Draft version May 10, 2018 Preprint typeset using LATEX style AASTeX6 v. 1.0 SPINUP AND DISRUPTION OF INTERSTELLAR ASTEROIDS BY MECHANICAL TORQUES, AND IMPLICATIONS FOR 1I/2017 U1 (‘OUMUAMUA) Thiem Hoang Korea Astronomy and Space Science Institute, Daejeon 34055, Korea, email: [email protected] and Korea University of Science and Technology, Daejeon, 34113, Korea Abraham Loeb Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, USA A. Lazarian Astronomy Department, University of Wisconsin, Madison, WI 53706, USA Jungyeon Cho Department of Astronomy and Space Science, Chungnam National University, Daejeon, Korea ABSTRACT The discovery of the first interstellar asteroid, 1I/2017 U1 (‘Oumuamua), has opened a new era for research on interstellar objects. In this paper, we study the rotational dynamics of interstellar asteroids (ISAs) of irregular shapes moving through the interstellar gas. We find that regular mechanical torques resulting from the bombardment of gas flow on the irregular body could be important for the dynamics and destruction of ISAs. Mechanical torques can spin up the ISA, resulting in the breakup of the original ISA into small binary asteroids when the rotation rate exceeds the critical frequency. We find that the breakup timescale is short for ISAs of highly irregular shapes and low tensile strength. We apply our results to the first observed ISA, ‘Oumuamua, and suggest that its extreme elongated shape may originate from a reassembly of the binary fragments due to gravity along its journey in the interstellar medium. The tumbling of ‘Oumuamua could have been induced by rotational disruption due to mechanical torques. Finally, we discuss the survival possibility of high-velocity asteroids presumably formed by tidal disruption of planetary systems by the black hole at the Galactic center. Keywords: asteroids: individual (1I/2017 U1 (‘Oumuamua)) meteorites, meteors, meteoroids arXiv:1802.01335v2 [astro-ph.GA] 8 May 2018 1. INTRODUCTION of the first ISA opens a new era of research on interstel- The detection of the first interstellar object, lar objects. 1I/2017 U1 (‘Oumuamua) by the Pan-STARRS sur- Rotational dynamics is critically important for under- vey (Bacci et al. 2017) implies an abundance popula- standing the formation and evolution of asteroids. For tion of similarly interstellar objects (Meech et al. 2017; solar system asteroids (SSAs), it is widely believed that Do et al. 2018). The elongated shape with an ex- the rotation of small asteroids is excited by Yarkovsky- treme axial ratio of & 5 : 1 of ‘Oumuamua is myste- OKeefe-Radzievskii-Paddack (YORP; Rubincam 2000; rious (Fraser et al. 2018; see also Jewitt et al. 2017 and see also Binzel 2003), while larger ones are driven by Gaidos et al. 2017). Bannister et al. (2017) and Gaidos random collisions with asteroids in the asteroid belt (see (2017) suggested that it may be an elongated object or Bottke et al. 2006 for a review). Tidal encounters be- a contact binary, while others speculate that the bizarre tween the asteroid and a planet system can disrupt large shape might be formed by violent events, such as colli- SSAs. sions during the planet formation stage. The discovery For ISAs such as ‘Oumuamua, the YORP effect is 2 Hoang et al. unlikely important because this ISA does not revolve grain can spin-up dust grains to suprathermal rotation, around the Sun as SSAs. Moreover, collisions of ISAs are i.e., rotating with velocity larger than the thermal ve- expected to be much less frequent due to the low density locity. Hoang et al. (2018) found that the efficiency of of the ISAs (see e.g., Feng & Jones 2018). Therefore, to spinup by mechanical torques depends on the irregular- understand the origin and evolution (in shape and size) ity of grain shapes, such that highly irregular shapes can of ISAs, it is necessary to study physical processes that be spun-up to suprathermal rotation, while axisymmet- can affect the dynamics of ISAs in the interplanetary ric grains experience negligible spinup. and interstellar medium (ISM). SSAs have different shapes, from highly irregular Photometric observations reveal that SSAs have dif- shapes to spheroidal shapes, and a number of SSAs ex- ferent shapes, from highly irregular shapes to spheroidal hibit simple shapes with large planar facets and sharp shapes. Thus, we expect ISAs to also have a variety of edges (Torppa et al. 2003), such as 2100 Ra-Shalom, 43 irregular shapes. Because of their motion through the Ariadne, and 694 Ekard (see Domokos et al. 2009). gas, ISAs should experience regular mechanical torques Therefore, to account for a wide range of shapes of in the same way as an helical dust grain or a wind- ISAs, we assume that the asteroid surface can be repre- mill gets spun-up by the gas flow. The original idea of sented by Nfc facets having random orientations. Thus, mechanical torques for irregular grain shapes was intro- an axisymmetric spheroid has Nfc , and the helical → ∞ duced by Lazarian & Hoang (2007) where the authors grain shape in Lazarian & Hoang (2007) has Nfc = 1. found that the gas flow can produce strong regular me- Let R be the effective size of the irregular asteroid with 5 chanical torques when interacting with the helical grain. mass density ρ. The inertia moment is Ii =8πρR αi/15 Numerical calculations in Das & Weingartner (2016) where αi are the dimensionless parameters of unity with demonstrated the spinup effect by mechanical torques αi = 1 for spherical asteroids. for dust grains of Gaussian random shapes. Recently, We consider an ISA drifting at speed vd through a gas Hoang et al. (2018) quantified the mechanical torques of hydrogen density nH and temperature Tgas. When for realistic grains and found that for highly irregular a stream of gas particles hit the asteroid surface, each shapes, mechanical torques are efficient in spinning-up facet can act as a mirror, acquires an amount of the grains to suprathermal rotation as well as aligning grains momentum due to reflection of the gas flow that provides with magnetic fields. a random contribution to the total torque. Following Due to the increase in mechanical torques with the Hoang et al. (2018), the mechanical torque (MAT) due object’s surface area, an ISA is expected to experience to specular reflection by a facet is approximately given large mechanical torques. As a result, ISAs would be by spun-up and disrupted when the rotation speed exceeds 4πR2 the critical threshold determined by the maximum ten- δΓMAT nH v γ (mHv R) , (1) ∼ N d r d sile strength of the material. Such a rotational disrup- fc tion would be an important process that breaks an ISA where γr is the reflection coefficient, and the grain sur- into small fragments in the ISM. face area is 4πR, nH is the gas number density. The structure of the paper is as follows. In Section 2 The net torque from Nfc facets can be calculated using we calculate the rotation velocity achieved by mechan- the random walk formula: ical torques for ISAs. In Section 3 we discuss the criti- Γ δΓ γ N cal speed and breakup timescale of ISAs by mechanical MAT MAT e fc ∼ 3 torques. These results are applied to ISAs in Section 4. p 2 R 4πγr√γenHmHvd , (2) Finally, 5 summarizes our main results. ∼ √Nfc where γe denotes the fraction of the grain surface area 2. ISA SPINUP BY MECHANICAL TORQUES AND that is substantially exposed to the stream of particles. IMPULSIVE TORQUES From Equations (2), we see that an arbitrary grain Rotational dynamics is important for understanding shape of Nfc facets has mechanical torques reduced by the wide range of sizes and rotation periods of small a factor √Nfc. Such a suppression arises from averag- SSAs, as well as the maximum observed rotation rate of ing individual torques of random facets, which we term large SSAs (Pravec & Harris 2000, see also Jewitt 2012). cancellation effect. For instance, a spheroidal or spher- We will first study the spinup of IASs by mechanical ical shape of Nfc 1 experience negligible mechanical ≫ torques. torques, as expected. The rotation frequency achieved after a time t is given 2.1. Spinup by mechanical torques by Lazarian & Hoang (2007) showed that mechanical Γ t torques due to the reflection a gas flow onto a helical Ω= MAT I1 Spinup and Disruption of interstellar asteroids 3 2 2 −5 sd nH 1km tGyr −1 Assuming that all interstellar dust mass is in 0.1 µm 10 − rads , (3) ≃ 10 30cm 3 R 1/2 particles and a dust-to-gas mass ratio of 0.01, the total α1Nfc number of collisions with 0.1 µm grains is 1/2 where sd = vd/vth with vth = (2kBTgas/mH) , and 9 0.01v tm πR2 the travel time tGyr is expressed in units of 10 yr. Here d H Ncoll −15 3 γe =1/6 is assumed. For perfect reflection, γr = 1, and ∼ (4π/3)ρ 10 cm 21 ×2 sticking collisions have the torque reduced by a factor of 10 sd,1R kmtGyr. (8) 2. ∼ The rotation period induced by mechanical torques is The rotation frequency after Ncoll (Eq. 8) collisions can now be calculated using the total angular momen- −3 1/2 −2 2 30cm α1Nfc tum ∆J from Equation (7): PMAT 175s R hr, (4) ≃ d,1 km n t H Gyr ∆J Ω Ncollδω where sd,1 = (sd/10), and R km is the asteroid radius in ∼ I1 ∼ km. p 3/2 −17 sd,1 1/2 3 −1 For an irregular shape with high helicity, e.g., Nfc 10 3 tGyra−5rads , (9) ∼ ≃ α1R km ! 10, a small asteroid of R =0.1 km moving at supersonic speed (s = 10) can be spun-up to PMAT 0.55 hr after which is negligible compared to the spinup by mechan- d ≈ t = 10 Gyr.