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Multiple Stellar Fly-Bys Sculpting the Circumstellar Architecture in Rw Aurigae

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Multiple Stellar Fly-Bys Sculpting the Circumstellar Architecture in Rw Aurigae DRAFT VERSION JUNE 11, 2018 Preprint typeset using LATEX style emulateapj v. 12/16/11 MULTIPLE STELLAR FLY-BYS SCULPTING THE CIRCUMSTELLAR ARCHITECTURE IN RW AURIGAE JOSEPH E. RODRIGUEZ1,RYAN LOOMIS1,SYLVIE CABRIT2,3,THOMAS J. HAWORTH4,STEFANO FACCHINI5,CATHERINE DOUGADOS3,RICHARD A. BOOTH6,ERIC L. N. JENSEN7,CATHIE J. CLARKE6,KEIVAN G. STASSUN8,9,WILLIAM R. F. DENT10, JER´ OMEˆ PETY11,2 1Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA 2Sorbonne Universite,´ Observatoire de Paris, Universite´ PSL, CNRS, LERMA, F75014 Paris, France 3Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France 4Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK 5Max-Planck-Institut fur¨ Extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany 6Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK 7Department of Physics and Astronomy, Swarthmore College, Swarthmore, PA 19081, USA 8Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA 9Department of Physics, Fisk University, Nashville, TN 37208, USA 10Joint ALMA Observatory, Alonso de Crdova 3107, 763-0355 Vitacura, Santiago, Chile and 11I.R.A.M., 300 rue de la Piscine, Domaine Universitaire, F-38406 Saint Martin dHres, France Draft version June 11, 2018 ABSTRACT We present high-resolution ALMA Band 6 and 7 observations of the tidally disrupted protoplane- tary disks of the RW Aurigae binary. Our observations reveal the presence of additional tidal streams to the previously observed tidal arm around RW Aur A. The observed configuration of tidal streams surrounding RW Aur A and B is incompatible with a single star–disk tidal encounter, suggesting that the RW Aurigae system has undergone multiple fly-by interactions. We also resolve the circumstellar disks around RW Aur A and B, with CO radii of 58 au and 38 au consistent with tidal truncation, and 2.5 times smaller dust emission radii. The disks appear misaligned by 12◦or 57◦. Using new pho- tometric observations from the American Association of Variable Star Observers (AAVSO) and All Sky Automated Survey for SuperNovae (ASAS-SN) archives, we have also identified an additional dimming event of the primary that began in late 2017 and is currently ongoing. With over a century of photometric observations, we are beginning to explore the same spatial scales as ALMA. Subject headings: stars: individual (RW Aur), stars: binaries: general, protoplanetary disks 1. INTRODUCTION pc away (van Leeuwen 2007) and comprised of at least two stellar objects, RW Aur A and B, separated by 1.5 The evolution of the circumstellar environment of a T ∼ 00 Tauri star (TTS) from gas and dust to planets involves (Herbig & Bell 1988; Ducheneˆ et al. 1999; Stout-Batalha complex dynamical processes that are directly influ- et al. 2000). RW Aur A has a large bipolar jet extend- enced by the presence of companions. It is known that ing out to 10000 and containing many emission knots most TTSs are in binaries (Ghez et al. 1993; Leinert et al. (Mundt & Eisl∼ offel¨ 1998; Lopez-Mart´ ´ın et al. 2003). Us- 1993; Richichi et al. 1994; Simon et al. 1995; Ghez et al. ing the Plateau de Bure Interferometer (PdBI), this sys- 1997), and the process of planet formation can be altered tem was mapped in 12CO and dust continuum by Cabrit and disrupted due to a stellar companion gravitation- et al. (2006) down to a resolution of 0.900 0.600. Sev- ally influencing the gas and dust within the star’s cir- eral unusual features were detected in these× observa- cumstellar disk. Specifically, strong binary interactions tions: a very compact rotating disk around RW Aur A, will stir up the disk, enhancing planetesimal collisions. a large 600 AU long 12CO tidal arm stretching from Additionally, binary interactions can excite the orbital ∼ arXiv:1804.09190v2 [astro-ph.SR] 8 Jun 2018 it, and a circumstellar structure with complex kinemat- eccentricities and inclinations of planetesimals as shown ics around RW Aur B. From comparison with early nu- by stellar fly-by models to explain the outer structure of merical simulations by Clarke & Pringle (1993), Cabrit the the Kuiper belt in our own solar system (Ida et al. et al. (2006) proposed that RW Aur B recently had a 2000). Theoretical models predict that young binary sys- close encounter with RW Aur A, tidally stripping the tems may truncate the surrounding circumstellar mate- original circumstellar disk around A. Although the disk rial at a distance of up to 3 times the orbital separation of around RW Aur A was not spatially resolved, compari- the two stars (Artymowicz & Lubow 1994). This might son of its CO profile with theoretical models for Keple- explain the deficiency of debris disks around main se- rian disks (Beckwith & Sargent 1993) indicated a small quence binaries with separation between 3–50 AU when radius of 40-57 AU, inclined 45◦ to 60◦ to the line of compared to other single star systems and other binaries sight. It was also speculated that the eccentric fly-by of (Trilling et al. 2007). RW Aur B possibly contributed to a temporary enhance- A demonstration of the impact of binary interactions 8 ment of the disk accretion rate onto RW Aur A (4 10− on disk evolution (and possibly planet formation) is the 7 1 × classical TTS, RW Aurigae. The RW Aur system is 140 - 2 10− M yr− , Hartigan et al. 1995; Facchini et al. × 2 Rodriguez et al. Band 6 Continuum Band 7 Continuum 8 50 1 1 ] ] 1 25 4 1 − − ] ] 00 00 [ [ δ 0 δ 0 ∆ ∆ [mJy bm 10 [mJy bm ⌫ ⌫ I I 1 3 1 1 − − 0 0 1 0 1 1 0 1 − − ∆↵ cos(δ)[00] ∆↵ cos(δ)[00] FIG. 1.— Synthesized images of the Band 6 and 7 mm dust continuum emission. Contours in both panels correspond to [5,10,20,40,80,160,320,640] s. The synthesized beam is 0. 19 0. 08 in the Band 6 image and 0. 27 0. 18 in the Band 7 image. The red, orange, × 00 × 00 00 × 00 and green crosses correspond to the center of the 12CO peaks for RW Aur A, B, and a third emission source, respectively. 2016). Whether the A disk will survive this enhanced encounter in the form of a star-disk fly-by, Dai et al. accretion episode is unclear: at the current rate, and (2015) used a series of hydrodynamic (SPH) simulations 4 with the disk mass estimated from dust (3 10− M to identify the orbital parameters that best reproduce all Andrews & Williams 2005), the disk would be× accreted the features of RW Aur inferred from the PdBI observa- in only 1000 yrs. tions. This included the length of the tidal arm, separa- To add to the mystery, RW Aur has also been exhibit- tion between RW Aur A and B, disk position angles, and ing strong occultation events. After being photometri- relative stellar proper motions found by Bisikalo et al. cally monitored for over a century (Beck & Simon 2001), (2012). Furthermore, they post-processed the hydrody- the RW Aur system faded by 2 mag in late 2010 for 180 namic simulations with the non-LTE radiative transfer days (Rodriguez et al. 2013).∼ From analyzing over 110 code TORUS (e.g. Harries 2000; Rundle et al. 2010) to years of B-band photometric observations (photoelec- compute synthetic molecular line and dust continuum tric, photographic, and visual observations), no event observations to compare with the observations from similar in duration and depth was seen but the nominal Cabrit et al. (2006), finding excellent agreement with the brightness of RW Aur does vary on decade timescales CO optical depths, kinematic signatures in the line pro- (Berdnikov et al. 2017). Using basic kinematics, it was files, and observed continuum and CO flux densities, determined that the occulting object was moving at a although CO emission around B is slightly underesti- 1 mated. The model also predicts that the line of sight to few km s− and was likely 0.3 AU in diameter. Three years later, another large dimming∼ (>4.5 mag) occurred, RW Aur A currently intersects a bridge of stripped off lasting 2 years (Petrov et al. 2015; Rodriguez et al. material between the two stars. Although of low col- ∼ umn density (N 10 4 g cm 2, i.e. A 0.1 mag), 2016; Lamzin et al. 2017). Near-IR observations of the H ≤ − − V ≤ RW Aurigae system suggest the occulting body con- it was argued that the bridge structure may have small sists of large grains ( 1 mm), causing gray absorption clumps of denser material able to occasionally dim the (Schneider et al. 2015).≥ During the more recent dim- central star (Dai et al. 2015). ming, there was an observed excess in the Near IR (L In this paper, we present new Atacama Large Mil- and M) attributed to hot dust ( 1000 K) in the inner limeter/submillimeter Array (ALMA) observations of disk (Shenavrin et al. 2015). VRI∼ polarimetry observa- the RW Aurigae system showing the aftermath of the tions during the 2014-2016 dimming show a 20-30% in- star–disk interaction at high angular resolution. The crease in polarization, consistent with what has been new observations show the presence of additional tidal seen for UX Ori stars (Lamzin et al. 2017). The high streams, suggesting the occurrence of multiple fly-bys accretion rate of RW Aur A remained constant during of RW Aur B. Using new photometric observations by both dimming events (Chou et al. 2013; Shenavrin et al. the American Association of Variable Star Observers 2015). The cause of these unexpected dimming events is (AAVSO) and the All Sky Automated Survey for Su- likely an occultation by a dust screen, but the origin of perNovae (ASAS-SN), we analyze the full 2014-2016, this screen is unclear and debated (Rodriguez et al.
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