Infrared Spectroscopy of HR 4796A's Bright Outer Cometary Ring + Tenuous Inner Hot Dust Cloud

摘要

We have obtained new NASA/IRTF SpeX spectra of the HR 4796A debris ring system. We find a unique red excess flux that extends out to ~9 μm in Spitzer IRS spectra, where thermal emission from cold, ~100 K dust from the system's ring at ~75 au takes over. Matching imaging ring photometry, we find the excess consists of NIR reflectance from the ring, which is as red as that of old, processed comet nuclei, plus a tenuous thermal emission component from close-in, T ~ 850 K circumstellar material evincing an organic/silicate emission feature complex at 7–13 μm. Unusual, emission-like features due to atomic Si, S, Ca, and Sr were found at 0.96–1.07 μm, likely sourced by rocky dust evaporating in the 850 K component. An empirical cometary dust phase function can reproduce the scattered light excess and 1:5 balance of scattered versus thermal energy for the ring with optical depth in an 8 au wide belt of 4 au vertical height and M dust > 0.1–0.7 M Mars. Our results are consistent with HR 4796A, consisting of a narrow shepherded ring of devolatilized cometary material associated with multiple rocky planetesimal subcores and a small steady stream of dust inflowing from this belt to a rock sublimation zone at ~1 au from the primary. These subcores were built from comets that have been actively emitting large, reddish dust for >0.4 Myr at ~100 K, the temperature at which cometary activity onset is seen in our solar system.