598 research outputs found
Stellar Activity and Coronal Heating: an overview of recent results
Observations of the coronae of the Sun and of solar-like stars provide
complementary information to advance our understanding of stellar magnetic
activity, and of the processes leading to the heating of their outer
atmospheres. While solar observations allow us to study the corona at high
spatial and temporal resolution, the study of stellar coronae allows us to
probe stellar activity over a wide range of ages and stellar parameters.
Stellar studies therefore provide us with additional tools for understanding
coronal heating processes, as well as the long-term evolution of solar X-ray
activity. We discuss how recent studies of stellar magnetic fields and coronae
contribute to our understanding of the phenomenon of activity and coronal
heating in late-type stars.Comment: Accepted for publication on Philosophical Transactions A. 29 pages, 5
figure
X-ray Spectroscopy of the Contact Binary VW Cephei
Short-period binaries represent extreme cases in the generation of stellar
coronae via a rotational dynamo. Such stars are important for probing the
origin and nature of coronae in the regimes of rapid rotation and activity
saturation. VW Cep (P=0.28 d) is a relatively bright, partially eclipsing, and
very active object. Light curves made from Chandra/HETGS data show flaring and
rotational modulation, but no eclipses. Velocity modulation of emission lines
indicates that one component dominates the X-ray emission. The emission measure
is highly structured, having three peaks. Helium-like triplet lines give
electron densities of about 3.0E+10 - 18.0E+10 /cm^3. We conclude that the
corona is predominantly on the polar regions of the primary star and compact.Comment: Accepted for publication in the Astropysical Journal, 23 June 2006;
22 pages, 15 figure
X-ray flare modeling in the single giant HR 9024
We analyze a Chandra-HETGS observation of the single G-type giant HR 9024.
The high flux allows us to examine spectral line and continuum diagnostics at
high temporal resolution, to derive plasma parameters (thermal distribution,
abundances, temperature, ...). A time-dependent 1D hydrodynamic loop model with
semi-length 10cm (), and impulsive footpoint heating
triggering the flare, satisfactorily reproduces the observed evolution of
temperature and emission measure, derived from the analysis of the strong
continuum emission. The observed characteristics of the flare appear to be
common features in very large flares in active stars (also pre-main sequence
stars), possibly indicating some fundamental physics for these very dynamic and
extreme phenomena in stellar coronae.Comment: 2 pages, 4 figures. To be included in the proceedings of the 'X-ray
universe 2005 meeting' held in San Lorenzo de El Escorial (Spain), 26-30 Sep
200
Chandra/HETGS Observations of the Capella System: the Primary as a Dominating X-ray Source
Using the Chandra/High Energy Transmission Grating Spectrometer (hereafter
HETGS) we have detected Doppler motion of Capella's X-ray emission lines in the
6 -- 25AA wave-band. The observed motion follows the expected orbital motion of
Capella's primary. This finding implies that the primary G8 III star, not the
secondary G1 III star in the Hertzsprung gap, has been the dominant source of
hot 10^{6.8} -- 10^{7}K plasma at least in the last six years. In addition, the
results demonstrate the long-term stability of the HETGS and demonstrate small
uncertainties of 25 and 33 km/s in the velocity determination with the HEG and
MEG, respectively.Comment: 4 pages, 4 figures, accepted by Astrophysical Journal Letter
X-ray Properties of Low-Mass Pre-Main Sequence Stars in the Orion Trapezium Cluster
The Chandra High Energy Transmission Gratings (HETG) Orion Legacy Project
(HOLP) is the first comprehensive set of observations of a very young massive
stellar cluster which provides high resolution X-ray spectra of very young
stars over a wide mass range (0.7 - 2.3 Msun). In this paper, we focus on the
six brightest X-ray sources with T Tauri stellar counterparts which are
well-characterized at optical and infra-red wavelengths. All stars show column
densities which are substantially smaller than expected from optical extinction
indicating that the sources are located on the near side of the cluster with
respect to the observer as well as that these stars are embedded in more dusty
environments. Stellar X-ray luminosities are well above erg/s, in
some cases exceeding erg/s for a substantial amount of time. The
stars during these observations show no flares but are persistently bright. The
spectra can be well fit with two temperature plasma components of 10 MK and 40
MK, of which the latter dominates the flux by a ratio 6:1 on average. The total
EMs range between 3 - 8 cm and are comparable to active
coronal sources. Limits on the forbidden to inter-combination line ratios in
the He-Like K-shell lines show that we observe a predominantely optically thin
plasma with electron densities below cm. Observed abundances
compare well with active coronal sources underlying the coronal nature of these
sources. The surface flux in this sample of 0.6 to 2.3 Msun classical T Tauri
stars shows that coronal activity and possibly coronal loop size increase
significantly between ages 0.1 to 10 Myrs.Comment: 13 pages, 12 figures, submitted to the Astrophysical Journa
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