5,244 research outputs found
Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars
We present a unique, homogeneous database of photometric measurements for
Classical T Tauri stars extending up to 20 years. The database contains more
than 21,000 UBVR observations of 72 CTTs. All the data were collected within
the framework of the ROTOR-program at Mount Maidanak Observatory (Uzbekistan)
and together they constitute the longest homogeneous, accurate record of TTS
variability ever assembled. We characterize the long term photometric
variations of 49 CTTs with sufficient data to allow a robust statistical
analysis and propose an empirical classification scheme. Several patterns of
long term photometric variability are identified. The most common pattern,
exhibited by a group of 15 stars which includes T Tau itself, consists of low
level variability (Delta(V)<=0.4mag) with no significant changes occurring from
season to season over many years. A related subgroup of 22 stars exhibits a
similar stable long term variability pattern, though with larger amplitudes (up
to Delta(V)~1.6 mag). Besides these representative groups, we identify three
smaller groups of 3-5 stars each which have distinctive photometric properties.
The long term variability of most CTTs is fairly stable and merely reflects
shorter term variability due to cold and hot surface spots. Only a small
fraction of CTTs undergo significant brightness changes on the long term
(months, years), which probably arise from slowly varying circumstellar
extinction.Comment: 16 pages, 11 figures. Astron. Astrophys., in pres
Constraints on the disk geometry of the T Tauri star AA Tau from linear polarimetry
We have simultaneously monitored the photometric and polarimetric variations
of the Classical T Tauri star AA Tau during the fall of 2002. We combine these
data with previously published polarimetric data covering two earlier epochs.
The phase coverage is complete, although not contiguous. AA Tau clearly shows
cyclic variations coupled with the rotation of the system. The star-disk system
produces a repeatable polarisation curve where the polarisation increases with
decreasing brightness. The data fit well with the model put forward by Bouvier
et al. (1999) where AA Tau is viewed almost edge-on and its disk is actively
dumping material onto the central star via magnetospheric accretion. The inner
edge of the disk is deformed by its interaction with the tilted magnetosphere,
producing eclipses as it rotates and occults the photosphere periodically. From
the shape of the polarisation curve in the QU-Plane we confirm that the
accretion disk is seen at a large inclination, almost edge-on, and predict that
its position angle is PA~90 deg., i.e., that the disk's major axis is oriented
in the East-West direction.Comment: Astron. Astrophys., in pres
The effect of landfills on rural residential property values: Some empirical evidence.
Abstract
The question of whether solid waste landfills affect residential property values has long been a subject of debate. Past research has resulted in mixed conclusions. The current study examines six landfills, which differ in size, operating status, and history of contamination. The effect of each landfill is estimated by the use of multiple regression. In five of the landfills, no statistically significant evidence of an effect was found. In the remaining case, evidence of an effect was found, indicating that houses in close proximity to this landfill suffered an average loss of about six percent in value
Angular momentum evolution of young low-mass stars and brown dwarfs: observations and theory
This chapter aims at providing the most complete review of both the emerging
concepts and the latest observational results regarding the angular momentum
evolution of young low-mass stars and brown dwarfs. In the time since
Protostars & Planets V, there have been major developments in the availability
of rotation period measurements at multiple ages and in different star-forming
environments that are essential for testing theory. In parallel, substantial
theoretical developments have been carried out in the last few years, including
the physics of the star-disk interaction, numerical simulations of stellar
winds, and the investigation of angular momentum transport processes in stellar
interiors. This chapter reviews both the recent observational and theoretical
advances that prompted the development of renewed angular momentum evolution
models for cool stars and brown dwarfs. While the main observational trends of
the rotational history of low mass objects seem to be accounted for by these
new models, a number of critical open issues remain that are outlined in this
review.Comment: 22 pages, 8 figures, accepted for publication in Protostars & Planets
VI, 2014, University of Arizona Press, eds. H. Beuther, R. Klessen, K.
Dullemond, Th. Hennin
Diamagnetic Blob Interaction Model of T Tauri Variability
Assuming a diamagnetic interaction between a stellar-spot originated
localized magnetic field and gas blobs in the accretion disk around a T- Tauri
star, we show the possibility of ejection of such blobs out of the disk plane.
Choosing the interaction radius and the magnetic field parameters in a suitable
way gives rise to closed orbits for the ejected blobs. A stream of matter
composed of such blobs, ejected on one side of the disk and impacting on the
other, can form a hot spot at a fixed position on the disk (in the frame
rotating with the star). Such a hot spot, spread somewhat by disk shear before
cooling, may be responsible in some cases for the lightcurve variations
observed in various T-Tauri stars over the years. An eclipse-based mechanism
due to stellar obscuration of the spot is proposed. Assuming high disk
inclination angles it is able to explain many of the puzzling properties of
these variations. By varying the field parameters and blob initial conditions
we obtain variations in the apparent angular velocity of the hot spot,
producing a constantly changing period or intermittent periodicity
disappearance in the models.Comment: 6 pages, 4 figures, aas2pp4 styl
The Origin of Enhanced Activity in the Suns of M67
We report the results of the analysis of high resolution photospheric line
spectra obtained with the UVES instrument on the VLT for a sample of 15
solar-type stars selected from a recent survey of the distribution of H and K
chromospheric line strengths in the solar-age open cluster M67. We find upper
limits to the projected rotation velocities that are consistent with solar-like
rotation (i.e., v sini ~< 2-3 km/s) for objects with Ca II chromospheric
activity within the range of the contemporary solar cycle. Two solar-type stars
in our sample exhibit chromospheric emission well in excess of even solar
maximum values. In one case, Sanders 1452, we measure a minimum rotational
velocity of vsini = 4 +/- 0.5 km/s, or over twice the solar equatorial
rotational velocity. The other star with enhanced activity, Sanders 747, is a
spectroscopic binary. We conclude that high activity in solar-type stars in M67
that exceeds solar levels is likely due to more rapid rotation rather than an
excursion in solar-like activity cycles to unusually high levels. We estimate
an upper limit of 0.2% for the range of brightness changes occurring as a
result of chromospheric activity in solar-type stars and, by inference, in the
Sun itself. We discuss possible implications for our understanding of angular
momentum evolution in solar-type stars, and we tentatively attribute the rapid
rotation in Sanders 1452 to a reduced braking efficiency.Comment: accepted by Ap
Dynamical Masses of Young Stars in Multiple Systems
We present recent measurements of the orbital motion in the young binaries DF
Tau and ZZ Tau, and the hierarchical triple Elias 12, that were obtained with
the Fine Guidance Sensors on the HST and at the Keck Observatory using adaptive
optics. Combining these observations with previous measurements from the
literature, we compute preliminary orbital parameters for DF Tau and ZZ Tau. We
find that the orbital elements cannot yet be determined precisely because the
orbital coverage spans only about 90 degr in position angle. Nonetheless, the
range of possible values for the period and semi-major axis already defines a
useful estimate for the total mass in DF Tau and ZZ Tau, with values of
0.90{+0.85}{-0.35} M_sun and 0.81{+0.44}{-0.25} M_sun, respectively, at a
fiducial distance of 140 pc.Comment: 26 pages, 9 figures, accepted for publication in A
Angular Momentum Evolution of Stars in the Orion Nebula Cluster
(Abridged) We present theoretical models of stellar angular momentum
evolution from the Orion Nebula Cluster (ONC) to the Pleiades and the Hyades.
We demonstrate that observations of the Pleiades and Hyades place tight
constraints on the angular momentum loss rate from stellar winds. The observed
periods, masses and ages of ONC stars in the range 0.2--0.5 M, and the
loss properties inferred from the Pleiades and Hyades stars, are then used to
test the initial conditions for stellar evolution models. We use these models
to estimate the distribution of rotational velocities for the ONC stars at the
age of the Pleiades (120 Myr). The modeled ONC and observed Pleiades
distributions of rotation rates are not consistent if only stellar winds are
included. In order to reconcile the observed loss of angu lar momentum between
these two clusters, an extrinsic loss mechanism such as protostar-accretion
disk interaction is required. Our model, which evolves the ONC stars with a
mass dependent saturation threshold normalized such that at 0.5 \m, and which includes a distribution of disk lifetimes
that is uniform over the range 0--6 Myr, is consistent with the Pleiades. This
model for disk-locking lifetimes is also consistent with inferred disk
lifetimes from the percentage of stars with infrared excesses observed in young
clusters. Different models, using a variety of initial period distributions and
different maximum disk lifetimes, are also compared to the Pleiades. For
disk-locking models that use a uniform distribution of disk lifetimes over the
range 0 to , the acceptable range of the maximum lifetime is Myr.Comment: 21 pages, 7 figures, submitted to Ap
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
Multiple protostellar systems. II. A high resolution near-infrared imaging survey in nearby star-forming regions
(abridged) Our project endeavors to obtain a robust view of multiplicity
among embedded Class I and Flat Spectrum protostars in a wide array of nearby
molecular clouds to disentangle ``universal'' from cloud-dependent processes.
We have used near-infrared adaptive optics observations at the VLT through the
H, Ks and L' filters to search for tight companions to 45 Class I and Flat
Spectrum protostars located in 4 different molecular clouds (Taurus-Auriga,
Ophiuchus, Serpens and L1641 in Orion). We complemented these observations with
published high-resolution surveys of 13 additional objects in Taurus and
Ophiuchus. We found multiplicity rates of 32+/-6% and 47+/-8% over the 45-1400
AU and 14-1400 AU separation ranges, respectively. These rates are in excellent
agreement with those previously found among T Tauri stars in Taurus and
Ophiuchus, and represent an excess of a factor ~1.7 over the multiplicity rate
of solar-type field stars. We found no non-hierarchical triple systems, nor any
quadruple or higher-order systems. No significant cloud-to-cloud difference has
been found, except for the fact that all companions to low-mass Orion
protostars are found within 100 AU of their primaries whereas companions found
in other clouds span the whole range probed here. Based on this survey, we
conclude that core fragmentation always yields a high initial multiplicity
rate, even in giant molecular clouds such as the Orion cloud or in clustered
stellar populations as in Serpens, in contrast with predictions of numerical
simulations. The lower multiplicity rate observed in clustered Class II and
Class III populations can be accounted for by a universal set of properties for
young systems and subsequent ejections through close encounters with unrelated
cluster members.Comment: 15 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
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