The ALMA view of MP Mus (PDS 66): A protoplanetary disk with no visible gaps down to 4 au scales^★

¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
e-mail: ar2193@cam.ac.uk
² European Southern Observatory, 3107, Alonso de Córdova, Vitacura, Santiago, Chile
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁴ Departamento de Física, Universidad de Santiago de Chile, Av. Victor Jara 3659, Santiago, Chile
⁵ Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Chile
⁶ Center for Interdisciplinary Research in Astrophysics and Space Exploration (CIRAS), Universidad de Santiago de Chile, Estación Central, Chile
⁷ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁸ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada
⁹ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
¹⁰ Núcleo Milenio de Formación Planetaria – NPF, Valparaíso, Chile
¹¹ Instituto de Astrofísica, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago RM, Chile
¹² Joint ALMA Observatory, Avenida Alonso de Córdova 3107, Vitacura, Santiago, Chile
¹³ Astronomy Department, University of California, Berkeley, CA 94720-3411, USA
¹⁴ Department of Astronomy and Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
¹⁵ Observatorio Astronómico Nacional (OAN, IGN), Calle Alfonso XII 3, 28014 Madrid, Spain
¹⁶ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

Received: 7 December 2022
Accepted: 17 February 2023

Abstract

Aims. We aim to characterize the protoplanetary disk around the nearby (d ~ 100 pc), young solar analog MP Mus (PDS 66) and to reveal any signs of planets or ongoing planet formation in the system.

Methods. We present new ALMA observations of MP Mus at 0.89 mm, 1.3 mm, and 2.2 mm with angular resolutions of ~1″, 0.05″, and 0.25″, respectively. These data probe the dust and gas in the disk with unprecedented detail and sensitivity.

Results. The disk appears smooth down to the 4 au resolution of the 1.3 mm observations, in contrast with most disks observed at comparable spatial scales. The dust disk has a radius of 60±5 au, a dust mass of 0.14_-0.06^+0.11 M_Jup, and a millimeter spectral index <2 in the inner 30 au, suggesting optically thick emission from grains with a high albedo in this region. Several molecular gas lines are also detected extending up to 130±15 au, similar to small grains traced by scattered light observations. Comparing the fluxes of different CO isotopologues with previous models yields a gas mass of 0.1–1 M_Jup, implying a gas-to-dust ratio of 1–10. We also measured a dynamical stellar mass of M_dyn = 1.30±0.08 M_⊙ and derived an age of 7–10 Myr.

Conclusions. The survival of large grains in an evolved disk without gaps or rings is surprising, and it is possible that existing substructures remain undetected due to optically thick emission at 1.3 mm. Alternatively, small structures may still remain unresolved with the current observations. Based on simple scaling relations for gap-opening planets and gap widths, this lack of substructures places upper limits to the masses of planets in the disk as low as 2 M_⊕−0.06 M_Jup at r > 40 au. The lack of millimeter emission at radii r > 60 au also suggests that the gap in scattered light between 30 and 80 au is likely not a gap in the disk density, but a shadow cast by a puffed-up inner disk.