736 research outputs found
Dipolar modes in luminous red giants
Lots of information on solar-like oscillations in red giants has been
obtained thanks to observations with CoRoT and Kepler space telescopes. Data on
dipolar modes appear most interesting. We study properties of dipolar
oscillations in luminous red giants to explain mechanism of mode trapping in
the convective envelope and to assess what may be learned from the new data.
Equations for adiabatic oscillations are solved by numerical integration down
to the bottom of convective envelope, where the boundary condition is applied.
The condition is based on asymptotic decomposition of the fourth order system
into components describing a running wave and a uniform shift of radiative
core. If the luminosity of a red giant is sufficiently high, for instance at M
= 2 Msun greater than about 100 Lsun, the dipolar modes become effectively
trapped in the acoustic cavity, which covers the outer part of convective
envelope. Energy loss caused by gravity wave emission at the envelope base is a
secondary or negligible source of damping. Frequencies are insensitive to
structure of the deep interior.Comment: 10 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Orbital Decay of the PSR J0045-7319/B Star Binary System: Age of Radio Pulsar and Initial Spin of Neutron Star
Recent timing observations of PSR J0045-7319 reveal that the neutron star/B
star binary orbit is decaying on a time scale of |\Porb/\dot\Porb|=0.5 Myr,
shorter than the characteristic age ( Myr) of the pulsar (Kaspi et
al.~1996a). We study mechanisms for the orbital decay. The standard weak
friction theory based on static tide requires far too short a viscous time to
explain the observed \dot\Porb. We show that dynamical tidal excitation of
g-modes in the B star can be responsible for the orbital decay. However, to
explain the observed short decay timescale, the B star must have some
significant retrograde rotation with respect to the orbit --- The retrograde
rotation brings lower-order g-modes, which couple much more strongly to the
tidal potential, into closer ``resonances'' with the orbital motion, thus
significantly enhancing the dynamical tide. A much less likely possibility is
that the g-mode damping time is much shorter than the ordinary radiative
damping time. The observed orbital decay timescale combined with a generic
orbital evolution model based on dynamical tide can be used as a ``timer'',
giving an upper limit of Myr for the age of the binary system since the
neutron star formation. Thus the characteristic age of the pulsar is not a good
age indicator. Assuming standard magnetic dipole braking for the pulsar and no
significant magnetic field decay on a timescale \lo 1 Myr, the upper limit
for the age implies that the initial spin of the neutron star at birth was
close to its current value.Comment: AASTeX, 9 pages, 3 ps figures. ApJ Letters, in pres
- …