P. Ventura

The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs ?

We present the results of a new study on the relationship between coronal X-ray emission and stellar rotation in late-type main-sequence stars. We have selected a sample of 259 dwarfs in the B V range 0.5-2.0, including 110 field stars and 149 members of the Pleiades, Hyades, Persei, IC 2602 and IC 2391 open clusters. All the stars have been observed with ROSAT, and most of them have photometrically-measured rotation periods available. Our results confirm that two emission regimes exist, one in which the rotation period is a good predictor of the total X-ray luminosity, and the other in which a constant saturated X-ray to bolometric luminosity ratio is attained; we present a quantitative estimate of the critical rotation periods below which stars of dierent masses (or spectral types) enter the saturated regime. In this work we have also empirically derived a characteristic time scale,e, which we have used to investigate the relationship between the X-ray emission level and an X-ray-based Rossby number Re = Prot=e: we show that our empirical time scalee resembles the theoretical convective turnover time for 0:4 M=M 1:2, but it also has the same functional dependence on B V as L 1=2 bol in the color range 0:5 B V 1:5. Our results imply that - for non-saturated coronae - the Lx - Prot relation is equivalent to the Lx=Lbol vs. Re relation.

Predictions for Self-Pollution in Globular Cluster Stars

Fully evolutionary models have been built to follow the phases of asymptotic giant branch evolution with mass loss for metal mass fractions from Z = 2 ? 10-4 to Z = 4 ? 10-3. The hot bottom burning at the base of the convective envelope is followed by fully coupled nuclear burning and noninstantaneous mixing. The models also show the occurrence of a spontaneous (i.e., not induced by overshooting) third dredge-up. For the first time, we find that temperatures close to or even larger than 108 K are achieved at low Z; the full CNO cycle operates at the base of the envelope, the 16O abundance for the most metal-poor models of 4 and 5 M? is drastically reduced, and sodium and aluminum production by proton capture on neon and magnesium can occur. Lithium is first largely produced in the envelope and then burned completely, so the average lithium abundance in the expelled envelope is a factor of up to 5 times smaller than the initial one, but it is never completely depleted. These results may be relevant for the evolution of primordial massive globular clusters; we suggest that the low-mass stars may have been polluted at the surface by accretion from the gas that was lost from the evolving intermediate-mass stars at early ages [(1-2) ? 108 yr]. In this hypothesis, we should expect that the polluted stars show smaller abundances of oxygen, larger abundances of products of advanced nucleosynthesis (as Na and Al), and lower, but never negligible, abundances of lithium. The abundance spreads should be smaller in clusters of higher metallicities, where the lithium in the polluted stars could be larger than in the nonpolluted stars.Full evolutionary models have been built to follow the phases of asymptotic giant branch evolution with mass loss for metal mass fractions from Z=0.0002 to Z=0.004. For the first time, we find that temperatures close to or even larger than 10^8 K are achieved at low Z; the full CNO cycle operates at the base of the envelope, the Oxygen abundance for the most metal-poor models of 4 and 5 solar masses is drastically reduced, and sodium and aluminum production by by proton capture on neon and magnesium can occur. These results may be relevant for the evolution of primordial massive globular clusters: we suggest that the low-mass stars may have been polluted at the surface by accretion from the gas that was lost from the evolving intermediate-mass stars at early ages.

Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars

Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell. Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion. Outstanding issues in our understanding of red giants include uncertainties in the amount of mass lost at the surface before helium ignition and the amount of internal mixing from rotation and other processes. Progress is hampered by our inability to distinguish between red giants burning helium in the core and those still only burning hydrogen in a shell. Asteroseismology offers a way forward, being a powerful tool for probing the internal structures of stars using their natural oscillation frequencies. Here we report observations of gravity-mode period spacings in red giants that permit a distinction between evolutionary stages to be made. We use high-precision photometry obtained by the Kepler spacecraft over more than a year to measure oscillations in several hundred red giants. We find many stars whose dipole modes show sequences with approximately regular period spacings. These stars fall into two clear groups, allowing us to distinguish unambiguously between hydrogen-shell-burning stars (period spacing mostly ∼50 seconds) and those that are also burning helium (period spacing ∼100 to 300 seconds).

Abundance patterns of multiple populations in globular clusters: a chemical evolution model based on yields from AGB ejecta

A large number of spectroscopic studies have provided evidence of the presence of multiple populations in globular clusters by revealing patterns in the stellar chemical abundances. This paper is aimed at studying the origin of these abundance patterns. We explore a model in which second generation (SG) stars form out of a mix of pristine gas and ejecta of the first generation of asymptotic giant branch stars. We first study the constraints imposed by the spectroscopic data of SG stars in globular clusters on the chemical properties of the asymptotic and super asymptotic giant branch ejecta. With a simple one-zone chemical model, we then explore the formation of the SG population abundance patterns focussing our attention on the Na-O, Al-Mg anticorrelations and on the helium distribution function. We carry out a survey of models and explore the dependence of the final SG chemical properties on the key parameters affecting the gas dynamics and the SG formation process. Finally, we use our chemical evolution framework to build specific models for NGC 2808 and M4, two Galactic globular clusters which show different patterns in the Na–O and Mg–Al anticorrelation and have different helium distributions. We find that the amount of pristine gas involved in the formation of SG stars is a key parameter to fit the observed O–Na and Mg–Al patterns. The helium distribution function for these models is in general good agreement with the observed one. Our models, by shedding light on the role of different parameters and their interplay in determining the final SG chemical properties, illustrate the basic ingredients, constraints and problems encountered in this self-enrichment scenario which must be addressed by more sophisticated chemical and hydrodynamic simulations.

Full computation of massive AGB evolution - I. The large impact of convection on nucleosynthesis

It is well appreciated that the description of overadiabatic convection affects the structure of the envelopes of luminous asymptotic giant branch (AGB) stars in the phase of “hot bottom burning” (HBB). We stress that this important uncertainty in the modeling plays a role which is much more dramatic than the role which can be ascribed, e.g., to the uncertainty in the nuclear cross-sections. Due to the role tentatively attributed today to the HBB nucleosynthesis as the site of self-enrichment of Globular Clusters stars, it is necessary to explore the difference in nucleosynthesis obtained by different prescriptions for convection. We present results of detailed evolutionary calculations of the evolution of stars of intermediate mass during the AGB phase for the metallicity typical of the Globular Clusters that show the largest spread in CNO abundances (

Theoretical amplitudes and lifetimes of non-radial solar-like oscillations in red giants

Z\sim 10^{-3}

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

). We follow carefully the nucleosynthesis at the base of the external convective region, showing that very different results can be obtained according to the presciption adopted to find out the temperature gradient within the instability regions. We discuss the uncertainties in the yields of the various chemical species and the role which these sources can play as polluters of the interstellar medium.

Massive AGB models of low metallicity: the implications for the self-enrichment scenario in metal-poor globular clusters

Context. Solar-like oscillations have been observed in numerous red giants from ground and from space. An important question arises: could we expect to detect non-radial modes probing the internal structure of these stars? Aims. We investigate under what physical circumstances non-radial modes could be observable in red giants; what would be their amplitudes, lifetimes and heights in the power spectrum (PS)? Methods. Using a non-radial non-adiabatic pulsation code including a non-local time-dependent treatment of convection, we compute the theoretical lifetimes of radial and non-radial modes in several red giant models. Next, using a stochastic excitation model, we compute the amplitudes of these modes and their heights in the PS. Results. Distinct cases appear. Case A corresponds to subgiants and stars at the bottom of the ascending giant branch. Our results show that the lifetimes of the modes are mainly proportional to the inertia I, which is modulated by the mode trapping. The predicted amplitudes are lower for non-radial modes. But the height of the peaks in the PS are of the same order for radial and non-radial modes as long as they can be resolved. The resulting frequency spectrum is complex. Case B corresponds to intermediate models in the red giant branch. In these models, the radiative damping becomes high enough to destroy the non-radial modes trapped in the core. Hence, only modes trapped in the envelope have significant heights in the PS and could be observed. The resulting frequency spectrum of detectable modes is regular for � = 0 and 2, but a little more complex for � = 1 modes because of less efficient trapping. Case C corresponds to models of even higher luminosity. In these models the radiative damping of non-radial modes is even larger than in the previous case and only radial and non-radial modes completely trapped in the envelope could be observed. The frequency pattern is very regular for these stars. The comparison between the predictions for radial and non-radial modes is very different if we consider the heights in the PS instead of the amplitudes. This is important as the heights (not the amplitudes) are used as detection criterion.

Asteroseismology of old open clusters with Kepler: direct estimate of the integrated red giant branch mass-loss in NGC 6791 and 6819

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.

The Hubble Space TelescopeUV Legacy Survey of Galactic Globular Clusters – V. Constraints on formation scenarios

Context. We present the physical and chemical properties of intermediate-mass star models of low metallicity, as they evolve along the thermal pulse phase. Aims. We extend to low metallicities, of Z = 1, 2a nd 6× 10 −4 , models previously computed for chemical compositions typical of globular clusters of an intermediate metallicity (Z = 0.001), and for the most metal-rich clusters found in our Galaxy (Z = 0.004). We aim to test in particular the self-enrichment scenario for metal-poor globular clusters. Methods. We calculated three grids of intermediate-mass models with metallicities Z = 10 −4 ,2 × 10 −4 ,a nd 6× 10 −4 , following their evolutionary sequences from the pre-main-sequence to the asymptotic giant branch phase, almost until the ejection of the entire envelope. We discuss the chemistry of the ejecta, and, in particular, the mass fractions of elements that have been studied in the numerous, deep, spectroscopic surveys of globular clusters. Results. Although oxygen and sodium data are scarce for low-metallicity globular clusters, the small amonut of data avalilable for the unevolved stars in NGC 6397 are compatible with the models. Furthermore, we find good agreement with the C–N anticorrelation of unevolved stars in the cluster M 15. In this cluster, however, no stars of low oxygen ([O/Fe] ∼− 1) abundance have been detected. The most massive, very metal-poor clusters, should contain such stars, according to the present models. At the lowest metallicity Z = 10 −4 , the ejecta of the most massive AGBs have C/O > 1, due to the dramatic decrease in the oxygen abundance. We discuss the possible implications of this prediction.