145 research outputs found
Flares observed with XMM-Newton and the VLA
We present lightcurves obtained in X-ray by the XMM-Newton EPIC cameras and
simultaneous radio lightcurves obtained with the VLA for five active M-type
flare stars. A number of flare events were observed, and by comparing radio
with X-ray data, we consider various possible flare mechanisms. In cases where
there seems to be a clear correlation between radio and X-ray activity, we use
an energy budget argument to show that the heating which leads to the X-ray
emission could be due to the same particles emitting in the radio. In cases
where there is radio activity without corresponding X-ray activity, we argue
that the radio emission is likely to arise from coherent processes involving
comparatively few particles. In one case, we are able to show from polarization
of the radio emission that this is almost certainly the case. Cases for which
X-ray activity is seen without corresponding radio activity are more difficult
to explain. We suggest that the heating particles may be accelerated to very
high energy, and the resulting synchrotron radio emission may be beamed in
directions other than the line of sight.Comment: 12 pages, 10 figure
X-rays from T Tau: A test case for accreting T Tauri stars
We test models for the generation of X-rays in accreting T Tauri stars (TTS),
using X-ray data from the classical TTS T Tau. High-resolution spectroscopy
from the Reflection Grating Spectrometers on XMM-Newton is used to infer
electron densities, element abundances and the thermal structure of the X-ray
source. We also discuss the ultraviolet light curve obtained by the Optical
Monitor, and complementary ground-based photometry. A high-resolution image
from Chandra constrains contributions from the two companions of T Tau N. The
X-ray grating spectrum is rich in emission lines, but shows an unusual mixture
of features from very hot (~30 MK) and very cool (1-3 MK) plasma, both emitted
by similar amounts of emission measure. The cool plasma confirms the picture of
a soft excess in the form of an enhanced OVII/OVIII Lya flux ratio, similar to
that previously reported for other accreting TTS. Diagnostics from lines formed
by this plasma indicate low electron densities (<~ 1E10 cm-3). The Ne/Fe
abundance ratio is consistent with a trend in pre-main sequence stars in which
this ratio depends on spectral type, but not on accretion. On the basis of line
density diagnostics, we conclude that the density of the cool ``soft-excess''
plasma is orders of magnitude below that predicted for an accretion shock,
assuming previously determined accretion rates of (3-6)E-8 M_sun/y. We argue
that loading of magnetic field lines with infalling material suppresses the
heating process in a part of the corona. We thus suggest that the X-ray
production of T Tau is influenced by the accretion process although the X-rays
may not form in the bulk of the accretion footpoints.Comment: 12 pages, 7 figures, A&A style. Accepted by A&A, to appear in a
special section/issue dedicated to the XMM-Newton Extended Survey of the
Taurus Molecular Cloud (XEST). See also
http://www.issibern.ch/teams/Taurus/papers.htm
A Chandra X-ray detection of the L dwarf binary Kelu-1: Simultaneous Chandra and Very Large Array observations
Magnetic activity in ultracool dwarfs, as measured in X-rays and H,
shows a steep decline after spectral type M7-M8. So far, no L dwarf has been
detected in X-rays. In contrast, L dwarfs may have higher radio activity than M
dwarfs. We observe L and T dwarfs simultaneously in X-rays and radio to
determine their level of magnetic activity in the context of the general
decline of magnetic activity with cooler effective temperatures. The field L
dwarf binary Kelu-1 was observed simultaneously with Chandra and the Very Large
Array. Kelu-1AB was detected in X-rays with erg/s, while it remained undetected in the radio down to a limit of erg/s/Hz. We argue that,
whereas the X-ray and H emissions decline in ultracool dwarfs with
decreasing effective temperature, the radio luminosity stays (more or less)
constant across M and early-L dwarfs. The radio surface flux or the luminosity
may better trace magnetic activity in ultracool dwarfs than the ratio of the
luminosity to the bolometric luminosity. Deeper radio observations (and at
short frequencies) are required to determine if and when the cut-off in radio
activity occurs in L and T dwarfs, and what kind of emission mechanism takes
place in ultracool dwarfs.Comment: Accepted for publication as a Letter in Astronomy & Astrophysic
Thermal evolution of protoplanetary disks: From β -cooling to decoupled gas and dust temperatures
Aims. We explore the long-term evolution of young protoplanetary disks with different approaches to computing the thermal structure determined by various cooling and heating processes in the disk and its surroundings. Methods. Numerical hydrodynamics simulations in the thin-disk limit were complemented with three thermal evolution schemes: a simplified β-cooling approach with and without irradiation, where the rate of disk cooling is proportional to the local dynamical time; a fiducial model with equal dust and gas temperatures calculated taking viscous heating, irradiation, and radiative cooling into account; and a more sophisticated approach allowing decoupled dust and gas temperatures. Results. We found that the gas temperature may significantly exceed that of dust in the outer regions of young disks thanks to additional compressional heating caused by the infalling envelope material in the early stages of disk evolution and slow collisional exchange of energy between gas and dust in low-density disk regions. However, the outer envelope shows an inverse trend, with the gas temperatures dropping below that of dust. The global disk evolution is only weakly sensitive to temperature decoupling. Nevertheless, separate dust and gas temperatures may affect the chemical composition, dust evolution, and disk mass estimates. Constant-β models without stellar and background irradiation fail to reproduce the disk evolution with more sophisticated thermal schemes because of the intrinsically variable nature of the β-parameter. Constant-β models with irradiation more closely match the dynamical and thermal evolution, but the agreement is still incomplete. Conclusions. Models allowing separate dust and gas temperatures are needed when emphasis is placed on the chemical or dust evolution in protoplanetary disks, particularly in subsolar metallicity environments. © ESO 2020.Austrian Science Fund, FWF: P31635-N27Austrian Science Fund, FWF17H06360, 17H02869Acknowledgements. We are thankful to the anonymous referee for constructive comments that helped to improve the manuscript. E.I.V. and M.G. acknowledge support from the Austrian Science Fund (FWF) under research grant P31635-N27. K.O and R.M acknowledge support work by MEXT/JSPS KAKENHI Grant Number17H01102, 17H02869, 17H06360. The simulations were performed on the Vienna Scientific Cluster
A Naive Bayes Source Classifier for X-ray Sources
The Chandra Carina Complex Project (CCCP) provides a sensitive X-ray survey
of a nearby starburst region over >1 square degree in extent. Thousands of
faint X-ray sources are found, many concentrated into rich young stellar
clusters. However, significant contamination from unrelated Galactic and
extragalactic sources is present in the X-ray catalog. We describe the use of a
naive Bayes classifier to assign membership probabilities to individual
sources, based on source location, X-ray properties, and visual/infrared
properties. For the particular membership decision rule adopted, 75% of CCCP
sources are classified as members, 11% are classified as contaminants, and 14%
remain unclassified. The resulting sample of stars likely to be Carina members
is used in several other studies, which appear in a Special Issue of the ApJS
devoted to the CCCP.Comment: Accepted for the ApJS Special Issue on the Chandra Carina Complex
Project (CCCP), scheduled for publication in May 2011. All 16 CCCP Special
Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
least. 19 pages, 7 figure
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