Abundance Anomalies Research Articles

A Survey for Pulsating Ap Stars from Naini Tal

Pulsation is ubiquitous among chemically normal A-type stars, but comparatively rare among chemically peculiar Am and Ap stars of the same temperature range. The conventional explanation for this is that diffusion produces the surface abundance anomalies in the Am and Ap stars, and also drains He from the He-II ionisation zone, thus quenching the κ-mechanism that drives δ Scuti pulsation. The pulsating Am and Ap stars exhibit dichotomous pulsation characteristics. The Am stars (and related stars) exhibit low-overtone δ Scuti pulsation, with amplitudes ranging from a few mmag to 0.1 mag. The pulsating Ap stars exhibit high-overtone pulsation with periods in the range 6-16 min and Johnson B semi-amplitudes typically ≤ 5 mmag. These stars are referred to as rapidly oscillating Ap stars, or ‘roAp’ stars (the see review by Martinez & Kurtz 1995).

Spectral and luminosity classification for the cool components in symbiotic stars

The near infrared spectra of 12 S-type symbiotic stars and 78 comparison stars have been observed with moderate dispersion in five runs from 1992 to 1997, the resolving power being R = lambda/Delta lambda > 2000, with a signal to noise ratio SIN > 100. The triple-headed absorption band of TiO (lambda lambda 8432, 8422 and 8452 Angstrom) emerges when a star is later than M2, and the depth of the TiO absorption band is very sensitive to the spectral type (ST) and insensitive to the luminosity class of the star. We fit a curve of spectral type against the index of the absorption depth of this band with a standard deviation sigma = 0.37 of a subdivision of one spectral type. The IR all triplet (lambda lambda 8498, 8542, 8662 Angstrom); Fe I 8689 Angstrom: and Fe I 8675 Angstrom are good luminosity indicators although the equivalent widths (EWs) of these lines clearly decrease fora star later than M3. When the star is a supergiant the lines have a smaller central residual intensity and broader wings than in the case of a normal giant. The Ca II 8662 Angstrom/Fe I 8675 Pi and Fe I 8689 Angstrom/Fe I 8675 A ratios are also good luminosity indicators for Ii-type giants. The latter is particularly useful when there are abundance anomalies. The metal-poor symbiotic star AG Dra is classified as a Ib or II giant, as is TX CVn, on the basis of Fe I 8689 Angstrom/Fe I 8675 Angstrom. 9 other symbiotic stars containing M-type cool components are classified as giants by direct comparison and quantitative analysis. Due to there being no known good ratio indicator of luminosity for M-type stars in the band studied and because there is no metal abundance data for the symbiotic stars studied by us except for AG Dra, the results for these 9 symbiotic stars are only preliminary. The infrared Ca II triplet of most symbiotic stars clearly varies between the different observing runs. The different luminosity classes given to the same symbiotic star are probably caused by the variability of the lines of ionized elements,: while in some cases they are affected by a low metal abundance.

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.

Very High Resolution Ultraviolet Spectroscopy of a Chemically Peculiar Star: Results of the χ Lupi Pathfinder Project

We summarize here the results of a major eight-year investigation of the extraordinarily detailed UV spectrum of the sharp-lined, nonmagnetic, main-sequence, chemically peculiar star χ Lupi (B9.5p HgMn + A2 Vm). The UV observations are composed of 345 Å of the spectrum acquired with the Goddard High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope at an average resolution of 0.023 Å. The complete set of echelle spectrograms is presented as an atlas in a companion paper. These data were supplemented by optical-wavelength spectra obtained at the Anglo-Australian Telescope. Quantitatively accurate analysis and theoretical interpretation of these data required major improvements in the accuracy and completeness of available atomic data—wavelengths, transition probabilities, hyperfine structure, and isotope shifts—for the lowest ionization states of many elements. A large, international group of theoretical and experimental atomic physicists has collaborated in this investigation, and their results are summarized or referenced in this paper. In turn, the GHRS observations of χ Lupi have become a useful source of data for atomic spectroscopy, displaying many transitions that are difficult to observe in a laboratory setting. Measured abundances or upper limits are presented for 72 ions of 51 chemical elements, spanning the periodic table. We have confirmed and refined previously identified isotopic abundance anomalies in mercury and platinum and have discovered similar isotopic anomalies in thallium and, tentatively, in lead. Large discrepancies among the LTE abundances derived, using a chemically homogeneous model atmosphere, from two or three ionization states of the same element are found to be common. In some cases these are due to departures from LTE in the ionization equilibria, but the largest such discrepancies probably result from chemical stratification within the photosphere. We find qualitative trends in the abundances of the elements that clearly signify radiatively driven diffusion and gravitational settling as the primary mechanism producing abundance anomalies. However, detailed non-LTE diffusion calculations for mercury and thallium show that there is insufficient unsaturated radiative force within the chemically enriched atmosphere to sustain the observed huge overabundances of these elements in equilibrium with gravity. Either other hydrodynamic processes, such as slow mass motions or unexpectedly strong stellar winds must assist radiation pressure in supporting the enriched material, or the observed abundance patterns simply provide a snapshot in time of a nonequilibrium, time-variable phenomenon.

Abundance Analyses of Field RV Tauri Stars. IV. AD Aquilae, DS Aquarii, V360 Cygni, AC Herculis, and V453 Ophiuchi

Abundance analyses are presented and discussed for five RV Tauri variables. Three stars—DS Aqr, V360 Cyg, and V453 Oph—are RV C stars by spectroscopic classification, i.e., metal lines are weak. They are shown to be metal poor with [Fe/H] from -1.0 to -2.2 with normal relative abundances of other elements. By contrast, AD Aql and AC Her are RV B stars with an odd abundance pattern: elements that condense into grains at a high temperatures are underabundant (i.e., [Fe/H] = -2.1 for AD Aql) but elements with a low condensation temperatures are much less underabundant (i.e., [S/H] = 0.0 and [Zn/H] = -0.1 for AD Aql). This abundance pattern is ascribed to a separation of dust and gas in the upper atmosphere of the star. The present analyses with previously published results are used to investigate the systematics of the dust-gas separation in RV Tauri variables. The process is apparently inoperative in stars with an initial metallicity of about [Fe/H] -1.0; RV C stars and similar variables in globular clusters are immune to the dust-gas separation. The process achieves more severe effects in RV B than in RV A stars. The strength of the abundance anomalies attributed to dust-gas separation is not correlated with reported infrared excesses. After correction for the effects of the dust-gas separation, there is no strong evidence from the abundances that evolution along the AGB and experience of the third dredge-up preceded the formation of the majority of the RV Tauri variables.

Extremely Metal Poor Stars. VI. The Heterogeneous Class of Lithium‐depleted Main‐Sequence Turnoff Dwarfs

We present abundances of 14 elements in the metal-poor main-sequence turnoff star G122-69 , which has at most one-tenth the Li abundance observed in most other stars of similar temperature and metallicity. The deficiency of Li is significant because of this element's role in constraining primordial fnucleosynthesis and the baryon density of the universe. Although we have examined elements of intermediate atomic mass, in the iron peak, and heavy neutron capture species, we find no other abundance anomalies in G122-69. Nor do we find any evidence of radial velocity variation at the 1 km s-1 level.

Physical Structure of a Coronal Streamer in the Closed‐Field Region as Observed from UVCS/SOHOand SXT/Yohkoh

We analyze a coronal helmet streamer observed on 1996 July 25 using instruments aboard two solar spacecraft, the Ultraviolet Coronagraph Spectrometer (UVCS) on board Solar and Heliospheric Observatory (SOHO) and the Soft X-Ray Telescope (SXT) on board Yohkoh. We derive temperatures and electron densities at 1.15 R☉ from SXT/Yohkoh observations. At this height, the streamer temperature is about log T (K) = 6.28 ± 0.05, and the electron density is about log ne(cm-3) = 8.09 ± 0.26, while at 1.5 R☉ a temperature of log T (K) = 6.2 and a density of log ne(cm-3) = 7.1 are obtained by UVCS/SOHO. Within the measurement uncertainty this suggests a constant temperature from the base of the streamer to 1.5 R☉. Electron density measurements suggest that the gas in the streamer core is close to hydrostatic equilibrium. Comparison with potential field models for the magnetic field suggests a plasma β larger than 1 in the closed-field region in the streamer. In deriving electron densities and temperatures from the SXT/Yohkoh data, we include the effects of abundance anomalies on the SXT filter response. We use the elemental abundances derived from the UVCS/SOHO observations to estimate the first ionization potential and gravitational settling effects. We then give the set of abundances for the solar corona, which agrees with our observations. In addition, we analyzed the SXT data from 6 consecutive days. We found that from 1996 July 22 to July 27, the physical properties of the streamer are nearly constant. We conclude that we may be observing the same loop system over 6 days.

Consistent Evolution of F Stars: Diffusion, Radiative Accelerations, and Abundance Anomalies

Consistent stellar evolution models of F stars (1.1-1.5 M☉) are calculated with radiative forces, opacities, and diffusion for all elements included in OPAL's opacity tables. The opacities and radiative forces are continuously recomputed during evolution from OPAL's monochromatic data (~1.5 Gbyte) in order to include all effects of abundance changes due to diffusion and nuclear evolution. TOPbase is also used for radiative accelerations. Iron surface overabundances occur in stars more massive than 1.3 M☉. Local overabundances of iron peak elements increase the Rosseland opacity in a region at the base of the convection zone by a factor of 3-6; this increases the mass of the convective zone by up to a factor of 5. It is important to follow Cr, Mn, and Ni independently of Fe, since they peak at different temperatures within the star. The predicted abundance anomalies are much larger than observed in most F-type stars of open clusters. This suggests that atomic diffusion is not the only process responsible for the Li gap in open clusters. The predicted iron peak element overabundances indicate trends that are compatible with those observed in Fm stars. They however tend to be larger than the observed overabundances, leaving room for some perturbing hydrodynamical process. The present models, devoid of free parameters, are a necessary step in constraining the additional hydrodynamical processes required to better reproduce observed surface abundances. Since the abundances of 28 elements are calculated, one may have 27 constraints on stellar hydrodynamics, once the relative abundances of all species have been determined observationally.

Consistent Solar Evolution Model Including Diffusion and Radiative Acceleration Effects

The solar evolution has been calculated including all the effects of the diffusion of helium and heavy elements. Monochromatic opacities are used to calculate radiative accelerations and Rosseland opacities at each evolution time step, taking into account the local abundance changes of all important (21) chemical elements. The OPAL monochromatic data are used for the opacities and the radiative accelerations. The Opacity Project data are needed to calculate how chemical species and electrons share the momentum absorbed from the radiation flux. A detailed evaluation of the impact of atomic diffusion on solar models is presented. On some elements thermal diffusion adds approximately 50% to the gravitational settling velocity. While gravitational settling had been included in previous solar models, this is the first time that the impact of radiative accelerations is considered. Radiative accelerations can be up to 40% of gravity below the solar convection zone and thus affect chemical element diffusion significantly, contrary to current belief. Up to the solar age, the abundances of most metals change by 7.5% if complete ionization is assumed, but by 8.5%-10% if detailed ionization of each species is taken into account. If radiative accelerations are included, intermediate values are obtained. Diffusion leads to a change of up to 8% in the Rosseland opacities, compared to those of the original mixture. Most of this effect can be taken into account by using tables with several values of Z. If one isolates the effects of radiative accelerations, the abundance changes they cause alter the Rosseland opacity by up to 0.5%; the density is affected by up to 0.2%; the sound speed is affected by at most 0.06%. The inclusion of radiative accelerations leads to a reduction of 3% of neutrino fluxes measured with 37Cl detectors and 1% measured with 71Ga detectors. The partial transformation of C and O into N by nuclear reactions in the core causes a ~1% change in the opacities that cannot be modeled by a change in Z alone. The evolution is allowed to proceed to 1010 yr in order to determine the impact at the end of the main-sequence life of solar-type stars. It is found that immediately below the convection zone, the radiative acceleration on some iron peak elements is within a few percent of gravity. The abundance anomalies reach 18% for He in the convection zone but are kept within 12% and 15% for most because of grad. They would have reached 18% in the absence of grad.

Fundamental Parameters of CP Stars

The quest for accurate fundamental parameters (M, R, Teff log g, v sin i) of CP stars has been a lasting problem ever since the nature and origin of these stars has been discussed. The main reason for the difficulty lies in the anomalous colours of these stars, linked with their abundance anomalies which are responsible for complicated blocking and backwarming effects. In the following, we review the Teff determinations proposed so far, and show how the Hipparcos results improve the radius and log g determinations of nearby Ap stars.

Stellar Evolution, Particle Transport, and the CP Phenomenon

In the light of recent results from helioseismology, the abundance anomalies seen at the surface of Bp, Ap, Am and Fm stars appear as the measurable superficial effect of a process that has much more general impact.

FLIERs and Other Microstructures in Planetary Nebulae. III.

Long-slit spectroscopic observations along the major axes of four planetary nebulae with interesting jets and FLIERs (Hb 4, IC 4634, NGC 6369, and NGC 7354) have been conducted with the Palomar 5 m telescope. Chemical abundances and physical conditions (n, T) in microstructures were derived along their structural axes. No evidence of conspicuous shock activity or N/O abundance anomalies is seen in most cases, unlike some earlier studies of similar features in other planetary nebulae. Microstructures seem to be a heterogeneous class of structures aside from their low ionization and generally supersonic motions.

Extremely Metal‐Poor Stars. III. The Lithium‐depleted Main‐Sequence Turnoff Dwarfs

We present abundances of 14 elements for the metal-poor, near-main-sequence turnoff stars G66-30, G139-8, and G186-26, which are well known to possess less than 1/5-1/10 the value of Li/H observed in field halo Spite Plateau dwarfs and are thought by many to have been produced in the currently accepted standard big bang cosmology. The stars have [Fe/H] = -1.53, -2.24, and -2.68, respectively, and for these values their collective abundance patterns are not abnormal. That is to say, there is no common abundance abnormality that one might associate with their Li deficiencies. In G186-26, we find an overabundance of the heavy neutron-capture elements that increases with atomic mass. We measure [Ba/Fe] = +0.35, whereas most stars of this metallicity have [Ba/Fe] ~ -0.5, together with smaller enhancements for Sr and possibly Y. Such supersolar, neutron-capture, element abundances, however, are exhibited by ~25% of stars having [Fe/H] ~ -2.7. No such enhancement is found in G66-30 or G139-8, and the simplest explanation for the data of the three stars is that there is no connection between Li depletion and the abundance patterns of the heavy neutron-capture elements. G66-30 and G186-26 appear to be multiple systems, and we discuss the possible role of binarity in producing the observed Li depletion. We concur with J. A. Thorburn that this group of objects is not the progeny of blue stragglers, and we discuss the alternative that some of the Li-depleted stars may be the Population II counterparts of the disk population subgiant CH, barium dwarf, and F str λ4077 stars. As suggested by others, the most likely explanation for the abundance anomalies in these objects, which includes universal Li deficiency, involves mass transfer across a binary during the asymptotic giant branch evolutionary phase of the erstwhile primary of the system. We note that while such mass transfer might be expected to produce Li depletion, variations in C, N, and the heavy neutron-capture elements are possible but not all necessary. Neither G66-30 nor G139-8 shows enhancement of C or the neutron-capture elements, and no information is available for N. Further work is necessary to settle the issue. If mass transfer is responsible for Li depletions, such stars justifiably could be excluded from investigations of the primordial Li abundance. In the absence of evidence for such mass transfer, however, the possibility remains that some of these objects, if not all, are extreme examples of a process that has affected all Plateau stars.

Nonvariability among λ Bootis stars

With asteroseismic techniques it is possible to investigate the interior and the evolutionary status of stars via their frequency spectrum. Both information would be very much needed for Bootis stars, a group of metal- poor Population I, A-type stars, since no conclusive the- ory exists explaining the observed abundance anomalies. Geneva and Stromgren photometry place these stars in- side the classical instability strip or at least very close to it. We therefore have started an extensive photometric survey for pulsation in Bootis stars and have discovered so far 13 new variables. In this paper we present results for stars which presum- ably are constant, because we are able to establish only an upper level for possible variability. A typical noise level of 3 mmag for Stromgren b was achieved in the relevant fre- quency domain up to 100 d 1 . Considering the given noise level of our survey, we con- clude that at least 50% of all investigated Bootis stars inside the instability strip are pulsating, making this group remarkable compared to stars with similar spectral types. This may suggest that a low (surface) metallicity has an influence on the pulsation behavior of stars. New models, as well as further observations are needed to clarify the Bootis phenomenon using asteroseismic techniques.

Oxygen λ7771–5 Triplet in Hyades A-Type Stars: A Quest for the Key to the Am Phenomenon

An extensive profile-fitting analysis of the OI 7771–5 triplet (along with the nearby FeI line) was carried out for thirty-one A-type stars, eighteen of them belonging to the Hyades group, in order to clarify the key factor which plays the decisive role in the appearance of abundance anomalies of metallic-line stars (Am), by examining whether any meaningful relation exists between the microturbulence (ξ), the oxygen abundance (log ∊O), the iron abundance (log ∊Fe), the projected rotational velocity (ve sin i), and the stellar atmospheric parameters. It was clearly found based on the Hyades results that the abundance of O/Fe, both being good indicators of the Am phenomenon, because of the negative/positive correlation with the metallicity estimated from colors, progressively increases/decreases (i.e., toward the direction of disappearance of anomaly) with an increase in ve sin i as well as with a decrease in log g. It was thus concluded that the most important driver of the Am phenomenon, producing a significant deficiency/excess of O/Fe, is the rotational velocity which appears to be responsible for not only the triggering, but also the extent of the peculiarity. Meanwhile, the observed log g-dependence of the abundance e anomalies is considered to be nothing but superficial, which may be interpreted as being due to the correlation between log g and ve sin i, i.e., rotation-induced lowering of the effective gravity.

HR 6094: A Young Solar-Type, Solar-Metallicity Barium Dwarf Star

The young solar-type star HR 6094 is found to be a barium dwarf, overabundant in the s-process elements as well as deficient in C. It is a member of the solar-metallicity, 0.3 Gyr old Ursa Major kinematical group. Measurements of radial velocity and ultraviolet flux do not support the attribution of such abundance anomalies to an unseen degenerate companion. A common proper motion, V = 10, DA white dwarf (WD), located 5360 AU away, however, strongly supports the explanation of the origin of this barium star by the process of mass transfer in a binary system, in which the secondary component accreted matter from the primary one (now the WD) when it was an asymptotic giant branch (AGB) star self-enriched in the s-process elements. The membership in the UMa group of another s-process-rich and C-deficient star, HR 2047, suggests that these stars could have formed a multiple system in the past, which was disrupted by the mass-loss episode of the former AGB star. Their [C/Fe] deficiency could be explained by the action of the hot-bottomed envelope burning process in the late AGB, thereby reconverting it from a C-rich to an O-rich star, depleting C while enriching its envelope with Li and neutron capture elements. This is the first identification of the barium phenomenon in a near-zero-age star, besides being the first barium system in which the remnant of the late AGB star responsible for the heavy-element enrichment may have been directly spotted.

Abundance anomalies in globular clusters

The galactic globular clusters presumably formed rapidly as high density concentrations at the centres of extensive star forming regions and the natural expectation is that they will be chemically homogeneous. In general, this is confirmed by observation — most globular clusters are extremely homogeneous with regard to most elements. (There are two exceptions: ω Cen and M22 both have intrinsic internal abundance ranges. While of considerable interest, e.g. Norris et al. 1996, this type of “abundance anomaly” will not be discussed here). But we have known for more than twenty years that the surface abundances, which are what we observe, of elements such as C, N and O can vary substantially from red giant to red giant within an individual globular cluster. Indeed it has become clear that “abundance anomalies” of this type are common in the galactic globular cluster population. Briefly, the observed anomalies can be summarized as: (i) the “anomalous” stars are depleted in C and enhanced in N. Depletions of O also often accompany the depletions in C. (ii) The C, N and O variations are usually accompanied by enhancements of Na and Al and when the O depletion and the Al enhancement are both large, Mg is depleted. No other elements, including r- and s-process indicators, vary.

Reddenings, Metallicities, and Possible Abundance Anomalies in Young Globular Clusters

Reddenings, Metallicities, and Possible Abundance Anomalies in Young Globular Clusters

The effect of rotation on RGB surface abundances

Pop II field and globular cluster giant stars (and, to a less extent, Pop I giants) exhibit chemical anomalies which are not predicted by standard stellar evolution theory. Two hypotheses have been proposed to explain these abundance variations, namely the primordial and the evolutionary explanations. A primordial origin for intracluster abundance anomalies (see e.g. Cottrel &amp; Da Costa 1981) would be related to inhomogeneities in the cluster material due to pollution by a prior generation of massive stars. In the evolutionary hypothesis, abundance variations would be due to nuclear and mixing processes internal to the giant stars themselves. Many good reviews exist on the subject (see e.g. Briley et al. 1994a, Kraft 1994), in which observational evidence supporting both hypotheses are presented. In this conference, Da Costa recalls the most recent observational data, and some excellent poster contributions bring essential clues to the subject.

Elemental abundance analyses with Complejo Astronómico El Leoncito REOSC echelle spectrograms. I. κ Cancri, HR 7245, and ξ Octantis

Elemental abundances are derived for three sharp-lined stars Cnc, HR 7245, and Oct using REOSC echelle spectrograms obtained at CASLEO. Comparisons are made with published equivalent widths. The derived abundances for Cnc and Oct are slightly larger than those obtained with other high dispersion spectrographs. HR 7245 exhibits a pattern of abundance anomalies similar to other Mercury-Manganese stars. The spectra of the two HgMn stars in the 4640 5100 region exhibit an interesting and useful variety of lines which can be used to supplement analyses of the photographic region.