| dc.description.abstract | Star formation in the Milky Way is confined to star-forming regions (OB associ-
ation, HII regions, and open clusters) in the Galactic plane. It is usually assumed
that these regions are found preferably along spiral arms, as is observed in other
spiral galaxies. However, young early-type stars are often found at high Galactic
latitudes, far away from their birthplaces in the Galactic disc. These stars are
called runaway stars, and it is believed that they were ejected from their birth-
places early in their lifetimes by one of two mechanisms: ejection from a binary
system following the destruction of the massive companion in a supernova type II
event (the binary ejection mechanism), or ejection from a dense cluster following
a close gravitational encounter between two close binaries (the dynamical ejection
mechanism).
The aims of our study were: to improve the current understanding of the nature
of high Galactic latitude runaway stars, in particular by investigating whether the
theoretical ejection mechanisms could explain the more extreme cases; to show
the feasibility of using high Galactic latitude stars as tracers of the spiral arms.
The main technique used in this investigation was the tracing of stellar orbits back
in time, given their present positions and velocities in 3D space. This technique
allowed the determination of the ejection velocities, flight times and birthplaces of
a sample of runaway stars. In order to obtain reasonable velocity estimates several
recent catalogues of proper motion data were used.
We found that the evolutionary ages of the vast majority of runaway stars is
consistent with the disc ejection scenario. However, we identified three outliers
which would need flight times much larger then their estimated ages in order to
reach their present positions in the sky. Moreover, the ejection velocity distribution
appears to be bimodal, showing evidence for two populations of runaway stars: a
“low” velocity population (89 per cent of the sample), with a maximum ejection
velocity of about 300 kms−1, and a “high” velocity population, with ejection velo-
cities of 400 – 500 kms−1. We argue that the observed bimodality and maximum
ejection velocity of 500 kms−1 can be interpreted as a natural consequence of
a variation of the binary ejection mechanism. A possible connection between the
“high” velocity population and the so-called hypervelocity stars is also explored,
resulting in the conclusion that some stars previously identified as hypervelocity
may be in fact runaway stars.
The feasibility of using stars as tracers of the spiral arms was tested on a local
sample, in order to obtain better quality data and larger numbers. We found that
the spiral arms pattern speeds estimated from this sample (24.9±5.2 kms−1 kpc−1)
and from a selected sample of runaways (22.8 ± 7.8 kms−1 kpc−1) are consistent
within the errors and also consistent with other published estimates. We concluded
that our estimates combined with the ones obtained in other studies suggest a value
in the range 20 − 25 kms−1 kpc−1 for the pattern speed. Moreover, we concluded
that an adequate representation of the spiral arms is obtained given the former
pattern speed estimate, even when applied to the sample of runaway stars. | en_US |
