4,037 research outputs found
Modelling spectral line profiles of wind-wind shock emissions from massive binary systems
One of the most intriguing spectral features of WR binary stars is the
presence of time-dependent line profiles. Long term observations of several
systems revealed the periodicity of this variability, synchronized with the
orbital movement. Several partially successful models have been proposed to
reproduce the observed data. The most promising assume that the origin of the
emission is the wind-wind interaction zone. In this scenario, two high velocity
and dense winds produce a strong shock layer, responsible for most of the
X-rays observed from these systems. As the secondary star moves along its
orbital path, the shock region of conical shape, changes its position with
relation to the line of sight. As a consequence, the stream measured Doppler
shift presents time variations resulting in position changes of the spectral
line. In our work, we present an alternative model, introducing turbulence in
the shock layer to account for the line broadening and opacity effects for the
asymmetry in the line profiles. We showed that the gas turbulence avoids the
need of an unnaturally large contact layer thickness to reproduce line
broadening. Also, we demonstrated that if the emission from the opposing cone
surface is absorbed, the result is a single peaked profile. This result fully
satisfies the recent data obtained from massive binary systems, and can help on
the determination of both winds and orbital parameters. We successfully applied
this model to the Br22 system and determined its orbital parameters.Comment: To appear in the MNRA
Asymmetries on red giant branch surfaces from CHARA/MIRC optical interferometry
Context. Red giant branch (RGB) stars are very bright objects in galaxies and
are often used as standard candles. Interferometry is the ideal tool to
characterize the dynamics and morphology of their atmospheres. Aims. We aim at
precisely characterising the surface dynamics of a sample of RGB stars.
Methods. We obtained interferometric observations for three RGB stars with the
MIRC instrument mounted at the CHARA interfer- ometer. We looked for
asymmetries on the stellar surfaces using limb-darkening models. Results. We
measured the apparent diameters of HD197989 (Epsilon Cyg) = 4.61+-0.02 mas,
HD189276 (HR7633) = 2.95+-0.01 mas, and HD161096 (Beta Oph) = 4.43+-0.01 mas.
We detected departures from the centrosymmetric case for all three stars with
the tendency of a greater effect for lower logg of the sample. We explored the
causes of this signal and conclude that a possible explanation to the
interferometric signal is the convection-related and/or the magnetic-related
surface activity. However, it is necessary to monitor these stars with new
observations, possibly coupled with spectroscopy, in order to firmly establish
the cause.Comment: Accepted for publication as a Letter in Astronomy and Astrophysics,
section 1. Letters to the Editor. The official date of acceptance is
06/03/201
Diffraction-limited near-IR imaging at Keck reveals asymmetric, time-variable nebula around carbon star CIT 6
We present multi-epoch, diffraction-limited images of the nebula around the
carbon star CIT 6 at 2.2 microns and 3.1 microns from aperture masking on the
Keck-I telescope. The near-IR nebula is resolved into two main components, an
elongated, bright feature showing time-variable asymmetry and a fainter
component about 60 milliarcseconds away with a cooler color temperature. These
images were precisely registered (~35 milliarcseconds) with respect to recent
visible images from the Hubble Space Telescope (Trammell et al. 2000), which
showed a bipolar structure in scattered light. The dominant near-IR feature is
associated with the northern lobe of this scattering nebula, and the
multi-wavelength dataset can be understood in terms of a bipolar dust shell
around CIT 6. Variability of the near-IR morphology is qualitatively consistent
with previously observed changes in red polarization, caused by varying
illumination geometry due to non-uniform dust production. The blue emission
morphology and polarization properties can not be explained by the above model
alone, but require the presence of a wide binary companion in the vicinity of
the southern polar lobe. The physical mechanisms responsible for the breaking
of spherical symmetry around extreme carbon stars, such as CIT 6 and IRC+10216,
remain uncertain.Comment: 18 pages, 5 figures (one in color), to appear in the Astrophysical
Journa
The Palomar kernel-phase experiment: testing kernel phase interferometry for ground-based astronomical observations
At present, the principal limitation on the resolution and contrast of astronomical imaging instruments comes from aberrations in the optical path, which may be imposed by the Earth's turbulent atmosphere or by variations in the alignment and shape of the telescope optics. These errors can be corrected physically,with active and adaptive optics, and in post-processing of the resulting image.Arecently developed adaptive optics post-processing technique, called kernelphase interferometry, uses linear combinations of phases that are self-calibrating with respect to small errors, with the goal of constructing observables that are robust against the residual optical aberrations in otherwise well-corrected imaging systems. Here, we present a direct comparison between kernel phase and the more established competing techniques, aperture masking interferometry, point spread function (PSF) fitting and bispectral analysis.We resolve the α Ophiuchi binary system near periastron, using the Palomar 200-Inch Telescope. This is the first case in which kernel phase has been used with a full aperture to resolve a system close to the diffraction limit with ground-based extreme adaptive optics observations. Excellent agreement in astrometric quantities is found between kernel phase and masking, and kernel phase significantly outperforms PSF fitting and bispectral analysis, demonstrating its viability as an alternative to conventional non-redundant masking under appropriate conditions
Thin shell morphology in the circumstellar medium of massive binaries
We investigate the morphology of the collision front between the stellar
winds of binary components in two long-period binary systems, one consisting of
a hydrogen rich Wolf-Rayet star (WNL) and an O-star and the other of a Luminous
Blue Variable (LBV) and an O-star. Specifically, we follow the development and
evolution of instabilities that form in such a shell, if it is sufficiently
compressed, due to both the wind interaction and the orbital motion. We use
MPI-AMRVAC to time-integrate the equations of hydrodynamics, combined with
optically thin radiative cooling, on an adaptive mesh 3D grid. Using parameters
for generic binary systems, we simulate the interaction between the winds of
the two stars. The WNL+O star binary shows a typical example of an adiabatic
wind collision. The resulting shell is thick and smooth, showing no
instabilities. On the other hand, the shell created by the collision of the O
star wind with the LBV wind, combined with the orbital motion of the binary
components, is susceptible to thin shell instabilities, which create a highly
structured morphology. We identify the nature of the instabilities as both
linear and non-linear thin-shell instabilities, with distinct differences
between the leading and the trailing parts of the collision front. We also find
that for binaries containing a star with a (relatively) slow wind, the global
shape of the shell is determined more by the slow wind velocity and the orbital
motion of the binary, than the ram pressure balance between the two winds. The
interaction between massive binary winds needs further parametric exploration,
to identify the role and dynamical importance of multiple instabilities at the
collision front, as shown here for an LBV+O star system.Comment: 10 pages, 13 figures. Accepted for publication in A&
Electronic transport in EuB
EuB is a magnetic semiconductor in which defects introduce charge
carriers into the conduction band with the Fermi energy varying with
temperature and magnetic field. We present experimental and theoretical work on
the electronic magnetotransport in single-crystalline EuB. Magnetization,
magnetoresistance and Hall effect data were recorded at temperatures between 2
and 300 K and in magnetic fields up to 5.5 T. The negative magnetoresistance is
well reproduced by a model in which the spin disorder scattering is reduced by
the applied magnetic field. The Hall effect can be separated into an ordinary
and an anomalous part. At 20 K the latter accounts for half of the observed
Hall voltage, and its importance decreases rapidly with increasing temperature.
As for Gd and its compounds, where the rare-earth ion adopts the same Hund's
rule ground state as Eu in EuB, the standard antisymmetric
scattering mechanisms underestimate the of this contribution by several
orders of magnitude, while reproducing its almost perfectly. Well below
the bulk ferromagnetic ordering at = 12.5 K, a two-band model
successfully describes the magnetotransport. Our description is consistent with
published de Haas van Alphen, optical reflectivity, angular-resolved
photoemission, and soft X-ray emission as well as absorption data, but requires
a new interpretation for the gap feature deduced from the latter two
experiments.Comment: 35 pages, 12 figures, submitted to PR
A Tale of Two Herbig Ae stars -MWC275 and AB Aurigae: Comprehensive Models for SED and Interferometry
We present comprehensive models for the Herbig Ae stars MWC275 and AB Aur
that aim to explain their spectral energy distribution (from UV to millimeter)
and long baseline interferometry (from near-infrared to millimeter)
simultaneously. Data from the literature, combined with new mid-infrared (MIR)
interferometry from the Keck Segment Tilting Experiment, are modeled using an
axisymmetric Monte Carlo radiative transfer code. Models in which most of the
near-infrared (NIR) emission arises from a dust rim fail to fit the NIR
spectral energy distribution (SED) and sub-milli-arcsecond NIR CHARA
interferometry. Following recent work, we include an additional gas emission
component with similar size scale to the dust rim, inside the sublimation
radius, to fit the NIR SED and long-baseline NIR interferometry on MWC275 and
AB Aur. In the absence of shielding of star light by gas, we show that the
gas-dust transition region in these YSOs will have to contain highly refractory
dust, sublimating at ~1850K. Despite having nearly identical structure in the
thermal NIR, the outer disks of MWC275 and AB Aur differ substantially. In
contrast to the AB Aur disk, MWC275 lacks small grains in the disk atmosphere
capable of producing significant 10-20micron emission beyond ~7AU, forcing the
outer regions into the "shadow" of the inner diskComment: Accepted for publication in Ap
Irregular dust features around intermediate-mass young stars with GPI: signs of youth or misaligned disks?
This is the final version. Available from the American Astronomical Society via the DOI in this recordWe are undertaking a large survey of over thirty disks using the Gemini Planet Imager (GPI) to
see whether the observed dust structures match spectral energy distribution (SED) predictions and
have any correlation with stellar properties. GPI can observe near-infrared light scattered from dust
in circumstellar environments using high-resolution Polarimetric Differential Imaging (PDI) with coronagraphy and adaptive optics. The data have been taken in J and H bands over two years, with inner
working angles of 0.08 ” and 0.11 ” respectively. Ahead of the release of the complete survey results,
here we present five objects with extended and irregular dust structures within 2 ” of the central star.
These objects are: FU Ori; MWC 789; HD 45677; Hen 3-365; and HD 139614. The observed structures are consistent with each object being a pre-main-sequence star with protoplanetary dust. The
five objects’ circumstellar environments could result from extreme youth and complex initial conditions, from asymmetric scattering patterns due to shadows cast by misaligned disks, or in some cases
from interactions with companions. We see complex Uφ structures in most objects that could indicate
multiple scattering or result from the illumination of companions. Specific key findings include the
first high-contrast observation of MWC 789 revealing a newly-discovered companion candidate and arc,
and two faint companion candidates around Hen 3-365. These two objects should be observed further
to confirm whether the companion candidates are co-moving. Further observations and modeling are
required to determine the causes of the structures.Science and Technology Facilities Council (STFC
Multiple spiral arms in the disk around intermediate-mass binary HD 34700A
This is the final version. Available from American Astronomical Society / IOP Publishing via the DOI in this record.We present the first images of the transition disk around the close binary system HD 34700A in
polarized scattered light using the Gemini Planet Imager instrument on Gemini South. The J and H
band images reveal multiple spiral-arm structures outside a large (R = 0.4900 = 175 au) cavity along
with a bluish spiral structure inside the cavity. The cavity wall shows a strong discontinuity and
we clearly see significant non-azimuthal polarization Uφ consistent with multiple scattering within a
disk at an inferred inclination ∼42◦
. Radiative transfer modeling along with a new Gaia distance
suggest HD 37400A is a young (∼5 Myr) system consisting of two intermediate-mass (∼2 M ) stars
surrounded by a transitional disk and not a solar-mass binary with a debris disk as previously classified.
Conventional assumptions of the dust-to-gas ratio would rule out a gravitational instability origin to the
spirals while hydrodynamical models using the known external companion or a hypothetical massive
protoplanet in the cavity both have trouble reproducing the relatively large spiral arm pitch angles
(∼ 30◦
) without fine tuning of gas temperature. We explore the possibility that material surrounding
a massive protoplanet could explain the rim discontinuity after also considering effects of shadowing
by an inner disk. Analysis of archival Hubble Space Telescope data suggests the disk is rotating
counterclockwise as expected from the spiral arm structure and revealed a new low-mass companion at
6.4500separation. We include an appendix which sets out clear definitions of Q, U, Qφ, Uφ, correcting
some confusion and errors in the literature.Science and Technology Facilities CouncilEuropean Research CouncilNASANational Science Foundation (NSF)University of Colorado Boulder Hale Fellowship progra
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