Type Ia Supernovae in semi-detached binary systems

We have considered scenarios for the evolution of close binaries resulting in the formation of semi-detached systems in which a white dwarf can accumulate Chandrasekhar mass by accretion from a main-sequence or subgiant companion with $M\sim 2 M_\odot$. These white dwarfs, probably, explode as type Ia supernovae or collapse with formation of neutron stars. We have carried out a population synthesis study for these systems and have estimated the occurrence rate of such events in the Galaxy, depending on the parameter of common envelopes, mass-exchange rate in the binary, reaction of the main-sequence component on accretion of helium in the intervening phase of evolution. We have found that the model occurrence rate of SNe Ia in semi-detached systems is $\simeq 0.2 \times 10^{-3}$ yr$^{-1}$, i. e., it does not exceed $\sim 10$ of the observational estimate of the Galactic occurrence rate of SNe Ia.Comment: Astronomy Letters, 2003, vol.29, no.12, in press. 22 pages, including 2 tables, 5 figure

Gravitational-wave confusion background from cosmological compact binaries: Implications for future terrestrial detectors

Increasing the sensitivity of a gravitational-wave (GW) detector improves our ability to measure the characteristics of detected sources. It also increases the number of weak signals that contribute to the data. Because GW detectors have nearly all-sky sensitivity, they can be subject to a confusion limit: Many sources which cannot be distinguished may be measured simultaneously, defining a stochastic noise floor to the sensitivity. For GW detectors operating at present and for their planned upgrades, the projected event rate is sufficiently low that we are far from the confusion-limited regime. However, some detectors currently under discussion may have large enough reach to binary inspiral that they enter the confusion-limited regime. In this paper, we examine the binary inspiral confusion limit for terrestrial detectors. We consider a broad range of inspiral rates in the literature, several planned advanced gravitational-wave detectors, and the highly advanced "Einstein Telescope" design. Though most advanced detectors will not be impacted by this limit, the Einstein Telescope with a very low frequency "seismic wall" may be subject to confusion noise. At a minimum, careful data analysis will be require to separate signals which will appear confused. This result should be borne in mind when designing highly advanced future instruments.Comment: 19 pages, 6 figures and 3 tables; accepted for publication in Phys. Rev.

Cosmological Gamma-Ray Bursts and Evolution of Galaxies

Evolution of the rate density of cosmological gamma-ray bursts (GRBs) is calculated and compared to the BATSE brightness distribution in the context of binary neutron-star mergers as the source of GRBs, taking account of the realistic star formation history in the universe and evolution of compact binary systems. We tried two models of the evolution of cosmic star formation rate (SFR): one is based on recent observations of SFRs at high redshifts, while the other is based on a galaxy evolution model of stellar population synthesis that reproduces the present-day colors of galaxies. It is shown that the binary merger scenario of GRBs naturally results in the comoving rate-density evolution of \propto (1+z)^{2-2.5} up to z ~ 1, that has been suggested independently from the compatibility between the number-brightness distribution and duration-brightness correlation. If the cosmic SFR has its peak at z ~ 1--2 as suggested by recent observations, the effective power-index of GRB photon spectrum, \alpha >~ 1.5$ is favored, that is softer than the recent observational determination of \alpha = 1.1 \pm 0.3. However, high redshift starbursts (z >~ 5) in elliptical galaxies, that have not yet been detected, can alleviate this discrepancy. The redshift of GRB970508 is likely about 2, just below the upper limit that is recently determined, and the absorption system at z = 0.835 seems not to be the site of the GRB.Comment: ApJ Lett. in press, very minor change just making clear that the predicted rate-density evolution is in a comoving sense. (Received 1997 May 15; Accepted 1997 July 2

KUV 01584-0939: A Helium-transferring Cataclysmic Variable with an Orbital Period of 10 Minutes

High speed photometry of KUV 01584-0939 (alias Cet3) shows that is has a period of 620.26 s. Combined with its hydrogen-deficient spectrum, this implies that it is an AM CVn star. The optical modulation is probably a superhump, in which case the orbital period will be slightly shorter than what we have observed.Comment: Published by PASP. See also the latest Early-Release Research Paper website of the PAS

Type Ia Supernova Scenarios and the Hubble Sequence

The dependence of the Type Ia supernova (SN Ia) rate on galaxy type is examined for three currently proposed scenarios: merging of a Chandrasekhar--mass CO white dwarf (WD) with a CO WD companion, explosion of a sub--Chandrasekhar mass CO WD induced by accretion of material from a He star companion, and explosion of a sub--Chandrasekhar CO WD in a symbiotic system. The variation of the SNe Ia rate and explosion characteristics with time is derived, and its correlation with parent population age and galaxy redshift is discussed. Among current scenarios, CO + He star systems should be absent from E galaxies. Explosion of CO WDs in symbiotic systems could account for the SNe Ia rate in these galaxies. The same might be true for the CO + CO WD scenario, depending on the value of the common envelope parameter. A testable prediction of the sub--Chandrasekhar WD model is that the average brightness and kinetic energy of the SN Ia events should increase with redshift for a given Hubble type. Also for this scenario, going along the Hubble sequence from E to Sc galaxies SNe Ia events should be brighter on average and should show larger mean velocities of the ejecta. The observational correlations strongly suggest that the characteristics of the SNe Ia explosion are linked to parent population age. The scenario in which WDs with masses below the Chandrasekhar mass explode appears the most promising one to explain the observed variation of the SN Ia rate with galaxy type together with the luminosity--expansion velocity trend.Comment: 16 pages uuencoded compressed Postscript, 2 figures included. ApJ Letters, in pres