4,491 research outputs found
A disk census for the nearest group of young stars: Mid-infrared observations of the TW Hydrae Association
A group of young, active stars in the vicinity of TW Hydrae has recently been
identified as a possible physical association with a common origin. Given its
proximity (50 pc), age (10 Myr) and abundance of binary systems,
the TW Hya Association is ideally suited to studies of diversity and evolution
of circumstellar disks. Here we present mid-infrared observations of 15
candidate members of the group, 11 of which have no previous flux measurements
at wavelengths longer than 2m. We report the discovery of a possible
10m excess in CD -337795, which may be due to a circumstellar
disk or a faint, as yet undetected binary companion. Of the other stars, only
TW Hya, HD 98800, Hen 3-600A, and HR 4796A -- all of which were detected by
IRAS -- show excess thermal emission. Our 10m flux measurements for the
remaining members of the Association are consistent with photospheric emission,
allowing us to rule out dusty inner disks. In light of these findings, we
discuss the origin and age of the TW Hya Association as well as implications
for disk evolution timescales.Comment: 10 pages and 1 PostScript figure, accepted for publication in The
Astrophysical Journal Letter
The radical cation of bacteriochlorophyll b. A liquid-phase endor and triple resonance study
The previous termradical cationnext term of bacterioehlorophyll b (BChl b) is investigated by ENDOR and TRIPLE resonance in liquid solution. The experimental hyperfine coupling constants, ten proton and three nitrogen couplings, are compared with the predictions from advanced molecular-orbital calculations (RHF INDO/SP). The detailed picture obtained of the spin density distribution is a prerequisite for the investigation of the primary electron donor previous termradical cationnext term in BChl b containing photosynthetic bacteria
Structural model correlation using large admissible perturbations incognate space
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76957/1/AIAA-10863-901.pd
Theoretical backgrounds of durability analysis by normalized equivalent stress functionals
Generalized durability diagrams and their properties are considered for a material under a multiaxial loading given by an arbitrary function of time. Material strength and durability under such loading are described in terms of durability, safety factor and normalized equivalent stress. Relations between these functionals are analysed. We discuss some material properties including time and load stability, self-degradation (ageing), and monotonic damaging. Phenomenological strength conditions are presented in terms of the normalized equivalent stress. It is shown that the damage based durability analysis is reduced to a particular case of such strength conditions. Examples of the reduction are presented for some known durability models. The approach is applicable to the strength and durability description at creep and impact loading and their combination
Families' social backgrounds matter : socio-economic factors, home learning and young children's language, literacy and social outcomes
Parental support with children's learning is considered to be one pathway through which socio-economic factors influence child competencies. Utilising a national longitudinal sample from the Millennium Cohort Study, this study examined the relationship between home learning and parents' socio-economic status and their impact on young children's language/literacy and socio-emotional competence. The findings consistently showed that, irrespective of socio-economic status, parents engaged with various learning activities (except reading) roughly equally. The socio-economic factors examined in this study, i.e., family income and maternal educational qualifications, were found to have a stronger effect on children's language/literacy than on social-emotional competence. Socio-economic disadvantage, lack of maternal educational qualifications in particular, remained powerful in influencing competencies in children aged three and at the start of primary school. For children in the first decade of this century in England, these findings have equity implications, especially as the socio-economic gap in our society widens
ESR, ENDOR and TRIPLE resonance studies of the primary donor radical cation P960+ in the photosynthetic bacterium Rhodopseudomonas viridis
The light-induced radical cation of the primary electron donor P960+âą in photosynthetic reaction centers from Rhodopseudomonas viridis has been investigated by ESR, ENDOR and TRIPLE techniques. Both the comparison with the cation radical of monomeric bacteriochlorophyll b (BChl b) and with molecular-orbital calculations performed on P960+âą using the results of an X-ray structure analysis, consistently show an asymmetric distribution of the unpaired electron over the two BChl b molecules which constitute P960+âą. The possible relevance of this result for the primary electron transfer step in the reaction center is briefly discussed
Evidence for ubiquitous carbon grain destruction in hot protostellar envelopes
Earth is deficient in carbon and nitrogen by up to orders of
magnitude compared with the Sun. Destruction of (carbon- and nitrogen-rich)
refractory organics in the high-temperature planet forming regions could
explain this deficiency. Assuming a refractory cometary composition for these
grains, their destruction enhances nitrogen-containing oxygen-poor molecules in
the hot gas (K) after the initial formation and sublimation of
these molecules from oxygen-rich ices in the warm gas (K). Using
observations of high-mass protostars with ALMA, we find that
oxygen-containing molecules (CHOH and HNCO) systematically show no
enhancement in their hot component. In contrast, nitrogen-containing,
oxygen-poor molecules (CHCN and CHCN) systematically show an
enhancement of a factor in their hot component, pointing to
additional production of these molecules in the hot gas. Assuming only thermal
excitation conditions, we interpret these results as a signature of destruction
of refractory organics, consistent with the cometary composition. This
destruction implies a higher C/O and N/O in the hot gas than the warm gas,
while, the exact values of these ratios depend on the fraction of grains that
are effectively destroyed. This fraction can be found by future chemical models
that constrain C/O and N/O from the abundances of minor carbon, nitrogen and
oxygen carriers presented here.Comment: Accepted for publication in ApJ Letter
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