13 research outputs found
The rates of Type Ia Supernovae. I. Analytical Formulations
This paper provides a handy tool to compute the impact of Type Ia Supernova
(SNIa) on the evolution of stellar systems. An effective formalism is presented
to couple the SNIa rate to the star formation history, which rests upon the
definition of two key properties of the progenitor's model: the realization
probability of the SNIa event from a single stellar generation and the
distribution function of the delay times. It is shown that the current SNIa
rate in late type galaxies implies that the realization probability is on the
order of 0.001. Analytical formulations for the distribution function of the
delay times for Single (SD) and Double Degenerate (DD) progenitors are derived,
based on stellar evolution arguments. These formulations, which agree well with
the results of Monte Carlo simulations for the evolution of close binaries,
have a built in parametrization of the key properties of the alternative
candidates. The various models for the progenitors have different impact on the
large scales. In particular, the paper examines the systematic trend of the
SNIa rate per unit mass with the color of the parent galaxy, and shows that the
recent observations favor the DD model. The SD scenario can reproduce the data
only if the distribution of the primordial mass ratios is flat, and the
accretion efficiency onto the WD is close to 100%. The timescale for the Fe
release from SNIa to the interstellar medium ranges between 0.3 and 3 Gyr for a
wide variety of hypothesis on the SNIa progenitors. (ABRIDGED)Comment: 28 pages, 18 figures, Astronomy and Astrophysics accepted, added 3
reference
A 2.3-Day Periodic Variability in the Apparently Single Wolf-Rayet Star WR 134: Collapsed Companion or Rotational Modulation?
We present the results of an intensive campaign of spectroscopic and
photometric monitoring of the peculiar Wolf-Rayet star WR 134 from 1989 to
1997.
This unprecedentedly large data set allows us to confirm unambiguously the
existence of a coherent 2.25 +/- 0.05 day periodicity in the line-profile
changes of He II 4686, although the global pattern of variability is different
from one epoch to another. This period is only marginally detected in the
photometric data set. Assuming the 2.25 day periodic variability to be induced
by orbital motion of a collapsed companion, we develop a simple model aiming at
investigating (i) the effect of this strongly ionizing, accreting companion on
the Wolf-Rayet wind structure, and (ii) the expected emergent X-ray luminosity.
We argue that the predicted and observed X-ray fluxes can only be matched if
the accretion on the collapsed star is significantly inhibited. Additionally,
we performed simulations of line-profile variations caused by the orbital
revolution of a localized, strongly ionized wind cavity surrounding the X-ray
source. A reasonable fit is achieved between the observed and modeled
phase-dependent line profiles of He II 4686. However, the derived size of the
photoionized zone substantially exceeds our expectations, given the observed
low-level X-ray flux. Alternatively, we explore rotational modulation of a
persistent, largely anisotropic outflow as the origin of the observed cyclical
variability. Although qualitative, this hypothesis leads to greater consistency
with the observations.Comment: 34 pages, 16 figures. Accepted by the Astrophysical Journa
The Evolution of Compact Binary Star Systems
We review the formation and evolution of compact binary stars consisting of
white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and
BHs are thought to be the primary astrophysical sources of gravitational waves
(GWs) within the frequency band of ground-based detectors, while compact
binaries of WDs are important sources of GWs at lower frequencies to be covered
by space interferometers (LISA). Major uncertainties in the current
understanding of properties of NSs and BHs most relevant to the GW studies are
discussed, including the treatment of the natal kicks which compact stellar
remnants acquire during the core collapse of massive stars and the common
envelope phase of binary evolution. We discuss the coalescence rates of binary
NSs and BHs and prospects for their detections, the formation and evolution of
binary WDs and their observational manifestations. Special attention is given
to AM CVn-stars -- compact binaries in which the Roche lobe is filled by
another WD or a low-mass partially degenerate helium-star, as these stars are
thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure