487 research outputs found
Binarity as a key factor in protoplanetary disk evolution: Spitzer disk census of the eta Chamaeleontis cluster
The formation of planets is directly linked to the evolution of the
circumstellar (CS) disk from which they are born. The dissipation timescales of
CS disks are, therefore, of direct astrophysical importance in evaluating the
time available for planet formation. We employ Spitzer Space Telescope spectra
to complete the CS disk census for the late-type members of the ~8 Myr-old eta
Chamaeleontis star cluster. Of the 15 K- and M-type members, eight show excess
emission. We find that the presence of a CS disk is anti-correlated with
binarity, with all but one disk associated with single stars. With nine single
stars in total, about 80% retain a CS disk. Of the six known or suspected close
binaries the only CS disk is associated with the primary of RECX 9. No
circumbinary disks have been detected. We also find that stars with disks are
slow rotators with surface values of specific angular momentum j = 2-15 j_sun.
All high specific angular momentum systems with j = 20-30 j_sun are confined to
the primary stars of binaries. This provides novel empirical evidence for
rotational disk locking and again demonstrates the much shorter disk lifetimes
in close binary systems compared to single star systems. We estimate the
characteristic mean disk dissipation timescale to be ~5 Myr and ~9 Myr for the
binary and single star systems, respectively.Comment: Accepted by ApJ
On the future of astrostatistics: statistical foundations and statistical practice
This paper summarizes a presentation for a panel discussion on "The Future of
Astrostatistics" held at the Statistical Challenges in Modern Astronomy V
conference at Pennsylvania State University in June 2011. I argue that the
emerging needs of astrostatistics may both motivate and benefit from
fundamental developments in statistics. I highlight some recent work within
statistics on fundamental topics relevant to astrostatistical practice,
including the Bayesian/frequentist debate (and ideas for a synthesis),
multilevel models, and multiple testing. As an important direction for future
work in statistics, I emphasize that astronomers need a statistical framework
that explicitly supports unfolding chains of discovery, with acquisition,
cataloging, and modeling of data not seen as isolated tasks, but rather as
parts of an ongoing, integrated sequence of analyses, with information and
uncertainty propagating forward and backward through the chain. A prototypical
example is surveying of astronomical populations, where source detection,
demographic modeling, and the design of survey instruments and strategies all
interact.Comment: 8 pp, 2 figures. To appear in "Statistical Challenges in Modern
Astronomy V," (Lecture Notes in Statistics, Vol. 209), ed. Eric D. Feigelson
and G. Jogesh Babu; publication planned for Sep 2012; see
http://www.springer.com/statistics/book/978-1-4614-3519-
Bright X-ray flares in Orion young stars from COUP: evidence for star-disk magnetic fields?
We have analyzed a number of intense X-ray flares observed in the Chandra
Orion Ultradeep Project (COUP), a 13 days observation of the Orion Nebula
Cluster (ONC). Analysis of the flare decay allows to determine the size, peak
density and magnetic field of the flaring structure. A total of 32 events (the
most powerful 1% of COUP flares), have sufficient statistics for the analysis.
A broad range of decay times (from 10 to 400 ks) are present in the sample.
Peak flare temperatures are often very high, with half of the flares in the
sample showing temperatures in excess of 100 MK. Significant sustained heating
is present in the majority of the flares. The magnetic structures which are
found, are in a number of cases very long, with semi-lengths up to 10^12 cm,
implying the presence of magnetic fields of hundreds of G extending to
comparable distance from the stellar photosphere. These very large sizes for
the flaring structures ($ >> R_*) are not found in more evolved stars, where,
almost invariably, the same type of analysis results in structures with L <=
R_*. As the majority of young stars in the ONC are surrounded by disks, we
speculate that the large magnetic structures which confine the flaring plasma
are actually the same type of structures which channel the plasma in the
magnetospheric accretion paradigm, connecting the star's photosphere with the
accretion disk.Comment: Accepted to ApJS, COUP special issu
X-ray flares in Orion young stars. I. Flare characteristics
Pre-main sequence (PMS) stars are known to produce powerful X-ray flares
which resemble magnetic reconnection solar flares scaled by factors up to 10^4.
However, numerous puzzles are present including the structure of X-ray emitting
coronae and magnetospheres, effects of protoplanetary disks, and effects of
stellar rotation. To investigate these issues in detail, we examine 216 of the
brightest flares from 161 PMS stars observed in the Chandra Orion Ultradeep
Project (COUP). These constitute the largest homogeneous dataset of PMS, or
indeed stellar flares at any stellar age, ever acquired. Our effort is based on
a new flare spectral analysis technique that avoids nonlinear parametric
modeling. It can be applied to much weaker flares and is more sensitive than
standard methods. We provide a catalog with >30 derived flare properties and an
electronic atlas for this unique collection of stellar X-ray flares. The
current study (Paper I) examines the flare morphologies, and provides general
comparison of COUP flare characteristics with those of other active X-ray stars
and the Sun. Paper II will concentrate on relationships between flare behavior,
protoplanetary disks, and other stellar properties. Several results are
obtained. First, the COUP flares studied here are among the most powerful,
longest, and hottest stellar X-ray flares ever studied. Second, no significant
statistical differences in peak flare luminosity or temperature distributions
are found among different morphological flare classes, suggesting a common
underlying mechanism for all flares. Third, comparison with the general
solar-scaling laws indicates that COUP flares may not fit adequately proposed
power-temperature and duration-temperature solar-stellar fits. Fourth, COUP
super-hot flares are found to be brighter but shorter than ... ABRIDGEDComment: Accepted for publication in ApJ (07/11/08); 63 pages, 16 figures, 4
table
The evolution of X-ray emission in young stars
We study the relation between age and magnetic activity in late-type pre-main
sequence (PMS) stars, for the first time using mass-stratified subsamples. The
effort is based on the Chandra Orion Ultradeep Project (COUP) which provides
very sensitive and homogenous X-ray data on a uniquely large sample of 481
optically well-characterized low-extinction low-mass members of the Orion
Nebula Cluster, for which individual stellar masses and ages could be
determined. More than 98 percent of the stars in this sample are detected as
X-ray sources. Within the PMS phase for stellar ages in the range
Myr, we establish a mild decay in activity with stellar age roughly as
. On longer timescales, when the Orion stars are
compared to main sequence stars, the X-ray luminosity decay law for stars in
the M mass range is more rapid with over the wide range of ages yr. The
magnetic activity history for M stars with masses is
distinctly different. Only a mild decrease in X-ray luminosity, and even a mild
increase in and , is seen over the 1-100 Myr
range, though the X-ray emission does decay over long timescales on the main
sequence. Together with COUP results on the absence of a rotation-activity
relation in Orion stars, we find that the activity-age decay is strong across
the entire history of solar-type stars but is not attributable to rotational
deceleration during the early epochs. A combination of tachocline and
distributed convective dynamos may be operative in young solar-type stars. The
results for the lowest mass stars are most easily understood by the dominance
of convective dynamos during both the PMS and main sequence phases.Comment: accepted for ApJS, COUP special issu
Chandra Discovery of a 100 kpc X-ray Jet in PKS 0637--752
The quasar PKS 0637-753, the first celestial X-ray target of the Chandra
X-ray Observatory, has revealed asymmetric X-ray structure extending from 3 to
12 arcsec west of the quasar, coincident with the inner portion of the jet
previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a
redshift of z=0.651, the jet is the largest (~100 kpc) and most luminous
(~10^{44.6} ergs/s) of the few so far detected in X-rays. This letter presents
a high resolution X-ray image of the jet, from 42 ks of data when PKS 0637-753
was on-axis and ACIS-S was near the optimum focus. For the inner portion of the
radio jet, the X-ray morphology closely matches that of new ATCA radio images
at 4.8 and 8.6 GHz. Observations of the parsec scale core using the VSOP space
VLBI mission show structure aligned with the X-ray jet, placing important
constraints on the X-ray source models. HST images show that there are three
small knots coincident with the peak radio and X-ray emission. Two of these are
resolved, which we use to estimate the sizes of the X-ray and radio knots. The
outer portion of the radio jet, and a radio component to the east, show no
X-ray emission to a limit of about 100 times lower flux.
The X-ray emission is difficult to explain with models that successfully
account for extra-nuclear X-ray/radio structures in other active galaxies. We
think the most plausible is a synchrotron self-Compton (SSC) model, but this
would imply extreme departures from the conventional minimum-energy and/or
homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung
models for the jet X-rays, unless multicomponent or ad hoc geometries are
invoked.Comment: 5 Pages, 2 Figures. Submitted to Ap. J. Letter
New distance measures for classifying X-ray astronomy data into stellar classes
The classification of the X-ray sources into classes (such as extragalactic
sources, background stars, ...) is an essential task in astronomy. Typically,
one of the classes corresponds to extragalactic radiation, whose photon
emission behaviour is well characterized by a homogeneous Poisson process. We
propose to use normalized versions of the Wasserstein and Zolotarev distances
to quantify the deviation of the distribution of photon interarrival times from
the exponential class. Our main motivation is the analysis of a massive dataset
from X-ray astronomy obtained by the Chandra Orion Ultradeep Project (COUP).
This project yielded a large catalog of 1616 X-ray cosmic sources in the Orion
Nebula region, with their series of photon arrival times and associated
energies. We consider the plug-in estimators of these metrics, determine their
asymptotic distributions, and illustrate their finite-sample performance with a
Monte Carlo study. We estimate these metrics for each COUP source from three
different classes. We conclude that our proposal provides a striking amount of
information on the nature of the photon emitting sources. Further, these
variables have the ability to identify X-ray sources wrongly catalogued before.
As an appealing conclusion, we show that some sources, previously classified as
extragalactic emissions, have a much higher probability of being young stars in
Orion Nebula.Comment: 29 page
The Properties of X-ray Luminous Young Stellar Objects in the NGC 1333 and Serpens Embedded Clusters
We present Chandra X-ray data of the NGC 1333 embedded cluster, combining
these data with existing Chandra data, Sptizer photometry and ground based
spectroscopy of both the NGC 1333 & Serpens North clusters to perform a
detailed study of the X-ray properties of two of the nearest embedded clusters
to the Sun. In NGC 1333, a total of 95 cluster members are detected in X-rays,
of which 54 were previously identified with Spitzer. Of the Spitzer sources, we
detect 23% of the Class I protostars, 53% of the Flat Spectrum sources, 52% of
the Class II, and 50% of the Transition Disk YSOs. Forty-one Class III members
of the cluster are identified, bringing the total identified YSO population to
178. The X-ray Luminosity Functions (XLFs) of the NGC 1333 and Serpens clusters
are compared to each other and the Orion Nebula Cluster. Based on this
comparison, we obtain a new distance for the Serpens cluster of 360+22/-13 pc.
The X-ray luminosity was found to depend on the bolometric luminosity as in
previous studies of other clusters, and that Lx depends primarily on the
stellar surface area. In the NGC 1333 cluster, the Class III sources have a
somewhat higher X-ray luminosity for a given surface area. We also find
evidence in NGC 1333 for a jump in the X-ray luminosity between spectral types
of M0 and K7, we speculate that this may result from the presence of radiative
zones in the K-stars. The gas column density vs. extinction in the NGC 1333 was
found to be N_H = 0.89 +/- 0.13 x 10^22 A_K, this is lower than expected of the
standard ISM but similar to that found previously in the Serpens Cloud Core.Comment: 58 pages, 14 figures, accepted by A
The Structure of the Star-forming Cluster RCW 38
We present a study of the structure of the high mass star-forming region
RCW~38 and the spatial distribution of its young stellar population. Spitzer
IRAC photometry 3-8um are combined with 2MASS near-IR data to identify young
stellar objects by IR-excess emission from their circumstellar material.
Chandra X-ray data are used to identify class III pre-main sequence stars
lacking circumstellar material. We identify 624 YSOs: 23 class 0/I and 90 flat
spectrum protostars, 437 Class II stars, and 74 Class III stars. We also
identify 29 (27 new) O star candidates over the IRAC field. Seventy-two stars
exhibit IR-variability, including seven class 0/I and 12 flat spectrum YSOs. A
further 177 tentative candidates are identified by their location in the IRAC
[3.6] vs. [3.6]-[5.8] cmd. We find strong evidence of subclustering in the
region. Three subclusters were identified surrounding the central cluster, with
massive and variable stars in each subcluster. The central region shows
evidence of distinct spatial distributions of the protostars and pre-main
sequence stars. A previously detected IR cluster, DB2001_Obj36, has been
established as a subcluster of RCW 38. This suggests that star formation in RCW
38 occurs over a more extended area than previously thought. The gas to dust
ratio is examined using the X-ray derived hydrogen column density, N_H and the
K-band extinction, and found to be consistent with the diffuse ISM, in contrast
with Serpens & NGC1333. We posit that the high photoionising flux of massive
stars in RCW 38 affects the agglomeration of the dust grains.Comment: 98 pages, 15 figure
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