Signatures of tidal disruption in the Milky Way globular cluster NGC 6981 (M72)

¹ Instituto Interdisciplinario de Ciencias Básicas (ICB), CONICET-UNCUYO, Padre J. Contreras 1300, M5502JMA Mendoza, Argentina
e-mail: andres.piatti@unc.edu.ar
² Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Godoy Cruz 2290, C1425FQB Buenos Aires, Argentina
³ Instituto de Astrofísica de La Plata (CONICET-UNLP), Buenos Aires, Argentina
⁴ Facultad de Ciencias Astronómicas y Geofísicas de La Plata (UNLP), La Plata, Argentina
⁵ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
⁶ European Southern Observatory, Alonso de Córdova 3107, Casilla, 19001 Santiago, Chile
⁷ Millenium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁸ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile

Received: 1 December 2020
Accepted: 4 January 2021

Abstract

We study the outer regions of the Milky Way globular cluster NGC 6981 based on publicly available BV photometry and new Dark Energy Camera (DECam) observations, both of which reach nearly 4 mag below the cluster main sequence (MS) turnoff. While the BV data sets reveal the present of extra-tidal features around the cluster, the much larger field of view of the DECam observations allowed us to identify some other tidal features, which extend from the cluster toward the opposite direction to the Milky Way center. Such structural features of clusters arise from stellar density maps built using MS stars, following a cleaning of the cluster color-magnitude diagram to remove the contamination of field stars. We also performed N-body simulations in order to help us to understand the spatial distribution of the extra-tidal debris. The outcomes reveal the presence of long trailing and leading tails that are mostly parallel to the direction of the cluster velocity vector. We find that the cluster loses most of its mass by tidal disruption during its perigalactic passages, each of which lasted nearly 20 Myr. Hence, a decrease in the density of escaping stars near the cluster is expected from our N-body simulations, which, in turn, means that stronger extra-tidal features could be found by exploring much larger areas around NGC 6891.