1,023 research outputs found
VLTI observations of IRS~3: The brightest compact MIR source at the Galactic Centre
The dust enshrouded star IRS~3 in the central light year of our galaxy was
partially resolved in a recent VLTI experiment. The presented observation is
the first step in investigating both IRS~3 in particular and the stellar
population of the Galactic Centre in general with the VLTI at highest angular
resolution. We will outline which scientific issues can be addressed by a
complete MIDI dataset on IRS~3 in the mid infrared.Comment: 4 pages, 3 figures, published in: The ESO Messenge
Subarcsecond Submillimeter Imaging of the Ultracompact HII Region G5.89-0.39
We present the first subarcsecond submillimeter images of the enigmatic
ultracompact HII region (UCHII) G5.89-0.39. Observed with the SMA, the 875
micron continuum emission exhibits a shell-like morphology similar to longer
wavelengths. By using images with comparable angular resolution at five
frequencies obtained from the VLA archive and CARMA, we have removed the
free-free component from the 875 micron image. We find five sources of dust
emission: two compact warm objects (SMA1 and SMA2) along the periphery of the
shell, and three additional regions further out. There is no dust emission
inside the shell, supporting the picture of a dust-free cavity surrounded by
high density gas. At subarcsecond resolution, most of the molecular gas tracers
encircle the UCHII region and appear to constrain its expansion. We also find
G5.89-0.39 to be almost completely lacking in organic molecular line emission.
The dust cores SMA1 and SMA2 exhibit compact spatial peaks in optically-thin
gas tracers (e.g. 34SO2), while SMA1 also coincides with 11.9 micron emission.
In CO(3-2), we find a high-velocity north/south bipolar outflow centered on
SMA1, aligned with infrared H2 knots, and responsible for much of the maser
activity. We conclude that SMA1 is an embedded intermediate mass protostar with
an estimated luminosity of 3000 Lsun and a circumstellar mass of ~1 Msun.
Finally, we have discovered an NH3 (3,3) maser 12 arcsec northwest of the UCHII
region, coincident with a 44 GHz CH3OH maser, and possibly associated with the
Br gamma outflow source identified by Puga et al. (2006).Comment: 41 pages, 11 figures, published in The Astrophysical Journal (2008)
Volume 680, Issue 2, pp. 1271-1288. An error in the registration of the
marker positions in Figure 11 has been corrected in this versio
The VLT/NaCo Large program to probe the occurrence of exoplanets and brown dwarfs in wide orbits: I- Sample definition and characterization
Young, nearby stars are ideal targets to search for planets using the direct
imaging technique. The determination of stellar parameters is crucial for the
interpretation of imaging survey results particularly since the luminosity of
substellar objects has a strong dependence on system age. We have conducted a
large program with NaCo at the VLT in order to search for planets and brown
dwarfs in wide orbits around 86 stars. A large fraction of the targets observed
with NaCo were poorly investigated in the literature. We performed a study to
characterize the fundamental properties (age, distance, mass) of the stars in
our sample. To improve target age determinations, we compiled and analyzed a
complete set of age diagnostics. We measured spectroscopic parameters and age
diagnostics using dedicated observations acquired with FEROS and CORALIE
spectrographs at La Silla Observatory. We also made extensive use of archival
spectroscopic data and results available in the literature. Additionally, we
exploited photometric time-series, available in ASAS and Super-WASP archives,
to derive rotation period for a large fraction of our program stars. We
provided updated characterization of all the targets observed in the VLT NaCo
Large program, a survey designed to probe the occurrence of exoplanets and
brown dwarfs in wide orbits. The median distance and age of our program stars
are 64 pc and 100 Myr, respectively. Nearly all the stars have masses between
0.70 and 1.50sun, with a median value of 1.01 Msun. The typical metallicity is
close to solar, with a dispersion that is smaller than that of samples usually
observed in radial velocity surveys. Several stars are confirmed or proposed
here to be members of nearby young moving groups. Eight spectroscopic binaries
are identified.Comment: 64 pages with Appendix, 15 figures, accepted to A&
Shadows cast on the transition disk of HD 135344B. Multiwavelength VLT/SPHERE polarimetric differential imaging
The protoplanetary disk around the F-type star HD 135344B (SAO 206462) is in
a transition stage and shows many intriguing structures both in scattered light
and thermal (sub-)millimeter emission which are possibly related to planet
formation processes. We study the morphology and surface brightness of the disk
in scattered light to gain insight into the innermost disk regions, the
formation of protoplanets, planet-disk interactions traced in the surface and
midplane layers, and the dust grain properties of the disk surface. We have
carried out high-contrast polarimetric differential imaging (PDI) observations
with VLT/SPHERE and obtained polarized scattered light images with ZIMPOL in R-
and I-band and with IRDIS in Y- and J-band. The scattered light images reveal
with unprecedented angular resolution and sensitivity the spiral arms as well
as the 25 au cavity of the disk. Multiple shadow features are discovered on the
outer disk with one shadow only being present during the second observation
epoch. A positive surface brightness gradient is observed in the stellar
irradiation corrected images in southwest direction possibly due to an
azimuthally asymmetric perturbation of the temperature and/or surface density
by the passing spiral arms. The disk integrated polarized flux, normalized to
the stellar flux, shows a positive trend towards longer wavelengths which we
attribute to large aggregate dust grains in the disk surface. Part of the the
non-azimuthal polarization signal in the Uphi image of the J-band observation
could be the result of multiple scattering in the disk. The detected shadow
features and their possible variability have the potential to provide insight
into the structure of and processes occurring in the innermost disk regions.Comment: Accepted for publication in A&A, 20 pages, 15 figure
Characterizing HR3549B using SPHERE
Aims. In this work, we characterize the low mass companion of the A0 field
star HR3549. Methods. We observed HR3549AB in imaging mode with the the NIR
branch (IFS and IRDIS) of SPHERE@VLT, with IFS in YJ mode and IRDIS in the H
band. We also acquired a medium resolution spectrum with the IRDIS long slit
spectroscopy mode. The data were reduced using the dedicated SPHERE GTO
pipeline, purposely designed for this instrument. We employed algorithms such
as PCA and TLOCI to reduce the speckle noise. Results. The companion was
clearly visible both with IRDIS and IFS.We obtained photometry in four
different bands as well as the astrometric position for the companion. Based on
our astrometry, we confirm that it is a bound object and put constraints on its
orbit. Although several uncertainties are still present, we estimate an age of
~100-150 Myr for this system, yielding a most probable mass for the companion
of 40-50MJup and T_eff ~300-2400 K. Comparing with template spectra points to a
spectral type between M9 and L0 for the companion, commensurate with its
position on the color-magnitude diagram.Comment: Accepted by A&A, 13 pages, 10 Figures (Figures 9 and 10 degraded to
reduce the dimension
Discovery of the Coldest Imaged Companion of a Sun-Like Star
We present the discovery of a brown dwarf or possible planet at a projected
separation of 1.9" = 29 AU around the star GJ 758, placing it between the
separations at which substellar companions are expected to form by core
accretion (~5 AU) or direct gravitational collapse (typically >100 AU). The
object was detected by direct imaging of its thermal glow with Subaru/HiCIAO.
At 10-40 times the mass of Jupiter and a temperature of 550-640 K, GJ 758 B
constitutes one of the few known T-type companions, and the coldest ever to be
imaged in thermal light around a Sun-like star. Its orbit is likely eccentric
and of a size comparable to Pluto's orbit, possibly as a result of
gravitational scattering or outward migration. A candidate second companion is
detected at 1.2" at one epoch.Comment: 5 pages, 3 figures, 2 tables. Accepted for publication in ApJ Letter
The Transition from Atomic to Molecular Hydrogen in Interstellar Clouds: 21cm Signature of the Evolution of Cold Atomic Hydrogen in Dense Clouds
We have investigated the time scale for formation of molecular clouds by
examining the conversion of HI to H2 using a time-dependent model. H2 formation
on dust grains and cosmic ray and photo destruction are included in
one-dimensional model slab clouds which incorporate time-independent density
and temperature distributions. We calculate 21cm spectral line profiles seen in
absorption against a background provided by general Galactic HI emission, and
compare the model spectra with HI Narrow Self-Absorption, or HINSA, profiles
absorbed in a number of nearby molecular clouds. The time evolution of the HI
and H2 densities is dramatic, with the atomic hydrogen disappearing in a wave
propagating from the central, denser regions which have a shorter H2 formation
time scale, to the edges, where the density is lower and the time scale for H2
formation longer. The model 21cm spectra are characterized by very strong
absorption at early times, when the HI column density through the model clouds
is extremely large. The minimum time required for a cloud to have evolved to
its observed configuration, based on the model spectra, is set by the
requirement that most of the HI in the outer portions of the cloud, which
otherwise overwhelms the narrow absorption, be removed. The characteristic time
that has elapsed since cloud compression and initiation of the HI to H2
conversion is a few x 10^{14} s or ~ 10^7 yr. This sets a minimum time for the
age of these molecular clouds and thus for the star formation that may take
place within them
The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs at wide orbits: II- Survey description, results and performances
In anticipation of the VLT/SPHERE planet imager guaranteed time programs, we
have conducted a preparatory survey of 86 stars between 2009 and 2013 in order
to identify new faint comoving companions to ultimately carry out a
comprehensive analysis of the occurence of giant planets and brown dwarf
companions at wide (10-2000 AU) orbits around young, solar-type stars. We used
NaCo at VLT to explore the occurrence rate of giant planets and brown dwarfs
between typically 0.1 and 8''. Diffraction-limited observations in H-band
combined with angular differential imaging enabled us to reach primary
star-companion brightness ratios as small as 10-6 at 1.5''. 12 systems were
resolved as new binaries, including the discovery of a new white dwarf
companion to the star HD8049. Around 34 stars, at least one companion candidate
was detected in the observed field of view. More than 400 faint sources were
detected, 90% of them in 4 crowded fields. With the exception of HD8049B, we
did not identify any new comoving companions. The survey also led to spatially
resolved images of the thin debris disk around HD\,61005 that have been
published earlier. Finally, considering the survey detection limits, we derive
a preliminary upper limit on the frequency of giant planets for semi-major axes
of [10,2000] AU: typically less than 15% between 100 and 500 AU, and less than
10% between 50 and 500 AU for exoplanets more massive than 5 MJup and 10 MJup
respectively, considering a uniform input distribution and with a confidence
level of 95%.Comment: 19 pages, 8 figures, 12 Tables, accepted to A&
- …