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 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* ∼ 20 L , solar abundance A1.5V star with little to no extinction, and excess emission from circumstellar dust detectable beyond 4.5 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 ∼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 ∼570 K and an unusual, broad SED maximum in the 6-9 m region, tracing dust at 1.1-2.2 au. An SED maximum at 6-9 m has been reported in just two other A-star systems, HD 131488 and HD 121191, both of ∼10 Myr age. From Spitzer, we have also identified the ∼12 Myr old A7V HD 148657 system as having similar 5-35 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 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.