36,215 research outputs found
Internal heating driven convection at infinite Prandtl number
We derive an improved rigorous bound on the space and time averaged
temperature of an infinite Prandtl number Boussinesq fluid contained
between isothermal no-slip boundaries thermally driven by uniform internal
heating. A novel approach is used wherein a singular stable stratification is
introduced as a perturbation to a non-singular background profile, yielding the
estimate where is the heat Rayleigh
number. The analysis relies on a generalized Hardy-Rellich inequality that is
proved in the appendix
"Ultimate state" of two-dimensional Rayleigh-Benard convection between free-slip fixed temperature boundaries
Rigorous upper limits on the vertical heat transport in two dimensional
Rayleigh-Benard convection between stress-free isothermal boundaries are
derived from the Boussinesq approximation of the Navier-Stokes equations. The
Nusselt number Nu is bounded in terms of the Rayleigh number Ra according to
uniformly in the Prandtl number Pr. This Nusselt
number scaling challenges some theoretical arguments regarding the asymptotic
high Rayleigh number heat transport by turbulent convection.Comment: 4 page
Spectroscopic Identification of the Infrared Counterpart to GX5-1
Using CGS4 on UKIRT, we have obtained a 1.95-2.45 micron infrared spectrum of
the primary candidate counterpart to the bright Z LMXB GX5-1. IR photometry by
Naylor, Charles, & Longmore (1992) and the astrometry of Jonker et al. (2000)
had previously identified this star as the most likely counterpart to GX5-1.
The spectrum presented here clearly shows Brackett gamma and He lines in
emission, for the first time confirming the identity of the counterpart.
Similar to our previous spectroscopy of the Z source LMXBs Sco X-1 and Sco X-2
(Bandyopadhyay et al. 1999), the K-band spectrum of GX5-1 shows emission lines
only. We briefly discuss the implications of this spectrum for the nature of
the Z sources.Comment: accepted for publication as a Letter in MNRA
Canonical Sasakian Metrics
Let be a closed manifold of Sasaki type. A polarization of is defined
by a Reeb vector field, and for one such, we consider the set of all Sasakian
metrics compatible with it. On this space, we study the functional given by the
squared -norm of the scalar curvature. We prove that its critical points,
or canonical representatives of the polarization, are Sasakian metrics that are
transversally extremal. We define a Sasaki-Futaki invariant of the
polarization, and show that it obstructs the existence of constant scalar
curvature representatives. For a fixed CR structure of Sasaki type, we define
the Sasaki cone of structures compatible with this underlying CR structure, and
prove that the set of polarizations in it that admit a canonical representative
is open.Comment: 36 pages, minor corrections made, example adde
The X-ray Transient XTE J2012+381
We present optical and infrared observations of the soft X-ray transient
(SXT) XTE J2012+381 and identify the optical counterpart with a faint red star
heavily blended with a brighter foreground star. The fainter star is coincident
with the radio counterpart and appears to show weak H alpha emission and to
have faded between observations. The RXTE/ASM lightcurve of XTE J2012+381 is
unusual for an SXT in that after an extended linear decay, it settled into a
plateau state for about 40 days before undergoing a weak mini-outburst. We
discuss the nature of the object and suggest similarities to long orbital
period SXTs.Comment: 5 pages, 7 postscript figures included, uses mn.sty. Accepted for
publication in MNRA
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