Abundance Anomalies Research Articles

Constraints on Mixing Processes from Abundance Anomalies

Abundance anomalies in the Sun, main sequence F, A and B stars, turnoff Pop II stars, the horizontal branch and white dwarfs are caused at least partly by particle transport processes. Detailed evolutionary models have been calculated for most of those objects taking into account the gravitational settling, thermal diffusion and radiative accelerations of 28 isotopes (24 atomic species). These will be used together with the observed abundances to put constraints on the mixing that rotationnally induced turbulence may lead to. The link between abundance anomalies and rotation on the HB is explained by the variation with log g of the ratio of the Eddington-Sweet and atomic diffusion velocities.

Abundance Anomalies and Rotation in Main Sequence OB Stars

We review the present status of our knowledge about observational evidence of the influence of rotation on the evolution and chemical surface enrichment of OB Main Sequence stars. We pay special attention to the CNO elements and show that luminosity class IV-V OB stars provide a good reference point for abundance analyses.While there is clear evidence of rotationally induced mixing in OB stars, we show that fast rotation is not a guarantee of enrichment. A comparison of observed data with present evolutionary models indicates that the observed facts can be explained assuming that the stars are born with a distribution of rotational velocities, and that depending on the stellar mass the surface abundance changes will be dominated by rotation or mass loss. Only the existence of a large number of B supergiants beyond the Main Sequence cannot be explained in this scenario. We show that current evolutionary models including rotation do not adequately account for the transfer of angular momentum from the interior to the surface.We point out that the difficulty in determining He abundances and in knowing the present or past binary status of many stars leads us to consider the above conclusions as preliminary.

FIP Effect and FIP-Dependent Bias in the Solar Corona

Using EUV spectra of an active region observed off the solar disk by the SOHO/SUMER spectrograph on the SOHO spacecraft, we investigate the dependence of the FIP effect on the height above the photosphere, and its relation to plasma magnetic structures present in the field of view. We also investigate the possibility of the FIP bias in the low-FIP elements to be FIP-dependent, so that different abundance anomalies must be found even within the low-FIP class of elements, which can provide important constraints on the FIP effect models.

Rotating Models for Evolved Low-Mass Stars

Low mass stars (< 2-2.5 M⊙) exhibit, at all the stages of their evolution, signatures of processes that require challenging modeling beyond the standard stellar theory. In this paper we focus on their peculiarities while they climb the red giant branch (RGB). We first compare the classical predictions for abundance variations due to the first dredge-up with observational data in various environments. We show how clear spectroscopic diagnostics probe the nucleosynthesis and the internal mixing mechanisms that drive RGB stars. Coherent data reveal in particular the existence of a non-standard mixing process that changes their surface abundances at the so-called RGB bump. By reviewing the models presented so far to explain the various abundance anomalies, we show that the occurrence of this extra-mixing process is certainly related to rotation. Finally we discuss the so-called Li-flash which is expected to occur at the very beginning of the extra-mixing episode.

Element Stratification in Main Sequence Stars and its Effect on Stellar Oscillations

AbstractElement settling due to the combined effects of gravity, thermal gradient, radiative acceleration and concentration gradient may lead to important abundance variations inside the stars that cannot be neglected in the computation of stellar structure. These processes were first introduced to account for abundance anomalies in “peculiar stars”, but their importance in the so-called “normal” stars is now fully acknowledged, specially after the evidence of helium settling in the Sun from helioseismology. These microscopic processes work in competition with macroscopic motions, such as rotation-induced mixing or mass loss, which increase the settling timescales. We have recently obtained clear evidence that asteroseismology of main sequence solar-type stars can give signatures of the chemical variations inside the stars and provide a better understanding of these processes.

Rotating Models of Low Mass Giants: Rotational Evolution and Surface Abundance Anomalies

We present some first results of stellar models of low mass, solar metallicity giants that include rotation and the mixing associated with it. We base our choice of initial conditions and input physics on observational constraints obtained for main sequence (MS) and horizontal branch (HB) stars, and obtain what can be regarded as a “best case scenario” for rotational mixing. These observations suggest local conservation of angular momentum in the cores and constant specific angular momentum in convective regions on the red giant branch (RGB). We show that under these circumstances and with moderate rotation rates at the turnoff it is possible to obtain enough mixing in order to reproduce the anomalous abundance ratios of the CNO species seen in giants. It is also shown that solid body rotation in the convective envelope fails to account for the observations. It is found that rotational mixing is inefficient on the lower RGB, so there is no need to invoke any inhibiting role of μ–gradients.

Rotation and Properties of A- and B-Type Chemically Peculiar Stars

Main-sequence chemically peculiar stars of spectral types A and B, which are characterised by photospheric abundance anomalies resulting from element segregation in the stellar outer layers, rotate slower than normal stars of similar temperatures. The mechanisms by which such slow rotation is achieved are not well understood yet; different processes may be involved for different types of chemical peculiarities, including stellar magnetic fields and multiplicity. Relevant existing observational data are reviewed.

The chemical composition and circumstellar environment of the variable star QY Sge

AbstractChemical composition studies of the variable star QY Sge show abundance anomalies suggestive of the pattern exhibited by the warmer RV Tau variables. Abundance analysis indicates a near-solar atmosphere with highly condensable elements depleted by several factors. Evidence of broad Na D emission features in its spectrum, a characteristic feature of R CrB stars taken at minimum, however, raises the question of its spectral classification. We address this question through a comparative analysis of its spectrum with those of γ Cyg 89 Her and R CrB taken at minimum, in addition to abundance analysis. We also discuss a possible physical scenario of the circumstellar environment accounting for the characteristics of the distinct spectroscopic features.

Thermal Stability of Rotating Low‐Mass Subgiants and Red Giants

A linear analysis of the thermal stability of rotating low-mass stars evolving from the zero-age main sequence (ZAMS) to the red giant branch (RGB) tip has been carried out. Two stellar models are considered: one for 1.2 M☉ with solar metallicity (Z = 0.0188), and the other for 0.8 M☉ with Z = 0.0005. An instability in a thermonuclear burning shell on the RGB in the first of these cases could potentially be related to the phenomenon of Li-rich red giants, while a hydrogen shell instability in the second model may help to explain the observed chemical abundance anomalies in globular cluster red giants. A range of surface rotational velocities in MS stars has been considered, and we have assumed that, beginning at the ZAMS, the stars (including their convective envelopes) rotate differentially with depth. This assumption, which leads to significantly higher centrifugal accelerations in the H-burning shell than with the case of solid-body rotation on the MS, is expected to favour the development of thermal instabilities. However, all of our models for rotating low-mass stars—even those that had much higher rotation rates on the MS than those observed for F-, G-, and K-type dwarfs—were found to be thermally stable throughout their evolution from the ZAMS to the RGB tip. Only when helium burning was ignited to end the first ascent of the giant branch did we find a thermal instability (the helium flash).

Abundance analysis of Am binaries and search for tidally driven abundance anomalies -- I. HD 33254, HD 178449 and HD 198391

It is suggested that if the tidally induced meridional circulation of Tassoul & Tassoul existed, it might successfully compete with diffusion processes and rotationally induced meridional circulation. This could affect the chemical composition of an Am binary component. The main goal of this paper is to start a systematic abundance analysis of a sample of Am binaries in order to search for possible observable abundance anomalies driven by tidal interaction in these binary systems. A synthetic spectrum analysis of CCD observations in two spectral regions (6400–6500 and 6660–6760 A) of HD 33254, HD 178449 and HD 198391 was carried out. Basic stellar properties, atmospheric parameters and abundance patterns were derived. HD 33254 is the star with pronounced Am anomalies, while HD 198391 is found to be an extremely sharp-lined hot Am star. HD 178449 is a controversial and extremely highly rotating star. We have succeeded in detecting a very faint secondary spectrum. The most probable explanation is that we have discovered the new spectroscopic Ab component of this spectroscopic and visual multiple system.

Geometrical structure of monobridged disilyene Si(H)SiH

By means of Fourier transform microwave spectroscopy of a supersonic molecular beam, we have detected the more abundant rare isotopic species of monobridged Si(H)SiH, and determined of its geometrical structure by isotopic substitution. Unexpected abundance anomalies of the singly deuterated isotopic species with respect to normal and Si(D)SiD were observed with equal mixtures of SiH4 and SiD4. This nonstatistical distribution is consistent with Si(H)SiH formation which is dominated by the reaction of a bare Si atom or ion with either SiH2 or SiH4. Dedicated laboratory searches were also undertaken for the remaining singlet isomer, H2SiSi, so far without success. From the absence of lines at the expected frequencies we conclude that H2SiSi is at least 50 times less abundant than Si(H)SiH. Because it is only slightly less stable than Si(H)SiH and because the barrier to isomerization is calculated to be quite low, H2SiSi may rearrange on the timescale of the supersonic expansion to Si(H)SiH.

Chemical fractionation and abundances in coronal plasma

Much of modern. astrophysics is grounded on the observed chemical compositions of stars and the diffuse plasma that pervades the space between stars, galaxies and clusters of galaxies. X-ray and EUV spectra of the hot plasma in the outer atmospheres of stars have demonstrated that these environments are subject to chemical fractionation in which the abundances of elements can be enhanced and depleted by an order of magnitude or more. These coronal abundance anomalies are discussed and some of the physical mechanisms that might be responsible for producing them are examined. It is argued that coronal abundances can provide important new diagnostics on physical processes at work in solar and stellar coronae. It seems likely that other hot astrophysical plasmas will be subject to similar effects.

A Reevaluation of the Super–LITHIUM‐rich Star in NGC 6633

We present a new abundance analysis of the super-Li-rich star J37 and a comparison star in NGC 6633. We confirm the result of Deliyannis et al. that J37 has a Li abundance well above the meteoritic value, and we also confirm that Al, S, Si, Ca, Fe, and Ni are supersolar and that C is subsolar. We additionally find that Na, Sc, Ti are supersolar, while O is subsolar. The abundance pattern of metals in J37 is generally consistent with the bulk composition of the Earth. We propose that accretion of circumstellar matter is the best explanation for these abundance anomalies, although we cannot rule out a secondary contribution from diffusion.

Abundance Anomalies in the X‐Ray Spectra of Planetary Nebulae NGC 7027 and BD +30o3639

We revisit Chandra observations of planetary nebulae NGC 7027 and BD +30°3639 in order to address the question of abundance anomalies in the X-ray-emitting gas. Enhanced abundances relative to solar of magnesium (Mg) for NGC 7027 and neon (Ne) for BD +30°3639 are required to fit their X-ray spectra, whereas observations at optical and infrared wavelengths show depleted Mg and Ne in these systems. We attribute the enhancement of Mg in NGC 7027 in the X-ray, relative to the optical, to the depletion of Mg onto dust grains within the optical nebula. For BD +30°3639, we speculate that the highly enhanced Ne comes from a white dwarf companion, which accreted a fraction of the wind blown by the asymptotic giant branch progenitor and went through a novalike outburst that enriched the X-ray-emitting gas with Ne.

X-ray spectroscopy of UZ Librae

UZ Librae is the giant primary of a close binary system with extremely fast rotation and revolution. Its distinctive characteristics include an optical light curve with a large amplitude and a high X-ray luminosity. UZ Librae was observed twice at five months interval in August 2000 and January 2001 by the XMM-Newton space observatory. Serie of lines of highly ion- ized Fe and several lines of the Ly and He series are visible in the reflection grating spectra, most notably from C, O and Ne. The corresponding ions are associated with plasma components with temperature in the range 3-8 MK. Bremstrahlung emission from plasma at high temperature (T > 3 × 10 7 K) indicates an intense flaring activity on UZ Librae. This is also supported by a Ne abundance enhancement relative to oxygen reminiscent of abundance anomalies observed during stellar and solar flares. In contrast to oxygen and neon, the abundance of Fe in the corona of UZ Librae is determined to be a tenth of the solar photospheric value. We conjecture that the large fluid kinetic helicity induced by the rapid rotation of the star currently gener- ates magnetic fields with two characteristic scales comparable with compact loops within solar active regions and with larger cooler loops interconnecting solar active regions. The X-ray emission of UZ Librae would be strongly enhanced not only due to the occurrence of these magnetic structures, but mainly due to their large surface coverage and to their permanent interactions responsible for heating UZ Librae plasma to high temperatures. A rotational modulation effect could have contributed to the luminosity variation between the two XMM-Newton observations since a large fraction of the X-ray emitting material might be located above large photospheric spots detected on Doppler images of the star.

Model atmospheres of red giants

Techniques and results of computations of model atmospheres are discussed for M and C giants and for giants with abundance anomalies. The SAM12 code, a modification of the ATLAS12 code developed by Kurucz, is used. Blanketing effects due to atomic and molecular line absorption are taken into account using the opacity-sampling approach. The computed model atmosphere for the Sun (G2V) is used as a test of the SAM12 code. The model red-giant atmospheres are compared with models reported in other studies. Comparisons between the computed and observed spectral energy distributions are given for the C giant WX Cyg (C-J6) and Sakurai's object (V4334 Sgr).

Surface Abundance Anomalies of Main Sequence Stars in Globular Clusters — A Probe into Interactions between Stars

In most of globular clusters, surface abundance anomalies are observed not only from red giant stars but also from main sequence stars. We discuss the possibility that the latter anomalies can be explained in terms the pollution due to mass transfer during close encounters with red giants, the latter of which have already developed the anomalies through their internal processes. If this is the case, the main sequence stars with the abundance anomalies may serve as a probe into the star-star interactions in dense cores of globular clusters.

Production of Aluminium and the Heavy Magnesium Isotopes in Asymptotic Giant Branch Stars

AbstractWe investigate the production of aluminium and magnesium in asymptotic giant branch models covering a wide range in mass and composition. We evolve models from the pre-main sequence, through all intermediate stages, to near the end of the thermally-pulsing asymptotic giant branch phase. We then perform detailed nucleosynthesis calculations from which we determine the production of the magnesium and aluminium isotopes as a function of the stellar mass and composition. We present the stellar yields of sodium and the magnesium and aluminium isotopes. We discuss the abundance predictions from the stellar models in reference to abundance anomalies observed in globular cluster stars.

Analysis of chemical abundances in the atmospheres of moderate barium stars

We used high-resolution spectra to compute model atmospheres to derive the atmospheric abundances of moderate barium stars. Comparing our results with analogous data for normal red giants, we find that the moderate barium stars appear to not differ systematically from normal red giants. Their chemical abundance anomalies show the same patterns and can be interpreted in terms of evolutionary effects: the evolutionary stage, mass, luminosity, and metallicity of the objects.

On the behavior of the C ii 4267.261, 6578.052 and 6582.882 Å lines in chemically peculiar and standard stars

With the aim of investigating the possible particular behavior of carbon in a sample of chemically peculiar stars of the main sequence without turning to modeling, we performed spectroscopic observations of three important and usually prominent single ionized carbon lines: 4267.261, 6578.052 and 6582.882 Å. In addition, we observed a large number of standard stars in order to define a kind of normality strip, useful for comparing the observed trend for the peculiar stars. We paid particular attention to the problem of the determination of fundamental atmospheric parameters, especially for the chemically peculiar stars for which the abundance anomalies change the flux distribution in such a way that the classical photometric methods to infer effective temperatures and gravities parameter cannot be applied. Regarding CP stars, we found a normal carbon abundance in Hg–Mn, Si (with some exceptions) and He strong stars. He weak stars are normal too, but with a large spread out of the data around the mean value. A more complicated behavior has been noted in the group of SrCrEu stars: four out of seven show a strong overabundance, being the others normal.