342 research outputs found
Four Brown Dwarfs in the Taurus Star-Forming Region
We have identified four brown dwarfs in the Taurus star-forming region. They
were first selected from and CCD photometry of 2.29 square degrees
obtained at the Canada-France-Hawaii Telescope. Subsequently, they were
recovered in the 2MASS second incremental data release point source catalog.
Low-resolution optical spectra obtained at the William Herschel telescope allow
us to derive spectral types in the range M7--M9. One of the brown dwarfs has
very strong H emission (EW=-340 \AA). It also displays Br
emission in an infrared spectrum obtained with IRCS on the Subaru telescope,
suggesting that it is accreting matter from a disk. The \ion{K}{1} resonance
doublet and the \ion{Na}{1} subordinate doublet at 818.3 and 819.5 nm in these
Taurus objects are weaker than in field dwarfs of similar spectral type,
consistent with low surface gravities as expected for young brown dwarfs. Two
of the objects are cooler and fainter than GG Tau Bb, the lowest mass known
member of the Taurus association. We estimate masses of only 0.03 M for
them. The spatial distribution of brown dwarfs in Taurus hints to a possible
anticorrelation between the density of stars and the density of brown dwarfs.Comment: ApJ Letters (in press
Nonlinear relaxation phenomena in metastable condensed matter systems
Nonlinear relaxation phenomena in three different systems of condensed matter are investigated. (i) First, the phase dynamics in Josephson junctions is analyzed. Specifically, a superconductor-graphene-superconductor (SGS) system exhibits quantum metastable states, and the average escape time from these metastable states in the presence of Gaussian and correlated fluctuations is calculated, accounting for variations in the the noise source intensity and the bias frequency. Moreover, the transient dynamics of a long-overlap Josephson junction (JJ) subject to thermal fluctuations and non-Gaussian noise sources is investigated. Noise induced phenomena are observed, such as the noise enhanced stability and the stochastic resonant activation. (ii) Second, the electron spin relaxation process in a n-type GaAs bulk driven by a fluctuating electric field is investigated. In particular, by using a Monte Carlo approach, we study the influence of a random telegraph noise on the spin polarized transport. Our findings show the possibility to raise the spin relaxation length by increasing the amplitude of the external fluctuations. Moreover, we find that, crucially, depending on the value of the external field strength, the electron spin depolarization length versus the noise correlation time increases up to a plateau. (iii) Finally, the stabilization of quantum metastable states by dissipation is presented. Normally, quantum fluctuations enhance the escape from metastable states in the presence of dissipation. We show that dissipation can enhance the stability of a quantum metastable system, consisting of a particle moving in a strongly asymmetric double well potential, interacting with a thermal bath. We find that the escape time from the metastable region has a nonmonotonic behavior versus the system- bath coupling and the temperature, producing a stabilizing effect
Spectroscopic characterization of 78 DENIS ultracool dwarf candidates in the solar neighborhood and the Upper Sco OB association
Aims: Low-resolution optical spectroscopic observations for 78 very low-mass
star and brown dwarf candidates that have been photometrically selected using
the DENIS survey point source catalogue. Methods: Spectral types are derived
for them using measurements of the PC3 index. They range from M6 to L4. H_alpha
emission and NaI subordinate doublet (818.3 nm and 819.9 nm) equivalent widths
are measured in the spectra. Spectroscopic indices of TiO, VO, CrH and FeH
molecular features are also reported. Results: A rule-of-thumb criterion to
select young very low-mass objects using the NaI doublet equivalent width is
given. It is used to confirm seven new members of the Upper Sco OB association
and two new members of the R Cr-A star-forming region. Four of our field
objects are also classified as very young, but are not members of any known
nearby young association. The frequency of lower-gravity young objects in our
field ultracool sample is 8.5%. Our results provide the first spectroscopic
classification for 38 ultracool dwarfs in the solar vicinity with
spectrophotometric distances in the range 17 pc to 65 pc (3 of them are new L
dwarfs within 20 pc).Comment: 25 pages, 10 figures, 7 tables, Accepted by A&
Optical spectra of selected Chamaeleon I young stellar objects
We present optical spectra of eight candidate brown dwarfs and a previously
known T Tauri star (Sz 33) of the Chamaeleon I dark cloud. We derived spectral
types based on the strength of the TiO or VO absorption bands present in the
spectra of these objects as well as on the PC3 index of Martin et al. (1999).
Photometric data from the literature are used to estimate the bolometric
luminosities for these sources. We apply D'Antona & Mazzitelli (1997) pre-main
sequence evolutionary tracks and isochrones to derive masses and ages. Based on
the presence of Halpha in emission, we confirm that most of the candidates are
young objects. Our sample however includes two sources for which we can only
provide upper limits for the emission in Halpha; whereas these two objects are
most likely foreground/background stars, higher resolution spectra are required
to confirm their true nature. Among the likely cloud members, we detect one new
sub-stellar object and three transition stellar/sub-stellar sources.Comment: 22 pages - manuscript forma
The substellar mass function in the central region of the open cluster Praesepe from deep LBT observations
Studies of the mass function (MF) of open clusters of different ages allow us
to probe the efficiency with which brown dwarfs (BDs) are evaporated from
clusters to populate the field. Surveys in old clusters (age > 100 Myr) do not
suffer so severely from several problems encountered in young clusters, such as
intra-cluster extinction and large uncertainties in BD models. Here we present
the results of a deep photometric survey to study the MF of the old open
cluster Praesepe (age 590 Myr and distance 190 pc), down to a 5 sigma detection
limit at i~25.6 mag (~40M_Jup). We identify 62 cluster member candidates, of
which 40 are substellar, from comparison with predictions from a dusty
atmosphere model. The MF rises from the substellar boundary until ~60M_Jup and
then declines. This is quite different from the form inferred for other open
clusters older than 50 Myr, but seems to be similar to those found in very
young open cluster, whose MFs peak at ~10M_Jup. Either Praesepe really does
have a different MF from other clusters or they had similar initial MFs but
have differed in their dynamical evolution. We further have identified six
foreground T dwarf candidates towards Praesepe, which require follow-up
spectroscopy to confirm their nature.Comment: 8 pages, 5 figures, to appear in the online proceedings of the Cool
Stars 16 conferenc
Near-infrared coronagraphic imaging of the circumstellar disk around TW Hydrae
We present ground-based near-infrared (H-band) imaging of the circumstellar
disk around the nearby classical T Tauri star TW Hydrae. The scattered light
image shows a face-on disk with radius 4 arcseconds (corresponding to 225 AU)
and a morphology which agrees with recent images from the Hubble Space
Telescope and the Very Large Array. The best fit power-law for the disk's
radial surface brightness profile obeys the law r^{-3.3\pm0.3}. We use our
image and published continuum flux densities to derive properties of the disk
with a simple model of emission from an optically thick, flat disk. The
best-fit values for disk mass and inner radius are 0.03 M_{\odot} and 0.3 AU;
best-fit values for temperature, density, and grain opacity power law exponents
(q, p, and \beta) are 0.7, 1.3, and 0.9, respectively. These properties are
similar to those of disks around classical T Tauri stars located in more
distant molecular clouds. Because of TW Hydrae's nearby location and pole-on
orientation, it is a uniquely favorable object for future studies of radial
disk structure at the classical T Tauri stage.Comment: 4 pages, 3 figures, accepted by ApJ
- …