898 research outputs found
Near-Infrared Time-Series Photometry in the Field of Cygnus OB2 Association I - Rotational Scenario For Candidate Members
In the last decades, the early pre main sequence stellar rotational evolution
picture has been constrained by studies targeting different young regions at a
variety of ages. Observational studies suggest a mass-rotation dependence, and
for some mass ranges a connection between rotation and the presence of a
circumstellar disk. Not still fully explored, though, is the role of
environmental conditions on the rotational regulation.
We investigate the rotational properties of candidate members of the young
massive association Cygnus OB2. The Stetson variability index, Lomb-Scargle
periodogram, Saunders statistics, string/rope length method, and visual
verification of folded light curves were applied to select 1224 periodic
variable stars. Completeness and contamination of the periodic sample was
derived from Monte Carlo simulations, out of which 894 periods were considered
reliable. Our study was considered reasonably complete for periods from 2 to 30
days.
The general rotational scenario seen in other young regions is confirmed by
Cygnus OB2 period distributions, with disked stars rotating on average slower
than non-disked stars. A mass-rotation dependence was also verified, but as in
NGC 6530, lower mass stars are rotating on average slower than higher mass
stars, with an excess of slow rotators among the lower mass population. The
effect of the environment on the rotational properties of the association was
investigated by re-analysing the results while taking into account the incident
UV radiation arising from O stars in the association. Results compatible with
the disk-locking scenario were verified for stars with low UV incidence, but no
statistical significant relation between rotation and disk presence was
verified for stars with high UV incidence suggesting that massive stars can
have an important role on regulating the rotation of nearby low mass stars.Comment: Submitted on December 23, 201
Dynamical star-disk interaction in the young stellar system V354 Mon
The main goal of this work is to characterize the mass accretion and ejection
processes of the classical T Tauri star V354 Mon, a member of the young stellar
cluster NGC 2264. In March 2008, photometric and spectroscopic observations of
V354 Mon were obtained simultaneously with the CoRoT satellite, the 60 cm
telescope at the Observat\'orio Pico dos Dias (LNA - Brazil) equipped with a
CCD camera and Johnson/Cousins BVRI filters, and the SOPHIE \'echelle
spectrograph at the Observatoire de Haute-Provence (CNRS - France). The light
curve of V354 Mon shows periodical minima (P = 5.26 +/- 0.50 days) that vary in
depth and width at each rotational cycle. From the analysis of the photometric
and spectroscopic data, it is possible to identify correlations between the
emission line variability and the light-curve modulation of the young system,
such as the occurrence of pronounced redshifted absorption in the H_alpha line
at the epoch of minimum flux. This is evidence that during photometric minima
we see the accretion funnel projected onto the stellar photosphere in our line
of sight, implying that the hot spot coincides with the light-curve minima. We
applied models of cold and hot spots and a model of occultation by
circumstellar material to investigate the source of the observed photometric
variations. We conclude that nonuniformly distributed material in the inner
part of the circumstellar disk is the main cause of the photometric modulation,
which does not exclude the presence of hot and cold spots at the stellar
surface. It is believed that the distortion in the inner part of the disk is
created by the dynamical interaction between the stellar magnetosphere,
inclined with respect to the rotation axis, and the circumstellar disk, as also
observed in the classical T Tauri star AA Tau and predicted by
magnetohydrodynamical numerical simulations.Comment: Accepted by Astronomy and Astrophysic
IV.3 The wealth of stellar variability
This book is dedicated to all the people interested in the CoRoT mission and the beautiful data that were delivered during its six year duration. Either amateurs, professional, young or senior researchers, they will find treasures not only at the time of this publication but also in the future twenty or thirty years. It presents the data in their final version, explains how they have been obtained, how to handle them, describes the tools necessary to understand them, and where to find them. It also highlights the most striking first results obtained up to now. CoRoT has opened several unexpected directions of research and certainly new ones still to be discovered
Dynamical Mass Constraints on Low-Mass Pre-Main-Sequence Stellar Evolutionary Tracks: An Eclipsing Binary in Orion with a 1.0 Msun Primary and an 0.7 Msun Secondary
We report the discovery of a double-lined, spectroscopic, eclipsing binary in
the Orion star-forming region. We analyze the system spectroscopically and
photometrically to empirically determine precise, distance-independent masses,
radii, effective temperatures, and luminosities for both components. The
measured masses for the primary and secondary, accurate to ~1%, are 1.01 Msun
and 0.73 Msun, respectively; thus the primary is a definitive pre-main-sequence
solar analog, and the secondary is the lowest-mass star yet discovered among
pre-main-sequence eclipsing binary systems. We use these fundamental
measurements to test the predictions of pre-main-sequence stellar evolutionary
tracks. None of the models we examined correctly predict the masses of the two
components simultaneously, and we implicate differences between the theoretical
and empirical effective temperature scales for this failing. All of the models
predict the observed slope of the mass-radius relationship reasonably well,
though the observations tend to favor models with low convection efficiencies.
Indeed, considering our newly determined mass measurements together with other
dynamical mass measurements of pre-main-sequence stars in the literature, as
well as measurements of Li abundances in these stars, we show that the data
strongly favor evolutionary models with inefficient convection in the stellar
interior, even though such models cannot reproduce the properties of the
present-day Sun.Comment: Accepted by Ap
Near-infrared time-series photometry in the field of Cygnus OB2 association II. Mapping the variability of candidate members
We present the results of a J, H, and K photometric variability survey of the
central 0.78 square degrees of the young OB association Cygnus OB2. We used
data observed with the Wide-Field CAMera at the United Kingdom Infrared
Telescope in 2007 (spanning 217 days) to investigate the light curves of 5083
low mass candidate members in the association and explore the occurrence and
main characteristics of their near-infrared variability. We identified 2529
stars (50 of the sample) with significant variability with
time-scales ranging from days to months. We classified the variable stars into
the following three groups according to their light curve morphology: periodic
variability (1697 stars), occultation variability (124 stars), and other types
of variability (726 stars). We verified that the disk-bearing stars in our
sample are significantly more variable in the near-infrared than diskless
stars, with a steep increase in the disk-fraction among stars with higher
variability amplitude. We investigated the trajectories described by variable
stars in the color-space and measured slopes for 335 stars describing linear
trajectories. Based on the trajectories in the color-space, we inferred that
the sample analyzed is composed of a mix of young stars presenting variability
due to hot and cold spots, extinction by circumstellar material, and changes in
the disk emission in the near-infrared. We contemplated using the use of
near-infrared variability to identify disk-bearing stars and verified that
53.4 of the known disk-bearing stars in our sample could have been
identified as such based solely on their variability. We present 18 newly
identified disk-bearing stars and 14 eclipsing binary candidates among CygOB2
lower-mass members.Comment: 19 pages, 21 Figure
On the Selection of Photometric Planetary Transits
We present a new method for differentiating between planetary transits and
eclipsing binaries based on the presence of the ellipsoidal light variations.
These variations can be used to detect stellar secondaries with masses ~0.2
M_sun orbiting sun-like stars at a photometric accuracy level which has already
been achieved in transit surveys. By removing candidates exhibiting this effect
it is possible to greatly reduce the number of objects requiring spectroscopic
follow up with large telescopes. Unlike the usual candidate selection method,
which are primarily based on the estimated radius of the orbiting object, this
technique is not biased against bona-fide planets and brown dwarfs with large
radii, because the amplitude of the effect depends on the transiting object's
mass and orbital distance. In many binary systems, where a candidate planetary
transit is actually due to the partial eclipse of two normal stars, the
presence of flux variations due to the gravity darkening effect will show the
true nature of these systems. We show that many of the recent OGLE-III
photometric transit candidates exhibit the presence of significant variations
in their light curves and are likely to be due to stellar secondaries. We find
that the light curves of white dwarf transits will generally not mimic those of
small planets because of significant gravitationally induced flux variations.
We discuss the relative merits of methods used to detect transit candidates
which are due to stellar blends rather than planets. We outline how photometric
observations taken in two bands can be used to detect the presence of stellar
blends.Comment: ApJ, 11 pages, 2 figures, 1 table, replaced with accepted versio
Modeling the Halpha line emission around classical T Tauri stars using magnetospheric accretion and disk wind models
Spectral observations of classical T Tauri stars show a wide range of line
profiles, many of which reveal signs of matter inflow and outflow. Halpha is
the most commonly observed line profile due to its intensity, and it is highly
dependent on the characteristics of the surrounding environment of these stars.
Our aim is to analyze how the Halpha line profile is affected by the various
parameters of our model which contains both the magnetospheric and disk wind
contributions to the Halpha flux. We used a dipolar axisymmetric stellar
magnetic field to model the stellar magnetosphere and a modified Blandford &
Payne model was used in our disk wind region. A three-level atom with continuum
was used to calculate the required Hydrogen level populations. We use the
Sobolev approximation and a ray-by-ray method to calculate the integrated line
profile. Through an extensive study of the model parameter space, we have
investigated the contribution of many of the model parameters on the calculated
line profiles. Our results show that the Halpha line is strongly dependent on
the densities and temperatures inside the magnetosphere and the disk wind
region. The bulk of the flux comes, most of the time, from the magnetospheric
component for standard classical T Tauri stars parameters, but the disk wind
contribution becomes more important as the mass accretion rate, the
temperatures and densities inside the disk wind increase. We have also found
that most of the disk wind contribution to the Halpha line is emitted at the
innermost region of the disk wind. Models that take into consideration both
inflow and outflow of matter are a necessity to fully understand and describe
classical T Tauri stars.Comment: 15 pages, 9 figures, accepted for publication in Astronomy &
Astrophysics. Revised version with English correction
Mapping accretion and its variability in the young open cluster NGC 2264: a study based on u-band photometry
We aim at characterizing the accretion properties of several hundred members
of the star-forming cluster NGC 2264 (3 Myr). We performed a deep u,g,r,i
mapping and a simultaneous u+r monitoring of the region with CFHT/MegaCam in
order to directly probe the accretion process from UV excess measurements.
Photometric properties and stellar parameters are determined homogeneously for
about 750 monitored young objects, spanning the mass range 0.1-2 Mo. About 40%
are classical (accreting) T Tauri stars, based on various diagnostics (H_alpha,
UV and IR excesses). The remaining non-accreting members define the
(photospheric+chromospheric) reference UV emission level over which flux excess
is detected and measured. We revise the membership status of cluster members
based on UV accretion signatures and report a new population of 50 CTTS
candidates. A large range of UV excess is measured for the CTTS population,
varying from a few 0.1 to 3 mag. We convert these values to accretion
luminosities and obtain mass accretion rates ranging from 1e-10 to 1e-7 Mo/yr.
Taking into account a mass-dependent detection threshold for weakly accreting
objects, we find a >6sigma correlation between mass accretion rate and stellar
mass. A power-law fit, properly accounting for upper limits, yields M_acc
M^{1.4+/-0.3}. At any given stellar mass, we find a large spread of
accretion rates, extending over about 2 orders of magnitude. The monitoring of
the UV excess on a timescale of a couple of weeks indicates that its
variability typically amounts to 0.5 dex, much smaller than the observed
spread. We suggest that a non-negligible age spread across the cluster may
effectively contribute to the observed spread in accretion rates at a given
mass. In addition, different accretion mechanisms (like, e.g., short-lived
accretion bursts vs. more stable funnel-flow accretion) may be associated to
different M_acc regimes.Comment: 24 pages, 21 figures, accepted for publication in Astronomy &
Astrophysic
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