arXiv subjects

Bernhard Aringer

Publications and source records attributed to Bernhard Aringer.

At least 19 recordsLinked to original sources

arXiv2024

Dimming events of evolved stars due to clouds of molecular gas. Scenarios based on 3D radiation-hydrodynamics simulations with CO5BOLD

The dramatic dimming episode of the red supergiant Betelgeuse in 2019/2020, caused by a partial darkening of the stellar disk, has highlighted gaps in the understanding of the evolution of massive stars. We analyzed numerical models to investigate the processes behind the formation of dark surface patches and the associated reduction in the disk-integrated stellar light. With the CO5BOLD code, we performed global 3D radiation-hydrodynamical simulations of evolved stars, including convection in the stellar interior, self-excited pulsations, and the resulting atmospheric dynamics with strong radiative shocks. We attribute dimming phenomena to obscuring clouds of cool gas in the lower atmosphere, forming according to three different scenarios. One process transports material outward in a strong shock, similar to what occurs in 1D simulations of radially pulsating AGB stars. Another mechanism is triggered by a large convective upflow structure, in combination with exceptionally strong radial pulsations. This induces Rayleigh-Taylor instabilities, causing plumes of material to be sent outward into the atmosphere. The third and rarest scenario involves large-amplitude convective fluctuations, leading to enhanced flows in deep downdrafts, which rebound and send material outward. In all cases, the dense gas above the stellar surface cools and darkens rapidly in visible light. AGB stars show localized dark patches regularly during intermediate phases of their large-amplitude pulsations, while more massive stars will only intermittently form such patches during luminosity minima. The episodic levitation of dense gas clumps above the stellar surface, followed by the formation of complex molecules in the cooling gas and possibly dust grains at a later stage, can account for the dark patches and strong dimming events of supergiant stars such as Betelgeuse.

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arXiv2023

Synthetic photometry for carbon-rich giants. V. Effects of grain-size-dependent dust opacities

The properties and the evolution of asymptotic giant branch (AGB) stars are strongly influenced by their mass loss through a stellar wind. This is believed to be caused by radiation pressure due to the absorption and scattering of the stellar radiation by the dust grains formed in the atmosphere. The optical properties of dust are often estimated using the small particle limit (SPL) approximation, and it has been used frequently in modelling AGB stellar winds when performing radiation-hydrodynamics (RHD) simulations. We aim to investigate the effects of replacing the SPL approximation by detailed Mie calculations of the size-dependent opacities for grains of amorphous carbon forming in C-rich AGB star atmospheres. We performed RHD simulations for a large grid of carbon star atmosphere+wind models with different effective temperatures, luminosities, stellar masses, carbon excesses, and pulsation properties. Also, a posteriori radiative transfer calculations for many radial structures (snapshots) of these models were done, resulting in spectra and filter magnitudes. We find, when giving up the SPL approximation, the wind models become more strongly variable and more dominated by gusts, although the average mass-loss rates and outflow speeds do not change significantly; the increased radiative pressure on the dust throughout its formation zone does, however, result in smaller grains and lower condensation fractions (and thus higher gas-to-dust ratios). The photometric K magnitudes are generally brighter, but at V the effects of using size-dependent dust opacities are more complex: brighter for low mass-loss rates and dimmer for massive stellar winds. Given the large effects on spectra and photometric properties, it is necessary to use the detailed dust optical data instead of the simple SPL approximation in stellar atmosphere+wind modelling where dust is formed.

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arXiv2022

Low-Temperature Gas Opacities with AESOPUS 2.0

This work introduces new low-temperature gas opacities, in the range 3.2 <= log(T/K) <= 4.5, computed with the AESOPUS code under the assumption of thermodynamic equilibrium (Marigo &_Aringer_2009). In comparison to the previous version AESOPUS 1.0, we updated and expanded molecular absorption to include 80 species, mostly using the recommended line lists currently available from the ExoMol and HITRAN databases. Furthermore, in light of a recent study, we revised the H- photodetachment cross section, added the free-free absorption of other negative ions of atoms and molecules, and updated the collision-induced absorption due to H2/H2, H2/H, H2/He, and H/He pairs. Using the new input physics, we computed tables of Rosseland mean opacities for several scaled-solar chemical compositions, including Magg et al. (2022)'s most recent one, as well as alpha-enhanced mixtures. The differences in opacity between the new AESOPUS 2.0 and the original AESOPUS 1.0 versions, as well as other sets of calculations, are discussed. The new opacities are released to the community via a dedicated web-page that includes both pre-computed tables for widely used chemical compositions, and a web-interface for calculating opacities on-the-fly for any abundance distribution.

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arXiv2021

Dynamic atmospheres and winds of cool luminous giants, II. Gradual Fe enrichment of wind-driving silicate grains

The winds observed around AGB stars are generally attributed to radiation pressure on dust formed in the dynamical atmospheres of these long-period variables. The composition of wind-driving grains is affected by a feedback between their optical properties and the resulting heating due to stellar radiation. We explore the gradual Fe enrichment of wind-driving silicate grains in M-type AGB stars to derive typical values for Fe/Mg and test the effects on wind properties and synthetic spectra. We present new DARWIN models that allow for the growth of silicate grains with a variable Fe/Mg ratio and predict mass-loss rates, wind velocities, and grain properties. Synthetic spectra and other observables are computed with the COMA code. The self-regulating feedback between grain composition and radiative heating, in combination with quickly falling densities in the stellar wind, leads to low values of Fe/Mg, typically a few percent. Nevertheless, the models show distinct silicate features around 10 and 18 microns. Fe enrichment affects visual and near-IR photometry moderately, and the new DARWIN models agree well with observations in (J-K) vs. (V-K) and Spitzer color-color diagrams. The enrichment of the silicate dust with Fe is a secondary process, taking place in the stellar wind on the surface of large Fe-free grains that have initiated the outflow. Therefore, the mass-loss rates are basically unaffected, while the wind velocities tend to be slightly higher than in corresponding models with Fe-free silicate dust. The gradual Fe enrichment of silicate grains in the inner wind region should produce signatures observable in mid-IR spectro-interferometrical measurements. Mass-loss rates derived from existing DARWIN models, based on Fe-free silicates, can be applied to stellar evolution models since the mass-loss rates are not significantly affected by the inclusion of Fe in the silicate grains.

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arXiv2020

Constraining the thermally pulsing asymptotic giant branch phase with resolved stellar populations in the Large Magellanic Cloud

Reliable models of the thermally pulsing asymptotic giant branch (TP-AGB) phase are of critical importance across astrophysics, including our interpretation of the spectral energy distribution of galaxies, cosmic dust production, and enrichment of the interstellar medium. With the aim of improving sets of stellar isochrones that include a detailed description of the TP-AGB phase, we extend our recent calibration of the AGB population in the Small Magellanic Cloud (SMC) to the more metal rich Large Magellanic Cloud (LMC). We model the LMC stellar populations with the TRILEGAL code, using the spatially-resolved star formation history derived from the VISTA survey. We characterize the efficiency of the third dredge-up by matching the star counts and the $K_{\rm s}$-band luminosity functions of the AGB stars identified in the LMC. In line with previous findings, we confirm that, compared to the SMC, the third dredge-up in AGB stars of the LMC is somewhat less efficient, as a consequence of the higher metallicity. The predicted range of initial mass of C-rich stars is between $M_{\rm i} \approx 1.7 - 3~\mathrm{M}_{\odot}$ at $Z_{\rm i} = 0.008$. We show how the inclusion of new opacity data in the carbon star spectra will improve the performance of our models. We discuss the predicted lifetimes, integrated luminosities and mass-loss rate distributions of the calibrated models. The results of our calibration are included in updated stellar isochrones publicly available.

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arXiv2020

Carbon star formation as seen through the non-monotonic initial-final mass relation

The initial-final mass relation (IFMR) links the birth mass of a star to the mass of the compact remnant left at its death. While the relevance of the IFMR across astrophysics is universally acknowledged, not all of its fine details have yet been resolved. A new analysis of a few carbon-oxygen white dwarfs in old open clusters of the Milky Way led us to identify a kink in the IFMR, located over a range of initial masses, $1.65 \lesssim M_{\rm i}/M_{\odot} \lesssim 2.10$. The kink's peak in WD mass of $\approx 0.70-0.75 \, M_{\odot}$ is produced by stars with $M_{\rm i} \simeq 1.8 - 1.9 \, M_{\odot}$, corresponding to ages of about $1.8 - 1.7 $ Gyr. Interestingly, this peak coincides with the initial mass limit between low-mass stars that develop a degenerate helium core after central hydrogen exhaustion, and intermediate-mass stars that avoid electron degeneracy. We interpret the IFMR kink as the signature of carbon star formation in the Milky Way. This finding is critical to constraining the evolution and chemical enrichment of low-mass stars, and their impact on the spectrophotometric properties of galaxies.

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arXiv2019

YBC, a stellar bolometric corrections database with variable extinction coefficients: an application to PARSEC isochrones

We present the \texttt{YBC} database of stellar bolometric corrections (BCs), available at \url{http://stev.oapd.inaf.it/YBC}. We homogenize widely-used theoretical stellar spectral libraries and provide BCs for many popular photometric systems, including the Gaia filters. The database can be easily extended to additional photometric systems and stellar spectral libraries. The web interface allows users to transform their catalogue of theoretical stellar parameters into magnitudes and colours of selected filter sets. The BC tables can be downloaded or also be implemented into large simulation projects using the interpolation code provided with the database. We compute extinction coefficients on a star-by-star basis, hence taking into account the effects of spectral type and non-linearity dependency on the total extinction. We illustrate the use of these BCs in \texttt{PARSEC} isochrones. We show that using spectral-type dependent extinction coefficients is quite necessary for Gaia filters whenever $\Av\gtrsim 0.5$\,mag. BC tables for rotating stars and tables of limb-darkening coefficients are also provided.

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arXiv2019

Evolutionary models for ultracool dwarfs

Ultracool dwarfs have emerged as key targets for searches of transiting exoplanets. Precise estimates of the host parameters (including mass, age, and radius) are fundamental to constrain the physical properties of orbiting exoplanets. We have extended our evolutionary code CLES (Code Liégeois d'Evolution Stellaire) to the ultracool dwarf regime. We include relevant equations of state for H, He, as well as C and O elements to cover the temperature-density regime of ultracool dwarf interiors. For various metallicities, we couple the interior models to two sets of model atmospheres as surface boundary conditions. We show that including C and O in the EOS has a significant effect close the H-burning limit mass. The typical systematic error associated with uncertainties in input physics in evolutionary models is $\sim 0.0005 M_\odot$. We test model results against observations for objects whose parameters have been determined from independent techniques. We are able to reproduce dynamical mass measurements of LSPM J1314+1320AB within $1σ$ with the condition of varying the metallicity (determined from calibrations) up to $2.5σ$. For GJ 65AB, a $2σ$ agreement is obtained between individual masses from differential astrometry and those from evolutionary models. We provide tables of ultracool dwarf models for various masses and metallicities that can be used as reference when estimating parameters for ultracool objects.

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arXiv2019

The mass-loss, expansion velocities and dust production rates of carbon stars in the Magellanic Clouds

The properties of carbon stars in the Magellanic Clouds (MCs) and their total dust production rates are predicted by fitting their spectral energy distributions (SED) over pre-computed grids of spectra reprocessed by dust. The grids are calculated as a function of the stellar parameters by consistently following the growth for several dust species in their circumstellar envelopes, coupled with a stationary wind. Dust radiative transfer is computed taking as input the results of the dust growth calculations. The optical constants for amorphous carbon are selected in order to reproduce different observations in the infrared and optical bands of \textit{Gaia} Data Release 2. We find a tail of extreme mass-losing carbon stars in the Large Magellanic Cloud (LMC) with low gas-to-dust ratios that is not present in the Small Magellanic Cloud (SMC). Typical gas-to-dust ratios are around $700$ for the extreme stars, but they can be down to $\sim160$--$200$ and $\sim100$ for a few sources in the SMC and in the LMC, respectively. The total dust production rate for the carbon star population is $\sim 1.77\pm 0.45\times10^{-5}$~M$_\odot$~yr$^{-1}$, for the LMC, and $\sim 2.52\pm 0.96 \times 10^{-6}$~M$_\odot$~yr$^{-1}$, for the SMC. The extreme carbon stars observed with the Atacama Large Millimeter Array and their wind speed are studied in detail. For the most dust-obscured star in this sample the estimated mass-loss rate is $\sim 6.3 \times 10^{-5}$~M$_\odot$~yr$^{-1}$. The grids of spectra are available at: https://ambrananni085.wixsite.com/ambrananni/online-data-1 and included in the SED-fitting python package for fitting evolved stars https://github.com/s-goldman/Dusty-Evolved-Star-Kit .

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arXiv2019

Carbon stars with increased oxygen and nitrogen abundances: hydrostatic dust-free model atmospheres

We have computed a grid of hydrostatic spherical COMARCS models for C stars covering metallicities from [Z/H]=0 to -2 and values of the carbon excess [C-O] from 6.41 to 9.15, plus some temperature sequences, where the amount of oxygen and nitrogen is increased relative to a scaled solar element mixture. Such abundance variations may appear during the late stages of stellar evolution. Our study covers changes of [O/Z] and [N/Z] going up to +0.5. Based on the atmospheric structures we have calculated synthetic spectra and photometry for all of the models in a consistent way. The sequences with changed [O/Z] and [N/Z] can be used to derive correction terms, which are applied to the colours predicted for a certain combination of effective temperature, surface gravity, metallicity and carbon excess. If one neglects these shifts in case of a variable oxygen amount, taking [C-O] instead of C/O gives much better results, since the first quantity dominates the formation of many important molecular species. For the warmer C giants with weaker pulsation it is in principle possible to determine [C-O], [O/Z] or [N/Z] from high resolution spectra, when the opacities in the radiative transfer calculations for the models and observable properties are treated consistently. The corresponding changes due to the abundances often become significantly larger than the deviations caused by uncertainties of the stellar parameters or by a optically thin dust shell. Photometric data and low or medium resolution spectra are not sufficient to derive the mentioned quantities.

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arXiv2019

Constraining the thermally-pulsing asymptotic giant branch phase with resolved stellar populations in the Small Magellanic Cloud

The thermally-pulsing asymptotic giant branch (TP-AGB) experienced by low- and intermediate-mass stars is one of the most uncertain phases of stellar evolution and the models need to be calibrated with the aid of observations. To this purpose, we couple high-quality observations of resolved stars in the Small Magellanic Cloud (SMC) with detailed stellar population synthesis simulations computed with the TRILEGAL code. The strength of our approach relies on the detailed spatially-resolved star formation history of the SMC, derived from the deep near-infrared photometry of the VISTA survey of the Magellanic Clouds, as well as on the capability to quickly and accurately explore a wide variety of parameters and effects with the COLIBRI code for the TP-AGB evolution. Adopting a well-characterized set of observations -- star counts and luminosity functions -- we set up a calibration cycle along which we iteratively change a few key parameters of the TP-AGB models until we eventually reach a good fit to the observations. Our work leads to identify two best-fitting models that mainly differ in the efficiencies of the third dredge-up and mass loss in TP-AGB stars with initial masses larger than about 3 M$_{\odot}$. On the basis of these calibrated models we provide a full characterization of the TP-AGB stellar population in the SMC in terms of stellar parameters (initial masses, C/O ratios, carbon excess, mass-loss rates). Extensive tables of isochrones including these improved models are publicly available.

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arXiv2017

Estimating the dust production rate of carbon stars in the Small Magellanic Cloud

We employ newly computed grids of spectra reprocessed by dust for estimating the total dust production rate (DPR) of carbon stars in the Small Magellanic Cloud (SMC). For the first time, the grids of spectra are computed as a function of the main stellar parameters, i.e. mass-loss rate, luminosity, effective temperature, current stellar mass and element abundances at the photosphere, following a consistent, physically grounded scheme of dust growth coupled with stationary wind outflow. The model accounts for the dust growth of various dust species formed in the circumstellar envelopes of carbon stars, such as carbon dust, silicon carbide and metallic iron. In particular, we employ some selected combinations of optical constants and grain sizes for carbon dust which have been shown to reproduce simultaneously the most relevant color-color diagrams in the SMC. By employing our grids of models, we fit the spectral energy distributions of $\approx$3100 carbon stars in the SMC, consistently deriving some important dust and stellar properties, i.e. luminosities, mass-loss rates, gas-to-dust ratios, expansion velocities and dust chemistry. We discuss these properties and we compare some of them with observations in the Galaxy and LMC. We compute the DPR of carbon stars in the SMC, finding that the estimates provided by our method can be significantly different, between a factor $\approx2-5$, than the ones available in the literature. Our grids of models, including the spectra and other relevant dust and stellar quantities, are publicly available at http://starkey.astro.unipd.it/web/guest/dustymodels

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arXiv2017

A new generation of PARSEC-COLIBRI stellar isochrones including the TP-AGB phase

We introduce a new generation of PARSEC-COLIBRI stellar isochrones that include a detailed treatment of the thermally-pulsing asymptotic giant branch (TP-AGB) phase, and covering a wide range of initial metallicities (0.0001<Zi<0.06). Compared to previous releases, the main novelties and improvements are: use of new TP-AGB tracks and related atmosphere models and spectra for M and C-type stars; inclusion of the surface H+He+CNO abundances in the isochrone tables, accounting for the effects of diffusion, dredge-up episodes and hot-bottom burning; inclusion of complete thermal pulse cycles, with a complete description of the in-cycle changes in the stellar parameters; new pulsation models to describe the long-period variability in the fundamental and first overtone modes; new dust models that follow the growth of the grains during the AGB evolution, in combination with radiative transfer calculations for the reprocessing of the photospheric emission. Overall, these improvements are expected to lead to a more consistent and detailed description of properties of TP-AGB stars expected in resolved stellar populations, especially in regard to their mean photometric properties from optical to mid-infrared wavelengths. We illustrate the expected numbers of TP-AGB stars of different types in stellar populations covering a wide range of ages and initial metallicities, providing further details on the C-star island that appears at intermediate values of age and metallicity, and about the AGB-boosting effect that occurs at ages close to 1.6 Gyr for populations of all metallicities. The isochrones are available through a new dedicated web server.

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arXiv2016

Constraining dust properties in Circumstellar Envelopes of C-stars in the Small Magellanic Cloud: optical constants and grain size of Carbon dust

We present a new approach aimed at constraining the typical size and optical properties of carbon dust grains in Circumstellar envelopes (CSEs) of carbon-rich stars (C-stars) in the Small Magellanic Cloud (SMC). To achieve this goal, we apply our recent dust growth description, coupled with a radiative transfer code to the CSEs of C-stars evolving along the TP-AGB, for which we compute spectra and colors. Then we compare our modeled colors in the near- and mid-infrared (NIR and MIR) bands with the observed ones, testing different assumptions in our dust scheme and employing several data sets of optical constants for carbon dust available in the literature. Different assumptions adopted in our dust scheme change the typical size of the carbon grains produced. We constrain carbon dust properties by selecting the combination of grain size and optical constants which best reproduces several colors in the NIR and MIR at the same time. The different choices of optical properties and grain size lead to differences in the NIR and MIR colors greater than two magnitudes in some cases. We conclude that the complete set of observed NIR and MIR colors are best reproduced by small grains, with sizes between $\sim$0.035 and $\sim$0.12~$μ$m, rather than by large grains between $\sim0.2$ and $0.7$~$μ$m. The inability of large grains to reproduce NIR and MIR colors seems independent of the adopted optical data set. We also find a possible trend of the grain size with mass-loss and/or carbon excess in the CSEs of these stars.

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arXiv2016

Dynamic atmospheres and winds of cool luminous giants, I. Al$_2$O$_3$ and silicate dust in the close vicinity of M-type AGB stars

High spatial resolution techniques have given valuable insights into the mass loss mechanism of AGB stars, which presumably involves a combination of atmospheric levitation by pulsation-induced shock waves and radiation pressure on dust. Observations indicate that Al$_2$O$_3$ condenses at distances of about 2 stellar radii or less, prior to the formation of silicates. Al$_2$O$_3$ grains are therefore prime candidates for producing the scattered light observed in the close vicinity of several M-type AGB stars, and they may be seed particles for the condensation of silicates at lower temperatures. We have constructed a new generation of Dynamic Atmosphere & Radiation-driven Wind models based on Implicit Numerics (DARWIN), including a time-dependent treatment of grain growth & evaporation for both Al$_2$O$_3$ and Fe-free silicates (Mg$_2$SiO$_4$). The equations describing these dust species are solved in the framework of a frequency-dependent radiation-hydrodynamical model for the atmosphere & wind structure, taking pulsation-induced shock waves and periodic luminosity variations into account. Condensation of Al$_2$O$_3$ at the close distances and in the high concentrations implied by observations requires high transparency of the grains in the visual and near-IR region to avoid destruction by radiative heating. For solar abundances, radiation pressure due to Al$_2$O$_3$ is too low to drive a wind. Nevertheless, this dust species may have indirect effects on mass loss. The formation of composite grains with an Al$_2$O$_3$ core and a silicate mantle can give grain growth a head start, increasing both mass loss rates and wind velocities. Furthermore, our experimental core-mantle grain models lead to variations of visual and near-IR colors during a pulsation cycle which are in excellent agreement with observations.

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arXiv2016

Synthetic photometry for M and K giants and stellar evolution: hydrostatic dust-free model atmospheres and chemical abundances

Based on a grid of hydrostatic spherical COMARCS models for cool stars we have calculated observable properties of these objects, which will be mainly used in combination with stellar evolution tracks and population synthesis tools. The high resolution opacity sampling and low resolution convolved spectra as well as bolometric corrections for a large number of filter systems are made electronically available. We exploit those data to study the effect of mass, C/O ratio and nitrogen abundance on the photometry of K and M giants. Depending on effective temperature, surface gravity and the chosen wavelength ranges variations of the investigated parameters cause very weak to moderate and, in the case of C/O values close to one, even strong shifts of the colours. For the usage with stellar evolution calculations they will be treated as correction factors applied to the results of an interpolation in the main quantities. When we compare the synthetic photometry to observed relations and to data from the Galactic Bulge, we find in general a good agreement. Deviations appear for the coolest giants showing pulsations, mass loss and dust shells, which cannot be described by hydrostatic models.

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arXiv2015

How important are metal-poor AGB stars as cosmic dust producers?

The efficiency of dust formation in oxygen-rich AGB stars should (in theory) be metallicity dependent since they are not producing their own raw material for dust production. Metal-poor carbon stars may not be very efficient dust producers either, because of more radiative heating of the grains forming in their atmospheres. We have just confirmed that inefficient dust and wind formation in simulations of metal-poor carbon stars is a real physical effect, albeit within the limitations of our simulations. Taken at face value, this implies that the amount of dust supplied by low-metallicity AGB stars to the build up of the cosmic dust component is clearly limited. Consequently, one may also ask how large a contribution AGB stars can make in general, when compared to recent observations of cosmic dust, which are suggesting major contributions from other sources?

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arXiv2015

Oxygen isotopic ratios in intermediate-mass red giants

Context. The abundances of the three main isotopes of oxygen are altered in the course of the CNO-cycle. When the first dredge-up mixes the burning products to the surface, the nucleosynthesis processes can be probed by measuring oxygen isotopic ratios. Aims. By measuring 16O/17O and 16O/18O in red giants of known mass we compare the isotope ratios with predictions from stellar and galactic evolution modelling. Methods. Oxygen isotopic ratios were derived from the K-band spectra of six red giants. The sample red giants are open cluster members with known masses of between 1.8 and 4.5 Msun . The abundance determination employs synthetic spectra calculated with the COMARCS code. The effect of uncertainties in the nuclear reaction rates, the mixing length, and of a change in the initial abundance of the oxygen isotopes was determined by a set of nucleosynthesis and mixing models using the FUNS code. Results. The observed 16O/17O ratios are in good agreement with the model results, even if the measured values do not present clear evidence of a variation with the stellar mass. The observed 16O/18O ratios are clearly lower than the predictions from our reference model. Variations in nuclear reaction rates and mixing length parameter both have only a very weak effect on the predicted values. The 12C/13C ratios of the K giants studied implies the absence of extra-mixing in these objects. Conclusions. A comparison with galactic chemical evolution models indicates that the 16O/18O abundance ratio underwent a faster decrease than predicted. To explain the observed ratios, the most likely scenario is a higher initial 18O abundance combined with a lower initial 16 O abundance. Comparing the measured 18 O/17 O ratio with the corresponding value for the ISM points towards an initial enhancement of 17O as well. Limitations imposed by the observations prevent this from being a conclusive result.

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