MNRAS 000,1{19 (2015) Preprint 4 July 2018 Compiled using MNRAS LATEX style file v3.0 A gap in the planetesimal disc around HD 107146 and asymmetric warm dust emission revealed by ALMA S. Marino,1?, J. Carpenter,2 M. C. Wyatt1, M. Booth3, S. Casassus4;5, V. Faramaz6, V. Guzman2, A. M. Hughes7, A. Isella8, G. M. Kennedy9, L. Matr`a10, L. Ricci11 and S. Corder2;12. 1Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK 2Joint ALMA Observatory (JAO), Alonso de Cordova 3107 Vitacura - Santiago de Chile, Chile 3Astrophysikalisches Institut und Universit¨atssternwarte, Friedrich-Schiller-Universit¨at Jena, Schillerg¨aßchen 2-3, 07745 Jena, Germany 4 ` Departamento de AstronomiIAa,, Universidad de Chile, Casilla 36-D, Santiago, Chile 5Millennium Nucleus ^aAIJProtoplanetary˘ Disks^aA˘I,_ Chile 6Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove drive, Pasadena CA 91109, USA. 7Department of Astronomy, Van Vleck Observatory, Wesleyan University, 96 Foss Hill Drive, Middletown, CT 06459, USA 8Department of Physics and Astronomy, Rice University, 6100 Main Street, MS-108, Houston, Texas 77005, USA 9Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK 10Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 11Department of Physics and Astronomy, California State University Northridge, 18111 Nordhoff Street, Northridge, CA 91130, USA 12National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA Accepted XXX. Received YYY; in original form ZZZ ABSTRACT While detecting low mass exoplanets at tens of au is beyond current instrumenta- tion, debris discs provide a unique opportunity to study the outer regions of planetary systems. Here we report new ALMA observations of the 80-200 Myr old Solar ana- logue HD 107146 that reveal the radial structure of its exo-Kuiper belt at wavelengths of 1.1 and 0.86 mm. We find that the planetesimal disc is broad, extending from 40 to 140 au, and it is characterised by a circular gap extending from 60 to 100 au in which the continuum emission drops by about 50%. We also report the non-detection of the CO J=3-2 emission line, confirming that there is not enough gas to affect the dust distribution. To date, HD 107146 is the only gas-poor system showing multiple rings in the distribution of millimeter sized particles. These rings suggest a similar distribution of the planetesimals producing small dust grains that could be explained invoking the presence of one or more perturbing planets. Because the disk appears axisymmetric, such planets should be on circular orbits. By comparing N-body sim- ulations with the observed visibilities we find that to explain the radial extent and depth of the gap, it would require the presence of multiple low mass planets or a single planet that migrated through the disc. Interior to HD 107146's exo-Kuiper belt we find extended emission with a peak at ∼ 20 au and consistent with the inner warm arXiv:1805.01915v2 [astro-ph.EP] 3 Jul 2018 belt that was previously predicted based on 22µm excess as in many other systems. This warm belt is the first to be imaged, although unexpectedly suggesting that it is asymmetric. This could be due to a large belt eccentricity or due to clumpy structure produced by resonant trapping with an additional inner planet. Key words: circumstellar matter - planetary systems - planets and satellites: dy- namical evolution and stability - techniques: interferometric - methods: numerical - stars: individual: HD 107146. 1 INTRODUCTION While exoplanet campaigns have discovered thousands of ? E-mail: [email protected] close in planets in the last decade, at separations greater © 2015 The Authors 2 S. Marino et al. than 10 au it has only been possible to detect a few gas gi- et al. 2011) and HD 131835 (Feldt et al. 2017). However, ants, mainly through direct imaging 1 (Marois et al. 2008; alternative scenarios without planets that could also repro- Lagrange et al. 2009; Rameau et al. 2013). Protoplane- duce the observed structure have not been ruled out yet tary disc observations, on the other hand, have shown that in these systems. For example, multiple ring structures can enough mass in both dust and gas to form massive planets arise from gas-dust interactions if gas and dust densities are resides at large stellocentric distances (see review by An- similar (Lyra & Kuchner 2013; Richert et al. 2017), which drews 2015). In addition, the detection of cold dusty debris might explain HD 131835's rings since large amounts of CO discs at tens of au shows that planetesimals can and do form gas (likely primordial origin) have been found in this sys- at tens and hundreds of au in extrasolar systems (e.g. Su tem (Mo´or et al. 2017). Moreover, the double ring structure et al. 2006; Hillenbrand et al. 2008; Wyatt 2008; Carpenter around HD 92945 and HD 131835 has only been identified in et al. 2009; Eiroa et al. 2013; Absil et al. 2013; Matthews scattered light images, tracing small dust grains whose dis- et al. 2014a; Thureau et al. 2014; Montesinos et al. 2016; tribution can be highly affected by radiation forces (Burns Hughes et al. 2018), although the exact planetesimal belt et al. 1979), therefore not necessarily tracing the distribution formation mechanism is a matter of debate (e.g. Matr`aet al. of planetesimals (e.g. Wyatt 2006). 2018a). Only HD 107146, an ∼ 80 − 200 Myr old G2V star It is natural then to wonder how far out can planets (Williams et al. 2004, and references therein) at a dis- form? In situ formation of the imaged distant gas giants is tance of 27:5 ± 0:3 pc (Gaia Collaboration et al. 2016a,b), challenging as the growth timescale of their cores can easily has a double debris ring structure tentatively identified take longer than the protoplanetary gas-rich phase (Pollack at longer wavelengths thanks to the Atacama Large Mil- et al. 1996; Rafikov 2004; Levison et al. 2010). Gravitational limeter/submillimeter Array (ALMA, Ricci et al. 2015a). instability was thought to be the only potential pathway At these wavelengths observations trace mm-sized dust for towards in-situ formation at tens of au (Boss 1997; Boley which radiation forces are negligible, therefore indicating 2009), but the revisited growth timescale of embryos through that the double ring structure is imprinted in the planetesi- pebble accretion could be fast enough to form ice giants or mal distribution as well. Moreover, these observations ruled the core of gas giants during the disc lifetime (Johansen out the presence of gas at densities high enough to be re- & Lacerda 2010; Ormel & Klahr 2010; Lambrechts & Jo- sponsible for such structure. The debris disc surrounding hansen 2012; Morbidelli & Nesvorny 2012; Bitsch et al. 2015; HD 107146 was first discovered by its infrared (IR) excess Johansen & Lambrechts 2017). Alternatively, the observed using IRAS data (Silverstone 2000), but it was not until re- giant planets at tens of au may have formed closer in and cently that the disc was resolved by the Hubble Space Tele- evolved to their current orbits by migrating outward (Crida scope (HST) in scattered light, revealing a nearly face on disc et al. 2009), as could be the case for HR 8799 with four gas with a surface brightness peak at 120 au and extending out giants in mean motion resonances (Marois et al. 2008, 2010) to ∼ 160 au (Ardila et al. 2004; Ertel et al. 2011; Schneider surrounded by an outer debris disc (Su et al. 2009; Matthews et al. 2014). Despite HST's high resolution, limitations in et al. 2014b; Booth et al. 2016), or may have been scattered subtracting the stellar emission or a smooth distribution of from closer in onto a highly eccentric orbit (Ford & Rasio small dust likely kept the double ring structure hidden. Us- 2008; Chatterjee et al. 2008; Juri´c& Tremaine 2008), as has ing ALMA's unprecedented sensitivity and resolution, Ricci been suggested for Fomalhaut b (Kalas et al. 2008, 2013; et al.(2015a) showed that this broad disc extended from Faramaz et al. 2015). On the other hand, after the disper- about 30 to 150 au, but that it had a decrease in the dust sal of gas and dust, planetesimals could continue growing to density at intermediate radii, which could correspond to a form icy planets at tens of au over 100 Myr timescales; how- gap produced by a planet of a few Earth masses clearing its ever, numerical studies show that once a Pluto size object orbit at 80 au through scattering. Finally, analysis of Spitzer is formed at 30-150 au within a disc of planetesimals, these spectroscopic and photometric data revealed the presence of are inevitably stirred, stopping growth and the formation of an extra unresolved warm dust component in the system, higher mass planets through oligarchic growth (Kenyon & at a temperature of ∼120 K and thus inferred to be located Bromley 2002, 2008, 2010). Thus, it is not yet clear how far between 5-15 au from the star (Morales et al. 2011; Kennedy from their stars planets can form. Moreover, the discovery & Wyatt 2014). of vast amounts of gas (possibly primordial) in systems with Despite the tentative evidence of planets producing low, debris-like levels of dust (so-called \hybrid discs", e.g. these gaps around their orbits, neither the HD 107146 Mo´or et al. 2017) has opened the possibility for long lived ALMA observations, nor the scattered light observations of gaseous discs that could facilitate the formation of both ice HD 92945 and HD 131835, ruled out alternative scenarios in and gas giant planets at tens of au.