180 research outputs found
Testing the Disk Regulation Paradigm with Spitzer Observations. II. A Clear Signature of Star-Disk Interaction in NGC 2264 and the Orion Nebula Cluster
Observations of PMS star rotation periods reveal slow rotators in young
clusters of various ages, indicating that angular momentum is somehow removed
from these rotating masses. The mechanism by which spin-up is regulated as
young stars contract has been one of the longest-standing problems in star
formation. Attempts to observationally confirm the prevailing theory that
magnetic interaction between the star and its circumstellar disk regulates
these rotation periods have produced mixed results. In this paper, we use the
unprecedented disk identification capability of the Spitzer Space Telescope to
test the star-disk interaction paradigm in two young clusters, NGC 2264 and the
Orion Nebula Cluster (ONC). We show that once mass effects and sensitivity
biases are removed, a clear increase in the disk fraction with period can be
observed in both clusters across the entire period range populated by cluster
members. We also show that the long-period peak (P 8 days) of the bimodal
distribution observed for high-mass stars in the ONC is dominated by a
population of stars possessing a disk, while the short-period peak (P 2
days) is dominated by a population of stars without a disk. Our results
represent the strongest evidence to date that star-disk interaction regulates
the angular momentum of these young stars. This study will make possible
quantitative comparisons between the observed period distributions of stars
with and without a disk and numerical models of the angular momentum evolution
of young stars.Comment: 31 pages, 7 figures, 2 tables. Accepted for publication in Ap
Discovery of Par 1802 as a Low-Mass, Pre-Main-Sequence Eclipsing Binary in the Orion Star-Forming Region
We report the discovery of a pre-main-sequence, low-mass, double-lined,
spectroscopic, eclipsing binary in the Orion star-forming region. We present
our observations including radial velocities derived from optical
high-resolution spectroscopy, and present an orbit solution that permits the
determination of precise empirical masses for both components of the system. We
measure that Par 1802 is composed of two equal mass (0.39+-0.03, 0.40+-0.03
Msun) stars in a circular, 4.7 day orbit. There is strong evidence, such as the
system exhibiting strong Li lines and a center-of-mass velocity consistent with
cluster membership, that this system is a member of the Orion star-forming
region and quite possibly the Orion Nebula Cluster, and therefore has an age of
only a few million years. As there are currently only a few empirical mass and
radius measurements for low-mass, PMS stars, this system presents an
interesting test for the predictions of current theoretical models of pre-main
sequence stellar evolution.Comment: 21 pages, 6 figures, 2 tables; Fig 2 caption edite
Photometric monitoring of the young star Par 1724 in Orion
We report new photometric observations of the 200000 year old naked weak-line
run-away T Tauri star Par 1724, located north of the Trapezium cluster in
Orion. We observed in the broad band filters B, V, R, and I using the 90cm
Dutch telescope on La Silla, the 80cm Wendelstein telescope, and a 25cm
telescope of the University Observatory Jena in Grossschwabhausen near Jena.
The photometric data in V and R are consistent with a 5.7 day rotation period
due to spots, as observed before between 1960ies and 2000. Also, for the first
time, we present evidence for a long-term 9 or 17.5 year cycle in photometric
data (V band) of such a young star, a cycle similar to that to of the Sun and
other active stars.Comment: AN in press (eps or ps files on
http://www.astro.uni-jena.de/Observations/gsh/gsh_papers.htm
The Angular Momentum Evolution of 0.1-10 Msun Stars From the Birthline to the Main Sequence
(Abridged) Projected rotational velocities (vsini) have been measured for a
sample of 145 stars with masses between 0.4 and >10 Msun (median mass 2.1 Msun)
located in the Orion star-forming complex. These measurements have been
supplemented with data from the literature for Orion stars with masses as low
as 0.1 Msun. The primary finding from analysis of these data is that the upper
envelope of the observed values of angular momentum per unit mass (J/M) varies
as M^0.25 for stars on convective tracks having masses in the range ~0.1 to ~3
Msun. This power law extends smoothly into the domain of more massive stars (3
to 10 Msun), which in Orion are already on the ZAMS. This result stands in
sharp contrast to the properties of main sequence stars, which show a break in
the power law and a sharp decline in J/M with decreasing mass for stars with M
<2 Msun. A second result of our study is that this break is seen already among
the PMS stars in our Orion sample that are on radiative tracks, even though
these stars are only a few million years old. A comparison of rotation rates
seen for stars on either side of the convective-radiative boundary shows that
stars do not rotate as solid bodies during the transition from convective to
radiative tracks.Comment: to appear in Ap
The Age of the Solar Neighbourhood
High-quality Hipparcos data for a complete sample of nearly 12000
main-sequence and subgiant stars, together with Padua isochrones, are used to
constrain the star-formation history of the solar neigbourhood and the
processes that stochastically accelerate disk stars. The velocity dispersion of
a coeval group of stars is found to increase with time from ~8 kms at birth as
t^{0.33}. In the fits, the slope of the IMF near 1 Msun proves to be degenerate
with the rate at which the star-formation rate declines. If the slope of the
IMF is to lie near Salpeter's value, -2.35, the star-formation rate has to be
very nearly constant. The age of the solar neighbourhood is found to be
11.2+/-0.75 Gyr with remarkably little sensitivity to variations in the assumed
metallicity distribution of old disk stars. This age is only a Gyr younger than
the age of the oldest globular clusters when the same isochrones and distance
scale are employed. It is compatible with current indications of the redshift
of luminous galaxy formation only if there is a large cosmological constant. A
younger age is formally excluded because it provides a poor fit to the number
density of red stars. Since this density is subject to a significantly
uncertain selection function, ages as low as 9 Gyr are plausible even though
they lie outside our formal error bars.Comment: 7 pages; typographical corrections onl
Three very young HgMn stars in the Orion OB1 Association
We report the detection of three mercury-manganese stars in the Orion OB1
association. HD 37886 and BD-0 984 are in the approximately 1.7 million year
old Orion OB1b. HD 37492 is in the approximately 4.6 million year old Orion
OB1c. Orion OB1b is now the youngest cluster with known HgMn star members. This
places an observational upper limit on the time scale needed to produce the
chemical peculiarities seen in mercury-manganese stars, which should help in
the search for the cause or causes of the peculiar abundances in HgMn and other
chemically peculiar upper main sequence stars.Comment: 8 pages including 1 figure. To appear in Astrophysical Journal
Letter
The distance to the Orion Nebula
We have used the Very Long Baseline Array to measure the trigonometric
parallax of several member stars of the Orion Nebula Cluster showing
non-thermal radio emission. We have determined the distance to the cluster to
be 414 +/- 7 pc. Our distance determination allows for an improved calibration
of luminosities and ages of young stars. We have also measured the proper
motions of four cluster stars which, when accurate radial velocities are
measured, will put strong constraints on the origin of the cluster.Comment: 13 pages, 3 figures, to appear in Astronomy & Astrophyisc
Russell Lecture: Dark Star Formation and Cooling Instability
Optically thin cooling gas at most temperatures above 30K will make
condensations by pressure pushing material into cool dense regions. This works
without gravity. Cooling condensations will flatten and become
planar/similarity solutions. Most star formation may start from cooling
condensations - with gravity only important in the later stages. The idea that
some of the dark matter could be pristine white dwarfs that condensed slowly on
to planetary sized seeds without firing nuclear reactions is found lacking.
However, recent observations indicate fifty times more halo white dwarfs than
have been previously acknowledged; enough to make the halo fraction observed as
MACHOS.
A cosmological census shows that only 1% of the mass of the Universe is of
known constitution.Comment: 32 Pages, Latex (uses aastex & natbib), 5 eps figures, submitted to
ApJ April 200
X-ray emitting young stars in the Orion Nebula
The Orion Nebula Cluster and the molecular cloud in its vicinity have been
observed with the ACIS-I detector on board the Chandra X-ray Observatory with
23 hours exposure. We detect 1075 X-ray sources: 91% are spatially associated
with known stellar members of the cluster, and 7% are newly identified deeply
embedded cloud members. This provides the largest X-ray study of a pre-main
sequence stellar population. We examine here the X-ray properties of Orion
young stars as a function of mass. Results include: (a) the discovery of rapid
variability in the O9.5 31 M_o star \theta^2A Ori, and several early B stars,
inconsistent with the standard model of X-ray production in small wind shocks;
(b) support for the hypothesis that intermediate-mass mid-B through A type
stars do not themselves produce significant X-ray emission; (c) confirmation
that low-mass G- through M-type T Tauri stars exhibit powerful flaring but
typically at luminosities considerably below the `saturation' level; (d)
confirmation that the presence or absence of a circumstellar disk has no
discernable effect on X-ray emission; (e) evidence that T Tauri plasma
temperatures are often very high with T >= 100 MK, even when luminosities are
modest and flaring is not evident; and (f) detection of the largest sample of
pre-main sequence very low mass objects showing high flaring levels and a
decline in magnetic activity as they evolve into L- and T-type brown dwarfs.Comment: 82 pages, 16 figures, 6 tables. To appear in the Astrophysical
Journal. For a version with high quality images and electronic tables, see
ftp://ftp.astro.psu.edu/pub/edf/orion1
A Correlation Between Pre-Main Sequence Stellar Rotation Rates and IRAC Excesses in Orion
Early observations of T Tauri stars suggested that stars with evidence of
circumstellar accretion disks rotated slower than stars without such evidence,
but more recent results are not as clear. Near-IR circumstellar disk
indicators, though the most widely available, are subject to uncertainties that
can result from inner disk holes and/or the system inclination. Mid-infrared
observations are less sensitive to such effects, but until now, these
observations have been difficult to obtain. The Spitzer Space Telescope now
easily enables mid-infrared measurements of large samples of PMS stars covering
a broad mass range in nearby star-forming regions. Megeath and collaborators
surveyed the Orion Molecular Clouds (~1 Myr) with the IRAC instrument (3.6,
4.5, 5.8, 8 microns) as part of a joint IRAC and MIPS GTO program. We examine
the relationship between rotation and Spitzer mid-IR fluxes for ~900 stars in
Orion for stars between 3 and 0.1 Msun. We find in these Spitzer data the
clearest indication to date that stars with longer periods are more likely than
those with short periods to have IR excesses suggestive of disks.Comment: 19 pages, 7 figures, accepted by Ap
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