17 research outputs found
Wind bubbles within H II regions around slowly moving stars
Interstellar bubbles around O stars are driven by a combination of the star's
wind and ionizing radiation output. The wind contribution is uncertain because
the boundary between the wind and interstellar medium is difficult to observe.
Mid-infrared observations (e.g., of the H II region RCW 120) show arcs of dust
emission around O stars, contained well within the H II region bubble. These
arcs could indicate the edge of an asymmetric stellar wind bubble, distorted by
density gradients and/or stellar motion. We present two-dimensional,
radiation-hydrodynamics simulations investigating the evolution of wind bubbles
and H II regions around massive stars moving through a dense (n=3000 cm^{-3}),
uniform medium with velocities ranging from 4 to 16 km/s. The H II region
morphology is strongly affected by stellar motion, as expected, but the wind
bubble is also very aspherical from birth, even for the lowest space velocity
considered. Wind bubbles do not fill their H II regions (we find filling
factors of 10-20%), at least for a main sequence star with mass M~30 Msun.
Furthermore, even for supersonic velocities the wind bow shock does not
significantly trap the ionization front. X-ray emission from the wind bubble is
soft, faint, and comes mainly from the turbulent mixing layer between the wind
bubble and the H II region. The wind bubble radiates <1 per cent of its energy
in X-rays; it loses most of its energy by turbulent mixing with cooler
photoionized gas. Comparison of the simulations with the H II region RCW 120
shows that its dynamical age is <=0.4 Myr and that stellar motion <=4 km/s is
allowed, implying that the ionizing source is unlikely to be a runaway star but
more likely formed in situ. The region's youth, and apparent isolation from
other O or B stars, makes it very interesting for studies of massive star
formation and of initial mass functions.Comment: 14 pages, 11 figures, accepted for publication in Astronomy and
Astrophysics (new version corrects an error in the simulation postprocessing,
figs 6,7,11 are modified slightly, conclusions unchanged
New Galactic Candidate Luminous Blue Variables and Wolf-Rayet Stars
We have undertaken a near-infrared spectral survey of stars associated with
compact mid-IR shells recently revealed by the MIPSGAL (24 micron) and GLIMPSE
(8 micron) Spitzer surveys, whose morphologies are typical of circumstellar
shells produced by massive evolved stars. Through spectral similarity with
known Luminous Blue Variable (LBV) and Wolf-Rayet (WR) stars, a large
population of candidate LBVs (cLBVs) and a smaller number of new WR stars are
being discovered. This significantly increases the Galactic cLBV population and
confirms that nebulae are inherent to most (if not all) objects of this class.
Keywords - stars: emission-line, Be, stars: mass loss, stars: winds,
outflows, stars: Wolf-RayetComment: 2 pages, 1 figure. To appear in IAU 282 proceedings "From Interacting
Binaries to Exoplanets: Essential Modeling Tools", Mercedes Richards \& Ivan
Hubeny, ed
X-rays observations of a super-Chandrasekhar object reveal an ONeMg and a CO white dwarf merger product embedded in a putative SN Iax remnant
The merger of two white dwarfs (WD) is a natural outcome from the evolution
of many binary stars. Recently, a WD merger product, IRAS 00500+6713, was
identified. IRAS 00500+6713 consists of a central star embedded in a circular
nebula. The analysis of the optical spectrum of the central star revealed that
it is hot, hydrogen and helium free, and drives an extremely fast wind with a
record breaking speed. The nebula is visible in infrared and in the [O III]
line images. No nebula spectroscopy was obtained prior to our observations.
Here we report the first deep X-ray imaging spectroscopic observations of IRAS
00500+6713. Both the central star and the nebula are detected in X-rays,
heralding the WD merger products as a new distinct type of strong X-ray
sources. Low-resolution X-ray spectra reveal large neon, magnesium, silicon,
and sulfur enrichment of the central star and the nebula. We conclude that IRAS
00500+6713 resulted from a merger of an ONe and a CO WD, which supports earlier
suggestion for a super-Chandrasekhar mass of this object. X-ray analysis
indicates that the merger was associated with an episode of carbon burning and
possibly accompanied by a SN Iax. In X-rays, we observe the point source
associated with the merger product while the surrounding diffuse nebula is a
supernova remnant. IRAS 00500+6713 will likely terminate its evolution with
another peculiar Type I supernova, where the final core collapse to a neutron
star might be induced by electron captures.Comment: accepted by A&A Letters, 9 pages including appendi
Very massive runaway stars from three-body encounters
Very massive stars preferentially reside in the cores of their parent
clusters and form binary or multiple systems. We study the role of tight very
massive binaries in the origin of the field population of very massive stars.
We performed numerical simulations of dynamical encounters between single
(massive) stars and a very massive binary with parameters similar to those of
the most massive known Galactic binaries, WR 20a and NGC 3603-A1. We found that
these three-body encounters could be responsible for the origin of high
peculiar velocities ( 70 km/s) observed for some very massive (
60-70 Msun) runaway stars in the Milky Way and the Large Magellanic Cloud
(e.g., Cep, BD+43 3654, Sk-67 22, BI 237, 30 Dor 016), which can
hardly be explained within the framework of the binary-supernova scenario. The
production of high-velocity massive stars via three-body encounters is
accompanied by the recoil of the binary in the opposite direction to the
ejected star. We show that the relative position of the very massive binary
R145 and the runaway early B-type star Sk-69 206 on the sky is consistent with
the possibility that both objects were ejected from the central cluster, R136,
of the star-forming region 30 Doradus via the same dynamical event -- a
three-body encounter.Comment: 10 pages, 12 figures, accepted to MNRA
Hyperfast pulsars as the remnants of massive stars ejected from young star clusters
Recent proper motion and parallax measurements for the pulsar PSR B1508+55
indicate a transverse velocity of ~1100 km/s, which exceeds earlier
measurements for any neutron star. The spin-down characteristics of PSR
B1508+55 are typical for a non-recycled pulsar, which implies that the velocity
of the pulsar cannot have originated from the second supernova disruption of a
massive binary system. The high velocity of PSR B1508+55 can be accounted for
by assuming that it received a kick at birth or that the neutron star was
accelerated after its formation in the supernova explosion. We propose an
explanation for the origin of hyperfast neutron stars based on the hypothesis
that they could be the remnants of a symmetric supernova explosion of a
high-velocity massive star which attained its peculiar velocity (similar to
that of the pulsar) in the course of a strong dynamical three- or four-body
encounter in the core of dense young star cluster. To check this hypothesis we
investigated three dynamical processes involving close encounters between: (i)
two hard massive binaries, (ii) a hard binary and an intermediate-mass black
hole, and (iii) a single star and a hard binary intermediate-mass black hole.
We find that main-sequence O-type stars cannot be ejected from young massive
star clusters with peculiar velocities high enough to explain the origin of
hyperfast neutron stars, but lower mass main-sequence stars or the stripped
helium cores of massive stars could be accelerated to hypervelocities. Our
explanation for the origin of hyperfast pulsars requires a very dense stellar
environment of the order of 10^6 -10^7 stars pc^{-3}. Although such high
densities may exist during the core collapse of young massive star clusters, we
caution that they have never been observed.Comment: 11 pages, 6 figures, 1 table, accepted to MNRA