A New Nearby Candidate Star Cluster in Ophiuchus at d = 170 pc

The recent discoveries of nearby star clusters and associations within a few hundred pc of the Sun, as well as the order of magnitude difference in the formation rates of the embedded and open cluster populations, suggests that additional poor stellar groups are likely to be found at surprisingly close distances to the Sun. Here I describe a new nearby stellar aggregate found by virtue of the parallel proper motions, similar trigonometric parallaxes, and consistent color-magnitude distribution of its early-type members. The 120 Myr-old group lies in Ophiuchus at $d$ $\simeq$ 170 pc, with its most massive member being the 4th-magnitude post-MS B8II-III star $\mu$ Oph. The group may have escaped previous notice due to its non-negligible extinction ($A_V$ $\simeq$ 0.9 mag). If the group was born with a normal initial mass function, and the nine B- and A-type systems represent a complete system of intermediate-mass stars, then the original population was probably of order $\sim$200 systems. The age and space motion of the new cluster are very similar to those of the Pleiades, $\alpha$ Per cluster, and AB Dor Moving Group, suggesting that these aggregates may have formed in the same star-forming complex some $\sim10^8$ yr ago.Comment: 23 pages, 3 figs., to appear in Nov. 2006 A

The nearest young moving groups

The latest results in the research of forming planetary systems have led several authors to compile a sample of candidates for searching for planets in the vicinity of the sun. Young stellar associations are indeed excellent laboratories for this study, but some of them are not close enough to allow the detection of planets through adaptive optics techniques. However, the existence of very close young moving groups can solve this problem. Here we have compiled the members of the nearest young moving groups, as well as a list of new candidates from our catalogue of late-type stars possible members of young stellar kinematic groups, studying their membership through spectroscopic and photometric criteria.Comment: Latex file with 16 pages, 4 figures. Available at http://www.ucm.es/info/Astrof/invest/actividad/skg/skg_sag.html Accepted for publication in: The Astrophysical Journal (ApJ

A New Association of Post-T Tauri Stars Near The Sun

Observing ROSAT sources in 20 x 25 deg centered at the high latitude active star ER Eri, we found evidences for a new young nearby association (~30Myr at~60pc), the Horologium Association (HorA), formed by at least 10 probable and 6 possible members, some being Post-T Tauri stars. We examine several requirements that characterize a young association and they, together, create a strong evidence for the reality of the HorA. In fact, the Li line intensities are between those of the oldest classical T Tauri stars and the ones of the Local Association stars. The space velocities 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. We suggest that some hotter and non-X-ray active stars, with similar space velocities, could be massive members of the HorA, among them, the nearby Be star Achernar. The maximum of the mass distribution function of the HorA is around 0.8 solar masses. At its distance, the projected size of the HorA, ~50 pc, would be larger than our surveyed area and many other members could have been missed. We also observed 3 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 2 very wide systems. The HorA is much more isolated from clouds and older 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 accreting disks. ER Eri itself was found to be a RS CVn-like system.Comment: 25 pages, 5 eps figures, to appear in Astron.

OxyEMG: an application for determination of the oxyspinel group end-members based on electron microprobe analyses

The Oxyspinel group End-Member Generator (OxyEMG) is an improved version of the EMG application. This new version allows for calculating, based on electron microprobe analysis (EMPA), the proportions of 31 end-member components in an oxyspinel composition. These components are MgAl2O4 (spinel), FeAl2O4 (hercynite), MnAl2O4 (galaxite), ZnAl2O4 (gahnite), NiAl2O4 (chihmingite), CuAl2O4 (thermaerogenite), MgFe2O4 (magnesioferrite), Fe3O4 (magnetite), MnFe2O4 (jacobsite), ZnFe2O4 (franklinite), NiFe2O4 (trevorite), CuFe2O4 (cuprospinel), FeMn2O4, MgMn2O4, Mn3O4 (hausmannite), ZnMn2O4 (hetaerolite), MgCr2O4 (magnesiochromite), FeCr2O4 (chromite), MnCr2O4 (manganochromite), ZnCr2O4 (zincochromite), NiCr2O4 (nichromite), CoCr2O4 (cochromite), MgV2O4 (magnesiocoulsonite), FeV2O4 (coulsonite), MnV2O4 (vuorelainenite), Co3O4 (guite), TiMg2O4 (qandilite), TiFe2O4 (ulvöspinel), SiMg2O4 (ringwoodite), SiFe2O4 (ahrensite) and GeFe2O4 (brunogeierite). Compared with the older version, OxyEMG allows for (a) calculating 12 additional oxyspinel group end-member compositions (chihmingite, thermaerogenite, hausmannite, hetaerolite, FeMn2O4, MgMn2O4, cuprospinel, cochromite, guite, ringwoodite, ahrensite and brunogeierite), (b) discriminating the cation valency not only for Fe2+–Fe3+ but also for Mn2+–Mn3+ and Co2+–Co3+, and (c) changing the method to calculate the components of the magnetite and ulvöspinel prisms. As in EMG, this new version is an application that does not require an installation process and was created with the purpose of performing calculations to obtain cation proportions (per formula unit, p.f.u.), end-members of the oxyspinel group, a ΣR3+ value, a ΣR2+ value, ΣR3+ / ΣR2+ ratios, redistribution proportions for the corresponding end-members in the magnetite or ulvöspinel prisms, and a data validation section to check the results.</p

A Comparison of the Chemical Evolutionary Histories of the Galactic Thin Disk and Thick Disk Stellar Populations

We have studied 23 long-lived G dwarfs that belong to the thin disk and thick disk stellar populations. Abundances have been derived for 24 elements: O, Na, Mg, Al, Si, Ca, Ti, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, and Eu. We find that the behavior of [alpha/Fe] and [Eu/Fe] vs. [Fe/H] are quite different for the two populations. As has long been known, the thin disk O, Mg, Si, Ca, and Ti ratios are enhanced relative to iron at the lowest metallicities, and decline toward solar values as [Fe/H] rises above -1.0. For the thick disk, the decline in [alpha/Fe] and [Eu/Fe] does not begin at [Fe/H] = -1.0, but at -0.4. Other elements share this behavior, including Sc, Co, and Zn, suggesting that at least in the chemical enrichment history of the thick disk, these elements were manufactured in similar-mass stars. Combining our results for the oldest and longest-lived stars with prior work, we find clear signs for an independent origin for the Galactic thick disk. (Abridged)Comment: 48 pages and 20 figures, accepted for publication in the Astronomical Journa

Rotational modulation of the photospheric and chromospheric activity in the young, single K2-dwarf PW And

High resolution echelle spectra of PW And (HD~1405) have been taken during eight observing runs from 1999 to 2002. The detailed analysis of the spectra allow us to determine its spectral type (K2V), mean heliocentric radial velocity (V_hel = -11.15 km/s) rotational velocity (vsin{i} = 22.6 km/s), and equivalent width of the lithium line 6707.8 AA (EW(LiI) = 273 mAA). The kinematic (Galactic Velocity (U, V, W)) confirms its membership of the Local Association moving group, in agreement with the age (30 to 80 Myrs) inferred from the color magnitude diagram and the lithium equivalent width. Photospheric activity (presence of cool spots that disturb the profiles of the photospheric lines) has been detected as changes in the the bisectors of the cross correlation function (CCF) resulting of cross-correlate the spectra of PW And with the spectrum of a non active star of similar spectral type. These variations of the CCF bisectors are related to the variations in the measured radial velocities and are modulated with a period similar to the photometric period of the star. At the same time, chromospheric activity has been analyzed, using the spectral subtraction technique and simultaneous spectroscopic observations of the H_alpha, H_beta, NaI D_1 and D_2$, HeI D_3, MgI b triplet, CaII H&K, and CaII infrared triplet lines. A flare was observed during the last observing run of 2001, showing an enhancement in the observed chromospheric lines. A less powerful flare was observed on 2002 August 23. The variations of the chromospheric activity indicators seem to be related to the photospheric activity. A correlation between radial velocity, changes in the CCF bisectors and equivalent width of different chromospheric lines is observed with a different behaviour between epochs 1999, 2001 and 2002.Comment: Latex file with 20 pages, 21 figures tar'ed gzip'ed. Full postscript (text, figures and tables) available at http://www.ucm.es/info/Astrof/users/dmg/pub_dmg.html Accepted for publication in: Astronomy & Astrophysics (A&A

Dynamical Masses of the Binary Brown Dwarf GJ 569Bab

We have obtained new images and high-resolution (R ~ 22400) near-infrared (1.2400-1.2575 micron) spectra of each component of the brown dwarf binary GJ 569Bab using the Adaptive Optics facility of the Keck II telescope and the NIRSPEC spectrometer. These data have allowed us to improve the determination of the astrometric orbit and to measure radial velocities of the components. We have used the astrometric and spectroscopic measurements to derive the dynamical mass of each brown dwarf and the systemic velocity of the pair by means of a chi^2 fitting technique. From various considerations, the mass of each component is likely in the range 0.034-0.070 Msol (GJ 569Bb) and 0.055-0.087 Msol (GJ 569Ba). This implies that the mass ratio, q, of the binary is greater than 0.4, being the most likely value q = 0.75-0.85. Adopting 0.072 Msol as the most conservative location of the substellar limit for solar metallicity, our analysis confirms GJ 569Bb as the first genuine brown dwarf known without any theoretical assumption. We have compared the dynamical masses of GJ 569Ba and Bb, and their effective temperatures and luminosities, to the predictions of state-of-the-art theoretical evolutionary isochrones, finding that models exhibit good performance in the regime of high substellar masses if the binary is about a few hundred million years old. However, the surface gravities of GJ 569Ba (M8.5V) and Bb (M9V) derived from our spectral analysis (the observed data have been compared to the latest synthetic spectra) appear to be smaller than the values provided by the evolutionary models.Comment: 36 pages (tables and figures included). Accepted for publication in Ap

Ages of A-type Vega-like stars from uvbyβ\beta Photometry

We have estimated the ages of a sample of A-type Vega-like stars by using Str\"{o}mgren \emph{uvby$\beta} photometric data and theoretical evolutionary tracks. We find that 13 percent of these A stars have been reported as Vega-like stars in the literature and that the ages of this subset run the gamut from very young (50~Myr) to old (1~Gyr), with no obvious age difference compared to those of field A stars. We clearly show that the fractional IR luminosity decreases with the ages of Vega-like stars.Comment: 4pages text, 3 tables, 3 figures, Accepted in Ap

Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics

While the strong anti-correlation between chromospheric activity and age has led to the common use of the Ca II H & K emission index (R'_HK = L_HK/L_bol) as an empirical age estimator for solar type dwarfs, existing activity-age relations produce implausible ages at both high and low activity levels. We have compiled R'_HK data from the literature for young stellar clusters, richly populating for the first time the young end of the activity-age relation. Combining the cluster activity data with modern cluster age estimates, and analyzing the color-dependence of the chromospheric activity age index, we derive an improved activity-age calibration for F7-K2 dwarfs (0.5 < B-V < 0.9 mag). We also present a more fundamentally motivated activity-age calibration that relies on conversion of R'_HK values through the Rossby number to rotation periods, and then makes use of improved gyrochronology relations. We demonstrate that our new activity-age calibration has typical age precision of ~0.2 dex for normal solar-type dwarfs aged between the Hyades and the Sun (~0.6-4.5 Gyr). Inferring ages through activity-rotation-age relations accounts for some color-dependent effects, and systematically improves the age estimates (albeit only slightly). We demonstrate that coronal activity as measured through the fractional X-ray luminosity (R_X = L_X/L_bol) has nearly the same age- and rotation-inferring capability as chromospheric activity measured through R'_HK. As a first application of our calibrations, we provide new activity-derived age estimates for the nearest 100 solar-type field dwarfs (d < 15 pc).Comment: 78 pages, 15 figures, ApJ, in press. Files can also be downloaded from http://www.cfa.harvard.edu/~emamajek/cahk