149 research outputs found
Formation of wind-captured discs in Supergiant X-ray binaries : consequences for Vela X-1 and Cygnus X-1
In Supergiant X-ray binaries (SgXB), a compact object captures a fraction of
the wind of an O/B supergiant on a close orbit. Proxies exist to evaluate the
efficiency of mass and angular momentum accretion but they depend so
dramatically on the wind speed that given the current uncertainties, they only
set loose constrains. Furthermore, they often bypass the impact of orbital and
shock effects on the flow structure. We study the wind dynamics and the angular
momentum gained as the flow is accreted. We identify the conditions for the
formation of a disc-like structure around the accretor and the observational
consequences for SgXB. We use recent results on the wind launching mechanism to
compute 3D streamlines, accounting for the gravitational and X-ray ionizing
influence of the compact companion on the wind. Once the flow enters the Roche
lobe of the accretor, we solve the hydrodynamics equations with cooling. A
shocked region forms around the accretor as the flow is beamed. For wind speeds
of the order of the orbital speed, the shock is highly asymmetric compared to
the axisymmetric bow shock obtained for a purely planar homogeneous flow. With
net radiative cooling, the flow always circularizes for wind speeds low enough.
Although the donor star does not fill its Roche lobe, the wind can be
significantly beamed and bent by the orbital effects. The net angular momentum
of the accreted flow is then sufficient to form a persistent disc-like
structure. This mechanism could explain the proposed limited outer extension of
the accretion disc in Cygnus X-1 and suggests the presence of a disc at the
outer rim of the neutron star magnetosphere in Vela X-1, with dramatic
consequences on the spinning up of the accretor
Triple-Star Candidates Among the Kepler Binaries
We present the results of a search through the photometric database of
eclipsing Kepler binaries (Prsa et al. 2011; Slawson et al. 2011) looking for
evidence of hierarchical triple star systems. The presence of a third star
orbiting the binary can be inferred from eclipse timing variations. We apply a
simple algorithm in an automated determination of the eclipse times for all
2157 binaries. The "calculated" eclipse times, based on a constant period
model, are subtracted from those observed. The resulting O-C (observed minus
calculated times) curves are then visually inspected for periodicities in order
to find triple-star candidates. After eliminating false positives due to the
beat frequency between the ~1/2-hour Kepler cadence and the binary period, 39
candidate triple systems were identified. The periodic O-C curves for these
candidates were then fit for contributions from both the classical Roemer delay
and so-called "physical" delay, in an attempt to extract a number of the system
parameters of the triple. We discuss the limitations of the information that
can be inferred from these O-C curves without further supplemental input, e.g.,
ground-based spectroscopy. Based on the limited range of orbital periods for
the triple star systems to which this search is sensitive, we can extrapolate
to estimate that at least 20% of all close binaries have tertiary companions.Comment: 19 pages, 13 figures, 3 tables; ApJ, 2013, 768, 33; corrected Fig. 7,
updated references, minor fixes to tex
Vela X-1 as a laboratory for accretion in High-Mass X-ray Binaries
Vela X-1 is an eclipsing high mass X-ray binary (HMXB) consisting of a 283s
accreting X-ray pulsar in a close orbit of 8.964 days around the B0.5Ib
supergiant HD77581 at a distance of just 2.4 kpc. The system is considered a
prototype of wind-accreting HMXB and it has been used as a baseline in
different theoretical or modelling studies.
We discuss the observational properties of the system and the use of the
observational data as laboratory to test recent developments in modelling the
accretion process in High-Mass X-ray Binaries (e.g., Sander et al. 2018; El
Mellah et al. 2018), which range from detailed descriptions of the wind
acceleration to modelling of the structure of the flow of matter close to the
neutron star and its variations.Comment: 4 pages, 2 figures, proceedings of the 12th INTEGRAL conference
"INTEGRAL looks AHEAD to Multimessenger astronomy" in Geneva (Switzerland) on
11-15 February 201
Radiography in high mass X-ray binaries -- Micro-structure of the stellar wind through variability of the column density
In high mass X-ray binaries (HMXBs), an accreting compact object orbits a
high mass star which loses mass through a dense and inhomogeneous wind. Using
the compact object as an X-ray backlight, the time variability of the absorbing
column density in the wind can be exploited in order to shed light on the
micro-structure of the wind and obtain unbiased stellar mass loss rates for
high mass stars. We explore the impact of clumpiness on the variability of the
column density with a simplified wind model. In particular, we focus on the
standard deviation of the column density and the characteristic duration of
enhanced absorption episodes, and compare them with analytical predictions
based on the porosity length. We identified the favorable systems and orbital
phases to determine the wind micro-structure. The coherence time scale of the
column density is shown to be the self-crossing time of a clump in front of the
compact object. We provide a recipe to get accurate measurements of the size
and of the mass of the clumps, purely based on the observable time variability
of the column density. The coherence time scale grants direct access to the
size of the clumps while their mass can be deduced separately from the
amplitude of the variability. If it is due to unaccreted passing-by clumps, the
high column density variations in some HMXBs requires high mass clumps to
reproduce the observed peak-to-peak amplitude and coherence time scales. These
clump properties are hardly compatible with the ones derived from first
principles. Alternatively, other components could contribute to the variability
of the column density: larger orbital scale structures produced by a mechanism
still to be identified, or a dense environment in the immediate vicinity of the
accretor such as an accretion disk, an outflow or a spherical shell around the
magnetosphere of the accreting neutron star
Possible Disintegrating Short-Period Super-Mercury Orbiting KIC 12557548
We report here on the discovery of stellar occultations, observed with
Kepler, that recur periodically at 15.685 hour intervals, but which vary in
depth from a maximum of 1.3% to a minimum that can be less than 0.2%. The star
that is apparently being occulted is KIC 12557548, a K dwarf with T_eff = 4400
K and V = 16. Because the eclipse depths are highly variable, they cannot be
due solely to transits of a single planet with a fixed size. We discuss but
dismiss a scenario involving a binary giant planet whose mutual orbit plane
precesses, bringing one of the planets into and out of a grazing transit. We
also briefly consider an eclipsing binary, that either orbits KIC 12557548 in a
hierarchical triple configuration or is nearby on the sky, but we find such a
scenario inadequate to reproduce the observations. We come down in favor of an
explanation that involves macroscopic particles escaping the atmosphere of a
slowly disintegrating planet not much larger than Mercury. The particles could
take the form of micron-sized pyroxene or aluminum oxide dust grains. The
planetary surface is hot enough to sublimate and create a high-Z atmosphere;
this atmosphere may be loaded with dust via cloud condensation or explosive
volcanism. Atmospheric gas escapes the planet via a Parker-type thermal wind,
dragging dust grains with it. We infer a mass loss rate from the observations
of order 1 M_E/Gyr, with a dust-to-gas ratio possibly of order unity. For our
fiducial 0.1 M_E planet, the evaporation timescale may be ~0.2 Gyr. Smaller
mass planets are disfavored because they evaporate still more quickly, as are
larger mass planets because they have surface gravities too strong to sustain
outflows with the requisite mass-loss rates. The occultation profile evinces an
ingress-egress asymmetry that could reflect a comet-like dust tail trailing the
planet; we present simulations of such a tail.Comment: 14 pages, 7 figures; submitted to ApJ, January 10, 2012; accepted
March 21, 201
The VLT/SPHERE view of the ATOMIUM cool evolved star sample. I. Overview:Sample characterization through polarization analysis
Aims. Through the ATOMIUM project, based on an ALMA large program, we aim to
present a consistent view of a sample of 17 nearby cool evolved stars
(Aymptotic Giant Branch and red supergiant stars).
Methods. Here we present VLT/SPHERE-ZIMPOL polarimetric maps obtained in the
visible of 14 out of the 17 ATOMIUM sources. They were obtained
contemporaneously with the ALMA high spatial resolution data. To help interpret
the polarized signal, we produced synthetic maps of light scattering by dust,
through 3D radiative transfer simulations with the RADMC3D code.
Results. The degree of linear polarization (DoLP) observed by ZIMPOL spreads
across several optical filters. We infer that it primarily probes dust located
just outside of the point spread function, and in or near the plane of the sky,
with a total optical depth close to unity in the line of sight, representing
only a fraction of the total circumstellar dust. The maximum DoLP ranges from
0.03-0.38 depending on the source, fractions that can be reproduced by our 3D
pilot models for grains composed of common dust species. The spatial structure
of the DoLP shows a diverse set of shapes. Only for three sources do we note a
correlation between the ALMA CO and SiO lines, which trace the gas density, and
the DoLP, which traces the dust.
Conclusion. The clumpiness of the DoLP and the lack of a consistent
correlation between the gas and the dust location show that, in the inner
circumstellar environment (CSE), dust formation occurs at very specific sites.
This has potential consequences for the derived mass-loss rates and dust-to-gas
ratio in the inner region of the CSE. Except for ~Gru and perhaps GY
Aql, we do not detect interactions between the circumstellar wind and the
hypothesized companions that shape the wind at larger scales. This suggests
that the orbits of any other companions are tilted out of the plane of the sky.Comment: Accepted for publication in Astronomy & Astrophysics. 22 pages, 15
figures, 5 table
The VLT/SPHERE view of the ATOMIUM cool evolved star sample. I. Overview: Sample characterization through polarization analysis
Aims. Through the ATOMIUM project, based on an ALMA large program, we aim to
present a consistent view of a sample of 17 nearby cool evolved stars
(Aymptotic Giant Branch and red supergiant stars).
Methods. Here we present VLT/SPHERE-ZIMPOL polarimetric maps obtained in the
visible of 14 out of the 17 ATOMIUM sources. They were obtained
contemporaneously with the ALMA high spatial resolution data. To help interpret
the polarized signal, we produced synthetic maps of light scattering by dust,
through 3D radiative transfer simulations with the RADMC3D code.
Results. The degree of linear polarization (DoLP) observed by ZIMPOL spreads
across several optical filters. We infer that it primarily probes dust located
just outside of the point spread function, and in or near the plane of the sky,
with a total optical depth close to unity in the line of sight, representing
only a fraction of the total circumstellar dust. The maximum DoLP ranges from
0.03-0.38 depending on the source, fractions that can be reproduced by our 3D
pilot models for grains composed of common dust species. The spatial structure
of the DoLP shows a diverse set of shapes. Only for three sources do we note a
correlation between the ALMA CO and SiO lines, which trace the gas density, and
the DoLP, which traces the dust.
Conclusion. The clumpiness of the DoLP and the lack of a consistent
correlation between the gas and the dust location show that, in the inner
circumstellar environment (CSE), dust formation occurs at very specific sites.
This has potential consequences for the derived mass-loss rates and dust-to-gas
ratio in the inner region of the CSE. Except for ~Gru and perhaps GY
Aql, we do not detect interactions between the circumstellar wind and the
hypothesized companions that shape the wind at larger scales. This suggests
that the orbits of any other companions are tilted out of the plane of the sky.Comment: Accepted for publication in Astronomy & Astrophysics. 22 pages, 15
figures, 5 table
(Sub)stellar companions shape the winds of evolved stars
Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe
ATOMIUM: ALMA tracing the origins of molecules in dust forming oxygen rich M-type stars: Motivation, sample, calibration, and initial results
This overview paper presents atomium, a Large Programme in Cycle 6 with the Atacama Large Millimeter/submillimeter Array (ALMA). The goal of atomium is to understand the dynamics and the gas phase and dust formation chemistry in the winds of evolved asymptotic giant branch (AGB) and red supergiant (RSG) stars. A more general aim is to identify chemical processes applicable to other astrophysical environments. Seventeen oxygen-rich AGB and RSG stars spanning a range in (circum)stellar parameters and evolutionary phases were observed in a homogeneous observing strategy allowing for an unambiguous comparison. Data were obtained between 213.83 and 269.71 GHz at high (0.025-0.050), medium (0.13-0.24), and low (~1) angular resolution. The sensitivity per ~1.3 km s-1 channel was 1.5-5 mJy beam-1, and the line-free channels were used to image the millimetre wave continuum. Our primary molecules for studying the gas dynamics and dust formation are CO, SiO, AlO, AlOH, TiO, TiO2, and HCN; secondary molecules include SO, SO2, SiS, CS, H2O, and NaCl. The scientific motivation, survey design, sample properties, data reduction, and an overview of the data products are described. In addition, we highlight one scientific result - the wind kinematics of the atomium sources. Our analysis suggests that the atomium sources often have a slow wind acceleration, and a fraction of the gas reaches a velocity which can be up to a factor of two times larger than previously reported terminal velocities assuming isotropic expansion. Moreover, the wind kinematic profiles establish that the radial velocity described by the momentum equation for a spherical wind structure cannot capture the complexity of the velocity field. In fifteen sources, some molecular transitions other than 12CO v = 0 J = 2 - 1 reach a higher outflow velocity, with a spatial emission zone that is often greater than 30 stellar radii, but much less than the extent of CO. We propose that a binary interaction with a (sub)stellar companion may (partly) explain the non-monotonic behaviour of the projected velocity field. The atomium data hence provide a crucial benchmark for the wind dynamics of evolved stars in single and binary star models
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