52 research outputs found
Spectral variability of classical T Tauri stars accreting in an unstable regime
Classical T Tauri stars (CTTSs) are variable in different time-scales. One
type of variability is possibly connected with the accretion of matter through
the Rayleigh-Taylor instability that occurs at the interface between an
accretion disc and a stellar magnetosphere. In this regime, matter accretes in
several temporarily formed accretion streams or `tongues' which appear in
random locations, and produce stochastic photometric and line variability. We
use the results of global three-dimensional magnetohydrodynamic simulations of
matter flows in both stable and unstable accretion regimes to calculate
time-dependent hydrogen line profiles and study their variability behaviours.
In the stable regime, some hydrogen lines (e.g. H-beta, H-gamma, H-delta,
Pa-beta and Br-gamma) show a redshifted absorption component only during a
fraction of a stellar rotation period, and its occurrence is periodic. However,
in the unstable regime, the redshifted absorption component is present rather
persistently during a whole stellar rotation cycle, and its strength varies
non-periodically. In the stable regime, an ordered accretion funnel stream
passes across the line of sight to an observer only once per stellar rotation
period while in the unstable regime, several accreting streams/tongues, which
are formed randomly, pass across the line of sight to an observer. The latter
results in the quasi-stationarity appearance of the redshifted absorption
despite the strongly unstable nature of the accretion. In the unstable regime,
multiple hot spots form on the surface of the star, producing the stochastic
light curve with several peaks per rotation period. This study suggests a CTTS
that exhibits a stochastic light curve and a stochastic line variability, with
a rather persistent redshifted absorption component, may be accreting in the
unstable accretion regime.Comment: 20 pages, 11 figures, 1 table, accepted for publication in MNRA
Radial and rotational velocities of young brown dwarfs and very low-mass stars in the Upper Scorpius OB association and the rho Ophiuchi cloud core
We present the results of a radial velocity (RV) survey of 14 brown dwarfs
(BDs) and very low-mass (VLM) stars in the Upper Scorpius OB association
(UScoOB) and 3 BD candidates in the rho Ophiuchi dark cloud core. We obtained
high-resolution echelle spectra at the Very Large Telescope using Ultraviolet
and Visual Echelle Spectrograph (UVES) at two different epochs for each object,
and measured the shifts in their RVs to identify candidates for binary/multiple
systems in the sample. The average time separation of the RV measurements is
21.6d, and our survey is sensitive to the binaries with separation < 0.1 au. We
found that 4 out of 17 objects (or 24^{+16}_{-13} per cent by fraction) show a
significant RV change in 4-33d time scale, and are considered as
binary/multiple `candidates.' We found no double-lined spectroscopic binaries
in our sample, based on the shape of cross-correlation curves. The RV
dispersion of the objects in UScoOB is found to be very similar to that of the
BD and VLM stars in Chamaeleon I (Cha I). We also found the distribution of the
mean rotational velocities (v sin i) of the UScoOB objects is similar to that
of the Cha I, but the dispersion of v sin i is much larger than that of the Cha
I objects.Comment: 10 pages, 5 figures, accepted for publication in MNRA
Linear spectropolarimetry of young and other emission line stars
The aim of this article is to demonstrate the useful role that can be played
by spectropolarimetric observations of young and evolved emission line stars
that analyse the linearly polarized component in their spectra. At the time of
writing, this demonstration has to be made on the basis of optical data since
there is no common-user infrared facility, in operation, that offers the
desired combination of spectral resolution and sensitivity. Here we focus on
what can be learned from linear spectropolarimetry alone at reasonably high
spectral resolution and at S/N. And we remind that the near
infrared (1--2 micron) has the potential to out-perform the optical as a domain
to work in because of the greatly reduced interstellar obscuration at these
wavelengths. This point has been reached at a time when theory, exploiting
flexible Monte Carlo methods, is fast becoming a powerful tool. In short we
have the complex phenomena, and the rise of the modelling capability to match
-- good data are the missing link.Comment: 11 pages, ESO Conference on High Resolution Infrared Spectroscop
Radiation-Driven Outflows in Active Galactic Nuclei
We review the results from multi-dimensional, time-dependent simulations of
gas dynamics in AGN. We will focus on two types of outflows powered by
radiation emitted from the AGN central engine: (i) outflows driven from the
innermost part of an accretion disk and (2) outflows driven from a large-scale
inflow that is likely the main supplier of material to the central engine. We
discuss the relevance of both types of outflows to the so-called AGN feedback
problem. However, the AGN feedback should not be considered separately from the
AGN physics. Therefore, we also discuss the issue whether the properties of the
same outflows are consistent with the gas properties in broad- and narrow-line
regions.Comment: 9 pages, 5 figures, in Recent Directions In Astrophysical
Quantitative Spectroscopy And Radiation Hydrodynamics: Proceedings of the
International Conference in Honor of Dimitri Mihalas for His Lifetime
Scientific Contributions on the Occasion of His 70th Birthday (AIP Conference
Proceedings 1171
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