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1. Redder than Red: Discovery of an Exceptionally Red L/T Transition Dwarf

   https://doi.org/10.3847/2041-8213/acb0cd  

   Schneider, Adam C.; Burgasser, Adam J.; Bruursema, Justice; Munn, Jeffrey A.; Vrba, Frederick J.; Caselden, Dan; Kabatnik, Martin; Rothermich, Austin; Sainio, Arttu; Bickle, Thomas P.; et al (January 2023, The Astrophysical Journal Letters)

   Abstract We present the discovery of CWISE J050626.96+073842.4 (CWISE J0506+0738), an L/T transition dwarf with extremely red near-infrared colors discovered through the Backyard Worlds: Planet 9 citizen science project. Photometry from UKIRT and CatWISE give a (J−K)_MKOcolor of 2.97 ± 0.03 mag and aJ_MKO− W2 color of 4.93 ± 0.02 mag, making CWISE J0506+0738 the reddest known free-floating L/T dwarf in both colors. We confirm the extremely red nature of CWISE J0506+0738 using Keck/NIRES near-infrared spectroscopy and establish that it is a low-gravity, late-type L/T transition dwarf. The spectrum of CWISE J0506+0738 shows possible signatures of CH₄absorption in its atmosphere, suggesting a colder effective temperature than other known, young, red L dwarfs. We assign a preliminary spectral type for this source of L8γ–T0γ. We tentatively find that CWISE J0506+0738 is variable at 3–5μm based on multiepoch WISE photometry. Proper motions derived from follow-up UKIRT observations combined with a radial velocity from our Keck/NIRES spectrum and a photometric distance estimate indicate a strong membership probability in theβPic moving group. A future parallax measurement will help to establish a more definitive moving group membership for this unusual object. 

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2. Tight Bounds on the Hardness of Learning Simple Nonparametric Mixtures

   Tai, Wai_Ming; Aragam, Bryon (July 2023, Proceedings of Thirty Sixth Conference on Learning Theory)

   We study the problem of learning nonparametric distributions in a finite mixture, and establish tight bounds on the sample complexity for learning the component distributions in such models.Namely, we are given i.i.d. samples from a pdf f where f=w1f1+w2f2,w1+w2=1,w1,w2>0 and we are interested in learning each component fi .Without any assumptions on fi , this problem is ill-posed.In order to identify the components fi , we assume that each fi can be written as a convolution of a Gaussian and a compactly supported density νi with supp(ν1)∩supp(ν2)=∅ .Our main result shows that (1ε)Ω(loglog1ε) samples are required for estimating each fi . The proof relies on a quantitative Tauberian theorem that yields a fast rate of approximation with Gaussians, which may be of independent interest. To show this is tight, we also propose an algorithm that uses (1ε)O(loglog1ε) samples to estimate each fi . Unlike existing approaches to learning latent variable models based on moment-matching and tensor methods, our proof instead involves a delicate analysis of an ill-conditioned linear system via orthogonal functions.Combining these bounds, we conclude that the optimal sample complexity of this problem properly lies in between polynomial and exponential, which is not common in learning theory. 

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3. Spatially resolved study of the SS 433/W 50 west region with Chandra: X-ray structure and spectral variation of non-thermal emission

   https://doi.org/10.1093/pasj/psac060  

   Kayama, Kazuho; Tanaka, Takaaki; Uchida, Hiroyuki; Tsuru, Takeshi_Go; Sudoh, Takahiro; Inoue, Yoshiyuki; Khangulyan, Dmitry; Tsuji, Naomi; Yamamoto, Hiroaki (August 2022, Publications of the Astronomical Society of Japan)

   Abstract The X-ray binary SS 433, embedded in the W 50 nebula (or supernova remnant W 50), shows bipolar jets that are ejected with mildly relativistic velocities and which extend toward the east and west out to scales of tens of parsecs. Previous X-ray observations revealed twin lobes along the jet precession axis that contain compact bright knots dominated by synchrotron radiation, which provide evidence of electron acceleration in this system. Particle acceleration in this system is substantiated by the recently detected gamma rays with energies up to at least 25 TeV. To elucidate the origin of the knots and particle acceleration sites in SS 433/W 50 further, we report here on detailed, spatially resolved X-ray spectroscopy of its western lobe with Chandra. We detect synchrotron emission along the jet precession axis, as well as optically thin thermal emission that is more spatially extended. Between the two previously known knots, w1 and w2, we discover another synchrotron knot, which we call w1.5. We find no significant synchrotron emission between SS 433 and the innermost X-ray knot (w1), suggesting that electrons only begin to be accelerated at w1. The X-ray spectra become gradually steeper from w1 to w2, and then rapidly so immediately outside of w2. Through comparison with a model taking into account electron transport and cooling along the jet, this result indicates that the magnetic field in w2 is substantially enhanced, which also explains its brightness. We discuss possible origins of the enhanced magnetic field of w2 as well as scenarios to explain the other two knots. 

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4. Metallicities from high-resolution spectra of 49 RR Lyrae variables

   https://doi.org/10.1093/mnras/stab857  

   Gilligan, Christina K; Chaboyer, Brian; Marengo, Massimo; Mullen, Joseph P; Bono, Giuseppe; Braga, Vittorio F; Crestani, Juliana; Dall’Ora, Massimo; Fiorentino, Giuliana; Monelli, Matteo; et al (April 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Accurate metallicities of RR Lyrae are extremely important in constraining period–luminosity–metallicity (PLZ) relationships, particularly in the near-infrared. We analyse 69 high-resolution spectra of Galactic RR Lyrae stars from the Southern African Large Telescope. We measure metallicities of 58 of these RR Lyrae stars with typical uncertainties of 0.15 dex. All but one RR Lyrae in this sample has accurate ($$\sigma _{\varpi }\lesssim 10{{\ \rm per\ cent}}$$) parallax from Gaia. Combining these new high-resolution spectroscopic abundances with similar determinations from the literature for 93 stars, we present new PLZ relationships in WISE W1 and W2 magnitudes, and the Wesenheit magnitudes W(W1, V − W1) and W(W2, V − W2). 

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5. Thirteen New M Dwarf + T Dwarf Pairs Identified with WISE/NEOWISE

   https://doi.org/10.3847/1538-4357/ad3f1d  

   Marocco, Federico; Kirkpatrick, J Davy; Schneider, Adam C; Meisner, Aaron M; Popinchalk, Mark; Gelino, Christopher R; Faherty, Jacqueline K; Burgasser, Adam J; Caselden, Dan; Gagné, Jonathan; et al (May 2024, The Astrophysical Journal)

   Abstract We present the discovery of 13 new widely separated T dwarf companions to M dwarf primaries, identified using Wide-field Infrared Survey Explorer/NEOWISE data by the CatWISE and Backyard Worlds: Planet 9 projects (hereafter BYW). This sample represents an ∼60% increase in the number of known M + T systems, and allows us to probe the most extreme products of binary/planetary system formation, a discovery space made available by the CatWISE2020 catalog and the BYW effort. Highlights among the sample are WISEP J075108.79-763449.6, a previously known T9 thought to be old due to its spectral energy distribution, which was found by Zhang et al. (2021b) to be part of a common proper motion pair with L34-26 A, a well-studied young M3 V star within 10 pc of the Sun; CWISE J054129.32-745021.5 B and 2MASS J05581644-4501559 B, two T8 dwarfs possibly associated with the very fast-rotating M4 V stars CWISE J054129.32745021.5 A and 2MASS J05581644-4501559 A; and UCAC3 52-1038 B, which is among the widest late-T companions to main-sequence stars, with a projected separation of ∼7100 au. The new benchmarks presented here are prime JWST targets, and can help us place strong constraints on the formation and evolution theory of substellar objects as well as on atmospheric models for these cold exoplanet analogs. 

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