117 research outputs found
VLTI/AMBER spectro-interferometric imaging of VX Sgr's inhomogenous outer atmosphere
Aims. We aim to explore the photosphere of the very cool late-type star VX
Sgr and in particular the existence and characterization of molecular layers
above the continuum forming photosphere. Methods. We obtained interferometric
observations with the VLTI/AMBER interferometer using the fringe tracker FINITO
in the spectral domain 1.45-2.50 micron with a spectral resolution of about 35
and baselines ranging from 15 to 88 meters.We perform independent image
reconstruction for different wavelength bins and fit the interferometric data
with a geometrical toy model.We also compare the data to 1D dynamical models of
Miras atmosphere and to 3D hydrodynamical simulations of red supergiant (RSG)
and asymptotic giant branch (AGB) stars. Results. Reconstructed images and
visibilities show a strong wavelength dependence. The H-band images display two
bright spots whose positions are confirmed by the geometrical toy model. The
inhomogeneities are qualitatively predicted by 3D simulations. At about 2,00
micron and in the region 2,35 - 2,50 micron, the photosphere appears extended
and the radius is larger than in the H band. In this spectral region, the
geometrical toy model locates a third bright spot outside the photosphere that
can be a feature of the molecular layers. The wavelength dependence of the
visibility can be qualitatively explained by 1D dynamical models of Mira
atmospheres. The best-fitting photospheric models show a good match with the
observed visibilities and give a photospheric diameter of theta = 8,82+-0,50
mas. The H2O molecule seems to be the dominant absorber in the molecular
layers. Conclusions. We show that the atmosphere of VX Sgr rather resembles
Mira/AGB star model atmospheres than RSG model atmospheres. In particular, we
see molecular (water) layers that are typical for Mira stars.Comment: 9 Pages, Accepted for publication on Astronomy & Astrophysics, two
references update
The Temperature and Cooling Age of the White-Dwarf Companion to the Millisecond Pulsar PSR B1855+09
We report on Keck and {\em Hubble Space Telescope} observations of the binary
millisecond pulsar PSR B1855+09. We detect its white-dwarf companion and
measure \mv=25.90\pm0.12 and \mi=24.19\pm0.11 (Vega system). From the
reddening-corrected color, (\mv-\mi)_0=1.06\pm0.21, we infer a temperature
\Teff=4800\pm800 K. The white-dwarf mass is known accurately from
measurements of the Shapiro delay of the pulsar signal,
\Mcomp=0.258^{+0.028}_{-0.016} \Msun. Hence, given a cooling model, one can
use the measured temperature to determine the cooling age. The main uncertainty
in the cooling models for such low-mass white dwarfs is the amount of residual
nuclear burning, which is set by the thickness of the hydrogen layer
surrounding the helium core. From the properties of similar systems, it has
been inferred that helium white dwarfs form with thick hydrogen layers, with
mass \simgt3\times10^{-3} \Msun, which leads to significant additional
heating. This is consistent with expectations from simple evolutionary models
of the preceding binary evolution. For PSR B1855+09, though, such models lead
to a cooling age of Gyr, which is twice the spin-down age of the
pulsar. It could be that the spin-down age were incorrect, which would call the
standard vacuum dipole braking model into question. For two other pulsar
companions, however, ages well over 10 Gyr are inferred, indicating that the
problem may lie with the cooling models. There is no age discrepancy for models
in which the white dwarfs are formed with thinner hydrogen layers
(\simlt3\times10^{-4} \Msun).Comment: 7 pages, 1 figure, aas4pp2.sty. Accepted for publication in ApJ
Do R Coronae Borealis Stars Form from Double White Dwarf Mergers?
A leading formation scenario for R Coronae Borealis (RCB) stars invokes the
merger of degenerate He and CO white dwarfs (WD) in a binary. The observed
ratio of 16O/18O for RCB stars is in the range of 0.3-20 much smaller than the
solar value of ~500. In this paper, we investigate whether such a low ratio can
be obtained in simulations of the merger of a CO and a He white dwarf. We
present the results of five 3-dimensional hydrodynamic simulations of the
merger of a double white dwarf system where the total mass is 0.9 Mdot and the
initial mass ratio (q) varies between 0.5 and 0.99. We identify in simulations
with a feature around the merged stars where the temperatures
and densities are suitable for forming 18O. However, more 16O is being
dredged-up from the C- and O-rich accretor during the merger than the amount of
18O that is produced. Therefore, on a dynamical time scale over which our
hydrodynamics simulation runs, a 16O/18O ratio of ~2000 in the "best" case is
found. If the conditions found in the hydrodynamic simulations persist for 10^6
seconds the oxygen ratio drops to 16 in one case studied, while in a hundred
years it drops to ~4 in another case studied, consistent with the observed
values in RCB stars. Therefore, the merger of two white dwarfs remains a strong
candidate for the formation of these enigmatic stars.Comment: 42 pages, 19 figures. Accepted for publication in the Astrophysical
Journa
The Unusual Binary Pulsar PSR J1744-3922: Radio Flux Variability, Near-infrared Observation and Evolution
PSR J1744-3922 is a binary pulsar exhibiting highly variable pulsed radio
emission. We report on a statistical multi-frequency study of the pulsed radio
flux variability which suggests that this phenomenon is extrinsic to the pulsar
and possibly tied to the companion, although not strongly correlated with
orbital phase. The pulsar has an unusual combination of characteristics
compared to typical recycled pulsars: a long spin period (172 ms); a relatively
high magnetic field strength (1.7x10^10 G); a very circular, compact orbit of
4.6 hours; and a low-mass companion (0.08 Msun). These spin and orbital
properties are likely inconsistent with standard evolutionary models. We find
similarities between the properties of the PSR J1744-3922 system and those of
several other known binary pulsar systems, motivating the identification of a
new class of binary pulsars. We suggest that this new class could result from
either: a standard accretion scenario of a magnetar or a high-magnetic field
pulsar; common envelope evolution with a low-mass star and a neutron star,
similar to what is expected for ultra-compact X-ray binaries; or, accretion
induced collapse of a white dwarf. We also report the detection of a possible
K'=19.30(15) infrared counterpart at the position of the pulsar, which is
relatively bright if the companion is a helium white dwarf at the nominal
distance, and discuss its implications for the pulsar's companion and
evolutionary history.Comment: 18 pages, 8 figures, accepted for publication of Ap
First AMBER/VLTI observations of hot massive stars
AMBER is the first near infrared focal instrument of the VLTI. It combines
three telescopes and produces spectrally resolved interferometric measures.
This paper discusses some preliminary results of the first scientific
observations of AMBER with three Unit Telescopes at medium (1500) and high
(12000) spectral resolution. We derive a first set of constraints on the
structure of the circumstellar material around the Wolf Rayet Gamma2 Velorum
and the LBV Eta Carinae
Three Binary Millisecond Pulsars in NGC 6266
We present rotational and astrometric parameters of three millisecond pulsars
located near the center of the globular cluster NGC 6266 (M62) resulting from
timing observations with the Parkes radio telescope. Their accelerations toward
the cluster center yield values of the cluster central density and
mass-to-light ratio consistent with those derived from optical data. The three
pulsars are in binary systems. One (spin period P=5.24 ms) is in a 3.5-day
orbit around a companion of minimum mass 0.2 Msun. The other two millisecond
pulsars (P=3.59 ms and 3.81 ms) have shorter orbital periods (3.4 hr and 5.0
hr) and lighter companions (minimum mass 0.12 Msun and 0.07 Msun respectively).
The pulsar in the closest system is the fifth member of an emerging class of
millisecond pulsars displaying irregular radio eclipses and having a relatively
massive companion. This system is a good candidate for optical identification
of the companion star. The lack of known isolated pulsars in NGC 6266 is also
discussed.Comment: 9 pages, 5 embedded figures, 2 tables, emulateapj style. Accepted for
publication in ApJ on 20 August 200
Transmission of MethicillinâResistant Staphylococcus aureus Infection Through Solid Organ Transplantation: Confirmation Via Whole Genome Sequencing
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109324/1/ajt12898.pd
Direct constraint on the distance of y2 Velorum from AMBER/VLTI observations
In this work, we present the first AMBER observations, of the Wolf-Rayet and
O (WR+O) star binary system y2 Velorum. The AMBER instrument was used with the
telescopes UT2, UT3, and UT4 on baselines ranging from 46m to 85m. It delivered
spectrally dispersed visibilities, as well as differential and closure phases,
with a resolution R = 1500 in the spectral band 1.95-2.17 micron. We interpret
these data in the context of a binary system with unresolved components,
neglecting in a first approximation the wind-wind collision zone flux
contribution. We show that the AMBER observables result primarily from the
contribution of the individual components of the WR+O binary system. We discuss
several interpretations of the residuals, and speculate on the detection of an
additional continuum component, originating from the free-free emission
associated with the wind-wind collision zone (WWCZ), and contributing at most
to the observed K-band flux at the 5% level. The expected absolute separation
and position angle at the time of observations were 5.1±0.9mas and
66±15° respectively. However, we infer a separation of
3.62+0.11-0.30 mas and a position angle of 73+9-11°. Our analysis thus
implies that the binary system lies at a distance of 368+38-13 pc, in agreement
with recent spectrophotometric estimates, but significantly larger than the
Hipparcos value of 258+41-31 pc
Multidrug-Resistant Staphylococcus aureus in US Meat and Poultry
We characterized the prevalence, antibiotic susceptibility profiles, and genotypes of Staphylococcus aureus among US meat and poultry samples (n = 136). S. aureus contaminated 47% of samples, and multidrug resistance was common among isolates (52%). S. aureus genotypes and resistance profiles differed significantly among sample types, suggesting food animalâspecific contamination
Arbitrarily Degenerate Helium White Dwarfs as Donors in AM CVn Binaries
We apply the Deloye & Bildsten (2003) isentropic models for donors in
ultracompact low-mass X-ray binaries to the AM CVn population of ultracompact,
interacting binaries. The mass-radius relations of these systems' donors in the
mass range of interest (M_2<0.1 \msun) are not single-valued, but
parameterized by the donor's specific entropy. This produces a range in the
relationships between system observables, such as orbital period, \Porb, and
mass transfer rate, \Mdot. For a reasonable range in donor specific entropy,
\Mdot can range over several orders of magnitude at fixed \Porb. We
determine the unique relation between \Mdot and in the AM CVn systems
with known donor to accretor mass ratios, . We use structural
arguments, as well as each system's photometric behavior, to place limits on
\Mdot and in each. Most systems allow a factor of about 3 variation in
\Mdot, although V803 Cen, if the current estimates of its are accurate,
is an exception and must have M_2 \approx 0.02 \msun and \Mdot \approx
10^{-10} \msun yr. Our donor models also constrain each donor's core
temperature, , range and correlate with . We examine how
variations in donor specific entropy across the white dwarf family
\citep{nele01a} of AM CVn systems affects this population's current galactic
distribution. Allowing for donors that are not fully degenerate produces a
shift in systems towards longer \Porb and higher \Mdot increasing the
parameter space in which these systems can be found. This shift increases the
fraction of systems whose \Porb is long enough that their gravity wave (GW)
signal is obscured by the background of detached double white dwarf binaries
that dominate the GW spectrum below a frequency mHz.Comment: 13 pages, 10 figures, uses emulateapj.cls. Accepted to Astrophysical
Journa
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