Is the escape velocity in star clusters linked to extended star formation histories? Using NGC 7252: W3 as a test case

Abstract

The colour-magnitude diagrams of some intermediate-age clusters (1-2 Gyr) star clusters show unexpectedly broad main-sequence turnoffs, raising the possibility that these clusters have experienced more than one episode of star formation. Such a scenario predicts the existence of an extended main sequence turn off (eMSTO) only in clusters with escape velocities above a certain threshold (> 15 km s −1 ), which would allow them to retain or accrete gas that eventually would fuel a secondary extended starformation episode. This paper presents a test of this scenario based on the study of the young and massive cluster NGC 7252: W3. We use the HST photometry from WFPC2 and WFC3 images obtained with UV and optical filters, as well as MagE echellette spectrograph data from the Las Campanas Clay 6.5m telescope, in order to construct the observed UV/optical SED of NGC 7252: W3. The observations are then compared with synthetic spectra based on different star formation histories consistent with those of the eMSTO clusters. We find that the SED of this cluster is best fitted by a synthetic spectrum with a single stellar population of age 570 +70 −62 Myr and mass 1.13 +0.14 −0.13 × 10 8 M ⊙ , confirming earlier works on NGC 7252:W3. We also estimate the lower limit on the central escape velocity of 193 km s −1 . We rule out extended star-formation histories, like those inferred for the eMSTO clusters in the Magellanic Clouds, at high confidence. We conclude that the escape velocity of a cluster does not dictate whether a cluster can undergo extended periods of star formation.

Extended Main-sequence Turn-offs in Intermediate-age Star Clusters: Stellar Rotation Diminishes, but Does Not Eliminate, Age Spreads

“…4.1.2). This may seem unexpected, given that a recent study of the very massive cluster W3 (M ∼ 10 8 M ⊙ , age ∼ 600 Myr, effective radius r eff = 17.5 pc) in the merger remnant galaxy NGC 7252 did not find any evidence for an extended star formation history (Cabrera-Ziri et al 2016). We suggest that this paradox might be explained if the ability of clusters to accrete and/or retain gas within its potential well during the first few 10 8 yr after their creation has a significant dependence on its environment, as shown by Conroy & Spergel (2011).…”

The search for multiple populations in Magellanic Cloud clusters -- IV. Coeval multiple stellar populations in the young star cluster NGC 1978

“…Such a scenario has been demonstrated to have several caveats. Among these, massive clusters should be forming stars for the first 10s to 100s Myr of their lives, while no clusters with ages of ∼ 10 Myr or older have been found to host current star formation events (Bastian et al 2013;Cabrera-Ziri et al 2016a). Additionally, there is a clear correlation between the inferred age spread and the age of the cluster, thus suggesting a stellar evolutionary effect is the cause (Niederhofer et al 2015).…”

A young cluster with an extended main-sequence turnoff: confirmation of a prediction of the stellar rotation scenario

“…The first is that massive clusters should show onc 2015 RAS going star-formation for the first few hundred Myr of their lives, whereas no clusters with ages beyond 10 Myr have been found with active star-formation (Bastian et al 20132016a;Niederhofer et al 2015a). Moreover, In order to form a second generation of stars, clusters must be able to retain and/or accrete material from their surroundings (e.g., Conroy & Spergel 2011).…”