Charles R. Cowley

Atomic Transition Probabilities in Tb II with Applications to Solar and Stellar Spectra

Absolute atomic transition probabilities are reported for many of the strongest lines in Tb II (the second spectra of terbium). Branching fractions were measured from Fourier transform spectrometer (FTS) data and combined with radiative lifetimes from recent laser-induced fluorescence measurements to determine these transition probabilities. Due to the absence of an extensive published classified line list for Tb II, accurate line positions in the FTS data as well as hyperfine structure patterns (described in the adjacent paper) were used to verify line classifications reported herein. These laboratory data are applied to determine photospheric terbium abundances in the Sun and the Galactic halo giant CS 22892-052.

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.

Rotationally modulated variations and the mean longitudinal magnetic field of the Herbig Ae star HD 101412

Despite the importance of magnetic fields to a full understanding of the properties of accreting Herbig Ae/Be stars, these fields have scarcely been studied until now over the rotation cycle. One reason for the paucity of these observations is the lack of knowledge of their rotation periods. The sharp-lined young Herbig Ae star HD 101412 with a strong surface magnetic field has become in the past few years one of the most well-studied targets among the Herbig Ae/Be stars. We present our multi-epoch polarimetric spectra of this star acquired with FORS 2 to search for a rotation period and constrain the geometry of the magnetic field. Methods: We measured longitudinal magnetic fields for 13 different epochs distributed over 62 days. These new measurements and our previous measurements of the magnetic field in this star were combined with available photometric observations to determine the rotation period. Results: We find the rotation period to be P = 42.076+/-0.017 d.

Lanthanide Rare Earths in Stellar Spectra With Emphasis on Chemically Peculiar Stars

This paper deals with stellar abundance patterns of the lanthanides. It is divided into two parts. In Part A we investigate the accuracy with which partition functions may be calculated from known energy levels. Corrections for missing levels are made, and current best estimates are tabulated. The current values are compared with partition functions built into the ATLAS code. The latter are generally rather good, except for Ce III. An increase of about a factor of two in published cerium abundances in many CP stars is indicated. In Part B, the systematics of the behavior of La II through Tm II are reviewed one-by-one. The emphasis is on CP stars, but late-type giants and dwarfs are also discussed. It is concluded that the overall typical pattern is tolerably well described by varying degrees of enhancement of the solar pattern, including the odd-even effect. Definite non-solar patterns do occur, however, such as the neodymium-samarium holes and the positive europium anomaly. From these abundance patterns we infer processes, nuclear or non-nuclear, that are distinct from those responsible for the lanthanide composition of the majority of stellar atmospheres.

Yttrium, barium, and the lanthanides in Ap and Am stars

This paper gives an element-by-element description of the spectra of yttrium, barium, and the lanthanides as they appear on 34 high-resolution spectra of 29 stars. Most of the stars are chemically peculiar (Ap or Am). Possible deviations from the odd--even effect are examined carefully, and two categories of such deviations are discussed. It is concluded that nuclear processes cannot be excluded on the basis of an odd-Z anomaly. An empirical two-component model for the lanthanide abundances is suggested. No resolution of the nuclear versus nonnuclear theories of abundance pecularies is proposed.

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.

On the possible presence of promethium in the spectra of HD 101065 (Przybylski's star) and HD 965

Traditional and statistical line-identification methods indicate the presence of Pm I and II, Tc I, and perhaps Tc II in the spectrum of the roAp star HD 101065. These methods also lead to the presence of Pm II and probably also Pm I in a related cool Ap star, HD 965. The spectroscopic evidence is strong enough that we would declare promethium to be present without hesitation, if any of its isotopes were stable. The longest-lived promethium isotope has a half-life of only 17.7 years. The presence of this element would mean that unrecognized processes - perhaps flare activities - are taking place in the atmospheres of these stars. The significance of such processes for galactic chemical evolution cannot be ruled out. We discuss the possibility that the highly improbable wavelength coincidences are due to chance, or due to contamination of the laboratory sources.

Metallic line profiles of the AO V star Vega

High dispersion ultrahigh signal to noise Reticon spectra of Vega were obtained with the coude spectrograph of the 1.2 m telescope. A mean signal to noise ratio of 2500 over the spectral region lambdas from 3825 to 5435 was achieved. Examination of the line profiles confirmed the presence of two different types of profiles which were previously seen in IIIaJ and lower signal to noise Reticon spectra. The profiles of the strong lines are essentially classical rotational profiles with enhanced wings which are slightly stronger than expected while those of weak lines are clearly flat-bottomed resulting in a trapezoidal appearance. A few possible theoretical explanations are presented.

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.

The application of wavelength coincidence statistics to line identification: HR 465 and HR 7575

A Monte Carlo technique for the experimental determinaiion of probabilities has been used to supplement standard line-identification procedures in the Ap stars HR 465 and HR 7575. The resulting identifications are divided into three categories ranging from definite to possible identifications. The identification of U II and Pt II in HR 465, thc possible idenitification of Te II, OsII. and Pm II in HR 465, and the possible identification of Se II and Cs II in HR 7575 are discussed in more detail. It is suggested that HR 465 has some of the properties usually associated with both the Hg-Mn and the Cr-Eu stars. (auth)