Emanuele Tognelli

Lithium is predicted, and observed, to be depleted in contracting low-mass pre-main-sequence (PMS) stars. Yet, these stars reach the zero-age main-sequence (ZAMS) with a spread in lithium abundance at a given effective temperature that is not predicted by standard stellar evolutionary models and which appears to be correlated with rotation. Using a homogeneous data set provided by the Gaia-ESO spectroscopic survey, we have followed the evolving photospheric lithium content of cohorts of stars destined to be ZAMS late G-, K-, and M-dwarfs, in clusters at ages of 2–300 Myr. We show that a dispersion in the Li i 6708 Å line strength develops in the lower mass stars after 10–20 Myr on the PMS, as soon as Li depletion begins, even in fully convective stars. A model based on a surface star-spot coverage varying from star-to-star, leading to a differential Li-burning rate, can explain this temporal behaviour and its mass dependence. However, to fully explain the magnitude of the Li dispersion and its correlation with rotation, the spot coverage during Li-burning would need to be a factor of 2 larger on average than measured in ZAMS clusters like the Pleiades and continue increasing with rotation in PMS stars beyond the usual ‘saturation limit’ observed for other magnetic activity indicators.

In this study we compute the equation of state and Rosseland mean opacity from temperatures of T ≃ 30,000 K down to T ≃ 400 K, pushing the capabilities of the ÆSOPUS code into the regime where solid grains can form. The GGchem code is used to solve the chemistry for temperatures less than ≃3000 K. Atoms, molecules, and dust grains in thermodynamic equilibrium are all included in the equation of state. To incorporate monochromatic atomic and molecular cross sections, an optimized opacity sampling technique is used. The Mie theory is employed to calculate the opacity of 43 grain species. Tables of Rosseland mean opacities for scaled-solar compositions are provided. Based on our computing resources, opacities for other chemical patterns, as well as various grain sizes, porosities, and shapes, can be easily computed upon user request to the corresponding author.

In this study we compute the equation of state and Rosseland mean opacity from temperatures of T~30000 K down to T~400 K, pushing the capabilities of the AESOPUS code (Marigo et al., 2022; Marigo & Aringer, 2009) into the regime where solid grains can form. The GGchem code (Woitke et al. 2018) is used to solve the chemistry for temperatures less than ~3000 K. Atoms, molecules, and dust grains in thermodynamic equilibrium are all included in the equation of state. To incorporate monochromatic atomic and molecular cross sections, an optimized opacity sampling technique is used. The Mie theory is employed to calculate the opacity of 43 grain species. Tables of Rosseland mean opacities for scaled-solar compositions are provided. Based on our computing resources, opacities for other chemical patterns, as well as various grain sizes, porosity, and shapes, can be easily computed upon user request to the corresponding author.

The luminosity of the tip of the red giant branch (TRGB) is instrumental for the construction of the distance ladder, and its accurate modelling is key for determining the local Hubble parameter. In this work, we present an extensive quantitative analysis of the TRGB luminosity, accounting for virtually all input physics that affect it, namely: chemical composition, opacity, diffusion, nuclear reaction rates, electron screening, neutrinos, convection efficiency, boundary conditions, and mass-loss, amongst others. Our analysis is based on a newly produced grid of ∼3 × 10⁶ TRGB models, evolved from pre-main sequence up to the helium ignition at the TRGB, and covering a wide range of metallicity (Z = 0.0001–0.02) and initial mass (M = 0.8–1.4 M_⊙). Through a Monte Carlo analysis, we study the systematic variation of the TRGB luminosity due to the combined effect of all above input physics, and show that a maximum theoretical uncertainty of about |$1.6 {{\ \rm per\ cent}}$| is still present on the current generation of models, dominated by systematics of radiative opacity. Results are also provided in several photometric bands. As a by-product of our analysis, we demonstrate robust evidence for the linear response of the tip luminosity to individual changes of input physics, which can significantly simplify future analyses. A comparison of our results with other stellar evolution codes shows excellent agreement. Our full grid of models is available upon reasonable requests, while a partial grid is available online on Zenodo.

Spectroscopy from the final internal data release of the Gaia–ESO Survey (GES) has been combined with Gaia EDR3 to assign membership probabilities to targets observed towards 63 Galactic open clusters and 7 globular clusters. The membership probabilities are based chiefly on maximum likelihood modelling of the 3D kinematics of the targets, separating them into cluster and field populations. From 43 211 observed targets, 13 985 are identified as highly probable cluster members (P > 0.9), with an average membership probability of 0.993. The addition of GES radial velocities successfully drives down the fraction of false positives and we achieve better levels of discrimination in most clusters over the use of astrometric data alone, especially those at larger distances. Since the membership selection is almost purely kinematic, the union of this catalogue with GES and Gaia is ideal for investigating the photometric and chemical properties of clusters as a function of stellar mass, age, and Galactic position.

We tested the capability of a Bayesian procedure to calibrate both the helium abundance and the mixing length parameter (α_ML), using precise photometric data for main sequence (MS) stars in a cluster with negligible reddening and well determined distance. The method has been applied first to a mock dataset generated to mimic Hyades MS stars and then to the real Hyades cluster. We tested the impact on the results of varying the number of stars in the sample, the photometric errors, and the estimated [Fe/H]. The analysis of the synthetic dataset shows that α_ML is recovered with a very good precision in all the analysed cases (with an error of few percent), while [Fe/H] and the helium-to-metal enrichment ratio ΔY/ΔZ are more problematic. If spectroscopic determinations of [Fe/H] are not available and thus [Fe/H] has to be recovered alongside with ΔY/ΔZ and α_ML, the well known degeneracy between [Fe/H]-ΔY/ΔZ-α_ML could result in a large uncertainty on the recovered parameters, depending on the portion of the MS used for the analysis. On the other hand, the prior knowledge of an accurate [Fe/H] value puts a strong constraint on the models, leading to a more precise parameters recovery. Using the current set of PISA models, the most recent [Fe/H] value and the Gaia photometry and parallaxes for the Hyades cluster, we obtained the average values <α_ML>=2.01 ± 0.05 and <ΔY/ΔZ>=2.03 ± 0.33, sensitively reducing the uncertainty in these important parameters.

Context. The cosmological lithium problem, that is, the discrepancy between the lithium abundance predicted by the Big Bang nucleosynthesis and the one observed for the stars of the “Spite plateau”, is one of the long standing problems of modern astrophysics. Recent hints for a possible solution involve lithium burning induced by protostellar mass accretion on Spite plateau stars. However, to date, most of the protostellar and pre-main sequence stellar models that take mass accretion into account have been computed at solar metallicity, and a detailed analysis on the impact of protostellar accretion on the lithium evolution in the metal-poor regime, which is relevant for stars in the Spite plateau, is completely missing. Aims. The purpose of this paper is to fill this gap, analysing, in detail, for the first time the effect of protostellar accretion on low metallicity low-mass stars with a focus on pre-main sequence lithium evolution. Methods. We computed the evolution from the protostar to the main-sequence phase of accreting models with final masses equal to 0.7 and 0.8 M_⊙, and three metallicities Z = 0.0001, Z = 0.0010, and Z = 0.0050, corresponding to [Fe/H] ∼ −2.1, −1.1 (typical of Spite plateau stars), and [Fe/H] ∼ −0.42, respectively. We followed the temporal evolution of the chemical composition by considering nuclear burning, convective mixing, and diffusion. The effects of changing some of the main parameters affecting accreting models, that is the accretion energy (i.e. cold versus hot accretion), the initial seed mass M_seed and radius R_seed, and the mass accretion rate ṁ (also considering episodic accretion), have been investigated in detail. Results. As for the main stellar properties and in particular the surface ⁷Li abundance, hot accretion models converge to standard non-accreting ones within 1 Myr, regardless of the actual value of M_seed, R_seed, and ṁ. Also, cold accretion models with a relatively large M_seed (≳10 M_J) or R_seed (≳1 R_⊙) converge to standard non-accreting ones in less than about 10−20 Myr. However, a drastically different evolution occurs whenever a cold protostellar accretion process starts from small values of M_seed and R_seed (M_seed ∼ 1 M_J, R_seed ≲ 1 R_⊙). These models almost entirely skip the standard Hayashi track evolution and deplete lithium before the end of the accretion phase. The exact amount of depletion depends on the actual combination of the accretion parameters (ṁ, M_seed, and R_seed), achieving in some cases the complete exhaustion of lithium in the whole star. Finally, the lithium evolution in models accounting for burst accretion episodes or for an initial hot accretion followed by a cold accretion phase closely resemble that of standard non-accreting ones. Conclusions. To significantly deplete lithium in low-mass metal poor stars by means of protostellar accretion, a cold accretion scenario starting from small initial M_seed and R_seed is required. Even in this extreme configuration leading to a non-standard evolution that misses almost entirely the standard Hayashi track, an unsatisfactory fine tuning of the parameters governing the accretion phase is required to deplete lithium in stars of different mass and metallicity – starting from the Big Bang nucleosynthesis abundance – in such a way as to produce the observed Spite plateau.

We collected over 6000 high-resolution spectra of four dozen field RR Lyrae (RRL) variables pulsating either in the fundamental (39 RRab) or in the first overtone (9 RRc) mode. We measured radial velocities (RVs) of four strong metallic and four Balmer lines along the entire pulsational cycle and derived RV amplitudes with accuracies better than 1–2 km s⁻¹. The new amplitudes were combined with literature data for 23 RRab and 3 RRc stars (total sample of 74 RRLs), which allowed us to investigate the variation of the Bailey diagram (photometric amplitude versus period) when moving from optical to mid-infrared bands and to recast the Bailey diagram in terms of RV amplitudes. We found that RV amplitudes for RRab are minimally affected by nonlinear phenomena (shocks) and multiperiodicity (Blazhko effect). The RV slope (logP–A(V _r )) when compared with the visual slope (logP–A(V)) is shallower, and the dispersion, at fixed period, decreases by a factor of two. We constructed homogeneous sets of horizontal branch evolutionary models and nonlinear, convective pulsation models of RRLs to constrain the impact of evolutionary effects on their pulsation properties. Evolution causes, on the Bailey diagram based on RV amplitudes, a modest variation in pulsation period and a large dispersion in amplitude. The broad dispersion in period of the Bailey diagram is mainly caused by variation in RRL intrinsic parameters (stellar mass, chemical composition). Empirical evidence indicates that RV amplitudes are an optimal diagnostic for tracing the mean effective temperature across the RRab instability strip.