5,344 research outputs found
Advertisement for Trapper Nelson\u27s Indian Pack Boards
This advertisement shows the type of hiking materials that were popular before the advent of plastics and polymers. In her book Not Just Trees, Dr. Jane Claire Dirks-Edmunds writes about the texture of the cloth and materials she used during fieldwork on Saddleback Mountain in her early research with Dr. James A. Macnab at Linfield College. Dirks-Edmunds, a 1937 graduate of Linfield, graduated from the University of Illinois in 1941; she returned to teach in the Biology department at Linfield from 1941-1974
Multigraded Cayley-Chow forms
We introduce a theory of multigraded Cayley-Chow forms associated to
subvarieties of products of projective spaces. Two new phenomena arise: first,
the construction turns out to require certain inequalities on the dimensions of
projections; and second, in positive characteristic the multigraded Cayley-Chow
forms can have higher multiplicities. The theory also provides a natural
framework for understanding multifocal tensors in computer vision.Comment: 20 pages, 1 figur
Changing Views on Curves and Surfaces
Visual events in computer vision are studied from the perspective of
algebraic geometry. Given a sufficiently general curve or surface in 3-space,
we consider the image or contour curve that arises by projecting from a
viewpoint. Qualitative changes in that curve occur when the viewpoint crosses
the visual event surface. We examine the components of this ruled surface, and
observe that these coincide with the iterated singular loci of the coisotropic
hypersurfaces associated with the original curve or surface. We derive
formulas, due to Salmon and Petitjean, for the degrees of these surfaces, and
show how to compute exact representations for all visual event surfaces using
algebraic methods.Comment: 31 page
The impact of systematic uncertainties in stellar parameters on integrated spectra of stellar populations
In this paper we investigate a hitherto unexplored source of potentially
significant error in stellar population synthesis (SPS) models, caused by
systematic uncertainties associated with the three fundamental stellar
atmospheric parameters; effective temperature T_eff, surface gravity g, and
iron abundance [Fe/H]. All SPS models rely on calibrations of T_eff, logg and
[Fe/H] scales, which are implicit in stellar models, isochrones and synthetic
spectra, and are explicitly adopted for empirical spectral libraries. We assess
the effect of a mismatch in scales between isochrones and spectral libraries
(the two key components of SPS models) and quantify the effects on 23 commonly
used diagnostic line indices. We find that typical systematic offsets of 100K
in T_eff, 0.15 dex in [Fe/H] and/or 0.25 dex in logg significantly alter
inferred absolute ages of simple stellar populations (SSPs) and that in some
circumstances, relative ages also change. Offsets in T_eff, logg and [Fe/H]
scales for a scaled-solar SSP produce deviations from the model which can mimic
the effects of altering abundance ratios to non-scaled-solar chemical
compositions, and could also be spuriously interpreted as evidence for a more
complex population, especially when multiple-index or full-SED fitting methods
are used. We stress that the behavior we find can potentially affect any SPS
models, whether using full integrated spectra or fitting functions to determine
line strengths. We present measured offsets in 23 diagnostic line indices and
urge caution in the over-interpretation of line-index data for stellar
populations.Comment: 14 pages, including 4 figures and 3 tables. Accepted for publication
in Ap
Evidence For A Mild Steepening And Bottom-Heavy IMF In Massive Galaxies From Sodium And Titanium-Oxide Indicators
We measure equivalent widths (EW) - focussing on two unique features (NaI and
TiO2) of low-mass stars (<0.3M\odot) - for luminous red galaxy spectra from the
the Sloan Digital Sky Survey (SDSS) and X-Shooter Lens Survey (XLENS) in order
to study the low-mass end of the initial mass function (IMF). We compare these
EWs to those derived from simple stellar population models computed with
different IMFs, ages, [{\alpha}/Fe], and elemental abundances. We find that
models are able to simultaneously reproduce the observed NaD {\lambda}5895 and
Na I {\lambda}8190 features for lower-mass (\sim {\sigma}\ast) early-type
galaxies (ETGs) but deviate increasingly for more massive ETGs, due do strongly
mismatching NaD EWs. The TiO2 {\lambda}6230 and the Na I {\lambda}8190 features
together appear to be a powerful IMF diagnostic, with age and metallicity
effects orthogonal to the effect of IMF. We find that both features correlate
strongly with galaxy velocity dispersion. The XLENS ETG (SDSSJ0912+0029) and an
SDSS ETG (SDSSJ0041-0914) appear to require both an extreme dwarf-rich IMF and
a high sodium enhancement ([Na/Fe] = +0.4). In addition, lensing constraints on
the total mass of the XLENS system within its Einstein radius limit a
bottom-heavy IMF with a power-law slope to x \leq 3.0 at the 90% C.L. We
conclude that NaI and TiO features, in comparison with state-of-the-art SSP
models, suggest a mildly steepening IMF from Salpeter (dn/dm \propto m-x with x
= 2.35) to x \approx 3.0 for ETGs in the range {\sigma} = 200 - 335 km s-1.Comment: 5 pages, 2 figures, 2 table
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