arXiv subjects

Jorge Melendez

Publications and source records attributed to Jorge Melendez.

At least 19 recordsLinked to original sources

arXiv2026

Large-scale time-series spectroscopy for stellar ages

To date, Galactic Astronomy has largely concerned itself with astrophysical processes, and with the locations, space motions and compositions of objects. Consider, for example, the elucidation of the components of the Galaxy over the past decades, its mapping as enabled by Gaia and its predecessors, the photometric and spectroscopic characterization of innumerable astrophysical objects in various wavelength ranges, both from the ground and from space, and the expanding discovery and characterization of exoplanets; all focused on the current, static Galaxy. This White Paper proposes a dedicated program to derive stellar ages from time-series spectroscopy to hasten the transformation of this static conception into a dynamical one with age-labeled objects and events.

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arXiv2024

Stellar Characterization and a Chromospheric Activity Analysis of a K2 Sample of Planet-Hosting Stars

Effective temperatures, surface gravities, and iron abundances were derived for 109 stars observed by the K2 mission using equivalent width measurements of Fe I and Fe II lines. Calculations were carried out in LTE using Kurucz model atmospheres. Stellar masses and radii were derived by combining the stellar parameters with Gaia DR3 parallaxes, V-magnitudes, and isochrones. The derived stellar and planetary radii have median internal precision of 1.8%, and 2.3%, respectively. The radius gap near $\rm R_{planet}\sim 1.9 R_\oplus$ was detected in this K2 sample. Chromospheric activity was measured from the Ca II H and K lines using the Values of $\log R^\prime_{\rm HK}$ were investigated as a function of stellar rotational period (P$_{rot}$) and we found that chromospheric activity decreases with increasing P$_{rot}$, although there is a large scatter in $\log R^\prime_{\rm HK}$ ($\sim$0.5) for a given P$_{rot}$. Activity levels in this sample reveal a paucity of F & G dwarfs with intermediate activity levels (Vaughan-Preston gap). The effect that stellar activity might have on the derivation of stellar parameters was investigated by including magnetically-sensitive Fe I lines in the analysis and we find no significant differences between parameters with and without magnetically-sensitive lines, although the more active stars ($\log R^\prime _{\rm HK}>-5.0$) exhibit a larger scatter in the differences in $T_{\rm eff}$ and [Fe/H].

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arXiv2020

Carbon, isotopic ratio $^{12}$C/$^{13}$C and nitrogen in solar twins: constraints for the chemical evolution of the local disc

Abundances of light elements in dwarf stars of different ages are important constraints for stellar yields, Galactic chemical evolution and exoplanet chemical composition studies. We have measured C and N abundances and $^{12}$C/$^{13}$C ratios for a sample of 63 solar twins spanning a wide range in age, based on spectral synthesis of a comprehensive list of CH\,A-X and CN\,B-X features using HARPS spectra. The analysis of 55 thin disc solar twins confirms the dependences of [C/Fe] and [N/Fe] on [Fe/H]. [N/Fe] is investigated as a function of [Fe/H] and age for the first time for these stars. Our derived correlation [C/Fe]-age agrees with works for solar-type stars and solar twins, but the [N/Fe]-age correlation does not. The relations [C,N/Fe]-[Fe/H] and [C,N/Fe]-age for the solar twins lay under-solar. $^{12}$C/$^{13}$C is found correlated with [Fe/H] and seems to have decreased along the evolution of the local thin disc. Predictions from chemical evolution models for the solar vicinity corroborate the relations [C,N/Fe]-[Fe/H], $^{12}$C/$^{13}$C-age and [N/O]-[O/H], but do not for the $^{12}$C/$^{13}$C-[Fe/H] and [C/O]-[O/H] relations. The N/O ratio in the Sun is placed at the high end of the homogeneous distribution of solar twins, which suggests uniformity in the N-O budget for the formation of icy planetesimals, watery super-earths and giant planets. C and N had different nucleosynthetic origins along the thin disc evolution, as shown by the relations of [C/N], [C/O] and [N/O] against [O/H] and age. [C/N] and [C/O] are particularly observed increasing in time for solar twins younger than the Sun.

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arXiv2020

The $R$-Process Alliance: Fourth Data Release from the Search for $r$-Process-Enhanced Stars in the Galactic Halo

This compilation is the fourth data release from the $R$-Process Alliance (RPA) search for $r$-process-enhanced stars, and the second release based on "snapshot" high-resolution ($R \sim 30,000$) spectra collected with the du Pont 2.5m Telescope. In this data release, we propose a new delineation between the $r$-I and $r$-II stellar classes at $\mathrm{[Eu/Fe]} = +0.7$, instead of the empirically chosen $\mathrm{[Eu/Fe]} = +1.0$ level previously in use, based on statistical tests of the complete set of RPA data released to date. We also statistically justify the minimum level of [Eu/Fe] for definition of the $r$-I stars, [Eu/Fe] $> +0.3$. Redefining the separation between $r$-I and $r$-II stars will aid in analysis of the possible progenitors of these two classes of stars and whether these signatures arise from separate astrophysical sources at all. Applying this redefinition to previous RPA data, the number of identified $r$-II and $r$-I stars changes to 51 and 121, respectively, from the initial set of data releases published thus far. In this data release, we identify 21 new $r$-II, 111 new $r$-I (plus three re-identified), and 7 new (plus one re-identified) limited-$r$ stars out of a total of 232 target stars, resulting in a total sample of 72 new $r$-II stars, 232 new $r$-I stars, and 42 new limited-$r$ stars identified by the RPA to date.

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arXiv2020

The R-process Alliance: First Magellan/MIKE Release from the Southern Search for R-Process-enhanced Stars

Extensive progress has been recently made into our understanding of heavy element production via the $r$-process in the Universe, specifically with the first observed neutron star binary merger (NSBM) event associated with the gravitational wave signal detected by LIGO, GW170817. The chemical abundance patterns of metal-poor $r$-process-enhanced stars provides key evidence into the dominant site(s) of the $r$-process, and whether NSBMs are sufficiently frequent or prolific $r$-process sources to be responsible for the majority of $r$-process material in the Universe. We present atmospheric stellar parameters (using a Non-Local Thermodynamic Equilibrium analysis) and abundances from a detailed analysis of 141 metal-poor stars, carried out as part of the $R$-Process Alliance (RPA) effort. We obtained high-resolution "snapshot" spectroscopy of the stars using the MIKE spectrograph on the 6.5m Magellan Clay telescope at Las Campanas Observatory in Chile. We find 10 new highly enhanced $r$-II (with [Eu/Fe] $> +1.0$), 62 new moderately enhanced $r$-I ($+0.3 < $ [Eu/Fe] $\le +1.0$) and 17 new limited-$r$ ([Eu/Fe] $< +0.3$) stars. Among those, we find 17 new carbon-enhanced metal-poor (CEMP) stars, of which five are CEMP-no. We also identify one new $s$-process-enhanced ([Ba/Eu ]$ > +0.5$), and five new $r/s$ ($0.0 < $ [Ba/Eu] $ < +0.5$) stars. In the process, we discover a new ultra metal-poor (UMP) star at [Fe/H]=$-$4.02. One of the $r$-II stars shows a deficit in $α$ and Fe-peak elements, typical of dwarf galaxy stars. Our search for $r$-process-enhanced stars by RPA efforts, has already roughly doubled the known $r$-process sample.

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arXiv2020

Rotation of solar analogs cross-matching Kepler and Gaia DR2

A major obstacle to interpreting the rotation period distribution for main-sequence stars from Kepler mission data has been the lack of precise evolutionary status for these objects. We address this by investigating the evolutionary status based on Gaia Data Release 2 parallaxes and photometry for more than 30,000 Kepler stars with rotation period measurements. Many of these are subgiants, and should be excluded in future work on dwarfs. We particularly investigate a 193-star sample of solar analogs, and report newly-determined rotation periods for 125 of these. These include 54 stars from a prior sample, of which can confirm the periods for 50. The remainder are new, and 10 of them longer than solar rotation period, suggesting that sun-like stars continue to spin down on the main sequence past solar age. Our sample of solar analogs could potentially serve as a benchmark for future missions such as PLATO, and emphasizes the need for additional astrometric, photometric, and spectroscopic information before interpreting the stellar populations and results from time-series surveys.

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arXiv2020

Li abundances for solar twins in the open cluster M67

We determine lithium (Li) abundances for solar twins in the M67 open cluster to add valuable information about the correlation between Li depletion and stellar age and, then, better understand stellar structure and evolution. We use high resolution and good signal-to-noise ratio spectra to characterize Li depletion in three solar twins from M67, using spectral synthesis in the region of the asymmetric 6707.75 Å\, Li I feature. The mean Li abundance value of A(Li)$=1.6\pm0.2$ dex for our sample of M67 solar twins (our three stars plus a fourth solar twin from a previous analysis in the literature) presents Li abundance expected for its age. Also, the scatter estimated from the standard deviation of the Li abundances in this work is similar to the typical scatter found in a sample of field solar twins presented in the literature.

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arXiv2019

Constraining Exoplanet Metallicities and Aerosols with ARIEL: An Independent Study by the Contribution to ARIEL Spectroscopy of Exoplanets (CASE) Team

Launching in 2028, ESA's Atmospheric Remote-sensing Exoplanet Large-survey (ARIEL) survey of $\sim$1000 transiting exoplanets will build on the legacies of Kepler and TESS and complement JWST by placing its high precision exoplanet observations into a large, statistically-significant planetary population context. With continuous 0.5--7.8~$μ$m coverage from both FGS (0.50--0.55, 0.8--1.0, and 1.0--1.2~$μ$m photometry; 1.25--1.95~$μ$m spectroscopy) and AIRS (1.95--7.80~$μ$m spectroscopy), ARIEL will determine atmospheric compositions and probe planetary formation histories during its 3.5-year mission. NASA's proposed Contribution to ARIEL Spectroscopy of Exoplanets (CASE) would be a subsystem of ARIEL's FGS instrument consisting of two visible-to-infrared detectors, associated readout electronics, and thermal control hardware. FGS, to be built by the Polish Academy of Sciences' Space Research Centre, will provide both fine guiding and visible to near-infrared photometry and spectroscopy, providing powerful diagnostics of atmospheric aerosol contribution and planetary albedo, which play a crucial role in establishing planetary energy balance. The CASE team presents here an independent study of the capabilities of ARIEL to measure exoplanetary metallicities, which probe the conditions of planet formation, and FGS to measure scattering spectral slopes, which indicate if an exoplanet has atmospheric aerosols (clouds and hazes), and geometric albedos, which help establish planetary climate. Our design reference mission simulations show that ARIEL could measure the mass-metallicity relationship of its 1000-planet single-visit sample to $>7.5σ$ and that FGS could distinguish between clear, cloudy, and hazy skies and constrain an exoplanet's atmospheric aerosol composition to $>5σ$ for hundreds of targets, providing statistically-transformative science for exoplanet atmospheres.

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arXiv2018

Thorium in solar twins: implications for habitability in rocky planets

We have investigated the thorium (Th) abundance in a sample of 53 thin disc solar twins covering a wide range of ages. These data provide constrains on the mantle energy budget of terrestrial planets that can be formed over the evolution of the Galaxy's thin disc. We have estimated Th abundances with an average precision of 0.025\,dex (in both [Th/H] and [Th/Fe]) through comprehensive spectral synthesis of a Th\,II line present at 4019.1290\,Å, using very high resolution (R\,=\,115,000) high quality HARPS spectra obtained at the ESO La Silla Observatory. We have confirmed that there is a large energy budget from Th decay for maintaining mantle convection inside potential rocky planets around solar twins, from the Galactic thin disc formation until now, because the pristine [Th/H]$_{\rm ZAMS}$ is super-solar on average under a uniform dispersion of 0.056\,dex (varying from +0.037 up to +0.138\,dex based on linear fits against isochrone stellar age). Comparing to neodymium (Nd) and europium (Eu), two others neutron-capture elements, the stellar pristine abundance of Th follows Eu along the Galactic thin disc evolution, but it does not follow Nd, probably because neodymium has a significant contribution from the $s$-process (about 60\,per\,cent).

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arXiv2018

The Chemical Homogeneity of Sun-like Stars in the Solar Neighborhood

The compositions of stars are a critical diagnostic tool for many topics in astronomy such as the evolution of our Galaxy, the formation of planets, and the uniqueness of the Sun. Previous spectroscopic measurements indicate a large intrinsic variation in the elemental abundance patterns of stars with similar overall metal content. However, systematic errors arising from inaccuracies in stellar models are known to be a limiting factor in such studies, and thus it is uncertain to what extent the observed diversity of stellar abundance patterns is real. Here we report the abundances of 30 elements with precisions of 2% for 79 Sun-like stars within 100 parsecs. Systematic errors are minimized in this study by focusing on solar twin stars and performing a line-by-line differential analysis using high-resolution, high-signal-to-noise spectra. We resolve [X/Fe] abundance trends in galactic chemical evolution at precisions of $10^{-3}$ dex Gyr$^{-1}$ and reveal that stars with similar ages and metallicities have nearly identical abundance patterns. Contrary to previous results, we find that the ratios of carbon-to-oxygen and magnesium-to-silicon in solar metallicity stars are homogeneous to within 10% throughout the solar neighborhood, implying that exoplanets may exhibit much less compositional diversity than previously thought. Finally, we demonstrate that the Sun has a subtle deficiency in refractory material relative to >80% of solar twins (at 2$σ$ confidence), suggesting a possible signpost for planetary systems like our own.

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arXiv2018

The R-Process Alliance: Chemical Abundances for a Trio of R-Process-Enhanced Stars -- One Strong, One Moderate, One Mild

We present detailed chemical abundances of three new bright (V ~ 11), extremely metal-poor ([Fe/H] ~ -3.0), r-process-enhanced halo red giants based on high-resolution, high-S/N Magellan/MIKE spectra. We measured abundances for 20-25 neutron-capture elements in each of our stars. J1432-4125 is among the most r-process rich r-II stars, with [Eu/Fe]= +1.44+-0.11. J2005-3057 is an r-I star with [Eu/Fe] = +0.94+-0.07. J0858-0809 has [Eu/Fe] = +0.23+-0.05 and exhibits a carbon abundance corrected for evolutionary status of [C/Fe]_corr = +0.76, thus adding to the small number of known carbon-enhanced r-process stars. All three stars show remarkable agreement with the scaled solar r-process pattern for elements above Ba, consistent with enrichment of the birth gas cloud by a neutron star merger. The abundances for Sr, Y, and Zr, however, deviate from the scaled solar pattern. This indicates that more than one distinct r-process site might be responsible for the observed neutron-capture element abundance pattern. Thorium was detected in J1432-4125 and J2005-3057. Age estimates for J1432-4125 and J2005-3057 were adopted from one of two sets of initial production ratios each by assuming the stars are old. This yielded individual ages of 12+-6 Gyr and 10+-6 Gyr, respectively.

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arXiv2018

A formation timescale of the Galactic halo from Mg isotopes in dwarf stars

We determine magnesium isotopic abundances of metal poor dwarf stars from the galactic halo, to shed light on the onset of asymptotic giant branch (AGB) star nucleossynthesis in the galactic halo and constrain the timescale of its formation. We observed a sample of eight new halo K dwarfs in a metallicity range of -1.9 < [Fe/H] < -0.9 and 4200 < $T_{eff}$(K) < 4950, using the HIRES spectrograph at the Keck Observatory (R~10^5 and 200 < S/N < 300). We obtain magnesium isotopic abundances by spectral synthesis on three MgH features and compare our results with galactic chemical evolution models. With the current sample, we almost double the number of metal poor stars with Mg isotopes determined from the literature. The new data allow us to determine the metallicity when the $^{26}$Mg abundances start to became important, $[Fe/H] \sim -1.4 \pm 0.1$. The data with $[Fe/H] > -1.4$ are somewhat higher (1-3 $σ$) than previous chemical evolution model predictions, indicating perhaps higher yields of the neutron-rich isotopes. Our results using only AGB star enrichment suggest a timescale for formation for the galactic halo of about 0.3 Gyr, but considering also supernova enrichment, the upper limit for the timescale formation is about 1.5 Gyr.

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arXiv2017

Kepler-11 is a Solar Twin: Revising the Masses and Radii of Benchmark Planets Via Precise Stellar Characterization

The six planets of the Kepler-11 system are the archetypal example of a population of surprisingly low-density transiting planets revealed by the Kepler mission. We have determined the fundamental parameters and chemical composition of the Kepler-11 host star to unprecedented precision using an extremely high quality spectrum from Keck-HIRES (R$\simeq$67,000, S/N per pixel$\simeq$260 at 600 nm). Contrary to previously published results, our spectroscopic constraints indicate that Kepler-11 is a young main-sequence solar twin. The revised stellar parameters and new analysis raise the densities of the Kepler-11 planets by between 20-95% per planet, making them more typical of the emerging class of "puffy" close-in exoplanets. We obtain photospheric abundances of 22 elements and find that Kepler-11 has an abundance pattern similar to that of the Sun with a slightly higher overall metallicity. We additionally analyze the Kepler lightcurves using a photodynamical model and discuss the tension between spectroscopic and transit/TTV-based stellar density estimates.

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arXiv2016

Planet signatures in the chemical composition of Sun-like stars

There are two possible mechanisms to imprint planet signatures in the chemical composition of Sun-like stars: i) dust condensation at the early stages of planet formation, causing a depletion of refractory elements in the gas accreted by the star in the late stages of its formation; ii) planet engulfment, enriching the host star in lithium and refractory elements. We discuss both planet signatures, the influence of galactic chemical evolution, and the importance of binaries composed of stellar twins as laboratories to verify abundance anomalies imprinted by planets.

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arXiv2016

The Solar Twin Planet Search. V. Close-in, low-mass planet candidates and evidence of planet accretion in the solar twin HIP 68468

[Methods]. We obtained high-precision radial velocities with HARPS on the ESO 3.6 m telescope and determined precise stellar elemental abundances (~0.01 dex) using MIKE spectra on the Magellan 6.5m telescope. [Results]. Our data indicate the presence of a planet with a minimum mass of 26 Earth masses around the solar twin HIP 68468. The planet is a super-Neptune, but unlike the distant Neptune in our solar system (30 AU), HIP 68468c is close-in, with a semi-major axis of 0.66 AU, similar to that of Venus. The data also suggest the presence of a super-Earth with a minimum mass of 2.9 Earth masses at 0.03 AU; if the planet is confirmed, it will be the fifth least massive radial velocity planet discovery to date and the first super-Earth around a solar twin. Both isochrones (5.9 Gyr) and the abundance ratio [Y/Mg] (6.4 Gyr) indicate an age of about 6 billion years. The star is enhanced in refractory elements when compared to the Sun, and the refractory enrichment is even stronger after corrections for Galactic chemical evolution. We determined a NLTE Li abundance of 1.52 dex, which is four times higher than what would be expected for the age of HIP 68468. The older age is also supported by the low log(R'HK) (-5.05) and low jitter. Engulfment of a rocky planet of 6 Earth masses can explain the enhancement in both lithium and the refractory elements. [Conclusions]. The super-Neptune planet candidate is too massive for in situ formation, and therefore its current location is most likely the result of planet migration that could also have driven other planets towards its host star, enhancing thus the abundance of lithium and refractory elements in HIP 68468. The intriguing evidence of planet accretion warrants further observations to verify the existence of the planets that are indicated by our data and to better constrain the nature of the planetary system around this unique star.

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arXiv2016

G64-12 and G64-37 are Carbon-Enhanced Metal-Poor Stars

We present new high-resolution chemical-abundance analyses for the well-known high proper-motion subdwarfs G64-12 and G64-37, based on very high signal-to-noise spectra (S/N ~ 700/1) with resolving power R ~ 95,000. These high-quality data enable the first reliable determination of the carbon abundances for these two stars; we classify them as carbon-enhanced metal-poor (CEMP) stars based on their carbonicities, which both exceed [C/Fe] = +1.0. They are sub-classified as CEMP- no Group-II stars, based on their location in the Yoon-Beers diagram of absolute carbon abundance, A(C) vs. [Fe/H], as well as on the conventional diagnostic [Ba/Fe]. The relatively low absolute carbon abundances of CEMP-no stars, in combination with the high effective temperatures of these two stars (Teff ~ 6500 K) weakens their CH molecular features to the point that accurate carbon abundances can only be estimated from spectra with very high S/N. A comparison of the observed abundance patterns with the predicted yields from massive, metal-free supernova models reduces the inferred progenitor masses by factors of ~ 2-3, and explosion energies by factors of ~ 10-15, compared to those derived using previously claimed carbon abundance estimates. There are certainly many more warm CEMP-no stars near the halo main-sequence turnoff that have been overlooked in past studies, directly impacting the derived frequencies of CEMP-no stars as a function of metallicity, a probe that provides important constraints on Galactic chemical evolution models, the initial mass function in the early Universe, and first-star nucleosynthesis.

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arXiv2016

GRACES observations of young [alpha/Fe]-rich stars

We measure chemical abundance ratios and radial velocities in four massive (i.e., young) [alpha/Fe]-rich red giant stars using high-resolution high-S/N spectra from ESPaDOnS fed by Gemini-GRACES. Our differential analysis ensures that our chemical abundances are on the same scale as the Alves-Brito et al. (2010) study of bulge, thin and thick disk red giants. We confirm that the program stars have enhanced [alpha/Fe] ratios and are slightly metal poor. Aside from lithium enrichment in one object, the program stars exhibit no chemical abundance anomalies when compared to giant stars of similar metallicity throughout the Galaxy. This includes the elements Li, O, Si, Ca, Ti, Cr, Ni, Cu, Ba, La, and Eu. Therefore, there are no obvious chemical signatures that can help to reveal the origin of these unusual stars. While our new observations show that only one star (not the Li-rich object) exhibits a radial velocity variation, simulations indicate that we cannot exclude the possibility that all four could be binaries. In addition, we find that two (possibly three) stars show evidence for an infrared excess, indicative of a debris disk. This is consistent with these young [alpha/Fe]-rich stars being evolved blue stragglers, suggesting their apparent young age is a consequence of a merger or mass transfer. We would expect a binary fraction of ~50% or greater for the entire sample of these stars, but the signs of the circumbinary disk may have been lost since these features can have short timescales. Radial velocity monitoring is needed to confirm the blue straggler origin.

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arXiv2016

Correlation between lithium abundances and ages of solar twin stars

We want to determine the lithium abundances of solar twin stars as a function of stellar age to provide constraints for stellar evolutions models and to investigate whether there is a connection between low Li abundance and the occurrence of planets. For a sample of 21 solar twins observed with the HARPS spectrograph at high spectral resolution (R~115.000) and very high signal-to-noise ratio (600 < S/N < 2400), precise lithium abundances were obtained by spectral synthesis of the LiI 6707.8 A line and compared to stellar ages, masses, and metallicities determined from a spectroscopic analysis of the same set of HARPS spectra. We show that for the large majority of the solar twins there is a strong correlation between lithium abundance and stellar age. As the age increases from 1 to 9 Gyr, the Li abundance decreases by a factor of ~ 50. The relation agrees fairly well with predictions from non-standard stellar evolution models of Li destruction at the bottom of the upper convection zone. Two stars deviate from the relation by having Li abundances enhanced by a factor of ~ 10, which may be due to planet engulfment. On the other hand, we find no indication of a link between planet hosting stars and enhanced lithium depletion.

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