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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.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Formation and Evolution of Planetary Systems: Properties of Debris Dust Around Solar-Type Stars
We present Spitzer photometric (IRAC and MIPS) and spectroscopic (IRSlow resolution) observations for 314 stars in the Formation andEvolution of Planetary Systems Legacy program. These data are used toinvestigate the properties and evolution of circumstellar dust aroundsolar-type stars spanning ages from approximately 3 Myr-3 Gyr.We identify 46 sources that exhibit excess infrared emission above thestellar photosphere at 24 μm, and 21 sources with excesses at 70μm. Five sources with an infrared excess have characteristics ofoptically thick primordial disks, while the remaining sources haveproperties akin to debris systems. The fraction of systems exhibiting a24 μm excess greater than 10.2% above the photosphere is 15% for ages< 300 Myr and declines to 2.7% for older ages. The upper envelope tothe 70 μm fractional luminosity appears to decline over a similar agerange. The characteristic temperature of the debris inferred from theIRS spectra range between 60 and 180 K, with evidence for the presenceof cooler dust to account for the strength of the 70 μm excessemission. No strong correlation is found between dust temperature andstellar age. Comparison of the observational data with disk modelscontaining a power-law distribution of silicate grains suggests that thetypical inner-disk radius is gsim 10 AU. Although the interpretation isnot unique, the lack of excess emission shortward of 16 μm and therelatively flat distribution of the 24 μm excess for ages lsim 300Myr is consistent with steady-state collisional models.
| The Formation and Evolution of Planetary Systems: Description of the Spitzer Legacy Science Database
We present the science database produced by the Formation and Evolutionof Planetary Systems (FEPS) Spitzer Legacy program. Data reduction andvalidation procedures for the IRAC, MIPS, and IRS instruments aredescribed in detail. We also derive stellar properties for the FEPSsample from available broadband photometry and spectral types, andpresent an algorithm to normalize Kurucz synthetic spectra to opticaland near-infrared photometry. The final FEPS data products include IRACand MIPS photometry for each star in the FEPS sample and calibrated IRSspectra.
| SIM PlanetQuest Key Project Precursor Observations to Detect Gas Giant Planets around Young Stars
We present a review of precursor observing programs for the SIMPlanetQuest Key Project devoted to detecting Jupiter-mass planets aroundyoung stars. In order to ensure that the stars in the sample are free ofvarious sources of astrometric noise that might impede the detection ofplanets, we have initiated programs to collect photometry, high-contrastimages, interferometric data, and radial velocities for stars in boththe northern and southern hemispheres. We have completed a high-contrastimaging survey of target stars in Taurus and the Pleiades and found nodefinitive common proper motion companions within 1" (140 AU) of the SIMtargets. Our radial velocity surveys have shown that many of the targetstars in Sco-Cen are fast rotators, and a few stars in Taurus and thePleiades may have substellar companions. Interferometric data of a fewstars in Taurus show no signs of stellar or substellar companions withseparations of 5-50 mas. The photometric survey suggests thatapproximately half of the stars initially selected for this program arevariable to a degree (1 σ > 0.1 mag) that would degrade theastrometric accuracy achievable for that star. While the precursorprograms are still a work in progress, we provide a comprehensive listof all targets and rank them according to their viability as a result ofthe observations taken to date. The observable that removes by far themost targets from the SIM young stellar object (YSO) program isphotometric variability.
| High-Dispersion Optical Spectra of Nearby Stars Younger Than the Sun
We present high-dispersion (R~16,000) optical (3900-8700 Å)spectra of 390 stars obtained with the Palomar 60 inch telescope. Themajority of stars observed are part of the Spitzer Legacy ScienceProgram ``The Formation and Evolution of Planetary Systems.'' Throughdetailed analysis we determine stellar properties for this sample,including radial and rotational velocities, Li I λ6708 andHα equivalent widths, the chromospheric activity indexR'HK, and temperature- and gravity-sensitive lineratios. Several spectroscopic binaries are also identified. From ourtabulations, we illustrate basic age- and rotation-related correlationsamong measured indices. One novel result is that Ca II chromosphericemission appears to saturate at vsini values above ~30 kms-1, similar to the well-established saturation of X-raysthat originate in the spatially separate coronal region.
| The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer
We provide an overview of the Spitzer Legacy Program, Formation andEvolution of Planetary Systems, that was proposed in 2000, begun in2001, and executed aboard the Spitzer Space Telescope between 2003 and2006. This program exploits the sensitivity of Spitzer to carry outmid-infrared spectrophotometric observations of solar-type stars. With asample of ~328 stars ranging in age from ~3 Myr to ~3 Gyr, we trace theevolution of circumstellar gas and dust from primordial planet-buildingstages in young circumstellar disks through to older collisionallygenerated debris disks. When completed, our program will help define thetimescales over which terrestrial and gas giant planets are built,constrain the frequency of planetesimal collisions as a function oftime, and establish the diversity of mature planetary architectures. Inaddition to the observational program, we have coordinated a concomitanttheoretical effort aimed at understanding the dynamics of circumstellardust with and without the effects of embedded planets, dust spectralenergy distributions, and atomic and molecular gas line emission.Together with the observations, these efforts will provide anastronomical context for understanding whether our solar system-and itshabitable planet-is a common or a rare circumstance. Additionalinformation about the FEPS project can be found on the team Web site.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| Formation and Evolution of Planetary Systems (FEPS): Primordial Warm Dust Evolution from 3 to 30 Myr around Sun-like Stars
We present data obtained with the Infrared Array Camera (IRAC) aboardthe Spitzer Space Telescope (Spitzer) for a sample of 74 young (t<30Myr old) Sun-like (0.7
| Post-T Tauri Stars in the Nearest OB Association
We present results of a spectroscopic survey of X-ray- andproper-motion-selected samples of late-type stars in the LowerCentaurus-Crux (LCC) and Upper Centaurus-Lupus (UCL) subgroups of thenearest OB association: Scorpius-Centaurus. The primary goals of thesurvey are to determine the star formation history of the OB subgroupsand to assess the frequency of accreting stars in a sample dominated by``post-T Tauri'' pre-main-sequence (PMS) stars. We investigate twosamples: (1) proper-motion candidates from the ACT Catalog and TychoReference Catalog (TRC) with X-ray counterparts in the ROSAT All-SkySurvey (RASS) Bright Source Catalog and (2) G- and K-type stars in theHipparcos catalog found to be candidate members by de Zeeuw et al. Weobtained optical spectra of 130 candidates with the Siding Spring 2.3 mdual-beam spectrograph. PMS stars were identified by (1) strong Liλ6707 absorption, (2) subgiant surface gravities, (3) propermotions consistent with Sco-Cen membership, and (4) H-R diagrampositions consistent with being PMS. We find 93% of the RASS-ACT/TRCstars to be probable PMS members, compared with 73% of the Hipparcoscandidates. We demonstrate that measuring the gravity-sensitive bandratio of Sr II λ4077 to Fe I λ4071 is a valuable means ofdiscriminating PMS and zero-age main-sequence (ZAMS) stars. Usingsecular parallaxes and Hipparcos, Tycho-2, and Two Micron All Sky Surveyphotometry, we construct an H-R diagram. Depending on the choice ofpublished evolutionary tracks, we find the mean ages of the PMSpopulations to range between 17 and 23 Myr for LCC and 15 and 22 Myr forUCL. Taking into account observational errors, it appears that 95% ofthe low-mass star formation in each subgroup must have occurred in lessthan 8 Myr (LCC) and 12 Myr (UCL). Using the Bertelli et al. tracks, wefind main-sequence turnoff ages for Hipparcos B-type members to be16+/-1 Myr for LCC and 17+/-1 Myr for UCL. Contrary to previousfindings, it appears that LCC is coeval with, or slightly older than,UCL. The secular parallaxes of the Sco-Cen PMS stars yield distances of85-215 pc, with 12 of the LCC members lying within 100 pc of the Sun.Only one out of 110 (0.9+2.1-0.8%; 1 σ) PMSsolar-type stars in the sample with ages of 13+/-1 (s.e.)+/-6 (1σ) Myr and masses of 1.3+/-0.2 (1 σ) Msolar showsboth enhanced Hα emission and a K-band excess indicative ofaccretion from a truncated circumstellar disk: the nearby (d~=86 pc)classical T Tauri star PDS 66.
| New "weak-line"--T Tauri stars in Lupus
We present first results obtained by a survey of the Lupus star formingregion in search of new T Tauri stars. This study has been performed onthe basis of deep pointed ROSAT observations in the Lupus dark clouds aswell as data from the ROSAT All-Sky-Survey in the surrounding, lessobscured regions. Our survey covers an area of about 230 square degrees,located between 15^h,6^m and 16^h,24^m$ in right ascension and between-47^\circ and -32^\circ in declination. Identification of ROSATAll-Sky-Survey sources in this area by means of optical spectroscopyrevealed 89 T Tauri stars, 86 of them "weak-line" T Tauri stars (WTTS)not known from previous studies of this region. Our pointed ROSATobservations led to the identification of 47 more T Tauri stars, givinga total of 136 new T Tauri stars. The large area of our study, ascompared with previous works, allows us to study the spatialdistribution of WTTS in this star forming region on a large scale. Wefind the new WTTS to be distributed over the whole area of our survey,indicating that their spatial distribution might extend well beyond ourstudy area. Contrary to the Lupus T Tauri stars known prior to thisstudy, the WTTS discovered by the ROSAT All-Sky-Survey are not clusteredin the regions of highest extinction, i.e. the dark clouds. Based onobservations collected at European Southern Observatory, La Silla, Chile(observing proposals ESO Nos. 49.7-0010, 50.7-0109, 51.7-0106,51.7-0029). Tables 5--12 are only available in electronic form at theCDS via anonymous ftp 130.79.128.5 or on www athttp://cdsweb.u-strasbg.fr/abstract.html.
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