665 research outputs found
Grids of white dwarf evolutionary models with masses from M= 0.1 to 1.2 Ms
We present detailed evolutionary calculations for carbon
- oxygen - and helium - core white dwarf (WD) models with masses ranging from
M= 0.1 to M= 1.2 solar masses and for metallicities Z= 0.001 and Z= 0. The
sequences cover a wide range of hydrogen envelopes as well. We employed a
detailed WD evolutionary code. In particular, the energy transport by
convectcion is treated within the formalism of the full spectrum turbulence
theory. The set of models presented here is very detailed and should be
valuable for the interpretation of the observational data on low - mass WDs
recently discovered in numerous binary configurations and also for the general
problem of determining the theoretical luminosity function for WDs. In this
context, we compare our cooling sequences with the observed WD luminosity
function recently improved by Leggett, Ruiz and Bergeron (1998) and we obtain
an age for the Galactic disc of approximately 8 Gyr. Finally, we applied the
results of this paper to derive stellar masses of a sample of low - mass white
dwarfs.Comment: 9 pages, 8 figures; accepted for publication in MNRAS; replaced with
minor corrections to tex
Self-bound models of compact stars and recent mass-radius measurements
The exact composition of a specific class of compact stars, historically
referred to as "neutron stars", is still quite unknown. Possibilities ranging
from hadronic to quark degrees of freedom, including self-bound versions of the
latter have been proposed. We specifically address the suitability of strange
star models (including pairing interactions) in this work, in the light of new
measurements available for four compact stars. The analysis shows that these
data might be explained by such an exotic equation of state, actually selecting
a small window in parameter space, but still new precise measurements and also
further theoretical developments are needed to settle the subject.Comment: To appear in PR
The potential of the variable DA white dwarf G117-B15A as a tool for Fundamental Physics
White dwarfs are well studied objects. The relative simplicity of their
physics allows to obtain very detailed models which can be ultimately compared
with their observed properties. Among white dwarfs there is a specific class of
stars, known as ZZ-Ceti objects, which have a hydrogen-rich envelope and show
periodic variations in their light curves. G117-B15A belongs to this particular
set of stars. The luminosity variations have been successfully explained as due
to g-mode pulsations. G117-B15A has been recently claimed to be the most stable
optical clock ever found, being the rate of change of its 215.2 s period very
small: \dot{P}= (2.3 +- 1.4)x10^{-15} s s^-1, with a stability comparable to
that of the most stable millisecond pulsars. The rate of change of the period
is closely related to its cooling timescale, which can be accurately computed.
In this paper we study the pulsational properties of G117-B15A and we use the
observed rate of change of the period to impose constraints on the axion
emissivity and, thus, to obtain a preliminary upper bound to the mass of the
axion. This upper bound turns out to be 4cos^{2}{\beta} meV at the 95%
confidence level. Although there are still several observational and
theoretical uncertainties, we conclude that G117-B15A is a very promising
stellar object to set up constraints on particle physics.Comment: 32 pages, 14 figures, accepted for publication in New Astronom
Preliminary results on tropospheric ZTD estimation by smartphone
The Global Navigation Satellite System (GNSS) receiver is one of the many sensors embedded in smartphones. The early versions of the Android operating system could only access limited information from the GNSS, allowing the related Application Program Interface (API) to obtain only the location. With the development of the Android 7.0 (Nougat) operating system in May 2016, raw measurements from the internal GNSS sensor installed in the smartphone could be accessed. This work aims to show an initial analysis regarding the feasibility of Zenith Total Delay (ZTD) estimation by GNSS measurements extracted from smartphones, evaluating the accuracy of estimation to open a new window on troposphere local monitoring. Two different test sites have been considered, and two different types of software for data processing have been used. ZTDs have been estimated from both a dual-frequency and a multi-constellation receiver embedded in the smartphone, and from a GNSS Continuously Operating Reference Station (CORS). The results have shown interesting performances in terms of ZTD estimation from the smartphone in respect of the estimations obtained with a geodetic receiver
Rydberg stabilization of atoms in strong fields: the "magic mountain" in the chaotic sea
We discuss the classical problem of an hydrogen atom interacting with a monochromatic field. We illustrate in particular, analytically and numerically, the stabilization mechanism and give theoretical expressions for the stabilization borders
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