1,172 research outputs found
Search and characterization of T-type planetary mass candidates in the sigma Orionis cluster
(Abridged) We aim to: i) confirm the presence of methane absorption in S Ori
73 (a T-type member candidate of the sig Orionis cluster, 3 Myr, 352 pc)
through methane imaging; ii) study S Ori 70 and 73 cluster membership via
photometric colors and accurate proper motion analysis; iii) perform a new
search to identify additional T-type sig Orionis member candidates with likely
masses below 7 Mjup. We obtained HAWK-I (VLT) J, H, and CH4off photometry of an
area of 119.15 sq. arcmin in sig Orionis down to Jcomp = 21.7 and Hcomp = 21
mag. Near-infrared data were complemented with optical photometry using images
acquired with OSIRIS (GTC) and VISTA as part of the VISTA Orion survey. We
derived proper motions by comparison of the new HAWK-I and VISTA images with
published near-infrared data taken 3.4 - 7.9 yr ago. S Ori 73 has a red
H-CH4off color indicating methane absorption in the H-band and a spectral type
of T4 +/- 1. S Ori 70 displays a redder methane color than S Ori 73 in
agreement with its latter spectral classification. Our proper motion
measurements are larger than the motion of sig Orionis, rendering S Ori 70 and
73 cluster membership uncertain. We identified one new photometric candidate
with J = 21.69 +/- 0.12 mag and methane color consistent with spectral type
greater than T8. S Ori 73 has colors similar to those of T3-T5 field dwarfs,
which in addition to its high proper motion suggests that it is probably a
field dwarf located at 170-200 pc. The origin of S Ori 70 remains unclear: it
can be a field, foreground mid- to late-T free-floating dwarf with peculiar
colors, or an orphan planet ejected through strong dynamical interactions from
sig Orionis or from a nearby star-forming region in Orion.Comment: Accepted for publication in A&
Discovery of a Low-Mass Brown Dwarf Companion of the Young Nearby Star G196-3
A substellar-mass object in orbit at about 300 astronomical units (AU) from
the young low-mass star G196-3 was detected by direct imaging. Optical and
infrared photometry and low- and intermediate-resolution spectroscopy of the
faint companion, hereafter referred to as G196-3B, confirms its cool atmosphere
and allows its mass to be estimated at 25^{+15}_{-10} Jupiter masses. The
separation between both objects and their mass ratio suggest the fragmentation
of a collapsing cloud as the most likely origin for G196-3B, but alternatively
it could have originated from a proto-planetary disc which has been dissipated.
Whatever the formation process was, the young age of the primary star (about
100 Myr) demonstrates that substellar companions can form in short time scales.Comment: Published in Science (13 Nov). One color figur
VLT X-shooter spectroscopy of the nearest brown dwarf binary
The aim of the project is to characterise both components of the nearest
brown dwarf sytem to the Sun, WISE J104915.57-531906.1 (=Luhman16AB) at optical
and near-infrared wavelengths. We obtained high signal-to-noise
intermediate-resolution (R~6000-11000) optical (600-1000 nm) and near-infrared
(1000-2480nm) spectra of each component of Luhman16AB, the closest brown dwarf
binary to the Sun, with the X-Shooter instrument on the Very Large Telescope.
We classify the primary and secondary of the Luhman16 system as L6-L7.5 and
T0+/-1, respectively, in agreement with previous measurements published in the
literature. We present measurements of the lithium pseudo-equivalent widths,
which appears of similar strength on both components (8.2+/-1.0 Angstroms and
8.4+/-1.5 Angstroms for the L and T components, respectively). The presence of
lithium (Lithium 7) in both components imply masses below 0.06 Msun while
comparison with models suggests lower limits of 0.04 Msun. The detection of
lithium in the T component is the first of its kind. Similarly, we assess the
strength of other alkali lines (e.g. pseudo-equivalent widths of 6-7 Angstroms
for RbI and 4-7 Angstroms for CsI) present in the optical and near-infrared
regions and compare with estimates for L and T dwarfs. We also derive effective
temperatures and luminosities of each component of the binary: -4.66+/-0.08 dex
and 1305(+180)(-135) for the L dwarf and -4.68+/-0.13 dex and 1320(+185)(-135)
for the T dwarf, respectively. Using our radial velocity determinations, the
binary does not appear to belong to any of the well-known moving group. Our
preliminary theoretical analysis of the optical and J-band spectra indicates
that the L- and T-type spectra can be reproduced with a single temperature and
gravity but different relative chemical abundances which impact strongly the
spectral energy distribution of L/T transition objects.Comment: 12 pages, 9 figure, 3 tables, accepted to A&
Studies of CMB structure at Dec=40. II: Analysis and cosmological interpretation
We present a detailed analysis of the cosmic microwave background structure
in the Tenerife Dec=+40 degrees data. The effect of local atmospheric
contributions on the derived fluctuation amplitude is considered, resulting in
an improved separation of the intrinsic CMB signal from noise. Our analysis
demonstrates the existence of common structure in independent data scans at 15
and 33 GHz. For the case of fluctuations described by a Gaussian
auto-correlation function, a likelihood analysis of our combined results at 15
and 33 GHz implies an intrinsic rms fluctuation level of 48^{+21}_{-15} uK on a
coherence scale of 4 degrees; the equivalent analysis for a
Harrison-Zel'dovitch model gives a power spectrum normalisation of Q_{rms-ps} =
22^{+10}_{-6} uK. The fluctuation amplitude is seen to be consistent at the 68%
confidence level with that reported for the COBE two-year data for primordial
fluctuations described by a power law model with a spectral index in the range
1.0 \le n \le 1.6. This limit favours the large scale CMB anisotropy being
dominated by scalar fluctuations rather than tensor modes from a gravitational
wave background. The large scale Tenerife and COBE results are considered in
conjunction with observational results from medium scale experiments in order
to place improved limits on the fluctuation spectral index; we find n=1.10 +/-
0.10 assuming standard CDM with H_{0}=50 kms^{-1}Mpc^{-1}.Comment: 10 pages LaTeX, including 8 PostScript figures. Accepted for
publication in MNRA
2MASS J154043.42-510135.7: a new addition to the 5 pc population
The aim of the project is to find the stars nearest to the Sun and to
contribute to the completion of the stellar and substellar census of the solar
neighbourhood. We identified a new late-M dwarf within 5 pc, looking for high
proper motion sources in the 2MASS-WISE cross-match. We collected astrometric
and photometric data available from public large-scale surveys. We complemented
this information with low-resolution optical and near-infrared spectroscopy
with instrumentation on the ESO NTT to confirm the nature of our candidate. We
also present a high-quality medium-resolution VLT/X-shooter spectrum covering
the 400 to 2500 nm wavelength range. We classify this new neighbour as an
M7.00.5 dwarf using spectral templates from the Sloan Digital Sky Survey
and spectral indices. Lithium absorption at 670.8 nm is not detected in the
X-shooter spectrum, indicating that the M7 dwarf is older than 600 Myr and more
massive than 0.06 M. We also derive a trigonometric distance of 4.4
pc, in agreement with the spectroscopic distance estimate, making
2MASS\,J154043.42510135.7 the nearest M7 dwarf to the Sun. This
trigonometric distance is somewhat closer than the 6 pc distance reported
by the ALLWISE team, who independently identified this object recently. This
discovery represents an increase of 25\% in the number of M7--M8 dwarfs already
known at distances closer than 8\,pc from our Sun. We derive a density of
\,=\,1.90.910\,pc for M7 dwarfs in the 8 pc
volume, a value similar to those quoted in the literature. This new ultracool
dwarf is among the 50 nearest systems to the Sun, demonstrating that our
current knowledge of the stellar census within the 5 pc sample remains
incomplete. 2M1540 represents a unique opportunity to search for extrasolar
planets around ultracool dwarfs due to its proximity and brightness.Comment: 8 pages, 5 figures. Acepted in Astronomy & Astrophysics (15/05/2005
- âŠ