Explore the vast range of titles available.

It has long been debated whether elements heavier than zinc are formed continually, for example in core-collapse supernovae, or in rare events, such as neutron star mergers; here, studies of element abundances in a local ultrafaint dwarf galaxy provide evidence that these elements are formed during rare yet prolific stellar events. Nucleosynthesis in dwarf galaxy Reticulum II The recently discovered Milky Way satellite Reticulum II, an ultra-faint dwarf galaxy, provides an ideal model for the study of stellar nucleosynthesis, the creation of heavier elements from hydrogen, helium and other lighter elements and particles. It has long been debated whether elements heavier than zinc are formed continually, for example in core-collapse supernovae, or in rare events, such as neutron star mergers. Alexander Ji et al . used high-resolution spectroscopy to determine element abundances in nine young stars in Reticulum II, and find that seven of the nine show strong enhancements in heavy neutron-capture elements with abundances that follow the universal r-process pattern above barium. The enhancement is several orders of magnitude greater than that seen in other ultra-faint dwarf galaxies, implying that a single rare event produced the r-process material. Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture processes 1 , 2 . The main site of production of the r-process elements (such as europium) has been debated for nearly 60 years 2 . Initial studies of trends in chemical abundances in old Milky Way halo stars suggested that these elements are produced continually, in sites such as core-collapse supernovae 3 , 4 . But evidence from the local Universe favours the idea that r-process production occurs mainly during rare events, such as neutron star mergers 5 , 6 . The appearance of a plateau of europium abundance in some dwarf spheroidal galaxies has been suggested as evidence for rare r-process enrichment in the early Universe 7 , but only under the assumption that no gas accretes into those dwarf galaxies; gas accretion 8 favours continual r-process enrichment in these systems. Furthermore, the universal r-process pattern 1 , 9 has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultrafaint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-process 10 , 11 . Reticulum II is one such galaxy 12 , 13 , 14 . The abundances of non-neutron-capture elements in this galaxy (and others like it) are similar to those in other old stars 15 . Here, we report that seven of the nine brightest stars in Reticulum II, observed with high-resolution spectroscopy, show strong enhancements in heavy neutron-capture elements, with abundances that follow the universal r-process pattern beyond barium. The enhancement seen in this ‘r-process galaxy’ is two to three orders of magnitude higher than that detected in any other ultrafaint dwarf galaxy 11 , 16 , 17 . This implies that a single, rare event produced the r-process material in Reticulum II. The r-process yield and event rate are incompatible with the source being ordinary core-collapse supernovae 18 , but consistent with other possible sources, such as neutron star mergers 19 .

We describe the discovery of a solar neighborhood (d = 468 pc) binary system with a main-sequence sunlike star and a massive noninteracting black hole candidate. The spectral energy distribution of the visible star is described by a single stellar model. We derive stellar parameters from a high signal-to-noise Magellan/MIKE spectrum, classifying the star as a main-sequence star with T eff = 5972 K, logg=4.54 , and M = 0.91 M ⊙. The spectrum shows no indication of a second luminous component. To determine the spectroscopic orbit of the binary, we measured the radial velocities of this system with the Automated Planet Finder, Magellan, and Keck over four months. We show that the velocity data are consistent with the Gaia astrometric orbit and provide independent evidence for a massive dark companion. From a combined fit of our spectroscopic data and the astrometry, we derive a companion mass of 11.39−1.31+1.51 M ⊙. We conclude that this binary system harbors a massive black hole on an eccentric (e = 0.46 ± 0.02), 185.4 ± 0.1 day orbit. These conclusions are independent of El-Badry et al., who recently reported the discovery of the same system. A joint fit to all available data yields a comparable period solution but a lower companion mass of 9.32−0.21+0.22M⊙ . Radial velocity fits to all available data produce a unimodal solution for the period that is not possible with either data set alone. The combination of both data sets yields the most accurate orbit currently available.

In this paper, we present a chemical and kinematic analysis of two ultrafaint dwarf galaxies (UFDs), Aquarius II (Aqu II) and Boötes II (Boo II), using Magellan/IMACS spectroscopy. We present the largest sample of member stars for Boo II (12), and the largest sample of red giant branch members with metallicity measurements for Aqu II (eight). In both UFDs, over 80% of targets selected based on Gaia proper motions turned out to be spectroscopic members. In order to maximize the accuracy of stellar kinematic measurements, we remove the identified binary stars and RR Lyrae variables. For Aqu II, we measure a systemic velocity of −65.3 ± 1.8 km s−1 and a metallicity of [Fe/H] = −2.57−0.17+0.17 . When compared with previous measurements, these values display a ∼6 km s−1 difference in radial velocity and a decrease of 0.27 dex in metallicity. Similarly for Boo II, we measure a systemic velocity of −130.4−1.1+1.4 km s−1, more than 10 km s−1 different from the literature, a metallicity almost 1 dex smaller at [Fe/H] = −2.71−0.10+0.11 , and a velocity dispersion 3 times smaller at σvhel=2.9−1.2+1.6 km s−1. Additionally, we derive systemic proper-motion parameters and model the orbits of both UFDs. Finally, we highlight the extremely dark-matter-dominated nature of Aqu II and compute the J-factor for both galaxies to aid searches of dark matter annihilation. Despite the small size and close proximity of Boo II, it is an intermediate target for the indirect detection of dark matter annihilation due to its low-velocity dispersion and corresponding low dark matter density.

In 2022 June, the Gaia mission released a catalog of astrometric orbital solutions for 168,065 binary systems, by far the largest such catalog to date. The catalog’s selection function is difficult to characterize because of choices made in its construction. Understanding the catalog’s selection function is required to model and interpret its contents. We use a combination of analytic and empirical prescriptions to construct a function that computes the probability that a binary with a given set of properties would have been published in the Gaia Data Release 3 astrometric orbit catalog. This is a complementary approach to the more accurate but significantly more computationally expensive approach of El-Badry et al. We also construct a binary population synthesis model to validate our characterization of the selection function, finding good agreement with the actual Gaia NSS catalog, with the exception of the orbital eccentricity distribution. The NSS catalog suggests high-eccentricity orbits are relatively uncommon at intermediate periods 100 ≲ Porb ≲ 1000 days. As an example application of the selection function, we estimate the Gaia DR3 detection probabilities of the star + BH binaries Gaia BH1 and BH2, and find them to be 0.38 and 0.27, respectively. Compared to the values obtained by detailed modeling in El-Badry et al., the probabilities are identical for BH1, and within a factor of 2 for BH2. We also estimate the population of Sun-like star + BH binaries in the Galaxy to be ∼3000 for 100 < Porb < 400 days, <800 for 400 < Porb < 1000 days, and <12,000 for 1000 < Porb < 1500 days.

The growing number of Milky Way satellites detected in recent years has introduced a new focus for stellar abundance analysis. Abundances of stars in satellites have been used to probe the nature of these systems and their chemical evolution. However, for most satellites, only centrally located stars have been examined. This paper presents an analysis of three stars in the Tucana V system, one in the inner region and two at ∼10′ (7–10 half-light radii) from the center. We find a remarkable chemical diversity between the stars. One star exhibits enhancements in rapid neutron-capture elements (an r-I star), and another is highly enhanced in C, N, and O but with low neutron-capture abundances (a CEMP-no star). The metallicities of the stars analyzed span more than 1 dex from [Fe/H] = −3.55 to −2.46. This, combined with a large abundance range of other elements like Ca, Sc, and Ni, confirms that Tuc V is an ultrafaint dwarf (UFD) galaxy. The variation in abundances, highlighted by [Mg/Ca] ratios ranging from +0.89 to −0.75, among the stars demonstrates that the chemical enrichment history of Tuc V was very inhomogeneous. Tuc V is only the second UFD galaxy in which stars located at large distances from the galactic center have been analyzed, along with Tucana II. The chemical diversity seen in these two galaxies, driven by the composition of the noncentral member stars, suggests that distant member stars are important to include when classifying faint satellites and that these systems may have experienced more complex chemical enrichment histories than previously anticipated.

We present deep Hubble Space Telescope photometry of 10 targets from Treasury Program GO-14734, including six confirmed ultrafaint dwarf (UFD) galaxies, three UFD candidates, and one likely globular cluster. Six of these targets are satellites of, or have interacted with, the Large Magellanic Cloud (LMC). We determine their structural parameters using a maximum-likelihood technique. Using our newly derived half-light radius (r h ) and V-band magnitude (M V ) values in addition to literature values for other UFDs, we find that UFDs associated with the LMC do not show any systematic differences from Milky Way UFDs in the magnitude–size plane. Additionally, we convert simulated UFD properties from the literature into the M V –r h observational space to examine the abilities of current dark matter (DM) and baryonic simulations to reproduce observed UFDs. Some of these simulations adopt alternative DM models, thus allowing us to also explore whether the M V –r h plane could be used to constrain the nature of DM. We find no differences in the magnitude–size plane between UFDs simulated with cold, warm, and self-interacting DM, but note that the sample of UFDs simulated with alternative DM models is quite limited at present. As more deep, wide-field survey data become available, we will have further opportunities to discover and characterize these ultrafaint stellar systems and the greater low surface-brightness universe.

Stellar-mass black holes (BHs) above 30M⊙ are predicted to form from low-metallicity progenitors, but direct detections of such systems in the Milky Way remain scarce. Motivated by the recent discovery of Gaia BH3, a 33M⊙ BH with a very metal-poor giant companion, we conduct a systematic search for additional systems. Approximately 900 candidates are identified with Gaia as having significant deviations from single-star astrometric motion, evidence of radial velocity (RV) variability, and low metallicities inferred from Gaia XP spectra. We obtain single epoch high-resolution spectra for over 600 of these sources with Magellan/Magellan Inamori Kyocera Echelle and Lick/Automated Planet Finder and measure independent RVs with ≈1 km s−1 precision. After removing contaminants such as hot stars, pulsators, eclipsing binaries, and hierarchical triples, we identify about 15 promising candidates with large RV amplitudes or offsets from the Gaia reported values. This program establishes a well-characterized sample of BH candidates for detailed orbital modeling once Gaia DR4 epoch astrometry and RVs are released in late 2026; multiepoch RV follow-up is ongoing. Together, the Gaia and ground-based data will place new constraints on the demographics of BHs with metal-poor companions and test theoretical predictions linking low metallicity to the formation of the most massive stellar remnants.

Among Neptunian mass exoplanets (20−50 M ⊕), puffy hot Neptunes are extremely rare, and their unique combination of low mass and extended radii implies very low density (ρ < 0.3 g cm−3). Over the last decade, only a few puffy planets have been detected and precisely characterized with both transit and radial velocity observations, most notably including WASP-107 b, TOI-1420 b, and WASP-193 b. In this paper, we report the discovery of TOI-1173 A b, a low-density ( ρ=0.195−0.017+0.018 g cm−3) super-Neptune with P = 7.06 days in a nearly circular orbit around the primary G-dwarf star in the wide binary system TOI-1173 A/B. Using radial velocity observations with the MAROON-X and HIRES spectrographs and transit photometry from TESS, we determine a planet mass of M p = 27.4 ± 1.7 M ⊕ and radius of R p = 9.19 ± 0.18 R ⊕. TOI-1173 A b is the first puffy super-Neptune planet detected in a wide binary system (projected separation ∼11,400 au). We explore several mechanisms to understand the puffy nature of TOI-1173 A b and show that tidal heating is the most promising explanation. Furthermore, we demonstrate that TOI-1173 A b likely has maintained its orbital stability over time and may have undergone von-Zeipel–Lidov–Kozai migration followed by tidal circularization, given its present-day architecture, with important implications for planet migration theory and induced engulfment into the host star. Further investigation of the atmosphere of TOI-1173 A b will shed light on the origin of close-in low-density Neptunian planets in field and binary systems, while spin–orbit analyses may elucidate the dynamical evolution of the system.

Milky way satellites: galaxies with a dark side Sophisticated optical astronomy projects such as the Sloan Digital Sky Survey are reaching a new threshold in detecting the least luminous galaxies in the Universe, and now at least twenty-three faint satellite galaxies are known in the region of the Milky Way. They range in luminosity from about a thousand to more than 100 million times that of the Sun. The velocities of the stars in these galaxies reveal that despite this variation in luminosity, each of the galaxies is similar in mass, at about 10 million times the mass of the Sun within their central 300 parsecs. The faintest of the Milky Way satellites are accordingly the most dark-matter-dominated galaxies known in the Universe. The Milky Way has at least twenty-three known satellite galaxies that shine with luminosities ranging from about a thousand to a billion times that of the Sun. Half of these galaxies were discovered 1 , 2 in the past few years in the Sloan Digital Sky Survey, and they are among the least luminous galaxies in the known Universe. A determination of the mass of these galaxies provides a test of galaxy formation at the smallest scales 3 , 4 and probes the nature of the dark matter that dominates the mass density of the Universe 5 . Here we use new measurements of the velocities of the stars in these galaxies 6 , 7 to show that they are consistent with them having a common mass of about 10 7 within their central 300 parsecs. This result demonstrates that the faintest of the Milky Way satellites are the most dark-matter-dominated galaxies known, and could be a hint of a new scale in galaxy formation or a characteristic scale for the clustering of dark matter.

Over the last decade, studies of large samples of binary systems have identified chemical anomalies and shown that they might be attributed to planet formation or planet engulfment. However, both scenarios have primarily been tested in pairs without known exoplanets. In this work, we explore these scenarios in the newly detected planet-hosting wide binary TOI-1173 A/B (projected separation ∼11,400 au), using high-resolution MAROON-X and ARCES spectra. We determined photospheric stellar parameters both by fitting stellar models and via the spectroscopic equilibrium approach. Both analyses agree and suggest that they are cool main-sequence stars located in the thin disk. A line-by-line differential analysis between the components (B−A) displays an abundance pattern in the condensation temperature plane, where the planet-hosting star TOI-1173 A is enhanced in refractory elements such as iron by more than 0.05 dex. This suggests the engulfment of ∼18 M ⊕ of rocky material in star A. Our hypothesis is supported by the dynamics of the system (detailed in our companion paper), which suggest that the super-Neptune TOI-1173 A b might have been delivered to its current short period (∼7 days) through circularization and von Zeipel–Lidov–Kozai mechanisms, thereby triggering the engulfment of inner rocky exoplanets.