345 research outputs found
Magnetic activity cycles in solar-like stars: The cross-correlation technique of p-mode frequency shifts
Aims. We aim studying the use of cross-correlation techniques to infer the
frequency shifts induced by changing magnetic fields in the p-mode frequencies
and provide precise estimation of the error bars. Methods. This technique and
the calculation of the associated errors is first tested and validated on the
Sun where the p-mode magnetic behaviour is very well known. These validation
tests are performed on 6000-day time series of Sun-as-a-star observations
delivered by the SoHO spacecraft. Errors of the frequency shifts are quantified
through Monte Carlo simulations. The same methodology is then applied to three
solar-like oscillating stars: HD 49933, observed by CoRoT, as well as KIC
3733735 and KIC 7940546 observed by Kepler. Results. We first demonstrate the
reliability of the error bars computed with the Monte Carlo simulations using
the Sun. From the three analyzed stars we confirm the presence of a magnetic
activity cycle with this methodology in HD 49933 and we unveil seismic
signature of on going magnetic variations in KIC 3733735. Finally, the third
star, KIC 7940546, seems to be in a quiet regime.Comment: Paper accepted in A&A. 7 pages and 11 figure
Comparing the performance of stellar variability filters for the detection of planetary transits
We have developed a new method to improve the transit detection of
Earth-sized planets in front of solar-like stars by fitting stellar
microvariability by means of a spot model. A large Monte Carlo numerical
experiment has been designed to test the performance of our approach in
comparison with other variability filters and fitting techniques for stars of
different magnitudes and planets of different radius and orbital period, as
observed by the space missions CoRoT and Kepler. Here we report on the results
of this experiment.Comment: 4 pages, 3 postscript figures, Transiting Planets Proceeding IAU
Symposium No.253, 200
CoRoT reveals a magnetic activity cycle in a Sun-like star
The 11-year activity cycle of the Sun is a consequence of a dynamo process
occurring beneath its surface. We analyzed photometric data obtained by the
CoRoT space mission, showing solar-like oscillations in the star HD49933, for
signatures of stellar magnetic activity. Asteroseismic measurements of global
changes in the oscillation frequencies and mode amplitudes reveal a modulation
of at least 120 days, with the minimum frequency shift corresponding to maximum
amplitude as in the Sun. These observations are evidence of a stellar magnetic
activity cycle taking place beneath the surface of HD49933 and provide
constraints for stellar dynamo models under conditions different from those of
the Sun.Comment: Brevia text and supporting online material, published in Scienc
Probing the Deep End of the Milky Way with New Oscillating Kepler Giants
The Kepler mission has been a success in both exoplanet search and stellar
physics studies. Red giants have actually been quite a highlight in the Kepler
scene. The Kepler long and almost continuous four-year observations allowed us
to detect oscillations in more than 15,000 red giants targeted by the mission.
However by looking at the power spectra of 45,000 stars classified as dwarfs
according to the Q1-16 Kepler star properties catalog, we detected red-giant
like oscillations in 850 stars. Even though this is a small addition to the
known red-giant sample, these misclassified stars represent a goldmine for
galactic archeology studies. Indeed they happen to be fainter (down to Kp~16)
and more distant (d>10kpc) than the known red giants, opening the possibility
to probe unknown regions of our Galaxy. The faintness of these red giants with
detected oscillations is very promising for detecting acoustic modes in red
giants observed with K2 and TESS. In this talk, I will present this new sample
of red giants with their revised stellar parameters derived from
asteroseismology. Then I will discuss about the distribution of their masses,
distances, and evolutionary states compared to the previously known sample of
red giants.Comment: 5 pages, 4 figures, Proceedings of the KASC9-TASC2 meetin
Gold and Silver joining technologies in the Moche Tombs âSeñor de SipĂĄnâ and âSeñora de Cao jewelery
About 200 gold and silver funerary ornaments from the Moche tombs âSeñor de SipĂĄnâ and âSeñora de Caoâ were analyzed to determine
their metallurgic characteristics. Of particular interest was the question about the gold-silver joining process. To this aim, following
methods were employed, all based on the use of X-rays:
- energy dispersive X-ray fluorescence;
- transmission of monoenergetic fluorescent X-rays;
- radiography.
At least three joining methods were possibly identified:
- of gluing gold and silver sheets;
- of brazing using a proper solder;
- of using a mercury amalgam
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