Resumen:

We conducted an extensive investigation into the stellar activity and variability of Iota-Horologii, a young star resembling our Sun. Our study employed a comprehensive multi-wavelength approach, combining data from various advanced instruments to obtain a comprehensive understanding of the star's behavior. To monitor the system, we utilized the ultra-stable spectropolarimeter/velocimeter HARPS at the ESO 3.6-m telescope for long-term spectropolarimetric observations. Additionally, we incorporated high-precision photometry data from the NASA Transiting Exoplanet Survey Satellite (TESS) and observations in the far- and near-ultraviolet spectral regions using the STIS instrument on the NASA/ESA Hubble Space Telescope (HST). Through the analysis of the gradient of the power spectra (GPS) derived from the TESS lightcurves, we were able to constrain the faculae-to-spot driver ratio (Sf/Ss) to 0.510 ± 0.023. This finding indicates that the stellar surface was primarily dominated by spots during the observation period. Furthermore, for the first time, we compared the photospheric activity properties obtained from the GPS method with a magnetic field map derived through Zeeman-Doppler imaging (ZDI) using simultaneous spectropolarimetric data. This comparative analysis provided valuable insights into the star's magnetic field and its influence on the observed activity. By employing different stellar activity proxies, we achieved a more comprehensive interpretation of the observed variability. Notably, we observed enhanced emission in the HST transition line diagnostics Civ and Ciii, suggesting the occurrence of a flaring event. Moreover, by analyzing TESS data simultaneously with the HST observations, we investigated the photometric variability during the precise moment of increased emission. This analysis allowed us to derive correlations between different observables, enabling us to probe the star's characteristics from its photosphere to its corona. Our extensive multi-wavelength campaign has provided a detailed understanding of the stellar activity and variability of Iota-Horologii.

Resumen:

The existence of a small percentage of lithium (Li)-rich giants, defined as those showing A(Li) > 1.5 dex in their surfaces, is a decades old problem that challenges stellar evolution. More recently, based on Li data for large samples of evolved stars delivered by large spectroscopic surveys, the case has been made for Li-rich giants on a much more generalized scale, suggesting that all low-mass clump giants in the field, with typical levels 0 < A(Li) < 1.0 dex, are also enriched in Li beyond what is to be expected from standard stellar evolution. This would be an extraordinary turn of events for many branches of astrophysics, and has prompted works that explore new production channels of Li inside low-mass giants, particularly associated to the helium flash. Although proposed for all low-mass stars, if the mechanism really operates in nature, it may also have something to do with the classic problem of Li-rich giants. I will review the evidence and arguments for and against this possibility of a new Li production channel ubiquitous to all low-mass stars, and will show the results of a population synthesis analysis that, by accounting for the correct mass distribution of the progenitor stars of today's giants as informed by asteroseismology, argues against these claims.

Resumen:

The mass-loss mechanism in asymptotic giant branch (AGB) stars is not yet fully understood. It is crucial to spatially resolve the region within several stellar radii, where the stellar wind accelerates. We present 20-milliarcsecond resolution ALMA imaging of the well-studied AGB star W Hya in molecular lines of SiO, SO$_2$, H$_2$O, SO, HCN, TiO, AlO, and AlOH at 250--269 GHz, including masers from SiO, H$_2$O, and SO$_2$. The spatial resolution is three times finer than the W Hya's millimeter continuum angular diameter of ~60 mas, allowing us to spatially resolve the emission extending to $\sim$100 mas ($\sim$5 stellar radii) as well as inhomogeneous absorption over the stellar disk. The emission is irregularly shaped with a plume extending in the NNW, a tail extending in the SSE, an extended atmosphere elongated in the ENE-WSW direction, and many clumpy structures. The spatially resolved spectra of some lines show outflow velocities close to the local escape velocity. Surprisingly, we detected prominent emission over the stellar disk--instead of pure absorption as expected--in Si$^{17}$O, $^{30}$SiO, H$_2$O, and SO$_2$ lines. The surface emission seen in the Si$^{17}$O and vibrationally excited H$_2$O (v$_2$=2, 268 GHz) lines is particularly strong, indicating maser actions. The H$_2$O masers are confined within $\sim$50 mas (2.4 stellar radii), and the spatially resolved maser spectra are very broad, ranging from about -10 to 14 km~s$^{-1}$. This can be explained by outflowing and infalling motion induced by the stellar pulsation. The visible polarimetric images taken contemporaneously with our ALMA data reveal good agreement in the spatial distribution between the H$_2$O maser emission and clumpy dust clouds, lending support to the picture that the H$_2$O masers trace cool and dense pockects, where dust can form.

Resumen:

The physical parameters of a sample composed of 28 late-type stars (spectral types G and K) were determined using recent parallaxes from Gaia DR3 together with analysis of the medium-resolution spectra (R approximately 21000) obtained in a dedicated observing program managed in the TIGRE telescope. Through comparisons with well-tested evolutionary tracks, mass values and stages of evolution were also derived, which in turn were put into perspective together with measurements of the S-index indicator of chromospheric magnetic activity. Four of the stars in the sample belong to the open cluster M41, so their coeval nature has made it possible to impose additional constraints to model their evolution, and thanks to this, the permanence of the activity exhibited during the relatively stable central helium-burning stage has been explored in good detail, whereas the non-cluster stars offer an insight into peripheral stages of evolution. Evidence points to a correlation between the state of evolutionary advancement within the horizontal branch and a sustained decline in activity, analogous to what has long been known to occur among cool main-sequence stars, and just as it was recently discovered to occur in the four Hyades K giants.

Resumen:

Most brown dwarfs show some level of photometric or spectrophotometric variability in different wavelength independently of their spectral type. Nevertheless, to date, we do not have an informed method to preselect the brown dwarfs that might show a higher variability amplitude for a thorough variability study. In Oliveros-Gomez, N., et.al. (2022), we designed and tested near-infrared spectral indices to preselect the most likely variable mid- and late-T dwarfs. In this work, we extend the method to preselect the most likely variable mid-L dwarfs (L4-L8). We used time-resolved near-infrared Hubble Space Telescope Wide Field Camera 3 spectra of a L6 dwarf, LP261-75B, to design our novel spectral indices. We compare the indices between them, which separate variable and non-variable spectra in different part of the index-index plots. In addition, we use a Machine Learning method to quantitively define areas where we most likely will find variable or non-variable brown dwarfs. We tested these spectral indices on 75 L4–L8 near-infrared SpeX/IRTF spectra, finding 30 new early- and mid-L variable candidates. We estimated the variability fraction of our sample as $\sim 54\%$, which agrees with the variability fractions provided by Metchev 2015 and Buenzli 2014 for L dwarfs. In addition, 11 of the 41 previously known variables in our sample of SpeX spectra are flagged as variable candidates by our indices. Similarly, 10 known non-variables in our sample are flagged as non-variable objects by our indices. These results suggest that our spectral indices might be used to find variable mid-L brown dwarf variables.

Resumen:

Stellar age is a key fundamental property for understanding the history, evolution, and future of stellar populations, the Milky Way, and exoplanets. In order to estimate precise ages, we also need to understand the evolution of other fundamental properties such as mass, radius and effective temperature. However, current theoretical models over-predict effective temperatures, and under-predict radii, compared to observations of low-mass stars, or M dwarfs (radius inflation problem). Solving this problem is essential for current and upcoming NASA missions such as such as the Transiting Exoplanet Survey Satellite (TESS) and the Nancy Grace Roman Space Telescope which are expected to discover and characterize Earth-like exoplanets orbiting the nearest M dwarfs. Magnetic activity has been shown to be correlated with radius inflation for binary stars, but no clear correlation was found for single stars. In this talk I will present a model independent method to estimate stellar radii and how we used it to study the problem of radius inflation. Taking advantage of the precise photometry from Gaia DR3 we calibrated the surface brightness-color relation (SBCR) for low-mass stars. With the Gaia DR3 parallaxes and our SBCR we estimated radii for M dwarfs with measurements of the H$\alpha$ spectral line –the first Balmer line– which in emission is a magnetic activity indicator. We found that our model independent SBCR estimates accurate radii by comparing our results with the literature. In addition, we found that radius inflation is correlated to magnetic activity for single stars, and we calibrated the percentage of radius inflation as a function of H$\alpha$ emission and mass. This correlation could explain the difference between models and observations for M dwarf radii, and get us a step closer to understanding M dwarf evolution.