arXiv subjects
Publications and source records attributed to E. Brocato.
We report new results on the LMC globular cluster NGC 1866 obtained by analyzing F555W and F814W images from WFPC2@HST. On the basis of the CMD we derive information on the cluster distance and constraints on stellar evolution theory. Evidence of mass segregation are found in the cluster core.
We present new theoretical evaluations of optical and near-IR Surface Brightness Fluctuations (SBF) magnitudes for single-burst stellar populations in the age range t=5-15 Gyr and metallicity from $Z_{\sun}/200$ to $2Z_{\sun}$. Our theoretical predictions can be successfully used to derive reliable distance evaluations. They also appear to be a new and valuable tool to trace the properties of unresolved stellar populations.
We discuss some clues on the large tilt observed in the Horizontal Branch of the metal-rich galactic globular clusters NGC 6388 and NGC 6441. This not yet understood feature is investigated from theoretical and observational sides.
Optical and near-infrared light curves of the Type IIn supernova 1999el in NGC 6951 are presented. A period of 220 days (416 days in the near-infrared) is covered from the first observation obtained a few days before maximum light. Spectroscopic observations are also discussed. Using as a distance calibrator the Type Ia SN 2000E, which occurred some months later in the same galaxy, and fitting a blackbody law to the photometric data we obtain a maximum bolometric luminosity for SN 1999el of $\sim 10^{44}$ erg s$^{-1}$. In general, the photometric properties of SN 1999el are very similar to those of SN 1998S, a bright and well studied Type IIn SN, showing a fast decline in all observed bands similar to those of Type II-L SNe. The differences with SN 1998S are analyzed and ascribed to the differences in a pre-existing circumstellar envelope in which dust was already present at the moment of the SN outburst. We infer that light echoes may play a possibly significant role in affecting the observed properties of the light curves, although improved theoretical models are needed to account for the data. We conclude that mass loss in the progenitor RG stars is episodic and occurs in an asymmetric way. This implies that collapsing massive stars appear as normal Type II SN if this occurs far from major mass loss episodes, whereas they appear as Type IIn SNe if a large mass loss episode is in progress.
We investigate the predicted Color-Magnitude distribution of metal-rich Horizontal Branch (HB) stars, discussing selected theoretical models computed under various assumptions about the star metallicity and the efficiency of super-adiabatic convection. We find that canonical Zero Age Horizontal Branches with metallicity larger or of the order of Z=0.002 should be all affected by a tilt, by an amount which increases when the metallicity is increased and/or the mixing length is decreased, reaching a tilt of $ΔV \sim$0.2 mag in the case of solar metallicity when a mixing length value $α$=1.6 is assumed ($ΔV$ is the magnitude difference between the top of the blue HB and the fainter magnitude reached by the red HB). Uncertainties in the luminosity of the red HB due to uncertainty in the mixing length value are discussed. We finally discuss the much larger tilt observed in the clusters NGC 6441 and NGC 6388, reporting additional evidence against suggested non-canonical evolutionary scenarios. Numerical experiments show that differential reddening could produce such sloped HBs. Further, HST-PC imaging of NGC 6441 gives clear indications about the occurrence of differential reddening across the cluster. However, the same imaging shows that the observed slope of the red HB {\em is not} an artifact of differential reddening. We finally show that sloping red HBs in metal rich clusters are a common occurrence not necessarily correlated with the appearance of extended blue HB.
Hubble Space Telescope V,I photometry of stars in the Large Magellanic Cloud Cluster NGC 1866 shows a well defined cluster main sequence down to V=25 mag, with little contamination from field or foreground stars. We use the main sequence fitting procedure to link the distance of NGC 1866 to the Hipparcos determination of the distance for the Hyades MS stars, making use of evolutionary prescriptions to allow for differences in the chemical composition. On this basis we find a true distance modulus for NGC 1866 of 18.35 +/- 0.05 mag. If the cluster is assumed to lie in the LMC plane then the LMC modulus is 0.02 mag less.
We present preliminary results of an incoming theoretical work concerning the integrated properties of the Population III clusters of stars. On the basis of synthetic Color-Magnitude Diagrams, we provide a grid of optical and near-IR colors of Simple Stellar Populations with very low metallicity (Z=10$^{-10}$ and Z=10$^{-6}$) and age which spans from 10 Myr to 15 Gyr. A comparison with higher metallicities up to 0.006 is also shown, disclosing sizable differences in the CMD morphology, integrated colors and Spectral Energy Distribution (SED).
This paper presents theoretical HST and broad band colours from population synthesis models based on an homogeneous set of stellar evolutionary tracks as computed under canonical (no overshooting) assumptions, covering the range of cluster ages from t=8 Myr to t=5 Gyr for three different metallicities (Z=0.02, 0.006, and 0.001). Statistical fluctuations in the cluster population have been investigated, assessing the predicted fluctuations of the various colours as a function of the cluster integrated absolute magnitude. We show that the red leak in HST UV filters deeply affects the predicted fluxes and colours. However, we find that for F152M-F307M < 0.5 and for F170M-F278M < 0.5 (which means ages lower than 1 Gyr) the HST UV colours can still be used to infer reliable indications on the age of distant clusters. Moreover, one finds that the age calibration of these colours is scarcely affected by the amount of original helium or by the assumed IMF. On this basis, we present a calibration of the HST UV two-colours (F152M-F307M vs F170M-F278M) in terms of cluster ages for the three above quoted metallicities. We suggest the combined use of HST UV colours and IR colours (V-K in particular) to disentangle the metallicity-age effect in integrated colours of young stellar populations (t< 1 Gyr).
We report on two X-ray observations of the transient source XTE J0421+560 performed by BeppoSAX, and on a series of observations performed by the 0.7m Teramo-Normale Telescope. Outburst peak occurrence time and duration depend on photon energy: the outburst peak is achieved first in the X-ray band, then in the optical and finally in the radio. An exponential decay law fits well the X-ray data except in the TOO2 0.5-1.0 keV band, where erratic time variability is detected. During TOO1 the e-folding time scale decreases with energy up to ~ 20 keV, when it achieves a saturation; during TOO2 it decreases up to ~ 2 keV and then increases. This change is correlated with a spectral change, characterized by the onset of a soft (< 2 keV) component in TOO2 (Orr et al. 1998). This component might originate from the relativistic jets, while the hard component is more likely associated to processes occurring in the circumstellar matter and/or near the compact object. Optical observations show that the object appears intrinsically red even during the outburst. The nature of the compact object is discussed.
We discuss theoretical predictions concerning the evolution of globular cluster Pop.II stars vis-a-vis current estimates of standard errors in the determination of nuclear burning rates. Numerical evaluations are given for the dependence of TO and HB luminosity from the rate of the relevant nuclear reactions. We conclude that evolutionary predictions appear rather solid in this respect, with a maximum 3$σ$ error of about 1 Gyr in the evaluation of cluster ages derived from the calibration of the difference in luminosity between HB and TO. However, current evaluation of the original He content, as given on the basis of the R parameter, should wait for a much better determination of the C12($α$,$γ$)O16 reaction before reaching a satisfactory accuracy.
Theoretical expectations concerning stars as neutrino sources are presented according to detailed evaluations of the stellar evolutionary histories for an extended grid of stellar masses. Neutrino fluxes and cumulative neutrino yields are given for both `thermo-nuclear' and `cooling' neutrinos all over the nuclear life of the stars and along the final cooling as White Dwarfs. Predictions concerning the galactic and the cosmic neutrino background are presented and discussed.
We present Very Low Mass stellar models as computed including non-grey model atmospheres for selected assumptions about the star metallicities. The role of atmospheres is discussed and the models are compared with models based on the Eddington approximation and with similar models appeared in the recent literature. Theoretical predictions concerning both the HR diagram location and the mass-luminosity relation are presented and discussed in terms of expectations in selected photometric bands. Comparison with available observational data concerning both galactic globular clusters and dwarfs in the solar neighborhood reveals a satisfactory agreement together with the existence of some residual mismatches.
We briefly review the observational and theoretical investigations on Very Low Mass (VLM) stars. New evolutionary computations, extending toward the lower main sequence previous computations already given for globular cluster stars, are presented. Comparisons with updated observational data both for globular cluster stars and VLM stars with known parallaxes are also shown. We discuss observational evidences supporting theoretical predictions concerning the Color - Magnitude (C-M) location of faint Main Sequence stars as far as concern the dependence of the lower main sequence on the metallicity.
We present preliminary results on the effects of SNIa explosions on the Spectral Energy Distribution (SED) of distant galaxies and the possible modifications which may occur in integrated spectra, magnitudes and colours of simple galaxy models of different ages and metallicities few days after a SNIa event.
We present a CCD investigation of the poorly known SMC young Globular Cluster NGC 458. The NTT data, presented here, allowed us to study in detail the more internal regions of the cluster which are less contaminated by the field. On the basis of theoretical isochrones, a preliminary evaluation of the age is also given.
In this paper we present new CCD investigations of RR Lyrae pulsators in the Oo.I globular cluster M5. B V curves of light for 15 RR Lyrae are presented. With the addition of further 11 curves of light by Storm, Carney and Beck (1991) one is dealing with a sample of 26 well studied cluster pulsators whose properties have been implemented with similar data for RR Lyrae in clusters M3, M15, M68 to allow a comparison with the theoretical scenario recently presented by Bono and Stellingwerf (1994). On this basis, we discuss the distribution of stars in the period amplitude diagram, disclosing a substantial reduction of Sandage's period shift. We suggest that theoretical constraints concerning periods and amplitudes could allow information on masses and luminosity of the pulsators directly from Bailey's diagram only. Static temperatures have been derived for all stars in the sample, discussing the dependence on the temperature of the observed pulsational properties.
Deep CCD photometry down to V=24.5 is presented for the old Magellanic Globular cluster NGC 2257. The main result cna be summarized as follows: i) the RGB Bump has been detected at V=18.7+-0.1, and is compatible with determination of this feature in GGCs; ii) NGC 2257 seems to be 2-3 Gyrs younger than a typical GGC; iii) the helium abundance has been estimated to be Y=0.21+-0.03, using the R- method as calibrated by Buzzoni et al., 1983.
We present a CCD investigation of the galactic globular M5 aimed to increase the statistical relevance of the available sample of evolving bright stars. Previous investigations, limited to the outer cluster region, have been extended toward the cluster center, more than doubling the number of observed luminous stars. On this basis, we discuss a statistically relevant sample, rich of 415 HB stars. The occurrence of a gap in the blue side of the HB is suggested. Comparison to the current evolutionary scenario discloses a good agreement concerning both the C-M diagram location and the relative abundance of stars in the advanced evolutionary phases, supporting our present knowledge of the evolution of low mass stars. Determination of the amount of the original helium content through the ratio R N(HB)/N(RGB) gives $Y = 0.22 \pm 0.02$.