217 research outputs found
Photometric Accretion Signatures Near the Substellar Boundary
Multi-epoch imaging of the Orion equatorial region by the Sloan Digital Sky
Survey has revealed that significant variability in the blue continuum persists
into the late-M spectral types, indicating that magnetospheric accretion
processes occur below the substellar boundary in the Orion OB1 association. We
investigate the strength of the accretion-related continuum veiling by
comparing the reddening-invariant colors of the most highly variable stars
against those of main sequence M dwarfs and evolutionary models. A gradual
decrease in the g band veiling is seen for the cooler and less massive members,
as expected for a declining accretion rate with decreasing mass. We also see
evidence that the temperature of the accretion shock decreases in the very low
mass regime, reflecting a reduction in the energy flux carried by the accretion
columns. We find that the near-IR excess attributed to circumstellar disk
thermal emission drops rapidly for spectral types later than M4. This is likely
due to the decrease in color contrast between the disk and the cooler stellar
photosphere. Since accretion, which requires a substantial stellar magnetic
field and the presence of a circumstellar disk, is inferred for masses down to
0.05 Msol we surmise that brown dwarfs and low mass stars share a common mode
of formation.Comment: 37 pages, 14 figures, accepted by A
The Mass Function of Newly Formed Stars (Review)
The topic of the stellar "original mass function" has a nearly 50 year
history,dating to the publication in 1955 of Salpeter's seminal paper. In this
review I discuss the many more recent results that have emerged on the initial
mass function (IMF), as it is now called, from studies over the last decade of
resolved populations in star forming regions and young open clusters.Comment: 9 pages, 1 figure; to appear in "The Dense Instellar Medium in
Galaxies -- 4'th Cologne-Bonn-Zermatt-Symposium" editted by S. Pfalzner, C.
Kramer, C. Straubmeier and A. Heithausen, Springer-Verlag (2004
A different look at the spin state of Co ions in CoO pyramidal coordination
Using soft-x-ray absorption spectroscopy at the Co- and O- edges,
we demonstrate that the Co ions with the CoO pyramidal
coordination in the layered SrCoOCl compound are unambiguously in the
high spin state. Our result questions the reliability of the spin state
assignments made so far for the recently synthesized layered cobalt
perovskites, and calls for a re-examination of the modeling for the complex and
fascinating properties of these new materials.Comment: 5 pages 3 figure
Accretion in Young Stellar/Substellar Objects
We present a study of accretion in a sample of 45 young, low mass objects in
a variety of star forming regions and young associations, about half of which
are likely substellar. Based primarily on the presence of broad, asymmetric
Halpha emission, we have identified 13 objects (~30% of our sample) which are
strong candidates for ongoing accretion. At least 3 of these are substellar. We
do not detect significant continuum veiling in most of the accretors with late
spectral types (M5-M7). Accretion shock models show that lack of measurable
veiling allows us to place an upper limit to the mass accretion rates of <~
10^{-10} Msun/yr. Using magnetospheric accretion models with appropriate
(sub)stellar parameters, we can successfully explain the accretor Halpha
emission line profiles, and derive quantitative estimates of accretion rates in
the range 10^{-12} < Mdot < 10^{-9} Msun/yr. There is a clear trend of
decreasing accretion rate with stellar mass, with mean accretion rates
declining by 3-4 orders of magnitude over ~ 1 - 0.05 Msun.Comment: 38 pages, including 8 figures and 6 tables, accepted by Ap
25 Orionis: A Kinematically Distinct 10 Myr Old Group in Orion OB1a
We report here on the photometric and kinematic properties of a well defined
group of nearly 200 low-mass pre-main sequence stars, concentrated within ~ 1
deg of the early-B star 25 Ori, in the Orion OB1a sub-association. We refer to
this stellar aggregate as the 25 Orionis group. The group also harbors the
Herbig Ae/Be star V346 Ori and a dozen other early type stars with photometry,
parallaxes, and some with IR excess emission, consistent with group membership.
The number of high and low-mass stars is in agreement with expectations from a
standard Initial Mass Function. The velocity distribution for the young stars
in 25 Ori shows a narrow peak centered at 19.7 km/s, very close to the velocity
of the star 25 Ori. Our results provide new and compelling evidence that the 25
Ori group is a distinct kinematic entity, and that considerable space and
velocity structure is present in the Ori OB1a sub-association. The low-mass
members follow a well defined band in the color-magnitude diagram, consistent
with an isochronal age of ~ 7-10 Myr, depending on the assumed evolutionary
model. The highest density of members is located near the star 25 Ori, but the
actual extent of the cluster cannot be well constrained with our present data.
In a simple-minded kinematic evolution scenario, the 25 Ori group may represent
the evolved counterpart of a younger aggregate like the sigma Ori cluster. The
25 Ori stellar aggregate is the most populous ~ 10 Myr sample yet known within
500 pc, setting it as an excellent laboratory to study the evolution of
solar-like stars and protoplanetary disks.Comment: 28 pages, 5 figures. Astrophysical Journal, in press. Abridged
abstrac
Discovery of the Optical Transient of the Gamma Ray Burst 990308
The optical transient of the faint Gamma Ray Burst 990308 was detected by the
QUEST camera on the Venezuelan 1-m Schmidt telescope starting 3.28 hours after
the burst. Our photometry gives , , , and for times ranging from 3.28 to 3.47
hours after the burst. The colors correspond to a spectral slope of close to
. Within the standard synchrotron fireball model,
this requires that the external medium be less dense than , the
electrons contain of the shock energy, and the magnetic field energy
must be less than 24% of the energy in the electrons for normal interstellar or
circumstellar densities. We also report upper limits of at 132 s
(with LOTIS), from 132-1029s (with LOTIS), at 28.2 min
(with Super-LOTIS), and a 8.5 GHz flux of at 110 days (with the
Very Large Array). WIYN 3.5-m and Keck 10-m telescopes reveal this location to
be empty of any host galaxy to and . The lack of a host
galaxy likely implies that it is either substantially subluminous or more
distant than a red shift of .Comment: ApJ Lett submitted, 5 pages, 2 figures, no space for 12 coauthor
Nature of the Low Field Transition in the Mixed State of High Temperature Superconductors
We have numerically studied the statics and dynamics of a model
three-dimensional vortex lattice at low magnetic fields. For the statics we use
a frustrated 3D XY model on a stacked triangular lattice. We model the dynamics
as a coupled network of overdamped resistively-shunted Josephson junctions with
Langevin noise. At low fields, there is a weakly first-order phase transition,
at which the vortex lattice melts into a line liquid. Phase coherence parallel
to the field persists until a sharp crossover, conceivably a phase transition,
near which develops at the same temperature as an infinite
vortex tangle. The calculated flux flow resistivity in various geometries near
closely resembles experiment. The local density of field induced
vortices increases sharply near , corresponding to the experimentally
observed magnetization jump. We discuss the nature of a possible transition or
crossover at (B) which is distinct from flux lattice melting.Comment: Updated references. 46 pages including low quality 25 eps figures.
Contact [email protected] or visit
http://www.physics.ohio-state.edu:80/~ryu/ for better figures and additional
movie files from simulations. To be published in Physical Review B1 01Jun9
Epidemiology in Latin America and the Caribbean: current situation and challenges
Background This article analyses the epidemiological research developments in Latin America and the Caribbean (LAC). It integrates the series commissioned by the International Epidemiological Association to all WHO Regions to identify global opportunities to promote the development of epidemiology
On Solving the Coronal Heating Problem
This article assesses the current state of understanding of coronal heating,
outlines the key elements of a comprehensive strategy for solving the problem,
and warns of obstacles that must be overcome along the way.Comment: Accepted by Solar Physics; Published by Solar Physic
New Quasars Detected via Variability in the QUEST1 Survey
By observing the high galactic latitude equatorial sky in drift scan mode
with the QUEST (QUasar Equatorial Survey Team) Phase 1 camera, multi-bandpass
photometry on a large strip of sky, resolved over a large range of time scales
(from hourly to biennially) has been collected. A robust method of ensemble
photometry revealed those objects within the scan region that fluctuate in
brightness at a statistically significant level. Subsequent spectroscopic
observations of a subset of those varying objects easily discriminated the
quasars from stars. For a 13-month time scale, 38% of the previously known
quasars within the scan region were seen to vary in brightness and subsequent
spectroscopic observation revealed that approximately 7% of all variable
objects in the scan region are quasars. Increasing the time baseline to 26
months increased the percentage of previously known quasars which vary to 61%
and confirmed via spectroscopy that 7% of the variable objects in the region
are quasars. This reinforces previously published trends and encourages
additional and ongoing synoptic searches for new quasars and their subsequent
analysis. During two spectroscopic observing campaigns, a total of 30 quasars
were confirmed, 11 of which are new discoveries and 19 of which were determined
to be previously known. Using the previously cataloged quasars as a benchmark,
we have found it possible to better optimize future variability surveys. This
paper reports on the subset of variable objects which were spectroscopically
confirmed as quasars.Comment: 22 pages, 3 figures, 4 tables. ApJ, submitted revised version: 19
pages, 4 figures, 2 tables, added clarifications, fixed typos, accepted by
ApJ 24 Jan 200
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