42 research outputs found
Predicted properties of Galactic and Magellanic Classical Cepheids in the SDSS filters
We present the first extensive and detailed theoretical scenario for the
interpretation of Cepheid properties observed in the SDSS filters. Three sets
of nonlinear convective pulsation models, corresponding to the chemical
compositions of Cepheids in the Milky Way, the Large Magellanic Cloud and the
Small Magellanic Cloud respectively, are transformed into the SDSS bands by
relying on updated model atmospheres. The resulting observables, namely the
instability strip boundaries and the light curves, as well as the
Period-Luminosity, the Wesenheit and the Period-Luminosity-Colour relations,
are discussed as a function of the metal content, for both the fundamental and
the first overtone mode. The fundamental PL relations are found to deviate from
linear relations when computed over the whole observed Cepheid period range,
especially at the shorter wavelenghts, confirming previous findings in the
Johnson-Cousins bands. The obtained slopes are found to be mildly steeper than
the ones of the semiempirical and the empirical relations available in the
literature and covering roughly the same period range, with the discrepancy
ranging from about 13% in u-band to about 3% in z.Comment: Accepted for publication in MNRA
RR Lyrae stars in Galactic globular clusters.III. Pulsational predictions for metal content Z=0.0001 to Z=0.006
The results of nonlinear, convective models of RR Lyrae pulsators with metal content Z=0.0001 to 0.006 are discussed and several predicted relations connecting pulsational (period and amplitude of pulsation) and evolutionary parameters (mass, absolute magnitude and color of the pulsator) are derived. These relations, when linked with the average mass of RR Lyrae stars, as suggested by horizontal branch evolutionary models, provide a ``pulsational'' route to the determination of the distance modulus, both apparent and intrinsic, of RR Lyrae rich globular clusters. Based on a preliminary set of synthetic horizontal branch simulations, we compare the predicted relations with observed variables in selected globular clusters (M2, M3, M5, M15, M55, M68, NGC 1851, NGC 3201, NGC 5466, NGC 6362, NGC 6934 and IC 4499). We show that the distance moduli inferred by the various theoretical relations are mutually consistent within the errors, provided that the value of the mixing-length parameter slightly increases from the blue to the red edge of the pulsation region. Moreover, we show that the relative ``pulsational'' distance moduli fit well previous empirical results and that the parallax of the prototype variable RR Lyr, as inferred by the predicted Period-Wasenheit relation, is in close agreement with the HST astrometric measurement
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
Stars caught in the braking stage in young Magellanic Clouds clusters
The color-magnitude diagrams of many Magellanic Cloud clusters (with ages up
to 2 billion years) display extended turnoff regions where the stars leave the
main sequence, suggesting the presence of multiple stellar populations with
ages which may differ even by hundreds million years (Mackey et al. 2008,
Milone et al. 2009, Girardi et al. 2011). A strongly debated question is
whether such an extended turnoff is instead due to populations with different
stellar rotations (Girardi et al. 2011, Goudfrooij et al. 2011, Rubele et al.
2013, Li et al. 2014). The recent discovery of a `split' main sequence in some
younger clusters (about 80--400Myr) added another piece to this puzzle. The
blue (red) side of the main sequence is consistent with slowly (rapidly)
rotating stellar models (D'Antona et al. 2015, Milone et al. 2016, Correnti et
al. 2017, Milone et al 2016), but a complete theoretical characterization of
the observed color-magnitude diagram appeared to require also an age spread
(Correnti et al. 2017). We show here that, in three clusters so far analyzed,
if the blue main sequence stars are interpreted with models that have been
always slowly rotating, they must be about 30% younger than the rest of the
cluster. If they are instead interpreted as stars initially rapidly rotating,
but that have later slowed down, the age difference disappears, and "braking"
also helps to explain the apparent age differences of the extended turnoff. The
age spreads in Magellanic Cloud clusters are a manifestation of rotational
stellar evolution. Observational tests are suggested.Comment: Accepted for publication and in state of Advance Online Publication
(from 24 July 2017) on Nature Astronom
Self-enrichment in Globular Clusters: the extreme He-rich population of NGC 2808
Almost several decades after the discovery of the first multiple populations
in galactic globular clusters (GC) the debate on their formation is still
extremely current and NGC2808 remains one of the best benchmark to test any
scenario for their origin and the evolution. In this work we focus on the
chemical composition of stars belonging to the extreme He-rich population
populated by stars with the most extreme abundance of Mg, Al, Na, O and Si. We
checked whether the most recent measures are consistent with the AGB yields of
stars of . These stars evolve on time scales of the order of
40-60 Myr and eject matter strongly enriched in helium, owing to a deep
penetration of the surface convective zone down to regions touched by CNO
nucleosynthesis occurring after the core He-burning phase. Since the big
unknown of the AGB phase of massive stars is the mass loss, we propose a new
approch that takes into account the effects of the radiation pressure on dust
particles. We show that this more realistic description is able to reproduce
the observed abundances of Mg, Al, Na and Si in these extreme stars. The large
spread in the oxygen abundances is explained by invoking deep mixing during the
RGB phase. It will be possible to check this work hypothesis as soon as the
oxygen measurements of the main sequence stars of NGC2808 will be available.Comment: 12 pages, 5 figures, accepted 2018 June 29 by MNRA
The dust production rate of AGB stars in the Magellanic Clouds
We compare theoretical dust yields for stars with mass 1 Msun < mstar < 8
Msun, and metallicities 0.001 < Z < 0.008 with observed dust production rates
(DPR) by carbon- rich and oxygen-rich Asymptotic Giant Branch (C-AGB and O-AGB)
stars in the Large and Small Magellanic Clouds (LMC, SMC). The measured DPR of
C-AGB in the LMC are reproduced only if the mass loss from AGB stars is very
efficient during the carbon-star stage. The same yields over-predict the
observed DPR in the SMC, suggesting a stronger metallicity dependence of the
mass-loss rates during the carbon- star stage. DPR of O-AGB stars suggest that
rapid silicate dust enrichment occurs due to efficient hot-bottom-burning if
mstar > 3 Msun and Z > 0.001. When compared to the most recent observations,
our models support a stellar origin for the existing dust mass, if no
significant destruction in the ISM occurs, with a contribution from AGB stars
of 70% in the LMC and 15% in the SMC.Comment: 12 pages, 8 figures, accepted by MNRA
Dust formation in the winds of AGBs: the contribution at low metallicities
We present new models for the evolution of stars with mass in the range 1Msun
< M < 7.5Msun, followed from the pre-main-sequence through the asymptotic giant
branch phase. The metallicity adopted is $Z=3*10^{-4} (which, with an
alpha-enhancement of +0.4, corresponds to [Fe/H]=-2). Dust formation is
described by following the growth of dust grains of various types as the wind
expands from the stellar surface.
Models with mass M>3Msun experience Hot Bottom Burning, thus maintaining the
surface C/O below unity. Unlike higher Z models, the scarcity of silicon
available in the envelope prevents the formation of silicates in meaningful
quantities, sufficient to trigger the acceleration of the wind via radiation
pressure on the dust grains formed. No silicate formation occurs below a
threshold metallicity of Z=10^{-3}.
Low--mass stars, with M< 2.5Msun become carbon stars, forming solid carbon
dust in their surroundings. The total dust mass formed depends on the uncertain
extent of the inwards penetration of the convective envelope during the Third
Dredge--Up episodes following the Thermal Pulses. Carbon grains have sizes 0.08
micron < a_C < 0.12 micron and the total amount of dust formed (increasing with
the mass of the star) is M_C=(2-6)*10^{-4}Msun.
Our results imply that AGB stars with Z=3*10^{-4} can only contribute to
carbon dust enrichment of the interstellar medium on relatively long
timescales, > 300 Myr, comparable to the evolutionary time of a 3Msun star. At
lower metallicities the scarcity of silicon available and the presence of Hot
Bottom Burning even in M< 2Msun, prevents the formation of silicate and carbon
grains. We extrapolate our conclusion to more metal--poor environments, and
deduce that at Z < 10^{-4} dust enrichment is mostly due to metal condensation
in supernova ejecta.Comment: 13 pages, 8 figures, accepted for publication on MNRA