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Published May 2007 | Published
Journal Article Open

Spitzer Observations of NGC 2362: Primordial Disks at 5 Myr


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 Hα 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_⊙ (B2-M5 spectral types) was drawn from known Hα 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% ± 2%, with another ~12% ± 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 μm. An upper limit for the fraction of stars hosting primordial, optically thick disks peaks near 10.7% ± 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 Hα 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 FGKM-type stars.

Additional Information

© 2007 The American Astronomical Society. Received 2006 August 6; accepted 2006 December 14. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407, the Chandra X-Ray Observatory, and the Gemini Multi- Object Spectrograph on Gemini-North. We have made use of the Digitized Sky Surveys, which were produced at the Space Telescope Science Institute under US government grant NAG W-2166, the SIMBAD database operated at CDS, Strasbourg, France, and the Two Micron All Sky Survey, a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the National Science Foundation. We gratefully acknowledge John Carpenter for his sound advice and recommendations for handling Spitzer IRAC data and Theodore Simon for reviewing the manuscript and providing many helpful suggestions that improved its quality. S. E. D. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST 05-02381.

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