The Quintuplet Cluster: Extended Structure and Tidal Radius

The Quintuplet star cluster is one of only three known young (<10 Myr) massive (M > 104 M o) clusters within ∼100 pc of the Galactic center (GC). In order to explore star cluster formation and evolution in this extreme environment, we analyze the Quintuplet's dynamical structure. Using the HST WFC3-IR instrument, we take astrometric and photometric observations of the Quintuplet covering a 120″ × 120″ field of view, which is 19 times larger than those of previous proper-motion studies of the Quintuplet. We generate a catalog of the Quintuplet region with multiband, near-infrared photometry, proper motions, and cluster membership probabilities for 10,543 stars. We present the radial density profile of 715 candidate Quintuplet cluster members with M ≈ 4.7 M o out to 3.2 pc from the cluster center. A 3σ lower limit of 3 pc is placed on the tidal radius, indicating the lack of a tidal truncation within this radius range. Only weak evidence for mass segregation is found, in contrast to the strong mass segregation found in the Arches cluster, a second and slightly younger massive cluster near the GC. It is possible that tidal stripping hampers a mass segregation signature, though we find no evidence of spatial asymmetry. Assuming that the Arches and Quintuplet clusters formed with comparable extent, our measurement of the Quintuplet's comparatively large core radius of pc provides strong empirical evidence that young massive clusters in the GC dissolve on a several-megayear timescale

The Unusual Initial Mass Function of the Arches Cluster

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 o between cluster radii of 0.25 and 3.0 pc. We achieve a cluster sample with just ∼6% field contamination, a significant improvement over photometrically selected samples that are severely compromised by the differential extinction across the field. Combining this sample with K-band spectroscopy of five 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 one-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 two-segment power-law model with a high-mass slope only slightly shallower than local star-forming regions but with a break at . 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

The Unusual Initial Mass Function of the Arches Cluster

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 o between cluster radii of 0.25 and 3.0 pc. We achieve a cluster sample with just ∼6% field contamination, a significant improvement over photometrically selected samples that are severely compromised by the differential extinction across the field. Combining this sample with K-band spectroscopy of five 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 one-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 two-segment power-law model with a high-mass slope only slightly shallower than local star-forming regions but with a break at . 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

The Optical/Near-infrared Extinction Law in Highly Reddened Regions

A precise extinction law is a critical input when interpreting observations of highly reddened sources such as young star clusters and the Galactic Center (GC). We use Hubble Space Telescope observations of a region of moderate extinction and a region of high extinction to measure the optical and near-infrared extinction law (0.8-2.2 μm). The moderate-extinction region is the young massive cluster Westerlund 1 (Wd1; A Ks ∼ 0.6 mag), where 453 proper-motion selected main-sequence stars are used to measure the shape of the extinction law. To quantify the shape, we define the parameter , which behaves similarly to a color-excess ratio, but is continuous as a function of wavelength. The high-extinction region is the GC (A Ks ∼ 2.5 mag), where 819 red clump stars are used to determine the normalization of the law. The best-fit extinction law is able to reproduce the Wd1 main-sequence colors, which previous laws misestimate by 10%-30%. The law is inconsistent with a single power law, even when only the near-infrared filters are considered, and has A F125W/A Ks and A F814W/A Ks values that are 18% and 24% higher than the commonly used Nishiyama et al. law, respectively. Using this law, we recalculate the Wd1 distance to be 3905 ± 422 pc from published observations of the eclipsing binary W13. This new extinction law should be used for highly reddened populations in the Milky Way, such as the Quintuplet cluster and Young Nuclear Cluster. A python code is provided to generate the law for future use

An Adaptive Optics Survey of Stellar Variability at the Galactic Center

We present an ≈11.5 yr adaptive optics (AO) study of stellar variability and search for eclipsing binaries in the central ∼0.4 pc (∼10″) of the Milky Way nuclear star cluster. We measure the photometry of 563 stars using the Keck II NIRC2 imager (K′-band, λ 0 = 2.124 μm). We achieve a photometric uncertainty floor of Δm K′ ∼ 0.03 (≈3%), comparable to the highest precision achieved in other AO studies. Approximately half of our sample (50% ± 2%) shows variability: 52% ±5% of known early-type young stars and 43% ±4% of known late-type giants are variable. These variability fractions are higher than those of other young, massive star populations or late-type giants in globular clusters, and can be largely explained by two factors. First, our experiment time baseline is sensitive to long-term intrinsic stellar variability. Second, the proper motion of stars behind spatial inhomogeneities in the foreground extinction screen can lead to variability. We recover the two known Galactic center eclipsing binary systems: IRS 16SW and S4-258 (E60). We constrain the Galactic center eclipsing binary fraction of known early-type stars to be at least 2.4% ±1.7%. We find no evidence of an eclipsing binary among the young S-stars nor among the young stellar disk members. These results are consistent with the local OB eclipsing binary fraction. We identify a new periodic variable, S2-36, with a 39.43 days period. Further observations are necessary to determine the nature of this source

Military Families : Topography of a Field

Over the past decades, debates revolving around the role and challenges of military families have developed into an important subfield in military sociology. Throughout history, military families have played an important role for military forces, and in the post-World War II era, the role of the family has shifted as a consequence of military professionalization. Research on military families explores the different demands placed upon service members from both the military organization and the family. More recently, such research has studied how the inclusion of women and gender minorities, operational deployments, and broader societal changes transformed the composition, stakes, and challenges of military families and the traditional idea of the military spouse