21 research outputs found
Reconstructing the evolution of double helium white dwarfs: envelope loss without spiral-in
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 . 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&
Properties of the close binary and circumbinary torus of the Red Rectangle
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.htm
Supernovae Rates: A Cosmic History
We discuss the cosmic history of supernovae on the basis of various
assumptions and recent data on the star formation history.
We show that supernova rates as a function of redshift can be used to place
significant constraints on progenitor models, on the star formation history,
and on the importance of dust obscuration.
We demonstrate that it is unlikely that the current observational indications
for the existence of a cosmological constant are merely an artifact of the
dominance of different progenitor classes at different redshift intervals.Comment: Accepted for publication in The Astrophysical Journa
Population synthesis for double white dwarfs I.Close detached systems
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 & Astrophysic
He-accreting WD: nucleosynthesis in the extreme binary system (1.02 + 0.30) Mâ
We investigate the evolutionary properties of AM CVn stars with massive white dwarf (WD) donors and accretors. As a representative of them we consider a binary initially composed by a 0.30 Mâ He WD and a 1.02 Mâ CO WD. We evaluate the time-dependent mass transfer rate from the donor and compute the evolution of the accretor, accounting for the effects of mass exchange on the evolution of orbital parameters. We model the thermal response of the accreting CO WD with the FUNS evolutionary code coupled to a full nuclear network, from H to Bi, including more than 700 isotopes linked by about 1000 nuclear processes. We find that accretors in these systems evolve through the stages of steady He-burning and mild and strong He-flashes and become at the end CO WDs capped by a massive (âŒ0.1 Mâ) He-rich buffer. During He-flashes (both mild and strong) the temperature in the He-shell increases above 3 Ă 108 K, so that the ^{22}Ne(α ,n)^{25}Mg reaction becomes efficient and n-rich isotopes can be produced. During the Roche lobe overflow episodes triggered by strong non-dynamical He-flashes matter enriched in α-elements and n-rich isotopes is ejected, polluting the interstellar medium. Our results strongly suggest that massive AM CVn systems with WD donors do not experience a final very strong dynamical He-flash driving an explosive event like SN .Ia. Though the ejected matter is highly enriched in heavy isotopes, the relative contribution of massive AM CVn systems to the Galactic chemical evolution is, most probably, negligible due to their expected paucity
Close Binary White Dwarf Systems: Numerous New Detections and Their Interpretation
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 styl