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Binary systems made by a low-mass CO WD and a He-donor represent possible progenitors of explosive events via He-detonation, producing low-luminosity thermonuclear Supernovae with a peculiar nucleosynthetis. Recently, the binary system PTF J223857.11+743015.1 has been suggested as one. We investigate the evolution of the PTF J223857.11+743015.1 system, composed by a 0.75Msun CO WD and a 0.390Msun subdwarf, capped by a thin H-rich layer, considering rotation of the WD component. We compute the evolution of two stars simultaneously, accounting for the possible evolution of the orbital parameters, as determined by mass transfer between components and by mass ejection from the system during RLOF episodes. We consider that the WD gains angular momentum due to accretion and we follow the evolution of the angular velocity profile as due to angular momentum transport via convection and rotation-induced instabilities. As the donor H-rich envelope is transferred, the WD experiences recurrent very strong H-flashes triggering RLOF episodes during which the entire accreted matter is lost from the system. Due to mixing of chemicals by rotation-induced instabilities during the accretion phase, H-flashes occur inside the original WD. Hence, pulse-by pulse, the accretor mass is reduced down to 0.7453Msun. When He-rich matter is transferred, He-detonation does not occur in the rotating WD, which undergoes 6 very strong He-flashes and subsequent RLOF episodes. Also in this case, due to rotation-induced mixing of the accreted layers with the underlying core, the WD is eroded. Finally, when the mass transfer rate from the donor decreases, a massive He-buffer is piled-up onto the accretor which ends its life as a cooling WD. The binary system PTF J2238+743015.1 as all those binaries having similar components masses and orbital parameters are not good candidates as thermonuclear explosions progenitors., Comment: 12 Pages, 9 Figures, 3 Tables. Accepted for the publication on A&A

We study the physical and evolutionary properties of the "WD family" of AM CVn stars by computing realistic models of IDD systems. We evaluate self-consistently both the mass transfer rate from the donor, as determined by GW emission and interaction with the binary companion, and the thermal response of the accretor to mass deposition. We find that, after the onset of mass transfer, all the considered systems undergo a strong non-dynamical He-flash. However, due to the compactness of these systems, the expanding accretors fill their Roche lobe very soon, thus preventing the efficient heating of the external layers of the accreted CO WDs. Moreover, due to the loss of matter from the systems, the orbital separations enlarge and mass transfer comes to a halt. The further evolution depends on the value of $\dot{M}$, after the donors fill again their lobe. On one hand, if the accretion rate, as determined by the actual value of (M$_{don}$,M$_{acc}$), is high enough, the accretors experience several He-flashes of decreasing strength and then quiescent He-burning sets in. Later on, since the mass transfer rate in IDD is a permanently decreasing function of time, accretors experience several recurrent strong flashes. On the other hand, for intermediate and low values of $\dot{M}$, the accretors enter the strong flashes accretion regime. As expected, in all the considered systems the last He-flash is the strongest one, even if a dynamical event never occurs. When the mass accretion rate decreases below (2-3)$\times 10^{-8} M_\odot yr^{-1}$, the compressional heating of the He-shell becomes less efficient than the neutrino cooling, so that all the accretors in the considered systems evolve into massive degenerate objects. Our results suggest that SNe .Ia or type Ia Supernovae due to Edge-Lit Detonation in the WD family of AM CVn stars should be much more rare than previously expected., Comment: 14 pages, 13 figures, 3 tables. Accepted for the publication on MNRAS. Corrected typos in the Abstract

We follow the evolution of helium stars of initial mass $(2.2 - 2.5) M_\odot$, and show that they undergo off-center carbon burning, which leaves behind ${\mathbf \sim 0.01 M_\odot}$ of unburnt carbon in the inner part of the core. When the carbon-oxygen core grows to Chandrasekhar mass, the amount of left-over carbon is sufficient to ignite thermonuclear runaway. At the moment of explosion, the star will possess an envelope of several $0.1 M_{\odot}$, consisting of He, C, and possibly some H, perhaps producing a kind of peculiar SN. Based on the results of Waldman and Barkat (2007) for accreting white dwarfs, we expect to get thermonuclear runaway at a broad range of $\rho_c \approx (1 - 6) \times 10^9 \mathrm{g cm^{-3}}$, depending on the amount of residual carbon. We verified the feasibility of this scenario by showing that in a close binary system with initial masses $(8.5 + 7.7) M_{\odot}$ and initial period of 150 day the primary produces a helium remnant of $2.3 M_{\odot}$ that evolves further like the model we considered., Comment: 4 pages, 2 figures, to appear in Hydrogen-Deficient Stars 2007

Thanks to the rapid increase of observations of Supernovae Ia, we may now claim that the Universe is accelerating. SN Ia are believed to be good standard candles, and after correcting the observations by various methods a precise Hubble diagram results. One of the most important problems to solve in astrophysics today is to find the progenitors of SN Ia. Candidates are to be found in close binary systems where one of the components may accumulate Chandrasekhar mass and then explode. We show that the AM CVn systems may contribute to the SNe population, but will not be the dominant contributor., Comment: 6 pages, 3 figures, EUROWD 2004 - 14th European Workshop on White Dwarfs

We discuss application of population synthesis for binary stars to progenitors of SN Ia. We show that the only candidate systems able to support the rate of SNe Ia $\sim 10^{-3} yr^{-1}$ both in old and young populations are merging white dwarfs. In young populations ($\sim 1$ Gyr) edge-lit detonations in semidetached systems with nondegenerate helium star donors are also able to support a similar rate. The estimated current Galactic rate of SN Ia with single-degenerate progenitors is $\sim 10^{-4} yr^{-1}$., Comment: 10 pages, 4 figires, to be published in White Dwarfs: Galactic and Cosmological Probes, Editors: E.M. Sion, H.L. Shipman and S. Vennes, Kluwer Academic Publishers

New diffraction-limited speckle images of the Red Rectangle in the wavelength range 2.1--3.3 microns with angular resolutions of 44--68 mas and previous speckle images at 0.7--2.2 microns revealed well-resolved bright bipolar outflow lobes and long X-shaped spikes originating deep inside the outflow cavities. This set of high-resolution images stimulated us to reanalyze all infrared observations of the Red Rectangle using our two-dimensional radiative transfer code. The new detailed modeling, together with estimates of the interstellar extinction in the direction of the Red Rectangle enabled us to more accurately determine one of the key parameters, the distance D=710 pc with model uncertainties of 70 pc, which is twice as far as the commonly used estimate of 330 pc. The central binary is surrounded by a compact, massive (M=1.2 Msun), very dense dusty torus with hydrogen densities reaching n_H=2.5x10^12 cm^-3 (dust-to-gas mass ratio rho_d/rho~0.01). The bright component of the spectroscopic binary HD 44179 is a post-AGB star with mass M*=0.57 Msun, luminosity L*=6000 Lsun, and effective temperature T*=7750 K. Based on the orbital elements of the binary, we identify its invisible component with a helium white dwarf with Mwd~0.35 Msun, Lwd~100 Lsun, and Twd~6x10^4 K. The hot white dwarf ionizes the low-density bipolar outflow cavities inside the dense torus, producing a small HII region observed at radio wavelengths. We propose an evolutionary scenario for the formation of the Red Rectangle nebula, in which the binary initially had 2.3 and 1.9 Msun components at a separation of 130 Rsun. The nebula was formed in the ejection of a common envelope after Roche lobe overflow by the present post-AGB star., Comment: 20 pages, 10 figures, accepted by Astronomy and Astrophysics, also available at http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.html

We model the population of double white dwarfs in the Galaxy and find a better agreement with observations compared to earlier studies, due to two modifications. The first is the treatment of the first phase of unstable mass transfer and the second the modelling of the cooling of the white dwarfs. A satisfactory agreement with observations of the local sample of white dwarfs is achieved if we assume that the initial binary fraction is ~ 50% and that the lowest mass white dwarfs (M < 0.3 Msun) cool faster than the most recently published cooling models predict. With this model we find a Galactic birth rate of close double white dwarfs of 0.05 yr^{-1}, a birth rate of AM CVn systems of 0.005 yr^{-1}, a merger rate of pairs with a combined mass exceeding the Chandrasekhar limit (which may be progenitors of SNe Ia) of 0.003 yr^{-1} and a formation rate of planetary nebulae of 1 yr^{-1}. We estimate the total number of double white dwarfs in the Galaxy as 2.5 10^8. In an observable sample with a limiting magnitude V_lim = 15 we predict the presence of ~855 white dwarfs of which ~220 are close pairs. Of these 10 are double CO white dwarfs of which one has a combined mass exceeding the Chandrasekhar limit and will merge within a Hubble time., Comment: Accepted for publication in Astronomy & Astrophysics

The unique core-mass - radius relation for giants with degenerate helium cores enables us to reconstruct the evolution of three observed double helium white dwarfs with known masses of both components. The last mass transfer phase in their evolution must have been a spiral-in. In the formalism proposed by Webbink (1984), we can constrain the efficiency of the deposition of orbital energy into the envelope to be $1 \la \alpha \la 6$, for an envelope structure parameter $\lambda=0.5$. We find that the two standard mass transfer types (stable mass transfer and spiral-in) are both unable to explain the first phase of mass transfer for these three binaries. We use a parametric approach to describe mass transfer in low-mass binaries, where both stars are of comparable mass and find that the orbital characteristics of the observed double helium white dwarfs can be well reproduced if the envelope of the primary is lost with ~1.5 times the specific angular momentum of the initial binary. In this case no substantial spiral-in occurs., Comment: 8 pages, accepted for publication in A&A

We describe radial velocity observations of a large sample of apparently single white dwarfs (WDs), obtained in a long-term effort to discover close, double-degenerate (DD) pairs which might comprise viable Type Ia Supernova (SN Ia) progenitors. We augment the WD sample with a previously observed sample of apparently single subdwarf B (sdB) stars, which are believed to evolve directly to the WD cooling sequence after the cessation of core helium burning. We have identified 18 new radial velocity variables, including five confirmed sdB+WD short-period pairs. Our observations are in general agreement with the predictions of the theory of binary star evolution. We describe a numerical method to evaluate the detection efficiency of the survey and estimate the number of binary systems not detected due to the effects of varying orbital inclination, orbital phase at the epoch of the first observation, and the actual temporal sampling of each object in the sample. Follow-up observations are in progress to solve for the orbital parameters of the candidate velocity variables., Comment: 30 pages (LaTeX) + 6 figures (Postscript), aaspp4 style

The evolution of the comoving cosmic merger-rate density of neutron star binaries n_c(z) is calculated using a distribution of their merging times provided by population-synthesis computations of binary stars. We adopt an exponential law for the star formation rate with various timescales for different morphological types of galaxies. For elliptical galaxies also an initial burst of star formation, lasting one Gyr, is considered. The resulting n_c(z) of most models agree with the form n_c(z) \propto (1+z)^{1.5-2} for z < 2, which has been proposed for the source population of gamma-ray bursts. Assuming a standard candle luminosity, the computed brightness distribution is consistent with the BATSE results if bursts at the peak flux threshold, P = 0.4 photons cm^{-2}/s, are located at a limiting redshift of 1.9 to 3.3. Progenitors of the systems producing gamma-ray bursts at small redshift (bright) are likely to host in spiral galaxies and star forming regions whereas these at high redshift (dim) reside mainly in elliptical galaxies. The location of a burst may be up to a Mpc away from the host galaxy., Comment: 4 pages letter including 4 figures, Latex; A&A, Letters in press

We examine the possibility that supernovae type Ia (SN Ia) are produced by white dwarfs accreting from Roche-lobe filling evolved companions, under the assumption that a strong optically thick stellar wind from accretor is able to stabilize the mass transfer. We show that if a mass transfer phase on a thermal timescale precedes a nuclear burning driven phase, then such systems (of which the supersoft X-ray sources are a subgroup) can account for about 10% of the inferred SN Ia rate. In addition, we examine the cosmic history of the supernova rate, and we show that the ratio of the rate of SN Ia to the rate of supernovae produced by massive stars (supernovae of types II, Ib, Ic) should increase from about z = 1 towards lower redshifts., Comment: 29 pages, Latex, 6 figures, aasms4.sty, psfig.sty, to appear in The Astrophysical Journal

By means of population synthesis we find that the expected Galactic number of black holes with massive helium star companions is \sim 100 and depends on an assumed threshold for M_{pre-BH}. The overwhelming majority of these systems has orbital periods in excess of 10 hr, with a maximum at \sim 100 hr, while under the Illarionov & Sunyaev (1975) disk formation criteria for accretion from the strong stellar wind of Wolf-Rayet star disk accretion is possible only for orbital periods below \sim 10 hr. However, the number of such short-period systems is vanishingly small. If the accretor in Cyg X-3 is a 10 M_sun black hole, then the accretion rate will be super-Eddington. Super-Eddington accretion may be responsible for the formation of jets in Cyg X-3 and may also support an X-ray luminosity as high as \sim 10^{39} erg/s. From the orbital period distribution for neutron stars with massive helium companions we find that if during the common envelope phase a neutron star accretes at Eddington rate and spins-up to the equilibrium period, then in most systems it acts as a ``propeller'' and accretion from the WR star wind is impossible. For the model with two massive helium stars as an immediate progenitor of Cyg X-3, requirement of accomodation of two WR stars in the post-common-envelope orbit combined with severe mass loss by them prevents formation of BH+WR systems with orbital periods less than several days., Comment: 8 pages, Latex, 2 figures, uses psfig.sty, to appear in Astronomy and Astrophysics

The formation and evolution of binaries which contain two neutron stars or a neutron star with a black hole are discussed in detail. The evolution of the distributions in orbital period and eccentricity for neutron star binaries are studied as a function of time. In the model which fits the observations of high mass binary pulsars best the deposition of orbital energy into common envelopes has to be very efficient and a kick velocity distribution has to contain a significant contribution of low velocity kicks. The estimated age of the population has to be between several 100 Myr and 1 Gyr. The birthrate of binary neutron stars is about 3.4 10^{-5} per year (assuming 100% binarity) and their merger rate is about 2 x 10^{-5} per year. The merger rate of neutron star binaries is consistent with the estimated rate of gamma-ray bursts, if the latter are beamed into an opening angle of a few degrees. We argue that PSR B2303+46 is possibly formed in a scenario in which the common envelope is avoided while for the other three known high-mass binary pulsars a common envelope is required to explain their orbital period., Comment: 18 pages including 9 figures, Latex; Accepted for publication in A&A