T. Ryabchikova

Influence of departures from LTE on oxygen abundance determination in the atmospheres of A-K stars

We have performed non-LTE calculations for O I with a multilevel model atom using currently available atomic data for a set of parameters corresponding to stars of spectral types from A to K. Departures from LTE lead to a strengthening of O I lines, and the difference between the non-LTE and LTE abundances (non-LTE correction) is negative. The non-LTE correction does not exceed 0.05 dex in absolute value for visible O I lines for main-sequence stars in the entire temperature range. For the infrared O I 7771 Å line, the non-LTE correction can reach −1.9 dex. The departures from LTE are enhanced with increasing temperature and decreasing surface gravity. We have derived the oxygen abundance for three A-type mainsequence stars with reliably determined parameters (Vega, Sirius, HD 32115). For each of the stars, allowance for the departures from LTE leads to a decrease in the difference between the abundances from infrared and visible lines, for example, for Vega from 1.17 dex in LTE to 0.14 dex when abandoning LTE. In the case of Procyon and the Sun, inelastic collisions with HI affect the statistical equilibrium of OI, and agreement between the abundances from different lines is achieved when using Drawin’s classical formalism. Based on the O I 6300, 6158, 7771-5, and 8446 Å lines of the solar spectrum, we have derived the mean oxygen abundance log ɛ = 8.74 ± 0.05 using a classical plane-parallel model solar atmosphere and log ɛ+3D = 8.78 ± 0.03 by applying the 3D corrections taken from the literature.

Accuracy of atmospheric parameters of FGK dwarfs determined by spectrum fitting

We performed extensive tests of the accuracy of atmospheric parameter determination for FGK stars based on the spectrum fitting procedure SpectroscopyMade Easy (SME). Our stellar sample consists of ...

Echography of young stars reveals their evolution

A finger on the pulse of young stars Adolescent stars quiver and quake before the onset of nuclear fusion in their cores. Zwintz et al. confirm theoretical predictions that the frequency of the seismic oscillations in a given star is tied to its evolutionary status (see the Perspective by Stahler and Palla). As the protostar evolves and contracts, growing hotter and denser, it pulsates faster. Though previously applied only to aging stars, asteroseismology now offers a powerful tool for discerning the ages of very young stars. The relative timing of star formation within young clusters especially benefits from this refinement, as stars there are often tagged with one blanket age. Science, this issue p. 550; see also p. 514 Pulsation measurements of stars not yet undergoing nuclear burning confirm a predicted correlation with evolutionary status. [Also see Perspective by Stahler and Palla] We demonstrate that a seismic analysis of stars in their earliest evolutionary phases is a powerful method with which to identify young stars and distinguish their evolutionary states. The early star that is born from the gravitational collapse of a molecular cloud reaches at some point sufficient temperature, mass, and luminosity to be detected. Accretion stops, and the pre–main sequence star that emerges is nearly fully convective and chemically homogeneous. It will continue to contract gravitationally until the density and temperature in the core are high enough to start nuclear burning of hydrogen. We show that there is a relationship for a sample of young stars between detected pulsation properties and their evolutionary status, illustrating the potential of asteroseismology for the early evolutionary phases.

Multi-element Doppler imaging of the CP2 star HD 3980

Diese Diplomarbeit behandelt die ungleichmasige Verteilung der Oberflachenhaufigkeiten von verschiedenen Elementen in der Atmosphare von HD 3980. Der Ursprung dieser Unter- und Uberhaufigkeiten ist atomare Diffusion in den stabilen Atmospharen von magnetischen Ap Sternen wie HD 3980. Um solche Veranderungen zu untersuchen, mussen hochaufgeloste Spektren in verschiedenen Phasen der Rotation des Sterns aufgenommen werden. Die Haufigkeitsverteilung der Elemente ist durch den Doppler-Effekt in den Linienprofilen im Spektrum des Sterns sichtbar. Eine gute Abdeckung der Rotationsphasen ist deshalb unabdingbar. Der erste Schritt hin zur Analyse des Spektrums ist die in Abschnitt 2.2 beschriebene Reduktion der Daten. In weiterer Folge ist es notig eine Modellatmosphare und anschliesend ein synthetisches Spektrum zu berechnen. Indem man die Beobachtung mit der Synthese vergleicht, konnen fur das Doppler Imaging geeignete Bereiche identifiziert werden. Die Bestimmung der atmospharischen Parameter ist in Abschnitt 3.4 beschrieben. Die atmospharischen Parameter wie, e.g., effektive Temperatur, Oberflachenbeschleunigung oder scheinbare Rotationsgeschwindigkeit des Sterns sind grundlegend fur solche Untersuchungen. Die Linienprofilvariationen wahrend der Umdrehung des Sterns wird vom Doppler Imaging Code INVERS12 zu einer Elementhaufigkeitsvereilung invertiert (siehe Abschnitt 4.1 fur weitere Informationen). Die Ergebnisse sind in 3 Gruppen, entsprechend dem Zusammenhang zwischen der Oberflachenverteilung und dem Magnetfeld, eingeteilt. Lithium, Sauerstoff, Mangan, Praseodym und Neodym sind hauptsachlich in der Gegend um die Pole des Magnetfelds konzentriert und unterhaufig in Regionen um den Magnetfeldaquator. Die Flecken mit hoher Silizium-,Lanthan-, Cerium-, Europium- und Gadolinium-Haufigkeit befinden sich zwischen den Magnetfeldpolen und dem Magnetfeldaquator. Lanthan bildet hier eine Ausnahme, da es sogar eine starke Unterhaufigkeit um die Magnetfeldpole besitzt. Ansonsten gibt es fur diese Gruppe keinen offensichtlichen Zusammenhang mit dem Magnetfeld. Kalcium, Chrom und Eisen zeigen eine erhohte Haufung entlang des Rotationsaquators und in der Gegend um die Pole des Magnetfelds. Der Schnittpunkt von Rotations- und Magnetfeldaquators bildet dabei, besonders fur Kalzium und Chrom welche dort unterhaufig sind, eine Ausnahme. Zwischen den theoretisch vorausgesagten und in dieser Arbeit nachgewiesenen Verteilungsmustern konnten keine offensichtlichen Zusammenhange entdeckt werden. Der Grund dafur kann einem Mangel an aktuellen theoretischen Modellen zugeschrieben werden.

Discovery of the longest period rapidly oscillating Ap star HD 177765

We present the discovery of a long-period, rapidly oscillating Ap (roAp) star, HD177765. Using high-resolution time-series observations obtained with the Ultraviolet and Visual Echelle Spectrograph at the European Southern Observatory Very Large Telescope, we found radial velocity variations with amplitudes 7-150 ms(-1) and a period of 23.6 min, exceeding that of any previously known roAp star. The largest pulsation amplitudes are observed for Eu III, Ce III and for the narrow core of H alpha. We derived the atmospheric parameters and chemical composition of HD177765, showing this star to be similar to other long-period roAp stars. Comparison with theoretical pulsational models indicates an advanced evolutionary state for HD177765. Abundance analyses of this and other roAp stars suggest a systematic variation with age of the rare-earth line anomalies seen in cool Ap stars.

gamma Doradus pulsation in two pre-main sequence stars discovered by CoRoT

Pulsations in pre-main sequence stars have been discovered several times within the last years. But nearly all of these pulsators are of delta Scuti-type. gamma Doradus-type pulsation in young stars has been predicted by theory, but lack observational evidence. We present the investigation of variability caused by rotation and (gammaDoradus-type) pulsation in two pre-main sequence members of the young open cluster NGC2264 using high-precision time series photometry from the CoRoT satellite and dedicated high-resolution spectroscopy. Time series photometry of NGC2264VAS20 and NGC 2264VAS87 was obtained by the CoRoT satellite during the dedicated short run SRa01 in March 2008. NGC2264VAS87 was re-observed by CoRoT during the short run SRa05 in December 2011 and January 2012. Frequency analysis was conducted using Period04 and SigSpec. The spectral analysis was performed using equivalent widths and spectral synthesis. The frequency analysis yielded 10 and 14 intrinsic frequencies for NGC2264VAS20 and NGC2264VAS 87, respectively, in the range from 0 to 1.5c/d which are attributed to be caused by a combination of rotation and pulsation. The effective temperatures were derived to be 6380

Discovery of new rapidly oscillating Ap pulsators in the UVES survey of cool magnetic Ap stars

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Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 - II. Two-dimensional magnetic Doppler imaging in all four Stokes parameters

150K for NGC2264VAS20 and 6220

Regular Frequency Patterns In The Young Delta Scuti Star HD 261711 Observed By The CoRoT And MOST Satellites

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A non-local thermodynamical equilibrium line formation for neutral and singly ionized titanium in model atmospheres of reference A–K stars

150K for NGC2264VAS87. Membership of the two stars to the cluster is confirmed independently using X-ray fluxes, radial velocity measurements and proper motions available in the literature. The derived Li abundances of log n(Li)=3.34 and 3.54 for NGC2264VAS20 and NGC2264VAS87, respectively, are in agreement with the Li abundance for other stars in NGC2264 of similar Teff reported in the literature. We conclude that the two objects are members of NGC2264 and therefore are in their pre-main sequence evolutionary stage. Assuming that part of their variability is caused by pulsation, these two stars might be the first pre-main sequence gamma Doradus candidates.