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.
Vera C. Rubin Observatory is a ground-based astronomical facility under construction, a joint project of the National Science Foundation and the U.S. Department of Energy, designed to conduct a multipurpose 10 yr optical survey of the Southern Hemisphere sky: the Legacy Survey of Space and Time. Significant flexibility in survey strategy remains within the constraints imposed by the core science goals of probing dark energy and dark matter, cataloging the solar system, exploring the transient optical sky, and mapping the Milky Way. The survey’s massive data throughput will be transformational for many other astrophysics domains and Rubin’s data access policy sets the stage for a huge community of potential users. To ensure that the survey science potential is maximized while serving as broad a community as possible, Rubin Observatory has involved the scientific community at large in the process of setting and refining the details of the observing strategy. The motivation, history, and decision-making process of this strategy optimization are detailed in this paper, giving context to the science-driven proposals and recommendations for the survey strategy included in this Focus Issue.
The Blanco DECam Bulge Survey (BDBS) imaged more than 200 square degrees of the Southern Galactic bulge using the ugrizY filters of the Dark Energy Camera, and produced point spread function photometry of approximately 250 million unique sources. In this paper, we present details regarding the construction and collation of survey catalogs, and also discuss the adopted calibration and dereddening procedures. Early science results are presented with a particular emphasis on the bulge metallicity distribution function and globular clusters. A key result is the strong correlation (σ ∼ 0.2 dex) between (u − i)o and [Fe/H] for bulge red clump giants. We utilized this relation to find that interior bulge fields may be well described by simple closed box enrichment models, but fields exterior to b ∼−6○ seem to require a secondary metal-poor component. Applying scaled versions of the closed box model to the outer bulge fields is shown to significantly reduce the strengths of any additional metal-poor components when compared to Gaussian mixture models. Additional results include: a confirmation that the u-band splits the sub-giant branch in M 22 as a function of metallicity, the detection of possible extra-tidal stars along the orbits of M 22 and FSR 1758, and additional evidence that NGC 6569 may have a small but discrete He spread, as evidenced by red clump luminosity variations in the reddest bands. We do not confirm previous claims that FSR 1758 is part of a larger extended structure.
The Blanco Dark Energy Camera (DECam) Bulge survey is a Vera Rubin Observatory (LSST) pathfinder imaging survey, spanning ∼200 sq. deg. of the Southern Galactic bulge, −2○ <b<−13○ and −11○ <l<+11○. We have employed the CTIO-4m telescope and the Dark Energy Camera (DECam) to image a contiguous ∼200 sq. deg. region of the relatively less reddened Southern Galactic bulge, in SDSS u + Pan-STARRSgrizy. Optical photometry with its large colour baseline will be used to investigate the age and metallicity distributions of the major structures of the bulge. Included in the survey footprint are 26 globular clusters imaged in all passbands. Over much of the bulge, we have Gaia DR2 matching astrometry to i ∼ 18, deep enough to reach the faint end of the red clump. This paper provides the background, scientific case, and description of the survey. We present an array of new reddening-corrected colour-magnitude diagrams that span the extent of Southern Galactic bulge. We argue that a population of massive stars in the blue loop evolutionary phase, proposed to lie in the bulge, are instead at ∼2 kpc from the Sun and likely red clump giants in the old disk. A bright red clump near (l, b) = ( + 8○, −4○) may be a feature in the foreground disk, or related to the long bar reported in earlier work. We also report the first map of the blue horizontal branch population spanning the BDBS field of regard, and our data does not confirm the reality of a number of proposed globular clusters in the bulge.
The Large Synoptic Survey Telescope (LSST) will provide for unbiased sampling of variability properties of objects with r mag < 24. This should allow for those objects whose variations reveal their orbital periods (P or b ), such as low mass X-ray binaries (LMXBs) and related objects, to be examined in much greater detail and with systematic sampling. However, the baseline LSST observing strategy has temporal sampling that is not optimised for such work in the Galaxy. Here we assess four candidate observing strategies for measurement of P or b in the range 10 minutes to 50 days. We simulate multi-filter quiescent LMXB lightcurves including ellipsoidal modulation and stochastic flaring, and then sample these using LSST's operations simulator (OpSim) over the (mag, P or b ) parameter space, and over five sightlines sampling a range of possible reddening values. The percentage of simulated parameter space with correctly returned periods ranges from ∼ 23 %, for the current baseline strategy, to ∼ 70 % for the two simulated strategies without reduced Galactic sampling. Convolving these results with a P or b distribution, a modelled Galactic spatial distribution and reddening maps, we conservatively estimate that the most recent version of the LSST baseline strategy (baseline2018a) will allow P or b determination for ∼ 18 % of the Milky Way's LMXB population, whereas strategies that do not reduce observations of the Galactic Plane can improve this dramatically to ∼ 32 %. This increase would allow characterisation of the full binary population by breaking degeneracies between suggested P or b distributions in the literature. Our results can be used in the ongoing assessment of the effectiveness of various potential cadencing strategies.
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