479 research outputs found
Characterization of the known T type dwarfs towards the Sigma Orionis cluster
(Abridged) A total of three T type candidates (SOri70, SOri73, and
SOriJ0538-0213) lying in the line of sight towards Sigma Orionis were
characterized by means of near-infrared photometric, astrometric, and
spectroscopic studies. H-band methane images were collected for all three
sources and an additional sample of 15 field T type dwarfs using LIRIS/WHT.
J-band spectra of resolution of ~500 were obtained for SOriJ0538-0213 with
ISAAC/VLT, and JH spectra of resolution of ~50 acquired with WFC3/HST were
employed for the spectroscopic classification of SOri70 and 73. Proper motions
with a typical uncertainty of +/-3 mas/yr and a time interval of ~7-9 yr were
derived. Using the LIRIS observations of the field T dwarfs, we calibrated this
imager for T spectral typing via methane photometry. The three SOri objects
were spectroscopically classified as T4.5+/-0.5 (SOri73), T5+/-0.5
(SOriJ0538-0213), and T7 (SOri70). The similarity between the
observed JH spectra and the methane colors and the data of field ultra-cool
dwarfs of related classifications suggests that SOri70, 73, and
SOriJ053804.65-021352.5 do not deviate significantly in surface gravity in
relation to the field. Additionally, the detection of KI at ~1.25 microns in
SOriJ0538-0213 points to a high-gravity atmosphere. Only the K-band reddish
nature of SOri70 may be consistent with a low gravity atmosphere. The proper
motions of SOri70 and 73 are measurable and are larger than that of the cluster
by >3.5 sigma. The proper motion of SOriJ0538-0213 is consistent with a null
displacement. These observations suggest that none of the three T dwarfs are
likely Sigma Orionis members, and that either planetary-mass objects with
masses below ~4 MJup may not exist free-floating in the cluster or they may lie
at fainter near-infrared magnitudes than those of the targets (this is H>20.6
mag), thus remaining unidentified to date.Comment: Accepted for publication in A&A (2014), corrected typo
Confirming the least massive members of the Pleiades star cluster
We present optical photometry (i- and Z-band) and low-resolution spectroscopy
(640-1015 nm) of very faint candidate members (J = 20.2-21.2 mag) of the
Pleiades star cluster (120 Myr). The main goal is to address their cluster
membership via photometric, astrometric, and spectroscopic studies, and to
determine the properties of the least massive population of the cluster through
the comparison of the data with younger and older spectral counterparts and
state-of-the art model atmospheres. We confirm three bona-fide Pleiades members
that have extremely red optical and infrared colors, effective temperatures of
~1150 K and ~1350 K, and masses in the interval 11-20 Mjup, and one additional
likely member that shares the same motion as the cluster but does not appear to
be as red as the other members with similar brightness. This latter object
requires further near-infrared spectroscopy to fully address its membership in
the Pleiades. The optical spectra of two bona-fide members were classified as
L6-L7 and show features of KI, a tentative detection of CsI, hydrides and water
vapor with an intensity similar to high-gravity dwarfs of related
classification despite their young age. The properties of the Pleiades L6-L7
members clearly indicate that very red colors of L dwarfs are not a direct
evidence of ages younger than ~100 Myr. We also report on the determination of
the bolometric corrections for the coolest Pleiades members. These data can be
used to interpret the observations of the atmospheres of exoplanets orbiting
stars.Comment: Accepted for publication in MNRAS (17 pages
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&
Trigonometric parallaxes of young field L dwarfs
(Abridged) We aim to determine the trigonometric parallaxes and proper
motions of a sample of ten field L0-L5 dwarfs with spectroscopic evidence for
low-gravity atmospheres. We obtained J and Ks imaging data using 2-4-m class
telescopes with a typical cadence of one image per month between 2010 January
and 2012 December. We also obtained low resolution optical spectra (R~300,
500-1100 nm) using the 10-m GTCs to assess the presence of lithium absorption
in four targets and confirm their young age. Trigonometric parallaxes and
proper motions were derived to typical accuracies of 1 mas and +/-10 mas/yr.
All ten L dwarfs have large motions, and are located at distances between 9 and
47 pc. They lie above and on the sequence of field dwarfs in the absolute J and
K_s magnitude versus spectral type and luminosity versus Teff diagrams,
implying ages similar to or smaller than those typical of the field. The
detection of atomic lithium in the atmosphere of 2MASS J00452143+1634446 is
reported for the first time. Three dwarfs have locations in the HR diagram
indicative of old ages and high masses consistent with the observed lithium
depletion previously published. We did not find evidence for the presence of
astrometric companions with minimum detectable masses typically >=25 Mjup and
face-on, circular orbits with periods between 60-90 d and 3 yr around eight
targets. The astrometric and spectroscopic data indicate that about 60-70% of
the field L-type dwarfs in our sample with evidence for low-gravity atmospheres
are indeed young-to-intermediate-age brown dwarfs of the solar neighborhood
with expected ages and masses in the intervals 10-500 Myr and 11-45 Mjup. The
peaked-shape of the H-band spectra of L dwarfs, a signpost of youth, appears to
be present up to ages of 120-500 Myr and intermediate-to-high gravities.Comment: Accepted for publication in A&
Keck NIRC Observations of Planetary-mass Candidate Members in the sigma Orionis Open Cluster
We present K-band photometry and low-resolution near-infrared spectroscopy
from 1.44 to 2.45 micron of isolated planetary-mass candidate members in the
sigma Orionis cluster found by Zapatero Osorio et al. The new data have been
obtained with NIRC at the Keck I telescope. All of our targets, except for one,
are confirmed as likely cluster members. Hence, we also confirm that the
planetary-mass domain in the cluster is well populated. Using our deep K-band
images we searched for companions to the targets in the separation range 0.3"
to 10" up to a maximum faint limit of K=19.5 mag. One suspected companion seems
to be an extremely red galaxy. The near-infrared colors of the sigma Orionis
substellar members indicate that dust grains condense and settle in their
atmospheres. We estimate that the surface temperatures range from 2500 K down
to 1500 K. The spectroscopic sequence covers the full range of L subclasses,
and the faintest object is tentatively classified as T0. These targets provide
a sequence of substellar objects of known age, distance and metallicity, which
can be used as benchmark for understanding the spectral properties of ultracool
dwarfs.Comment: Accepted for publication in ApJ Letters. 9 pages, 4 figures include
S Ori J053825.4-024241: A Classical T Tauri-like object at the substellar boundary
We present a spectrophotometric analysis of S Ori J053825.4-024241, a
candidate member close to the substellar boundary of the young (1-8 Myr),
nearby (~360 pc) sigma Orionis star cluster. Our optical and near-infrared
photometry and low-resolution spectroscopy indicate that S Ori J053825.4-024241
is a likely cluster member with a mass estimated from evolutionary models at
0.06+0.07-0.02 Msol, which makes the object a probable brown dwarf. The radial
velocity of S Ori J053825.4-024241 is similar to the cluster systemic velocity.
This target, which we have classified as an M 6.0+-1.0 low-gravity object,
shows excessemission in the near-infrared and anomalously strong photometric
variability for its type (from the blue to the J band), suggesting the presence
of a surrounding disc. The optical spectroscopic observations show a continuum
excess at short wavelengths and a persistent and resolved Halpha emission
(pseudo-equivalent width of ~-250 AA) in addition to the presence of other
forbidden and permitted emission lines, which we interpret as indicating
accretion from the disc and possibly mass loss. We conclude that despite the
low mass of S Ori J053825.4-024241, this object exhibits some of the properties
typical of active classical T Tauri stars.Comment: 12 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysics, section 5. Galactic structure, stellar clusters and
populations. The official date of acceptance is 24/08/2005. Acknowledgements
of the use of telescopes, instruments, catalogues and software are also give
Activity at the Deuterium-Burning Mass Limit in Orion
We report very intense and variable Halpha emission (pseudo-equivalent widths
of ~180, 410 A) of S Ori 55, a probable free-floating, M9-type substellar
member of the young sigma Orionis open star cluster. After comparison with
state-of-the-art evolutionary models, we infer that S Ori 55 is near or below
the cluster deuterium-burning mass borderline, which separates brown dwarfs and
planetary-mass objects. We find its mass to be 0.008-0.015 Msun for ages
between 1 Myr and 8 Myr, with ~0.012 Msun the most likely value at the cluster
age of 3 Myr. The largest Halpha intensity reached the saturation level of log
L(Halpha)/L(bol) = -3. We discuss several possible scenarios for such a strong
emission. We also show that sigma Orionis M and L dwarfs have in general more
Halpha emission than their older field spectral counterparts. This could be due
to a decline in the strength of the magnetic field with age in brown dwarfs and
isolated planetary-mass objects, or to a likely mass accretion from disks in
the very young sigma Orionis substellar members.Comment: Accepted for publication in ApJ Letters. Nine pages (figures
included
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