Luca Casagrande

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.

While standard disk instability theory predicts thermal, steady, outburst-free disks in nova-like variables due to their high mass-transfer rates, mass-transfer variations are prevailingly invoked to explain the occasional flaring and fading phenomena observed in these systems. Here we report the observational evidence for a dynamical collapse of a thermally stable accretion disk, captured serendipitously by TESS during a fading episode of the VY Scl–type nova-like system MASTER OT J072703.91-631952.8, and traced by the emergence of an unusual negative superhump that evolves toward its orbital frequency. Simultaneously, the system underwent an anomalous eruptive event featuring a remarkably symmetric 45 day light-curve profile, indicative of a mild energy-release mechanism fundamentally distinct from documented eruptive events in cataclysmic variables. Notably, the concurrence of the eruption and the disk collapse is difficult to reconcile with the paradigm of mass-transfer variations: the eruption implies enhanced mass transfer, whereas the disk collapse indicates a decline. The morphological disk evolution over ∼700 days, characterized by both negative and positive superhumps, indicates a transition from a circular disk to an eccentric disk, followed by a tilted state, and ultimately a minimal disk configuration. This evolutionary sequence provides evidence for a previously unrecognized dynamics-driven cycle operating in the thermally stable accretion disk of a VY Scl–type nova-like star.

Aims. We present time-series radial velocities of the G8 subgiant star β Aql obtained in 2022 and 2023 using SONG-Tenerife and, for the first time, SONG-Australia. We also analyse a sector of TESS photometry that overlapped with the 2022 SONG data. Methods. We processed the time series to assign weights and to remove bad data points. The resulting power spectrum clearly shows solar-like oscillations centred at 430 μHz. The TESS light curve shows the oscillations at lower signal-to-noise, reflecting the fact that photometric measurements are much more affected by the granulation background than are radial velocities. Results. The simultaneous observations in velocity and photometry represent the best such measurements for any star apart from the Sun. They allowed us to measure the ratio between the bolometric photometric amplitude and the velocity amplitude to be 26.6 ± 3.1 ppm/ms⁻¹. We measured this ratio for the Sun from published SOHO data to be 19.5 ± 0.7 ppm/ms⁻¹ and, after accounting for the difference in effective temperatures of β Aql and the Sun, these values align with expectations. In both the Sun and β Aql, the photometry-to-velocity ratio appears to be a function of frequency. We also measured the phase shift of the oscillations in β Aql between SONG and TESS to be −113° ±7°, which agrees with the value for the Sun and also with a 3D simulation of a star with similar properties to β Aql. Importantly for exoplanet searches, we argue that simultaneous photometry can be used to predict the contribution of oscillations to radial velocities. We measured frequencies for 22 oscillation modes in β Aql and carried out asteroseismic modelling, yielding an excellent fit to the frequencies. We derived accurate values for the mass and age, and were able to place quite strong constraints on the mixing-length parameter. Finally, we show that the oscillation properties of β Aql are very similar to stars in the open cluster M67.

Split main-sequences and extended main-sequence turn-offs (eMSTOs) have been observed in nearly all Magellanic Clouds clusters younger than 2 Gyr. More recently, Hubble Space Telescope (HST) ultraviolet photometry uncovered a puzzling new population of UV-absorbed stars, dubbed ‘UVdim’, in five Magellanic Clouds clusters aged between 40 and 200 Myr, as well as in one 1.5 Gyr-old cluster. These UVdim stars predominantly lie on the blue MS, which is composed of slow rotators, and their distinct UV properties are believed to stem from dusty circumstellar discs. Although eMSTOs are common in both Magellanic Clouds and Galactic open clusters (OCs) of comparable ages, UVdim stars have not yet been investigated in Galactic OCs. In this work, we fill that gap by combining Swift/Ultraviolet-Optical Telescope, SkyMapper, and Gaia photometry to extend the search for UVdim stars to 35 Galactic OCs younger than 2 Gyr. By constructing colour–colour diagrams analogous to those employed with HST WFC3/UVIS, we find no evidence of UVdim-like stars in most Galactic open clusters and identify possible UVdim candidates in only five systems.The rarity of UVdim stars in young OCs suggests a potential difference between Magellanic Cloud clusters and their Milky Way counterparts, although the underlying reason remains unclear.