179 research outputs found
Quiescent and flaring X-ray emission from the nearby M/T dwarf binary SCR 1845-6357
We investigate an XMM-Newton observation of SCR 1845-6357, a nearby,
ultracool M8.5/T5.5 dwarf binary. The binary is unresolved in the XMM
detectors, however the X-ray emission is very likely from the M8.5 dwarf. We
compare its flaring emission to those of similar very low mass stars and
additionally present an XMM observation of the M8 dwarf VB 10. We detect
quasi-quiescent X-ray emission from SCR 1845-6357 at soft X-ray energies in the
0.2-2.0 keV band, as well as a strong flare with a count rate increase of a
factor of 30 and a duration of only 10 minutes. The quasi-quiescent X-ray
luminosity of log L_x = 26.2 erg/s and the corresponding activity level of log
L_x/L_bol = -3.8 point to a fairly active star. Coronal temperatures of up to 5
MK and frequent minor variability support this picture. During the flare, that
is accompanied by a significant brightening in the near-UV, plasma temperatures
of 25-30 MK are observed and an X-ray luminosity of L_x= 8 x 10^27 erg/s is
reached. SCR 1845-6357 is a nearby, very low mass star that emits X-rays at
detectable levels in quasi-quiescence, implying the existence of a corona. The
high activity level, coronal temperatures and the observed large flare point to
a rather active star, despite its estimated age of a few Gyr.Comment: Accepted by A&A, 6 pages, 5 figure
Coronal activity cycles in nearby G and K stars - XMM-Newton monitoring of 61 Cygni and Alpha Centauri
We use X-ray observations of the nearby binaries 61 Cyg A/B (K5V and K7V) and
Alpha Cen A/B (G2V and K1V) to study the long-term evolution of magnetic
activity in weakly to moderately active G + K dwarfs over nearly a decade.
Specifically we search for X-ray activity cycles and related coronal changes
and compare them to the solar behavior. For 61 Cyg A we find a regular coronal
activity cycle analog to its 7.3 yr chromospheric cycle. The X-ray brightness
variations are with a factor of three significantly lower than on the Sun, yet
the changes of coronal properties resemble the solar behavior with larger
variations occurring in the respective hotter plasma components. 61 Cyg B does
not show a clear cyclic coronal trend so far, but the X-ray data matches the
more irregular chromospheric cycle. Both Alpha Cen stars exhibit significant
long-term X-ray variability. Alpha Cen A shows indications for cyclic
variability of an order of magnitude with a period of about 12-15 years; the
Alpha Cen B data suggests an X-ray cycle with an amplitude of about six to
eight and a period of 8-9 years. The sample stars exhibit X-ray luminosities
ranging between Lx < 1x10^26 - 3x10^27 erg s^-1 in the 0.2-2.0 keV band and
have coronae dominated by cool plasma with variable average temperatures of
around 1.0-2.5 MK. We find that coronal activity cycles are apparently a common
phenomenon in older, slowly rotating G and K stars. The spectral changes of the
coronal X-ray emission over the cycles are solar-like in all studied targets.Comment: 11 pages, 9 figures, accepted by Astronomy and Astrophysic
X-ray emission from the remarkable A-type star HR 8799
We present a Chandra observation of the exceptional planet bearing A5V star
HR 8799, more precisely classified as a kA5hF0mA5 star and search for intrinsic
X-ray emission. We clearly detect HR 8799 at soft X-ray energies with the
ACIS-S detector in a 10 ks exposure; minor X-ray brightness variability is
present during the observation. The coronal plasma is described well by a model
with a temperature of around 3 MK and an X-ray luminosity of about Lx = 1.3 x
10^28 erg/s in the 0.2-2.0 keV band, corresponding to an activity level of log
Lx/Lbol ~ -6.2. Altogether, these findings point to a rather weakly active and
given a RASS detection, long-term stable X-ray emitting star. The X-ray
emission from HR 8799 resembles those of a late A/early F-type stars, in
agreement with its classification from hydrogen lines and effective temperature
determination and thus resolving the apparent discrepancy with the standard
picture of magnetic activity that predicts mid A-type stars to be virtually
X-ray dark.Comment: 4 pages, 3 figures, accepted by A&
Spatially resolved X-ray emission of EQ Pegasi
We present an analysis of an XMM-Newton observation of the M dwarf binary EQ
Pegasi with a special focus on the the spatial structure of the X-ray emission
and the analysis of light curves. Making use of data obtained with EPIC
(European Photon Imaging Camera) we were for the first time able to spatially
resolve the two components in X-rays and to study the light curves of the
individual components of the EQ Peg system. During the observation a series of
moderate flares was detected, where it was possible to identify the respective
flaring component.Comment: 6 pages, 11 figures, accepted by A&
Coronal properties of planet-bearing stars
Do extrasolar planets affect the activity of their host stars? Indications
for chromospheric activity enhancement have been found for a handful of
targets, but in the X-ray regime, conclusive observational evidence is still
missing. We want to establish a sound observational basis to confirm or reject
major effects of Star-Planet Interactions (SPI) in stellar X-ray emissions. We
therefore conduct a statistical analysis of stellar X-ray activity of all known
planet-bearing stars within 30pc distance for dependencies on planetary
parameters such as mass and semimajor axis. We find that in our sample, there
are no significant correlations of X-ray luminosity or the activity indicator
L_X/L_bol with planetary parameters which cannot be explained by selection
effects. Coronal SPI seems to be a phenomenon which might only manifest itself
as a strong effect for a few individual targets, but not to have a major effect
on planet-bearing stars in general.Comment: accepted by A&
DN Tauri - coronal activity and accretion in a young low-mass CTTS
We present a deep XMM-Newton observation of DN Tau, a young M0-type accreting
CTTS and investigate its X-ray properties and X-ray generating mechanisms.
Specifically we examine the presence of X-ray emission from magnetic activity
and accretion shocks and compare our new X-ray data with UV data taken
simultaneously and with X-ray/UV observations performed before. We find that
the X-ray emission from DN Tau is dominated by coronal plasma generated via
magnetic activity, but also clearly detect a contribution of the accretion
shocks to the cool plasma component at about 2 MK as consistently inferred from
density and temperature analysis. Typical phenomena of active coronae like
flaring, the presence of very hot plasma at 30 MK and an abundance pattern
showing the inverse FIP effect are seen on DN Tau. Strong variations in the
emission measure of the cooler plasma components between the 2005 and 2010 XMM
data point to accretion related changes. The UV light curve taken
simultaneously is in general not related to the X-ray brightness, but exhibits
clear counterparts during the observed X-ray flares. The X-ray properties of DN
Tau are similar to those of more massive CTTS, but its low mass and large
radius shift the accretion shocks to lower temperatures, reducing their imprint
in the X-ray regime.Comment: 11 pages, 8 figures, accepted for publication in A&
51 Pegasi - a planet-bearing Maunder minimum candidate
We observed 51 Peg, the first detected planet-bearing star, in a 55 ks
XMM-Newton pointing and in 5 ks pointings each with Chandra HRC-I and ACIS-S.
The star has a very low count rate in the XMM observation, but is clearly
visible in the Chandra images due to the detectors' different sensitivity at
low X-ray energies. This allows a temperature estimate for 51 Peg's corona of
T<1MK; the detected ACIS-S photons can be plausibly explained by emission lines
of a very cool plasma near 200eV. The constantly low X-ray surface flux and the
flat-activity profile seen in optical CaII data suggest that 51 Peg is a
Maunder minimum star; an activity enhancement due to a Hot Jupiter, as proposed
by recent studies, seems to be absent. The star's X-ray fluxes in different
instruments are consistent with the exception of the HRC Imager, which might
have a larger effective area below 200eV than given in the calibration.Comment: accepted by A&
Solar and Stellar Activity: Diagnostics and Indices
We summarize the fifty-year concerted effort to place the "activity" of the
Sun in the context of the stars. As a working definition of solar activity in
the context of stars, we adopt those globally-observable variations on time
scales below thermal time scales, of ~ 100,000 yr for the convection zone. So
defined, activity is dominated by magnetic-field evolution, including the
22-year Hale cycle, the typical time it takes for the quasi-periodic reversal
in which the global magnetic-field takes place. This is accompanied by sunspot
variations with 11 year periods, known since the time of Schwabe, as well as
faster variations due to rotation of active regions and flaring. "Diagnostics
and indices" are terms given to the indirect signatures of varying magnetic-
fields, including the photometric (broad-band) variations associated with the
sunspot cycle, and variations of the accompanying heated plasma in higher
layers of stellar atmospheres seen at special optical wavelengths, and UV and
X-ray wavelengths. Our attention is also focussed on the theme of the Symposium
by examining evidence for deep and extended minima of stars, and placing the
70-year long solar Maunder Minimum into a stellar context.Comment: Invited keynote paper for IAU Symposium No 286, 2012. Comparative
Magnetic Minima: Characterizing quiet times in the Sun and star
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