Photospheric Chemical Abundances Research Articles

White Dwarf Photospheric Abundances in Cataclysmic Variables. III. Five Dwarf Novae with an Evolved Secondary Donor Star* * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of University for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

In the last two decades infrared spectroscopy has brought mounting evidence, in the form of weak CO features together with enhanced 13C, of the presence of CNO-processed material in the atmosphere of the donor star of some nonmagnetic cataclysmic variables. Some of these donors also exhibit a temperature too high for their binary orbital period, indicating that they evolved off the main sequence before mass transfer began. The ultraviolet spectra of evolved donor systems exhibit strong N v (λ1240) and the almost complete absence of C iv (λ1550) emission lines. We present here an archival Hubble Space Telescope ultraviolet spectral analysis of five systems containing an evolved donor star. We derive their white dwarf masses, effective temperatures, and photospheric chemical abundances. The [N/C] ratio is very large, of the order 102–103 (in solar units) for the short-period systems V485 Cen, GZ Cet, and QZ Ser, and of the order 20 for the longer-period systems HS 0218 and EY Cyg. Silicon ([S/H]) is solar for GZ Cet and QZ Ser, suprasolar for V485 Cen, and subsolar for HS 0218 and EY Cyg. We also derive abundances of O, Mg, Al, Si, P, S, Ca, and Fe, which vary from system to system. The abundances we derived are consistent with the more evolved nature of the donor star (metal enriched, hydrogen depleted). It is impossible to confirm hydrogen deficiency for these systems, since at these wavelengths (1100–2000 Å) white dwarf spectra show little dependency on the [He/H] ratio, unless it is extremely large ([He/H] ≫ 1).

Understanding dust production and mass loss in the AGB phase using post-AGB stars in the Magellanic Clouds

Context. The asymptotic giant branch (AGB) phase of evolution in low- and intermediate-mass stars is governed by poorly understood physical mechanisms, such as convection, mixing, dust production and mass loss, which play a crucial role in determining the internal structure and the evolution of these stars. The spectra of post-asymptotic giant branch (post-AGB) stars hold critical chemical fingerprints that serve as exquisite tracers of the evolution, nucleosynthesis, and dust production during the AGB phase. Aims. We aim to understand the variation in the surface chemistry that occurs during the AGB phase by analysing results from observations of single post-AGB stars in the Magellanic Clouds. We also aim to reconstruct dust-formation processes, which are active in the circumstellar envelope of AGB stars, occurring towards the end of the AGB phase and during the subsequent course of evolution when contraction to the post-AGB has begun. Methods. We study likely single post-AGB sources in the Magellanic Clouds that exhibit a double-peaked (shell-type) spectral energy distribution (SED). We interpret their SED by comparing with results from radiative transfer calculations to derive the luminosity and the dust content of the individual sources. Additionally, we compare the observationally derived stellar parameters and the photospheric chemical abundances of the target sample with results from stellar evolution modelling of AGB and post-AGB stars. This allows for the characterization of the individual sources in terms of the initial mass and formation epoch of the progenitors. The theoretically derived dust mineralogy and optical depth is used to assess when dust formation ceases and to determine the propagation velocity of the dust-gas system during post-AGB evolution. Results. We find that amongst our target sample of 13 likely single post-AGB stars with shell-type SED, eight objects are carbon stars descending from ∼1−2.5 M⊙ progenitors. Five of the 13 objects are of lower mass, descending from M < 1 M⊙ stars. Based on the dust mineralogy, we find that these five stars are surrounded by silicate dust, and thus failed to become carbon stars. The dust optical depth and the luminosity of the stars are correlated, owing to the faster evolutionary timescale of brighter stars, which makes the dusty layer closer to the central object. From our detailed analysis of the SEDs, we deduce that the dust currently observed around post-AGB stars was released after the onset of the central star contraction and an increase in the effective temperature to ∼3500−4000 K.

Orbital and atmospheric parameters of two wide O-type subdwarf binaries: BD−11o162 and Feige 80

Context. There are 23 long-period binary systems discovered to date that contain a B-type hot subdwarf whose orbital parameters have been fully solved. They evolve into O-type subdwarfs (sdO) once the helium burning transitions from the core to the He shell. Their study will help constraint parameters on the formation and evolution of these binaries and explain some of their puzzling features. Aims. In this study, we aim to solve orbital and atmospheric parameters of two long-period sdO binaries and, for the first time, investigate the chemical composition of their main-sequence (MS) companions. Methods. HERMES high-resolution spectra are used to obtain radial velocities and solve their orbits. The GSSP code is used to derive the atmospheric parameters and photospheric chemical abundances of the MS companions. Stellar evolution models (MIST) are fitted to the companion atmospheric parameters to derive masses. Results. The orbital and atmospheric parameters have been fully solved. Masses of the companions and the sdOs were obtained. The photospheric chemical abundances of the MS stars for elements with available lines in the optical range have been derived. They match general trends expected from Galactic chemical evolution but show a depletion of yttrium in both systems and an enrichment of carbon in the BD−11o162 MS. Conclusions. In the bimodal period-eccentricity diagram, the orbital parameters indicate that Feige 80 matches the same correlation as the majority of the systems. The analysis suggests that Feige 80 has a canonical subdwarf mass and followed a standard formation channel. However, BD−11o162 is an exceptional system with a lower mass. It also shows a C overabundance, which could be caused by a higher progenitor mass. The Y depletion in the MS companions could indicate the existence of a circumbinary disk in these systems’ pasts. Nevertheless, a chemical analysis of a larger sample is necessary to draw strong conclusions.

Discovery of new mercury–manganese stars, including a fast rotator

ABSTRACT The origin of mercury–manganese (HgMn) stars is still poorly understood and the statistical analysis of an extensive sample of well characterized objects would be very valuable. Nevertheless, the compilation of a clean and reliable list of all known HgMn stars, and complete characterization of those objects, is still a pending issue. We report the identification of 13 new HgMn stars from our own spectroscopic observations and from publicly available spectral libraries. In addition, we perform a thorough literature search to give a list of 46 confirmed HgMn stars (plus a further nine suspected HgMn stars) discovered over the last years or neglected by other compilations. When required, we performed specific observations to confirm doubtful cases. For the 13 newly identified HgMn stars we derived photospheric chemical abundances by spectral synthesis. Besides Hg and Mn, which are overabundant in the whole sample, P, Ga, and Xe are overabundant in the hottest stars of the sample, and Sr, Zr, and Pt among the coolest; Y is present in most stars of the sample. Three of these new HgMn stars are members of open clusters: HD 93549 in IC 2602, HD 96473 in NGC 3532, and HD 322659 in NGC 6281. Furthermore, our sample includes HD 145842, an object remarkable for its high rotation. In fact, its projected rotational velocity of ve sin i = 124 km s−1 is the highest measured in any HgMn star and is more than 30 per cent above the theoretical limit predicted by the diffusion theory, which makes HD 145842 a challenging test case for the current models.

The EUV spectrum of the Sun: Quiet- and active-Sun irradiances and chemical composition

We benchmark new atomic data against a selection of irradiances obtained from medium-resolution quiet-Sun spectra in the extreme ultraviolet (EUV), from 60 to 1040 Å. We used as a baseline the irradiances measured during solar minimum on 2008 April 14 by the prototype (PEVE) of the Solar Dynamics Observatory Extreme ultraviolet Variability Experiment (EVE). We took into account some inconsistencies in the PEVE data, using flight EVE data and irradiances we obtained from Solar and Heliospheric Observatory (SoHO) Coronal Diagnostics Spectrometer (CDS) data. We performed a differential emission measure and find overall excellent agreement (to within the accuracy of the observations, about 20%) between predicted and measured irradiances in most cases, although we point out several problems with the currently available ion charge-state distributions. We used the photospheric chemical abundances of Asplund et al. (2009, ARA&A, 47, 481). The new atomic data are nearly complete in this spectral range for medium-resolution irradiance spectra. Finally, we used observations of the active Sun in 1969 to show that the composition of the solar corona up to 1 MK is nearly photospheric in this case as well. Variations of a factor of 2 are present for higher-temperature plasma, which is emitted within active regions. These results are in excellent agreement with our previous findings.

A volume-limited survey of mCP stars within 100 pc – I. Fundamental parameters and chemical abundances

We present the first results of a volume-limited survey of main sequence (MS) magnetic chemically peculiar (mCP) stars. The sample consists of all identified intermediate-mass MS stars (mCP and non-mCP) within a heliocentric distance of $100\,{\rm pc}$ as determined using Hipparcos parallaxes. The two populations are compared in order to determine the unique properties that allow a small fraction of MS stars with masses $\gtrsim1.4\,M_\odot$ to host strong, large scale magnetic fields. A total of 52 confirmed mCP stars are identified using published magnetic, spectroscopic, and photometric observations along with archived and newly obtained spectropolarimetric (Stokes $V$) observations. We derive the fundamental parameters (effective temperatures, luminosities, masses, and evolutionary states) of the mCP and non-mCP populations using homogeneous analyses. A detailed analysis of the mCP stars is performed using the {\sc llmodels} code, which allows observed spectral energy distributions to be modeled while incorporating chemical peculiarities and magnetic fields. The surface gravities and mean chemical abundances are derived by modelling averaged spectra using the {\sc gssp} and {\sc zeeman} spectral synthesis codes. Masses and stellar ages are derived using modern, densely calculated evolutionary model grids. We confirm a number of previously reported evolutionary properties associated with mCP stars including a conspicuously high incidence of middle-aged MS stars with respect to the non-mCP subsample; the incidence of mCP stars is found to sharply increase with mass from $0.3$~per~cent at $1.5\,M_\odot$ to $\approx11$~per~cent at $3.8\,M_\odot$. Finally, we identify clear trends in the mean photospheric chemical abundances with stellar age.

Lighting up stars in chemical evolution models: the CMD of Sculptor

We present a novel approach to draw the synthetic color-magnitude diagram of galaxies, which can provide - in principle - a deeper insight in the interpretation and understanding of current observations. In particular, we `light up' the stars of chemical evolution models, according to their initial mass, metallicity and age, to eventually understand how the assumed underlying galaxy formation and evolution scenario affects the final configuration of the synthetic CMD. In this way, we obtain a new set of observational constraints for chemical evolution models beyond the usual photospheric chemical abundances. The strength of our method resides in the very fine grid of metallicities and ages of the assumed database of stellar isochrones. In this work, we apply our photo-chemical model to reproduce the observed CMD of the Sculptor dSph and find that we can reproduce the main features of the observed CMD. The main discrepancies are found at fainter magnitudes in the main sequence turn-off and sub-giant branch, where the observed CMD extends towards bluer colors than the synthetic one; we suggest that this is a signature of metal-poor stellar populations in the data, which cannot be captured by our assumed one-zone chemical evolution model.

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.

Absolute dimensions of detached eclipsing binaries – III. The metallic-lined system YZ Cassiopeiae

The bright binary system YZ Cassiopeiae is a remarkable laboratory for studying the Am phenomenon. It consists of a metallic-lined A2 star and an F2 dwarf on a circular orbit, which undergo total and annular eclipses. We present an analysis of 15 published light curves and 42 new high-quality échelle spectra, resulting in measurements of the masses, radii, effective temperatures and photospheric chemical abundances of the two stars. The masses and radii are measured to 0.5 per cent precision: MA = 2.263 ± 0.012 M⊙, MB = 1.325 ± 0.007 M⊙, RA = 2.525 ± 0.011 R⊙ and RB = 1.331 ± 0.006 R⊙. We determine the abundance of 20 elements for the primary star, of which all except scandium are supersolar by up to 1 dex. The temperature of this star (9520 ± 120 K) makes it one of the hottest Am stars. We also measure the abundances of 25 elements for its companion (Teff = 6880 ± 240 K), finding all to be solar or slightly above solar. The photospheric abundances of the secondary star should be representative of the bulk composition of both stars. Theoretical stellar evolutionary models are unable to match these properties: the masses, radii and temperatures imply a half-solar chemical composition (Z = 0.009 ± 0.003) and an age of 490–550 Myr. YZ Cas therefore presents a challenge to stellar evolutionary theory.

Chemical peculiarities in magnetic and non-magnetic pre-main sequence A and B stars

AbstractIn A- and late B-type stars, strong magnetic fields are always associated with Ap and Bp chemical peculiarities. However, it is not clear at what point in a star's evolution those peculiarities develop. Strong magnetic fields have been observed in pre-main sequence A and B stars (Herbig Ae and Be stars), and these objects have been proposed to be the progenitors of Ap and Bp stars. However, the photospheric chemical abundances of these magnetic Herbig stars have not been studied carefully, and furthermore the chemical abundances of 'normal' non-magnetic Herbig stars remain poorly characterized. To investigate this issue, we have studied the photospheric compositions of 23 Herbig stars, four of which have confirmed magnetic fields. Surprisingly, we found that half the non-magnetic stars in our sample show λ Bootis chemical peculiarities to varying degrees. For the stars with detected magnetic fields, we find one chemically normal star, one star with λ Boo peculiarities, one star displaying weak Ap/Bp peculiarities, and one somewhat more evolved star with somewhat stronger Ap/Bp peculiarities. These results suggests that Ap/Bp peculiarities are preceded by magnetic fields, and that these peculiarities develop over the pre-main sequence lives of A and B stars. The incidence of λ Boo stars we find is much higher than that seen on the main sequence. We argue that a selective accretion model for the formation of λ Boo peculiarities is a natural explanation for this remarkably large incidence.

Abundances and search for vertical stratification in the atmospheres of four HgMn stars

Using high resolution, high-S/N archival UVES spectra, we have performed a detailed spectroscopic analysis of 4 chemically peculiar HgMn stars (HD 71066, HD 175640, HD 178065 and HD 221507). Using spectrum synthesis, mean photospheric chemical abundances are derived for 22 ions of 16 elements. We find good agreement between our derived abundances and those published previously by other authors. For the 5 elements that present a sufficient number of suitable lines, we have attempted to detect vertical chemical stratification by analyzing the dependence of derived abundance as a function of optical depth. For most elements and most stars we find no evidence of chemical stratification with typical 3\sigma upper limits of \Delta\log N_elem/N_tot~0.1-0.2 dex per unit optical depth. However, for Mn in the atmosphere of HD 178065 we find convincing evidence of stratification. Modeling of the line profiles using a two-step model for the abundance of Mn yields a local abundance varying approximately linearly by ~0.7 dex through the optical depth range log \tau_5000=-3.6 to -2.8.

Fast-ion-beam laser-fluorescence measurements of oscillator strengths in the lanthanides

Atomic oscillator-strength data are of great interest to astronomers studying photospheric chemical abundances, stellar interiors and nucleosynthesis. We review the classic methods of measurement and then discuss our approach, which is based on fast-ion-beam laser-fluorescence techniques for both lifetime and branching-fraction measurements. Recently obtained large data sets in singly ionized lanthanides are discussed, with several examples illustrating the major advantages of this method: unambiguous identification of the upper level of a transition and greatly reduced spectral line blending.

Abundances anomalies and meridional circulation in horizontal branch stars

Context. Photospheric chemical abundances on the horizontal branch (HB) show some striking variations with effective temperature (Teff). The most straightforward explanation is that these anomalies develop through diffusion processes, in particular gravitational settling and radiative levitation. However, the abrupt disappearance of strong abundance anomalies as one moves below about 11 000 K on the HB suggests that another factor plays an important role. Aims. We test an extension to the HB of the diffusion model for main-sequence HgMn stars, where strong anomalies can only develop in the slower rotators. In these rotators the gravitational settling of helium leads to the disappearance of its superficial convection zone, so that chemical separation by radiative levitation can occur all the way to the photosphere. Methods. More specifically, we calculate the critical rotational velocity at which He settling is prevented by rotationally-induced meridional circulation, in a suite of stellar models spanning the zero-age HB. Helium settling serves as the measure of the atomic diffusion of all species. Results. Our abundance evolution calculations show that, for models with Teff less than about 11 500 K, corresponding to stars typically observed with the same metal composition as giants, meridional circulation is efficient enough to suppress He settling for rotational velocities, in good agreement with observed values. Once the meridional circulation profile of a star rotating as a near rigid body has been adopted, no adjustable parameter is involved. Conclusions. The Teff dependence of abundance anomalies observed on the HB can be explained by atomic diffusion transport if one introduces the competition of meridional circulation with the observed Teff dependence of rotation velocity of HB stars.

Stellar Parameters and Elemental Abundances of Late-G Giants

Abstract The properties of 322 intermediate-mass late-G giants (comprising 10 planet-host stars) selected as the targets of the Okayama Planet Search Program, many of which are red-clump giants, were comprehensively investigated by establishing their various stellar parameters (atmospheric parameters, including turbulent velocity fields, metallicity, luminosity, mass, age, projected rotational velocity, etc.), and their photospheric chemical abundances for 17 elements, in order to study their mutual dependence, connection with the existence of planets, and possible evolution-related characteristics. The metallicity distribution of planet-host giants was found to be almost the same as that of non-planet-host giants, making marked contrast to the case of planet-host dwarfs tending to be metal-rich. Generally, the metallicities of these comparatively young (typical age of $\sim 10^{9}$ yr) giants tend to be somewhat lower than those of dwarfs at the same age, and super-metal-rich ([Fe$/$H] $\gt$ 0.2) giants appear to be lacking. Apparent correlations were found between the abundances of C, O, and Na, suggesting that the surface compositions of these elements have undergone appreciable changes due to dredge-up of H-burning products by evolution-induced deep envelope mixing, which becomes more efficient for higher mass stars.

An abundance analysis of a chemically peculiar B star – JL 87

Aims. The aim of this study is to understand the nature and origin of a chemically peculiar star JL 87 by measuring its physical parameters and chemical abundances. Methods. Physical parameters – effective temperature, surface gravity and helium abundance were measured from a moderate resolution optical spectrum using fully line-blanketed LTE model atmospheres. The effective temperature and extinction were verified by comparing FUSE, IUE spectrophotometry and optical/IR broadband photometry with theoretical flux distributions from LTE model atmospheres. The photospheric chemical abundances were measured from a high-resolution optical spectrum using LTE model atmospheres and spectral synthesis. Results. On the basis of its physical parameters and chemical abundances, we confirm that JL 87 is a chemically peculiar subluminous B star. It is significantly cooler, has a lower surface gravity and is more helium-rich than previously believed. It is moderately enriched in carbon and nitrogen, but its overall metallicity is slightly subsolar. Conclusions. The shallow-mixing model of a late core-flash on a white-dwarf cooling track currently provides the most consistent agreement with the observable properties of JL 87.

A–type stars: evolution, rotation and binarity

We discuss the internal structure of stars in the mass range 1.5 to 4 from the PMS to the subgiant phase with a particular emphasis on the convective core and the convective superficial layers. Different physical aspects are considered such as overshooting, treatment of convection, microscopic diffusion and rotation. Their influence on the internal structure and on the photospheric chemical abundances is briefly described.The role of binarity in determining the observed properties and as a tool to constrain the internal structure is also introduced and the current limits of theories of orbital evolution and of available binary datasets are discussed.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

White Dwarfs in Cataclysmic Variables

ABSTRACTPrior to the last 15 years, the literature on cataclysmic variables (CVs) contained little on the properties of the underlying white dwarf accreter other than estimates of their masses. They were regarded simply as nondescript potential wells for studies of accretion disk structure and stability. Estimates of their masses and associated core compositions were discussed, but only in the context of thermonuclear runaway theory. With the advent of space spectroscopy, especially HST, IUE, EUVE, and HUT, direct spectroscopic observations of exposed white dwarfs in CVs were carried out during dwarf nova quiescence or low brightness states of nova‐like variables and magnetic CVs when accretion rates were very low. This review covers new insights and physical properties of white dwarfs in nonmagnetic and magnetic CVs, including surface temperatures, heating and cooling measurements, rotational velocities, CV white dwarf masses (including Einstein redshift masses), photospheric chemical abundances (including composition relics of ancient novae), accretion belts, the physics of accretion heating, and long‐term CV evolution. For an ensemble of 37 CV degenerates with secure temperatures, the average Teff=20,800 K, for nonmagnetic CV degenerates Teff=24,100 K, and for magnetic CV degenerates Teff=16,400 K. The lowest Teff values are associated with magnetic CVs and with systems whose orbital periods are less than 2 hr. Thermal e‐folding times of the white dwarf envelope in response to accretion heating are in the range 6–600 days, photospheric chemical abundances range from moderately subsolar to greatly above solar, and rotational velocities lie within the range 50 km s−1<Vsin i<1200 km s−1. If pre‐CV white dwarfs had time to cool to 10-3L⊙ prior to the onset of Roche lobe overflow, then their average lower limit lifetime is 2.5 × 108 yr, and they have been accretion heated to an average luminosity log (L/L⊙) = 1.90 or, on average, by ∼11,000 K since the end of their pre‐CV evolution phase.

Post-AGB Evolution

This review focuses on the evolution of single and binary optically bright post-AGB stars and the relation between the photospheric chemical abundances and the circumstellar material.Surface abundance determinations of optically bright post-AGB stars offer a powerful tool, not only to clarify the evolutionary nature of the objects themselves, but also to constrain theoretical calculations for chemical evolution. However, a survey of recent results shows that the chemical patterns observed are very diverse and that several different classes can be distinguished. The most striking result is that only a minor fraction, namely the 21 μm objects, are conform to standard post third dredge-up theory.In recent years it became clear that binarity can influence the late evolutionary stages considerably. A second part therefore discusses the varied and fundamental impact of the presence of a companion on the evolution of post-AGB systems.

Spectrophotometric Data of the Central Star of the Large Magellanic Cloud Planetary Nebula N66: Quantitative Analysis of Its WN‐Type Spectrum

Hubble Space Telescope, IUE, and ground-based observations of the central star of the LMC planetary nebula N66 (CS N66), obtained in different epochs, are presented. Since 1990, CS N66 has displayed remarkable short- and long-term spectroscopic and photometric changes amounting to more than 3 mag in the optical. Expanding model atmospheres have been constructed to fit observations from different epochs. Fits provide the chemical composition, the fundamental stellar parameters L*, T*, R*, the mass-loss rate, and the wind velocity. From our best models we find that CS N66 is a very luminous He star (X/Y ≤ 0.1), with a small amount of N, undergoing a violent and unstable mass-loss event. The photospheric chemical abundances correspond to the equilibrium CNO nuclear burning values, while the nebula has a normal chemical composition. Models fitting data from different epochs indicate that the fundamental stellar parameters remain constant with time, with values log (L*/L☉) = 4.53 ± 0.10, T* = 93,300 K, and R* = 0.71 R☉. The short- and long-term stellar variations are produced by large changes in the mass-loss rate, which varies by large factors, from ≤ 8 × 10−7 M☉ yr-1 in 1983 (preoutburst epoch) to =2.5 × 10−5 M☉ yr-1 in early 1995 (maximum stellar brightness). No evidence was found to support the suggestion that the outburst was due to a late thermal pulse. We propose that the event taking place in CS N66 was produced by an atmospheric instability similar to that triggering the giant eruptions of Population I luminous blue variable stars. The possible mechanism causing the atmospheric instability is briefly discussed.