arXiv subjects

Marcelo M. Miller Bertolami

Publications and source records attributed to Marcelo M. Miller Bertolami.

At least 37 records · Page 2Linked to original sources

arXiv2022

A Red Giants' Toy Story II: Understanding the Red-Giant Branch Bump

The Red-Giant Branch Bump (RGBB) is one of the most noteworthy features in the red-giant luminosity function of stellar clusters. It is caused by the passage of the hydrogen-burning shell through the composition discontinuity left at the point of the deepest penetration by the convective envelope. When crossing the discontinuity the usual trend in increasing luminosity reverses for a short time before it increases again, causing a zig-zag in the evolutionary track. In spite of its apparent simplicity the actual physical reason behind the decrease in luminosity is not well understood and several different explanations have been offered. Here we use a recently proposed simple toy-model for the structure of low-mass red giants, together with previous results, to show beyond reasonable doubt that the change in luminosity at the RGBB can be traced to the change in the mean molecular weight of the layers on top of the burning shell. And that these changes happen on a nuclear timescale. The change in the effective mean molecular weight, as the burning shell approaches the discontinuity, causes a drop in the temperature of the burning shell which is attenuated by the consequent feedback contraction of the layers immediately below the burning shell. Our work shows that, when applied correctly, including the feedback on the structure of the core together with of the increase in the mass of the core, shell-source homology relations do a great quantitative job in explaining the properties of full evolutionary models at the RGBB.

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arXiv2022

A Red Giants' Toy Story

In spite of the spectacular progress accomplished by stellar evolution theory some simple questions remain unanswered. One of these questions is ``Why do stars become Red Giants?''. Here we present a relatively simple analytical answer to this question. We validate our analysis by constructing a quantitative toy-model of a red giant and comparing its predictions to full stellar evolutionar models. We find that the envelope forces the value of $\nabla=d \ln T/d \ln P$ at, and above, the burning shell into a very narrow range of possible values. Together with the fact that the stellar material at the burning shell both provides and transports most of the stellar luminosity, this leads to tight relations between the thermodynamic variables at the burning shell and the mass and radius of the core -- $T_s(M_c,R_s)$, $P_s(M_c,R_s)$ and $ρ_s(M_c,R_s)$. When complemented by typical mass-radius relations of the helium cores, this implies that for all stellar masses the evolution of the core dictates the values of $T_s$, $P_s$ and $ρ_s$. We show that for all stellar masses evolution leads to an increase in the pressure and density contrasts between the shell and the core, forcing a huge expansion of the layers on top of the burning shell. Besides explaining why stars become red giants our analysis also offers a mathematical demonstration of the so-called shell homology relations, and provides simple quantitative answers to some properties of low-mass red giants.

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arXiv2022

An evolutionary channel for CO-rich and pulsating He-rich subdwarfs

Recently a new class of hot subluminous stars strongly enriched in C and O have been discovered (CO-sdOs). These stars show abundances very similar to those observed in PG1159 stars but at lower temperatures. Moreover, it has been recently suggested that C and O enrichment might be the key ingredient driving the pulsations in He-rich hot subdwarf stars (He-sdBVs). Here we argue that these two types of rare stars can be explained by a variant of one of the main channels forming hot subdwarf stars. The scenario involves the formation and merging of a He-core white dwarf and a less massive CO-core white dwarf. We have constructed a simple merger models and computed their subsequent evolution. The merger products are in agreement with the surface parameters and composition of CO-sdOs. In addition, we have performed simulations including the effects of element diffusion and the excitation of pulsations. These simulations show that less massive merger products can form stellar structures that have surface parameters, abundances, and pulsation periods similar to those displayed by He-sdBVs. We conclude that the proposed scenario, or some variant of it, offers a very plausible explanation for the formation of CO-sdOs, pulsating He-sdBs and low-luminosity PG1159 stars.

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arXiv2022

The composition of massive white dwarfs and their dependence on the C-burning modeling

Recent computations of the interior composition of ultra-massive white dwarfs (WD) have suggested that some white dwarfs could be composed of neon (Ne)-dominated cores. This result is at variance with our previous understanding of the chemical structure of massive white dwarfs, where oxygen is the predominant element. In addition, it is not clear whether some hybrid carbon (C) oxygen (O)-Ne white dwarfs might form when convective boundary mixing is accounted for during the propagation of the C-flame, in the C-burning stage. Both Ne-dominated and hybrid CO-Ne core would have measurable consequences for asteroseismological studies based on evolutionary models. In this work we explore in detail to which extent differences in the adopted micro- and macro-physics can explain the different final white dwarf compositions that have been found by different authors. Additionally, we explored the impact of such differences in the cooling times, crystallization and pulsational properties of pulsating WDs. We explore the impact of the intensity of convective boundary mixing during the C-flash, extreme mass-loss rates, and the size of the adopted nuclear networks on the final composition, age, crystallization and pulsational properties of white dwarfs. Based on the insight coming from 3D hydro-dynamical simulations, we expect that the very slow propagation of the carbon flame will be altered by turbulent entrainment affecting the inward propagation of the flame. Also, we find that Ne-dominated chemical profiles of massive WDs recently reported appear in their modeling due to the overlooking of a key nuclear reaction. We find that the inaccuracies in the chemical composition of ultra-massive white dwarfs recently reported lead to differences of 10% in the cooling times and degree of crystallization and about 8% in the period spacing of the models once they reach the ZZ Ceti instability strip.

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arXiv2021

Long live the disk: lifetimes of protoplanetary disks in hierarchical triple star systems and a possible explanation for HD 98800 B

The gas dissipation from a protoplanetary disk is one of the key processes affecting planet formation, and it is widely accepted that it happens on timescales of a few million years for disks around single stars. Over the last years, several protoplanetary disks have been discovered in multiple star systems, and despite the complex environment in which they find themselves, some of them seem to be quite old, a situation that may favor planet formation. A clear example of this is the disk around HD 98800 B, a binary in a hierarchical quadruple stellar system, which at a $\sim$10 Myr age seems to still be holding significant amounts of gas. Here we present a 1D+1D model to compute the vertical structure and gas evolution of circumbinary disks in hierarchical triple star systems considering different stellar and disk parameters. We show that tidal torques due to the inner binary together with the truncation of the disk due to the external companion strongly reduce the viscous accretion and expansion of the disk. Even allowing viscous accretion by tidal streams, disks in these kind of environments can survive for more than 10 Myr, depending on their properties, with photoevaporation being the main gas dissipation mechanism. We particularly apply our model to the circumbinary disk around HD 98800 B and confirm that its longevity, along with the current non-existence of a disk around the companion binary HD 98800 A, can be explained with our model and by this mechanism.

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arXiv2021

Weighing stars from birth to death: mass determination methods across the HRD

The mass of a star is the most fundamental parameter for its structure, evolution, and final fate. It is particularly important for any kind of stellar archaeology and characterization of exoplanets. There exists a variety of methods in astronomy to estimate or determine it. In this review we present a significant number of such methods, beginning with the most direct and model-independent approach using detached eclipsing binaries. We then move to more indirect and model-dependent methods, such as the quite commonly used isochrone or stellar track fitting. The arrival of quantitative asteroseismology has opened a completely new approach to determine stellar masses and to complement and improve the accuracy of other methods. We include methods for different evolutionary stages, from the pre-main sequence to evolved (super)giants and final remnants. For all methods uncertainties and restrictions will be discussed. We provide lists of altogether more than 200 benchmark stars with relative mass accuracies between $[0.3,2]\%$ for the covered mass range of $M\in [0.1,16]\,\msun$, $75\%$ of which are stars burning hydrogen in their core and the other $25\%$ covering all other evolved stages. We close with a recommendation how to combine various methods to arrive at a "mass-ladder" for stars.

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arXiv2020

A first asteroseismological analysis on WD J1916+3938, the ZZ Ceti star discovered in the Kepler mission field

Asteroseismology of DAV stars (ZZ Ceti variables) can provide valuable clues about the origin, structure and evolution of DA (atmospheres rich in H) white dwarfs. Recently, a new DAV star, WD J191643.83+393849.7, has been discovered in the field of the Kepler spacecraft. It is expected that further monitoring of this star in the next years will enable astronomers to obtain the best lightcurve of a pulsating DA white dwarf ever recorded, and thus to know with unprecedented precision the hidden details of the internal structure of this star. In this paper, we perform a first asteroseismological analysis of WD J191643.83+393849.7 on the basis of fully evolutionary DA white-dwarf models. Specifically, we employ a complete set of evolutionary DA white-dwarf structures covering a wide range of effective temperatures, stellar masses, and H envelope thicknesses. These models have been obtained on the basis of a complete treatment of the evolutionary history of progenitors stars. We compute g-mode adiabatic pulsation periods for this set of models and compare them with the pulsation periods exhibited by WD J191643.83+393849.7. Based on a tentative estimation of the mean period spacing of the star, we find that the stellar mass should be substantially large ($\gtrsim 0.80 M_{\odot}$), in agreement with the spectroscopically derived stellar mass. Also, from period-to-period fits we find an asteroseismological model characterised by a low effective temperature, rather high stellar mass and a thin H envelope. The possibility that this rather massive pulsating white dwarf can be further monitored with Kepler with a high degree of detail turns the star WD J191643.83+393849.7 into a promising and unique object to study the physics of crystallization and carbon/oxygen phase diagrams at high densities. ALL THIS ANALYSIS IS OUTDATED AND INVALID

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arXiv2020

Asteroseismic signatures of the helium-core flash

All evolved stars with masses $M_\star\lesssim 2M_\odot$ undergo a helium(He)-core flash at the end of their first stage as a giant star. Although theoretically predicted more than 50 years ago, this core-flash phase has yet to be observationally probed. We show here that gravity modes (g modes) stochastically excited by He-flash driven convection are able to reach the stellar surface, and induce periodic photometric variabilities in hot-subdwarf stars with amplitudes of the order of a few mmag. As such they can now be detected by space-based photometry with the Transiting Exoplanet Survey Satellite (TESS) in relatively bright stars (e.g. magnitudes $I_C\lesssim 13$). The range of predicted periods spans from a few thousand seconds to tens of thousand seconds, depending on the details of the excitation region. In addition, we find that stochastically excited pulsations reproduce the pulsations observed in a couple of He-rich hot subdwarf stars. These stars, and in particular the future TESS target Feige 46, are the most promising candidates to probe the He-core flash for the first time.

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arXiv2020

Impact of Convective Boundary Mixing on the TP-AGB

The treatment of convective boundaries remains an important source of uncertainty within stellar evolution, with drastic implications for the thermally-pulsing stars on the Asymptotic Giant Branch (AGB). Various sources are taken as motivation for the incorporation of convective boundary mixing during this phase, from s-process nucleosynthesis to hydrodynamical models. In spite of the considerable evidence in favour of the existence of convective boundary mixing on the pre-AGB evolution, this mixing is not universally included in models of TP-AGB stars. The aim of this investigation is to ascertain the extent of convective boundary mixing, which is compatible with observations when considering full evolutionary models. Additionally, we investigate a theoretical argument that has been made that momentum-driven overshooting at the base of the pulse-driven convection zone should be negligible. We show that, while the argument holds,estimations based on the picture of turbulent entrainment suggest that mixing is possible at both convective boundaries. We demonstrate that additional mixing at convective boundaries during core-burning phases prior to the thermally-pulsing Asymptotic Giant Branch has an impact on the later evolution, changing the mass range at which the third dredge-up and hot-bottom burning occur, and thus also the final surface composition. In addition, an effort has been made to constrain the efficiency of convective boundary mixing at the different convective boundaries, using observational constraints. Our study suggests a strong tension between different constraints that makes it impossible to reproduce all observables simultaneously within the framework of an exponentially decaying overshooting. This result calls for a reassessment of both the models of convective boundary mixing and the observational constraints.

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arXiv2019

Revised Simulations of the Planetary Nebulae Luminosity Function

We describe a revised procedure for the numerical simulation of planetary nebulae luminosity functions (PNLF), improving on previous work (Méndez & Soffner 1997). The procedure now is based on new H-burning post-AGB evolutionary tracks (Miller Bertolami 2016). For a given stellar mass, the new central stars are more luminous and evolve faster. We have slightly changed the distribution of the [OIII] 5007 intensities relative to those of H$β$ and the generation of absorbing factors, while still basing their numerical modeling on empirical information extracted from studies of galactic planetary nebulae (PNs) and their central stars. We argue that the assumption of PNs being completely optically thick to H-ionizing photons leads to conflicts with observations and show that to account for optically thin PNs is necessary. We then use the new simulations to estimate a maximum final mass, clarifying its meaning, and discuss the effect of internal dust extinction as a possible way of explaining the persistent discrepancy between PNLF and surface brightness fluctuation (SBF) distances. By adjusting the range of minimum to maximum final mass, it is also possible to explain the observed variety of PNLF shapes at intermediate magnitudes. The new PN formation rates are calculated to be slightly lower than suggested by previous simulations based on older post-AGB evolutionary tracks.

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arXiv2019

On the recent parametric determination of an asteroseismological model for the DBV star KIC 08626021

Asteroseismology of white dwarf (WD) stars is a powerful tool that allows to reveal the hidden chemical structure of WD and infer details about their evolution by comparing the observed periods with those obtained from stellar models. A recent asteroseismological study has reproduced the period spectrum of the helium rich pulsating WD KIC 08626021 with an unprecedented precision. The chemical structure derived from that analysis is notably different from that expected for a WD according to currently accepted formation channels, thus posing a challenge to the theory of stellar evolution. We explore the relevant micro- and macro-physics processes acting during the formation and evolution of KIC 08626021 that could lead to a chemical structure similar to that found through asteroseismology. We quantify to which extent is necessary to modify the physical processes that shapes the chemical structure, in order to reproduce the most important features of the asteroseismic model. We model the previous evolution of KIC 08626021 by exploring specific changes in the 12C+alpha reaction rate, screening processes, microscopic diffusion, as well as convective boundary mixing during core-He burning. We find that, in order to reproduce the core chemical profile derived for KIC 0862602, the 12C+alpha nuclear reaction rate has to be increased by a factor of $\sim$ 10 during the helium-core burning, and reduced by a factor of $\sim$ 1000 during the following helium-shell burning, as compared with the standard predictions for this rate. In addition, the main chemical structures derived for KIC 0862602 cannot be reconciled with our present knowledge of white dwarf formation. We find that within our current understanding of white dwarf formation and evolution, it is difficult to reproduce the most important asteroseismologically-derived features of the chemical structure of KIC 08626021.

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arXiv2019

Pulsating white dwarfs: new insights

White dwarf stars constitute the final evolutionary stage for more than 95 per cent of all stars. The Galactic population of white dwarfs conveys a wealth of information about several fundamental issues and are of vital importance to study the structure, evolution and chemical enrichment of our Galaxy and its components ---including the star formation history of the Milky Way. In addition, white dwarfs are tracers of the evolution of planetary systems along several phases of stellar evolution. Also, white dwarfs are used as laboratories for astro-particle physics, being their interest focused on physics beyond the standard model. The last decade has witnessed a great progress in the study of white dwarfs. In particular, a wealth of information of these stars from different surveys has allowed us to make meaningful comparison of evolutionary models with observations. While some information like surface chemical composition, temperature and gravity of isolated white dwarfs can be inferred from spectroscopy, and the total mass and radius can be derived as well when they are in binaries, the internal structure of these compact stars can be unveiled only by means of asteroseismology, an approach based on the comparison between the observed pulsation periods of variable stars and the periods predicted by appropriate theoretical models. The asteroseismological techniques allow us to infer details of the internal chemical stratification, the total mass, and even the stellar rotation profile. In this review, we first revise the evolutionary channels currently accepted that lead to the formation of white-dwarf stars, and then, we give a detailed account of the different sub-types of pulsating white dwarfs known so far, emphasizing the recent observational and theoretical advancements in the study of these fascinating variable stars.

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arXiv2019

On the evolution of ultra-massive white dwarfs

Ultra-massive white dwarfs are powerful tools to study various physical processes in the Asymptotic Giant Branch (AGB), type Ia supernova explosions and the theory of crystallization through white dwarf asteroseismology. Despite the interest in these white dwarfs, there are few evolutionary studies in the literature devoted to them. Here, we present new ultra-massive white dwarf evolutionary sequences that constitute an improvement over previous ones. In these new sequences, we take into account for the first time the process of phase separation expected during the crystallization stage of these white dwarfs, by relying on the most up-to-date phase diagram of dense oxygen/neon mixtures. Realistic chemical profiles resulting from the full computation of progenitor evolution during the semidegenerate carbon burning along the super-AGB phase are also considered in our sequences. Outer boundary conditions for our evolving models are provided by detailed non-gray white dwarf model atmospheres for hydrogen and helium composition. We assessed the impact of all these improvements on the evolutionary properties of ultra-massive white dwarfs, providing up-dated evolutionary sequences for these stars. We conclude that crystallization is expected to affect the majority of the massive white dwarfs observed with effective temperatures below $40\,000\, \rm K$. Moreover, the calculation of the phase separation process induced by crystallization is necessary to accurately determine the cooling age and the mass-radius relation of massive white dwarfs. We also provide colors in the GAIA photometric bands for our H-rich white dwarf evolutionary sequences on the basis of new models atmospheres. Finally, these new white dwarf sequences provide a new theoretical frame to perform asteroseismological studies on the recently detected ultra-massive pulsating white dwarfs.

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arXiv2019

Spectral analysis of the binary nucleus of the planetary nebula Hen 2-428 - first results

Identifying progenitor systems for the double-degenerate scenario is crucial to check the reliability of type Ia super-novae as cosmological standard candles. Santander-Garcia et al. (2015) claimed that Hen 2-428 has a double-degenerate core whose combined mass significantly exceeds the Chandrasekhar limit. Together with the short orbital period (4.2 hours), the authors concluded that the system should merge within a Hubble time triggering a type Ia supernova event. Garcia-Berro et al. (2016) explored alternative scenarios to explain the observational evidence, as the high mass conclusion is highly unlikely within predictions from stellar evolution theory. They conclude that the evidence supporting the supernova progenitor status of the system is premature. Here we present the first quantitative spectral analysis of Hen 2-428 which allows us to derive the effective temperatures, surface gravities and helium abundance of the two CSPNe based on state-of-the-art, non-LTE model atmospheres. These results provide constrains for further studies of this particularly interesting system.

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arXiv2018

Revealing the True Nature of Hen 2-428

The nucleus of Hen 2-428 is a short orbital period (4.2 h) spectroscopic binary, whose status as potential supernovae type Ia progenitor has raised some controversy in the literature. We present preliminary results of a thorough analysis of this interesting system, which combines quantitative non-local thermodynamic equilibrium spectral modelling (non-LTE), radial velocity analysis,multi-band light curve fitting, and state-of-the art stellar evolutionary calculations. Importantly, we find that the dynamical system mass that is derived by using all available He II lines does not exceed the Chandrasekhar mass limit. Furthermore, the individual masses of the two central stars are too small to lead to an SN Ia in case of a dynamical explosion during the merger process.

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arXiv2018

The inside-out planetary nebula around a born-again star

Planetary nebulae are ionized clouds of gas formed by the hydrogen-rich envelopes of low- and intermediate-mass stars ejected at late evolutionary stages. The strong UV flux from their central stars causes a highly stratified ionization structure, with species of higher ionization potential closer to the star. Here we report on the exceptional case of HuBi 1, a double-shell planetary nebula whose inner shell presents emission from low-ionization species close to the star and emission from high-ionization species farther away. Spectral analysis demonstrates that the inner shell of HuBi 1 is excited by shocks, whereas its outer shell is recombining. The anomalous excitation of these shells can be traced to its low-temperature [WC10] central star whose optical brightness has declined continuously by 10 magnitudes in a period of 46 years. Evolutionary models reveal that this star is the descendent of a low-mass star ($\simeq$1.1 $M_\odot$) that has experienced a born-again event whose ejecta shock-excite the inner shell. HuBi 1 represents the missing link in the formation of metal-rich central stars of planetary nebulae from low-mass progenitors, offering unique insight regarding the future evolution of the born-again Sakurai's object. Coming from a solar-mass progenitor, HuBi 1 represents a potential end-state for our Sun.

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arXiv2018

The mysterious cut-off of the Planetary Nebula Luminosity Function

Planetary Nebulae (PN) emit enormous amount of energy in several emission lines. Measuring the line-flux for PNe in a given stellar population, the Planetary Nebula Luminosity Function (PNLF) can be compiled. Surveys of PNe revealed that the faint-end of the PNLF can be approximated by a simple exponential dependency expected for an expanding spherical shell. However at the bright-end there exists a steep cut-off which was unexpected and remains unexplained. Interestingly, the cut-off value appears to be nearly the same for different stellar populations as young spiral galaxies and old elliptical galaxies and, despite the lack of understanding, became an extragalactic distance estimator. Here we show that the recently computed post-AGB evolutionary tracks are capable to explain the decades old mystery. All new models with ages between 1 and 7 Gyr (progenitor masses between 2.0 and 1.1 of solar mass) evolve fast enough to ionize the PN, and at similar post-AGB luminosity which allows the [O III] 500.7nm line to reach nearly the same magnitude. The new models predict that the Sun at the end of its life will form a rather faint PN.

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arXiv2018

Pulsational instabilities driven by the $ε$ mechanism in hot pre-horizontal branch stars I. The Hot-Flasher Scenario

The $ε$ mechanism is a self-excitation mechanism of stellar pulsations which acts in regions inside the star where nuclear burning takes place. It has been shown that the $ε$ mechanism can excite pulsations in models of hot pre-horizontal branch stars, and that the shortest periods of LS IV-14$^{\circ}$116 could be explained that way. We aim to study the $ε$ mechanism in stellar models appropriate for hot pre-horizontal branch stars to predict their pulsational properties and the instability domain in the $\log g-\log T_{\rm eff}$ plane. We perform detailed computations of non-adiabatic non-radial pulsations on stellar models during the helium subflashes just before the helium-core burning phase. We find an instability domain of long-period gravity modes due to the $ε$ mechanism in the $\log g-\log T_{\rm eff}$ plane at roughly $22000\,{\rm K} \lesssim T_{\rm eff}\lesssim 50000\,$K and $4.67 \lesssim \log g \lesssim 6.15$. Consequently, we find instabilities due to the $ε$ mechanism on pre-extreme horizontal branch stellar models ($T_{\rm eff}\gtrsim 22000\,$K), but not on pre-blue horizontal branch stellar models ($T_{\rm eff}\lesssim 21000\,$K). The periods of excited modes range between $\sim 200$ and $\sim2000\,$s. Comparison with the three known pulsating He-rich subdwarfs shows that $ε$ mechanism can excite gravity modes in stars with similar surface properties, but in our models it is only able to excite modes in the range of the shortest observed periods. We predict a new instability strip for hot-subdwarf stars of which LS IV-14$^{\circ}$116 could be the first inhabitant. Based on simple estimates we expect 1 to 10 stars in the current samples of hot-subdwarf stars to be pulsating by the $ε$ mechanism. Our results could constitute a theoretical basis for future searches of pulsators in the Galactic field.

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