201 research outputs found
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.Comment: 15 pages, 3 postscript figures. Accepted for publication in ApJ
Letters, uses aasms4.st
White dwarf cooling sequences and cosmochronology
The evolution of white dwarfs is a simple gravothermal process. This means
that their luminosity function, i.e. the number of white dwarfs per unit
bolometric magnitude and unit volume as a function of bolometric magnitude, is
a monotonically increasing function that decreases abruptly as a consequence of
the finite age of the Galaxy. The precision and the accuracy of the white dwarf
luminosity functions obtained with the recent large surveys together with the
improved quality of the theoretical models of evolution of white dwarfs allow
to feed the hope that in a near future it will be possible to reconstruct the
history of the different Galactic populations.Comment: Proceedings of the 40th Liege International Astrophysical Colloquium:
Aging low mass stars: from red giants to white dwarf
The explosion of supernova 2011fe in the frame of the core-degenerate scenario
We argue that the properties of the Type Ia supernova (SN Ia) SN 2011fe can
be best explained within the frame of the core-degenerate (CD) scenario. In the
CD scenario a white dwarf (WD) merges with the core of an asymptotic giant
branch (AGB) star and forms a rapidly rotating WD, with a mass close to and
above the critical mass for explosion. Rapid rotation prevents immediate
collapse and/or explosion. Spinning down over a time of 0-10 Gyr brings the WD
to explosion. A very long delayed explosion to post-crystallization phase,
which lasts for about 2 Gyr leads to the formation of a highly carbon-enriched
outer layer. This can account for the carbon-rich composition of the
fastest-moving ejecta of SN 2011fe. In reaching the conclusion that the CD
scenario best explains the observed properties of SN 2011fe we consider both
its specific properties, like a very compact exploding object and carbon rich
composition of the fastest-moving ejecta, and the general properties of SNe Ia.Comment: Accepted by MNRAS Letter
Advanced production systems: new challenges, new opportunities
The future lies in a balanced model where the industry and services reinforce each othe
The ages of very cool hydrogen-rich white dwarfs
The evolution of white dwarfs is essentially a cooling process that depends
primarily on the energy stored in their degenerate cores and on the
transparency of their envelopes. In this paper we compute accurate cooling
sequences for carbon-oxygen white dwarfs with hydrogen dominated atmospheres
for the full range of masses of interest. For this purpose we use the most
accurate available physical inputs for both the equation of state and opacities
of the envelope and for the thermodynamic quantities of the degenerate core. We
also investigate the role of the latent heat in the computed cooling sequences.
We present separately cooling sequences in which the effects of phase
separation of the carbon-oxygen binary mixture upon crystallization have been
neglected, and the delay introduced in the cooling times when this mechanism is
properly taken into account, in order to compare our results with other
published cooling sequences which do not include a treatment of this
phenomenon. We find that the cooling ages of very cool white dwarfs with pure
hydrogen atmospheres have been systematically underestimated by roughly 1.5 Gyr
at log(L/Lo)=-4.5 for an otherwise typical 0.6 Mo white dwarf, when phase
separation is neglected. If phase separation of the binary mixture is included
then the cooling ages are further increased by roughly 10%. Cooling tracks and
cooling isochrones in several color-magnitude diagrams are presented as well.Comment: 8 Pages; ApJ, accepted for publicatio
Monte Carlo simulations of the halo white dwarf population
The interpretation of microlensing results towards the Large Magellanic Cloud
(LMC) still remains controversial. Whereas white dwarfs have been proposed to
explain these results and, hence, to contribute significantly to the mass
budget of our Galaxy, there are as well several constraints on the role played
by white dwarfs. In this paper we analyze self-consistently and simultaneously
four different results, namely, the local halo white dwarf luminosity function,
the microlensing results reported by the MACHO team towards the LMC, the
results of Hubble Deep Field (HDF) and the results of the EROS experiment, for
several initial mass functions and halo ages. We find that the proposed
log-normal initial mass functions do not contribute to solve the problem posed
by the observed microlensing events and, moreover, they overproduce white
dwarfs when compared to the results of the HDF and of the EROS survey. We also
find that the contribution of hydrogen-rich white dwarfs to the dynamical mass
of the halo of the Galaxy cannot be more than .Comment: 17 pages, 10 figures; accepted for publication in Astronomy and
Astrophysic
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