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A Revised Age for Upper Scorpius and the Star Formation History among the F-type Members of the Scorpius-Centaurus OB Association
We present an analysis of the ages and star formation history of theF-type stars in the Upper Scorpius (US), Upper Centaurus-Lupus (UCL),and Lower Centaurus-Crux (LCC) subgroups of Scorpius-Centaurus(Sco-Cen), the nearest OB association. Our parent sample is thekinematically selected Hipparcos sample of de Zeeuw et al., restrictedto the 138 F-type members. We have obtained classification-resolutionoptical spectra and have also determined the spectroscopic accretiondisk fraction. With Hipparcos and 2MASS photometry, we estimate thereddening and extinction for each star and place the candidate memberson a theoretical H-R diagram. For each subgroup we construct empiricalisochrones and compare to published evolutionary tracks. We find that(1) our empirical isochrones are consistent with the previouslypublished age-rank of the Sco-Cen subgroups; (2) subgroups LCC and UCLappear to reach the main-sequence turn-on at spectral types ~F4 and ~F2,respectively. An analysis of the A-type stars shows US reaching the mainsequence at about spectral type ~A3. (3) The median ages for thepre-main-sequence members of UCL and LCC are 16 Myr and 17 Myr,respectively, in agreement with previous studies, however we find that(4) Upper Sco is much older than previously thought. The luminosities ofthe F-type stars in US are typically a factor of ~2.5 less luminous thanpredicted for a 5 Myr old population for four sets of evolutionarytracks. We re-examine the evolutionary state and isochronal ages for theB-, A-, and G-type Upper Sco members, as well as the evolved Msupergiant Antares, and estimate a revised mean age for Upper Sco of 11± 1 ± 2 Myr (statistical, systematic). Using radialvelocities and Hipparcos parallaxes we calculate a lower limit on thekinematic expansion age for Upper Sco of >10.5 Myr (99% confidence).However, the data are statistically consistent with no expansion. Wereevaluate the inferred masses for the known substellar companions inUpper Sco using the revised age and find that the inferred masses aretypically ~20%-70% higher than the original estimates which had assumeda much younger age; specifically, we estimate the mass of 1RXSJ1609-2105b to be 14+2 -3 M Jup,suggesting that it is a brown dwarf rather than a planet. Finally, wefind the fraction of F-type stars exhibiting H? emission and/or aK-band excess consistent with accretion to be 0/17 (<19% 95% CL) inUS at ~11 Myr, while UCL has 1/41 (2+5 -1%68% CL) accretors and LCC has 1/50 (2+4 -1%68% CL) accretors at ~16 Myr and ~17 Myr, respectively.
| A Magellan MIKE and Spitzer MIPS Study of 1.5-1.0 M sun Stars in Scorpius-Centaurus
We obtained Spitzer Space Telescope Multiband Imaging Photometer forSpitzer (MIPS) 24 ?m and 70 ?m observations of 182 nearby,Hipparcos F- and G-type common proper motion single and binary systemsin the nearest OB association, Scorpius-Centaurus. We also obtainedMagellan/MIKE R ~ 50,000 visual spectra at 3500-10500 Å for 181candidate ScoCen stars in single and binary systems. Combining our MIPSobservations with those of other ScoCen stars in the literature, weestimate 24 ?m F+G-type disk fractions of 9/27 (33% ± 11%),21/67 (31% ± 7%), and 25/71 (35% ± 7%) for Upper Scorpius(~10 Myr), Upper Centaurus Lupus (~15 Myr), and Lower Centaurus Crux(~17 Myr), respectively. We confirm previous IRAS and MIPS excessdetections and present new discoveries of 41 protoplanetary and debrisdisk systems, with fractional infrared luminosities ranging from LIR/L * = 10-5 to10-2 and grain temperatures ranging from Tgr = 40-300 K. We searched for an increase in 24 ?m excessat an age of 15-20 Myr, consistent with the onset of debris productionpredicted by coagulation N-body simulations of outer planetary systems.We found such an increase around 1.5 M sun stars butdiscovered a decrease in the 24 ?m excess around 1.0 M sunstars. We additionally discovered that the 24 ?m excess around 1.0 Msun stars is larger than predicted by self-stirred models.Finally, we found a weak anti-correlation between fractional infraredluminosity (L IR/L *) and chromospheric activity(R'HK), that may be the result of differences in stellarproperties, such as mass, luminosity, and/or winds.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
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Observation and Astrometry data
| Constellation: | Zentaur |
| Right ascension: | 12h31m12.62s |
| Declination: | -51°41'49.7" |
| Apparent magnitude: | 10.677 |
| Proper motion RA: | -24.2 |
| Proper motion Dec: | 2.6 |
| B-T magnitude: | 11.078 |
| V-T magnitude: | 10.711 |
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