3,717 research outputs found
The Page-R{\'e}nyi parking process
In the Page parking (or packing) model on a discrete interval (also known as
the discrete R{\'e}nyi packing problem or the unfriendly seating problem), cars
of length two successively park uniformly at random on pairs of adjacent
places, until only isolated places remain. We give a probabilistic proof of the
(known) fact that the proportion of the interval covered by cars goes to
1-exp(-2) , when the length of the interval goes to infinity. We obtain some
new consequences, and also study a version of this process defined on the
infinite line
On the algebraic numbers computable by some generalized Ehrenfest urns
This article deals with some stochastic population protocols, motivated by
theoretical aspects of distributed computing. We modelize the problem by a
large urn of black and white balls from which at every time unit a fixed number
of balls are drawn and their colors are changed according to the number of
black balls among them. When the time and the number of balls both tend to
infinity the proportion of black balls converges to an algebraic number. We
prove that, surprisingly enough, not every algebraic number can be "computed"
this way
Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38
We used the ARO 12m antenna to observe emission from the J=1-0 lines of
carbon monoxide, \hcop\ and HNC and the J=2-1 line of CS toward and around the
continuum peak used for absorption studies and we compare them with CH, HNC,
C\p\ and other absorption spectra from PRISMAS. We develop a kinematic analysis
that allows a continuous description of the spectral properties and relates
them to viewing geometry in the Galaxy. As for CH, HF, C\p, \hcop\ and other
species observed in absorption, mm-wave emission in CO, \hcop, HNC and CS is
continuous over the full velocity range expected for material between the Sun
and W31 4.95 kpc away. CO emission is much stronger than average in the
Galactic molecular ring and the mean \HH\ density derived from CH, 4 \pccc
\la 2 \la 10 \pccc at 4 \la R \la 6.4 kpc, is similarly
elevated. The CO-\HH\ conversion factor falls in a narrow range \XCO\ =
1-2\times10^{20}~\HH\ \pcc~({\rm K}-\kms)^{-1} if the emitting gas is mostly
on the near side of the sub-central point, as we suggest. The brightnesses of
\hcop, HNC, and CS are comparable (0.83\%, 0.51\% and 1.1\% respectively
relative to CO) and have no variation in galactocentric radius with respect to
CO. Comparison of the profile-averaged \hcop\ emission brightness and optical
depth implies local densities n(H) \approx 135\pm25\pccc with most of
excitation of \hcop\ from electrons. At such density, a consistent picture of
the \HH-bearing gas, accounting also for the CO emission, has a volume filling
factor 3\% and a 5 pc clump or cloud size.Comment: Accepted to A&
Construction of a short path in high dimensional First Passage Percolation
For First Passage Percolation in Z^d with large d, we construct a path
connecting the origin to {x_1 =1}, whose passage time has optimal order \log
d/d. Besides, an improved lower bound for the "diagonal" speed of the cluster
combined with a result by Dhar (1988) shows that the limiting shape in FPP with
exponential passage times (and thus that of Eden model) is not the euclidian
ball in dimension larger than 35
Stellar Feedback in the ISM Revealed by Wide-Field Far-Infrared Spectral-Imaging
The radiative and mechanical interaction of stars with their environment
drives the evolution of the ISM and of galaxies as a whole. The far-IR emission
(lambda ~30 to 350 microns) from atoms and molecules dominates the cooling of
the warm gas in the neutral ISM, the material that ultimately forms stars.
Far-IR lines are thus the most sensitive probes of stellar feedback processes,
and allow us to quantify the deposition and cycling of energy in the ISM. While
ALMA (in the (sub)mm) and JWST (in the IR) provide astonishing sub-arcsecond
resolution images of point sources and their immediate environment, they cannot
access the main interstellar gas coolants, nor are they designed to image
entire star-forming regions (SFRs). Herschel far-IR photometric images of the
interstellar dust thermal emission revealed the ubiquitous large-scale
filamentary structure of SFRs, their mass content, and the location of
thousands of prestellar cores and protostars. These images, however, provide a
static view of the ISM: not only they dont constrain the cloud dynamics,
moreover they cannot reveal the chemical composition and energy transfer within
the cloud, thus giving little insight into the regulation process of star
formation by stellar feedback. In this white paper we emphasize the need of a
space telescope with wide-field spectral-imaging capabilities in the critical
far-IR domain.Comment: White Paper submitted to the Astro 2020 Decadal Survey on Astronomy
and Astrophysics (National Academies of Science, Engineering, and Medicine
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