207 research outputs found
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
Naar een architectuur van vereenvoudiging? Victor Horta’s lezingen over Amerikaanse architectuur in de jaren twintig
The Belgian architect Victor Horta (1861-1947) spent most of the First World War in the United States. Over the course of three years, from December 1915 to January 1919, he explored how the American skyscrapers, standardized dwellings and ingenious urban planning might serve as a model for a modern, post-war Belgium.
Yet Horta’s memoirs had very little to say about his discussion of American architecture or any influence his travels might have had on his post-war work. This article consequently breaks new ground in examining the various talks on the subject that Horta gave in the 1920s. Horta’s lectures actually provide a good picture of how he started to see American architecture as a model for the future. In American architectural practice, serial production, standardization and economies of scale facilitated a simplification of the design – a solution Horta also proposed for war-torn Belgium.
By way of illustration, the article describes the affinity between Horta’s more classical formal language of the 1920s, as in his 1925 pavilion for the Exposition international des Arts décoratifs et industriels modernes, and the architecture of the United States. It also shows that Horta was an important proponent of American architecture in Belgium, and also played a pioneering role in the introduction of some of its defining features, such as high-rise construction.De Belgische architect Victor Horta (1861-1947) verbleef het grootste deel van de Eerste Wereldoorlog in de Verenigde Staten. Gedurende drie jaar, van december 1915 tot januari 1919, onderzocht hij daar hoe de Amerikaanse wolkenkrabbers, gestandaardiseerde woningen en ingenieuze stedenbouw als model konden dienen voor een modern, naoorlogs België.
In zijn memoires was Horta echter erg summier over zijn bespreking van de Amerikaanse architectuur, en de mogelijkse invloed die zijn reis heeft gehad op zijn naoorlogs werk. Dit artikel onderzoekt daarom voor de eerste keer de verschillende lezingen die Horta in de jaren twintig over het onderwerp heeft gegeven. Horta’s voordrachten laten namelijk goed zien hoe hij de Amerikaanse architectuur begon te zien als een model voor de toekomst. Serieproductie, standaardisatie en schaalvergroting werkten in de Amerikaanse architectuurpraktijk een vereenvoudiging van ontwerp in de hand – een oplossing die Horta ook voorstelde in het door de oorlog geruïneerde België.
Om dit te illustreren beschrijven de auteurs in dit artikel hoe Horta’s klassiekere vormentaal van de jaren twintig, zoals zichtbaar in zijn paviljoen voor de Exposition internationale des Arts décoratifs et industriels modernes uit 1925, verwant was aan de architectuur in de Verenigde Staten. Het toont daarnaast aan hoe Horta een belangrijke rol heeft gespeeld in het propageren van de Amerikaanse architectuur in België, en ook een pioniersrol heeft gespeeld in de introductie van enkele van haar voornaamste principes, zoals hoogbouw
Reproduction of Crassula helmsii by seed in western Europe
The amphibious plant species Crassula helmsii is a widely established and still-spreading alien in various parts of Europe, where it is considered invasive as its dense swards stress the viability of local biota. The species was considered to exclusively reproduce through vegetative means, until ex situ germination was recorded from a single locality in Belgium. We assessed whether this seed viability holds on a wider scale, by testing 16 populations from The Netherlands, Belgium, northern France, eastern England and northern Germany in a greenhouse germination experiment. Seedlings were observed from all populations but two, and from each of the five countries. Although most fruits were lacking seeds and the inferred germination percentages were overall low, germinable seed numbers are considerable given the high density of flowering stems. An in situ test revealed seeds to make it through normal winter conditions without signs of physical damage and with retention of germinability. Our results suggest that reproduction by seed is a relatively cryptic but widespread phenomenon throughout western Europe. The persistency of seed banks requires further investigation. Nonetheless, these findings already challenge the efficacy of techniques currently applied in C. helmsii control
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
Gamma-Ray Bursts from tidally spun-up Wolf-Rayet stars?
The collapsar model requires rapidly rotating Wolf-Rayet stars as progenitors
of long gamma-ray bursts. However, Galactic Wolf-Rayet stars rapidly lose
angular momentum due to their intense stellar winds. We investigate whether the
tidal interaction of a Wolf-Rayet star with a compact object in a binary system
can spin up the Wolf-Rayet star enough to produce a collapsar. We compute the
evolution of close Wolf-Rayet binaries, including tidal angular momentum
exchange, differential rotation of the Wolf-Rayet star, internal magnetic
fields, stellar wind mass loss, and mass transfer. The Wolf-Rayet companion is
approximated as a point mass. We then employ a population synthesis code to
infer the occurrence rates of the various relevant binary evolution channels.
We find that the simple scenario -- i.e., the Wolf-Rayet star being tidally
spun up and producing a collapsar -- does not occur at solar metallicity and
may only occur with low probability at low metallicity. It is limited by the
widening of the binary orbit induced by the strong Wolf-Rayet wind or by the
radius evolution of the Wolf-Rayet star that most often leads to a binary
merger. The tidal effects enhance the merger rate of Wolf-Rayet stars with
black holes such that it becomes comparable to the occurrence rate of long
gamma-ray bursts.Comment: 9 pages, 11 figures, accepted for publication in A&
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
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
Cabomba caroliniana Gray (Cabombaceae) invades major waterways in Belgium
Recently, unattached viable stem fragments of the invasive macrophyte Cabomba caroliniana Gray were found at several locations in two canals in northern Belgium. A subsequent survey of the canal Zuid-Willemsvaart revealed one site with rooted plants and the presence of floating stem fragments along the entire length of the canal (25 km). These findings indicate that C. caroliniana is already well established in the Zuid-Willemsvaart and probably considerably more widespread than previous records from isolated lentic water bodies indicate. We have summarised the new records and discuss the potential spread and risks associated with further establishment and spread of C. caroliniana in Belgium
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