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1.
The probability of discovering contact close binary stars in early spectral classes (CE systems) as eclipsing variables is
calculated as a function of the mass of the principal component, the mass ratio, and the angle of inclination of the orbit.
The case of total limb darkening of the star's disk (hypothesis “D”) is examined. A comparison with previous results for uniformly
bright stellar disks (hypothesis “U”) shows that the difference between the two cases is small.
__________
Translated from Astrofizika, Vol. 51, No. 2, pp. 285–294 (May 2008). 相似文献
2.
The probabilities of discovering detached close binary (type DM) stars as eclipsing variables are calculated as a function
of the mass of the main component, mass ratio, major semiaxis, and angle of inclination of the orbit. The case of total limb
darkening (hypothesis “D”) is examined. This is compared with earlier results for uniformly bright stellar disks (hypothesis
“U”). Based on data from Svechnikov and Kuznetsova’s Catalog of Approximate Photometric and Absolute Elements of Eclipsing
Variables, the spatial density of stars of this type in the neighborhood of the sun is estimated to be ≈ 460 · 10
−6
pc−3.
__________
Translated from Astrofizika, Vol. 49, No. 1, pp. 151–169 (February 2006). 相似文献
3.
A magnetic field model is constructed for the extremely slow rotator γEqu based on measurements of its magnetic field over
many years and using the “magnetic charge” method. An analysis of γEqu and of all the data accumulated up to the present on
the magnetic field parameters of chemically peculiar stars leads to some interesting conclusions, of which the main ones are:
the fact that the axis of rotation and the dipole axis are not parallel in γEqu and the other slowly rotating magnetic stars
which we have studied previously is one of the signs that the braking of CP stars does not involve the participation of the
magnetic field as they evolve “to the main sequence.” The axes of the magnetic field dipole in slow rotators are oriented
arbitrarily with respect to their axes of rotation. The substantial photometric activity of these CP stars also argues against
these axes being close. The well-known absence of sufficiently strong magnetic fields in the Ae/Be Herbig stars also presents
difficulties for the hypothesis of “magnetic braking” in the “pre-main sequence” stages of evolution. The inverse relation
between the average surface magnetic field Bs and the rotation period P is yet another fact in conflict with the idea that
the magnetic field is involved in the braking of CP stars. We believe that angular momentum loss involving the magnetic field
can hardly have taken place during evolution immediately prior “to the main sequence,” rather the slow rotation of CP stars
most likely originates from protostellar clouds with low angular momentum. Some of the slowly rotating stars have a central
dipole magnetic field configuration, while others have a displaced dipole configuration, where the displacement can be toward
the positive or the negative magnetic pole.
__________
Translated from Astrofizika, Vol. 49, No. 2, pp. 251–262 (May 2006). 相似文献
4.
The Hertzsprung-Russell diagram of the Large Magellanic Cloud compiled recently by Fitzpatrick & Garmany (1990) shows that
there are a number of supergiant stars immediately redward of the main sequence although theoretical models of massive stars
with normal hydrogen abundance predict that the region 4.5 ≤ logT
eff ≤ 4.3 should be un-populated (“gap”). Supergiants having surface enrichment of helium acquired for example from a previous
phase of accretion from a binary companion, however, evolve in a way so that the evolved models and observed data are consistent
— an observation first made by Tuchman & Wheeler (1990). We compare the available optical data on OB supergiants with computed
evolutionary tracks of massive stars of metallicity relevant to the LMC with and without helium-enriched envelopes and conclude
that a large fraction (≈ 60 per cent) of supergiant stars may occur in binaries. As these less evolved binaries will later evolve into massive X-ray
binaries, the observed number and orbital period distribution of the latter can constrain the evolutionary scenarios of the
supergiant binaries. The distributions of post main sequence binaries and closely related systems like WR + O stars are bimodal-consisting
of close and wide binaries in which the latter type is numerically dominating. When the primary star explodes as a supernova
leaving behind a neutron star, the system receives a kick and in some cases can lead to runaway O-stars. We calculate the
expected space velocity distribution for these systems. After the second supernova explosion, the binaries in most cases,
will be disrupted leading to two runaway neutron stars. In between the two explosions, the first born neutron star’s spin
evolution will be affected by accretion of mass from the companion star. We determine the steady-state spin and radio luminosity
distributions of single pulsars born from the massive stars under some simple assumptions. Due to their great distance, only
the brightest radio pulsars may be detected in a flux-limited survey of the LMC. A small but significant number of observable
single radio pulsars arising out of the disrupted massive binaries may appear in the short spin period range. Most pulsars
will have a low velocity of ejection and therefore may cluster around the OB associations in the LMC. 相似文献
5.
We propose a kinematic approach to searching for the stars that could be formed with the Sun in a common “parent” open cluster.
The approach consists in preselecting suitable candidates by the closeness of their space velocities to the solar velocity
and analyzing the parameters of their encounters with the solar orbit in the past in a time interval comparable to the lifetime
of stars. We consider stars from the Hipparcos catalog with available radial velocities. The Galactic orbits of stars have
been constructed in the Allen-Santillan potential by taking into account the perturbations from the spiral density wave. We
show that two stars, HIP 87382 and HIP 47399, are of considerable interest in our problem. Their orbits oscillate near the
solar orbit with an amplitude of ≈250 pc; there are short-term close encounters to distances <10 pc. Both stars have an evolutionary
status and metallicity similar to the solar ones. 相似文献
6.
Robert W. Slawson Zoran Ninkov Elliott P. Horch 《Astrophysics and Space Science》2007,312(3-4):171-187
We have obtained and analyzed UBVRI
CCD frames of the young, 4–10 Myr, open cluster NGC 3293 and the surrounding field in order to study its stellar content and determine the cluster’s IMF. We found significantly fewer lower mass stars, M≤2.5M
⊙, than expected. This is particularly so if a single age for the cluster of 4.6 Myr is adopted as derived from fitting evolutionary
models to the upper main sequence. Some intermediate-mass stars near the main sequence in the HR diagram imply an age for the cluster of about 10 Myr. When compared with the Scalo (The stellar initial mass function. ASP
conference series, vol. 24, p. 201, 1998) IMF scaled to the cluster IMF in the intermediate mass range, 2.5≤M/M
⊙≤8.0 where there is good agreement, the high mass stars have a distinctly flatter IMF, indicating an over abundance of these stars, and there is a sharp turnover in the distribution at lower masses. The radial
density distribution of cluster stars in the massive and intermediate mass regimes indicate that these stars are more concentrated
to the cluster core whereas the lower-mass stars show little concentration. We suggest that this is evidence supporting the
formation of massive stars through accretion and/or coagulation processes in denser cluster cores at the expense of the lower
mass proto-stars.
R.W. Slawson and E.P. Horch are guest investigators at the University of Toronto Southern Observatory, Las Campanas, Chile. 相似文献
7.
We have performed photometric B, V, and R observations of nine disk galaxies that presumably have abnormally low total mass-to-light (M/L) ratios for given color indices. Our data on surface photometry are used to analyze the possible causes of anomalous M/L estimates. In many cases, these can be the result of errors in photometry or rotational velocity determination but can also
reflect the real peculiarities of the stellar composition of galaxies. Comparison of the photometric and dynamical disk mass
estimates obtained by analyzing the rotational velocities shows that low M/L values for a given color index are probably real for some of the galaxies. This is primarily true of NGC 4826 (Sab), NGC
5347 (Sab), and NGC 6814 (Sb). The small number of such galaxies suggests that the stellar initial mass function is universal.
However, a small fraction of galaxies probably may have a non-typical mass function “depleted” in low-mass stars. Such galaxies
require a more careful study. 相似文献
8.
Klaus G. Strassmeier 《Astrophysics and Space Science》2006,304(1-4):333-336
With the advent of 8–12m-class telescopes and powerful new spectrographs, we can now extend the Doppler-imaging technique to the cool (and faint) end of the main sequence. At a spectral type of approximately M2, stars are thought to become fully convective and cannot possess an overshoot layer between a radiative core and a convective envelope which, as in the case of the Sun and similar stars, likely harbors the dynamo. Therefore, one could expect a fundamentally different magnetic-field topology than on the Sun and thus a qualitatively different surface temperature distribution with new, hitherto unknown, magnetic activity phenomena. Unfortunately, most single M stars do not rotate sufficiently fast for Doppler imaging and one has to “use” binaries or pre-main-sequence stars in which M stars appear spun up or, in binaries, synchronized to the orbital motion. 相似文献
9.
D. Koester 《Astronomy and Astrophysics Review》2002,11(1):33-66
Summary. During the last decade white dwarfs have become important as tools in many areas beyond traditional stellar physics: from
the age determination of the stars in the solar neighborhood to the dating of open clusters and the distance determination
of globular clusters. They are primary candidates for the MACHO microlensing events, possibly for a stellar component of the
dark halo, and for the supernova Ia progenitors. The recent developments in these areas are reviewed, but some highlights
from more “mature” areas such as stellar parameters, mass distributions, magnetic, and pulsating white dwarfs are also summarized
briefly.
Received 5 October 2001 / Published online 11 January 2002 相似文献
10.
H.J. Habing 《Astronomy and Astrophysics Review》1996,7(2):97-207
Summary.
Red giants are sometimes surrounded by envelopes, the
result of the ejection of stellar matter at a large rate
(/yr)
and at a low velocity (10 km/s). In this review
the envelopes are discussed and the relation between stars and
envelope: what stars combine with what envelopes?
The envelope emits radiation by various processes and has been
detected at all wavelengths between the visual and the microwave
range. I review the observations of continuum radiation emitted by
dust particles and of rotational transitions of molecules, where
these molecules have been excited by thermal or by non–thermal
(“maser”) processes. I discuss mainly the oxygen–rich
stars, those of spectral type M, and only briefly the closely related
carbon–rich stars.
By and large the density in the envelope is well described by
spherically symmetric outflow at a constant velocity; on the time
scale needed to flow from stellar surface to the outermost layers,
i.e.
yr, the loss of mass is sometimes interrupted suddenly
after which the envelope becomes “detached” from the star. The
temperature decreases when moving outward; heat input is by
friction between dust particles and gas and cooling occurs by line
radiation by various molecules, especially by
HO. The molecular
composition is determined by formation in an equilibrium process
deep in the atmosphere and by destruction in the outer parts of the
outflow by interstellar UV radiation
(H, CO, HO) or by
depletion due to condensation on dust grains (SiO); dust particles
of silicate material solidify where the radiation temperature is
decreased to about 1000 K, and this is at a few stellar radii.
The various continuum spectra produced by the dust particles in
different stars are well modelled by a simple model of the density
and dust temperature distribution plus the assumption that the
particles consist of “dirty silicate”, i.e. silicate with Fe and Al
ions added. A large range of optical depths,
, is observed:
from 0.01 to 10. In envelopes with large optical depth the star
itself can no longer be detected directly. Model calculations also
show that the momentum in the outflow, i.e.
is
provided by radiation pressure on the dust particles followed by
the complete transfer of this momentum to the gas. The mass–loss
rate itself,
, is not determined by radiation pressure but by
dynamic processes in the region below the dust condensation layer.
When
is sufficiently large its measurement, that of the
stellar luminosity,
and that of the outflow velocity,
,
permit the determination of
, i.e. the total
outflow rate, without making assumptions about the abundance of the dust
particles or of the molecular gases. Detached envelopes have been
seen in a few cases.
Thermal molecular radiation is faint compared to the maser emission
but has been measured in distant stars, e.g. in stars near the
galactic center. Different molecules outline different “spheres”
around the star. The largest sphere (a radius of 0.1 pc) is
outlined by an emission line belonging to the
CO()
transition. Higher rotational transitions of CO give smaller
diameters. A comparison of CO
() and () fluxes
in stars with very thick envelopes leads to the conclusion that an
abrupt decrease in the mass–loss rate occurred some ten thousand
years ago.
Three molecules produce each several maser lines: SiO,
HO and
OH. Several new
HO lines have recently been discovered; their
exploration has hardly been started. The high intensity of the
maser lines makes interferometry possible and hence detailed
mapping. The SiO lines are formed deep in the envelope, below the
dust condensation layer. OH maser lines are produced farthest out,
HO lines in
between. The excitation mechanisms for most maser
lines is understood globally, but detailed models are lacking,
largely because the problem is non–linear and the solution of the
radiative transfer equation requires a highly anisotropic geometry.
The geometrical and kinematical properties of the 1612 MHz OH
maser, which in many objects is very strong, are explained by a
thin shell of OH; because the angular diameter of the shell can be
measured directly and the linear diameter can be determined from
the difference in the time of maximum flux of blue and red maser
peaks, the distance of the shell and of the star can be measured.
The presence or absence of individual maser lines appears to depend
on the value of
and is well described by a sequence called
“Lewis' chronology”.
The central star is a long–period variable with a period of 300
days or longer and with a large luminosity amplitude
().
Evidence is given that each star has the
maximum luminosity it will reach during its evolution and that it
is a thermally–pulsing Asymptotic–Giant–Branch star (TP–AGB)
with a main–sequence mass between 1 and 6
. Stars of the same
main–sequence mass,
,
have different mass–loss rates, in
some cases by a factor of 10. The mass–loss rate probably
increases with time, and the highest mass–loss rates are reached
toward the end of the evolution. Stars with higher
ultimately
reach higher mass–loss rates. The calibration of the
main–sequence mass is reviewed. Most Mira variables with mass loss
have a mass between 1.0 and 1.2
. OH/IR stars with periods
over 1000 days have no counterparts among the carbon stars and thus
have .
Stars as discussed in this review have been
found only in the thin galactic disk and in the bulge.
Finally I review several recently proposed scenarios for TP–AGB
evolution in which mass loss is taken into account. These scenarios
represent the observations quite well; their major short–coming is
the lack of an explanation why the central stars are always
large–amplitude, long–period variables and why such stars are the
ones with high mass–loss rates.
Received: 10 January 1996 相似文献
11.
Pranab Ghosh 《Journal of Astrophysics and Astronomy》1995,16(2):289-305
I review our understanding of the evolution of the spin periods of neutron stars in binary stellar systems, from their birth
as fast, spin-powered pulsars, through their middle life as accretion-powered pulsars, upto their recycling or “rebirth” as
spin-powered pulsars with relatively low magnetic fields and fast rotation. I discuss how the new-born neutron star is spun
down by electromagnetic and “propeller” torques, until accretion of matter from the companion star begins, and the neutron
star becomes an accretion-powered X-ray pulsar. Detailed observations of massive radio pulsar binaries like PSR 1259-63 will
yield valuable information about this phase of initial spindown. I indicate how the spin of the neutron star then evolves
under accretion torques during the subsequent phase as an accretion-powered pulsar. Finally, I describe how the neutron star
is spun up to short periods again during the subsequent phase of recycling, with the accompanying reduction in the stellar
magnetic field, the origins of which are still not completely understood. 相似文献
12.
We derive a perturbation inside a rotating star that occurs when the star is accelerated by orbiting bodies. If a fluid element
has rotational and orbital components of angular momentum with respect to the inertially fixed point of a planetary system
that are of opposite sign, then the element may have potential energy that could be released by a suitable flow. We demonstrate
the energy with a very simple model in which two fluid elements of equal mass exchange positions, calling to mind a turbulent
field or natural convection. The exchange releases potential energy that, with a minor exception, is available only in the
hemisphere facing the barycenter of the planetary system. We calculate its strength and spatial distribution for the strongest
case (“vertical”) and for weaker horizontal cases whose motions are all perpendicular to gravity. The vertical cases can raise
the kinetic energy of a few well positioned convecting elements in the Sun’s envelope by a factor ≤7. This is the first physical
mechanism by which planets can have a nontrivial effect on internal solar motions. Occasional small mass exchanges near the
solar center and in a recently proposed mixed shell centered at 0.16R
s would carry fresh fuel to deeper levels. This would cause stars like the Sun with appropriate planetary systems to burn somewhat
more brightly and have shorter lifetimes than identical stars without planets. The helioseismic sound speed and the long record
of sunspot activity offer several bits of evidence that the effect may have been active in the Sun’s core, its envelope, and
in some vertically stable layers. Additional proof will require direct evidence from helioseismology or from transient waves
on the solar surface. 相似文献
13.
Douglas C. Leonard 《Astrophysics and Space Science》2011,336(1):117-122
Theory holds that a star born with an initial mass between about 8 and 140 times the mass of the Sun will end its life through
the catastrophic gravitational collapse of its iron core to a neutron star or black hole. This core collapse process is thought
to usually be accompanied by the ejection of the star’s envelope as a supernova. This established theory is now being tested
observationally, with over three dozen core-collapse supernovae having had the properties of their progenitor stars directly
measured through the examination of high-resolution images taken prior to the explosion. Here I review what has been learned
from these studies and briefly examine the potential impact on stellar evolution theory, the existence of “failed supernovae”,
and our understanding of the core-collapse explosion mechanism. 相似文献
14.
V. N. Frolov Yu. K. Ananjevskaja D. L. Gorshanov E. V. Polyakov 《Astronomy Letters》2010,36(5):338-348
We present the results of our comprehensive study of the Galactic open star cluster NGC 6866. The positions of stars in the
investigated region have been obtained with the “Fantasy” automatic measuring machine from 10 plates of the normal astrograph
at the Pulkovo Astronomical Observatory. The size of the investigated field is 40′ × 40′, the limiting magnitude is B ∼ 16·
m
6, and the maximum epoch difference is 79 yr. For 1202 field stars, we have determined the relative proper motions with an
rms error of 2.5 mas yr−1. Out of them, 423 stars may be considered cluster members with a probability P > 70% according to the astrometric criterion. Photometric diagrams have been used as an additional criterion. We have performed
two-color BV CCD photometry of stars with the Pulkovo ZA-320M mirror astrograph. The U magnitudes from the literature have also been used to construct the two-color diagrams. A total of 267 stars have turned
out to be members of NGC 6866 according to the two criteria. We present refined physical parameters of the cluster and its
age estimate (5.6 × 108 yr). The cluster membership of red and blue giants, variable, double, and multiple stars is considered. We have found an
almost complete coincidence of the positions of one of the stars in the region (a cluster nonmember) and a soft X-ray source
in the ROSAT catalog. The “Fantasy” automatic measuring machine is described in the Appendix. 相似文献
15.
We report observations of 24 stars in the fields of the three Seyfert galaxies MCG +08-23-067, Mrk 817, and Mrk 290. The observations
were made with a CCD array photometer in the BVRcIc bands. The V magnitudes of the observed stars ranged from 13.5 and 17.2.
For stars of approximate magnitude 15, the typical photometric errors are 0.010, 0.011, 0.008 and 0.011 magnitude in the BVRI
bands, respectively. The BVRI magnitudes of all these stars were not known previously. In the field of the galaxy Mrk 290
a star has been found that probably has a periodic brightness variation with P=1.518 days and the mean value V=14.80 and which
may belong to the class of “spotted” stars. 14′x14′ charts are supplied for identifying the stars. These results can be used
for differential photometry of active galactic nuclei in the BVRI bands.
__________
Translated from Astrofizika, Vol. 51, No. 1, pp. 41–50 (February 2008). 相似文献
16.
L. P. Ossipkov 《Astrophysics》2008,51(4):428-442
The work of V. A. Ambartsumyan on stellar kinematics and dynamics is analyzed briefly. Its significance in the discussions
of cosmogony during the 1930's is emphasized, especially as related to the victory of the “short” scale for galactic evolution.
The theory of the destruction of star systems through evaporation of stars, which was founded by Ambartsumyan, is discussed.
Ambartsumyan's theory of the equilibrium and evolution of binary stars in the galactic field and its subsequent development
are outlined briefly. 相似文献
17.
Claude Catala 《Experimental Astronomy》2009,23(1):329-356
The PLAnetary Transits and Oscillations of stars Mission (PLATO), presented to ESA in the framework of its “Cosmic Vision” programme, will detect and characterize exoplanets
by means of their transit signature in front of a very large sample of bright stars, and measure the seismic oscillations
of the parent stars orbited by these planets in order to understand the properties of the exoplanetary systems. PLATO is the
next-generation planet finder, building on the accomplishments of CoRoT and Kepler: i) it will observe significantly more stars, ii) its targets will be 2 to 3 magnitudes brighter (hence the precision of the measurements will be correspondingly greater
as will be those of post-detection investigations, e.g. spectroscopy, asteroseismology, and eventually imaging), iii) it will be capable of observing significantly smaller exoplanets. The space-based observations will be complemented by ground-
and space-based follow-up observations. These goals will be achieved by a long-term (4 years), high-precision, high-time-resolution,
high-duty-cycle monitoring in visible photometry of a sample of more than 100,000 relatively bright (m
V
≤ 12) stars and another 400,000 down to m
V
= 14. Two different mission concepts are proposed for PLATO: i) a “staring” concept with 100 small, very wide-field telescopes, assembled on a single platform and all looking at the same 26° diameter field,
and ii) a “spinning” concept with three moderate-size telescopes covering more than 1400 degree2.
See for The PLATO Consortium. 相似文献
18.
G. F. Burgio M. Baldo O. E. Nicotra H.-J. Schulze 《Astrophysics and Space Science》2007,308(1-4):387-394
We study the structure of protoneutron stars within the finite-temperature Brueckner–Bethe–Goldstone many-body theory. If
nucleons, hyperons, and leptons are present in the stellar core, we find that neutrino trapping stiffens considerably the
equation of state, because hyperon onsets are shifted to larger baryon density. However, the value of the critical mass turns
out to be smaller than the “canonical” value 1.44M
⊙. We find that the inclusion of a hadron-quark phase transition increases the critical mass and stabilizes it at about 1.5–1.6M
⊙.
相似文献
19.
Alan H. Batten 《Astrophysics and Space Science》2005,296(1-4):3-15
H.N. Russell and Z. Kopal both liked the metaphor of the “Royal Road” to scientific discovery; I discuss which one used it
first. I present some personal reminiscences of Professor Kopal and then consider his attitude to the determination of the
elements of eclipsing binaries from their observed light changes, comparing it with that of Russell. This leads me to discuss
Kopal’s work on the evolution of binary stars and his opposition to the prevailing belief in the importance of mass transfer
between the components. Kopal’s attitudes on these matters puzzle me, but I suggest that at least part of his motivation was
to act as a critic of “normal science” within the paradigm that most of us have accepted. 相似文献
20.
The oldest open clusters in our Galaxy set the lower limit to the age of the Galactic Disk (9–10 Gyr). Although they appear
to be very rich now, it is clear that their primordial populations were much larger. Often considered as transitional objects,
these populous open clusters show structural differences with respect to globular clusters so their dynamics and characteristic
evolutionary time scales can also be different. On the other hand, their large membership lead to different dynamical evolution
as compared with average open clusters. In this paper, the differential features of the evolution of rich open clusters are
studied using N-body simulations, including several of the largest (104 stars) published direct collisional N-body calculations so far, which were performed on a CRAY YMP. The disruption rate of rich open clusters is analysed in detail
and the effect of the initial spatial distribution of the stars in the cluster on its dynamics is studied. The results show
that cluster life-time depends on this initial distribution, decreasing when it is more concentrated. The effect of stellar
evolution on the dynamical evolution of rich clusters is an important subject that also has been considered here. We demonstrate
that the cluster's life-expectancy against evaporation increases because of mass loss by evolving high-mass stars.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献