Ivan Ramirez

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.

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.

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.

Oxygen abundances in nearby stars - Clues to the formation and evolution of the Galactic disk

The abundances of iron and oxygen are homogeneously determined in a sample of 523 nearby (d 5500 K and [Fe/H] −0.3, we find no obvious indication of a sudden decrease (i.e., a “knee”) in the [O/Fe] vs. [Fe/H] pattern of thick-disk stars that would connect the thick and thin disk trends at a high metallicity. We conclude that Type Ia supernovae (SN Ia) did not contribute significantly to the chemical enrichment of the thick disk. In the −0.8 < [Fe/H] < +0.3 range, thin-disk stars show decreasing [O/Fe] ratios from about 0.4 to 0.0 that require a SN Ia contribution. The implications of these results for studies of the formation and evolution of the Galactic disk are discussed.

CONFIRMATION OF ERRORS IN HIPPARCOS PARALLAXES FROM HUBBLE SPACE TELESCOPE FINE GUIDANCE SENSOR ASTROMETRY OF THE PLEIADES

We present absolute trigonometric parallaxes and relative proper motions for three members of the Pleiades, obtained with the Hubble Space Telescopes Fine Guidance Sensor 1r, a white-light interferometer. We estimate spectral types and luminosity classes of the stars comprising the astrometric reference frame from R ? 2000 spectra, VJHK photometry, and reduced proper motions. From these we derive estimates of absolute parallaxes and introduce them into our model as observations with error. We constrain the three cluster members to have a 1 ? dispersion in distance less than 6.4 pc and find an average ?abs = 7.43 ? 0.17 ? 0.20 mas, where the second error is systematic due to member placement within the cluster. This parallax corresponds to a distance of 134.6 ? 3.1 pc or a distance modulus of m - M = 5.65 ? 0.05 for these three Pleiades stars, presuming a central location. This result agrees with three other independent determinations of the Pleiades distance. Presuming that the cluster depth systematic error can be significantly reduced because of the random placement of these many members within the cluster, these four independent measures yield a best-estimate Pleiades distance of ?abs = 7.49 ? 0.07 mas, corresponding to a distance of 133.5 ? 1.2 pc or a distance modulus of m - M = 5.63 ? 0.02. This resolves the dispute between the main-sequence fitting and the Hipparcos distance moduli in favor of main-sequence fitting.

Large Excess of Heavy Nitrogen in Both Hydrogen Cyanide and Cyanogen from Comet 17P/Holmes

From millimeter and optical observations of the Jupiter-family comet 17P/Holmes performed soon after its huge outburst of 2007 October 24, we derive 14N/15N = 139 ± 26 in HCN and 14N/15N = 165 ± 40 in CN, establishing that HCN has the same nonterrestrial isotopic composition as CN. The same conclusion is obtained for the long-period comet C/1995 O1 (Hale-Bopp) after a reanalysis of previously published measurements. These results are compatible with HCN being the prime parent of CN in cometary atmospheres. The15N excess relative to the Earths atmospheric value indicates that N-bearing volatiles in the solar nebula underwent important N isotopic fractionation at some stage of solar system formation. HCN molecules never isotopically equilibrated with the main nitrogen reservoir in the solar nebula before being incorporated in Oort Cloud and Kuiper Belt comets. The 12C/13C ratios in HCN and CN are measured to be consistent with the terrestrial value.

The remarkable solar twin HIP 56948: a prime target in the quest for other Earths

Context. The Sun shows abundance anomalies relative to most solar twins. If the abundance peculiarities are due to the formation of inner rocky planets, that would mean that only a small fraction of solar type stars may host terrestrial planets. Aims. In this work we study HIP 56948, the best solar twin known to date, to determine with an unparalleled precision how similar it is to the Sun in its physical properties, chemical composition and planet architecture. We explore whether the abundances anomalies may be due to pollution from stellar ejecta or to terrestrial planet formation. Methods. We perform a differential abundance analysis (both in LTE and NLTE) using high resolution (R ~ 100 000) high S/N (600–650) Keck HIRES spectra of the Sun (as reflected from the asteroid Ceres) and HIP 56948. We use precise radial velocity data from the McDonald and Keck observatories to search for planets around this star. Results. We achieve a precision of σ ≲ 0.003 dex for several elements. Including errors in stellar parameters the total uncertainty is as low as σ ≃ 0.005 dex (1%), which is unprecedented in elemental abundance studies. The similarities between HIP 56948 and the Sun are astonishing. HIP 56948 is only 17 ± 7 K hotter than the Sun, and log g, [Fe/H] and microturbulence velocity are only + 0.02 ± 0.02 dex, +0.02 ± 0.01 dex and +0.01 ± 0.01 km s^(-1) higher than solar, respectively. Our precise stellar parameters and a differential isochrone analysis shows that HIP 56948 has a mass of 1.02 ± 0.02 M_⊙ and that it is ~1 Gyr younger than the Sun, as constrained by isochrones, chromospheric activity, Li and rotation. Both stars show a chemical abundance pattern that differs from most solar twins, but the refractory elements (those with condensation temperature T_(cond) ≳ 1000 K) are slightly (~0.01 dex) more depleted in the Sun than in HIP 56948. The trend with T_(cond) in differential abundances (twins − HIP 56948) can be reproduced very well by adding ~3 M_⊕ of a mix of Earth and meteoritic material, to the convection zone of HIP 56948. The element-to-element scatter of the Earth/meteoritic mix for the case of hypothetical rocky planets around HIP 56948 is only 0.0047 dex. From our radial velocity monitoring we find no indications of giant planets interior to or within the habitable zone of HIP 56948. Conclusions. We conclude that HIP 56948 is an excellent candidate to host a planetary system like our own, including the possible presence of inner terrestrial planets. Its striking similarity to the Sun and its mature age makes HIP 56948 a prime target in the quest for other Earths and SETI endeavors.

Reappraising the Spite Lithium Plateau: Extremely Thin and Marginally Consistent with WMAP Data

The lithium abundance in 62 halo dwarfs is determined from accurate equivalent widths reported in the literature and an improved infrared flux method temperature scale. The Li abundance of 41 plateau stars (those with Teff > 6000 K) is found to be independent of temperature and metallicity, with a star-to-star scatter of only 0.06 dex over a broad range of temperatures (6000 K < Teff < 6800 K) and metallicities (-3.4 < [Fe/H] < -1), thus imposing stringent constraints on depletion by mixing and production by Galactic chemical evolution. We find a mean Li plateau abundance of ALi = 2.37 dex (7Li/H = 2.34 ? 10-10), which, considering errors of the order of 0.1 dex in the absolute abundance scale, is just in borderline agreement with the constraints imposed by the theory of primordial nucleosynthesis and Wilkinson Microwave Anisotropy Probe data (2.51 dex < A < 2.66 dex).The lithium abundance in 62 halo dwarfs is determined from accurate equivalent widths reported in the literature and an improved infrared flux method (IRFM) temperature scale. The Li abundance of 41 plateau stars (those with Teff > 6000 K) is found to be independent of temperature and metallicity, with a star-to-star scatter of only 0.06 dex over a broad range of temperatures (6000 K < Teff < 6800 K) and metallicities (−3.4 < [Fe/H] < −1), thus imposing stringent constraints on depletion by mixing and production by Galactic chemical evolution. We find a mean Li plateau abundance of ALi = 2.37 dex ( Li/H = 2.34 ×10), which, considering errors of the order of 0.1 dex in the absolute abundance scale, is just in borderline agreement with the constraints imposed by the theory of primordial nucleosynthesis and WMAP data (2.51 < A Li < 2.66 dex). Subject headings: cosmology: observations stars: abundances stars: Population IIThe lithium abundance in 62 halo dwarfs is determined from accurate equivalent widths reported in the literature and an improved infrared flux method temperature scale. The Li abundance of 41 plateau stars (those with Teff > 6000 K) is found to be independent of temperature and metallicity, with a star-to-star scatter of only 0.06 dex over a broad range of temperatures (6000 K < Teff < 6800 K) and metallicities (-3.4 < [Fe/H] < -1), thus imposing stringent constraints on depletion by mixing and production by Galactic chemical evolution. We find a mean Li plateau abundance of ALi = 2.37 dex (7Li/H = 2.34 × 10-10), which, considering errors of the order of 0.1 dex in the absolute abundance scale, is just in borderline agreement with the constraints imposed by the theory of primordial nucleosynthesis and Wilkinson Microwave Anisotropy Probe data (2.51 dex < A < 2.66 dex).

HIP 56948: A Solar Twin with a Low Lithium Abundance

For more than a decade, 18 Sco (HD 146233) has been considered the star that most closely resembles the Sun, even though significant differences such as its Li content, which is about 3 times solar, exist. Using high-resolution, high-S/N spectra obtained at McDonald Observatory, we show that the stars HIP 56948 and HIP 73815 are very similar to the Sun in both stellar parameters and chemical composition, including a low Li abundance, which was previously thought to be peculiar in the Sun. HIP 56948, in particular, has stellar parameters identical to solar within the observational uncertainties, being thus the best solar twin known to date. HIP 56948 is also similar to the Sun in its lack of hot Jupiters. Considering the age of this star (~1 ± 1 Gyr older than the Sun) and its location and orbit around the Galaxy, if terrestrial planets exist around it, they may have had enough time to develop complex life, making it a prime target for SETI.For more than a decade, 18 Sco (HD 146233) has been considered the star that most closely resembles the Sun, even though significant differences such as its Li content, which is about 3 times solar, exist. Using high-resolution, high-S/N spectra obtained at McDonald Observatory, we show that the stars HIP 56948 and HIP 73815 are very similar to the Sun in both stellar parameters and chemical composition, including a low Li abundance, which was previously thought to be peculiar in the Sun. HIP 56948, in particular, has stellar parameters identical to solar within the observational uncertainties, being thus the best solar twin known to date. HIP 56948 is also similar to the Sun in its lack of hot Jupiters. Considering the age of this star (~1 ? 1 Gyr older than the Sun) and its location and orbit around the Galaxy, if terrestrial planets exist around it, they may have had enough time to develop complex life, making it a prime target for SETI.

OXYGEN ABUNDANCES IN LOW- AND HIGH-alpha FIELD HALO STARS AND THE DISCOVERY OF TWO FIELD STARS BORN IN GLOBULAR CLUSTERS

Oxygen abundances of 67 dwarf stars in the metallicity range ?1.6 < [Fe/H] < ?0.4 are derived from a non-LTE analysis of the 777?nm O I triplet lines. These stars have precise atmospheric parameters measured by Nissen and Schuster, who find that they separate into three groups based on their kinematics and ?-element (Mg, Si, Ca, Ti) abundances: thick disk, high-? halo, and low-? halo. We find the oxygen abundance trends of thick-disk and high-? halo stars very similar. The low-? stars show a larger star-to-star scatter in [O/Fe] at a given [Fe/H] and have systematically lower oxygen abundances compared to the other two groups. Thus, we find the behavior of oxygen abundances in these groups of stars similar to that of the ? elements. We use previously published oxygen abundance data of disk and very metal-poor halo stars to present an overall view (?2.3 < [Fe/H] < +0.3) of oxygen abundance trends of stars in the solar neighborhood. Two field halo dwarf stars stand out in their O and Na abundances. Both G53-41 and G150-40 have very low oxygen and very high sodium abundances, which are key signatures of the abundance anomalies observed in globular cluster (GC) stars. Therefore, they are likely field halo stars born in GCs. If true, we estimate that at least 3% ? 2% of the local field metal-poor star population was born in GCs.