arXiv subjects

Jordi Isern

Publications and source records attributed to Jordi Isern.

At least 37 records · Page 2Linked to original sources

arXiv2013

White dwarf constraints on a varying $G$

A secular variation of $G$ modifies the structure and evolutionary time scales of white dwarfs. Using an state-of-the-art stellar evolutionary code, an up-to-date pulsational code, and a detailed population synthesis code we demonstrate that the effects of a running $G$ are obvious both in the properties of individual white dwarfs, and in those of the white dwarf populations in clusters. Specifically, we show that the white dwarf evolutionary sequences depend on both the value of $\dot G/G$, and on the value of $G$ when the white dwarf was born. We show as well that the pulsational properties of variable white dwarfs can be used to constrain $\dot G/G$. Finally, we also show that the ensemble properties of of white dwarfs in clusters can also be used to set upper bounds to $\dot G/G$. Precisely, the tightest bound --- $\dot G/G \sim -1.8 10^{-12}$ yr$^{-1}$ --- is obtained studying the population of the old, metal-rich, well populated, open cluster NGC 6791. Less stringent upper limits can be obtained comparing the theoretical results obtained taking into account the effects of a running $G$ with the measured rates of change of the periods of two well studied pulsating white dwarfs, G117--B15A and R548. Using these white dwarfs we obtain $\dot G/G\sim -1.8\times 10^{-10}$ yr$^{-1}$, and $\dot G/G\sim -1.3\times 10^{-10}$ yr$^{-1}$, respectively, which although less restrictive than the previous bound, can be improved measuring the rate of change of the period of massive white dwarfs.

astro-ph.SR↗

arXiv2013

White Dwarfs constrain Dark Forces

The white dwarf luminosity function, which provides information about their cooling, has been measured with high precision in the past few years. Simulations that include well known Standard Model physics give a good fit to the data. This leaves little room for new physics and makes these astrophysical objects a good laboratory for testing models beyond the Standard Model. It has already been suggested that white dwarfs might provide some evidence for the existence of axions. In this work we study the constraints that the white dwarf luminosity function puts on physics beyond the Standard Model involving new light particles (fermions or bosons) that can be pair-produced in a white dwarf and then escape to contribute to its cooling. We show, in particular, that we can severely constrain the parameter space of models with dark forces and light hidden sectors (lighter than a few tens of keV). The bounds we find are often more competitive than those from current lab searches and those expected from most future searches.

hep-ph↗

arXiv2013

Detonations in white dwarf dynamical interactions

In old, dense stellar systems collisions of white dwarfs are a rather frequent phenomenon. Here we present the results of a comprehensive set of Smoothed Particle Hydrodynamics simulations of close encounters of white dwarfs aimed to explore the outcome of the interaction and the nature of the final remnants for different initial conditions. Depending on the initial conditions and the white dwarf masses, three different outcomes are possible. Specifically, the outcome of the interaction can be either a direct or a lateral collision or the interaction can result in the formation of an eccentric binary system. In those cases in which a collision occurs, the infalling material is compressed and heated such that the physical conditions for a detonation may be reached during the most violent phases of the merger. While we find that detonations occur in a significant number of our simulations, in some of them the temperature increase in the shocked region rapidly lifts degeneracy, leading to the quenching of the burning. We thus characterize under which circumstances a detonation is likely to occur as a result of the impact of the disrupted star on the surface of the more massive white dwarf. Finally, we also study which interactions result in bound systems, and in which ones the more massive white dwarf is also disrupted as a consequence of the dynamical interaction. The sizable number of simulations performed in this work allows to find how the outcome of the interaction depends on the distance at closest approach, and on the masses of the colliding white dwarfs, and which is the chemical pattern of the nuclearly processed material. Finally, we also discuss the influence of the masses and core chemical compositions of the interacting white dwarfs and the different kinds of impact in the properties of the remnants.

astro-ph.SR↗

arXiv2013

Magnetic white dwarfs with debris discs

It has long been accepted that a possible mechanism for explaining the existence of magnetic white dwarfs is the merger of a binary white dwarf system, as there are viable mechanisms for producing sustainable magnetic fields within the merger product. However, the lack of rapid rotators in the magnetic white dwarf population has been always considered a problematic issue of this scenario. Smoothed Particle Hydrodynamics simulations show that in mergers in which the two white dwarfs have different masses a disc around the central compact object is formed. If the central object is magnetized it can interact with the disc through its magnetosphere. The torque applied by the disc changes the spin of the star, whereas the transferred angular momentum from the star to the disc determines the properties of the disc. In this work we build a model for the disc evolution under the effect of magnetic accretion, and for the angular momentum evolution of the star, which can be compared with the observations. Our model predicts that the magnetospheric interaction of magnetic white dwarfs with their discs results in a significant spin down, and we show that for magnetic white dwarfs with relatively strong fields (larger than 10 MG) the observed rotation periods of the magnetic white dwarf population can be reproduced. We also investigate whether turbulence can be sustained during the late phases of the evolution of the system. When a critical temperature below which turbulence is not sustained is introduced into the model, the periods of the three fast rotating, strongly magnetic, massive white dwarfs in the solar neighborhood are recovered.

astro-ph.SR↗

arXiv2012

An independent limit on the axion mass from the variable white dwarf star R548

Pulsating white dwarfs with hydrogen-rich atmospheres, also known as DAV stars, can be used as astrophysical laboratories to constrain the properties of fundamental particles like axions. Comparing the measured cooling rates of these stars with the expected values from theoretical models allows us to search for sources of additional cooling due to the emission of weakly interacting particles. In this paper, we present an independent inference of the mass of the axion using the recent determination of the evolutionary cooling rate of R548, the DAV class prototype. We employ a state-of-the-art code which allows us to perform a detailed asteroseismological fit based on fully evolutionary sequences. Stellar cooling is the solely responsible of the rates of change of period with time ($\dotΠ$) for the DAV class. Thus, the inclusion of axion emission in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DAV stars. This allows us to compare the theoretical $\dotΠ$ values to the corresponding empirical rate of change of period with time of R548 to discern the presence of axion cooling. We found that if the dominant period at 213.13 s in R548 is associated with a pulsation mode trapped in the hydrogen envelope, our models indicate the existence of additional cooling in this pulsating white dwarf, consistent with axions of mass $m_{\rm a} \cos^2 β\sim 17.1$ meV at a 2$σ$ confidence level. This determination is in agreement with the value inferred from another well-studied DAV, G117$-$B15A. We now have two independent and consistent estimates of the mass of the axion obtained from DAVs, although additional studies of other pulsating white dwarfs are needed to confirm this value of the axion mass.

astro-ph.SR↗

arXiv2012

Magnetic white dwarfs with debris disks

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2011

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

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 carbon-oxygen binary mixture. 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. 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), which presents an azeotrope, and the phase diagram of Segretain & Chabrier (1993), which is of the spindle form. We computed the evolution of 0.593 and 0.878M_sun 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. (2010) is used. This translates into time delays that are on average a factor about 2 smaller than those obtained when the phase diagram of Segretain & Chabrier (1993) is employed. 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.

astro-ph.SR↗

arXiv2011

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

A variation of the gravitational constant over cosmological ages modifies the main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art stellar evolutionary code we compute the effects of a secularly varying G on the main sequence ages and, employing white dwarf cooling ages computed taking into account the effects of a running G, we place constraints on the rate of variation of Newton's constant. This is done using the white dwarf luminosity function and the distance of the well studied open Galactic cluster NGC 6791. We derive an upper bound G'/G ~ -1.8 10^{-12} 1/yr. This upper limit for the secular variation of the gravitational constant compares favorably with those obtained using other stellar evolutionary properties, and can be easily improved if deep images of the cluster allow to obtain an improved white dwarf luminosity function.

gr-qc↗

arXiv2010

Evolutionary and pulsational properties of white dwarf stars

Abridged. White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. The study of white dwarfs has potential applications to different fields of astrophysics. In particular, they 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. They 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. In this work, we review the essentials of the physics of white dwarf stars. 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.

astro-ph.SR↗

arXiv2010

Evolution of white dwarf stars with high-metallicity progenitors: the role of 22Ne diffusion

Motivated by the strong discrepancy between the main sequence turn-off age and the white dwarf cooling age in the metal-rich open cluster NGC 6791, we compute a grid of white dwarf evolutionary sequences that incorporates for the first time the energy released by the processes of 22Ne sedimentation and of carbon/oxygen phase separation upon crystallization. The grid covers the mass range from 0.52 to 1.0 Msun, and it is appropriate for the study of white dwarfs in metal-rich clusters. The evolutionary calculations are based on a detailed and self-consistent treatment of the energy released from these two processes, as well as on the employment of realistic carbon/oxygen profiles, of relevance for an accurate evaluation of the energy released by carbon/oxygen phase separation. We find that 22Ne sedimentation strongly delays the cooling rate of white dwarfs stemming from progenitors with high metallicities at moderate luminosities, whilst carbon/oxygen phase separation adds considerable delays at low luminosities. Cooling times are sensitive to possible uncertainties in the actual value of the diffusion coefficient of 22Ne. Changing the diffusion coefficient by a factor of 2, leads to maximum age differences of approx. 8-20% depending on the stellar mass. We find that the magnitude of the delays resulting from chemical changes in the core is consistent with the slow down in the white dwarf cooling rate that is required to solve the age discrepancy in NGC 6791.

astro-ph.SR↗

arXiv2010

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

NGC 6791 is a well studied open cluster1 that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (~8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (~6 Gyr) are significantly different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallise and phase separation of the main constituents of the core of a typical white dwarf, 12C and 16O, is expected to occur. This sequence of events is expected to introduce significant delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, solving the age discrepancy for NGC 6791.

astro-ph.SR↗

arXiv2002

High proper motion white dwarfs and halo dark matter

The interpretation of the old, cool white dwarfs recently found by Oppenheimer et al. (2001) is still controversial. Whereas these authors claim that they have finally found the elusive ancient halo white dwarf population that contributes significantly to the mass budget of the galactic halo, there have been several other contributions that argue that these white dwarfs are not genuine halo members but, instead, thick disk stars. We show here that the interpretation of this sample is based on the adopted distances, which are obtained from a color--magnitude calibration, and we demonstrate that when the correct distances are used a sizeable fraction of these putative halo white dwarfs belong indeed to the disk population. We also perform a maximum likelihood analysis of the remaining set of white dwarfs and we find that they most likely belong to the thick disk population. However, another possible explanation is that this sample of white dwarfs has been drawn from a 1:1 mixture of the halo and disk white dwarf populations.

astro-ph↗

arXiv2000

The Chemical Composition of Carbon Stars II: The J-type stars

Abundances of Li, heavy elements and carbon isotope ratios have been measured in 12 J-type galactic carbon stars. The abundance analysis shows that in these stars the abundances of s-process elements with respect to the metallicity are nearly normal. Tc is not present in most of them, although upper limits have been derived for WZ Cas and WX Cyg, perhaps two SC-type rather than J-type carbon stars. The Rb abundances, obtained from the resonance 7800 Å Rb I line, are surprisingly low, probably due to strong non-LTE effects in the formation of this line in cool carbon-rich stars. Lithium and $^{13}$C are found to be enhanced in all the stars. These results together with the nitrogen abundances and oxygen isotope ratios measured by Lambert et al. (1986) and Harris et al. (1987) are used to discuss the origin of J-stars. The luminosity and variability class of the stars studied would indicate that they are low mass (M$\leq 2-3$ M$_\odot$), less evolved objects than the normal carbon stars, although the presence of some luminous (M$_{bol}<-5.5$) J-stars in our galaxy (WZ Cas may be an example) and in other galaxies, suggests the existence of at least two types of J-stars, with different formation scenarios depending upon the initial mass of the parent star. Standard evolutionary AGB models are difficult to reconcile with all the observed chemical characteristics. In fact, they suggest the existence of an extra-mixing mechanism which transports material from the convective envelope down to hotter regions where some nuclear burning occurs. This mechanism would act preferably on the early-AGB phase in low-mass stars. Mixing at the He-core flash and the binary system hypothesis are also discussed as alternatives to the above scenario.

astro-ph↗

arXiv1999

The chemical composition of the rare J-type carbon stars

Abundances of lithium, heavy elements and carbon isotope ratios have been derived in 12 J-type galactic carbon stars. The abundance analysis shows that in these stars the abundances of s-process elements with respect to the metallicity are nearly solar. Tc is not present in most of them. The Rb abundances, obtained from the resonance 7800 Å RbI line, are surprisingly low, probably due to stroong non-LTE effects. Lithium and $^{13}$C are found to be enhanced in all the stars. These results are used to discuss the origin of J-stars.

astro-ph↗

arXiv1999

Asymptotic Giant Brach Stars as Astroparticle Laboratories

We show that the inclusion of axion emission during stellar evolution introduces important changes into the evolutionary behaviour of AGB stars. The mass of the resulting C/O white dwarf is much lower than the equivalent obtained from standard evolution. This implies a deficit in luminous AGB stars and in massive WDs. Moreover the total mass processed in the nuclear burning shells that is dredged-up to the surface ($3^{rd} D_{up}$) increases when axion emission is included, modifying the chemical composition of the photosphere. We conclude that the AGB is a promising phase to put constraints on particle physics

astro-ph↗

arXiv1998

Neural Network identification of halo white dwarfs

The white dwarf luminosity function has proven to be an excellent tool to study some properties of the galactic disk such as its age and the past history of the local star formation rate. The existence of an observational luminosity function for halo white dwarfs could provide valuable information about its age, the time that the star formation rate lasted, and could also constrain the shape of the allowed Initial Mass Functions (IMF). However, the main problem is the scarce number of white dwarfs already identified as halo stars. In this Letter we show how an artificial intelligence algorithm can be succesfully used to classify the population of spectroscopically identified white dwarfs allowing us to identify several potential halo white dwarfs and to improve the significance of its luminosity function.

astro-ph↗

arXiv1998

Production of the LiBeB isotopes with time-dependent GCR fluxes

It is well known that no time-independent galactic cosmic ray (GCR) flux model can account for the observed linear relationship of Be and B abundances with metallicity. In this contribution we solve the time-dependent CR propagation equation using simple analytic time dependences for the CR spectrum at the source, the galactic CR escape-length and gas density evolution. The CR fluxes so obtained are used to study the early evolution of the LiBeB abundances in the Galaxy.

astro-ph↗