2,869 research outputs found
The Dearth of UV-Bright Stars in M32: Implications for Stellar Evolution Theory
Using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope,
we have obtained deep far-ultraviolet images of the compact elliptical galaxy
M32. When combined with earlier near-ultraviolet images of the same field,
these data enable the construction of an ultraviolet color-magnitude diagram of
the hot horizontal branch (HB) population and other hot stars in late phases of
stellar evolution. We find few post-asymptotic giant branch (PAGB) stars in the
galaxy, implying that these stars either cross the HR diagram more rapidly than
expected, and/or that they spend a significant fraction of their time
enshrouded in circumstellar material. The predicted luminosity gap between the
hot HB and its AGB-Manque (AGBM) progeny is less pronounced than expected,
especially when compared to evolutionary tracks with enhanced helium
abundances, implying that the presence of hot HB stars in this metal-rich
population is not due to (Delta Y)/(Delta Z) > 4. Only a small fraction (~2%)
of the HB population is hot enough to produce significant UV emission, yet most
of the UV emission in this galaxy comes from the hot HB and AGBM stars,
implying that PAGB stars are not a significant source of UV emission even in
those elliptical galaxies with a weak UV excess.Comment: Accepted for publication in The Astrophysical Journal. Latex, 18
pages, 18 black & white figures, in emulate-ApJ format. Figures 11 & 16 have
been degraded due to size constraints; the high-quality version of the paper
is at http://www.stsci.edu/~tbrown/research/m32fuv.pd
Cohesion, team mental models, and collective efficacy: Towards an integrated framework of team dynamics in sport
A nomological network on team dynamics in sports consisting of a multi-framework perspective is introduced and tested. The aim was to explore the interrelationship among cohesion, team mental models (TMM), collective-efficacy (CE), and perceived performance potential (PPP). Three hundred and forty college-aged soccer players representing 17 different teams (8 female and 9 male) participated in the study. They responded to surveys on team cohesion, TMM, CE and PPP. Results are congruent with the theoretical conceptualization of a parsimonious view of team dynamics in sports. Specifically, cohesion was found to be an exogenous variable predicting both TMM and CE beliefs. TMM and CE were correlated and predicted PPP, which in turn accounted for 59% of the variance of objective performance scores as measured by teams’ season record. From a theoretical standpoint, findings resulted in a parsimonious view of team dynamics, which may represent an initial step towards clarifying the epistemological roots and nomological network of various team-level properties. From an applied standpoint, results suggest that team expertise starts with the establishment of team cohesion. Following the establishment of cohesiveness, teammates are able to advance team-related schemas and a collective sense of confidence. Limitations and key directions for future research are outlined
Observation of enhanced rate coefficients in the H + H H + H reaction at low collision energies
The energy dependence of the rate coefficient of the H reaction has been measured in the range of
collision energies between K and
mK. A clear deviation of the rate coefficient from the value expected on the
basis of the classical Langevin-capture behavior has been observed at collision
energies below K, which is attributed to the joint
effects of the ion-quadrupole and Coriolis interactions in collisions involving
ortho-H molecules in the rotational level, which make up 75% of the
population of the neutral H molecules in the experiments. The experimental
results are compared to very recent predictions by Dashevskaya, Litvin, Nikitin
and Troe (J. Chem. Phys., in press), with which they are in agreement.Comment: 14 pages, 3 figure
Casimir Energy of a Spherical Shell
The Casimir energy for a conducting spherical shell of radius is computed
using a direct mode summation approach. An essential ingredient is the
implementation of a recently proposed method based on Cauchy's theorem for an
evaluation of the eigenfrequencies of the system. It is shown, however, that
this earlier calculation uses an improper set of modes to describe the waves
exterior to the sphere. Upon making the necessary corrections and taking care
to ensure that no mathematically ill-defined expressions occur, the technique
is shown to leave numerical results unaltered while avoiding a longstanding
criticism raised against earlier calculations of the Casimir energy.Comment: LaTeX, 14 pages, 1 figur
Post-translational Claisen Condensation and Decarboxylation en Route to the Bicyclic Core of Pantocin A
Pantocin A (PA) is a member of the growing family of ribosomally encoded and post-translationally modified peptide natural products (RiPPs). PA is much smaller than most known RiPPs, a tripeptide with a tight bicyclic core that appears to be cleaved from the middle of a larger 30-residue precursor peptide. We show here that the enzyme PaaA catalyzes the double dehydration and decarboxylation of two glutamic acid residues in the 30-residue precursor PaaP. Further truncates of PaaP leader and follower peptide sequences demonstrate the different impacts of these two regions on PaaA-mediated tailoring and delineate an essential role for the follower sequence in the decarboxylation step. The crystal structure of apo PaaA is reported, allowing identification of structural features that set PaaA apart from other homologous enzymes that typically do not catalyze such extended post-translational chemistry. Together, these data reveal how additional chemistry can be extracted from a ubiquitous enzyme family toward ribosomally derived peptide natural product biosynthesis and suggest that more examples of such enzymes likely exist in untapped genomic space
Anisotropic static solutions in modelling highly compact bodies
Einstein field equations for anisotropic spheres are solved and exact
interior solutions obtained. This paper extends earlier treatments to include
anisotropic models which accommodate a wider variety of physically viable
energy densities. Two classes of solutions are possible. The first class
contains the limiting case for the energy density which
arises in many astrophysical applications. In the second class the singularity
at the center of the star is not present in the energy density. The models
presented in this paper allow for increasing and decreasing profiles in the
behavior of the energy density.Comment: 9 pages, to appear in Pramana - J. Phy
On Possibilities of Studying of Supernova Neutrinos at BAKSAN
We consider the possibilities of studying a supernova collapse neutrino burst
at Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy
of Sciences) using the prposed 5-kt target-mass liquid scintillation
spectrometer. Attention is given to the influence of mixing angle
on the expected rates and spectra of neutrino events
HUT observations of carbon monoxide in the coma of Comet Levy (1990c)
Observations of comet Levy (1990c) were made with the Hopkins Ultraviolet Telescope during the Astro-1 Space Shuttle mission on 10 Dec. 1990. The spectrum, covering the wavelength range 415 to 1850 A at a spectral emission of 3 A (in first order), shows the presence of carbon monoxide and atomic hydrogen, carbon, and sulfur in the coma. Aside from H I Lyman-beta, no cometary features are detected below 1200 A, although cometary O I and O II would be masked by the same emissions present in the day airglow spectrum. The 9.4 x 116 arcsec aperture corresponds to 12,000 x 148,000 km at the comet. The derived production rate of CO relative to water, 0.13 + or - 0.02, compared with the same ratio derived from IUE observations (made in Sep. 1990) which sample a much smaller region of the coma, 0.04 + or - 0.01, suggests the presence of an extended source of CO, as was found in comet Halley. Upper limits on Ne and Ar abundance are within an order of magnitude or solar abundances
Comparative Direct Analysis of Type Ia Supernova Spectra. IV. Postmaximum
A comparative study of optical spectra of Type Ia supernovae (SNe Ia)
obtained near 1 week, 3 weeks, and 3 months after maximum light is presented.
Most members of the four groups that were defined on the basis of maximum light
spectra in Paper II (core normal, broad line, cool, and shallow silicon)
develop highly homogeneous postmaximum spectra, although there are interesting
exceptions. Comparisons with SYNOW synthetic spectra show that most of the
spectral features can be accounted for in a plausible way. The fits show that 3
months after maximum light, when SN Ia spectra are often said to be in the
nebular phase and to consist of forbidden emission lines, the spectra actually
remain dominated by resonance scattering features of permitted lines, primarily
those of Fe II. Even in SN 1991bg, which is said to have made a very early
transition to the nebular phase, there is no need to appeal to forbidden lines
at 3 weeks postmaximum, and at 3 months postmaximum the only clear
identification of a forbidden line is [Ca II] 7291, 7324. Recent studies of SN
Ia rates indicate that most of the SNe Ia that have ever occurred have been
"prompt" SNe Ia, produced by young (100,000,000 yr) stellar populations, while
most of the SNe Ia that occur at low redshift today are "tardy", produced by an
older (several Gyrs) population. We suggest that the shallow silicon SNe Ia
tend to be the prompt ones.Comment: Accepted by PAS
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