14,334 research outputs found
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
On the White Dwarf distances to Galactic Globular Clusters
We analyze in detail various possible sources of systematic errors on the
distances of globular clusters derived by fitting a local template DA white
dwarf sequence to the cluster counterpart (the so-called WD-fitting technique).
We find that the unknown thickness of the hydrogen layer of white dwarfs in
clusters plays a non negligible role. For reasonable assumptions - supported by
the few sparse available observational constraints - about the unknown mass and
thickness of the hydrogen layer for the cluster white dwarfs, a realistic
estimate of the systematic error on the distance is within +-0.10 mag. However,
particular combinations of white dwarf masses and envelope thicknesses - which
at present cannot be excluded a priori - could produce larger errors.
Contamination of the cluster DA sequence by non-DA white dwarfs introduces a
very small systematic error of about -0.03 mag in the Mv/(V-I) plane, but in
the Mv/(B-V) plane the systematic error amounts to ~ +0.20 mag. Contamination
by white dwarfs with helium cores should not influence appreciably the
WD-fitting distances. Finally, we obtain a derivative D((m-M)v)/D(E(B-V))~ -5.5
for the WD-fitting distances, which is very similar to the dependence found
when using the Main Sequence fitting technique.Comment: 12 pages, 11 figures A&A, accepted for publicatio
Axions and White Dwarfs
White dwarfs are almost completely degenerate objects that cannot obtain
energy from the thermonuclear sources and their evolution is just a
gravothermal process of cooling. The simplicity of these objects, the fact that
the physical inputs necessary to understand them are well identified, although
not always well understood, and the impressive observational background about
white dwarfs make them the most well studied Galactic population. These
characteristics allow to use them as laboratories to test new ideas of physics.
In this contribution we discuss the robustness of the method and its
application to the axion case.Comment: 4 pages, 1 figure, to appear in the Proceedings for the 6th Patras
meeting on Axions, WIMPs and WISP
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