86 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
[EUV Spectroscopy of MrK 421]
In accordance with our proposal, we carried out and analyzed a successful Extreme Ultraviolet Explorer spectroscopic observation of the BL Lacertae object Mrk 421 in April/May 1995 (approx. 242 ksec useful time). During this period, the source underwent a flare that was detected also in simultaneous observations in the X-ray and TeV (gamma)ray energy bands. Our EUVE data, however, provided the best continuous coverage of the flare and therefore played a key role in the analysis of the multiwavelength observation campaign (Buckley et al. 1996). Our spectral analysis benefited from an efficient procedure that we employed for properly assessing the background contamination at the shortest wavelengths. As a result, we were able to identify strong absorption features at the shortest observed EUV wavelengths (approx. 65-75 A), which were missed in earlier, on axis EUVE observations that extended only down to approx. 75 A (Fruscione et al. 1996). Our observations and interpretation of Mrk 421 were described in Kartje et al. 1997 (see attached reprint). The results were also presented in a poster at the AGN session in the 18th Texas Symposium on Relativistic Astrophysicsheld (Chicago, December 1996). Our interpretation of the observations has formed the basis for a unified model of outflows in BL Lac objects and Broad Absorption Line QSOs (Kartje & Konigl 1997). Preliminary accounts of this work, which have included a description of the EUVE observations of BL Lac objects, were presented in posters in IAU Colloquium No. 163 on Accretion Phenomena and Associated Outflows (Port Douglas, July 1996) and in the Carnegie Observatories Workshop on Ejection of Matter from AGNs (Pasadena, February 1997)
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
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
X-ray emission from young stars in Taurus-Auriga-Perseus: Luminosity functions and the rotation-activity-age relation
We report on a systematic search for X-ray emission from pre-main sequence
and young main sequence stars in the Taurus-Auriga-Perseus region. Our stellar
sample consists of all T Tauri stars from the Taurus-Auriga region, and all
late-type stars from the Pleiades and Hyades clusters which have been observed
by the ROSAT PSPC in pointed observations. We present the X-ray parameters for
all observed stars in tables, and study the connection between coronal X-ray
activity and stellar parameters for different subgroups of our sample. In
particular we compile X-ray luminosity functions (XLF), and discuss the
relations between X-ray emission and spectral type, age, and rotation, on the
largest sample so far.Comment: 19 pages, plus 6 tables, accepted for publication in A&
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&
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
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