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.     

Comprehensive study of the magnetic stars HD 5797 and HD 40711 with large chromium and iron overabundances

We present the results of a comprehensive study of the chemically peculiar stars HD 5797 and HD 40711. The stars have the same effective^temperature, T _eff = 8900 K, and a similar chemical composition with large iron (+1.5 dex) and chromium (+3 dex) overabundances compared to the Sun. The overabundance of rare-earth elements typically reaches +3 dex. We have measured the magnetic field of HD 5797. The longitudinal field component B _e has been found to vary sinusoidally between −100 and +1000 G with a period of 69 days. Our estimate of the evolutionary status of the stars suggests that HD 5797 and HD 40711, old objects with an age t ≈ 5 × 10⁸ yr, are near the end of the core hydrogen burning phase.     

Evolution of magnetic fields in stars across the upper main sequence: II. Observed distribution of the magnetic field geometry

We re‐discuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that magnetic stars of mass M < 3 M_⊙ are concentrated towards the centre of the main‐sequence band. In contrast, stars with masses M > 3 M_⊙ seem to be concentrated closer to the ZAMS. The study of a few known members of nearby open clusters with accurate Hipparcos parallaxes confirms these conclusions. Stronger magnetic fields tend to be found in hotter, younger and more massive stars, as well as in stars with shorter rotation periods. The longest rotation periods are found only in stars which spent already more than 40% of their main sequence life, in the mass domain between 1.8 and 3 M_⊙ and with log g values ranging from 3.80 to 4.13. No evidence is found for any loss of angular momentum during the main‐sequence life. The magnetic flux remains constant over the stellar life time on the main sequence. An excess of stars with large obliquities β is detected in both higher and lower mass stars. It is quite possible that the angle β becomes close to 0. in slower rotating stars of mass M > 3 M_⊙ too, analog to the behaviour of angles β in slowly rotating stars of M < 3 M_⊙. The obliquity angle distribution as inferred from the distribution of r ‐values appears random at the time magnetic stars become observable on the H‐R diagram. After quite a short time spent on the main sequence, the obliquity angle β tends to reach values close to either 90. or 0. for M < 3 M_⊙. The evolution of the obliquity angle β seems to be somewhat different for low and high mass stars. While we find a strong hint for an increase of β with the elapsed time on the main sequence for stars with M > 3 M_⊙, no similar trend is found for stars with M < 3 M_⊙. However, the predominance of high values of β at advanced ages in these stars is notable. As the physics governing the processes taking place in magnetised atmospheres remains poorly understood, magnetic field properties have to be considered in the framework of dynamo or fossil field theories. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The exceptional Herbig Ae star HD 101412: The first detection of resolved magnetically split lines and the presence of chemical spots in a Herbig star

In our previous search for magnetic fields in Herbig Ae stars, we pointed out that HD 101412 possesses the strongest magnetic field among the Herbig Ae stars and hence is of special interest for follow‐up studies of magnetism among young pre‐main‐sequence stars. We obtained high‐resolution, high signal‐to‐noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD 101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting and several spectral lines observed in unpolarized light at high dispersion are resolved into magnetically split components. The measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8 kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The period determination was done using the Lomb‐Scargle method. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period P_rot = 13.86 d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non‐statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current‐driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate‐mass stars could be an alternative to a frozen‐in fossil field (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photometry and spectrophotometry of magnetic stars

A review of selected photometric and spectrophotometric properties of magnetic stars is given. It is shown that unreddened B–V of magnetic stars around B9 type can be transformed to log T_e using the calibration for normal stars and no systematic errors are introduced. Mechanisms of light variability are discussed. It is concluded that different mechanisms must be operating in different stars. Recently obtained photometric observations of short-period variability by KURTZ and Hβ variability by MADEJ et al. are discussed. An information about internal structure and the atmospheric structure of these stars can be derived from these observations.     

Magnetic field evolution in OBA stars

The applications of the spectral analysis methods discovered by Kirchhoff for the investigation of stellar magnetic fields are considered. The statistical properties of the mean magnetic fields for OBA stars have been investigated by analyzing data from two catalogs of magnetic fields. It is shown that the mean effective magnetic field ℬ of a star can be used as a statistically significant characteristic of its magnetic field. The magnetic field distribution functions F(ℬ) have been constructed for B-type and chemically peculiar (CP) stars, which exhibit a power-law dependence on ℬ. A sharp decrease in F(ℬ) in the range of weak magnetic fields has been found. The statistical properties of the magnetic fluxes for main-sequence stars, white dwarfs, and neutron stars are analyzed.     

Chemical evolution of the galactic halo

The chemical enrichment in the galactic halo is studied, on the basis of the numerical model developed in Paper I, with paricular attention to the overabundances of O and light elements with respect to Fe shown by metal poor stars. Some representative nucleosynthesis pictures for stars of both Population I and Population II are considered and their yields are compared with observations of relative abundances in the Sun and in the halo, to identify the possible reasons of the observed compositional differences. It is found that solar elemental ratios can be reproduced if intermediate mass stars are allowed to give some contribution to the production of Fe by type-I supernovae, while the ratios of abundances observed in the halo are more similar to the relative yields produced by massive stars. These features are shared by all the nucleosynthesis schemes which have been considered. Using the best model of Paper I, we show that the steep star formation induced by the collapse has a decisive effect in maintaining the overabundances of light elements during the whole evolution of the halo. The relevance of this conclusion is discussed also in the light of a possible interpretation of the differences between the two abundance scales for globular clusters.     

Magnetic survey of emission line B‐type stars with FORS 1 at the VLT

We report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission‐line B‐type star υ Sgr, and in a sample of fourteen members of the open young cluster NGC3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B‐type stars and He‐strong/He‐weak stars. Nine Be stars have been studied with magnetic field time series obtained over ∼1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi‐mode instrument FORS1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD 62367, μ Cen, o Aqr, and ε Tuc. The strongest longitudinal magnetic field, 〈B_z〉 = 117 ± 38 G, was detected in the Be star HD 62367. Among the Be stars studied with time series, one Be star, λ Eri, displays cyclic variability of the magnetic field with a period of 21.12 min. The binary star υ Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field 〈B_z〉 = –102 ± 10 G at MJD 54333.018 was measured using hydrogen lines. The cluster NGC3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B‐type stars out of the observed fourteen cluster members (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Yields of r-Process Elements and Chemical Evolution of the Galaxy

The supernova yields of r-process elements are obtained as a function of the mass of their progenitor stars from the abundance patterns of extremely metal-poor stars on the left-side [{Ba/Mg}]--[{Mg/H}] boundary with a procedure proposed by Tsujimoto and Shigeyama. The ejected masses of r-process elements associated with stars of progenitor mass M _ms ≤ 18 M _⊙ are infertile sources and the SNe II with 20 M _⊙ ≤ M _ms ≤ 40 M _⊙ are the dominant source of r-process nucleosynthesis in the Galaxy. The ratio of these stars 20 M _⊙ ≤ M _ms ≤ 40 M _⊙ with compared to the all massive stars is about∼ 18%. In this paper, we present a simple model that describes a star's [r/Fe] in terms of the nucleosynthesis yields of r-process elements and the number of SN II explosions. Combined the r-process yields obtained by our procedure with the scatter model of the Galactic halo, the observed abundance patterns of the metal-poor stars can be well reproduced.     

Long-Term Starspot Activity of the Eclipsing Variable System CG Cyg

Photometric observations of the eclipsing variable system CG Cyg (G9+K3, P=0 ^d .63), which belongs to a group of short-period, chromospherically active RS CVn type stars with the same type of activity as the sun, have been made over two seasons in 2003 and 2004. The spotting of the star’s surface is modelled using the original observations together with photometric data published since 1965. In every season the spots were concentrated near the equator and covered up to 18% of the star’s entire surface, while the temperature difference between the quiescent photosphere and the spotted regions was 2100 K. Cyclical variations in the total area and average latitude of the spots, as well as flip-flop of the active longitudes, are suspected. Latitudinal drift of the spots during a cycle and differential rotation of the star are discovered. The following orbital elements are obtained: M₁ = 0.93 M⊙, M₂ = 0.81 M⊙, R₁ = 1.01 R⊙, and R₂ = 0.82 R⊙.__________Translated from Astrofizika, Vol. 48, No. 3, pp. 349–363 (August 2005).     

Radii of some Ap,Am and A stars

Based on the observed energy curves of nine Ap stars, three Am stars, four normal A stars and one F0 V magnetic star, their radii have been estimated.Thence, the bolometric magnitudesM _bo1 have been obtained and a plot between logT _e andM _bo1 of these stars shows that a majority of Ap and Am stars are a little above the zero-age Main-Sequence, suggesting that they are slightly more evolved as compared to the normal A stars.The bolometric corrections derived from the aboveM _bo1 are much closer to those computed by Mihalas than to the ones given by Davis and Webb, the latter being about O ^m 1 more negative than the former.     

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

The 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)     

Lithium in chemically peculiar CP stars with magnetic fields

The problem of lithium in chemically peculiar Ap-CP stars has been the subject of debate for many years. The main reason for this is a lack of spectral observations of Ap stars in the neighborhood of the lithium resonance doublet Li I 6708 Å. An international cooperation project on “Lithium in cool CP stars with magnetic fields” was started in 1996. Systematic observations of CP stars in spectral regions of the 6708 Å and 6103 Å lines at the ZTSh (CrAO), CAT (ESO), Feros (ESO), and the 74″ telescope of the Mount Stromlo Observatory (Australia) have been used to analyze spectra of several CP stars studied by the way the 6708 Å lithium line varies with the stars’ rotational phase. Monitoring of the spectra of the oscillating CP stars (group I) HD 83368, HD 60435, and HD 3980, for which significant Doppler shifts of the Li I 6708 Å line are observed led to the discovery of “lithium spots” on the surface of these stars whose positions are related to the magnetic field structure. Models of the surfaces of these stars with the special program “ROTATE” based on the profiles of the Li I 6708 Å line are used to estimate the size of the spots, their positions on the stars’ surface, and the lithium abundances in these spots. A detailed analysis and modelling of the spectra of slowly rotating oscillating CP stars with strong, invariant lithium 6708 Å emission, including blending with lines of the rare earth elements, reveals an enhanced lithium abundance, with the abundance determined from the lithium 6103 Å line being higher than that determined from the 6708 Å line for all the stars. This may indicate vertical stratification of lithium in the atmospheres of CP stars with an anomalous isotopic composition (⁶Li/⁷Li = 0.2–0.5). HD 101065, an ultraslow rotator (vsini ≈ 1.5) visible from the poles and with powerful oscillations which cause pulsating line broadening in its spectrum, is unique among these stars. The amount of lithium in the atmosphere of HD 101065 logN(Li) = 3.1 on a scale of logN(H) = 12.0 and the isotope ratio ⁶Li/⁷Li ≈ 0.3. The high estimates of ⁶Li/⁷Li may be explained by the production of lithium in spallation reactions and the preservation of surface ⁶Li and ⁷Li by strong magnetic fields in the upper layers of the atmosphere near the magnetic poles. __________ Translated from Astrofizika, Vol. 50, No. 3, pp. 463–492 (August 2007).     

A search for the age-dependency of Ap star parameters

Some observational data of the sample of the magnetic chemically peculiar stars (MCP stars) are investigated statistically. For the MCP stars of spectral types later than A2 both the frequency distribution and the R ⋅ sin i-values suggest the existence of a linear relation between stellar diameter and rotation period. The MCP stars of spectral types earlier than B9 show an overpopulation of small R ⋅ sin i which may indicate the existence of a second group with smaller radius in this sample. The equatorially symmetric rotator is used as the magnetic model. With respect to its temporal behaviour the effective magnetic field is separated into dipolar and quadrupolar contribution. Both signs of the axisymmetric quadrupole moment appear with equal frequency. The dipole moment which produces the amplitude of the B_eff(t) curve forms for longer periods two groups which are separated by a distinct gap. Both of the groups exhibit magnetic fields which are the stronger the greater the stellar radius is, contrary to what is expected for frozen-in fields. The dominance of magnetic curves without polarity reversal for longer-period stars is in accordance with predictions of the dynamo theory.     

Comparison of the four catalogues by pairs,the Hipparcos,new Hipparcos,EOC‐2 and IL,by using μδ for the common 1120 stars

We present comparison results of our Independent Latitude (IL) catalogue of μ_δ determinations for 1120 bright stars with the Hipparcos, new Hipparcos and Earth Orientation Catalogue (EOC‐2) values. Also, we took into consideration the EOC3 and EOC4 (recent versions of EOC catalogues). Our μ_δ values are based on zenith telescope observations from seven Independent Latitude (IL) observatories. The IL measures are spanning a time baseline of up to 90 years which is the key advantage to the accurate determination of μ_δ. The short interval of the Hipparcos satellite observations is a disadvantage for a good accuracy of stellar proper motion, especially in the case of double and multiple stars. For this reason many astrometric catalogues have appeared after the publication of the Hipparcos including our IL catalogue. These catalogues are an appropriate combination of the Hipparcos satellite and ground‐based data which yields more accurate stellar coordinates and/or their proper motions. Among various types of ground‐based observations the latitude and universal time variations obtained from several million observations of stars reduced to the Hipparcos reference system were used for this purpose. These observations were obtained during almost the entire last century and were originally used to determine the Earth Orientation Parameters. It is also possible to use these data in the inverse task of checking the accuracy of stellar coordinates and/or their proper motions listed in the Hipparcos Catalogue. Such latitude and universal time variations data are the basis of the EOC and IL catalogues. In this paper, we computed the differences in μ_δ values between pairs of catalogues and analyzed the results to characterize the μ_δ errors for the four catalogues with a special focus on our IL catalogue. The standard errors of μ_δ for IL stars observed over more than 20 years are mostly smaller than or equal to the Hipparcos errors, and close to the accuracy level of the EOC‐2 (EOC‐3, EOC‐4) and the new Hipparcos. The resulting investigations of errors of differences of μ_δ, show that all four catalogues have relatively small random and systematic errors which are close to each other meaning that the corresponding μ_δ values have a high accuracy. Our sample also contains detected double and multiple stars for which the effects of the orbital and proper motions are difficult to separate. The differences of μ_δ values for these stars generally exceed those obtained for single stars. Also, these discrepancies could be attributed to effect of possible, still unrecognized, astrometric binaries. These investigations about the proper motions and double stars are in line with the activity of the IAU Working Group on Astrometry by Small Ground‐Based Telescopes. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Discovery of magnetic fields in three He variable Bp stars with He and Si spots

It is essential for the understanding of stellar structure models of high mass stars to explain why constant stars, nonpulsating chemically peculiar hot Bp stars and pulsating stars co‐exist in the slowly pulsating B stars and β Cephei instability strips. We have conducted a search for magnetic fields in the four Bp stars HD55522, HD105382, HD131120, and HD138769 which previously have been wrongly identified as slowly pulsating B stars. A recent study of these stars using the Doppler Imaging technique revealed that the elements He and Si are inhomogeneously distributed on the stellar surface, causing the periodic variability. Using FORS 1 in spectropolarimetric mode at the VLT, we have acquired circular polarisation spectra to test the presence of a magnetic field in these stars. A variable magnetic field is clearly detected in HD55522 and HD105382, but no evidence for the existence of a magnetic field was found in HD131120. The presence of a magnetic field in HD138769 is suggested by one measurement at 3σ level. We discuss the occurrence of magnetic B stars among the confirmed pulsating B stars and find strong magnetic fields of order kG and oscillations to be mutually exclusive. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic fields of chemically peculiar stars. I. The catalog of magnetic CP stars

This is the first paper of the series dedicated to the analysis of the magnetism of chemically peculiar (CP) stars of the upper Main Sequence. We use our own measurements and published data to compile a catalog of magnetic CP stars containing a total of 326 objects with confidently detected magnetic fields and 29 stars which are very likely to possess magnetic field. We obtained the data on the magnetism of the overwhelming majority of the stars solely based on the analysis of longitudinal field component B _e . The surface magnetic field, B _s , has been measured for 49 objects. Our analysis shows that the number of magnetic CP stars decreases with increasing field strength in accordance with exponential law, and stars with B _e exceeding 5kG occur rarely (about 3% objects of our list).     

Flare stars: Postflare behaviour of emission lines

The problem of the postflare behaviour of chromospheric emission lines in UV Cet-type flare stars is considered. It is shown that the postflare behaviours of different emission lines differ essentially from one another. In particular, the intensity of hydrogen and helium lines must be greatest at flare peak in continuum, while the intensities of 2800 Mg_II and H and K Ca_II lines reach their maximum magnitudes in the intermediate period between two flares. Theoretical postflare light curves are derived for the most important chromospheric lines: helium, hydrogen ionized magnesium and calcium (Figures 8, 9, 11, and 16). The definite regularities in sequence of these light curves are established (Figure 17). Methods of obtaining the mass concentration,n _*, in the chromospheres of flare stars based on the analysis of hydrogen and calcium light curves are elaborated. Values ofn _* obtained for a group of UV Cet-type flare stars are listed in Table VII. The conditions of the intensification of emission lines during the flare are examined, as is the possibility of the existence of an empirical relationship between chromospheric concentration,n _* and flare frequency,f _U .     

A search for magnetic fields in cool sdB stars

Hot cluster horizontal branch (HB) stars and field subdwarf B (sdB) stars are core helium burning stars that exhibit abundance anomalies that are believed to be due to atomic diffusion. Diffusion can be effective in these stars because they are slowly rotating. In particular, the slow rotation of the hot HB stars (T_eff > 11000 K), which show abundance anomalies, contrasts with the fast rotation of the cool HB stars, where the observed abundances are consistent with those of red giants belonging to the same cluster. The reason why sdB stars and hot HB stars are rotating slowly is unknown. In order to assess the possible role of magnetic fields on abundances and rotation, we investigated the occurrence of such fields in sdB stars with T_eff < 30 000 K, whose temperatures overlap with those of the hot HB stars. We conclude that large‐scale organised magnetic fields of kG order are not generally present in these stars but at the achieved accuracy, the possibility that they have fields of a few hundred Gauss remains open. We report the marginal detection of such a field in SB 290; further observations are needed to confirm it (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Some Comments on the Magnetic Braking of CP Stars

The low rotation velocities of magnetic CP stars are discussed. Arguments against the involvement of the magnetic field in the loss of angular momentum are given: (1) the fields are not strong enough in young stars in the stage of evolution prior to the main sequence; (2) there is no significant statistical correlation between the magnetic field strength and the rotation period of CP stars; (3) stars with short periods have the highest fields; (4) a substantial number of stars with very low magnetic fields (B _e < 500 G) have rotation speeds that are typical of other CP stars; (5) simulations of the magnetic fields by Leroy and the author show that the orientation of dipoles inside rotating stars, both slow and fast, is consistent with an arbitrary orientation of the dipoles; and, (6) slow rotators with P>25 days, which form 12% of the total, probably lie at the edge of the velocity distribution for low mass stars. All of these properties conflict with the hypothesis of magnetic braking of CP stars.