183 research outputs found
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
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
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
The formation of multiple populations in the globular cluster 47 Tuc
We use the combination of photometric and spectroscopic data of 47 Tuc stars
to reconstruct the possible formation of a second generation of stars in the
central regions of the cluster, from matter ejected from massive Asymptotic
Giant Branch stars, diluted with pristine gas. The yields from massive AGB
stars with the appropriate metallicity (Z=0.004, i.e. [Fe/H]=-0.75) are
compatible with the observations, in terms of extension and slope of the
patterns observed, involving oxygen, nitrogen, sodium and aluminium. Based on
the constraints on the maximum helium of 47 Tuc stars provided by photometric
investigations, and on the helium content of the ejecta, we estimate that the
gas out of which second generation stars formed was composed of about one-third
of gas from intermediate mass stars, with M>= 5Mo and about two-thirds of
pristine gas. We tentatively identify the few stars whose Na, Al and O
abundances resemble the undiluted AGB yields with the small fraction of 47 Tuc
stars populating the faint subgiant branch. From the relative fraction of first
and second generation stars currently observed, we estimate that the initial FG
population in 47 Tuc was about 7.5 times more massive than the cluster current
total mass.Comment: Accepted for publication in 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
On the nature of the most obscured C-rich AGB stars in the Magellanic Clouds
The stars in the Magellanic Clouds with the largest degree of obscuration are
used to probe the highly uncertain physics of stars in the asymptotic giant
branch (AGB) phase of evolution. Carbon stars in particular, provide key
information on the amount of third dredge-up (TDU) and mass loss. We use two
independent stellar evolution codes to test how a different treatment of the
physics affects the evolution on the AGB. The output from the two codes are
used to determine the rates of dust formation in the circumstellar envelope,
where the method used to determine the dust is the same for each case. The
stars with the largest degree of obscuration in the LMC and SMC are identified
as the progeny of objects of initial mass and , respectively. This difference in mass is motivated by the
difference in the star formation histories of the two galaxies, and offers a
simple explanation of the redder infrared colours of C-stars in the LMC
compared to their counterparts in the SMC. The comparison with the Spitzer
colours of C-rich AGB stars in the SMC shows that a minimum surface carbon mass
fraction must have been reached by stars of initial
mass around . Our results confirm the necessity of adopting
low-temperature opacities in stellar evolutionary models of AGB stars. These
opacities allow the stars to obtain mass-loss rates high enough () to produce the amount of dust needed to reproduce the
Spitzer coloursComment: 14 pages, 5 figures, 1 table; accepted for publication in MNRAS Main
Journa
Probing O-enrichment in C-rich dust planetary nebulae
The abundance of O in planetary nebulae (PNe) has been historically used as a
metallicity indicator of the interstellar medium (ISM) where they originated;
e.g., it has been widely used to study metallicity gradients in our Galaxy and
beyond. However, clear observational evidence for O self enrichment in
low-metallicity Galactic PNe with C-rich dust has been recently reported. Here
we report asymptotic giant branch (AGB) nucleosynthesis predictions for the
abundances of the CNO elements and helium in the metallicity range Zsun/4 < Z <
2Zsun. Our AGB models, with diffusive overshooting from all the convective
borders, predict that O is overproduced in low-Z low-mass (~1-3 Msun) AGB stars
and nicely reproduce the recent O overabundances observed in C-rich dust PNe.
This confirms that O is not always a good proxy of the original ISM metallicity
and another chemical elements such as Cl or Ar should be used instead. The
production of oxygen by low-mass stars should be thus considered in
galactic-evolution models.Comment: Accepted for publication in MNRAS Letters (5 pages, 1 figure, and 1
table
The Large Magellanic Cloud as a laboratory for Hot Bottom Burning in massive Asymptotic Giant Branch stars
We use Spitzer observations of the rich population of Asymptotic Giant Branch
stars in the Large Magellanic Cloud (LMC) to test models describing the
internal structure and nucleosynthesis of the most massive of these stars, i.e.
those with initial mass above . To this aim, we compare
Spitzer observations of LMC stars with the theoretical tracks of Asymptotic
Giant Branch models, calculated with two of the most popular evolution codes,
that are known to differ in particular for the treatment of convection.
Although the physical evolution of the two models are significantly different,
the properties of dust formed in their winds are surprisingly similar, as is
their position in the colour-colour (CCD) and colour-magnitude (CMD) diagrams
obtained with the Spitzer bands. This model independent result allows us to
select a well defined region in the () plane,
populated by AGB stars experiencing Hot Bottom Burning, the progeny of stars
with mass . This result opens up an important test of the
strength hot bottom burning using detailed near-IR (H and K bands)
spectroscopic analysis of the oxygen-rich, high luminosity candidates found in
the well defined region of the colour-colour plane. This test is possible
because the two stellar evolution codes we use predict very different results
for the surface chemistry, and the C/O ratio in particular, owing to their
treatment of convection in the envelope and of convective boundaries during
third dredge-up. The differences in surface chemistry are most apparent when
the model stars reach the phase with the largest infrared emission.Comment: 11 pages, 14 figures, accepted for publication in MNRA
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