4,635 research outputs found
A measurement of the turbulence-driven density distribution in a non-star-forming molecular cloud
Molecular clouds are supersonically turbulent. This turbulence governs the initial mass function and the star formation rate. In order to understand the details of star formation, it is therefore essential to understand the properties of turbulence, in particular the probability distribution of density in turbulent clouds. We present H2CO volume density measurements of a non-star-forming cloud along the line of sight toward W49A. We use these measurements in conjunction with total mass estimates from 13CO to infer the shape of the density probability distribution function. This method is complementary to measurements of turbulence via the column density distribution and should be applicable to any molecular cloud with detected CO. We show that turbulence in this cloud is probably compressively driven, with a compressive-to-total Mach number ratio . We measure the standard deviation of the density distribution, constraining it to the range 1.5 < σ s < 1.9, assuming that the density is lognormally distributed. This measurement represents an essential input into star formation laws. The method of averaging over different excitation conditions to produce a model of emission from a turbulent cloud is generally applicable to optically thin line observations
Sofic-Dyck shifts
We define the class of sofic-Dyck shifts which extends the class of
Markov-Dyck shifts introduced by Inoue, Krieger and Matsumoto. Sofic-Dyck
shifts are shifts of sequences whose finite factors form unambiguous
context-free languages. We show that they correspond exactly to the class of
shifts of sequences whose sets of factors are visibly pushdown languages. We
give an expression of the zeta function of a sofic-Dyck shift
Hypervelocity Stars: Predicting the Spectrum of Ejection Velocities
The disruption of binary stars by the tidal field of the black hole in the
Galactic Center can produce the hypervelocity stars observed in the halo. We
use numerical models to simulate the full spectrum of observable velocities of
stars ejected into the halo by this binary disruption process. Our model
includes a range of parameters for binaries with 3-4 M_Solar primaries,
consideration of radial orbits of the ejected stars through an approximate mass
distribution for the Galaxy, and the impact of stellar lifetimes. We calculate
the spectrum of ejection velocities and reproduce previous results for the mean
ejection velocity at the Galactic center. The model predicts that the full
population of ejected stars includes both the hypervelocity stars with
velocities large enough to escape from the Galaxy and a comparable number of
ejected, but bound, stars of the same stellar type. The predicted median speeds
of the population of ejected stars as a function of distance in the halo are
consistent with current observations. Combining the model with the data also
shows that interesting constraints on the properties of binaries in the
Galactic Center and on the mass distribution in the Galaxy can be obtained even
with modest samples of ejected stars.Comment: 26 pages, including 6 figures, accepted for publication in the
Astrophysical Journa
Hypervelocity Stars. I. The Spectroscopic Survey
We discuss our targeted search for hypervelocity stars (HVSs), stars
traveling with velocities so extreme that dynamical ejection from a massive
black hole is their only suggested origin. Our survey, now half complete, has
successfully identified a total of four probable HVSs plus a number of other
unusual objects. Here we report the most recently discovered two HVSs: SDSS
J110557.45+093439.5 and possibly SDSS J113312.12+010824, traveling with
Galactic rest-frame velocities at least +508+-12 and +418+-10 km/s,
respectively. The other late B-type objects in our survey are consistent with a
population of post main-sequence stars or blue stragglers in the Galactic halo,
with mean metallicity [Fe/H]=-1.3 and velocity dispersion 108+-5 km/s.
Interestingly, the velocity distribution shows a tail of objects with large
positive velocities that may be a mix of low-velocity HVSs and high-velocity
runaway stars. Our survey also includes a number of DA white dwarfs with
unusually red colors, possibly extremely low mass objects. Two of our objects
are B supergiants in the Leo A dwarf, providing the first spectroscopic
evidence for star formation in this dwarf galaxy within the last ~30 Myr.Comment: 10 pages, uses emulateapj, accepted by Ap
Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center
We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars
traveling with such extreme velocities that dynamical ejection from a massive
black hole (MBH) is their only suggested origin. We also detect a population of
possibly bound HVSs. The significant asymmetry we observe in the velocity
distribution -- we find 26 stars with v_rf > 275 km/s and 1 star with v_rf <
-275 km/s -- shows that the HVSs must be short-lived, probably 3 - 4 Msun main
sequence stars. Any population of hypervelocity post-main sequence stars should
contain stars falling back onto the Galaxy, contrary to the observations. The
spatial distribution of HVSs also supports the main sequence interpretation:
longer-lived 3 Msun HVSs fill our survey volume; shorter-lived 4 Msun HVSs are
missing at faint magnitudes. We infer that there are 96 +- 10 HVSs of mass 3 -
4 Msun within R < 100 kpc, possibly enough HVSs to constrain ejection
mechanisms and potential models. Depending on the mass function of HVSs, we
predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs
favor a continuous ejection process, although a ~120 Myr-old burst of HVSs is
also allowed.Comment: 10 pages, 8 figures, accepted to ApJ, minor revision
On the Commutative Equivalence of Context-Free Languages
The problem of the commutative equivalence of context-free and regular languages is studied. In particular conditions ensuring that a context-free language of exponential growth is commutatively equivalent with a regular language are investigated
Soft gluon radiation and energy dependence of total hadronic cross-sections
An impact parameter representation for soft gluon radiation is applied to
obtain both the initial decrease of the total cross-section ()
for proton-proton collisions as well as the later rise of with
energy for both and . The non-perturbative soft part of the
eikonal includes only limited low energy gluon emission and leads to the
initial decrease in the proton-proton cross- section. On the other hand, the
rapid rise in the hard, perturbative jet part of the eikonal is tamed into the
experimentally observed mild increase by soft gluon radiation whose maximum
energy rises slowly with energy.Comment: 30 pages, 6 figures. Version accepted for publication in Physical
Review D. Additional section with explanatory material added making the paper
more self contained and two figures changed to have a complete summary of the
available accelerator dat
Antichain cutsets of strongly connected posets
Rival and Zaguia showed that the antichain cutsets of a finite Boolean
lattice are exactly the level sets. We show that a similar characterization of
antichain cutsets holds for any strongly connected poset of locally finite
height. As a corollary, we get such a characterization for semimodular
lattices, supersolvable lattices, Bruhat orders, locally shellable lattices,
and many more. We also consider a generalization to strongly connected
hypergraphs having finite edges.Comment: 12 pages; v2 contains minor fixes for publicatio
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