26,742 research outputs found
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&
X-ray activity cycle on the active ultra-fast rotator AB Dor A? Implication of correlated coronal and photometric variability
Although chromospheric activity cycles have been studied in a larger number
of late-type stars for quite some time, very little is known about coronal
activity-cycles in other stars and their similarities or dissimilarities with
the solar activity cycle. While it is usually assumed that cyclic activity is
present only in stars of low to moderate activity, we investigate whether the
ultra-fast rotator AB Dor, a K dwarf exhibiting signs of substantial magnetic
activity in essentially all wavelength bands, exhibits a X-ray activity cycle
in analogy to its photospheric activity cycle of about 17 years and possible
correlations between these bands. We analysed the combined optical photometric
data of AB Dor A, which span ~35 years. Additionally, we used ROSAT and
XMM-Newton X-ray observations of AB Dor A to study the long-term evolution of
magnetic activity in this active K dwarf over nearly three decades and searched
for X-ray activity cycles and related photometric brightness changes. AB Dor A
exhibits photometric brightness variations ranging between 6.75 < Vmag < 7.15
while the X-ray luminosities range between 29.8 < log LX [erg/s] < 30.2 in the
0.3-2.5 keV. As a very active star, AB Dor A shows frequent X-ray flaring, but,
in the long XMM-Newton observations a kind of basal state is attained very
often. This basal state probably varies with the photospheric activity-cycle of
AB Dor A which has a period of ~17 years, but, the X-ray variability amounts at
most to a factor of ~2, which is, much lower than the typical cycle amplitudes
found on the Sun.Comment: 10 page
A magnetic cycle of tau Bootis? The coronal and chromospheric view
Tau Bootis is a late F-type main sequence star orbited by a Hot Jupiter.
During the last years spectropolarimetric observations led to the hypothesis
that this star may host a global magnetic field that switches its polarity once
per year, indicating a very short activity cycle of only one year duration. In
our ongoing observational campaign, we have collected several X-ray
observations with XMM-Newton and optical spectra with TRES/FLWO in Arizona to
characterize tau Boo's corona and chromosphere over the course of the supposed
one-year cycle. Contrary to the spectropolarimetric reconstructions, our
observations do not show indications for a short activity cycle.Comment: 4 pages, 2 figures, appeared in Astronomical Notes 333, 1, 26-29
(2012
The Chandra X-ray view of the power sources in Cepheus A
The central part of the massive star-forming region Cepheus A contains
several radio sources which indicate multiple outflow phenomena, yet the
driving sources of the individual outflows have not been identified. We present
a high-resolution Chandra observation of this region that shows the presence of
bright X-ray sources, consistent with active pre-main sequence stars, while the
strong absorption hampers the detection of less luminous objects. A new source
has been discovered located on the line connecting H_2 emission regions at the
eastern and western parts of Cepheus A. This source could be the driving source
of HH 168. We present a scenario relating the observed X-ray and radio
emission.Comment: 7 pages, 6 figures, accepted for publication in 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&
The evolution of the X-ray emission of HH 2 - Investigating heating and cooling processes
Young stellar objects often drive powerful bipolar outflows which evolve on
time scales of a few years. An increasing number of these outflows has been
detected in X-rays implying the existence of million degree plasma almost
co-spatial with the lower temperature gas observed in the optical and near-IR.
The details of the heating and cooling processes of the X-ray emitting part of
these so-called Herbig-Haro objects are still ambiguous, e.g., whether the
cooling is dominated by expansion, radiation or thermal conduction.
We present a second epoch Chandra observation of the first X-ray detected
Herbig-Haro object (HH 2) and derive the proper-motion of the X-ray emitting
plasma and its cooling history. We argue that the most likely explanation for
the constancy of the X-ray luminosity, the alignment with the optical emission
and the proper-motion is that the cooling is dominated by radiative losses
leading to cooling times exceeding a decade. We explain that a strong shock
caused by fast material ramming into slower gas in front of it about ten years
ago can explain the X-ray emission while being compatible with the available
multi-wavelength data of HH 2.Comment: 5 pages with 4 figures; accepted for publication by Astronomy and
Astrophysic
Transit observations of the Hot Jupiter HD 189733b at X-ray wavelengths
We present new X-ray observations obtained with Chandra ACIS-S of the HD
189733 system, consisting of a K-type star orbited by a transiting Hot Jupiter
and an M-type stellar companion. We report a detection of the planetary transit
in soft X-rays with a significantly larger transit depth than observed in the
optical. The X-ray data favor a transit depth of 6-8%, versus a broadband
optical transit depth of 2.41%. While we are able to exclude several possible
stellar origins for this deep transit, additional observations will be
necessary to fully exclude the possibility that coronal inhomogeneities
influence the result. From the available data, we interpret the deep X-ray
transit to be caused by a thin outer planetary atmosphere which is transparent
at optical wavelengths, but dense enough to be opaque to X-rays. The X-ray
radius appears to be larger than the radius observed at far-UV wavelengths,
most likely due to high temperatures in the outer atmosphere at which hydrogen
is mostly ionized. We furthermore detect the stellar companion HD 189733B in
X-rays for the first time with an X-ray luminosity of log LX = 26.67 erg/s. We
show that the magnetic activity level of the companion is at odds with the
activity level observed for the planet-hosting primary. The discrepancy may be
caused by tidal interaction between the Hot Jupiter and its host star.Comment: 15 pages, accepted for publication in The Astrophysical Journa
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