1,777 research outputs found
Age spreads in star forming regions?
Rotation periods and projected equatorial velocities of pre-main-sequence
(PMS) stars in star forming regions can be combined to give projected stellar
radii. Assuming random axial orientation, a Monte-Carlo model is used to
illustrate that distributions of projected stellar radii are very sensitive to
ages and age dispersions between 1 and 10 Myr which, unlike age estimates from
conventional Hertzsprung-Russell diagrams, are relatively immune to
uncertainties due to extinction, variability, distance etc. Application of the
technique to the Orion Nebula cluster reveals radius spreads of a factor of
2--3 (FWHM) at a given effective temperature. Modelling this dispersion as an
age spread suggests that PMS stars in the ONC have an age range larger than the
mean cluster age, that could be reasonably described by the age distribution
deduced from the Hertzsprung-Russell diagram. These radius/age spreads are
certainly large enough to invalidate the assumption of coevality when
considering the evolution of PMS properties (rotation, disks etc.) from one
young cluster to another.Comment: To appear in "The Ages of Stars", E.E. Mamajek, D.R. Soderblom,
R.F.G. Wyse (eds.), IAU Symposium 258, CU
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
Near-Infrared Photometric Variability of Stars Toward the Chamaeleon I Molecular Cloud
We present the results of a J, H, and K_s photometric monitoring campaign of
a 0.72 x 6 sq deg. area centered on the Chamaeleon I star forming region. Data
were obtained on 15 separate nights over a 4 month time interval using the
2MASS South telescope. Out of a total of 34,539 sources brighter than the
photometric completeness limits (J=16.0, H=15.2, K_s=14.8), 95 exhibit
near-infrared variability in one or more bands. The variables can be grouped
into a population of bright, red objects that are associated with the
Chamaeleon I association, and a population of faint, blue variables that are
dispersed over the full 6 deg of the survey and are likely field stars or older
pre-main-sequence stars unrelated to the present-day Chamaeleon I molecular
cloud. Ten new candidate members of Chamaeleon I, including 8 brown dwarf
candidates, have been identified based on variability and/or near-infrared
excess emission in the J-H vs. H-K_s color-color-diagram. We also provide a
compendium of astrometry and J, H, and K_s photometry for previously identified
members and candidate members of Chamaeleon I.Comment: To appear in AJ; see
http://www.astro.caltech.edu/~jmc/variables/cham1
Evidence for Mass-dependent Circumstellar Disk Evolution in the 5 Myr Old Upper Scorpius OB Association
We present 4.5, 8, and 16 ”m photometry from the Spitzer Space Telescope for 204 stars in the Upper Scorpius OB
association. The data are used to investigate the frequency and properties of circumstellar disks around stars with masses between ~0.1 and 20 M_â at an age of ~5 Myr. We identify 35 stars that have emission at 8 or 16 ”m in excess of the, stellar photosphere. The lower mass stars (~0.1â1.2M_â) appear surrounded by primordial optically thick disks based on, the excess emission characteristics. Starsmoremassive than ~1.8M_â have lower fractional excess luminosities suggesting, that the inner ~10 AU of the disk has been largely depleted of primordial material. None of the G and F stars (~1.2â1.8 M_â) in our sample have an infrared excess at wavelengths â€16 ”m. These results indicate that the mechanisms for, dispersing primordial optically thick disks operate less efficiently, on average, for low-mass stars, and that longer timescales are available for the buildup of planetary systems in the terrestrial zone for stars with masses âŸ1 M_â
VIRIS: A Visual-Infrared Imaging System for the Lick Observatory 1-M Telescope
We describe a system in use at the Lick Observatory 1-m Nickel telescope for
near-simultaneous imaging at optical and near-infrared wavelengths. The
combined availability of a CCD and a NICMOS-3 camera makes the system
well-suited for photometric monitoring from 0.5-2.2 microns of a variety of
astrophysical objects. Our science program thus far has concentrated on
studying variability trends in young stellar objects.Comment: 11 pages LaTex, 3 Postscript figure, Pub. Astr. Soc. Pac. 1998, in
pres
The low-mass Initial Mass Function in the Orion Nebula cluster based on HST/NICMOS III imaging
We present deep HST/NICMOS Camera 3 F110W and F160W imaging of a 26'x33',
corresponding to 3.1pcx3.8pc, non-contiguous field towards the Orion Nebula
Cluster (ONC). The main aim is to determine the ratio of low--mass stars to
brown dwarfs for the cluster as a function of radius out to a radial distance
of 1.5pc. The sensitivity of the data outside the nebulous central region is
F160W=21.0 mag, significantly deeper than previous studies of the region over a
comparable area. We create an extinction limited sample and determine the ratio
of low-mass stars (0.08-1Msun) to brown dwarfs (0.02-0.08Msun and
0.03-0.08Msun) for the cluster as a whole and for several annuli. The ratio
found for the cluster within a radius of 1.5pc is
R(02)=N(0.08-1Msun)/N(0.02-0.08Msun)=1.7+-0.2, and
R(03)=N(0.08-1Msun)/N(0.03-0.08Msun)=2.4+-0.2, after correcting for field
stars. The ratio for the central 0.3pcx0.3pc region down to 0.03Msun was
previously found to be R(03)=3.3+0.8-0.7, suggesting the low-mass content of
the cluster is mass segregated. We discuss the implications of a gradient in
the ratio of stars to brown dwarfs in the ONC in the context of previous
measurements of the cluster and for other nearby star forming regions. We
further discuss the current evidence for variations in the low-mass IMF and
primordial mass segregation.Comment: Accepted to A&
High-resolution Spectroscopy of [Ne II] Emission from TW Hya
We present high-resolution echelle spectra of [Ne II] 12.81 micron emission
from the classical T Tauri star (CTTS) TW Hya obtained with MICHELLE on Gemini
North. The line is centered at the stellar radial velocity and has an intrinsic
FWHM of 21\pm 4 km/s. The line width is broader than other narrow emission
lines typically associated with the disk around TW Hya. If formed in a disk,
the line broadening could result from turbulence in a warm disk atmosphere,
Keplerian rotation at an average distance of 0.1 AU from the star, or a
photoevaporative flow from the optically-thin region of the disk. We place
upper limits on the [Ne II] emission flux from the CTTSs DP Tau and BP Tau.Comment: Accepted by ApJ. 18 pages, including 2 figures and 2 table
Searching for young Jupiter analogs around AP Col: L-band high-contrast imaging of the closest pre-main sequence star
The nearby M-dwarf AP Col was recently identified by Riedel et al. 2011 as a
pre-main sequence star (age 12 - 50 Myr) situated only 8.4 pc from the Sun. The
combination of its youth, distance, and intrinsically low luminosity make it an
ideal target to search for extrasolar planets using direct imaging. We report
deep adaptive optics observations of AP Col taken with VLT/NACO and Keck/NIRC2
in the L-band. Using aggressive speckle suppression and background subtraction
techniques, we are able to rule out companions with mass m >= 0.5 - 1M_Jup for
projected separations a>4.5 AU, and m >= 2 M_Jup for projected separations as
small as 3 AU, assuming an age of 40 Myr using the COND theoretical
evolutionary models. Using a different set of models the mass limits increase
by a factor of ~2. The observations presented here are the deepest
mass-sensitivity limits yet achieved within 20 AU on a star with direct
imaging. While Doppler radial velocity surveys have shown that Jovian bodies
with close-in orbits are rare around M-dwarfs, gravitational microlensing
studies predict that ~17% of these stars host massive planets with orbital
separations of 1-10 AU. Sensitive high-contrast imaging observations, like
those presented here, will help to validate results from complementary
detection techniques by determining the frequency of gas giant planets on wide
orbits around M-dwarfs.Comment: Accepted for publication in ApJ, 6 pages text ApJ style (incl.
references), 4 figures, 1 tabl
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