1,041 research outputs found
Modeling the HD32297 Debris Disk with Far-IR Herschel Data
HD32297 is a young A-star (~30 Myr) 112 pc away with a bright edge-on debris
disk that has been resolved in scattered light. We observed the HD32297 debris
disk in the far-infrared and sub-millimeter with the Herschel Space Observatory
PACS and SPIRE instruments, populating the spectral energy distribution (SED)
from 63 to 500{\mu}m. We aimed to determine the composition of dust grains in
the HD32297 disk through SED modeling, using geometrical constraints from the
resolved imaging to break degeneracies inherent in SED modeling. We found the
best fitting SED model has 2 components: an outer ring centered around 110 AU,
seen in the scattered light images, and an inner disk near the habitable zone
of the star. The outer disk appears to be composed of grains > 2{\mu}m
consisting of silicates, carbonaceous material, and water ice with an abundance
ratio of 1:2:3 respectively and 90% porosity. These grains appear consistent
with cometary grains, implying the underlying planetesimal population is
dominated by comet-like bodies. We also discuss the 3.7{\sigma} detection of [C
II] emission at 158{\mu}m with the Herschel PACS Spectrometer, making HD32297
one of only a handful of debris disks with circumstellar gas detected.Comment: 11 pages, 4 figures, accepted for publication in The Astrophysical
Journa
Measuring the extent of convective cores in low-mass stars using Kepler data: towards a calibration of core overshooting
Our poor understanding of the boundaries of convective cores generates large
uncertainties on the extent of these cores and thus on stellar ages. Our aim is
to use asteroseismology to consistently measure the extent of convective cores
in a sample of main-sequence stars whose masses lie around the mass-limit for
having a convective core. We first test and validate a seismic diagnostic that
was proposed to probe in a model-dependent way the extent of convective cores
using the so-called ratios, which are built with and
modes. We apply this procedure to 24 low-mass stars chosen among Kepler targets
to optimize the efficiency of this diagnostic. For this purpose, we compute
grids of stellar models with both the CESAM2k and MESA evolution codes, where
the extensions of convective cores are modeled either by an instantaneous
mixing or as a diffusion process. Among the selected targets, we are able to
unambiguously detect convective cores in eight stars and we obtain seismic
measurements of the extent of the mixed core in these targets with a good
agreement between the CESAM2k and MESA codes. By performing optimizations using
the Levenberg-Marquardt algorithm, we then obtain estimates of the amount of
extra-mixing beyond the core that is required in CESAM2k to reproduce seismic
observations for these eight stars and we show that this can be used to propose
a calibration of this quantity. This calibration depends on the prescription
chosen for the extra-mixing, but we find that it should be valid also for the
code MESA, provided the same prescription is used. This study constitutes a
first step towards the calibration of the extension of convective cores in
low-mass stars, which will help reduce the uncertainties on the ages of these
stars.Comment: 27 pages, 15 figures, accepted in A&
Starspot Jitter in Photometry, Astrometry and Radial Velocity Measurements
Analytical relations are derived for the amplitude of astrometric,
photometric and radial velocity perturbations caused by a single rotating spot.
The relative power of the star spot jitter is estimated and compared with the
available data for Ceti and HD 166435, as well as with numerical
simulations for Ceti and the Sun. A Sun-like star inclined at
i=90\degr at 10 pc is predicted to have a RMS jitter of 0.087 \uas in its
astrometric position along the equator, and 0.38 m s in radial
velocities. If the presence of spots due to stellar activity is the ultimate
limiting factor for planet detection, the sensitivity of SIM Lite to Earth-like
planets in habitable zones is about an order of magnitude higher that the
sensitivity of prospective ultra-precise radial velocity observations of nearby
stars.Comment: accepted in ApJ Letters, Nov. 200
Spectra disentangling applied to the Hyades binary Theta^2 Tau AB: new orbit, orbital parallax and component properties
Theta^2 Tauri is a detached and single-lined interferometric-spectroscopic
binary as well as the most massive binary system of the Hyades cluster. The
system revolves in an eccentric orbit with a periodicity of 140.7 days. The
secondary has a similar temperature but is less evolved and fainter than the
primary. It is also rotating more rapidly. Since the composite spectra are
heavily blended, the direct extraction of radial velocities over the orbit of
component B was hitherto unsuccessful. Using high-resolution spectroscopic data
recently obtained with the Elodie (OHP, France) and Hermes (ORM, La Palma,
Spain) spectrographs, and applying a spectra disentangling algorithm to three
independent data sets including spectra from the Oak Ridge Observatory (USA),
we derived an improved spectroscopic orbit and refined the solution by
performing a combined astrometric-spectroscopic analysis based on the new
spectroscopy and the long-baseline data from the Mark III optical
interferometer. As a result, the velocity amplitude of the fainter component is
obtained in a direct and objective way. Major progress based on this new
determination includes an improved computation of the orbital parallax. Our
mass ratio is in good agreement with the older estimates of Peterson et al.
(1991, 1993), but the mass of the primary is 15-25% higher than the more recent
estimates by Torres et al. (1997) and Armstrong et al. (2006). Due to the
strategic position of the components in the turnoff region of the cluster,
these new determinations imply stricter constraints for the age and the
metallicity of the Hyades cluster. The location of component B can be explained
by current evolutionary models, but the location of the more evolved component
A is not trivially explained and requires a detailed abundance analysis of its
disentangled spectrum.Comment: in press, 13 pages, 10 Postscript figures, 5 tables. Table~4 is
available as online material. Keywords: astrometry - techniques: high angular
resolution - stars: binaries: visual - stars: binaries: spectroscopic -
stars: fundamental parameter
Modeling the HD 32297 Debris Disk With Far-Infrared Herschel Data
HD 32297 is a young A-star (approx. 30 Myr) 112 pc away with a bright edge-on debris disk that has been resolved in scattered light. We observed the HD 32297 debris disk in the far-infrared and sub-millimeter with the Herschel Space Observatory PACS and SPIRE instruments, populating the spectral energy distribution (SED) from 63 to 500 micron..We aimed to determine the composition of dust grains in the HD 32297 disk through SED modeling, using geometrical constraints from the resolved imaging to break the degeneracies inherent in SED modeling. We found the best fitting SED model has two components: an outer ring centered around 110 AU, seen in the scattered light images, and an inner disk near the habitable zone of the star. The outer disk appears to be composed of grains>2 micron consisting of silicates, carbonaceous material, and water ice with an abundance ratio of 1:2:3 respectively and 90% porosity. These grains appear consistent with cometary grains, implying the underlying planetesimal population is dominated by comet-like bodies. We also discuss the 3.7 sigma detection of [C ii] emission at 158 micron with the Herschel PACS instrument, making HD 32297 one of only a handful of debris disks with circumstellar gas detecte
Discordance in the minimal inhibitory concentrations of ertapenem for Enterobacter cloacae: Vitek 2 system versus Etest and agar dilution methods
Our objective was to compare the ertapenem minimal inhibitory concentrations (MICs) for Enterobacter cloacae isolates categorized intermediate or resistant to ertapenem when measured with the Vitek 2 system, with the MICs for these isolates when measured by two methods performed in agar medium: the Etest and agar plate dilution method (APDM). Overall, 50 E. cloacae isolates were included in the study. The mean MIC of ertapenem was 2.92±1.77μg/ml according to the Vitek 2 system, 0.94±0.84μg/ml according to the Etest strips, and 0.93±0.62μg/ml according to the APDM. Furthermore, the MICs determined by the Vitek 2 system were higher than the MICs determined by the two other methods for 96% of strains. Lastly, according to the Etest strips and APDM, 42% of E. cloacae were susceptible to ertapenem. No carbapenemase was identified by the screening method used. Using the Vitek 2 system to determine ertapenem MICs for E. cloacae can have potential consequences in terms of additional carbapenemase-detecting tests and antimicrobial therapy. It would be interesting to determine if the Vitek 2 system is more effective for the detection of carbapenemase producers with low-level carbapenem resistance than the two methods performed in agar medium
Hipparcos open clusters and stellar evolution
By relying on recently improved Hipparcos parallaxes for the Hyades, Pleiades
and Ursa Major clusters we find that stellar models with updated physical
inputs nicely reproduce the location in the color magnitude diagram of main
sequence stars of different metallicities. Stars in the helium burning phase
are also discussed, showing that the luminosity of giants in the Hyades,
Praesepe and Ursa Major clusters appears to be in reasonable agreement with
theoretical predictions. A short discussion concerning the current evolutionary
scenarios closes the paper.Comment: 5 pages, 6 Postscript figures, accepted by MNRA
- …