J. Meléndez

Universidade de São Paulo

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Co-Authors

M. Asplund

Australian Academy of Science

140

I. Ramírez

Heliatek (Germany)

108

David Yong

Kolon Industries (South Korea)

Alan Alves-Brito

Universidade Federal do Rio Grande do Sul

Megan Bedell

Simons Foundation

L. Spina

Osservatorio Astronomico di Padova

Jacob L. Bean

University of Chicago

T. Bensby

Lund University

L. Casagrande

Australian National University

S. Feltzing

Lund University

Cooperative Institutions

Centre National de la Recherche Scientifique

127

Max Planck Institute for Astronomy

115

Max Planck Society

101

Universidade de São Paulo

University of Padua

Observatoire de Paris

Osservatorio Astronomico di Padova

California Institute of Technology

European Southern Observatory

National Institute for Astrophysics

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Modelling the solar twin 18 Scorpii

Astronomy and Astrophysics (2018)

M. Bazot O. L. Creevey J. Christensen‐Dalsgaard J. Meléndez

Solar twins are objects of great interest in that they allow us to understand better how stellar evolution and structure are affected by variations of the stellar mass, age and chemical composition in the vicinity of the commonly accepted solar values. We aim to use the existing spectrophotometric, interferometric and asteroseismic data for the solar twin 18 Sco to constrain stellar evolution models. 18 Sco is the brightest solar twin and is a good benchmark for the study of solar twins. The goal is to obtain realistic estimates of its physical characteristics (mass, age, initial chemical composition, mixing-length parameter) and realistic associated uncertainties using stellar models. We set up a Bayesian model that relates the statistical properties of the data to the probability density of the stellar parameters. Special care is given to the modelling of the likelihood for the seismic data, using Gaussian mixture models. The probability densities of the stellar parameters are approximated numerically using an adaptive MCMC algorithm. From these approximate distributions we proceeded to a statistical analysis. We also performed the same exercise using local optimisation. The precision on the mass is approximately 6%. The precision reached on X0 and Z0 and the mixing-length parameter are respectively 6%, 9%, and 35%. The posterior density for the age is bimodal, with modes at 4.67 Gyr and 6.95 Gyr, the first one being slightly more likely. We show that this bimodality is directly related to the structure of the seismic data. When asteroseismic data or interferometric data are excluded, we find significant losses of precision for the mass and the initial hydrogen-mass fraction. Our final estimates of the uncertainties from the Bayesian analysis are significantly larger than values inferred from local optimization.

Bimodality

Data set

Stellar mass

10.1051/0004-6361/201834058

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Citations (17)

The Solar Twin Planet Search

Astronomy and Astrophysics (2014)

I. Ramírez J. Meléndez Jacob L. Bean M. Asplund Megan Bedell

Context. We are carrying out a search for planets around a sample of solar twin stars using the HARPS spectrograph. The goal of this project is to exploit the advantage offered by solar twins to obtain chemical abundances of unmatched precision. This survey will enable new studies of the stellar composition – planet connection.

10.1051/0004-6361/201424244

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Citations (155)

Abundance analysis of the bulge globular clusters NGC 6553 and NGC 6528

B. Barbuy J. Meléndez S. Ortolani M. Zoccali E. Bica

A detailed abundance analysis of 5 giants of the metal-rich bulge globular cluster NGC 6553 was carried out using high resolution infrared spectra in the H band, obtained at the Gemini-South 8m telescope. High resolution spectra of 3 stars in the bulge globular cluster NGC 6528 were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph. The present analysis provides a metallicity [Fe/H] = −0.15±0.2 and [α/Fe] ≈ +0.2 for NGC 6528, and [Fe/H] = –0.20±0.10 and an oxygen overabundance of [O/Fe] = +0.20 for NGC 6553, resulting in an overall metallicity Z ≈ Z for both clusters

Very Large Telescope

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Oxygen Abundances in Metal‐poor Stars (−2.2 < [Fe/H] < −1.2) from Infrared OH Lines

The Astrophysical Journal (2001)

J. Meléndez B. Barbuy F. Spite

Infrared OH lines at 1.55 - 1.56 um in the H-band were obtained with the Phoenix high-resolution spectrograph at the 2.1m telescope of the Kitt Peak National Observatory for a sample of 14 metal-poor stars. Detailed analyses of the sample stars have been carried out, deriving stellar parameters based on two methods: (a) spectroscopic parameters; (b) IRFM effective temperatures, trigonometric gravities and metallicities from Fe II lines. The Fe I lines present in the H-band region observed showed to be well fitted by the stellar parameters within $\Delta$[Fe/H] < 0.15 dex. The oxygen abundances were derived from fits of spectrum synthesis calculations to the infrared OH lines. CO lines in the H- and K-bands were obtained for a subsample in order to determine their carbon abundances. Adopting the spectroscopic parameters a mean oxygen-to-iron ratio of [O/Fe] ~ +0.52 is obtained, whereas using the IRFM temperatures, Hipparcos gravities and [FeII/H], [O/Fe] ~ +0.25 is found. A mean of the two methods gives a final value of [O/Fe] ~ +0.4 for the metallicity range -2.2 < [Fe/H] < -1.2 of the sample metal-poor stars.

10.1086/321624

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Citations (58)

Solar twins in M 67: evolutionary status and lithium abundance

Astronomy and Astrophysics (2010)

M. Castro J.-D. do Nascimento K. Biazzo J. Meléndez J. R. De Medeiros

Aims. We determine the age and mass of the three best solar twin candidates in open cluster M 67 through lithium evolutionary models.

10.1051/0004-6361/201015034

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Citations (27)

The Solar Twin Planet Search IV. The Sun as a typical rotator and evidence for a new rotational braking law for Sun-like stars

Astronomy and Astrophysics (2016)

Leonardo A. Dos Santos J. Meléndez J.-D. do Nascimento Megan Bedell I. Ramírez

Context. It is still unclear how common the Sun is when compared to other similar stars in regards to some of its physical properties, such as rotation. Considering that gyrochronology relations are widely used today to estimate ages of stars in the main sequence, and that the Sun is used to calibrate it, it is crucial to assess whether these procedures are acceptable.Aims. We analyze the rotational velocities, limited by the unknown rotation axis inclination angle, of an unprecedented large sample of solar twins to study the rotational evolution of Sun-like stars, and assess whether the Sun is a typical rotator.Methods. We used high-resolution (R = 115 000) spectra obtained with the HARPS spectrograph and the 3.6 m telescope at La Silla Observatory. The projected rotational velocities for 81 solar twins were estimated by line profile fitting with synthetic spectra. Macroturbulence velocities were inferred from a prescription that accurately reflects their dependence with effective temperature and luminosity of the stars.Results. Our sample of solar twins include some spectroscopic binaries with enhanced rotational velocities, and we do not find any nonspectroscopic binaries with unusually high rotation velocities. We verified that the Sun does not have a peculiar rotation, but the solar twins exhibit rotational velocities that depart from the Skumanich relation.Conclusions. The Sun is a regular rotator when compared to solar twins with a similar age. Additionally, we obtain a rotational braking law that better describes the stars in our sample (v ∝ t -0.6 ) in contrast to previous, often-used scalings.

Rotation period

Solar rotation

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The age and abundance structure of the stellar populations in the central sub-kpc of the Milky Way

Proceedings of the International Astronomical Union (2017)

T. Bensby S. Feltzing Andrew Gould Jennifer C. Yee Jennifer A. Johnson

The four main findings about the age and abundance structure of the Milky Way bulge based on microlensed dwarf and subgiant stars are: (1) a wide metallicity distribution with distinct peaks at [Fe/H]=-1.09, -0.63, -0.20, +0.12, +0.41; (2) a high fraction of intermediate-age to young stars where at [Fe/H]>0 more than 35 % are younger than 8 Gyr, (3) several episodes of significant star formation in the bulge 3, 6, 8, and 11 Gyr ago; (4) the `knee' in the alpha-element abundance trends of the sub-solar metallicity bulge appears to be located at a slightly higher [Fe/H] (about 0.05 to 0.1 dex) than in the local thick disk.

Subgiant

Thick disk

10.1017/s1743921317007426

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HD 183579b: a warm sub-Neptune transiting a solar twin detected by TESS

Monthly Notices of the Royal Astronomical Society (2021)

Tianjun Gan Megan Bedell Sharon X. Wang Daniel Foreman-Mackey J. Meléndez

We report the discovery and characterization of a transiting warm sub-Neptune planet around the nearby bright ($V=8.75$ mag, $K=7.15$ mag) solar twin HD 183579, delivered by the Transiting Exoplanet Survey Satellite (TESS). The host star is located $56.8\pm0.1$ pc away with a radius of $R_{\ast}=0.97\pm0.02\ R_{\odot}$ and a mass of $M_{\ast}=1.03\pm0.05\ M_{\odot}$. We confirm the planetary nature by combining space and ground-based photometry, spectroscopy, and imaging. We find that HD 183579b (TOI-1055b) has a radius of $R_{p}=3.53\pm0.13\ R_{\oplus}$ on a $17.47$ day orbit with a mass of $M_{p}=11.2\pm5.4\ M_{\oplus}$ ($3\sigma$ mass upper limit of $27.4\ M_{\oplus}$). HD 183579b is the fifth brightest known sub-Neptune planet system in the sky, making it an excellent target for future studies of the interior structure and atmospheric properties. By performing a line-by-line differential analysis using the high resolution and signal-to-noise ratio HARPS spectra, we find that HD 183579 joins the typical solar twin sample, without a statistically significant refractory element depletion.

Neptune

Solar mass

10.1093/mnras/stab2224

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First high-precision differential abundance analysis of extremely metal-poor stars

Astronomy and Astrophysics (2015)

Henrique Reggiani J. Meléndez David Yong I. Ramírez M. Asplund

Context: Studies of extremely metal-poor stars indicate that chemical abundance ratios [X/Fe] have an rms scatter as low as 0.05 dex (12 \%). It remains unclear whether this reflects observational uncertainties or intrinsic astrophysical scatter arising from physical conditions in the ISM at early times. Aims: Measure differential chemical abundance ratios in extremely metal-poor stars to investigate the limits of precision and to understand whether cosmic scatter or observational errors are dominant. Methods: We used high resolution (R $\sim 95,000$) and high S/N (S/N $= 700$ at 5000$\AA$) HIRES/Keck spectra, to determine high precision differential abundances between two extremely metal-poor stars through a line-by-line differential approach. We determined stellar parameters for the star G64-37 with respect to the standard star G64-12. We performed EW measurements for the two stars for the lines recognized in both stars and performed spectral synthesis to study the carbon abundances. Results: The differential approach allowed us to obtain errors of $\sigma$(T$_{eff}$ ) $=$ 27 K, $\sigma$(log $g$) $=$ 0.06 dex, $\sigma$([Fe/H]) $=$ 0.02 dex and $\sigma$(v$_{t}$ ) $=$ 0.06 kms$^{-1}$. We estimated relative chemical abundances with a precision as low as $\sigma$([X/Fe]) $\approx$ 0.01 dex. The small uncertainties demonstrate that there are genuine abundance differences larger than the measurement errors. The observed Li difference can not be explained by the difference in mass, because the less massive star has more Li. Conclusions: It is possible to achieve an abundance precision around $\approx$ 0.01-0.05 dex for extremely metal-poor stars, opening new windows on the study of the early chemical evolution of the Galaxy.

Root mean square

Square root

10.1051/0004-6361/201527439

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Citations (18)

The Solar Twin Planet Search III. The [Y/Mg] clock: estimating stellar ages of solar-type stars

arXiv (Cornell University) (2016)

Marcelo Tucci Maia I. Ramírez J. Meléndez Megan Bedell Jacob L. Bean

Solar twins are stars with similar stellar (surface) parameters to the Sun that can have a wide range of ages. This provide an opportunity to analyze the variation of their chemical abundances with age. Nissen (2015) recently suggested that the abundances of the s-process element Y and the $\alpha$-element Mg could be used to estimate stellar ages. This paper aims to determine with high precision the Y, Mg, and Fe abundances for a sample of 88 solar twins that span a broad age range ($0.3-10.0$\,Gyr) and investigate their use for estimating ages. We obtained high-quality Magellan Inamori Kyocera Echelle (MIKE) spectra and determined Y and Mg abundances using equivalent widths and a line-by-line differential method within a 1D LTE framework. Stellar parameters and iron abundances were measured in Paper I of this series for all stars, but a few (three) required a small revision. The [Y/Mg] ratio shows a strong correlation with age. It has a slope of -0.041$\pm$0.001 dex/Gyr and a significance of 41 $\sigma$. This is in excellent agreement with the relation first proposed by Nissen (2015). We found some outliers that turned out to be binaries where mass transfer may have enhanced the yttrium abundance. Given a precise measurement of [Y/Mg] with typical error of 0.02 dex in solar twins, our formula can be used to determine a stellar age with $\sim$0.8 Gyr precision in the 0 to 10 Gyr range.

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