HgMn Stars Research Articles

Statistical analysis from recent abundance determinations in HgMn stars

Statistical analysis from recent abundance determinations in HgMn stars

Spectroscopic study of the HgMn star HD 49606: the quest for binarity, abundance stratifications and magnetic field

Spectroscopic study of the HgMn star HD 49606: the quest for binarity, abundance stratifications and magnetic field

CHEMICAL COMPOSITION OF INTERMEDIATE-MASS STAR MEMBERS OF THE M6 (NGC 6405) OPEN CLUSTER

ABSTRACT We present here the first abundance analysis of 44 late B-, A-, and F-type members of the young open cluster M6 (NGC 6405, age about 75 Myr). Low- and medium-resolution spectra, covering the 4500–5840 Å wavelength range, were obtained using the FLAMES/GIRAFFE spectrograph attached to the ESO Very Large Telescopes. We determined the atmospheric parameters using calibrations of the Geneva photometry and by adjusting the H β profiles to synthetic ones. The abundances of up to 20 chemical elements, from helium to mercury, were derived for 19 late B, 16 A, and 9 F stars by iteratively adjusting synthetic spectra to the observations. We also derived a mean cluster metallicity of [Fe/H] = 0.07 ± 0.03 dex from the iron abundances of the F-type stars. We find that for most chemical elements, the normal late B- and A-type stars exhibit larger star-to-star abundance variations than the F-type stars probably because of the faster rotation of the B and A stars. The abundances of C, O, Mg, Si, and Sc appear to be anticorrelated with that of Fe, while the opposite holds for the abundances of Ca, Ti, Cr, Mn, Ni, Y, and Ba as expected if radiative diffusion is efficient in the envelopes of these stars. In the course of this analysis, we discovered five new peculiar stars: one mild Am, one Am, and one Fm star (HD 318091, CD-32 13109, GSC 07380-01211, CP1), one HgMn star (HD 318126, CP3), and one He-weak P-rich (HD 318101, CP4) star. We also discovered a new spectroscopic binary, most likely a SB2. We performed a detailed modeling of HD 318101, the new He-weak P-rich CP star, using the Montréal stellar evolution code XEVOL which self-consistently treats all particle transport processes. Although the overall abundance pattern of this star is properly reproduced, we find that detailed abundances (in particular the high P excess) resisted modeling attempts even when a range of turbulence profiles and mass-loss rates were considered. Solutions are proposed which are still under investigation.

Magnetic, chemically peculiar (CP2) stars in the SuperWASP survey

AbstractThe magnetic chemically peculiar (CP2) stars of the upper main sequence are well‐suited for investigating the impact of magnetic fields on the surface layers of stars, which leads to abundance inhomogeneities (spots) resulting in photometric variability. The light changes are explained in terms of the oblique rotator model; the derived photometric periods thus correlate with the rotational periods of the stars. CP2 stars exhibiting this kind of variability are classified as α2 Canum Venaticorum (ACV) variables. We have analysed around 3850000 individual photometric WASP measurements of magnetic chemically peculiar (CP2) stars and candidates selected from the catalogue of Ap, HgMn, and Am stars, with the ultimate goal of detecting new ACV variables. In total, we found 80 variables, from which 74 are reported here for the first time. The data allowed us to establish variability for 23 stars which had been reported as probably constant in the literature before. Light curve parameters were obtained for all stars by a least‐squares fit with the fundamental sine wave and its first harmonic. Because of the scarcity of Strömgren uvbyβ measurements and the lack of parallax measurements with an accuracy better than 20%, we are not able to give reliable astro‐physical parameters for the investigated objects. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A search for photometric variability in magnetic chemically peculiar stars using ASAS-3 data

The (magnetic) chemically peculiar (CP) stars of the upper main sequence are well-suited laboratories for investigating the influence of magnetic fields on the stellar surface because they produce abundance inhomogeneities (spots), which results in photometric variability that is explained in terms of the oblique rotator model. CP stars exhibiting this phenomenon are normally classified as alpha2 Canum Venaticorum (ACV) variables. It is important to increase the sample of known rotational periods among CP stars by discovering new ACV variables. The ASAS-3 data were cross-correlated with the Catalogue of Ap, HgMn, and Am stars in order to analyse the light curves of bona fide CP and related stars. The light curves were downloaded and cleaned of outliers and data points with a flag indicating bad quality. Promising candidates showing a larger scatter than observed for constant stars in the corresponding magnitude range were searched for periodic signals using a standard Fourier technique. In total, we found 323 variables, from which 246 are reported here for the first time, and 77 were probably wrongly classified before. The observed variability pattern of most stars is in accordance with an ACV classification. For some cases, it is difficult to distinguish between the light curves of double-waved ACVs and the variability induced by orbital motion (ellipsoidal variables/eclipsing variables), especially for objects exhibiting very small amplitudes and/or significant scatter in their light curves. Thus, some eclipsing or rotating ellipsoidal variables might be present. However, we are confident that the given periods are the correct ones. There seems to be a possible weak correlation between the rotational period and colour, in the sense that cooler magnetic CP stars rotate more slowly. However, this correlation seems to disappear when correcting for the interstellar reddening.

Precision age indicators that exploit chemically peculiar stars

We would like to find a way to improve the determination of galaxy star formation history from integrated light spectroscopy. To this end, several classes of chemically peculiar (CP) stars arise during the course of normal evolution in single stars and noninteracting binary stars. An aging stellar population has periods of time in which CP stars contribute to the integrated light, and others in which the contributions fade. The HgMn stars, for example, occupy a narrow temperature range of 10500 to 16000 K, which maps to a narrow range of ages. Wolf-Rayet stars, He-poor stars, Bp-Ap stars, Am-Fm stars, and C stars all become very common in a normal stellar population at various ages between zero and several Gyr, fading in and out in a way that is analogous to features used in stellar spectral classification. We examine population fractions and light fractions in order to assess the feasibility of using CP stars as age tracers. We find that, even though CP stars do not usually dominate in number, there are enough of them so that the CP spectral features are detectable in high-quality integrated spectra of young and intermediate age stellar populations. The new technique should be calibratable and useful. Furthermore, using CP signatures as age dating tools sidesteps reliance on photometry that is susceptible to dust and Balmer features that are susceptible to nebular fill-in.

New Mn II energy levels from the STIS-HST spectrum of the HgMn star HD 175640

The NIST database lists several Mn II lines that were observed in the laboratory but not classified. They cannot be used in spectrum synthesis because their atomic line data are unknown. These lines are concentrated in the 2380-2700 A interval. We aimed to assign energy levels and log gf values to these lines. Semi-empirical line data for Mn II computed by Kurucz were used to synthesize the ultraviolet spectrum of the slow-rotating, HgMn star HD 175640. The spectrum was compared with the high-resolution spectrum observed with the HST-STIS equipment. A UVES spectrum covering the 3050-10000 A region was also examined. We determined a total of 73 new energy levels, 58 from the STIS spectrum of HD 175640 and another 15 from the UVES spectrum. The new energy levels give rise to numerous new computed lines. We have identified more than 50% of the unclassified lines listed in the NIST database and have changed the assignement of another 24 lines. An abundance analysis of the star HD 175640, based on the comparison of observed and computed ultraviolet spectra in the 1250-3040 A interval, is the by-product of this study on Mn II.

Four new HgMn stars: HD 18104, HD 30085, HD 32867, and HD 53588

We have detected four new HgMn stars, while monitoring a sample of apparently slowly rotating superficially normal bright late B and early A stars in the northern hemisphere. Important classification lines of Hg II and Mn II are found as conspicuous features in the high resolution SOPHIE spectra of these stars (R = 75000). Several lines of Hg II, Mn II and Fe II have been synthesized using model atmospheres and the spectrum synthesis code SYNSPEC48 including hyperfine structure of various isotopes when relevant. These synthetic spectra have been compared to high resolution high signal-to-noise observations of these stars in order to derive abundances of these key elements. The four stars are found to have distinct enhancements of Hg and Mn which show that these stars are not superficially normal B and A stars, but actually are new HgMn stars and should reclassified as such.

Element spots in Ap and Hg‐Mn stars from current‐driven diffusion

AbstractThe stars of the middle main sequence often have spot‐like chemical structures at their surfaces. We consider the diffusion process caused by electric currents that can lead to the formation of such chemical spots. Diffusion is considered using the partial momentum equations derived by the Chapman‐Enskog method. We argue that diffusion caused by electric currents can substantially change the surface chemistry of stars and form spotted chemical structures even in a relatively weak magnetic field. The considered mechanism can be responsible for a formation of element spots in Hg‐Mn and Ap‐stars. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A non-LTE spectral analysis of the3He and4He isotopes in the HgMn starκCancri

We present a pilot study on non-local thermodynamic equilibrium (NLTE) line-formation computations for the isotopes 3He and 4He in the mercury-manganese star kappa Cancri. The impact of NLTE effects on the determination of isotopic abundances and the vertical stratification of helium in the atmosphere is investigated. Modern NLTE line-formation computations were employed to analyse a high-resolution and high signal-to-noise ratio ESO-VLT/UVES spectrum of kap Cnc. The atmospheric parameters were determined from fitting the hydrogen Balmer lines and the spectral energy distribution. Multiple HeI lines were investigated, including HeI 4921A and 6678A, which show the widest isotopic splits. Half of the observed HeI lines in the spectrum of kap Cnc show significant NLTE strengthening, the effects are strongest in the red lines HeI 5875A and HeI 6678A. NLTE abundances from individual HeI lines are up to a factor of about 3 lower than LTE values. Helium is found to be stratified in the atmosphere of kap Cnc. While the LTE analysis indicates a step-like profile of the helium abundance, a gradual decrease with height is indicated by the NLTE analysis. A 3He/4He ratio of about 0.25-0.30 is found. With the available data it cannot be decided whether the two isotopes follow the same stratification profile, or not. This work implies that NLTE effects may be ubiquitous in the atmospheres of HgMn stars and may have a significant impact on abundance determinations and the interpretation of the vertical abundance stratification of elements.

Abundance analysis, spectral variability, and search for the presence of a magnetic field in the typical PGa star HD 19400

The aim of this study is to carry out an abundance determination, to search for spectral variability and for the presence of a weak magnetic field in the typical PGa star HD19400. High-resolution, high signal-to-noise HARPS spectropolarimetric observations of HD19400 were obtained at three different epochs in 2011 and 2013. For the first time, we present abundances of various elements determined using an ATLAS12 model, including the abundances of a number of elements not analysed by previous studies, such as Ne I, Ga II, and Xe II. Several lines of As II are also present in the spectra of HD19400. To study the variability, we compared the behaviour of the line profiles of various elements. We report on the first detection of anomalous shapes of line profiles belonging to Mn and Hg, and the variability of the line profiles belonging to the elements Hg, P, Mn, Fe, and Ga. We suggest that the variability of the line profiles of these elements is caused by their non-uniform surface distribution, similar to the presence of chemical spots detected in HgMn stars. The search for the presence of a magnetic field was carried out using the moment technique and the SVD method. Our measurements of the magnetic field with the moment technique using 22 Mn II lines indicate the potential existence of a weak variable longitudinal magnetic field on the first epoch. The SVD method applied to the Mn II lines indicates <B_z>=-76+-25G on the first epoch, and at the same epoch the SVD analysis of the observations using the Fe II lines shows <B_z>=-91+-35G. The calculated false alarm probability values, 0.008 and 0.003, respectively, are above the value 10^{-3}, indicating no detection.

Superheavy elements in the atmospheres of magnetic stars

The studies of the abundances of superheavy chemical elements in magnetic stars conducted at the Crimean Astrophysical Observatory were facilitated by the launch of the Astron space station in 1983. This spacecraft observed the ultraviolet spectra of such stars. The present brief review is focused on the abundances of superheavy elements (Pt, Au, Hg, Tl, Pb, Bi, Th, and U) in the atmospheres of magnetic Ap and HgMn stars. These results were obtained basing on the visible and ultraviolet spectra of stars. The data accumulated over more than 30 years show that these stars are characterized by significant overabundances (up to 6–7 dex) of such elements. The following important fact is noted: the superheavy element anomalies follow the trend in the anomalies that are characteristic of less heavy elements. Therefore, it may be assumed that all these anomalies (including the significant overabundances of superheavy elements) share a common explanation. Certain unresolved problems are discussed briefly.

Elemental Abundance Analyses with DAO Spectrograms. XXXIV. A Three-Dimensional Graphical Examination of the Elemental Abundances of the Mercury-Manganese and Metallic-Line Stars

Detailed analyses of high-dispersion, high signal-to-noise spectra enable astronomers to infer many stellar properties. We study nonmagnetic normal and chemically peculiar B, A, and F stars to understand the details of their optical region abundances via graphical techniques using two kinds of figures for 32 elements. By characterizing the anomalies of the mercury-manganese (HgMn) and the metallic-line (Am) stars, we provide major theoretical tests. We confirmed the known Hg dichotomy between HgMn stars, which are greatly overabundant, and the Am stars with normal abundances. Further P, Ga, Xe, Pt, and Au values were only overabundant for some HgMn stars, and lines of the rare earth elements, such as Sm and Eu, were seen only in some Am and normal stars. These observations might be due in some cases to changes in the major ionization state of atoms in the relevant stellar atmosphere. That some HgMn stars with large Ga overabundances have positions close in the H-R diagram to HgMn stars that lack Ga II lines in the optical region suggests a dichotomy similar to Hg with a boundary close to, but not identical, to that for Hg. The spread of the abundance anomalies for a given element tends to be smaller among the Am stars than among the HgMn stars. Star-to-star differences are superimposed upon abundance trends.

Physical and Chemical Parameters of HgMn Stars Based on Available Data

HgMn stars are among the quietest stars of the main sequence. They represent the A2-B5 subclasses, being slow rotators, and they are more often in binary systems than in normal stars. Their effective temperatures are in the range from 10000 K to 16000 K, and the existence of the magnetic field in their atmospheres is still the subject of debates. HgMn stars belong to the group of Chemically Peculiar (CP) stars, and as a separate subgroup they are characterized by unusually strong lines of HgII and MnII. The HgMn stars often show mild helium deficiencies, and sometimes the presence of other peculiarities has been reported: for instance, isotopic anomalies in platinum and mercury are mentioned in some of them. To see the trends of chemical abundances in those stars, I used the element abundances expressed by solar values and the final results are compared with the ones compiled by Smith [1]. For the first time, such kind of search Astrophysics, Vol. 57, No. 1, March , 2014

On the nature of sn stars

The sn stars present sharp Balmer lines, sharp metallic lines and broad coreless He I lines. Initially Abt & Levato proposed a shell-like nature to explain the sn stars, although this scenario was subsequently questioned. We aim to derive abundances for a sample of 9 stars, including sn and non-sn stars, to determine the possible relation between sn and CP stars. We analysed the photospheric chemical composition of sn stars and show that approximately 40% of them display chemical peculiarities (such as He-weak and HgMn stars) within a range of temperature of 10300 - 14500 K. We have studied the possible contribution of different processes to the broad He I lines present in the sn stars. Although NLTE effects could not be completely ruled out, it seems that NLTE is not directly related to the broad He I profiles observed in the sn stars. The broad-line He I 4026 is the clearest example of the sn characteristics in our sample. We succesfully fit this line in 4 out of 7 sn stars by using the appropriate Stark broadening tables, while small differences appear in the other 3 stars. Studying the plots of abundance vs depth for the He I lines resulted in some sn stars probably being stratified in He. However, a further study of variability in the He I lines would help for determining whether a possible non-uniform He superficial distribution could also play a role in these sn stars. We conclude that the broad He I lines that characterize the sn class could be modelled (at least in some of these stars) by the usual radiative transfer process with Stark broadening, without needing another broadening mechanism. The observed line broadening in sn stars seems to be related to the "normal" He line formation that originates in these atmospheres. (abridged)

On the sodium versus iron correlation in late B-type stars

Abstract With an aim to study whether the close correlation between [Na/H] and [Fe/H] recently found in A-type stars further persists in the regime of B-type stars, the abundances of Na were determined for 30 selected sharp-lined late B-type stars (10000 K ≲ Teff ≲ 14000 K) from the Na i 5890/5896 doublet. These Na abundances were then compared with the O and Fe abundances (derived from the O i 6156–8 and Fe ii 6147/6149 lines) showing anti-correlated peculiarities. It turned out that, unlike in the case of A-type stars, [Na/H] is roughly constant at a slightly subsolar level {[Na/H] ∼ −0.2 (±0.2)} without any significant correlation with [Fe/H], which shows considerable dispersion ranging from ∼ −0.6 to ∼ +1.0. This may serve as an important observational constraint for understanding the abundance peculiarities along with the physical mechanism of atomic diffusion in upper main-sequence stars of late A- through late B-type including Am and HgMn stars.

HD 161701, a chemically peculiar binary with a HgMn primary and an Ap secondary

Context. In recent years, HgMn stars have attracted attention after in several of them surface chemical spots of different elements were discovered, whose origin and possible connection with binarity and magnetic fields is not clear yet. Aims. We perform a complete study of the HgMn binary HD 161701, including the determination of physical parameters and photospheric chemical abundances. Methods. We analyzed time series of high-resolution spectroscopic observations obtained with ESO’s spectrograph FEROS and the spectro-polarimeter HARPS, in addition to a near-IR spectrum obtained with CRIRES. Radial velocity curves for both stars were combined with additional spectroscopic information to estimate absolute stellar parameters. A spectral separation technique was applied, which enabled us to conduct a detailed spectral analysis of both the HgMn primary and the low-luminosity secondary component, which turned out to be a peculiar star as well. Chemical abundances were determined by spectral synthesis. Using HARPS and FEROS spectra obtained at 11 rotational phases, we detected line profile variations in both stellar companions, indicating a nonuniform surface distribution of chemical composition. Results. HD 161701 is a binary system formed by two chemically peculiar stars in an almost circular orbit with a period of 12.451 d.

A search for pulsations in the HgMn star HD 45975 with CoRoT photometry and ground-based spectroscopy

The existence of pulsations in HgMn stars is still being debated. To provide the first unambiguous observational detection of pulsations in this class of chemically peculiar objects, the bright star HD 45975 was monitored for nearly two months by the CoRoT satellite. Independent analyses of the light curve provides evidence of monoperiodic variations with a frequency of 0.7572 c/d and a peak-to-peak amplitude of ~2800 ppm. Multisite, ground-based spectroscopic observations overlapping the CoRoT observations show the star to be a long-period, single-lined binary. Furthermore, with the notable exception of mercury, they reveal the same periodicity as in photometry in the line moments of chemical species exhibiting strong overabundances (e.g., Mn and Y). In contrast, lines of other elements do not show significant variations. As found in other HgMn stars, the pattern of variability consists in an absorption bump moving redwards across the line profiles. We argue that the photometric and spectroscopic changes are more consistent with an interpretation in terms of rotational modulation of spots at the stellar surface. In this framework, the existence of pulsations producing photometric variations above the ~50 ppm level is unlikely in HD 45975. This provides strong constraints on the excitation/damping of pulsation modes in this HgMn star.

Attempt to detect the granulation signature in the HgMn star HD 175640 from CoRoT★ light curves

HgMn stars are affected by atomic diffusion processes. Because theoretical models based on atomic diffusion require that the atmospheres of ApBp stars have to be much more stable than those of normal stars, the detection or the absence of detection of granulation in them should be an interesting information for the modelling. We would like to address the question of the stability of the atmospheres of HgMn stars by analysing the light curves provided by the Convection Rotation and planetary Transits (CoRoT) satellite. We used the CoRoT light curves of the bright HgMn star HD 175640 observed through asteroseismology channel, to look for granulation signature. We have developed a new quasi-automatic idl procedure, which allows us to correct abnormal jumps and variations which exist in CoRoT N2 data. The earlier results show that this procedure is an appropriate tool for the analysis of data obtained through asteroseismology and exoplanet channels. Calculations we carried out for this target show that granulation signature in this HgMn star is not detected from CoRoT light curves. On the other hand, we do not detect clear pulsation signal for this star. That shows that even if some HgMn stars are pulsating, this is not a general property of the group.

Chemically peculiar stars in the orion OB1 association. I. occurrence frequency, spatial distribution, and kinematics

A total of 85 CP stars of various types are identified among 814 members of the OriOB1 association. The fraction of CP stars decreases with age for different cluster subgroups: from 15.1% in the youngest subgroup (b) to 7.7% in the oldest one (a). Individual comments are provided for each of the 85 stars, where we analyze the physical parameters and distance of the objects. All the 23 Am stars identified as a result of this study are found to have heliocentric distances between 100 and 300 pc and appear not to be members of the OrionOB1 association, but foreground objects. We identified 59 Bp stars, which account for 13.4% of the total number of B-type stars in the association. The fraction of peculiar B-type stars in the OriOB1 association is found to be twice higher than that of peculiar A-type stars. The same is true for field stars. The association contains 22 magnetic stars, out of which 21 are Bp stars and only one is an Ap star. Seventeen of these stars are objects with anomalous helium lines. Magnetic stars show a well-defined tendency to concentrate in the central region of the association (in Orion’s Belt), which contains most of these objects. No significant differences are found between the field strengths in the B-type stars of the association and Bp-type field stars, although there is a noticeable trend for He-rich stars to have stronger fields compared to He-weak stars. We identified 17 binaries, which make up 20% of the total number of peculiar stars studied, which is the standard ratio for CP stars. Except for one HgMn star (HD35548), the radial velocities and proper motions of our identified objects are consistent with the corresponding parameters of normal B-type stars.