121 research outputs found
The Effect of Tides on the Population of PN from Interacting Binaries
We have used the tidal equations of Zahn to determine the maximum orbital
distance at which companions are brought into Roche lobe contact with their
giant primary, when the primary expands during the giant phases. This is a key
step when determining the rates of interaction between giants and their
companions. Our stellar structure calculations are presented as maximum radii
reached during the red and asymptotic giant branch (RGB and AGB, respectively)
stages of evolution for masses between 0.8 and 4.0 Mo (Z=0.001 - 0.04) and
compared with other models to gauge the uncertainty on radii deriving from
details of these calculations. We find overall tidal capture distances that are
typically 1-4 times the maximum radial extent of the giant star, where
companions are in the mass range from 1 Jupiter mass to a mass slightly smaller
than the mass of the primary. We find that only companions at initial orbital
separations between ~320 and ~630 Ro will be typically captured into a Roche
lobe-filling interaction or a common envelope on the AGB. Comparing these
limits with the period distribution for binaries that will make PN, we deduce
that in the standard scenario where all ~1-8 Mo stars make a PN, at most 2.5
per cent of all PN should have a post-common envelope central star binary, at
odds with the observational lower limit of 15-20 per cent. The observed
over-abundance of post-interaction central stars of PN cannot be easily
explained considering the uncertainties. We examine a range of explanations for
this discrepancy.Comment: 19 pages, 16 figures, accepted by Monthly Notices of the Royal
Astronomical Societ
The radial metallicity gradient and the history of elemental enrichment in M81 through emission-line probes
We present a new set of weak-line abundances of HII regions in M81, based on
Gemini Multi-Object Spectrograph (GMOS) observations. The aim is to derive
plasma and abundance analysis for a sizable set of emission-line targets to
study the galactic chemical contents in the framework of galactic metallicity
gradients. We used the weak-line abundance approach by deriving electron
density and temperatures for several HII regions in M81. Gradient analysis is
based on oxygen abundances.Together with a set of HII region abundances
determined similarly by us with Multi-Mirror Telescope (MMT) spectra, the new
data yield to a radial oxygen gradient of -0.0880.013 dex kpc,
which is steeper than the metallicity gradient obtained for planetary nebulae
(-0.0440.007 dex kpc). This result could be interpreted as gradient
evolution with time: Models of galactic evolution with inside-out disk
formation associated to pre-enriched gas infall would produce such difference
of gradients, although stellar migration effects would also induce a difference
in the metallicity gradients between the old and young populations. By
comparing the M81 metallicity gradients with those of other spiral galaxies,
all consistently derived from weak-line analysis, we can infer that similar
gradient difference is common among spirals. The metallicity gradient slopes
for HII regions and PNe seem to be steeper in M81 than in other galactic disks,
which is likely due to the fact that M81 belongs to a galaxy group. We also
found that M81 has experienced an average oxygen enrichment of 0.140.08
dex in the spatial domain defined by the observations. Our data are compatible
with a break in the radial oxygen gradient slope around R as inferred by
other authors both in M81 and in other galaxies.Comment: Astronomy and Astrophysics, in pres
Hot Jupiters and Cool Stars
Close-in planets are in jeopardy as their host stars evolve off the main
sequence to the subgiant and red giant phases. In this paper, we explore the
influences of the stellar mass (in the range 1.5--2\Mso ), mass-loss
prescription, planet mass (from Neptune up to 10 Jupiter masses), and
eccentricity, on the orbital evolution of planets as their parent stars evolve
to become subgiants and Red Giants. We find that planet engulfment during the
Red Giant Branch is not very sensitive to the stellar mass or mass-loss rates
adopted in the calculations, but quite sensitive to the planetary mass. The
range of initial separations for planet engulfment increases with decreasing
mass-loss rates or stellar mass and increasing planetary masses. Regarding the
planet's orbital eccentricity, we find that as the star evolves into the red
giant phase, stellar tides start to dominate over planetary tides. As a
consequence, a transient population of moderately eccentric close-in Jovian
planets is created, that otherwise would have been expected to be absent from
main sequence stars. We find that very eccentric and distant planets do not
experience much eccentricity decay, and that planet engulfment is primarily
determined by the pericenter distance and the maximum stellar radius.Comment: 38 pages, 15 figures, accepted for publication in Ap
Foretellings of Ragnarök: World-engulfing asymptotic giants and the inheritance of white dwarfs
The Astrophysical Journal 761.2 (2012): 21, reproduced by permission of the AASThe search for planets around white dwarf stars, and evidence for dynamical instability around them in the form of atmospheric pollution and circumstellar disks, raises questions about the nature of planetary systems that can survive the vicissitudes of the asymptotic giant branch (AGB). We study the competing effects, on planets at several AU from the star, of strong tidal forces arising from the star's large convective envelope, and of the planets' orbital expansion due to stellar mass loss. We study, for the first time, the evolution of planets while following each thermal pulse on the AGB. For Jovian planets, tidal forces are strong, and can pull into the envelope planets initially at ∼3 AU for a 1 M⊙star and ∼5 AU for a 5 M⊙ star. Lower-mass planets feel weaker tidal forces, and terrestrial planets initially within 1.5-3 AU enter the stellar envelope. Thus, low-mass planets that begin inside the maximum stellar radius can survive, as their orbits expand due to mass loss. The inclusion of a moderate planetary eccentricity slightly strengthens the tidal forces experienced by Jovian planets. Eccentric terrestrial planets are more at risk, since their eccentricity does not decay and their small pericenter takes them inside the stellar envelope. We also find the closest radii at which planets will be found around white dwarfs, assuming that any planet entering the stellar envelope is destroyed. Planets are in that case unlikely to be found inside ∼1.5 AU of a white dwarf with a 1 M⊙ progenitor and ∼10 AU of a white dwarf with a 5 M⊙ progenitor.This work is funded by the Spanish National Plan of R&D grant AYA2010-20630, “Planets and stellar evolution.” E.V. also acknowledges the support provided by the Marie Curie grant FP7-People-RG268111
The Survival of Planetary Nebulae in the Intracluster Medium
The stellar population stripped from galaxies in clusters evolve under the
extreme conditions imposed by the intracluster (IC) medium. Intracluster stars
generally suffer very high systemic velocities, and evolve within a rarefied
and extremely hot IC medium. We present numerical simulations which aim to
explore the evolution and survival of IC Asymptotic Giant Branch (AGB)
envelopes and Planetary Nebula (PN) shells. Our models reflect the evolution of
a low-mass star under the observed conditions in the Virgo IC medium. We find
that the integrated hydrogen-recombination line emission of a PN is dominated
by the inner dense shell, whose evolution is unaffected by the environment. Ram
pressure stripping affects mainly the outermost IC PN shell, which hardly
influences the emission when the PN is observed as a point source. More
importantly, we find that a PN with progenitor mass of 1 Msun fades to ~30% and
10% of its maximum emission, in 5,000 and 10,000 yr respectively, disclosing an
actual PN lifetime t_PN several times shorter to what is usually adopted
(25,000 yr). This result affects the theoretical calculation of the
luminosity-specific density of IC PNe, which scales with t_PN. For t_PN=10,000
yr, our more conservative estimate, we obtain that the luminosity-specific
density of PNe is in fair agreement with the value obtained from Red Giants.
With our more realistic PN lifetime we infer a higher fraction (above 15%) of
IC starlight in the Virgo core than current estimates.Comment: Accepted for publication in the Astrophysical Journal 14 pages,
including 2 figure
The Weihai Observatory search for close-in planets orbiting giant stars
Planets are known to orbit giant stars, yet there is a shortage of planets
orbiting within ~0.5 AU (P<100 days). First-ascent giants have not expanded
enough to engulf such planets, but tidal forces can bring planets to the
surface of the star far beyond the stellar radius. So the question remains: are
tidal forces strong enough in these stars to engulf all the missing planets? We
describe a high-cadence observational program to obtain precise radial
velocities of bright giants from Weihai Observatory of Shandong University. We
present data on the planet host Beta Gem (HD 62509), confirming our ability to
derive accurate and precise velocities; our data achieve an rms of 7.3 m/s
about the Keplerian orbit fit. This planet-search programme currently receives
~100 nights per year, allowing us to aggressively pursue short-period planets
to determine whether they are truly absent.Comment: Accepted for publication in PAS
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