M. Di Criscienzo

Multiple Stellar Populations in 47 Tucanae

We use Hubble Space Telescope (HST) and ground-based imaging to study the multiple populations of 47 Tucanae (47 Tuc), combining high-precision photometry with calculations of synthetic spectra. Using filters covering a wide range of wavelengths, our HST photometry splits the main sequence into two branches, and we find that this duality is repeated in the subgiant and red giant regions, and on the horizontal branch. We calculate theoretical stellar atmospheres for main-sequence stars, assuming different chemical composition mixtures, and we compare their predicted colors through the HST filters with our observed colors. We find that we can match the complex of observed colors with a pair of populations, one with primeval abundance and another with enhanced nitrogen and a small helium enhancement, but with depleted C and O. We confirm that models of red giant and red horizontal branch stars with that pair of compositions also give colors that fit our observations. We suggest that the different strengths of molecular bands of OH, CN, CH, and NH, falling in different photometric bands, are responsible for the color splits of the two populations. Near the cluster center, in each portion of the color-magnitude diagram the population with primeval abundances makes up only ~20% of the stars, a fraction that increases outward, approaching equality in the outskirts of the cluster, with a fraction ~30% averaged over the whole cluster. Thus the second, He/N-enriched population is more concentrated and contributes the majority of the present-day stellar content of the cluster. We present evidence that the color-magnitude diagram of 47 Tuc consists of intertwined sequences of the two populations, whose separate identities can be followed continuously from the main sequence up to the red giant branch, and thence to the horizontal branch. A third population is visible only in the subgiant branch, where it includes ~8% of the stars.

Yields of AGB and SAGB models with chemistry of low- and high-metallicity globular clusters

We present yields from stars of mass in the range Mo<M<8Mo of metallicities Z=0.0003 and Z=0.008, thus encompassing the chemistry of low- and high-Z Globular Clusters. The yields are based on full evolutionary computations, following the evolution of the stars from the pre-Main Sequence through the Asymptotic Giant Branch phase, until the external envelope is lost. Independently of metallicity, stars with M<3Mo are dominated by Third Dredge-Up, thus ejecting into their surroundings gas enriched in carbon and nitrogen. Conversely, Hot Bottom Burning is the main responsible for the modification of the surface chemistry of more massive stars, whose mass exceeds 3Mo: their gas shows traces of proton-capture nucleosynthesis. The extent of Hot Bottom Burning turns out to be strongly dependent on metallicity. In this paper we analyze the consequences of this fact. These results can be used to understand the role played by intermediate mass stars in the self-enrichment scenario of globular clusters: the results from spectroscopic investigations of stars belonging to the second generation of clusters with different metallicity will be used as an indirect test of the reliability of the present yields. The treatment of mass loss and convection are confirmed as the main uncertainties affecting the results obtained in the context of the modeling of the thermal pulses phase. An indirect proof of this comes from the comparison with other investigations in the literature, based on a different prescription for the efficiency of convection in transporting energy and using a different recipe to determine the mass loss rate.

Does Kepler unveil the mystery of the Blazhko effect? First detection of period doubling in Kepler Blazhko RR Lyrae stars

The first detection of the period doubling phenomenon is reported in the Kepler RR Lyrae stars RR Lyr, V808 Cyg and V355 Lyr. Interestingly, all these pulsating stars show Blazhko modulation. The period doubling manifests itself as alternating maxima and minima of the pulsational cycles in the light curve, as well as through the appearance of half-integer frequencies located halfway between the main pulsation period and its harmonics in the frequency spectrum. The effect was found to be stronger during certain phases of the modulation cycle. We were able to reproduce the period-doubling bifurcation in our non-linear RR Lyrae models computed by the Florida-Budapest hydrocode. This enabled us to trace the origin of this instability in RR Lyrae stars to a resonance, namely a 9:2 resonance between the fundamental mode and a high-order (ninth) radial overtone showing strange-mode characteristics. We discuss the connection of this new type of variation to the mysterious Blazhko effect and argue that it may give us fresh insights into solving this century-old enigma.

FIRST KEPLER RESULTS ON RR LYRAE STARS

We present the first results of our analyses of selected RR Lyrae stars for which data have been obtained by the Kepler Mission. As expected, we find a significant fraction of the RRab stars to show the Blazhko effect, a still unexplained phenomenon that manifests itself as periodic amplitude and phase modulations of the light curve, on timescales of typically tens to hundreds of days. The long time span of the Kepler Mission of 3.5 yr and the unprecedentedly high precision of its data provide a unique opportunity for the study of RR Lyrae stars. Using data of a modulated star observed in the first roll as a showcase, we discuss the data, our analyses, findings, and their implications for our understanding of RR Lyrae stars and the Blazhko effect. With at least 40% of the RR Lyrae stars in our sample showing modulation, we confirm the high incidence rate that was only found in recent high-precision studies. Moreover, we report the occurrence of additional frequencies, beyond the main pulsation mode and its modulation components. Their half-integer ratio to the main frequency is reminiscent of a period doubling effect caused by resonances, observed for the first time in RR Lyrae stars.

Flavours of variability: 29 RR Lyrae stars observed with Kepler

We present our analysis of Kepler observations of 29 RR Lyrae stars, based on 138 d of observation. We report precise pulsation periods for all stars. Nine of these stars had incorrect or unknown periods in the literature. 14 of the stars exhibit both amplitude and phase Blazhko modulations, with Blazhko periods ranging from 27.7 to more than 200 d. For V445 Lyr, a longer secondary variation is also observed in addition to its 53.2-d Blazhko period. The unprecedented precision of the Kepler photometry has led to the discovery of the the smallest modulations detected so far. Moreover, additional frequencies beyond the well-known harmonics and Blazhko multiplets have been found. These frequencies are located around the half-integer multiples of the main pulsation frequency for at least three stars. In four stars, these frequencies are close to the first and/or second overtone modes. The amplitudes of these periodicities seem to vary over the Blazhko cycle. V350 Lyr, a non-Blazhko star in our sample, is the first example of a double-mode RR Lyrae star that pulsates in its fundamental and second overtone modes.

Dust formation around AGB and SAGB stars: a trend with metallicity?

We calculate the dust formed around asymptotic giant branch (AGB) and super-AGB stars of metallicity Z = 0.008 by following the evolution of models with masses in the range 1 M⊙ ≤ M ≤ 8 M⊙ through the thermal pulses phase, assuming that dust forms via condensation of molecules within a wind expanding isotropically from the stellar surface. We find that, because of the strong hot bottom burning (HBB) experienced, high-mass models produce silicates, whereas lower mass objects are predicted to be surrounded by carbonaceous grains; the transition between the two regimes occurs at a threshold mass of 3.5 M⊙. These findings are consistent with the results presented in a previous investigation, for Z = 0.001. However, in the present higher metallicity case, the production of silicates in the more massive stars continues for the whole AGB phase, because the HBB experienced is softer at Z = 0.008 than at Z = 0.001; thus, the oxygen in the envelope, essential for the formation of water molecules, is never consumed completely. The total amount of dust formed for a given mass experiencing HBB increases with metallicity, because of the higher abundance of silicon, and the softer HBB, both factors favouring a higher rate of silicates production. This behaviour is not found in low-mass stars, because the carbon enrichment of the stellar surface layers, due to repeated third dredge-up episodes, is almost independent of the metallicity. Regarding cosmic dust enrichment by intermediate-mass stars, we find that the cosmic yield at Z = 0.008 is a factor of ∼5 larger than at Z = 0.001. In the lower metallicity case carbon dust dominates after ∼300 Myr, but at Z = 0.008 the dust mass is dominated by silicates at all times, with a prompt enrichment occurring after ∼40 Myr, associated with the evolution of stars with masses M ∼ 7.5–8 M⊙. These conclusions are partly dependent on the assumptions concerning the two important macrophysics inputs needed to describe the AGB phase, and still unknown from first principles: the treatment of convection, which determines the extent of the HBB experienced and of the third dredge-up following each thermal pulse, and mass-loss, essential in fixing the time-scale on which the stellar envelope is lost from the star.

Dust from asymptotic giant branch stars: relevant factors and modelling uncertainties

The dust formation process in the winds of Asymptotic Giant Branch stars is discussed, based on full evolutionary models of stars with mass in the range 1M⊙ 6M6 8M⊙, and metallicities 0.001 < Z < 0.008. Dust grains are assumed to form in an isotropically expanding wind, by growth of pre–existing seed nuclei. Convection, for what concerns the treatment of convective borders and the efficiency of the schematization adopted, turns out to be the physical ingredient used to calculate the evolutionary sequences with the highest impact on the results obtained. Low–mass stars with M6 3M⊙ produce carbon type dust with also traces of silicon carbide. The mass of solid carbon formed, fairly independently of metallicity, ranges from a few 10 −4 M⊙, for stars of initial mass 1 1.5M⊙, to � 10 −2 M⊙ for M� 2 2.5M⊙; the size of dust particles is in the range 0.1µm6 aC 6 0.2µm. On the contrary, the production of silicon carbide (SiC) depends on metallicity. For 10 −3 6 Z 6 8×10 −3 the size of SiC grains varies in the range 0.05µm < aSiC < 0.1µm, while the mass of SiC formed is 10 −5 M⊙ < MSiC < 10 −3 M⊙. Models of higher mass experience Hot Bottom Burning, which prevents the formation of carbon stars, and favours the formation of silicates and corundum. In this case the results scale with metallicity, owing to the larger silicon and aluminium contained in higher–Z models. At Z=8×10 −3 we find that the most massive stars produce dust masses md � 0.01M⊙, whereas models of smaller mass produce a dust mass ten times smaller. The main component of dust are silicates, although corundum is also formed, in not negligible quantities (� 10 20%).

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.

RR Lyrae stars in Galactic globular clusters - VI. The period-amplitude relation

Aims. This work uses nonlinear convective models of RR Lyrae stars and evolutionary predictions of low-mass helium burning stellar structures to constrain the properties of cluster and field RR Lyrae variables. In particular, we address two problems: is the PeriodAmplitude (PAV ) plane of fundamental (RRab) variables a good diagnostic for the metal abundance? Is the MV (RR)-[Fe/H] relation of field and cluster variables linear over the whole metal abundance range of [Fe/H] ∼− 2.5 to ∼0? Methods. We perform a detailed comparison between theory and observations for fundamental RR Lyrae variables in the solar neighborhood and in both Oosterhoff type I (OoI) and type II (OoII) Galactic globular clusters. Results. We show that the distribution of cluster RRab variables in the PAV plane depends not only on the metal abundance, but also on the cluster Horizontal Branch (HB) morphology. We find that on average the observed pulsation parameter kpuls connecting the period to the visual amplitude increases when moving from metal-poor to metal-rich GGCs. However, this parameter shows marginal changes among OoI clusters with intermediate to red HB types and iron abundances −1.8 ≤ [Fe/H] ≤− 1.1, whereas its value decreases in OoII clusters with the bluer HB morphology, although these clusters are also the less metal-poor ones of the group.

RR Lyrae-based calibration of the Globular Cluster Luminosity Function

We test whether the peak absolute magnitude M v (TO) of the Globular Cluster Luminosity Function (GCLF) can be used for reliable extragalactic distance determination. Starting with the luminosity function of the Galactic Globular Clusters listed in Harris catalogue, we determine M v (TO) either using current calibrations of the absolute magnitude M v (RR) of RR Lyrae stars as a function of the cluster metal content [Fe/H] and adopting selected cluster samples. We show that the peak magnitude is slightly affected by the adopted M v (RR)-[Fe/H] relation, with the exception of that based on the revised Baade-Wesselink method, while it depends on the criteria to select the cluster sample. Moreover, grouping the Galactic Globular Clusters by metallicity, we find that the metal-poor (MP) ([Fe/H] -1.0, ( ∼ -0.6) one, in substantial agreement with the theoretical metallicity effect suggested by synthetic Globular Cluster populations with constant age and mass function. Moving outside the Milky Way, we show that the peak magnitude of the MP clusters in M31 appears to be consistent with that of Galactic clusters with similar metallicity, once the same M V (RR)-[Fe/H] relation is used for distance determination. As for the GCLFs in other external galaxies, using Surface Brightness Fluctuations (SBF) measurements we give evidence that the luminosity functions of the blue (MP) Globular Clusters peak at the same luminosity within ∼0.2 mag, whereas for the red (MR) samples the agreement is within ∼0.5 mag even accounting for the theoretical metallicity correction expected for clusters with similar ages and mass distributions. Then, using the SBF absolute magnitudes provided by a Cepheid distance scale calibrated on a fiducial distance to Large Magellanic Cloud (LMC), we show that the M V (TO) value of the MP clusters in external galaxies is in excellent agreement with the value of both Galactic and M31 ones, as inferred by an RR Lyrae distance scale referenced to the same LMC fiducial distance. Eventually, adopting μ 0 (LMC) = 18.50 mag, we derive that the luminosity function of MP clusters in the Milky Way, M31, and external galaxies peak at M V (TO) = -7.66 ± 0.11, - 7.65 ± 0.19 and -7.67 ± 0.23 mag, respectively. This would suggest a value of -7.66 ± 0.09 mag (weighted mean), with any modification of the LMC distance modulus producing a similar variation of the GCLF peak luminosity.