Ten micron observations of nearby young stars

We present new 10 mum photometry of 21 nearby young stars obtained at the Palomar 5 m and at the Keck I 10 m telescopes as part of a program to search for dust in the habitable zone of young stars. Thirteen of the stars are in the F - K spectral type range (solar analogs''), four have B or A spectral types, and four have spectral type M. We confirm existing IRAS 12 mum and ground-based 10 mum photometry for 10 of the stars and present new insight into this spectral regime for the rest. Excess emission at 10 mum is not found in any of the young solar analogs, except for a possible 2.4 sigma detection in the G5 V star HD 88638. The G2 V star HD 107146, which does not display a 10 mum excess, is identified as a new Vega-like candidate, based on our 10 mum photospheric detection, combined with previously unidentified 60 and 100 mum IRAS excesses. Among the early-type stars, a 10 mum excess is detected only in HD 109573A (HR 4796A), confirming prior observations; among the M dwarfs, excesses are confirmed in AATau, CD - 40degrees8434, and Hen 3- 600A. A previously suggested N-band excess in the M3 dwarf CD - 33degrees7795 is shown to be consistent with photospheric emission. We calculate infrared to stellar bolometric luminosity ratios for all stars exhibiting mid-infrared excesses and infer the total mass of orbiting dust in the cases of optically thin disks. For a derived median photometric precision of +/- 0.11 mag, we place an upper limit of M-dust approximate to 2 x 10(-5) M-. on the dust mass (assuming a dust temperature of 300 K) around solar analogs not seen in excess at 10 mum. Our calculations for the nearby K1 V star HD 17925 show that it may have the least massive debris disk known outside our solar system (M-dust greater than or similar to7 x 10(-6) M-.). Our limited data confirm the expected tendency of decreasing fractional dust excess f(d) = L-IR/L-* with increasing stellar age. However, we argue that estimates of f(d) suffer from a degeneracy between the temperature and the amount of circumstellar dust M-dust, and we propose a relation of M-dust as a function of age instead.