84 research outputs found
Study of KIC 8561221 observed by Kepler: an early red giant showing depressed dipolar modes
The continuous high-precision photometric observations provided by the CoRoT
and Kepler space missions have allowed us to better understand the structure
and dynamics of red giants using asteroseismic techniques. A small fraction of
these stars shows dipole modes with unexpectedly low amplitudes. The reduction
in amplitude is more pronounced for stars with higher frequency of maximum
power. In this work we want to characterize KIC 8561221 in order to confirm
that it is currently the least evolved star among this peculiar subset and to
discuss several hypotheses that could help explain the reduction of the dipole
mode amplitudes. We used Kepler short- and long-cadence data combined with
spectroscopic observations to infer the stellar structure and dynamics of KIC
8561221. We then discussed different scenarios that could contribute to the
reduction of the dipole amplitudes such as a fast rotating interior or the
effect of a magnetic field on the properties of the modes. We also performed a
detailed study of the inertia and damping of the modes. We have been able to
characterize 37 oscillations modes, in particular, a few dipole modes above
nu_max that exhibit nearly normal amplitudes. We have inferred a surface
rotation period of around 91 days and uncovered the existence of a variation in
the surface magnetic activity during the last 4 years. As expected, the
internal regions of the star probed by the l = 2 and 3 modes spin 4 to 8 times
faster than the surface. With our grid of standard models we are able to
properly fit the observed frequencies. Our model calculation of mode inertia
and damping give no explanation for the depressed dipole modes. A fast rotating
core is also ruled out as a possible explanation. Finally, we do not have any
observational evidence of the presence of a strong deep magnetic field inside
the star.Comment: Accepted in A&A. 17 pages, 16 figure
The rapid rotation and complex magnetic field geometry of Vega
The recent discovery of a weak surface magnetic field on the normal
intermediate-mass star Vega raises the question of the origin of this magnetism
in a class of stars that was not known to host magnetic fields. We aim to
confirm the field detection and provide additional observational constraints
about the field characteristics, by modelling the magnetic geometry of the star
and by investigating the seasonal variability of the reconstructed field. We
analyse a total of 799 circularly-polarized spectra collected with the NARVAL
and ESPaDOnS spectropolarimeters during 2008 and 2009. We employ a
cross-correlation procedure to compute, from each spectrum, a mean polarized
line profile with a signal-to-noise ratio of about 20,000. The technique of
Zeeman-Doppler Imaging is then used to determine the rotation period of the
star and reconstruct the large-scale magnetic geometry of Vega at two different
epochs. We confirm the detection of circularly polarized signatures in the mean
line profiles. The amplitude of the signatures is larger when spectral lines of
higher magnetic sensitivity are selected for the analysis, as expected for a
signal of magnetic origin. The short-term evolution of polarized signatures is
consistent with a rotational period of 0.732 \pm 0.008 d. The reconstructed
magnetic topology unveils a magnetic region of radial field orientation,
closely concentrated around the rotation pole. This polar feature is
accompanied by a small number of magnetic patches at lower latitudes. No
significant variability in the field structure is observed over a time span of
one year. The repeated observation of a weak photospheric magnetic field on
Vega suggests that a previously unknown type of magnetic stars exists in the
intermediate-mass domain. Vega may well be the first confirmed member of a much
larger, as yet unexplored, class of weakly-magnetic stars.Comment: Accepted by Astronomy & Astrophysics. Abstract shortened to respect
the arXiv limit of 1920 character
Why are some A stars magnetic, while most are not?
A small fraction of intermediate-mass main sequence (A and B type) stars have strong, organised magnetic fields. The large majority of such stars, however, show no evidence for magnetic fields, even when observed with very high precision. In this paper we describe a simple model, motivated by qualitatively new observational results, that provides a natural physical explanation for the small fraction of observed magnetic stars
A Brief Review of the Piroplasms, with Special Reference to the Types Found in Australian "Redwater"
New evidence regarding hormone replacement therapies is urgently required. Transdermal postmenopausal hormone therapy differs from oral hormone therapy in risks and benefits
Controversies about the safety of different postmenopausal hormone therapies (HTs) started 30 years ago and reached a peak in 2003 after the publication of the results from the Women Health Initiative (WHI) trial and the Million Women Study (MWS) [Writing group for the women's health initiative investigations. Risks and benefits of estrogen plus progestin in healthy postmenopausal women. JAMA 2002;288:321–33; Million women study collaborators. Breast cancer and hormone-replacement therapy in the million women study. Lancet 2003;362:419–27]. The single HT formulation used in the WHI trial for non hysterectomized women—an association of oral conjugated equine estrogens (CEE–0.625 mg/day) and a synthetic progestin, medroxyprogesterone acetate (MPA–2.5 mg/day)—increases the risks of venous thromboembolism, cardiovascular disease, stroke and breast cancer. The MWS, an observational study, showed an increased breast cancer risk in users of estrogens combined with either medroxyprogesterone acetate (MPA), norethisterone, or norgestrel. It is unclear and questionable to what extent these results might be extrapolated to other HRT regimens, that differ in their doses, compositions and administration routes, and that were not assessed in the WHI trial and the MWS. Significant results were achieved with the publication of the WHI estrogen-only arm study [Anderson GL, Limacher M, Assaf AR, et al. Effects of conjugated equine estrogen in postmenopausal women with hysterectomy: the Women's Health Initiative randomized controlled trial. JAMA 2004;291:1701–1712] in which hormone therapy was reserved to women who had carried out hysterectomy. What emerged from this study will allow us to have some important argument to develop
New evidence regarding hormone replacement therapies is urgently required - Transdermal postmenopausal hormone therapy differs from oral hormone therapy in risks and benefits
Controversies about the safety of different postmenopausal hormone therapies (HTs) started 30 years ago and reached a peak in 2003 after the publication of the results from the Women Health Initiative (WHI) trial and the Million Women Study (MWS) [Writing group for the women´s health initiative investigations. Risks and benefits of estrogen plus progestin in healthy postmenopausal women. JAMA 2002;288:321-33; Million women study collaborators. Breast cancer and hormone-replacement therapy in. the million women study. Lancet 2003;362:419-27]. The single HT formulation used in the WHI trial for non hysterectomized women-an association of oral conjugated equine estrogens (CEE-0.625 mg/day) and a synthetic progestin, medroxyprogesterone acetate (MPA-2.5 mg/day)-increases the risks of venous thromboembolism, cardiovascular disease, stroke and breast cancer. The MWS, an observational study, showed an increased breast cancer risk in users of estrogens combined with either medroxyprogesterone acetate (MPA), norethisterone, or norgestrel. It is unclear and questionable to what extent these results might be extrapolated to other HRT regimens, that differ in their doses, compositions and administration routes, and that were not assessed in the WHI trial and the MWS. Significant results were achieved with the publication of the WHI estrogen-only arm study [Anderson GL, Limacher M, Assaf AR, et al. Effects of conjugated equine estrogen in postmenopausal women with hysterectomy: the Women´s Health Initiative randomized controlled trial. JAMA 2004;291:1701-1712] in which hormone therapy was reserved to women who had carried out hysterectomy. What emerged from this study will allow us to have some important argument to develop
Frequency regularities of acoustic modes and multi-colour mode identification in rapidly rotating stars
International audienceContext. Mode identification has remained a major obstacle in the interpretation of pulsation spectra in rapidly rotating stars. This has motivated recent work on calculating realistic multi-colour mode visibilities in this type of star.Aims. We would like to test mode identification methods and seismic diagnostics in rapidly rotating stars, using oscillation spectra that are based on these new theoretical predictions.Methods. We investigate the auto-correlation function and Fourier transform of theoretically calculated frequency spectra, in which modes are selected according to their visibilities. Given that intrinsic mode amplitudes are determined by non-linear saturation and cannot currently be theoretically predicted, we experimented with various ad-hoc prescriptions for setting the mode amplitudes, including using random values. Furthermore, we analyse the ratios between mode amplitudes observed in different photometric bands to see up to what extent they can identify modes.Results. When non-random intrinsic mode amplitudes are used, our results show that it is possible to extract a mean value for the large frequency separation or half its value and, sometimes, twice the rotation rate, from the auto-correlation of the frequency spectra. Furthermore, the Fourier transforms are mostly sensitive to the large frequency separation or half its value. The combination of the two methods may therefore measure and distinguish the two types of separations. When the intrinsic mode amplitudes include random factors, which seems more representative of real stars, the results are far less favourable. It is only when the large separation or half its value coincides with twice the rotation rate, that it might be possible to detect the signature of a frequency regularity. We also find that amplitude ratios are a good way of grouping together modes with similar characteristics. By analysing the frequencies of these groups, it is possible to constrain mode identification, as well as determine the large frequency separation and the rotation rate
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