2,445 research outputs found
The mass ratio and the orbital parameters of the sdOB binary AA Doradus
The time sequence of 105 spectra covering one full orbital period of AA Dor
has been analyzed. Direct determination of Vsini for the sdOB component from 97
spectra outside of the eclipse for the lines MgII 4481 A and SiIV 4089 A
clearly indicated a substantially smaller value than estimated before. Detailed
modelling of line profile variations for 8 spectra during the eclipse for the
MgII 4481 A line, combined with the out-of-eclipse fits, gave Vsini =
31.8+/-1.8 km/s. The previous determinations of Vsini, based on the HeII 4686 A
line, appear to be invalid because of the large natural broadening of the line.
With the assumption of the solid-body, synchronous rotation of the sdOB
primary, the measured values of the semi-amplitude K1 and Vsini lead to the
mass ratio q = 0.213+/-0.013 which in turn gives K2 and thus the masses and
radii of both components. The sdOB component appears to be less massive than
assumed before, M1 = 0.25+/-0.05 Msol, but the secondary has its mass-radius
parameters close to theoretically predicted for a brown dwarf, M2 =
0.054+/-0.010 Msol and R2 = 0.089+/-0.005 Rsol. Our results do not agree with
the recent determination of Vuckovic et al. 2008 based on a K2 estimate from
line-profile asymmetries.Comment: Accepted by MNRA
Hyperbolic formulations of General Relativity with Hamiltonian structure
With the aim of deriving symmetric hyperbolic free-evolution systems for GR
that possess Hamiltonian structure and allow for the popular puncture gauge
condition we analyze the hyperbolicity of Hamiltonian systems. We develop
helpful tools which are applicable to either the first order in time, second
order in space or the fully second order form of the equations of motion. For
toy models we find that the Hamiltonian structure can simplify the proof of
symmetric hyperbolicity. In GR we use a special structure of the principal part
to prove symmetric hyperbolicity of a formulation that includes gauge
conditions which are very similar to the puncture gauge.Comment: Our mathematica scripts are available at
http://na.mathematik.uni-tuebingen.de/~richter
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