56 research outputs found
Tidal Flows in asynchronous binaries: The beta-factor
We discuss the potential role that tidal flows in asynchronous binary stars
may play in transporting chemically enriched material from deep layers towards
the surface and the corresponding observational consequences of these
processes. We suggest that the time-dependent velocity field induced by
asynchronous rotation may contribute significantly to the mixing, thus
providing a channel for the formation of chemically enriched slowly rotating
massive stars.Comment: in Setting New Standards in Binary Star Research, A. Tkachenko & K.
Pavlovski (eds), EAS Publication Series, in Pres
Long uninterrupted photometric observations of the Wolf-Rayet star EZ CMa by the Toronto {\em{BRITE}} satellite reveal a very fast apsidal motion
Context. The variability of the Wolf-Rayet star EZ CMa has been documented
for close to half a century, and a clear periodicity of 3.7 days is
established. However, all attempts to prove that it is a binary have failed
because the photometric, spectroscopic, and polarimetric variations are not
coherent over more than a few orbital cycles.
Aims. In this letter we show that the lack of coherence in the variability
can be explained with a very rapid apsidal motion in a binary orbit.}
Methods. We measured the times of minima in a recently published
exceptionally long photometric light curve obtained by the Toronto
{\emph{BRITE}} satellite. The apsidal motion and the system eccentricity are
determined from the length of the time intervals between these minima, which
alternate in their duration, following a pattern that is clearly associated
with apsidal motion. These minima are superposed on brightness enhancements of
the emission from a shock zone, which occur at about the times of periastron
phases.
Results. We determine the orbital periodicity, d, and the
period of the apsidal motion, d, which together yield an
average sidereal period of d. The eccentricity is found to be
close to 0.1. The rate of periapsis retreat changes significantly over the
period of observation and is determined to be at
the beginning of the observing period and at the
end.
Conclusions. We demonstrate that by introducing a fast apsidal motion, the
basic photometric variability is very well explained. The binary nature of EZ
CMa is now established. This might imply that other apparently single
Wolf-Rayet stars that emit hard X-rays, similar to EZ CMa, are also binaries.Comment: A&A Letter in press, 5 pages, 3 figure
The nature of the companion in the Wolf-Rayet system EZ Canis Majoris
EZ Canis Majoris is a classical Wolf-Rayet star whose binary nature has been
debated for decades. It was recently modeled as an eccentric binary with a
periodic brightening at periastron of the emission originating in a shock
heated zone near the companion. The focus of this paper is to further test the
binary model and to constrain the nature of the unseen close companion by
searching for emission arising in the shock-heated region. We analyze over 400
high resolution the International Ultraviolet Explorer spectra obtained between
1983 and 1995 and XMM-Newton observations obtained in 2010. The light curve and
radial velocity (RV) variations were fit with the eccentric binary model and
the orbital elements were constrained. We find RV variations in the primary
emission lines with a semi-amplitude K30 km/s in 1992 and 1995, and a
second set of emissions with an anti-phase RV curve with K150 km/s. The
simultaneous model fit to the RVs and the light curve yields the orbital
elements for each epoch. Adopting a Wolf-Rayet mass M20 M leads
to M3-5 M, which implies that the companion could be a late
B-type star. The eccentric (e=0.1) binary model also explains the hard X-ray
light curve obtained by XMM-Newton and the fit to these data indicates that the
duration of maximum is shorter than the typical exposure times. The anti-phase
RV variations of two emission components and the simultaneous fit to the RVs
and the light curve are concrete evidence in favor of the binary nature of EZ
Canis Majoris. The assumption that the emission from the shock-heated region
closely traces the orbit of the companion is less certain, although it is
feasible because the companion is significantly heated by the WR radiation
field and impacted by the WR wind.Comment: 16 pages, 18 figure
Planet heating prevents inward migration of planetary cores
Planetary systems are born in the disks of gas, dust and rocky fragments that
surround newly formed stars. Solid content assembles into ever-larger rocky
fragments that eventually become planetary embryos. These then continue their
growth by accreting leftover material in the disc. Concurrently, tidal effects
in the disc cause a radial drift in the embryo orbits, a process known as
migration. Fast inward migration is predicted by theory for embryos smaller
than three to five Earth masses. With only inward migration, these embryos can
only rarely become giant planets located at Earth's distance from the Sun and
beyond, in contrast with observations. Here we report that asymmetries in the
temperature rise associated with accreting infalling material produce a force
(which gives rise to an effect that we call "heating torque") that counteracts
inward migration. This provides a channel for the formation of giant planets
and also explains the strong planet-metallicity correlation found between the
incidence of giant planets and the heavy-element abundance of the host stars.Comment: 19 pages, 4 figure
FUSE observations of HD 5980: The wind structure of the eruptor
HD 5980 is a unique system containing one massive star (star A) that is
apparently entering the luminous blue variable phase, and an eclipsing
companion (star B) that may have already evolved beyond this phase to become a
Wolf-Rayet star. In this paper we present the results from FUSE observations
obtained in 1999, 2000, and 2002 and one far-UV observation obtained by
ORFEUS/BEFS in 1993 shortly before the first eruption of HD 5980. The eight
phase-resolved spectra obtained by FUSE in 2002 are analyzed in the context of
a wind-eclipse model. This analysis shows that the wind of the eruptor obeyed a
very fast velocity law in 2002, which is consistent with the line-driving
mechanism. Large amplitude line-profile variations on the orbital period are
shown to be due to the eclipse of star B by the wind of star A, although the
eclipse due to gas flowing in the direction of star B is absent. This can only
be explained if the wind of star A is not spherically symmetric, or if the
eclipsed line radiation is "filled-in" by emission originating from somewhere
else in the system, e.g., in the wind-wind collision region. Except for a
slightly lower wind speed, the ORFEUS/BEFS spectrum is very similar to the
spectrum obtained by FUSE at the same orbital phase: there is no indication of
the impending eruption. However, the trend for decreasing wind velocity
suggests the occurrence of the "bi-stability" mechanism, which in turn implies
that the restructuring of the circumbinary environment caused by the transition
from "fast, rarefied wind" to "slow, dense wind" was observed as the eruptive
event. The underlying mechanism responsible for the long-term decrease in wind
velocity that precipitated this change remains an open issue.Comment: 19 pages, 13 figure
Tidal effects on the radial velocity curve of HD77581 (Vela X-1)
The mass of the neutron star in Vela X-1 has been found to be more massive
than the canonical 1.5 Mo. This result relies on the assumption that the
amplitude of the optical component's measured radial velocity curve is not
seriously affected by the interactions in the system. In this paper we explore
the effect on the radial velocity curve caused by surface motions excited by
tidal interactions. We use a calculation from first principles that involves
solving the equations of motion of a Lagrangian grid of surface elements. The
velocities on the visible surface of the star are projected along the
line-of-sight to the observer to obtain the absorption-line profile in the
observer's reference frame. The centroid of the line-profiles for different
orbital phases is then measured and a simulated RV curve constructed. Models
are run for the "standard" (vsini=116 km/s) and "slow" (56 km/s) supergiant
rotation velocities. We find that the surface velocity field is complex and
includes fast, small-spatial scale structures. It leads to strong variability
in the photospheric line profiles which, in turn, causes significant deviations
from a Keplerian RV curve. The peak-to-peak amplitudes of model RV curves are
in all cases larger than the amplitude of the orbital motion. Keplerian fits to
RV curves obtained with the "standard" rotation velocity imply a neutron star
>1.7 Mo. However, a similar analysis of the "slow" rotational velocity models
allows for m_ns ~ 1.5 Mo. Thus, the stellar rotation plays an important role in
determining the characteristics of the perturbed RV curve. Given the
observational uncertainty in GP Vel's projected rotation velocity and the
strong perturbations seen in the published and the model RV curves, we are
unable to rule out a small (~1.5 Mo) mass for the neutron star companion.Comment: 14 pages, 16 figures; A&A, accepte
Internal circulation in tidally locked massive binary stars -- Consequences for double black hole formation
Steady-state circulation currents are predicted in tidally deformed binary
stars, which are believed to be progenitors of double black-hole merger events.
This work aims to quantitatively characterise the steady-state circulation
currents in components of a tidally locked binary system and to explore the
effects of such currents on numerical models. Previous results describing the
circulation in a single rotating star and a binary star are used to deduce a
new prescription for the internal circulation in tidally locked binaries. We
explore the effect of this prescription numerically for binary systems with
primary masses between 25 and 100 solar masses. When comparing circulation
velocities in the radial direction for the single rotating star and binary
star, it is found that the average circulation velocity in the binary star may
be described as an enhancement to the circulation velocity in a single rotating
star. This velocity enhancement is a simple function depending on the masses of
the binary components and amounts to a factor of approximately two when the
components have equal masses. It is found that the ehancement causes the
formation of double helium stars through efficient mixing to occur for systems
with higher initial orbital periods, lower primary masses and lower mass
ratios, compared to the standard circulation scenario. Taking into account
appropriate distributions for primary mass, initial period and mass ratio,
models with enhanced mixing predict 2.4 times more double helium stars being
produced in the parameter space than models without. We conclude that the
effects of companion-induced circulation have strong implications for the
formation of close binary black holes. Not only do the predicted detection
rates increase but double black-hole systems with mass ratios as low as 0.8 may
be formed when companion-induced circulation is taken into account.Comment: 17 pages, 10 figures, accepted for publication in A&
- …