Bacham E. Reddy

Parent Stars of Extrasolar Planets. VI. Abundance Analyses of 20 New Systems

The results of new spectroscopic analyses of 20 recently reported extrasolar planet parent stars are presented. The companion of one of these stars, HD 10697, has recently been shown to have a mass in the brown dwarf regime; we find [Fe/H] = +0.16 for it. For the remaining sample, we derive [Fe/H] estimates ranging from -0.41 to +0.37, with an average value of +0.18 ± 0.19. If we add the 13 stars included in the previous papers of this series and six other stars with companions below the 11 MJ limit from the recent studies of Santos et al., we derive [Fe/H] = +0.17 ± 0.20. Among the youngest stars with planets with F or G0 spectral types, [Fe/H] is systematically larger than young field stars of the same Galactocentric distance by 0.15 to 0.20 dex. This confirms the recent finding of Laughlin that the most massive stars with planets are systematically more metal-rich than field stars of the same mass. We interpret these trends as supporting a scenario in which these stars accreted high-Z material after their convective envelopes shrunk to near their present masses. Correcting these young star metallicities by 0.15 dex still does not fully account for the difference in mean metallicity between the field stars and the full parent stars sample. The stars with planets appear to have smaller [Na/Fe], [Mg/Fe], and [Al/Fe] values than field dwarfs of the same [Fe/H]. They do not appear to have significantly different values of [O/Fe], [Si/Fe], [Ca/Fe], or [Ti/Fe], though. The claim made in Paper V that stars with planets have low [C/Fe] is found to be spurious, due to unrecognized systematic differences among published studies. When corrected for these differences, they instead display slightly enhanced [C/Fe] (but not significantly so). If these abundance anomalies are due to the accretion of high-Z matter, it must have a composition different from that of the Earth.

Lithium abundances of the local thin disc stars

Lithium abundances are presented for a sample of 181 nearby F and G dwarfs with accurate Hipparcos parallaxes. The stars are on circular orbits about the Galactic centre and, hence, are identified as belonging to the thin disc. This sample is combined with two published surveys to provide a catalogue of lithium abundances, metallicities ([Fe/H]), masses, and ages for 451 F-G dwarfs, almost all belonging to the thin disc. The lithium abundances are compared and contrasted with published lithium abundances for F and G stars in local open clusters. The field stars span a larger range in [Fe/H] than the clusters for which [Fe/H] ≃ 0.0 ± 0.2. The initial (i.e. interstellar) lithium abundance of the solar neighbourhood, as derived from stars for which astration of lithium is believed to be unimportant, is traced from log ∈(Li) = 2.2 at [Fe/H] = -1 to log ∈(Li) = 3.2 at +0.1. This form for the evolution is dependent on the assumption that astration of lithium is negligible for the stars defining the relation. An argument is advanced that this latter assumption may not be entirely correct, and, the evolution of lithium with [Fe/H] may be flatter than previously supposed. A sharp Hyades-like Li dip is not seen among the field stars and appears to be replaced by a large spread among lithium abundances of stars more massive than the lower mass limit of the dip. Astration of lithium by stars of masses too low to participate in the Li dip is discussed. These stars show little to no spread in lithium abundance at a given [Fe/H] and mass.

Origin of Lithium Enrichment in K Giants

In this Letter, we report on a low-resolution spectroscopic survey for Li-rich K giants among 2000 low-mass (M ≤ 3 M ☉) giants spanning the luminosity range from below to above the luminosity of the clump. Fifteen new Li-rich giants including four super Li-rich K giants (log (Li) ≥3.2) were discovered. A significant finding is that there is a concentration of Li-rich K giants at the luminosity of the clump or red horizontal branch. This new finding is partly a consequence of the fact that our low-resolution survey is the first large survey to include giants well below and above the red giant branch (RGB) bump and clump locations in the H-R diagram. Origin of the lithium enrichment may be plausibly attributed to the conversion of 3He via 7Be to 7Li by the Cameron-Fowler mechanism but the location for the onset of the conversion is uncertain. Two possible opportunities to effect this conversion are discussed: the bump in the first ascent of the RGB and the He-core flash at the tip of the RGB. The finite luminosity spread of the Li-rich giants serves to reject the idea that Li enhancement is, in general, a consequence of a giant swallowing a large planet.

A search for 6Li in stars with planets

Using very high-resolution (R ∼ 125 000) and high-quality (signal-to-noise ratio ≥ 350) spectra, we have searched for 6 Li in stars hosting extrasolar planets. From detailed profile-fitting of the Li I resonance line at 6707.7 A, we find no significant amount of 6 Li relative to 7 Li for any of eight planet-bearing stars ( 6 Lι/ 7 Li ≤ 0.0-4.03) with strong Li I lines. In particular, we do not confirm the presence of 6 Li with 6 Lι/ 7 Li = 0.13 reported by Israelian et al. for HD 82943, a star with two known planets. Several of the eight stars plus HD 219542 A, the planetless primary of a binary, have been identified in the literature as possible recipients of accreted terrestrial material. For all of the planet-hosting stars and an additional five planetless stars, we find no 6 Li.

Spectroscopic Analysis of Two Carbon-rich Post-Asymptotic Giant Branch Stars

The chemical compositions of the C-rich post-asymptotic giant branch (pAGB) stars IRAS 05113+1347 and IRAS 22272+5424 are determined from high-resolution optical spectra using standard LTE model atmosphere-based techniques. The stars are C, N, and s-process enriched, suggesting efficient operation of the third dredge-up in the AGB star following a first dredge-up that increased the N abundance. Lithium is present with an abundance requiring Li manufacture. With this pair, abundance analyses are now available for 11 C-rich pAGBs. A common history is indicated and, in particular, the s-abundances, especially the relative abundances of light to heavy s-process elements, follow recent predictions for the third dredge-up in AGB stars.

An Abundance Analysis of Two Carbon-rich Proto-Planetary Nebulae: IRAS Z02229+6208 and IRAS 07430+1115

In this paper, we present an LTE abundance analysis of two new proto-planetary nebulae, IRAS Z02229 + 6208 and IRAS 07430+1115, based on high-resolution (R ≈ 55,000) optical echelle spectra. Results show that both stars are metal-poor ([Fe/H] = -0.5) and overabundant in C, N, and s-process elements. The average elemental abundances are [C/Fe] = +0.8, [N/Fe] = +1.2, and [s-process/Fe] = + 1.4 for IRAS Z02229 + 6208, and [C/Fe] = + 0.6, [N/Fe] = + 0.4, and [s-process/ Fe] = +1.6 for IRAS 07430+1115. These abundances suggest that the stars have experienced nucleo-synthesis on the asymptotic giant branch (AGB), and the resultant products of CNO, 3α, and s-process reactions were brought to the photosphere during shell flashes and deep mixing episodes during the AGB phase of their evolution. Of major significance is the measurement of a high Li abundance in both stars, log ∈(Li) ≈ 2.3 and 2.4 for IRAS Z02229 + 6208 and IRAS 07430 + 1115, respectively. This may be the result of hot bottom burning, below the deep convective zone. We also present an analysis of the circumstellar molecular (C2 and CN) and atomic (Na I and K I) absorption spectra of both stars. We derive rotational temperatures, column densities, and envelope expansion velocities using molecular C2 Phillips and CN Red system bands. The values derived for expansion velocities, 8-14 km s-1, are typical of the values found for post-AGB stars. IRAS 07430 + 1115 is unusual in that it shows P Cygni-shaped C2 emission profiles in the spectra of the circumstellar envelope. A minimum distance for IRAS Z02229 + 6208, determined from interstellar Na I lines, suggests that it is evolved from an intermediate-mass star. Including these two stars, the number of post-AGB stars for which clear C, N, and s-process elemental overabundances are found rises to eight. IRAS Z02229 + 6208 is known to possess the 21 μm emission feature in its mid-infrared spectrum; these results support the idea that all 21 μm emission stars are carbon-rich post-AGB stars.

Abundance Analyses of Field RV Tauri Stars. VI. An Extended Sample

An abundance analysis is presented and discussed for a sample of 14 RV Tauri stars. The present abundance data and those from our previous papers and by other workers are combined in an attempt to further understanding of the dust-gas separation process that afflicts many RV Tauri variables. We propose that a stars intrinsic (i.e., initial) metallicity is given by the photospheric zinc abundance. Variables warmer than about 5000 K and with an initial metallicity [Fe/H] ≥ -1 are affected by dust-gas separation. Variables of all metallicities and cooler than about Teff 5000 K are unaffected by dust-gas separation. The RV Tauri variables show a spread in their C abundances, with the lower boundary of the points in the C versus Zn plane falling close to the predicted trend for giants after the first dredge-up. The upper boundary is inhabited by a few stars that are carbon-rich. The O abundances in the mean follow the predicted trend from unevolved stars, in line with the expectation that photospheric O abundance is unaffected by the first dredge-up. An evolutionary scenario involving mass loss by a first-ascent or early-AGB red giant, the primary star of a binary, is sketched.

Searching for the metal-weak thick disc in the solar neighbourhood

An abundance analysis is presented of 60 metal-poor stars drawn from catalogues of nearby stars provided by Arifyanto et al. and Schuster et al. In an attempt to isolate a sample of metal-weak thick disc stars, we applied the kinematic criteria V rot ≥ 100 km s −1 , |ULSR |≤ 140 km s −1 (LSR – local standard of rest) and |W LSR |≤ 100 km s −1 . 14 stars satisfying these criteria and having [Fe/H] ≤− 1.0 are included in the sample of 60 stars. Eight of the 14 have [Fe/H] ≥− 1.3 and may be simply thick disc stars of slightly lower than average [Fe/H]. The other six have [Fe/H] from −1.3 to −2.3 and are either metal-weak thick disc stars or halo stars with kinematics mimicking those of the thick disc. The sample of 60 stars is completed by eight thick disc stars, 20 stars of a hybrid nature (halo or thick disc stars) and 18 stars with kinematics distinctive of the halo.

HD 77361: A NEW CASE OF SUPER Li-RICH K GIANT WITH ANOMALOUS LOW 12C/13C RATIO

Results from high-resolution spectroscopic analysis of HD 77361 are reported. LTE analysis shows that HD 77361 is a K giant of atmospheric parameters: T eff = 4580 ? 75 K, log g = 2.5 ? 0.1, and ?t = 1.40 ? 0.5 km s?1. We found that the atmosphere of HD 77361 is highly enriched in Li with log (Li) = 3.82 ? 0.1. With this finding the total number of super Li-rich K giants (log (Li) ? 3.3 ISM value) known to date reached six. Contrary to first dredge-up, extra-deep mixing and the associated cool bottom processing, and other recent predictions for K giants on the red giant branch (RGB) luminosity bump phase, HD 77361 shows a very low value of 12C/13C = 4.3 ? 0.5, having, simultaneously, very large amount of Li. Also, HD 77361 is the only Population I low-luminosity (log L/L ? = 1.66 ? 0.1) low-mass K giant (M = 1.5 ? 0.2 M ?) among the known super Li-rich K giants that has a very low 12C/13C ratio. Results of HD 77361 further constrain our theoretical understanding of Li enhancement in the atmospheres of RGB stars.

Three Li-rich K Giants: IRAS 12327−6523, 13539−4153, and 17596−3952

We report on spectroscopic analyses of three K giants previously suggested to be Li-rich: IRAS 12327-6523, 13539-4153, and 17596-3952. High-resolution optical spectra and the LTE model atmospheres are used to derive the stellar parameters (Teff, log g, [Fe/H]), elemental abundances, and the isotopic ratio 12C/13C. IRAS 13539-4153 shows an extremely high Li abundance of log (Li) ≈ 4.2, a value 10 times greater than the present Li abundance in the local interstellar medium. This is the third highest Li abundance yet reported for a K giant. IRAS 12327-6523 shows a Li abundance of log (Li) ≈ 1.4. IRAS 17596-3952 is a rapidly rotating (V sin i ≈ 35 km s-1) K giant with log (Li) ≈ 2.2. Infrared photometry shows the presence of an IR excess, suggesting mass loss. A comparison is made between these three stars and previously recognized Li-rich giants.