41 research outputs found
A Surprising Reversal of Temperatures in the Brown-Dwarf Eclipsing Binary 2MASS J05352184-0546085
The newly discovered brown-dwarf eclipsing binary 2MASS J05352184-0546085
provides a unique laboratory for testing the predictions of theoretical models
of brown-dwarf formation and evolution. The finding that the lower-mass brown
dwarf in this system is hotter than its higher-mass companion represents a
challenge to brown-dwarf evolutionary models, none of which predict this
behavior. Here we present updated determinations of the basic physical
properties of 2M0535-05, bolstering the surprising reversal of temperatures
with mass in this system. We compare these measurements with widely used
brown-dwarf evolutionary tracks, and find that the temperature reversal can be
explained by some models if the components of 2M0535-05 are mildly non-coeval,
possibly consistent with dynamical simulations of brown-dwarf formation.
Alternatively, a strong magnetic field on the higher-mass brown dwarf might
explain its anomalously low surface temperature, consistent with emerging
evidence that convection is suppressed in magnetically active, low-mass stars.
Finally, we discuss future observational and theoretical work needed to further
characterize and understand this benchmark system.Comment: 31 pages, 7 figures, accepted by Ap
Spatially-resolved Chandra Imaging Spectroscopy of the classical/weak-lined T Tauri system V710 Tau
We present spatially-resolved X-ray observations of the binary T Tauri star
system V710 Tau. Using Chandra's Advanced CCD Imaging Spectrometer (ACIS), we
imaged this 3.2'' separation binary system, consisting of a classical T Tauri
star, V710 Tau N, and a weak-lined T Tauri star, V710 Tau S. The Chandra
ACIS-S3 images -- obtained in two 9 ks exposures separated by about three
months (2004 December and 2005 April) -- cleanly resolve the V710 Tau binary,
demonstrating that both stars emit X-rays and thereby enabling the first
spectral/temporal study of the individual components of this mixed (classical
and weak-lined) T Tauri star binary system. The northern component, V710 Tau N,
appears to have been in a flaring state during the first (2004 December)
exposure. During this flare event, the X-ray flux of the classical T Tauri star
hardened significantly. Single-component plasma models with plasma temperatures
in the range kT ~ 0.7-1.1 keV are adequate to fit the observed X-ray spectra of
V710 Tau S in 2004 December and both stars in 2005 April. The 2004 December
flare-state observation of V710 Tau N requires a higher-temperature plasma
component (kT ~ 2.5 - 3.0 keV) in addition to the soft component (kT ~ 0.5 keV)
and is better fit by a model that includes a slightly enhanced Ne/Fe abundance
ratio. These results are generally consistent with statistical contrasts
between the X-ray emission properties of classical (rapidly accreting) vs.
weak-lined (weakly accreting or non-accreting) T Tauri stars.Comment: accepted to Ap
Towards the Rosetta Stone of planet formation
Transiting exoplanets (TEPs) observed just about 10 Myrs after formation of
their host systems may serve as the Rosetta Stone for planet formation
theories. They would give strong constraints on several aspects of planet
formation, e.g. time-scales (planet formation would then be possible within 10
Myrs), the radius of the planet could indicate whether planets form by
gravitational collapse (being larger when young) or accretion growth (being
smaller when young). We present a survey, the main goal of which is to find and
then characterise TEPs in very young open clusters.Comment: Poster contribution to Detection and Dynamics of Transiting
Exoplanets (Haute Provence Observatory Colloquium, 23-27 August 2010
Constraints on a second planet in the WASP-3 system
There have been previous hints that the transiting planet WASP-3 b is
accompanied by a second planet in a nearby orbit, based on small deviations
from strict periodicity of the observed transits. Here we present 17 precise
radial velocity measurements and 32 transit light curves that were acquired
between 2009 and 2011. These data were used to refine the parameters of the
host star and transiting planet. This has resulted in reduced uncertainties for
the radii and masses of the star and planet. The radial-velocity data and the
transit times show no evidence for an additional planet in the system.
Therefore, we have determined the upper limit on the mass of any hypothetical
second planet, as a function of its orbital period.Comment: Accepted for publication in The Astronomical Journa
A Spitzer Survey of Mid-Infrared Molecular Emission from Protoplanetary Disks II: Correlations and LTE Models
We present an analysis of Spitzer-IRS observations of H2O, OH, HCN, C2H2, and
CO2 emission, and Keck-NIRSPEC observations of CO emission, from a diverse
sample of T Tauri and Herbig Ae/Be circumstellar disks. We find that detections
and strengths of most mid-IR molecular emission features are correlated with
each other, suggesting a common origin and similar excitation conditions. We
note that the line detection efficiency is anti-correlated with the 13/30 um
SED spectral slope, which is a measure of the degree of grain settling in the
disk atmosphere. We also note a correlation between detection efficiency and
H-alpha equivalent width, and tentatively with accretion rate, suggesting that
accretional heating contributes to line excitation. If detected, H2O line
fluxes are correlated with the mid-IR continuum flux, and other co-varying
system parameters, such as L_star. However, significant sample variation,
especially in molecular line ratios, remains. LTE models of the H2O emission
show that line strength is primarily related to the best-fit emitting area, and
this accounts for most source-to-source variation in H2O emitted flux. Best-fit
temperatures and column densities cover only a small range of parameter space,
near 10^{18} cm-2 and 450 K for all sources, suggesting a high abundance of H2O
in many planet-forming regions. Other molecules have a range of excitation
temperatures from ~500-1500 K, also consistent with an origin in planet-forming
regions. We find molecular ratios relative to water of ~10^{-3} for all
molecules, with the exception of CO, for which n(CO)/n(H2O)~1. However, LTE
fitting caveats and differences in the way thermo-chemical modeling results are
reported make comparisons with such models difficult, and highlight the need
for additional observations coupled with the use of line-generating radiative
transfer codes
XMM-Newton survey of two Upper Scorpius regions
We study X-ray emission from young stars by analyzing deep XMM-Newton
observations of two regions of the Upper Scorpius association, having an age of
5 Myr. Based on near infrared and optical photometry we identify 22 Upper
Scorpius photometric members among the 224 detected X-ray sources. We derive
coronal properties of Upper Scorpius stars by performing X-ray spectral and
timing analysis. The study of four strong and isolated stellar flares allows us
to derive the length of the flaring loops. Among the 22 Upper Scorpius stars,
13 are identified as Upper Scorpius photometric members for the first time. The
sample includes 7 weak-line T Tauri stars and 1 classical T Tauri star, while
the nature of the remaining sources is unknown. Except for the intermediate
mass star HD 142578, all the detected USco sources are low mass stars of
spectral type ranging from G to late M. The X-ray emission spectrum of the most
intense Upper Scorpius sources indicates metal depleted plasma with temperature
of ~10 MK, resembling the typical coronal emission of active main sequence
stars. At least 59% of the detected members of the association have variable
X-ray emission, and the flaring coronal structures appear shorter than or
comparable to the stellar radii already at the Upper Scorpius age. We also find
indication of increasing plasma metallicity (up to a factor 20) during strong
flares. We identify a new galaxy cluster among the 224 X-ray source detected:
the X-ray spectrum of its intra cluster medium indicates a redshift of
0.41+/-0.02.Comment: 27 pages, 15 postscript figures, accepted for publication in
Astronomy and Astrophysics. A complete version of the paper, containing
better qaulity figures and Appendices B & C, is available at
http://www.astropa.unipa.it/Library/preprint.htm
Multi-site campaign for transit timing variations of WASP-12 b: possible detection of a long-period signal of planetary origin
The transiting planet WASP-12 b was identified as a potential target for
transit timing studies because a departure from a linear ephemeris was reported
in the literature. Such deviations could be caused by an additional planet in
the system. We attempt to confirm the existence of claimed variations in
transit timing and interpret its origin. We organised a multi-site campaign to
observe transits by WASP-12 b in three observing seasons, using 0.5-2.6-metre
telescopes. We obtained 61 transit light curves, many of them with
sub-millimagnitude precision. The simultaneous analysis of the best-quality
datasets allowed us to obtain refined system parameters, which agree with
values reported in previous studies. The residuals versus a linear ephemeris
reveal a possible periodic signal that may be approximated by a sinusoid with
an amplitude of 0.00068+/-0.00013 d and period of 500+/-20 orbital periods of
WASP-12 b. The joint analysis of timing data and published radial velocity
measurements results in a two-planet model which better explains observations
than single-planet scenarios. We hypothesize that WASP-12 b might be not the
only planet in the system and there might be the additional 0.1 M_Jup body on a
3.6-d eccentric orbit. A dynamical analysis indicates that the proposed
two-planet system is stable over long timescales.Comment: Accepted for publication in A&