7,076 research outputs found
On the release of binding energy and accretion power in core collapse-like environments
All accretion models of gamma-ray bursts share a common assumption: accretion
power and gravitational binding energy is released and then dissipated locally,
with the mass of its origin. This is equivalent to the Shakura-Sunyaev 1973
(SS73) prescription for the dissipation of accretion power and subsequent
conversion into radiate output. Since their seminal paper, broadband
observations of quasars and black hole X-ray binaries insist that the SS73
prescription cannot wholly describe their behavior. In particular, optically
thick black hole accretion flows are almost universally accompanied by coronae
whose relative power by far exceeds anything seen in studies of stellar
chromospheric and coronal activity. In this note, we briefly discuss the
possible repercussions of freeing accretion models of GRBs from the SS73
prescription. Our main conclusion is that the efficiency of converting
gravitational binding energy into a GRB power can be increased by an order of
magnitude or more.Comment: 6 page Aspen conference proceedin
Warm Jupiters Need Close "Friends" for High-Eccentricity Migration -- A Stringent Upper Limit on the Perturber's Separation
We propose a stringent observational test on the formation of warm Jupiters
(gas-giant planets with 10 d <~ P <~ 100 d) by high-eccentricity (high-e)
migration mechanisms. Unlike hot Jupiters, the majority of observed warm
Jupiters have pericenter distances too large to allow efficient tidal
dissipation to induce migration. To access the close pericenter required for
migration during a Kozai-Lidov cycle, they must be accompanied by a strong
enough perturber to overcome the precession caused by General Relativity (GR),
placing a strong upper limit on the perturber's separation. For a warm Jupiter
at a ~ 0.2 AU, a Jupiter-mass (solar-mass) perturber is required to be <~ 3 AU
(<~ 30 AU) and can be identified observationally. Among warm Jupiters detected
by Radial Velocities (RV), >~ 50% (5 out of 9) with large eccentricities (e >~
0.4) have known Jovian companions satisfying this necessary condition for
high-e migration. In contrast, <~ 20 % (3 out of 17) of the low-e (e <~ 0.2)
warm Jupiters have detected additional Jovian companions, suggesting that
high-e migration with planetary perturbers may not be the dominant formation
channel. Complete, long-term RV follow-ups of the warm-Jupiter population will
allow a firm upper limit to be put on the fraction of these planets formed by
high-e migration. Transiting warm Jupiters showing spin-orbit misalignments
will be interesting to apply our test. If the misalignments are solely due to
high-e migration as commonly suggested, we expect that the majority of warm
Jupiters with low-e (e <~0.2) are not misaligned, in contrast with low-e hot
Jupiters.Comment: Minor changes, Accepted by ApJ
Protrepticus
A new translation and edition of Aristotle's Protrepticus (with critical comments on the fragments)
Welcome
The Protrepticus was an early work of Aristotle, written while he was still a member of Plato's Academy, but it soon became one of the most famous works in the whole history of philosophy. Unfortunately it was not directly copied in the middle ages and so did not survive in its own manuscript tradition. But substantial fragments of it have been preserved in several works by Iamblichus of Chalcis, a third century A.D. neo-Pythagorean philosopher and educator. On the basis of a close study of Iamblichus' extensive use and excerption of Aristotle's Protrepticus, it is possible to reconstruct the backbone of the lost work, and then to flesh it out with the other surviving reports about the work from antiquity (for example in Alexander of Aphrodisias and other ancient commentators on Aristotle). It is also possible to identify several papyrus fragments of the work, and many references and literary allusions in later authors, especially Cicero, whose own lost dialogue Hortensius was a defense of philosophy modeleld on Aristotle's
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