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 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: = 1.80 0.05 (stat) 0.06 (sys), where dN/dm ∝ m− , 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.
ABSTRACT
The Blanco DECam Bulge Survey (BDBS) imaged more than 200 sq deg 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 catalogues, 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 subgiant branch in M22 as a function of metallicity, the detection of possible extratidal 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.
Abstract
Perhaps the most exciting promise of the Rubin Observatory Legacy Survey of Space and Time (LSST) is its capability to discover phenomena never before seen or predicted: true astrophysical ...novelties; but the ability of LSST to make these discoveries will depend on the survey strategy. Evaluating candidate strategies for true novelties is a challenge both practically and conceptually. Unlike traditional astrophysical tracers like supernovae or exoplanets, for anomalous objects, the template signal is by definition unknown. We approach this problem by assessing survey completeness in a phase space defined by object color and flux (and their evolution), and considering the volume explored by integrating metrics within this space with the observation depth, survey footprint, and stellar density. With these metrics, we explore recent simulations of the Rubin LSST observing strategy across the entire observed spatial footprint and in specific Local Volume regions: the Galactic Plane and Magellanic Clouds. Under our metrics, observing strategies with greater diversity of exposures and time gaps tend to be more sensitive to genuinely new transients, particularly over time-gap ranges left relatively unexplored by previous surveys. To assist the community, we have made all of the tools developed publicly available. While here we focus on transients, an extension of the scheme to include proper motions and the detection of associations or populations of interest will be communicated in Paper II of this series. This paper was written with the support of the Vera C. Rubin LSST Transients and Variable Stars and Stars, Milky Way, Local Volume Science Collaborations.
ABSTRACT
SMC X-1 is a high-mass X-ray binary showing superorbital modulation with an unstable period. Previous monitoring shows three excursion events in 1996–1998, 2005–2007, and 2014–2016. The ...superorbital period drifts from ≳60 to ≲40 d and then evolves back during an excursion. Here, we report a new excursion event of SMC X-1 in 2020–2021, indicating that the superorbital modulation has an unpredictable, chaotic nature. We trace the spin-period evolution and find that the spin-up rate accelerated 1 yr before the onset of this new excursion, which suggests a possible inside-out process connecting the spin-up acceleration and the superorbital excursion. This results in a deviation of the spin-period residual, similar to the behaviour of the first excursion in 1996–1998. In further analysis of the pulse profile evolution, we find that the pulsed fraction shows a long-term evolution and may be connected to the superorbital excursion. These discoveries deepen the mystery of SMC X-1 because they cannot be solely interpreted by the warped-disc model. Upcoming pointed observations and theoretical studies may improve our understanding of the detailed accretion mechanisms taking place.
The Quintuplet star cluster is one of only three known young (<10 Myr) massive (M > 104 M ) 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 out to 3.2 pc from the cluster center. A 3 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.
Abstract Transient low-mass X-ray binaries (LMXBs) are discovered largely by X-ray and gamma-ray all-sky monitors. The X-ray outburst is also accompanied by an optical brightening, which empirically ...can precede the detection of X-rays. Newly sensitive optical synoptic surveys may offer a complementary pathway for discovery and potential for insight into the initial onset and propagation of the thermal instability that leads to the ionization of the accretion disk. We use the Zwicky Transient Facility (ZTF) alert stream to perform a comprehensive search at optical wavelengths for previously undiscovered outbursting LMXBs. Our pipeline first crossmatches the positions of the alerts to cataloged X-ray sources, and then analyzes the 30 day lightcurve of matched alerts by thresholding on differences with an 8 day exponentially weighted moving average. In addition to a 19 month long live search, we ran our pipeline over 4 yr of ZTF archival data, recovering four known LMXBs. We also independently detected an outburst of MAXI J1957+032 in the live search and found the first outburst of Swift J1943.4+0228, an unclassified X-ray transient, in 10 yr. Using Monte Carlo simulations of the Galactic LMXB population, we estimate that 29% of outbursting LMXBs are detectable by ZTF and that 4.4% of LMXBs would be present in the crossmatched X-ray catalogs, giving an estimated Galactic population of 3390 − 1930 + 3980 . We estimate that our current pipeline can detect 1.3% of all outbursting LMXBs, including those previously unknown, but that Rubin Observatory’s Legacy Survey of Space and Time will be able to detect 43% of outbursting LMXBs.
ABSTRACT
Globular clusters (GCs) have been at the heart of many longstanding questions in many sub-fields of astronomy and, as such, systematic identification of GCs in external galaxies has immense ...impacts. In this study, we take advantage of M87’s well-studied GC system to implement supervised machine learning (ML) classification algorithms – specifically random forest and neural networks – to identify GCs from foreground stars and background galaxies, using ground-based photometry from the Canada–France–Hawaii Telescope (CFHT). We compare these two ML classification methods to studies of ‘human-selected’ GCs and find that the best-performing random forest model can reselect 61.2 per cent ± 8.0 per cent of GCs selected from HST data (ACSVCS) and the best-performing neural network model reselects 95.0 per cent ± 3.4 per cent. When compared to human-classified GCs and contaminants selected from CFHT data – independent of our training data – the best-performing random forest model can correctly classify 91.0 per cent ± 1.2 per cent and the best-performing neural network model can correctly classify 57.3 per cent ± 1.1 per cent. ML methods in astronomy have been receiving much interest as Vera C. Rubin Observatory prepares for first light. The observables in this study are selected to be directly comparable to early Rubin Observatory data and the prospects for running ML algorithms on the upcoming data set yields promising results.
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 m). The moderate-extinction region is the young massive cluster Westerlund 1 (Wd1; AKs ∼ 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 (AKs ∼ 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 AF125W/AKs and AF814W/AKs 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.
ABSTRACT
We present photometric metallicity measurements for a sample of 2.6 million bulge red clump stars extracted from the Blanco DECam Bulge Survey (BDBS). Similar to previous studies, we find ...that the bulge exhibits a strong vertical metallicity gradient, and that at least two peaks in the metallicity distribution functions appear at b < −5°. We can discern a metal-poor (Fe/H ∼ −0.3) and metal-rich (Fe/H ∼ +0.2) abundance distribution that each show clear systematic trends with latitude, and may be best understood by changes in the bulge’s star formation/enrichment processes. Both groups exhibit asymmetric tails, and as a result we argue that the proximity of a star to either peak in Fe/H space is not necessarily an affirmation of group membership. The metal-poor peak shifts to lower Fe/H values at larger distances from the plane while the metal-rich tail truncates. Close to the plane, the metal-rich tail appears broader along the minor axis than in off-axis fields. We also posit that the bulge has two metal-poor populations – one that belongs to the metal-poor tail of the low latitude and predominantly metal-rich group, and another belonging to the metal-poor group that dominates in the outer bulge. We detect the X-shape structure in fields with |Z| > 0.7 kpc and for stars with Fe/H > −0.5. Stars with Fe/H < −0.5 may form a spheroidal or ‘thick bar’ distribution while those with Fe/H $\gtrsim$ −0.1 are strongly concentrated near the plane.
Abstract
We present photometric evidence for multiple stellar populations (MPs) in 14 globular clusters (GCs) toward the southern Galactic bulge. The photometric data come as part of the Blanco DECam ...Bulge Survey, which is a deep, wide-field near-UV-near-IR (
ugriz
Y) survey of the southern Galactic bulge. Here, we present the first systematic study of bulge GC multiple populations with deep photometry including the
u
band, which is a crucial indicator of the abundance of CNO-bearing molecules in stellar atmospheres. We identify cluster members using Gaia EDR3 proper motion measurements, and then isolate red giant branch stars using
r
versus
u
−
r
color–magnitude diagrams. We find evidence suggesting all 14 clusters host at least two populations, and NGC 6441, NGC 6626, and NGC 6656 appear to have at least three populations. Many of these clusters are not part of the Hubble Space Telescope (HST) surveys nor do they have comprehensive spectroscopic analyses so we are presenting the first evidence of MPs in several clusters. Not only do we find a strong anticorrelation between the fraction of first-generation stars and cluster absolute
V
magnitude, but the correlation coefficient and cluster-to-cluster scatter are similar to the results obtained from HST. Our ground-based data extend to much larger radial distances than similar HST observations, enabling a reliable estimate of the global fraction of first-generation stars in each cluster. This study demonstrates that ground-based
u
-band photometry as provided by DECam will prove powerful in the study of multiple populations in resolved GCs.
PDF
