478 research outputs found
Lithium enrichment on the single active K1-giant DI Piscium -- Possible joint origin of differential rotation and Li enrichment
We investigate the surface spot activity of the rapidly rotating,
lithium-rich active single K-giant DI Psc to measure the surface differential
rotation and understand the mechanisms behind the Li-enrichment. Doppler
imaging was applied to recover the surface temperature distribution of DI Psc
in two subsequent rotational cycles using the individual mapping lines Ca I
6439, Fe I 6430, Fe I 6421 and Li I 6708. Surface differential rotation was
derived by cross-correlation of the subsequent maps. Difference maps are
produced to study the uniformity of Li-enrichment on the surface. These maps
are compared with the rotational modulation of the Li I 6708 line equivalent
width. Doppler images obtained for the Ca and Fe mapping lines agree well and
reveal strong polar spottedness, as well as cool features at lower latitudes.
Cross-correlating the consecutive maps yields antisolar differential rotation
with shear coefficient -0.083 +- 0.021. The difference of the average and the
Li maps indicates that the lithium abundance is non-activity related. There is
also a significant rotational modulation of the Li equivalent width.Comment: 8 pages, 7 figures, accepted in A&
Magnitude-range brightness variations of overactive K giants
We study three representative, overactive spotted K giants (IL Hya, XX Tri,
and DM UMa) known to exhibit V-band light variations between 0.65-1.05 mags.
Our aim is to find the origin of their large brightness variation. We employ
long-term phase-resolved multicolor photometry, mostly from automatic
telescopes, covering 42 yr for IL Hya, 28 yr for XX Tri, and 34 yr for DM UMa.
For one target, IL Hya, we present a new Doppler image from NSO data taken in
late 1996. Effective temperatures for our targets are determined from all
well-sampled observing epochs and are based on a V-I_C color-index calibration.
The effective temperature change between the extrema of the rotational
modulation for IL Hya and XX Tri is in the range 50-200 K. The bolometric flux
during maximum of the rotational modulation, i.e., the least spotted states,
varied by up to 39% in IL Hya and up to 54% in XX Tri over the course of our
observations. We emphasize that for IL Hya this is just about half of the total
luminosity variation that can be explained by the photospheric temperature
(spots/faculae) changes, while for XX Tri it is even about one third. The
long-term, 0.6 mag V-band variation of DM UMa is more difficult to explain
because little or no B-V color index change is observed on the same timescale.
Placing the three stars with their light and color variations into H-R
diagrams, we find that their overall luminosities are generally too low
compared to predictions from current evolutionary tracks. A change in the
stellar radius due to strong and variable magnetic fields during activity
cycles likely plays a role in explaining the anomalous brightness and
luminosity of our three targets. At least for IL Hya, a radius change of about
9% is suggested from m_bol and T_eff, and is supported by independent vsin(i)
measurements.Comment: 13 pages, 8 figures, accepted in A&
Time-series Doppler images and surface differential rotation of the effectively-single rapidly-rotating K-giant KU Pegasi
According to most stellar dynamo theories, differential rotation (DR) plays a
crucial role for the generation of toroidal magnetic fields. Numerical models
predict surface differential rotation to be anti-solar for rapidly-rotating
giant stars, i.e., their surface angular velocity could increase with stellar
latitude. However, surface differential rotation has been derived only for a
handful of individual giant stars to date.
The spotted surface of the K-giant KU Pegasi is investigated in order to
detect its time evolution and quantify surface differential rotation.
We present altogether 11 Doppler images from spectroscopic data collected
with the robotic telescope STELLA between 2006--2011. All maps are obtained
with the surface reconstruction code iMap. Differential rotation is extracted
from these images by detecting systematic (latitude-dependent) spot
displacements. We apply a cross-correlation technique to find the best
differential rotation law.
The surface of KU Peg shows cool spots at all latitudes and one persistent
warm spot at high latitude. A small cool polar spot exists for most but not all
of the epochs. Re-identification of spots in at least two consecutive maps is
mostly possible only at mid and high latitudes and thus restricts the
differential-rotation determination mainly to these latitudes. Our
cross-correlation analysis reveals solar-like differential rotation with a
surface shear of , i.e., approximately five times weaker
than on the Sun. We also derive a more accurate and consistent set of stellar
parameters for KU Peg including a small Li abundance of ten times less than
solar.Comment: 13 pages, 12 figures, accepted for publication in A&
Multiple and changing cycles of active stars II. Results
We study the time variations of the cycles of 20 active stars based on
decades-long photometric or spectroscopic observations. A method of
time-frequency analysis, as discussed in a companion paper, is applied to the
data. Fifteen stars definitely show multiple cycles; the records of the rest
are too short to verify a timescale for a second cycle. The cycles typically
show systematic changes. For three stars, we found two cycles in each of them
that are not harmonics, and which vary in parallel, indicating that a common
physical mechanism arising from a dynamo construct. The positive relation
between the rotational and cycle periods is confirmed for the inhomogeneous set
of active stars. Stellar activity cycles are generally multiple and variable.Comment: Accepted to Astronomy and Astrophysic
Investigating magnetic activity in very stable stellar magnetic fields: long-term photometric and spectroscopic study of the fully convective M4 dwarf V374 Peg
The ultrafast-rotating () fully convective
single M4 dwarf V374 Peg is a well-known laboratory for studying intense
stellar activity in a stable magnetic topology. As an observable proxy for the
stellar magnetic field, we study the stability of the light curve, and thus the
spot configuration. We also measure the occurrence rate of flares and coronal
mass ejections (CMEs). We analyse spectroscopic observations,
photometry covering 5 years, and additional photometry that expands the
temporal base over 16 years. The light curve suggests an almost rigid-body
rotation, and a spot configuration that is stable over about 16 years,
confirming the previous indications of a very stable magnetic field. We
observed small changes on a nightly timescale, and frequent flaring, including
a possible sympathetic flare. The strongest flares seem to be more concentrated
around the phase where the light curve indicates a smaller active region.
Spectral data suggest a complex CME with falling-back and re-ejected material,
with a maximal projected velocity of 675km/s. We observed a CME rate
much lower than expected from extrapolations of the solar flare-CME relation to
active stars.Comment: 15 figures, 4 tables, accepted for publication in A&
Cytotoxic drug sensitivity of Epstein-Barr virus transformed lymphoblastoid B-cells.
BACKGROUND: Epstein-Barr virus (EBV) is the causative agent of immunosuppression
associated lymphoproliferations such as post-transplant lymphoproliferative
disorder (PTLD), AIDS related immunoblastic lymphomas (ARL) and immunoblastic
lymphomas in X-linked lymphoproliferative syndrome (XLP). The reported overall
mortality for PTLD often exceeds 50%. Reducing the immunosuppression in
recipients of solid organ transplants (SOT) or using highly active antiretroviral
therapy in AIDS patients leads to complete remission in 23-50% of the PTLD/ARL
cases but will not suffice for recipients of bone marrow grafts. An additional
therapeutic alternative is the treatment with anti-CD20 antibodies (Rituximab) or
EBV-specific cytotoxic T-cells. Chemotherapy is used for the non-responding cases
only as the second or third line of treatment. The most frequently used
chemotherapy regimens originate from the non-Hodgkin lymphoma protocols and there
are no cytotoxic drugs that have been specifically selected against EBV induced
lymphoproliferative disorders. METHODS: As lymphoblastoid cell lines (LCLs) are
well established in vitro models for PTLD, we have assessed 17 LCLs for cytotoxic
drug sensitivity. After three days of incubation, live and dead cells were
differentially stained using fluorescent dyes. The precise numbers of live and
dead cells were determined using a custom designed automated laser confocal
fluorescent microscope. RESULTS: Independently of their origin, LCLs showed very
similar drug sensitivity patterns against 29 frequently used cytostatic drugs.
LCLs were highly sensitive for vincristine, methotrexate, epirubicin and
paclitaxel. CONCLUSION: Our data shows that the inclusion of epirubicin and
paclitaxel into chemotherapy protocols against PTLD may be justified
The fly's eye project: Sidereal tracking on a hexapod mount
The driving objective of the Fly's Eye Project is a high resolution, high coverage time-domain survey in multiple optical passbands: our goal is to cover the entire visible sky above the 30° horizontal altitude with a cadence of ~ 3 min. Imaging is intended to perform with 19 wide-field cameras mounted on a hexapod platform. The essence of the hexapod allows us to build an instrument that does not require any kind of precise alignment and, in addition, the similar mechanics can be involved independently of the geographical location of the device. Here we summarize our early results with a single camera, focusing on the sidereal tracking as it is performed with the hexapod built by our group
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