Sarah L. Martell

We report the discovery, validation, and characterization of 44 new symbiotic stars (SySts) accreting at the lowest detectable levels (L _acc ≲3 L _⊙ ). Candidates were initially selected from the Galah survey among non-pulsating M-type red giants exhibiting H α excess. Between 2021 and 2025, these targets were subjected to intensive multi-wavelength campaigns, including ground-based, high-resolution Echelle spectroscopy over 544 nights, multi-epoch flickering searches down to 1 mmag, and UV/X-ray observations with the Swift satellite. Our high-resolution spectra allowed for a detailed investigation of H α flux and line-profile variability. Crucially, the radial velocities measured for the M-giant components yielded reliable spectroscopic orbits for 22 objects and preliminary orbits for an additional eight, corroborated by binary-induced modulations in ASAS-SN photometry. Measured accretion levels vary by factors of up to 20×, emphasizing the need for high-sensitivity, long-term monitoring. Swift X-ray data exclude neutron star companions in all targets, consistent with predominance of white-dwarf degenerates in known SySts. Combined with the 33 objects from Paper I, this sample of 77 new SySts significantly exceeds the number of previously known, accreting-only Galactic symbiotic stars, constituting the bulk of the low-accretion (LowAcc) variety discovered to date and representing >10% of all known SySts in the Galaxy. As SySts likely spend most of their lifetimes in LowAcc faint states, characterizing this population is essential for estimating the total Galactic SySt population and evaluating the role of SySts as Type Ia supernova progenitors or sources of UV excess in elliptical galaxies.

Context. The Stellar Observations Network Group (SONG) network has operated for more than a decade, providing long-baseline high-cadence spectroscopic observations of bright stars and the Sun. The observations from 2014 through 2025 constitute a substantial archive of high-resolution spectra and precise radial velocities suitable for a broad range of time-domain stellar astrophysics. Aims. We present an overview of the current status, instrumentation, and scientific capabilities of the SONG network and describe the scope and accessibility of the SONG data archive (SODA). We further illustrate the breadth of science enabled by SONG observations, including asteroseismology, stellar variability studies, binary-star characterisation, and exoplanet research. Methods. We summarise the operational status and observing strategies of the SONG facilities, describe the available data products and archive infrastructure, and outline procedures for accessing archival observations and proposing new observations within the SONG community framework. Results. The SODA archive currently contains more than 580, 000 spectra of 3091 stars obtained with SONG using either iodine-cell or thorium–argon wavelength calibration. The archive spans more than a decade of observations and includes extensive high-cadence time-series data for bright targets spanning a wide range of stellar types and variability classes. Access to the archive is available to members of the SONG community, which remains open to new participants who agree to follow the community policies. Conclusions. The SONG archive has developed into a major long-baseline resource for stellar spectroscopy and radial velocity time-series analysis. Continued expansion of the archive, together with coordinated observations obtained contemporaneously with TESS and future PLATO observations, is expected to enable new studies of stellar oscillations, variability, and exoplanet host stars through combined radial velocity and photometric analyses.

We present results of a spectroscopic campaign around the diffuse dwarf galaxy Crater 2 (Cra2) and its tidal tails as part of the Southern Stellar Stream Spectroscopic Survey ( S ⁵ ). Cra2 is a Milky Way dwarf spheroidal satellite with extremely cold kinematics, but a huge size similar to the Small Magellanic Cloud, which may be difficult to explain within collisionless cold dark matter. We identify 143 Cra2 members, of which 114 belong to the galaxy’s main body, and 29 are deemed part of its stellar stream. We confirm that Cra2 is dynamically cold (central velocity dispersion 2.5 1 − 0.30 + 0.33 km s − 1 ) and also discover a ≈7 σ velocity gradient consistent with its tidal debris track. We separately estimate the stream’s internal velocity dispersion to be 5.7 4 − 0.83 + 0.98 km s − 1 . We develop a suite of N -body simulations with both cuspy and cored density profiles on a realistic Cra2 orbit to compare with S ⁵ observations. We find that the velocity dispersion ratio between the Cra2 stream and galaxy ( 2.3 0 − 0.35 + 0.41 ) is difficult to reconcile with a cuspy halo with fiducial concentration and an initial mass predicted by standard stellar mass–halo mass relationships. Instead, either a cored halo with a relatively small core radius or a low-concentration cuspy model can reproduce this ratio. Despite tidal mass loss, Cra2 is metal-poor (〈[Fe/H]〉 = −2.16 ± 0.04) compared to the stellar mass–metallicity relation for its luminosity. Other diffuse dwarf galaxies similar to Cra2 in the Local Group (Antlia 2 and Andromeda 19) also challenge galaxy formation models. Finally, we discuss possible formation scenarios for Cra2, including ram-pressure stripping of a gas-rich progenitor combined with tides.

We present a comprehensive data-driven spectroscopic analysis of 357,415 red giant stars using Gaia DR3 Radial Velocity Spectrometer (RVS) spectra (8460–8700 Å; R ≈ 11, 500), aimed at deriving precise stellar parameters and elemental abundances (collectively referred to as stellar labels). We employ The Cannon, a generative model based on 2747 giants in common with GALAH DR4, adopting GALAH labels (R ≈ 28, 000) for training. The resulting model predicts eleven stellar labels for RVS giants: effective temperature (Teff), surface gravity (log g), projected rotational velocity (vsin i), and abundances of [Fe/H], [Ca/Fe], [Si/Fe], [Ni/Fe], [Ti/Fe], as well as the neutron-capture elements [Zr/Fe], [Ce/Fe], and [Nd/Fe]. Building on these results, we develop a probabilistic framework to chemically identify debris from the Gaia–Sausage–Enceladus (GSE) accretion event. A logistic regression classifier, optimised via Markov Chain Monte Carlo sampling and trained on a small reference sample of GSE members and comparison stars, identifies stars with high GSE membership probabilities based solely on their chemical abundances, with the resulting candidates exhibiting distinctive abundance-ratio patterns, including [Ca/Ti], [Ti/Ce], and [Nd/Zr]. Applying independent kinematic constraints yields a robust sample of GSE candidates, demonstrating that the characteristic chemical signatures remain consistent after applying these constraints. This work demonstrates the power of data-driven analysis techniques to extract detailed chemical information from medium-resolution spectra and establishes a framework for tracing Galactic accretion events using chemical abundances.

Context . Recent studies have revealed that the Milky Way’s stellar halo is a composite of stellar populations of different origins, including multiple accretion events. To better understand how the Milky Way and other spiral galaxies were formed, it is necessary to thoroughly characterize the chemical and kinematic properties of these structures. Aims . We search for kinematic structures of the stellar halo to find any substructures within them (when indeed present) and characterize the chemodynamical properties of the identified groups with the GALAH DR4 and Gaia surveys. Methods . We applied wavelet transforms in the space defined by a square root of radial action ( J _r ) and azimuthal action ( L _z ) to search for kinematic overdensities. Then, we selected stars in the detected structures and investigated their elemental abundances to determine their origin. Additionally, we checked for any contamination from other stellar populations within the detected groups with the unsupervised machine learning algorithm t-Distributed Stochastic Neighbor Embedding (t-SNE), for which we performed chemical tagging in a high-dimensional parameter space using 15 elemental abundances as input. Results . We recovered five kinematic structures in the action space with the wavelet transform. These groups are the Galactic disk, Splash, Gaia -Sausage-Enceladus (GSE), Thamnos1, and Thamnos2. We found that GSE has two peaks with the wavelet transform. One of these peaks is located at J _r ≃ 25 kpc km s ⁻¹ and is a result of contamination from disk stars. The other peak corresponds to the ‘cleanest’ GSE population and is located above J _r ≃ 40 kpc km s ⁻¹ . We also detected three peaks in Thamnos. We linked two of them to Thamnos1, while the peak with the stars on the most retrograde orbits was linked to Thamnos2. The t-SNE algorithm confirmed these findings. We also analyzed individual elemental abundances of each group and found that Thamnos2 has a higher [ α /Fe] ratio than the other groups and that iron-peak elements are more abundant in the Splash than in the halo groups, while the halo structures retain a higher r -process signature than the splashed disk. Conclusions . A multiply peaked substructure we observe in action space in GSE and Thamnos suggests that the splashed disk extends beyond the borders of prograde orbits. Each of the four halo groups studied in this paper have unique chemodynamical properties that confirm their extra-galactic origin.

Stellar streams are sensitive laboratories for understanding the small-scale structure in our Galaxy’s gravitational field. Here, we analyze the morphology of the 300 S stellar stream, which has an eccentric, retrograde orbit and thus could be an especially powerful probe of both baryonic and dark substructures within the Milky Way. Due to extensive background contamination from the Sagittarius stream (Sgr), we perform an analysis combining Dark Energy Camera Legacy Survey photometry, ???????????????? DR3 proper motions, and spectroscopy from the Southern Stellar Stream Spectroscopic Survey ( ???? 5 ). We redetermine the stream coordinate system and distance gradient, then apply two approaches to describe 300 S ’s morphology. In the first, we analyze stars from ???????????????? using proper motions to remove Sgr. In the second, we generate a simultaneous model of 300 S and Sgr based purely on photometric information. Both approaches agree within their respective domains and describe the stream over a region spanning 33 ∘ . Overall, 300 S has three well-defined density peaks and smooth variations in stream width. Furthermore, 300 S has a possible gap of ∼ 4.7 ∘ and a kink. Dynamical modeling of the kink implies that 300 S was dramatically influenced by the Large Magellanic Cloud. This is the first model of 300 S ’s morphology across its entire known footprint, opening the door for deeper analysis to constrain the structures of the Milky Way.

Globular clusters (GCs) are powerful tracers of galaxy assembly, frequently used to identify accreted substructure and reconstruct hierarchical merger histories. With advances in GC formation models and cosmological simulations, we can now better quantify the information about galaxy evolution encoded in present-day GCs. Here, we investigate how GC kinematics evolve over cosmic time and assess the extent to which GCs retain memory of the past of their host galaxy. Using a GC formation model applied to five Milky Way (MW) analogues from the Latte suite of the FIRE-2 simulations, we track the evolution of kinematic properties. At z = 0, in-situ and ex-situ GCs exhibit substantial overlap in kinematic space, indicating that these populations are not clearly separable. We find that a subset of kinematic properties evolve in an ordered fashion across both in-situ and ex-situ populations, whereas others are dominated by stochastic variations. As a result, by the present day, most memory of the progenitor of an accreted GC is erased and only a few correlations persist. These correlations link progenitor halo mass to the total mass and number of a GC population, and the galactocentric distance of GC substructure to progenitor maximum circular velocity. These results highlight how both deterministic and stochastic processes driven by galaxy evolution shape GC kinematics and demonstrate the limits of reconstructing the assembly history of a galaxy from present-day GC orbits alone.

We present a detailed chemical abundance analysis of the three brightest known stars from each of the Turranburra and Willka Yaku stellar streams using high-resolution Magellan/Magellan Inamori Kyocera Echelle spectra. Abundances for 27 elements, ranging from carbon to dysprosium, were derived. Our results support the original classification that Turranburra, with a low average metallicity of [Fe/H] = −2.45 ± 0.07, likely originates from a dwarf galaxy progenitor. Willka Yaku has a low average metallicity of [Fe/H] = −2.35 ± 0.03 with a small scatter in the abundances, consistent with a globular cluster progenitor as suggested by previous studies. Both streams exhibit mild enhancements in neutron-capture elements, with averages of [Eu II /Fe] = 0.47 ± 0.09 for Turranburra and 0.44 ± 0.05 for Willka Yaku, consistent with enrichment from an r -process event. A similar enrichment is observed in other stellar streams, and we further discuss this signature as it relates to the potential enrichment histories of these two streams.

We present the first high-resolution abundance study of ESO 280-SC06, one of the least luminous and most metal-poor gravitationally bound Milky Way globular clusters. Using Magellan/MIKE spectroscopy for ten stars, we confirm the cluster’s low metallicity as [Fe/H] = − 2.54 ± 0.06 and the presence of a nitrogen-enhanced star enriched by binary mass transfer. We determine abundances or abundance upper limits for 21 additional elements from the light, alpha, odd-Z, iron peak, and neutron-capture groups for all ten stars. We find no spread in neutron-capture elements, unlike previous trends identified in some metal-poor globular clusters such as M15 and M92. Eight of the ten stars have light-element abundance patterns consistent with second-population globular cluster stars, which is a significantly larger second-population fraction than would be expected from the low present-day mass of 10 4.1 Msun. We estimate the initial mass of the cluster as 10 5.4 − 5.7 Msun based on its orbit in the Milky Way. A preferential loss of first-population stars could explain the high fraction of second-population stars at the present time. Our results emphasize the importance of considering mass loss when studying globular clusters and their enrichment patterns.