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1. An Updated Reference Frame for the Galactic Inner Parsec

   https://doi.org/10.3847/1538-4357/acec71  

   Darling, Jeremy ; Paine, Jennie ; Reid, Mark J. ; Menten, Karl M. ; Sakai, Shoko ; Ghez, Andrea ( September 2023 , The Astrophysical Journal)

   Infrared observations of stellar orbits about Sgr A* probe the mass distribution in the inner parsec of the Galaxy and provide definitive evidence for the existence of a massive black hole. However, the infrared astrometry is relative and is tied to the radio emission from Sgr A* using stellar SiO masers that coincide with infrared-bright stars. To support and improve this two-step astrometry, we present new astrometric observations of 15 stellar SiO masers within 2 pc of Sgr A*. Combined with legacy observations spanning 25.8 yr, we reanalyze the relative offsets of these masers from Sgr A* and measure positions and proper motions that are significantly improved compared to the previously published reference frame. Maser positions are corrected for epoch-specific differential aberration, precession, nutation, and solar gravitational deflection. Omitting the supergiant IRS 7, the mean position uncertainties are 0.46 mas and 0.84 mas in R.A. and decl., and the mean proper motion uncertainties are 0.07 mas yr⁻¹and 0.12 mas yr⁻¹, respectively. At a distance of 8.2 kpc, these correspond to position uncertainties of 3.7 and 6.9 au and proper motion uncertainties of 2.7 and 4.6 km s⁻¹. The reference frame stability, the uncertainty in the variance-weighted mean proper motion of the maser ensemble, is 8μas yr⁻¹(0.30 km s⁻¹) in R.A. and 11μas yr⁻¹(0.44 km s⁻¹) in decl., which represents a 2.3-fold improvement over previous work and a new benchmark for the maser-based reference frame.

    

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2. VVV J165507.19-421755.5: A Nearby T Dwarf Hidden in the Galactic Plane

   https://doi.org/10.3847/2515-5172/ac9141  

   Schapera, Noah ; Caselden, Dan ; Meisner, Aaron M. ; Burgasser, Adam J. ; Schneider, Adam C. ; Humphreys, Austin ; Hsu, Chih-Chun ; Softich, Emma ; Smith, Leigh C. ; Lucas, Philip W. ; et al ( September 2022 , Research Notes of the AAS)

   We present the discovery of VVV J165507.19−421755.5, a mid-T dwarf found through ongoing unWISE-based proper motion searches. A near-infrared spectrum of this object obtained with the NIRES instrument on the Keck II telescope indicates a spectral classification of T5. Using data from the VISTA Variables in the Via Lactea (VVV) catalog with a 9 year baseline, we measure a proper motion of (${\mu }_{\alpha }\cos \left(\delta \right)$,μ_δ) = (−631.0 ± 1.3, −315.0 ± 1.4) mas yr⁻¹and a trigonometric parallax ofπ_abs = 66.0 ± 4.8 mas, corresponding to a distance of 15.2 ± 1.1 pc. The trigonometric parallax agrees well with our photometric distance estimate (16.1${}_{-3.9}^{+5.1}$pc) assuming that VVV J165507.19−421755.5 is a single T5 dwarf. VVV J165507.19−421755.5 is a new member of the 20 parsec census.

    

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3. Pegasus IV: Discovery and Spectroscopic Confirmation of an Ultra-faint Dwarf Galaxy in the Constellation Pegasus

   https://doi.org/10.3847/1538-4357/aca1c3  

   Cerny, W. ; Simon, J. D. ; Li, T. S. ; Drlica-Wagner, A. ; Pace, A. B. ; Martínez-Vázquez, C. E. ; Riley, A. H. ; Mutlu-Pakdil, B. ; Mau, S. ; Ferguson, P. S. ; et al ( January 2023 , The Astrophysical Journal)

   Abstract We report the discovery of Pegasus IV, an ultra-faint dwarf galaxy found in archival data from the Dark Energy Camera processed by the DECam Local Volume Exploration Survey. Pegasus IV is a compact, ultra-faint stellar system ( r 1 / 2 = 41 − 6 + 8 pc; M V = −4.25 ± 0.2 mag) located at a heliocentric distance of 90 − 6 + 4 kpc . Based on spectra of seven nonvariable member stars observed with Magellan/IMACS, we confidently resolve Pegasus IV’s velocity dispersion, measuring σ v = 3.3 − 1.1 + 1.7 km s −1 (after excluding three velocity outliers); this implies a mass-to-light ratio of M 1 / 2 / L V , 1 / 2 = 167 − 99 + 224 M ⊙ / L ⊙ for the system. From the five stars with the highest signal-to-noise spectra, we also measure a systemic metallicity of [Fe/H] = − 2.63 − 0.30 + 0.26 dex, making Pegasus IV one of the most metal-poor ultra-faint dwarfs. We tentatively resolve a nonzero metallicity dispersion for the system. These measurements provide strong evidence that Pegasus IV is a dark-matter-dominated dwarf galaxy, rather than a star cluster. We measure Pegasus IV’s proper motion using data from Gaia Early Data Release 3, finding ( μ α * , μ δ ) = (0.33 ± 0.07, −0.21 ± 0.08) mas yr −1 . When combined with our measured systemic velocity, this proper motion suggests that Pegasus IV is on an elliptical, retrograde orbit, and is currently near its orbital apocenter. Lastly, we identify three potential RR Lyrae variable stars within Pegasus IV, including one candidate member located more than 10 half-light radii away from the system’s centroid. The discovery of yet another ultra-faint dwarf galaxy strongly suggests that the census of Milky Way satellites is still incomplete, even within 100 kpc. 

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4. Early-forming Massive Stars Suppress Star Formation and Hierarchical Cluster Assembly

   https://doi.org/10.3847/1538-4357/acb0c5  

   Lewis, Sean C. ; McMillan, Stephen L. W. ; Low, Mordecai-Mark Mac ; Cournoyer-Cloutier, Claude ; Polak, Brooke ; Wilhelm, Martijn J. C. ; Tran, Aaron ; Sills, Alison ; Zwart, Simon Portegies ; Klessen, Ralf S. ; et al ( February 2023 , The Astrophysical Journal)

   Feedback from massive stars plays an important role in the formation of star clusters. Whether a very massive star is born early or late in the cluster formation timeline has profound implications for the star cluster formation and assembly processes. We carry out a controlled experiment to characterize the effects of early-forming massive stars on star cluster formation. We use the star formation software suiteTorch, combining self-gravitating magnetohydrodynamics, ray-tracing radiative transfer,N-body dynamics, and stellar feedback, to model four initially identical 10⁴M_⊙giant molecular clouds with a Gaussian density profile peaking at 521.5 cm⁻³. Using theTorchsoftware suite through theAMUSEframework, we modify three of the models, to ensure that the first star that forms is very massive (50, 70, and 100M_⊙). Early-forming massive stars disrupt the natal gas structure, resulting in fast evacuation of the gas from the star-forming region. The star formation rate is suppressed, reducing the total mass of the stars formed. Our fiducial control model, without an early massive star, has a larger star formation rate and total efficiency by up to a factor of 3, and a higher average star formation efficiency per freefall time by up to a factor of 7. Early-forming massive stars promote the buildup of spatially separate and gravitationally unbound subclusters, while the control model forms a single massive cluster.

    

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5. Finding r-II Sibling Stars in the Milky Way with the Greedy Optimistic Clustering Algorithm

   https://doi.org/10.3847/1538-4357/acb93b  

   Hattori, Kohei ; Okuno, Akifumi ; Roederer, Ian U. ( March 2023 , The Astrophysical Journal)

   R-process enhanced stars with [Eu/Fe] ≥ +0.7 (so-calledr-II stars) are believed to have formed in an extremely neutron-rich environment in which a rare astrophysical event (e.g., a neutron-star merger) occurred. This scenario is supported by the existence of an ultra-faint dwarf galaxy, Reticulum II, where most of the stars are highly enhanced inr-process elements. In this scenario, some small fraction of dwarf galaxies around the Milky Way wererenhanced. When each r-enhanced dwarf galaxy accreted to the Milky Way, it deposited manyr-II stars in the Galactic halo with similar orbital actions. To search for the remnants of ther-enhanced systems, we analyzed the distribution of the orbital actions ofN= 161r-II stars in the solar neighborhood by using Gaia EDR3 data. Since the observational uncertainty is not negligible, we applied a newly developed greedy optimistic clustering method to the orbital actions of our sample stars. We found six clusters ofr-II stars that have similar orbits and chemistry, one of which is a new discovery. Given the apparent phase-mixed orbits of the member stars, we interpret that these clusters are good candidates for remnants of completely disruptedr-enhanced dwarf galaxies that merged with the ancient Milky Way.

    

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