Luigi Pulone

Helium-burning evolutionary phases in population II stars. I Breathing pulses in horizontal branch stars

The result of an investigation into the evolutionary characteristics of a typical horizontal-branch (HB) model are presented. A new treatment of semiconvection has been used which overlaps Robertson and Faulkners prescription in the major phase of central He burning and which allows a meaningful treatment of the last phases of He exhaustion at the center. The occurrence of convective instabilities near the He exhaustion in the central core is confirmed, finding that three major convection pulses occur before the exhaustion of He. Consequences regarding HB lifetimes and post-HB evolution are briefly discussed. 19 references.

Why haven't loose globular clusters collapsed yet?

We report on the discovery of a surprising observed correlation between the slope of the low-mass stellar global mass function (GMF) of globular clusters (GCs) and their central concentration parameter c = log(rt/rc), i.e., the logarithmic ratio of tidal and core radii. This result is based on the analysis of a sample of 20 Galactic GCs with solid GMF measurements from deep HST or VLT data. All the high-concentration clusters in the sample have a steep GMF, most likely reflecting their initial mass function. Conversely, low-concentration clusters tend to have a flatter GMF, implying that they have lost many stars via evaporation or tidal stripping. No GCs are found with a flat GMF and high central concentration. This finding appears counterintuitive, since the same two-body relaxation mechanism that causes stars to evaporate and the cluster to eventually dissolve should also lead to higher central density and possibly core collapse. Therefore, more concentrated clusters should have lost proportionately more stars and have a shallower GMF than low-concentration clusters, contrary to what is observed. It is possible that severely depleted GCs have also undergone core collapse and have already recovered a normal radial density profile. It is, however, more likely that GCs with a flat GMF have a much denser and smaller core than that suggested by their surface brightness profile and may well be undergoing collapse at present. In either case, we may have so far seriously underestimated the number of post-core collapse clusters, and many may be lurking in the Milky Way.

The Mass Function of Main-Sequence Stars in NGC 6397 from Near-Infrared and Optical High-Resolution Hubble Space Telescope Observations

We have investigated the properties of the stellar mass function in the globular cluster NGC 6397 through the use of a large set of Hubble Space Telescope (HST) observations. The latter include existing WFPC 2 images in the V and I bands, obtained at ~45 and 10 radial distances, as well as a series of deep images in the J and H bands obtained with the NIC 2 and NIC 3 cameras of the NICMOS instrument pointed, respectively, to regions located ~45 and ~32 from the center. These observations span the region from ~1 to ~3 times the clusters half-light radius (rhl 3) and have been subjected to the same, homogeneous data processing so as to guarantee that the ensuing results could be directly compared to one another. We have built color-magnitude diagrams that we use to measure the luminosity function of main-sequence stars extending from just below the turnoff all the way down to the hydrogen-burning limit. All luminosity functions derived in this way show the same, consistent behavior in that they all increase with decreasing luminosity up to a peak at MI 8.5 or MH 7 and then drop precipitously well before photometric incompleteness becomes significant. Within the observational uncertainties, at MI 12 or MH 10.5 (~0.09 M☉) the luminosity functions are compatible with zero. The direct comparison of our NIC 2 field with previous WFPC 2 observations of the same area shows that down to MH 11 there are no more faint, red stars than those already detected by the WFPC 2, thus excluding a significant population of faint, low-mass stars at the bottom of the main sequence. By applying the best available mass-luminosity relation appropriate to the metallicity of NGC 6397 and consistent with our color-magnitude diagrams to both the optical and the IR data, we obtain a mass function that shows a break in slope at ~0.3 M☉. No single-exponent power-law distribution is compatible with these data, regardless of the value of the exponent. We find that a dynamical model of the cluster can simultaneously reproduce the luminosity functions observed in the core, at ~32, 45, and 10 away from the center, as well as the surface brightness and velocity dispersion profiles of red giant stars, only if the model initial mass function (IMF) rises as m-1.6±0.2 in the range 0.8-0.3 M☉ and then drops as m0.2±0.1 below ~0.3 M☉. Adopting a more physical lognormal distribution for the IMF, we find that all these data taken together imply a best-fit distribution with mc 0.3 and σ 1.8.

The Lower Main Sequence of ω Centauri from Deep Hubble Space Telescope NICMOS Near-Infrared Observations

A 20 × 20 field located ~7 from the center of the massive galactic globular cluster ω Centauri (NGC 5139) was observed by the NIC2 camera of the Near-Infrared Camera and Multiobject Spectrometer on board the Hubble Space Telescope (HST) through the F110W and F160W broadband filters centered at 1.1 and 1.6 μm for a total of 3000 and 4000 s for the two filters, respectively. Standard photometric analysis of the resulting images yields 340 stars with a signal above a 10 σ threshold in both filters, covering the range of HST m160 magnitudes between 20 and 26, the deepest probe yet of a globular cluster in this wavelength region. These objects form a well-defined sequence in the m160 versus m110-m160 plane that is consistent with the theoretical near-IR color-magnitude diagram expected from recent low-mass stellar model calculations. The resulting stellar luminosity function increases steadily with increasing magnitude up to a peak at m16025, where it turns over and drops slowly down to the detection limit set by the incompleteness limit of 60% at m16026. With the theoretical mass-luminosity relationship that provides the best fit to the IR color-magnitude diagram, we obtain an excellent fit to the observed luminosity function down to a mass of ~0.2 M with a power-law mass function having a slope of α=-1.

Convective cores and carbon ignition in intermediate-mass stars

It is found that in intermediate-mass stars, convective overshooting during central He burning is responsible for the overgrowth of the convective core. As a consequence, the upper mass limit for stars developing a degenerate C-O core is strongly reduced. It is shown that, when such an effect is taken into account, some previous disagreements between theory and observation are spontaneously solved. Finally, the implications convective overshooting has on supernovae triggered by electron capture are discussed briefly. 13 references.

Theoretical models for helium-burning stars in intermediate-metallicity globular clusters

Un modele devolution des etoiles brulant de lhelium, sur la branche horizontale et sur la branche geante asymptotique, dans un amas globulaire galactique a metallicite intermediaire est developpe. Les diagrammes de la luminosite, la temperature effective et la composition chimique en fonction du temps sont presentes

Tidal Disruption and the Tale of Three Clusters

How well can we tell whether a globular cluster will survive the Galaxys tidal forces? This is conceptually easy to do if we know the clusters total mass, mass structure and space motion parameters. This information is used in models that predict the probability of disruption due to tidal stripping, disc and bulge shocking. But just how accurate is the information that goes into these models and, therefore, how reliable are their predictions? To understand the virtues and weaknesses of these models, we have studied in detail three globular clusters (NGC 6397, NGC 6712, NGC 6218) whose predicted interaction with the galaxy is very different. We have used deep HST and VLT data to measure the luminosity function of stars throughout the clusters in order to derive a solid global mass function, which is the best tell-tale of the strength and extent of tidal stripping operated by the Galaxy. We indeed find that the global mass functions of the three clusters are different, but not in the way predicted by the models. [abridged]