3,690 research outputs found

### Cataloguing PL 4-manifolds by gem-complexity

We describe an algorithm to subdivide automatically a given set of PL
n-manifolds (via coloured triangulations or, equivalently, via
crystallizations) into classes whose elements are PL-homeomorphic. The
algorithm, implemented in the case n=4, succeeds to solve completely the
PL-homeomorphism problem among the catalogue of all closed connected PL
4-manifolds up to gem-complexity 8 (i.e., which admit a coloured triangulation
with at most 18 4-simplices). Possible interactions with the (not completely
known) relationship among different classification in TOP and DIFF=PL
categories are also investigated. As a first consequence of the above PL
classification, the non-existence of exotic PL 4-manifolds up to gem-complexity
8 is proved. Further applications of the tool are described, related to
possible PL-recognition of different triangulations of the K3-surface.Comment: 25 pages, 5 figures. Improvements suggested by the refere

### Computing Matveev's complexity via crystallization theory: the boundary case

The notion of Gem-Matveev complexity has been introduced within
crystallization theory, as a combinatorial method to estimate Matveev's
complexity of closed 3-manifolds; it yielded upper bounds for interesting
classes of such manifolds. In this paper we extend the definition to the case
of non-empty boundary and prove that for each compact irreducible and
boundary-irreducible 3-manifold it coincides with the modified Heegaard
complexity introduced by Cattabriga, Mulazzani and Vesnin. Moreover, via
Gem-Matveev complexity, we obtain an estimation of Matveev's complexity for all
Seifert 3-manifolds with base $\mathbb D^2$ and two exceptional fibers and,
therefore, for all torus knot complements.Comment: 27 pages, 14 figure

### PL 4-manifolds admitting simple crystallizations: framed links and regular genus

Simple crystallizations are edge-coloured graphs representing PL 4-manifolds
with the property that the 1-skeleton of the associated triangulation equals
the 1-skeleton of a 4-simplex. In the present paper, we prove that any
(simply-connected) PL $4$-manifold $M$ admitting a simple crystallization
admits a special handlebody decomposition, too; equivalently, $M$ may be
represented by a framed link yielding $\mathbb S^3$, with exactly $\beta_2(M)$
components ($\beta_2(M)$ being the second Betti number of $M$). As a
consequence, the regular genus of $M$ is proved to be the double of
$\beta_2(M)$. Moreover, the characterization of any such PL $4$-manifold by
$k(M)= 3 \beta_2(M)$, where $k(M)$ is the gem-complexity of $M$ (i.e. the
non-negative number $p-1$, $2p$ being the minimum order of a crystallization of
$M$) implies that both PL invariants gem-complexity and regular genus turn out
to be additive within the class of all PL $4$-manifolds admitting simple
crystallizations (in particular: within the class of all "standard"
simply-connected PL 4-manifolds).Comment: 14 pages, no figures; this is a new version of the former paper "A
characterization of PL 4-manifolds admitting simple crystallizations

### High Resolution Near-Infrared Spectra of Protostars

We present new high resolution (R = 21,000) near-infrared (2 microns)
spectroscopic observations of a sample of Class I and flat-spectrum
protostellar objects in the rho Ophiuchi dark cloud. None of the five Class I
spectra show CO v = 0 -- 2 absorption features, consistent with high K-band
continuum veilings, 4 <= r_k <= 20 and fast stellar rotation, assuming that the
underlying protostellar photospheres are of late spectral type, as is suggested
by the low luminosities of most of these objects. Two of the flat-spectrum
protostellar objects also show no absorption features and are likely to be
highly veiled. The remaining two flat-spectrum sources show weak, broad
absorptions which are consistent with an origin in quickly rotating (v sin i ~
50 km / s) late-type stellar photospheres which are also strongly veiled, r_k =
3 - 4. These observations provide further evidence that: 1)-Class I sources are
highly veiled at near-infrared wavelengths, confirming previous findings of
lower resolution spectroscopic studies; and 2)- flat-spectrum protostars rotate
more rapidly than classical T Tauri stars (Class II sources), supporting
findings from a recent high resolution spectroscopic study of other
flat-spectrum sources in this cloud. In addition our observations are
consistent with the high rotation rates derived for two of the Class I
protostellar objects in our sample from observations of variable hard X-ray
emission obtained with the ASCA satellite. These observations suggest that
certain Class I sources can rotate even more rapidly than flat-spectrum
protostars, near breakup velocity.Comment: 16 pages including 2 tables and 2 figures (AASTeX 5.x) to be
published in The Astronomical Journal July 200

### Supercritical Carbon Dioxide Treatment Of Natural Biomaterials For Tissue Engineering Applications

The number of people requiring an organ transplant in the United States has increased considerably over the past 25 years, but the number of organ donations has stagnated; over 8,000 people now die annually while awaiting a transplant or become too sick to receive one. Tissue engineering (TE), the design and production of artificial tissues and organs in vitro, has been proposed to alleviate this problem. Though synthetic polymers offer tunable mechanical and biochemical properties, natural biomaterials have recently garnered attention in TE for their high degree of biocompatibility and ability to direct cell proliferation and constructive tissue remodeling. Yet scaffold processing remains challenging and a need for novel treatment and fabrication methods still exists.
One underexplored method for creating TE scaffolds is treatment with supercritical fluids (SCFs). SCFs are appealing for treating biomaterials because of their desirable solvent properties; liquid-like densities and gas-like viscosities allow supercritical fluids to wet and penetrate matrices easily without damaging surface tension effects. Supercritical carbon dioxide (scCO2) is of particular interest. scCO2 is a non-toxic, non-flammable substance that is relatively inert and can be used to process biomaterials at physiologic temperatures and mild pressures. scCO2 treatment avoids organic solvents, does not leave cytotoxic residue, and has already been utilized in similar biomedical applications, including sterilization, pasteurization, biomolecule extraction, and removal of endotoxins, bioburden, and allergenic proteins.
Supercritical CO2 has been used in foaming of synthetic polymer scaffolds, but it is almost completely unexplored in treatment of natural biomaterials for TE. In this dissertation, the potential of scCO2 in natural biomaterial TE is extensively explored. Two commonly-studied natural TE scaffold biomaterials were examined: a single-component biomaterial, type I collagen, and a multi-component biomaterial, extracellular matrix (ECM) obtained by decellularization of porcine aorta. Both biomaterials were studied at the fundamental and applied level.
First, the chemical compatibility of collagen and liquid and scCO2 was assessed. Compatibility was determined based on changes in four biochemical properties: thermal stability, molecular weight, secondary structure, and overall appearance. For scCO2, no significant differences were observed, indicating chemical compatibility. Liquid CO2 treatment caused significant denaturing, though it was hypothesized that the apparent incompatibility may be a result of treatment conditions rather than total incompatibility.
After chemical compatibility between collagen and scCO2 was established, scCO2 was applied to crosslinked collagen films to extract residual glutaraldehyde after crosslinking. After 1 hr of scCO2/ethanol treatment, over 95% of residual glutaraldehyde was removed, reducing the concentration below 1 ppm. Differential scanning calorimetry analysis showed a high degree of crosslinking and a denaturation temperature of about 63Â°C both before and after scCO2 treatment. Tensile testing did reveal a significant increase in both stiffness and tensile strength caused by scCO2 treatment, likely resulting from dehydration caused by the ethanol additive. However, this dehydration is preventable and less disruptive than heat-based removal of residual glutaraldehyde.
Decellularized ECM is also commonly used as a TE scaffolds. Current decellularization methods often utilize chemical detergents, which are residually cytotoxic and can damage ECM composition and ultrastructure. scCO2 has been proposed as a decellularizing agent, but added ethanol severely dehydrates the matrix. The second half of this dissertation explores how scCO2 can decellularize a tissue without dehydrating it. To prevent dehydration, a novel presaturation method was developed where scCO2 and water are thoroughly mixed before treatment. Presaturation with water led to mass retention of over 99% in a model hydrogel and over 97% in porcine aorta during scCO2 treatment, compared to only 46% and 78%, respectively, when dry (pure) scCO2 was used, proving that dehydration during scCO2 treatment is easily prevented.
Finally, scCO2 was used to decellularize porcine aorta. Contrary to a previous report, scCO2 alone was unable to achieve complete cell removal, even with a polar additive. However, when an SDS pretreatment step was used, the same scCO2 treatment completely decellularized porcine aorta as indicated by histology and DNA quantitation. Presaturation of scCO2 with water maintained the hydration state of the matrix, better maintaining the mechanical properties of the native tissue.
This dissertation confirms the potential of supercritical CO2 as a processing method for naturally-derived biomaterial scaffolds. Further work can be performed to determine the efficacy of CO2 on different scaffold compositions and morphologies as well as decellularization of other tissue types. More complex treatments may also be possible, such as simultaneous sterilization and decellularization. These studies provide insight into the mechanisms and applications of scCO2 in TE and offer a springboard for impactful discoveries in the future

### VIRIS: A Visual-Infrared Imaging System for the Lick Observatory 1-M Telescope

We describe a system in use at the Lick Observatory 1-m Nickel telescope for
near-simultaneous imaging at optical and near-infrared wavelengths. The
combined availability of a CCD and a NICMOS-3 camera makes the system
well-suited for photometric monitoring from 0.5-2.2 microns of a variety of
astrophysical objects. Our science program thus far has concentrated on
studying variability trends in young stellar objects.Comment: 11 pages LaTex, 3 Postscript figure, Pub. Astr. Soc. Pac. 1998, in
pres

### A Companion Galaxy to the Post-Starburst Quasar UN J1025-0040

UN J1025-0040 is a quasar at z = 0.6344 that shows an extremely bright post
starburst population of age ~ 400 Myr (Brotherton et al. 1999). Images of UN
J1025-0040 show a nearly stellar object 4.2 arcseconds SSW of the quasar. We
present imaging and spectroscopy that confirm that this object is a companion
galaxy at redshift z = 0.6341. We estimate an age of ~ 800 Myr for the dominant
stellar population in the companion. The companion appears to be interacting
with the quasar host galaxy, and this interaction may have triggered both the
starburst and the quasar activity in UN J1025-0040.Comment: 8 pages plus 4 figures (2 postscript, 2 jpg). Postscript version
available at http://www.ifa.hawaii.edu/~canaguby/preprints.html To appear in
the January 2000 issue of A

### PG 1700+518 Revisited: Adaptive Optics Imaging and a Revised Starburst Age for the Companion

We present the results of adaptive-optics imaging of the z=0.2923 QSO PG
1700+518 in the J and H bands. The extension to the north of the QSO is clearly
seen to be a discrete companion with a well-defined tidal tail, rather than a
feature associated with the host galaxy of PG 1700+518 itself. On the other
hand, an extension to the southwest of the QSO (seen best in deeper, but
lower-resolution, optical images) does likely comprise tidal material from the
host galaxy. The SED derived from images in J, H, and two non-standard optical
bands indicates the presence of dust intermixed with the stellar component. We
use our previously reported Keck spectrum of the companion, the SED found from
the imaging data, and updated spectral-synthesis models to constrain the
stellar populations in the companion and to redetermine the age of the
starburst. While our best-fit age of 0.085 Gyr is nearly the same as our
earlier determination, the fit of the new models is considerably better. This
age is found to be remarkably robust with respect to different assumptions
about the nature of the older stellar component and the effects of dust.Comment: 11 pages; includes two eps figures. Latex (AASTEX). Two additional
figures in gif format. Postscript version including all figs. (424 kb) can be
obtained from http://www.ifa.hawaii.edu/~canaguby/preprints.html To appear in
ApJ. Letter

### Uniform random colored complexes

We present here random distributions on $(D+1)$-edge-colored, bipartite
graphs with a fixed number of vertices $2p$. These graphs are dual to
$D$-dimensional orientable colored complexes. We investigate the behavior of
quantities related to those random graphs, such as their number of connected
components or the number of vertices of their dual complexes, as $p \to
\infty$. The techniques involved in the study of these quantities also yield a
Central Limit Theorem for the genus of a uniform map of order $p$, as $p \to
\infty$.Comment: 36 pages, 9 figures, minor additions and correction

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