Mixing along the Red Giant Branch in Metal-poor Field Stars

We have determined Li, C, N, O, Na, and Fe abundances, and 12C/13C isotopic ratios for a sample of 62 field metal-poor stars (plus 43 taken from the literature). This large sample was used to show that small mass lower-RGB stars (i.e., fainter than the RGB bump) have abundances of light elements in agreement with theoretical predictions from classical evolutionary models. A second, distinct mixing episode occurs just after the RGB bump, reaching regions of incomplete CNO burning. No O-Na anticorrelation, as observed in globular cluster stars, is found in field stars. This means that the mixing episode is not deep enough to reach regions where ON-burning occurs.Comment: 6 pages, 3 encapsulated figures, LateX, uses crckapb.sty; invited talk, in "The Chemical Evolution of the Milky Way: Stars vs Clusters, Vulcano (Italy), 20-24 September 1999, F. Matteucci and F. Giovannelli eds, Kluwer, in pres

The Age of the Galactic Disk

I review different methods devised to derive the age of the Galactic Disk, namely the Radio-active Decay (RD), the Cool White Dwarf Luminosity Function (CWDLF), old opne clusters (OOC) and the Color Magnitude Diagram (CMD) of the stars in the solar vicinity. I argue that the disk is likely to be 8-10 Gyr old. Since the bulk of globulars has an age around 13 Gyr, the possibility emerges that the Galaxy experienced a minimum of Star Formation at the end of the halo/bulge formation. This minimum might reflect the time at which the Galaxy started to acquire material to form the disk inside-out.Comment: 10 pages, 4 figure, invited review, in "The chemical evolution of the Milky Way : Stars vs Clusters, Vulcano (Italy), 20-24 September 199

Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters

Recent progress in studies of globular clusters has shown that they are not simple stellar populations, being rather made of multiple generations. Evidence stems both from photometry and spectroscopy. A new paradigm is then arising for the formation of massive star clusters, which includes several episodes of star formation. While this provides an explanation for several features of globular clusters, including the second parameter problem, it also opens new perspectives about the relation between globular clusters and the halo of our Galaxy, and by extension of all populations with a high specific frequency of globular clusters, such as, e.g., giant elliptical galaxies. We review progress in this area, focusing on the most recent studies. Several points remain to be properly understood, in particular those concerning the nature of the polluters producing the abundance pattern in the clusters and the typical timescale, the range of cluster masses where this phenomenon is active, and the relation between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review

Globular Cluster Distance Determinations

The present status of the distance scale to Galactic globular clusters is reviewed. Six distance determination techniques which are deemed to be most reliable are discussed in depth. These different techniques are used to calibrate the absolute magnitude of the RR Lyrae stars. The various calibrations fall into three groups. Main sequence fitting using Hipparcos parallaxes, theoretical HB models and the RR Lyrae in the LMC all favor a bright calibration, implying a `long' globular cluster distance scale. White dwarf fitting and the astrometric distances yield a somewhat fainter RR Lyrae calibration, while the statistical parallax solution yields faint RR Lyrae stars implying a `short' distance scale to globular clusters. Various secondary distance indicators discussed all favor the long distance scale. The `long' and `short' distance scales differ by (0.31+/-0.16) mag. Averaging together all of the different distance determinations yields Mv(RR) = (0.23+/-0.04)([Fe/H] + 1.6) + (0.56+/-0.12) mag.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles', A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in pres

Distances and ages of globular clusters using Hipparcos parallaxes of local subdwarfs

We discuss the impact of Population II and Globular Cluster (GCs) stars on the derivation of the age of the Universe, and on the study of the formation and early evolution of galaxies, our own in particular. The long-standing problem of the actual distance scale to Population II stars and GCs is addressed, and a variety of different methods commonly used to derive distances to Population II stars are briefly reviewed. Emphasis is given to the discussion of distances and ages for GCs derived using Hipparcos parallaxes of local subdwarfs. Results obtained by different authors are slightly different, depending on different assumptions about metallicity scale, reddenings, and corrections for undetected binaries. These and other uncertainties present in the method are discussed. Finally, we outline progress expected in the near future.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles', A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 22 pages including 3 tables and 2 postscript figures, uses Kluwer's crckapb.sty LaTeX style file, enclose

The Hubble Constant from Observations of the Brightest Red Giant Stars in a Virgo-Cluster Galaxy

The Virgo and Fornax clusters of galaxies play central roles in determining the Hubble constant H_0. A powerful and direct way of establishing distances for elliptical galaxies is to use the luminosities of the brightest red-giant stars (the TRGB luminosity, at M_I = -4.2). Here we report the direct observation of the TRGB stars in a dwarf elliptical galaxy in the Virgo cluster. We find its distance to be 15.7 +- 1.5 Megaparsecs, from which we estimate a Hubble constant of H_0 = 77 +- 8 km/s/Mpc. Under the assumption of a low-density Universe with the simplest cosmology, the age of the Universe is no more than 12-13 billion years.Comment: 12 pages, LaTeX, with 2 postscript figures; in press for Nature, July 199

Rapidly rotating second-generation progenitors for the blue hook stars of {\omega} Cen

Horizontal Branch stars belong to an advanced stage in the evolution of the oldest stellar galactic population, occurring either as field halo stars or grouped in globular clusters. The discovery of multiple populations in these clusters, that were previously believed to have single populations gave rise to the currently accepted theory that the hottest horizontal branch members (the blue hook stars, which had late helium-core flash ignition, followed by deep mixing) are the progeny of a helium-rich "second generation" of stars. It is not known why such a supposedly rare event (a late flash followed by mixing) is so common that the blue hook of {\omega} Cen contains \sim 30% of horizontal branch stars 10 , or why the blue hook luminosity range in this massive cluster cannot be reproduced by models. Here we report that the presence of helium core masses up to \sim 0.04 solar masses larger than the core mass resulting from evolution is required to solve the luminosity range problem. We model this by taking into account the dispersion in rotation rates achieved by the progenitors, whose premain sequence accretion disc suffered an early disruption in the dense environment of the cluster's central regions where second-generation stars form. Rotation may also account for frequent late-flash-mixing events in massive globular clusters.Comment: 44 pages, 8 figures, 2 tables in Nature, online june 22, 201

The RR Lyrae Distance Scale

We review seven methods of measuring the absolute magnitude M_V of RR Lyrae stars in light of the Hipparcos mission and other recent developments. We focus on identifying possible systematic errors and rank the methods by relative immunity to such errors. For the three most robust methods, statistical parallax, trigonometric parallax, and cluster kinematics, we find M_V (at [Fe/H] = -1.6) of 0.77 +/- 0.13, 0.71 +/- 0.15, 0.67 +/- 0.10. These methods cluster consistently around 0.71 +/- 0.07. We find that Baade-Wesselink and theoretical models both yield a broad range of possible values (0.45-0.70 and 0.45-0.65) due to systematic uncertainties in the temperature scale and input physics. Main-sequence fitting gives a much brighter M_V = 0.45 +/- 0.04 but this may be due to a difference in the metallicity scales of the cluster giants and the calibrating subdwarfs. White-dwarf cooling-sequence fitting gives 0.67 +/- 0.13 and is potentially very robust, but at present is too new to be fully tested for systematics. If the three most robust methods are combined with Walker's mean measurement for 6 LMC clusters, V_{0,LMC} = 18.98 +/- 0.03 at [Fe/H] = -1.9, then mu_{LMC} = 18.33 +/- 0.08.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles', A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 21 pages including 1 table; uses Kluwer's crckapb.sty LaTeX style file, enclose

The Distances of the Magellanic Clouds

The present status of our knowledge of the distances to the Magellanic Clouds is evaluated from a post-Hipparcos perspective. After a brief summary of the effects of structure, reddening, age and metallicity, the primary distance indicators for the Large Magellanic Cloud are reviewed: The SN 1987A ring, Cepheids, RR Lyraes, Mira variables, and Eclipsing Binaries. Distances derived via these methods are weighted and combined to produce final "best" estimates for the Magellanic Clouds distance moduli.Comment: Invited review article to appear in ``Post Hipparcos Cosmic Candles'', F. Caputo & A. Heck (Eds.), Kluwer Academic Publ., Dordrecht, in pres

Does Speaking Two Dialects in Daily Life Affect Executive Functions? An Event-Related Potential Study

Whether using two languages enhances executive functions is a matter of debate. Here, we take a novel perspective to examine the bilingual advantage hypothesis by comparing bidialect with mono-dialect speakers’ performance on a non-linguistic task that requires executive control. Two groups of native Chinese speakers, one speaking only the standard Chinese Mandarin and the other also speaking the Southern-Min dialect, which differs from the standard Chinese Mandarin primarily in phonology, performed a classic Flanker task. Behavioural results showed no difference between the two groups, but event-related potentials recorded simultaneously revealed a number of differences, including an earlier P2 effect in the bi-dialect as compared to the mono-dialect group, suggesting that the two groups engage different underlying neural processes. Despite differences in the early ERP component, no between-group differences in the magnitude of the Flanker effects, which is an index of conflict resolution, were observed in the N2 component. Therefore, these findings suggest that speaking two dialects of one language does not enhance executive functions. Implications of the current findings for the bilingual advantage hypothesis are discussed