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  • Monte Carlo Simulations of the Population of Single and Binary White Dwarfs of our Galaxy  Open access

     Camacho Díaz, Judit
    Universitat Politècnica de Catalunya
    Theses

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    Degut a que les nanes blanques són l'etapa final de l'evolució de la gran majoria de les estrelles, aporten informació molt important sobre l'evolució química de la nostra galàxia, la taxa de formació d'estrelles, i la seva estructura i dinàmica. En aquesta tesi es presta atenció a dos problemes astrofísics relacionats tot i que diferents que involucren les nanes blanques.El primer d'aquests problemes es refereix a la naturalesa i la real ubicació dels successos de microlents en la direcció del Gran Núvols de Magallanes (LMC) que continua essent un misteri. Els principals grups d'observació, MACHO i EROS, es troben en disputa entre si, tot i que s'ha aconseguit cert consens en els punts més importants.El segon dels problemes que abordem en aquesta tesi és també un problema obert. Els sistemes binaris compactes representen el punt clau de diversos fenòmens interessants en la nostra galàxia i en altres galàxies. De fet, la gran majoria de binàries compactes es formen a través d'almenys un episodi d'embolcall comú. Tot i que els conceptes bàsics de l'evolució durant la fase d'embolcall comú són més aviat simples, els detalls encara estan lluny de ser ben entesos.Per aclarir aquests problemes, s'ha utilitzat un simulador Monte Carlo ja existent. Hem ampliat aquest simulador incloent-hi els models més recents de refredament de nanes blanques, així com un model detallat de la nostra galàxia i del LMC per tal d'emular els experiments dels equips MACHO i EROS. A més, s'ha inclòs la població de nanes vermelles i s'ha realitzat una anàlisi conjunta de les aportacions d'ambdues poblacions al contingut total de matèria fosca de la nostra galàxia. D'altra banda, s'ha estudiat la contribució de la subpoblació de nanes blanques amb atmosferes d'hidrogen deficient .D'altra banda, el nostre codi de Monte Carlo s'ha ampliat per analitzar els sistemes compostos per una nana blanca i un estel de la seqüència principal que han evolucionat a través d'una fase d'embolcall comú. S'ha implementat de forma detallada diversos processos físics, incloent-hi una descripció completa de l'episodi de transferència de massa, un tractament complet de l'episodi de desbordament del lòbul de Roche, marees gravitacionals i l'evolució orbital del sistema binari. Així mateix, hem inclòs detalladament tots els diferents criteris de selecció i biaixos observacionals.Els resultats de les nostres simulacions de Monte Carlo dels experiments de microlents mostren un bon acord amb els resultats dels equips EROS i MACHO. Les nostres troballes mostren que ni les nanes blanques ni les nanes vermelles poden ser els principals contribuents a la profunditat òptica dels microlents en la direcció de la LMC. Aquests fets reforcen la idea, prèviament assenyalada per altres estudis, que la profunditat òptica trobada per l'equip MACHO és molt probablement una sobreestimació, potser a causa de la contaminació de microlents de la pròpia LMC, entre d'altres possibles explicacions .Pel que fa al segon punt d'aquesta tesi, les nostres simulacions Monte Carlo reprodueixen correctament les propietats de la població de WD + MS PCEBs. El millor ajust s'obté per models amb fraccions de l'energia interna inferiors al ~20% , i valors per al paràmetre d'eficiència de l'embolcall comú inferiors a ~0,3.En conclusió , el treball presentat en aquesta tesi suposa un important pas endavant no només en la limitació de la naturalesa dels descobriments de microlents , sinó també en la validació dels models per a les poblacions de nanes blanques observades de la nostra Galàxia. Per no esmentar el fet que posa les bases per a moltes altres missions en curs i previstes , i com ajuda als observadors en les seves prediccions.

    Since white dwarfs are the final stage of the evolution of the vast majority of stars, they carry important information about the chemical evolution of our Galaxy, its star formation rate, and its structure and dynamics. This thesis pays attention to two related but distinct astrophysical problems involving white dwarfs. The first of these problems concern the nature and the location of the microlensing events towards the Large Magellanic Clouds (LMC), which still remains a mystery. The main observational groups, MACHO and EROS, are in dispute each, yet agreement has now been reached in some of the most important points. The second of the problems we address in this thesis is an open problem as well. Close compact binaries are at the heart of several interesting phenomena in our Galaxy as well. Close compact binaries are formed through at least one common envelope episode. Even though the basics concepts of the evolution during a common envelope phase are rather simple, the details are still far from being well understood. To shed light on these problems, we used an existing Monte Carlo simulator. We expanded this simulator integrating the most up-to-date white dwarfs cooling models as well as detailed modeling of our Galaxy and the LMC in order to mimic both the MACHO and EROS experiments. Additionally, we included the red dwarf population and performed a joint analysis of the contributions of both populations to the dark matter content of our Galaxy. Moreover, we studied the contribution of the subpopulation of white dwarfs with hydrogen-deficient atmospheres. On the other hand, our Monte Carlo code has been expanded to deal with those systems composed by a white dwarf and a main sequence star, which have evolved through a common episode. A detailed implementation of several different physical processes, including a full description of the mass transfer episode, a complete treatment of the Roche lobe overflow episode, gravitational tides and orbital evolution of the binary system, was performed. Furthermore, in our treatment we carefully included all the different selection criteria and observational biases. This allowed us to make a meaningful comparison with the available data, besides examining the role played by the binding energy parameter and by the common envelope parameter, not to mention the role played by the distribution of secondary masses of the binary systems. The results of our Monte Carlo simulations of the microlensing experiments show agreement with the findings of the EROS and MACHO survey. Our findings show that neither white dwarfs nor red dwarfs can be major contributors to the microlensing depth towards the LMC. These facts reinforce the idea, previously pointed by others studies, that the optical depth found by the MACHO survey is highly likely an overestimate, probably due to contamination of self-lensing objects, amid other possible explanations. Concerning the second point of this thesis, our Monte Carlo simulations correctly reproduce the properties of the observed population of post-common envelope white dwarf plus main sequence binaries, once biases are taken into account. The best-fit models are obtained with fractions less than ~20% of the internal energy contributing to the ejection of the star progenitor¿s envelope, and values for the common-envelope efficiency parameter less than ~0.3. To conclude, the work presented in this thesis poses an important step forward not only in constraining the microlensing discoveries, but also in validating models for the observed white-dwarf populations of our Galaxy.

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    A population synthesis study of the luminosity function of hot white dwarfs  Open access

     Torres Gil, Santiago; Garcia-berro Montilla, Enrique; Krzesinski, Jurek; Kleinman, Scot J.
    Astronomy and astrophysics
    Vol. 563, num. A&A, p. 1-6
    DOI: 10.1051/0004-6361/201323256
    Date of publication: 2014-03-05
    Journal article

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    Context. We present a coherent and detailed Monte Carlo simulation of the population of hot white dwarfs. We assess the statistical significance of the hot end of the white dwarf luminosity function and the role played by the bolometric corrections of hydrogen-rich white dwarfs at high effective temperatures. Aims. We use the most up-to-date stellar evolutionary models and implement a full description of the observational selection biases to obtain realistic simulations of the observed white dwarf population. Methods. Our theoretical results are compared with the luminosity function of hot white dwarfs obtained from the Sloan Digital Sky Survey (SDSS), for both DA and non-DA white dwarfs. Results. We find that the theoretical results are in excellent agreement with the observational data for the population of white dwarfs with hydrogen deficient atmospheres (non-DA white dwarfs). For the population of white dwarfs with hydrogen-rich atmospheres (white dwarfs of the DA class), our simulations show some discrepancies with the observations for the brightest luminosity bins, namely those corresponding to luminosities larger than ~10 L.These discrepancies can be attributed to the way in which the masses of the white dwarfs contributing to this luminosity bin have been computed, as most of them have masses smaller than the theoretical lower limit for carbon-oxygen white dwarfs. Conclusions. We conclude that the way in which the observational luminosity function of hot white dwarfs is obtained is very sensitive to the particular implementation of the method used to derive the masses of the sample. We also provide a revised luminosity function for hot white dwarfs with hydrogen-rich atmospheres. © ESO, 2014.

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    Monte Carlo simulations of post-common-envelope white dwarf + main sequence binaries: The effects of including recombination energy  Open access

     Zorotovic, Monica; Garcia-berro Montilla, Enrique; Schreiber, Mathias R.; Camacho Díaz, Judit; Torres Gil, Santiago; Rebassa Mansergas, Alberto; Gänsicke, Boris T.
    Astronomy and astrophysics
    Vol. 568, num. id. A68, p. 1-10
    DOI: 10.1051/0004-6361/201323039
    Date of publication: 2014-08-20
    Journal article

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    Context. Detached white dwarf + main sequence (WD+MS) post-common-envelope binaries (PCEBs) are perhaps the most suitable objects for testing predictions of close-compact binary-star evolution theories, in particular, common-envelope (CE) evolution. Consequently, the population of WD+MS PCEBs has been simulated by several authors in the past and the predictions have been compared with the observations. However, most of those theoretical predictions did not take into account the possible contributions to the envelope ejection from additional sources of energy (mostly recombination energy) stored in the envelope. Aims. Here we update existing binary population models of WD+MS PCEBs by assuming that in addition to a fraction aCE of the orbital energy, a fraction arec of the recombination energy available within the envelope contributes to ejecting the envelope. Methods. We performed Monte Carlo simulations of 107 MS+MS binaries for 9 different combinations of aCE and arec using standard assumptions for the initial primary mass function, binary separations, and initial-mass-ratio distribution and evolved these systems using the publicly available binary star evolution (BSE) code. Results. Including a fraction of the recombination energy leads to a clear prediction of a large number of long orbital period (¿10 days) systems mostly containing high-mass WDs. The fraction of systems with He-core WD primaries (MWD ¿ 0.5 M¿) increases with the CE efficiency and the existence of very low-mass He WDs (¿0.3 M¿) is only predicted for high values of the CE efficiency, i.e. aCE ¿ 0.5. All models predict on average longer orbital periods for PCEBs containing C/O-core WDs (MWD ¿ 0.5 M¿) than for PCEBs containing He WDs. This effect increases with increasing values of both efficiencies, i.e., aCE and arec. Longer periods after the CE phase are also predicted for systems containing more massive secondary stars. The initial-mass-ratio distribution affects the distribution of orbital periods, especially the distribution of secondary star masses. Conclusions. Our simulations, in combination with a large and homogeneous observational sample, can provide constraints on the values of aCE and arec, as well as on the initial-mass-ratio distribution for MS+MS binary stars.

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    The effects of metallicity on the Galactic disk population of white dwarfs  Open access

     Cojocaru, Elena Ruxandra; Torres Gil, Santiago; Isern Vilaboy, Jordi; Garcia-berro Montilla, Enrique
    Astronomy and astrophysics
    Vol. 566, num. id.A81
    DOI: 10.1051/0004-6361/201423396
    Date of publication: 2014-06
    Journal article

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    Context. It has been known for a long time that stellar metallicity plays a significant role in the determination of the ages of the different Galactic stellar populations, when main sequence evolutionary tracks are employed. Aims. Here, we analyze the role that metallicity plays on the white dwarf luminosity function of the Galactic disk, which is often used to determine its age. Methods. We employ a Monte Carlo population synthesis code that accounts for the properties of the population of Galactic disk white dwarfs. Our code incorporates the most up-to-date evolutionary cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres for both carbon-oxygen and oxygen-neon cores. We use two different models to assess the evolution of the metallicity: the first, where the adopted metallicity is constant with time but with a moderate dispersion, and the second where the metallicity increases with time. Results. We found that our theoretical results are in a very satisfactory agreement with the observational luminosity functions obtained from the Sloan Digital Sky Survey (SDSS) and from the SuperCOSMOS Sky Survey (SSS). These results are independent of the adopted age-metallicity law. In particular, we found that the age-metallicity law has no noticeable impact in shaping the bright branch of the white dwarf luminosity function and that the position of its cut-off is almost insensitive to the adopted age-metallicity relationship. Conclusions. Because the shape of the bright branch of the white dwarf luminosity function is insensitive to the age-metallicity law, it can be safely employed to test the theoretical evolutionary sequences, while its location provides a robust indicator of the age of the Galactic disk due to the limited sensitivity of the position of the drop-off to the distribution of metallicities.

    Context. It has been known for a long time that stellar metallicity plays a significant role in the determination of the ages of the different Galactic stellar populations, when main sequence evolutionary tracks are employed. Aims. Here, we analyze the role that metallicity plays on the white dwarf luminosity function of the Galactic disk, which is often used to determine its age. Methods. We employ a Monte Carlo population synthesis code that accounts for the properties of the population of Galactic disk white dwarfs. Our code incorporates the most up-to-date evolutionary cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres for both carbon-oxygen and oxygen-neon cores. We use two different models to assess the evolution of the metallicity: the first, where the adopted metallicity is constant with time but with a moderate dispersion, and the second where the metallicity increases with time. Results. We found that our theoretical results are in a very satisfactory agreement with the observational luminosity functions obtained from the Sloan Digital Sky Survey (SDSS) and from the SuperCOSMOS Sky Survey (SSS). These results are independent of the adopted age-metallicity law. In particular, we found that the age-metallicity law has no noticeable impact in shaping the bright branch of the white dwarf luminosity function and that the position of its cut-off is almost insensitive to the adopted age-metallicity relationship. Conclusions. Because the shape of the bright branch of the white dwarf luminosity function is insensitive to the age-metallicity law, it can be safely employed to test the theoretical evolutionary sequences, while its location provides a robust indicator of the age of the Galactic disk due to the limited sensitivity of the position of the drop-off to the distribution of metallicities

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    Monte Carlo simulations of post-common-envelope white dwarf plus main sequence binaries: comparison with the SDSS DR7 observed sample  Open access

     Camacho Díaz, Judit; Torres Gil, Santiago; Garcia-berro Montilla, Enrique; Zorotovic, Monica; Schreiber, Mathias R.; Rebassa-Mansergas, Alberto; Nebot Gómez-Moran, Ada; Gänsicke, Boris T.
    Astronomy and astrophysics
    Vol. 566, p. 14
    DOI: 10.1051/0004-6361/201323052
    Date of publication: 2014-06-01
    Journal article

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    Context. Detached white dwarf + main sequence (WD+MS) systems represent the simplest population of post-common envelope binaries (PCEBs). Since the ensemble properties of this population carries important information about the characteristics of the common-envelope (CE) phase, it deserves close scrutiny. However, most population synthesis studies do not fully consider the effects of the observational selection biases of the samples used to compare with the theoretical simulations.; Aims. Here we present the results of a set of detailed Monte Carlo simulations of the population of WD+MS binaries in the Sloan Digital Sky Survey (SDSS) Data Release 7.; Methods. We used up-to-date stellar evolutionary models, a complete treatment of the Roche lobe overflow episode, and a full implementation of the orbital evolution of the binary systems. Moreover, in our treatment we took the selection criteria and all the known observational biases into account.; Results. Our population synthesis study allowed us to make a meaningful comparison with the available observational data. In particular, we examined the CE efficiency, the possible contribution of internal energy, and the initial mass ratio distribution (IMRD) of the binary systems. We find that our simulations correctly reproduce the properties of the observed distribution of WD+MS PCEBs. In particular, we find that once the observational biases are carefully considered, the distribution of orbital periods and of masses of the WD and MS stars can be correctly reproduced for several choices of the free parameters and different IMRDs, although models in which a moderate fraction (<= 10%) of the internal energy is used to eject the CE and in which a low value of CE efficiency is used (<= 0.3) seem to fit the observational data better. We also find that systems with He-core WDs are over-represented in the observed sample, because of selection effects.; Conclusions. Although our study represents an important step forward in modeling the population of WD+MS PCEBs, the still scarce observational data preclude deriving a precise value of the several free parameters used to compute the CE phase without ambiguity or ascertaining which the correct IMRD might be.

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    Explaining the Type Ia supernova PTF 11kx with a violent prompt merger scenario  Open access

     Garcia-berro Montilla, Enrique; Torres Gil, Santiago; Camacho Diaz, Judit; Soker, Noam; Kashi, Amit
    Monthly notices of the Royal Astronomical Society
    Vol. 431, num. 2, p. 1541-1546
    DOI: 10.1093/mnras/stt271
    Date of publication: 2013-05
    Journal article

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    We argue that the multiple shells of circumstellar material (CSM) and the supernovae (SNe) ejecta interaction with the CSM starting 59 d after the explosion of the Type Ia SN PTF 11kx are best described by a violent prompt merger. In this prompt merger scenario, the common envelope (CE) phase is terminated by a merger of a white dwarf (WD) companion with the hot core of a massive asymptotic giant branch star. In most cases, the WD is disrupted and accreted on to the more massive core. However, in the rare cases, where the merger takes place when the WD is denser than the core, the core will be disrupted and accreted on to the cooler WD. In such cases, the explosion might occur with no appreciable delay, i.e. months to years after the termination of the CE phase. This, we propose, might be the evolutionary route that could lead to the explosion of PTF 11kx. This scenario can account for the very massive CSM within similar to 1000 au of the exploding PTF 11kx star, for the presence of hydrogen, and for the presence of shells in the CSM.

  • Enanas blancas: progenitores, evolución, pulsaciones y explosiones

     Diaz Aguiló, Marc; Renedo Rouco, Isabel; Sala Cladellas, Gloria; Escartin Vigo, Jose Antonio; Loren Aguilar, Pablo; Garcia Senz, Domingo; Torres Gil, Santiago; Camacho Díaz, Judit; Garcia-berro Montilla, Enrique
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  • Monte Carlo simulations of the luminosity function of hot white dwarfs

     Torres Gil, Santiago; Garcia-berro Montilla, Enrique; Krzesinski, Jurek; Kleinman, Scot J.
    European White Dwarf Workshop
    p. 109-112
    Presentation's date: 2012-08
    Presentation of work at congresses

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    We present a detailed Monte Carlo simulation of the population of the hot branch of the white dwarf luminosity function. We used the most up-to-date stellar evolutionary models and we implemented a full description of the observational selection biases. Our theoretical results are compared with the luminosity function of hot white dwarfs obtained from the Sloan Digital Sky Survey (SDSS), for both DA and non-DA white dwarfs. For non-DA white dwarfs we find an excellent agreement with the observational data, while for DA white dwarfs our simulations show some discrepancies with the observations for the brightest luminosity bins, those corresponding to L greater than or similar to 10 L-circle dot.

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    Double degenerate mergers as progenitors of high-field magnetic white dwarfs  Open access

     Garcia-berro Montilla, Enrique; Loren Aguilar, Pablo; Aznar Siguan, Gabriela; Torres Gil, Santiago; Camacho Díaz, Judit; Althaus, Leandro G.; Córsico, Alejando Hugo; Kulebi, Baybar; Isern Vilaboy, Jordi
    Astrophysical journal
    Vol. 749, num. 1, p. 1-6
    DOI: 10.1088/0004-637X/749/1/25
    Date of publication: 2011-04-10
    Journal article

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    High-field magnetic white dwarfs have been long suspected to be the result of stellar mergers. However, the nature of the coalescing stars and the precise mech anism that produces the magnetic field are still unknown. Here we show that the hot, convective, diffe rentially rotating corona present in the outer layers of the remnant of the merger of two degenerate cores is able to produce magnetic fields of the required strength that do not decay for long timescales. We also show, using an state-of-the-art Monte Carlo simulator, that the expected number of high-field magn etic white dwarfs produced in this way is consistent with that found in the Solar neighborhood.

  • The evolution of white dwarfs with a varying gravitational constant

     Althaus, Leandro G.; Corsico, Alejandro H.; Torres Gil, Santiago; Loren Aguilar, Pablo; Isern Vilaboy, Jordi; Garcia-berro Montilla, Enrique
    Astronomy and astrophysics
    Vol. 527, num. A72, p. 1-5
    DOI: 10.1051/0004-6361/201015849
    Date of publication: 2011-03
    Journal article

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  • The Cooling of White Dwarfs and a Varying Gravitational Constant

     Garcia-berro Montilla, Enrique; Althaus, Leandro G.; Torres Gil, Santiago; Loren Aguilar, Pablo; Corsico, Alejandro H.; Isern Vilaboy, Jordi
    Astrophysics and space science proceedings
    p. 47-57
    DOI: 10.1007/978-3-642-19397-2_6
    Date of publication: 2011
    Journal article

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  • An upper limit to the secular variation of the gravitational constant from white dwarf stars

     Garcia-berro Montilla, Enrique; Loren Aguilar, Pablo; Torres Gil, Santiago; Althaus, Leandro G.; Isern Vilaboy, Jordi
    Journal of cosmology and astroparticle physics
    Vol. 2011, num. 5, p. 1-15
    DOI: 10.1088/1475-7516/2011/05/021
    Date of publication: 2011-05
    Journal article

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    The white-dwarf cooling sequence of NGC 6791: a unique tool for stellar evolution  Open access

     Garcia-berro Montilla, Enrique; Torres Gil, Santiago; Renedo Rouco, Isabel; Camacho Díaz, Judit; Althaus, Leandro G.; Córsico, Alejandro H.; Salaris, Mauricio; Isern Vilaboy, Jordi
    Astronomy and astrophysics
    Vol. 533, num. A31, p. 1-8
    DOI: 10.1051/0004-6361/201116499
    Date of publication: 2011-09
    Journal article

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    Context. NGC 6791 is a well-studied, metal-rich open cluster that is so close to us that it can be imaged down to luminosities fainter than that of the termination of its white-dwarf cooling sequence, thus allowing for an in-depth study of its white dwarf population. Aims. White dwarfs carry important information about the history of the cluster. We use observations of the white-dwarf cooling sequence to constrain important properties of the cluster stellar population, such as the existence of a putative population of massive helium-core white dwarfs, and the properties of a large population of unresolved binary white dwarfs. We also investigate the use of white dwarfs to disclose the presence of cluster subpopulations with a different initial chemical composition, and we obtain an upper bound to the fraction of hydrogen-deficient white dwarfs. Methods. We use a Monte Carlo simulator that employs up-to-date evolutionary cooling sequences for white dwarfs with hydrogenrich and hydrogen-deficient atmospheres, with carbon-oxygen and helium cores. The cooling sequences for carbon-oxygen cores account for the delays introduced by both 22Ne sedimentation in the liquid phase and by carbon-oxygen phase separation upon crystallization. Results. We do not find evidence for a substantial fraction of helium-core white dwarfs, and hence our results support the suggestion that the origin of the bright peak of the white-dwarf luminosity function can only be attributed to a population of unresolved binary white dwarfs. Moreover, our results indicate that if this hypothesis is at the origin of the bright peak, the number distribution of secondary masses of the population of unresolved binaries has to increase with increasing mass ratio between the secondary and primary components of the progenitor system. We also find that the observed cooling sequence appears to be able to constrain the presence of progenitor subpopulations with different chemical compositions and the fraction of hydrogen-deficient white dwarfs. Conclusions. Our simulations place interesting constraints on important characteristics of the stellar populations of NGC 6791. In particular, we find that the fraction of single helium-core white dwarfs must be smaller than 5%, that a subpopulation of stars with zero metallicity must be <∼12%, while if the adopted metallicity of the subpopulation is solar the upper limit is ∼8%. Finally, we also find that the fraction of hydrogen-deficient white dwarfs in this particular cluster is surprinsingly small (<∼6%).

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    Population Synthesis of Post-Common Envelope White-Dwarf-Main-Sequence Binaries in the Sloan Digital Sky Survey  Open access

     Torres Gil, Santiago; Camacho Díaz, Judit; Rebassa-Mansergas, Alberto; Zorotovic, Monica; Schreiber, Mathias R.; Garcia-berro Montilla, Enrique
    Evolution of Compact Binaries
    p. 193-194
    Presentation's date: 2011-05-06
    Presentation of work at congresses

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  • Physics of compact objects: I.Theorical modeling and observations of stellar explosions

     Torres Gil, Santiago; Garcia Senz, Domingo; Bravo Guil, Eduardo; Garcia-berro Montilla, Enrique; Renedo Rouco, Isabel; Camacho Díaz, Judit; Sala Cladellas, Gloria; Loren Aguilar, Pablo; Casanova Bustamante, Jordi; Longland, Richard Leigh; Parikh, Anuj Ramesh; José Pont, Jordi
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  • Study of binary systems after a common envelope episode

     Camacho Díaz, Judit; Torres Gil, Santiago; Zorotovic, Monica; Schreiber, Mathias R.
    Competitive project

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  • Physics of Compact Objects: explosive nucleosynthesis and evolution

     José Pont, Jordi; Camacho Díaz, Judit; Torres Gil, Santiago; Renedo Rouco, Isabel; Garcia Senz, Domingo; Sala Cladellas, Gloria; Casanova Bustamante, Jordi; Garcia-berro Montilla, Enrique
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    The effects of the Observational Selection Criteria on the post common envelope white dwarf-main sequence binary population  Open access

     Camacho Díaz, Judit; Torres Gil, Santiago; Garcia-berro Montilla, Enrique; Zorotovic, Mónica; Schreiber, Mathias R.; Rebassa-Mansergas, Alberto
    17th European Workshop on White Dwarfs
    p. 390-393
    DOI: 10.1063/1.3527848
    Presentation's date: 2010
    Presentation of work at congresses

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    We present a detailed Monte Carlo simulator of the Galactic population of binary stars. Preliminary results are presented for the white dwarf-main sequence (WD+MS) binary population resulting from a common envelope (CE) episode. We also study the effects of the observational selection criteria on different color-color diagrams, for different binding energy parameters, l , and common envelope efficiencies, aCE. Finally, we also compare our results with the identified population of white dwarf-main sequence binaries in the SDSS

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    The white dwarf population of tri-axial haloes  Open access

     Torres Gil, Santiago; Camacho Díaz, Judit; Garcia-berro Montilla, Enrique
    DOI: 10.1063/1.3527846
    Date of publication: 2010
    Book chapter

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    The nature of the several microlensing events observed by the MACHO team towards the Large Magellanic Cloud (LMC) is still a controversial subject. Substellar objects and stars with masses larger than ∼ 1M⊙ have been ruled out as major components of a Massive Astrophysical Halo Object (MACHO) Galactic halo. Stars of near half-solar mass, in particular white dwarfs, appear to be the best candidates to explain the observed microlensing events. On the other hand, observational evidence based on the structure of the debris of the Sagittarius dwarf galaxy suggest that the Galactic halo is non-spherical. We use a tri-axial halo model and advanced Monte Carlo techniques to study the contribution of the halo white dwarf population to the dark matter content of the Galaxy.

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  • Monte Carlo simulations of the post-common envelope white dwarf-main sequence binary population

     Camacho Díaz, Judit; Torres Gil, Santiago; Garcia-berro Montilla, Enrique; Zorotovic, Mónica; Schreiber, Mathias R.
    DOI: 10.1063/1.3536374
    Date of publication: 2010
    Book chapter

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  • The effects of the observational selection criteria on the post-common envelope white dwarf-main sequence binary population

     Camacho Díaz, Judit; Torres Gil, Santiago; Garcia-berro Montilla, Enrique; Zorotovic, Monica; Schreiber, Mathias R.; Rebassa-Mansergas Mansergas, Alberto
    DOI: 10.1063/1.3527848
    Date of publication: 2010
    Book chapter

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    White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy  Open access

     Torres Gil, Santiago; Camacho Díaz, Judit; Isern Vilaboy, Jordi; Garcia-berro Montilla, Enrique
    Astronomy and astrophysics
    Vol. 511, p. 1-11
    DOI: 10.1051/0004-6361/200912905
    Date of publication: 2010-02
    Journal article

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  • Monte Carlo simulations of the post-common-envelope white-dwarf main-sequence binary population

     Camacho Díaz, Judit; Torres Gil, Santiago; Garcia-berro Montilla, Enrique; Zorotovic, Monica; Schreiber, Mathias R.
    AIP Conference proceedings
    Vol. 1314, p. 22-23
    DOI: 10.1063/1.3536374
    Date of publication: 2010
    Journal article

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  • A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes

     Garcia-berro Montilla, Enrique; Torres Gil, Santiago; Althaus, Leandro Gabriel; Renedo Rouco, Isabel; Loren Aguilar, Pablo; Corsico, Alejandro H.; Rohrmann, Rene Daniel; Salaris, Mauricio; Isern Vilaboy, Jordi
    Nature
    Vol. 465, num. 7295, p. 194-196
    DOI: 10.1038/nature09045
    Date of publication: 2010-05-13
    Journal article

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  • The white dwarf cooling age of NGC 6791

     Garcia-berro Montilla, Enrique; Althaus, Leandro G.; Torres Gil, Santiago; Loren Aguilar, Pablo; Renedo Rouco, Isabel; Corsico, Alejandro H.; Isern Vilaboy, Jordi; Salaris, Maurizio; Rohrmann, Rene Daniel
    European White Dwarf Workshop
    p. 129-134
    Presentation's date: 2010-08
    Presentation of work at congresses

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    The white dwarf population of tri-axial haloes  Open access

     Torres Gil, Santiago; Camacho Díaz, Judit; Garcia-berro Montilla, Enrique
    European White Dwarf Workshop
    p. 37-40
    DOI: 10.1063/1.3527846
    Presentation's date: 2010
    Presentation of work at congresses

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    The nature of the several microlensing events observed by the MACHO team towards the Large Magellanic Cloud (LMC) is still a controversial subject. Substellar objects and stars with masses larger than ~ 1M¿ have been ruled out as major components of a Massive Astrophysical Halo Object (MACHO) Galactic halo. Stars of near half-solar mass, in particular white dwarfs, appear to be the best candidates to explain the observed microlensing events. On the other hand, observational evidence based on the structure of the debris of the Sagittarius dwarf galaxy suggest that the Galactic halo is non-spherical. We use a tri-axial halo model and advanced Monte Carlo techniques to study the contribution of the halo white dwarf population to the dark matter content of the Galaxy

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    Monte Carlo simulations of the post-common envelope white dwarf-main sequence binary population  Open access

     Camacho Díaz, Judit; Torres Gil, Santiago; Garcia-berro Montilla, Enrique; Zorotovic, Monica; Schreiber, Mathias R.
    International Conference on Binaries: in celebration of Ron Webbink's 65th birthday
    p. 22-23
    DOI: 10.1063/1.3536374
    Presentation's date: 2010-06
    Presentation of work at congresses

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  • Two challenges for modern stellar evolution: supernova progenitors and AGB stars

     Camacho Díaz, Judit; Garcia-berro Montilla, Enrique; Torres Gil, Santiago
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  • DOS RETOS PARA LA EVOLUCION ESTELAR MODERNA:PROGENITORES DE SUPERNOVA Y ESTRELLAS AGB

     Torres Gil, Santiago; Garcia Senz, Domingo; Renedo Rouco, Isabel; Cabezón Gómez, Rubén Martín; González Villafranca, Alberto; Camacho Díaz, Judit; Escartin Vigo, Jose Antonio; Sala Cladellas, Gloria; Loren Aguilar, Pablo; Casanova Bustamante, Jordi; Garcia-berro Montilla, Enrique
    Competitive project

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  • Astronomy and Astrophysics, Consolidated Research Group

     Camacho Díaz, Judit; Garcia-berro Montilla, Enrique; Torres Gil, Santiago
    Competitive project

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  • GRUP D'ASTRONOMIA I ASTROFÍSICA

     Garcia Senz, Domingo; Riera Mora, M. Angeles; José Pont, Jordi; Diaz Aguiló, Marc; Bravo Guil, Eduardo; Cabezón Gómez, Rubén Martín; González Villafranca, Alberto; Renedo Rouco, Isabel; Camacho Díaz, Judit; Escartin Vigo, Jose Antonio; Sala Cladellas, Gloria; Loren Aguilar, Pablo; Torres Gil, Santiago; Casanova Bustamante, Jordi; Garcia-berro Montilla, Enrique
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    Monte Carlo simulations of the binary white dwarf population: a progress report  Open access

     Camacho Díaz, Judit; Torres Gil, Santiago; Garcia-berro Montilla, Enrique
    Journal of physics: conference series
    Vol. 172, num. 1, p. 1-5
    DOI: 10.1088/1742-6596/172/1/012030
    Date of publication: 2009
    Journal article

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    We present a detailed Monte Carlo simulator of the population of binary stars within the solar neighborhood. We have used the most updated models for stellar evolution, a complete treatment of the Roche lobe overflow episodes, as well as a full implementation of the orbital evolution. Special emphasis has been placed on processes leading to the formation of binary systems in which one of the members is a white dwarf.

  • Axions and the white dwarf luminosity function

     Isern Vilaboy, Jorge; Catalán Ruiz, Silvia; Garcia-berro Montilla, Enrique; Torres Gil, Santiago
    Journal of physics: conference series
    Vol. 172, num. 1
    DOI: 10.1088/1742-6596/172/1/012005
    Date of publication: 2009-06-01
    Journal article

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  • On the origin of white dwarfs with carbon-dominated atmospheres: the case of H1504+65

     Althaus, Lg; Corsico, Ah; Torres Gil, Santiago; Garcia-berro Montilla, Enrique
    Astronomy and astrophysics
    Vol. 494, num. 3, p. 1021-1024
    Date of publication: 2009-02
    Journal article

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  • White dwarfs, red dwarfs and halo dark matter

     Garcia-berro Montilla, Enrique; Torres Gil, Santiago; Camacho Díaz, Judit; Isern Vilaboy, Jorge
    Journal of physics: conference series
    Vol. 172, num. 1, p. 12003
    DOI: 10.1088/1742-6596/172/1/012003
    Date of publication: 2009
    Journal article

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    The nature of the microlensing events observed by the MACHO team towards the LMC still remains controversial. Low-mass substellar objects and stars with masses larger than ~ 1M¿ have been ruled out, while stars of ~ 0.5 M¿ are the most probable candidates. This means that the microlenses should be either red or white dwarfs. Consequently, we assess jointly the relative contributions of both types of stars to the mass budget of the Galactic halo. We use a Monte Carlo code that incorporates up-to-date evolutionary sequences of both red dwarfs and white dwarfs as well as detailed descriptions of both our Galaxy and the LMC and we compare the synthetic populations obtained with our simulator with the results obtained by the MACHO and EROS experiments. We find that the contribution of the red dwarf population is not enough to explain the number of events measured by the MACHO team. Even though, the optical depth obtained in our simulations almost doubles that obtained when taking into account the white dwarf population alone. Finally, we also find that the contribution to the halo dark matter of the entire population under study is smaller than 10%, at the 95% confidence level.

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    The contribution of red dwarfs and white dwarfs to the halo dark matter  Open access

     Torres Gil, Santiago; Camacho Díaz, Judit; Isern Vilaboy, Jordi; Garcia-berro Montilla, Enrique
    Astronomy and astrophysics
    Vol. 486, p. 427-435
    DOI: 10.1051/0004-6361:200809805
    Date of publication: 2008
    Journal article

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  • Axions and the cooling of white dwarfs stars

     Garcia-berro Montilla, Enrique; Torres Gil, Santiago
    Astrophysical journal
    Vol. 682, p. L109-L112
    Date of publication: 2008-01
    Journal article

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    Monte Carlo simulations of the binary white dwarf population: a progress report  Open access

     Camacho Díaz, Judit; Torres Gil, Santiago; Garcia-berro Montilla, Enrique
    16th European White Dwarfs Workshop
    p. 12030-
    DOI: 10.1088/1742-6596/172/1/012030
    Presentation's date: 2008-06
    Presentation of work at congresses

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    We present a detailed Monte Carlo simulator of the population of binary stars within the solar neighborhood. We have used the most updated models for stellar evolution, a complete treatment of the Roche lobe overflow episodes, as well as a full implementation of the orbital evolution. Special emphasis has been placed on processes leading to the formation of binary systems in which one of the members is a white dwarf.

  • White dwarfs, red dwarfs and halo dark matter

     Garcia-berro Montilla, Enrique; Torres Gil, Santiago; Camacho Díaz, Judit; Isern Vilaboy, Jorge
    16th European White Dwarfs Workshop
    Presentation's date: 2008
    Presentation of work at congresses

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  • Análisis de la aplicación de nuevas metodologias docentes en ingenieria

     Bruna Escuer, Pere; Calvete Manrique, Daniel; Crespo Artiaga, Daniel; Pilar, Gil; Gutierrez, Jordi; Torres Gil, Santiago; Alonso Maleta, Maria Aranzazu
    VI Taller Iberoamericano de Educación en Ciencia e Ingeniería de Materiales (TIECIM)
    Presentation's date: 2008-12-02
    Presentation of work at congresses

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    Monte Carlo simulations of the formation channels of SNeIb. PoS(SUPERNOVA)008  Open access

     Camacho Díaz, Judit; Torres Gil, Santiago; Garcia-berro Montilla, Enrique
    Date of publication: 2007
    Book chapter

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    We present a Monte Carlo simulator of the population of binary stars within the solar neighborhood. We have used the most updated models for stellar evolution, a complete treatment of the Roche lobe over ow episodes, as well as a full implementation of the orbital evolution. Special emphasis has been placed on processes leading to the formation of binary systems in which one of the members is a white dwarf. As a preliminary application of our Monte Carlo simulator we have performed a statistical study of the scenarios leading to type Ib supernovae events within our Galaxy

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    The contribution of Oxygen-Neon white dwarfs to the MACHO content of the Galactic halo  Open access

     Camacho Díaz, Judit; Torres Gil, Santiago; Villaboy, Isern J; Althaus, L; Garcia-berro Montilla, Enrique
    Astronomy and astrophysics
    Vol. 471, num. 1, p. 151-158
    DOI: 10.1051/0004-6361:20077714
    Date of publication: 2007-08
    Journal article

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    Context. The interpretation of microlensing results towards the Large Magellanic Cloud (LMC) still remains controversial. White dwarfs have been proposed to explain these results and, hence, to contribute significantly to the mass budget of our Galaxy. However, several constraints on the role played by regular carbon-oxygen white dwarfs exist. Aims. Massive white dwarfs are thought to be made of a mixture of oxygen and neon. Correspondingly, their cooling rate is larger than those of typical carbon-oxygen white dwarfs and they fade to invisibility in short timescales. Consequently, they constitute a good candidate for explaining the microlensing results. Methods. Here, we examine in detail this hypothesis by using the most recent and up-to-date cooling tracks for massive white dwarfs and a Monte Carlo simulator which takes into account the most relevant Galactic inputs. Results.We find that oxygen-neon white dwarfs cannot account for a substantial fraction of the microlensing depth towards the LMC,independently of the adopted initial mass function, although some microlensing events could be due to oxygen-neon white dwarfs. Conclusions.The white dwarf population contributes at most a 5% to the mass of the Galactic halo.

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    The white dwarf luminosity function - II. The effect of the measurement errors and other biases  Open access

     Torres Gil, Santiago; Garcia-berro Montilla, Enrique; Isern Vilaboy, Jordi
    Monthly notices of the Royal Astronomical Society
    Vol. 378, num. 4, p. 1461-1470
    Date of publication: 2007-07
    Journal article

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    Context. The interpretation of microlensing results towards the Large Magellanic Cloud (LMC) still remains controversial. White dwarfs have been proposed to explain these results and, hence, to contribute significantly to the mass budget of our Galaxy. However, several constraints on the role played by regular carbon-oxygen white dwarfs exist. Aims. Massive white dwarfs are thought to be made of a mixture of oxygen and neon. Correspondingly, their cooling rate is larger than those of typical carbon-oxygen white dwarfs and they fade to invisibility in short timescales. Consequently, they constitute a good candidate for explaining the microlensing results. Methods. Here, we examine in detail this hypothesis by using the most recent and up-to-date cooling tracks for massive white dwarfs and a Monte Carlo simulator which takes into account the most relevant Galactic inputs. Results.We find that oxygen-neon white dwarfs cannot account for a substantial fraction of the microlensing depth towards the LMC,independently of the adopted initial mass function, although some microlensing events could be due to oxygen-neon white dwarfs. Conclusions.The white dwarf population contributes at most a 5% to the mass of the Galactic halo.

  • Monte Carlo simulations of the Galactic binary population. Monte Carlo simulations of the formation channels of SNeIb

     Camacho Díaz, Judit; Torres Gil, Santiago; Garcia-berro Montilla, Enrique
    Supernovae: lights in the darkness (XXIII trobades Científiques de la Mediterrània)
    Presentation's date: 2007-08
    Presentation of work at congresses

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  • The Late stages of stellar evolution : stellar explosions and non-interacting white dwarfs in the UV

     Hernanz Carbó, Margarita; Dominguez, I.; Garcia-berro Montilla, Enrique; Garcia Senz, Domingo; Barstow, M.A.; Bravo Guil, Eduardo; Gonzalez-Riestra, R.; Isern, J.; José Pont, Jordi; Torres Gil, Santiago
    Date of publication: 2006-12
    Book chapter

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    The white dwarf luminosity function . I. Statistical errors and alternatives  Open access

     Geijo, E M; Torres Gil, Santiago; Vilaboy, Isern I J; Garcia-berro Montilla, Enrique
    Monthly notices of the Royal Astronomical Society
    Vol. 369, num. 3, p. 1654-1666
    Date of publication: 2006-07
    Journal article

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    The white dwarf luminosity function is an important tool for the study of the solar neighbourhood, since it allows the determination of the age of the Galactic disc. Over the years, several methods have been proposed to compute galaxy luminosity functions, from the most simple ones – counting sample objects inside a given volume – to very sophisticated ones – like the C- method, the STY method or the Choloniewski method, among others. However, only the 1/Vmax method is usually employed in computing the white dwarf luminosity function and other methods have not been applied so far to the observational sample of spectroscopically identified white dwarfs – in sharp contrast with the situation when galaxy luminosity functions are derived from a large variety of samples. Moreover, the statistical significance of the white dwarf luminosity function has also received little attention and a thorough study still remains to be done. In this paper, we study, using a controlled synthetic sample of white dwarfs generated using a Monte Carlo simulator, which is the statistical significance of the white dwarf luminosity function and which are the expected biases. We also present a comparison between different estimators for computing the white dwarf luminosity function. We find that for sample sizes large enough the 1/Vmax method provides a reliable characterization of the white dwarf luminosity function, provided that the input sample is selected carefully. Particularly, the 1/Vmax method recovers well the position of the cut-off of the white dwarf luminosity function. However, this method turns out to be less robust than the Choloniewski method when the possible incompletenesses of the sample are taken into account. We also find that the Choloniewski method performs better than the method in estimating the overall density of white dwarfs, but misses the exact location of the cut-off of the white dwarf luminosity function.

  • The white dwarf luminosity function: measurement errors and estimators

     Torres Gil, Santiago; Garcia-berro Montilla, Enrique
    15th European Workshop on White Dwarfs
    p. 73-76
    Presentation of work at congresses

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  • Astronomy and Astrophysics, Consolidated Research Group

     Torres Gil, Santiago; Garcia-berro Montilla, Enrique
    Competitive project

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  • Las poblaciones fósiles de la galaxia: progenitores, propiedades y aplicaciones.

     Torres Gil, Santiago; Garcia-berro Montilla, Enrique; Bravo Guil, Eduardo
    Competitive project

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  • Ayudas de investigación a grupos consolidados de la Generalitat de Catalunya

     Torres Gil, Santiago; Garcia-berro Montilla, Enrique
    Competitive project

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