The origin of runaway stars

Milli-arcsecond astrometry provided by Hipparcos and by radio observations makes it possible to retrace the orbits of some of the nearest runaway stars and pulsars to determine their site of origin. The orbits of the runaways AE Aurigae and mu Columbae and of the eccentric binary iota Orionis intersect each other about 2.5 Myr ago in the nascent Trapezium cluster, confirming that these runaways were formed in a binary-binary encounter. The path of the runaway star zeta Ophiuchi intersects that of the nearby pulsar PSR J1932+1059, about 1 Myr ago, in the young stellar group Upper Scorpius. We propose that this neutron star is the remnant of a supernova that occurred in a binary system which also contained zeta Oph, and deduce that the pulsar received a kick velocity of about 350 km/s in the explosion. These two cases provide the first specific kinematic evidence that both mechanisms proposed for the production of runaway stars, the dynamical ejection scenario and the binary-supernova scenario, operate in nature.Comment: 5 pages, including 2 eps-figures and 1 table, submitted to the ApJ Letters. The manuscript was typeset using aaste

Radial velocity measurements of B stars in the Scorpius-Centaurus association

We derive single-epoch radial velocities for a sample of 56 B-type stars members of the subgroups Upper Scorpius, Upper Centaurus Lupus and Lower Centaurus Crux of the nearby Sco-Cen OB association. The radial velocity measurements were obtained by means of high-resolution echelle spectra via analysis of individual lines. The internal accuracy obtained in the measurements is estimated to be typically 2-3 km/s, but depends on the projected rotational velocity of the target. Radial velocity measurements taken for 2-3 epochs for the targets HD120307, HD142990 and HD139365 are variable and confirm that they are spectroscopic binaries, as previously identified in the literature. Spectral lines from two stellar components are resolved in the observed spectra of target stars HD133242, HD133955 and HD143018, identifying them as spectroscopic binaries.Comment: accepted for publication in A&

On the Theory of Gamma Ray Bursts and Hypernovae: The Black Hole Soft X-ray Transient Sources

We show that a common evolutionary history can produce the black hole binaries in the Galaxy in which the black holes have masses of ~ 5-10 M_sun. In with low-mass, <~ 2.5 M_sun, ZAMS (zero age main sequence) companions, the latter remain in main sequence during the active stage of soft X-ray transients (SXTs), most of them being of K or M classification. In two intermediate cases, IL Lupi and Nova Scorpii with ZAMS ~ 2.5 M_sun companions the orbits are greatly widened because of large mass loss in the explosion forming the black hole, and whereas these companions are in late main sequence evolution, they are close to evolving. Binaries with companion ZAMS masses >~ 3 M_sun are initially "silent" until the companion begins evolving across the Herzsprung gap. We provide evidence that the narrower, shorter period binaries, with companions now in main sequence, are fossil remnants of gamma ray bursters (GRBs). We also show that the GRB is generally accompanied by a hypernova explosion (a very energetic supernova explosion). We further show that the binaries with evolved companions are good models for some of the ultraluminous X-ray sources (ULXs) recently seen by Chandra in other galaxies. The great regularity in our evolutionary history, especially the fact that most of the companions of ZAMS mass <~ 2.5 M_sun remain in main sequences as K or M stars can be explained by the mass loss in common envelope evolution to be Case C; i.g., to occur only after core He burning has finished. Since our argument for Case C mass transfer is not generally understood in the community, we add an appendix, showing that with certain assumptions which we outline we can reproduce the regularities in the evolution of black hole binaries by Case C mass transfer.Comment: 59 pages, 12 figures, review articl

Comparing the Job Satisfaction and Intention to Leave of Different Categories of Health Workers in Tanzania, Malawi, and South Africa.

Job satisfaction is an important determinant of health worker motivation, retention, and performance, all of which are critical to improving the functioning of health systems in low- and middle-income countries. A number of small-scale surveys have measured the job satisfaction and intention to leave of individual health worker cadres in different settings, but there are few multi-country and multi-cadre comparative studies. The objective of this study was to compare the job satisfaction and intention to leave of different categories of health workers in Tanzania, Malawi, and South Africa. We undertook a cross-sectional survey of a stratified cluster sample of 2,220 health workers, 564 from Tanzania, 939 from Malawi, and 717 from South Africa. Participants completed a self-administered questionnaire, which included demographic information, a 10-item job satisfaction scale, and one question on intention to leave. Multiple regression was used to identify significant predictors of job satisfaction and intention to leave. There were statistically significant differences in job satisfaction and intention to leave between the three countries. Approximately 52.1% of health workers in South Africa were satisfied with their jobs compared to 71% from Malawi and 82.6% from Tanzania (χ2=140.3, p<0.001). 18.8% of health workers in Tanzania and 26.5% in Malawi indicated that they were actively seeking employment elsewhere, compared to 41.4% in South Africa (χ2=83.5, p<0.001). The country differences were confirmed by multiple regression. The study also confirmed that job satisfaction is statistically related to intention to leave. We have shown differences in the levels of job satisfaction and intention to leave between different groups of health workers from Tanzania, Malawi, and South Africa. Our results caution against generalising about the effectiveness of interventions in different contexts and highlight the need for less standardised and more targeted HRH strategies than has been practised to date

Nova Sco and coalescing low mass black hole binaries as LIGO sources

Double neutron star binaries, analogous to the well known Hulse--Taylor pulsar PSR 1913+16, are guaranteed-to-exist sources of high frequency gravitational radiation detectable by LIGO. There is considerable uncertainty in the estimated rate of coalescence of such systems, with conservative estimates of ~1 per million years per galaxy, and optimistic theoretical estimates one or more magnitude larger. Formation rates of low-mass black hole-neutron star binaries may be higher than those of NS-NS binaries, and may dominate the detectable LIGO signal rate. We estimate the enhanced coalescence rate for BH-BH binaries due to weak asymmetric kicks during the formation of low mass black holes like Nova Sco, and find they may contribute significantly to the LIGO signal rate, possibly dominating the phase I detectable signals if the range of BH masses for which there is significant kick is broad enough. For a standard Salpeter IMF, assuming mild natal kicks, we project that the R6 merger rate of BH-BH systems is ~0.5, smaller than that of NS-NS systems. However, the higher chirp mass of these systems produces a signal nearly four times greater, on average, with a commensurate increase in search volume. The BH-BH coalescence channel considered here also predicts that a substantial fraction of BH-BH systems should have at least one component with near-maximal spin (a/M ~ 1).The waveforms produced by the coalescence of such a system should produce a clear spin signature, so this hypothesis could be directly tested by LIGO.Comment: 16 pages, LaTeX/AASTeX, 5 figure

Structure and Evolution of Nearby OB Associations

We present the first results of a comprehensive census of the stellar content of the nearby OB associations based on Hipparcos positions, proper motions and parallaxes for 12842 candidate member stars distributed over 21 fields on the sky. We use a new method to identify moving groups in these fields (see de Bruijne et al., these proceedings). Previously, astrometric membership in nearly all the nearby OB associations was known only for stars with spectral types earlier than B5. The Hipparcos measurements now allow us to identify members down to late F. This census provides a firm basis for studies of galactic and extragalactic star forming regions.Comment: 6 pages, 4 Postscript figures, LaTeX using venice97.sty, twocolumn.sty; to appear in Proceedings of the Hipparcos Venice '97 Symposium, ESA-SP 40

Evidence for Evolution Among Primordial Disks in the 5 Myr Old Upper Scorpius OB Association

Moderate-resolution, near-infrared spectra between 0.8 and 5.2 microns were obtained for 12 late-type (K0-M3) disk-bearing members of the ~5 Myr old Upper Scorpius OB association using SpeX on the NASA Infrared Telescope Facility. For most sources, continuum excess emission first becomes apparent between ~2.2 and 4.5 microns and is consistent with that produced by single-temperature blackbodies having characteristic temperatures ranging from ~500 to 1300 K. The near-infrared spectra for 5 of 12 Upper Scorpius sources exhibit Pa-gamma, Pa-beta and Br-gamma emission, indicators of disk accretion. Using a correlation between Pa-beta and Br-gamma emission line luminosity and accretion luminosity, mass accretion rates (Mdot) are derived for these sources that range from Mdot = 3.5 X 10^{-10} to 1.5 X 10^{-8} MSun per yr. Merging the SpeX observations with Spitzer Space Telescope mid-infrared (5.4-37.0 micron) spectroscopy and 24 and 70 micron broadband photometry, the observed spectral energy distributions are compared with those predicted by two-dimensional, radiative transfer accretion disk models. Of the 9 Upper Scorpius sources examined in this analysis, 3 exhibit spectral energy distributions that are most consistent with models having inner disk radii that substantially exceed their respective dust sublimation radii. The remaining Upper Scorpius members possess spectral energy distributions that either show significant dispersion among predicted inner disk radii or are best described by models having inner disk rims coincident with the dust sublimation radius.Comment: 35 pages, 5 figures, accepted for publication in the Astronomical Journa

XMM-Newton observation of PSR B2224+65 and its jet

We have investigated the pulsar PSR B2224+65 and its X-ray jet with XMM-Newton. Apart from the long X-ray jet which is almost perpendicular to the direction of proper motion, a putative extended feature at the pulsar position, which oriented in the opposite direction of the proper motion, is also suggested by this deep X-ray imaging. Non-detection of any coherent X-ray pulsation disfavors the magnetospheric origin of the X-rays observed from the position of PSR B2224+65 and hence suggest that the interpretation of pulsar wind nebula is more viable. We have also probed the origin of PSR B2224+65 and identified a runaway star, which possibly originated from the Cygnus OB9 association, as a candidate for the former binary companion of the neutron star's progenitor.Comment: 24 pages, 8 figures, 3 tables, accepted for publication in Ap