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We present a novel deep-learning-based cosmic ray (CR) rejection routine for HST that is applicable across all HST WFC3/UVIS filters. Our label-free training approach circumvents the need for a dedicated training set, but instead utilizes dark frames and the science images requiring CR removal themselves. During training, CRs present in dark frames are added to the science images, which the network is then trained to identify. In turn, the trained deepCR model can then be applied to identify CRs originally present in the science images. (See full abstract in the title link)

We report the first star formation history study of the Milky Ways nuclear star cluster (NSC), which includes observational constraints from a large sample of stellar metallicity measurements from Gemini and VLT. Photometry and spectroscopy of 770 late-type stars are forward modeled with a Bayesian inference, finding the best fit with a two-component model. The dominant metal-rich component is ~5 Gyr old, notably younger by ~3 Gyr than previous estimates. This suggests in-situ formation, while challenging the co-evolutionary models of NSCs and supermassive black holes. The minor metal-poor group favors globular cluster or dwarf galaxy infall. (See full abstract in the title link)

We report new infrared measurements of the supermassive black hole at the Galactic Center, Sgr A*, over a decade that was previously inaccessible at these wavelengths. This enables a variability study that addresses variability timescales that are 10 times longer than earlier published studies. We report that the brightness and its variability is consistent over 22 yr. We note that the 2001 periapse of the extended, dusty object G1 had no apparent effect on the infrared emission from accretion flow onto Sgr A*. (See full abstract in the title link)

We present the results of the first systematic search for spectroscopic binaries within the central 2 × 3 arcsec^2 around the supermassive black hole at the center of the Milky Way galaxy. We find no significant periodic signals in our sample of 28 stars, of which 16 are massive, young (main-sequence B) stars and 12 are low-mass, old (M and K giant) stars. This result is consistent with a scenario in which the central supermassive black hole drives nearby stellar binaries to merge or be disrupted, and it may have important implications for the production of gravitational waves and hypervelocity stars. (See full abstract in the title link)

We report new Keck Telescope observations showing that the Galactic center supermassive black hole, Sgr A*, reached much brighter infrared flux levels in 2019 than ever measured. Sgr A* reached twice the flux level of the previously observed peak flux from >13,000 measurements over 130 nights. We also observe a factor of 75 change in flux over a 2 hr time span. Potential physical origins may be from changes in the accretion flow as a result of the star S0-2’s closest passage to the black hole in 2018, or from a delayed reaction to the approach of the dusty object G2 in 2014. (See full abstract in the title link)

The general theory of relativity predicts that a star passing close to a supermassive black hole should exhibit a relativistic redshift. We used observations of the Galactic Center 16-year-orbit star S0-2 to test this prediction. We combined the measurements from March to September 2018, which cover three events during S0-2’s closest approach to the black hole with existing measurements from 1995–2017. We detected a combination of special relativistic and gravitational redshift. (See full abstract in the title link)

We present new measurements of [M/H] and the first α-element abundances [α/Fe] of two subsolar-metallicity stars in the Galactic Center. The stars have an [α/Fe] in-between the [α/Fe] of globular clusters and dwarf galaxies at similar [M/H] values. Their abundances are very different than the bulk of the stars in the nuclear star cluster. These results indicate that the sub-solar metallicity population in the nuclear star cluster likely originated from infalling dwarf galaxies or globular clusters and are unlikely to have formed in-situ. (See full abstract in the title link)

Using 5.7 Ms Chandra observations, we provide the first abundance measurement of Si, S, Ar, Ca, and Fe, in four prominent diffuse X-ray features located in the central parsec of the Galaxy, which are the manifestation of shock-heated hot gas. If the light elements had a solar-like abundance, the heavy elements have a fitted abundance of ∼1–2 solar. The α/Fe abundance ratio is mostly supersolar. Implications of the measured abundances for the Galactic Centre environment are addressed. (See full abstract in the title link)