5,173 research outputs found
Integrated Light 2MASS IR Photometry of Galactic Globular Clusters
We have mosaiced 2MASS images to derive surface brightness profiles in JHK
for 104 Galactic globular clusters. We fit these with King profiles, and show
that the core radii are identical to within the errors for each of these IR
colors, and are identical to the core radii at V in essentially all cases. We
derive integrated light colors V-J, V-H, V-K_s, J-H and J-K_s for these
globular clusters. Each color shows a reasonably tight relation between the
dereddened colors and metallicity. Fits to these are given for each color. The
IR--IR colors have very small errors due largely to the all-sky photometric
calibration of the 2MASS survey, while the V-IR colors have substantially
larger uncertainties. We find fairly good agreement with measurements of
integrated light colors for a smaller sample of Galactic globular clusters by
Aaronson, Malkan & Kleinmann from 1977. Our results provide a calibration for
the integrated light of distant single burst old stellar populations from very
low to Solar metallicities. A comparison of our dereddened measured colors with
predictions from several models of the integrated light of single burst old
populations shows good agreement in the low metallicity domain for V-K_s
colors, but an offset at a fixed [Fe/H] of ~0.1 mag in J-K_s, which we ascribe
to photometric system transformation issues. Some of the models fail to
reproduce the behavior of the integrated light colors of the Galactic globular
clusters near Solar metallicity.Comment: Accepted for publication in the A
Comparison of an h- and hp-Adaptive Finite Element Solver for Chemo-Mechanically Coupled Battery Electrode Particles
Numerical investigations of mechanical stresses for phase transforming battery electrode materials on the particle scale are computationally highly demanding. The limitations are mainly induced by the strongly varying spatial and temporal scales of the underlying phase field model, which require an ultra fine mesh and time resolution, however, solely at specific stages in space and time. To overcome these numerical difficulties we present a general-purpose space and time adaptive solution algorithm based on an hp-adaptive finite element method and a variable-step, variable-order time integrator. At the example of a chemo-mechanical electrode particle model we demonstrate the computational savings gained by the hp-adaptivity. In particular, we compare the results to an h-adaptive finite element method and show the reduction of computational complexity
CCD Photometry of Galactic Globular Clusters. IV. The NGC 1851 RR Lyraes
The variable star population of the galactic globular cluster NGC 1851
(C0512-400) has been studied by CCD photometry, from observations made in the
B, V, and I bands during 1993-4. Light curves are presented for 29 variables,
seven of which are new discoveries. The behavior of the RR lyraes in the
period-temperature diagram appears normal when compared to clusters which
bracket the NGC 1851 metallicity. Reddening and metallicity are re-evaluated,
with no compelling evidence to change from accepted values. Photometry for
stars within an annulus with radii 80 and 260 arcsec agrees to better than 0.02
mag in all colors with extensive earlier photometry, to at least V = 18.5.
Instability strip boundary positions for several clusters shows a trend for the
red boundary to move to redder colors as the metallicity increases.Comment: 29 pages, 9 figures, accepted by A.
Theoretical models for classical Cepheids. VIII. Effects of helium and heavy elements abundance on the Cepheid distance scale
Previous nonlinear fundamental pulsation models for classical Cepheids with
metal content Z <= 0.02 are implemented with new computations at super-solar
metallicity (Z=0.03, 0.04) and selected choices of the helium-to-metal
enrichment ratio DeltaY/Delta Z. On this basis, we show that the location into
the HR diagram of the Cepheid instability strip is dependent on both metal and
helium abundance, moving towards higher effective temperatures with decreasing
the metal content (at fixed Y) or with increasing the helium content (at fixed
Z). The contributions of helium and metals to the predicted Period-Luminosity
and Period-Luminosity-Color relations are discussed, as well as the
implications on the Cepheid distance scale. Based on these new results, we
finally show that the empirical metallicity correction suggested by Cepheid
observations in two fields of the galaxy M101 may be accounted for, provided
that the adopted helium-to-metal enrichment ratio is reasonably high (Delta
Y/Delta Z ~ 3.5).Comment: 23 pages, including 6 postscript figures, accepted for publication on
Ap
The shape of the Red Giant Branch Bump as a diagnostic of partial mixing processes in low-mass stars
We suggest to use the shape of the Red Giant Branch (RGB) Bump in metal-rich
globular clusters as a diagnostic of partial mixing processes between the base
of the convective envelope and the H-burning shell. The Bump located along the
differential luminosity function of cluster RGB stars is a key observable to
constrain the H-profile inside these structures. In fact, standard evolutionary
models that account for complete mixing in the convective unstable layers and
radiative equilibrium in the innermost regions do predict that the first
dredge-up lefts over a very sharp H-discontinuity at the bottom of the
convective region. Interestingly enough we found that both atomic diffusion and
a moderate convective overshooting at the base of the convective region
marginally affects the shape of the RGB Bump in the differential Luminosity
Function (LF). As a consequence, we performed several numerical experiments to
estimate whether plausible assumptions concerning the smoothing of the
H-discontinuity, due to the possible occurrence of extra-mixing below the
convective boundary, affects the shape of the RGB Bump. We found that the
difference between the shape of RGB Bump predicted by standard and by smoothed
models can be detected if the H-discontinuity is smoothed over an envelope
region whose thickness is equal or larger than 0.5 pressure scale heights.
Finally, we briefly discuss the comparison between theoretical predictions and
empirical data in metal-rich, reddening free Galactic Globular Clusters (GGCs)
to constrain the sharpness of the H-profile inside RGB stars.Comment: 15 pages, 8 postscript figures, ApJ in pres
Artifacts at 4.5 and 8.0 um in Short Wavelength Spectra from the Infrared Space Observatory
Spectra from the Short Wavelength Spectrometer (SWS) on ISO exhibit artifacts
at 4.5 and 8 um. These artifacts appear in spectra from a recent data release,
OLP 10.0, as spurious broad emission features in the spectra of stars earlier
than ~F0, such as alpha CMa. Comparison of absolutely calibrated spectra of
standard stars to corresponding spectra from the SWS reveals that these
artifacts result from an underestimation of the strength of the CO and SiO
molecular bands in the spectra of sources used as calibrators by the SWS.
Although OLP 10.0 was intended to be the final data release, these findings
have led to an additional release addressing this issue, OLP 10.1, which
corrects the artifacts.Comment: 14 pages, AASTex, including 5 figures. Accepted by ApJ Letter
Simulation of the Deformation for Cycling Chemo-Mechanically Coupled Battery Active Particles with Mechanical Constraints
Next-generation lithium-ion batteries with silicon anodes have positive characteristics due to higher energy densities compared to state-of-the-art graphite anodes. However, the large volume expansion of silicon anodes can cause high mechanical stresses, especially if the battery active particle cannot expand freely. In this article, a thermodynamically consistent continuum model for coupling chemical and mechanical effects of electrode particles is extended by a change in the boundary condition for the displacement via a variational inequality. This switch represents a limited enlargement of the particle swelling or shrinking due to lithium intercalation or deintercalation in the host material, respectively. For inequality constraints as boundary condition a smaller time step size is need as well as a locally finer mesh. The combination of a primal-dual active set algorithm, interpreted as semismooth Newton method, and a spatial and temporal adaptive algorithm allows the efficient numerical investigation based on a finite element method. Using the example of silicon, the chemical and mechanical behavior of one- and two-dimensional representative geometries for a charge-discharge cycle is investigated. Furthermore, the efficiency of the adaptive algorithm is demonstrated. It turns out that the size of the gap has an significant influence on the maximal stress value and the slope of the increase. Especially in two dimension, the obstacle can cause an additional region with a lithium-poor phase
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