What are the best analogs for exocomets?

Matthew M. Knight (University of Maryland, USA) Contact: [email protected]

Temperatures Overview The theoretical blackbody temperature � is defined Suspected exocomets are often described for an object having an albedo � at a distance � from as being “star-grazing”, yet owing to a star of temperature �∗ and radius �∗: differences between their host stars and �∗ the Sun, these are not necessarily � = � (1 − �) . analogous to the “sungrazing” comets seen ∗ 2� in our own solar system. Here I give a brief Theoretical blackbody temperatures for an object overview of our solar system’s near-Sun with a cometary albedo of 0.04 for main sequence comet populations and calculate some star types are shown to the right. The curves are relevant properties for main sequence stars calculated using the minimum temperature and and suspected exocometary systems. The radius for a given classification, so stars of type O best exocomet analogs seem to be the through K are above the indicated line. M type stars poorly understood “sunskirting” comets. are below the K line and a representative curve is shown.

Group/Comet Perihelion Perihelion Perihelion Period Radius Notes Near-Sun Comet Overview (AU) (�⊙) Temp (K) (yr) (km) More than 3700 comets have been discovered in Sungrazers: Kreutz group ~0.006 ~1.2 3685 >500 0.001- Includes C/1965 S1 Ikeya-Seki, SOHO and STEREO images since 1996 (Battams & The Kreutz group includes several historically 10 C/2011 W3 Lovejoy Knight 2017; pie chart below). The vast majority bright comets seen from the ground (Marsden C/2012 S1 (ISON) 0.012 2.6 2517 >105 <1 Dynamically new sungrazer are members of one of several groups produced 1967, Sekanina 2002). While the brightest Meyer group ~0.036 ~8 1453 ? <0.1 [?] Presumed long period by the cascading fragmentation of a single parent members of the group (e.g., C/1965 S1 Ikeya-Seki) Kracht group ~0.045 10 1300 5-6 <0.1 [?] Fragments of 96P comet (e.g., Marsden 2005). Except in a handful of are likely 1-10 km in size, the vast majority are <30 Marsden group ~0.048 10 1259 5-6 <0.1 [?] Fragments of 96P cases, none are seen beyond the fields of view of m with a total mass of such small comets much Kracht-2 group ~0.054 12 1187 4.0 <0.3 322P seen from ground SOHO and STEREO. less than a single large fragment (Knight et al. 96P/Machholz 1 0.124 27 783 5.3 3.4 Lowest q “regular” comet 2010). None of the small Kreutz survive perihelion 1I/’Oumuamua 0.256 55 545 N/A ~0.2 Interstellar object due, presumably, to the combination of intense 2P/Encke 0.336 72 476 3.3 2.4 Shortest period “regular” comet sublimation and tidal fragmentation. 1P/Halley 0.586 126 360 75.3 5.5 “Halley-type” comet Sunskirters: C/1995 O1 Hale-Bopp 0.917 197 288 2456 37 Oort cloud comet The Marsden and Kracht groups are descended 67P/C-G 1.245 268 247 6.5 1.8 Rosetta’s target comet from comet 96P/Machholz 1 (Ohtsuka et al. 2003, 133P/Elst-Pizarro 2.665 573 169 5.62 1.9 Main asteroid belt comet Sekanina & Chodas 2005). Machholz is ~3.4 km in 29P/S-W 1 5.719 1230 115 14.7 15 Active centaur radius and is seen regularly at “normal” cometary Table 1: Notable comets in our solar system distances near 1 AU (Eisner et al. 2019). Many of the Marsden and Kracht comets survive perihelion and are presumed to be 10s to 100s of m in size. System Periastron Periastron Spectral Roche References (R*) Temp (K) type limit (R*) Meyer group objects are dynamically linked to β Pic (population D) 9±3 1879 A6V 2.3 Keifer et al. 2014 each other but not to any other known objects. β Pic (population S) 18±4 1328 A6V 2.3 Keifer et al. 2014 SOHO discoveries by group (number of The brighter objects survive perihelion (Lamy et al. 49 Ceti <22 >1327 A1V 2.5 Miles et al. 2016 detections in parenthesis) as of the end of 2018. 2013), suggesting they are fragments of a long KIC 3542116 <138 >412 F2V 2.5 Rappaport et al. 2018, Kennedy et al. 2019 Statistics from K. Battams (private comm. 2019). period comet or asteroid. They are estimated to KIC 11084727 <35 >807 F[?] 2.5 Rappaport et al. 2018, Kennedy et al. 2019 be 10s to 100s of m in size, but with a total mass Below, I summarize these groups and adopt the KIC 8027456 <201 >445 A[?] 2.9 Kennedy et al. 2019 consistent with the breakup of <10 km object nomenclature of Jones et al. (2018) for defining KIC 8462852 <84 >514 F[?] 2.5 Wyatt et al. 2018 (Battams & Knight 2017). types of orbits. Objects are “sungrazing” if they EPIC 205718330 13* 918 K 4.0 *assumed; Ansdell et al. 2019 reach perihelion inside the Sun’s Roche limit for a Table 2: Estimated parameters for selected exocometary systems The primary object in the Kracht-2 group is typical comet density of 500 kg/m3 (3.45 � = ⊙ 322P/SOHO 1, which is 150-320 m in diameter 0.016 AU). Objects are “sunskirting” if they reach (Knight et al. 2016). 322P is inactive when near 1 perihelion from 3.45-33 � (0.016-0.15 AU). References • References ⊙ AU, suggesting it may be an asteroid which is only Objects with perihelion distances inside the orbit • Ansdell et al. MNRAS 483, 3579 (2019) • Marsden, B.G. AJ 72, 2270 (1967) active due to extreme thermal processes near of Mercury (66 � = 0.31 AU) are termed “near- • Battams & Knight, Phil. Trans. R. Soc. A. • Marsden, B.G. Annu. Rev. Astron. Astrophys. 43, 75 ⊙ perihelion. 375:20160257 (2017) (2005) Sun”. • Eisner et al., AJ 157, 186 (2019) • Miles et al. ApJ 824, 126 (2016) Non-group objects are not known to be linked to • Jones et al., SSR 214, 20 (2018) • Ohtsuka et al., PASJ 55, 321 (2003) any other objects and have unknown sizes. • Keifer et al., Nature 514, 462 (2014) • Rappaport et al. MNRAS 474, 1453 (2018) Acknowledgements • Kennedy et al. MNRAS 482, 5587 (2019) • Sekanina, Z. ApJ 566, 577 (2002) • Knight et al., AJ 139, 926 (2010) • Sekanina & Chodas, ApJSS 161, 551 (2005) This work was supported by NASA Near Earth Group properties are summarized in Table 1. Object Observations grant NNX17AK15G. • Knight et al., ApJL 823, L6 (2016) • Wyatt et al. MNRAS 473, 5286 (2018) • Lamy et al., Icarus 226, 1350 (2013)