arXiv subjects

Scilla Degl'Innocenti

Publications and source records attributed to Scilla Degl'Innocenti.

7 recordsLinked to original sources

arXiv2026

Two stellar clocks, one history: a Gaia colour-magnitude-diagram reconstruction of the solar-neighbourhood star-formation history tested against white dwarfs

The solar-neighbourhood star-formation history (SFH) can be constrained from the colour-magnitude diagram (CMD) of non-degenerate stars and tested independently with the white-dwarf luminosity function (WDLF). These two tracers sample complementary evolutionary phases and provide a consistency check on both the recovered SFH and the adopted WD cooling models. We reconstruct the local SFH from the Gaia DR3 CMD and test whether the resulting history reproduces the observed 40 pc WDLF. We fit non-degenerate stars in a cylinder of radius 200 pc and vertical extent $\pm40$ pc using the SFERA synthetic-population code. The fit is performed with the PISA and PARSEC stellar libraries. Each CMD-derived solution is then used to generate a synthetic WD population for comparison with the 40 pc WDLF, adopting the same initial-final mass relation and four WD cooling model sets: BaSTI, MIST, La Plata, and Montreal. The two CMD-based solutions agree on the main features of the local SFH: low activity at the oldest ages, an early episode around 9-11 Gyr ago, reduced activity between about 4 and 9 Gyr, and enhanced star formation over the last few Gyr. The predicted WDLFs reproduce the bright and intermediate part of the observed distribution for all cooling grids, but differ substantially around the WDLF maximum and along the faint tail. MIST and BaSTI shift too much weight to magnitudes fainter than the observed maximum, whereas La Plata gives the closest match to the faint tail but leaves larger residuals around the maximum. Montreal provides the best overall compromise. On this cooling scale, progressively removing the oldest synthetic WDs shows that the faint tail disfavours a substantial contribution from WDs with total ages older than about 11 Gyr.

astro-ph.GA↗

arXiv2026

Bayesian analysis of proton-proton fusion in chiral effective field theory

The astrophysical $S$-factor for the proton-proton fusion is calculated in the low-energy regime for a variety of nuclear interactions and consistent nuclear currents, derived within chiral effective field theory. We estimate, for the first time, the theoretical uncertainty on the $S$-factor due to the truncation of the chiral expansion of the currents using a Bayesian analysis. In order to reach an accuracy at the percent level in the calculation, the electromagnetic potential includes contributions beyond the leading Coulomb interaction, such as two-photon exchange and vacuum polarization. The initial proton-proton state is expanded in partial waves and only the ${}^1S_0$ contribution is included, as it is known that the other partial-waves effects are negligible. The low-energy constant entering the contact term in the weak axial current operator is calibrated to reproduce the Gamow-Teller matrix element in Tritium $β$-decay. The value $S(0)$ is found to be $S(0)=(4.068 \pm 0.025)\times 10^{-25} \: \text{MeV}\: \text{b}$.

nucl-th↗

arXiv2019

When the tale comes true: multiple populations and wide binaries in the Orion Nebula Cluster

The high-quality OmegaCAM photometry of the 3x3 deg around the Orion Nebula Cluster (ONC) in r, and i filters by Beccari et al.(2017) revealed three well-separated pre-main sequences in the color-magnitude diagram (CMD). The objects belonging to the individual sequences are concentrated towards the center of the ONC. The authors concluded that there are two competitive scenarios: a population of unresolved binaries and triples with an exotic mass ratio distribution, or three stellar populations with different ages. We use Gaia DR2 in combination with the photometric OmegaCAM catalog to test and confirm the presence of the putative three stellar populations. We also study multiple stellar systems in the ONC for the first time using Gaia DR2. We confirm that the second and third sequence members are more centrally concentrated towards the center of the ONC. In addition we find an indication that the parallax and proper motion distributions are different among the members of the stellar sequences. The age difference among stellar populations is estimated to be 1-2 Myr. We use Gaia measurements to identify and remove as many unresolved multiple system candidates as possible. Nevertheless we are still able to recover two well-separated sequences with evidence for the third one, supporting the existence of the three stellar populations. We were able to identify a substantial number of wide binary objects (separation between 1000-3000 au). This challenges previously inferred values that suggested no wide binary stars exist in the ONC. Our inferred wide-binary fraction is approx 5%. We confirm the three populations correspond to three separated episodes of star formation. Based on this result, we conclude that star formation is not happening in a single burst in this region. (abridged)

astro-ph.SR↗

arXiv2015

Cumulative theoretical uncertainties in lithium depletion boundary age

We performed a detailed analysis of the main theoretical uncertainties affecting the age at the lithium depletion boundary (LDB). To do that we computed almost 12000 pre-main sequence models with mass in the range [0.06, 0.4] M_sun by varying input physics (nuclear reaction cross-sections, plasma electron screening, outer boundary conditions, equation of state, and radiative opacity), initial chemical elements abundances (total metallicity, helium and deuterium abundances, and heavy elements mixture), and convection efficiency (mixing length parameter, alpha_ML). As a first step, we studied the effect of varying these quantities individually within their extreme values. Then, we analysed the impact of simultaneously perturbing the main input/parameters without an a priori assumption of independence. Such an approach allowed us to build for the first time the cumulative error stripe, which defines the edges of the maximum uncertainty region in the theoretical LDB age. We found that the cumulative error stripe is asymmetric and dependent on the adopted mixing length value. For alpha_ML = 1.00, the positive relative age error ranges from 5 to 15 per cent, while for solar-calibrated mixing length, the uncertainty reduces to 5-10 per cent. A large fraction of such an error (about 40 per cent) is due to the uncertainty in the adopted initial chemical elements abundances.

astro-ph.SR↗

arXiv2011

Photometric determination of the mass accretion rates of pre-main sequence stars. II. NGC346 in the Small Magellanic Cloud

[Abridged] We have studied the properties of the stellar populations in the field of the NGC346 cluster in the Small Magellanic Cloud, using a novel self-consistent method that allows us to reliably identify pre-main sequence (PMS) objects actively undergoing mass accretion, regardless of their age. The method does not require spectroscopy and combines broad-band V and I photometry with narrow-band Halpha imaging to identify all stars with excess Halpha emission and derive the accretion luminosity Lacc and mass accretion rate Macc for all of them. The application of this method to existing HST/ACS photometry of the NGC346 field has allowed us to identify and study 680 bona-fide PMS stars with masses from ~0.4 to ~4 Msolar and ages in the range from ~1 to ~30 Myr. This is the first study to reveal that, besides a young population of PMS stars (~ 1 Myr old), in this field there is also an older population of PMS objects with a median age of ~20 Myr. We provide for all of them accurate physical parameters. We study the evolution of the mass accretion rate as a function of stellar parameters and find that logMacc ~ -0.6 Log t + Log m + c, where t is the age of the star, m its mass and c a quantity that is higher at lower metallicity. The high mass accretion rates that we find suggest that a considerable fraction of the stellar mass is accreted during the PMS phase and that PMS evolutionary models that do not account for this effect will systematically underestimate the true age when compared with the observations.

astro-ph.SR↗

arXiv2010

ΔY/ ΔZ from the analysis of local K dwarfs

The stellar helium-to-metal enrichment ratio, ΔY/ΔZ, is a widely studied astrophysical quantity. However, its value is still not precisely constrained. This paper is focused on the study of the main sources of uncertainty which affect the ΔY/ΔZ derived from the analysis of the low-main sequence (MS) stars in the solar neighborhood. The possibility to infer the value of ΔY/ΔZ from the study of low-MS stars relies on the dependence of the stellar luminosity and effective temperature on the initial Y and Z. The ΔY/ΔZ ratio is obtained by comparing the magnitude difference between the observed stars and a reference theoretical zero age main sequence (ZAMS) with the related theoretical magnitude differences computed from a new set of stellar models with up-to-date input physics and a fine grid of chemical compositions. A Monte Carlo approach has been used to evaluate the impact on the result of different sources of uncertainty, i.e. observational errors, evolutionary effects, systematic uncertainties of the models. As a check of the procedure, the method has been applied to a different data set, namely the low-MS of the Hyades. Once a set of ZAMS and atmosphere models have been chosen, we found that the inferred value of ΔY/ΔZ is sensitive to the age of the stellar sample, even if we restricted the data set to low luminosity stars. The lack of an accurate age estimate of low mass field stars leads to an underestimate of the inferred ΔY/ΔZ of ~2 units. On the contrary the method firmly recovers the ΔY/ΔZ value for not evolved samples of stars such as the Hyades low-MS. Adopting a solar calibrated mixing-length parameter and the PHOENIX GAIA v2.6.1 atmospheric models, we found ΔY/ΔZ = 5.3 +/- 1.4 once the age correction has been applied. The Hyades sample provided a perfectly consistent value.

astro-ph.GA↗

arXiv2006

Monte Carlo Simulations of Metal-Poor Star Clusters

Metal-poor globular clusters (GCs) can provide a probe of the earliest epoch of star formation in the Universe, being the oldest stellar systems observable. In addition, young and intermediate-age low-metallicity GCs are present in external galaxies. Nevertheless, inferring their evolutionary status by using integrated properties may suffer from large \emph{intrinsic} uncertainty caused by the discrete nature of stars in stellar systems, especially in the case of faint objects. In this paper, we evaluate the \emph{intrinsic} uncertainty (due to statistical effects) affecting the integrated colours and mass--to--light ratios as a function of the cluster integrated visual magnitude ($M_V^{tot}$), which represents a quantity directly measured. Our approach is based on Monte Carlo techniques for randomly generating stars distributed according to the cluster's mass function. Integrated colours and mass--to--light ratios in different photometric bands are checked to be in good agreement with the observational values of low-metallicity Galactic clusters. We present integrated colours and mass--to--light ratios as a function of age for different assumptions on the cluster total $V$ magnitude. We find that the emph{intrinsic} uncertainty cannot be neglected. In particular, in models with $M_V^{tot}=-4$ the broad-band colours show an \emph{intrinsic} uncertainty so high as to prevent precise age evaluation of the cluster. Finally, the present predictions are compared with recent results available in the literature, showing in some cases non-negligible differences.

astro-ph↗