Spectral Types of Field and Cluster O-Stars

The recent catalog of spectral types of Galactic O-type stars by Mai'z-Apella'niz et al. has been used to study the differences between the frequencies of various subtypes of O-type stars in the field, in OB associations and among runaway stars. At a high level of statistical significance the data show that O-stars in clusters and associations have earlier types (and hence presumably larger masses and/or younger ages) than those that are situated in the general field. Furthermore it is found that the distribution of spectral subtypes among runaway O-stars is indistinguishable from that among field stars, and differs significantly from that of the O-type stars that are situated in clusters and associations. The difference is in the sense that runaway O-stars, on average, have later subtypes than do those that are still located in clusters and associations.Comment: To be published in the October 2004 issue of the Astronomical Journal Included Figure 1, page

Decomposition of select expressions

A select operation that is part of an expression applying to a relational database is decomposed into one or more independent select operations for the purpose of optimising the relational expression. The select expression is treated as a logical expression. From the canonical form of this expression an optimal conjunctive form is obtained which can be decomposed into separate select operations. These separate selects can then be moved to the most effective place within the relational expression. The method also eliminates redundancy in the original expression. A prototype has been used in developing the optimisation method; from this prototype an implementation for use in an actual system has been derived

A refurbished convergent point method for finding moving groups in the Hipparcos Catalogue

The Hipparcos data allow a major step forward in the research of `moving groups' in the Solar neighbourhood, as the common motion of group members causes converging proper motions. Previous knowledge on these coherent structures in velocity space has always been limited by the availability, reliability, and accuracy of ground-based proper motion measurements. A refurbishment of Jones' convergent point method is presented which takes full advantage of the quality of the Hipparcos data. The original implementation of this method determines the maximum likelihood convergent point on a grid on the sky and simultaneously selects group members from a given set of stars with positions and proper motions. The refurbished procedure takes into account the full covariance matrix of the Hipparcos measurements instead of standard errors only, allows for internal motions of the stars, and replaces the grid-based approach by a direct minimization. The method is tested on Monte Carlo simulations of moving groups, and applied to the Hyades. Despite the limited amount of data used by the convergent point method, the results for stars in and around the cluster- centre region agree very well with those of the recent comprehensive study by Perryman et al. (1998).Comment: 14 pages, 7 Postscript figures, LaTeX using mn.sty and psfig.sty; accepted for publication in MNRA

Radial velocities of early-type stars in the Perseus OB2 association

We present radial velocities for 29 B- and A-type stars in the field of the nearby association Perseus OB2. The velocities are derived from spectra obtained with AURELIE, via cross correlation with radial velocity standards matched as closely as possible in spectral type. The resulting accuracy is ~2 - 3 km s$^{-1}$. We use these measurements, together with published values for a few other early-type stars, to study membership of the association. The mean radial velocity (and measured velocity dispersion) of Per OB2 is 23.5 \pm 3.9 km s$^{-1}$, and lies ~15 km s$^{-1}$ away from the mean velocity of the local disk field stars. We identify a number of interlopers in the list of possible late-B- and A-type members which was based on Hipparcos parallaxes and proper motions, and discuss the colour-magnitude diagram of the association.Comment: 20 pages, 9 figures, accepted for publication in A&A, minor revision

The ISM Interactions of a Runaway LBV Nebula in the LMC

New observations of the Magellanic Cloud Luminous Blue Variable candidate S119 (HD269687) show the relationship of the star to its environs. Echelle spectroscopy and high-resolution HST imagery reveal an expanding bubble centered on the star. This bubble appears in both Halpha and [NII] and is noticeably brighter on the near (blue-shifted) side. The systemic velocity of both the expanding bubble and the star itself (as seen by the very broad Halpha emission feature in the stellar spectrum) is V_hel=160 km/s whereas the velocity of the superposed LMC ISM is 250-300 km/s. ISM absorption features seen in FUSE spectra reveal components at both stellar and LMC velocities. Thus we conclude that S119 is located within the LMC ISM and that the bubble is interacting strongly with the ISM in a bow shock.Comment: 5 pages in EmulateApJ format, 3 figures Accepted by ApJL See http://fuse.pha.jhu.edu/~danforth/s119

Samen sta je sterker!

Presentatie gegeven op een symposium van delict en multiproblem van de Hanzehogeschool (120) te Groningen. Verdienen delictplegers een tweede kans

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