Enzo Brocato

Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.

Context. The surface brightness fluctuation (SBF) method is one of the most reliable and efficient ways of measuring distances to galaxies within 100 Mpc, a matter of crucial importance for all fields of astrophysics. While recent implementations have increasingly relied on space-based observations, SBF remains effective when applied to ground-based data. In particular, deep, wide-field imaging surveys with sub-arcsecond seeing conditions allows us to make accurate SBF measurements across large samples of galaxies. With the upcoming next generation of wide-area imaging surveys, the thousands of galaxies suitable for SBF measurements will give us the opportunity to constrain the 3D structure of the local Universe. Aims. We present FAST-SBF, a new Python-based pipeline for measuring SBF, developed to support the analysis of large datasets from upcoming wide-field imaging surveys such as LSST, Euclid , and Roman . The procedure, still in the testing and development stage, is designed for automation and minimal user intervention, offering a fast and flexible approach to SBF distance estimation. Methods. We validated the performance of the procedure on high-quality imaging data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP), a precursor to LSST, analyzing a sample of both luminous early-type galaxies and fainter dwarfs. Our measurements were also compared with recent results from the Next Generation Virgo Cluster Survey (NGVS) and with the SPoT stellar population synthesis models. Results. The results show excellent agreement with published distances, with the capability of measuring the SBF signal also for faint dwarf galaxies. The pipeline allows the user to completely analyze a galaxy in relatively short time (≈ minutes) and significantly reduces the need for user intervention. The FAST-SBF tool is planned for public release to support the community in using SBF as a distance indicator in next-generation surveys.

In this paper we analyse the spectro-photometric properties of the Type II supernova SN 2024bch, which exploded in NGC 3206 at a distance of 19.9 Mpc. Its early spectra are characterised by narrow high-ionisation emission lines, which are often interpreted as signatures of ongoing interaction between rapidly expanding ejecta and a confined dense circumstellar medium. However, we provide a model for the bolometric light curve of the transient that does not require sources of energy other than radioactive decays and H recombination. Our model can reproduce the bolometric light curve of SN 2024bch if we adopt an ejected mass of M _bulk ≃ 5 M _⊙ surrounded by an extended envelope of only 0.2 M _⊙ with an outer radius R _env = 7.0 × 10 ¹³ cm. Accurate modelling focused on the radioactive part of the light curve, which accounts for incomplete γ -ray trapping, gives a ⁵⁶ Ni mass of 0.048 M _⊙ . We propose that narrow lines are powered by Bowen fluorescence that is induced by scattering of He II Ly α photons. Simple light travel time calculations based on the maximum phase of the narrow emission lines place the inner radius of the H-rich, un-shocked shell at a radius ≃ 4.4 × 10 ¹⁵ cm, consistent with no interaction occurring between the ejecta and the surrounding circumstellar material during the initial weeks of evolution. Possible signatures of interaction appear only ∼ 69 days after the explosion, although the resulting conversion of kinetic energy into radiation does not seem to contribute significantly to the total luminosity of the transient.

Several missions to Venus have recently been selected for launch, opening a new era for the exploration of the planet. A key question these missions seek to address is whether Venus is geologically active today. Detecting active volcanism on Venus would contribute to a better understanding of volcanic and atmospheric processes on Earth. The Analogs for VENus' GEologically Recent Surfaces (AVENGERS) initiative provides a comparative framework for the identification and analysis of terrestrial analog sites relevant to recent and possibly ongoing volcanic activity on Venus. Although this review focuses on terrestrial analogs for Venus exploration, the AVENGERS initiative also includes investigations that use Venus and Earth as analogs to better understand the evolution of terrestrial exoplanets. In addition, the AVENGERS initiative facilitates international collaboration by connecting Venus mission teams with the broader planetary and exoplanetary science communities. This review describes the scientific context of AVENGERS, details the selection criteria for terrestrial analog sites, and summarizes a set of suitable locations. Methodological approaches in analog studies that support the interpretation of Venus observations are also discussed.

We present the result of a sample of B-stars in the Large Magellanic Cloud young double stellar cluster NGC 1850 A and NGC 1850 B, observed with the integral-field spectrograph at the Very Large Telescope, the Multi Unit Spectroscopic Explorer. We compare the observed equivalent widths (EWs) of four He lines (4922 Å 5015 Å 6678 Å and 7065 Å) with the ones determined from synthetic spectra computed with different He mass fraction (Y=0.25, 0.27, 0.30 and 0.35) with the code SYNSPEC, that takes into account the non-LTE effect. From this comparison, we determined the He mass fraction of the B stars, finding a not homogeneous distribution. The stars can be divided in three groups, He-weak (Y< 0.24) and the He-normal (0.24≤Y≤ 0.26) belonging to the MS of NGC 1850 A, and the He-rich stars 0.33≤Y≤0.38) situated in the MS associated to NGC 1850 B. We have analyzed the stellar rotation as possible responsible of the anomalous features of the He lines in the He-rich stars. We provide a simple analysis of the differences between the observed EWs and the ones obtained from the theoretical models with different rotation velocity (Vsin i = 0 and 250 Km/s). The resolution of the MUSE spectra do not allow to get a conclusive result, however our analysis support the He-enhanced hypothesis.

The Vera C. Rubin Legacy Survey of Space and Time (LSST) holds the potential to revolutionize time domain astrophysics, reaching completely unexplored areas of the Universe and mapping variability time scales from minutes to a decade. To prepare to maximize the potential of the Rubin LSST data for the exploration of the transient and variable Universe, one of the four pillars of Rubin LSST science, the Transient and Variable Stars Science Collaboration, one of the eight Rubin LSST Science Collaborations, has identified research areas of interest and requirements, and paths to enable them. While our roadmap is ever-evolving, this document represents a snapshot of our plans and preparatory work in the final years and months leading up to the survey’s first light.

We present the results from a dense multi-wavelength (optical/UV, near-infrared (IR), and X-ray) follow-up campaign of the nuclear transient AT2017gge, covering a total of 1698 days from the transient's discovery. The bolometric lightcurve, the black body temperature and radius, the broad H and He I $\lambda$5876 emission lines and their evolution with time, are all consistent with a tidal disruption event (TDE) nature. A soft X-ray flare is detected with a delay of $\sim$200 days with respect to the optical/UV peak and it is rapidly followed by the emergence of a broad He II $\lambda$4686 and by a number of long-lasting high ionization coronal emission lines. This indicate a clear connection between a TDE flare and the appearance of extreme coronal line emission (ECLEs). An IR echo, resulting from dust re-radiation of the optical/UV TDE light is observed after the X-ray flare and the associated near-IR spectra show a transient broad feature in correspondence of the He I $\lambda$10830 and, for the first time in a TDE, a transient high-ionization coronal NIR line (the [Fe XIII] $\lambda$10798) is also detected. The data are well explained by a scenario in which a TDE occurs in a gas and dust rich environment and its optical/UV, soft X-ray, and IR emission have different origins and locations. The optical emission may be produced by stellar debris stream collisions prior to the accretion disk formation, which is instead responsible for the soft X-ray flare, emitted after the end of the circularization process.

Aims. We study the accretion properties of pre-main sequence (PMS) low-mass stars in the LH 91 association within the Large Magellanic Clouds. Methods. Using optical multiband photometry obtained with the Hubble Space Telescope, we identify 75 candidates showing Hα excess emission above the 3σ level with equivalent width EWH_α ⩾ 10 Å. We estimate the physical parameters (effective temperature, luminosity, age, mass, accretion luminosity, and mass accretion rate) of the PMS stellar candidates. Results. The age distribution suggests a period of active star formation ranging from a few million years up to ∼60 Myr with a gap between ∼5 Myr and 10 Myr. The masses of the PMS candidates span from 0.2 M_⊙ for the cooler objects to 1.0 M_⊙ with a median of ∼0.80 M_⊙. The median value of the accretion luminosity of our 75 PMS stars is about 0.12 L_⊙, the median value of the mass accretion rate is about 4.8 × 10⁻⁹ M_⊙ yr⁻¹ with higher values for the younger population (∼1.2 × 10⁻⁸ M_⊙ yr⁻¹), and lower values for the older candidates (∼4.7 × 10⁻⁹ M_⊙ yr⁻¹). We compare our results with findings for LH 95, the closest region to LH 91 for which accretion properties of PMS candidates were previously derived. An interesting qualitative outcome is that LH 91 seems to be in a more evolved stage. Moreover, we find that the PMS candidates are distributed homogeneously, without any evidence of clumps around more massive stars.

The discovery of the electromagnetic counterpart to the binary neutron star (NS) merger GW170817 has opened the era of gravitational-wave multimessenger astronomy. Rapid identification of the optical/infrared kilonova enabled a precise localization of the source, which paved the way to deep multiwavelength follow-up and its myriad of related science results. Fully exploiting this new territory of exploration requires the acquisition of electromagnetic data from samples of NS mergers and other gravitational-wave sources. After GW170817, the frontier is now to map the diversity of kilonova properties and provide more stringent constraints on the Hubble constant, and enable new tests of fundamental physics. The Vera C. Rubin Observatory’s Legacy Survey of Space and Time can play a key role in this field in the 2020s, when an improved network of gravitational-wave detectors is expected to reach a sensitivity that will enable the discovery of a high rate of merger events involving NSs (∼tens per year) out to distances of several hundred megaparsecs. We design comprehensive target-of-opportunity observing strategies for follow-up of gravitational-wave triggers that will make the Rubin Observatory the premier instrument for discovery and early characterization of NS and other compact-object mergers, and yet unknown classes of gravitational-wave events.