482 research outputs found
Benchmark ages for the Gaia benchmark stars
In the era of large-scale surveys of stars in the Milky Way, stellar ages are
crucial for studying the evolution of the Galaxy. But determining ages of field
stars is notoriously difficult; therefore, we attempt to determine benchmark
ages for the extensively studied Gaia benchmark stars which can be used for
validation purposes. By searching the literature for age estimates from
different methods and deriving new ages based on Bayesian isochrone fitting, we
are able to put reliable limits on the ages of 16 out of the 33 benchmark
stars. The giants with well-defined ages are all young, and an expansion of the
sample to include older giants with asteroseismic ages would be beneficial.
Some of the stars have surface parameters inconsistent with isochrones younger
than 16 Gyr. Including -enhancement in the models when relevant
resolves some of these cases, but others clearly highlight discrepancies
between the models and observations. We test the impact of atomic diffusion on
the age estimates by fitting to the actual surface metallicity of the models
instead of the initial value and find that the effect is negligible except for
a single turn-off star. Finally, we show that our ability to determine
isochrone-based ages for large spectroscopic surveys largely mirrors our
ability to determine ages for these benchmark stars, except for stars with
dex since their location in the HR diagram is almost age
insensitive. Hence, isochrone fitting does not constrain their ages given the
typical uncertainties of spectroscopic stellar parameters.Comment: Accepted in MNRAS. 69 pages (18 for main text, 11 for appendix, and
40 for extra figures
Long-term stability of the HR 8799 planetary system without resonant lock
HR 8799 is a star accompanied by four massive planets on wide orbits. The
observed planetary configuration has been shown to be unstable on a timescale
much shorter than the estimated age of the system (~ 30 Myr) unless the planets
are locked into mean motion resonances. This condition is characterised by
small-amplitude libration of one or more resonant angles that stabilise the
system by preventing close encounters. We simulate planetary systems similar to
the HR 8799 planetary system, exploring the parameter space in separation
between the orbits, planetary masses and distance from the Sun to the star. We
find systems that look like HR 8799 and remain stable for longer than the
estimated age of HR 8799. None of our systems are forced into resonances. We
find, with nominal masses and in a narrow range of orbit separations, that 5 of
100 systems match the observations and lifetime. Considering a broad range of
orbit separations, we find 12 of 900 similar systems. The systems survive
significantly longer because of their slightly increased initial orbit
separations compared to assuming circular orbits from the observed positions. A
small increase in separation leads to a significant increase in survival time.
The low eccentricity the orbits develop from gravitational interaction is
enough for the planets to match the observations. With lower masses, but still
comfortably within the estimated planet mass uncertainty, we find 18 of 100
matching and long-lived systems in a narrow orbital separation range. In the
broad separation range, we find 82 of 900 matching systems. Our results imply
that the planets in the HR 8799 system do not have to be in strong mean motion
resonances.Comment: Accepted for publication in A&
Supernova enrichment and dynamical histories of solar-type stars in clusters
We use N-body simulations of star cluster evolution to explore the hypothesis
that short-lived radioactive isotopes found in meteorites, such as 26-Al, were
delivered to the Sun's protoplanetary disc from a supernova at the epoch of
Solar System formation. We cover a range of star cluster formation parameter
space and model both clusters with primordial substructure, and those with
smooth profiles. We also adopt different initial virial ratios - from cool,
collapsing clusters to warm, expanding associations. In each cluster we place
the same stellar population; the clusters each have 2100 stars, and contain one
massive 25M_Sun star which is expected to explode as a supernova at about
6.6Myr. We determine the number of Solar (G)-type stars that are within 0.1 -
0.3pc of the 25M_Sun star at the time of the supernova, which is the distance
required to enrich the protoplanetary disc with the 26-Al abundances found in
meteorites. We then determine how many of these G-dwarfs are unperturbed
`singletons'; stars which are never in close binaries, nor suffer sub-100au
encounters, and which also do not suffer strong dynamical perturbations.
The evolution of a suite of twenty initially identical clusters is highly
stochastic, with the supernova enriching over 10 G-dwarfs in some clusters, and
none at all in others. Typically only ~25 per cent of clusters contain
enriched, unperturbed singletons, and usually only 1 - 2 per cluster (from a
total of 96 G-dwarfs in each cluster). The initial conditions for star
formation do not strongly affect the results, although a higher fraction of
supervirial (expanding) clusters would contain enriched G-dwarfs if the
supernova occurred earlier than 6.6Myr. If we sum together simulations with
identical initial conditions, then ~1 per cent of all G-dwarfs in our
simulations are enriched, unperturbed singletons.Comment: 14 pages, 5 figures, accepted for publication in MNRA
Close encounters involving free-floating planets in star clusters
Instabilities in planetary systems can result in the ejection of planets from
their host system, resulting in free-floating planets (FFPs). If this occurs in
a star cluster, the FFP may remain bound to the star cluster for some time and
interact with the other cluster members until it is ejected. Here, we use
-body simulations to characterise close star-planet and planet-planet
encounters and the dynamical fate of the FFP population in star clusters
containing single or binary star members. We find that FFPs ejected
from their planetary system at low velocities typically leave the star cluster
40% earlier than their host stars, and experience tens of close ( AU)
encounters with other stars and planets before they escape. The fraction of
FFPs that experiences a close encounter depends on both the stellar density and
the initial velocity distribution of the FFPs. Approximately half of the close
encounters occur within the first 30 Myr, and only 10% occur after 100 Myr. The
periastron velocity distribution for all encounters is well-described by a
modified Maxwell-Bolzmann distribution, and the periastron distance
distribution is linear over almost the entire range of distances considered,
and flattens off for very close encounters due to strong gravitational
focusing. Close encounters with FFPs can perturb existing planetary systems and
their debris structures, and they can result in re-capture of FFPs. In
addition, these FFP populations may be observed in young star clusters in
imaging surveys; a comparison between observations and dynamical predictions
may provide clues to the early phases of stellar and planetary dynamics in star
clusters.Comment: Accepted for publication in MNRAS; 18 pages, 12 figure
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