715 research outputs found
Detection of a sub-arcsecond dust shell around the Wolf-Rayet star WR 112
A lunar occultation event of the Wolf-Rayet star WR 112 (type WC9) has been
observed simultaneously from two independent telescopes at lambda = 2.2microns,
allowing us to investigate this source with an angular resolution of approx
0.003 arc-seconds. We have detected a circumstellar dust envelope whose
brightness distribution can be approximately fitted by a gaussian with a FWHM
of approx 0.06 arc-seconds (approx 10^15 cm). We present and discuss the
reconstructed brightness profile, which shows an asymmetry in the radial dust
distribution. The derived dust grain temperature at the inner dust zone of
approx 1150 K is consistent with available model calculations. There is no
signature of the central star from our observations, providing a direct
confirmation that the circumstellar shell emission dominates over the
photospheric emission at 2.2microns as predicted by fits to the spectral energy
distribution. Further lunar occultation observations at different position
angles are essential to reconstruct the 2--D image of the dust shell around WR
112. The current series of lunar occultations of WR 112 will continue to the
end of 1999 and will be visible for all equatorial and southern latitude
observatories.Comment: 5 pages, 1 figur
First result with AMBER+FINITO on the VLTI: The high-precision angular diameter of V3879 Sgr
Our goal is to demonstrate the potential of the interferometric AMBER
instrument linked with the Very Large Telescope Interferometer (VLTI)
fringe-tracking facility FINITO to derive high-precision stellar diameters. We
use commissioning data obtained on the bright single star V3879 Sgr. Locking
the interferometric fringes with FINITO allows us to record very low contrast
fringes on the AMBER camera. By fitting the amplitude of these fringes, we
measure the diameter of the target in three directions simultaneously with an
accuracy of 25 micro-arcseconds. We showed that V3879 Sgr has a round
photosphere down to a sub-percent level. We quickly reached this level of
accuracy because the technique used is independent from absolute calibration
(at least for baselines that fully span the visibility null). We briefly
discuss the potential biases found at this level of precision. The proposed
AMBER+FINITO instrumental setup opens several perspectives for the VLTI in the
field of stellar astrophysics, like measuring with high accuracy the oblateness
of fast rotating stars or detecting atmospheric starspots
Evidence of asymmetries in the Aldebaran photosphere from multiwavelength lunar occultations
We have recorded three lunar occultations of Aldebaran (α Tau) at different telescopes and using various band-passes, from the ultraviolet to the far red. The data have been analysed using both model-dependent and model-independent methods. The derived uniform-disc angular diameter values have been converted to limb-darkened values using model atmosphere relations and are found in broad agreement among themselves and with previous literature values. The limb-darkened diameter is about 20.3 mas on average. However, we have found indications that the photospheric brightness profile of Aldebaran may have not been symmetric, a finding already reported by other authors for this and for similar late-type stars. At the sampling scale of our brightness profile, between 1 and 2 mas, the uniform and limb-darkened disc models may not be a good description for Aldebaran. The asymmetries appear to differ with wavelength and over the 137-d time span of our measurements. Surface spots appear as a likely explanation for the differences between observations and the models
First Lunar Occultation Results from the 2.4 m Thai National Telescope equipped with ULTRASPEC
The recently inaugurated 2.4\,m Thai National Telescope (TNT) is equipped,
among other instruments, with the ULTRASPEC low-noise, frame-transfer EMCCD
camera. At the end of its first official observing season, we report on the use
of this facility to record high time resolution imaging using small detector
subarrays with sampling as fast as several \,Hz. In particular, we have
recorded lunar occultations of several stars which represent the first
contribution to this area of research made from South-East Asia with a
telescope of this class. Among the results, we discuss an accurate measurement
of ~Cnc, which has been reported previously as a suspected close
binary. Attempts to resolve this star by several authors have so far met with a
lack of unambiguous confirmation. With our observation we are able to place
stringent limits on the projected angular separation (<0\farcs003) and
brightness () of a putative companion. We also present a
measurement of the binary {HR~7072}, which extends considerably the time
coverage available for its yet undetermined orbit. We discuss our precise
determination of the flux ratio and projected separation in the context of
other available data. We conclude by providing an estimate of the performance
of ULTRASPEC at TNT for lunar occultation work. This facility can help to
extend the lunar occultation technique in a geographical area where no
comparable resources were available until now.Comment: Accepted for publication in Astronomical Journa
Novel synthetic approach to heteroatom doped polycyclic aromatic hydrocarbons: Optimizing the bottom-up approach to atomically precise doped nanographenes
The success of the rational bottom-up approach to nanostructured carbon materials and the discovery of the importance of their doping with heteroatoms puts under the spotlight all synthetic organic approaches to polycyclic aromatic hydrocarbons. The construction of atomically precise heteroatom doped nanographenes has evidenced the importance of controlling its geometry and the position of the doping heteroatoms, since these parameters influence their chemical–physical properties and their applications. The growing interest towards this research topic is testified by the large number of works published in this area, which have transformed a once “fundamental research” into applied research at the cutting edge of technology. This review analyzes the most recent synthetic approaches to this class of compounds
Limb-Darkening of a K Giant in the Galactic Bulge: PLANET Photometry of MACHO 97-BLG-28
We present the PLANET photometric dataset for the binary-lens microlensing
event MACHO 97-BLG-28 consisting of 696 I and V-band measurements, and analyze
it to determine the radial surface brightness profile of the Galactic bulge
source star. The microlensed source, demonstrated to be a K giant by our
independent spectroscopy, crossed the central isolated cusp of the lensing
binary, generating a sharp peak in the light curve that was well-resolved by
dense (3 - 30 minute) and continuous monitoring from PLANET sites in Chile,
South Africa, and Australia. Our modeling of these data has produced stellar
profiles for the source star in the I and V bands that are in excellent
agreement with those predicted by stellar atmospheric models for K giants. The
limb-darkening coefficients presented here are the first derived from
microlensing, among the first for normal giants by any technique, and the first
for any star as distant as the Galactic bulge. Modeling indicates that the
lensing binary has a mass ratio q = 0.23 and an (instantaneous) separation in
units of the angular Einstein ring radius of d = 0.69 . For a lens in the
Galactic bulge, this corresponds to a typical stellar binary with a projected
separation between 1 and 2 AU. If the lens lies closer, the separation is
smaller, and one or both of the lens objects is in the brown dwarf regime.
Assuming that the source is a bulge K2 giant at 8 kpc, the relative lens-source
proper motion is mu = 19.4 +/- 2.6 km/s /kpc, consistent with a disk or bulge
lens. If the non-lensed blended light is due to a single star, it is likely to
be a young white dwarf in the bulge, consistent with the blended light coming
from the lens itself.Comment: 32 Pages, including 1 table and 9 postscript figures. (Revised
version has slightly modified text, corrected typo, and 1 new figure.)
Accepted for publication in 1999 Astrophysical Journal; data are now
available at http://www.astro.rug.nl/~plane
Angular Radii of Stars via Microlensing
We outline a method by which the angular radii of giant and main sequence
stars in the Galactic bulge can be measured to a few percent accuracy. The
method combines ground-based photometry of caustic-crossing bulge microlensing
events, with a handful of precise astrometric measurements of the lensed star
during the event, to measure the angular radius of the source, theta_*. Dense
photometric coverage of one caustic crossing yields the crossing timescale dt.
Less frequent coverage of the entire event yields the Einstein timescale t_E
and the angle phi of source trajectory with respect to the caustic. The
photometric light curve solution predicts the motion of the source centroid up
to an orientation on the sky and overall scale. A few precise astrometric
measurements therefore yield theta_E, the angular Einstein ring radius. Then
the angular radius of the source is obtained by theta_*=theta_E(dt/t_E)
sin(phi). We argue that theta_* should be measurable to a few percent accuracy
for Galactic bulge giant stars using ground-based photometry from a network of
small (1m-class) telescopes, combined with astrometric observations with a
precision of ~10 microarcsec to measure theta_E. We find that a factor of ~50
times fewer photons are required to measure theta_E to a given precision for
binary-lens events than single-lens events. Adopting parameters appropriate to
the Space Interferometry Mission (SIM), ~7 min of SIM time is required to
measure theta_E to ~5% accuracy for giant sources in the bulge. For
main-sequence sources, theta_E can be measured to ~15% accuracy in ~1.4 hours.
With 10 hrs of SIM time, it should be possible to measure theta_* to ~5% for
\~80 giant stars, or to 15% for ~7 main sequence stars. A byproduct of such a
campaign is a significant sample of precise binary-lens mass measurements.Comment: 13 pages, 3 figures. Revised version, minor changes, required SIM
integration times revised upward by ~60%. Accepted to ApJ, to appear in the
March 20, 2003 issue (v586
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