Program: GN-2009B-C-10

We have had two nights on Michelle and one night on T-ReCS through the Keck/Gemini Exchange in the past semester and two upcoming T-ReCS nights in June 2009. We have acquired much useful data and now request additional nights with Michelle (2) and T-ReCS (1) to continue follow-up observations of some very dusty IRAS and Spitzer stars. The initial motivation of our program, going back to LWS in 2001, was verification of warm dust IRAS excess candidates through N- and Q-band photometry. Since its inception this program has produced a steady stream of ApJ, Nature and AJ papers (Song et al 2002, Weinberger et al 2004, Song et al 2005, Rhee et al 2007, Rhee et al 2008), including identification of the two dustiest main sequence stars known (BD+20 307 and Pleiad HD 23514), one of which might be as old as the Sun (Zuckerman et al 2008). Warm dust in the terrestrial planet zone indicative of rocky planet formation around additional stars is analyzed in Carl Melis' PhD thesis (2009, in prep.) and in Melis et al. (in prep.). As a consequence of our recent Keck/Gemini mid-IR time allocations, a new class of stars -- first-ascent giants with both dusty disks and gaseous mass accretion -- has been identified (Melis et al. 2009, C. Melis PhD thesis, Melis et al, in prep.). We now seek to continue to characterize the dusty emission around these various types of stars through spectroscopic and additional photometric observations in the mid-IR. For most targets mid-IR features will be carried by silicate grains. Spectroscopy will enable determination of the dominant dust species within the disks and degree of dust processing for various host star ages and evolutionary states. As we have learned in previous observations, even 10 ?m spectroscopy can reveal a great deal about the dominant dust species and the nature of the dust grains orbiting a star (e.g., Rhee et al. 2008). For some stars, specifically the dusty first-ascent giants, we expect to find PAH emission. PAHs have been found in the IRS spectra of two dusty giant stars (Jura et al. 2006, Sloan et al. 2007), and our recent Michelle 10 ?m spectroscopy of a few dusty accreting giants also reveals strong PAH emission. We will continue to observe dusty giants spectroscopically for potential PAH emission to learn about formation of organic material within disks and in extreme environments. We will obtain spectra of dusty dwarfs to learn about the nature of terrestrial planet zone, dust-forming, events. Unlike main sequence stars with orbiting cold dust, of which numerous examples are known, examples of stars orbited by warm dust still remain quite sparse (Rhee et al 2007 and 2008). Photometric measurements of our stars at longer wavelengths (~20 ?m) will enable more accurate characterization of dusty disks. For some targets with cold Kuiper Belt-like dust, we will attempt to spatially resolve, at 20 ?m, the region of dust emission. Previously, such observations have been quite fruitful for determination of dust distribution and other properties (Moerchen et al 2007a & b, Smith et al 2009). We will also continue to obtain verification photometry for newly identified warm excess candidates. We are actively identifying new serendipitous candidates in our Spitzer Archival search (for which new data become public every day) and through auxiliary searches of the IRAS catalogs. (Note: We are requesting two nights on Michelle because a previously awarded Michelle classical night (09A) was transferred to T-ReCS due to undersubscription of Michelle at GEMINI-N.)