Effect of Radiation Drag on Hoyle-Lyttleton Accretion

Hoyle-Lyttleton type accretion is investigated, by taking account of not only the effect of radiation pressure but the effect of radiation drag. We calculate the trajectories of particles for three cases: only the effect of gravity is considered (case A); the effect of radiation pressure is taken into account (case B); the effect of radiation drag as well as radiation pressure is taken into account (case C). The accretion radii for former two cases are $2GM/v_{\infty}^2$ for case A and $2GM(1-\Gamma)/v_{\infty}^2$ for case B, where M is the mass of the accreted object, $v_{\infty}$ the relative velocity, and Gamma the normalized luminosity of the accreted object. We found that the accretion radius for case C is in between those of cases A and B under the present approximation; i.e., the accretion radius decreases due to radiation pressure while it increases due to radiation drag. In addition, the accretion radius for case C becomes larger as the incident velocity becomes fast. The effect of radiation drag becomes more and more important when the velocity of the incident particle is comparable to the light speed.Comment: 11 pages, LaTeX with 6 eps figures, accepted by Publications of the Astronomical Society of Japa

Comments on "Angular Momentum Transport in Quasi-Keplerian Accretion Disks"

Subramanian, Pujari and Becker (2004) claim that the correct expression for the angular momentum transport in an accretion disc, which is proportional to dOmega/dR, can be derived on the basis of the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disc. We study their argument and show that their derivation contains several fundamental errors: 1) the biased choice of the desired formula from an infinite number of formulae; 2) the biased choice of parcel trajectories; and 3) confusion regarding the reference frames. Following 1) we could derive, for example, a (invalid) formula in which the angular momentum transport is proportional to dv_phi/dR, and from 2) we could even prove that the angular momentum transport is either inward or null. We present the correct approach to the problem of angular momentum transport in an accretion disc in terms of mean free path theory.Comment: 9 pages, 2 figures, to appear in Prog. Theor. Phys. Vol.113, No.6, 200

Making sense of the Indo-Pacific strategy : an inheritance from the past

For more about the East-West Center, see http://www.eastwestcenter.org/Takuya Matsuda, PhD Student at King's College in London, explains that "'Indo-Pacific' is a concept that emerged as a culmination of policy choices made since the mid-1990s to incorporate India into the US strategic framework in the Western Pacific and to encourage allies including Japan to upgrade their roles in international security.