The Nearby Young Moving Groups (NYMGs) of stars are ideal for the study of evolution circumstellar disks in which planets may form because their ages range from a few Myr to about 100 Myr, about the same as the interval over which planets are thought
to form. Their stars are distributed over large regions of the sky. Hence, the Wide Field Infrared Survey Explorer (WISE) which scanned the entire sky in four bands from 3.4 to 22.1 mu provides a database well-suited for the study of members of the NYMGs, particularly those identified after the eras of the IRAS and Spitzer observatories. We report our study of the stars in the epsilon and eta Cha, TW Hya, beta Pic, Tuc-Hor, and AB Dor NYMGs. The WISE Preliminary Release Source Catalog, which covers 57% of the sky, contains data for 64% of the stars in our search lists. WISE detected the 11.6 and 22.1 mu emission of all the previously known disks except for the coldest one, AU Mic. WISE detected no disks in the Tuc-Hor and AB Dor groups, the two oldest in our sample; the frequency of disks detected by WISE decreases rapidly with age of the group. WISE detected a circumstellar disk associated with 2M J0820-8003, a pre-main sequence star with episodic accretion in the ~ 6 Myr old eta Cha cluster. The inner radius of the disk extends close to the star, ~0.02 AU and its luminosity is about a tenth that of the star. The episodic accretion is probably powered by the circumstellar disk discussed here.
We report angular diameters of HIP 560 and 21547, two F spectral type pre-main sequence members of the beta Pic Moving Group. We used the East-West 314-m long baseline of the CHARA Array. The measured limb-darkened angular diameters of HIP 560 and 21
547 are 0.492+-0.032 and 0.518+-0.009 mas, respectively. The corresponding stellar radii are 2.1 and 1.6 Rsun for HIP 560 and HIP 21547 respectively. These values indicate that the stars are truly young. Analyses using the evolutionary tracks calculated by Siess, Dufour, and Forestini and the tracks of the Yonsei-Yale group yield consistent results. Analyzing the measurements on an angular diameter vs color diagram we find that the ages of the two stars are indistinguishable; their average value is 13+-2 MY. The masses of HIP 560 and 21547 are 1.65+-0.02 and 1.75+-0.05 Msun, respectively. However, analysis of the stellar parameters on a Hertzsprung-Russell Diagram yields ages at least 5 MY older. Both stars are rapid rotators. The discrepancy between the two types of analyses has a natural explanation in gravitational darkening.
We model the detectability of exoplanets around stars in the Beta Pic Moving Group (BPMG) using the Gemini Planet Imager (GPI), a coronagraphic instrument designed to detect companions by imaging. Members of the BPMG are considered promising targets
for exoplanet searches because of their youth (~12 MY) and proximity (median distance ~35 pc). We wrote a modeling procedure to generate hypothetical companions of given mass, age, eccentricity, and semi-major axis, and place them around BPMG members that fall within the V-band range of the GPI. We count as possible detections companions lying within the GPIs field of view and H-band fluxes that have a host-companion flux ratio placing them within its sensitivity. The fraction of companions that could be detected depends on their brightness at 12 Myr, and hence formation mechanism, and on their distribution of semi-major axes. We used brightness models for formation by disk instability and core-accretion. We considered the two extreme cases of the semi-major axis distribution - the log-normal distribution of the nearby F and G type stars and a power-law distribution indicated by the exoplanets detected by the radial velocity technique. We find that the GPI could detect exoplanets of all the F and G spectral type stars in the BPMG sample with a probability that depends on the brightness model and semi-major axis distribution. At spectral type K to M1, exoplanet detectability depends on brightness and hence distance of the host star. GPI will be able to detect the companions of M stars later than M1 only if they are closer than 10 pc. Of the four A stars in BPMG sample, only one has V-band brightness in the range of GPI; the others are too bright.
We report an analysis of twins of spectral types F or later in the 9th Catalog of Spectroscopic Binaries (SB9). Twins, the components of binaries with mass ratio within 2% of 1.0, are found among the binaries with primaries of F and G spectral type.
They are most prominent among the binaries with periods less than 43 days, a cutoff first identified by Lucy. Within the subsample of binaries with P<43 days, the twins do not differ from the other binaries in their distributions of periods (median P~7d), masses, or orbital eccentricities. Combining the mass ratio distribution in the SB9 in the mass range 0.6 to 0.85 Msun with that measured by Mazeh et al. for binaries in the Carney-Latham high proper motion survey, we estimate that the frequency of twins in a large sample of spectroscopic binaries is about 3%. Current theoretical understanding indicates that accretion of high specific angular momentum material by a protobinary tends to equalize its masses. We speculate that the excess of twins is produced in those star forming regions where the accretion processes were able to proceed to completion for a minority of protobinaries. This predicts that the components of a young twin may appear to differ in age and that, in a sample of spectroscopic binaries in a star formation region, the twins are, on average, older than the binaries with mass ratios much smaller than 1.
