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 ($m_{\rm F702W}=25.21\pm0.07$; 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