Rotation and stellar activity
Activity, rotation, and star-planet interaction of CoRoT-6
Modeling of the photospheric activity of the star and use the maps of the active regions to study stellar differential rotation and the star-planet interaction. Individual active regions have lifetimes up to 30-40 days. Most of them form and decay within five active longitudes whose different migration rates are attributed to the stellar differential rotation with a lower limit of ~0.12.
See also "Host stars"
Evidence for granulation in early A-type stars, Kallinger & Matthews (ApJ, 711, L35, 2010)
Original (grey curve) and residual gap-filled (black curve) power spectra of HD 174936 after pre-whitening the 10 most significant pulsation frequencies. The colored lines represent a global fit plotted with (blue) and without (red) the pulsation component of the model consisting of 2 power laws (dashed lines), white noise and a pulsation power excess, approximated by a Gaussian. Bottom panel: The above power spectrum corrected for the granulation signal (red line in top panel), which should now include only the pulsation signal. The dashed line indicates a signal-to-background-noise ratio of 4. For details consult the mentioned paper.
Stars with spectral types earlier than about F0 on (or close) to the main sequence have long been believed to lack observable surface convection, although evolutionary models of A-type stars do predict very thin surface convective zones. teh signature of granulation has been observed in two delta Scuti stars of spectral type A2 observed by CoRoT: HD 174936 (SRc01) and HD50844 (IRa01). Recent analyses of the data indicated up to some 1000 frequencies which were interpreted as individual pulsation modes. This interpretation requires a large number of nonradial modes of very high degree (l) but which should have very low amplitudes due to cancellation effects in disk-integrated photometry. An alternative model assumes that most of the peaks in the Fourier spectra are the signature of non-white granulation background noise, which leaves less than about 100 frequencies as candidates for stellar p-modes. This would be consistent with only low degree observable pulsation modes. Furthermore, the granulation time scales are consistent with scaling relations for cooler stars with known surface convection.
Very special system of two late B stars
Ref.:HD 174884: a strongly eccentric, short-period early-type binary system discovered by CoRoT, Maceroni, C.; Montalbán, J.; Michel, E.; Harmanec, P.; Prsa, A.; Briquet, M.; Niemczura, E.; Morel, T.; Ladjal, D.; Auvergne, M.; and 5 coauthors doi:10.1051/0004-6361/200913311
The CoRoT lightcurve of HD 174884, a seismological target which was found to be an astrophysically important double-lined eclipsing binary in an eccentric orbit (e~0.3) and with a short (3.65705d) orbital period. The high eccentricity, coupled with the orientation of the binary orbit in space, explains the very unusual observed light curve with strongly unequal primary and secondary eclipses, having the depth ratio of 1-to-100 in the CoRoT "seismo" passband.
The analysis of the CoRoT photometry and the result of the spectroscopic follow-up program, provided a consistent picture of the system, composed of two late B stars and stringent constraints on the theoretical models. In particular it was evidenced the need of stellar models including rotation to recover the agreement between theory and observations.
The analysis of the light curve fit residuals evidences the presence of additional periodic variability other than that due to the orbit, and the Fourier decomposition yields two components, multiples of the orbital frequency, which are interpreted as tidally induced pulsations.
A sample of 6 CTTS light curves from the CoRoT observation of NGC 2264. F0 corresponds to the maximum flux value of each LC. Light curves a and b have been classified as spot-like, c and d as AA Tau-like and e and f as irregular.
Ref.: Astronomy and Astrophysics accepted
The young cluster NGC 2264 was observed with the CoRoT satellite for 23 days uninterruptedly in March 2008 with unprecedent photometric accuracy.
AA Tau-like light curves are found to be fairly common, with a frequency of at least ~ 30 to 40% in young stars with inner dusty disks. The temporal evolution of the light curves indicates that the structure of the inner disk warp, located close to the corotation radius and responsible for the obscuration episodes, varies over a timescale of a few (~1-3) rotational periods. This probably reflects the highly dynamical nature of the star-disk magnetospheric interaction.
Processed lightcurve of the star 102694749 (COROT-ID), mV=12.05, observed with the exoplanets CCDs (Additional Programme). The dots are the observed data points, the red line is the fitted light curve after reduction. This star has been identified as HgMn star and presents a clear photometric variation with an amplitude of about 1.6 mmag. The main period is 4.3 days. This variation is compatible with a non-radial g-modes pulsation predicted by models (Alecian et al. 2009).
The chemically peculiar stars of the main sequence in the HR diagram are known to exhibit large abundance anomalies of heavy elements. They are divided in several groups: AmFm stars for the coolest of them (F and A type stars) with moderate anomalies, magnetic Ap stars and HgMn stars (A and late-B type stars) with the strongest anomalies (for instance, up to 5 orders of magnitude overabundance of Hg with respect to the cosmic abundance). These abundance anomalies are thought to be the result of the stratifications of chemical elements due to atomic diffusion processes.
A subgroup among the magnetic Ap stars group is constituted by the roAp stars. These stars have well established short period pulsations (typically around 10 minutes) in addition to the photometric rotational modulations (property common to the parent group) that are related to inhomogeneous element distributions over their surface (inhomogeneities due to atomic diffusion in strong magnetic fields). Pulsations in other groups of CP stars - like HgMn stars - are strongly suspected but not firmly established.