygen white dwarfs. The full grid covers the entire range of
observed nova characteristics, even those of peculiar objects, which have not
been numerically reproduced until now. Most remarkably, runs for very low Mdot
lead to very high values for some characteristics, such as outburst amplitude
A>~20, high super-Eddington luminosities at maximum, heavy element abundance of
the ejecta Z_{ej}~0.63 and high ejected masses m_{ej}~7e-4 Msun.Comment: Accepted for publication in ApJ - 04/2005. Complete grid results
(Tables 2+3) data available at: http://geophysics.tau.ac.il/personal/oferya/
under "Publications
A Modeling of the Super-Eddington Luminosity in Nova Outbursts: V1974 Cygni
We have modeled nova light curves exceeding the Eddington luminosity. It has
been suggested that a porous structure develops in nova envelopes during the
super Eddington phase and the effective opacity is much reduced for such a
porous atmosphere. Based on this reduced opacity model, we have calculated
envelope structures and light curves of novae. The optically thick wind model
is used to simulate nova winds. We find that the photospheric luminosity and
the wind mass-loss rate increase inversely proportional to the reducing factor
of opacities, but the wind velocity hardly changes. We also reproduce the
optical light curve of V1974 Cygni (Nova Cygni 1992) in the super-Eddington
phase, which lasts 13 days from the optical peak 1.7 mag above the Eddington
luminosity.Comment: 8 pages, 4 figures, to appear in ApJ
The Early Palomar Program (1950-1955) for the Discovery of Classical Novae in M81: Analysis of the Spatial Distribution, Magnitude Distribution, and Distance Suggestion
Data obtained in the 1950-1955 Palomar campaign for the discovery of
classical novae in M81 are set out in detail. Positions and apparent B
magnitudes are listed for the 23 novae that were found. There is modest
evidence that the spatial distribution of the novae does not track the B
brightness distribution of either the total light or the light beyond an
isophotal radius that is 70\arcsec from the center of M81. The nova
distribution is more extended than the aforementioned light, with a significant
fraction of the sample appearing in the outer disk/spiral arm region. We
suggest that many (perhaps a majority) of the M81 novae that are observed at
any given epoch (compared with say years ago) are daughters of
Population I interacting binaries. The conclusion that the present day novae
are drawn from two population groups, one from low mass white dwarf secondaries
of close binaries identified with the bulge/thick disk population, and the
other from massive white dwarf secondaries identified with the outer thin
disk/spiral arm population, is discussed. We conclude that the M81 data are
consistent with the two population division as argued previously from (1) the
observational studies on other grounds by Della Valle et al. (1992, 1994),
Della Valle & Livio (1998), and Shafter et al. (1996) of nearby galaxies, (2)
the Hatano et al. (1997a,b) Monte Carlo simulations of novae in M31 and in the
Galaxy, and (3) the Yungelson et al. (1997) population synthesis modeling of
nova binaries. Two different methods of using M81 novae as distance indicators
give a nova distance modulus for M81 as , consistent with the
Cepheid modulus that is the same value.Comment: 24 pages, 7 figures, accepted to PAS
Cooling of young stars growing by disk accretion
In the initial formation stages young stars must acquire a significant
fraction of their mass by accretion from a circumstellar disk that forms in the
center of a collapsing protostellar cloud. Throughout this period mass
accretion rates through the disk can reach 10^{-6}-10^{-5} M_Sun/yr leading to
substantial energy release in the vicinity of stellar surface. We study the
impact of irradiation of the stellar surface produced by the hot inner disk on
properties of accreting fully convective low-mass stars, and also look at
objects such as young brown dwarfs and giant planets. At high accretion rates
irradiation raises the surface temperature of the equatorial region above the
photospheric temperature T_0 that a star would have in the absence of
accretion. The high-latitude (polar) parts of the stellar surface, where disk
irradiation is weak, preserve their temperature at the level of T_0. In
strongly irradiated regions an almost isothermal outer radiative zone forms on
top of the fully convective interior, leading to the suppression of the local
internal cooling flux derived from stellar contraction (similar suppression
occurs in irradiated ``hot Jupiters''). Properties of this radiative zone
likely determine the amount of thermal energy that gets advected into the
convective interior of the star. Total intrinsic luminosity integrated over the
whole stellar surface is reduced compared to the non-accreting case, by up to a
factor of several in some systems (young brown dwarfs, stars in quasar disks,
forming giants planets), potentially leading to the retardation of stellar
contraction. Stars and brown dwarfs irradiated by their disks tend to lose
energy predominantly through their cool polar regions while young giant planets
accreting through the disk cool through their whole surface.Comment: 14 pages, 6 figures, submitted to Ap
Stellar growth by disk accretion: the effect of disk irradiation on the protostellar evolution
Young stars are expected to gain most of their mass by accretion from a disk
that forms around them as a result of angular momentum conservation in the
collapsing protostellar cloud. Accretion initially proceeds at high rates of
10^{-6}-10^{-5} M_Sun/yr resulting in strong irradiation of the stellar surface
by the hot inner portion of the disk and leading to the suppression of the
intrinsic stellar luminosity. Here we investigate how this luminosity
suppression affects evolution of the protostellar properties. Using simple
model based on the energy balance of accreting star we demonstrate that disk
irradiation causes only a slight increase of the protostellar radius, at the
level of several per cent. Such a weak effect is explained by a minor role
played by the intrinsic stellar luminosity (at the time when it is
significantly altered by irradiation) in the protostellar energy budget
compared to the stellar deuterium burning luminosity and the inflow of the
gravitational potential energy brought in by the freshly accreted material. Our
results justify the neglect of irradiation effects in previous studies of the
protostellar growth via disk accretion. Evolution of some other actively
accreting objects such as young brown dwarfs and planets should also be only
weakly sensitive to the effects of disk irradiation.Comment: 6 pages, 5 figures, submitted to Ap
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