318 research outputs found
Massive runaway stars in the Large Magellanic Cloud
The origin of massive field stars in the Large Magellanic Cloud (LMC) has
long been an enigma. The recent measurements of large offsets (~100 km/s)
between the heliocentric radial velocities of some very massive (O2-type) field
stars and the systemic LMC velocity provides a possible explanation of this
enigma and suggests that the field stars are runaway stars ejected from their
birth places at the very beginning of their parent cluster's dynamical
evolution. A straightforward way to prove this explanation is to measure the
proper motions of the field stars and to show that they are moving away from
one of the nearby star clusters or OB associations. This approach however is
complicated by the large distance to the LMC, which makes accurate proper
motion measurements difficult. We use an alternative approach for solving the
problem, based on the search for bow shocks produced by runaway stars. The
geometry of detected bow shocks would allow us to infer the direction of
stellar motion and thereby to determine their possible parent clusters. In this
paper we present the results of a search for bow shocks around six massive
field stars which were suggested in the literature as candidate runaway stars.
Using archival (Spitzer Space Telescope) data, we found a bow shock associated
with one of our program stars, the O2 V((f*)) star BI 237, which is the
first-ever detection of bow shocks in the LMC. Orientation of the bow shock
suggests that BI 237 was ejected from the OB association LH 82 (located at ~120
pc in projection from the star). A by-product of our search is the detection of
bow shocks generated by four OB stars in the field of the LMC and an arc-like
structure attached to the candidate luminous blue variable R81 (HD 269128). The
geometry of two of these bow shocks is consistent with the possibility that
their associated stars were ejected from the 30 Doradus star forming complex.Comment: 5 pages, 7 figures, accepted for publication in A&
On the origin of hyperfast neutron stars
We propose an explanation for the origin of hyperfast neutron stars (e.g. PSR
B1508+55, PSR B2224+65, RX J0822-4300) based on the hypothesis that they could
be the remnants of a symmetric supernova explosion of a high-velocity massive
star (or its helium core) which attained its peculiar velocity (similar to that
of the neutron star) in the course of a strong three- or four-body dynamical
encounter in the core of a young massive star cluster. This hypothesis implies
that the dense cores of star clusters (located either in the Galactic disk or
near the Galactic centre) could also produce the so-called hypervelocity stars
-- the ordinary stars moving with a speed of ~1000 km/s.Comment: 2 pages, to appear in Dynamical Evolution of Dense Stellar Systems,
Proceed. of the IAU Symp. 246 (Capri, Sept. 2007), eds. E.Vesperini, M.
Giersz, and A. Sill
High-velocity runaway stars from three-body encounters
We performed numerical simulations of dynamical encounters between hard
massive binaries and a very massive star (VMS; formed through runaway mergers
of ordinary stars in the dense core of a young massive star cluster), in order
to explore the hypothesis that this dynamical process could be responsible for
the origin of high-velocity (\geq 200-400 km/s) early or late B-type stars. We
estimated the typical velocities produced in encounters between very tight
massive binaries and VMSs (of mass of \geq 200 Msun) and found that about 3-4
per cent of all encounters produce velocities of \geq 400 km/s, while in about
2 per cent of encounters the escapers attain velocities exceeding the Milky
Ways's escape velocity. We therefore argue that the origin of high-velocity
(\geq 200-400 km/s) runaway stars and at least some so-called hypervelocity
stars could be associated with dynamical encounters between the tightest
massive binaries and VMSs formed in the cores of star clusters. We also
simulated dynamical encounters between tight massive binaries and single
ordinary 50-100 Msun stars. We found that from 1 to \simeq 4 per cent of these
encounters can produce runaway stars with velocities of \geq 300-400 km/s
(typical of the bound population of high-velocity halo B-type stars) and
occasionally (in less than 1 per cent of encounters) produce hypervelocity
(\geq 700 km/s) late B-type escapers.Comment: 4 pages, 2 figure, to appear in Star Clusters -- Basic Galactic
Building Blocks throughout Time and Space, Proceed. of the IAU Symp. 266,
eds. R. de Grijs and J. Lepin
Massive runaway stars in the Small Magellanic Cloud
Using archival Spitzer Space Telescope data, we identified for the first time
a dozen runaway OB stars in the Small Magellanic Cloud (SMC) through the
detection of their bow shocks. The geometry of detected bow shocks allows us to
infer the direction of motion of the associated stars and to determine their
possible parent clusters and associations. One of the identified runaway stars,
AzV 471, was already known as a high-velocity star on the basis of its high
peculiar radial velocity, which is offset by ~40 km/s from the local systemic
velocity. We discuss implications of our findings for the problem of the origin
of field OB stars. Several of the bow shock-producing stars are found in the
confines of associations, suggesting that these may be "alien" stars
contributing to the age spread observed for some young stellar systems. We also
report the discovery of a kidney-shaped nebula attached to the early WN-type
star SMC-WR3 (AzV 60a). We interpreted this nebula as an interstellar structure
created owing to the interaction between the stellar wind and the ambient
interstellar medium.Comment: Accepted by A&
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