5,309 research outputs found
Using seismic inversions to obtain an internal mixing processes indicator for main-sequence solar-like stars
Determining accurate and precise stellar ages is a major problem in
astrophysics. These determinations are either obtained through empirical
relations or model-dependent approaches. Currently, seismic modelling is one of
the best ways of providing accurate ages. However, current methods are affected
by simplifying assumptions concerning mixing processes. In this context,
providing new structural indicators which are less model-dependent and more
sensitive to such processes is crucial. We build a new indicator for core
conditions on the main sequence, which should be more sensitive to structural
differences and applicable to older stars than the indicator t presented in a
previous paper. We also wish to analyse the importance of the number and type
of modes for the inversion, as well as the impact of various constraints and
levels of accuracy in the forward modelling process that is used to obtain
reference models for the inversion. First, we present a method to obtain new
structural kernels and use them to build an indicator of central conditions in
stars and test it for various effects including atomic diffusion, various
initial helium abundances and metallicities, following the seismic inversion
method presented in our previous paper. We then study its accuracy for 7
different pulsation spectra including those of 16CygA and 16CygB and analyse
its dependence on the reference model by using different constraints and levels
of accuracy for its selection We observe that the inversion of the new
indicator using the SOLA method provides a good diagnostic for additional
mixing processes in central regions of stars. Its sensitivity allows us to test
for diffusive processes and chemical composition mismatch. We also observe that
octupole modes can improve the accuracy of the results, as well as modes of low
radial order.Comment: Accepted for publication in Astronomy and Astrophysic
Constraints on the structure of 16 Cyg A and 16 Cyg B using inversion techniques
Constraining mixing processes and chemical composition is a central problem
in stellar physics as their impact on stellar age determinations leads to
biases in our studies of stellar evolution, galactic history and exoplanetary
systems. In two previous papers, we showed how seismic inversion techniques
could offer strong constraints on such processes by pointing out weaknesses in
theoretical models. We now apply our technique to the solar analogues 16CygA
and 16CygB, being amongst the best targets in the Kepler field to test the
diagnostic potential of seismic inversions. The combination of various seismic
indicators helps to provide more constrained and accurate fundamendal
parameters for these stars. We use the latest seismic, spectroscopic and
interferometric observational constraints in the litterature for this system to
determine reference models independently for both stars. We carry out seismic
inversions of the acoustic radius, the mean density and a core conditions
indicator. We note that a degeneracy exists for the reference models. Namely,
changing the diffusion coefficient or the chemical composition within the
observational values leads to 5% changes in mass, 3% changes in radius and up
to 8% changes in age. We use acoustic radius and mean density inversions to
improve our reference models then carry out inversions for a core conditions
indicator. Thanks to its sensitivity to microscopic diffusion and chemical
composition mismatches, we are able to reduce the mass dispersion to 2%, namely
[0.96, 1.0] M_sun, the radius dispersion to 1%, namely [1.188, 1.200] R_sun and
the age dispersion to 3%, namely [7.0, 7.4] Gy, for 16CygA. For 16CygB, we can
check the consistency of the models but not reduce independently the age
dispersion. Nonetheless, assuming consistency with the age of 16CygA helps to
further constrain its mass and radius.Comment: Submitted to Astronomy and Astrophysic
Recent measures of the latitude and longitude of jupiter's red spot
Latitude and longitude of Jupiter red spot measured from photographic plate
Vibration analysis of a circular disc backed by a cylindrical cavity
This paper describes the free vibration analysis of a thin disc vibrating and interacting with an acoustic medium contained in a cylindrical duct. The effects of structural-acoustic coupling are studied by means of an analytical-numerical method that is based upon classical theory and the Galerkin method. The coupling effects are discussed, and results obtained from the analysis are compared with corresponding values obtained both experimentally and from a finite element analysis. There is good agreement between the three sets of results
Latitude and longitude measurements of Jovian features in 1967-68
Photographic measurements of latitude and longitude of Jovian feature
Star formation history in the solar neighborhood: the link between stars and cosmology
Using a cosmological galactic evolutionary approach to model the Milky Way,
we calculate the star formation history (SFH) of the solar neighborhood. The
good agreement we obtain with the observational inferences suggests that our
physical model describes accurately the long term/large spatial trends of the
local and global Milky Way SFH. In this model, star formation is triggered by
disk gravitational instabilities and self-regulated by an energy balance in the
ISM. The drivers of the SFH are the cosmological gas infall rate and the gas
surface density determined by the primordial spin parameter. A LambdaCDM
cosmology was used throughout.Comment: 8 pages, uses kluwer.cls. Invited talk, to appear in "New Quests in
Stellar Astrophysics: The link between Stars and Cosmology", eds. M. Chavez,
A. Bressan, A. Buzzoni & D. Mayya, Kluwer Academic Publisher
Spectral analysis of Swift long GRBs with known redshift
We study the spectral and energetics properties of 47 long-duration gamma-ray
bursts (GRBs) with known redshift, all of them detected by the Swift satellite.
Due to the narrow energy range (15-150 keV) of the Swift-BAT detector, the
spectral fitting is reliable only for fitting models with 2 or 3 parameters. As
high uncertainty and correlation among the errors is expected, a careful
analysis of the errors is necessary. We fit both the power law (PL, 2
parameters) and cut--off power law (CPL, 3 parameters) models to the
time-integrated spectra of the 47 bursts, and present the corresponding
parameters, their uncertainties, and the correlations among the uncertainties.
The CPL model is reliable only for 29 bursts for which we estimate the nuf_nu
peak energy Epk. For these GRBs, we calculate the energy fluence and the rest-
frame isotropic-equivalent radiated energy, Eiso, as well as the propagated
uncertainties and correlations among them. We explore the distribution of our
homogeneous sample of GRBs on the rest-frame diagram E'pk vs Eiso. We confirm a
significant correlation between these two quantities (the "Amati" relation) and
we verify that, within the uncertainty limits, no outliers are present. We also
fit the spectra to a Band model with the high energy power law index frozen to
-2.3, obtaining a rather good agreement with the "Amati" relation of non-Swift
GRBs.Comment: 16 pages. To appear in MNRAS. Minor changes were introduced in this
last versio
Tethers in space handbook
The handbook provides a list and description of ongoing tether programs. This includes the joint U.S.-Italy demonstration project, and individual U.S. and Italian studies and demonstration programs. An overview of the current activity level and areas of emphasis in this emerging field is provided. The fundamental physical principles behind the proposed tether applications are addressed. Four basic concepts of gravity gradient, rotation, momentum exchange, and electrodynamics are discussed. Information extracted from literature, which supplements and enhances the tether applications is also presented. A bibliography is appended
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