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

Abstract

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, ~100K 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 um. 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 vs. thermal energy for the ring with optical depth <τ> ≥ 0.10 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 sheparded 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 ~100K, the temperature at which cometary activity onset is seen in our Solar System.

Characterization of the Nucleus, Morphology, and Activity of Interstellar Comet 2I/Borisov by Optical and Near-infrared GROWTH, Apache Point, IRTF, ZTF, and Keck Observations

“…The study of interstellar objects (ISOs) is presently the best opportunity to directly observe the contents of extrasolar circumstellar disks at larger than centimeter-sized scales. Present-day observations are limited to observing the micronsized (e.g., Lisse et al 2012Lisse et al , 2017 to millimeter-sized (MacGregor et al 2019) dust contents of extrasolar disks. Indirect observations of macroscopic objects and their volatile contents in debris disks can be obtained through the massive amounts of dust produced by their collision with each other (Meng et al 2014;Su et al 2019), their presence around young stars (Chen et al 2006), or sometimes by their transit of stars (Rappaport et al 2018), but observing and obtaining the physical properties and volatile contents of specific bodies from other stars has remained elusive.…”

Multiband Polarimetric Imaging of HR 4796A with the Gemini Planet Imager

“…The disk is exceptionally bright, with an infrared excess f=L IR / L * =5×10 −3 (Jura 1991), which has made it a popular target for subsequent debris disk studies. Since its discovery, the disk has been imaged in many wavelengths, including the submillimeter (Sheret et al 2004), the millimeter (Greaves et al 2000), midinfrared (Koerner et al 1998;Lisse et al 2017), near-infrared (Schneider et al 1999;Perrin et al 2014;Milli et al 2017), and visible (Schneider et al 2009(Schneider et al , 2014Milli et al 2019). These multiwavelength observations have allowed for extensive modeling of the spectral energy distribution (SED) in order to understand the dust composition of the disk (Li & Lunine 2003;Rodigas et al 2015).…”

APO Time-resolved Color Photometry of Highly Elongated Interstellar Object 1I/‘Oumuamua

“…Lisse et al 2007;Öberg et al 2015;Nomura et al 2016;Lisse et al 2017). However, these observations have consisted of distant collections of millions of objects spanning large ranges of temperature, astrocentric distance, and composition.…”