TY - JOUR
T1 - Infrared Spectroscopy of HR 4796A's Bright Outer Cometary Ring + Tenuous Inner Hot Dust Cloud
AU - Lisse, C. M.
AU - Sitko, M. L.
AU - Marengo, M.
AU - Vervack, R. J.
AU - Fernandez, Y. R.
AU - Mittal, T.
AU - Chen, C. H.
N1 - Funding Information:
The SPEX data used in this work were obtained by the authors as Visiting Astronomers at the Infrared Telescope Facility, which is operated by the University of Hawaii under Contract with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program. Our NIRDS observations take advantage of and add to the SpeX spectral library of ∼200 cool FGKM stars (Rayner et al. 2009), and we are deeply indebted to J. Rayner for building SpeX and for providing his observing time and expertise to this project. The authors would like to thank T. Rodigas and M. Perrin for their many inputs and suggestions that have made the paper more robust. The authors wish to recognize and acknowledge the very significant role and reverence that the summit of Mauna Kea has within the indigenous Hawaiian community; we are most fortunate to have the opportunity to conduct observations from this mountain. C.L. would also like to gratefully acknowledge the support and input of the NASA Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate. R.V. and Y.F. gratefully acknowledge their support, in part, by NSF grant AST-1109855 in performing this work.
Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved..
PY - 2017/11
Y1 - 2017/11
N2 - 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.
AB - 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.
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U2 - 10.3847/1538-3881/aa855e
DO - 10.3847/1538-3881/aa855e
M3 - Article
AN - SCOPUS:85034581537
SN - 0004-6256
VL - 154
JO - Astronomical Journal
JF - Astronomical Journal
IS - 5
M1 - 182
ER -