360 research outputs found
The impact of binary-star yields on the spectra of galaxies
One of the complexities in modelling integrated spectra of stellar populations is the effect of interacting binary stars besides Type Ia supernovae (SNeIa). These include common envelope systems, cataclysmic variables, novae, and are usually ignored in models predicting the chemistry and spectral absorption line strengths in galaxies. In this paper, predictions of chemical yields from populations of single and binary stars are incorporated into a galactic chemical evolution model to explore the significance of the effects of these other binary yields. Effects on spectral line strengths from different progenitor channels of SNeIa are also explored. Small systematic effects are found when the yields from binaries, other than SNeIa, are included, for a given star formation history. These effects are, at present, within the observational uncertainties on the line strengths. More serious differences can arise in considering different types of SNIa models, their rates and contributions
Planetary nebulae after common-envelope phases initiated by low-mass red giants
It is likely that at least some planetary nebulae are composed of matter
which was ejected from a binary star system during common-envelope (CE)
evolution. For these planetary nebulae the ionizing component is the hot and
luminous remnant of a giant which had its envelope ejected by a companion in
the process of spiralling-in to its current short-period orbit. A large
fraction of CE phases which end with ejection of the envelope are thought to be
initiated by low-mass red giants, giants with inert, degenerate helium cores.
We discuss the possible end-of-CE structures of such stars and their subsequent
evolution to investigate for which structures planetary nebulae are formed. We
assume that a planetary nebula forms if the remnant reaches an effective
temperature greater than 30 kK within 10^4 yr of ejecting its envelope. We
assume that the composition profile is unchanged during the CE phase so that
possible remnant structures are parametrized by the end-of-CE core mass,
envelope mass and entropy profile. We find that planetary nebulae are expected
in post-CE systems with core masses greater than about 0.3 solar masses if
remnants end the CE phase in thermal equilibrium. We show that whether the
remnant undergoes a pre-white dwarf plateau phase depends on the prescribed
end-of-CE envelope mass. Thus, observing a young post-CE system would constrain
the end-of CE envelope mass and post-CE evolution.Comment: Published in MNRAS. 12 pages, 12 figures. Minor changes to match
published versio
The s-process in stellar population synthesis: a new approach to understanding AGB stars
Thermally pulsating asymptotic giant branch (AGB) stars are the main
producers of slow neutron capture (s-) process elements, but there are still
large uncertainties associated with the formation of the main neutron source,
13C, and with the physics of these stars in general. Observations of s-process
element enhancements in stars can be used as constraints on theoretical models.
For the first time we apply stellar population synthesis to the problem of
s-process nucleosynthesis in AGB stars, in order to derive constraints on free
parameters describing the physics behind the third dredge-up and the properties
of the neutron source. We utilize a rapid evolution and nucleosynthesis code to
synthesize different populations of s-enhanced stars, and compare them to their
observational counterparts to find out for which values of the free parameters
in the code the synthetic populations fit best to the observed populations.
These free parameters are the amount of third dredge-up, the minimum core mass
for third dredge-up, the effectiveness of 13C as a source of neutrons and the
size in mass of the 13C pocket. We find that galactic disk objects are
reproduced by a spread of a factor of two in the effectiveness of the 13C
neutron source. Lower metallicity objects can be reproduced only by lowering by
at least a factor of 3 the average value of the effectiveness of the 13C
neutron source needed for the galactic disk objects. Using observations of
s-process elements in post-AGB stars as constraints we find that dredge-up has
to start at a lower core mass than predicted by current theoretical models,
that it has to be substantial ( >~ 0.2) in stars with mass M <~ 1.5
M_sun and that the mass of the 13C pocket must be about 1/40 that of the
intershell region.Comment: 16 pages, 15 figures, accepted for publication in Astronomy &
Astrophysic
Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. II. Statistical analysis of a sample of 67 CEMP- stars
Many observed CEMP stars are found in binary systems and show enhanced
abundances of -elements. The origin of the chemical abundances of these
CEMP- stars is believed to be accretion in the past of enriched material
from a primary star in the AGB phase. We investigate the mechanism of mass
transfer and the process of nucleosynthesis in low-metallicity AGB stars by
modelling the binary systems in which the observed CEMP- stars were formed.
For this purpose we compare a sample of CEMP- stars with a grid of
binary stars generated by our binary evolution and nucleosynthesis model. We
classify our sample CEMP- stars in three groups based on the observed
abundance of europium. In CEMP stars the europium-to-iron ratio is more
than ten times higher than in the Sun, whereas it is lower than this threshold
in CEMP stars. No measurement of europium is currently available for
CEMP- stars. On average our models reproduce well the abundances observed
in CEMP- stars, whereas in CEMP- stars and CEMP- stars the
abundances of the light- elements are systematically overpredicted by our
models and in CEMP- stars the abundances of the heavy- elements are
underestimated. In all stars our modelled abundances of sodium overestimate the
observations. This discrepancy is reduced only in models that underestimate the
abundances of most of the -elements. Furthermore, the abundance of lead is
underpredicted in most of our model stars. These results point to the
limitations of our AGB nucleosynthesis model, particularly in the predictions
of the element-to-element ratios. Finally, in our models CEMP- stars are
typically formed in wide systems with periods above 10000 days, while most of
the observed CEMP- stars are found in relatively close orbits with periods
below 5000 days.Comment: 23 pages, 8 figures, accepted for publication on Astronomy &
Astrophysic
Third Dredge-up in Low Mass Stars: Solving the LMC Carbon Star Mystery
A long standing problem with asymptotic giant branch (AGB) star models has
been their inability to produce the low-luminosity carbon stars in the Large
and Small Magellanic Clouds. Dredge-up must begin earlier and extend deeper. We
find this for the first time in our models of LMC metallicity. Such features
are not found in our models of SMC metallicity. The fully implicit and
simultaneous stellar evolution code STARS has been used to calculate the
evolution of AGB stars with metallicities of Z=0.008 and Z=0.004, corresponding
to the observed metallicities of the Large and Small Magellanic Clouds,
respecitively. Third dredge-up occurs in stars of 1Msol and above and carbon
stars were found for models between 1Msol and 3Msol. We use the detailed models
as input physics for a population synthesis code and generate carbon star
luminosity functions. We now find that we are able to reproduce the carbon star
luminosity function of the LMC without any manipulation of our models. The SMC
carbon star luminosity function still cannot be produced from our detailed
models unless the minimum core mass for third dredge-up is reduced by 0.06Msol.Comment: 6 pages, 5 figures. Accepted for publication in MNRA
Reaction rate uncertainties and the operation of the NeNa and MgAl chains during HBB in intermediate-mass AGB stars
We test the effect of proton-capture reaction rate uncertainties on the
abundances of the Ne, Na, Mg and Al isotopes processed by the NeNa and MgAl
chains during hot bottom burning (HBB) in asymptotic giant branch (AGB) stars
of intermediate mass between 4 and 6 solar masses and metallicities between
Z=0.0001 and 0.02. We provide uncertainty ranges for the AGB stellar yields,
for inclusion in galactic chemical evolution models, and indicate which
reaction rates are most important and should be better determined. We use a
fast synthetic algorithm based on detailed AGB models. We run a large number of
stellar models, varying one reaction per time for a very fine grid of values,
as well as all reactions simultaneously. We show that there are uncertainties
in the yields of all the Ne, Na, Mg and Al isotopes due to uncertain
proton-capture reaction rates. The most uncertain yields are those of 26Al and
23Na (variations of two orders of magnitude), 24Mg and 27Al (variations of more
than one order of magnitude), 20Ne and 22Ne (variations between factors 2 and
7). In order to obtain more reliable Ne, Na, Mg and Al yields from IM-AGB stars
the rates that require more accurate determination are: 22Ne(p,g)23Na,
23Na(p,g)24Mg, 25Mg(p,g)26Al, 26Mg(p,g)27Al and 26Al(p,g)27Si. Detailed
galactic chemical evolution models should be constructed to address the impact
of our uncertainty ranges on the observational constraints related to HBB
nucleosynthesis, such as globular cluster chemical anomalies.Comment: accepted for publication on Astronomy & Astrophysic
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
Models of the circumstellar medium of evolving, massive runaway stars moving through the Galactic plane
At least 5 per cent of the massive stars are moving supersonically through
the interstellar medium (ISM) and are expected to produce a stellar wind bow
shock. We explore how the mass loss and space velocity of massive runaway stars
affect the morphology of their bow shocks. We run two-dimensional axisymmetric
hydrodynamical simulations following the evolution of the circumstellar medium
of these stars in the Galactic plane from the main sequence to the red
supergiant phase. We find that thermal conduction is an important process
governing the shape, size and structure of the bow shocks around hot stars, and
that they have an optical luminosity mainly produced by forbidden lines, e.g.
[OIII]. The Ha emission of the bow shocks around hot stars originates from near
their contact discontinuity. The H emission of bow shocks around cool
stars originates from their forward shock, and is too faint to be observed for
the bow shocks that we simulate. The emission of optically-thin radiation
mainly comes from the shocked ISM material. All bow shock models are brighter
in the infrared, i.e. the infrared is the most appropriate waveband to search
for bow shocks. Our study suggests that the infrared emission comes from near
the contact discontinuity for bow shocks of hot stars and from the inner region
of shocked wind for bow shocks around cool stars. We predict that, in the
Galactic plane, the brightest, i.e. the most easily detectable bow shocks are
produced by high-mass stars moving with small space velocities.Comment: 22 pages, 24 figure
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