William J. Schuster

Two distinct halo populations in the solar neighborhood - Evidence from stellar abundance ratios and kinematics

Aims. Precise abundance ratios are determined for 94 dwarf stars with 5200 < Teff < 6300 K, −1.6 < [Fe/H] < −0.4, and distances D < 335 pc. Most of them have halo kinematics, but 16 thick-disk stars are included. Methods. Equivalent widths of atomic lines are measured from VLT/UVES and NOT/FIES spectra with resolutions R � 55 000 and R � 40 000, respectively. An LTE abundance analysis based on MARCS models is applied to derive precise differential abundance ratios of Na, Mg, Si, Ca, Ti, Cr, and Ni with respect to Fe. Results. The halo stars fall into two populations, clearly separated in [α/Fe], where α refers to the average abundance of Mg, Si, Ca, and Ti. Differences in [Na/Fe] and [Ni/Fe] are also present with a remarkably clear correlation between these two abundance ratios. Conclusions. The “high-α” stars may be ancient disk or bulge stars “heated” to halo kinematics by merging satellite galaxies or they could have formed as the first stars during the collapse of a proto-Galactic gas cloud. The kinematics of the “low-α” stars suggest that they have been accreted from dwarf galaxies, and that some of them may originate from the ω Cen progenitor galaxy.

Observational evidence for a broken Li Spite plateau and mass-dependent Li depletion

We present NLTE Li abundances for 88 stars in the metallicity range −3.5 −2. 5f all into two well-def ined plateaus of ALi = 2.18 (σ = 0.04) and ALi = 2.27 (σ = 0.05), respectively. We show that the two plateaus are flat, unlike previous claims for a steep monotonic decrease in Li abundances with decreasing metallicities. At all metallicities we uncover a fine-structure in the Li abundances of Spite plateau stars, which we trace to Li depletion that depends on both metallicity and mass. Models including atomic diffusion and turbulent mixing seem to reproduce the observed Li depletion assuming a primordial Li abundance ALi = 2.64, which agrees well with current predictions (ALi = 2.72) from standard Big Bang nucleosynthesis. Adopting the Kurucz overshooting model atmospheres increases the Li abundance by +0.08 dex to ALi = 2.72, which perfectly agrees with BBN+WMAP.

Two distinct halo populations in the solar neighborhood - III. Evidence from stellar ages and orbital parameters

Context. In Papers I and II of this series, we have found clear indications of the existence of two distinct populations of stars in the solar neighborhood belonging to the metal-rich end of the halo metallicity distribution function. Based on high-resolution, high S/N spectra, it is possible to distinguish between “high-alpha” and “low-alpha” components using the [α/Fe] versus [Fe/H] diagram. Aims. Precise relative ages and orbital parameters are determined for 67 halo and 16 thick-disk stars having metallicities in the range −1.4 < [Fe/H]< −0.4 to better understand the context of the two halo populations in the formation and evolution of the Galaxy. Methods. Ages are derived by comparing the positions of stars in the logTeff–log g diagram with isochrones from the Y 2 models interpolated to the exact [Fe/H] and [α/Fe] values of each star. The stellar parameters have been adopted from the preceding spectroscopic analyses, but possible systematic errors in Teff and log g are considered and corrected. With space velocities from Paper I as initial conditions, orbital integrations have been carried out using a detailed, observationally constrained Milky Way model including a bar and spiral arms. Results. The “high-alpha” halo stars have ages 2–3 Gyr larger than the “low-alpha” ones, with some probability that the thick-disk stars have ages intermediate between these two halo components. The orbital parameters show very distinct differences between the “high-alpha” and “low-alpha” halo stars. The “low-alpha” ones have rmax’s to 30–40 kpc, zmax’s to ≈18 kpc, and emax’s clumped at values greater than 0.85, while the “high-alpha” ones, rmax’s to about 16 kpc, zmax’s to 6–8 kpc, and emax values more or less uniformly distributed over 0.4–1.0. Conclusions. A dual in situ-plus-accretion formation scenario best explains the existence and characteristics of these two metal-rich halo populations, but one remaining defect is that this model is not consistent regarding the rmax’s obtained for the in situ “high-alpha” component; the predicted values are too small. It appears that ω Cen may have contributed in a significant way to the existence of the “low-alpha” component; recent models, including dynamical friction and tidal stripping, have produced results consistent with the present mass and orbital characteristics of ω Cen, while at the same time including extremes in the orbital parameters as great as those of the “low-alpha” component.

Carbon and oxygen abundances in stellar populations

Abundances of C, O, and Fe are determined for F and G main-sequence stars in the solar neighborhood in order to study trends and systematic differences in the C/Fe, O/Fe, and C/O ratios for stellar populations. Carbon abundances are determined from the CI lines at 5052 and 5380 AA and oxygen abundances from the OI triplet at 7774 AA and the [OI] line at 6300 AA. MARCS model atmospheres are applied and non-LTE corrections for the OI triplet are included. Systematic differences between high- and low-alpha halo stars and between thin- and thick-disk stars are seen in the trends of [C/Fe] and [O/Fe]. The two halo populations and thick-disk stars show the same trend of [C/O] versus [O/H], whereas thin-disk stars are shifted to higher [C/O]. Furthermore, we find some evidence of higher C/O and C/Fe ratios in stars hosting planets than in stars for which no planets have been detected. The results suggest that C and O in both high- and low-alpha halo stars and in thick-disk stars are made mainly in massive stars, whereas thin-disk stars have an additional carbon contribution from low-mass AGB and massive stars of high metallicity causing a rising trend of C/O with increasing metallicity. However, the C/O ratio does not exceed 0.8, which seems to exclude formation of carbon planets if proto-planetary disks have the same composition as their parent stars.

THE UBV(RI)(C) COLORS OF THE SUN

Photometric data in the UBV(RI)C system have been acquired for 80 solar analog stars for which we have previously derived highly precise atmospheric parameters T eff, log g, and [Fe/H] using high-resolution, high signal-to-noise ratio spectra. UBV and (RI)C data for 46 and 76 of these stars, respectively, are published for the first time. Combining our data with those from the literature, colors in the UBV(RI)C system, with 0.01 mag precision, are now available for 112 solar analogs. Multiple linear regression is used to derive the solar colors from these photometric data and the spectroscopically derived T eff, log g, and [Fe/H] values. To minimize the impact of systematic errors in the model-dependent atmospheric parameters, we use only the data for the 10 stars that most closely resemble our Sun, i.e., the solar twins, and derive the following solar colors: (B – V)☉ = 0.653 ± 0.005, (U – B)☉ = 0.166 ± 0.022, (V – R)☉ = 0.352 ± 0.007, and (V – I)☉ = 0.702 ± 0.010. These colors are consistent, within the 1σ errors, with those derived using the entire sample of 112 solar analogs. We also derive the solar colors using the relation between spectral-line-depth ratios and observed stellar colors, i.e., with a completely model-independent approach, and without restricting the analysis to solar twins. We find (B – V)☉ = 0.653 ± 0.003, (U – B)☉ = 0.158 ± 0.009, (V – R)☉ = 0.356 ± 0.003, and (V – I)☉ = 0.701 ± 0.003, in excellent agreement with the model-dependent analysis.

Kinematics, ages and metallicities for F- and G-type stars in the solar neighbourhood

A new metallicity distribution and an age–metallicity relation are presented for 437 nearby F and G turn-off and sub-giant stars selected from radial velocity data of Nidever et al. Photometric metallicities are derived from uvby− Hβ photometry, and the stellar ages from the isochrones of Bergbusch & VandenBerg as transformed to uvby photometry using the methods of Clem et al. The X (stellar population) criterion of Schuster et al., which combines both kinematic and metallicity information, provides 22 thick-disc stars. σW= 32 ± 5 km s−1, 〈Vrot〉= 154 ± 6 km s−1 and 〈[M/H]〉=−0.55 ± 0.03 dex for these thick-disc stars, which is in agreement with values from previous studies of the thick disc. α-element abundances which are available for some of these thick-disc stars show the typical α-element signatures of the thick disc, supporting the classification procedure based on the X criterion. Both the scatter in metallicity at a given age and the presence of old, metal-rich stars in the age–metallicity relation make it difficult to decide whether or not an age–metallicity relation exists for the older thin-disc stars. For ages greater than 3 Gyr, our results agree with the other recent studies that there is almost no correlation between age and metallicity, Δ([M/Fe])/Δ(age) =−0.01 ± 0.005 dex Gyr−1. For the 22 thick-disc stars there is a range in ages of 7–8 Gyr, but again almost no correlation between age and metallicity. For the subset of main-sequence stars with extra-solar planets, the age–metallicity relation is very similar to that of the total sample, very flat, the main difference being that these stars are mostly metal-rich, [M/H]≳−0.2 dex. However, two of these stars have [M/H]∼−0.6 dex and have been classified as thick-disc stars. As for the total sample, the range in ages for these stars with extra-solar planetary systems is considerable with a nearly uniform distribution over 3 ≲ age ≲ 13 Gyr.

uvby–β photometry of solar twins - The solar colors, model atmospheres, and the Teff and metallicity scales

Aims. Solar colors have been determined on the uvby–β photometric system to test absolute solar fluxes, to examine colors predicted by model atmospheres as a function of stellar parameters (Teff ,l ogg ,[ Fe/H]), and to probe zero-points of Teff and metallicity scales. Methods. New uvby–β photometry is presented for 73 solar-twin candidates. Most stars of our sample have also been observed spectroscopically to obtain accurate stellar parameters. Using the stars that most closely resemble the Sun, and complementing our data with photometry available in the literature, the solar colors on the uvby–β system have been inferred. Our solar colors are compared with synthetic solar colors computed from absolute solar spectra and from the latest Kurucz (ATLAS9) and MARCS model atmospheres. The zero-points of different Teff and metallicity scales are verified and corrections are proposed. Results. Our solar colors are (b − y)� = 0.4105 ± 0.0015, m1,� = 0.2122 ± 0.0018, c1,� = 0.3319 ± 0.0054, and β� = 2.5915 ± 0.0024. The (b − y)� and m1,� colors obtained from absolute spectrophotometry of the Sun agree within 3-σ with the solar colors derived here when the photometric zero-points are determined from either the STIS HST observations of Vega or an ATLAS9 Vega model, but the c1,� and β� synthetic colors inferred from absolute solar spectra agree with our solar colors only when the zero-points based on the ATLAS9 model are adopted. The Kurucz solar model provides a better fit to our observations than the MARCS model. For photometric values computed from the Kurucz models, (b − y)� and m1,� are in excellent agreement with our solar colors independently of the adopted zero-points, but for c1,� and β� agreement is found only when adopting the ATLAS9 zero-points. The c1,� color computed from both the Kurucz and MARCS models is the most discrepant, probably revealing problems either with the models or observations in the u band. The Teff calibration of Alonso and collaborators has the poorest performance (∼140 K off), while the relation of Casagrande and collaborators is the most accurate (within 10 K). We confirm that the Ramirez & Melendez uvby metallicity calibration, recommended by Arnadottir and collaborators to obtain [Fe/H] in F, G, and K dwarfs, needs a small (∼10%) zero-point correction to place the stars and the Sun on the same metallicity scale. Finally, we confirm that the c1 index in solar analogs has a strong metallicity sensitivity.

The Gaia spectrophotometric standard stars survey – I. Preliminary results†

We describe two ground-based observing campaigns aimed at building a grid of approximately 200 spectrophotometric standard stars (SPSS), with an internal ≃1 per cent precision and tied to Vega within ≃3 per cent, for the absolute flux calibration of data gathered by Gaia, the European Space Agency (ESA) astrometric mission. The criteria for the selection and a list of candidates are presented, together with a description of the survey strategy and the adopted data analysis methods. We also discuss a short list of notable rejected SPSS candidates and difficult cases, based on identification problems, literature discordant data, visual companions and variability. In fact, all candidates are also monitored for constancy (within ±5 mmag, approximately). In particular, we report on a CALSPEC standard, 1740346, that we found to be a δ Scuti variable during our short-term monitoring (1-2 h) campaign. Based on data obtained within the Gaia Data Processing and Analysis Consortium (DPAC) - and coordinated by the Ground-based Observations for Gaia (GBOG) working group - at various telescopes; see acknowledgments.

Two distinct halo populations in the solar neighborhood - IV. Lithium abundances

Context. A previous study of F and G main-sequence stars in the solar neighborhood has revealed the existence of two distinct halo populations with a clear separation in [α/Fe] for the metallicity range −1.4 < [Fe/H] < −0.7. Taking into account the kinematics and ages of the stars, some Galactic formation models suggest that the “high-alpha” halo stars were formed in situ, whereas the “lowalpha” stars have been accreted from satellite galaxies. Aims. In this paper we investigate if there is a systematic difference in the lithium abundances of stars belonging to the high- and low-alpha halo populations. Methods. Equivalent widths of the Lii 6707.8 A resonance line are measured from high resolution VLT/UVES and NOT/FIES spectra and used to derive Li abundances on the basis of MARCS model atmospheres. Furthermore, masses of the stars are determined from the logTeff –l ogg diagram by interpolating between evolutionary tracks based on Yonsei-Yale models. Results. There is no significant systematic difference in the lithium abundances of high- and low-alpha stars. For the large majority of stars with masses 0.7 < M/M� < 0.9 and heavy-element mass fractions 0.001 < Z < 0.006, the lithium abundance is well fitted by a relation A(Li) = a0 + a1 M + a2 Z + a3 MZ ,w herea0, a1, a2 ,a nda3 are constants. Extrapolating this relation to Z = 0 leads to a lithium abundance close to the primordial value predicted from standard Big Bang nucleosynthesis calculations and the WMAP baryon density. The relation, however, does not apply to stars with metallicities below [Fe/H] �− 1.5. Conclusions. We suggest that metal-rich halo stars were formed with a lithium abundance close to the primordial value, and that lithium in their atmospheres has been depleted in time with an approximately linear dependence on stellar mass and Z. The lack of a systematic difference in the Li abundances of high- and low-alpha stars indicates that an environmental effect is not important for the destruction of lithium.

Optical variability of the Seyfert nucleus NGC 7469 in timescales from days to minutes

We report optical photometric observations of the Seyfert nucleus NGC 7469. During the first observing run in 1989, night-to-night measurements were done during 13 photometric nights spanning November 3-30. The largest variation, Δm=0 m .403, within a 20 arcsec diaphragm, occurred in eleven days and a decrease in brightness of Δm=0 m .337 (∼70% of the luminosity of the unresolved nuclear component) occurred in only five days. During the second run in August 1990, all-night monitoring was done during five photometric nights (∼29 hr) using differential photometry techniques