HgMn Stars Research Articles

Radiative accelerations in stellar atmospheres

We present calculations of radiative accelerations obtained with the general purpose model atmosphere code PHOENIX. The accelerations are computed simultaneously for all the elements He–Ga; Kr–Nb; Ba; La, using the opacity sampling method. The calculations are mainly performed with the LTE approximation. Tests are made to evaluate the influence of non-LTE effects and the validity of the radiative flux obtained through the diffusion approximation. It is shown that the abundances supported in the atmospheres of HgMn and He-weak stars are generally consistent with those observed.

An Abundance Study in the Hg-Mn Star 46 Aquilae (HD 186122) with the SUBARU/HDS

Abstract A detailed abundance analysis has been carried out for the Hg-Mn star 46 Aql using high-resolution spectra in the visual region (5100–6400 ${Å}$) obtained with the High Dispersion Spectrograph of the Subaru Telescope. Our attention has been mainly focused on those elements which have not been analyzed previously. He, C, N, and O are underabundant, while Na shows the solar abundance. We have confirmed previously reported underabundances of Mg, Al, and Si, and have found an overabundances of P ($+1.5\,\mathrm{dex}$) and Ti ($+1.0 \,\mathrm{dex}$) and a significant underabundance of S ($-1.8 \,\mathrm{dex}$). Fe is overabundant by $+0.8 \pm 0.1 \,\mathrm{dex}$ and a microturbulent velocity of $0.3 \,\mathrm{km} \,\mathrm{s}^{-1}$ was found from an analysis of the Fe ii lines. Among a few hundred unidentified absorption lines, we identified lines of As ii and Xe ii and a very large ($+4.0 \,\mathrm{dex}$) overabundance of Xe is obtained. We have confirmed emission lines of Mn ii and Ti ii near 6100 ${Å}$, while those of Cr ii cannot have been seen.

An elemental abundance analysis of the mercury manganese star HD 29647

The sharp-lined mercury manganese (HgMn) star HD 29647, which is located behind the outer envelope of the Taurus Molecular Cloud 1, is of interest for both its stellar properties and its utility as a probe of interstellar gas and dust along its line of sight. In this paper we review the properties of the star, summarize its line identifications and present an abundance analysis based on spectrograms obtained at the McDonald Observatory and the Observatoire de Haute-Provence. This star has elemental abundances similar to those of other HgMn stars except that its He/H ratio is closer to solar, possibly indicating a young age, and that it, like HR 7775, is overabundant in selected elements including the rare earths. The stellar radial velocity, unfortunately, closely matches that of the foreground cloud.

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 $\log(m)< $ $\approx-1.5$.

New results of magnetic field diagnosis in HgMn stars and normal late B-type stars

We suggested in a previous paper that three HgMn stars, HD 175640, HD 178065, and HD 186122, may be suspected to possess a magnetic eld that could be larger than 2 kG. We report here new observations of these three stars, three more HgMn stars, and four normal late B-type stars. The search was carried out by measuring the equivalent width of the Fe II 6147:7 A line relative to the equivalent width of the Fe II 6149:2 A line. The observed relative dierences between the equivalent widths of these Fe II lines are compared with those derived from synthetic spectra computed by neglecting magnetic eld eects. To investigate the eect of oscillator strength uncertainties on the results, we computed equivalent widths by using both Fe II log gf-values taken from Kurucz & Bell (1995) and Fe II log gf-values taken from Raassen & Uylings (2000). The comparison of the computed and observed equivalent widths based on the Kurucz & Bell (1995) atomic data leads us to conclude that all the stars of our sample, except HD 175640, are very likely to possess a magnetic eld. On the other hand, the comparison of the computed and observed equivalent widths based on the Raassen & Uylings (2000) log gf-values suggests the possible presence of magnetic elds only in three stars, the HgMn star HD 16717 and the two normal B-type stars HD 179761 and HD 186568. The latter two are those in the sample with the largest v sini (15 km s 1 and 18 km s 1 , respectively), so that the results for them are the most uncertain ones.

Non-LTE gallium abundance in HgMn stars

We present, for the first time, the Non-LTE gallium equivalent widths for the most prominent gallium transitions as identified in real spectra and in (hot) mercury-manganese star. The common feature of the departure coefficients is to decrease near the stellar surface, the collision rates are dominant in many cases and the Non-LTE equivalent widths are generally smaller. In particular, the abundance difference as derived from UV and visual lines is reduced. The photoionization cross sections were computed by means of standard R-matrix formalism.

The Bismuth Abundance in the HgMn Stars χ Lupi and HR 7775 and Improved Atomic Data for Selected Transitions of Bii, Biii, and Biiii

High-resolution spectra of the chemically peculiar HgMn stars chi Lupiand HR 7775, obtained with the Hubble Space Telescope/Goddard HighResolution Spectrograph, are investigated for their abundance of bismuthby comparison with LTE synthetic spectrum modeling. HR 7775, previouslyknown from International Ultraviolet Explorer spectra to display stronglines of Bi II, is determined to have bismuth present at an enhancementlevel of nearly 5 orders of magnitude from the lines Bi IIlambdalambda1436, 1902 and Bi III lambda1423. The bismuthenhancement for chi Lupi is found to be near a level of 1.5 dex, andan ionization anomaly between Bi+ and Bi++ isapparent. HR 7775 abundance enhancements of the heavy elements platinum,[Pt/H]=4.7 dex, and gold, [Au/H]=3.8 dex, have also been determined. Newlaboratory measurements for wavelengths and hyperfine structure patternsof Bi I/Bi II/Bi III lines are presented, as well as the results ofcalculations for hyperfine structure constants and oscillator strengthsfor selected lines of Bi II and Bi III. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS5-26555. (Less)

Spectroscopic investigations of classical Cepheids and main-sequence stars in galactic open clusters and associations

The small-amplitude Cepheid SU Cas and four members of the association Cas OB2 (HD 16893, HD 17327a and b, HD 17443) were investigated, using high-resolution CCD spectra. The following results were obtained: 1) All these objects have the same metallicity values, close to that of the Sun; 2) Elemental abundance indicates that SU Cas is a post first dredge-up star with an age from 1 10 to 1.45 10 yr, and it is not crossing the Cepheid instability strip for the first time. The mean value of log g = 2.35 corresponds to pulsations in the fundamental tone, although errors in gravity estimations provide overtone pulsations. The questions about its pulsational mode and membership in Cas OB2 remained open; 3) HD 17327a is a slowly rotating HgMn–star with the highest helium content among such objects, while HD 16893 also has a manganese overabundance and might be classified as an Am–star; 4) HD 17327b and HD 17443 are rapidly rotating main-sequence stars, while HD 17443 has a helium content comparable with that of the Sun.

Elemental abundance analyses with DAO spectrograms

Elemental abundances analyses are performed for the Mercury-Manganese stars Her, Her, and HR 7018 consistent with previous studies of this series using spectrograms obtained with Reticon and CCD detectors. Comparisons of the rst two analyses with those performed using coadded photographic plates show the general consistency of the derived elemental abundances. For Her and for Her, abundances were newly found for O, and for Al, V, Zn, and Ce, respectively. HR 7018 is discovered to be a single-lined spectroscopic binary. Its Sc abundance is the smallest of any class member with derived abundances and its Sr abundance the largest of any known HgMn star. A correlation analysis of the most complete abundance sets for 20 HgMn stars shows that the abundances of some elements are correlated with one another and some are functions of the stellar eective temperature.

The Origin of M[CLC]n[/CLC] [CSC]ii[/CSC] Emission in the Spectra of Chemically Peculiar Stars

Emission from Mn II multiplet 13 (λλ6122-6132) in the spectrum of the 3He star 3 Centauri A and the hot, mild, HgMn star 46 Aquilae can be naturally explained by interlocked non-LTE effects. However, reproduction of the strength of the Mn II emission in both stars requires vertical stratification of the manganese abundance, with manganese concentrated high in the photosphere (column mass 10-2 g cm-2). If this formation picture is correct, several additional transitions of Mn II with λ > 8000 A should also be present in emission in the spectrum of 3 Cen A. The wide range in the strength of Mn II multiplet 13 among upper main-sequence stars (ranging from absorption to emission) is made possible by the interplay in the non-LTE radiative transfer solution of the stellar Teff, manganese abundance, and manganese stratification profile. In particular, emission is strongly suppressed by a large manganese overabundance in the photosphere. This explains why the hot, mild, HgMn star 46 Aql, which has only a modest manganese enhancement, is detected in emission in Mn II multiplet 13 while other HgMn stars of similar Teff but with large photospheric manganese overabundances, such as κ Cancri, present Mn II multiplet 13 in absorption.

Neon abundances in normal late-B and mercury--manganese stars

ABSTRA C T We make new non-local thermodynamic equilibrium calculations to deduce the abundances of neon from visible-region echelle spectra of selected Ne i lines in seven normal stars and 20 HgMn stars. We find that the best strong blend-free Ne line that can be used at the lower end of the effective temperature Teff range is l6402, although several other potentially useful Ne i lines are found in the red region of the spectra of these stars. The mean neon abundance in the normal starsOlog Aa 8:10U is in excellent agreement with the standard abundance of neon (8.08). However, in HgMn stars neon is almost universally underabundant, ranging from marginal deficits of 0.1‐0.3 dex to underabundances of an order of magnitude or more. In many cases, the lines are so weak that only upper limits can be established. The most extreme example found is y Her with an underabundance of at least 1.5 dex. These underabundances are qualitatively expected from radiative acceleration calculations, which show that Ne has a very small radiative acceleration in the photosphere, and that it is expected to undergo gravitational settling if the mixing processes are sufficiently weak and there is no strong stellar wind. According to theoretical predictions, the low Ne abundances place an important constraint on the intensity of such stellar winds, which must be less than 10 214 M( yr 21 if they are non-turbulent.

An Optical Region Elemental Abundance Analysis of the HgMn Type Star HR 7775

Optical region spectra of the chemically peculiar HgMn type star HR 7775 have been analyzed for the determination of elemental abundances and spectral line identification as a complementary study to the analysis of Hubble Space Telescope Goddard High Resolution Spectrograph spectra. The spectra were obtained with the SOFIN echelle spectrograph on the Nordic Optical Telescope at nominal resolving powers of R = 25,000, 80,000, and 170,000 and signal-to-noise ratios typically in excess of 100 : 1. The overall nature of the elemental abundance distribution is similar to other cool, HgMn stars; somewhat enhanced abundances for the iron-group elements, with progressively stronger enhancements for heavier elements. Of particular note are the presence of lines from the rare-earth spectra Nd III and Pr III, and the heavy elements Pt, Au, Hg, Tl, and Bi. The isotopic mixture for several lines of mercury and platinum has been investigated. For mercury a possible difference is found between the mixtures as derived from lines of Hg I and Hg II. Three lines of Pt II display the same mixture, which is different from that found from lower excitation lines at ultraviolet wavelengths. We present new laboratory Fourier Transform spectrometer measurements for isotopic and hyperfine structure components of Hg I λ4046 and 4358 and Hg II λ3984 A.

Opacity Sampling in Radiative Acceleration Calculations

The accuracy requirements on atomic data for the calculation of stellar evolution with atomic diffusion are determined. In particular, the density of frequency grids needed for precise radiative acceleration (grad) calculations via the sampling method are presented. In order to minimize the number of frequency points needed for precise grad calculations, frequency grids that are more refined in the regions of the spectrum where the radiative flux is large are suggested. The following number of frequency points are needed for opacity table calculations to be used in stellar evolutionary codes including diffusion: 50,000 points for 4.20 ≤ log T ≤ 4.5, 30,000 points for 4.5 5.5. These opacity tables would render possible the study atomic diffusion in the exterior regions of certain chemically peculiar stars such as Ap or HgMn stars. In the sampling method, correction factors can be applied after the basic integrations over sampled spectra to include such effects as ion velocity averaging, redistribution of momentum among ions, and electron recoil during photoionization; these corrections are evaluated and illustrated for a few typical stellar models. Silicon is used as an example to show that the corrections are important mainly for T < 50,000 K. These corrections are used in stellar evolution calculations based on OPAL monochromatic opacity tables.

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.

Spectroscopic Investigations of the Main-Sequence B Stars in the Association Cas OB2 and the Open Cluster Platais 1 (C2128+488)

AbstractA spectroscopic investigation of four main-sequence stars in the association Cas OB2 (HD 17327A and B, HD 16893, HD 17443) and of two stars in the open cluster Platais 1 (No. 1 (1921) and No. 111 (1600)) was carried out using high-resolution CCD spectra, obtained with echelle-spectrometer ”LYNX” (6-m telescope, Special Astrophysical Observatory, Russian Academy of Sciences). We have determined the atmospheric parameters for the program stars, their projected rotational velocities and elemental abundances in their atmospheres. Two new HgMn stars - HD 17327A and Platais No. 1 - were detected. HD 17443 is not a Be star.

Oxygen 6156–8 Å Triplet in Chemically Peculiar Stars of t he Upper Main Sequence: Do HgMn Stars Show an Oxygen Anomaly?

An extensive spectrum-fitting analysis in the λ ∼ 6150 Å region was performed for forty late-B and A chemically peculiar (HgMn, Ap, Am, weak-lined) and normal stars of the upper main sequence, in order to quantitatively establish the abundance of oxygen from the O I 6156-8 triplet and to study its behavior/anomaly for each peculiarity group, where special attention was paid to HgMn-type stars. Magnetic Ap variables (Si or SrCrEu type) generally show a remarkably large oxygen depletion (by ∼ −1.6 dex to ∼ −0.4 dex relative to the solar abundance), and classical Am stars also show a clear underabundance (by ∼ −0.6 dex on the average). In contrast, the oxygen abundance in HgMn stars, exhibiting only a mild deficiency relative to the Sun typically by ∼ 0.3−0.4 dex, is not markedly different from the tendency of normal stars, also showing a subsolar abundance ([O/H] ⋍ −0.2). In view of the recent observational implication that the present solar oxygen abundance is enriched by ∼ 0.2−0.3 dex relative to the interstellar gas (from which young stars are formed), and thus unsuitable for the comparison standard, we concluded that the extent of the average O-deficiency in HgMn stars is appreciably reduced down to only ∼ 0.1−0.2 dex (i.e., not much different from the initial composition), which contrasts with their well-known drastically large N-depletion. Yet, as can be seen from the delicate and tight O vs. Fe anticorrelation, this marginal deficiency should be real, and thus some physical process simultaneously producing O-depletion/Fe-enrichment must have actually worked in the atmosphere of HgMn stars.

Abundances of the Elements in Sharp‐lined Early‐Type Stars fromIUEHigh‐Dispersion Spectrograms. II. The Nitrogen Deficiency in Mercury‐Manganese Stars

We determine nitrogen abundances from co-added IUE high-dispersion SWP spectrograms of four HgMn stars and five normal or superficially normal main-sequence B and A stars. We find N deficiencies in the HgMn stars greater than previously reported (depletion factors of 135-400 relative to the Sun). N abundance discrepancies from UV and IR studies of normal stars are discussed in light of possible non-LTE effects. Our data set for our sample of HgMn stars (observed with a consistent strategy to maximize the benefits of co-additions) is an improvement over the single or few images previously used to derive N abundances for most of these stars.

Mercury Elemental and Isotopic Abundances in Mercury‐Manganese Stars

Hg II abundances have been determined for 42 mercury-manganese (HgMn) stars by fitting synthetic spectra to observed spectra of the 3984 A Hg II line. Twenty of the stars had lines sharp enough to allow their Hg isotopic abundance mixes to be estimated. The Hg abundance is reported for more HgMn stars here than in any other single work. No correlation was found between Hg II abundance and Teff or the mean central wavelength of HgMn λ3984 stars. The mean central wavelength of λ3984 , an indicator of the Hg isotopic mix, is loosely correlated with Teff: stars with primarily heavy Hg isotopes tend to be cooler, although one star, 46 Aql, has almost pure 204Hg and Teff in about the middle of the temperature range for HgMn stars. We find that there is no evidence that any of the HgMn stars have 196Hg or 198Hg. For the very sharp-lined stars, the 204Hg abundance decreases with increasing Teff. No correlation is seen between the mean central wavelength and the surface gravity. No correlation was found between the projected rotational velocity and the Hg II abundance or the central wavelength of λ3984, although this result may be biased by the selection of stars with low reported v sin i. Hg I λ4358 was measured at high spectral resolution for seven HgMn stars. The isotopic shifts are too small, and the hyperfine components are too weak to allow unambiguous isotopic abundance ratios to be found. Hg I abundances correlate fairly well with Hg II abundances. Some of the Hg isotopic mixtures are difficult to explain using only diffusion. HR 7245 has approximately equal abundances of 199Hg, 200Hg, 202Hg, and 204Hg but very little 201Hg, and 11 Per has Hg that is mostly 199Hg and 204Hg. Calculations show that hyperfine splitting of 201Hg changes the radiative forces it feels compared with other isotopes, which may alter diffusion of that isotope enough to explain its absence in HR 7245, but we have found no possible explanation for the Hg isotopic mix found in 11 Per. These are the first very high resolution measurements of Hg II λ3984 for HR 7245 and 11 Per. Although diffusion may be acting in HgMn stars, either there are one or more other mechanisms acting to help produce the overabundances and isotopic mixtures seen or our understanding of diffusion is lacking on some important point.

Diffusion of iron-group elements in the envelopes of HgMn stars

The observed abundance anomalies for iron-group elements in atmospheres of HgMn stars are due to diffusive movements which are driven by radiation-pressure forces and which persist in the stellar envelopes, going down to regions with temperatures of about 106 K. Studies of diffusion in the envelopes are required both in order to understand the observed atmospheric abundances and in order to calculate the changes in opacities that result from changes in abundances. Let τ be the Rosseland-mean optical depth. It is shown that one can define an upper boundary, ττu, such that one can obtain solutions for the diffusive movements in the region of τ≥τu without any knowledge of what happens in the higher layers of τ<τu. The paper is concerned with a description of the numerical methods that can be used to obtain such solutions. For Cr and Mn we are able to follow the diffusion for times of order 108 yr with τu=1. For Fe we are also able to obtain some estimates of abundances at τ=1 allowing for diffusion processes. For Mn, Cr and Fe we attempt some comparisons of abundances computed for τu=1 with observed atmospheric abundances and obtain results that are not discouraging. For Fe and Ni, larger values of τu are required as the diffusion proceeds (after 108 yr we require τu≃8 for Fe and τu≃70 for Ni). For the outer regions with τ<τu it will be necessary to obtain solutions allowing for outflows of iron-group elements at the stellar surfaces. In such work it should be possible to match the outer-region solutions to the envelope solutions obtained using the methods described in this paper. The diffusive movements lead to changes in Rosseland-mean opacities by factors of up to 4. For Fe there is a build-up in concentrations in the region of log(T)≃5.1, where the dominant ionization stages are near Ar-like. This leads to the Z-bump in opacities being shifted from its normal position at log(T)≃5.3 to lower values of log(T) and becoming more sharply peaked. There is also a large build-up in Ni concentrations in the outer parts of the envelopes, leading to opacity enhancements. In the present work we allow neither for the normal main-sequence evolution of the stars nor for the modifications in that evolution which will result from changes in opacities. Solution for both envelopes and outer regions will eventually be required.

Three Very Young H[CLC]g[/CLC]M[CLC]n[/CLC] Stars in the Orion OB1 Association

We report the detection of three mercury-manganese stars in the Orion OB1 association. HD 37886 and BD-0 984 are in the approximately 1.7 million year old Orion OB1b. HD 37492 is in the approximately 4.6 million year old Orion OB1c. Orion OB1b is now the youngest cluster with known HgMn star members. This places an observational upper limit on the time scale needed to produce the chemical peculiarities seen in mercury-manganese stars, which should help in the search for the cause or causes of the peculiar abundances in HgMn and other chemically peculiar upper main sequence stars.