2,297 research outputs found
Discovery of kHz Fluctuations in Centaurus X-3: Evidence for Photon Bubble Oscillations (PBO) and Turbulence in a High Mass X-ray Binary Pulsar
We report the discovery of kHz fluctuations, including quasi-periodic
oscillations (QPO) at ~330 Hz and ~760 Hz and a broadband kHz continuum in the
power density spectrum of the high mass X-ray binary pulsar Centaurus X-3.
These observations of Cen X-3 were carried out with the Rossi X-ray Timing
Explorer (RXTE). The fluctuation spectrum is flat from mHz to a few Hz, then
steepens to behavior between a few Hz and ~100 Hz. Above a hundred Hz,
the spectrum shows the QPO features, plus a flat continuum extending to ~1200
Hz and then falling out to ~1800 Hz. These results, which required the
co-adding three days of observations of Cen X-3, are at least as fast as the
fastest known variations in X-ray emission from an accreting compact object
(kHz QPO in LMXB sources) and probably faster since extension to ~1800 Hz is
indicated by the most likely parameterization of the data.
Multi-dimensional radiation hydrodynamics simulations of optically thick
plasma flow onto the magnetic poles of an accreting neutron star show that the
fluctuations at frequencies above 100 Hz are consistent with photon bubble
turbulence and oscillations (PBO) previously predicted to be observable in this
source. For a polar cap opening angle of 0.25 radians, we show that the
spectral form above 100 Hz is reproduced by the simulations, including the
frequencies of the QPO and the relative power in the QPO and the kHz continuum.
This has resulted in the first model-dependent measurement of the polar cap
size of an X-ray pulsar.Comment: received ApJ: April 1, 1999 accepted ApJ: September 1, 199
Radiation Pressure in Massive Star Formation
Stars with masses of >~ 20 solar masses have short Kelvin times that enable
them to reach the main sequence while still accreting from their natal clouds.
The resulting nuclear burning produces a huge luminosity and a correspondingly
large radiation pressure force on dust grains in the accreting gas. This effect
may limit the upper mass of stars that can form by accretion. Indeed,
simulations and analytic calculations to date have been unable to resolve the
mystery of how stars of 50 solar masses and up form. We present two new ideas
to solve the radiation pressure problem. First, we use three-dimensional
radiation hydrodynamic adaptive mesh refinement simulations to study the
collapse of massive cores. We find that in three dimensions a configuration in
which radiation holds up an infalling envelope is Rayleigh-Taylor unstable,
leading radiation driven bubbles to collapse and accretion to continue. We also
present Monte Carlo radiative transfer calculations showing that the cavities
created by protostellar winds provides a valve that allow radiation to escape
the accreting envelope, further reducing the ability of radiation pressure to
inhibit accretion.Comment: To be appear in "IAU 227: Massive Star Birth: A Crossroads of
Astrophysics"; 6 pages, 1 figur
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