368,778 research outputs found
A VLT/FLAMES survey for massive binaries in Westerlund 1: II. Dynamical constraints on magnetar progenitor masses from the eclipsing binary W13
Westerlund 1 is a young, massive Galactic starburst cluster that contains a
rich coeval population of Wolf-Rayet stars, hot- and cool-phase transitional
supergiants, and a magnetar. We use spectroscopic and photometric observations
of the eclipsing double-lined binary W13 to derive dynamical masses for the two
components, in order to determine limits for the progenitor masses of the
magnetar CXOU J164710.2-455216 and the population of evolved stars in Wd1. W13
has an orbital period of 9.2709+/-0.0015 days and near-contact configuration.
The shallow photometric eclipse rules out an inclination greater than 65
degrees, leading to lower limits for the masses of the emission-line optical
primary and supergiant optical secondary of 21.4+/-2.6Msun and 32.8+/-4.0Msun
respectively, rising to 23.2 +3.3/-3.0Msun and 35.4 +5.0/-4.6 Msun for our
best-fit inclination 62 +3/-4 degrees. Comparison with theoretical models of
Wolf-Rayet binary evolution suggest the emission-line object had an initial
mass in excess of 35Msun, with the most likely model featuring highly
non-conservative late-Case-A/Case-B mass transfer and an initial mass in excess
of 40Msun. This confirms the high magnetar progenitor mass inferred from its
membership in Wd1, and represents the first dynamical constraint on the
progenitor mass of any magnetar. The red supergiants in Wd1 must have similar
progenitor masses to W13 and are therefore amongst the most massive stars to
undergo a red supergiant phase, representing a challenge for population models
that suggest stars in this mass range end their redwards evolution as yellow
hypergiants. [ABRIDGED]Comment: Accepted for publication in A&A. 8 pages, 5 figures. See also
http://www.eso.org/public/news/eso1034/ from noon (CEST) Wed 18th Augus
Supernova Type Ia progenitors from merging double white dwarfs: Using a new population synthesis model
The study of Type Ia supernovae (SNIa) has lead to greatly improved insights
into many fields in astrophysics, however a theoretical explanation of the
origin of these events is still lacking. We investigate the potential
contribution to the SNIa rate from the population of merging double
carbon-oxygen white dwarfs. We aim to develope a model that fits the observed
SNIa progenitors as well as the observed close double white dwarf population.
We differentiate between two scenarios for the common envelope (CE) evolution;
the alpha-formalism based on the energy equation and the gamma-formalism that
is based on the angular momentum equation. In one model we apply the
alpha-formalism always. In the second model the gamma-formalism is applied,
unless the binary contains a compact object or the CE is triggered by a tidal
instability for which the alpha-formalism is used. The binary population
synthesis code SeBa was used to evolve binary systems from the zero-age main
sequence to the formation of double white dwarfs and subsequent mergers. SeBa
has been thoroughly updated since the last publication of the content of the
code. The limited sample of observed double white dwarfs is better represented
by the simulated population using the gamma-formalism than the alpha-formalism.
For both CE formalisms, we find that although the morphology of the simulated
delay time distribution matches that of the observations within the errors, the
normalisation and time-integrated rate per stellar mass are a factor 7-12 lower
than observed. Furthermore, the characteristics of the simulated populations of
merging double carbon-oxygen white dwarfs are discussed and put in the context
of alternative SNIa models for merging double white dwarfs.Comment: 16 pages (including 4 pages appendix), 15 figure
Massive binaries and the enrichment of the interstellar medium in globular clusters
Abundance anomalies observed in globular cluster stars indicate pollution
with material processed by hydrogen burning. Two main sources have been
suggested: asymptotic giant branch stars and massive stars rotating near the
break-up limit. We discuss the potential of massive binaries as an interesting
alternative source of processed material.
We discuss observational evidence for mass shedding from interacting
binaries. In contrast to the fast, radiatively driven winds of massive stars,
this material is typically ejected with low velocity. We expect that it remains
inside the potential well of a globular cluster and becomes available for the
formation or pollution of a second generation of stars. We estimate that the
amount of processed low-velocity material that can be ejected by massive
binaries is larger than the contribution of two previously suggested sources
combined.Comment: 6 pages, 2 figures, to appear in the proceedings of IAU Symposium
266, "Star Clusters - Basic Galactic Building Blocks throughout Time and
Space", 10-14 August 2009, at the general assembly in Rio de Janeiro, Brazi
Episodic mass loss in binary evolution to the Wolf-Rayet phase: Keck and HST proper motions of RY Scuti's nebula
Binary mass transfer via Roche-lobe overflow (RLOF) is a key channel for
producing stripped-envelope Wolf-Rayet (WR) stars and may be critical to
account for SN Ib/c progenitors. RY Scuti is an extremely rare example of a
massive binary star caught in this brief but important phase. Its toroidal
nebula indicates equatorial mass loss during RLOF, while the mass-gaining star
is apparently embedded in an opaque accretion disk. RY Scuti's toroidal nebula
has two components: an inner ionised double-ring system, and an outer dust
torus that is twice the size of the ionised rings. We present two epochs of
Lband Keck NGS-AO images of the dust torus, plus three epochs of HST images of
the ionised gas rings. Proper motions show that the inner ionised rings and the
outer dust torus came from two separate ejection events roughly 130 and 250 yr
ago. This suggests that RLOF in massive contact binaries can be accompanied by
eruptive and episodic burst of mass loss, reminiscent of LBVs. We speculate
that the repeating outbursts may arise in the mass gainer from instabilities
associated with a high accretion rate. If discrete mass-loss episodes in other
RLOF binaries are accompanied by luminous outbursts, they might contribute to
the population of extragalactic optical transients. When RLOF ends for RY
Scuti, the overluminous mass gainer, currently surrounded by an accretion disk,
will probably become a B[e] supergiant and may outshine the hotter mass-donor
star that should die as a Type Ib/c supernova.Comment: 15 pages, 7 figures, submitted to MNRA
On the origin of the O and B-type stars with high velocities II Runaway stars and pulsars ejected from the nearby young stellar groups
We use milli-arcsecond accuracy astrometry (proper motions and parallaxes)
from Hipparcos and from radio observations to retrace the orbits of 56 runaway
stars and nine compact objects with distances less than 700 pc, to identify the
parent stellar group. It is possible to deduce the specific formation scenario
with near certainty for two cases. (i) We find that the runaway star zeta
Ophiuchi and the pulsar PSR J1932+1059 originated about 1 Myr ago in a
supernova explosion in a binary in the Upper Scorpius subgroup of the Sco OB2
association. The pulsar received a kick velocity of about 350 km/s in this
event, which dissociated the binary, and gave zeta Oph its large space
velocity. (ii) Blaauw & Morgan and Gies & Bolton already postulated a common
origin for the runaway-pair AE Aur and mu Col, possibly involving the massive
highly-eccentric binary iota Ori, based on their equal and opposite velocities.
We demonstrate that these three objects indeed occupied a very small volume
\sim 2.5Myr ago, and show that they were ejected from the nascent Trapezium
cluster.
We identify the parent group for two more pulsars: both likely originate in
the 50 Myr old association Per OB3, which contains the open cluster alpha
Persei. At least 21 of the 56 runaway stars in our sample can be linked to the
nearby associations and young open clusters. These include the classical
runaways 53 Arietis (Ori OB1), xi Persei (Per OB2), and lambda Cephei (Cep
OB3), and fifteen new identifications, amongst which a pair of stars running
away in opposite directions from the region containing the lambda Ori cluster.
Other currently nearby runaways and pulsars originated beyond 700 pc, where our
knowledge of the parent groups is very incomplete.Comment: Accepted for publication in the A&A. 29 pages, 19 figure
The Past and Future History of Regulus
We show how the recent discovery of a likely close white dwarf companion to
the well known star Regulus, one of the brightest stars in the sky, leads to
considerable insight into the prior evolutionary history of this star,
including the cause of its current rapid rotation. We infer a relatively narrow
range for the initial masses of the progenitor system: M_{10} = 2.3 +/- 0.2
M_sun and M_{20} = 1.7 +/- 0.2 M_sun, where M_{10} and M_{20} are the initial
masses of the progenitors of the white dwarf and Regulus, respectively. In this
scenario, the age of the Regulus system would exceed 1 Gyr. We also show that
Regulus, with a current orbital period of 40 days, has an interesting future
ahead of it. This includes (i) a common envelope phase, and, quite possibly,
(ii) an sdB phase, followed by (iii) an AM CVn phase with orbital periods < 1
hr. Binary evolution calculations are presented in support of this scenario. We
also discuss alternative possibilities, emphasizing the present uncertainties
in binary evolution theory. Thus, this one particular star system illustrates
many different aspects of binary stellar evolution.Comment: PDFLaTeX, 9 pages with 8 figure
On the nature of the Be star HR 7409 (7 Vul)
HR 7409 (7 Vul) is a newly identified Be star possibly part of the Gould Belt
and is the massive component of a 69-day spectroscopic binary. The binary
parameters and properties of the Be star measured using high-dispersion spectra
obtained at Ondrejov Observatory and at Rozhen Observatory imply the presence
of a low mass companion (~ 0.5-0.8 M_sun). If the pair is relatively young
(<50-80 Myr), then the companion is a K V star, but, following another, older
evolutionary scenario, the companion is a horizontal-branch star or possibly a
white dwarf star. In the latter scenario, a past episode of mass transfer from
an evolved star onto a less massive dwarf star would be responsible for the
peculiar nature of the present-day, fast-rotating Be star.Comment: Accepted for publication in MNRA
Constraining the mass transfer in massive binaries through progenitor evolution models of Wolf-Rayet+O binaries
Since close WR+O binaries are the result of a strong interaction of both
stars in massive close binary systems, they can be used to constrain the highly
uncertain mass and angular momentum budget during the major mass transfer
phase. We explore the progenitor evolution of the three best suited WR+O
binaries HD 90657, HD 186943 and HD 211853, which are characterized by a WR/O
mass ratio of 0.5 and periods of 6..10 days. We are doing so at three
different levels of approximation: predicting the massive binary evolution
through simple mass loss and angular momentum loss estimates, through full
binary evolution models with parametrized mass transfer efficiency, and through
binary evolution models including rotation of both components and a physical
model which allows to compute mass and angular momentum loss from the binary
system as function of time during the mass transfer process. All three methods
give consistently the same answers. Our results show that, if these systems
formed through stable mass transfer, their initial periods were smaller than
their current ones, which implies that mass transfer has started during the
core hydrogen burning phase of the initially more massive star. Furthermore,
the mass transfer in all three cases must have been highly non-conservative,
with on average only 10% of the transferred mass being retained by the
mass receiving star. This result gives support to our system mass and angular
momentum loss model, which predicts that, in the considered systems, about 90%
of the overflowing matter is expelled by the rapid rotation of the mass
receiver close to the -limit, which is reached through the accretion of
the remaining 10%.Comment: accepted A&A version of paper with better quality plots available at
http://www.astro.uu.nl/~petrovi
On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking into account
the effects induced by the fast rotation on their fundamental parameters. All
program stars were observed in the BCD spectrophotometric system in order to
minimize the perturbations produced by the circumstellar environment on the
spectral photospheric signatures. This is one of the first attempts at
determining stellar masses and ages by simultaneously using model atmospheres
and evolutionary tracks, both calculated for rotating objects. The stellar ages
() normalized to the respective inferred time that each rotating star can
spend in the main sequence phase () reveal a mass-dependent
trend. This trend shows that: a) there are Be stars spread over the whole
interval 0 \la \tau/\tau\_{\rm MS} \la 1 of the main sequence evolutionary
phase; b) the distribution of points in the () diagram indicates that in massive stars (M \ga
12M\_{\odot}) the Be phenomenon is present at smaller
age ratios than for less massive stars (M \la 12M\_{\odot}). This
distribution can be due to: ) higher mass-loss rates in massive objets,
which can act to reduce the surface fast rotation; ) circulation time
scales to transport angular momentum from the core to the surface, which are
longer the lower the stellar mass.Comment: 18 pages, 6 figures, A&A, in pres
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