J. F. González

Magnetic field measurements of O stars with FORS 1 at the VLT.

Context. The presence of magnetic fields in O-type stars has been suspected for a long time. The discovery of these fields would explain a wide range of well documented enigmatic phenomena in massive stars, in particular cyclical wind variability, Ha emission variations, chemical peculiarity, narrow X-ray emission lines, and non-thermal radio/X-ray emission. Aims. To investigate the incidence of magnetic fields in O stars, we acquired 38 new spectropolarimetric observations with FORS 1 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-m Kueyen telescope of the VLT. Methods. Spectropolarimetric observations were obtained at different phases for a sample of 13 O stars. Ten stars were observed in the spectral range 348-589 nm, HD36879 and HD148937 were observed in the spectral region 325-621 nm, and HD155806 was observed in both settings. To prove the feasibility of the FORS1 spectropolarimetric mode for the measurements of magnetic fields in hot stars, we present in addition 12 FORS 1 observations of the mean longitudinal magnetic field in theta(1) OriC and compare them with measurements obtained with the MuSiCoS, ESPaDOnS, and Narval spectropolarimeters. Results. Most stars in our sample, which were observed on different nights, show a change of the magnetic field polarity, but a field at a significance level of 3 sigma was detected in only four stars, HD36879, HD148937, HD152408, and HD164794. The largest longitudinal magnetic field, = -276 +/- 88 G, was detected in the Of?p star HD148937. We conclude that large-scale organized magnetic fields with polar field strengths larger than 1 kG are not widespread among O-type stars.

Inhomogeneous surface distribution of chemical elements in the eclipsing binary AR Aur: a new challenge for our understanding of HgMn stars ★

We present the results of a high spectral resolution study of the eclipsing binary AR Aur. AR Aur is the only known eclipsing binary with a HgMn primary star exactly on the zero-age main sequence (ZAMS) and a secondary star still contracting towards the ZAMS. We detect, for the first time in the spectra of the primary star, that for many elements the line profiles are variable over the rotation period. The strongest profile variations are found for the elements Pt, Hg, Sr, Y, Zr, He and Nd, while the line profiles of O, Na, Mg, Si, Ca, Ti and Fe show only weak distortions over the rotation period. The slight variability of He and Y is also confirmed by the study of high-resolution spectra of another HgMn star: a And. A preliminary modelling of the inhomogeneous distribution has been carried out for Sr and Y. Our analysis shows that these elements are very likely concentrated in a fractured ring along the rotational equator. It may be an essential clue for the explanation of the origin of the chemical anomalies in HgMn stars (which are very frequently found in binary and multiple systems) that one large fraction of the ring is missing exactly on the surface area which is permanently facing the secondary, and another small one on the almost opposite side. The results presented about the inhomogeneous distribution of various chemical elements over the stellar surface of the primary suggest new directions for investigations to solve the question of the origin of abundance anomalies in B-type stars with HgMn peculiarity.

Abundances in the Herbig Ae star HD 101412 Abundance anomalies; λ Boo-Vega characteristics?

Context. Recent attention has been directed to abundance variations among very young stars. Aims. We perform a detailed abundance study of the Herbig Ae star HD 101412, taking advantage of its unusually sharp spectral lines. Methods. High-resolution spectra are measured for accurate wavelengths and equivalent widths. Balmer-line fits and ionization equlibria give a relation between Teff, and log(g). Abundance anomalies and uncertain reddening preclude the use of spectral type or photometry to fix Teff. Excitation temperatures are used to break the degeneracy between Teff and log(g). Results. Strong lines are subject to an anomalous saturation that cannot be removed by assuming a low microturbulence. By restricting the analysis to weak (≤20 mA) lines, we find consistent results for neutral and ionized species, based on a model with Teff = 8300 K, and log(g) = 3.8. The photosphere is depleted in the most refractory elements, while volatiles are normal or, in the case of nitrogen, overabundant with respect to the sun. The anomalies are unlike those of Ap or Am stars. Conclusions. We suggest the anomalous saturation of strong lines arises from heating of the upper atmospheric layers by infalling material from a disk. The overall abundance pattern may be related to those found for the λ Boo stars, though the depletions of the refractory elements are milder, more like those of Vega. However, the intermediate volatile zinc is depleted, precluding a straightforward interpretation of the abundance pattern in terms of gas-grain separation.

Discovery of pulsational line profile variations in the δ Scuti star HD 21190 and in the Ap Sr star HD 218994

Asteroseismology has the potential to provide new insights into the physics of stellar interiors. We have obtained UVES high time resolution observations of theScuti star HD21190 and of the Ap Sr star HD218994 to search for pulsational line profile variations. We report the discovery of a new roAp star, HD218994, with a pulsation period of 14.2min. This is one of the most evolved roAp stars. No rapid pulsations have been found in the spectra of the cool Ap star - � Scuti star HD21190. However, we detect with unprecedented clarity for aSct star moving peaks in the cores of spectral lines that indicate the presence of high degree non-radial pulsations in this star.

Heavy calcium in CP stars

Large wavelength shifts of infrared triplet lines of Ca II have been observed in the spectra of HgMn and magnetic Ap stars. They have been attributed to the heavy calcium isotopes, including 48 Ca. One member of the triplet, λ8542, had been either unavailable, or of poor quality in earlier spectra. The present material shows conclusively that the stellar λ8542 shifts are consistent with an interpretation in terms of 48 Ca. We find no relation between isotopic shifts of the Ca II triplet lines, and those of Hg II λ3984. There is a marginal indication that the shifts are anticorrelated with the surface field strengths of the magnetic stars. We see sparse evidence for 48 Ca in other chemically peculiar stars, for example, Am stars, metal-poor stars or chemically peculiar red giants. However, the sample is still very small, and the wavelengths of all three triplet lines, including those in the Sun, show slight positive shifts with respect to terrestrial positions. Some profiles of the Ca II infrared triplet in the magnetic stars show extensive wings beyond a well-defined core. We can obtain reasonable fits to these profiles using a stratified calcium abundance similar to that used by previous workers. There is no indication that either the stratification or the Zeeman effect significantly disturbs the measurement of isotope shifts.

AO Velorum: a young quadruple system with a ZAMS eclipsing BpSi primary

Using recent spectroscopic observations, we show that the triple system AOVel with an eclipsing BpSi primary is in fact a remarkable quadruple system formed by two double-lined spectroscopic binaries with components close to the ZAMS. All available data have been reanalyzed to derive proper orbital parameters for both binary systems and to calculate absolute parameters of the eclipsing system. For the first time, direct determination of the radius and the mass have been obtained for a BpSi star. The study of the physical parameters of this unique system is especially important since it can be used as a test of evolutionary models for very young stars of intermediate mass.

Stratification and isotope separation in CP stars

We investigate the elemental and isotopic stratification in the atmospheres of selected chemically peculiar (CP) stars of the upper main sequence. Reconfiguration of the UV-Visual Echelle Spectrograph in 2004 has made it possible to examine all three lines of the Ca II infrared (IR) triplet. Much of the material analysed was obtained in 2008. We support the claim of Ryabchikova, Kochukhov & Bagnulo (RKB) that the calcium isotopes have distinct stratification profiles for the stars 10 Aql, HR 1217 and HD 122970, with the heavy isotope concentrated towards the higher layers. Better observations are needed to learn the extent to which 40 Ca dominates in the deepest layers of all or most CP stars that show the presence of 48 Ca. There is little evidence for 40 Ca in the spectra of some HgMn stars, and the infrared (IR) triplet in the magnetic star HD 101065 is well fit by pure 48 Ca. In HR 5623 (HD 133792) and HD 217522, it is likely that the heavy isotope dominates, though models are possible where this is not the case. While elemental stratification is surely needed in many cases, we point out the importance of including adjustments in the assumed T eff and log (g) values, in attempts to model stratification. We recommend emphasis on profiles of the strongest lines, where the influence of stratification is most evident. Isotopic mixtures, involving the four stable calcium nuclides with masses between 40 and 48 are plausible, but are not emphasized.

AO Vel: the role of multiplicity in the development of chemical peculiarities in late B-type stars★

We present high-resolution, high signal-to-noise UVES spectra of AO Vel, a quadruple system containing an eclipsing BpSi star. From these observations, we reconstruct the spectra of the individual components and perform an abundance analysis of all four stellar members. We found that all components are chemically peculiar with different abundance patterns. In particular, the two less massive stars show typical characteristics of HgMn stars. The two most massive stars in the system show variable line profiles indicating the presence of chemical spots. Given the youth of the system and the notable chemical peculiarities of their components, this system could give important insights in the origin of chemical anomalies.

Line identification in high-resolution, near-infrared CRIRES spectra of chemically peculiar and Herbig Ae stars

Context. Contrary to the late-type stars, our knowledge of atomic transitions in intermediate-mass stars is still very poor. The recent availability of ESO’s high-resolution spectrograph CRIRES offers now the opportunity to study numerous spectral features in the near-IR in intermediate-mass main-sequence and pre-main-sequence stars. Aims. The aim of the study is to explore the diagnostic potential of near-IR spectral regions. We carry out the first line identification in a few spectral regions for the two strongly magnetic Ap stars γ Equ and HD 154708, and their potential precursors two pre-main sequence Herbig Ae/Be stars HD 101412 and 51 Oph. Methods. High-resolution CRIRES spectra were obtained in three spectral regions, two regions around 1 μm and one region around 1.57 μm containing magnetically sensitive Fe i lines. To study the spectral line variability in the Herbig Ae star HD 101412, the observations were collected on six different rotation phases. All currently available atomic line lists were involved to properly identify the detected spectral features. Results. The largest number of near-IR spectral features was detected and identified in the well-studied magnetic Ap star γ Equ. Nearly 30% of the spectral lines in the Ap star HD 154708 with one of the strongest magnetic fields known among the Ap stars of the order of 25 kG, remain unidentified due to a lack of atomic data. Only very few lines belonging to the rare earth element group have been identified in both Ap stars. A number of spectral lines including the Ce iii and Dy ii lines appear magnetically split due to the presence of a strong magnetic field in their atmospheres. The content of the spectra of the Herbig Ae/Be stars HD 101412 and 51 Oph is rather unexciting. Variable behaviour of lines of the elements He, N, Mg, Si, and Fe over the rotation period in the spectra of HD 101412 confirm our previous finding of variability in the optical region. Due to the very fast rotation of 51 Oph, only a few spectral lines have been identified with certainty.

Spectroscopic signatures of magnetospheric accretion in Herbig Ae/Be stars - I. The case of HD 101412

Models of magnetically-driven accretion and outflows reproduce many observational properties of T Tauri stars. This concept is not well established for the more massive Herbig Ae/Be stars. We intend to examine the magnetospheric accretion in Herbig Ae/Be stars and search for rotational modulation using spectroscopic signatures, in this first paper concentrating on the well-studied Herbig Ae star HD101412. We used near-infrared spectroscopic observations of the magnetic Herbig Ae star HD101412 to test the magnetospheric character of its accretion disk/star interaction. We reduced and analyzed 30 spectra of HD101412, acquired with the CRIRES and X-shooter spectrographs installed at the VLT (ESO, Chile). The spectroscopic analysis was based on the He I lambda 10,830 and Pa gamma lines, formed in the accretion region. We found that the temporal behavior of these diagnostic lines in the near-infrared spectra of HD101412 can be explained by rotational modulation of line profiles generated by accreting gas with a period P = 20.53+-1.68 d. The discovery of this period, about half of the magnetic rotation period P_m = 42.076 d previously determined from measurements of the mean longitudinal magnetic field, indicates that the accreted matter falls onto the star in regions close to the magnetic poles intersecting the line-of-sight two times during the rotation cycle. We intend to apply this method to a larger sample of Herbig Ae/Be stars.