32 research outputs found
On the interpretation of the beta(sub p) relation in interstellar clouds
Troland and Heiles (1986) have recently presented an updated compilation of observational data concerning the relationship between the interstellar magnetic field strength B and the gas density rho (or, equivalently, the particle density n). One of the main findings of their survey was that B remains constant over the density range 0.1 - approx. 100 cu. cm and shows evidence for increase only a higher densities. They compared this result with theoretical predictions based on the Parker-instability scenario for the formation and evolution of interstellar clouds in the presence of the galactic magnetic field. In this picture, low-density gas is driven by the magnetic Rayleigh-Taylor instability into magnetic valleys, where it accumulates into denser concentrations. The gas initially flows along the magnetic field lines and there is little increase of the field strength with density; B only starts to rise when n becomes large enough for self-gravity to begin competing with the magnetic stresses. For a cloud mass of approx. 1,000 sub M and the measured background field strength, the critical density for contraction in approx. 75 cu. cm. Troland and Heiles therefore concluded that this scenario is basically consistent with the observations. This conclusion is debated
Magnetic braking in weakly ionized circumstellar disks
Recent observations of disk-like mass distributions around newly formed stars have provided evidence for rapid rotation on scales similar to less than 0.1pc with specific angular momenta much higher than typical stellar values. A likely mechanism for the extraction of angular momentum from these regions is magnetic braking by means of Alfven waves that propagate into the lower-density ambient medium. However, because of the relatively high particle densities and the correspondingly low implied ionization fractions in these apparent disks, their constituent ions and neutrals need not be well coupled to each other and could develop large relative drift velocities. For this reason, previous treatments of magnetic braking that assumed perfect coupling between ions and neutrals have to be modified in this case. In particular, one has to take into account both the azimuthal drift that develops because only the ions are directly coupled to the magnetic field and the radial drift (or ambipolar diffusion) which leads to a redistribution (and leakage) of the magnetic flux. The results of a preliminary analysis of these effects are described
Enhanced X-ray variability from V1647 Ori, the young star in outburst illuminating McNeil's Nebula
We report a ~38 ks X-ray observation of McNeil's Nebula obtained with XMM on
2004 April 4. V1647 Ori, the young star in outburst illuminating McNeil's
Nebula, is detected with XMM and appears variable in X-rays. We investigate the
hardness ratio variability and time variations of the event energy distribution
with quantile analysis, and show that the large increase of the count rate from
V1647 Ori observed during the second half of the observation is not associated
with any large plasma temperature variations as for typical X-ray flares from
young low-mass stars. X-ray spectral fitting shows that the bulk (~75%) of the
intrinsic X-ray emission in the 0.5-8 keV energy band comes from a soft plasma
component (0.9 keV) reminiscent of the X-ray spectrum of the classical T Tauri
star TW Hya, for which X-ray emission is believed to be generated by an
accretion shock onto the photosphere of a low-mass star. The hard plasma
component (4.2 keV) contributes ~25% of the total X-ray emission, and can be
understood only in the framework of plasma heating sustained by magnetic
reconnection events. We find a hydrogen column density of NH=4.1E22 cm-2, which
points out a significant excess of hydrogen column density compared to the
value derived from optical/IR observations, consistent with the picture of the
rise of a wind/jet unveiled from ground optical spectroscopy. The X-ray flux
observed with XMM ranges from roughly the flux observed by Chandra on 2004
March 22 (~10 times greater than the pre-outburst X-ray flux) to a value two
times greater than that caught by Chandra on 2004 March 7 (~200 times greater
than the pre-outburst X-ray flux). We have investigated the possibility that
V1647 Ori displays a periodic variation in X-ray brightness as suggested by the
combined Chandra+XMM data set (abridged).Comment: 11 pages and 8 Figures. Accepted for publication by Astronomy &
Astrophysic
Modeling the Halpha line emission around classical T Tauri stars using magnetospheric accretion and disk wind models
Spectral observations of classical T Tauri stars show a wide range of line
profiles, many of which reveal signs of matter inflow and outflow. Halpha is
the most commonly observed line profile due to its intensity, and it is highly
dependent on the characteristics of the surrounding environment of these stars.
Our aim is to analyze how the Halpha line profile is affected by the various
parameters of our model which contains both the magnetospheric and disk wind
contributions to the Halpha flux. We used a dipolar axisymmetric stellar
magnetic field to model the stellar magnetosphere and a modified Blandford &
Payne model was used in our disk wind region. A three-level atom with continuum
was used to calculate the required Hydrogen level populations. We use the
Sobolev approximation and a ray-by-ray method to calculate the integrated line
profile. Through an extensive study of the model parameter space, we have
investigated the contribution of many of the model parameters on the calculated
line profiles. Our results show that the Halpha line is strongly dependent on
the densities and temperatures inside the magnetosphere and the disk wind
region. The bulk of the flux comes, most of the time, from the magnetospheric
component for standard classical T Tauri stars parameters, but the disk wind
contribution becomes more important as the mass accretion rate, the
temperatures and densities inside the disk wind increase. We have also found
that most of the disk wind contribution to the Halpha line is emitted at the
innermost region of the disk wind. Models that take into consideration both
inflow and outflow of matter are a necessity to fully understand and describe
classical T Tauri stars.Comment: 15 pages, 9 figures, accepted for publication in Astronomy &
Astrophysics. Revised version with English correction
The enigmatic young brown dwarf binary FU Tau: accretion and activity
FU Tau belongs to a rare class of young, wide brown dwarf binaries. We have
resolved the system in a Chandra X-ray observation and detected only the
primary, FU Tau A. Hard X-ray emission, presumably from a corona, is present
but, unexpectedly, we detect also a strong and unusually soft component from FU
Tau A. Its X-ray properties, so far unique among brown dwarfs, are very similar
to those of the T Tauri star TW Hya. The analogy with TW Hya suggests that the
dominating soft X-ray component can be explained by emission from accretion
shocks. However, the typical free-fall velocities of a brown dwarf are too low
for an interpretation of the observed X-ray temperature as post-shock region.
On the other hand, velocities in excess of the free-fall speed are derived from
archival optical spectroscopy, and independent pieces of evidence for strong
accretion in FU Tau A are found in optical photometry. The high X-ray
luminosity of FU Tau A coincides with a high bolometric luminosity confirming
an unexplained trend among young brown dwarfs. In fact, FU Tau A is
overluminous with respect to evolutionary models while FU Tau B is on the 1 Myr
isochrone suggesting non-contemporaneous formation of the two components in the
binary. The extreme youth of FU Tau A could be responsible for its peculiar
X-ray properties, in terms of atypical magnetic activity or accretion.
Alternatively, rotation and magnetic field effects may reduce the efficiency of
convection which in turn affects the effective temperature and radius of FU Tau
A shifting its position in the HR diagram. Although there is no direct prove of
this latter scenario so far we present arguments for its plausibility.Comment: Accepted for publication in MNRAS; 9 pages, 5 figure
On the observability of T Tauri accretion shocks in the X-ray band
Context. High resolution X-ray observations of classical T Tauri stars
(CTTSs) show a soft X-ray excess due to high density plasma (n_e=10^11-10^13
cm^-3). This emission has been attributed to shock-heated accreting material
impacting onto the stellar surface. Aims. We investigate the observability of
the shock-heated accreting material in the X-ray band as a function of the
accretion stream properties (velocity, density, and metal abundance) in the
case of plasma-beta<<1 in the post-shock zone. Methods. We use a 1-D
hydrodynamic model describing the impact of an accretion stream onto the
chromosphere, including the effects of radiative cooling, gravity and thermal
conduction. We explore the space of relevant parameters and synthesize from the
model results the X-ray emission in the [0.5-8.0] keV band and in the resonance
lines of O VII (21.60 Ang) and Ne IX (13.45 Ang), taking into account the
absorption from the chromosphere. Results. The accretion stream properties
influence the temperature and the stand-off height of the shocked slab and its
sinking in the chromosphere, determining the observability of the shocked
plasma. Our model predicts that X-ray observations preferentially detect
emission from low density and high velocity shocked accretion streams due to
the large absorption of dense post-shock plasma. In all the cases examined, the
post-shock zone exhibits quasi-periodic oscillations due to thermal
instabilities, but in the case of inhomogeneous streams and beta<<1, the shock
oscillations are hardly detectable. Conclusions. We suggest that, if accretion
streams are inhomogeneous, the selection effect introduced by the absorption on
observable plasma components may explain the discrepancy between the accretion
rate measured by optical and X-ray data as well as the different densities
measured using different He-like triplets in the X-ray band.Comment: 12 pages, 7 figures. Accepted for publication on A&
RACE-OC Project: Rotation and variability in the epsilon Chamaeleontis, Octans, and Argus stellar associations
We aim at determining the rotational and magnetic-related activity properties
of stars at different stages of evolution. We focus our attention primarily on
members of young stellar associations of known ages. Specifically, we extend
our previous analysis in Paper I (Messina et al. 2010, A&A 520, A15) to 3
additional young stellar associations beyond 100 pc and with ages in the range
6-40 Myr: epsilon Chamaeleontis (~6 Myr), Octans (~20 Myr), and Argus (~40
Myr). Additional rotational data of eta Chamaeleontis and IC2391 clusters are
also considered. Rotational periods were determined from photometric
time-series data obtained by the All Sky Automated Survey (ASAS) and the Wide
Angle Search for Planets (SuperWASP) archives. With the present study we have
completed the analysis of the rotational properties of the late-type members of
all known young loose associations in the solar neighborhood. Considering also
the results of Paper I, we have derived the rotation periods of 241 targets:
171 confirmed, 44 likely, 26 uncertain. The period of the remaining 50 stars
known to be part of loose associations still remains unknown. This rotation
period catalogue, and specifically the new information presented in this paper
at ~6, 20, and 40 Myr, contributes significantly to a better observational
description of the angular momentum evolution of young stars.Comment: Accepted by Astronomy & Astrophysics. Onlines figures will be
available at CD
150 keV Emission from PKS2149-306 with BeppoSAX
A BeppoSAX observation of the z=2.34 quasar PKS2149-306 produced a strong
signal in the high energy PDS instrument up to a maximum observed energy of
nearly 50 keV, 150 keV in the quasar frame. The Beppo-SAX spectrum spans almost
3 decades (0.3-150 keV, quasar frame) and shows an extremely hard
(alpha=0.4+/-0.05) X-ray spectrum above 3 keV (comparable to the X-ray
background slope), and either a softer (alpha=1.0(+0.6, -0.3)) low energy
component, or an ionized absorber at zero redshift. No evidence is seen of an
Fe-K emission line (EW<167 eV at 6.5 keV quasar frame) or a Compton hump
(R<0.3). A bremsstrahlung fit gives kT(rest)=46(+32, -16) keV, similar to the
X-ray background value, and a high energy cut-off power law requires E(cut)>120
keV (quasar frame).
The SED of PKS 2149-306 shows two peaks at ~ 10(12+/-0.5}Hz and
\~10(21+/-1.0)Hz (~ 0.3 mm and ~ 4 MeV), strongly resembling a low energy
cutoff BL~Lac object (LBL). The ratio of the two peaks shows an extreme Compton
dominance (C_D=1.4+/- 0.4), as in flat spectrum radio quasars (FSRQs). The
presence of an additional `optical/UV big bump' component may provide photons
that cool the jet, suppressing the radio emission.Comment: 18 pages, 3 figures. LaTeX, with AAS .sty file aasms4. Revised to
correct a stupid mistake affecting the Fe-K EW. The results now agree with
the ASCA data from Yaqoob et a
Accretion of Planetary Material onto Host Stars
Accretion of planetary material onto host stars may occur throughout a star's
life. Especially prone to accretion, extrasolar planets in short-period orbits,
while relatively rare, constitute a significant fraction of the known
population, and these planets are subject to dynamical and atmospheric
influences that can drive significant mass loss. Theoretical models frame
expectations regarding the rates and extent of this planetary accretion. For
instance, tidal interactions between planets and stars may drive complete
orbital decay during the main sequence. Many planets that survive their stars'
main sequence lifetime will still be engulfed when the host stars become red
giant stars. There is some observational evidence supporting these predictions,
such as a dearth of close-in planets around fast stellar rotators, which is
consistent with tidal spin-up and planet accretion. There remains no clear
chemical evidence for pollution of the atmospheres of main sequence or red
giant stars by planetary materials, but a wealth of evidence points to active
accretion by white dwarfs. In this article, we review the current understanding
of accretion of planetary material, from the pre- to the post-main sequence and
beyond. The review begins with the astrophysical framework for that process and
then considers accretion during various phases of a host star's life, during
which the details of accretion vary, and the observational evidence for
accretion during these phases.Comment: 18 pages, 5 figures (with some redacted), invited revie