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Star Mass (star + mass)

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Physics and Astronomy100%

Selected Abstracts

Equilibrium states of magnetized toroid,central compact object systems

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2009
Jun Otani
ABSTRACT Equilibrium configurations of self-gravitating magnetized toroid,central compact object systems have been constructed in the framework of the Newtonian gravity. We have succeeded in including not only poloidal but also toroidal magnetic fields under the ideal magnetohydrodynamic approximation. We find two new and interesting results about the critical equilibrium states of such systems beyond which no equilibrium states are allowed to exist. First, there appear critical distances from the central compact objects to the inner surfaces of the magnetized toroids. Furthermore, these critical distances are much larger than the distances of the innermost stable circular orbits. It implies that even if these systems would be treated in the framework of general relativity, there would appear cusp structures of the effective total potential of the gravitational and magnetic forces for strongly magnetized toroids which are different from the general relativistic cusp structures. Secondly, since the strength of the magnetic field for the critical equilibrium configurations is roughly 1015 G if the mass of the central object is 1.4 M, and the maximum density of the toroid is 1011 g cm,3, the existence of equilibrium states of toroids around compact objects seems to set limit to the maximum value of the magnetic field of the system to be ,1015 G, i.e. no stronger magnetic fields can be realized for the systems consisting of magnetized toroids and central compact objects with the masses around the typical neutron star mass. The value of the maximum density of the toroid, 1011 g cm,3, is taken from the theoretical computational results of binary neutron star merging simulations in full general relativity. [source]

Two stellar mass functions combined into one by the random sampling model of the initial mass function

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2000
Bruce G. Elmegreen
The turnover in the stellar initial mass function (IMF) at low mass suggests the presence of two independent mass functions that combine in different ways above and below a characteristic mass given by the thermal Jeans mass in the cloud. In the random sampling model introduced earlier, the Salpeter IMF at intermediate to high mass follows primarily from the hierarchical structure of interstellar clouds, which is sampled by various star formation processes and converted into stars at the local dynamical rate. This power-law part is independent of the details of star formation inside each clump and therefore has a universal character. The flat part of the IMF at low mass is proposed here to result from a second, unrelated, physical process that determines only the probability distribution function for final star mass inside a clump of a given mass, and is independent of both this clump mass and the overall cloud structure. Both processes operate for all potentially unstable clumps in a cloud, regardless of mass, but only the first shows up above the thermal Jeans mass, and only the second shows up below this mass. Analytical and stochastic models of the IMF that are based on the uniform application of these two functions for all masses reproduce the observations well. [source]

The estimations of neutron star mass and radius by the kHz QPOs

ASTRONOMISCHE NACHRICHTEN, Issue 4 2009
C.M. Zhang
Abstract The kHz quasi-periodic oscillations (QPOs) have been detected by the RXTE satellite in about thirty neutron stars (NSs) in low mass X-ray binaries (LMXBs), which are usually interpreted to be related to the Keplerian motions in the orbit close to NS surface where the accreted matter is sucked onto the star. Based on the MHD Alfvén wave oscillation model and the relativistic precession model for the neutron star (NS) kHz QPOs, estimations of mass M and radius R of some NSs are given, which can give clues to evaluate the models. Furthermore, comparisons with theoretical M - R relations by stellar equations of state (EOSs) are presented (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

PSR J1829+2456: a relativistic binary pulsar

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2004
D. J. Champion
ABSTRACT We report the discovery of a new binary pulsar, PSR J1829+2456, found during a mid-latitude drift-scan survey with the Arecibo telescope. Our initial timing observations show the 41-ms pulsar to be in a 28-h, slightly eccentric, binary orbit. The advance of periastron yr,1 is derived from our timing observations spanning 200 d. Assuming that the advance of periastron is purely relativistic and a reasonable range of neutron star masses for PSR J1829+2456, we constrain the companion mass to be between 1.22 and 1.38 M,, making it likely to be another neutron star. We also place a firm upper limit on the pulsar mass of 1.38 M,. The expected coalescence time due to gravitational wave emission is long (,60 Gyr), and this system will not significantly impact upon calculations of merger rates that are relevant to upcoming instruments such as LIGO. [source]