We present the first near-IR scattered light detection of the transitional disk associated with the Herbig Ae star MWC 758 using data obtained as part of the Strategic Exploration of Exoplanets and Disks with Subaru, and 1.1µm HST/NICMOS data. While sub-millimeter studies suggested there is a dustdepleted cavity with r=0. ′′ 35, we find scattered light as close as 0.1 ′′ (20-28 AU) from the star, with no visible cavity at H, K', or K s . We find two small-scaled spiral structures which asymmetrically shadow the outer disk. We model one of the spirals using spiral density wave theory, and derive a disk aspect ratio of h∼0.18, indicating a dynamically warm disk. If the spiral pattern is excited by a perturber, we estimate its mass to be 5 +3 −4 M J , in the range where planet filtration models predict accretion continuing onto the star. Using a combination of non-redundant aperture masking data at L ′ and angular differential imaging with Locally Optimized Combination of Images at K ′ and K s , we exclude stellar or massive brown dwarf companions within 300 mas of the Herbig Ae star, and all but planetary mass companions exterior to 0. ′′ 5. We reach 5-σ contrasts limiting companions to planetary masses, 3-4 M J at 1. ′′ 0 and 2 M J at 1. ′′ 55 using the COND models. Collectively, these data strengthen the case for MWC 758 already being a young planetary system. Subject headings: circumstellar matter instrumentation: high angular resolution polarization planetary systems: protoplanetary disks stars: individual (MWC 758) waves

We present preliminary trigonometric parallaxes of 184 late-T and Y dwarfs using observations from Spitzer (143), USNO (18), NTT (14), and UKIRT (9). To complete the 20-pc census of ≥T6 dwarfs, we combine these measurements with previously published trigonometric parallaxes for an additional 44 objects and spectrophotometric distance estimates for another 7. For these 235 objects, we estimate temperatures, sift into five 150K-wide T eff bins covering the range 300-1050K, determine the completeness limit for each, and compute space densities. To anchor the high-mass end of the brown dwarf mass spectrum, we compile a list of earlyto mid-L dwarfs within 20 pc. We run simulations using various functional forms of the mass function passed through two different sets of evolutionary code to compute predicted distributions in T eff . The best fit of these predictions to our L, T, and Y observations is a simple power-law model with α ≈ 0.6 (where dN/dM ∝ M −α ), meaning that the slope of the field substellar mass function is in rough agreement with that found for brown dwarfs in nearby star forming regions and young clusters. Furthermore, we find that published versions of the log-normal form do not predict the steady rise seen in the space densities from 1050K to 350K. We also find that the low-mass cutoff to formation, if one exists, is lower than ∼5 M Jup , which corroborates findings in young, nearby moving groups and implies that extremely low-mass objects have been forming over the lifetime of the Milky Way.

We present results from a mid-infrared imaging survey of the $5 Myr old cluster NGC 2362 carried out with the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope. The archival mid-infrared data were merged with extant H emission data, optical and near-infrared photometry, and moderate-resolution optical spectroscopy to identify the remnant disk-bearing population of the cluster and to estimate the fraction of stars that still retain primordial circumstellar disks. The principal sample of 232 suspected cluster members with masses ranging from $10 to 0.3 M (B2YM5 spectral types) was drawn from known H emission stars, X-ray-detected stars from a single 100 ks archival Chandra observation, and established lithium-rich stars. A second sample of 153 stars over a similar mass range whose membership status was based on optical photometry alone was also examined. Measured fluxes in the optical and infrared passbands were fitted with synthetic, low-resolution spectra created using the NextGen atmospheric models, permitting the detection of infrared excesses relative to predicted stellar photospheric fluxes. Using the measured slope of the stellar spectral energy distribution through the four IRAC channels to characterize disk emission for the 195 out of 232 activity/lithium-selected stars and the 105 out of 153 photometric membership candidates having complete IRAC photometry, we derive an upper limit for the primordial, optically thick disk fraction of NGC 2362 of $7% AE 2%, with another $12% AE 3% of suspected members exhibiting infrared excesses indicative of weak or optically thin disk emission. The presence of circumstellar disks among candidate members of NGC 2362 is strongly mass-dependent, such that no stars more massive than $1.2 M exhibit significant infrared excess shortward of 8 m. An upper limit for the fraction of stars hosting primordial, optically thick disks peaks near 10:7% AE 4% for stars with masses between 1.05 and 0.6 M , but the Spitzer IRAC survey is sensitivity-limited below $0.3 M . From H emission-line strengths, an upper limit for the accretion fraction of the cluster is estimated at $5%, with most suspected accretors associated with primordial, optically thick disks identified with Spitzer. The presence of primordial disk-bearing stars in NGC 2362, some of which are suspected of still experiencing gaseous accretion, may imply that even within dense cluster environments, sufficient numbers of inner disks survive to ages consistent with core accretion models of giant planet formation to account for the observed frequency of exoplanets within 5 AU of all FGKMtype stars.

An H emission survey of the young cluster NGC 2264 in the Mon OB1 association resulted in the detection of 490 H emission stars in a 25 0 ; 40 0 field approximately centered between the O7 V multiple star S Mon and the Cone Nebula. The survey was carried out with the wide-field grism spectrograph (WFGS) on the University of Hawaii 2.2 m telescope on Mauna Kea. X-ray observations made with the European Photon Imaging Camera on board the European Space Agency's XMM-Newton satellite observatory will be discussed in a subsequent paper. Optical (BVR C I C ) photometry was obtained for selected fields to supplement similar data from the literature. Spectra covering the 6000-8000 8 region at a resolution of R $ 3000 (adequate for the determination of Li i k6708 line strengths) were obtained for 150 H and X-ray emission sources with the Gemini Multi-Object Spectrograph. Nearinfrared spectra (1-2.5 m) of a number of T Tauri stars (TTSs), X-ray sources, and LH25 (W90) were also obtained using SpeX on the Infrared Telescope Facility. Ages and masses for the H emitters were inferred from the isochrones and evolutionary tracks of D'Antona & Mazzitelli. The median age for the TTS population is about 1.1 Myr, but a considerable dispersion, from 0.1 to 5 Myr, exists for individual objects. Several fields in the cluster were observed with the WFGS on more than one occasion, permitting an examination of H variability over long baselines in time. About 90% of the classical T Tauri stars (CTTSs) showed changes in W(H) of at least 10%, while 57% varied at levels of 50% or more. No evidence was found for a significant pool of dormant H emitters. Summing the masses of the TTSs and the OB stellar population of NGC 2264, a lower limit for the total stellar mass content of the cluster is about 430 M . This is less than 1% of the total mass of the atomic and molecular gas believed to be associated with NGC 2264. Evidence for hierarchical structure within the cluster is suggested by the spatial distribution of TTSs. Four concentrations of H emitters are evident: two near S Mon and two near the Cone Nebula. The median age of the TTSs in the immediate vicinity of S Mon was found to be greater than that of the TTSs near Allen's infrared source ( IRS-1), but a significant dispersion is present. From the rotational data of Lamm et al. and Makidon et al., 241 of the TTSs are periodic variables, 150 weak-line T Tauri stars ( WTTSs) and 91 CTTSs, while 123 stars are irregular variables (30 WTTSs and 93 CTTSs). A weak-to-moderate positive correlation is found between H À K color and P rot for the CTTSs, in the sense that stars having longer periods tend to have larger H À K colors. A similar positive correlation is found between L H and P rot among the CTTSs. No statistically significant correlation is found between P rot and theoretical age or between P rot and L X . Other topics discussed include the fraction of H emitters that are WTTSs, f (WTTS) ¼ N (WTTS)=N (TTS), for clusters of different ages; the relative detectability of H emissio...

High-resolution optical and moderate-resolution near-infrared spectra were obtained for 40 near solar mass (∼2.0-0.5 M ) members of the 2-3 Myr old cluster IC 348 in order to examine established accretion diagnostics and the coupling between inner disk gas and hot, micron-sized dust grains inferred from thermal and mid-infrared excesses. The stellar sample was drawn from the cluster census of Luhman in 2003 with membership being confirmed by radial velocity analysis and the presence of strong Li i λ6708 absorption. Of the stars included in this survey, 12 were classified by Lada in 2006 as hosting primordial, optically thick circumstellar disks, 5 as weak or transition disk systems, and 23 as non-excess stars using the measured slope of the stellar spectral energy distribution (SED) through the four Infrared Array Camera channels (3.6-8.0 µm) of Spitzer Space Telescope. Using the velocity width of Hα as an accretion indicator, we find that 11 primordial disk candidates are suspected accretors, suggesting a strong correlation between gaseous inner disks and optically thick dust emission. Of the five weak or transition disk systems observed, two (L21 and L67) exhibit spectroscopic features indicative of accretion. The presence of gas within the inner disk of these systems, which are free of infrared excess emission shortward of ∼4.5 µm, may place constraints upon the physical mechanism responsible for inner disk clearing. Mass accretion rates (Ṁ) were determined for all suspected accretors using continuum excess measurements near λ6500 and established relationships between He i λ5876, Hα, Ca ii λ8542, Paβ, and Brγ line fluxes and accretion luminosity.Ṁ values were found to range from logṀ = −8.7 to −7.2 M yr −1 , with a median value of −8.1 M yr −1 . Magnetospheric accretion models of Hα, Paβ, and Brγ emission by Muzerolle et al. and Kurosawa et al. are found to be in relative agreement with observed fluxes and derivedṀ estimates. He i λ10830 with its metastable lower level is confirmed to be a critical indicator of magnetospheric accretion, although deep subcontinuum absorption profiles often associated with its emission are not fully understood. No statistically significant correlation is found betweenṀ and the slope of the SED from 3.6 to 8.0 µm or from 8.0 to 24.0 µm; however, the small number of suspected accretors examined does not allow broader conclusions to be drawn.