N. A. Drake

Multi-element abundance Doppler imaging of the rapidly oscillating Ap star HR 3831

We investigate magnetic field geometry and surface distribution of chemical elements in the rapidly oscillating Ap star HR 3831. Results of the model atmosphere analysis of the spectra of this star are combined with the Hipparcos parallax and evolutionary models to obtain new accurate estimates of the fundamental stellar parameters: Teff = 7650 K, log L/L� = 1.09, M/M� = 1.77 and an inclination angle i = 68 ◦ of the stellar axis of rotation. We find that the variation of the longitudinal magnetic field of HR 3831 and the results of our analysis of the magnetic intensification of Fe  lines in the spectrum of this star are consistent with a dipolar magnetic topology with a magnetic obliquity β = 87 ◦ and a polar strength Bp = 2. 5k G. We apply a multi-element abundance Doppler imaging inversion code for the analysis of the spectrum variability of HR 3831, and recover surface distributions of 17 chemical elements, including Li, C, O, Na, Mg, Si, Ca, Ti, Cr, Mn, Fe, Co, Ba, Y, Pr, Nd, Eu. Our study represents the most thorough examination of the surface chemical structure in a magnetic Ap star and provides important observational constraints for modelling radiative diffusion in magnetic stars. The exceedingly high quality of some of our spectroscopic data allowed us to reconstruct unprecedented details of abundance distributions, demonstrating a high level of complexity in the surface structure down to the resolution limit of the Doppler maps. The Doppler imaging analysis of HR 3831 forms a basis for subsequent detailed observational investigations and theoretical modelling of non-radial oscillations in this star. We discuss the compound effect of the chemical nonuniformities and pulsational velocity field on the rapid line profile variations, and assess the possibility of identifying pulsation modes by using spatial filtering produced by an inhomogeneous abundance distribution. The results of our study of the surface chemical structure suggest that differences in pulsational behaviour of lines of different ions observed for HR 3831 are not a consequence of horizontal atmospheric inhomogeneities, but predominantly a depth effect.

Rapidly Rotating Lithium-rich K Giants: The New Case of the Giant PDS 365

PDS 365 is a newly detected, rapidly rotating (v sin i = 20 km s-1), single, low-mass giant star that with HD 233517 and HD 219025 forms a remarkable ensemble of single K giants with the unique properties of rapid rotation, very strong Li lines, an asymmetrical Hα profile, and a large far-infrared excess. Their v sin i values are between 18 and 23 km s-1, and their LTE Li abundances, log e(Li), are between 2.9 and 3.9. Detailed analysis of PDS 365 reveals it to be a ~1 M⊙ giant with a value of 12C/13C approximately equal to 12. A clear relation between high rotational velocities and very high Li abundances for K giant stars is found only when asymmetrical Hα profiles and large far-infrared excesses are present. If we consider single K giants, we find that among rapid (v sin i ≥ 8 km s-1) rotators, a very large proportion (~50%) are Li-rich giants. This proportion is in contrast with a very low proportion (~2%) of Li-rich stars among the much more common slowly rotating K giants. This striking difference is discussed in terms of proposed mechanisms for Li enrichment.

Chemical analysis of giant stars in the young open cluster NGC 3114

Context. Open clusters are very useful targets for examining possibl e trends in galactocentric distance and age, especially whe n young and old open clusters are compared. Aims. We carried out a detailed spectroscopic analysis to derive t he chemical composition of seven red giants in the young open cluster NGC 3114. Abundances of C, N, O, Li, Na, Mg, Al, Ca, Si, Ti, Ni, Cr, Y, Zr, La, Ce, and Nd were obtained, as well as the carbon isotopic ratio. Methods. The atmospheric parameters of the studied stars and their ch emical abundances were determined using high-resolution optical spectroscopy. We employed the local-thermodynamic-equilibrium model atmospheres of Kurucz and the spectral analysis codemoog. The abundances of the light elements were derived using the spectral synthesis technique. Results. We found that NGC 3114 has a mean metallicity of [Fe/H] =−0.01±0.03. The isochrone fit yielded a turn-o ff mass of 4.2 M⊙. The [N/C] ratio is in good agreement with the models predicted by firs t dredge-up. We found that two stars, HD 87479 and HD 304864, have high rotational velocities of 15.0 km s −1 and 11.0 km s −1 ; HD 87526 is a halo star and is not a member of NGC 3114. Conclusions. The carbon and nitrogen abundance in NGC 3114 agree with the fi eld and cluster giants. The oxygen abundance in NGC 3114 is lower compared to the field giants. The [O/Fe] ratio is similar to the giants in young clusters. We detec ted sodium enrichment in the analyzed cluster giants. As far as the othe r elements are concerned, their [X/Fe] ratios follow the same trend seen in giants with the same metallicity.

Direct detection of a magnetic field in the photosphere of the single M giant EK Bootis: How common is magnetic activity among M giants?

Aims. We study the fast rotating M 5 giant EK Boo by means of spectropolarimetry to obtain direct and simultaneous measurements of both the magnetic field and activity indicators, in order to infer the origin of the activity in this fairly evolved giant. Methods. We used the new spectropolarimeter NARVAL at the Bernard Lyot Telescope (Observatoire du Pic du Midi, France) to obtain a series of Stokes I and Stokes V profiles for EK Boo. Using the least square deconvolution (LSD) technique we were able to detect the Zeeman signature of the magnetic field. We measured its longitudinal component by means of the averaged Stokes V and Stokes I profiles. The spectra also permitted us to monitor the Ca ii K&H chromospheric emission lines, which are well known as indicators of stellar magnetic activity.

High-resolution spectroscopic observations of two chemically peculiar metal-poor stars: HD 10613 and BD+04°2466

Aims. We determined the atmospheric parameters and abundance pattern of two chemically peculiar metal-poor stars: HD 10613 and BD+04 ◦ 2466 in order to better understand their evolutionary state and the nature of the s-element enhancement of these stars. Methods. We used high resolution optical spectroscopy. Atmospheric parameters and abundances were determined in the localthermodynamic-equilibrium model atmospheres of Kurucz using the spectral analysis code MOOG. Results. We conclude that HD 10613 is another metal-poor barium star with C/O = 0.52 and [Fe/H] = −0.82, while BD+04 ◦ 2466 is a CH star with C/O = 3.6 and [Fe/H] = −1.92 rather than a metal-deficient barium star as it was previously classified. BD+04 ◦ 2466 appears to be enriched in lead with [Pb/Ce] =+ 0.85 and [Pb/La] =+ 0.72. For BD+04 ◦ 2466 the abundance of lead is in agreement with predictions from AGB models. Due to the low luminosity of these two stars, their observed s-process overabundance is better explained by mass-transfer in the past from an AGB star.

LIGHT ELEMENT ABUNDANCES IN TWO CHEMICALLY PECULIAR STARS: HD 104340 AND HD 206983*

We derive carbon, nitrogen, oxygen, and lithium abundances as well as 12C/13C isotopic ratios in two chemically peculiar red giant stars: HD 104340 and HD 206983. The abundances were determined via spectrum synthesis of optical high-resolution spectra containing C2, CH, CN, [O I], and Li I lines. Our results indicate that HD 206983 is a barium star, while HD 104340, although showing enhancements of s-process elements, should not be considered as a classical barium star: its barium star nature can be explained by internal nucleosynthesis. The low metallicity giant HD 104340 can experience deeper convective mixing and, consequently, a larger dredge-up of CNO-cycle products compared to normal red giants. The light element abundance pattern of HD 104340 resembles anomalies resulting from the appearance on the stellar surface of material enriched by triple-α and CNO cycling.

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.

Chemical abundances and kinematics of barium stars

In this paper we present an homogeneous analysis of photospheric abundances based on high-resolution spectroscopy of a sample of 182 barium stars and candidates. We determined atmospheric parameters, spectroscopic distances, stellar masses, ages, luminosities and scale height, radial velocities, abundances of the Na, Al, alphaelements, iron-peak elements, and s-process elements Y, Zr, La, Ce, and Nd. We employed the local-thermodynamic-equilibrium model atmospheres of Kurucz and the spectral analysis code moog. We found that the metallicities, the temperatures and the surface gravities for barium stars can not be represented by a single gaussian distribution. The abundances of alpha-elements and iron peak elements are similar to those of field giants with the same metallicity. Sodium presents some degree of enrichment in more evolved stars that could be attributed to the NeNa cycle. As expected, the barium stars show overabundance of the elements created by the s-process. By measuring the mean heavy-element abundance pattern as given by the ratio [s/Fe], we found that the barium stars present several degrees of enrichment. We also obtained the [hs/ls] ratio by measuring the photospheric abundances of the Ba-peak and the Zrpeak elements. Our results indicated that the [s/Fe] and the [hs/ls] ratios are strongly anti-correlated with the metallicity. Our kinematical analysis showed that 90% of the barium stars belong to the thin disk population. Based on their luminosities, none of the barium stars are luminous enough to be an AGB star, nor to become self-enriched in the s-process elements. Finally, we determined that the barium stars also follow an age-metallicity relation.

CD-62°1346: an extreme halo or hypervelocity CH star?

Context. High-velocity halo stars provide important information about the properties of the extreme Galactic halo. The study of unbound and bound Population II stars permits us to better estimate the mass of the halo. Aims. We carried out a detailed spectroscopic and kinematic study and have significantly refined the distance and the evolutionary state of the star. Methods. Its atmospheric parameters, chemical abundances and kinematical properties were determined using high-resolution optical spectroscopy and employing the local-thermodynamic-equilibrium model atmospheres of Kurucz and the spectral analysis code moog. Results. We found that CD-62 ◦ 1346 is a metal-poor ([Fe/H] = −1.6) evolved giant star with Teff = 5300 K and log g = 1.7. The star exhibits high carbon and s-element abundances typical of CH stars. It is also a lead star. Our kinematic analysis of its 3D space motions shows that this star has a highly eccentric (e = 0.91) retrograde orbit with an apogalactic distance of ∼100 kpc, exceeding by a factor of two the distance of the Magellanic Clouds. The star travels with very high velocity relative to the Galactocentric reference frame (VGRF = 570 km s −1 ). Conclusions. CD-62 ◦ 1346 is an evolved giant star and not a subgiant star, as was considered earlier. Whether it is bound or unbound to the Galaxy depends on the assumed mass and on the adopted Galactic potential. We also show that the star HD 5223 is another example of a high-velocity CH star that exceeds the Galactic escape velocity. Possible origins of these two high-velocity stars are briefly discussed. CD-62 ◦ 1346 and HD 5223 are the first red giant stars to join the restricted group of hypervelocity stars.

HIGH-RESOLUTION SPECTROSCOPIC OBSERVATIONS OF A NEW CH SUBGIANT STAR: BD–03°3668

This work reports on the discovery of a new CH subgiant star BD-03{sup 0}3668 with a significant overabundance of the elements created by s-process nucleosynthesis, such as barium and lead. Its atmospheric parameters and abundance pattern are determined by employing the local thermodynamic equilibrium model atmospheres of Kurucz using the spectral analysis code MOOG. Due to the low luminosity of this star, the s-process overabundance is better explained by mass transfer from an asymptotic giant branch star in the past.