Uncovering the structure and kinematics of the ionized core of M 2-9 with ALMA

¹ Centro de Astrobiología (CAB), CSIC-INTA, ESAC-campus, Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada, Madrid, Spain
² Department of Space, Earth, Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
³ Observatorio Astronómico Nacional (IGN), Alfonso XII No 3, 28014 Madrid, Spain
⁴ Institut de Radioastronomie Millimetrique, 300 rue de la Piscine, 38406 Saint Martin d’Heres, France
⁵ Observatorio Astronómico Nacional IGN), Ap 112, 28803 Alcalá de Henares, Madrid, Spain

^★ Corresponding author; csanchez@cab.inta-csic.es

Received: 26 July 2024
Accepted: 31 October 2024

Abstract

We present interferometric observations at 1 and 3 mm with the Atacama Large Millimeter Array (ALMA) of the free-free continuum and millimeter(mm)-wavelength recombination line (mRRL) emission of the ionized core (within ≲130 au) of the young planetary nebula (PN) candidate M 2-9. These inner regions are concealed in the vast majority of similar objects. A spectral index for the mm-to-centimeter(cm) continuum of ~0.9 indicates predominantly free-free emission from an ionized wind, with a minor contribution from warm dust. The mm continuum emission in M 2-9 reveals an elongated structure along the main symmetry axis of the large-scale bipolar nebula with a C-shaped curvature surrounded by a broad-waisted component. This structure is consistent with an ionized, bent jet and a perpendicular compact dusty disk. The presence of a compact equatorial disk (of radius ~50 au) is also supported by redshifted CO and ¹³CO absorption profiles observed from the base of the receding northern lobe against the compact background continuum. The redshift observed in the CO absorption profiles likely signifies gas infall movements from the disk toward a central source. The mRRLs exhibit velocity gradients along the axis, implying systematic expansion in the C-shaped bipolar outflow. The highest expansion velocities (~80 km s⁻¹) are found in two diagonally opposed compact regions along the axis, referred to as the high-velocity spots or shells (HVSs), indicating either rapid wind acceleration or shocks at radial distances of ~0.″02–0.″04 (~ 15–25 au) from the center. A subtle velocity gradient perpendicular to the lobes is also found, suggesting rotation. Our ALMA observations detect increased brightness and broadness in the mRRLs compared to previously observed profiles, implying variations in wind kinematics and physical conditions on timescales of less than two years, which is in agreement with the extremely short kinematic ages (≲0.5–1 yr) derived from observed velocity gradients in the compact ionized wind. Radiative transfer modeling indicates an average electron temperature of ~15 000 K and reveals a nonuniform density structure within the ionized wind, with electron densities ranging from n_e≈10⁶ to 10⁸ cm⁻³. These results potentially reflect a complex bipolar structure resulting from the interaction of a tenuous companion-launched jet and the dense wind of the primary star.