77 research outputs found
On the Dynamic Stability of Cool Supergiant Atmospheres
We have developed a new formalism to compute the thermodynamic coefficient
Gamma1 in the theory of stellar and atmospheric stability. We generalize the
classical derivation of the first adiabatic index, which is based on the
assumption of thermal ionization and equilibrium between gas and radiation
temperature, towards an expression which incorporates photo-ionization due to
radiation with a temperature T_rad different from the local kinetic gas
temperature.Our formalism considers the important non-LTE conditions in the
extended atmospheres of supergiant stars. An application to the Kurucz grid of
cool supergiant atmospheres demonstrates that models with T_rad =~ T_eff
between 6500 K and 7500 K become most unstable against dynamic perturbations,
according to Ledoux' stability integral . This results from Gamma1 and
acquiring very low values, below 4/3, throughout the entire stellar
atmosphere, which causes very high gas compression ratios around these
effective temperatures. Based on detailed NLTE-calculations, we discuss
atmospheric instability of pulsating massive yellow supergiants, like the
hypergiant rho Cas (Ia+), which exist in the extension of the Cepheid
instability strip, near the Eddington luminosity limit.Comment: 54 pages including figures and the Appendix, 7 figures, Accepted for
The Astrophysical Journal, Main Journal, 558, Sept. 200
Interaction Between Convection and Pulsation
This article reviews our current understanding of modelling convection
dynamics in stars. Several semi-analytical time-dependent convection models
have been proposed for pulsating one-dimensional stellar structures with
different formulations for how the convective turbulent velocity field couples
with the global stellar oscillations. In this review we put emphasis on two,
widely used, time-dependent convection formulations for estimating pulsation
properties in one-dimensional stellar models. Applications to pulsating stars
are presented with results for oscillation properties, such as the effects of
convection dynamics on the oscillation frequencies, or the stability of
pulsation modes, in classical pulsators and in stars supporting solar-type
oscillations.Comment: Invited review article for Living Reviews in Solar Physics. 88 pages,
14 figure
How might acupuncture work? A systematic review of physiologic rationales from clinical trials
BACKGROUND: Scientific interest in acupuncture has led numerous investigators to conduct clinical trials to test the efficacy of acupuncture for various conditions, but the mechanisms underlying acupuncture are poorly understood. METHODS: The author conducted a PubMed search to obtain a fair sample of acupuncture clinical trials published in English in 2005. Each article was reviewed for a physiologic rationale, as well as study objectives and outcomes, experimental and control interventions, country of origin, funding sources and journal type. RESULTS: Seventy-nine acupuncture clinical trials were identified. Twenty-six studies (33%) offered no physiologic rationale. Fifty-three studies (67%) posited a physiologic basis for acupuncture: 33 (62% of 53) proposed neurochemical mechanisms, 2 (4%) segmental nervous system effects, 6 (11%) autonomic nervous system regulation, 3 (6%) local effects, 5 (9%) effects on brain function and 5 (9%) other effects. No rationale was proposed for stroke; otherwise having a rationale was not associated with objective, positive or negative findings, means of intervention, country of origin, funding source or journal type. The dominant explanation for how acupuncture might work involves neurochemical responses and is not reported to be dependent on treatment objective, specific points, means or method of stimulation. CONCLUSION: Many acupuncture trials fail to offer a meaningful rationale, but proposing a rationale can help investigators to develop and test a causal hypothesis, choose an appropriate control and rule out placebo effects. Acupuncture may stimulate self-regulatory processes independent of the treatment objective, points, means or methods used; this would account for acupuncture's reported benefits in so many disparate pathologic conditions
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