UBVI and Ha Photometry of the h & chi Persei cluster

UBVI and Ha photometry is presented for 17319 stars in vicinity of the young double cluster h & chi Persei. Our photometry extends over a 37arcmin x 1arcdeg field centered on the association. We construct reddening contours within the imaged field. We find that the two clusters share a common distance modulus of 11.75$\pm$0.05 and ages of log age(yr) = 7.1$\pm$0.1. From the V-Ha colour, a measure of the Ha emission strength, we conduct a survey for emission line objects within the association. We detect a sample of 33 Be stars, 8 of which are new detections. We present a scenario of evolutionary enhancement of the Be phenomenon to account for the peak in Be fraction towards the top of the main-sequence in the population of h & chi Persei and similar young clusters.Comment: 20 pages, 9 figures, AJ Jul

Abundances of metal-weak thick-disc candidates

High resolution spectra of 5 candidate metal-weak thick-disc stars suggested by Beers & Sommer-Larsen (1995) are analyzed to determine their chemical abundances. The low abundance of all the objects has been confirmed with metallicity reaching [Fe/H]=-2.9. However, for three objects, the astrometric data from the Hipparcos catalogue suggests they are true halo members. The remaining two, for which proper-motion data are not available, may have disc-like kinematics. It is therefore clear that it is useful to address properties of putative metal-weak thick-disc stars only if they possess full kinematic data. For CS 22894-19 the abundance pattern similar to those of typical halo stars is found, suggesting that chemical composition is not a useful discriminant between thick-disc and halo stars. CS 29529-12 is found to be C enhanced with [C/Fe]=+1.0; other chemical peculiarities involve the s process elements: [Sr/Fe]=-0.65 and [Ba/Fe]=+0.62, leading to a high [Ba/Sr] considerably larger than what is found in more metal-rich carbon-rich stars, but similar to LP 706-7 and LP 625-44 discussed by Norris et al (1997a). Hipparcos data have been used to calculate the space velocities of 25 candidate metal-weak thick-disc stars, thus allowing us to identify 3 bona fide members, which support the existence of a metal-poor tail of the thick-disc, at variance with a claim to the contrary by Ryan & Lambert (1995).Comment: to be published in MNRA

Complex Dynamics and Multistability in a Damped Harmonic Oscillator with Delayed Negative Feedback

A center manifold reduction and numerical calculations are used to demonstrate the presence of limit cycles, two-tori, and multistability in the damped harmonic oscillator with delayed negative feedback. This model is the prototype of a mechanical system operating with delayed feedback. Complex dynamics are thus seen to arise in very plausible and commonly occurring mechanical and neuromechanical feedback systems

Corticosteroid suppression of lipoxin A4 and leukotriene B4from alveolar macrophages in severe asthma

<p>Abstract</p> <p>Background</p> <p>An imbalance in the generation of pro-inflammatory leukotrienes, and counter-regulatory lipoxins is present in severe asthma. We measured leukotriene B₄(LTB₄), and lipoxin A₄(LXA₄) production by alveolar macrophages (AMs) and studied the impact of corticosteroids.</p> <p>Methods</p> <p>AMs obtained by fiberoptic bronchoscopy from 14 non-asthmatics, 12 non-severe and 11 severe asthmatics were stimulated with lipopolysaccharide (LPS,10 μg/ml) with or without dexamethasone (10^-6M). LTB₄and LXA₄were measured by enzyme immunoassay.</p> <p>Results</p> <p>LXA₄biosynthesis was decreased from severe asthma AMs compared to non-severe (p < 0.05) and normal subjects (p < 0.001). LXA₄induced by LPS was highest in normal subjects and lowest in severe asthmatics (p < 0.01). Basal levels of LTB₄were decreased in severe asthmatics compared to normal subjects (p < 0.05), but not to non-severe asthma. LPS-induced LTB₄was increased in severe asthma compared to non-severe asthma (p < 0.05). Dexamethasone inhibited LPS-induced LTB₄and LXA₄, with lesser suppression of LTB₄in severe asthma patients (p < 0.05). There was a significant correlation between LPS-induced LXA₄and FEV₁(% predicted) (r_s= 0.60; p < 0.01).</p> <p>Conclusions</p> <p>Decreased LXA₄and increased LTB₄generation plus impaired corticosteroid sensitivity of LPS-induced LTB₄but not of LXA₄support a role for AMs in establishing a pro-inflammatory balance in severe asthma.</p