231 research outputs found
Optical spectroscopy of EX Lupi during quiescence and outburst: Infall, wind, and dynamics in the accretion flow
We explore the accretion mechanisms in EX Lupi, prototype of EXor variables,
during its quiescence and outburst phases. We analyse high-resolution optical
spectra taken before, during, and after its 2008 outburst. In quiescence and
outburst, the star presents many permitted emission lines, including typical
CTTS lines and numerous neutral and ionized metallic lines. During the
outburst, the number of emission lines increases to over a thousand, with
narrow plus broad component structure (NC+BC). The BC profile is highly
variable on short timescales (24-72h). An active chromosphere can explain the
metallic lines in quiescence and the outburst NC. The dynamics of the BC line
profiles suggest an origin in a hot, dense, non-axisymmetric, and non-uniform
accretion column that suffers velocity variations along the line-of-sight on
timescales of days. Assuming Keplerian rotation, the emitting region would be
located at ~0.1-0.2 AU, consistent with the inner disk rim, but the velocity
profiles of the lines reveal a combination of rotation and infall. Line ratios
of ions and neutrals can be reproduced with a temperature of T~6500 K for
electron densities of a few times 10cm in the line-emitting
region. The data confirm that the 2008 outburst was an episode of increased
accretion, albeit much stronger than previous EX Lupi and typical EXors
outbursts. The line profiles are consistent with the infall/rotation of a
non-axisymmetric structure that could be produced by clumpy accretion during
the outburst phase. A strong inner disk wind appears in the epochs of higher
accretion. The rapid recovery of the system after the outburst and the
similarity between the pre-outburst and post-outburst states suggest that the
accretion channels are similar during the whole period, and only the accretion
rate varies, providing a superb environment for studying the accretion
processes.Comment: 15 pages plus 26 pages online material, accepted by A&
Cluster investigation of mixed O76:H19 Shiga toxin-producing Escherichia coli and atypical enteropathogenic E. coli infection in a Spanish household
A Spanish household was identified through a Public Health follow up on a Shiga toxin-producing Escherichia coli (STEC)-positive 14-month-old girl reporting bloody diarrhoea, with the four household members experiencing either symptomatic or asymptomatic STEC and/or atypical enteropathogenic E. coli (aEPEC) shedding. In total, two different O76:H19 STEC strains and six aEPEC strains belonging to multiple serotypes were isolated and characterized in the household during a 5-month period. Prolonged asymptomatic shedding of O76:H19 STEC and O51:H49 aEPEC was detected in two family members. Although there was no conclusive evidence, consumption of vegetables fertilized with sheep manure was the suspected source of infection. This study highlights the risk of cross-infections posed by prolonged asymptomatic carriage and close household contact between family members, and illustrates the importance of molecular epidemiology in understanding disease clusters.We thank José Manuel Luquin and Gemma Poignonfor facilitating the follow-up sampling of the house-hold members and relatives. We thank DanielEibach for critically reviewing the manuscript. Wealso thank Flemming Scheutz for conventional O:Hserotyping the strains. Sergio Sánchez acknowledgesthe Juan de la Cierva programme from theMinisterio de Economía y Competitividad for hisresearch contract. This study was supported by theMadrid Regional Government (P2009/AGR-1489)
Magnetically Controlled Accretion Flows onto Young Stellar Objects
(abridged) Accretion from disks onto young stars is thought to follow
magnetic field lines from the inner disk edge to the stellar surface. The
accretion flow thus depends on the geometry of the magnetic field. This paper
extends previous work by constructing a collection of orthogonal coordinate
systems, including the corresponding differential operators, where one
coordinate traces the magnetic field lines. This formalism allows for an
(essentially) analytic description of the geometry and the conditions required
for the flow to pass through sonic points. Using this approach, we revisit the
problem of magnetically controlled accretion flow in a dipole geometry, and
then generalize the treatment to consider magnetic fields with multiple
components, including dipole, octupole, and split monopole contributions. This
approach can be generalized further to consider more complex magnetic field
configurations. Observations indicate that accreting young stars have
substantial dipole and octupole components, and that accretion flow is
transonic. If the effective equation of state for the fluid is too stiff, the
flow cannot pass smoothly through the sonic points in steady state. For a
multipole field of order \ell, we derive a constraint on the polytropic index,
n>\ell+3/2, required for steady transonic flow to reach free-fall velocities.
For octupole fields, inferred on surfaces of T Tauri stars, n>9/2, so that the
flow must be close to isothermal. The inclusion of octupole field components
produces higher densities at the stellar surface and smaller hot spots, which
occur at higher latitudes; the magnetic truncation radius is also modified.
This contribution thus increases our understanding of magnetically controlled
accretion for young stellar objects and can be applied to a variety of
additional astrophysical problems.Comment: 50 pages, 8 figures, accepted to Ap
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
On the origin of [Ne II] emission in young stars: mid-infrared and optical observations with the Very Large Telescope
{Abridged version for ArXiv}. We provide direct constraints on the origin of
the [Ne II] emission in 15 young stars using high-spatial and spectral
resolution observations with VISIR at the VLT that allow us to study the
kinematics of the emitting gas. In addition we compare the [Ne II] line with
optical forbidden lines observed for three stars with UVES. The [Ne II] line
was detected in 7 stars, among them the first confirmed detection of [Ne II] in
a Herbig Be star, V892 Tau. In four cases, the large blueshifted lines indicate
an origin in a jet. In two stars, the small shifts and asymmetric profiles
indicate an origin in a photo-evaporative wind. CoKu Tau 1, seen close to
edge-on, shows a spatially unresolved line centered at the stellar rest
velocity, although cross-dispersion centroids move within 10 AU from one side
of the star to the other as a function of wavelength. The line profile is
symmetric with wings extending up to about +-80 km/s. The origin of the [Ne II]
line could either be due to the bipolar jet or to the disk. For the stars with
VLT-UVES observations, in several cases, the optical forbidden line profiles
and shifts are very similar to the profile of the [Ne II] line, suggesting that
the lines are emitted in the same region. A general trend observed with VISIR
is a lower line flux when compared with the fluxes obtained with Spitzer. We
found no correlation between the line full-width at half maximum and the line
peak velocity. The [Ne II] line remains undetected in a large part of the
sample, an indication that the emission detected with Spitzer in those stars is
likely extended.Comment: Accepted for publication in Astronomy & Astrophysics; revised
version: corrected minor typos, corrected center values (col 3) for CoKuTau1
in Table
Tracing the origins of permitted emission lines in RU Lupi down to AU scales
Most of the observed emission lines and continuum excess from young accreting
low mass stars (Classical T Tauri stars -- CTTSs) take place in the star-disk
or inner disk region. These regions have a complex emission topology still
largely unknown. In this paper the magnetospheric accretion and inner wind
contributions to the observed permitted He and H near infrared (NIR) lines of
the bright southern CTTS RU Lupi are investigated for the first time. Previous
optical observations of RU Lupi showed a large H-alpha profile, due to the
emission from a wind in the line wings, and a micro-jet detected in forbidden
lines. We extend this analysis to NIR lines through seeing-limited high
spectral resolution spectra taken with VLT/ISAAC, and adaptive optics (AO)
aided narrow-band imaging and low spectral resolution spectroscopy with
VLT/NACO. Using spectro-astrometric analysis we investigate the presence of
extended emission down to very low spatial scales (a few AU). The HeI 10830
line presents a P Cygni profile whose absorption feature indicates the presence
of an inner stellar wind. Moreover the spectro-astrometric analysis evidences
the presence of an extended emission superimposed to the absorption feature and
likely coming from the micro-jet detected in the optical. On the contrary, the
origin of the Hydrogen Paschen and Brackett lines is difficult to address. We
tried tentatively to explain the observed line profiles and flux ratios with
both accretion and wind models showing the limits of both approaches. The lack
of spectro-astrometric signal indicates that the HI emission is either compact
or symmetric. Our analysis confirms the sensitivity of the HeI line to the
presence of faint extended emission regions in the close proximity of the star.Comment: 11 pages, 4 figures, accepted for publication on A&
Accretion dynamics in the classical T Tauri star V2129 Oph
We analyze the photometric and spectroscopic variability of the classical T
Tauri star V2129 Oph over several rotational cycles to test the dynamical
predictions of magnetospheric accretion models. The photometric variability and
the radial velocity variations in the photospheric lines can be explained by
rotational modulation due to cold spots, while the radial velocity variations
of the He I (5876 \AA) line and the veiling variability are due to hot spot
rotational modulation. The hot and cold spots are located at high latitudes and
about the same phase, but the hot spot is expected to sit at the chromospheric
level, while the cold spot is at the photospheric level. Using the
dipole+octupole magnetic-field configuration previously proposed in the
literature for the system, we compute 3D MHD magnetospheric simulations of the
star-disk system. We use the simulation's density, velocity and scaled
temperature structures as input to a radiative transfer code, from which we
calculate theoretical line profiles at all rotational phases. The theoretical
profiles tend to be narrower than the observed ones, but the qualitative
behavior and the observed rotational modulation of the H\alpha and H\beta
emission lines are well reproduced by the theoretical profiles. The
spectroscopic and photometric variability observed in V2129 Oph support the
general predictions of complex magnetospheric accretion models with
non-axisymmetric, multipolar fields.Comment: Accepted by Astronomy and Astrophysic
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