187 research outputs found
A near-infrared survey for new low-mass members in Alpha Per
We present a near-infrared (K'-band) survey of 0.7 square degree area in the
Alpha Persei open cluster (age = 90 Myr, distance = 182 pc) carried out with
the Omega-Prime camera on the Calar Alto 3.5-m telescope. Combining optical
data (Rc and Ic) obtained with the KPNO/MOSA detector and presented in Stauffer
et al. (1999) with the K' observations, a sample of new candidate members has
been extracted from the optical-infrared colour-magnitude diagram. The location
of these candidates in the colour-colour diagram suggests that two-thirds of
them are actually reddened background giants. About 20 new candidate members
with masses between 0.3 and 0.04 Msun are added to the 400 known Alpha Per
cluster members. If they are indeed Alpha Per members, four of the new
candidates would be brown dwarfs. We discuss the advantages and drawbacks of
the near-infrared survey as compared to the optical selection method. We also
describe the outcome of optical spectroscopy obtained with the Twin
spectrograph on the Calar Alto 3.5-m telescope for about 30 candidates,
including selected members from the optical sample presented in Barrado y
Navascues et al. (2002) and from our joint optical/infrared catalogue. These
results argue in favour of the optical selection method for this particular
cluster.Comment: 13 pages, 10 figures, 3 tables and 1 electronic table, accepted for
publication in A&
Spectroscopy of Very Low Mass Stars and Brown Dwarfs in the Lambda Orionis Star Forming Region
Context. Most observational studies so far point towards brown dwarfs sharing
a similar formation mechanism as the one accepted for low mass stars. However,
larger databases and more systematic studies are needed before strong
conclusions can be reached. Aims. In this second paper of a series devoted to
the study of the spectroscopic properties of the members of the Lambda Orionis
Star Forming Region, we study accretion, activity and rotation for a wide set
of spectroscopically confirmed members of the central star cluster Collinder 69
to draw analogies and/or differences between the brown dwarf and stellar
populations of this cluster. Moreover, we present comparisons with other star
forming regions of similar and different ages to address environmental effects
on our conclusions. Methods. We study prominent photospheric lines to derive
rotational velocities and emission lines to distinguish between accretion
processes and chromospheric activity. In addition, we include information about
disk presence and X-ray emission. Results. We report very large differences in
the disk fractions of low mass stars and brown dwarfs (~58%) when compared to
higher mass stars (26+4-3%) with 0.6 Msun being the critical mass we find for
this dichotomy. As a byproduct, we address the implications of the spatial
distribution of disk and diskless members in the formation scenario of the
cluster itself. We have used the Halpha emission to discriminate among
accreting and non-accreting sources finding that 38+8-7% of sources harboring
disks undergo active accretion and that his percentage stays similar in the
substellar regime. For those sources we have estimated accretion rates.
Finally, regarding rotational velocities, we find a high dispersion in vsin(i)
which is even larger among the diskless population.Comment: Accepted for publication in A&A. 18 figs including the Appendix and
an online tabl
A Constraint on brown dwarf formation via ejection: radial variation of the stellar and substellar mass function of the young open cluster IC2391
Using the Wide Field Imager (WFI) at the ESO 2.2m telescope at La Silla and
the CPAPIR camera at the CTIO 1.5m telescope at Cerro Tololo, we have performed
an extensive, multiband photometric survey of the open cluster IC2391 (D~146pc,
age~50Myr, solar metallicity). Here we present the results from our photometric
survey and from a spectroscopic follow-up of the central part of the survey.Comment: 4 pages, 3 figures, to appear in the proceedings of the Cool Stars 15
conferenc
A mid-infrared study of very low mass stars and brown dwarfs in Upper Scorpius
We report the results of mid-IR observations with VISIR at the VLT of 10
ultracool dwarfs members of the nearby Upper Scorpius OB association in four
filters ranging between 8.59 (PAH1) to 12.8 m (Ne II), and one brown dwarf
with Spitzer between 3.6 and 24 m. Seven of our targets are detected in at
least one of the bands, and we derive upper limits on the fluxes of the
remaining 4. These results combined with previous studies from the literature
lead to an improved disk frequency of 5012%. This frequency is
significantly higher than that of accretors (16.3%6.2%). Only one object
showing mid-IR excess also has H emission at a level indicating that it
must be accreting. Four of the detected targets are multiple system candidates.
The observed disk frequency for sub-stellar objects in the Upper Scorpius
association is similar to that of stars, consistent with a common formation
scenario. It is also similar to the disk fractions observed in younger
clusters, suggesting that the disk lifetimes might be longer for ultracool
dwarfs than for higher-mass stars.Comment: 10 pages, 4 figures, accepted for A&
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
Large Amplitude Photometric Variability of the Candidate Protoplanet TMR-1C
In their HST/NICMOS observations, Terebey et al. 1998 detected a candidate
protoplanet, TMR-1C, that lies at a separation of about 10" (~1000 AU) from the
Class I protobinary TMR-1 (IRAS 04361+2547). A narrow filament-like structure
was observed extending south-east from the central proto-binary system towards
TMR-1C, suggesting a morphology in which the candidate protoplanet may have
been ejected from the TMR-1 system. Follow-up low-resolution spectroscopy could
not confirm if this object is a protoplanet or a low-luminosity background
star. We present two epochs of near-infrared photometric observations obtained
at the CFHT of TMR-1C. The time span of ~7 years between the two sets of
observations provides with an opportunity to, (a) check for any photometric
variability similar to that observed among young stellar objects, which would
indicate the youth of this source, and, (b) determine the proper motion. TMR-1C
displays large photometric variability between 1 and 2 mag in both the H- and
Ks-bands. From our 2002 observations, we find a (H-Ks) color of 0.3 mag, which
is much bluer than the value of 1.3 mag reported by T98 from HST observations.
Also, we observe brightening in both the H- and Ks-bands when the colors are
bluer, i.e. the object gets redder as it becomes fainter. We have explored the
possible origins for the observed variability, and find extinction due to the
presence of circumstellar material to be the most likely scenario. The observed
large-amplitude photometric variations, and the possible presence of a
circumstellar disk, are strong arguments against this object being an old
background star.Comment: Accepted in A&
Rotation and accretion of very low mass objects in the SigmaOri cluster
We report on two photometric monitoring campaigns of Very Low Mass (VLM)
objects in the young open cluster around SigmaOrionis. Our targets were
pre-selected with multi-filter photometry in a field of 0.36 sqdeg. For 23 of
these objects, spanning a mass range from 0.03 to 0.7 MS, we detect periodic
variability. Of these, 16 exhibit low-level variability, with amplitudes of
less than 0.2 mag in the I-band, which is mostly well-approximated by a sine
wave. These periodicities are probably caused by photospheric spots co-rotating
with the objects. In contrast, the remaining variable targets show high-level
variability with amplitudes ranging from 0.25 to 1.1 mag, consisting of a
periodic light variation onto which short-term fluctuations are superimposed.
This variability pattern is very similar to the photometric behaviour of
solar-mass, classical T Tauri stars. Low-resolution spectra of a few of these
objects reveal strong Halpha and Ca-triplet emission, indicative of ongoing
accretion processes. This suggests that 5-7% of our targets still possess a
circumstellar disk. In combination with previous results for younger objects,
this translates into a disk lifetime of 3-4 Myr, significantly shorter than for
solar mass stars. The highly variable objects rotate on average slower than the
low-amplitude variables, which is expected in terms of a disk-locking scenario.
There is a trend towards faster rotation with decreasing mass, which might be
caused by shortening of the disk lifetimes or attenuation of magnetic fields.Comment: 19 pages, 14 figures, A&A, in pres
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
On the age of the TW Hydrae Association and 2M1207334-393254
We have estimated the age of the young moving group TW Hydrae Association, a
cohort of a few dozen stars and brown dwarfs located near the Sun which share
the same kinematic properties and, presumably, the same origin and age. The
chronology has been determined by analyzing different properties (magnitudes,
colors, activity, lithium) of its members and comparing them with several
well-known star forming regions and open clusters, as well as theoretical
models. In addition, by using medium-resolution optical spectra of two M8
members of the association (2M1139 and 2M1207 -an accreting brown dwarf with a
planetary mass companion), we have derived spectral types and measured H(alpha)
and lithium equivalent widths. We have also estimated their effective
temperature and gravity, which were used to produce an independent age
estimation for these two brown dwarfs. We have also collected spectra of
2M1315, a candidate member with a L5 spectral type and measured its H(alpha)
equivalent width. Our age estimate for the association, 10 Myr (in the range
3-20 Myr), agrees with previous values cited in the literature. In the case of
the two brown dwarfs, we have derived an age of 15 Myr (in the range 5-30 Myr),
which also agree with our estimate for the whole group. We compared our results
with recent articles published on the same subject using other techniques, and
discuss the limits of the age-dating techniques.Comment: Astronomy & Astrophysics, accepte
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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