372 research outputs found
Time evolution of X-ray coronal activity in PMS stars; a possible relation with the evolution of accretion disks
We investigate the evolution of X-ray stellar activity from the age of the
youngest known star forming regions (SFR), < 1Myr, to about 100 Myr, i.e. the
zero age main sequence (ZAMS) for a ~ 1M_sun star. We consider five SFR of
varying age (Rho Ophiuchi, the Orion Nebula Cluster, NGC 2264, Chamaeleon I,
and Eta Chamaeleontis) and two young clusters (the Pleiades and NGC 2516).
Optical and X-ray data for these regions are retrieved both from archival
observations and recent literature, and reanalyzed here in a consistent manner
so to minimize systematic differences in the results. We study trends of L_X
and L_X/L_bol as a function of stellar mass and association age. For low mass
stars (M < 1M_sun) we observe an increase in L_X/L_bol in the first 3-4 Myr and
a subsequent leveling off at the saturation level (L_X/L_bol ~ -3). Slowly
evolving very low mass stars then retain saturated levels down to the oldest
ages here considered, while for higher mass stars activity begins to decline at
some age after ~10^7 years. We find our data consistent with the following
tentative picture: low mass PMS stars with no circumstellar accretion disk have
saturated activity, consistently with the activity-Rossby number relation
derived for MS stars. Accretion and/or the presence of disks somehow lowers the
observed activity levels; disk dissipation and/or the decrease of mass
accretion rate in the first few Myrs of PMS evolution is therefore responsible
for the observed increase of L_X/L_bol with time.Comment: 17 pages, 8 figures, accepted for publication in A&
X-ray observations of IC348 in light of an updated cluster census
IC348 is an excellent laboratory for studies of low-mass star formation being
nearby, compact and rich. A Chandra observation was carried out early in the
satellite's lifetime. The extensive new data in optical and infrared
wavelengths accumulated in subsequent years have changed the cluster census
calling for a re-analysis of the X-ray data.Comment: poster paper to appear in Proc. of the 15th Workshop on Cool Stars,
Stellar Systems and the Su
The History of the Mysterious Eclipses of KH 15D: Asiago Observatory, 1967-1982
We are gathering archival observations to determine the photometric history
of the unique and unexplained eclipses of the pre-main-sequence star KH 15D.
Here we present a light curve from 1967-1982, based on photographic plates from
Asiago Observatory. During this time, the system alternated periodically
between bright and faint states, as observed today. However, the bright state
was 0.9 mag brighter than the modern value, and the fractional variation
between bright and faint states (Delta I = 0.7 mag) was smaller than observed
today (3.5 mag). A possible explanation for these findings is that the system
contains a second star that was previously blended with the eclipsing star, but
is now completely obscured.Comment: Accepted to AJ. 24 pages, 10 figures, 2 tables. v2: Phase error
corrected in figures 8 and 1
Observational clues for a role of circumstellar accretion in PMS X-ray activity
We revisit the published analyses of ROSAT X-ray observations of the star forming regions NGC 2264 and Chamaeleon I (~3 and ~5 Myr old respectively) in the light of newly published optical data. At odds with previous results on Chamaeleon I members, we find that low mass stars in both regions have near-saturated emission levels. Similarly to what previously found in the Orion Nebula Cluster, Weak Line T-Tauri Stars in NGC 2264 and in the Chamaeleon I cloud have higher X-ray activity levels respect to Classical T Tauri Stars, arguing in favor of a role of the disk and/or accretion in determining X-ray emission
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
A statistical analysis of X-ray variability in pre-main sequence objects of the Taurus Molecular Cloud
This work is part of a systematic X-ray survey of the Taurus star forming
complex with XMM-Newton. We study the time series of all X-ray sources
associated with Taurus members, to statistically characterize their X-ray
variability, and compare the results to those for pre-main sequence stars in
the Orion Nebula Cluster and to expectations arising from a model where all the
X-ray emission is the result of a large number of stochastically occurring
flares. We find that roughly half of the detected X-ray sources show
variability above our sensitivity limit, and in ~ 26 % of the cases this
variability is recognized as flares. Variability is more frequently detected at
hard than at soft energies. The variability statistics of cTTS and wTTS are
undistinguishable, suggesting a common (coronal) origin for their X-ray
emission. We have for the first time applied a rigorous maximum likelihood
method in the analysis of the number distribution of flare energies on pre-main
sequence stars. In its differential form this distribution follows a power-law
with index alpha = 2.4 +- 0.5, in the range typically observed on late-type
stars and the Sun. The flare energy distribution is probably steep enough to
explain the heating of stellar coronae by nano-flares (alpha > 2), albeit
associated with a rather large uncertainty that leaves some doubt on this
conclusion.Comment: accepted for publication in Astronomy & Astrophysics; to appear in a
Special Section dedicated to the XMM-Newton Extended Survey of the Taurus
Molecular Cloud (XEST
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