203 research outputs found
Graphs with tiny vector chromatic numbers and huge chromatic numbers
Karger, Motwani, and Sudan [J. ACM, 45 (1998), pp. 246-265] introduced the notion of a vector coloring of a graph. In particular, they showed that every k-colorable graph is also vector k-colorable, and that for constant k, graphs that are vector k-colorable can be colored by roughly Δ^(1 - 2/k) colors. Here Δ is the maximum degree in the graph and is assumed to be of the order of n^5 for some 0 < δ < 1. Their results play a major role in the best approximation algorithms used for coloring and for maximum independent sets. We show that for every positive integer k there are graphs that are vector k-colorable but do not have independent sets significantly larger than n/Δ^(1- 2/k) (and hence cannot be colored with significantly fewer than Δ^(1-2/k) colors). For k = O(log n/log log n) we show vector k-colorable graphs that do not have independent sets of size (log n)^c, for some constant c. This shows that the vector chromatic number does not approximate the chromatic number within factors better than n/polylogn.
As part of our proof, we analyze "property testing" algorithms that distinguish between graphs that have an independent set of size n/k, and graphs that are "far" from having such an independent set. Our bounds on the sample size improve previous bounds of Goldreich, Goldwasser, and Ron [J. ACM, 45 (1998), pp. 653-750] for this problem
Numerical studies on the link between radioisotopic signatures on Earth and the formation of the Local Bubble. I. 60Fe transport to the solar system by turbulent mixing of ejecta from nearby supernovae into a locally homogeneous ISM
The discovery of radionuclides like 60Fe with half-lives of million years in
deep-sea crusts and sediments offers the unique possibility to date and locate
nearby supernovae. We want to quantitatively establish that the 60Fe
enhancement is the result of several supernovae which are also responsible for
the formation of the Local Bubble, our Galactic habitat. We performed
three-dimensional hydrodynamic adaptive mesh refinement simulations (with
resolutions down to subparsec scale) of the Local Bubble and the neighbouring
Loop I superbubble in different homogeneous, self-gravitating environments. For
setting up the Local and Loop I superbubble, we took into account the time
sequence and locations of the generating core-collapse supernova explosions,
which were derived from the mass spectrum of the perished members of certain
stellar moving groups. The release of 60Fe and its subsequent turbulent mixing
process inside the superbubble cavities was followed via passive scalars, where
the yields of the decaying radioisotope were adjusted according to recent
stellar evolution calculations. The models are able to reproduce both the
timing and the intensity of the 60Fe excess observed with rather high
precision, provided that the external density does not exceed 0.3 cm-3 on
average. Thus the two best-fit models presented here were obtained with
background media mimicking the classical warm ionised and warm neutral medium.
We also found that 60Fe (which is condensed onto dust grains) can be delivered
to Earth via two physical mechanisms: either through individual fast-paced
supernova blast waves, which cross the Earth's orbit sometimes even twice as a
result of reflection from the Local Bubble's outer shell, or, alternatively,
through the supershell of the Local Bubble itself, injecting the 60Fe content
of all previous supernovae at once, but over a longer time range.Comment: 14 pages, 7 figures, accepted for publication in A&
Numerical studies on the link between radioisotopic signatures on Earth and the formation of the Local Bubble. II. Advanced modeling of interstellar 26Al, 53Mn, 60Fe, and 244Pu influxes as traces of past supernovae in the solar neighborhood
Measurements of long-lived radioisotopes provide a means, completely
independent of other observational channels, to draw conclusions about
near-Earth supernovae (SNe) and thus the origin of the Local Bubble (LB). First
and foremost in this context is 60Fe, which has already been detected across
the Earth and on the Moon. Using Gaia EDR3, we identified 14 SN explosions,
with 13 occurring in UCL/LCC, and one in V1062 Sco, all being subgroups of the
Sco-Cen OB association. The timing of these explosions was obtained by us
through interpolation of rotating stellar evolution tracks via the initial
masses of the already exploded massive stars. We further developed a new Monte
Carlo-type approach for deriving the trajectories of the SN progenitors. We
then performed 3D hydrodynamic simulations based on these initial conditions to
explore the evolution of the LB in an inhomogeneous local interstellar medium
and the transport of radioisotopes to Earth. The simulations include the
stellar winds from the SN progenitors and additional radioisotopes (26Al, 53Mn,
and 244Pu) besides 60Fe. We find that (i) our simulations are consistent with
measurements of 60Fe, in particular, a peak 2-3 Myr before present, as well as
26Al, 53Mn, and 244Pu data, (ii) stellar winds contribute to the distribution
of radioisotopes and also to the dynamics of the LB, (iii) the solar system
(SS) entered the LB about 4.6 Myr ago, and (iv) the measured recent influx of
60Fe can be naturally explained by turbulent radioisotopic transport. Our
simulations not only support the recent hypothesis that the LB triggered star
formation in the solar vicinity through its expansion, but also suggest that
the second, separate 60Fe peak measured at 6-9 Myr ago was generated by the
passage of the SS through a neighboring superbubble (SB), possibly the
Orion-Eridanus SB, prior to its current residence in the LB.Comment: 30 pages, 13 figures, accepted for publication in A&
Implementation of higher-order absorbing boundary conditions for the Einstein equations
We present an implementation of absorbing boundary conditions for the
Einstein equations based on the recent work of Buchman and Sarbach. In this
paper, we assume that spacetime may be linearized about Minkowski space close
to the outer boundary, which is taken to be a coordinate sphere. We reformulate
the boundary conditions as conditions on the gauge-invariant
Regge-Wheeler-Zerilli scalars. Higher-order radial derivatives are eliminated
by rewriting the boundary conditions as a system of ODEs for a set of auxiliary
variables intrinsic to the boundary. From these we construct boundary data for
a set of well-posed constraint-preserving boundary conditions for the Einstein
equations in a first-order generalized harmonic formulation. This construction
has direct applications to outer boundary conditions in simulations of isolated
systems (e.g., binary black holes) as well as to the problem of
Cauchy-perturbative matching. As a test problem for our numerical
implementation, we consider linearized multipolar gravitational waves in TT
gauge, with angular momentum numbers l=2 (Teukolsky waves), 3 and 4. We
demonstrate that the perfectly absorbing boundary condition B_L of order L=l
yields no spurious reflections to linear order in perturbation theory. This is
in contrast to the lower-order absorbing boundary conditions B_L with L<l,
which include the widely used freezing-Psi_0 boundary condition that imposes
the vanishing of the Newman-Penrose scalar Psi_0.Comment: 25 pages, 9 figures. Minor clarifications. Final version to appear in
Class. Quantum Grav
Isolation, establishment, and characterization of ex vivo equine melanoma cell cultures
Gray horses spontaneously develop metastatic melanomas that resemble human disease, and this is often accompanied with metastasis to other organs. Unlike in other species, the establishment of primary equine melanoma cultures that could be used to develop new therapeutic approaches has remained a major challenge. The purpose of the study was to develop a protocol for routine isolation and cultivation of primary equine melanocytes. Melanoma tissues were excised from 13 horses under local anesthesia, mainly from the perianal area. The melanoma cells were isolated from the melanoma tissue by serial enzymatic digestion using dispase and collagenase. Out of the 13 excised melanomas, cell cultures from eight melanomas were established, which corresponded to a success rate 62%. These cells showed different degrees of melanin pigmentation. Characterization of these cells using confocal microscopy, FACs analysis and western blotting showed that they expressed melanoma-associated antigens; Melan-A, MAGE-1, and MAGE-3, and PCNA expression was higher in fast-proliferating isolates. The protocol we developed and established proved successful for routine isolation and cultivation of primary equine melanoma cells. This method provided a large number of primary equine melanoma cells that could be used to study new therapeutic approaches for treatment of equine melanoma
Gemini GNIRS near-infrared spectroscopy of 50 quasars at z>~5.7
We report initial results from a large Gemini program to observe z>~5.7
quasars with GNIRS near-IR spectroscopy. Our sample includes 50 quasars with
simultaneous ~0.85-2.5 micron spectra covering the rest-frame ultraviolet and
major broad emission lines from Ly-alpha to MgII. We present spectral
measurements for these quasars and compare to their lower-redshift counterparts
at z=1.5-2.3. We find that when quasar luminosity is matched, there are no
significant differences between the rest-UV spectra of z>~5.7 quasars and the
low-z comparison sample. High-z quasars have similar continuum and emission
line properties and occupy the same region in the black hole mass and
luminosity space as the comparison sample, accreting at an average Eddington
ratio of ~0.3. There is no evidence for super-Eddington accretion or
hypermassive (>10^10 Msun) black holes within our sample. We find a mild excess
of quasars with weak CIV lines relative to the control sample. Our results,
corroborating earlier studies but with better statistics, demonstrate that
these high-z quasars are already mature systems of accreting supermassive black
holes operating with the same physical mechanisms as those at lower redshifts.Comment: replaced with the accepted version (ApJ); improved the fitting
results and replaced all figures and tables (w/ minor changes); conclusions
unchange
Probing the interstellar medium and star formation of the Most Luminous Quasar at z=6.3
We report new IRAM/PdBI, JCMT/SCUBA-2, and VLA observations of the
ultraluminous quasar SDSSJ010013.02+280225.8 (hereafter, J0100+2802) at z=6.3,
which hosts the most massive supermassive black hole (SMBH) of 1.24x10^10 Msun
known at z>6. We detect the [C II] 158 m fine structure line and molecular
CO(6-5) line and continuum emission at 353 GHz, 260 GHz, and 3 GHz from this
quasar. The CO(2-1) line and the underlying continuum at 32 GHz are also
marginally detected. The [C II] and CO detections suggest active star formation
and highly excited molecular gas in the quasar host galaxy. The redshift
determined with the [C II] and CO lines shows a velocity offset of ~1000 km/s
from that measured with the quasar Mg II line. The CO (2-1) line luminosity
provides direct constraint on the molecular gas mass which is about
(1.0+/-0.3)x10^10 Msun. We estimate the FIR luminosity to be (3.5+/-0.7)x10^12
Lsun, and the UV-to-FIR spectral energy distribution of J0100+2802 is
consistent with the templates of the local optically luminous quasars. The
derived [C II]-to-FIR luminosity ratio of J0100+2802 is 0.0010+/-0.0002, which
is slightly higher than the values of the most FIR luminous quasars at z~6. We
investigate the constraint on the host galaxy dynamical mass of J0100+2802
based on the [C II] line spectrum. It is likely that this ultraluminous quasar
lies above the local SMBH-galaxy mass relationship, unless we are viewing the
system at a small inclination angle.Comment: 19 pages, 4 figures, published by the Astrophysical Journal, minimal
changes in acknowledgement to match the published versio
The Final SDSS High-Redshift Quasar Sample of 52 Quasars at z>5.7
We present the discovery of nine quasars at identified in the Sloan
Digital Sky Survey (SDSS) imaging data. This completes our survey of
quasars in the SDSS footprint. Our final sample consists of 52 quasars at
, including 29 quasars with mag selected from
11,240 deg of the SDSS single-epoch imaging survey (the main survey), 10
quasars with selected from 4223 deg of the SDSS
overlap regions (regions with two or more imaging scans), and 13 quasars down
to mag from the 277 deg in Stripe 82. They span a
wide luminosity range of . This well-defined sample
is used to derive the quasar luminosity function (QLF) at . After
combining our SDSS sample with two faint ( mag) quasars from
the literature, we obtain the parameters for a double power-law fit to the QLF.
The bright-end slope of the QLF is well constrained to be
. Due to the small number of low-luminosity quasars, the
faint-end slope and the characteristic magnitude are
less well constrained, with and
mag. The spatial density of luminous quasars,
parametrized as , drops rapidly
from to 6, with . Based on our fitted QLF and assuming
an IGM clumping factor of , we find that the observed quasar population
cannot provide enough photons to ionize the IGM at \%
confidence. Quasars may still provide a significant fraction of the required
photons, although much larger samples of faint quasars are needed for more
stringent constraints on the quasar contribution to reionization.Comment: 20 pages, 12 figures, Accepted for publication in The Astrophysical
Journa
Model Factory for Additive Manufacturing of Mechatronic Products: Interconnecting World-class Technology Partnerships with Leading AM Players
AbstractThe additive manufacturing (AM) model factory's aim is to establish a leading-edge learning academy for the digital and generative production of innovative mechatronic products, where the complete value chain is integrated on a single site. Short courses and deep dives enable easier access to the state of the art technologies and increase the awareness for their potentials. Anchored in key industries such as automotive, aerospace, and medical by major OEMs and regional SMEs, the AM model factory cooperates with world-class partners and leading market players. This paper displays the model factory's setup, selected technologies, exemplary courses, and benefits
Analysis of Neptune's 2017 Bright Equatorial Storm
We report the discovery of a large (8500 km diameter) infrared-bright
storm at Neptune's equator in June 2017. We tracked the storm over a period of
7 months with high-cadence infrared snapshot imaging, carried out on 14 nights
at the 10 meter Keck II telescope and 17 nights at the Shane 120 inch reflector
at Lick Observatory. The cloud feature was larger and more persistent than any
equatorial clouds seen before on Neptune, remaining intermittently active from
at least 10 June to 31 December 2017. Our Keck and Lick observations were
augmented by very high-cadence images from the amateur community, which
permitted the determination of accurate drift rates for the cloud feature. Its
zonal drift speed was variable from 10 June to at least 25 July, but remained a
constant m s from 30 September until at least 15
November. The pressure of the cloud top was determined from radiative transfer
calculations to be 0.3-0.6 bar; this value remained constant over the course of
the observations. Multiple cloud break-up events, in which a bright cloud band
wrapped around Neptune's equator, were observed over the course of our
observations. No "dark spot" vortices were seen near the equator in HST imaging
on 6 and 7 October. The size and pressure of the storm are consistent with
moist convection or a planetary-scale wave as the energy source of convective
upwelling, but more modeling is required to determine the driver of this
equatorial disturbance as well as the triggers for and dynamics of the observed
cloud break-up events.Comment: 42 pages, 14 figures, 6 tables; Accepted to Icaru
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