352 research outputs found
Prefix Codes: Equiprobable Words, Unequal Letter Costs
Describes a near-linear-time algorithm for a variant of Huffman coding, in
which the letters may have non-uniform lengths (as in Morse code), but with the
restriction that each word to be encoded has equal probability. [See also
``Huffman Coding with Unequal Letter Costs'' (2002).]Comment: proceedings version in ICALP (1994
Hydrodynamic Simulations of the Interaction between an AGB Star and a Main Sequence Companion in Eccentric Orbits
The Rotten Egg Nebula has at its core a binary composed of a Mira star and an
A-type companion at a separation >10 au. It has been hypothesized to have
formed by strong binary interactions between the Mira and a companion in an
eccentric orbit during periastron passage ~800 years ago. We have performed
hydrodynamic simulations of an asymptotic giant branch star interacting with
companions with a range of masses in orbits with a range of initial
eccentricities and periastron separations. For reasonable values of the
eccentricity, we find that Roche lobe overflow can take place only if the
periods are <<100 years. Moreover, mass transfer causes the system to enter a
common envelope phase within several orbits. Since the central star of the
Rotten Egg nebula is an AGB star, we conclude that such a common envelope phase
must have lead to a merger, so the observed companion must have been a tertiary
companion of a binary that merged at the time of nebula ejection. Based on the
mass and timescale of the simulated disc formed around the companion before the
common envelope phase, we analytically estimate the properties of jets that
could be launched. Allowing for super-Eddington accretion rates, we find that
jets similar to those observed are plausible, provided that the putative lost
companion was relatively massive.Comment: accepted for publication in MNRA
Mixing Time Scales in a Supernova-Driven Interstellar Medium
We study the mixing of chemical species in the interstellar medium (ISM).
Recent observations suggest that the distribution of species such as deuterium
in the ISM may be far from homogeneous. This raises the question of how long it
takes for inhomogeneities to be erased in the ISM, and how this depends on the
length scale of the inhomogeneities. We added a tracer field to the
three-dimensional, supernova-driven ISM model of Avillez (2000) to study mixing
and dispersal in kiloparsec-scale simulations of the ISM with different
supernova (SN) rates and different inhomogeneity length scales. We find several
surprising results. Classical mixing length theory fails to predict the very
weak dependence of mixing time on length scale that we find on scales of
25--500 pc. Derived diffusion coefficients increase exponentially with time,
rather than remaining constant. The variance of composition declines
exponentially, with a time constant of tens of Myr, so that large differences
fade faster than small ones. The time constant depends on the inverse square
root of the supernova rate. One major reason for these results is that even
with numerical diffusion exceeding physical values, gas does not mix quickly
between hot and cold regions.Comment: 23 pages, 14 figures that include 7 simulation images and 19 plots,
accepted for publication at Ap
Variations in Arterial Blood Pressure after Kidney Transplantation
The course of hypertension within the first 2 months after kidney transplantation was correlated with renal function, plasma renin activity (PRA), and the daily maintenance dose of prednisone in 18 homograft recipients. During acute rejection blood pressure (BP) closely correlated with PRA. Patients with normal homograft function showed an increase in BP early after transplantation which in most returned to normal 3-8 weeks later. In the latter group no correlation could be found between the level of BP and PRA, however the BP correlated closely with the dose of prednisone. These observations suggest that during acute rejection the increase in BP may at least partly be mediated by a renal pressor mechanism, whereas with normal renal function the high dose of glucocorticoids may play an important role in the development of hypertension.</jats:p
Recovery from Hepatorenal Syndrome after Orthotopic Liver Transplantation
Three patients with progressive renal failure and advanced hepatic insufficiency due to cirrhosis of the liver underwent orthotopic liver transplantation. All three patients had immediate improvement in hepatic function and within two weeks after liver replacement regained nearly normal kidney function. However, the renal recovery was delayed in each case, and its course was not uniform. Plasma renin activity was high, and renin substrate was low before transplantation in one case in which these measurements were obtained; both returned to normal soon after liver replacement. (N Engl J Med 289:1155–1159, 1973). © 1973, Massachusetts Medical Society. All rights reserved
HST Images Flash Ionization of Old Ejecta by the 2011 Eruption of Recurrent Nova T Pyxidis
T Pyxidis is the only recurrent nova surrounded by knots of material ejected
in previous outbursts. Following the eruption that began on 2011 April 14.29,
we obtained seven epochs (from 4 to 383 days after eruption) of Hubble Space
Telescope narrowband Ha images of T Pyx . The flash of radiation from the nova
event had no effect on the ejecta until at least 55 days after the eruption
began. Photoionization of hydrogen located north and south of the central star
was seen 132 days after the beginning of the eruption. That hydrogen recombined
in the following 51 days, allowing us to determine a hydrogen atom density of
at least 7e5 cm^-3 - at least an order of magnitude denser than the previously
detected, unresolved [NII] knots surrounding T Pyx. Material to the northwest
and southeast was photoionized between 132 and 183 days after the eruption
began. 99 days later that hydrogen had recombined. Both then (282 days after
outburst) and 101 days later, we detected almost no trace of hydrogen emission
around T Pyx. There is a large reservoir of previously unseen, cold diffuse
hydrogen overlapping the previously detected, [NII] - emitting knots of T Pyx
ejecta. The mass of this newly detected hydrogen is probably an order of
magnitude larger than that of the [NII] knots. We also determine that there is
no significant reservoir of undetected ejecta from the outer boundaries of the
previously detected ejecta out to about twice that distance, near the plane of
the sky. The lack of distant ejecta is consistent with the Schaefer et al
(2010) scenario for T Pyx, in which the star underwent its first eruption
within five years of 1866 after many millennia of quiescence, followed by the
six observed recurrent nova eruptions since 1890. This lack of distant ejecta
is not consistent with scenarios in which T Pyx has been erupting continuously
as a recurrent nova for many centuries or millennia.Comment: 27 pages, 10 figures, submitted to the Astrophysical Journa
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