603 research outputs found
The Formation of Brown Dwarfs: Observations
We review the current state of observational work on the formation of brown
dwarfs, focusing on their initial mass function, velocity and spatial
distributions at birth, multiplicity, accretion, and circumstellar disks. The
available measurements of these various properties are consistent with a common
formation mechanism for brown dwarfs and stars. In particular, the existence of
widely separated binary brown dwarfs and a probable isolated proto-brown dwarf
indicate that some substellar objects are able to form in the same manner as
stars through unperturbed cloud fragmentation. Additional mechanisms such as
ejection and photoevaporation may play a role in the birth of some brown
dwarfs, but there is no observational evidence to date to suggest that they are
the key elements that make it possible for substellar bodies to form.Comment: Protostars and Planets V, in pres
Adaptive Optics Observations of Vega: Eight Detected Sources and Upper Limits to Planetary-Mass Companions
From adaptive optics observations with the Palomar 5-meter telescope we place
upper limits on the masses of any planetary companions located between ~30-230
AU away from Vega, where our data are sensitive to depths ranging from H=12.5
mag to H=19.0 mag fainter than Vega itself. Our observations cover a
plus-shaped area with two 25"x57" elements, excluding 7"x7" centered on the
star. We have identified 2 double and 4 single point sources. These projected
companions are 14.9-18.9 mag fainter than Vega, and if physically associated
would have masses ranging from 4 to 35 Mjup and orbital radii 170-260 AU.
Recent simulations of dusty rings around Vega predict the presence of a
perturbing body with mass <2-3 Mjup and orbital radius ~40-100 AU, though more
massive (~10 Mjup) planets cannot be excluded. None of the detected objects are
this predicted planet. Based on a color-magnitude, spectroscopic, and proper
motion analysis, all objects are consistent with being background sources.
Given the glare of Vega, a 2 Mjup object near the expected orbital radii would
not have been visible at the 5-sigma level in our data, though any >10 Mjup
brown dwarf could have been seen at separation >80 AU.Comment: 16 pages, 1 table, 4 figures. Accepted for publication in Ap
A Long-Lived Accretion Disk Around a Lithium-Depleted Binary T Tauri Star
We present a high dispersion optical spectrum of St 34 and identify the
system as a spectroscopic binary with components of similar luminosity and
temperature (both M3+/-0.5). Based on kinematics, signatures of accretion, and
location on an H-R diagram, we conclude that St 34 is a classical T Tauri star
belonging to the Taurus-Auriga T Association. Surprisingly, however, neither
component of the binary shows LiI 6708 A, absorption, the most universally
accepted criterion for establishing stellar youth. In this uniquely known
instance, the accretion disk appears to have survived longer than the lithium
depletion timescale. We speculate that the long-lived accretion disk is a
consequence of the sub-AU separation companion tidally inhibiting, though not
preventing, circumstellar accretion. Comparisons with pre-main sequence
evolutionary models imply, for each component of St 34, a mass of 0.37+/-0.08
Msun and an isochronal age of 8+/-3 Myr, which is much younger than the
predicted lithium depletion timescale of ~ 25 Myr. Although a distance 38%
closer than that of Taurus-Auriga or a hotter temperature scale could reconcile
this discrepancy at 21-25 Myr, similar discrepancies in other systems and the
implications of an extremely old accreting Taurus-Auriga member suggest instead
a possible problem with evolutionary models. Regardless, the older age implied
by St 34's depleted lithium abundance is the first compelling evidence for a
substantial age spread in this region. Additionally, since St 34's coeval
co-members with early M spectral types would likewise fail the lithium test for
youth, current membership lists may be incomplete.Comment: 4 pages, including 2 figures. Accepted for publication in ApJ Let
Multiplicity at the Stellar/Substellar Boundary in Upper Scorpius
We present the results of a high-resolution imaging survey of 12 brown dwarfs
and very low mass stars in the closest (~145 pc) young (~5 Myr) OB association,
Upper Scorpius. We obtained images with the Advanced Camera for Surveys/High
Resolution Camera on HST through the F555W (V), F775W (i'), and F850LP (z')
filters. This survey discovered three new binary systems, including one
marginally resolved pair with a projected separation of only 4.9 AU, resulting
in an observed binary fraction of 25+/-14% at separations >4 AU. After
correcting for detection biases assuming a uniform distribution of mass ratios
for q>0.6, the estimated binary fraction is 33+/-17%. The binary fraction is
consistent with that inferred for higher-mass stars in Upper Sco, but the
separation and mass ratio distributions appear to be different. All three
low-mass binary systems in Upper Sco are tight (<18 AU) and of similar mass
(q>0.6), consistent with expectations based on previous multiplicity studies of
brown dwarfs and very low mass stars in the field and in open clusters. The
implication is that the distinct separation and mass ratio distributions of
low-mass systems are set in the formation process or at very young ages, rather
than by dynamical disruption of wide systems at ages >5 Myr. Finally, we
combine the survey detection limits with the models of Burrows et al. (1997) to
show that there are no planets or very low-mass brown dwarfs with masses >10
M_J at projected separations >20 AU, or masses >5 M_J at projected separations
>40 AU orbiting any of the low-mass (0.04-0.10 M_sun) objects in our sample.Comment: Accepted for publication in ApJ; 10 pages, 4 figures in emulateapj
forma
Keck NIRSPEC Radial Velocity Observations of Late-M dwarfs
We present the results of an infrared spectroscopic survey of 23 late-M
dwarfs with the NIRSPEC echelle spectrometer on the Keck II telescope. Using
telluric lines for wavelength calibration, we are able to achieve measurement
precisions of down to 45 m/s for our late-M dwarfs over a one to four year-long
baseline. Our sample contains two stars with RV variations of >1000 m/s. While
we require more measurements to determine whether these RV variations are due
to unseen planetary or stellar companions or are the result of starspots known
to plague the surface of M dwarfs, we can place upper limits of <40 MJsini on
the masses of any companions around those two M dwarfs with RV variations of
<160 m/s at orbital periods of 10-100 days. We have also measured the
rotational velocities for all the stars in our late-M dwarf sample and offer
our multi-order, high-resolution spectra over 2.0 to 2.4 micron to the
atmospheric modeling community to better understand the atmospheres of late-M
dwarfs.Comment: Accepted to Ap
Multiepoch Radial Velocity Observations of L Dwarfs
We report on the development of a technique for precise radial-velocity
measurements of cool stars and brown dwarfs in the near infrared. Our technique
is analogous to the Iodine (I2) absorption cell method that has proven so
successful in the optical regime. We rely on telluric CH4 absorption features
to serve as a wavelength reference, relative to which we measure Doppler shifts
of the CO and H2O features in the spectra of our targets. We apply this
technique to high-resolution (R~50,000) spectra near 2.3 micron of nine L
dwarfs taken with the Phoenix instrument on Gemini-South and demonstrate a
typical precision of 300 m/s. We conduct simulations to estimate our expected
precision and show our performance is currently limited by the signal-to-noise
of our data. We present estimates of the rotational velocities and systemic
velocities of our targets. With our current data, we are sensitive to
companions with M sin i > 2MJ in orbits with periods less than three days. We
identify no companions in our current data set. Future observations with
improved signal-to-noise should result in radial-velocity precision of 100 m/s
for L dwarfs.Comment: Accepted for publication in ApJ, 24 pages, 7 figure
HDE 233517: Lithium and Excess Infrared Emission in Giant Stars
Recent studies have identified a small class of moderately rapidly rotating, chromospherically active, single giants, some of which are lithium rich. We present evidence suggesting the peculiar K-type star HDE 233517 is one such object. Previously, HDE 233517 has been suggested to be a young star, consistent with its large far-infrared excess and our log Δ(Li) ~ 3.3. However, our high-resolution spectroscopic observations show it is likely a single, post-main-sequence K2 giant with v sin i = 15 km s-1 and modest Ca II H and K emission. The giant status of HDE 233517 is determined directly from luminosity-sensitive line ratios and a lack of significant line wings, and is further supported by a large radial velocity (46.5 km s-1), small proper motion, and the presence of interstellar absorption features. Interpretation of the data in the context of a recent mass outflow model for giant stars proposed by de la Reza and coworkers indicates that HDE 233517 has the largest mass-loss rate, ~3 Ă 10-7 Mâ yr-1, of any known luminosity class III giant. We suggest that the processes causing rapid rotation, large lithium abundance, and infrared excess are triggered at the base of the giant branch when the convection zone reaches the rapidly rotating core of low-mass stars
High-Dispersion Optical Spectra of Nearby Stars Younger Than The Sun
We present high-dispersion (R~16,000) optical (3900-8700 A) spectra of 390
stars obtained with the Palomar 60 inch telescope. The majority of stars
observed are part of the Spitzer Legacy Science Program "The Formation and
Evolution of Planetary Systems." Through detailed analysis we determine stellar
properties for this sample, including radial and rotational velocities, Li I
6708 and Ha equivalent widths, the chromospheric activity index R'_HK, and
temperature- and gravity-sensitive line ratios. Several spectroscopic binaries
are also identified. From our tabulations, we illustrate basic age- and
rotation-related correlations among measured indices. One novel result is that
Ca II chromospheric emission appears to saturate at vsini values above ~30
km/s, similar to the well established saturation of X-rays that originate in
the spatially separate coronal regions.Comment: 1 electronic table; published in the Astronomical Journa
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