Licio da Silva

A New Association of Post-T Tauri Stars near the Sun*

Observing ROSAT sources in an area 20° × 25° centered at the high-latitude (b = -59°) active star ER Eri, we found evidences for a nearby association, that we call the Horologium association (HorA), formed by at least 10 very young stars, some of them being bona fide post–T Tauri stars. We suggest other six stars as possible members of this proposed association. We examine several requirements that characterize a young stellar association. Although no one of them, isolated, gives an undisputed prove of the existence of the HorA, all together practically create a strong evidence for it. In fact, the Li line intensities are between those of the older classical T Tauri stars and the ones of the Local Association stars. The space velocity components of the HorA relative to the Sun (U = -9.5 ± 1.0, V = -20.9 ± 1.1, W = -2.1 ± 1.9) are not far from those of the Local Association, so that it could be one of its last episodes of star formation. In this region of the sky there are some hotter and non–X-ray active stars, with similar space velocities, that could be the massive members of the HorA, among them, the nearby Be star Achernar. The maximum of the mass distribution function of its probable members is around 0.7–0.9 M☉. We estimate its distance as ~60 pc and its size as ~50 pc. If spherical, this size would be larger than the surveyed area, and many other members could have been missed. ER Eri itself was found to be not a member, but a background RS CVn–like system. We also observed three control regions, two at northern and southern Galactic latitudes and a third one in the known TW Hya association (TWA), and the properties and distribution of their young stars strengthen the reality of the HorA. Contrary to the TWA, the only known binaries in the HorA are two very wide systems. The HorA is much more isolated from clouds and older (~30 Myr) than the TWA and could give some clues about the lifetime of the disks around T Tauri stars. Actually, none of the proposed members is an IRAS source indicating an advanced stage of the evolution of their primitive accreting disks.

Rapidly Rotating Lithium-rich K Giants: The New Case of the Giant PDS 365

PDS 365 is a newly detected, rapidly rotating (v sin i = 20 km s-1), single, low-mass giant star that with HD 233517 and HD 219025 forms a remarkable ensemble of single K giants with the unique properties of rapid rotation, very strong Li lines, an asymmetrical Hα profile, and a large far-infrared excess. Their v sin i values are between 18 and 23 km s-1, and their LTE Li abundances, log e(Li), are between 2.9 and 3.9. Detailed analysis of PDS 365 reveals it to be a ~1 M⊙ giant with a value of 12C/13C approximately equal to 12. A clear relation between high rotational velocities and very high Li abundances for K giant stars is found only when asymmetrical Hα profiles and large far-infrared excesses are present. If we consider single K giants, we find that among rapid (v sin i ≥ 8 km s-1) rotators, a very large proportion (~50%) are Li-rich giants. This proportion is in contrast with a very low proportion (~2%) of Li-rich stars among the much more common slowly rotating K giants. This striking difference is discussed in terms of proposed mechanisms for Li enrichment.

Sacy — A Search for Associations Containing Young Stars

The scientific goal of the SACY (Search for Associations Containing Young-stars) was to identify possible associations of stars younger than the Pleiades Association among optical counterparts of the ROSAT X-ray bright sources. High-resolution spectra for possible optical counterparts later than G0 belonging to HIPPARCOS and/or TYCHO-2 catalogs were obtained in order to assess both the youth and the spatial motion of each target. More than 1000 ROSAT sources were observed, covering a large area in the Southern Hemisphere. The newly identified young stars present a patchy distribution in UVW and XYZ, revealing the existence of huge nearby young associations. Here we present the associations identified in this survey.

On 7Li Enrichment by Low-Mass Metal-Poor Red Giant Branch Stars.

First-ascent red giants with strong and very strong Li lines have just been discovered in globular clusters. Using the stellar internal prompt (7)Li enrichment-mass-loss scenario, we explore the possibility of (7)Li enrichment in the interstellar matter of the globular cluster M3 produced by these Li-rich giants. We found that enrichment as large as 70% or more compared to the initial (7)Li content of M3 can be obtained during the entire life of this cluster. However, because M3 will cross into the Galactic plane several times, the new (7)Li will be very probably removed by ram pressure into the disk. Globular clusters appear then as possible new sources of (7)Li in the Galactic disk. It is also suggested that the known Na/Al variations in stars of globular clusters could be somehow related to the (7)Li variations and that the cool bottom process mixing mechanism acting in the case of (7)Li could also play a role in the case of Na and Al surface enrichments.

On The Physical Existence of The Zeta Reticuli Moving Group: A Chemical Composition Analysis

We report the spectroscopic analysis of six kinematical members of the Zeta Reticuli Moving Group, one of them for the first time. We confirm the existence of the Group by establishing a common abundance pattern for four kinematical members. High resolution spectra yielded abundances of Si, Ca, Fe, Ni and Ba, and others. Effective temperatures were derived from the excitation & ionization equilibria of Fe lines of four stars. For these, and the remaining two members, temperatures were derived from colors and the fitting of theoretical spectra to the Hα line, and ages and masses were estimated from theoretical HR diagrams. We suggest that the Group is physical being metal-poor and ~6 Gyr old.

sacy : open clusters connected to young nearby associations

In recent years. we have spectroscopically observed thousands of counterparts of selected ROSAT bright sources in the sacy project (Search for Associations Containing Young stars). We demonstrated that hundreds of the young stars found this way belong to nearby young, loose associations, some detected only in sacy , with ages in the range from 5 to 100 Myr. For their ages, these associations show no trace of their primitive nurseries, which poses some difficulties as to their origins. Nevertheless, there are some associations clearly associated with young open clusters, such as the ϵ Cha association (6 Myr) with the η Cha cluster, and the Argus association with IC 2391 (40 Myr). There seem to exist also more subtle connections, such as the AB Dor association (75 Myr) with the Pleiades. Here, we present evidence of the connections of young associations to open clusters, to understand their origin and their possible insertion in the cluster infant-mortality and Gould Belt scenarios.

Lithium abundances in southern associations containing young stars

In a recent paper, da Silva et al. (2009), we report results of Li abundance analysis for nine young stellar associations, defined in Torres et al. (2008), from a high-resolution optical spectroscopic survey searching for associations containing young stars (SACY), among optical counterparts of ROSAT All-Sky X-ray sources in the Southern Hemisphere. They have applied a convergence method in the (UVW) velocity space and have determined nine nearby young associations in the sample. As they are young and with different ages, those associations form an interesting laboratory to test the Li depletion theory, as a function of the star age.

The Li Abundance and the Age of AB Dor Association

In a recent paper (Torres et al. 2006) we report the results from a highresolution optical spectroscopic survey searching for associations containing young stars (SACY), among optical counterparts of ROSAT All-Sky X-ray sources in the Southern Hemisphere. We have applied a convergence method in the (UVW) velocity space and have found several nearby young associations in the sample. As they are young and with different ages, those associations form an interesting laboratory to test the Li depletion theory, as a function of the star age. We present here our determination of Li abundance for two of them, the β Pic and AB Dor Associations.

The age of the Galactic thin disk from Th/Eu nucleocosmochronology: extended sample

We report the determination of the age of the Galactic thin disk by means of Th/Eu nucleocosmochronology. This method is only weakly dependent on stellar evolutions models, therefore allowing an important verification of the most used dating techniques, which are the fitting of isochrones to the oldest Galactic open clusters, and the calculation of white dwarf cooling sequences. This work builds upon our previous determination (del Peloso et al . 2005a, 2005b), by including 7 new objects to the sample which was originally composed of 19 disk dwarfs/subgiants of F5 to G8 spectral types – a 37% extension. The obtained result, (8.8±1.7)Gyr, corroborates the most recent white dwarf ages determined via cooling sequence calculations, which indicate a low age (

Analysis of Very Rich K Giant Stars

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