Stefano Bagnulo

This study presents phase-resolved spectroscopy and spectropolarimetry of the DAHe white dwarf LP 705-64. Here, two apparently discrete chromospheric regions are observed to alternate in visibility and magnetic polarity with rotation, with field strengths of 9.5 and 22 MG based on the measured Zeeman splitting in Hα. As in the class prototype GD 356, emission line equivalent widths vary in antiphase with the broadband light curve. The 9.5 MG field is visible for 60 per cent of the stellar rotation and stays nearly constant, whereas the 22 MG field increases modestly during the remaining phases. Notably, the time-series transitions between magnetic regions are abrupt and transit-like in morphology, requiring at least one magnetic region to be highly localised and significantly smaller than the areas inferred for the dark surface spots. Both the intensity and the Stokes V/I spectra indicate a field configuration significantly more complex than an offset dipole, but higher-cadence data are necessary to distinguish between a global field and localised magnetic regions. All data are consistent with magnetic, chromospheric, and dark surface regions that are co-located and thus physically related. These findings support a model of chromospheric emission that is intrinsic and localised, consistent with predictions of dynamo-driven electrical currents that heat the upper atmosphere near magnetic poles.

White dwarfs that show atmospheric metal enrichment offer a direct window into the bulk composition of exoplanetary material and provide some of the most direct constraints on the building blocks of rocky exoplanetary systems. Recent work has suggested that magnetic fields may influence how this material is accreted, potentially channelling debris along field lines and retaining it near magnetic poles rather than allowing it to be redistributed uniformly across the stellar surface. Here we report the discovery of photometric, spectroscopic, and spectropolarimetric variability in the magnetic DZ white dwarf WD 1532+129, revealing a rotation period of about 289 d, by far the longest directly measured for any degenerate star thus far. The observed period may result from the combined effect of angular-momentum loss during the evolution leading to the white-dwarf stage and an unusually slowly rotating progenitor, perhaps a magnetic Ap star. The white dwarf exhibits surface abundance inhomogeneities, with metals concentrated near both magnetic poles, and the observed modulation is consistent with localised metal-rich regions aligned with the magnetic geometry. WD 1532 + 129 is the third of four known magnetic DZ white dwarfs within 20 pc to show evidence of polar metal enhancement, indicating that magnetically guided inhomogeneities are a common property of magnetic metal-enriched white dwarfs.

The VSTPOL project aims to transform the VLT Survey Telescope (VST) at Cerro Paranal into one of the first wide-field optical polarimetric facility which might be dedicated to high-sensitivity, large-area sky surveys. Building on the VST’s established performance as a 2.6-m, one-degree-field imaging telescope, VSTPOL introduces a new optomechanical module designed to deliver alternatively imaging or polarimetric capabilities across the full OmegaCAM 1deg2 field of view. The project’s primary scientific drivers include mapping interstellar magnetic fields, characterizing polarized emission and scattering from solar-system, Galactic and extragalactic sources, and enabling time-domain polarimetry of transients such as supernovae and gamma-ray burst afterglows, even before their accurate localization. Being colocated with CTAO-South, VSTPOL will be ideal for optical and polarimetric follow-up of the CTAO sources. The VSTPOL optomechanical device, comprising a rotating polarimetric filter installed within the field corrector in a relatively large mounting structure, has now been fully realized following detailed optical, mechanical and electronic design, and system integration. Laboratory characterization demonstrates that the instrument meets the optical and system requirements derived from the science case. Work is currently progressing on the control software, calibration pipeline, and integration interfaces with the existing VST infrastructure. Installation at Paranal is planned within 2026. The integration phase will include full system verification, followed by on-sky commissioning and the development of polarimetric calibration procedures tailored to the temporally modulated single-beam implementation, and to its wide-field operations. VSTPOL is expected to open a new domain of survey-scale optical polarimetry, complementing photometric and spectroscopic facilities and providing the community with an unprecedented resource for probing astrophysical magnetic fields and emission mechanisms at large angular scales.

We report spectropolarimetric and broadband polarimetric observations of the near-Earth asteroid (NEA) 2025 FA22 during its close approach on 18 September 2025 (about two Moon distances). With a diameter estimated between 130 and 290 m, 2025 FA22 is among the largest NEAs observable at such proximity, prompting an International Asteroid Warning Network (IAWN) rapid-response campaign. Although early orbital solutions indicated a possible impact in 2089, further follow-up astrometric observations ruled out a collision hazard. The favourable geometry of this close encounter enabled dense coverage of the positive part of the phase-polarisation curve, from the high polarisation domain (high phase angles), to near the inversion angle where the linear polarisation fraction vanishes. The spectropolarimetric observations provided the wavelength dependence of the polarisation fraction. Using empirical relationships, an estimate of the geometric albedo could be drawn from the slope of the phase-polarisation curve at an inversion angle p _v = 0.17 ± 0.04 in the V band. This value, together with the spectropolarimetric trend, provides constraints on the taxonomic class, with the results being most consistent with an M (or Xc)-type classification. These results demonstrate the value of polarimetry and spectropolarimetry for the rapid characterisation of newly discovered NEAs in planetary defence campaigns.

Our understanding of dust in the diffuse interstellar medium remains incomplete with regard to the structure, composition, size distribution, and alignment properties of its grains. Joint observations of reddening, starlight polarisation spectra, and polarised dust emission for individual sightlines provide essential constraints on such properties. We studied a far-UV-selected sample of 96 reddening curves, for which optical linear polarisation spectra were obtained with FORS at the VLT as part of the Large Interstellar Polarisation Survey. Starlight polarisation spectra for 60 stars are presented in this work. These data are combined with Gaia distance estimates and Planck thermal dust emission. A three-component dust model is made publicly available. It consists of nanoparticles, amorphous grains, and micrometre-sized dust agglomerates, with varying axial ratios, porosities, sizes, element abundances, and alignment efficiencies that match observations. The diversity of reddening and polarisation spectra is well reproduced by prolate grains with typical axial ratios of two, a porosity of 10%, and high alignment efficiencies. Such efficiencies can be achieved with radiative torque alignment theory but not with imperfect Davis-Greenstein alignment, except when adjusting the magnetic-field orientation to maximise the polarisation. Micrometre-sized dust contributes wavelength-independent grey extinction in the optical and accounts for about one-third of the visual extinction and one-third of the dust mass. A follow-up sub-millimetre survey with high-resolution polarimetry will further constrain grain shapes and alignment physics.

In this paper, we present a description and characterisation of the 2-Channel Focal Reducer Rozhen (FoReRo2), with an emphasis on its polarimetric and spectropolarimetric observation modes. FoReRo2 is a multimode instrument mounted at the Ritchey-Chrétien (RC) focus of the 2-meter Ritchey-Chrétien-Coude (RCC) telescope at the Bulgarian National Astronomical Observatory (BNAO) - Rozhen. The primary targets of this instrument include novae, Be/X-ray binaries, symbiotic stars, asteroids, and comets. Standard stars with a high degree of polarisation are an essential part of the polarimetric observations. In our sample, HD 204827 shows shortterm variability in the position angle (PA), making it unsuitable for calibration. HD 183143 displays intrinsic polarisation variability, but remains stable in terms of the PA. For the first time, we present a statistical study of the K and λ _max parameters of Serkowski’s law, including the mean value, standard deviation, and distribution. To demonstrate FoReRo2's polarimetric capabilities, we present several examples. The recurrent nova RS Oph shows the variability of Serkowski’s K parameter, which is due to dust formation within the first few days after the 2021 outburst. These examples also include the imaging polarimetry of comet C/2019 Y4 (ATLAS) and spectropolarimetric observations of the symbiotic star Z And.

The small DAHe and DAe spectral classes comprise isolated, hydrogen-dominated atmosphere white dwarfs that exhibit variable photometric flux and Balmer line emission. These mysterious systems offer unique insight into the complex interplay between magnetic fields, stellar rotation and atmospheric activity in single white dwarfs. DAHe stars have detectable magnetic fields through Zeeman-split spectral lines, whereas DAe stars lack such splitting. We report the first discovery and characterisation of magnetism in the DAe white dwarf WD J165335.21−100116.33 with new time-resolved spectropolarimetry from FORS2. We detect a weak but variable longitudinal magnetic field with values 〈B_z〉 > −9.2 ± 2.4 kG and 〈B_z〉 < −2.2 ± 1.0 kG. Independent ZTF and ATLAS photometry reveal a consistent period of P = 80.3070 ± 0.0007 h. Time-resolved optical spectroscopy obtained with six ground-based instruments demonstrates strong modulation in the strength of the Hα and Hβ Balmer line emission with P = 80.2922 ± 0.0108 h. The photometric flux and Balmer emission strength vary in antiphase, with the strongest magnetic detections coinciding with phases of low photometric flux and strong line emission. These characteristics support the theory that a magnetically active, temperature-inverted spot/region is producing an optically thin chromospheric emission region. Comparison with other DAe and DAHe white dwarfs reveals all systems have a strikingly similar antiphase phenomenology, reinforcing the theory that they are subject to a unified physical mechanism. With the detection of a weak magnetic field, we reclassify WD J165335.21−100116.33 as a low-field DAHe white dwarf.

Earthshine observations offer a unique opportunity to study Earth as an exoplanet seen from the Moon. As the Sun-Earth-Moon geometry changes, Earth can be observed as a spatially unresolved exoplanet at different phase angles, providing important context for future observations of Earth-like exoplanets. Here, we present a catalog of Earthshine polarization spectra obtained with FORS2 on the VLT, covering diverse scenes, surface conditions, cloud properties, and weather patterns for over a decade. For the first time, we model this extensive dataset in detail using a homogeneous modeling framework. Previous efforts to model some of these spectra struggled to reproduce the observed polarization continuum, even with advanced 3D radiative transfer models incorporating satellite-derived surface and atmospheric data. We improve upon this with a state-of-the-art 3D model that includes subgrid cloud variability, wavelength-dependent surface albedo maps, and an accurate treatment of ocean glint. Our simulations successfully reproduce most observed spectra to a much higher precision than previously possible. Additionally, our statistical analysis reveals that the spectral slope in the visible can distinguish between ocean and mixed surfaces in both reflected and polarized light, which is not possible using broadband filters alone. Polarized light at large phase angles, beyond the Rayleigh scattering regime, is particularly effective in differentiating oceans from land, unlike reflected light. While the vegetation red edge (VRE) is more pronounced in reflectance, it remains detectable in polarization. We also identify correlations between cloud optical thickness and the polarized spectral slope, and between cloud cover and broadband B-R differences in reflected light, demonstrating the diagnostic power of these observations. This catalog and its modeling highlight the potential of polarization for characterizing Earth-like exoplanets. From polarization alone, we can infer oceans, vegetation, and an active water cycle, key indicators of a habitable planet.