3,057 research outputs found
Higgs bundles over elliptic curves
In this paper we study -Higgs bundles over an elliptic curve when the
structure group is a classical complex reductive Lie group. Modifying the
notion of family, we define a new moduli problem for the classification of
semistable -Higgs bundles of a given topological type over an elliptic curve
and we give an explicit description of the associated moduli space as a finite
quotient of a product of copies of the cotangent bundle of the elliptic curve.
We construct a bijective morphism from this new moduli space to the usual
moduli space of semistable -Higgs bundles, proving that the former is the
normalization of the latter. We also obtain an explicit description of the
Hitchin fibration for our (new) moduli space of -Higgs bundles and we study
the generic and non-generic fibres
Lithium-7 surface abundance in pre-MS stars. Testing theory against clusters and binary systems
The disagreement between theoretical predictions and observations for surface
lithium abundance in stars is a long-standing problem, which indicates that the
adopted physical treatment is still lacking in some points. However, thanks to
the recent improvements in both models and observations, it is interesting to
analyse the situation to evaluate present uncertainties. We present a
consistent and quantitative analysis of the theoretical uncertainties affecting
surface lithium abundance in the current generation of models. By means of an
up-to-date and well tested evolutionary code, FRANEC, theoretical errors on
surface 7Li abundance predictions, during the pre-main sequence (pre-MS) and
main sequence (MS) phases, are discussed in detail. Then, the predicted surface
7Li abundance was tested against observational data for five open clusters,
namely Ic 2602, \alpha Per, Blanco1, Pleiades, and Ngc 2516, and for four
detached double-lined eclipsing binary systems. Stellar models for the
aforementioned clusters were computed by adopting suitable chemical
composition, age, and mixing length parameter for MS stars determined from the
analysis of the colour-magnitude diagram of each cluster. We restricted our
analysis to young clusters, to avoid additional uncertainty sources such as
diffusion and/or radiative levitation efficiency. We confirm the disagreement,
within present uncertainties, between theoretical predictions and 7Li
observations for standard models. However, we notice that a satisfactory
agreement with observations for 7Li abundance in both young open clusters and
binary systems can be achieved if a lower convection efficiency is adopted
during the pre-MS phase with respect to the MS one.Comment: 10 pages, 5 figures. Accepted for publication in A&
Accreting pre-main sequence models and abundance anomalies in globular clusters
We investigated the possibility of producing helium enhanced stars in
globular clusters by accreting polluted matter during the pre-main sequence
phase. We followed the evolution of two different classes of pre-main sequence
accreting models, one which neglects and the other that takes into account the
protostellar evolution.
We analysed the dependence of the final central helium abundance, of the
tracks position in the HR diagram and of the surface lithium abundance
evolution on the age at which the accretion of polluted material begins and on
the main physical parameters that govern the protostellar evolution. The later
is the beginning of the late accretion and the lower are both the central
helium and the surface lithium abundances at the end of the accretion phase and
in ZAMS (Zero Age Main Sequence). In order to produce a relevant increase of
the central helium content the accretion of polluted matter should start at
ages lower than 1 Myr. The inclusion of the protostellar evolution has a strong
impact on the ZAMS models too. The adoption of a very low seed mass (i.e. 0.001
M) results in models with the lowest central helium and surface
lithium abundances. The higher is the accretion rate and the lower is the final
helium content in the core and the residual surface lithium. In the worst case
-- i.e. seed mass 0.001 M and accretion rate M
yr -- the central helium is not increased at all and the surface lithium
is fully depleted in the first few million years.Comment: Accepted for pubblication in MNRAS. 19 pages, 15 figures, 2 table
Betti numbers of the moduli space of rank 3 parabolic Higgs bundles
We compute the Betti numbers of the moduli space of rank 3 parabolic Higgs
bundles, using Morse theory. A key point is that certain critical submanifolds
of the Morse function can be identified with moduli spaces of parabolic
triples. These moduli spaces come in families depending on a real parameter and
we study their variation with this parameter.Comment: 78 pages. Extended version. Added a section with the fixed
determinant case. To appear in Memoirs of the AM
Dust photophoretic transport around a T Tauri star: Implications for comets composition
There is a growing body of evidences for the presence of crystalline material
in comets. These crystals are believed to have been annealed in the inner part
of the proto-solar nebula, while comets should have been formed in the outer
regions. Several transport processes have been proposed to reconcile these two
facts; among them a migration driven by photophoresis. The primarily goal of
this work is to assess whether disk irradiation by a Pre-Main Sequence star
would influence the photophoretic transport. To do so, we have implemented an
evolving 1+1D model of an accretion disk, including advanced numerical
techniques, undergoing a time-dependent irradiation, consistent with the
evolution of the proto-Sun along the Pre-Main Sequence. The photophoresis is
described using a formalism introduced in several previous works. Adopting the
opacity prescription used in these former studies, we find that the disk
irradiation enhances the photophoretic transport: the assumption of a disk
central hole of several astronomical units in radius is no longer strictly
required, whereas the need for an ad hoc introduction of photoevaporation is
reduced. However, we show that a residual trail of small particles could
annihilate the photophoretic driven transport via their effect on the opacity.
We have also confirmed that the thermal conductivity of transported aggregates
is a crucial parameter which could limit or even suppress the photophoretic
migration and generate several segregation effects
Theoretical uncertainties on the radius of low- and very-low mass stars
We performed an analysis of the main theoretical uncertainties that affect
the radius of low- and very-low mass-stars predicted by current stellar models.
We focused on stars in the mass range 0.1-1Msun, on both the zero-age
main-sequence (ZAMS) and on 1, 2 and 5 Gyr isochrones. First, we quantified the
impact on the radius of the uncertainty of several quantities, namely the
equation of state, radiative opacity, atmospheric models, convection efficiency
and initial chemical composition. Then, we computed the cumulative radius error
stripe obtained by adding the radius variation due to all the analysed
quantities. As a general trend, the radius uncertainty increases with the
stellar mass. For ZAMS structures the cumulative error stripe of very-low mass
stars is about and percent, while at larger masses it increases
up to and percent. The radius uncertainty gets larger and age
dependent if isochrones are considered, reaching for Msun about
percent at an age of 5 Gyr. We also investigated the radius
uncertainty at a fixed luminosity. In this case, the cumulative error stripe is
the same for both ZAMS and isochrone models and it ranges from about
percent to and () percent. We also showed that the sole
uncertainty on the chemical composition plays an important role in determining
the radius error stripe, producing a radius variation that ranges between about
and percent on ZAMS models with fixed mass and about
and percent at a fixed luminosity.Comment: 18 pages, 20 figures, 1 table; accepted for publication in MNRA
Effect of planet ingestion on low-mass stars evolution: the case of 2MASS J08095427--4721419 star in the Gamma Velorum cluster
We analysed the effects of planet ingestion on the characteristics of a
pre-MS star similar to the Gamma Velorum cluster member 2MASS
J08095427--4721419 (#52). We discussed the effects of changing the age at
which the accretion episode occurs, the mass of the ingested planet and its
chemical composition. We showed that the mass of the ingested planet required
to explain the current [Fe/H]^#52 increases by decreasing the age and/or
by decreasing the Iron content of the accreted matter.
We compared the predictions of a simplified accretion method -- where only
the variation of the surface chemical composition is considered -- with that of
a full accretion model that properly accounts for the modification of the
stellar structure. We showed that the two approaches result in different
convective envelope extension which can vary up to 10 percent. We discussed the
impact of the planet ingestion on a stellar model in the colour-magnitude
diagram, showing that a maximum shift of about 0.06 dex in the colour and 0.07
dex in magnitude are expected and that such variations persist even much later
the accretion episode. We also analysed the systematic bias in the stellar mass
and age inferred by using a grid of standard non accreting models to recover
the characteristics of an accreting star. We found that standard non accreting
models can safely be adopted for mass estimate, as the bias is <= 6 percent,
while much more caution should be used for age estimate where the differences
can reach about 60 percent.Comment: Accepted for publication in MNRAS. 13 pages, 3 tables, 9 figure
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