13 research outputs found
Magnetic Field Generation in Stars
Enormous progress has been made on observing stellar magnetism in stars from
the main sequence through to compact objects. Recent data have thrown into
sharper relief the vexed question of the origin of stellar magnetic fields,
which remains one of the main unanswered questions in astrophysics. In this
chapter we review recent work in this area of research. In particular, we look
at the fossil field hypothesis which links magnetism in compact stars to
magnetism in main sequence and pre-main sequence stars and we consider why its
feasibility has now been questioned particularly in the context of highly
magnetic white dwarfs. We also review the fossil versus dynamo debate in the
context of neutron stars and the roles played by key physical processes such as
buoyancy, helicity, and superfluid turbulence,in the generation and stability
of neutron star fields.
Independent information on the internal magnetic field of neutron stars will
come from future gravitational wave detections. Thus we maybe at the dawn of a
new era of exciting discoveries in compact star magnetism driven by the opening
of a new, non-electromagnetic observational window.
We also review recent advances in the theory and computation of
magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo
theory. These advances offer insight into the action of stellar dynamos as well
as processes whichcontrol the diffusive magnetic flux transport in stars.Comment: 41 pages, 7 figures. Invited review chapter on on magnetic field
generation in stars to appear in Space Science Reviews, Springe
Measuring the cooling of the neutron star in Cassiopeia A with all Chandra X-ray Observatory detectors
The thermal evolution of young neutron stars (NSs) reflects the neutrino emission properties of their cores. Heinke & Ho (2010) measured a 3.6+/-0.6% decay in the surface temperature of the Cassiopeia A (Cas A) NS between 2000 and 2009, using archival data from the Chandra X-ray Observatory ACIS-S detector in Graded mode. Page et al. (2011) and Shternin et al. (2011) attributed this decay to enhanced neutrino emission from a superfluid neutron transition in the core. Here we test this decline, combining analysis of the Cas A NS using all Chandra X-ray detectors and modes (HRC-S, HRC-I, ACIS-I, ACIS-S in Faint mode, and ACIS-S in Graded mode) and adding a 2012 May ACIS-S Graded mode observation, using the most current calibrations (CALDB 4.5.5.1). We measure the temperature changes from each detector separately and test for systematic effects due to the nearby filaments of the supernova remnant. We find a 0.92%-2.0% decay over 10 years in the effective temperature, inferred from HRC-S data, depending on the choice of source and background extraction regions, with a best-fit decay of 1.0+/-0.7%. In comparison, the ACIS-S Graded data indicate a temperature decay of 3.1%–5.0% over 10 years, with a best-fit decay of 3.5+/-0.4%. Shallower observations using the other detectors yield temperature decays of 2.6+/-1.9% (ACIS-I), 2.1+/-1.0%(HRC-I), and 2.1+/-1.9% (ACIS-S Faint mode) over 10 years. Our best estimate indicates a decline of 2.9+/-0.5stat+/-1.0sys% over 10 years. The complexity of the bright and varying supernova remnant background makes a definitive interpretation of archival Cas A Chandra observations difficult. A temperature decline of 1–3.5% over 10 years would indicate extraordinarily fast cooling of the NS that can be regulated by superfluidity of nucleons in the stellar core
Femtosecond Time-Resolved Photofragment Rotational Angular Momentum Alignment in Electronic Predissociation Dynamics
This Letter presents an experimental and theoretical study of femtosecond time-resolved vector correlations in methyl iodide (CHI) electronic predissociation via the second absorption B-band at 201.2 nm. The time evolution of the phenomenological anisotropy parameters β was determined from time-resolved photofragment angular distributions obtained by means of the femtosecond laser pump-probe technique coupled with velocity map imaging detection of vibrational ground-state CH( ν = 0) fragments and spin-orbit excited I∗(P) atoms. Theoretical interpretation of the experimental results was performed on the basis of a fitting procedure using quasiclassical theory, which elucidates vector correlations in photodissociation of symmetric top molecules. The results of the fitting are in very good agreement with the experimental data and demonstrate the important role of molecular excited-state lifetimes, parent molecule and methyl fragment rotations, and methyl fragment angular momentum alignment on the time-dependent electronic predissociation dynamics.Peer Reviewe