The structural properties of multiple populations in globular clusters: the instructive case of NGC 3201
arxiv(2024)
摘要
All multiple population (MP) formation models in globular clusters (GCs)
predict that second population (SP) stars form more centrally concentrated than
the first population (FP). As dynamical evolution proceeds, differences are
progressively erased, and only dynamically young clusters are expected to still
retain a partial memory of the initial structural differences. In recent years,
this picture has been supported by observations of the MP radial distributions
of both Galactic and extragalactic GCs. However, recent observations have
suggested that in some systems, FPs might actually form more centrally
segregated, with NGC 3201 being one significant example of such a possibility.
Here we present a morphological and kinematic characterization of the MPs in
NGC 3201 based on a combination of photometric and astrometric data. We show
that the distribution of the SP is bimodal. Specifically, the SP is
significantly more centrally concentrated than the FP within 1.3 cluster's
half-mass radius. Beyond this point, the SP fraction increases again, likely
due to asymmetries in the spatial distributions of the two populations. The
central concentration of the SP observed in the central regions implies that it
formed more centrally concentrated than the FP, even more so than what is
observed in the present-day. This interpretation is supported by the MP
kinematic properties. Indeed, we find that the FP is isotropic across all the
sampled cluster extension, while the velocity distribution of the SP becomes
radially anisotropic in the cluster's outer regions, as expected for the
dynamical evolution of SP stars formed more centrally concentrated than the FP.
The combination of spatial and kinematic observations provide key insights into
the dynamical properties of this cluster and lend further support to scenarios
in which the SP forms more centrally concentrated than the FP.
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