THE DISCOVERY OF NEW WARM DEBRIS DISKS AROUND F-TYPE STARS

We report the discovery of four rare debris disks with warm excesses around F stars, significantly increasing the number of such systems known in the solar neighborhood. Three of the disks are consistent with the predictions of steady-state planetesimal disk evolution models. The oldest source, HD 169666, displays a dust fractional luminosity too high to be in a steady state and we suggest that this system recently underwent a transient event of dust production. In addition, two spectra of this star separated by approximately three years show silicate emission features, indicative of submicron- to micron-sized grains. We argue that such small grains would be rapidly depleted and their presence in both spectra suggests that the production of small dust is continuous over a timescale of at least a few years. We predict that systems showing variable mid-infrared spectra, if they exist, will provide valuable help in distinguishing the possible scenarios proposed for dust replenishment.