The Cooling of CO White Dwarfs: Influence of the Internal Chemical Distribution

et al. 2010

Astron Astrophys Rev

Self Cite

380

372

White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. Since the coolest white dwarfs are very old objects, the present population of white dwarfs contains a wealth of information on the evolution of stars from birth to death, and on the star formation rate throughout the history of our Galaxy. Thus, the study of white dwarfs has potential applications to different fields of astrophysics. In particular, white dwarfs can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, like our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow to use these stars as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. Last but not least, since many white dwarf stars undergo pulsational instabilities, the study of their properties constitutes a powerful tool for applications beyond stellar astrophysics. In particular, white dwarfs can be used to constrain fundamental properties of elementary particles such as axions and neutrinos, and to study problems related to the variation of fundamental constants.These potential applications of white dwarfs have led to a renewed interest in the calculation of very detailed evolutionary and pulsational models for these stars. In this work, we review the essentials of the physics of white dwarf stars. We enumerate the reasons that make these stars excellent chronometers and we describe why white dwarfs provide tools for a wide variety of applications. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational instability strips. The existence of these instability strips provides astronomers with an unique opportunity to peer into their internal structure that would otherwise remain hidden from observers. We will show that this allows to measure with unprecedented precision the stellar masses and to infer their envelope thicknesses, to probe the core chemical stratification, and to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white dwarfs and the most recent applications of white dwarf asteroseismology.

Section: Da White Dwarf Models and Asteroseismological Studies Of Davsmentioning

confidence: 99%

Evolutionary and pulsational properties of white dwarf stars

Althaus

Córsico

et al. 2010

Astron Astrophys Rev

Self Cite

380

372

“…These improved cooling sequences are based on the Salaris et al (1997) but including an accurate treatment of the crystallization process of the carbon-oxygen core, considering phase separation upon crystallization, together with up-to-date input physics suitable for computing white dwarf evolution. The chemical profiles of Salaris et al (2000) consider carbon-oxygen white dwarfs in the mass range from 0.5 to 1.0 M ⊙ and take into account the presence of large abundances of oxygen in the central regions due to the high rates of the 12 C(α, γ) 16 O reaction.…”

Section: A2 Carbon-oxygen White Dwarfsmentioning

confidence: 99%

“…The high oxygen abundances in the central regions of the white dwarf minimize the effects of chemical differentiation upon crystallization as compared with the models in which a homogeneous 50/50 carbon and oxygen distributions throughout the star are adopted. For the sake of comparison, Salaris et al (1997) also considered a lower rate for 12 C(α, γ) 16 O reaction, which gives a smaller oxygen content at the center of the white dwarf. This leads to a larger heat capacity and therefore to a slower cooling rate.…”

Section: A2 Carbon-oxygen White Dwarfsmentioning

confidence: 99%

Testing the Initial–Final Mass Relationship of White Dwarfs

Catalán

Astrophysics and Space Science Proceedings

Garcı́a–Berro

et al. 2010

113

184

A mis padres y hermana AgraïmentsEn primer lloc vull agrair als meus directors de tesi, Jordi Isern i Enrique García-Berro, que em donessin l'oportunitat de realitzar el doctorat a l'IEEC (al CSIC i a la UPC), així com que em proporcionessin tot el suport necessari. Crec que he après moltes coses importants en aquestsúltims anys, no només en la part científica, i això en partés una conseqüència d'haver treballat amb ells. Els vull donar les gràcies també pel suport i la confiança dipositada en mi, especialment durant aquestúltim any de feina.Durant tota aquestaèpoca he tingut la sort de tenir uns companys fantàstics que m'han fet molt més fàcil la feina. Primer vaig conèixer l'Alina, la meva companya de "fatigas" i després el Jose, el Pep i el Miquel. Més tard s'hi van afegir l'Ignasi, la Violeta, l'Anna C., l'Anna A., el Carlo, el Moi, el Nil, l'Anaïs. . . Tots els moments que hem compartit dins i fora de l'IEEC han estat molt importants per a mi i m'emporto un gran record de tota aquestaèpoca, primer al Nexus, després a la UAB i finalment a l'ETSE. Ara que tinc la oportunitat, els hi vull agrair als meus companys, o més ben dit, amics, tots elsànims i suport que m'han donat. També els hi vull donar les gràcies al personal d'administració i a la resta de companys que he tingut durant aquests anys.En especial, l'Ignasi es mereix un gran gràcies. . . Primer de tot, per venir a l'IEEC en un moment en que la meva feina necessitava d'un observador amb experiència. Per tenir la generositat de compartir els seus coneixements amb mi durant aquests anys, per acompanyar-me en les observacions, per tenir sempre tanta confiança en mi, molta més que jo mateixa i per ajudar-me sempre que se li presentés la oportunitat. Els seusànims i el seu suport han estat essencials per a que arribés aquest moment i per tant, només em queda donar-li el meu agraïment més sincer.También le debo un agradecimiento especial a Carlos Allende Prieto, que sin ni siquiera conocerme en persona aceptó acompañarme a observar al McDonald Observatory durante 10 días. Su ayuda y dedicación en la distancia, especialmente en el análisis de las compañeras de las enanas blancas, han sido también vitales para el desarrollo de esta tesis.Durante mi doctorado he tenido la oportunidad de disfrutar de estancias en otros centros. Le quiero dar las gracias a Inma Domínguez, Carlos Abia y Olga Zamora de la Universidad de Granada por la atención que me dispensaron cuando estuve allí. También quisiera dar las gracias a Roberto Silvotti y Juan Alcalá por la ayuda y el buen trato que me dieron durante el tiempo que pasé en el Osservatorio Astronomico di Capodimonte. Gracias también al resto de personas que trabajan en el Osservatorio y que hicieron mi estancia muy agradable.A mi profesor Manolo Segura le quiero agredecer que en su día me hiciera creer en mis posibilidades y despertara mi afán por intentar superarme poco a poco. Muchas gracias también por la "contingencia" y por la confianza dipositada en mí.En un momento para mí tan importante comoéste no me puedo olvi...

“…As the oxygen-rich solid core grows at the center of the white dwarf, the lighter carbon-rich liquid mantle left behind is efficiently redistributed by Rayleigh-Taylor instabilities (Isern et al 1997). This process releases gravitational energy, and this additional energy source has a substantial impact in the computed cooling times of cool white dwarfs (Segretain et al 1994;Salaris et al 1997;Montgomery et al 1999;Salaris et al 2000;Isern et al 2000;Renedo et al 2010).…”

Section: Introductionmentioning

confidence: 99%

New phase diagrams for dense carbon-oxygen mixtures and white dwarf evolution

Althaus

Garcı́a–Berro

et al. 2011

A&A

Context. Cool white dwarfs are reliable and independent stellar chronometers. The most common white dwarfs have carbon-oxygen dense cores. Consequently, the cooling ages of very cool white dwarfs sensitively depend on the adopted phase diagram of the carbonoxygen binary mixture. Aims. A new phase diagram of dense carbon-oxygen mixtures appropriate for white dwarf interiors has been recently obtained using direct molecular dynamics simulations. In this paper, we explore the consequences of this phase diagram in the evolution of cool white dwarfs. Methods. To do this we employ a detailed stellar evolutionary code and accurate initial white dwarf configurations, derived from the full evolution of progenitor stars. We use two different phase diagrams, that of Horowitz et al. (2010, Phys. Rev. Lett., 104, 231101), which presents an azeotrope, and the phase diagram of Segretain & Chabrier (1993, A&A, 271, L13), which is of the spindle form. Results. We computed the evolution of 0.593 and 0.878 M white dwarf models during the crystallization phase, and we found that the energy released by carbon-oxygen phase separation is smaller when the new phase diagram of Horowitz et al. is used. This translates into time delays that are on average a factor ∼2 smaller than those obtained when the phase diagram of Segretain & Chabrier is employed. Conclusions. Our results have important implications for white dwarf cosmochronology, because the cooling ages of very old white dwarfs are different for the two phase diagrams. This may have a noticeable impact on the age determinations of very old globular clusters, for which the white dwarf color-magnitude diagram provides an independent way of estimating their age.