3 research outputs found
Accretion-Induced Lithium Line Enhancements in Classical T Tauri Stars: RW Aur
It is widely accepted that much of the stochastic variability of T Tauri
stars is due to accretion by a circumstellar disk. The emission line spectrum
as well as the excess continuum emission are common probes of this process. In
this communication, we present additional probes of the circumstellar
environment in the form of resonance lines of low ionization potential
elements. Using a set of 14 high resolution echelle observations of the
classical T Tauri star (CTTS), RW Aur, taken between 1986 and 1996, we
carefully measure the continuum veiling at each epoch by comparing more than
500 absorption lines with those of an appropriate template. This allows us to
accurately subtract out the continuum emission and to recover the underlying
photospheric spectrum. In doing so, we find that selected photospheric lines
are enhanced by the accretion process, namely the resonance lines of LiI and
KI. A resonance line of TiI and a low excitation potential line of CaI also
show weak enhancements. Simple slab models and computed line bisectors lead us
to propose that these line enhancements are markers of cool gas at the
beginning of the accretion flow which provides an additional source of line
opacity. These results suggest that published values of surface lithium
abundances of classical T Tauri stars are likely to be overestimated. This
would account for the various reports of surface lithium abundances in excess
of meteoritic values among the extreme CTTS. Computing LTE lithium abundances
of RW Aur in a low and then high accretion state yields abundances which vary
by one order of magnitude. The low accretion state lithium abundance is
consistent with theoretical predictions for a star of this age and mass while
the high accretion state spectrum yields a super-meteoritic lithium abundance.Comment: 28 pages, 8 figures, accepted by Ap
Eclipses by circumstellar material in the T Tauri star AA Tau. II. Evidence for non-stationary magnetospheric accretion
We report the results of a synoptic study of the photometric and
spectroscopic variability of the classical T Tauri star AA Tau on timescales
ranging from a few hours to several weeks. Emission lines show both infall and
outflow signatures and are well reproduced by magnetospheric accretion models
with moderate mass accretion rates and high inclinations. The veiling shows
variations that indicate the presence of 2 rotationally modulated hot spots
corresponding to the two magnetosphere poles. It correlates well with the HeI
line flux, with B-V and the V excess flux. We have indications of a time delay
between the main emission lines and veiling, the lines formed farther away
preceding the veiling changes. The time delay we measure is consistent with
accreted material propagating downwards the accretion columns at free fall
velocity from a distance of about 8 Rstar. We also report periodic radial
velocity variations of the photospheric spectrum which might point to the
existence of a 0.02 Msun object orbiting the star at a distance of 0.08 AU.
During a few days, the variability of the system was strongly reduced and the
line fluxes and veiling severely depressed. We argue that this episode of
quiescence corresponds to the temporary disruption of the magnetic
configuration at the disk inner edge. The radial velocity variations of inflow
and outflow diagnostics in the Halpha profile yield further evidence for large
scale variations of the magnetic configuration on a timescale of a month. These
results may provide the first clear evidence for large scale instabilities
developping in T Tauri magnetospheres as the magnetic field lines are twisted
by differential rotation between the star and the inner disk.Comment: 25 pages, Astron. Astrophys., in pres