Dahm, S. E. and Carpenter, John M. (2009) Spitzer spectroscopy of circumstellar disks in the 5 Myr old upper Scorpius OB association. Astronomical Journal, 137 (4). pp. 4024-4045. ISSN 0004-6256 http://resolver.caltech.edu/CaltechAUTHORS:20090410-110057116
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We present mid-infrared spectra between 5.2 and 38 μm for 26 disk-bearing members of the ~5 Myr old Upper Scorpius OB association obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. We find clear evidence for changes in the spectral characteristics of dust emission between the early-type (B+A) and late-type (K+M) infrared excess stars. The early-type members exhibit featureless continuum excesses that become apparent redward of ~8 μm. In contrast, 10 and 20 μm silicate features or polycyclic aromatic hydrocarbon emission are present in all but one of the late-type excess members of Upper Scorpius. The strength of silicate emission among late-type Upper Scorpius members is spectral-type dependent, with the most prominent features being associated with K5-M2-type stars. By fitting the spectral energy distributions (SED) of a representative sample of low-mass stars with accretion disk models, we find that the SEDs are consistent with models having inner disk radii ranging from ~0.2 to 1.2 AU. Complementary high-resolution (R ~ 33,000) optical (λλ4800-9200) spectra for the Upper Scorpius excess stars were examined for signatures of gaseous accretion. Of the 35 infrared excess stars identified in Upper Scorpius, only seven (all late-type) exhibit definitive signatures of accretion. Mass-accretion rates (M) for these stars were estimated to range from 10^–11 to 10^–8.9 M⊙ yr^–1. Compared to Class II sources in Taurus-Auriga, the disk population in Upper Scorpius exhibits reduced levels of near- and mid-infrared excess emission and an order of magnitude lower mass-accretion rates. These results suggest that the disk structure has changed significantly over the 2-4 Myr in age separating these two stellar populations. The ubiquity of depleted inner disks in the Upper Scorpius excess sample implies that such disks are a common evolutionary pathway that persists for some time.
|Additional Information:||© 2009 The American Astronomical Society. Received 2008 October 21; accepted 2009 January 26; published 2009 March 10. The authors gratefully acknowledge Eric Mamajek for his significant contributions in sample selection for the Upper Scorpius survey. We would also like to thank an anonymous referee whose constructive comments improved this manuscript. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory (JPL), California Institute of Technology, under NASA contract 1407. 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 (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center (IPAC)/California Institute of Technology, funded by NASA and the National Science Foundation. S.E.D. was partially supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-0502381.|
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|Deposited By:||Tony Diaz|
|Deposited On:||16 Apr 2009 18:16|
|Last Modified:||26 Dec 2012 10:56|
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