1,428 research outputs found
A Simple Calculation in Service of Constraining the Rate of FU Orionis Outburst Events from Photometric Monitoring Surveys
An enigmatic and rare type of young stellar object is the FU Orionis class.
The members are interpreted as "outbursting," that is, currently in a state of
enhanced accretion by several orders of magnitude relative to the more modest
disk-to-star accretion rates measured in typical T Tauri stars. They are key to
our understanding of the history of stellar mass assembly and pre-main sequence
evolution, as well as critical to consider in the chemical and physical
evolution of the circumstellar environment -- where planets form. A common
supposition is that *all* T Tauri stars undergo repeated such outbursts, more
frequently in their earlier evolutionary stages when the disks are more
massive, so as to build up the requisite amount of stellar mass on the required
time scale. However, the actual data supporting this traditional picture of
episodically enhanced disk accretion are limited, and the observational
properties of the known sample of FU Ori objects quite diverse. To improve our
understanding of these rare objects, we outline the logic for meaningfully
constraining the rate of FU Ori outbursts and present numbers to guide
parameter choices in the analysis of time domain surveys.Comment: accepted for publication in Ap
Adaptive Optics Echelle Spectroscopy of [Fe II] 1.644 um in the RW Aur Jet: A Narrow Slice Down the Axis of the Flow
We present new adaptive optics echelle spectra of the near-infrared [Fe II]
lines in the redshifted and blueshifted jets from the T Tauri star RW Aur. The
spectra have an unprecedented combination of high spatial and spectral
resolution that makes it possible to trace the dynamics of the flow to a
projected distance of only 10 AU from the source. As noted by previous studies,
the redshifted flow is much slower than its fainter blueshifted counterpart.
Our observations clearly show that both the radial velocities and the emission
line widths are larger closer to the source on both sides of the jet. The line
widths are 20% - 30% of the jet velocity on both sides of the flow,
significantly larger than would be produced by a divergent constant velocity
flow. The observed line widths could arise from a layered velocity structure in
the jet or from magnetic waves. A bright knot in the redshifted jet has no
concomitant increase in line width, implying that it is not heated by a bow
shock. Alternate heating mechanisms include planar shocks, ambipolar diffusion
and magnetic reconnection
An optical spectroscopic H-R diagram for low-mass stars and brown dwarfs in Orion
The masses and temperatures of young low mass stars and brown dwarfs in star-
forming regions are not yet well established because of uncertainties in the
age of individual objects and the spectral type vs. temperature scale
appropriate for objects with ages of only a few Myr. Using multi-object optical
spectroscopy, 45 low-mass stars and brown dwarfs in the Trapezium Cluster in
Orion have been classified and 44 of these confirmed as bona fide cluster
members. The spectral types obtained have been converted to effective
temperatures using a temperature scale intermediate between those of dwarfs and
giants, which is suitable for young pre-main sequence objects. The objects have
been placed on an H-R diagram overlaid with theoretical isochrones. The low
mass stars and the higher mass substellar objects are found to be clustered
around the 1 Myr isochrone, while many of the lower mass substellar objects are
located well above this isochrone. An average age of 1 Myr is found for the
majority of the objects. Assuming coevality of the sources and an average age
of 1 Myr, the masses of the objects have been estimated and range from 0.018 to
0.44Msun. The spectra also allow an investigation of the surface gravity of the
objects by measurement of the sodium doublet equivalent width. With one
possible exception, all objects have low gravities, in line with young ages,
and the Na indices for the Trapezium objects lie systematically below those of
young stars and brown dwarfs in Chamaeleon, suggesting that the 820 nm Na index
may provide a sensitive means of estimating ages in young clusters.Comment: 19 pages, accepted by MNRA
Embedded Clusters and the IMF
Despite valiant efforts over nearly five decades, attempts to determine the
IMF over a complete mass range for galactic field stars and in open clusters
have proved difficult. Infrared imaging observations of extremely young
embedded clusters coupled with Monte Carlo modeling of their luminosity
functions are improving this situation and providing important new
contributions to our fundamental knowledge of the IMF and its universality in
both space and time.Comment: 6 pages, 2 figures to appear in "The IMF@50", Kluwer Academic Press,
eds. C. Corbelli, F. Palla, & Hans Zinnecke
B- and A-Type Stars in the Taurus-Auriga Star Forming Region
We describe the results of a search for early-type stars associated with the
Taurus-Auriga molecular cloud complex, a diffuse nearby star-forming region
noted as lacking young stars of intermediate and high mass. We investigate
several sets of possible O, B and early A spectral class members. The first is
a group of stars for which mid-infrared images show bright nebulae, all of
which can be associated with stars of spectral type B. The second group
consists of early-type stars compiled from (i) literature listings in SIMBAD;
(ii) B stars with infrared excesses selected from the Spitzer Space Telescope
survey of the Taurus cloud; (iii) magnitude- and color-selected point sources
from the 2MASS; and (iv) spectroscopically identified early-type stars from the
SDSS coverage of the Taurus region. We evaluated stars for membership in the
Taurus-Auriga star formation region based on criteria involving: spectroscopic
and parallactic distances, proper motions and radial velocities, and infrared
excesses or line emission indicative of stellar youth. For selected objects, we
also model the scattered and emitted radiation from reflection nebulosity and
compare the results with the observed spectral energy distributions to further
test the plausibility of physical association of the B stars with the Taurus
cloud. This investigation newly identifies as probable Taurus members three
B-type stars: HR 1445 (HD 28929), tau Tau (HD 29763), 72 Tau (HD 28149), and
two A-type stars: HD 31305 and HD 26212, thus doubling the number of stars A5
or earlier associated with the Taurus clouds. Several additional early-type
sources including HD 29659 and HD 283815 meet some, but not all, of the
membership criteria and therefore are plausible, though not secure, members.Comment: 31 pages, 18 figures, 6 tables. Accepted for publication in The
Astrophysical Journa
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
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
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