2,037 research outputs found
Binary sdB Stars with Massive Compact Companions
Original paper can be found at: http://astrosociety.org/pubs/cs/381.html Copyright ASPThe masses of compact objects like white dwarfs, neutron stars and black holes are fundamental to astrophysics, but very difficult to measure. We present the results of an analysis of subluminous B (sdB) stars in close binary systems with unseen compact companions to derive their masses and clarify their nature. Radial velocity curves were obtained from time resolved spectroscopy. The atmospheric parameters were determined in a quantitative spectral analysis. Based on high resolution spectra we were able to measure the projected rotational velocity of the stars with high accuracy. In the distribution of projected rotational velocities signs of tidal locking with the companions are visible. By detecting ellipsoidal variations in the lightcurve of an sdB binary we were able to show that subdwarf binaries with orbital periods up to 0.6 d are most likely synchronized. In this case, the inclination angles and companion masses of the binaries can be tightly constrained. Five invisible companions have masses that are compatible with that of normal white dwarfs or late type main sequence stars. However, four sdBs have compact companions massive enough to be heavy white dwarfs (> 1M⊙), neutron stars or even black holes. Such a high fraction of massive compact companions is not expected from current models of binary evolution
Internal rotation of subdwarf B stars: limiting cases and asteroseismological consequences
Observations of the rotation rates of horizontal branch (HB) stars show
puzzling systematics. In particular, cooler HB stars often show rapid rotation
(with velocities in excess of 10 km/s), while hotter HB stars typically show
much smaller rotation velocities. Simple models of angular momentum evolution
of stars from the main sequence through the red giant branch fail to explain
these effects. In general, evolutionary models in all cases preserve a rapidly
rotating core. The observed angular velocities of HB stars require that some of
the angular momentum stored in the core reaches the surface.
To test the idea that HB stars contain such a core, one can appeal to
detailed computations of trace element abundences and rotational mixing.
However, a more direct probe is available to test these limiting cases of
angular momentum evolution. Some of the hottest horizontal branch stars are
members of the pulsating sdB class. They frequently show rich pulsation spectra
characteristic of nonradially pulsating stars. Thus their pulsations probe the
internal rotation of these stars, and should show the effects of rapid rotation
in their cores. Using models of sdB stars that include angular momentum
evolution, we explore this possibility and show that some of the sdB pulsators
may indeed have rapidly rotating cores.Comment: accepted for publication in The Astrophysical Journa
Interpretation of increased energetic particle flux measurements by SEPT aboard the STEREO spacecraft and contamination
Context. Interplanetary (IP) shocks are known to be accelerators of energetic
charged particles observed in-situ in the heliosphere. However, the
acceleration of near-relativistic electrons by shocks in the interplanetary
medium is often questioned. On 9 August 2011 a Corotating Interaction Region
(CIR) passed STEREO B (STB) that resulted in a flux increase in the electron
and ion channels of the Solar Electron and Proton Telescope (SEPT). Because
electron measurements in the few keV to several 100 keV range rely on the
so-called magnet foil technique, which is utilized by SEPT, ions can contribute
to the electron channels. Aims. We aim to investigate whether the flux increase
in the electron channels of SEPT during the CIR event on 9 August 2011 is
caused by ion contamination only. Methods. We compute the SEPT response
functions for protons and helium utilizing an updated GEANT4 model of SEPT. The
CIR energetic particle ion spectra for protons and helium are assumed to follow
a Band function in energy per nucleon with a constant helium to proton ratio.
Results. Our analysis leads to a helium to proton ratio of 16.9% and a proton
flux following a Band function with the parameters /
(cm2 s sr MeV/nuc.), keV/nuc. and spectral indices of and which are in good agreement with measurements by
the Suprathermal Ion Telescope (SIT) aboard STB. Conclusions. Since our results
explain the SEPT measurements, we conclude that no significant amount of
electrons were accelerated between keV and keV by the CIR
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