566 research outputs found
A New Pleiades Member at the Lithium Substellar Boundary
We present the discovery of an object in the Pleiades open cluster, named
Teide 2, with optical and infrared photometry which place it on the cluster
sequence slightly below the expected substellar mass limit. We have obtained
low- and high-resolution spectra that allow us to determine its spectral type
(M6), radial velocity and rotational broadening; and to detect H in
emission and Li I 670.8 nm in absorption. All the observed properties strongly
support the membership of Teide 2 into the Pleiades. This object has an
important role in defining the reappearance of lithium below the substellar
limit in the Pleiades. The age of the Pleiades very low-mass members based on
their luminosities and absence or presence of lithium is constrained to be in
the range 100--120 Myr.Comment: 17 pages, 3 figure
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&
Brown Dwarfs in the Pleiades Cluster Confirmed by the Lithium Test
We present 10 m Keck spectra of the two Pleiades brown dwarfs Teide 1 and
Calar 3 showing a clear detection of the 670.8 nm Li resonance line. In Teide
1, we have also obtained evidence for the presence of the subordinate line at
812.6 nm. A high Li abundance (log N(Li) >= 2.5), consistent with little if any
depletion, is inferred from the observed lines. Since Pleiades brown dwarfs are
unable to burn Li the significant preservation of this fragile element confirms
the substellar nature of our two objects. Regardless of their age, their low
luminosities and Li content place Teide 1 and Calar 3 comfortably in the
genuine brown dwarf realm. Given the probable age of the Pleiades cluster,
their masses are estimated at 55 +- 15 Jupiter masses.Comment: 14 pages gzipped and uuencoded. Figures are included. Also available
at http://www.iac.es/. Accepted for publication in ApJ Letter
Membership and Multiplicity among Very Low-Mass Stars and Brown Dwarfs in the Pleiades Cluster
We present near-infrared photometry and optical spectroscopy of very low-mass
stars and brown dwarf candidates in the Pleiades open cluster. The membership
status of these objects is assessed. Eight objects out of 45 appear to be
non-members. A search for companions among 34 very low-mass Pleiades members
(M0.09 M) in high-spatial resolution images obtained with the
Hubble Space Telescope and the adaptive optics system of the
Canada-France-Hawaii telescope produced no resolved binaries with separations
larger than 0.2 arcsec (a ~ 27 AU; P ~ 444 years). Nevertheless, we find
evidence for a binary sequence in the color-magnitude diagrams, in agreement
with the results of Steele & Jameson (1995) for higher mass stars. We compare
the multiplicity statistics of the Pleiades very low-mass stars and brown
dwarfs with that of G and K-type main sequence stars in the solar neighborhood
(Duquennoy & Mayor 1991). We find that there is some evidence for a deficiency
of wide binary systems (separation >27 AU) among the Pleiades very low-mass
members. We briefly discuss how this result can fit with current scenarios of
brown dwarf formation. We correct the Pleiades substellar mass function for the
contamination of cluster non-members found in this work. We find a
contamination level of 33% among the brown dwarf candidates identified by
Bouvier et al. (1998). Assuming a power law IMF across the substellar boundary,
we find a slope dN/dM ~ M^{-0.53}, implying that the number of objects per mass
bin is still rising but the contribution to the total mass of the cluster is
declining in the brown dwarf regime.Comment: to be published in The Astrophysical Journa
The substellar mass function in sigma Orionis. II. Optical, near-infrared and IRAC/Spitzer photometry of young cluster brown dwarfs and planetary-mass objects
We investigate the mass function in the substellar domain down to a few
Jupiter masses in the young sigma Orionis open cluster (3+/-2 Ma, d =
360^+70_-60 pc). We have performed a deep IJ-band search, covering an area of
790 arcmin^2 close to the cluster centre. This survey was complemented with an
infrared follow-up in the HKs- and Spitzer 3.6-8.0 mum-bands. Using
colour-magnitude diagrams, we have selected 49 candidate cluster members in the
magnitude interval 16.1 mag < I < 23.0 mag. Accounting for flux excesses at 8.0
mum and previously known spectral features of youth, 30 objects are bona fide
cluster members. Four are first identified from our optical-near infrared data.
Eleven have most probable masses below the deuterium burning limit and are
classified as planetary-mass object candidates. The slope of the substellar
mass spectrum (Delta N / Delta M = a M^-alpha) in the mass interval 0.11 Msol M
< 0.006 Msol is alpha = +0.6+/-0.2. Any opacity mass-limit, if these objects
form via fragmentation, may lie below 0.006 Msol. The frequency of sigma
Orionis brown dwarfs with circumsubstellar discs is 47+/-15 %. The continuity
in the mass function and in the frequency of discs suggests that very low-mass
stars and substellar objects, even below the deuterium-burning mass limit, may
share the same formation mechanism.Comment: Accepted for publication in A&A (12/04/2007). It has not been edited
for language ye
A Methane Isolated Planetary Mass Object in Orion
We report on the discovery of a free-floating methane dwarf toward the
direction of the young star cluster sigma Orionis. Based on the object's
far-red optical and near-infrared photometry and spectroscopy, we conclude that
it is a possible member of this association. We have named it as S Ori
J053810.1-023626 (S Ori 70 is the abridged name). If it is a true member of
sigma Orionis, the comparison of the photometric and spectroscopic properties
of S Ori 70 with state-of-the-art evolutionary models yields a mass of 3
(+5/-1) Jupiter mass for ages between 1 Myr and 8 Myr. The presence of such a
low-mass object in our small search area (55.4 sq. arcmin) would indicate a
rising substellar initial mass function in the sigma Orionis cluster even for
planetary masses.Comment: Accepted for publication in the ApJ. Twelve pages, figures and tables
include
New Brown Dwarfs in the Pleiades Cluster
We present intermediate and low resolution optical spectroscopy (650-915 nm)
of seven faint, very red objects (20 > I >= 17.8, I-Z >= 0.5) discovered in a
CCD-based IZ survey covering an area of 1 sq. deg in the central region of the
Pleiades open cluster. The observed spectra show that these objects are very
cool dwarfs, having spectral types in the range M6-M9. Five out of the seven
objects can be considered as Pleiades members on the basis of their radial
velocities, Halpha emission and other gravity sensitive atomic features like
the NaI doublet at 818.3 and 819.5 nm. According to current evolutionary models
the masses of these new objects range from roughly 80 MJup for the hottest in
the sample down to 45 MJup for Roque 4, the coolest and faintest confirmed
member. These observations prove that the cloud fragmentation process extends
well into the brown dwarf realm, suggesting a rise in the initial mass function
below the substellar limit.Comment: 16 pages, 2 figures, aasms4 macro in LateX. 1997 ApJ Letters,
accepte
The Substellar Mass Function in sigma Orionis
We combine results from imaging searches for substellar objects in the sigma
Orionis cluster and follow-up photometric and spectroscopic observations to
derive a census of the brown dwarf population in a region of 847 arcmin^2. We
identify 64 very low-mass cluster member candidates in this region. We have
available three color (IZJ) photometry for all of them, spectra for 9 objects,
and K photometry for 27% of our sample. These data provide a well defined
sequence in the I vs I-J, I-K color magnitude diagrams, and indicate that the
cluster is affected by little reddening despite its young age (~5 Myr). Using
state-of-the-art evolutionary models, we derive a mass function from the
low-mass stars (0.2 Msol) across the complete brown dwarf domain (0.075 Msol to
0.013 Msol), and into the realm of free-floating planetary-mass objects (<0.013
Msol). We find that the mass spectrum (dN/dm ~ m^{-alpha}) increases toward
lower masses with an exponent alpha = 0.8+/-0.4. Our results suggest that
planetary-mass isolated objects could be as common as brown dwarfs; both kinds
of objects together would be as numerous as stars in the cluster. If the
distribution of stellar and substellar masses in sigma Orionis is
representative of the Galactic disk, older and much lower luminosity
free-floating planetary-mass objects with masses down to about 0.005 Msol
should be abundant in the solar vicinity, with a density similar to M-type
stars.Comment: Accepted for publication in ApJ. 19 pages, 3 figures include
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