112 research outputs found
New Calculations of Stellar Wind Torques
Using numerical simulations of magnetized stellar winds, we carry out a
parameter study to find the dependence of the stellar wind torque on observable
parameters. We find that the power-law dependencies of the torque on parameters
is significantly different than what has been used in all spin evolution models
to date.Comment: To appear in the proceedings for the 15th Cambridge Workshop on Cool
Stars, Stellar Systems, and the Sun. 4 page poster contributio
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
Stellar or Non-Stellar Light? Determining Near-Infrared Contamination in Low Mass X-ray Binaries
Low-mass X-ray binary (LMXB) systems are comprised of a low-mass, K or M dwarflike star orbiting a compact object. Stellar black hole masses and their distributions are important inputs for binary evolution and supernova models. Currently, the main limiting factor in determining accurate black hole masses in LMXBs is the uncertainty of the orbital inclination angle due to an unknown amount of contaminating light in the near infrared. If present, this light dilutes the ellipsoidal variations of the low-mass secondary star, and thus gives the appearance of a lower orbital inclination system. It has been generally thought that the near infrared ellipsoidal light curves of these systems were relatively uncontaminated and represented primarily the light from the low-mass secondary star; however, recent disk and jet models have thrust this thinking into question. We combine our data from the Spitzer Space Telescope with our ground-based optical and near infrared data for several LMXBs to characterize and derive the amount of light contaminating the near-infrared ellipsoidal variations of the low-mass secondary star
Testing evolutionary tracks of Pre-Main Sequence stars: the case of HD113449
Evolutionary tracks are of key importance for the understanding of star
formation. Unfortunately, tracks published by various groups differ so that it
is fundamental to have observational tests. In order to do this, we intend to
measure the masses of the two components of the Pre-Main Sequence binary
HD113449 by combining radial velocity measurements taken with HARPS, with
infrared interferometric data using AMBER on the VLTI. The spectroscopic orbit
that has already been determined, combined with the first AMBER measurement,
allows us to obtain a very first estimation of the inclination of the binary
system and from this the masses of the two stars. More AMBER measurements of HD
113449 are needed to improve the precision on the masses: in the ESO period P82
two new measurements are scheduled.Comment: 4 pages, 3 figures; to appear in proceedings of Cool Star 15
conference, St.Andrews 200
Methane T-Dwarf Candidates in the Star Forming Region IC 348
IC 348 is a young (t3Myr) and nearby (d340pc) star forming region
in the Perseus molecular cloud. We performed a deep imaging survey using the
MEGACAM (z-band) and WIRCAM (JHK and narrowband CH on/off) wide-field
cameras on the Canada-France-Hawaii Telescope. From the analysis of the
narrowband CH on/off deep images, we report 4 T-dwarf candidates, of
which 3 clearly lie within the limits of the IC 348 cluster. An upper limit on
the extinction was estimated for each candidate from colour-magnitude diagrams,
and found consistent with extinction maps of the cloud. Initial comparisons
with T-dwarf spectral models suggest these candidates have a spectral type
between T3 and T5, and perhaps later, potentially making these among the lowest
mass isolated objects detected in a young star forming region so far
Photometric/spectroscopic analyses and magnetic activity in young late-type stars
We present the work in progress of a study based on photometric and
spectroscopic observations of young Weak-line T Tauri and Post T Tauri stars
just attiving on the Zero Age Main Sequence. This study is part of a project
based on high-resolution spectra obtained with FOCES@CAHA (Spain) and SARG@TNG
(Spain) and contemporaneous photometry performed at Catania (Italy) and Ege
(Turkey) observatories. The main aim is to investigate the topology of magnetic
active regions at photospheric and chromospheric levels in young single stars.
Since our targets are slow rotators (vsini < 25 km/s), corresponding to
rotation periods larger than about 2 days, we are able to apply the
spectroscopic technique based on line-depth ratio for the measure of the
photospheric temperature modulation. These stars, possible members of Stellar
Kinematic Groups, display emission cores in the CaII H&K and IRT lines, as well
as a conspicuous filling-in of the Halpha core. Moreover, we detect absorption
of the HeI-D3 line, coming from the upper chromospheric layers, derive the
lithium abundance (age indicator), and measure the rotational and radial
velocities. We find a clear rotational modulation, due to photospheric spots,
both in the light and the temperature curves. The Halpha and the CaII-IRT
emissions display a fair variation correlated with the rotation. Finally, we
are developing a spot/plage model to reproduce the data and derive the spot
parameters (namely, filling factor and temperature) and to recover information
about the chromospheric inhomogeneities (flux contrast and filling factor).
This study is very important to explore the correlations between global stellar
parameters (e.g., surface gravity, effective temperature) and spot/plage
characteristics in stars with different activity level and evolutionary stage.Comment: 4 pages, 4 figures, poster paper presented at the Cool Stars 15 (St.
Andrews, Scotland), to be published in the Conference Proceedings Series of
the American Institute of Physics (AIP
The ultracool eld dwarfs luminosity function from the Canada-France Brown Dwarf Survey
The Canada-France Brown Dwarf Survey is a wide eld survey for cool brown
dwarfs conducted with the MegaCam camera on the CFHT telescope. Our objectives
are to nd ultracool brown dwarfs and to constrain the eld brown dwarf mass
function from a large and homogeneous sample of L and T dwarfs. We identify
candidates in CFHT/Megacam i' and z' images and follow them up with pointed NIR
imaging on several telescopes. Our survey has to date found 50 T dwarfs
candidates and 170 L or late M dwarf candidates drawn from a larger sample of
1300 candidates with typical ultracool dwarfs i'-z' colours, found in 900
square degrees. We currently have completed the NIR follow-up on a large part
of the survey for all candidates from the latest T dwarfs known to the late L
color range. This allows us to build on a complete and well de ned sample of
ultracool dwarfs to investigate the luminosity function of eld L and T dwarfs.Comment: Cool Stars XV conference. to appear in proceedings of Cool Stars XV
Conferenc
The pre-main sequence spectroscopic binary UZ Tau East: improved orbital parameters and accretion phase dependence
We present radial-velocity measurements obtained using high- and
intermediate-resolution spectroscopic observations of the classical T Tauri
star UZ Tau East obtained from 1994 to 1996. We also provide measurements of
H equivalent widths and optical veiling. Combining our radial-velocity
data with those recently reported by Prato et al. (2002), we improve the
orbital elements for this spectroscopic binary. The orbital period is
18.9790.007 days and the eccentricity is e=0.14. We find variability in
the H emission and veiling, signposts of accretion, but at periastron
passage the accretion is not as clearly enhanced as in the case of the binary
DQ Tau. The difference in the behaviour of these two binaries is consistent
with the hydrodynamical models of accretion from circumbinary disks because UZ
Tau East has lower eccentricity than DQ Tau. It seems that enhanced periastron
accretion may occur only in systems with very high eccentricity (e0.5).Comment: accepted for publication in A&
Interpretation of the Veiling of the Photospheric Spectrum for T Tauri Stars in Terms of an Accretion Model
The problem on heating the atmospheres of T Tauri stars by radiation from an
accretion shock has been solved. The structure and radiation spectrum of the
emerging so-called hot spot have been calculated in the LTE approximation. The
emission not only in continuum but also in lines has been taken into account
for the first time when calculating the spot spectrum. Comparison with
observations has shown that the strongest of these lines manifest themselves as
narrow components of helium and metal emission lines, while the weaker ones
decrease significantly the depth of photospheric absorption lines, although
until now, this effect has been thought to be due to the emission continuum
alone. The veiling by lines changes the depth of different photospheric lines
to a very different degree even within a narrow spectral range. Therefore, the
nonmonotonic wavelength dependence of the degree of veiling r found for some
CTTS does not suggest a nontrivial spectral energy distribution of the veiling
continuum. In general, it makes sense to specify the degree of veiling r only
by providing the set of photospheric lines from which this quantity was
determined. We show that taking into account the contribution of lines to the
veiling of the photospheric spectrum can cause the existing estimates of the
accretion rate onto T Tauri stars to decrease by several times, with this being
also true for stars with a comparatively weakly veiled spectrum. Neglecting the
contribution of lines to the veiling can also lead to appreciable errors in
determining the effective temperature, interstellar extinction, radial
velocity, and vsin(i)
X-ray emission in the Outer Galaxy: the Star Forming Region NGC 1893
A key issue of the star formation process is its independence from the environmental conditions. In particular, it is not clear whether star formation in the outer Galaxy occurs in the same way as in the inner Galaxy. We present preliminary results of the analysis of a âŒ440âKs ACISâChandra observation of NGC1893, a young cluster (âŒ3â4âMyrs), far away from the Galactic Center with the aim to study star formation in the outer region of the Galaxy and investigate the coronal properties of the cluster stars. We detect more than 1000 Xâray sources, most of which are likely cluster members. We present also a preliminary analysis of Xâray variability of the cluster stars
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