914 research outputs found
Hunting for brown dwarf binaries and testing atmospheric models with X-Shooter
The determination of the brown dwarf binary fraction may contribute to the
understanding of the substellar formation mechanisms. Unresolved brown dwarf
binaries may be revealed through their peculiar spectra or the discrepancy
between optical and near-infrared spectral type classification.
We obtained medium-resolution spectra of 22 brown dwarfs with these
characteristics using the X-Shooter spectrograph at the VLT.
We aimed to identify brown dwarf binary candidates, and to test if the
BT-Settl 2014 atmospheric models reproduce their observed spectra.
To find binaries spanning the L-T boundary, we used spectral indices and
compared the spectra of the selected candidates to single spectra and synthetic
binary spectra. We used synthetic binary spectra with components of same
spectral type to determine as well the sensitivity of the method to this class
of binaries.
We identified three candidates to be combination of L plus T brown dwarfs. We
are not able to identify binaries with components of similar spectral type. In
our sample, we measured minimum binary fraction of .
From the best fit of the BT-Settl models 2014 to the observed spectra, we
derived the atmospheric parameters for the single objects. The BT-Settl models
were able to reproduce the majority of the SEDs from our objects, and the
variation of the equivalent width of the RbI (794.8 nm) and CsI (852.0 nm)
lines with the spectral type. Nonetheless, these models did not reproduce the
evolution of the equivalent widths of the NaI (818.3 nm and 819.5 nm) and KI
(1253 nm) lines with the spectral type.Comment: Accepted for publication in MNRA
The near-infrared spectral energy distribution of {\beta} Pictoris b
A gas giant planet has previously been directly seen orbiting at 8-10 AU
within the debris disk of the ~12 Myr old star {\beta} Pictoris. The {\beta}
Pictoris system offers the rare opportunity to study the physical and
atmospheric properties of an exoplanet placed on a wide orbit and to establish
its formation scenario. We obtained J (1.265 {\mu}m), H (1.66 {\mu}m), and M'
(4.78 {\mu}m) band angular differential imaging of the system between 2011 and
2012. We detect the planetary companion in our four-epoch observations. We
estimate J = 14.0 +- 0.3, H = 13.5 +- 0.2, and M' = 11.0 +- 0.3 mag. Our new
astrometry consolidates previous semi-major axis (sma=8-10 AU) and excentricity
(e <= 0.15) estimates of the planet. These constraints, and those derived from
radial velocities of the star provides independent upper limits on the mass of
{\beta} Pictoris b of 12 and 15.5 MJup for semi-major axis of 9 and 10 AU. The
location of {\beta} Pictoris b in color-magnitude diagrams suggests it has
spectroscopic properties similar to L0-L4 dwarfs. This enables to derive
Log10(L/Lsun) = -3.87 +- 0.08 for the companion. The analysis with 7
PHOENIX-based atmospheric models reveals the planet has a dusty atmosphere with
Teff = 1700 +- 100 K and log g = 4.0+- 0.5. "Hot-start" evolutionary models
give a new mass of 10+3-2 MJup from Teff and 9+3-2 MJup from luminosity.
Predictions of "cold-start" models are inconsistent with independent
constraints on the planet mass. "Warm-start" models constrain the mass to M >=
6MJup and the initial entropies to values (Sinit >= 9.3Kb/baryon), intermediate
between those considered for cold/hot-start models, but likely closer to those
of hot-start models.Comment: 19 pages, accepted in Astronomy and Astrophysic
A brown dwarf companion to the intermediate-mass star HR6037
In the course of an imaging survey we have detected a visual companion to the
intermediate-mass star HR 6037. In this letter, we present two epoch
observations of the binary with NACO/VLT, and near-IR spectroscopy of the
secondary with ISAAC/VLT. The NACO observations allow us to confirm HR 6037B as
a co-moving companion. Its J and H band ISAAC spectra suggest the object has an
spectral type of M9+-1, with a surface gravity intermediate between that of 10
Myr dwarfs and field dwarfs with identical spectral type. The comparison of its
Ks-band photometry with evolutionary tracks allows us to derive a mass,
effective temperature, and surface gravity of 62+-20 MJup, Teff = 2330+-200 K,
and log g = 5.1+-0.2, respectively. The small mass ratio of the binary, -0.03,
and its long orbital period, -5000 yr, makes HR 6037 a rare and uncommon binary
system.Comment: (5 pages, 4 figures, accepted for publication in A&A Letters
OTS44: Disk and accretion at the planetary border
We discover that the very low-mass brown dwarf OTS44 (M9.5, ~12 M_Jup) has
significant accretion and a substantial disk, which demonstrates that the
processes that accompany canonical star formation occur down to a central mass
of a few Jupiter masses. We discover in VLT/SINFONI spectra that OTS44 has
strong, broad, and variable Paschen beta emission that is evidence for active
accretion at the planetary border. We also detect strong Halpha emission of
OTS44 in a literature spectrum and determine an Halpha EW (-141 A) that is
indicative of active accretion. Both the Pa beta and Halpha emission lines have
broad profiles with wings extending to velocities of about +/-200 km/s. We
determine the mass accretion rate of OTS44 based on Halpha to 7.6x10^{-12}
Msun/yr, which shows that OTS44 has a relatively high mass-accretion rate
considering its small central mass. This mass rate is nevertheless consistent
with the general decreasing trend found for stars of several solar masses down
to brown dwarfs. Furthermore, we determine the properties of the disk
surrounding OTS44 through radiative transfer modeling of flux measurement from
the optical to the far-IR (Herschel) by applying a Bayesian analysis. We find
that OTS44 has a highly flared disk (beta >1.2) with a mass of 9.1x10^{-5}
M_Sun, i.e. about 0.1 M_Jup or 30 M_Earth. We show that the ratio of
disk-to-central-mass of about 10^{-2} found for objects between 0.03 Msun and
14 Msun is also valid for OTS44 at a mass of ~0.01 M_Sun. Our observations are
in line with an isolated star-like mode of the formation of brown dwarfs down
to 0.01 M_Sun.Comment: Accepted for publication in Astronomy and Astrophysics Lette
Orbital characterization of the \beta Pictoris b giant planet
In June 2010, we confirmed the existence of a giant planet in the disk of the
young star Beta Pictoris, located between 8 AU and 15 AU from the star. This
young planet offers the rare opportunity to monitor a large fraction of the
orbit using the imaging technique over a reasonably short timescale. Using the
NAOS-CONICA adaptive-optics instrument (NACO) at the Very Large Telescope
(VLT), we obtained repeated follow-up images of the Bpic system in the Ks and
L' filters at four new epochs in 2010 and 2011. Complementing these data with
previous measurements, we conduct a homogeneous analysis, which covers more
than eight yrs, to accurately monitor the Bpic b position relative to the star.
On the basis of the evolution of the planet's relative position with time, we
derive the best-fit orbital solutions for our measurements. More reliable
results are found with a Markov-chain Monte Carlo approach. The solutions favor
a low-eccentricity orbit e < 0.17, with semi-major axis in the range 8--9 AU
corresponding to orbital periods of 17--21 yrs. Our solutions favor a highly
inclined solution with a peak around i=88.5+-1.7 deg, and a longitude of
ascending node tightly constrained at Omega = -147.5+-1.5 deg. These results
indicate that the orbital plane of the planet is likely to be above the
midplane of the main disk, and compatible with the warp component of the disk
being tilted between 3.5 deg and 4.0 deg. This suggests that the planet plays a
key role in the origin of the inner warped-disk morphology of the Bpic disk.
Finally, these orbital parameters are consistent with the hypothesis that the
planet is responsible for the transit-like event observed in November 1981, and
also linked to the cometary activity observed in the Bpic system.Comment: 10 pages, 12 figures, accepted to A&
Discovery of a probable 4-5 Jupiter-mass exoplanet to HD 95086 by direct-imaging
Direct imaging has just started the inventory of the population of gas giant
planets on wide-orbits around young stars in the solar neighborhood. Following
this approach, we carried out a deep imaging survey in the near-infrared using
VLT/NaCo to search for substellar companions. We report here the discovery in
L' (3.8 microns) images of a probable companion orbiting at 56 AU the young
(10-17 Myr), dusty, and early-type (A8) star HD 95086. This discovery is based
on observations with more than a year-time-lapse. Our first epoch clearly
revealed the source at 10 sigma while our second epoch lacked good observing
conditions hence yielding a 3 sigma detection. Various tests were thus made to
rule out possible artifacts. This recovery is consistent with the signal at the
first epoch but requires cleaner confirmation. Nevertheless, our astrometric
precision suggests the companion to be comoving with the star, with a 3 sigma
confidence level. The planetary nature of the source is reinforced by a
non-detection in Ks-band (2.18 microns) images according to its possible
extremely red Ks - L' color. Conversely, background contamination is rejected
with good confidence level. The luminosity yields a predicted mass of about
4-5MJup (at 10-17 Myr) using "hot-start" evolutionary models, making HD 95086 b
the exoplanet with the lowest mass ever imaged around a star.Comment: accepted for publication to APJ
The 2008 outburst in the young stellar system ZCMa: I. Evidence of an enhanced bipolar wind on the AU-scale
Accretion is a fundamental process in star formation. Although the time
evolution of accretion remains a matter of debate, observations and modelling
studies suggest that episodic outbursts of strong accretion may dominate the
formation of the protostar. Observing young stellar objects during these
elevated accretion states is crucial to understanding the origin of unsteady
accretion. ZCMa is a pre-main-sequence binary system composed of an embedded
Herbig Be star, undergoing photometric outbursts, and a FU Orionis star. The
Herbig Be component recently underwent its largest optical photometric outburst
detected so far. We aim to constrain the origin of this outburst by studying
the emission region of the HI Brackett gamma line, a powerful tracer of
accretion/ejection processes on the AU-scale in young stars. Using the
AMBER/VLTI instrument at spectral resolutions of 1500 and 12 000, we performed
spatially and spectrally resolved interferometric observations of the hot gas
emitting across the Brackett gamma emission line, during and after the
outburst. From the visibilities and differential phases, we derive
characteristic sizes for the Brackett gamma emission and spectro-astrometric
measurements across the line, with respect to the continuum. We find that the
line profile, the astrometric signal, and the visibilities are inconsistent
with the signature of either a Keplerian disk or infall of matter. They are,
instead, evidence of a bipolar wind, maybe partly seen through a disk hole
inside the dust sublimation radius. The disappearance of the Brackett gamma
emission line after the outburst suggests that the outburst is related to a
period of strong mass loss rather than a change of the extinction along the
line of sight. Based on these conclusions, we speculate that the origin of the
outburst is an event of enhanced mass accretion, similar to those occuring in
EX Ors and FU Ors.Comment: Accepted for publication in Astronomy and Astrophysics Letter
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