755 research outputs found
Moment Approach for Determining the Orbital Elements of an Astrometric Binary with Low Signal-to-noise Ratio
A moment approach for orbit determinations of an astrometric binary with low
signal-to-noise ratio from astrometric observations alone is proposed,
especially aiming at a close binary system with a short orbital period such as
Cyg-X1 and also at a star wobbled by planets. As an exact solution to the
nonlinearly coupled equation system, the orbital elements are written in terms
of the second and third moments of projected positions that are measured by
astrometry. This may give a possible estimation of the true orbit.Comment: 18 pages, 5 figures, 1 table; accepted by PAS
Numerical Simulations of Equatorially-Asymmetric Magnetized Supernovae: Formation of Magnetars and Their Kicks
A series of numerical simulations on magnetorotational core-collapse
supernovae are carried out. Dipole-like configurations which are offset
northward are assumed for the initially strong magnetic fields together with
rapid differential rotations. Aims of our study are to investigate effects of
the offset magnetic field on magnetar kicks and on supernova dynamics. Note
that we study a regime where the proto-neutron star formed after collapse has a
large magnetic field strength approaching that of a ``magnetar'', a highly
magnetized slowly rotating neutron star. As a result, equatorially-asymmetric
explosions occur with a formation of the bipolar jets. Resultant magnetar's
kick velocities are km s. We find that the acceleration
is mainly due to the magnetic pressure while the somewhat weaker magnetic
tension works toward the opposite direction, which is due to stronger magnetic
field in the northern hemisphere. Noted that observations of magnetar's proper
motions are very scarce, our results supply a prediction for future
observations. Namely, magnetars possibly have large kick velocities, several
hundred km s, as ordinary neutron stars do, and in an extreme case they
could have those up to 1000 km s.Comment: 36 pages, 9 figures, accepted by the Astrophysical Journa
The criterion of supernova explosion revisited: the mass accretion history
By performing neutrino-radiation hydrodynamic simulations in spherical
symmetry (1D) and axial symmetry (2D) with different progenitor models by
Woosley & Heger (2007) from 12 to 100 , we find that all
1D runs fail to produce an explosion and several 2D runs succeed. The
difference in the shock evolutions for different progenitors can be interpreted
by the difference in their mass accretion histories, which are in turn
determined by the density structures of progenitors. The mass accretion history
has two phases in the majority of the models: the earlier phase in which the
mass accretion rate is high and rapidly decreasing and the later phase with a
low and almost constant accretion rate. They are separated by the so-called
turning point, the origin of which is a change of the accreting layer. We argue
that shock revival will most likely occur around the turning point and hence
that its location in the - plane will be a good measure for the
possibility of shock revival: if the turning point lies above the critical
curve and the system stays there for a long time, shock revival will obtain. In
addition, we develop a phenomenological model to approximately evaluate the
trajectories in the - plane, which, after calibrating free
parameters by a small number of 1D simulations, reproduces the location of the
turning point reasonably well by using the initial density structure of
progenitor alone. We suggest the application of the phenomenological model to a
large collection of progenitors in order to infer without simulations which
ones are more likely to explode.Comment: 17 pages, 24 figures, 2 tables; accepted for publication in Ap
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