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Spitzer IRS Spectroscopy of IRAS-discovered Debris Disks
We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS)5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60μm excess. The spectra of five objects possess spectral features thatare well-modeled using micron-sized grains and silicates withcrystalline mass fractions 0%-80%, consistent with T Tauri and HerbigAeBe stars. With the exception of η Crv, these objects are youngwith ages <=50 Myr. Our fits require the presence of a cool blackbodycontinuum, Tgr=80-200 K, in addition to hot, amorphous, andcrystalline silicates, Tgr=290-600 K, suggesting thatmultiple parent body belts are present in some debris disks, analogousto the asteroid and Kuiper belts in our solar system. The spectra forthe majority of objects are featureless, suggesting that the emittinggrains probably have radii a>10 μm. We have modeled the excesscontinua using a continuous disk with a uniform surface densitydistribution, expected if Poynting-Robertson and stellar wind drag arethe dominant grain removal processes, and using a single-temperatureblackbody, expected if the dust is located in a narrow ring around thestar. The IRS spectra of many objects are better modeled with asingle-temperature blackbody, suggesting that the disks possess innerholes. The distribution of grain temperatures, based on our blackbodyfits, peaks at Tgr=110-120 K. Since the timescale for icesublimation of micron-sized grains with Tgr>110 K is afraction of a Myr, the lack of warmer material may be explained if thegrains are icy. If planets dynamically clear the central portions ofdebris disks, then the frequency of planets around other stars isprobably high. We estimate that the majority of debris disk systemspossess parent body masses, MPB<1 M⊕. Thelow inferred parent body masses suggest that planet formation is anefficient process.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.
| Nearby Debris Disk Systems with High Fractional Luminosity Reconsidered
By searching the IRAS and ISO databases, we compiled a list of 60 debrisdisks that exhibit the highest fractional luminosity values(fd>10-4) in the vicinity of the Sun (d<120pc). Eleven out of these 60 systems are new discoveries. Special carewas taken to exclude bogus disks from the sample. We computed thefractional luminosity values using available IRAS, ISO, and Spitzer dataand analyzed the Galactic space velocities of the objects. The resultsrevealed that stars with disks of high fractional luminosity oftenbelong to young stellar kinematic groups, providing an opportunity toobtain improved age estimates for these systems. We found thatpractically all disks with fd>5×10-4 areyounger than 100 Myr. The distribution of the disks in the fractionalluminosity versus age diagram indicates that (1) the number of oldsystems with high fd is lower than was claimed before, (2)there exist many relatively young disks of moderate fractionalluminosity, and (3) comparing the observations with a currenttheoretical model of debris disk evolution, a general good agreementcould be found.
| 51 Eridani and GJ 3305: A 10-15 Myr old Binary Star System at 30 Parsecs
Following the suggestion of Zuckerman and coworkers, we consider theevidence that 51 Eri (spectral type F0) and GJ 3305 (M0), historicallyclassified as unrelated main-sequence stars in the solar neighborhood,are instead a wide physical binary system and members of the youngβ Pic moving group (BPMG). The BPMG is the nearest (d<~50 pc) ofseveral groups of young stars with ages around 10 Myr that arekinematically convergent with the Oph-Sco-Cen association (OSCA), thenearest OB star association. Combining South African AstronomicalObservatory optical photometry, Hobby-Eberly Telescope high-resolutionspectroscopy, Chandra X-Ray Observatory data, and Second US NavalObservatory CCD Astrograph Catalog kinematics, we confirm with highconfidence that the system is indeed extremely young. GJ 3305 itselfexhibits very strong magnetic activity but has rapidly depleted most ofits lithium. The 51 Eri/GJ 3305 system is the westernmost known memberof the OSCA, lying 110 pc from the main subgroups. The system is similarto the BPMG wide binary HD 172555/CD -64 1208 and the HD 104237 quintet,suggesting that dynamically fragile multiple systems can survive theturbulent environments of their natal giant molecular cloud complexes,while still having high dispersion velocities imparted. Nearby youngsystems such as these are excellent targets for evolved circumstellardisk and planetary studies, having stellar ages comparable to that ofthe late phases of planet formation.
| X-ray emission of brown dwarfs: towards constraining the dependence on age, luminosity, and temperature
Aims.We observed brown dwarfs in different evolutionary stages with theChandra X-ray Observatory with the aim to disentangle the influence ofdifferent stellar parameters on the X-ray emission of substellarobjects. The ages of our three targets (HR 7329 B, Gl 569 Bab, and HD130948 BC) are constrained by them being companions to main-sequencestars of known age. With both known age and effective temperature orbolometric luminosity, the mass can be derived from evolutionary models.Methods.Combining the new observations with previous studies presentedin the literature yields a brown dwarf sample that covers the age rangefrom ~1 Myr to ~1 Gyr. Since the atmospheric temperature of brown dwarfsis approximately constant at young ages, a sample with a large agespread is essential for investigating the possible influence ofeffective temperature on X-ray activity. Results.Two out of three browndwarfs are detected with Chandra, with variable lightcurves andcomparatively soft spectra. Combining our results with published dataallows us to consider a subsample of high-mass brown dwarfs (with0.05-0.07 M_ȯ), thus eliminating mass from the list of freeparameters. We find evidence that X-ray luminosity declines withdecreasing bolometric luminosity steeper than expected from thecanonical relation for late-type stars (L_x/L_bol =10-3...-5). Effective temperature is identified as a likelyparameter responsible for the additional decline of X-ray activity inthe more evolved (and therefore cooler) brown dwarfs of the "high-mass"sample. In another subsample of brown dwarfs characterized by similareffective temperature, the X-ray luminosity scales with the bolometricluminosity without indications for a deviation from the canonical rangeof 10-3...-5 observed for late-type stars.Conclusions.Ourfindings support the idea that effective temperature plays a criticalrole for the X-ray activity in brown dwarfs. This underlines an earliersuggestion based on observations of chromospheric Hα emission inultracool dwarfs that the low ionization fraction in the cool browndwarf atmospheres may suppress magnetic activity.
| An Infrared Coronagraphic Survey for Substellar Companions
We have used the F160W filter (1.4-1.8 μm) and the coronagraph on theNear-Infrared Camera and Multi-Object Spectrometer (NICMOS) on theHubble Space Telescope to survey 45 single stars with a median age of0.15 Gyr, an average distance of 30 pc, and an average H magnitude of 7mag. For the median age we were capable of detecting a 30MJcompanion at separations between 15 and 200 AU. A 5MJ objectcould have been detected at 30 AU around 36% of our primaries. Forseveral of our targets that were less than 30 Myr old, the lower masslimit was as low as 1MJ, well into the high mass planetregion. Results of the entire survey include the proper-motionverification of five low-mass stellar companions, two brown dwarfs(HR7329B and TWA5B), and one possible brown dwarf binary (Gl 577B/C).
| A substellar companion around the intermediate-mass giant star HD 11977
We report the discovery of a substellar companion to theintermediate-mass star HD 11977 (G5 III). Radialvelocities of this star have been monitored for five years with FEROS atthe 1.52-m ESO and later at the 2.2-m MPG/ESO telescope in La Silla,Chile. Based on the collected data we calculated an orbital solutionwith a period of P=711 days, a semi-amplitude of K1=105 ms-1, and an eccentricity of e=0.4. The period of theradial-velocity variation is longer than that of the estimated stellarrotation, rendering it unlikely that rotational modulation is the sourceof the variation in the radial velocity. This hypothesis is supported bythe absence of a correlation between stellar activity indicators andradial-velocity variation. By determining a primary stellar mass ofMstar=1.91 Mȯ, the best-fit minimum mass ofthe companion and semi-major axis of the orbit arem2sin{i}=6.54 MJup and a2=1.93 AU,respectively. An upper limit for the mass of the companion ofm2 65.5 MJup has been calculated fromHipparcos astrometric measurements. Although the possibility of abrown-dwarf companion cannot be excluded, HD 11977 B is one of the fewplanet candidates detected around an intermediate-mass star. Theprogenitor main-sequence star of HD 11977 is probablyan A-type star. This discovery gives an indirect evidence for planetarycompanions around early type main-sequence stars.
| Young Stars Near the Sun
Until the late 1990s the rich Hyades and the sparse UMa clusters werethe only coeval, comoving concentrations of stars known within 60 pc ofEarth. Both are hundreds of millions of years old. Then beginning in thelate 1990s the TW Hydrae Association, the Tucana/Horologium Association,the Pictoris Moving Group, and the AB Doradus Moving Group wereidentified within 60 pc of Earth, and the Chamaeleontis cluster wasfound at 97 pc. These young groups (ages 8 50 Myr), along with othernearby, young stars, will enable imaging and spectroscopic studies ofthe origin and early evolution of planetary systems.
| Initial Results from the Palomar Adaptive Optics Survey of Young Solar-Type Stars: A Brown Dwarf and Three Stellar Companions
We present first results from the Palomar Adaptive Optics Survey ofYoung Stars conducted at the Hale 5 m telescope. Through direct imagingwe have discovered a brown dwarf and two low-mass stellar companions tothe young solar-type stars HD 49197, HD 129333 (EK Dra), and V522 Perand confirmed a previously suspected companion to RX J0329.1+0118(Sterzik et al.), at respective separations of 0.95" (43 AU), 0.74" (25AU), 2.09" (400 AU), and 3.78" (380 AU). Physical association of eachbinary system is established through common proper motion and/orlow-resolution infrared spectroscopy. Based on the companion spectraltypes, we estimate their masses at 0.06, 0.20, 0.13, and 0.20Msolar, respectively. From analysis of our imaging datacombined with archival radial velocity data, we find that the spatiallyresolved companion to HD 129333 is potentially identical to thepreviously identified spectroscopic companion to this star (Duquennoy& Mayor). However, a discrepancy with the absolute magnitudesuggests that the two companions could also be distinct, with theresolved one being the outermost component of a triple system. The browndwarf HD 49197B is a new member of a growing list of directly imagedsubstellar companions at 10-1000 AU separations from main-sequencestars, indicating that such brown dwarfs may be more common thaninitially speculated.
| Constraining the Lifetime of Circumstellar Disks in the Terrestrial Planet Zone: A Mid-Infrared Survey of the 30 Myr old Tucana-Horologium Association
We have conducted an N-band survey of 14 young stars in the ~30 Myr oldTucana-Horologium association to search for evidence of warm,circumstellar dust disks. Using the MIRAC-BLINC camera on the Magellan I(Baade) 6.5 m telescope, we find that none of the stars have astatistically significant N-band excess compared to the predictedstellar photospheric flux. Using three different sets of assumptions,this null result rules out the existence of the following around thesepost-T Tauri stars: (1) optically thick disks with inner hole radii of<~0.1 AU, (2) optically thin disks with masses of less than10-6 M⊕ (in ~1 μm sized grains) within<~10 AU of these stars, and (3) scaled-up analogs of the solar systemzodiacal dust cloud with more than 4000 times the emitting area. Oursurvey was sensitive to dust disks in the terrestrial planet zone withfractional luminosity oflog(Ldust/L*)~10-2.9, yet none werefound. Combined with results from previous surveys, these data suggestthat circumstellar dust disks become so optically thin as to beundetectable at N band before age ~20 Myr. We also present N-bandphotometry for several members of other young associations and asubsample of targets that will be observed with the Spitzer SpaceTelescope by the Formation and Evolution of Planetary Systems LegacyScience Program. Finally, we present an absolute calibration ofMIRAC-BLINC for four filters (L, N, 11.6, and Qs) on theCohen-Walker-Witteborn system.
| New Aspects of the Formation of the β Pictoris Moving Group
In a previous work, we explored the possibility that the β Pictorismoving group (BPMG), consisting of low-mass post-T Tauri stars, wasformed near the Scorpius-Centaurus OB association. The cause of theformation could be a Type II supernova exploding either in LowerCentaurus Crux (LCC) or the Upper Centaurus Lupus (UCL), the two oldersubgroups of that association. Here we present new results for BPMG. Amore detailed analysis of the orbit confinement in this group leads to astar distribution pattern at birth that can be considered as arepresentation of the density distribution in the natal cloud. We alsopropose a plausible origin for the supernova that could have triggeredthe star formation in BPMG by finding the past position of the runawaystar HIP 46950. We find that this scenario is capable of explaining theorigin of all the members of BPMG proposed by Zuckerman and coworkersand by Song and coworkers, with the exception of HIP 79881, which isprobably an old main-sequence interloper.
| S Orionis 70: Just a Foreground Field Brown Dwarf?
We examine recent claims that the T-type brown dwarf S Ori053810.1-203626 (S Ori 70) is a spectroscopically verified low-mass(3+5-1 MJup) member of the 1-8 Myrσ Orionis cluster. Comparative arguments by Martín &Zapatero Osorio asserting that S Ori 70 exhibits low surface gravityspectral features indicative of youth and low mass are invalidated bythe fact that their comparison object was not the field T dwarf 2MASS0559-1404, but rather a nearby background star. Instead, we find thatthe 1-2.5 μm spectra of S Ori 70 are well matched to older (age~fewGyr) field T6-T7 dwarfs. Moreover, we find that spectral model fits tolate-type field T dwarf spectra tend to yield low surface gravities(logg=3.0-3.5), and thus young ages (<~5 Myr) and low masses (<~3MJup), inconsistent with expected and/or empirical values.Finally, we show that the identification of one T dwarf in the fieldimaged by Zapatero Osorio et al. is statistically consistent with theexpected foreground contamination. Based on the reexamined evidence, weconclude that S Ori 70 may simply be an old, massive (30-60MJup) field brown dwarf lying in the foreground of theσ Orionis cluster. This interpretation should be considered beforepresuming the existence of so-called ``cluster planets.''
| Dusty Debris Disks as Signposts of Planets: Implications for Spitzer Space Telescope
Submillimeter and near-infrared images of cool dusty debris disks andrings suggest the existence of unseen planets. At dusty but nonimagedstars, semimajor axes of associated planets can be estimated from thedust temperature. For some young stars these semimajor axes are greaterthan 1" as seen from Earth. Such stars are excellent targets forsensitive near-infrared imaging searches for warm planets. To probe thefull extent of the dust and hence of potential planetary orbits, Spitzerobservations should include measurements with the 160 μm filter.
| Infrared spectroscopy of a brown dwarf companion candidate near the young star GSC 08047-00232 in Horologium
We present infrared H- and K-band spectra of a companion candidate 3''north of the young star GSC 08047-00232, a probable member of the nearbyyoung Horologium association. From previously obtained JHK-band colorsand the magnitude difference between primary and companion candidate,the latter could well be substellar (Neuhäuser et al.\cite{Neuh03}) with the spectral type being roughly M 7-L 9 from the JHKcolors (Chauvin et al. \cite{Chauvin03}). With the H- and K-band spectranow obtained with ISAAC at the VLT, the spectral type of the companioncandidate is found to be M 6-9.5. Assuming the same age and distance asfor the primary star ({} 35 Myr, 50 to 85 pc), this yields a massof {} 25 Jupiter masses for the companion, hence indeed substellar.After TWA-5 B and HR 7329 B, this is the third brown dwarf companionaround a nearby (≤100 pc) young (≤100 Myr) star. A total ofthree confirmed brown dwarf companions (any mass, separation ≥50 AU)around 79 stars surveyed in three young nearby associations correspondsto a frequency of 6 ± 4% (with a correction for missingcompanions which are almost on the same line-of-sight as the primarystar instead of being separated well), consistent with the expectation,if binaries have the same mass function as field stars. Hence, it seemsthat there is no brown dwarf desert at wide separations.Based on observations obtained on Cerro Paranal, Chile, in ESO programs65.L-0144 and 71.C-0206C.
| Companions to Young Stars
Brown dwarfs occupy the important region in the mass range between starsand planets. Their existence, ambiguous until only recently, and theirproperties give insight into stellar and planetary formation. We presentstatistical results of an infrared, coronagraphic survey of young,nearby stars that includes probable companions to three young G-typestars, Gl 503.2 (G2V), HD 102982 (G3V), and Gl 577 (G5V). The companionto Gl 577 is a possible binary brown dwarf, according to evolutionarymodels. A dynamical determination of the components' masses will beachievable in the near future and will provide an excellent test of thepredictive ability of the evolutionary models.
| New Members of the TW Hydrae Association, β Pictoris Moving Group, and Tucana/Horologium Association
We have identified five new members of the TW Hydrae association (TWA),11 new members of the β Pic moving group, and 11 newTucana/Horologium association members. These are the three youngest(<~30 Myr) known kinematic stellar groups near the Earth. Newlyidentified β Pic group members are located mostly in the northernhemisphere, and they have a slightly different U-component of Galacticvelocity compared to that of previously known members. Tracing themotion of β Pic members backward in time for 12 Myr indicates thatthey might have formed in a small region with an initial velocitydispersion of ~8 km s-1. A couple of mid-M spectral typeβ Pic members show emission features [He Iλ5876+λ6678) and Na D λ5890+λ5896)] seenamong earlier spectral type stars in the TWA and β Pic groups. Toderive the distances of the non-Hipparcos members of these groups, wehave constructed a V-K versus MK color-magnitude diagram thatis very useful in separating young K/M stars from older main-sequencecounterparts and constraining theoretical pre-main-sequence evolutionarytracks. All newly identified K- and M-type members of the three groupsshow saturated X-ray activity(LX/Lbol~10-3). One newly identifiedTWA member, SSS 101727-5354, is estimated to be only 22 pc away fromEarth. Its extreme youth, late spectral type (~M5), and proximity toEarth make SSS 101727-5354 perhaps the best target for direct imagingdetection of cooling planets.
| An infrared imaging search for low-mass companions to members of the young nearby β Pic and Tucana/Horologium associations
We present deep high dynamic range infrared images of young nearby starsin the Tucana/Horologium and β Pic associations, all 10 to 35Myrs young and at 10 to 60 pc distance. Such young nearby starsare well-suited for direct imaging searches for brown dwarf and evenplanetary companions, because young sub-stellar objects are stillself-luminous due to contraction and accretion. We performed ourobservations at the ESO 3.5m NTT with the normal infrared imagingdetector SofI and the MPE speckle camera Sharp-I. Three arc sec north ofGSC 8047-0232 in Horologium a promising brown dwarf companion candidateis detected, which needs to be confirmed by proper motion and/orspectroscopy. Several other faint companion candidates are alreadyrejected by second epoch imaging. Among 21 stars observed inTucana/Horologium, there are not more than one to five brown dwarfcompanions outside of 75 AU (1.5'' at 50 pc); most certainly only <=5% of the Tuc/HorA stars have brown dwarf companions (13 to 78 Jupitermasses) outside of 75 AU. For the first time, we can report an upperlimit for the frequency of massive planets ( 10 Mjup) atwide separations ( 100 AU) using a meaningfull and homogeneoussample: Of 11 stars observed sufficiently deep in β Pic (12 Myrs),not more than one has a massive planet outside of 100 AU, i.e.massive planets at large separations are rare (<= 9%).Based on observations obtained on La Silla, Chile, in ESO programs65.L-0144(B), 66.D-0135, 66.C-0310(A), 67.C-0209(B), 67.C-0213(A),68.C-0008(A), and 68.C-0009(A)} }
| Adaptive optics imaging survey of the Tucana-Horologium association
We present the results of an adaptive optics (AO) imaging survey of thecommon associations of Tucana and Horologium, carried out at the ESO 3.6m telescope with the ADONIS/SHARPII system. Based on our observations oftwo dozen probable association members, HIP 1910 and HIP 108422 appearto have low-mass stellar companions, while HIP 6856 and GSC 8047-0232have possible sub-stellar candidate companions. Astrometricmeasurements, performed in November 2000 and October 2001, indicate thatHIP 1910 B likely is bound to its primary, but are inconclusive in thecase of the candidate companion to HIP 6856. Additional observations areneeded to confirm the HIP 6856 companionship as well as for HIP 108422and GSC 8047-0232.
| Astrophysics in 2001
During the year, astronomers provided explanations for solar topicsranging from the multiple personality disorder of neutrinos tocannibalism of CMEs (coronal mass ejections) and extra-solar topicsincluding quivering stars, out-of-phase gaseous media, black holes ofall sizes (too large, too small, and too medium), and the existence ofthe universe. Some of these explanations are probably possibly true,though the authors are not betting large sums on any one. The data oughtto remain true forever, though this requires a careful definition of``data'' (think of the Martian canals).
| The status of Galactic field λ Bootis stars in the post-Hipparcos era
The λ Bootis stars are Population I, late B- to early F-typestars, with moderate to extreme (up to a factor 100) surfaceunderabundances of most Fe-peak elements and solar abundances of lighterelements (C, N, O and S). To put constraints on the various existingtheories that try to explain these peculiar stars, we investigate theobservational properties of λ Bootis stars compared with areference sample of normal stars. Using various photometric systems andHipparcos data, we analyse the validity of standard photometriccalibrations, elemental abundances, and Galactic space motions. Therecrystallizes a clear picture of a homogeneous group of Population Iobjects found at all stages of their main-sequence evolution, with apeak at about 1 Gyr. No correlation of astrophysical parameters such asthe projected rotational velocities or elemental abundances with age isfound, suggesting that the a priori unknown mechanism, which createsλ Bootis stars, works continuously for late B- to early F-typestars in all stages of main-sequence evolution. Surprisingly, the sodiumabundances seem to indicate an interaction between the stars and theirlocal environment.
| Discovery of Reflection Nebulosity around Five Vega-like Stars
Coronagraphic optical observations of six Vega-like stars revealreflection nebulosities, five of which were previously unknown. Thenebulosities illuminated by HD 4881, HD 23362, HD 23680, HD 26676, andHD 49662 resemble that of the Pleiades, indicating an interstellarorigin for dust grains. The reflection nebulosity around HD 123160 has adouble-arm morphology, but no disklike feature is seen as close as 2.5"from the star in K-band adaptive optics data. We demonstrate that auniform density dust cloud surrounding HD 23362, HD 23680, and HD 123160can account for the observed 12-100 μm spectral energy distributions.For HD 4881, HD 26676, and HD 49662, an additional emission source, suchas from a circumstellar disk or nonequilibrium grain heating, isrequired to fit the 12-25 μm data. These results indicate that insome cases, particularly for Vega-like stars located beyond the LocalBubble (>100 pc), the dust responsible for excess thermal emissionmay originate from the interstellar medium rather than from a planetarydebris system.
| Deep infrared imaging and spectroscopy of the nearby late M-dwarf DENIS-P J104814-395606
We obtained deep H- and K-band images of DENIS-P J104814-395606 usingSofI and the speckle camera SHARP-I at the ESO-3.5m-NTT as well as QUIRCat the Mauna Kea 2.2m telescope between December 2000 and June 2001. Thetarget was recently discovered as nearby M9-dwarf among DENIS sources(Delfosse et al. 2001). We detect parallactic motion on our images anddetermine the distance to be 4.6 +/- 0.3 pc, more precise thanpreviously known. From the available colors, the distance, and thespectral type, we conclude from theoretical models that the star has amass of ~ 0.075 to 0.09 Msun and an age of ~ 1 to 2 Gyrs.We also obtained H- and K-band spectra of this star with ISAAC at theVLT. A faint companion candidate is detected 6'' NNW of thestar, which is 6.4 +/- 0.5 mag fainter in H. However, according toanother image taken several month later, the companion candidate is notco-moving with the M9-dwarf. Instead, it is a non-moving backgroundobject. Limits for undetected companion candidates are such that we canexclude any stellar companions outside of ~ 0.25'' (1 AU),any brown dwarf companions (above the deuterium burning mass limit)outside of ~ 2'' (9 AU), and also any companion down to ~40 M_jup with >= 0.15'' (0.7 AU) separation, allcalculated for an age of 2 Gyrs. Our observations show that directimaging of sub-stellar companions near the deuterium burning mass limitin orbit around nearby ultra-cool dwarfs is possible, even withseparations that are smaller than the semi-major axis of the outermostplanet in our solar system, namely a few tens of AU.
| Infrared imaging and spectroscopy of companion candidates near the young stars HD 199143 and HD 358623 in Capricornius
We present JHK images of the young ( ~ 20 Myrs) nearby ( ~ 48 pc) starsHD 199143 and HD 358623 (van den Ancker et al. \cite{van00}) with highsensitivity and high dynamic range in order to search for (sub-)stellarcompanions around them. The images were obtained in JHK with the specklecamera SHARP-I in July 2001 and in H with the infrared imaging cameraSofI in December 2000, July 2001, and December 2001, all at the ESO 3.5mNTT. We present a companion candidate with a 2 arcsec offset beingalmost 2 mag fainter than HD 358623 with proper motion (over one yearbaseline) consistent with the known proper motion of the primary star HD358623 A and 6 sigma deviant from the assumption that it is a non-movingbackground object. Then, we obtained a spectrum in the H-band (with SofIat the NTT) of this companion showing that it has spectral type M2 (+/-1), consistent with its JHK colors (for negligible extinction) and withbeing a companion (i.e. at same age and distance) of HD 358623 A(K7-M0), given the magnitude difference. Also, a companion candidatewith a 1 arcsec offset being 2 mag fainter than HD 199143 is detected,but clearly resolved from the primary only with SHARP-I, so that we haveno proper motion information. Also, we could not obtain a spectrum withSofI due to the high dynamic range. The JHK colors of this candidatesand the magnitude difference between primary (F8) and companioncandidate are consistent with a spectral type M0-2. This companioncandidate was predicted by van den Ancker et al. (\cite{van00}) toexplain some unusual properties of the primary star HD 199143 A. Wedetermine the ages and masses of all four objects from theoreticaltracks and isochrones all four stars appear to be co-eval with ~ 20Myrs. Both these companions were presented previously by Jayawardhana& Brandeker (\cite{Jay01}), but only with single-epoch images in Jand K obtained in May 2001. Our limits for additional detectable butundetected companions are such that we would have detected all stellarcompanions with separations ge 0.5{' '} (24 AU at 48 pc).Based on observations obtained on La Silla, Chile, in ESO programs66.D-0135, 67.C-0209, 67.C-0213, 68.C-0009.
| Dusty Circumstellar Disks
Dusty circumstellar disks in orbit around main-sequence stars werediscovered in 1983 by the infrared astronomical satellite. It was thefirst time material that was not another star had been seen in orbitaround a main-sequence star other than our Sun. Since that time,analyses of data from the infrared astronomical satellite, the infraredspace observatory, and ground-based telescopes have enabled astronomersto paint a picture of dusty disks around numerous main-sequence andpost-main-sequence stars. This review describes, primarily in anevolutionary framework, the properties of some dusty disks orbiting,first, pre-main-sequence stars, then main-sequence andpost-main-sequence stars, and ending with white dwarfs.
| The β Pictoris Moving Group
Following the 1983 IRAS detection and subsequent imaging of itsextensive dusty circumstellar disk, β Pictoris became theprototypical and most studied example of a potential forming planetarysystem. Here we report the identification of 17 star systems, each withone or more characteristics indicative of extreme youth, that are movingthrough space together with β Pic. This diverse set of ~12 millionyr old star systems, which includes a ~35 Jupiter mass brown dwarf, anda wide assortment of dusty circumstellar disks, is the comoving,youthful group closest to Earth. Their unique combination of youth andproximity to Earth makes group members-many of which have masses similarto that of the Sun-prime candidates for imaging of warm planets anddusty circumstellar disks with ground- and space-based telescopes.
| Tucana Association
Among star clusters, only the sparse Ursa Major nucleus is closer toEarth than the recently identified Tucana association. Based on newphotometric VRI magnitudes, we construct a color-magnitude diagramcomposed of likely and possible Tucana members. The implied age of theTucana association stars, <~40 Myr, is consistent with the agedetermined from a previous analysis by Stelzer & Neuhäuser ofthe X-ray luminosities of plausible association members. Based primarilyon space motions and X-ray fluxes, we identify potential new members ofthe Tucana association not considered in earlier studies. Torres andcoworkers recently identified a group of post-T Tauri stars, theHorologium association, which is located near the Tucana association inright ascension and declination. Because the Horologium stars have thesame space motions, age, distance from Earth, volume density, and rangeof spectral types as Tucana stars, we suggest that, rather than beingcharacterized as a separate group, it would be appropriate andeconomical to subsume the Horologium association stars into the Tucanastream.
| Ages of A-Type Vega-like Stars from uvbyβ Photometry
We have estimated the ages of a sample of A-type Vega-like stars byusing Strömgren uvbyβ photometric data and theoreticalevolutionary tracks. We find that 13% of these A stars have beenreported as Vega-like stars in the literature and that the ages of thissubset run the gamut from very young (50 Myr) to old (1 Gyr), with noobvious age difference compared to those of field A stars. We clearlyshow that the fractional IR luminosity decreases with the ages ofVega-like stars.
| Discovery of 14 Nearby Double Stars
We report the discovery of 14 double-star systems with main-sequenceprimaries, most of which are M type. Infrared imaging indicates that thesecondaries are most likely late-type dwarfs. The fact that most of thestars are within 25 pc underscores the incompleteness of our presentcensus of our nearest neighbors.
| Comparing Dynamical Ranges of Direct Imaging Planet Detection
Direct imaging of extra-solar planets in orbit around stars is difficultbecause of the problem of dynamic range: Planets are much fainter thentheir parent stars and very close to them. One way around this problemis searching for giant planets around young nearby stars, where theplanets are still self-luminous due to their own thermal emssion becauseof on-going contraction and accretion. We are activelly searching foryoung planets around young (up to 100 Myrs) nearby (up to 100 pc) starsusing space- and ground-based facilities. Previously, we did detect oneplanet candidate near TWA-7, which could have been a few jupiter masscompanion, if at the same distance and age as TWA-7. However, ourfollow-up H-band ISAAC/VLT spectrum has shown that this object is abackground K-type star. Then, we also confirmed two brown dwarfs ascompanions to TWA-5 and HR 7329 by both spectroscopy and proper motion.We will show and compare the dynamical ranges (detected or detectablemagnitude difference versus separation from the primary star) for HSTWFPC, HST Nicmos (without coronograph), Calar Alto 3.5m ALFA (AO), ESO3.6m Adonis (AO, with and without coronograph), ESO 3.5m NTT with theMPE speckle camera SHARP 1, ESO 3.5m NTT with the IR camera SOFI, ESO8.2m VLT with IR camera ISAAC, and Hbar okbar upa'a (AO) at the 8.3mGemini-North on Mauna Kea. We will also discuss the capabilities ofCONICA-NAOS, soon to be available at the VLT.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
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