86 research outputs found
Limits to the Mass and the Radius of the Compact Star in SAX J1808.4--3658 and Their Implications
We show that a survey of equations of state and observations of X-ray
pulsations from SAX J1808.4-3658 give 2.27 solar mass as the upper limit of the
compact star mass. The corresponding upper limit of the radius comes out to be
9.73 km. We also do a probabilistic study to estimate the lower limit of the
mass of the compact star. Such a limit puts useful constraints on equations of
state. We also discuss the implications of the upper mass limit for the the
evolutionary history of the source, as well as the detection of it in radio
frequencies. We envisage that the possible observation of radio-eclipse may be
able to rule out several soft equation of state models, by setting a moderately
high value for the lower limit of inclination angle.Comment: 7 pages, 1 table, 2 figures, accepted for publication in ApJ Letter
Is KPD 1930+2752 a good SN Ia progenitor?
We investigate the evolution of a binary system which initially has an orbital period of 2^h 17^m and contains a 0.5 M_sun helium star with a white dwarf companion of 0.97 M_sun, similarly to suggested SN Ia candidate progenitor KPD 1930+2752. We show that the helium star completes core helium burning and becomes a white dwarf before components merge. The most probable outcome of the merger of components is formation of a massive white dwarf, despite initially the total mass of the system is above the Chandrasekhar mass
Equation of state and opacities for hydrogen atmospheres of magnetars
The equation of state and radiative opacities of partially ionized, strongly
magnetized hydrogen plasmas, presented in a previous paper [ApJ 585, 955
(2003), astro-ph/0212062] for the magnetic field strengths 8.e11 G < B < 3.e13
G, are extended to the field strengths 3.e13 G < B < 1.e15 G, relevant for
magnetars. The first- and second-order thermodynamic functions and radiative
opacities are calculated and tabulated for 5.e5 < T < 4.e7 K in a wide range of
densities. We show that bound-free transitions give an important contribution
to the opacities in the considered range of B in the outer neutron-star
atmosphere layers. Unlike the case of weaker fields, bound-bound transitions
are unimportant.Comment: 7 pages, 6 figures, LaTeX using emulateapj.cls (included). Accepted
by Ap
The Convective Urca Process with Implicit Two-Dimensional Hydrodynamics
Consideration of the role of the convective flux in the thermodymics of the
convective Urca neutrino loss process in degenerate, convective, quasi-static,
carbon-burning cores shows that the convective Urca process slows down the
convective current around the Urca-shell, but, unlike the "thermal" Urca
process, does not reduce the entropy or temperature for a given convective
volume. Here we demonstrate these effects with two-dimensional numerical
hydrodynamical calculations. These two-dimensional implicit hydrodynamics
calculations invoke an artificial speeding up of the nuclear and weak rates.
They should thus be regarded as indicative, but still qualitative. We find
that, compared to a case with no Urca-active nuclei, the case with Urca effects
leads to a higher entropy in the convective core because the energy released by
nuclear burning is confined to a smaller volume by the effective boundary at
the Urca shell. All else being equal, this will tend to accelerate the
progression to dynamical runaway. We discuss the open issues regarding the
impact of the convective Urca process on the evolution to the "smoldering
phase" and then to dynamical runaway.Comment: 22 pages, 11 figures, accepted for publication in the Astrophysical
Journa
Transient radio emisison from SAX J1808.4-3658
We report on the detection of radio emission from the accretion-powered X-ray
millisecond pulsar SAX J1808.4-3658, using the Australia Telescope Compact
Array. We detected a ~0.8 mJy source at the position of SAX J1808.4-3658 on
1998 April 27, approximately one day after the onset of a rapid decline in the
X-ray flux; no such source was seen on the previous day. We consider this
emission to be related to the radio emission from other X-ray binaries, and is
most likely associated with an ejection of material from the system. No radio
emission was detected at later epochs, indicating that if SAX J1808.4-3658 is a
radio pulsar during X-ray quiescence then its monochromatic luminosity must be
less than L(1.4 GHz) ~6 mJy/kpc^2.Comment: 6 pages, uses emulateapj.sty, one embedded PS figure. Accepted to ApJ
Letter
Optical Observations of the Binary Pulsar System PSR B1718-19: Implications for Tidal Circularization
We report on Keck and Hubble Space Telescope optical observations of the
eclipsing binary pulsar system PSR B1718-19, in the direction of the globular
cluster NGC 6342. These reveal a faint star (; Vega
system) within the pulsar's 0\farcs5 radius positional error circle. This may
be the companion. If it is a main-sequence star in the cluster, it has radius
\rcomp\simeq0.3 \rsun, temperature \teff\simeq3600 K, and mass
\mcomp\simeq0.3 \msun. In many formation models, however, the pulsar (spun up
by accretion or newly formed) and its companion are initially in an eccentric
orbit. If so, for tidal circularization to have produced the present-day highly
circular orbit, a large stellar radius is required, i.e., the star must be
bloated. Using constraints on the radius and temperature from the Roche and
Hayashi limits, we infer from our observations that \rcomp\simlt0.44 \rsun
and \teff\simgt3300 K. Even for the largest radii, the required efficiency of
tidal dissipation is larger than expected for some prescriptions.Comment: 10 pages, 2 figures, aas4pp2.sty. Accepted for publication in Ap
Mass Limits For Black Hole Formation
We present a series of two-dimensional core-collapse supernova simulations
for a range of progenitor masses and different input physics. These models
predict a range of supernova energies and compact remnant masses. In
particular, we study two mechanisms for black hole formation: prompt collapse
and delayed collapse due to fallback. For massive progenitors above 20 solar
masses, after a hydrodynamic time for the helium core (a few minutes to a few
hours), fallback drives the compact object beyond the maximum neutron star mass
causing it to collapse into a black hole. With the current accuracy of the
models, progenitors more massive than 40 solar masses form black holes directly
with no supernova explosion (if rotating, these black holes may be the
progenitors of gamma-ray bursts). We calculate the mass distribution of black
holes formed, and compare these predictions to the observations, which
represent a small biased subset of the black hole population. Uncertainties in
these estimates are discussed.Comment: 15 pages total, 4 figures, Modifications in Conclusion, accepted by
Ap
Carbon-poor stellar cores as supernova progenitors
Exploring stellar models which ignite carbon off-center (in the mass range of
about 1.05 - 1.25 Msun, depending on the carbon mass fraction) we find that
they may present an interesting SN I progenitor scenario, since whereas in the
standard scenario runaway always takes place at the same density of about 2 X
10^9 gr/cm^3, in our case, due to the small amount of carbon ignited, we get a
whole range of densities from 1 X 10^9 up to 6 X 10^9 gr/cm^3. These results
could contribute in resolving the emerging recognition that at least some
diversity among SNe I exists, since runaway at various central densities is
expected to yield various outcomes in terms of the velocities and composition
of the ejecta, which should be modeled and compared to observations.Comment: 49 pages, 20 figure
The most massive progenitors of neutron stars: CXO J164710.2-455216
The evolution leading to the formation of a neutron star in the very young
Westerlund 1 star cluster is investigated. The turnoff mass has been estimated
to be 35 Msun, indicating a cluster age ~ 3-5 Myr. The brightest X-ray source
in the cluster, CXO J164710.2-455216, is a slowly spinning (10 s) single
neutron star and potentially a magnetar. Since this source was argued to be a
member of the cluster, the neutron star progenitor must have been very massive
(M_zams > 40 Msun) as noted by Muno et al. (2006). Since such massive stars are
generally believed to form black holes (rather than neutron stars), the
existence of this object poses a challenge for understanding massive star
evolution. We point out while single star progenitors below M_zams < 20 Msun
form neutron stars, binary evolution completely changes the progenitor mass
range. In particular, we demonstrate that mass loss in Roche lobe overflow
enables stars as massive as 50-80 Msun, under favorable conditions, to form
neutron stars. If the very high observed binary fraction of massive stars in
Westerlund 1 (> 70 percent) is considered, it is natural that CXO
J164710.2-455216 was formed in a binary which was disrupted in a supernova
explosion such that it is now found as a single neutron star. Hence, the
existence of a neutron star in a given stellar population does not necessarily
place stringent constraints on progenitor mass when binary interactions are
considered. It is concluded that the existence of a neutron star in Westerlund
1 cluster is fully consistent with the generally accepted framework of stellar
evolution.Comment: 5 pages of text and 4 figures (submitted to Astrophysical Journal
The Galactic Population of Low- and Intermediate-Mass X-ray Binaries
(abridged) We present the first study that combines binary population
synthesis in the Galactic disk and detailed evolutionary calculations of low-
and intermediate-mass X-ray binaries (L/IMXBs). We show that the formation
probability of IMXBs with initial donor masses of 1.5--4 Msun is typically >~5
times higher than that of standard LMXBs, and suggest that the majority of the
observed systems may have descended from IMXBs. Distributions at the current
epoch of the orbital periods, donor masses, and mass accretion rates have been
computed, as have orbital-period distributions of BMPs. Several significant
discrepancies between the theoretical and observed distributions are discussed.
The orbital-period distribution of observed BMPs strongly favors cases where
the envelope of the neutron-star progenitor is more easily ejected during the
common-envelope phase. However, this leads to a >~100-fold overproduction of
the theoretical number of luminous X-ray sources relative to the total observed
number of LMXBs. X-ray irradiation of the donor star may result in a dramatic
reduction in the X-ray active lifetime of L/IMXBs, thus possibly resolving the
overproduction problem, as well as the long-standing BMP/LMXB birthrate
problem.Comment: 12 pages, emulateapj, submitted to Ap
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