The Unusual Initial Mass Function of the Arches Cluster

Abstract

As a young massive cluster in the Central Molecular Zone, the Arches cluster is a valuable probe of the stellar Initial Mass Function (IMF) in the extreme Galactic Center environment. We use multi-epoch Hubble Space Telescope observations to obtain high-precision proper motion and photometric measurements of the cluster, calculating cluster membership probabilities for stars down to ∼1.8 M between cluster radii of 0.25 pc -3.0 pc. We achieve a cluster sample with just ∼6% field contamination, a significant improvement over photometrically-selected samples which are severely compromised by the differential extinction across the field. Combining this sample with K-band spectroscopy of 5 cluster members, we forward model the Arches cluster to simultaneously constrain its IMF and other properties (such as age and total mass) while accounting for observational uncertainties, completeness, mass segregation, and stellar multiplicity. We find that the Arches IMF is best described by a 1-segment power law that is significantly top-heavy: α = 1.80 ± 0.05 (stat) ± 0.06 (sys), where dN/dm ∝ m −α , though we cannot discount a 2-segment power law model with a high-mass slope only slightly shallower than local star forming regions (α = 2.04 +0.14 −0.19 ± 0.04) but with a break at 5.8 +3.2 −1.2 ± 0.02 M . In either case, the Arches IMF is significantly different than the standard IMF. Comparing the Arches to other young massive clusters in the Milky Way, we find tentative evidence for a systematically top-heavy IMF at the Galactic Center.

Making top-heavy IMFs from canonical IMFs near the Galactic Centre

“…Therefore the observation by Hosek et al (2019) that the Arches cluster close to the GC has a top-heavy IMF might be telling us that those stars formed with a top-heavy IMF, or that they formed with a canonical IMF and dynamical evolution has stripped significant numbers of lower-mass stars. A search within 20 pc of the Arches for low-mass stars of the same age as the Arches would be required to say for sure.…”

“…Of particular interest to us in this paper are observations of IMFs in the Galactic Centre (GC). Hosek et al (2019) apparantly find a top-heavy IMF for the Arches cluster (see also Kim et al 2006), and the Quintuplet cluster also seems to have a flatter MF than the canonical IMF (Hußmann et al 2012).…”

PopSyCLE: A New Population Synthesis Code for Compact Object Microlensing Events

“…PyPopStar is a software package that generates singleage, single-metallicity populations (i.e. star clusters) using adjustable initial mass functions (IMFs), multiplicity distributions, stellar evolution and atmosphere grids, and extinction laws; for a more detailed description see §4 of Hosek et al (2019). 3 In summary, IMFs are parametrized as piecewise functions consisting of multiple power-laws and generated according to the prescription from Pflamm-Altenburg & Kroupa (2006).…”

“…The PyPopStar software as described in §4 of Hosek et al (2019) would discard stars that had evolved into compact objects. To be able to perform population synthesis, we have updated PyPopStar with an initialfinal mass relation (IFMR) that will properly account for compact objects.…”

High-Energy Particles and Radiation in Star-Forming Regions

“…There is a rationale to examine the dependence of the 22 Ne/ 20 Ne ratio from the parameters, such as IMF power law index γ (there is evidence that in some YMSCs it can be significantly different from Salpeter's value -see, e.g., Hosek et al (2019), Lim et al (2013)) and the velocity of rotation (it can influence dramatically the evolution of the star). The results of the calculations are presented in Fig.…”