88 research outputs found
Dall’Osservatorio astronomico universitario all’Osservatorio astronomico di Bologna: 1957-1985
No full textViene presentata la storia istituzionale e scientifica dell'astronomia bolognese dal secondo dopoguerra alla nascita dell'Osservatorio Astronomic
The Globular Cluster NGC 1978 in the Large Magellanic Cloud
No full textWe have used deep, high-resolution Hubble Space Telescope ACS observations to image the cluster NGC 1978 in the Large Magellanic Cloud. This high-quality photometric data set allowed us to confirm the high ellipticity (?~0.30+/-0.02) of this stellar system. The derived color-magnitude diagram allowed a detailed study of the main evolutionary sequences; in particular, we have detected the so-called bump along the red giant branch (at V555=19.10+/-0.10). This is the first detection of this feature in an intermediate-age cluster. Moreover, the morphology of the evolutionary sequence and the population ratios have been compared with the expectations of different theoretical models (namely, BaSTI, Pisa Evolutionary Library [PEL], and Padua) in order to quantify the effect of convective overshooting. The best agreement (in terms of both morphology and star counts) has been found within the PEL isochrone, with Z=0.008 (consistent with the most recent determination of the cluster metallicity, [M/H]=-0.37 dex) and a mild overshooting efficiency (?os=0.1). By adopting this theoretical set an age of ?=1.9+/-0.1 Gyr has been obtained
The correlation between blue straggler and binary fractions in the core of Galactic globular clusters
No full textContext: Blue stragglers stars (BSSs) are thought to form in globular clusters by two main formation channels: i) mergers induced by stellar collisions; and ii) coalescence or mass transfer between companions in binary systems. The detailed study of the BSS properties is therefore crucial for understanding the binary evolution mechanisms and the complex interplay between dynamics and stellar evolution in dense stellar systems. Aims: We present the first comparison between the BSS specific frequency and the binary fraction in the core of a sample of Galactic globular clusters, with the aim of investigating the relative efficiency of the two proposed formation mechanisms. Methods: We derived the frequency of BSSs in the core of thirteen low-density Galactic globular clusters by using deep ACS@HST observations and investigated its correlation with the binary fraction and various other cluster parameters. Results: We observed a correlation between the BSS specific frequency and the binary fraction. The significance of the correlation increases by including a further dependence on the cluster's central velocity dispersion. Conclusions: We conclude that the unperturbed evolution of primordial binaries could be the dominant BSS formation process, at least in low-density environments
The fraction of binary systems in the core of 13 low-density Galactic globular clusters
No full textWe used deep observations collected with Advanced Camera for Surveys (ACS) at Hubble Space Telescope (HST) to derive the fraction of binary systems in a sample of 13 low-density Galactic globular clusters. By analysing the colour distribution of main-sequence stars we derived the minimum fraction of binary systems required to reproduce the observed colour-magnitude diagram morphologies. We found that all the analysed globular clusters contain a minimum binary fraction larger than 6 per cent within the core radius. The estimated global fractions of binary systems range from 10 to 50 per cent depending on the cluster. A dependence of the relative fraction of binary systems on the cluster age has been detected, suggesting that the binary disruption process within the cluster core is active and can significantly reduce the binary content in time
The fraction of binary systems in the core of five Galactic open clusters
No full textWe used deep wide-field photometric observations to derive the fraction of binary systems in a sample of five high-latitude Galactic open clusters. By analysing the colour distribution of main-sequence stars, we derived the minimum fraction of binary systems required to reproduce the observed colour–magnitude diagram morphologies. We found that all the analysed clusters contain a minimum binary fraction larger than 11 per cent within the core radius. The estimated global fractions of binary systems range from 35 to 70 per cent depending on the cluster. The comparison with homogeneous estimates performed in globular clusters indicates that open clusters hold a significantly higher fraction of binary systems, as predicted by theoretical models and N-body simulations. A dependence of the relative fraction of binary systems on the cluster mass has been detected, suggesting that the binary disruption within the cluster core is the dominant process that drives the fraction of binaries in stellar systems
The First Empirical Mass-Loss Law for Population II Giants
No full textUsing the Spitzer IRAC camera we have obtained mid-IR photometry of the red giant branch stars in the Galactic globular cluster 47 Tuc. About 100 stars show an excess of mid-IR light above that expected from their photospheric emission. This is plausibly due to dust formation in mass flowing from these stars. This mass loss extends down to the level of the horizontal branch and increases with luminosity. The mass loss is episodic, occurring in only a fraction of stars at a given luminosity. Using a simple model and our observations we derive mass-loss rates for these stars. Finally, we obtain the first empirical mass-loss formula calibrated with observations of Population II stars. The dependence on luminosity of our mass-loss rate is considerably shallower than the widely used Reimers law. The results presented here are the first from our Spitzer survey of a carefully chosen sample of 17 Galactic globular clusters, spanning the entire metallicity range from about one hundredth up to almost solar
The peculiar horizontal branch of NGC 2808
No full textWe present an accurate analysis of the peculiar horizontal branch (HB) of the massive Galactic globular cluster NGC2808, based on high-resolution far-UV and optical images of the central region of the cluster obtained with the Hubble Space Telescope. We confirm the multimodal distribution of stars along the HB: four sub-populations separated by gaps are distinguishable. The detailed comparison with suitable theoretical models showed that (i) it is not possible to reproduce the luminosity of the entire HB with a single helium abundance, while an appropriate modelling is possible for three HB groups by assuming different helium abundances in the range 0.24 < deltaY < 0.4 that are consistent with the multiple populations observed in the main sequence; and (ii) canonical HB models are not able to properly match the observational properties of the stars populating the hottest end of the observed HB distribution, the so-called blue hook region. These objects are probably `hot flashers', stars that peel off the red giant branch before reaching the tip and ignite helium at high Teff. Both of these conclusions are based on the luminosity of the HB in the optical and ultraviolet bands and do not depend on specific assumptions about mass loss
DUST IS FORMING ALONG THE RED GIANT BRANCH OF 47 Tuc
No full textWe present additional evidence that dust is really forming along the red giant branch (RGB) of 47 Tuc at luminosities ranging from above the horizontal branch to the RGB tip. The presence of dust had been inferred from an infrared excess in the (K - 8) color, with K measured from high spatial resolution ground-based near-IR photometry and "8" referring to Spitzer-Infrared Array Camera (IRAC) 8 μm photometry. We show how (K - 8) is a far more sensitive diagnostic for detecting tiny circumstellar envelopes around warm giants than colors using only the Spitzer-IRAC bands, for example, the (3.6 - 8) color used by Boyer et al. In addition, we also show high-resolution Hubble Space Telescope Advanced Camera for Surveys I-band images of the giant stars that have (K - 8) color excess. These images clearly demonstrate that the Boyer et al. statement that our detections of color excess associated with stars below the RGB tip arise from blends and artifacts is simply not valid
L'Universo in Evoluzione - dal Big Bang alla Vita
No full textMostra presso Sala Borsa (Bologna), laboratori, conferenze. Tutte le iniziative alla pagina web:
http://www.boastro2009.it/paginab.htm
Fast Rotating Blue Stragglers in the Globular Cluster M4
No full textWe have used high-resolution spectra obtained with the spectrograph FLAMES at the European Southern Observatory Very Large Telescope to determine the kinematical properties and the abundance patterns of 20 blue straggler stars (BSSs) in the globular cluster (GC) M4. We found that ~40% of the measured BSSs are fast rotators (with rotational velocities >50 km s-1). This is the largest frequency of rapidly rotating BSSs ever detected in a GC. In addition, at odds with what has been found in 47 Tucanae, no evidence of carbon and/or oxygen depletion has been revealed in the sample of 11 BSSs for which we were able to measure the abundances. This could be due to either low statistics, or a different BSS formation process acting in M4.
Based on observations collected at the ESO-VLT under program 081.D-0356. Also based on observations collected at the ESO-MPI Telescope under program 69.D-0582, and with the NASA/ESA HST, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555
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