The Abundance of S-Process Elements: Temporal and Spatial Trends from Open Cluster Observations

Spectroscopic observations of stars belonging to open clusters, with well-determined ages and distances, are a unique tool for constraining stellar evolution, nucleosynthesis, mixing processes, and, ultimately, Galactic chemical evolution. Abundances of slow (s) process neutron capture elements in stars that retain their initial surface composition open a window into the processes that generated them. In particular, they give us information on their main site of production, i.e., the low-and intermediate-mass Asymptotic Giant Branch (AGB) stars. In the present work, we review some observational results obtained during the last decade that contributed to a better understanding of the AGB phase: the growth of s-process abundances at recent epochs, i.e., in the youngest stellar populations; the different relations between age and [s/Fe] in distinct regions of the disc; and finally the use of s-process abundances combined with those of α elements, [s/α], to estimate stellar ages. We revise some implications that these observations had both on stellar and Galactic evolution, and on our ability to infer stellar ages

Red Horizontal Branch stars: an asteroseismic perspective

Robust age estimates of red giant stars are now possible thanks to the precise inference of their mass based on asteroseismic constraints. However, there are cases where such age estimates can be highly precise yet very inaccurate. An example is giants that have undergone mass loss or mass transfer events that have significantly altered their mass. In this context, stars with "apparent" ages significantly higher than the age of the Universe are candidates as stripped stars, or stars that have lost more mass than expected, most likely via interaction with a companion star, or because of the poorly understood mass-loss mechanism along the red-giant branch. In this work we identify examples of such objects among red giants observed by $\textit{Kepler}$, both at low ([Fe/H] $\lesssim -0.5$) and solar metallicity. By modelling their structure and pulsation spectra, we find a consistent picture confirming that these are indeed low-mass objects consisting of a He core of $\approx 0.5 \, M_\odot$ and an envelope of $\approx 0.1 - 0.2 \, M_\odot$. Moreover, we find that these stars are characterised by a rather extreme coupling ($q \gtrsim 0.4$) between the pressure-mode and gravity-mode cavities, i.e. much higher than the typical value for red clump stars, providing thus a direct seismic signature of their peculiar structure. The complex pulsation spectra of these objects, if observed with sufficient frequency resolution, hold detailed information about the structural properties of likely products of mass stripping, hence can potentially shed light on their formation mechanism. On the other hand, our tests highlight the difficulties associated with measuring reliably the large frequency separation, especially in shorter datasets, with impact on the reliability of the inferred masses and ages of low-mass Red Clump stars with e.g. K2 or TESS data.Comment: Accepted for publication in A&A Letter

The evolution of the Milky Way's thin disc radial metallicity gradient with K2 asteroseismic ages

The radial metallicity distribution of the Milky Way's disc is an important observational constraint for models of the formation and evolution of our Galaxy. It informs our understanding of the chemical enrichment of the Galactic disc and the dynamical processes therein, particularly radial migration. We investigate how the metallicity changes with guiding radius in the thin disc using a sample of red-giant stars with robust astrometric, spectroscopic and asteroseismic parameters. Our sample contains $668$ stars with guiding radii $4$ kpc < $R_\mathrm{g}$ < $11$ kpc and asteroseismic ages covering the whole history of the thin disc with precision $\approx 25\%$. We use MCMC analysis to measure the gradient and its intrinsic spread in bins of age and construct a hierarchical Bayesian model to investigate the evolution of these parameters independently of the bins. We find a smooth evolution of the gradient from $\approx -0.07$ dex/kpc in the youngest stars to $\approx -0.04$ dex/kpc in stars older than $10$ Gyr, with no break at intermediate ages. Our results are consistent with those based on asteroseismic ages from CoRoT, with that found in Cepheid variables for stars younger than $1$ Gyr, and with open clusters for stars younger than $6$ Gyr. For older stars we find a significantly lower metallicity in our sample than in the clusters, suggesting a survival bias favouring more metal-rich clusters. We also find that the chemical evolution model of Chiappini (2009) is too metal-poor in the early stages of disc formation. Our results provide strong new constraints for the growth and enrichment of the thin disc and radial migration, which will facilitate new tests of model conditions and physics.Comment: 15 pages, 16 figures. Accepted for publication in MNRA

How Magnetic Activity Alters What We Learn from Stellar Spectra

Magnetic fields and stellar spots can alter the equivalent widths of absorption lines in stellar spectra, varying during the activity cycle. This also influences the information that we derive through spectroscopic analysis. In this study, we analyze high-resolution spectra of 211 sunlike stars observed at different phases of their activity cycles, in order to investigate how stellar activity affects the spectroscopic determination of stellar parameters and chemical abundances. We observe that the equivalent widths of lines can increase as a function of the activity index log R'HK during the stellar cycle, which also produces an artificial growth of the stellar microturbulence and a decrease in effective temperature and metallicity. This effect is visible for stars with activity indexes log RHK -5.0 (i.e., younger than 4-5 Gyr), and it is more significant at higher activity levels. These results have fundamental implications on several topics in astrophysics that are discussed in the paper, including stellar nucleosynthesis, chemical tagging, the study of Galactic chemical evolution, chemically anomalous stars, the structure of the Milky Way disk, stellar formation rates, photoevaporation of circumstellar disks, and planet hunting.L.S. and A.I.K. acknowledge financial support from the Australian Research Council (Discovery Project 170100521). A.R.C. acknowledges the support from the Australian Research Council (DECRA 190100656). J.M. thanks support by FAPESP (2018/04055-8) and CNPq (Bolsa de Produtividade). J.Y.G. acknowledges the support from CNPq. This research was supported by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE17010001

Determination of stellar parameters for Ariel targets: a comparison analysis between different spectroscopic methods

Ariel has been selected as the next ESA M4 science mission and it is expected to be launched in 2028. During its 4-year mission, Ariel will observe the atmospheres of a large and diversified population of transiting exoplanets. A key factor for the achievement of the scientific goal of Ariel is the selection strategy for the definition of the input target list. A meaningful choice of the targets requires an accurate knowledge of the planet hosting star properties and this is necessary to be obtained well before the launch. In this work, we present the results of a bench-marking analysis between three different spectroscopic techniques used to determine stellar parameters for a selected number of targets belonging to the Ariel reference sample. We aim to consolidate a method that will be used to homogeneously determine the stellar parameters of the complete Ariel reference sample. Homogeneous, accurate and precise derivation of stellar parameters is crucial for characterising exoplanet-host stars and in turn is a key factor for the accuracy of the planet properties

The homogeneous characterisation of Ariel host stars

The Ariel mission will characterise the chemical and thermal properties of the atmospheres of about a thousand exoplanets transiting their host star(s). The observation of such a large sample of planets will allow to deepen our understanding of planetary and atmospheric formation at the early stages, providing a truly representative picture of the chemical nature of exoplanets, and relating this directly to the type and chemical environment of the host star. Hence, the accurate and precise determination of the host star fundamental properties is essential to Ariel for drawing a comprehensive picture of the underlying essence of these planetary systems. We present here a structured approach for the characterisation of Ariel stars that accounts for the concepts of homogeneity and coherence among a large set of stellar parameters. We present here the studies and benchmark analyses we have been performing to determine robust stellar fundamental parameters, elemental abundances, activity indices, and stellar ages. In particular, we present results for the homogeneous estimation of the activity indices S and log (RHK') , and preliminary results for elemental abundances of Na, Al, Mg, Si, C, N. In addition, we analyse the variation of a planetary spectrum, obtained with Ariel, as a function of the uncertainty on the stellar effective temperature. Finally, we present our observational campaign for precisely and homogeneously characterising all Ariel stars in order to perform a meaningful choice of final targets before the mission launch

Enabling planetary science across light-years. Ariel Definition Study Report

Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was adopted as the fourth medium-class mission in ESA's Cosmic Vision programme to be launched in 2029. During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. The payload consists of an off-axis Cassegrain telescope (primary mirror 1100 mm x 730 mm ellipse) and two separate instruments (FGS and AIRS) covering simultaneously 0.5-7.8 micron spectral range. The satellite is best placed into an L2 orbit to maximise the thermal stability and the field of regard. The payload module is passively cooled via a series of V-Groove radiators; the detectors for the AIRS are the only items that require active cooling via an active Ne JT cooler. The Ariel payload is developed by a consortium of more than 50 institutes from 16 ESA countries, which include the UK, France, Italy, Belgium, Poland, Spain, Austria, Denmark, Ireland, Portugal, Czech Republic, Hungary, the Netherlands, Sweden, Norway, Estonia, and a NASA contribution

Effect of angiotensin-converting enzyme inhibitor and angiotensin receptor blocker initiation on organ support-free days in patients hospitalized with COVID-19

IMPORTANCE Overactivation of the renin-angiotensin system (RAS) may contribute to poor clinical outcomes in patients with COVID-19. Objective To determine whether angiotensin-converting enzyme (ACE) inhibitor or angiotensin receptor blocker (ARB) initiation improves outcomes in patients hospitalized for COVID-19. DESIGN, SETTING, AND PARTICIPANTS In an ongoing, adaptive platform randomized clinical trial, 721 critically ill and 58 non–critically ill hospitalized adults were randomized to receive an RAS inhibitor or control between March 16, 2021, and February 25, 2022, at 69 sites in 7 countries (final follow-up on June 1, 2022). INTERVENTIONS Patients were randomized to receive open-label initiation of an ACE inhibitor (n = 257), ARB (n = 248), ARB in combination with DMX-200 (a chemokine receptor-2 inhibitor; n = 10), or no RAS inhibitor (control; n = 264) for up to 10 days. MAIN OUTCOMES AND MEASURES The primary outcome was organ support–free days, a composite of hospital survival and days alive without cardiovascular or respiratory organ support through 21 days. The primary analysis was a bayesian cumulative logistic model. Odds ratios (ORs) greater than 1 represent improved outcomes. RESULTS On February 25, 2022, enrollment was discontinued due to safety concerns. Among 679 critically ill patients with available primary outcome data, the median age was 56 years and 239 participants (35.2%) were women. Median (IQR) organ support–free days among critically ill patients was 10 (–1 to 16) in the ACE inhibitor group (n = 231), 8 (–1 to 17) in the ARB group (n = 217), and 12 (0 to 17) in the control group (n = 231) (median adjusted odds ratios of 0.77 [95% bayesian credible interval, 0.58-1.06] for improvement for ACE inhibitor and 0.76 [95% credible interval, 0.56-1.05] for ARB compared with control). The posterior probabilities that ACE inhibitors and ARBs worsened organ support–free days compared with control were 94.9% and 95.4%, respectively. Hospital survival occurred in 166 of 231 critically ill participants (71.9%) in the ACE inhibitor group, 152 of 217 (70.0%) in the ARB group, and 182 of 231 (78.8%) in the control group (posterior probabilities that ACE inhibitor and ARB worsened hospital survival compared with control were 95.3% and 98.1%, respectively). CONCLUSIONS AND RELEVANCE In this trial, among critically ill adults with COVID-19, initiation of an ACE inhibitor or ARB did not improve, and likely worsened, clinical outcomes. TRIAL REGISTRATION ClinicalTrials.gov Identifier: NCT0273570

Near-Infared Photometry of the Galactic globular cluster M30

Galactic Globular Clusters (GGCs) are gravitationally bound stellar clusters present in the halo and the bulge of the Milky Way. The stars of the clusters have the same metal abundance, the distance from us and very similar ages (within few 10^8 yr with respect to the cluster ages of the order of 10 Gyr). They constitute a benchmark for stellar evolution and dynamics of stellar system theories. Moreover, they provide a robust constraint to the formation time scale and to the early chemical and dynamical history of the Milky Way. Among the Galactic Globular Clusters, M30 is particularly interesting because is a very metal-poor GGC ([Fe/H]~-2.33) and therefore it is one of the oldest GCCs. My thesis project is divided in two part. In the main part, I have reduced and analysed the near-infrared photometric data of the Globular Cluster M30 (NGC 7099) in J and Ks bands, never analysed before. The data were collected with three different instruments: SOFI (Son OF ISAAC, Infrared Spectrometer And Array Camera) mounted on the NTT telescope (3.58m) located at La Silla in Chile, HAWKI (High Acuity Wide field K-band Imager) and MAD (Multi-conjugate Adaptive optics Demonstrator) installed at the UT4 (8.2m) and the UT3 (8.2m) telescopes of the VLT located at Paranal in Chile, respectively. The data collected with MAD are taken with the Adaptive Optics (AO), a technology which was developed in order to overcome the impact of the atmospheric turbulence on the image formation for ground-based measurements. The stellar photometry in globular clusters is quite difficult because they are very crowded stellar systems. Indeed, the stellar density can reach 10^5 stars/pc^3. In order to perform a very accurate and detailed photometry, I used Point Spread Function Photometry method by means of DAOPHOT/ALLSTAR and ALLFRAME software and I obtained accurate and deep near-infrared colour-magnitude diagrams. To fully characterize the stellar system, I also cross-correlated the near-infrared catalogue with the optical ones available in the literature, in particular, data collected from the space by ACS (Advanced Camera for Surveys), mounted on HST (Hubble Space Telescope) and from an ensemble of the ground-based telescopes. For the second part, the comparison between theoretical models and observed color-magnitude diagrams (CMD) was performed. The distance modulus, the reddening and the age of M30 were estimated by means of recent isochrones (the theoretical models which describe the stellar population of a cluster) compared with observed data. The age suggested from the theory-observations comparison provided a value of 12+/-1 Gyr, in good agreement with the recent literature. Among the evolutionary features, I focussed on the RGB bump, a star clustering along the red giant branch on the color-magnitude diagram, due to the crossing of the same luminosity range for three times by the stars during their evolution. This happens when the H-burning shell of a red giant star reaches the discontinuity in the chemical composition left by the convective envelope which had sunk inside the star in previous phases. The analysis of the bump position reveals a discrepancy between theory and observations, particularly evident for low-metallicity clusters. This fact, already discussed in the literature, is probably due to the uncertainties of the predicted external convective transport, mechanism for which we still have a limited knowledge. From the data, it was possible to estimate the RGB bump magnitude from the luminosity function in different photometric bands. Since the RGB bump position is affected by the uncertainties of the distance modulus and the reddening, the parameter DeltaV^{bump}_{HB}=V_{bump}-V_{HB} is used, because it is independent of them. DeltaV^{bump}_{HB}=V_{bump}-V_{HB} is the difference between the apparent visual magnitude of the bump and the apparent visual magnitude of the horizontal branch at the luminosity level of RR Lyrae. Comparing the predicted and the observed parameter, I found that the predicted values are ~ 0.41 mag fainter the observed one, suggesting a clear discrepancy between predictions and observations, independently of the used optical band (this discrepancy is investigated only for visual band in literature). This confirm previous results found in the literature for other globular clusters which seems to indicate a problem in theoretical models to be analysed in much more details