HR 4796A: A Nearby System Hosting a Dense Bright Ring of Active Comets C.M. Lisse1, M.L. Sitko2, M. Marengo3 R. J. Vervack Jr.1, Y.R. Fernandez4, T. Mittal5 1JHU-APL, 11100 Johns Hopkins Road, Laurel, MD 20723 car-ey.lisse@jhuapl.edu, ron.vervack@jhuapl.edu 2Space Science Institute, 475Walnut St., Suite205, Boulder, CO80301 sitko@spacescience.org 3Department of Physics and Astronomy, 12 Physics Hall, Iowa State University, Ames, IA 50010 4 Department of Physics, University of Central Florida, Orlando, FL 32186-2385 yan@physics.ucf.edu 5 Department of Earth and Planetary Sciences, McCone Hall, University of California at Berkeley, Berkeley, CA 94720 tmittal2@berkeley.edu

Abstract. Using the NASA/IRTF SpeX 0.8-5.0 µm spectrometer we have obtained new R~1000 NIR ob-servations of the HR 4796A debris ring system. We find a uniquely red excess flux increasing with wave-length from 2-5 µm; the trend is consistent with Debes' 7-band system photometry [1]. Combining the SpeX measurements with archival Spitzer IRS spectroscopy we find a strong scattering trend extending out to ~9 µm, where thermal emission at ~100 K takes over. A 1.5-3% total energy excess is observed, with scattering from material in the known ~75 AU circumstellar ring sourcing 0.5-2% and thermal re-radiation sourcing ~1% of the A0.5V primary's starlight. The NIR reflec-tance spectrum we derive is as red as the reddest Solar System objects, and only matches that of old, active comet nuclei in the Solar System. We conclude that the HR 4796A ring consists of a narrow sheparded belt of comets associated with a planet-building event 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.

Introduction. With a fractional IR luminosity of 5 × 10−3, the circumstellar belt around HR 4796A is one of the brightest and most famous known. HR 4796A is a star of spectral type A0.5V (effective temperature Teff ~ 9350 K, M* = 2.2 MSun, R* = 1.7 RSun, L* = 23 LSun, [Fe/H] = -0.03) located at 67.1±1.7 pc, with estimated age of 10 ± 3 Myr and a known infrared (IR) excess [2-5]. Its young age places HR 4796A at a transitional stage between massive gaseous protostellar disks and evolved and tenuous debris disks around main se-quence stars [6-7]. The belt has maximum brightness at its inner edge, at ~75 AU from the primary and a radial extent of ~5 AU. It is inclined by 75.8◦ with respect to pole-on. HST/NICMOS and GPI coronographic high-angular-resolution images of this system show that the disk ring is narrow, with steep inner and outer edges [8-9]. HR 4796A has been studied in detail for over 20 years, but there is still debate about what constitutes its circumstellar belt. We present new 0.8 – 5.0 µm, R ~ 1000 spectra of the HR 4796A system taken as part of the 100+ hours Near InfraRed Debris disk Survey (NIRDS; 45 systems to date [10]) of northern debris disks using the NASA/IRTF 3m’s SPEX instrument [11-15]. Our spectrum is novel in that it covers the entire wavelength range from 0.8 - 5.0 µm; previous studies have only used small snippets of this range and have not gone past 2.5 µm. The HR 4796A spectrum

Figure 1 - Combined SpeX 0.8 - 5.0 and Spitzer 5.3 - 35 µm SED for HR 4796A (blue). Excess flux after photosphere model (gold) subtraction in lower left (black). with its strong linear red excess from 2-9 µm is unique amongst the NIRDS dataset, including the 15 star sys-tems that evince SEDs produced by the stellar photo-sphere + cold outer system dust and three YSO/Transition disks of similar age to HR 4796A.

Observations. We observed the HR 4796A system at 2.2 - 5.0 µm on 04 Feb 2012 at 13:56 UT, and from 0.8 – 2.5 µm on 30 May 2013 at 05:44 UT, from the NASA/IRTF 3m telescope on the summit of Mauna Kea, HI. Our 0.8 - 5.0 µm SpeX results for HR 4796A are shown in Fig. 1 in blue, combined with a re-extracted and calibrated Spitzer IRS R ~ 100 spectrum of the system taken by [4,16]. The independently cali-brated spectra agree well, and are consistent with ALLWISE, AKARI, and Spitzer photometry of HR 4796A, and with saturation-corrected 2MASS and Ty-cho synthetic photometry [17-18]. The overall appear-ance is of a combined SED dominated by stellar photo-spheric emission at the shorter wavelengths, with a small but growing excess flux extending out to ~9 µm, then a rapid increase in flux due to a cold thermal dust source peaking around 30 µm. There are no detectable HBrγ or CO emission lines due to fluorescing circum-stellar gas or accretion, or HeI 1.083 µm, FeII 1.256/1.644 µm, and H2 S(1) 1-0 2.121 µm emission lines due to strong wind outflow within the noise of the measurement.

NIR Excess Due to Scattering. We determine that the observed HR4796A NIR excess is due to scattered light from the ~75 AU dust ring as the excess spectrum shows a clear flux excess at the few % level over pho-

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tospheric emission that grows approximately linearly with increasing wavelength, and has the same absorp-tion line emission width pattern as the A0V primary. The excess does not match the exponential Wien law trends we see in warm (200-400K) dust-dominated systems we have observed (e.g., HD113766, HD145263, HD98800B). If we assume the excess is due to thermal emission, its color temperature is 3100 to 6100K, depending on the model normalization, in a regime where solid dust is not stable in vacuo. We rule out an NIR excess sourced by a close-in small stellar or sub-stellar companion, because the SpeX excess spectrum shows neither of the strong 1.31 µm AlI and 2.20 µm NaI absorption features characteristic of K or M dwarf stars, nor the CH4/H2O absorptions typical of brown dwarfs or young hot Jupiters [12, 19-21]. The "kink" at ~1.5 µm in K- and M-star photo-spheric continua due to H- absorption is also lacking [12]. Further, a point source object with ~ 2% of the primary's luminosity and 10% of its 2 µm flux would have been easily detected by GPI in the NIR [9], a close-in low mass young M-star or brown dwarf com-panion would have been easily detected in the x-ray, as HR 4796B was by Chandra [22], and Hipparcos long-term astrometry [23] shows no evidence for perturba-tions due to an unresolved close-in companion.

Source of the NIR Scattering Excess. The scattered ring light reflectance we find is very red (Fig. 2). There are 15 other photosphere + cold Kuiper Belt dust sys-tems in the NIRDS survey that do not evince such be-havior, including Fomalhaut (A1-3V) and HD 32297 (A4V) and three other early-to-mid type A stars of high luminosity (Fig. 2). Our reflectance spectrum only matches that of active comets observed in our Solar System (Fig. 3). It definitely does not match KBO or distant, inactive Centaur spectra, and only matches that of the active and famously red Centaur 5145 Pholus out to ~2 µm - these objects have surface reflectance spectra dominated by spectrally flat and neutral ices at longer wavelengths [24-25]. An active comet source makes sense when we note that the temperature of the ring dust's thermal emission in the SED is ~95 – 100 K, the temperature at which two cometary activity drivers in our Solar System become important: the onset of amorphous water ice crystallization and the crossing of the CO2 "ice line" [26-28]. Also, a come-tary dust phase function [29] at i = 17o can reproduce the 0.3-2% total excess and 0.5-2:1 balance of scat-tered vs. thermal energy in a narrow belt with GPI's reported 8 AU vertical thickness [9], optical depth 0.5 ≥ <τ> ≥ 0.08, and minimum dust mass of 0.1-0.8 MMars. Because the observed narrow ring structure is likely due to shepherding by nearby planetisimals [9,30], given the low 10 ± 3 Myr age of the HR4796A system [2,5] and the belt's planetary-scale minimum mass, it is likely that tit is associated with a recent bought of planet formation within the last few Myr.

Figure 2 - Spectral dependence of the HR 4796A excess reflectance (black/blue) vs. other NIRDS survey A-stars.

Figure 3 - NIR reflectance spectra for comets, Centaurs, and Trojan small bodies, the reddest objects in the Solar System.

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