arXiv subjects

Jorge Melendez

Publications and source records attributed to Jorge Melendez.

At least 55 records · Page 3Linked to original sources

arXiv2007

New Extremely Metal-Poor Stars in the Galactic Halo

We present a detailed abundance analysis based on high resolution and high signal-to-noise spectra of eight extremely metal poor (EMP) stars with [Fe/H] < -3.5$ dex, four of which are new. Only stars with 4900 < Teff< 5650 K are included. Two stars of the eight are outliers in each of several abundance ratios. The most metal poor star in this sample, HE1424-0241, has [Fe/H] ~ -4 dex and is thus among the most metal poor stars known in the Galaxy. It has highly anomalous abundance ratios unlike those of any other known EMP giant, with very low Si, Ca and Ti relative to Fe, and enhanced Mn and Co, again relative to Fe. Only (low) upper limits for C and N can be derived from the non-detection of the CH and NH molecular bands. HE0132$-$2429, another sample star, has excesses of N and Sc with respect to Fe. The strong outliers in abundance ratios among the Fe-peak elements in these C-normal stars, not found at somewhat higher metallicities, are definitely real. They suggest that at such low metallicities we are beginning to see the anticipated and long sought stochastic effects of individual supernova events contributing to the Fe-peak material within a single star. A detailed comparison of the results of the analysis procedures adopted by our 0Z project compared to those of the First Stars VLT Large Project finds a systematic difference for [Fe/H] of ~0.3 dex, our values always being higher.

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arXiv2007

Magnesium Isotopes in Halo Stars

We have determined Mg isotope ratios in halo field dwarfs and giants in the globular cluster M71 based on high S/N high spectral resolution (R = 10$^5$) Keck HIRES spectra. Unlike previous claims of an important contribution from intermediate-mass AGB stars to the Galactic halo, we find that our $^{26}$Mg/$^{24}$Mg ratios can be explained by massive stars.

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arXiv2007

A New Type of Extremely Metal Poor Star

We present an abundance analysis for the extremely metal poor star HE1424-0241 based on high dispersion spectra from HIRES at Keck. This star is a giant on the lower red giant branch with [Fe/H] ~ -4.0 dex. Relative to Fe, HE1424-0241 has normal Mg, but it shows a very large deficiency of Si, with epsilon(Si)/epsilon(Fe) ~ 1/10 and epsilon(Si)/epsilon(Mg) ~ 1/25 that of all previously known extremely metal poor giants or dwarfs. It also has a moderately large deficiency of Ca and a smaller deficit of Ti, combined with enhanced Mn and Co and normal or low C. We suggest that in HE1424-0241 we see the effect of a very small number of contributing supernovae, and that the SNII contributing to the chemical inventory of HE1424-0241 were biased in progenitor mass or in explosion characteristics so as to reproduce its abnormal extremely low Si/Mg ratio. HE1424-0241 shows a deficiency of the explosive alpha-burning elements Si, Ca and Ti coupled with a ratio [Mg/Fe] normal for EMP stars; Mg is produced via hydrostatic alpha-burning. The latest models of nucleosynthesis in SNII fail to reproduce the abundance ratios seen in HE1424-0241 for any combination of the parameter space of core-collapse explosions they explore.

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arXiv2007

Magnesium Isotopes in Metal-Poor Dwarfs, the Rise of AGB Stars and the Formation Timescale of the Galactic Halo

We have determined magnesium isotopic ratios (^{25,26}Mg/Mg) in metal-poor (-2.6 [Fe/H] -1.3) halo dwarfs employing high S/N (90-280) high spectral resolution (R = 10^5) Keck HIRES spectra. Unlike previous claims of an important contribution from intermediate-mass AGB stars at low metallicities, we find that the rise of the AGB contribution in the Galactic halo did not occur until intermediate metallicities ([Fe/H] ~> -1.5).

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arXiv2006

HD 98618: A Star Closely Resembling our Sun

Despite the observational effort carried out in the last few decades, no perfect solar twin has been found to date. An important milestone was achieved a decade ago by Porto de Mello & da Silva, who showed that 18 Sco is almost a solar twin. In the present work, we use extremely high resolution (R = 10^5) high S/N Keck HIRES spectra to carry out a differential analysis of 16 solar twin candidates. We show that HD 98618 is the second-closest solar twin and that the fundamental parameters of both HD 98618 and 18 Sco are very similar (within a few percent) to the host star of our solar system, including the likelihood of hosting a terrestrial planet within their habitable zone. We suggest that these stars should be given top priority in exoplanet and SETI surveys.

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arXiv2006

Carbon Stars in the Hamburg/ESO Survey: Abundances

We have carried out a detailed abundance analysis for a sample of 16 carbon stars found among candidate extremely metal-poor (EMP) stars from the Hamburg/ESO Survey. We find that the Fe-metallicities for the cooler C-stars (Teff ~ 5100K) have been underestimated by a factor of ~10 by the standard HES survey tools. The results presented here provided crucial supporting data used by Cohen et al (2006) to derive the frequency of C-stars among EMP stars. C-enhancement in these EMP C-stars appears to be independent of Fe-metallicity and approximately constant at ~1/5 the solar C/H. The mostly low C12/C13 ratios (~4) and the high N abundances in many of these stars suggest that material which has been through proton burning via the CN cycle comprises most of the stellar envelope. C-enhancement is associated with strong enrichment of heavy nuclei beyond the Fe-peak for 12 of the 16 stars. The remaining C-stars from the HES, which tend to be the most Fe-metal poor, show no evidence for enhancement of the heavy elements. Very high enhancements of lead are detected in some of the C-stars with highly enhanced Ba. (We show that) the s-process is responsible for the enhancement of the heavy elements for the majority of the C-stars in our sample. We suggest that both the s-process rich and Ba-normal C-stars result from phenomena associated with mass transfer in binary systems. This leads directly to the progression from C-stars to CH stars and then to Ba stars as the Fe-metallicity increases. (abridged and slightly edited to shorten)

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arXiv2005

The Frequency of Carbon Stars Among Extremely Metal-Poor Stars

We demonstrate that there are systematic scale errors in the [Fe/H] values determined by the Hamburg/ESO Survey (and by inference by the HK Survey in the past) for certain extremely metal poor highly C-enhanced giants. The consequences of these scale errors are that a) the fraction of carbon stars at extremely low metallicities has been overestimated in several papers in the recent literature b) the number of extremely metal poor stars known is somewhat lower than has been quoted in the recent literature c) the yield for extremely metal poor stars by the HES Survey is somewhat lower than is stated in the recent literature. A preliminary estimate for the frequency of Carbon stars among the giants in the HES sample with -4 < [Fe/H] < -2.0 dex is 7.4 +-2.9%; adding an estimate for the C-enhanced giants with [C/Fe] > 1.0 dex without detectable C2 bands raises the fraction to 14 +-4$%. We rely on the results of an extensive set of homogeneous detailed abundance analyses of stars expected to have [Fe/H] < -3.0 dex selected from the HES to establish these claims. We have found that the Fe-metallicity of the cooler (Teff < 5200K) C-stars as derived from spectra taken with HIRES at Keck are a factor of ~10 higher than those obtained via the algorithm used by the HES project to analyze the moderate resolution follow-up spectra, which is identical to that used until very recently by the HK Survey. This error in Fe-abundance estimate for C-stars arises from a lowering of the emitted flux in the continuum bandpasses of the KP (3933 A line of CaII) and particularly the HP2 (Hdelta) indices used to estimate [Fe/H] due to absorption from strong molecular bands.

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arXiv2005

The Temperature Scale of Metal-Poor Dwarfs: Lithium and Oxygen Abundances

We employ a new Infrared Flux Method (IRFM) temperature scale (Ramirez & Melendez 2005, ApJ 626, 446; ApJ 626, 465) in order to determine Li, O, and Fe NLTE abundances in a sample of relatively unevolved (dwarfs, turn-off, subgiants) metal-poor stars. We show that the analysis of the permitted OI triplet and FeII lines leads to a plateau in [OI/FeII] over the broad metallicity range -3.2 < [Fe/H] < -0.7, independent of temperature and metallicity, and with a star-to-star scatter of only 0.1 dex. The Li abundance in halo stars is also found to be independent of temperature and metallicity (Spite plateau), with a star-to-star scatter of just 0.06 dex over the metallicity range -3.4 < [Fe/H] < -1. Our Li abundance (Melendez & Ramirez 2004, ApJ 615, L33) is higher than previously reported values, but still lower than the primordial abundance suggested by WMAP data and BBN.

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arXiv2005

The Frequency of Carbon Rich Stars Among Extremely Metal Poor Stars

We demonstrate that there are systematic scale errors in the [Fe/H] values determined by the Hamburg/ESO Survey (and by the HK Survey by inference) for certain extremely metal poor (EMP) highly C-enhanced giants. The consequences of these scale errors are that a) the fraction of carbon stars at extremely low metallicities has been substantially overestimated in several papers in the recent literature b) the number of EMP stars known is somewhat lower than has been quoted in the recent literature c) the yield for EMP stars by the HK and the HES Survey is somewhat lower than is stated in the recent literature. A preliminary estimate for the frequency of Carbon stars among the giants in the HES sample with -4 < [Fe/H] < -2.0 dex is 7.4 \pm 2.9%, and for C-rich giants with [C/Fe] > +1.0 dex is 14.4 \pm 4%. Here we present the key results of detailed abundance analyses of 14 C-stars selected from the HES as candidate EMP stars, of ~50 such stars analyzed. About 80% of such C-stars show highly enhanced Ba as well, with C enhanced by a factor of about 100, and [Ba/C] roughly Solar. These stars often show prominent lead lines, The remaining 20% of the C-stars do not show an enhancemement of the s-process neutron capture elements around the Ba peak. They tend to be the most metal-poor stars studied. We suggest that all these EMP C-stars are the remnants of the secondary in a mass transfer binary system where the former primary was an AGB star, which transferred substantial mass at that evolutionary stage. (published abstract will be shorter due to space limitations)

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arXiv2005

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 parameters 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 vs. 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 vs. 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, -4.0<[Fe/H]<+0.5, have Teff homogeneously determined with the IRFM. (Abridged)

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arXiv2005

The effective temperature scale of FGK stars. II. Teff : color : [Fe/H] calibrations

We present up-to-date metallicity-dependent temperature vs. color calibrations for main sequence and giant stars based on temperatures derived with the infrared flux method (IRFM). Seventeen colors in the following photometric systems: UBV, uvby, Vilnius, Geneva, RI(Cousins), DDO, Hipparcos-Tycho, and 2MASS, have been calibrated. The spectral types covered 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 K 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. (Abridged)

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arXiv2004

Outer Versus Inner Halo Globular Clusters: NGC 7492 Abundances

We have carried out a detailed abundance analysis for 21 elements in a sample of four RGB stars in the outer halo globular cluster NGC 7492 (R{GC} 25 kpc); we find [Fe/H] = -1.82 dex inferred from FeI lines (-1.79 from FeII) using high dispersion (R=35,000) spectra obtained with HIRES at the Keck Observatory. Most elements show no sign of star-to-star variation within our limited sample. We have, however, detected an anti-correlation between O and Na abundances similar to that seen in our previous analyses of inner halo GCs as well as in studies of relatively nearby GCs by others. We compare the abundance ratios in NGC 7492 with those we previously determined for the much closer old halo GCs M3 and M13. After making corrections for trends of abundance ratio with metallicity characteristic of halo stars, we find that for for these three GCs for each of the elements in common we deduce identical abundance ratios with respect to Fe to within the probable measurement uncertainties. Thus, the chemical history of the outer halo as exemplified by the metal-poor outer halo globular cluster NGC 7492 is indistinguishable from that of the inner halo, exemplified by M3 and M13, at least through the epoch of formation of these old globular clusters. This applies to the neutron capture processes as well.

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arXiv2004

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 and in a sample of 13 stars distributed from the tip to the base of the RGB in the GC M3. 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. We have detected an anti-correlation 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. 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. Star I-5 in M13 has large excesses of Y and of Ba, with no strong enhancement of Eu, suggesting an s-process event contributed. The mean abundance ratios for M3 and for M13 are identical to within the errors. The abundance ratios for 13 Galactic GCs with recent detailed abundance analyses are compared to 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.

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arXiv2004

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

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 A(Li) = 2.37 dex (7Li/H = 2.34 X 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 WMAP data (2.51 < A(Li)[WMAP] < 2.66 dex).

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arXiv2004

Spectroscopic Equilibrium of Iron in Metal-Rich Dwarfs

We analyze twenty five nearby metal-rich G and late-F dwarfs in order to verify whether the spectroscopic equilibrium (LTE) of iron lines satisfy the observational constraints imposed by the Infrared Flux Method (angular diameters) and Hipparcos parallaxes. The atmospheric parameters derived from iron lines (assuming LTE and employing 1D Kurucz model atmospheres) do not satisfy simultaneously both observational constraints, probably because classical modeling fails to reproduce the detailed line formation of FeI lines.

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arXiv2004

A Low Solar Oxygen Abundance from the First Overtone OH Lines

An extremely high-resolution (> 10^5) high-S/N (> 10^3) solar spectrum has been used to measure 15 very weak first overtone (Delta v = 2) infrared OH lines, resulting in a low solar abundance of A(O) ~ 8.6 when MARCS, 3D, and spatially and temporally averaged 3D model atmospheres are used. A higher abundance is obtained with Kurucz (A(O) ~ 8.7) and Holweger & Muller (A(O) ~ 8.8) model atmospheres. The low solar oxygen abundance obtained in this work is in good agreement with a recent 3D analysis of [OI], OI, OH fundamental (Delta v = 1) vibration-rotation and OH pure rotation lines (Asplund et al. 2004). The present result brings further support for a low solar metallicity, and although using a low solar abundance with OPAL opacities ruins the agreement between the calculated and the helioseismic measurement of the depth of the solar convection zone, recent results from the OP project show that the opacities near the base of the solar convection zone are larger than previously thought, bringing further confidence for a low solar oxygen abundance.

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arXiv2004

Cooler and bigger than thought? Planetary host stellar parameters from the InfraRed Flux Method

Effective temperatures and radii for 92 planet-hosting stars as determined from the InfraRed Flux Method (IRFM) are presented and compared with those given by other authors using different approaches. The IRFM temperatures we have derived are systematically lower than those determined from the spectroscopic condition of excitation equilibrium, the mean difference being as large as 110 K. They are, however, consistent with previous IRFM studies and with the colors derived from Kurucz and MARCS model atmospheres. Comparison with direct measurements of stellar diameters for 7 dwarf stars, which approximately cover the range of temperatures of the planet-hosting stars, suggest that the IRFM radii and temperatures are reliable in an absolute scale. A better understanding of the fundamental properties of the stars with planets will be achieved once this discrepancy between the IRFM and the spectroscopic temperature scales is resolved.

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arXiv2002

IRFM Temperature Calibrations for the Vilnius, Geneva, RI(C) and DDO Photometric Systems

We have used the infrared flux method (IRFM) temperatures of a large sample of late type dwarfs given by Alonso et al. (1996a) to calibrate empirically the relations Teff = f (colour, [Fe/H]) for the Vilnius, Geneva, RI(C) and DDO photometric systems. The resulting temperature scale and intrinsic colour-colour diagrams for these systems are also obtained. From this scale, the solar colours are derived and compared with those of the solar twin 18 Sco. Since our work is based on the same Teff and [Fe/H] values used by Alonso et al. (1996b) to calibrate other colours, we now have an homogeneous calibration for a large set of photometric systems.

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