arXiv subjects

Marcelo M. Miller Bertolami

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

At least 55 records · Page 3Linked to original sources

arXiv2017

Chemical Abundances of Planetary Nebulae in the Substructures of M31 -- II. The Extended Sample and A Comparison Study with the Outer-disk Group

We report deep spectroscopy of ten planetary nebulae (PNe) in the Andromeda Galaxy (M31) using the 10.4m GTC. Our targets reside in different regions of M31, including halo streams and dwarf satellite M32, and kinematically deviate from the extended disk. The temperature-sensitive [O III] 4363 auroral line is observed in all targets. For four PNe, the GTC spectra extend beyond 1 micron, enabling explicit detection of the [S III] 6312 and 9069,9531 lines and thus determination of the [S III] temperature. Abundance ratios are derived and generally consistent with AGB model predictions. Our PNe probably all evolved from low-mass (<2 M_sun) stars, as analyzed with the most up-to-date post-AGB evolutionary models, and their main-sequence ages are mostly ~2-5 Gyr. Compared to the underlying, smooth, metal-poor halo of M31, our targets are uniformly metal-rich ([O/H]> -0.4), and seem to resemble the younger population in the stream. We thus speculate that our halo PNe formed in the Giant Stream's progenitor through extended star formation. Alternatively, they might have formed from the same metal-rich gas as did the outer-disk PNe, but was displaced into their present locations as a result of galactic interactions. These interpretations are, although speculative, qualitatively in line with the current picture, as inferred from previous wide-field photometric surveys, that M31's halo is the result of complex interactions and merger processes. The behavior of N/O of the combined sample of the outer-disk and our halo/substructure PNe signifies that hot bottom burning might actually occur at <3 M_sun, but careful assessment is needed.

astro-ph.GA↗

arXiv2016

New models for the evolution of central stars of planetary nebulae: Faster and Brighter

The post-asymptotic giant branch (AGB) phase is arguably one of the least understood phases of the evolution of low- and intermediate- mass stars. The recent post-AGB evolutionary sequences computed by Miller Bertolami (2016) are at least three to ten times faster than those previously published by Vassiliadis & Wood (1994) and Bloecker (1995) which have been used in a large number of studies. This is true for the whole mass and metallicity range. The new models are also $\sim$0.1-0.3 dex brighter than the previous models with similar remnant masses. In this short article we comment on the main reasons behind these differences, and discuss possible implications for other studies of post-AGB stars or planetary nebulae.

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arXiv2016

An asteroseismic constraint on the mass of the axion from the period drift of the pulsating DA white dwarf star L19-2

We employ an asteroseismic model of L19-2, a relatively massive ($M_{\star} \sim 0.75 M_{\sun}$) and hot ($T_{\rm eff} \sim 12\,100$ K) pulsating DA (H-rich atmosphere) white dwarf star (DAV or ZZ Ceti variable), and use the observed values of the temporal rates of period change of its dominant pulsation modes ($Π\sim 113$ s and $Π\sim 192$ s), to derive a new constraint on the mass of the axion, the hypothetical non-barionic particle considered as a possible component of the dark matter of the Universe. If the asteroseismic model employed is an accurate representation of L19-2, then our results indicate hints of extra cooling in this star, compatible with emission of axions of mass $m_{\rm a} \cos^2 β\lesssim 25$ meV or an axion-electron coupling constant of $g_{\rm ae} \lesssim 7 \times 10^{-13}$.

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arXiv2016

The effect of $^{22}$Ne diffusion in the evolution and pulsational properties of white dwarfs with solar metallicity progenitors

Because of the large neutron excess of $^{22}$Ne, this isotope rapidly sediments in the interior of the white dwarfs. This process releases an additional amount of energy, thus delaying the cooling times of the white dwarf. This influences the ages of different stellar populations derived using white dwarf cosmochronology. Furthermore, the overabundance of $^{22}$Ne in the inner regions of the star, modifies the Brunt-Väisälä frequency, thus altering the pulsational properties of these stars. In this work, we discuss the impact of $^{22}$Ne sedimentation in white dwarfs resulting from Solar metallicity progenitors ($Z=0.02$). We performed evolutionary calculations of white dwarfs of masses $0.528$, $0.576$, $0.657$ and $0.833$ M$_{\sun}$, derived from full evolutionary computations of their progenitor stars, starting at the Zero Age Main Sequence all the way through central hydrogen and helium burning, thermally-pulsing AGB and post-AGB phases. Our computations show that at low luminosities ($\log(L/L_{\sun})\la -4.25$), $^{22}$Ne sedimentation delays the cooling of white dwarfs with Solar metallicity progenitors by about 1~Gyr. Additionally, we studied the consequences of $^{22}$Ne sedimentation on the pulsational properties of ZZ~Ceti white dwarfs. We find that $^{22}$Ne sedimentation induces differences in the periods of these stars larger than the present observational uncertainties, particularly in more massive white dwarfs.

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arXiv2016

On the relevance of bubbles and potential flows for stellar convection

Recently Pasetto et al. have proposed a new method to derive a convection theory appropriate for the implementation in stellar evolution codes. Their approach is based on the simple physical picture of spherical bubbles moving within a potential flow in dynamically unstable regions, and a detailed computation of the bubble dynamics. Based on this approach the authors derive a new theory of convection which is claimed to be parameter free, non-local and time-dependent. This is a very strong claim, as such a theory is the holy grail of stellar physics. Unfortunately we have identified several distinct problems in the derivation which ultimately render their theory inapplicable to any physical regime. In addition we show that the framework of spherical bubbles in potential flows is unable to capture the essence of stellar convection, even when equations are derived correctly.

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arXiv2015

White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up

We present new white dwarf evolutionary sequences for low-metallicity progenitors. White dwarf sequences have been derived from full evolutionary calculations that take into account the entire history of progenitor stars, including the thermally-pulsing and the post-asymptotic giant branch phases. We show that for progenitor metallicities in the range 0.00003--0.001, and in the absence of carbon enrichment due to the occurrence of a third dredge-up episode, the resulting H envelope of the low-mass white dwarfs is thick enough to make stable H burning the most important energy source even at low luminosities. This has a significant impact on white dwarf cooling times. This result is independent of the adopted mass-loss rate during the thermally-pulsing and post-AGB phases, and the planetary nebulae stage. We conclude that in the absence of third dredge-up episodes, a significant part of the evolution of low-mass white dwarfs resulting from low-metallicity progenitors is dominated by stable H burning. Our study opens the possibility of using the observed white dwarf luminosity function of low-metallicity globular clusters to constrain the efficiency of third dredge up episodes during the thermally-pulsing AGB phase of low-metallicity progenitors.

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arXiv2014

Testing Fundamental Particle Physics with the Galactic White Dwarf Luminosity Function

Recent determinations of the white dwarf luminosity function (WDLF) from very large surveys have extended our knowledge of the WDLF to very high luminosities. It has been shown that the shape of the luminosity function of white dwarfs (WDLF) is a powerful tool to test the possible properties and existence of fundamental weakly interacting subelectronvolt particles. This, together with the availability of new full evolutionary white dwarf models that are reliable at high luminosities, have opened the possibility of testing particle emission in the core of very hot white dwarfs. We use the available WDLFs from the Sloan Digital Sky Survey and the SuperCOSMOS Sky Survey to constrain the values of the neutrino magnetic dipole moment ($μ_ν$) and the axion-electron coupling constant ($g_{ae}$) of DFSZ-axions.

hep-ph↗

arXiv2014

Post-Asymptotic Giant Branch Evolution of Low- and Intermediate-Mass Stars. Preliminary Results

Preliminary results from an ongoing project to compute a grid of post-AGB models is presented. Our preliminary results show that stellar evolution computations that include an updated treatment of the microphysics predict post-AGB timescales that are several times shorter that predicted by older models. Also the mass-luminosity relation of post-AGB models deviates from that of older grids. In addition, our results suggest only a slight metallicity dependence of the post-AGB timescales. We expect these results to have significant consequences for models of the formation of planetary nebulae and their luminosity function.

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arXiv2014

Revisiting the axion bounds from the Galactic white dwarf luminosity function

It has been shown that the shape of the luminosity function of white dwarfs (WDLF) is a powerful tool to check for the possible existence of DFSZ-axions, a proposed but not yet detected type of weakly interacting particles. With the aim of deriving new constraints on the axion mass, we compute in this paper new theoretical WDLFs on the basis of WD evolving models that incorporate for the feedback of axions on the thermal structure of the white dwarf. We find that the impact of the axion emission into the neutrino emission can not be neglected at high luminosities ($M_{\rm Bol}\lesssim 8$) and that the axion emission needs to be incorporated self-consistently into the evolution of the white dwarfs when dealing with axion masses larger than $m_a\cos^2β\gtrsim 5$ meV (i.e. axion-electron coupling constant $g_{ae}\gtrsim 1.4\times 10^{-13}$). We went beyond previous works by including 5 different derivations of the WDLF in our analysis. Then we have performed $χ^2$-tests to have a quantitative measure of the assessment between the theoretical WDLFs ---computed under the assumptions of different axion masses and normalization methods--- and the observed WDLFs of the Galactic disk. While all the WDLF studied in this work disfavour axion masses in the range suggested by asteroseismology ($m_a\cos^2β\gtrsim 10$ meV; $g_{ae}\gtrsim 2.8\times 10^{-13}$) lower axion masses can not be discarded from our current knowledge of the WDLF of the Galactic Disk. A larger set of completely independent derivations of the WDLF of the galactic disk as well as a detailed study of the uncertainties of the theoretical WDLFs is needed before quantitative constraints on the axion-electron coupling constant can be made.

hep-ph↗

arXiv2014

The white dwarf cooling sequence of 47 Tucanae

47 Tucanae is one of the most interesting and well observed and theoretically studied globular clusters. This allows us to study the reliability of our understanding of white dwarf cooling sequences, to confront different methods to determine its age, and to assess other important characteristics, like its star formation history. Here we present a population synthesis study of the cooling sequence of the globular cluster 47 Tucanae. In particular, we study the distribution of effective temperatures, the shape of the color-magnitude diagram, and the corresponding magnitude and color distributions. We do so using an up-to-date population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases. We find a good agreement between our theoretical models and the observed data. Thus, our study, rules out previous claims that there are still missing physics in the white dwarf cooling models at moderately high effective temperatures. We also derive the age of the cluster using the termination of the cooling sequence, obtaining a good agreement with the age determinations using the main-sequence turn-off. Finally, we find that the star formation history of the cluster is compatible with that btained using main sequence stars, which predict the existence of two distinct populations. We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main sequence stars provides an unique tool to model the properties of globular clusters.

astro-ph.GA↗

arXiv2014

Constraining the neutrino magnetic dipole moment from white dwarf pulsations

Pulsating white dwarf stars can be used as astrophysical laboratories to constrain the properties of weakly interacting particles. Comparing the cooling rates of these stars with the expected values from theoretical models allows us to search for additional sources of cooling due to the emission of axions, neutralinos, or neutrinos with magnetic dipole moment. In this work, we derive an upper bound to the neutrino magnetic dipole moment using an estimate of the rate of period change of the pulsating DB white dwarf star PG 1351+489. By comparing the theoretical rate of change of period expected for this star with the rate of change of period with time of PG 1351+489, we assess the possible existence of additional cooling by neutrinos with magnetic dipole moment. Our models suggest the existence of some additional cooling in this pulsating DB white dwarf, consistent with a non-zero magnetic dipole moment. Our upper limit for the neutrino magnetic dipole moment is somewhat less restrictive than, but still compatible with, other limits inferred from the white dwarf luminosity function or from the color-magnitude diagram of the Globular cluster M5. Further improvements of the measurement of the rate of period change of the dominant pulsation mode of PG 1351+489 will be necessary to confirm our bound.

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arXiv2013

Quiescent nuclear burning in low-metallicity white dwarfs

We discuss the impact of residual nuclear burning in the cooling sequences of hydrogen-rich DA white dwarfs with very low metallicity progenitors ($Z=0.0001$). These cooling sequences are appropriate for the study of very old stellar populations. The results presented here are the product of self-consistent, fully evolutionary calculations. Specifically, we follow the evolution of white dwarf progenitors from the zero-age main sequence through all the evolutionary phases, namely the core hydrogen-burning phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. This is done for the most relevant range of main sequence masses, covering the most usual interval of white dwarf masses --- from $0.53\, M_{\sun}$ to $0.83\, M_{\sun}$. Due to the low metallicity of the progenitor stars, white dwarfs are born with thicker hydrogen envelopes, leading to more intense hydrogen burning shells as compared with their solar metallicity counterparts. We study the phase in which nuclear reactions are still important and find that nuclear energy sources play a key role during long periods of time, considerably increasing the cooling times from those predicted by standard white dwarf models. In particular, we find that for this metallicity and for white dwarf masses smaller than about $0.6\, M_{\sun}$, nuclear reactions are the main contributor to the stellar luminosity for luminosities as low as $\log(L/L_{\sun})\simeq -3.2$. This, in turn, should have a noticeable impact in the white dwarf luminosity function of low-metallicity stellar populations.

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arXiv2013

New evolutionary sequences for extremely low mass white dwarfs: Homogeneous mass and age determinations, and asteroseismic prospects

We provide a fine and homogeneous grid of evolutionary sequences for He-core white dwarfs with masses 0.15-0.45 Msun, including the mass range for ELM white dwarfs (<0.20Msun). The grid is appropriate for mass and age determination, and to study their pulsational properties. White dwarf sequences have been computed by performing full evolutionary calculations that consider the main energy sources and processes of chemical abundance changes during white dwarf evolution. Initial models for the evolving white dwarfs have been obtained by computing the non-conservative evolution of a binary system consisting of a Msun ZAMS star and a 1.4 Msun neutron star for various initial orbital periods. To derive cooling ages and masses for He-core white dwarf we perform a least square fitting of the M(Teff, g) and Age(Teff, g) relations provided by our sequences by using a scheme that takes into account the time spent by models in different regions of the Teff-g plane. This is useful when multiple solutions for cooling age and mass determinations are possible in the case of CNO-flashing sequences. We also explore the adiabatic pulsational properties of models near the critical mass for the development of CNO flashes (~0.2 Msun). This is motivated by the discovery of pulsating white dwarfs with stellar masses near this threshold value. We obtain reliable and homogeneous mass and cooling age determinations for 58 very low-mass white dwarfs, including 3 pulsating stars. Also, we find substantial differences in the period spacing distributions of g-modes for models with stellar masses ~ 0.2 Msun, which could be used as a seismic tool to distinguish stars that have undergone CNO flashes in their early cooling phase from those that have not. Finally, for an easy application of our results, we provide a reduced grid of values useful to obtain masses and ages of He-core white dwarf.

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arXiv2012

Synthetic Light Curves for Born Again Events: Preliminary Results

The development of surveys which will be able to cover a large region of the sky several times per year will allow the massive detection of transient events taking place in timescales of years. In addition, the projected full digitalization of the Harvard plate collection will open a new window to identify slow transients taking place in timescales of centuries. In particular, these projects will allow the detection of stars undergoing slow eruptions as those expected during late helium flashes in the post-AGB evolution. In order to identify those transients which correspond with late helium flashes the development of synthetic light curves of those events is mandatory. In this connection we present preliminary results of a project aimed at computing grids of theoretical light curves of born again stars.

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arXiv2012

Pulsations driven by the $ε$-mechanism in post-merger remnants: first results

Helium-rich subdwarfs are a rare subclass of hot subdwarf stars which constitute a small and inhomogeneous group showing varying degrees of helium enrichment. Only one star, LS IV $^\circ$14 116 has been found to show multiperiodic luminosity variations. The variability of LS IV $^\circ$14 116 has been explained as the consequence of nonradial g-mode oscillations, whose excitation is difficult to understand within the frame of the standard $κ$-mechanism driving pulsations in sdBV stars. In a recent study, we have proposed that the pulsations of LS IV $^\circ$14 116 might be driven through the $ε$-mechanism acting in unstable He-burning zones in the interior of the star, that appear before the quiescent He-burning phase. One of the few accepted scenarios for the formation of He-rich subdwarfs is the merger of two He-core white dwarfs. As part of this project, we present a study of the $ε$-mechanism in post-merger remnants, and discuss the results in the light of the pulsations exhibited by LS IV $^\circ$14 116.

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arXiv2012

The rate of cooling of the pulsating white dwarf star G117$-$B15A: a new asteroseismological inference of the axion mass

We employ a state-of-the-art asteroseismological model of G117-B15A, the archetype of the H-rich atmosphere (DA) white dwarf pulsators (also known as DAV or ZZ Ceti variables), and use the most recently measured value of the rate of period change for the dominant mode of this pulsating star to derive a new constraint on the mass of axion, the still conjectural non-barionic particle considered as candidate for dark matter of the Universe. Assuming that G117-B15A is truly represented by our asteroseismological model, and in particular, that the period of the dominant mode is associated to a pulsation g-mode trapped in the H envelope, we find strong indications of the existence of extra cooling in this star, compatible with emission of axions of mass m_a \cos^2 β= 17.4^{+2.3}_{-2.7} meV.

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arXiv2012

A public database for white dwarf asteroseismology with fully evolutionary models: I. Chemical profiles and pulsation periods of ZZ Ceti (DAV) stars

We present a large bank of chemical profiles and pulsation periods suited for asteroseismological studies of ZZ Ceti (or DAV) variable stars. Our background equilibrium DA white dwarf models are the result of fully evolutionary computations that take into account the complete history of the progenitor stars from the ZAMS. The models are characterized by self-consistent chemical structures from the centre to the surface, and cover a wide range of stellar masses, effective temperatures, and H envelope thicknesses. We present dipole and quadrupole pulsation $g$-mode periods comfortably covering the interval of periods observed in ZZ Ceti stars. Complete tabulations of chemical profiles and pulsation periods to be used in asteroseismological period fits, as well as other quantities of interest, can be freely downloaded from our website (\url{http://www.fcaglp.unlp.edu.ar/evolgroup}

astro-ph.SR↗

arXiv2012

Asteroseismology of the Kepler V777 Her variable white dwarf with fully evolutionary models

DBV stars are pulsating white dwarfs with atmospheres rich in He. Asteroseismology of DBV stars can provide valuable clues about the origin, structure and evolution of hydrogen-deficient white dwarfs, and may allow to study neutrino and axion physics. Recently, a new DBV star, KIC 8626021, has been discovered in the field of the \emph{Kepler} spacecraft. It is expected that further monitoring of this star in the next years will enable astronomers to determine its detailed asteroseismic profile. We perform an asteroseismological analysis of KIC 8626021 on the basis of fully evolutionary DB white-dwarf models. We employ a complete set of evolutionary DB white-dwarf structures covering a wide range of effective temperatures and stellar masses. They 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 properties exhibited by KIC 8626021. On the basis of the mean period spacing of the star, we found that the stellar mass should be substantially larger than spectroscopy indicates. From period-to-period fits we found an asteroseismological model characterized by an effective temperature much higher than the spectroscopic estimate. In agreement with a recent asteroseismological analysis of this star by other authors, we conclude that KIC 8626021 is located near the blue edge of the DBV instability strip, contrarily to spectroscopic predictions. We also conclude that the mass of KIC 8626021 should be substantially larger than thought.

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