The Young Massive Star Cluster Westerlund 2 Observed with MUSE. I. First Results on the Cluster Internal Motion from Stellar Radial Velocities

Abstract

Westerlund 2 (Wd2) is the central ionizing star cluster of the H II region RCW 49 and the second most massive young star cluster (M = (3.6 ± 0.3) × 10 4 M ) in the Milky Way. Its young age (∼ 2 Myr) and close proximity to the Sun (∼ 4 kpc) makes it a perfect target to study stars emerging from their parental gas cloud, the large number of OB-stars and their feedback onto the gas, and the gas dynamics. We combine high-resolution multi-band photometry obtained in the optical and near-infrared with the Hubble Space Telescope (HST), and VLT/MUSE integral field spectroscopy to study the gas, the stars, and their interactions, simultaneously. In this paper we focus on a small, 64 × 64 arcsec 2 region North of the main cluster center, which we call the Northern Bubble (NB), a circular cavity carved into the gas of the cluster region. Using MUSE data, we determined the spectral types of 17 stars in the NB from G9III to O7.5. With the estimation of these spectral types we add 2 O and 5 B-type stars to the previously published census of 37 OB-stars in Wd2. To measure radial velocities we extracted 72 stellar spectra throughout Wd2, including the 17 of the NB, and show that the cluster member stars follow a bimodal velocity distribution centered around (8.10±1.53) km s −1 and (25.41 ± 1.57) km s −1 with a dispersion of (4.52 ± 1.78) km s −1 and (3.46 ± 1.29) km s −1 , respectively. These are in agreement with CO(J = 1-2) studies of RCW 49 leaving cloud-cloud collision as a viable option for the formation scenario of Wd2. The bimodal distribution is also detected in the Gaia DR2 proper motions.

Star Clusters Near and Far

“…New instruments like large integral field units (IFUs), such as the Multi Object Spectrographic Explorer (MUSE, Bacon et al 2010) mounted at the VLT, the Gaia satellite (Prusti et al 2016;Brown et al 2018), and long baseline photometric observations allow us, for the first time, to study the detailed 3D dynamics of the majority of stars in these resolved star clusters, including the dynamics of the gas (e.g., Kamann et al 2013;McLeod et al 2015;Zeidler et al 2018;Lennon et al 2018;Wright and Mamajek 2018;Ward and Kruijssen 2018;Ward et al 2019;Getman et al 2019;Zari et al 2019). This provides insights into the star cluster formation modes: Do star clusters form hierarchically, following the structure of the giant molecular cloud (GMC) (e.g., Kruijssen et al 2012b;Parker et al 2014;Longmore et al 2014;Walker et al 2015Walker et al , 2016Barnes et al 2019;Ward et al 2019), or do they form in monolithic, central starburst-like events (e.g., Lada et al 1984;Bastian and Goodwin 2006;Banerjee and Kroupa 2015)?…”

“…Although multiple populations have not been detected in any other YSCs, mainly due to observational limitations, the majority of clusters and star-forming regions is still highly sub-structured showing multiple smaller clumps and are far from a spherical shape. In Wd2, Zeidler et al (2018) recently discovered that the cluster stellar population shows multiple velocity components using MUSE observation to measure stellar radial velocities (RVs). These components appear to be spatially correlated with its two coeval subclumps (Hur et al 2015;Zeidler et al 2015), suggesting that they are, given the young age (∼ 1 Myr, Zeidler et al 2015), an imprint of the formation history of the cluster.…”

“…Smith et al 2016). The early feedback originating within the star cluster could be the main driver for possible, triggered secondary star formation events (McLeod et al 2018;Zeidler et al 2018) in the shell of gas and dust still surrounding the cluster, which emphasized the necessity of a detailed analysis of the gas.…”

The young massive SMC cluster NGC 330 seen by MUSE

“…Similar approaches were proposed by e.g. Kerton et al (1999) by Zeidler et al (2018) In order to calibrate the classification scheme we use the standard stars listed in Gray & Corbally (2009). For all stars, observations obtained with the HERMES spectrograph mounted at the Mercator telescope in La Palma (Raskin et al 2011) are available.…”

Resolving stellar populations with integral field spectroscopy

“…Alfaro‐Cuello et al () have observed the central cluster M54 of the Sagittarius dwarf galaxy with 4 × 4 MUSE pointings, resulting in a total of 55,000 spectra, ∼6600 of which were analyzed with ULySS (Koleva et al ) to show the presence of three distinct stellar populations, whose ages and metallicities were explained with accretion and merger of GCs, and the recent encounter of the dwarf galaxy in the process of merging with the Milky Way. As for another class of objects, Zeidler et al () have performed MUSE observations of the young massive star cluster Westerlund 2 to classify a total of 72 spectra into spectral types O (4), B (7), and A…G (6), while 55 stars remained unclassified, and to measure radial velocities for all of the 72 spectra. It is useful to recall that all of these studies were performed by relying on PampelMuse as the tool to extract spectra from the datacubes.…”