1,277 research outputs found
The Distance to the M31 Globular Cluster System
The distance to the centroid of the M31 globular cluster system is determined
by fitting theoretical isochrones to the observed red-giant branches of
fourteen globular clusters in M31. The mean true distance modulus of the M31
globular clusters is found to be 24.47 +/- 0.07 mag. This is consistent with
distance modulii for M31 that have been obtained using other distance
indicators.Comment: 11 pages, 2 postscript figures, uses aaspp4.sty, to be published in
the May 1998 Astronomical Journa
An evaluation of the implementation of Georgia's Pre-k program: Report of the findings from the Georgia Early Childhood Study (2002-03)
After ten years, Georgia continues to lead the nation in providing full day, publicly subsidized Pre-K to four-year-olds whose parents choose to enroll them. In this report, we assess the extent to which differences in the way Pre-K is implemented affect children's development. Do teachers with higher levels of education have more positive impacts on children's development? Do teaching styles make a difference in terms of children's outcomes by the end of kindergarten? Do children taught using certain curricula fare better than those taught using others? Answers to questions such as these can assist Pre-K administrators in refining Georgia's program and inform those in other states who are developing or expanding their prekindergarten programs
Constant-pH simulations with the polarizable atomic multipole AMOEBA force field
Accurately predicting protein behavior across diverse pH environments remains a significant challenge in biomolecular simulations. Existing constant-pH molecular dynamics (CpHMD) algorithms are limited to fixed-charge force fields, hindering their application to biomolecular systems described by permanent atomic multipoles or induced dipoles. This work overcomes these limitations by introducing the first polarizable CpHMD algorithm in the context of the Atomic Multipole Optimized Energetics for Biomolecular Applications (AMOEBA) force field. Additionally, our implementation in the open-source Force Field X (FFX) software has the unique ability to handle titration state changes for crystalline systems including flexible support for all 230 space groups. The evaluation of constant-pH molecular dynamics (CpHMD) with the AMOEBA force field was performed on 11 crystalline peptide systems that span the titrating amino acids (Asp, Glu, His, Lys, and Cys). Titration states were correctly predicted for 15 out of the 16 amino acids present in the 11 systems, including for the coordination of Z
Surface Enhanced Second Harmonic Generation from Macrocycle, Catenane, and Rotaxane Thin Films: Experiments and Theory
Surface enhanced second harmonic generation (SE SHG) experiments on molecular structures, macrocycles, catenanes, and rotaxanes, deposited as monolayers and multilayers by vacuum sublimation on silver, are reported. The measurements show that the molecules form ordered thin films, where the highest degree of order is observed in the case of macrocycle monolayers and the lowest in the case of rotaxane multilayers. The second harmonic generation activity is interpreted in terms of electric field induced second harmonic (EFISH) generation where the electric field is created by the substrate silver atoms. The measured second order nonlinear optical susceptibility for a rotaxane thin film is compared with that obtained by considering only EFISH contribution to SHG intensity. The electric field on the surface of a silver layer is calculated by using the Delphi4 program for structures obtained with TINKER molecular mechanics/dynamics simulations. An excellent agreement is observed between the calculated and the measured SHG susceptibilities.
A self-filling microfluidic device for noninvasive and time-resolved single red blood cell experiments
Existing approaches to red blood cell (RBC) experiments on the single-cell level usually rely on chemical or physical manipulations that often cause difficulties with preserving the RBC's integrity in a controlled microenvironment. Here, we introduce a straightforward, self-filling microfluidic device that autonomously separates and isolates single RBCs directly from unprocessed human blood samples and confines them in diffusion-controlled microchambers by solely exploiting their unique intrinsic properties. We were able to study the photo-induced oxygenation cycle of single functional RBCs by Raman microscopy without the limitations typically observed in optical tweezers based methods. Using bright-field microscopy, our noninvasive approach further enabled the time-resolved analysis of RBC flickering during the reversible shape evolution from the discocyte to the echinocyte morphology. Due to its specialized geometry, our device is particularly suited for studying the temporal behavior of single RBCs under precise control of their environment that will provide important insights into the RBC's biomedical and biophysical properties
Protein design in a lattice model of hydrophobic and polar amino acids
A general strategy is described for finding which amino acid sequences have
native states in a desired conformation (inverse design). The approach is used
to design sequences of 48 hydrophobic and polar aminoacids on three-dimensional
lattice structures. Previous studies employing a sequence-space Monte-Carlo
technique resulted in the successful design of one sequence in ten attempts.
The present work also entails the exploration of conformations that compete
significantly with the target structure for being its ground state. The design
procedure is successful in all the ten cases.Comment: RevTeX, 12 pages, 1 figur
Steric constraints in model proteins
A simple lattice model for proteins that allows for distinct sizes of the
amino acids is presented. The model is found to lead to a significant number of
conformations that are the unique ground state of one or more sequences or
encodable. Furthermore, several of the encodable structures are highly
designable and are the non-degenerate ground state of several sequences. Even
though the native state conformations are typically compact, not all compact
conformations are encodable. The incorporation of the hydrophobic and polar
nature of amino acids further enhances the attractive features of the model.Comment: RevTex, 5 pages, 3 postscript figure
Age and Abundance Discrimination in Old Stellar Populations Using Mid-Ultraviolet Colors
The restframe mid-ultraviolet spectral region (2000-3200 A) is important in
analyzing the stellar populations of the "red envelope" systems observed at
high redshifts. Here, we explore the usefulness of the mid-UV for determining
ages and abundances of old populations. A mid-UV to optical/IR wavelength
baseline provides good separation of population components because the main
sequence turnoff dominates the integrated light between 2500 and 4000 A. We
find a six magnitude difference in the mid-UV continuum level over the
metallicity range -1.5 < log Z/Z_o < +0.5 and a comparable difference (per unit
log t) for ages in the range 4-16 Gyr. Logarithmic derivatives of mid-UV colors
with respect to age or metal abundance are 3-10 times larger than for the UBV
region. Most of the spectral information on old populations therefore resides
below 4000 A. We investigate the capability of UBV and mid-UV broad-band colors
to separately determine age and abundance, taking into account precision in the
color measurements. We find that the mid-UV improves resolution in (log t,log
Z) space by about a factor of 3 for a given observational precision.
Contamination by hot horizontal branch phases can seriously affect mid-UV
spectra, reaching over 80% in some cases. However, this is straightforward to
remove as long as far-UV measurements are available. Finally, we show that a 4
Gyr, solar abundance model based on empirical spectra provides an excellent fit
to the mid-UV spectrum of the E galaxy M32. This indicates that the poorer
results obtained from theoretical spectra arise from limitations of the
synthesis models for individual stars. [Condensed]Comment: 38 pages, 8 figures. Accepted by ApJ. Full resolution figures
available at: http://www.astro.virginia.edu/~rwo/miduv-fullresfigs.ps.g
The Sloan Digital Sky Survey Reverberation Mapping Project: Technical Overview
The Sloan Digital Sky Survey Reverberation Mapping project (SDSS-RM) is a
dedicated multi-object RM experiment that has spectroscopically monitored a
sample of 849 broad-line quasars in a single 7 deg field with the SDSS-III
BOSS spectrograph. The RM quasar sample is flux-limited to i_psf=21.7 mag, and
covers a redshift range of 0.1<z<4.5. Optical spectroscopy was performed during
2014 Jan-Jul dark/grey time, with an average cadence of ~4 days, totaling more
than 30 epochs. Supporting photometric monitoring in the g and i bands was
conducted at multiple facilities including the CFHT and the Steward Observatory
Bok telescopes in 2014, with a cadence of ~2 days and covering all lunar
phases. The RM field (RA, DEC=14:14:49.00, +53:05:00.0) lies within the CFHT-LS
W3 field, and coincides with the Pan-STARRS 1 (PS1) Medium Deep Field MD07,
with three prior years of multi-band PS1 light curves. The SDSS-RM 6-month
baseline program aims to detect time lags between the quasar continuum and
broad line region (BLR) variability on timescales of up to several months (in
the observed frame) for ~10% of the sample, and to anchor the time baseline for
continued monitoring in the future to detect lags on longer timescales and at
higher redshift. SDSS-RM is the first major program to systematically explore
the potential of RM for broad-line quasars at z>0.3, and will investigate the
prospects of RM with all major broad lines covered in optical spectroscopy.
SDSS-RM will provide guidance on future multi-object RM campaigns on larger
scales, and is aiming to deliver more than tens of BLR lag detections for a
homogeneous sample of quasars. We describe the motivation, design and
implementation of this program, and outline the science impact expected from
the resulting data for RM and general quasar science.Comment: 25 pages, submitted to ApJS; project website at http://www.sdssrm.or
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