93 research outputs found
Generation of an optimal target list for the Exoplanet Characterisation Observatory (EChO)
The Exoplanet Characterisation Observatory (EChO) has been studied as a space
mission concept by the European Space Agency in the context of the M3 selection
process. Through direct measurement of the atmospheric chemical composition of
hundreds of exoplanets, EChO would address fundamental questions such as: What
are exoplanets made of? How do planets form and evolve? What is the origin of
exoplanet diversity?
More specifically, EChO is a dedicated survey mission for transit and eclipse
spectroscopy capable of observing a large, diverse and well-defined planetary
sample within its four to six year mission lifetime.
In this paper we use the end-to-end instrument simulator EChOSim to model the
currently discovered targets, to gauge which targets are observable and assess
the EChO performances obtainable for each observing tier and time. We show that
EChO would be capable of observing over 170 relativity diverse planets if it
were launched today, and the wealth of optimal targets for EChO expected to be
discovered in the next 10 years by space and ground-based facilities is simply
overwhelming.
In addition, we build on previous molecular detectability studies to show
what molecules and abundances will be detectable by EChO for a selection of
real targets with various molecular compositions and abundances.
EChO's unique contribution to exoplanetary science will be in identifying the
main constituents of hundreds of exoplanets in various mass/temperature
regimes, meaning that we will be looking no longer at individual cases but at
populations. Such a universal view is critical if we truly want to understand
the processes of planet formation and evolution in various environments.
In this paper we present a selection of key results. The full results are
available online (http://www.ucl.ac.uk/exoplanets/echotargetlist/).Comment: Accepted for publication in Experimental Astronomy, 20 pages, 10
figures, 3 table
Membership of the Orion Nebula Population from the Chandra Orion Ultradeep Project
The Chandra Orion Ultradeep project (COUP) observation described in a
companion paper by Getman et al. provides an exceptionally deep X-ray survey of
the Orion Nebula Cluster and associated embedded young stellar objects.
Membership of the region is important for studies of the stellar IMF, cluster
dynamics, and star formation. The COUP study detected 1616 X-ray sources. In
this study we confirm cloud membership for 1315 stars, identify 16 probable
foreground field stars having optical counterparts with discrepant proper
motions, and classify the remaining 285 X-ray sources, of which 51 are lightly
and 234 heavily obscured. The 51 lightly obscured sources without known
counterparts fall into three groups. (i) Sixteen are likely new members of the
Orion Nebula Cluster. (ii) Two with unusually soft and non-flaring X-ray
emission appear to be associated with nebular shocks, and may be new examples
of X-rays produced at the bow shocks of Herbig-Haro outflows. (iii) The
remaining thirty three are very weak uncertain sources, possibly spurious. Out
of 234 heavily absorbed sources without optical or near-infrared counterparts
75 COUP sources are likely new embedded cloud members (with membership for 42
confirmed by powerful X-ray flares), and the remaining 159 are likely
extragalactic AGN seen through the molecular cloud, as argued by a careful
simulation of the extragalactic background population. Finally, a few new
binary companions to Orion stars may have been found, but most cases of
proximate COUP sources can be attributed to chance superpositions in this
crowded field.Comment: 49 pages, 6 figures, 5 tables. Accepted for publication in ApJS,
special issue dedicated to Chandra Orion Ultradeep Project. A version with
high quality figures can be found at
http://www.astro.psu.edu/users/gkosta/COUP_Membership.pd
Non-thermal processes in colliding-wind massive binaries: the contribution of Simbol-X to a multiwavelength investigation
Several colliding-wind massive binaries are known to be non-thermal emitters
in the radio domain. This constitutes strong evidence for the fact that an
efficient particle acceleration process is at work in these objects. The
acceleration mechanism is most probably the Diffusive Shock Acceleration (DSA)
process in the presence of strong hydrodynamic shocks due to the
colliding-winds. In order to investigate the physics of this particle
acceleration, we initiated a multiwavelength campaign covering a large part of
the electromagnetic spectrum. In this context, the detailed study of the hard
X-ray emission from these sources in the SIMBOL-X bandpass constitutes a
crucial element in order to probe this still poorly known topic of
astrophysics. It should be noted that colliding-wind massive binaries should be
considered as very valuable targets for the investigation of particle
acceleration in a similar way as supernova remnants, but in a different region
of the parameter space.Comment: 4 pages, 2 figures, to appear in Proc. of the Second Internqtionql
Simbol-X Symposium, held in Paris (France
Activity and rotation of the X-ray emitting Kepler stars
The relation between magnetic activity and rotation in late-type stars
provides fundamental information on stellar dynamos and angular momentum
evolution. Rotation/activity studies found in the literature suffer from
inhomogeneity in the measure of activity indexes and rotation periods. We
overcome this limitation with a study of the X-ray emitting late-type
main-sequence stars observed by XMM-Newton and Kepler. We measure rotation
periods from photometric variability in Kepler light curves. As activity
indicators, we adopt the X-ray luminosity, the number frequency of white-light
flares, the amplitude of the rotational photometric modulation, and the
standard deviation in the Kepler light curves. The search for X-ray flares in
the light curves provided by the EXTraS (Exploring the X-ray Transient and
variable Sky) FP-7 project allows us to identify simultaneous X-ray and
white-light flares. A careful selection of the X-ray sources in the Kepler
field yields 102 main-sequence stars with spectral types from A to M. We find
rotation periods for 74 X-ray emitting main-sequence stars, 22 of which without
period reported in the previous literature. In the X-ray activity/rotation
relation, we see evidence for the traditional distinction of a saturated and a
correlated part, the latter presenting a continuous decrease in activity
towards slower rotators. For the optical activity indicators the transition is
abrupt and located at a period of ~ 10 d but it can be probed only marginally
with this sample which is biased towards fast rotators due to the X-ray
selection. We observe 7 bona-fide X-ray flares with evidence for a white-light
counterpart in simultaneous Kepler data. We derive an X-ray flare frequency of
~ 0.15 d^{-1} , consistent with the optical flare frequency obtained from the
much longer Kepler time-series.Comment: Accepted for publication in A&A. 31 pages, 19 figure
Multi-wavelength Data Analysis of ONC X-ray Sources
To take advantage of the increasing amount of available multi-wavelength astronomical
data, we are statistically merging data from several wavelength regimes to analyze astronomical
objects. This simultaneous analysis of emission across a wide range of wavelengths will help
to provide a composite understanding of young stellar objects. A statistical clustering technique
coupled with fused multi-wavelength data from the optical, infrared, and X-ray can provide insight
into the physical mechanisms responsible for the intense emission from young stars in different
wavelength regimes and can be used to view trends and correlations between those regimes. We
present some details of the data fusion followed by our results thus far analyzing spectral data from
pre-main sequence (PMS) stars in the Orion Nebula Cluster (ONC)
X-ray Photoevaporation-starved T Tauri Accretion
X-ray luminosities of accreting T Tauri stars are observed to be
systematically lower than those of non-accretors. There is as yet no widely
accepted physical explanation for this effect, though it has been suggested
that accretion somehow suppresses, disrupts or obscures coronal X-ray activity.
Here, we suggest that the opposite might be the case: coronal X-rays modulate
the accretion flow. We re-examine the X-ray luminosities of T Tauri stars in
the Orion Nebula Cluster and find that not only are accreting stars
systematically fainter, but that there is a correlation between mass accretion
rate and stellar X-ray luminosity. We use the X-ray heated accretion disk
models of Ercolano et al. to show that protoplanetary disk photoevaporative
mass loss rates are strongly dependent on stellar X-ray luminosity and
sufficiently high to be competitive with accretion rates. X-ray disk heating
appears to offer a viable mechanism for modulating the gas accretion flow and
could be at least partially responsible for the observed correlation between
accretion rates and X-ray luminosities of T Tauri stars.Comment: 4 pages 3 figures, ApJ Letters, in pres
X-ray emission from young brown dwarfs in the Orion Nebula Cluster
We use the sensitive X-ray data from the Chandra Orion Ultradeep Project
(COUP) to study the X-ray properties of 34 spectroscopically-identified brown
dwarfs with near-infrared spectral types between M6 and M9 in the core of the
Orion Nebula Cluster. Nine of the 34 objects are clearly detected as X-ray
sources. The apparently low detection rate is in many cases related to the
substantial extinction of these brown dwarfs; considering only the BDs with
mag, nearly half of the objects (7 out of 16) are detected in
X-rays. Our 10-day long X-ray lightcurves of these objects exhibit strong
variability, including numerous flares. While one of the objects was only
detected during a short flare, a statistical analysis of the lightcurves
provides evidence for continuous (`quiescent') emission in addition to flares
for all other objects. Of these, the M9 brown dwarf COUP 1255 = HC 212
is one of the coolest known objects with a clear detection of quiescent X-ray
emission. The X-ray properties (spectra, fractional X-ray luminosities, flare
rates) of these young brown dwarfs are similar to those of the low-mass stars
in the ONC, and thus there is no evidence for changes in the magnetic activity
around the stellar/substellar boundary, which lies at M6 for ONC
sources. Since the X-ray properties of the young brown dwarfs are also similar
to those of M6--M9 field stars, the key to the magnetic activity in very cool
objects seems to be the effective temperature, which determines the degree of
ionization in the atmosphere.Comment: accepted for ApJS, COUP special issu
Debris Disks of Members of the Blanco 1 Open Cluster
We have used the Spitzer Space Telescope to obtain Multiband Imaging
Photometer for Spitzer (MIPS) 24 um photometry for 37 members of the ~100 Myr
old open cluster Blanco 1. For the brightest 25 of these stars (where we have
3sigma uncertainties less than 15%), we find significant mid-IR excesses for
eight stars, corresponding to a debris disk detection frequency of about 32%.
The stars with excesses include two A stars, four F dwarfs and two G dwarfs.
The most significant linkage between 24 um excess and any other stellar
property for our Blanco 1 sample of stars is with binarity. Blanco 1 members
that are photometric binaries show few or no detected 24 um excesses whereas a
quarter of the apparently single Blanco 1 members do have excesses. We have
examined the MIPS data for two other clusters of similar age to Blanco 1 -- NGC
2547 and the Pleiades. The AFGK photometric binary star members of both of
these clusters also show a much lower frequency of 24 um excesses compared to
stars that lie near the single-star main sequence. We provide a new
determination of the relation between V-Ks color and Ks-[24] color for main
sequence photospheres based on Hyades members observed with MIPS. As a result
of our analysis of the Hyades data, we identify three low mass Hyades members
as candidates for having debris disks near the MIPS detection limit.Comment: Accepted to Ap
X-Ray flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary Disks
We study the properties of powerful X-ray flares from 161 pre-main sequence
(PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula
region. Relationships between flare properties, protoplanetary disks and
accretion are examined in detail to test models of star-disk interactions at
the inner edge of the accretion disks. Previous studies had found no
differences in flaring between diskfree and accreting systems other than a
small overall diminution of X-ray luminosity in accreting systems. The most
important finding is that X-ray coronal extents in fast-rotating diskfree stars
can significantly exceed the Keplerian corotation radius, whereas X-ray loop
sizes in disky and accreting systems do not exceed the corotation radius. This
is consistent with models of star-disk magnetic interaction where the inner
disk truncates and confines the PMS stellar magnetosphere. We also find two
differences between flares in accreting and diskfree PMS stars. First, a
subclass of super-hot flares with peak plasma temperatures exceeding 100 MK are
preferentially present in accreting systems. Second, we tentatively find that
accreting stars produce flares with shorter durations. Both results may be
consequences of the distortion and destabilization of the stellar magnetosphere
by the interacting disk. Finally, we find no evidence that any flare types,
even slow-rise flat-top flares are produced in star-disk magnetic loops. All
are consistent with enhanced solar long-duration events with both footprints
anchored in the stellar surface.Comment: Accepted for publication in ApJ (07/17/08); 46 pages, 14 figures, 2
table
Classification of Chandra X-Ray Sources in Cygnus OB2
© 2023. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/We have devised a predominantly Naive Bayes−based method to classify X-ray sources detected by Chandra in the Cygnus OB2 association into members, foreground objects, and background objects. We employ a variety of X-ray, optical, and infrared characteristics to construct likelihoods using training sets defined by well-measured sources. Combinations of optical photometry from the Sloan Digital Sky Survey (riz) and Isaac Newton Telescope Photometric Hα Survey (r I i I Hα), infrared magnitudes from United Kingdom Infrared Telescope Deep Sky Survey and Two-Micron All Sky Survey (JHK), X-ray quantiles and hardness ratios, and estimates of extinction A v are used to compute the relative probabilities that a given source belongs to one of the classes. Principal component analysis is used to isolate the best axes for separating the classes for the photometric data, and Gaussian component separation is used for X-ray hardness and extinction. Errors in the measurements are accounted for by modeling as Gaussians and integrating over likelihoods approximated as quartic polynomials. We evaluate the accuracy of the classification by inspection and reclassify a number of sources based on infrared magnitudes, the presence of disks, and spectral hardness induced by flaring. We also consider systematic errors due to extinction. Of the 7924 X-ray detections, 5501 have a total of 5597 optical/infrared matches, including 78 with multiple counterparts. We find that ≈6100 objects are likely association members, ≈1400 are background objects, and ≈500 are foreground objects, with an accuracy of 96%, 93%, and 80%, respectively, with an overall classification accuracy of approximately 95%.Peer reviewe
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