1,392 research outputs found
Binaries are the best single stars
Stellar models of massive single stars are still plagued by major
uncertainties. Testing and calibrating against observations is essential for
their reliability. For this purpose one preferably uses observed stars that
have never experienced strong binary interaction, i.e. "true single stars".
However, the binary fraction among massive stars is high and identifying "true
single stars" is not straight forward. Binary interaction affects systems in
such a way that the initially less massive star becomes, or appears to be,
single. For example, mass transfer results in a widening of the orbit and a
decrease of the luminosity of the donor star, which makes it very hard to
detect. After a merger or disruption of the system by the supernova explosion,
no companion will be present.
The only unambiguous identification of "true single stars" is possible in
detached binaries, which contain two main-sequence stars. For these systems we
can exclude the occurrence of mass transfer since their birth. A further
advantage is that binaries can often provide us with direct measurements of the
fundamental stellar parameters. Therefore, we argue these binaries are worth
the effort needed to observe and analyze them. They may provide the most
stringent test cases for single stellar models.Comment: 5 pages, 1 figure, contribution to the proceedings of "The
multi-wavelength view of hot, massive stars", 39th Li`ege Int. Astroph.
Coll., 12-16 July 201
Evolution of Mass Functions of Coeval Stars through Wind Mass Loss and Binary Interactions
Accurate determinations of stellar mass functions and ages of stellar
populations are crucial to much of astrophysics. We analyse the evolution of
stellar mass functions of coeval main sequence stars including all relevant
aspects of single- and binary-star evolution. We show that the slope of the
upper part of the mass function in a stellar cluster can be quite different to
the slope of the initial mass function. Wind mass loss from massive stars leads
to an accumulation of stars which is visible as a peak at the high mass end of
mass functions, thereby flattening the mass function slope. Mass accretion and
mergers in close binary systems create a tail of rejuvenated binary products.
These blue straggler stars extend the single star mass function by up to a
factor of two in mass and can appear up to ten times younger than their parent
stellar cluster. Cluster ages derived from their most massive stars that are
close to the turn-off may thus be significantly biased. To overcome such
difficulties, we propose the use of the binary tail of stellar mass functions
as an unambiguous clock to derive the cluster age because the location of the
onset of the binary tail identifies the cluster turn-off mass. It is indicated
by a pronounced jump in the mass function of old stellar populations and by the
wind mass loss peak in young stellar populations. We further characterise the
binary induced blue straggler population in star clusters in terms of their
frequency, binary fraction and apparent age.Comment: 21 pages, 22 figures, accepted for publication in Ap
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
The rotation rates of massive stars: the role of binary interaction through tides, mass transfer and mergers
Rotation is thought to be a major factor in the evolution of massive stars,
especially at low metallicity, with consequences for their chemical yields,
ionizing flux and final fate. Determining the natal rotation-rate distribution
of stars is of high priority given its importance as a constraint on theories
of massive star formation and as input for models of stellar populations in the
local Universe and at high redshift. Recently, it has become clear that the
majority of massive stars interact with a binary companion before they die. We
investigate how this affects the distribution of rotation rates.
For this purpose, we simulate a massive binary-star population typical for
our Galaxy assuming continuous star formation. We find that, because of binary
interaction, 20^+5_-10% of all massive main-sequence stars have projected
rotational velocities in excess of 200km/s. We evaluate the effect of uncertain
input distributions and physical processes and conclude that the main
uncertainties are the mass transfer efficiency and the possible effect of
magnetic braking, especially if magnetic fields are generated or amplified
during mass accretion and stellar mergers.
The fraction of rapid rotators we derive is similar to that observed. If
indeed mass transfer and mergers are the main cause for rapid rotation in
massive stars, little room remains for rapidly rotating stars that are born
single. This implies that spin down during star formation is even more
efficient than previously thought. In addition, this raises questions about the
interpretation of the surface abundances of rapidly rotating stars as evidence
for rotational mixing. Furthermore, our results allow for the possibility that
all early-type Be stars result from binary interactions and suggest that
evidence for rotation in explosions, such as long gamma-ray bursts, points to a
binary origin.Comment: 14 pages, 5 figures, accepted for publication in ApJ., no changes
with v1 apart from fixed typos/ref
Probing the topological exciton condensate via Coulomb drag
The onset of exciton condensation in a topological insulator thin film was
recently predicted. We calculate the critical temperature for this transition,
taking into account screening effects. Furthermore, we show that the proximity
to this transition can be probed by measuring the Coulomb drag resistivity
between the surfaces of the thin film as a function of temperature. This
resistivity shows an upturn upon approaching the exciton-condensed state.Comment: 4 pages, 3 figure
Spin transport in a unitary Fermi gas close to the BCS transition
We consider spin transport in a two-component ultracold Fermi gas with
attractive interspecies interactions close to the BCS pairing transition. In
particular, we consider the spin-transport relaxation rate and the
spin-diffusion constant. Upon approaching the transition, the scattering
amplitude is enhanced by pairing fluctuations. However, as the system
approaches the transition, the spectral weight for excitations close to the
Fermi level is decreased by the formation of a pseudogap. To study the
consequence of these two competing effects, we determine the spin-transport
relaxation rate and the spin-diffusion constant using both a Boltzmann approach
and a diagrammatic approach. The former ignores pseudogap physics and finite
lifetime effects. In the latter, we incorporate the full pseudogap physics and
lifetime effects, but we ignore vertex corrections, so that we effectively
calculate single-particle relaxation rates instead of transport relaxation
rates. We find that there is qualitative agreement between these two approaches
although the results for the transport coefficients differ quantitatively.Comment: 9 pages, 10 figure
Modelling the evolution and nucleosynthesis of carbon-enhanced metal-poor stars
We present the results of binary population simulations of carbon-enhanced
metal-poor (CEMP) stars. We show that nitrogen and fluorine are useful tracers
of the origin of CEMP stars, and conclude that the observed paucity of very
nitrogen-rich stars puts strong constraints on possible modifications of the
initial mass function at low metallicity. The large number fraction of CEMP
stars may instead require much more efficient dredge-up from low-metallicity
asymptotic giant branch stars.Comment: 6 pages, 1 figure, to appear in the proceedings of IAU Symposium 252
"The Art of Modelling Stars in the 21st Century", April 6-11, 2008, Sanya,
Chin
An ultrahigh-speed digitizer for the Harvard College Observatory astronomical plates
A machine capable of digitizing two 8 inch by 10 inch (203 mm by 254 mm)
glass astrophotographic plates or a single 14 inch by 17 inch (356 mm by 432
mm) plate at a resolution of 11 microns per pixel or 2309 dots per inch (dpi)
in 92 seconds is described. The purpose of the machine is to digitize the
\~500,000 plate collection of the Harvard College Observatory in a five year
time frame. The digitization must meet the requirements for scientific work in
astrometry, photometry, and archival preservation of the plates. This paper
describes the requirements for and the design of the subsystems of the machine
that was developed specifically for this task.Comment: 12 pages, 9 figures, 1 table; presented at SPIE (July, 2006) and
published in Proceeding
Light elements in massive single and binary stars
We highlight the role of the light elements (Li, Be, B) in the evolution of
massive single and binary stars, which is largely restricted to a diagnostic
value, and foremost so for the element boron. However, we show that the boron
surface abundance in massive early type stars contains key information about
their foregoing evolution which is not obtainable otherwise. In particular, it
allows to constrain internal mixing processes and potential previous mass
transfer event for binary stars (even if the companion has disappeared). It may
also help solving the mystery of the slowly rotating nitrogen-rich massive main
sequence stars.Comment: 10 pages, 8 figures, to appear in proc. IAU-Symp. 268. C. Charbonnel
et al., eds
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