Draft version May 18, 2021 Typeset using LATEX twocolumn style in AASTeX63 Thermophysical Modeling of 20 Themis Family Asteroids with WISE/NEOWISE Observations Haoxuan Jiang1 and Jianghui Ji1, 2 1CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China 2CAS Center for Excellence in Comparative Planetology, Hefei 230026, China (Received March 2, 2021) Submitted to AJ ABSTRACT Themis family is one of the largest and oldest asteroid populations in the main-belt. Water-ice may widely exist on the parent body (24) Themis. In this work, we employ the Advanced Thermophysical Model as well as mid-infrared measurements from NASA's Wide-Field Infrared Survey Explorer to explore thermal parameters of 20 Themis family members. Here we show that the average thermal inertia and geometric albedo are 39:5 ± 26:0 Jm−2s−1=2K−1 and 0:067 ± 0:018, respectively. The family members have a relatively moderate roughness fraction on their surfaces. We find that the relatively low albedos of Themis members are consistent with the typical values of B-type and C-type asteroids. As aforementioned, Themis family bears a very low thermal inertia, which indicates a fine and mature regolith on their surfaces. The resemblance of thermal inertia and geometric albedo of Themis members may reveal their close connection in origin and evolution. In addition, we present the compared results of thermal parameters for several prominent families. Keywords: minor planets, asteroid | | thermal | individual, Themis family 1. INTRODUCTION 7 Themis family asteroids, and found that the spectra The Themis family is located in the outer region of of carbonaceous chondrite meteorites can fit the gen- the asteroid belt at a mean distance of 3.13 AU from eral spectral shapes and trends of their Themis fam- the Sun, and is one of the oldest asteroid family, hav- ily members. Based on NASA 3.0-m Infrared Telescope ing been predicted to be formed ∼ 2:5 ± 1:0 Gyr ago Facility (IRTF), Rivkin & Emery(2010) and Campins (Broˇzet al. 2013) by a collisional event from its parent et al.(2010) reported the spectroscopic detection of body (24) Themis. In Nesvorn´yHierarchical Clustering water-ice and organic material on the family's parent Method (HCM) Asteroid Family Catalog, Themis fam- body (24) Themis, indicating the widespread presence of ily is one of the largest asteroid populations, which in- water-ice in asteroidal surfaces and interiors of Themis. cludes more than 4700 family members (Nesvorny 2012) IRTF's spectra showed the presence of a constant depth in 3:1−µm absorption due to the existence of water-ice, with proper orbital elements 3:08 ≤ ap ≤ 3:24 AU, ◦ as well as absorptions between 3.3 and 3.6 µm that are 0:09 ≤ ep ≤ 0:22 and ip ≤ 3 , where ap, ep, and ip closely matched by organic compounds, implying that arXiv:2105.08017v1 [astro-ph.EP] 17 May 2021 are proper semi-major axis, eccentricity and inclination, respectively. This population is known as one of the the ice and organic are widespread and may be evenly most statistically reliable asteroid family in the main distributed over the surface of (24) Themis (Campins belt (Fornasier et al. 2016). et al. 2010). If (24) Themis had experienced cometary Most of the Themis family members are recognized as activities or impact event, the surface ice may be replen- C-type asteroids (Moth´e-Dinizet al. 2005). Ziffer et al. ished by a sub-surface reservoir (Campins et al. 2010). (2011) presented near-infrared spectra (0:8 − 2:4 µm) of Moreover, Fornasier et al.(2016) presented the outcomes of visible and near-infrared spectroscopic survey of 22 Themis family members, and found these asteroids have Corresponding author: Jianghui Ji diverse spectral behaviors including blue/neutral and [email protected], [email protected] moderately red spectra, which 4 of them showed absorp- 2 Jiang & Ji tion bands centered at 0:68 − 0:73 µm, indicating the mal inertia. Thus, we need to employ a more sophisti- presence of aqueous alteration. Besides, Marsset et al. cated thermal model to understand thermal features of (2016) analysed near-infrared spectral properties of 15 Themis family. In this work, we investigate 20 Themis- Themis family members, which are found to be consis- tians from the perspective of thermal physics. Thus we tent with that of chondritic porous interplanetary dust aim to derive their geometric albedos, thermal inertia, particles, and ultra-fine grained materials are found to effective diameters, roughness fraction, and obtain their be the dominant constituents, thereby inferring a par- distribution characteristics. The results may be consid- ent body accreted from a mixture of ice and anhydrous ered to assess whether the asteroids are "interlopers", silicates. thereby revealing the homogeneity/heterogeneity of the Furthermore, water-ice was also discovered on two family members. More recently, by using the data of main belt comets (MBCs), 133P/Elst-Pizarro and Subaru and Herschel telescopes, O'Rourke et al.(2020) 176P/LINEAR, which connected with Themis family showed that the thermal inertia and geometric albedo +25 −2 −1=2 −1 from viewpoints of dynamical evolution and spectral re- of (24) Themis to be ΓThemis = 20−10 Jm s K , flectance (Licandro et al. 2012; Hsieh & Jewitt 2006). pv;Themis = 0:07 ± 0:01 , respectively, with a diameter +10 Dynamical analysis showed that MBCs are more likely DThemis = 192−7 km. Moreover, the asteroid families to have formed in situ in the main belt, rather than are formed from the impact events in a wide variety of originate from the outer solar system (Fern´andezet al. ages and heliocentric distances, thereby making spacial 2002). In particular, if they are the fragments of a environment of the families diversified, which may in collisional family, the activities of MBCs are driven by turn have induced an evolution of thermal process. The water-ice sublimation, implying that a plenty of aster- physical nature of the family members are also deter- oids from Themis family may have water-ice under the mined by the materials of parent body and the impactor. surface. As a matter of fact, after water-ice on (24) Therefore, by comparing the variations in thermal pa- Themis was detected by Rivkin & Emery(2010) and rameters of individual family, one may infer the colli- Campins et al.(2010), there are also similar water- sional scenario of the families and the characteristics of ice detections for other MBAs, along with 4 Themis the parent body. In addition, thermal inertia distribu- family members (Takir & Emery 2012; Hargrove et al. tion among individual asteroid family may be a crucial 2015). Moreover, the visible spectra of 133P/Elst- evidence for the existence of asteroidal differentiation Pizarro and 176P/LINEAR have resemblance to those (Matter et al. 2013). As iron meteorites have higher of three Themistians, indicating the two MBCs may be thermal conductivity than ordinary and carbonaceous the member of Themis family (Licandro et al. 2011). chondrites (Opeil et al. 2010), a metal iron-rich regolith With the data of Spitzer Space Telescope, Hsieh et al. is expected to have larger thermal inertia, thus thermal (2009) determined the geometric R-band albedos and inertia can help us distinguish iron-rich or iron-poor as- effective diameters of two MBCs, pR;133P = 0:05 ± 0:02, teroids (Delbo et al. 2015). Table1 lists the target aster- D133P = 3:8±0:3 km and pR;176P = 0:06±0:02, D176P = oids in this work, where includes the orbital and physical 4:0 ± 0:2 km. Recently, Yu et al.(2020) derived the parameters as well as the spectral type. From Table1, geometric albedo and effective diameter of 133P/Elst- we note that except (1633) Chimay and (1687) Glarona, +0:4 Pizarro to be 0:074 ± 0:013 and 3:9−0:3 km , respec- the remaining bodies are B-type or C-type asteroids, and tively, and evaluated the thermal inertia of 133P/Elst- (2592) Hunan is a slow rotator with a rotation period of Pizarro to be 25 Jm−2s−1=2K−1. The geometric albedos approximately 50 hours when compared to others. of two MBCs correspond roughly to the typical values This paper is structured as follows. In Section 2 we in- of Themistians. Therefore, the Themis family members troduce the Advanced Thermophysical Model (ATPM), may contain crucial clue to catastrophic event and inte- which can be used to calculate the theoretical flux of the rior characteristics of their parent body. target asteroids. The radiometric results for 20 Themis- The cometary activities of Themistians may be partly tians under study and their analysis are presented in involved in the surface temperature, which can be deter- Section 3. In Section 4, we show the distribution of ther- mined by certain thermophysical models. Masiero et al. mal parameters and compare those with other asteroid (2013) applied the Near Earth Asteroid Thermal Model families. Section 5 summarizes the results. (NEATM) (Harris 1998) to investigate thermal charac- teristics for a wide variety of asteroid families. However, 2. ADVANCED THERMOPHYSICAL MODEL AND the NEATM model can simply obtain the albedo and di- WISE OBSERVATIONS ameter of the asteroid, whereas the heat conduction and As mentioned in Delbo et al.(2015), asteroid ther- temperature variation procedure is dominated by ther- mophysical modeling aims to calculate the temperature Thermophysical modeling of Themis Family 3 of asteroids' surface by using specific thermophysical where t is time, x is the depth below the asteroid surface, model, then the theoretical flux emitted by the asteroid κ, ρ and C are given as in Eq.1. Considering the upper can be obtained from Planck function.