Jorge Melendez

New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s) - Improved astrophysical parameters for the Geneva-Copenhagen Survey

We present a re-analysis of the Geneva-Copenhagen survey, which benefits from the infrared flux method to improve the accuracy of the derived stellar effective temperatures and uses the latter to build a consistent and improved metallicity scale. Metallicities are calibrated on high-resolution spectroscopy and checked against four open clusters and a moving group, showing excellent consistency. The new temperature and metallicity scales provide a better match to theoretical isochrones, which are used for a Bayesian analysis of stellar ages. With respect to previous analyses, our stars are on average 100 K hotter and 0.1 dex more metal rich, which shift the peak of the metallicity distribution function around the solar value. From Stromgren photometry we are able to derive for the first time a proxy for [alpha/Fe] abundances, which enables us to perform a tentative dissection of the chemical thin and thick disc. We find evidence for the latter being composed of an old, mildly but systematically alpha-enhanced population that extends to super solar metallicities, in agreement with spectroscopic studies. Our revision offers the largest existing kinematically unbiased sample of the solar neighbourhood that contains full information on kinematics, metallicities, and ages and thus provides better constraints on the physical processes relevant in the build-up of the Milky Way disc, enabling a better understanding of the Sun in a Galactic context. (Less)

An absolutely calibrated T eff scale from the infrared flux method. Dwarfs and subgiants

Various effective temperature scales have been proposed over the years. Despite much work and the high internal precision usually achieved, systematic differences of order 100 K (or more) among various scales are still present. We present an investigation based on the Infrared Flux Method aimed at assessing the source of such discrepancies and pin down their origin. We break the impasse among different scales by using a large set of solar twins, stars which are spectroscopically and photometrically identical to the Sun, to set the absolute zero point of the effective temperature scale to within few degrees. Our newly calibrated, accurate and precise temperature scale applies to dwarfs and subgiants, from super-solar metallicities to the most metal-poor stars currently known. At solar metallicities our results validate spectroscopic effective temperature scales, whereas for [Fe/H]<-2.5 our temperatures are roughly 100 K hotter than those determined from model fits to the Balmer lines and 200 K hotter than those obtained from the excitation equilibrium of Fe lines. Empirical bolometric corrections and useful relations linking photometric indices to effective temperatures and angular diameters have been derived. Our results take full advantage of the high accuracy reached in absolute calibration in recent years and are further validated by interferometric angular diameters and space based spectrophotometry over a wide range of effective temperatures and metallicities.

The Effective Temperature Scale of FGK Stars. II. Teff:Color:[Fe/H] Calibrations

We present up-to-date metallicity-dependent temperature versus color calibrations for main-sequence and giant stars based on temperatures derived with the infrared flux method (IRFM). Seventeen colors in the photometric systems UBV, uvby, Vilnius, Geneva, RI(Cousins), DDO, Hipparcos-Tycho, and Two Micron All Sky Survey (2MASS) have been calibrated. The spectral types covered by the calibrations range from F0 to K5 (7000 K Teff 4000 K) with some relations extending below 4000 K or up to 8000 K. Most of the calibrations are valid in the metallicity range -3.5 [Fe/H] 0.4, although some of them extend to as low as [Fe/H] ~ -4.0. All fits to the data have been performed with more than 100 stars; standard deviations range from 30 to 120 K. Fits were carefully performed and corrected to eliminate the small systematic errors introduced by the calibration formulae. Tables of colors as a function of Teff and [Fe/H] are provided. This work is largely based on the study by A. Alonso and collaborators; thus, our relations do not significantly differ from theirs except for the very metal-poor hot stars. From the calibrations, the temperatures of 44 dwarf and giant stars with direct temperatures available are obtained. The comparison with direct temperatures confirms our finding in Paper I that the zero point of the IRFM temperature scale is in agreement, to the 10 K level, with the absolute temperature scale (that based on stellar angular diameters) within the ranges of atmospheric parameters covered by those 44 stars. The colors of the Sun are derived from the present IRFM Teff scale and they compare well with those of five solar analogs. It is shown that if the IRFM Teff scale accurately reproduces the temperatures of very metal-poor stars, systematic errors of the order of 200 K, introduced by the assumption of (V - K) being completely metallicity independent when studying very metal-poor dwarf stars, are no longer acceptable. Comparisons with other Teff scales, both empirical and theoretical, are also shown to be in reasonable agreement with our results, although it seems that both Kurucz and MARCS synthetic colors fail to predict the detailed metallicity dependence, given that for [Fe/H] = -2.0, differences as high as approximately ±200 K are found.

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars - V. Evidence for a wide age distribution and a complex MDF

Based on high-resolution spectra obtained during gravitational microlensing events we present a detailed elemental abundance analysis of 32 dwarf and subgiant stars in the Galactic bulge. Combined with the sample of 26 stars from the previous papers in this series, we now have 58 microlensed bulge dwarfs and subgiants that have been homogeneously analysed. The main characteristics of the sample and the findings that can be drawn are: (i) the metallicity distribution (MDF) is wide and spans all metallicities between [Fe/H] = −1.9 to +0.6; (ii) the dip in the MDF around solar metallicity that was apparent in our previous analysis of a smaller sample (26 microlensed stars) is no longer evident; instead it has a complex structure and indications of multiple components are starting to emerge. A tentative interpretation is that there could be different stellar populations at interplay, each with a different scale height: the thin disk, the thick disk, and a bar population; (iii) the stars with [Fe/H] ≲ −0.1 are old with ages between 10 and 12 Gyr; (iv) the metal-rich stars with [Fe/H] ≳ −0.1 show a wide variety of ages, ranging from 2 to 12 Gyr with a distribution that has a dominant peak around 4−5 Gyr and a tail towards higher ages; (v) there are indications in the [α/Fe]−[Fe/H] abundance trends that the “knee” occurs around [Fe/H] = −0.3 to −0.2, which is a slightly higher metallicity as compared to the “knee” for the local thick disk. This suggests that the chemical enrichment of the metal-poor bulge has been somewhat faster than what is observed for the local thick disk. The results from the microlensed bulge dwarf stars in combination with other findings in the literature, in particular the evidence that the bulge has cylindrical rotation, indicate that the Milky Way could be an almost pure disk galaxy. The bulge would then just be a conglomerate of the other Galactic stellar populations (thin disk, thick disk, halo, and ...?), residing together in the central parts of the Galaxy, influenced by the Galactic bar.

The Peculiar Solar Composition and Its Possible Relation to Planet Formation

We have conducted a differential elemental abundance analysis of unprecedented accuracy (~0.01 dex) of the Sun relative to 11 solar twins from the Hipparcos catalog and 10 solar analogs from planet searches. We find that the Sun shows a characteristic signature with a 20% depletion of refractory elements relative to the volatile elements in comparison with the solar twins. The abundance differences correlate strongly with the condensation temperatures of the elements. This peculiarity also holds in comparisons with solar analogs known to have close-in giant planets while the majority of solar analogs found not to have such giant planets in radial velocity monitoring show the solar abundance pattern. We discuss various explanations for this peculiarity, including the possibility that the differences in abundance patterns are related to the formation of planetary systems like our own, in particular to the existence of terrestrial planets.

Accurate abundance patterns of solar twins and analogs - Does the anomalous solar chemical composition come from planet formation?

We derive the abundance of 19 elements in a sample of 64 stars with fundamental parameters very similar to solar, which minimizes the impact of systematic errors in our spectroscopic 1D-LTE differential analysis, using high-resolution (R � 60 000), high signalto-noise ratio (S /N � 200) spectra. The estimated errors in the elemental abundances relative to solar are as small as � 0.025 dex. The abundance ratios [X/ Fe] as af unction of [Fe/H] agree closely with previously established patterns of Galactic thin-disk chemical evolution. Interestingly, the majority of our stars show a significant correlation between [X/Fe] and condensation temperature (TC). In the sample of 22 stars with parameters closest to solar, we find that, on average, low TC elements are depleted with respect to high TC elements in the solar twins relative to the Sun by about 0.08 dex (� 20%). An increasing trend is observed for the abundances as a function of TC for 900 < TC < 1800 K, while abundances of lower TC elements appear to be roughly constant. We speculate that this is a signature of the planet formation that occurred around the Sun but not in the majority of solar twins. If this hypothesis is correct, stars with planetary systems like ours, although rare (frequency of � 15%), may be identified through a very detailed inspection of the chemical compositions of their host stars.

The Effective Temperature Scale of FGK Stars. I. Determination of Temperatures and Angular Diameters with the Infrared Flux Method

The infrared flux method (IRFM) has been applied to a sample of 135 dwarf and 36 giant stars covering the following regions of the atmospheric parameter space: (1) the metal-rich ([Fe/H] 0) end (consisting mostly of planet-hosting stars), (2) the cool (Teff 5000 K) metal-poor (-1 [Fe/H] -3) dwarf region, and (3) the very metal-poor ([Fe/H] -2.5) end. These stars were especially selected to cover gaps in previous works on Teff versus color relations, particularly the IRFM Teff scale of A. Alonso and collaborators. Our IRFM implementation was largely based on the Alonso et al. study (absolute infrared flux calibration, bolometric flux calibration, etc.) with the aim of extending the ranges of applicability of their Teff versus color calibrations. In addition, in order to improve the internal accuracy of the IRFM Teff scale, we recomputed the temperatures of almost all stars from the Alonso et al. work using updated input data. The updated temperatures do not significantly differ from the original ones, with few exceptions, leaving the Teff scale of Alonso et al. mostly unchanged. Including the stars with updated temperatures, a large sample of 580 dwarf and 470 giant stars (in the field and in clusters), which cover the ranges 3600 K Teff 8000 K and -4.0 [Fe/H] +0.5, have Teff homogeneously determined with the IRFM. The mean uncertainty of the temperatures derived is 75 K for dwarfs and 60 K for giants, which is about 1.3% at solar temperature and 4500 K, respectively. It is shown that the IRFM temperatures are reliable in an absolute scale given the consistency of the angular diameters resulting from the IRFM with those measured by long baseline interferometry, lunar occultation, and transit observations. Using the measured angular diameters and bolometric fluxes, a comparison is made between IRFM and direct temperatures, which shows excellent agreement, with the mean difference being less than 10 K for giants and about 20 K for dwarf stars (the IRFM temperatures being larger in both cases). This result was obtained for giants in the ranges 3800 K < Teff < 5000 K and -0.7 < [Fe/H] < 0.2 and dwarfs in the ranges 4000 K < Teff < 6500 K and -0.55 < [Fe/H] < 0.25; thus, the zero point of the IRFM Teff scale is essentially the absolute one (that derived from angular diameters and bolometric fluxes) within these limits. The influence of the bolometric flux calibration adopted is explored and it is shown that its effect on the Teff scale, although systematic, is conservatively no larger than 50 K. Finally, a comparison with temperatures derived with other techniques is made. Agreement is found with the temperatures from Balmer line profile fitting and the surface brightness technique. The temperatures derived from the spectroscopic equilibrium of Fe I lines are differentially consistent with the IRFM, but a systematic difference of about 100 and 65 K (the IRFM temperatures being lower) is observed in the metal-rich dwarf and metal-poor giant Teff scales, respectively.

Abundances in a Large Sample of Stars in M3 and M13

We have carried out a detailed abundance analysis for 21 elements in a sample of 25 stars with a wide range in luminosity from luminous giants to stars near the main-sequence turnoff in the globular cluster M13 ([Fe/H] = -1.50 dex) and in a sample of 13 stars distributed from the tip to the base of the red giant branch (RGB) in the globular cluster M3 ([Fe/H] = -1.39 dex). The analyzed spectra, obtained with HIRES at the Keck Observatory, are of high dispersion (R = ?/?? = 35,000). Most elements, including Fe, show no trend with Teff and low scatter around the mean between the top of the RGB and near the main-sequence turnoff, suggesting that at this metallicity, non-LTE effects and gravitationally induced heavy-element diffusion are not important for this set of elements over the range of stellar parameters spanned by our sample. We have detected an anticorrelation between O and Na abundances, observed previously among the most luminous RGB stars in both of these clusters, in both M3 and in M13 over the full range of luminosity of our samples, i.e., in the case of M13 to near the main-sequence turnoff. M13 shows a larger range in both O and Na abundance than does M3 at all luminosities, in particular having a few stars at its RGB tip with unusually strongly depleted O. We detect a correlation between Mg abundance and O abundance among the stars in the M13 sample. We also find a decrease in the mean Mg abundance as one moves toward lower luminosity, which we tentatively suggest is due to our ignoring non-LTE effects in Mg. Although CN burning must be occurring in both M3 and in M13, and ON burning is required for M13, we combine our new O abundances with published C and N abundances to confirm with quite high precision that the sum of C+N+O is constant near the tip of the giant branch, and we extend this down to the bump in the luminosity function. The same holds true for a smaller sample in M3, with somewhat larger variance. Star I-5 in M13 has large excesses of Y and of Ba, with no strong enhancement of Eu, suggesting that an s-process event contributed to its heavy-element abundances. The mean abundance ratios for M3 and for M13 are identical to within the errors. They show the typical pattern for metal-poor globular clusters of scatter among the light elements, with the odd atomic number elements appearing in the mean enhanced. The Fe-peak elements, where the odd atomic number elements are excessively depleted, do not show any detectable star-to-star variations in either cluster. The abundance ratios for 13 Galactic globular clusters with recent detailed abundance analyses, obtained by combining our samples with published data, are compared with those of published large surveys of metal-poor halo field stars. For most elements, the agreement is very good, suggesting a common chemical history for the halo field and cluster stars.

Chemical similarities between Galactic bulge and local thick disk red giants: O, Na, Mg, Al, Si, Ca, and Ti

Context. The formation and evolution of the Galactic bulge and its relationship with the other Galactic populations is still poorly understood. Aims. To establish the chemical differences and similarities between the bulge and other stellar populations, we performed an elemental abundance analysis of α- (O, Mg, Si, Ca, and Ti) and Z-odd (Na and Al) elements of red giant stars in the bulge as well as of local thin disk, thick disk and halo giants. Methods. We use high-resolution optical spectra of 25 bulge giants in Baade’s window and 55 comparison giants (4 halo, 29 thin disk and 22 thick disk giants) in the solar neighborhood. All stars have similar stellar parameters but cover a broad range in metallicity (−1.5 < [Fe/H] < +0.5). A standard 1D local thermodynamic equilibrium analysis using both Kurucz and MARCS models yielded the abundances of O, Na, Mg, Al, Si, Ca, Ti and Fe. Our homogeneous and differential analysis of the Galactic stellar populations ensured that systematic errors were minimized. Results. We confirm the well-established differences for [α/Fe] at a given metallicity between the local thin and thick disks. For all the elements investigated, we find no chemical distinction between the bulge and the local thick disk, in agreement with our previous study of C, N and O but in contrast to other groups relying on literature values for nearby disk dwarf stars. For −1.5 < [Fe/H] < −0.3 exactly the same trend is followed by both the bulge and thick disk stars, with a star-to-star scatter of only 0.03 dex. Furthermore, both populations share the location of the knee in the [α/Fe] vs. [Fe/H] diagram. It still remains to be confirmed that the local thick disk extends to super-solar metallicities as is the case for the bulge. These are the most stringent constraints to date on the chemical similarity of these stellar populations. Conclusions. Our findings suggest that the bulge and local thick disk stars experienced similar formation timescales, star formation rates and initial mass functions, confirming thus the main outcomes of our previous homogeneous analysis of [O/Fe] from infrared spectra for nearly the same sample. The identical α-enhancements of thick disk and bulge stars may reflect a rapid chemical evolution taking place before the bulge and thick disk structures we see today were formed, or it may reflect Galactic orbital migration of inner disk/bulge stars resulting in stars in the solar neighborhood with thick-disk kinematics.

Chemical similarities between galactic bulge and local thick disk red giant stars

Context. The evolution of the Milky Way bulge and its relationship with the other Galactic populations is still poorly understood. The bulge has been suggested to be either a merger-driven classical bulge or the product of a dynamical instability of the inner disk. Aims. To probe the star formation history, the initial mass function and stellar nucleosynthesis of the bulge, we performed an elemental abundance analysis of bulge red giant stars. We also completed an identical study of local thin disk, thick disk and halo giants to establish the chemical differences and similarities between the various populations. Methods. High-resolution infrared spectra of 19 bulge giants and 49 comparison giants in the solar neighborhood were acquired with Gemini/Phoenix. All stars have similar stellar parameters but cover a broad range in metallicity. A standard 1D local thermodynamic equilibrium analysis yielded the abundances of C, N, O and Fe. A homogeneous and differential analysis of the bulge, halo, thin disk and thick disk stars ensured that systematic errors were minimized. Results. We confirm the well-established differences for [O/Fe] (at a given metallicity) between the local thin and thick disks. For the elements investigated, we find no chemical distinction between the bulge and the local thick disk, which is in contrast to previous studies relying on literature values for disk dwarf stars in the solar neighborhood. Conclusions. Our findings suggest that the bulge and local thick disk experienced similar, but not necessarily shared, chemical evolution histories. We argue that their formation timescales, star formation rates and initial mass functions were similar.