13 research outputs found
Orbital eccentricities of binary systems with a former AGB star
Many binary stellar systems in which the primary star is beyond the
asymptotic giant branch (AGB) evolutionary phase show significant orbital
eccentricities whereas current binary interaction models predict their orbits
to be circularised. We analyse how the orbital parameters in a system are
modified under mass loss and mass exchange among its binary components and
propose a model for enhanced mass-loss from the AGB star due to tidal
interaction with its companion, which allows a smooth transition between the
wind and Roche-lobe overflow mass-loss regimes. We explicitly follow its effect
along the orbit on the change of eccentricity and orbital semi-major axis, as
well as the effect of accretion by the companion. We calculate timescales for
the variation of these orbital parameters and compare them to the tidal
circularisation timescale. We find that in many cases, due to the enhanced mass
loss of the AGB component at orbital phases closer to the periastron, the net
eccentricity growth rate in one orbit is comparable to the rate of tidal
circularisation. We show that with this eccentricity enhancing mechanism it is
possible to reproduce the orbital period and eccentricity of the Sirius system,
which under the standard assumptions of binary interaction is expected to be
circularised. We also show that this mechanism may provide an explanation for
the eccentricities of most barium star systems, which are expected to be
circularised due to tidal dissipation. By proposing a tidally enhanced model of
mass loss from AGB stars we find a mechanism which efficiently works against
the tidal circularisation of the orbit, which explains the significant
eccentricities observed in binary systems containing a white dwarf and a less
evolved companion, such as Sirius and systems with barium stars.Comment: 9 pages, 5 figures, accepted for publication in Astronomy and
Astrophysics on 24th of October of 200
Type Ia Supernovae: An Examination of Potential Progenitors and the Redshift Distribution
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 Journa
Jets and Tori in Proto-Planetary Nebulae
We investigate the time sequence for the appearance of jets and molecular
tori in the transition of stars from the Asymptotic Giant Branch to the
planetary nebula phase. Jets and tori are prominent features of this evolution,
but their origins are uncertain. Using optical and millimeter line kinematics,
we determine the ejection history in a sample of well-observed cases. We find
that jets and tori develop nearly simultaneously. We also find evidence that
jets typically appear slightly later than tori, with a lag time of a few
hundred years. These characteristics provide strong evidence that jets and tori
are physically related, and they set new constraints on theories of jet
formation. The ejection of a discrete torus followed by jets on a short time
scale favors the class of models in which a companion interacts with the
central star. Models with long time scales, or with jets followed by a torus,
are ruled out.Comment: 24 pages, 2 figures, to be published in Ap
Transients Among Binaries with Evolved Low-Mass Companions
We show that stable disk accretion should be very rare among low-mass X-ray
binaries and cataclysmic variables whose evolution is driven by the nuclear
expansion of the secondary star on the first giant branch. Stable accretion is
confined to neutron-star systems where the secondary is still relatively
massive, and some supersoft white dwarf accretors. All other systems, including
all black-hole systems, appear as soft X-ray transients or dwarf novae. All
long-period neutron-star systems become transient well before most of the
envelope mass is transferred, and remain transient until envelope exhaustion.
This complicates attempts to compare the numbers of millisecond pulsars in the
Galactic disk with their LMXB progenitors, and also means that the pulsar spin
rates are fixed in systems which are transient rather than steady, contrary to
common assumption. The long-period persistent sources Sco X-2, LMC X-2, Cyg X-2
and V395 Car must have minimum companion masses > 0.75 Msun if they contain
neutron stars, and still larger masses if they contain black holes. The
companion in the neutron-star transient GRO J1744-2844 must have a mass <0.87
Msun. The existence of any steady sources at all at long periods supports the
ideas that (a) the accretion disks in many, if not all, LMXBs are strongly
irradiated by the central source, and (b) mass transfer is thermally unstable
in long-period supersoft X-ray sources.Comment: 10 pages, Latex, 1 ps figure, Ap.J., accepted Feb. 15, 199
Spectroscopic binaries among Hipparcos M giants III. The eccentricity-period diagram and mass-transfer signatures
This paper is the third one in a series devoted to studying the properties of
binaries involving M giants. We use a new set of orbits to construct the first
(e-logP) diagram of an extensive sample of M giant binaries, to obtain their
mass-function distribution, and to derive evolutionary constraints for this
class of binaries and related systems. The orbital properties of binaries
involving M giants were analysed and compared with those of related families of
binaries (K giants, post-AGB stars, barium stars, Tc-poor S stars). The orbital
elements of post-AGB stars and M giants are not different, which may very
indicate that, for the considered sample of post-AGB binaries, the post-AGB
star left the AGB at quite an early stage (M4 or so). Neither are the orbital
elements of post-mass-transfer binaries like barium stars very different from
those of M giants, suggesting that the mass transfer did not alter the orbital
elements much, contrary to current belief. Finally, we show that binary systems
with e < 0.4 log P - 1 (with periods expressed in days) are predominantly
post-mass-transfer systems, because (i) the vast majority of barium and S
systems match this condition, and (ii) these systems have companion masses
peaking around 0.6 solar mass, as expected for white dwarfs. The latter
property has been shown to hold as well for open-cluster binaries involving K
giants, for which a lower bound on the companion mass may easily be set.Comment: 14 pages, 12 figures, accepted for publication in A&A, language
editing changes onl
Spitzer Mid-Infrared Observations of Seven Bipolar Planetary Nebulae
We have investigated the mid-infrared (MIR) and visual structures of seven
bipolar planetary nebulae (BPNe), using imaging and spectroscopy acquired using
the Spitzer Space Telescope (SST), and the Observatorio Astronomico Nacional in
Mexico. The results show that the sources are more extended towards longer MIR
wavelengths, as well as having higher levels of surface brightness in the 5.8
and 8.0 microns bands. It is also noted that the 5.8/4.5 and 8.0/4.5 microns
flux ratios increase with increasing distance from the nuclei of the sources.
All of these latter trends may be attributable to emission by polycyclic
aromatic hydrocarbons (PAHs) and/or warm dust continua within circum-nebular
photo-dissociation regions (PDRs). A corresponding decrease in the flux ratios
8.0/5.8 microns may, by contrast, arise due to changes in the properties of the
PAH emitting grains. We note evidence for possible 8.0 microns ring-like
structures in the envelope of NGC 2346, located in a region beyond the minor
axis limits of the ionized envelope. An analysis of the inner two rings shows
that whilst they have higher surface brightnesses at longer MIR wavelengths,
they are relatively stronger (compared to underlying emission) at 3.6 and 4.5
microns. There is also evidence for point reflection symmetry along the major
axis of the outflow.Comment: 27 pages, 22 figures, Accepted for publication in MNRAS. 69 pages in
arXi
Formation of contact in massive close binaries
We present evolutionary calculations for 74 close binaries systems with
initial primary masses in the range 12...25 M_sun, and initial secondary masses
between 6 and 24 M_sun. The initial periods were chosen such that mass overflow
starts during the core hydrogen burning phase of the primary (Case A), or
shortly thereafter (Case B). We assume conservative evolution for contact-free
systems, i.e., no mass or angular momentum loss from those system except due to
stellar winds. We investigate the borderline between contact-free evolution and
contact, as a function of the initial system parameters. We also investigate
the effect of the treatment of convection, and found it relevant for contact
and supernova order in Case A systems, particularly for the highest considered
masses. For Case B systems we find contact for initial periods above
approximate 10 days and below. However, in that case (and for not too large
periods) contact occurs only after the mass ratio has been reversed, due to the
increased fraction of the donor's convective envelope. As most In all Cases we
find contact for mass ratios below approximate 0.65. We derive the observable
properties of our systems after the major mass transfer event, where the mass
gainer is a main sequence or supergiant O or early B type star, and the mass
loser is a helium star. We point out that the assumption of conservative
evolution for contact-free systems could be tested by finding helium star
companions to O stars.Comment: 19 pages, 14 figures, accepted by A&
An incisive look at the symbiotic star SS Leporis -- Milli-arcsecond imaging with PIONIER/VLTI
Context. Determining the mass transfer in a close binary system is of prime
importance for understanding its evolution. SS Leporis, a symbiotic star
showing the Algol paradox and presenting clear evidence of ongoing mass
transfer, in which the donor has been thought to fill its Roche lobe, is a
target particularly suited to this kind of study. Aims. Since previous
spectroscopic and interferometric observations have not been able to fully
constrain the system morphology and characteristics, we go one step further to
determine its orbital parameters, for which we need new interferometric
observations directly probing the inner parts of the system with a much higher
number of spatial frequencies. Methods. We use data obtained at eight different
epochs with the VLTI instruments AMBER and PIONIER in the H- and K-bands. We
performed aperture synthesis imaging to obtain the first model-independent view
of this system. We then modelled it as a binary (whose giant is spatially
resolved) that is surrounded by a circumbinary disc. Results. Combining these
interferometric measurements with previous radial velocities, we fully
constrain the orbit of the system. We then determine the mass of each star and
significantly revise the mass ratio. The M giant also appears to be almost
twice smaller than previously thought. Additionally, the low spectral
resolution of the data allows the flux of both stars and of the dusty disc to
be determined along the H and K bands, and thereby extracting their
temperatures. Conclusions. We find that the M giant actually does not stricto
sensus fill its Roche lobe. The mass transfer is more likely to occur through
the accretion of an important part of the giant wind. We finally rise the
possibility for an enhanced mass loss from the giant, and we show that an
accretion disc should have formed around the A star.Comment: 11 pages, 5 figures, published in A&A Appendix presenting reduced
data and extracted parameters Reduced data can be found on the CD
BINSTAR: a new binary stellar evolution code. Tidal interactions
We provide a detailed description of a new stellar evolution code, BINSTAR, which has been developed to study interacting binaries. Based on the stellar evolution code STAREVOL, it is specifically designed to study low- and intermediate-mass binaries. We describe the state-of-the-art input physics, which includes treatments of tidal interactions, mass transfer and angular momentum exchange within the system. A generalised Henyey method is used to solve simultaneously the stellar structure equations of each component as well as the separation and eccentricity of the orbit. Test simulations for cases A and B mass transfer are presented and compared with available models. The results of the evolution of Algol systems are in remarkable agreement with the calculations of the Vrije Universiteit Brussel (VUB) group, thus validating our code.We also computed a large grid of models for various masses (2 ≤ M/M ≤ 20) and seven metallicities (Z = 0.0001, 0.001, 0.004, 0.008, 0.01, 0.02, 0.03) to provide a useful analytical parameterisation of the tidal torque constant E2, which allows the determination of the circularisation and synchronisation timescales for stars with a radiative envelope and convective core. The evolution of E2 during the main sequence shows noticeable differences compared to available models. In particular, our new calculations indicate that the circularisation timescale is constant during core hydrogen burning. We also show that E2 weakly depends on core overshooting but is substantially increased when the metallicity becomes lower. © ESO 2013.SCOPUS: ar.jinfo:eu-repo/semantics/publishe