278 research outputs found
Orbit of the Mercury-Manganese binary 41 Eridani
Context. Mercury-manganese (HgMn) stars are a class of slowly rotating
chemically peculiar main-sequence late B-type stars. More than two-thirds of
the HgMn stars are known to belong to spectroscopic binaries.
Aims. By determining orbital solutions for binary HgMn stars, we will be able
to obtain the masses for both components and the distance to the system.
Consequently, we can establish the position of both components in the
Hertzsprung-Russell diagram and confront the chemical peculiarities of the HgMn
stars with their age and evolutionary history.
Methods. We initiated a program to identify interferometric binaries in a
sample of HgMn stars, using the PIONIER near-infrared interferometer at the
VLTI on Cerro Paranal, Chile. For the detected systems, we intend to obtain
full orbital solutions in conjunction with spectroscopic data.
Results. The data obtained for the SB2 system 41 Eridani allowed the
determination of the orbital elements with a period of just five days and a
semi-major axis of under 2 mas. Including published radial velocity
measurements, we derived almost identical masses of 3.17 +/- 0.07 M_Sun for the
primary and 3.07 +/- 0.07 M_Sun for the secondary. The measured magnitude
difference is less than 0.1 mag. The orbital parallax is 18.05 +/- 0.17 mas,
which is in good agreement with the Hipparcos trigonometric parallax of 18.33
+/- 0.15 mas. The stellar diameters are resolved as well at 0.39 +/- 0.03 mas.
The spin rate is synchronized with the orbital rate.Comment: 5 pages, 4 figure
SearchCal: a Virtual Observatory tool for searching calibrators in optical long baseline interferometry. I: The bright object case
In long baseline interferometry, the raw fringe contrast must be calibrated
to obtain the true visibility and then those observables that can be
interpreted in terms of astrophysical parameters. The selection of suitable
calibration stars is crucial for obtaining the ultimate precision of
interferometric instruments like the VLTI. We have developed software SearchCal
that builds an evolutive catalog of stars suitable as calibrators within any
given user-defined angular distance and magnitude around the scientific target.
We present the first version of SearchCal dedicated to the bright-object case
V<=10; K<=5). Star catalogs available at the CDS are consulted via web
requests. They provide all the useful information for selecting of calibrators.
Missing photometries are computed with an accuracy of 0.1 mag and the missing
angular diameters are calculated with a precision better than 10%. For each
star the squared visibility is computed by taking the wavelength and the
maximum baseline of the foreseen observation into account.} SearchCal is
integrated into ASPRO, the interferometric observing preparation software
developed by the JMMC, available at the address: http://mariotti.fr
Evidence of an asymmetrical Keplerian disk in the Br{\gamma} and He I emission lines around the Be star HD 110432
Context. HD 110432 was classified as a "\gamma Cas X-ray analog" since it has
similar peculiar X-ray and optical characteristics, i.e. a hard-thermal X-ray
variable emission and an optical spectrum affected by an extensive disk. Lopes
de Oliveira et al. (2007) suggest that it might be a Be star harboring an
accreting white dwarf or that the X-rays may come from an interaction between
the surface of the star and its disk. Aims. To investigate the disk around this
Be star we used the VLTI/AMBER instrument, which combines high spectral
(R=12000) and high spatial (\theta min =4 mas) resolutions. Methods. We
constrain the geometry and kinematics of its circumstellar disk from the
highest spatial resolution ever achieved on this star. Results. We obtain a
disk extension in the Br{\gamma} line of 10.2 D\ast and 7.8 D\ast in the He I
line at 2.05 \mu m assuming a Gaussian disk model. The disk is clearly
following a Keplerian rotation. We obtained an inclination angle of 55\degree,
and the star is a nearly critical rotator with Vrot /Vc =1.000.2. This
inclination is greater than the value found for \gamma Cas (about 42\degree,
Stee et al. 2012), and is consistent with the inference from optical Fe II
emission profiles by Smith & Balona (2006) that the inclination should be more
than the \gamma Cas value. In the near-IR continuum, the disk of HD 110432 is 3
times larger than \gamma Cas's disk. We have no direct evidence of a companion
around HD 110432, but it seems that we have a clear signature for disk
inhomogeneities as detected for {\zeta} Tau. This asymmetrical disk detection
may be interpreted within the one-armed oscillation viscous disk framework.
Another finding is that the disk size in the near-IR is similar to other Be
stars with different spectral types and thus may be independent of the stellar
parameters, as found for classical Be stars.Comment: 9 page
The inner circumstellar disk of the UX Ori star V1026 Sco
The UX Ori type variables (named after the prototype of their class) are
intermediate-mass pre-main sequence objects. One of the most likely causes of
their variability is the obscuration of the central star by orbiting dust
clouds. We investigate the structure of the circumstellar environment of the
UX~Ori star V1026 Sco (HD 142666) and test whether the disk inclination is
large enough to explain the UX Ori variability. We observed the object in the
low-resolution mode of the near-infrared interferometric VLTI/AMBER instrument
and derived H- and K-band visibilities and closure phases. We modeled our AMBER
observations, published Keck Interferometer observations, archival MIDI/VLTI
visibilities, and the spectral energy distribution using geometric and
temperature-gradient models. Employing a geometric inclined-ring disk model, we
find a ring radius of 0.15 +- 0.06 AU in the H band and 0.18 +- 0.06 AU in the
K band. The best-fit temperature-gradient model consists of a star and two
concentric, ring-shaped disks. The inner disk has a temperature of
1257^{+133}_{-53} K at the inner rim and extends from 0.19 +- 0.01 AU to 0.23
+- 0.02 AU. The outer disk begins at 1.35^{+0.19}_{-0.20} AU and has an inner
temperature of 334^{+35}_{-17} K. The derived inclination of
48.6^{+2.9}_{-3.6}deg approximately agrees with the inclination derived with
the geometric model (49 +- 5deg in the K band and 50 +- 11deg in the H band).
The position angle of the fitted geometric and temperature-gradient models are
163 +- 9deg (K band; 179 +- 17deg in the H band) and 169.3^{+4.2}_{-6.7}deg,
respectively. The narrow width of the inner ring-shaped model disk and the disk
gap might be an indication for a puffed-up inner rim shadowing outer parts of
the disk. The intermediate inclination of ~50deg is consistent with models of
UX Ori objects where dust clouds in the inclined disk obscure the central star
User-friendly imaging algorithms for interferometry
OPTICON currently supports a Joint Research Activity (JRA) dedicated to providing easy to use image reconstruction algorithms for optical/IR interferometric data. This JRA aims to provide state-of-the-art image reconstruction methods with a common interface and comprehensive documentation to the community. These tools will provide the capability to compare the results of using different settings and algorithms in a consistent and unified way. The JRA is also providing tutorials and sample datasets to introduce the principles of image reconstruction and illustrate how to use the software products. We describe the design of the imaging tools, in particular the interface between the graphical user interface and the image reconstruction algorithms, and summarise the current status of their implementation.European Community’s Seventh Framework Programme (FP7/2013–2016) (Grant ID: 312430 (OPTICON))This is the author accepted manuscript. The final version is available from SPIE via http://dx.doi.org/10.1117/12.223338
Keck Adaptive Optics Imaging of Nearby Young Stars: Detection of Close Multiple Systems
Using adaptive optics on the Keck II 10-meter telescope on Mauna Kea, we have
surveyed 24 of the nearest young stars known in search of close companions. Our
sample includes members of the MBM 12 and TW Hydrae young associations and the
classical T Tauri binary UY Aurigae in the Taurus star-forming region. We
present relative photometry and accurate astrometry for 10 close multiple
systems. The multiplicity frequency in the TW Hydrae and MBM 12 groups are high
in comparison to other young regions, though the significance of this result is
low because of the small number statistics. We resolve S 18 into a triple
system including a tight 63 mas (projected separation of 17 AU at a distance of
275 pc) binary for the first time, with a hierarchical configuration
reminiscent of VW Chamaeleontis and T Tauri. Another tight binary in our sample
-- TWA 5Aab (54 mas or 3 AU at 55 pc) -- offers the prospect of dynamical mass
measurement using astrometric observations within a few years, and thus could
be important for testing pre-main sequence evolutionary models. Our
observations confirm with 9-sigma confidence that the brown dwarf TWA 5B is
bound to TWA 5A. We find that the flux ratio of UY Aur has changed
dramatically, by more than a magnitude in the H-band, possibly as a result of
variable extinction. With a smaller flux ratio, the system may once again
become detectable as an optical binary, as it was at the time of its discovery
in 1944. Taken together, our results demonstrate that adaptive optics on large
telescopes is a powerful tool for detecting tight companions, and thus
exploring the frequency and configurations of close multiple systems.Comment: accepted for publication in The Astronomical Journa
Continuum and CO/HCO+ Emission from the Disk Around the T Tauri Star LkCa 15
We present OVRO Millimeter Array lambda = 3.4 - 1.2 mm dust continuum and
spectral line observations of the accretion disk encircling the T Tauri star
LkCa 15. The 1.2 mm dust continuum emission is resolved, and gives a minimum
diameter of 190 AU and an inclination angle of 57+/-5 degrees. There is a
noticeable, but at present poorly constrained, decrease in the continuum
spectral slope with frequency that may result from the coupled processes of
grain growth and dust settling. Imaging of the fairly intense emission from the
lowest rotational transitions of CO, 13CO and HCO+ reveals a rotating disk and
emission extends to 750 AU and the characteristic radius of the disk is
determined to be around 425 AU (HWHM) based on model fits to the CO velocity
field. The disk mass derived from the CO isotopologues with ``typical'' dense
cloud abundances is still nearly two orders of magnitude less than that
inferred from the dust emission, which is probably due to extensive molecular
depletion in the cold, dense disk midplane. N2H+ 1-0 emission has also been
detected which, along with HCO+, sets a lower limit to the fractional
ionization of 10^{-8} in the near-surface regions of protoplanetary disks. This
first detection of N2H+ in circumstellar disks has also made possible a
determination of the N2/CO ratio (~2) that is at least an order of magnitude
larger than those in the envelopes of young stellar objects and dense clouds.
The large N2/CO ratio indicates that our observations probe disk layers in
which CO is depleted but some N2 remains in the gas phase. Such differential
depletion can lead to large variations in the fractional ionization with height
in the outer reaches of circumstellar disks, and may help to explain the
relative nitrogen deficiency observed in comets.Comment: Submitted to ApJ, 28 pages, 7 figure
Panchromatic observations and modeling of the HV Tau C edge-on disk
We present new high spatial resolution (<~ 0.1") 1-5 micron adaptive optics
images, interferometric 1.3 mm continuum and 12CO 2-1 maps, and 350 micron, 2.8
and 3.3 mm fluxes measurements of the HV Tau system. Our adaptive optics images
reveal an unusually slow orbital motion within the tight HV Tau AB pair that
suggests a highly eccentric orbit and/or a large deprojected physical
separation. Scattered light images of the HV Tau C edge-on protoplanetary disk
suggest that the anisotropy of the dust scattering phase function is almost
independent of wavelength from 0.8 to 5 micron, whereas the dust opacity
decreases significantly over the same range. The images further reveal a marked
lateral asymmetry in the disk that does not vary over a timescale of 2 years.
We further detect a radial velocity gradient in the disk in our 12CO map that
lies along the same position angle as the elongation of the continuum emission,
which is consistent with Keplerian rotation around an 0.5-1 Msun central star,
suggesting that it could be the most massive component in the triple system. We
use a powerful radiative transfer model to compute synthetic disk observations
and use a Bayesian inference method to extract constraints on the disk
properties. Each individual image, as well as the spectral energy distribution,
of HV Tau C can be well reproduced by our models with fully mixed dust provided
grain growth has already produced larger-than-interstellar dust grains.
However, no single model can satisfactorily simultaneously account for all
observations. We suggest that future attempts to model this source include more
complex dust properties and possibly vertical stratification. (Abridged)Comment: 26 pages, 11 figures, editorially accepted for publication in Ap
Dynamical Masses of T Tauri Stars and Calibration of PMS Evolution
We have used the high sensitivity and resolution of the IRAM interferometer
to produce sub-arcsecond 12CO 2-1 images of 9 protoplanetary disks surrounding
T Tauri stars in the Taurus-Auriga cloud (7 singles and 2 binaries). The images
demonstrate the disks are in Keplerian rotation around their central stars.
Using the least square fit method described in Guilloteau and Dutrey (1998), we
derive the disk properties, in particular its inclination angle and rotation
velocity, hence the dynamical mass. Since the disk mass is usually small, this
is a direct measurement of the stellar mass. Typically, we reach an internal
precision of 10% in the determinations of stellar mass. The over-all accuracy
is limited by the uncertainty in the distance to a specific star. In a distance
independent way, we compare the derived masses with theoretical tracks of
pre-main-sequence evolution. Combined with the mean distance to the Taurus
region (140 pc), for stars with mass close to 1 Msun, our results tend to favor
the tracks with cooler photospheres (higher masses for a given spectral type).
We find that in UZ Tau E the disk and the spectroscopic binary orbit appear to
have different inclinations.Comment: 32 pages, 5 figure
Circumstellar Dust Disks in Taurus-Auriga: The Submillimeter Perspective
We present a sensitive, multiwavelength submillimeter continuum survey of 153
young stellar objects in the Taurus-Auriga star formation region. The
submillimeter detection rate is 61% to a completeness limit of ~10 mJy
(3-sigma) at 850 microns. The inferred circumstellar disk masses are
log-normally distributed with a mean mass of ~0.005 solar masses and a large
dispersion (0.5 dex). Roughly one third of the submillimeter sources have disk
masses larger than the minimal nebula from which the solar system formed. The
median disk to star mass ratio is 0.5%. The empirical behavior of the
submillimeter continuum is best described as F_nu ~ nu^(2.0 +/- 0.5) between
350 microns and 1.3 mm, which we argue is due to the combined effects of the
fraction of optically thick emission and a flatter frequency behavior of the
opacity compared to the ISM. This latter effect could be due to a substantial
population of large dust grains, which presumably would have grown through
collisional agglomeration. In this sample, the only stellar property that is
correlated with the outer disk is the presence of a companion. We find evidence
for significant decreases in submillimeter flux densities, disk masses, and
submillimeter continuum slopes along the canonical infrared spectral energy
distribution evolution sequence for young stellar objects. The fraction of
objects detected in the submillimeter is essentially identical to the fraction
with excess near-infrared emission, suggesting that dust in the inner and outer
disk are removed nearly simultaneously.Comment: accepted by Ap
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