53 research outputs found
Stellar Populations and Variable Stars in the Core of the Globular Cluster M5
We report the discovery of a variable blue straggler in the core of the
globular cluster M5, based on a 12-hour long series of images obtained with the
Planetary Camera aboard the Hubble Space Telescope. In addition, we present the
light curves of 28 previously unknown or poorly studied large-amplitude
variable stars (all but one are RR Lyrae) in the cluster core. A (V, U-I)
color-magnitude diagram shows 24 blue stragglers within 2 core radii of the
cluster center. The blue straggler population is significantly more centrally
concentrated than the horizontal branch and red giant stars.Comment: 22 pages, 18 postscript figures, 1 postscript table, AJ, in press
(Feb 1998
Exploring the origin of neutron star magnetic field: magnetic properties of the progenitor OB stars
Ferrario & Wickramasinghe (2006) explored the hypothesis that the magnetic
fields of neutron stars are of fossil origin. In this context, they predicted
the field distribution of the progenitor OB stars, finding that 5 per cent of
main sequence massive stars should have fields in excess of 1kG. We have
carried out sensitive ESPaDOnS spectropolarimetric observations to search for
direct evidence of such fields in all massive B- and O-type stars in the Orion
Nebula Cluster star-forming region. We have detected unambiguous Stokes V
Zeeman signatures in spectra of three out of the eight stars observed (38%).
Using a new state-of-the-art Bayesian analysis, we infer the presence of strong
(kG), organised magnetic fields in their photospheres. For the remaining five
stars, we constrain any dipolar fields in the photosphere to be weaker than
about 200G. Statistically, the chance of finding three ~kG fields in a sample
of eight OB stars is quite low (less than 1%) if the predictions of Ferrario &
Wickramasinghe are correct. This implies that either the magnetic fields of
neutron stars are not of fossil origin, that the flux-evolution model of
Ferrario & Wickramasinghe is incomplete, or that the ONC has unusual magnetic
properties. We are undertaking a study of other young star clusters, in order
to better explore these possibilities.Comment: 40 Years of Pulsars conference: Millisecond Pulsars, Magnetars and
More. McGill University, Montreal, Canada, August 12-17, 2007. 5 pages, 4
figure
A Near-Infrared Survey of the Inner Galactic Plane for Wolf-Rayet Stars I. Methods and First Results: 41 New WR Stars
The discovery of new Wolf-Rayet (WR) stars in our Galaxy via large-scale
narrowband optical surveys has been severely limited by dust extinction. Recent
improvements in infrared technology have made narrowband-broadband imaging
surveys viable again. We report a new J, K and narrow-band imaging survey of
300 square degrees of the plane of the Galaxy, spanning 150 degrees in Galactic
longitude and reaching 1 degree above and below the Galactic plane. The survey
has a useful limiting magnitude of K = 15 over most of the observed Galactic
plane, and K = 14 within a few degrees of the Galactic center. Thousands of
emission line candidates have been detected. In spectrographic follow-ups of
173 WR star candidates we have discovered 41 new WR stars, 15 of type WN and 26
of type WC. Star subtype assignments have been confirmed with K band spectra,
and distances approximated using the method of spectroscopic parallax. A few of
the new WR stars are amongst the most distant known in our Galaxy. The
distribution of these new WR stars is seen to follow that of previously known
WR stars along the spiral arms of the Galaxy. Tentative radial velocities were
also measured for most of the new WR stars.Comment: 55 pages, 23 figures, 7 tables, accepted to Astronomical Journa
Wolf-Rayet Stars in M33 II: Optical Spectroscopy of emission-line stars in Giant Hii Regions
We present optical spectra of 14 emission-line stars in M33s giant HII
regions NGC 592, NGC 595 and NGC 604: five of them are known WR stars, for
which we present a better quality spectrogram, eight were WR candidates based
on narrow-band imagery and one is a serendipitous discovery. Spectroscopy
confirms the power of interference filter imagery to detect emission-line stars
down to an equivalent width of about 5 A in crowded fields. We have also used
archival HST/WFPC2 images to correctly identify emission-line stars in NGC 592
and NGC 588. emission-line stars in NGC 592 and NGC 588.Comment: 9 pages, 14 figures, revised version submitted to MNRA
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