Astron. J., 154, 182-182 (2017/November-0)
Infrared spectroscopy of HR 4796A's bright outer cometary ring + tenuous inner hot dust cloud.
LISSE C.M., SITKO M.L., MARENGO M., VERVACK R.J., FERNANDEZ Y.R., MITTAL T. and CHEN C.H.
Abstract (from CDS):
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 <τ>0.10 in an 8 au wide belt of 4 au vertical height and Mdust > 0.1-0.7 MMars. 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.
© 2017. The American Astronomical Society. All rights reserved.
infrared: stars - scattering - radiation mechanisms: thermal - circumstellar matter - planetary systems - techniques: spectroscopic - techniques: spectroscopic
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