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The Sizes of the Nearest Young Stars
We present moderate resolution (R ~ 3575) optical spectra of 19 known orsuspected members of the AB Doradus and ? Pictoris Moving Groups,obtained with the DeVeny Spectrograph on the 72 inch Perkins telescopeat Lowell Observatory. For four of five recently proposed members,signatures of youth such as Li I 6708 Å absorption and H?emission further strengthen the case for youth and membership. The lackof detected lithium in the proposed ? Pic member TYC 2211-1309-1implies that it is older than all other K-type members and weakens thecase for membership. Effective temperatures are determined via lineratio analyses for the 11 F, G, and early-K stars observed, and viaspectral comparisons for the eight late-K and M stars observed. Weassemble updated candidate membership lists for these moving groups thataccount for known binarity. Currently, the AB Dor Moving Group contains127 proposed members and the ? Pic Moving Group holds 77 proposedmembers. We then use temperature, luminosity, and distance estimates topredict angular diameters for these stars; the motivation is to identifystars that can be spatially resolved with long-baseline optical/infraredinterferometers in order to improve age estimates for these groups andto constrain evolutionary models at young ages. Considering the portionof the sky accessible to northern hemisphere facilities (decl. >- 30), six stars have diameters large enough to be spatiallyresolved (? > 0.4 mas) with the CHARA Array, which currentlyhas the world's longest baseline of 331 m; this subsample includes thelow-mass M2.5 member of AB Dor, GJ 393, which is likely to still bepre-main sequence. For southern hemisphere facilities (decl. < + 30),18 stars have diameters larger than this limiting size, including thelow-mass debris disk star AU Mic (0.72 mas). However, the longestbaselines of southern hemisphere interferometers (160 m) are only ableto resolve the largest of these, the B6 star ? Gru (1.17 mas)proposed long-baseline stations may alleviate the current limitations.
| Cool Young Stars in the Northern Hemisphere: ? Pictoris and AB Doradus Moving Group Candidates
As part of our continuing effort to identify new, low-mass members ofnearby, young moving groups (NYMGs), we present a list of young,low-mass candidates in the northern hemisphere. We used our provenproper-motion selection procedure and ROSAT X-ray and GALEX-UV activityindicators to identify 204 young stars as candidate members of the? Pictoris and AB Doradus NYMGs. Definitive membership assignmentof a given candidate will require a measurement of its radial velocityand distance. We present a simple system of indices to characterize theyoung candidates and help prioritize follow-up observations. New groupmembers identified in this candidate list will be high priority targetsfor (1) exoplanet direct imaging searches, (2) the study of post-T-Tauriastrophysics, (3) understanding recent local star formation, and (4) thestudy of local galactic kinematics. Information available now allows usto identify eight likely new members in the list. Two of these, a late-Kand an early-M dwarf, we find to be likely members of the ? Picgroup. The other six stars are likely members of the AB Dor movinggroup. These include an M dwarf triple system, and three very coolobjects that may be young brown dwarfs, making them the lowest-mass,isolated objects proposed in the AB Dor moving group to date.
| Potential Members of Stellar Kinematic Groups within 30 pc of the Sun
We analyze the kinematic histories of stars within 30 pc of the Sun, forwhich three-dimensional spatial coordinates and three-dimensionalvelocity vectors are available. From this sample, we extract members ofstellar kinematic groups (SKGs) in the following manner. First, weconsider in the three-dimensional velocity space centered on the localstandard of rest, a sphere with a radius of 8 km s-1centered on the mean velocity vector of a particular SKG. Around eachSKG velocity center, we have found a significant excess of starscompared to background field stars. For each candidate, in thethree-dimensional spatial coordinate space, its trajectory is tracedback in time by the age of the relevant SKG to obtain the estimateddistance from the SKG center at the time of the SKG's birth by theepicyclic approximation and harmonic vertical motion. It often happensthat a star is a candidate member of multiple SKGs. Then we rank thecandidacy to multiple SKGs based on the smallness of distanceseparations. In this manner, we have kinematically selected 238candidates. We further impose at least one of the following qualitativecriteria for being a member: spectral type A or B, variability, or EUVand X-ray emission. We have finally selected 137 candidate members ofSKGs out of a sample of 966 stars.
| ? Pictoris and AB Doradus Moving Groups: Likely New Low-mass Members
We present results from our continuing program to identify new,low-mass, members of the nearby young moving groups (NYMGs) using aproper motion selection algorithm and various observational techniques.We have three goals: (1) to provide high priority targets for exoplanetsearches by direct imaging, (2) to complete the census of the membershipin the NYMGs down to ~0.1 M sun, and thus (3) provide awell-characterized sample of nearby (median distances at least twice asclose as the Taurus and Ophiuchus star-forming regions), young (8-50Myr) stars for detailed study of their physical properties andmultiplicity. Our program proceeds as follows: we apply the selectionalgorithm to a proper motion catalog where initial selection cuts ofcandidate members are based on the mean motion of known NYMG members andthe proper motions and photometric distances of the candidates. NYMGmembership is investigated further using possible signs of youth,including H? emission and X-ray flux, and then verified throughradial velocity measurements. We identify TYC 1766-1431-1 (M3), TYC1208-468-1 and 2 (K3), TYC 7558-655-1 (K5), and PM I04439+3723W and E(M3) as likely members of the ? Pictoris moving group (BPMG) andTYC 1741-2117-1N and S (K7), TYC 1752-63-1 (K7), TYC 523-573-1 (K7), andTYC 4943-192-1 (M0) as likely members of the AB Doradus moving group(ABDMG). We also rule out the membership of several BPMG and ABDMGcandidates. To date our program has identified 16 new NYMG members ofspectral type K3 or later.
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