485 research outputs found
A test for asymptotic giant branch evolution theories: Planetary Nebulae in the Large Magellanic Cloud
We used a new generation of asymptotic giant branch (AGB) stellar models that
include dust formation in the stellar winds to find the links between
evolutionary models and the observed properties of a homogeneous sample of
Large Magellanic Cloud (LMC) planetary nebulae (PNe). Comparison between the
evolutionary yields of elements such as CNO and the corresponding observed
chemical abundances is a powerful tool to shed light on evolutionary processes
such as hot bottom burning (HBB) and third dredge-up (TDU). We found that the
occurrence of HBB is needed to interpret the nitrogen-enriched (log(N/H)+12>8)
PNe. In particular, N-rich PNe with the lowest carbon content are nicely
reproduced by AGB models of mass M >=6 Mo, whose surface chemistry reflects the
pure effects of HBB. PNe with log(N/H)+12<7.5 correspond to ejecta of stars
that have not experienced HBB, with initial mass below about 3 Mo. Some of
these stars show very large carbon abundances, owing to the many TDU episodes
experienced. We found from our LMC PN sample that there is a threshold to the
amount of carbon accumulated at AGB surfaces, log(C/H)+12<9. Confirmation of
this constraint would indicate that, after the C-star stage is reached,AGBs
experience only a few thermal pulses, which suggests a rapid loss of the
external mantle, probably owing to the effects of radiation pressure on
carbonaceous dust particles present in the circumstellar envelope. The
implications of these findings for AGB evolution theories and the need to
extend the PN sample currently available are discussed.Comment: 12 pages, 4 figures, 1 table, accepted for publication in MNRAS (2015
July 13; in original form 2015 June 9
Evolved stars in the Local Group galaxies. I. AGB evolution and dust production in IC 1613
We used models of thermally-pulsing asymptotic giant branch (AGB) stars, that
also describe the dust-formation process in the wind, to interpret the
combination of near- and mid-infrared photometric data of the dwarf galaxy IC
1613. This is the first time that this approach is extended to an environment
different from the Milky Way and the Magellanic Clouds (MCs). Our analysis,
based on synthetic population techniques, shows a nice agreement between the
observations and the expected distribution of stars in the colour-magnitude
diagrams obtained with JHK and Spitzer bands. This allows a characterization of
the individual stars in the AGB sample in terms of mass, chemical composition,
and formation epoch of the progenitors. We identify the stars exhibiting the
largest degree of obscuration as carbon stars evolving through the final AGB
phases, descending from 1-1.25Msun objects of metallicity Z=0.001 and from
1.5-2.5Msun stars with Z=0.002. Oxygen-rich stars constitute the majority of
the sample (65%), mainly low mass stars (<2Msun) that produce a negligible
amount of dust (<10^{-7}Msun/yr). We predict the overall dust-production rate
from IC 1613, mostly determined by carbon stars, to be 6x10^{-7}Msun/yr with an
uncertainty of 30%. The capability of the current generation of models to
interpret the AGB population in an environment different from the MCs opens the
possibility to extend this kind of analysis to other Local Group galaxies.Comment: 14 pages, 6 figures, accepted for publication on MNRA
Planetary Nebulae in the Small Magellanic Cloud
We analyse the planetary nebulae (PNe) population of the Small Magellanic
Cloud (SMC), based on evolutionary models of stars with metallicities in the
range and mass , evolved through the asymptotic giant branch (AGB) phase. The models
used account for dust formation in the circumstellar envelope. To characterise
the PNe sample of the SMC, we compare the observed abundances of the various
species with the final chemical composition of the AGB models: this study
allows us to identify the progenitors of the PNe observed, in terms of mass and
chemical composition. According to our interpretation, most of the PNe descend
from low-mass () stars, which become carbon rich, after
experiencing repeated third dredge-up episodes, during the AGB phase. A
fraction of the PNe showing the signature of advanced CNO processing are
interpreted as the progeny of massive AGB stars, with mass above , undergoing strong hot bottom burning. The differences with the
chemical composition of the PNe population of the Large Magellanic Cloud (LMC)
is explained on the basis of the diverse star formation history and
age-metallicity relation of the two galaxies. The implications of the present
study for some still highly debated points regarding the AGB evolution are also
commented.Comment: Accepted for publication in MNRAS, 11 pages, 4 figure
Gas and dust from solar metallicity AGB stars
We study the asymptotic giant branch (AGB) evolution of stars with masses
between . We focus on stars with a solar chemical
composition, which allows us to interpret evolved stars in the Galaxy. We
present a detailed comparison with models of the same chemistry, calculated
with a different evolution code and based on a different set of physical
assumptions. We find that stars of mass experience hot
bottom burning at the base of the envelope. They have AGB lifetimes shorter
than yr and eject into their surroundings gas contaminated
by proton-capture nucleosynthesis, at an extent sensitive to the treatment of
convection. Low mass stars with become
carbon stars. During the final phases the C/O ratio grows to . We find
a remarkable agreement between the two codes for the low-mass models and
conclude that predictions for the physical and chemical properties of these
stars, and the AGB lifetime, are not that sensitive to the modelling of the AGB
phase. The dust produced is also dependent on the mass: low-mass stars produce
mainly solid carbon and silicon carbide dust, whereas higher mass stars produce
silicates and alumina dust. Possible future observations potentially able to
add more robustness to the present results are also discussed.Comment: 27 pages, 24 figures; accepted for publication in MNRA
Magnesium isotopes: a tool to understand self-enrichment in Globular Clusters
A critical issue in the asymptotic giant branch (AGB) self-enrichment
scenario for the formation of multiple populations in Globular Clusters (GCs)
is the inability to reproduce the magnesium isotopic ratios, despite the model
in principle can account for the depletion of magnesium. In this work we
analyze how the uncertainties on the various p-capture cross sections affect
the results related to the magnesium content of the ejecta of AGB stars. The
observed distribution of the magnesium isotopes and of the overall Mg-Al trend
in M13 and NGC 6752 are successfully reproduced when the proton-capture rate by
25Mg at the temperatures 100 MK, in particular the 25Mg(p, gamma)26Alm channel,
is enhanced by a factor 3 with respect to the most recent experimental
determinations. This assumption also allows to reproduce the full extent of the
Mg spread and the Mg-Si anticorrelation observed in NGC 2419. The uncertainties
in the rate of the 25Mg(p,gamma)26Alm reaction at the temperatures of interest
here leave space for our assumption and we suggest that new experimental
measurements are needed to settle this problem. We also discuss the competitive
model based on the super massive star nucleosynthesis.Comment: accepted for publication on MNRA
AGB stars in the SMC: evolution and dust properties based on Spitzer observations
We study the population of asymptotic giant branch (AGB) stars in the Small
Magellanic Cloud (SMC) by means of full evolutionary models of stars of mass
1Msun < M < 8Msun, evolved through the thermally pulsing phase. The models also
account for dust production in the circumstellar envelope. We compare Spitzer
infrared colours with results from theoretical modelling. We show that ~75% of
the AGB population of the SMC is composed by scarcely obscured objects, mainly
stars of mass M < 2.5Msun at various metallicity, formed between 700 Myr and 5
Gyr ago; ~ 70% of these sources are oxygen--rich stars, while ~ 30% are
C-stars. The sample of the most obscured AGB stars, accounting for ~ 25% of the
total sample, is composed almost entirely by carbon stars. The distribution in
the colour-colour ([3.6]-[4.5], [5.8]-[8.0]) and colour-magnitude ([3.6]-[8.0],
[8.0]) diagrams of these C-rich objects, with a large infrared emission, traces
an obscuration sequence, according to the amount of carbonaceous dust in their
surroundings. The overall population of C-rich AGB stars descends from
1.5-2Msun stars of metallicity Z=0.004, formed between 700 Myr and 2 Gyr ago,
and from lower metallicity objects, of mass below 1.5Msun, 2-5 Gyr old. We also
identify obscured oxygen-rich stars (M ~ 4-6Msun) experiencing hot bottom
burning. The differences between the AGB populations of the SMC and LMC are
also commented.Comment: 18, pages, 11 figures, accepted for publication on MNRA
On the alumina dust production in the winds of O-rich Asymptotic Giant Branch stars
The O-rich Asymptotic Giant Branch (AGB) stars experience strong mass loss
with efficient dust condensation and they are major sources of dust in the
interstellar medium. Alumina dust (AlO) is an important dust component
in O-rich circumstellar shells and it is expected to be fairly abundant in the
winds of the more massive and O-rich AGB stars. By coupling AGB stellar
nucleosynthesis and dust formation, we present a self-consistent exploration on
the AlO production in the winds of AGB stars with progenitor masses
between 3 and 7 M and metallicities in the range 0.0003 Z
0.018. We find that AlO particles form at radial distances from
the centre between and 4 R (depending on metallicity), which is in
agreement with recent interferometric observations of Galactic O-rich AGB
stars. The mass of AlO dust is found to scale almost linearly with
metallicity, with solar metallicity AGBs producing the highest amount (about
10 M) of alumina dust. The AlO grain size decreases
with decreasing metallicity (and initial stellar mass) and the maximum size of
the AlO grains is 0.075 for the solar metallicity models.
Interestingly, the strong depletion of gaseous Al observed in the
low-metallicity HBB AGB star HV 2576 seems to be consistent with the formation
of AlO dust as predicted by our models. We suggest that the content of
Al may be used as a mass (and evolutionary stage) indicator in AGB stars
experiencing HBB.Comment: 13 pages, 8 figures, accepted for publication in MNRA
- …