T. A. A. Sigut

A highly sensitive search for magnetic fields in B, A and F stars

Circular spectropolarimetric observations of 74 stars were obtained in an attempt to detect magnetic fields via the longitudinal Zeeman eect in their spectral lines. The sample observed includes 22 normal B, A and F stars, four emission-line Ba nd As tars, 25 Am stars, 10 HgMn stars, two Boo stars and 11 magnetic Ap stars. Using the Least-Squares Deconvolution multi-line analysis approach (Donati et al. 1997), high precision Stokes I and V mean signatures were extracted from each spectrum. We find absolutely no evidence for magnetic fields in the normal, Am and HgMn stars, with upper limits on longitu- dinal field measurements usually considerably smaller than any previously obtained for these objects. We conclude that if any magnetic fields exist in the photospheres of these stars, these fields are not ordered as in the magnetic Ap stars, nor do they resemble the fields of active late-type stars. We also detect for the first time a field in the A2pSr star HD 108945 and make new precise measurements of longitudinal fields in five previously known magnetic Ap stars, but do not detect fields in five other stars classified as Ap SrCrEu. We also report new results for several binary systems, including a newv sini for the rapidly rotating secondary of the Am- Del SB2 HD 110951.

PREDICTED Fe ii EMISSION-LINE STRENGTHS FROM ACTIVE GALACTIC NUCLEI

We present theoretical Fe ii emission line strengths for physical conditions typical of active galactic nuclei with broad-line regions. The Fe ii line strengths were computed with a precise treatment of radiative transfer using extensive and accurate atomic data from the Iron Project. Excitation mechanisms for the Fe ii emission included continuum fluorescence, collisional excitation, self-fluorescence among the Fe ii transitions, and fluorescent excitation by Lyand Ly� . A large Fe ii atomic model consisting of 827 fine structure levels (including states to E � 15 eV) was used to predict fluxes for approximately 23,000 Fe ii transitions, covering most of the UV, optical, and IR wavelengths of astrophysical interest. Spectral synthesis for wavelengths from 1600 Ato 1.2 lm is presented. Applications of present theoretical templates to the analysis of observa- tions are described. In particular, we discuss recent observations of near-IR Fe ii lines in the 8500 A ˚ -1 lm region which are predicted by the Lyfluorescence mechanism. We also compare our UV spectral synthesis with an empirical iron template for the prototypical, narrow-line Seyfert galaxy I Zw 1. The theoretical Fe ii template presented in this work should also be applicable to a variety of objects with Fe ii spectra formed under similar excitation conditions, such as supernovae and symbiotic stars. Subject headings: line: formation — line: identification — quasars: emission lines — supernovae: general On-line material: machine-readable table

Magnetic topology and surface differential rotation on the K1 subgiant of the RS CVn system HR 1099

We present here spectropolarimetric observations of the RS CVn system HR 1099 (V711 Tau) secured from 1998 February to 2002 January with the spectropolarimeter MuSiCoS at the Telescope Bernard Lyot (Observatoire du Pic du Midi, France). We apply Zeeman–Doppler imaging and reconstruct surface brightness and magnetic topologies of the K1 primary subgiant of the system, at five different epochs. We confirm the presence of large, axisymmetric regions where the magnetic field is mainly azimuthal, providing further support to the hypothesis that dynamo processes may be distributed throughout the whole convective zone in this star. n n n nWe study the short-term evolution of surface structures from a comparison of our images with observations secured at close-by epochs by Donati et al. at the Anglo-Australian Telescope. We conclude that the small-scale brightness and magnetic patterns undergo major changes within a time-scale of 4–6 weeks, while the largest structures remain stable over several years. n n n nWe report the detection of a weak surface differential rotation (both from brightness and magnetic tracers) indicating that the equator rotates faster than the pole with a difference in rotation rate between the pole and the equator about four times smaller than that of the Sun. This result suggests that tidal forces also affect the global dynamic equilibrium of convective zones in cool active stars.

Atmospheric velocity fields in tepid main sequence stars

Context. The line profiles of the stars with ve sin i below a few km s −1 can reveal direct signatures of local velocity fields such as convection in stellar atmospheres. This effect is well established in cool main sequence stars, and has been detected and studied in three A stars. Aims. This paper reports observations of main sequence B, A and F stars (1) to identify additional stars with suffi ciently low values of ve sin i to search for spectral line profile signatures of local veloc ity fields, and (2) to explore how the signatures of the local v elocity fields in the atmosphere depend on stellar parameters such as effective temperature and peculiarity type. Methods. We have carried out a spectroscopic survey of B and A stars of low ve sin i at high resolution. Comparison of model spectra with those observed allows us to detect signatures of the loc al velocity fields such as asymmetric excess line wing absorp tion, best-fit ve sin i parameter values that are found to be larger for strong lines than for weak lines, and discrepancies between observed and modelled line profile shapes. Results. Symptoms of local atmospheric velocity fields are always det ected through a non-zero microturbulence parameter for main sequence stars having Te below about 10000 K, but not for hotter stars. Direct line profile tracers of the atmospheric velocity field are found in six very sharp-lined stars in addition to the three r eported earlier. Direct signatures of velocity fields are fo und to occur in A stars with and without the Am chemical peculiarities, although the amplitude of the effects seems larger in Am stars. Velocity fields are also directly detected in spectral line profiles of A and e arly F supergiants, but without significant line asymmetrie s. Conclusions. We confirm that several atmospheric velocity field signature s, particularly excess line wing absorption which is strong er in the blue line wing than in the red, are detectable in the spe ctral lines of main sequence A stars of suffi ciently low ve sin i. We triple the sample of A stars known to show these effects, which are found both in Am and normal A stars. We argue that the observed line distortions are probably due to convective motions reachin g the atmosphere. These data still have not been satisfactor ily explained by models of atmospheric convection, including numerical simulations.

A PARAMETER STUDY OF CLASSICAL Be STAR DISK MODELS CONSTRAINED BY OPTICAL INTERFEROMETRY

We have computed theoretical models of circumstellar disks for the classical Be stars κ Dra, β Psc, and υ Cyg. Models were constructed using a non-LTE radiative transfer code developed by Sigut & Jones (2007), which incorporates a number of improvements over previous treatments of the disk thermal structure, including a realistic chemical composition. Our models are constrained by direct comparison with long-baseline optical interferometric observations of the Hα-emitting regions and by contemporaneous Hα line profiles. Detailed comparisons of our predictions with Hα interferometry and spectroscopy place very tight constraints on the density distributions for these circumstellar disks.

BE STAR DISK MODELS IN CONSISTENT VERTICAL HYDROSTATIC EQUILIBRIUM

A popular model for the circumstellar disks of Be stars is that of a geometrically thin disk with a density in the equatorial plane that drops as a power law of distance from the star. It is usually assumed that the vertical structure of such a disk (in the direction parallel to the stellar rotation axis) is governed by the hydrostatic equilibrium set by the vertical component of the stars gravitational acceleration. Previous radiative equilibrium models for such disks have usually been computed assuming a fixed density structure. This introduces an inconsistency as the gas density is not allowed to respond to temperature changes and the resultant disk model is not in vertical, hydrostatic equilibrium. In this work, we modify the BEDISK code of Sigut & Jones so that it enforces a hydrostatic equilibrium consistent with the temperature solution. We compare the disk densities, temperatures, Hα line profiles, and near-IR excesses predicted by such models with those computed from models with a fixed density structure. We find that the fixed models can differ substantially from the consistent hydrostatic models when the disk density is high enough that the circumstellar disk develops a cool (T 10, 000 K) equatorial region close to the parent star. Based on these new hydrostatic disks, we also predict an approximate relation between the (global) density-averaged disk temperature and the T eff of the central star, covering the full range of central Be star spectral types.

Theoretical Fe I-III Emission-Line Strengths from Active Galactic Nuclei with Broad-Line Regions

We present theoretical iron emission line strengths for physical conditions typical of active galactic nuclei (AGNs) with broad-line regions. The non–local thermodynamic equilibrium (NLTE) models include a new and extensive treatment of radiative transfer in the Fe iii ion, complementing the Fe ii emission line strengths predicted in our earlier works. We also briefly present preliminary results for the Fe i emission from AGNs using a reduced atom model. We can satisfactorily reproduce the empirical UV Fe iii emission line template of Vestergaard & Wilkes for the prototypical narrow-line Seyfert 1 galaxy I Zw 1, both in terms of the general Fe iii flux distribution and the relative strength of the Fe iii and Fe ii emission. However, a number of detailed features are still not matched; the most prominent example is the strongest single Fe iii feature observed in the I Zw 1 spectrum, UV47: it is predicted to be strong only in models suppressing Fe-H charge exchange reactions. We examine the role of variations in cloud turbulent velocity and iron abundance and carry out Monte Carlo simulations to demonstrate the effect of uncertainties in atomic data on the computed spectra. Subject headingg s: atomic data — line: formation — line: identification — quasars: emission lines — supernovae: general

Emission Lines in the Spectrum of the 3He Star 3 Centauri A

Emission in the 4d-->4f transitions of Mn ii (multiplet 13, lambdalambda6122-6132), in the 4f-->6g transitions of P ii, and in lambda6149.5 of Hg ii has been detected in the spectrum of the helium-weak star 3 Centauri A (B5 III-IVp). Weaker emission from the same Mn ii multiplet is also seen in the hot, mild HgMn star 46 Aquilae (B9 III). It is suggested that the emission is of photospheric origin and may be evidence for the stratification of manganese, phosphorus, and mercury in the photosphere of 3 Cen A and of manganese in 46 Aql.

The interpretation of Mn II emission from late-type B stars

The photospheric Mnii emission lines Wahlgren & Hubrig ([CITE]) detect in the spectra of several late-type B and HgMn stars and attribute to fluorescent excitation are more naturally explained by interlocked non-LTE effects acting in a photosphere in which the manganese abundance is stratified with depth. The case is particularly strong for HD 186122 (46 Aql) and HD 179761 both of which require the manganese overabundance to be concentrated to column masses of

THE EFFECT OF DENSITY ON THE THERMAL STRUCTURE OF GRAVITATIONALLY DARKENED Be STAR DISKS

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