Spectral Evidence for an Inner Carbon-rich Circumstellar Belt in the Young HD 36546 A-star System

Using the NASA/IRTF SpeX and BASS spectrometers we have obtained 0.7-13 mu m observations of the newly imaged 3-10 Myr old HD 36546 disk system. The SpeX spectrum is most consistent with the photospheric emission expected from an L* similar to 20 L-circle dot, solar abundance A1.5V star with little to no extinction, and excess emission from circumstellar dust detectable beyond 4.5 mu m. Non-detections of CO emission lines and accretion signatures point to the gas-poor circumstellar environment of a very old transition disk. Combining the SpeX + BASS spectra with archival WISE/AKARI/IRAS/Herschel photometry, we find an outer cold dust belt at similar to 135 K and 20-40 au from the primary, likely coincident with the disk imaged by Subaru, and a new second inner belt with a temperature similar to 570 K and an unusual, broad SED maximum in the 6-9 mu m region, tracing dust at 1.1-2.2 au. An SED maximum at 6-9 mu m has been reported in just two other A-star systems, HD 131488 and HD 121191, both of similar to 10 Myr age. From Spitzer, we have also identified the similar to 12 Myr old A7V HD 148657 system as having similar 5-35 mu m excess spectral features. The Spitzer data allows us to rule out water emission and rule in carbonaceous materials-organics, carbonates, SiC-as the source of the 6-9 mu m excess. Assuming a common origin for the four young A-star systems' disks, we suggest they are experiencing an early era of carbon-rich planetesimal processing.