3,566 research outputs found
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
X-ray emitting MHD accretion shocks in classical T Tauri stars. Case for moderate to high plasma-beta values
AIMS. We investigate the stability and dynamics of accretion shocks in CTTSs,
considering the case of beta >= 1 in the post-shock region. In these cases the
1D approximation is not valid and a multi-dimensional MHD approach is
necessary.
METHODS. We model an accretion stream propagating through the atmosphere of a
CTTS and impacting onto its chromosphere, by performing 2D axisymmetric MHD
simulations. The model takes into account the stellar magnetic field, the
gravity, the radiative cooling, and the thermal conduction (including the
effects of heat flux saturation).
RESULTS. The dynamics and stability of the accretion shock strongly depends
on the plasma beta. In the case of shocks with beta > 10, violent outflows of
shock-heated material (and possibly MHD waves) are generated at the base of the
accretion column and strongly perturb the surrounding stellar atmosphere and
the accretion column itself (modifying, therefore, the dynamics of the shock).
In shocks with beta ~ 1, the post-shock region is efficiently confined by the
magnetic field. The shock oscillations induced by cooling instability are
strongly influenced by beta: for beta > 10, the oscillations may be rapidly
dumped by the magnetic field, approaching a quasi-stationary state, or may be
chaotic with no obvious periodicity due to perturbation of the stream induced
by the post-shock plasma itself; for beta ~ 1 the oscillations are
quasi-periodic, although their amplitude is smaller and the frequency higher
than those predicted by 1D models.Comment: 12 pages, 10 Figures; accepted for publication on A&A. Version with
full resolution images can be found at
http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_13565.pd
Emission-line profile modelling of structured T Tauri magnetospheres
We present hydrogen emission line profile models of magnetospheric accretion
onto Classical T Tauri stars. The models are computed under the Sobolev
approximation using the three-dimensional Monte Carlo radiative-transfer code
TORUS. We have calculated four illustrative models in which the accretion flows
are confined to azimuthal curtains - a geometry predicted by
magneto-hydrodynamical simulations. Properties of the line profile variability
of our models are discussed, with reference to dynamic spectra and
cross-correlation images. We find that some gross characteristics of observed
line profile variability are reproduced by our models, although in general the
level of variability predicted is larger than that observed. We conclude that
this excessive variability probably excludes dynamical simulations that predict
accretion flows with low degrees of axisymmetry.Comment: 14 pages, 12 figures. Published in MNRA
Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph
From observations collected with the ESPaDOnS spectropolarimeter, we report
the discovery of magnetic fields at the surface of the mildly accreting
classical T Tauri star V2129 Oph. Zeeman signatures are detected, both in
photospheric lines and in the emission lines formed at the base of the
accretion funnels linking the disc to the protostar, and monitored over the
whole rotation cycle of V2129 Oph. We observe that rotational modulation
dominates the temporal variations of both unpolarized and circularly polarized
line profiles. We reconstruct the large-scale magnetic topology at the surface
of V2129 Oph from both sets of Zeeman signatures simultaneously. We find it to
be rather complex, with a dominant octupolar component and a weak dipole of
strengths 1.2 and 0.35 kG, respectively, both slightly tilted with respect to
the rotation axis. The large-scale field is anchored in a pair of 2-kG unipolar
radial field spots located at high latitudes and coinciding with cool dark
polar spots at photospheric level. This large-scale field geometry is unusually
complex compared to those of non-accreting cool active subgiants with moderate
rotation rates. As an illustration, we provide a first attempt at modelling the
magnetospheric topology and accretion funnels of V2129 Oph using field
extrapolation. We find that the magnetosphere of V2129 Oph must extend to about
7R* to ensure that the footpoints of accretion funnels coincide with the
high-latitude accretion spots on the stellar surface. It suggests that the
stellar magnetic field succeeds in coupling to the accretion disc as far out as
the corotation radius, and could possibly explain the slow rotation of V2129
Oph. The magnetospheric geometry we derive produces X-ray coronal fluxes
typical of those observed in cTTSs.Comment: MNRAS, in press (18 pages, 17 figures
The absence of sub-minute periodicity in classical T Tauri stars
Classical T Tauri stars (CTTS) are young, late-type objects, that still
accrete matter from a circumstellar disk. Analytical treatments and numerical
simulations predict instabilities of the accretion shock on the stellar
surface. We search for variability on timescales below a few minutes in the
CTTS TW Hya and AA Tau. TW Hya was observed with SALTICAM on the Southern
African Large Telescope (SALT) in narrow-band filters around the Balmer jump.
The observations were performed in slit mode, which provides a time resolution
of about 0.1 s. For AA Tau we obtained observations with OPTIMA, a single
photon-counting device with even better time resolution. Small-scale
variability typically lasts a few seconds, however, no significant periodicity
is detected. We place a 99 % confidence upper limit on the pulsed fraction of
the lightcurves. The relative amplitude is below 0.001 for TW Hya in the
frequency range 0.02-3 Hz in the 340 nm filter and 0.1-3 Hz in the 380 nm
filter. The corresponding value for AA Tau is an amplitude of 0.005 for 0.02-50
Hz. The relevant timescales indicate that shock instabilites should not be seen
directly in our optical and UV observations, but the predicted oscialltions
would induce observable variations in the reddening. We discuss how the
magnetic field could stabilise the accretion shock.Comment: 8 pages, 9 figures, accepted by A&
An algorithm for Monte-Carlo time-dependent radiation transfer
A new Monte-Carlo algorithm for calculating time-dependent radiative-transfer
under the assumption of LTE is presented. Unlike flux-limited diffusion the
method is polychromatic, includes scattering, and is able to treat the
optically thick and free-streaming regimes simultaneously. The algorithm is
tested on a variety of 1-d and 2-d problems, and good agreement with benchmark
solutions is found. The method is used to calculate the time-varying spectral
energy distribution from a circumstellar disc illuminated by a protostar whose
accretion luminosity is varying. It is shown that the time lag between the
optical variability and the infrared variability results from a combination of
the photon travel time and the thermal response in the disc, and that the lag
is an approximately linear function of wavelength.Comment: 10 pages, 10 figures, accepted for publication by MNRA
Outflows in rho Ophiuchi as Seen with the Spitzer Infrared Array Camera
Using the IRAC images from the Spitzer c2d program, we have made a survey of
mid-infrared outflows in the rho Ophiuchi molecular cloud. Extended objects
that have prominent emission in IRAC channel 2 (4.5 micron) compared to IRAC
channel 1 (3.6 micron) and stand out as green objects in the three-color images
(3.6 micron in blue, 4.5 micron in green, 8.0 micron in red) are identified as
mid-infrared outflows. As a result, we detected 13 new outflows in the rho
Ophiuchi molecular cloud that have not been previously observed in optical or
near-infrared. In addition, at the positions of previously observed HH objects
or near-infrared emission, we detected 31 mid-infrared outflows, among which
seven correspond to previously observed HH objects and 30 to near-infrared
emission. Most of the mid-infrared outflows detected in the rho Ophiuchi cloud
are concentrated in the L1688 dense core region. In combination with the survey
results for Young Stellar Objects (YSOs) and millimeter and sub-millimeter
sources, the distribution of mid-infrared outflows in the rho Ophiuchi
molecular complex hints a propagation of star formation in the cloud in the
direction from the northwest to the southeast as suggested by previous studies
of the region.Comment: 23 pages and 43 figure
Accretion-powered chromospheres in classical T Tauri stars
(Abridged) Optical spectra of classical T Tauri stars (cTTS) are rich in
emission lines of low-excitation species that are composed of narrow and broad
components, related to two regions with different kinematics, densities, and
temperatures. The photospheric spectrum is often veiled by an excess continuous
emission. This veiling is usually attributed to radiation from a heated region
beneath the accretion shock. The aim of this research is to clarify the nature
of the veiling, and whether the narrow chromospheric lines of Fe I and other
metals represent a standard chromosphere of a late-type star, or are induced by
mass accretion. From high-resolution spectroscopy of DR Tauri we found that the
amount of veiling in this star varies from practically nothing to factors more
than 10 times the stellar continuum intensity, and that the veiling is caused
by both a non-photospheric continuum and chromospheric line emission filling in
the photospheric absorption lines. This effect can be shown to exist in several
other T Tauri stars. We conclude that enhanced chromospheric emission in cTTS
is linked not only to solar-like magnetic activity, but is powered to a greater
extent by the accreting gas. We suggest that the area of enhanced chromospheric
emission is induced by mass accretion, which modifies the local structure of
stellar atmosphere in an area that is more extended than the hot accretion
spot. The narrow emission lines from this extended area are responsible for the
extra component in the veiling through line-filling of photospheric absorption
lines.Comment: 11 pages, 13 figure
The mass ratio and formation mechanisms of Herbig Ae/Be star binary systems
We present B and R band spectroastrometry of a sample of 45 Herbig Ae/Be
stars in order to study their binary properties. All but one of the targets
known to be binary systems with a separation of ~0.1-2.0 arcsec are detected by
a distinctive spectroastrometric signature. Some objects in the sample exhibit
spectroastrometric features that do not appear attributable to a binary system.
We find that these may be due to light reflected from dusty halos or material
entrained in winds. We present 8 new binary detections and 4 detections of an
unknown component in previously discovered binary systems. The data confirm
previous reports that Herbig Ae/Be stars have a high binary fraction, 74+/-6
per cent in the sample presented here. We use a spectroastrometric
deconvolution technique to separate the spatially unresolved binary spectra
into the individual constituent spectra. The separated spectra allow us to
ascertain the spectral type of the individual binary components, which in turn
allows the mass ratio of these systems to be determined. In addition, we
appraise the method used and the effects of contaminant sources of flux. We
find that the distribution of system mass ratios is inconsistent with random
pairing from the Initial Mass Function, and that this appears robust despite a
detection bias. Instead, the mass ratio distribution is broadly consistent with
the scenario of binary formation via disk fragmentation.Comment: Accepted for publication in MNRAS, minor changes made in proof stag
HDE 245059: A Weak-Lined T Tauri Binary Revealed by Chandra and Keck
We present the Chandra High Energy Transmission Grating Spectrometer (HETGS)
and Keck observations of HDE 245059, a young weak-lined T Tauri star (WTTS),
member of the pre-main sequence group in the Lambda Orionis Cluster. Our high
spatial resolution, near-infrared observations with Keck reveal that HDE 245059
a binary separated by 0.87". Based on this new information we have obtained an
estimate of the masses of the binary components; 3M_{sun} and 2.5M_{sun} for
the north and south components, respectively. We have estimated the age of the
system to be ~2-3 Myr. We detect both components of the binary in the zeroth
order Chandra image and in the grating spectra. Our fits to the spectrum of the
binary have shown that the emission is dominated by a plasma between 8 and 15
MK, a soft component at 4 MK and a hard component at 50 MK are also detected.
The value of the hydrogen column density was low, 8 x 10^{19} cm^{-2}, likely
due to the clearing of the inner region of the Lambda Orionis cloud. The
abundance pattern shows an inverse First Ionization Potential (FIP) effect for
all elements from O to Fe, the only exception being Ca. A 3-T model was fitted
to the individual zeroth order spectra using the abundances derived for the
binary. We have also obtained several lines fluxes from the grating spectra.
The fits to the triplets show no evidence of high densities. We conclude that
the X-ray properties of the weak-lined T Tau binary HDE 245059 are similar to
those generally observed in other weak-lined T Tau stars. Although its
accretion history may have been affected by the clearing of the interstellar
material around Lambda Ori, its coronal properties appears not to have been
strongly modified.Comment: 12 pages, 10 figures Accepted for publication in Ap
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