Resumen:

Lithium is an essential element for Stellar Astrophysics because it is depleted at somewhat low temperatures which are easily reached in the interior of stars. According to standard models, the base of the convection zone of main sequence solar-type stars does not reach deep enough inside the star to burn Li. However, it is observed that older field solar analogs present a lower Li content, indicating that part of the Li present in the stars is taken towards inner regions through non-standard mixture processes. In this work, we determined Li abundances for a sample of 153 solar analogs, exploring the dependence of Li with age, mass and convective mass. The high-quality (R $=$ 115000; 270 $\leq$ SNR $\leq$ 1000) spectra used in this project were taken with ESO’s HARPS spectrograph. We employed the method of high precision differential spectroscopy, measuring equivalent widths of spectral lines to determine atmospheric parameters and infer masses and ages. The Li abundance was determined via spectral synthesis. We found a strong correlation between lithium abundance and age, and also a clear link between low lithium abundances and lower masses (and higher convective masses). We added another high-precision HARPS sample of solar analogs studied in our group to consider the effect of planets on Li in a sample of 194 stars and we report that planet hosts have on average Li abundances -0.3 dex lower than non-planet hosts with similar parameters, with a significance above 99\% for our results.

Resumen:

Deriving chemical abundances from spectra is one of the great jobs in astronomy. The chemical evolution of the Galaxy can be studied through the abundance of chemical elements of stars of different stellar populations. The applications are diverse, deriving abundances in a spectrum can help us predict, for example, which is the future of stars like the Sun will be like. Each star has a chemical signature depending on the conditions of its formation, its mass and age. The analysis is a job that demands time and precision. Commonly, for a robust work, it is necessary to analyse hundreds of spectral lines, which can take days or months to adjust all of them. For each line, it is necessary to run programs that generate synthetic spectra of stars several times, changing the chemical abundance of the element being analysed in order to better fit the model with the observed spectrum. MEAFS (Multiple Element Abundance Fitting Software) is a code being developed in \textit{Python} and \textit{C} that aims to fit abundances automatically and for multiple elements in a single execution. It is also able to adjust the convolution, the continuum and the wavelength shift of the synthetic spectrum for the best fit with the observed spectrum. Not only does it faster than the manual way, it is in principle also more accurate than an average person. The code is under development and currently MEAFS can run the \textit{Turbospectrum} (Plez, 2012) spectrum generator. The version to use the code \textit{PFANT} (Barbuy et al., 2018) is aso in progress. The current version with \textit{Turbospectrum} was tested with spectra of the star CS 31082-001 and the analysis of 260 spectral lines takes around two hours. The code is validated for clear and unblended lines as shown in Ernandes et al. (2023).

Resumen:

Although the theoretical bases of line-driven winds were set around half a century ago, it is still uncertain what are the real mass-loss rates among different luminosity classes of O stars. One severe issue is the so-called weak wind phenomenon, where the empirical mass-loss rates ($\dot{M}$) of late O stars (O8-9V and O8-9III) are lower by $10^{2}$ than theoretical predictions from Vink’s mass loss recipe. In this work, we present our ongoing results on weak winds in late O dwarfs and giants using hydrodynamical simulations with the code HYDWIND. These hydrodynamical results are used as input parameters in state-of-the-art non-LTE radiative transfer codes, CMFGEN and FASTWIND, to calculate synthetic spectra and test them against observations in the UV and visible regions. In Fig. 1, we show our preliminary results based on H$\alpha$ spectroscopy for the weak wind star HD 46202 (O9V). The observed H$\alpha$ line profile is well reproduced using both codes with the solved hydrodynamics by HYDWIND. This happens when we consider the last fast wind solution, that is, increasing the value of the line-force parameter $\delta$ (from the CAK-theory) before obtaining a $\delta$-slow wind solution ($\delta >$ 0.25-0.30). In this case, our derived $\dot{M}$ for this star is $\sim$$3\mathrm{E}{-10}$ $M_\odot$ yr\textsuperscript{-1} (weak wind regime). For comparison, this value is about $10^{2}$ lower than the one from Vink’s mass loss recipe. In conclusion, these results indicate that weak winds could be explained from first principles (solving the wind motion equation), deserving additional constraints from spectral regions other than the UV and visible. We also discuss our perspectives of exploring weak winds in the infrared region, especially using the Br$\alpha$ line profile, since we had allocated time to observe late O dwarfs and giants with the GEMINI/GNIRS and ESO/CRIRES instruments, to be performed during the second semester of 2023.

Resumen:

Thanks to space-borne missions like Kepler and TESS, and new asteroseismic techniques developed in recent years, the rotation rate in stellar interiors was obtained for hundreds of stars in different evolutionary phases. This revealed a strong disagreement between stellar models and observations, pointing to a missing physical process in stellar evolution models with rotation. The treatment of rotation in stellar interiors affects both the transport of angular momentum and chemical elements, and hence can change several outputs of stellar models such as the surface chemical composition, surface rotational velocities, and even affect the predicted rotational periods of neutron stars and spins of black holes. Therefore improving our understanding of angular momentum transport processes in stellar interiors is fundamental to improve stellar models and make better predictions. In this talk I will review the status of the problem with emphasis on recent constraints obtained with asteroseismic techniques and its interplay with spectroscopic data. I will then present the main physical candidates proposed to solve the inconsistency between models and observations, with particular emphasis on magnetic fields present in stellar interiors.

Resumen:

The globular clusters in the innermost regions of the Milky Way are affected by high, differential extinction. Their physical parameters are therefore not as reliably measured as their counterparts of the outer halo. Extinction effects are highly diminished in near-infrared observations, like the ones provided by the Vista Variables in the Via Lactea (VVV) survey and its extension, the VVV-X. More than 50 known Galactic globular clusters located towards the inner Milky Way lie in the region surveyed by the VVV and the VVV-X. Their multi-epoch observations allow us to search and characterize the pulsating stars contained in these star clusters. The tight near-infrared period-luminosity-metallicity relations of these variable stars allow a better parametrization of the globular clusters to which they belong. In my contribution, I will present our current, ongoing analyses of the pulsating stars in some of these poorly known objects within the framework of the VVV and VVV-X.