463 research outputs found
High Angular Resolution Stellar Imaging with Occultations from the Cassini Spacecraft II: Kronocyclic Tomography
We present an advance in the use of Cassini observations of stellar
occultations by the rings of Saturn for stellar studies. Stewart et al. (2013)
demonstrated the potential use of such observations for measuring stellar
angular diameters. Here, we use these same observations, and tomographic
imaging reconstruction techniques, to produce two dimensional images of complex
stellar systems. We detail the determination of the basic observational
reference frame. A technique for recovering model-independent brightness
profiles for data from each occulting edge is discussed, along with the
tomographic combination of these profiles to build an image of the source star.
Finally we demonstrate the technique with recovered images of the {\alpha}
Centauri binary system and the circumstellar environment of the evolved
late-type giant star, Mira.Comment: 8 pages, 8 figures, Accepted by MNRA
The Keck Aperture Masking Experiment: Dust Enshrouded Red Giants
While the importance of dusty asymptotic giant branch (AGB) stars to galactic
chemical enrichment is widely recognised, a sophisticated understanding of the
dust formation and wind-driving mechanisms has proven elusive due in part to
the difficulty in spatially-resolving the dust formation regions themselves. We
have observed twenty dust-enshrouded AGB stars as part of the Keck Aperture
Masking Experiment, resolving all of them in multiple near-infrared bands
between 1.5 microns and 3.1 microns. We find 45% of the targets to show
measurable elongations that, when correcting for the greater distances of the
targets, would correspond to significantly asymmetric dust shells on par with
the well-known cases of IRC+10216 or CIT6. Using radiative transfer models, we
find the sublimation temperature of 1130 +- 90 K and 1170 +- 60 K for silicates
and amorphous carbon respectively, both somewhat lower than expected from
laboratory measurements and vastly below temperatures inferred from the inner
edge of YSO disks. The fact that O-rich and C-rich dust types showed the same
sublimation temperature was surprising as well. For the most optically-thick
shells (tau > 2 at 2.2 microns), the temperature profile of the inner dust
shell is observed to change substantially, an effect we suggest could arise
when individual dust clumps become optically-thick at the highest mass-loss
rates.Comment: accepted to Monthly Notices of the Royal Astronomical Societ
The Keck Aperture Masking Experiment: spectro-interferometry of 3 Mira Variables from 1.1 to 3.8 microns
We present results from a spectro-interferometric study of the Miras o Cet, R
Leo and W Hya obtained with the Keck Aperture Masking Experiment from 1998 Sep
to 2002 Jul. The spectrally dispersed visibility data permit fitting with
circularly symmetric brightness profiles such as a simple uniform disk. The
stellar angular diameter obtained over up to ~ 450 spectral channels spaning
the region 1.1-3.8 microns is presented. Use of a simple uniform disk
brightness model facilitates comparison between epochs and with existing data
and theoretical models. Strong size variations with wavelength were recorded
for all stars, probing zones of H2O, CO, OH, and dust formation. Comparison
with contemporaneous spectra extracted from our data show a strong
anti-correlation between the observed angular diameter and flux. These
variations consolidate the notion of a complex stellar atmosphere consisting of
molecular shells with time-dependent densities and temperatures. Our findings
are compared with existing data and pulsation models. The models were found to
reproduce the functional form of the wavelength vs. angular diameter curve
well, although some departures are noted in the 2.8-3.5 micron range.Comment: 10 pages, 10 figures Accepted to Ap
Born Again Protoplanetary Disk Around Mira B
The Mira AB system is a nearby (~107 pc) example of a wind accreting binary
star system. In this class of system, the wind from a mass-losing red giant
star (Mira A) is accreted onto a companion (Mira B), as indicated by an
accretion shock signature in spectra at ultraviolet and X-ray wavelengths.
Using novel imaging techniques, we report the detection of emission at
mid-infrared wavelengths between 9.7 and 18.3 m from the vicinity of Mira
B but with a peak at a radial position about 10 AU closer to the primary Mira
A. We interpret the mid-infrared emission as the edge of an optically-thick
accretion disk heated by Mira A. The discovery of this new class of accretion
disk fed by M-giant mass loss implies a potential population of young planetary
systems in white-dwarf binaries which has been little explored, despite being
relatively common in the solar neighborhood.Comment: Accepted for Ap
Sparse aperture masking at the VLT I. Faint companion detection limits for the two debris disk stars HD 92945 and HD 141569
Observational data on companion statistics around young stellar systems is
needed to flesh out the formation pathways for extrasolar planets and brown
dwarfs. Aperture masking is a new technique that is able to address an
important part of this discovery space. We observed the two debris disk systems
HD 92945 and HD 141569 with sparse aperture masking (SAM), a new mode offered
on the NaCo instrument at the VLT. A search for faint companions was performed
using a detection strategy based on the analysis of closure phases recovered
from interferograms recorded on the Conica camera. Our results demonstrate that
SAM is a very competitive mode in the field of companion detection. We obtained
5 sigma high-contrast detection limits at lambda/D of 2.5x10^{-3} (\Delta L' =
6.5) for HD 92945 and 4.6x10^{-3} (\Delta L' = 5.8) for HD 141569. According to
brown dwarf evolutionary models, our data impose an upper mass boundary for any
companion for the two stars to, respectively, 18 and 22 Jupiter masses at
minimum separations of 1.5 and 7 AU. The detection limits is mostly independent
of angular separation, until reaching the diffraction limit of the telescope.
We have placed upper limits on the existence of companions to our target
systems that fall close to the planetary mass regime. This demonstrates the
potential for SAM mode to contribute to studies of faint companions. We
furthermore show that the final dynamic range obtained is directly proportional
to the error on the closure phase measurement. At the present performance
levels of 0.28 degree closure phase error, SAM is among the most competitive
techniques for recovering companions at scales of one to several times the
diffraction limit of the telescope. Further improvements to the detection
threshold can be expected with more accurate phase calibration.Comment: accepted in A&A, 9 page
Optical Interferometry of early-type stars with PAVO@CHARA. I. Fundamental stellar properties
We present interferometric observations of 7 main-sequence and 3 giant stars
with spectral types from B2 to F6 using the PAVO beam combiner at the CHARA
array. We have directly determined the angular diameters for these objects with
an average precision of 2.3%. We have also computed bolometric fluxes using
available photometry in the visible and infrared wavelengths, as well as
space-based ultraviolet spectroscopy. Combined with precise \textit{Hipparcos}
parallaxes, we have derived a set of fundamental stellar properties including
linear radius, luminosity and effective temperature. Fitting the latter to
computed isochrone models, we have inferred masses and ages of the stars. The
effective temperatures obtained are in good agreement (at a 3% level) with
nearly-independent temperature estimations from spectroscopy. They validate
recent sixth-order polynomial (B-V)- empirical relations
\citep{Boyajian2012a}, but suggest that a more conservative third-order
solution \citep{vanBelle2009} could adequately describe the
(V-K)- relation for main-sequence stars of spectral type A0 and
later. Finally, we have compared mass values obtained combining surface gravity
with inferred stellar radius (\textit{gravity mass}) and as a result of the
comparison of computed luminosity and temperature values with stellar
evolutionary models (\textit{isochrone mass}). The strong discrepancy between
isochrone and gravity mass obtained for one of the observed stars,
\,Lyr, suggests that determination of the stellar atmosphere parameters
should be revised.Comment: 13 pages, 9 figures, accepted for publication in MNRA
Precision Masses of the low-mass binary system GJ 623
We have used Aperture Masking Interferometry and Adaptive Optics (AO) at the
Palomar 200'' to obtain precise mass measurements of the binary M dwarf GJ 623.
AO observations spread over 3 years combined with a decade of radial velocity
measurements constrain all orbital parameters of the GJ 623 binary system
accurately enough to critically challenge the models. The dynamical masses
measured are m_{1}=0.371\pm0.015 M_{\sun} (4%) and m_{2}=0.115\pm0.0023
M_{\sun} (2%) for the primary and the secondary respectively. Models are not
consistent with color and mass, requiring very low metallicities.Comment: 7 pages, 5 figures. Accepted for Ap
A dusty torus around the luminous young star LkHa 101
A star forms when a cloud of dust and gas collapses. It is generally believed
that this collapse first produces a flattened rotating disk, through which
matter is fed onto the embryonic star at the center of the disk. When the
temperature and density at the center of the star pass a critical threshold,
thermonuclear fusion begins. The remaining disk, which can still contain up to
0.3 times the mass of the star, is then sculpted and eventually dissipated by
the radiation and wind from the newborn star. Unfortunately this picture of the
structure and evolution of the disk remains speculative because of the lack of
morphological data of sufficient resolution and uncertainties regarding the
underlying physical processes. Here we present resolved images of a young star,
LkHa 101 in which the structure of the inner accretion disk is resolved. We
find that the disk is almost face-on, with a central gap (or cavity) and a hot
inner edge. The cavity is bigger than previous theoretical predictions, and we
infer that the position of the inner edge is probably determined by sublimation
of dust grains by direct stellar radiation, rather than by disk reprocessing or
the viscous heating processes as usually assumed.Comment: 7 pages, 1 figure. Appears in Nature, 22 Feb, 2001 (Vol 409
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