1,561 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
New and Noteworthy Plants of Texas
Rhexia alifanus (Melastomataceae) and Rhynchospora inundatum (Cyperaceae), both previously reported in the state, are documented as part of the flora of the state. Ratibida pinnata (Asteraceae), Tradescantia fluminensis (Commelinaceae) and Saccharum ravennae (Poaceae) are reported as new to Texas. Phytologia 92(2): 249-255 (August 2, 2010).Waller Creek Working Grou
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
A Systematic Survey of the Effects of Wind Mass Loss Algorithms on the Evolution of Single Massive Stars
Mass loss is a key uncertainty in the evolution of massive stars. Stellar
evolution calculations must employ parametric algorithms for mass loss, and
usually only include stellar winds. We carry out a parameter study of the
effects of wind mass loss on massive star evolution using the open-source
stellar evolution code MESA. We provide a systematic comparison of wind mass
loss algorithms for solar-metallicity, nonrotating, single stars in the initial
mass range of . We consider combinations drawn from two hot
phase algorithms, three cool phase algorithms, and two Wolf-Rayet algorithms.
We consider linear wind efficiency scale factors of , , and to
account for reductions in mass loss rates due to wind inhomogeneities. We find
that the initial to final mass mapping for each zero-age main-sequence (ZAMS)
mass has a uncertainty if all algorithm combinations and wind
efficiencies are considered. The ad-hoc efficiency scale factor dominates this
uncertainty. While the final total mass and internal structure of our models
vary tremendously with mass loss treatment, final observable parameters are
much less sensitive for ZAMS mass . This indicates that
uncertainty in wind mass loss does not negatively affect estimates of the ZAMS
mass of most single-star supernova progenitors from pre-explosion observations.
Furthermore, we show that the internal structure of presupernova stars is
sensitive to variations in both main sequence and post main-sequence mass loss.
We find that the compactness parameter varies by as much as
for a given ZAMS mass evolved with different wind efficiencies and mass
loss algorithm combinations. [abridged]Comment: Accepted for publication on A&A, 22 pages + 2 appendixes, 12 figures,
online input parameters available at https://stellarcollapse.org/renzo2017
and data at https://zenodo.org/record/292924#.WK0q2tWi6W
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
Low-metallicity massive single stars with rotation. Evolutionary models applicable to I Zwicky 18
Massive rotating single stars with an initial metal composition appropriate
for the dwarf galaxy I Zw 18 ([Fe/H]=1.7) are modelled during hydrogen
burning for initial masses of 9-300 M and rotational velocities of
0-900 km s. Internal mixing processes in these models were calibrated
based on an observed sample of OB-type stars in the Magellanic Clouds. Even
moderately fast rotators, which may be abundant at this metallicity, are found
to undergo efficient mixing induced by rotation resulting in quasi
chemically-homogeneous evolution. These homogeneously-evolving models reach
effective temperatures of up to 90 kK during core hydrogen burning. This,
together with their moderate mass-loss rates, make them Transparent Wind
Ultraviolet INtense stars (TWUIN star), and their expected numbers might
explain the observed HeII ionizing photon flux in I Zw 18 and other
low-metallicity HeII galaxies. Our slowly rotating stars above 80
M evolve into late B- to M-type supergiants during core hydrogen
burning, with visual magnitudes up to 19 at the distance of I Zw
18. Both types of stars, TWUIN stars and luminous late-type supergiants, are
only predicted at low metallicity. Massive star evolution at low metallicity is
shown to differ qualitatively from that in metal-rich environments. Our grid
can be used to interpret observations of local star-forming dwarf galaxies and
high-redshift galaxies, as well as the metal-poor components of our Milky Way
and its globular clusters.Comment: accepted for publication in A\&
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