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P. Marigo

Publications and source records attributed to P. Marigo.

At least 37 records · Page 2Linked to original sources
arXiv2021

On the effects of the Initial Mass Function on Galactic chemical enrichment

There is mounting evidence that the stellar initial mass function (IMF) could extend much beyond the canonical Mi ~100, Msun limit, but the impact of such hypothesis on the chemical enrichment of galaxies still remains to be clarified. We aim to address this question by analysing the observed abundances of thin- and thick-disc stars in the Milky Way with chemical evolution models that account for the contribution of very massive stars dying as pair-instability supernovae. We built new sets of chemical yields from massive and very massive stars up to Mi ~ 350 Msun, by combining the wind ejecta extracted from our hydrostatic stellar evolution models with explosion ejecta from the literature. Using a simple chemical evolution code we analyse the effects of adopting different yield tables by comparing predictions against observations of stars in the solar vicinity. After several tests, we focus on the [O/Fe] ratio which best separates the chemical patterns of the two Milky Way components. We find that with a standard IMF, truncated at Mi ~ 100 Msun, we can reproduce various observational constraints for thin-disc stars, but the same IMF fails to account for the [O/Fe] ratios of thick-disc stars. The best results are obtained by extending the IMF up to Mi = 350 Msun and including the chemical ejecta of very massive stars, in the form of winds and pair-instability supernova explosions.Our study indicates that PISN played a significant role in shaping the chemical evolution of the Milky Way thick disc. By including their chemical yields it is easier to reproduce not only the level of the alpha-enhancement but also the observed slope of thick-disc stars in the [O/Fe] vs [Fe/H] diagram. The bottom line is that the contribution of very massive stars to the chemical enrichment of galaxies is potentially quite important and should not be neglected in chemical evolution models.

astro-ph.GA↗
arXiv2020

Setup commissioning for an improved measurement of the D(p,gamma)3He cross section at Big Bang Nucleosynthesis energies

Among the reactions involved in the production and destruction of deuterium during Big Bang Nucleosynthesis, the deuterium-burning D(p,gamma)3He reaction has the largest uncertainty and limits the precision of theoretical estimates of primordial deuterium abundance. Here we report the results of a careful commissioning of the experimental setup used to measure the cross-section of the D(p,gamma)3He reaction at the Laboratory for Underground Nuclear Astrophysics of the Gran Sasso Laboratory (Italy). The commissioning was aimed at minimising all sources of systematic uncertainty in the measured cross sections. The overall systematic error achieved (< 3 %) will enable improved predictions of BBN deuterium abundance.

physics.ins-det↗
arXiv2020

A new approach to monitor 13C-targets degradation in situ for 13C(alpha,n)16O cross-section measurements at LUNA

Direct measurements of reaction cross-sections at astrophysical energies often require the use of solid targets able to withstand high ion beam currents for extended periods of time. Thus, monitoring target thickness, isotopic composition, and target stoichiometry during data taking is critical to account for possible target modifications and to reduce uncertainties in the final cross-section results. A common technique used for these purposes is the Nuclear Resonant Reaction Analysis (NRRA), which however requires that a narrow resonance be available inside the dynamic range of the accelerator used. In cases when this is not possible, as for example the 13C(alpha,n)16O reaction recently studied at low energies at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy, alternative approaches must be found. Here, we present a new application of the shape analysis of primary gamma rays emitted by the 13C(p,g)14N radiative capture reaction. This approach was used to monitor 13C target degradation {\em in situ} during the 13C(alpha,n)16O data taking campaign. The results obtained are in agreement with evaluations subsequently performed at Atomki (Hungary) using the NRRA method. A preliminary application for the extraction of the 13C(alpha,n)16O reaction cross-section at one beam energy is also reported.

physics.ins-det↗
arXiv2019

Period-luminosity diagram of long period variables in the Magellanic Clouds. New aspects revealed from Gaia Data Release 2

Context: The period-luminosity diagram (PLD) has proven to be a powerful tool for studying populations of pulsating red giants. Gaia Data Release 2 (DR2) provides a large data set including many long-period variables (LPVs) on which this tool can be applied. Aims: We investigate the location of LPVs from the Large and Small Magellanic Clouds in the PLD using various optical and infrared luminosity indicators from Gaia and 2MASS, respectively. We thereby distinguish between stars of different masses and surface chemistry. Methods: The data set taken from the Gaia DR2 catalogue of LPVs allows for a homogeneous study from low- to high-mass LPVs. These sources are divided into sub-populations of asymptotic giant branch (AGB) stars according to their mass and their O- or C-rich nature using the Gaia-2MASS diagram developed by our group. This diagram uses a Wesenheit index Wrp based on Wesenheit functions in the Gaia and 2MASS photometric bands. Four different luminosity indicators are used to study the period-luminosity (P-L) relations. Results: We provide the first observational evidence of a P-L relation offset for both fundamental and 1O pulsators between low- and intermediate-mass O-rich stars, in agreement with published pulsation predictions. Among the luminosity indicators explored, sequence C' is the narrowest in the P-Wrp diagram, and is thus to be preferred over the other PLDs for the determination of distances using LPVs. The majority of massive asymptotic giant branch (AGB) stars and red supergiants form a smooth extension of sequence C of low- and intermediate-mass AGB stars in the P-Wrp diagram, suggesting that they pulsate in the fundamental mode. All results are similar in the two Magellanic Clouds.

astro-ph.SR↗
arXiv2019

The evolution of DARWIN: current status of wind models for AGB stars

The slow, dense winds observed in evolved asymptotic giant branch (AGB) stars are usually attributed to a combination of dust formation in the dynamical inner atmosphere and momentum transfer from stellar photons interacting with the newly formed dust particles. Wind models calculated with the DARWIN code, using this mass-loss scenario, have successfully produced outflows with dynamical and photometric properties compatible with observations, for both C-type and M-type AGB stars. Presented here is an overview of the DARWIN models currently available and what output these models produce, as well as future plans.

astro-ph.SR↗
arXiv2019

A Dramatic Decrease in Carbon Star Formation in M31

We analyze resolved stellar near-infrared photometry of 21 HST fields in M31 to constrain the impact of metallicity on the formation of carbon stars. Observations of nearby galaxies show that the carbon stars are increasingly rare at higher metallicity. Models indicate that carbon star formation efficiency drops due to the decrease in dredge-up efficiency in metal-rich thermally-pulsing Asymptotic Giant Branch (TP-AGB) stars, coupled to a higher initial abundance of oxygen. However, while models predict a metallicity ceiling above which carbon stars cannot form, previous observations have not yet pinpointed this limit. Our new observations reliably separate carbon stars from M-type TP-AGB stars across 2.6-13.7 kpc of M31's metal-rich disk using HST WFC3/IR medium-band filters. We find that the ratio of C to M stars (C/M) decreases more rapidly than extrapolations of observations in more metal-poor galaxies, resulting in a C/M that is too low by more than a factor of 10 in the innermost fields and indicating a dramatic decline in C star formation efficiency at metallicities higher than [M/H] $\approx$ -0.1 dex. The metallicity ceiling remains undetected, but must occur at metallicities higher than what is measured in M31's inner disk ([M/H] $\gtrsim$ +0.06 dex).

astro-ph.SR↗
arXiv2019

Astro2020 Science White Paper: The fundamentals of outflows from evolved stars

Models of the chemical evolution of the interstellar medium, galaxies, and the Universe rely on our understanding of the amounts and chemical composition of the material returned by stars and supernovae. Stellar yields are obtained from stellar-evolution models, which currently lack predictive prescriptions of stellar mass loss, although it significantly affects stellar lifetimes, nucleosynthesis, and chemical ejecta. Galaxy properties are derived from observations of the integrated light of bright member stars. Stars in the late stages of their evolution are among the infrared-brightest objects in galaxies. An unrealistic treatment of the mass-loss process introduces significant uncertainties in galaxy properties derived from their integrated light. We describe current efforts and future needs and opportunities to characterize AGB outflows: driving mechanisms, outflow rates, underlying fundamental physical and chemical processes such as dust grain formation, and dependency of these on metallicity.

astro-ph.SR↗
arXiv2019

Carbon star wind models at solar and sub-solar metallicities: a comparative study. I. Mass loss and the properties of dust-driven winds

The atmospheres and winds of C-type AGB stars are modeled with the 1D spherically symmetric radiation-hydrodynamical code DARWIN. To explore the metallicity-dependence of mass loss we calculate model grids at three different chemical abundances. Since carbon may be dredged up during the thermal pulses as AGB stars evolve, we keep the carbon abundance as a free parameter. The models in these three different grids all have a current mass of one solar mass; effective temperatures of 2600K, 2800K, 3000K, or 3200K; and stellar luminosities equal to log(L/Lsun)=3.70, 3.85, or 4.00. The models show that mass loss in carbon stars is facilitated by high luminosities, low effective temperatures, and a high carbon excess (C-O) at both solar and subsolar metallicities. Similar combinations of effective temperature, luminosity, and carbon excess produce outflows at both solar and subsolar metallicities. There are no large systematic differences in the mass-loss rates and wind velocities produced by these wind models with respect to metallicity, nor any systematic difference concerning the distribution of grain sizes or how much carbon is condensed into dust. DARWIN models at subsolar metallicity have approximately 15% lower mass-loss rates compared to DARWIN models at solar metallicity with the same stellar parameters and carbon excess. For both solar and subsolar environments typical grain sizes range between 0.1 and 0.5 micron, the degree of condensed carbon varies between 5% and 40%, and the gas-to-dust ratios between 500 and 10000. Furthermore, stellar evolution models can use the mass-loss rates calculated from DARWIN models at solar metallicity when modeling the AGB phase at subsolar metallicities if carbon excess is used as the critical abundance parameter instead of the C/O ratio.

astro-ph.SR↗
arXiv2018

Direct capture cross section and the $E_p$ = 71 and 105 keV resonances in the $^{22}$Ne($p,γ$)$^{23}$Na reaction

The $^{22}$Ne($p,γ$)$^{23}$Na reaction, part of the neon-sodium cycle of hydrogen burning, may explain the observed anticorrelation between sodium and oxygen abundances in globular cluster stars. Its rate is controlled by a number of low-energy resonances and a slowly varying non-resonant component. Three new resonances at $E_p$ = 156.2, 189.5, and 259.7 keV have recently been observed and confirmed. However, significant uncertainty on the reaction rate remains due to the non-resonant process and to two suggested resonances at $E_p$ = 71 and 105 keV. Here, new $^{22}$Ne($p,γ$)$^{23}$Na data with high statistics and low background are reported. Stringent upper limits of 6$\times$10$^{-11}$ and 7$\times$10$^{-11}$\,eV (90\% confidence level), respectively, are placed on the two suggested resonances. In addition, the off-resonant S-factor has been measured at unprecedented low energy, constraining the contributions from a subthreshold resonance and the direct capture process. As a result, at a temperature of 0.1 GK the error bar of the $^{22}$Ne($p,γ$)$^{23}$Na rate is now reduced by three orders of magnitude.

nucl-ex↗
arXiv2018

A new method to identify subclasses among AGB stars using Gaia and 2MASS photometry

Aims: We explore the wealth of high quality photometric data provided by data release 2 of the Gaia mission for long period variables (LPVs) in the Large Magellanic Cloud. Our goal is to identify stars of various types and masses along the Asymptotic Giant Branch. Methods: For this endeavour, we developed a new multi-band approach combining Wesenheit functions W_{RP,BP-RP} and W_{K_s,J-K_s} in the Gaia BP, RP and 2MASS J, K_s spectral ranges, respectively, and use a new diagram (W_{RP,BP-RP}-W_{K_s,J-K_s}) versus K_s to distinguish between different kinds of stars in our sample of LPVs. We used stellar population synthesis models to validate our approach. Results:We demonstrate the ability of the new diagram to discriminate between O-rich and C-rich objects, and to identify low-mass, intermediate-mass and massive O-rich red giants, as well as extreme C-rich stars. Stellar evolution and population synthesis models guide the interpretation of the results, highlighting the diagnostic power of the new tool to discriminate between stellar initial masses, chemical properties and evolutionary stages.

astro-ph.SR↗
arXiv2018

A high-efficiency gas target setup for underground experiments, and redetermination of the branching ratio of the 189.5 keV $\mathbf{^{22}Ne(p,γ)^{23}Na}$ resonance

The experimental study of nuclear reactions of astrophysical interest is greatly facilitated by a low-background, high-luminosity setup. The Laboratory for Underground Nuclear Astrophysics (LUNA) 400 kV accelerator offers ultra-low cosmic-ray induced background due to its location deep underground in the Gran Sasso National Laboratory (INFN-LNGS), Italy, and high intensity, 250-500 $μ$A, proton and $α$ ion beams. In order to fully exploit these features, a high-purity, recirculating gas target system for isotopically enriched gases is coupled to a high-efficiency, six-fold optically segmented bismuth germanate (BGO) $γ$-ray detector. The beam intensity is measured with a beam calorimeter with constant temperature gradient. Pressure and temperature measurements have been carried out at several positions along the beam path, and the resultant gas density profile has been determined. Calibrated $γ$-intensity standards and the well-known $E_p$ = 278 keV $\mathrm{^{14}N(p,γ)^{15}O}$ resonance were used to determine the $γ$-ray detection efficiency and to validate the simulation of the target and detector setup. As an example, the recently measured resonance at $E_p$ = 189.5 keV in the $^{22}$Ne(p,$γ$)$^{23}$Na reaction has been investigated with high statistics, and the $γ$-decay branching ratios of the resonance have been determined.

nucl-ex↗
arXiv2017

PLATO as it is: a legacy mission for Galactic archaeology

Deciphering the assembly history of the Milky Way is a formidable task, which becomes possible only if one can produce high-resolution chrono-chemo-kinematical maps of the Galaxy. Data from large-scale astrometric and spectroscopic surveys will soon provide us with a well-defined view of the current chemo-kinematical structure of the Milky Way, but will only enable a blurred view on the temporal sequence that led to the present-day Galaxy. As demonstrated by the (ongoing) exploitation of data from the pioneering photometric missions CoRoT, Kepler, and K2, asteroseismology provides the way forward: solar-like oscillating giants are excellent evolutionary clocks thanks to the availability of seismic constraints on their mass and to the tight age-initial-mass relation they adhere to. In this paper we identify five key outstanding questions relating to the formation and evolution of the Milky Way that will need precise and accurate ages for large samples of stars to be addressed, and we identify the requirements in terms of number of targets and the precision on the stellar properties that are needed to tackle such questions. By quantifying the asteroseismic yields expected from PLATO for red-giant stars, we demonstrate that these requirements are within the capabilities of the current instrument design, provided that observations are sufficiently long to identify the evolutionary state and allow robust and precise determination of acoustic-mode frequencies. This will allow us to harvest data of sufficient quality to reach a 10% precision in age. This is a fundamental pre-requisite to then reach the more ambitious goal of a similar level of accuracy, which will only be possible if we have to hand a careful appraisal of systematic uncertainties on age deriving from our limited understanding of stellar physics, a goal which conveniently falls within the main aims of PLATO's core science.

astro-ph.SR↗
arXiv2017

Modelling the UV to radio SEDs of nearby star-forming galaxies: new Parsec SSP for Grasil

By means of the updated PARSEC database of evolutionary tracks of massive stars, we compute the integrated stellar light, the ionizing photon budget and the supernova rates of young simple stellar populations (SSPs), for different metallicities and IMF upper mass limits. Using CLOUDY we compute and include in the SSP spectra the neb- ular emission contribution. We also revisit the thermal and non-thermal radio emission contribution from young stars. Using GRASIL we can thus predict the panchromatic spectrum and the main recombination lines of any type of star-forming galaxy, including the effects of dust absorption and re-emission. We check the new models against the spectral energy distributions (SEDs) of selected well-observed nearby galaxies. From the best-fit models we obtain a consistent set of star formation rate (SFR) calibrations at wavelengths ranging from ultraviolet (UV) to radio. We also provide analytical calibrations that take into account the dependence on metallcity and IMF upper mass limit of the SSPs. We show that the latter limit can be well constrained by combining information from the observed far infrared, 24 μm, 33 GHz and Hα luminosities. Another interesting property derived from the fits is that, while in a normal galaxy the attenuation in the lines is significantly higher than that in the nearby continuum, in individual star bursting regions they are similar, supporting the notion that this effect is due to an age selective extinction. Since in these conditions the Balmer decrement method may not be accurate, we provide relations to estimate the attenuation from the observed 24 μm or 33 GHz fluxes. These relations can be useful for the analysis of young high redshift galaxies.

astro-ph.GA↗
arXiv2016

$^{22}$Ne and $^{23}$Na ejecta from intermediate-mass stars: The impact of the new LUNA rate for $^{22}$Ne(p,$γ$)$^{23}$Na

We investigate the impact of the new LUNA rate for the nuclear reaction $^{22}$Ne$(p,γ)^{23}$Na on the chemical ejecta of intermediate-mass stars, with particular focus on the thermally-pulsing asymptotic giant branch (TP-AGB) stars that experience hot-bottom burning. To this aim we use the PARSEC and COLIBRI codes to compute the complete evolution, from the pre-main sequence up to the termination of the TP-AGB phase, of a set of stellar models with initial masses in the range $3.0\,M_{\odot} - 6.0\,M_{\odot}$, and metallicities $Z_{\rm i}=0.0005$, $Z_{\rm i}=0.006$, and $Z_{\rm i} = 0.014$. We find that the new LUNA measures have much reduced the nuclear uncertainties of the $^{22}$Ne and $^{23}$Na AGB ejecta, which drop from factors of $\simeq 10$ to only a factor of few for the lowest metallicity models. Relying on the most recent estimations for the destruction rate of $^{23}$Na, the uncertainties that still affect the $^{22}$Ne and $^{23}$Na AGB ejecta are mainly dominated by evolutionary aspects (efficiency of mass-loss, third dredge-up, convection). Finally, we discuss how the LUNA results impact on the hypothesis that invokes massive AGB stars as the main agents of the observed O-Na anti-correlation in Galactic globular clusters. We derive quantitative indications on the efficiencies of key physical processes (mass loss, third dredge-up, sodium destruction) in order to simultaneously reproduce both the Na-rich, O-poor extreme of the anti-correlation, and the observational constraints on the CNO abundance. Results for the corresponding chemical ejecta are made publicly available.

astro-ph.SR↗
arXiv2016

Shell and explosive hydrogen burning

The nucleosynthesis of light elements, from helium up to silicon, mainly occurs in Red Giant and Asymptotic Giant Branch stars and Novae. The relative abundances of the synthesized nuclides critically depend on the rates of the nuclear processes involved, often through non-trivial reaction chains, combined with complex mixing mechanisms. In this review, we summarize the contributions made by LUNA experiments in furthering our understanding of nuclear reaction rates necessary for modeling nucleosynthesis in AGB stars and Novae explosions.

nucl-ex↗
arXiv2016

The ALMA detection of CO rotational line emission in AGB stars in the Large Magellanic Cloud

Context: Low- and intermediate-mass stars lose most of their stellar mass at the end of their lives on the asymptotic giant branch (AGB). Determining gas and dust mass-loss rates (MLRs) is important in quantifying the contribution of evolved stars to the enrichment of the interstellar medium. Aims: Attempt to, for the first time, spectrally resolve CO thermal line emission in a small sample of AGB stars in the Large Magellanic Cloud. Methods: ALMA was used to observe 2 OH/IR stars and 4 carbon stars in the LMC in the CO J= 2-1 line. Results: We present the first measurement of expansion velocities in extragalactic carbon stars. All four C-stars are detected and wind expansion velocities and stellar velocities are directly measured. Mass-loss rates are derived from modelling the spectral energy distribution and Spitzer/IRS spectrum with the DUSTY code. Gas-to-dust ratios are derived that make the predicted velocities agree with the observed ones. The expansion velocities and MLRs are compared to a Galactic sample of well-studied relatively low MLRs stars supplemented with "extreme" C-stars that have properties more similar to the LMC targets. Gas MLRs derived from a simple formula are significantly smaller than derived from the dust modelling, indicating an order of magnitude underestimate of the estimated CO abundance, time-variable mass loss, or that the CO intensities in LMC stars are lower than predicted by the formula derived for Galactic objects. This could be related to a stronger interstellar radiation field in the LMC. Conclusions: Although the LMC sample is small and the comparison to Galactic stars is non-trivial because of uncertainties in their distances it appears that for C stars the wind expansion velocities in the LMC are lower than in the solar neighbourhood, while the MLRs appear similar. This is in agreement with dynamical dust-driven wind models.

astro-ph.SR↗
arXiv2013

Is there a metallicity ceiling to form carbon stars? - A novel technique reveals a scarcity of C stars in the inner M31 disk

We use medium-band near-infrared (NIR) Hubble Space Telescope WFC3 photometry with model NIR spectra of Asymptotic Giant Branch (AGB) stars to develop a new tool for efficiently distinguishing carbon-rich (C-type) AGB stars from oxygen-rich (M-type) AGB stars in galaxies at the edge of and outside the Local Group. We present the results of a test of this method on a region of the inner disk of M31, where we find a surprising lack of C stars, contrary to the findings of previous C star searches in other regions of M31. We find only 1 candidate C star (plus up to 6 additional, less certain C stars candidates), resulting in an extremely low ratio of C to M stars (C/M = (3.3(+20,-0.1))x10^-4) that is 1-2 orders of magnitude lower than other C/M estimates in M31. The low C/M ratio is likely due to the high metallicity in this region which impedes stars from achieving C/O > 1 in their atmospheres. These observations provide stringent constraints evolutionary models of metal-rich AGB stars and suggest that there is a metallicity threshold above which M stars are unable to make the transition to C stars, dramatically affecting AGB mass loss and dust production and, consequently, the observed global properties of metal-rich galaxies.

astro-ph.SR↗
arXiv2013

Catching the fish - Constraining stellar parameters for TX Psc using spectro-interferometric observations

Stellar parameter determination is a challenging task when dealing with galactic giant stars. The combination of different investigation techniques has proven to be a promising approach. We analyse archive spectra obtained with the Short-Wavelength-Spectrometer (SWS) onboard of ISO, and new interferometric observations from the Very Large Telescope MID-infrared Interferometric instrument (VLTI/MIDI) of a very well studied carbon-rich giant: TX Psc. The aim of this work is to determine stellar parameters using spectroscopy and interferometry. The observations are used to constrain the model atmosphere, and eventually the stellar evolutionary model in the region where the tracks map the beginning of the carbon star sequence. Two different approaches are used to determine stellar parameters: (i) the 'classic' interferometric approach where the effective temperature is fixed by using the angular diameter in the N-band (from interferometry) and the apparent bolometric magnitude; (ii) parameters are obtained by fitting a grid of state-of-the-art hydrostatic models to spectroscopic and interferometric observations. We find a good agreement between the parameters of the two methods. The effective temperature and luminosity clearly place TX Psc in the carbon-rich AGB star domain in the H-R-diagram. Current evolutionary tracks suggest that TX Psc became a C-star just recently, which means that the star is still in a 'quiet' phase compared to the subsequent strong-wind regime. This is in agreement with the C/O ratio being only slightly larger than 1.

astro-ph.SR↗