21 research outputs found
Enhancement of the helium resonance lines in the solar atmosphere by suprathermal electron excitation II: non-Maxwellian electron distributions
In solar EUV spectra the He I and He II resonance lines show unusual
behaviour and have anomalously high intensities compared with other transition
region lines. The formation of the helium resonance lines is investigated
through extensive non-LTE radiative transfer calculations. The model
atmospheres of Vernazza, Avrett & Loeser are found to provide reasonable
matches to the helium resonance line intensities but significantly
over-estimate the intensities of other transition region lines. New model
atmospheres have been developed from emission measure distributions derived by
Macpherson & Jordan, which are consistent with SOHO observations of transition
region lines other than those of helium. These models fail to reproduce the
observed helium resonance line intensities by significant factors. The
possibility that non-Maxwellian electron distributions in the transition region
might lead to increased collisional excitation rates in the helium lines is
studied. Collisional excitation and ionization rates are re-computed for
distribution functions with power law suprathermal tails which may form by the
transport of fast electrons from high temperature regions. Enhancements of the
helium resonance line intensities are found, but many of the predictions of the
models regarding line ratios are inconsistent with observations. These results
suggest that any such departures from Maxwellian electron distributions are not
responsible for the helium resonance line intensities.Comment: 23 pages, 11 figures, accepted to appear in MNRAS, LaTeX uses mn.st
Molecular chemistry and the missing mass problem in PNe
Detections of molecular lines, mainly from H2$ and CO, reveal molecular
material in planetary nebulae. Observations of a variety of molecules suggest
that the molecular composition in these objects differs from that found in
interstellar clouds or in circumstellar envelopes. The success of the models,
which are mostly devoted to explain molecular densities in specific planetary
nebulae, is still partial, however. The present study aims at identifying the
influence of stellar and nebular properties on the molecular composition of
planetary nebulae by means of chemical models. A comparison of theoretical
results with those derived from the observations may provide clues to the
conditions that favor the presence of a particular molecule. A self-consistent
photoionization numerical code was adapted to simulate cold molecular regions
beyond the ionized zone. The code was used to obtain a grid of models and the
resulting column densities are compared with those inferred from observations.
Our models show that the inclusion of an incident flux of X-rays is required to
explain the molecular composition derived for planetary nebulae. We also obtain
a more accurate relation for the N(CO)/N(H2) ratio in these objects. Molecular
masses obtained by previous works in the literature were then recalculated,
showing that these masses can be underestimated by up to three orders of
magnitude. We conclude that the problem of the missing mass in planetary
nebulae can be solved by a more accurate calculation of the molecular mass.Comment: 12 pages, 11 figure
A new speech analysis system: ASSESS (Automatic Statistical Summary of Elementary Speech Structures)
Fibroblast growth factor-2 induced chondrocyte cluster formation in experimentally wounded articular cartilage is blocked by soluble Jagged-1
SummaryIntroductionBasic fibroblast growth factor (FGF2) is a mitogen for articular chondrocytes. Cell death frequently occurs upon cartilage wounding and is evident during the progression of osteoarthritis. We hypothesised that incubation of wounded articular cartilage with exogenously added FGF2 would enhance cartilage repair, replacing dead cells through increased cell proliferation.MethodsArticular cartilage from the metacarapalphalangeal joint of immature bovine steers was wounded in situ, then incubated in vitro in the continual presence or absence of FGF2. Cellular proliferation was expressed as a ratio of cell density of a fixed area between wounded and adjacent cartilage. Immunolabelling revealed the incorporation of bromodeoxyuridine and localisation of collagen type VI and Notch1 epitopes. γ-secretase inhibitor N-[N-(3,5-Difluorophenacetyl-l-alanyl)]-S-phenylglycine t-butyl ester and soluble Jagged1 ligand (sJ1) were used to analyse the function of Notch signalling in this wound model.ResultsFGF2 induced cellular proliferation at the margins of wounded articular cartilage, where proliferative chondrocytes adopted a cluster configuration. Collagen type VI protein was expressed by chondrocytes in clusters, as was Notch1. Cellular proliferation was not affected by inhibition of γ-secretase dependent Notch1 signalling. Binding of sJ1 to Notch1 receptors in FGF2 treated cartilage inhibited proliferation.ConclusionAddition of FGF2 induces rapid chondrocyte proliferation in wounded cartilage, chondrocytes adopt a cluster morphology and also express Notch1. Binding of sJ1 to Notch1 causes apoptosis overriding a proliferative response. This study may shed some light on the significance of increased Notch1 expression and its localisation in chondrocyte clusters in osteoarthritic cartilage