The Hottest Horizontal-Branch Stars in omega Centauri - Late Hot Flasher vs. Helium Enrichment

UV observations of some massive globular clusters uncovered a significant population of very hot stars below the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either as results of the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population recently postulated to exist in some clusters. Moderately high resolution spectra of stars at the hot end of the blue HB in omega Cen were analysed for atmospheric parameters and abundances using LTE and Non-LTE model atmospheres. In the temperature range 30,000K to 50,000K we find that 35% of our stars are helium-poor (log(n_He/n_H) < -2), 51% have solar helium abundance within a factor of 3 (-1.5 <= log(n_He/n_H) <= -0.5) and 14% are helium-rich (log(n_He/n_H)> -0.4). We also find carbon enrichment in step with helium enrichment, with a maximum carbon enrichment of 3% by mass. At least 14% of the hottest HB stars in omega Cen show helium abundances well above the highest predictions from the helium enrichment scenario (Y = 0.42 corresponding to log(n_He/n_H) ~ -0.74). In addition, the most helium-rich stars show strong carbon enrichment as predicted by the late hot flasher scenario. We conclude that the helium-rich HB stars in omega Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence. (Abridged)Comment: 4 pages, 3 figures, uses aa.cls (enclosed), accepted as A&A Lette

On a new theoretical framework for RR Lyrae stars I: the metallicity dependence

We present new nonlinear, time-dependent convective hydrodynamical models of RR Lyrae stars computed assuming a constant helium-to-metal enrichment ratio and a broad range in metal abundances (Z=0.0001--0.02). The stellar masses and luminosities adopted to construct the pulsation models were fixed according to detailed central He burning Horizontal Branch evolutionary models. The pulsation models cover a broad range in stellar luminosity and effective temperatures and the modal stability is investigated for both fundamental and first overtones. We predict the topology of the instability strip as a function of the metal content and new analytical relations for the edges of the instability strip in the observational plane. Moreover, a new analytical relation to constrain the pulsation mass of double pulsators as a function of the period ratio and the metal content is provided. We derive new Period-Radius-Metallicity relations for fundamental and first-overtone pulsators. They agree quite well with similar empirical and theoretical relations in the literature. From the predicted bolometric light curves, transformed into optical (UBVRI) and near-infrared (JHK) bands, we compute the intensity-averaged mean magnitudes along the entire pulsation cycle and, in turn, new and homogenous metal-dependent (RIJHK) Period-Luminosity relations. Moreover, we compute new dual and triple band optical, optical--NIR and NIR Period-Wesenheit-Metallicity relations. Interestingly, we find that the optical Period-W(V,B-V) is independent of the metal content and that the accuracy of individual distances is a balance between the adopted diagnostics and the precision of photometric and spectroscopic datasets.Comment: 51 pages, 20 figures, 9 tables, accepted for publication on Ap

Improvement of the CORS method for Cepheids radii determination based on Stromgren photometry

In this paper we present a modified version of the CORS method based on a new calibration of the Surface Brightness function in the Stromgren photometric system. The method has been tested by means of synthetic light and radial velocity curves derived from nonlinear pulsation models. Detailed simulations have been performed to take into account the quality of real observed curves as well as possible shifts between photometric and radial velocity data. The method has been then applied to a sample of Galactic Cepheids with Stromgren photometry and radial velocity data to derive the radii and a new PR relation. As a result we find log R = (1.19 +- 0.09) + (0.74 +- 0.11) log P (r.m.s=0.07). The comparison between our result and previous estimates in the literature is satisfactory. Better results are expected from the adoption of improved model atmosphere grids.Comment: 13 pages including 12 postscript figures and 3 jpeg figures; accepted for publication on A&

Predicted properties of RR Lyrae stars in the SDSS photometric system

The luminosities and effective temperatures, as well as the whole bolometric lightcurves of nonlinear convective RR Lyrae models with 0.0001<Z<0.006 are transformed into the SDSS photometric system. The obtained ugriz lightcurves, mean magnitudes and colors, pulsation amplitudes and color-color loops are shown and analytical relations connecting pulsational to intrinsic stellar parameters, similarly to the ones currently used in the Johnson-Cousins filters, are derived. Finally the behaviour in the color-color planes is compared with available observations in the literature and possible systematic uncertainties affecting this comparison are discussed.Comment: accepted for publication in MNRA

The Carina dSph galaxy: where is the edge?

Recent cosmological N-body simulations suggest that current empirical estimates of tidal radii in dSphs might be underestimated by at least one order of magnitude. To constrain the plausibility of this theoretical framework, we undertook a multiband (U,B,V,I) survey of the Carina dSph. Deep B,V data of several fields located at radial distances from the Carina center ranging from 0.5 to 4.5 degrees show a sizable sample of faint blue objects with the same magnitudes and colors of old, Turn-Off stars detected across the center. We found that the (U-V,B-I) color-color plane is a robust diagnostic to split stars from background galaxies. Unfortunately, current U,I-band data are too shallow to firmly constrain the real extent of Carina.Comment: To be published on the proceedings of the XLIX meeting of the Italian Astronomical Society. Requires mem.cl