Ulisse Munari

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.

In the course of selecting targets for a spectroscopic survey of faint planetary-nebula nuclei, we encountered two anomalous cases of late-type variable stars surrounded by nebulosities discovered by amateur astronomers. We obtained follow-up deep narrow-band images of the two nebulae: StDr Objet 14 (diameter ∼ 1 . ′ 5 ) and StDr 4 (diameter ∼ 2 . ′ 5). StDr Objet 14 is found to be a low-excitation nebula, bright only in H α +[N ii ], and showing a unique double-ring “bull’s-eye” morphology. StDr 4 exhibits a bow-shock structure, bright in H α and [S ii ], embedded in an extended emission nebula. Both central stars, respectively GR Cyg and [D75] 141, are found to be pulsating semiregular variable stars. Our spectroscopic observations show GR Cyg to be a cool carbon star of type C9,2, with emission lines of the Balmer series, [O ii ] and [O iii ], [Ne iii ], [S ii ], and Fe ii . A synthetic narrow-band image of GR Cyg shows that the source of its strong [O ii ] λ 3727 emission line is a knot displaced from the star by about 1 . ″ 6 , indicative of a recent collimated ejection event. [D75] 141 is a M6 III red giant, with weak Balmer-series emission. Both stars show a blue excess, and GR Cyg was detected in the near-UV by the GALEX spacecraft. These data establish both stars as new symbiotic binaries, containing a late-type red giant and a hot companion accreting from its stellar wind. We speculate that the double rings surrounding GR Cyg were ejected during helium-shell thermal pulses (or conceivably from recurring nova outbursts), while the nebula around [D75] 141 may be the result of a collision of the symbiotic star’s ejecta with an interstellar cloud. Our accidental discoveries of these two binaries because they are surrounded by faint nebulae supports the view that there is a significant population of “missing” symbiotic stars yet to be discovered.

T Coronae Borealis (T CrB, HD 143454) is a symbiotic recurrent nova consisting of a red giant and white dwarf with recent eruptions in 1866 and 1946 and an anticipated eruption in the mid-2020s. We report Center for High Angular Resolution Astronomy (or CHARA) Array observations collected with MIRC-X ( H band) and MYSTIC ( K band) in 2022–2025. We fit limb-darkened disk models constrained with literature limb-darkening coefficients to the squared visibilities as only the first visibility lobe is sampled. The average limb-darkened diameter of the star across these epochs is 0.70 ± 0.04 mas in the H band and 0.72 ± 0.07 mas in the K band. Adopting a distance of 91 4 − 22 + 24 pc, the corresponding stellar radius is 69 ± 5 R _⊙ in the H band and 71 ± 8 R _⊙ in the K band. These values are consistent with the Roche-lobe volume radius of 71 R _⊙ inferred from published orbital solutions. These measurements provide a preeruption angular diameter and support a Roche-lobe-filling donor.

Context. Among galactic novae, many remain poorly characterized due to heterogeneous data, observational challenges, or heavy reddening along the Galactic plane. A detailed characterization is particularly crucial for novae exhibiting peculiar features, such as those belonging to the hybrid class. Aims. We present the spectroscopic and photometric evolution of the heavily reddened Nova Vul 2024 (=V615 Vul) from discovery on 29 July 2024 to well into its nebular phase. Methods. We obtained daily optical, absolute-fluxed spectroscopy in both low- and high-resolution echelle modes, providing a detailed account of the spectral evolution. Photometric data were primarily sourced from AAVSO, while refined astrometry was performed to assess the positional coincidence with a potential progenitor. Robust determinations of t ₃ and E _{B  −  V} were achieved, allowing for the application of the Maximum Magnitude–Rate of Decline (MMRD) relation; the resulting distance is in excellent agreement with 3D Galactic extinction maps. Results. Nova Vul 2024 is a very fast nova ( t ₃ = 10.7 ± 0.5 d) located at a distance of 5.0 ± 1.0 kpc, suffering from a large reddening ( E _{B  −  V} = 1.6 ± 0.1). Around maximum light, it exhibited a Fe II-type spectrum with very broad emission lines (FWZI ≈ 5800 km s ⁻¹ ) and high-velocity P-Cygni absorptions. Following t ₃ , coinciding with the emergence of hard X-ray emission, the nova displayed pronounced photometric oscillations primarily driven by continuum variations. Simultaneously, He/N features developed alongside Fe II lines, classifying V615 Vul as a rare hybrid nova. During the nebular phase, the ionization level increased, reaching [Fe VII] and likely [Fe X]. The ejecta showed no neon overabundance and expanded ballistically, as indicated by the constant widths, profiles, and castellation of the high-resolution emission lines.

Context. R Aquarii (R Aqr) is a well-known symbiotic binary that attracted renewed interest during its recent periastron passage, an event that occurs only once every ∼40 years. This passage marks the first to be observed with modern, state-of-the-art instruments. Aims We investigated the inner, sub-arcsecond active region of R Aqr during this recent periastron passage, with the goal of gaining insight into the jet-launching mechanisms at work in this system. Methods. We analysed H α speckle interferometric images obtained one month apart using Fourier techniques. These are complemented by high-resolution optical spectra in the same emission line. Results. Our speckle imaging reveals a newborn two-sided jet orientated in the north–south direction. Its proper motion, 66 ± 19 mas yr ⁻¹ , confirms that it was launched around 2020 January 7, at the onset of the periastron passage. Further analysis of the elongated central structure reveals a knot in the southern counterpart of the jet moving away from the binary with μ = 27 ± 17 mas yr ⁻¹ at a position angle of 187° and an ejection time around 2019 October 28. This interpretation is further supported by our high-resolution spectroscopic data. In addition, we update the expansion parallax distance of R Aqr to 260 pc.

Aims . The nebular stage is that interval in the expansion of classical nova ejecta when the transparency of the matter permits a comprehensive view of the structure of the ejecta. We aim to study the radiative response of nova ejecta to illumination variations in the post-outburst white dwarf (WD). Methods . We used medium-resolution (R ≈ 10 ⁴ ) optical spectra, a substantial fraction of which were flux-calibrated. Time series were obtained for profile and flux variations supplemented by Swift X-ray Telescope (XRT) spectrophotometry. The ejecta structure and density stratification were modeled assuming nebular conditions with biconical geometries, and densities were derived using the [O III] lines and scaled for ejecta expansion. Results . Antiphase variations were observed for the C IV 5808 Å recombination line and coronal transitions of [Fe VII] and [Ca V] coincident with X-ray flaring activity of the central WD. In the earlier transition stage to nebular conditions, similar profile and flux variations were observed for [N II] 5755 Å, although no X-rays were detected during that time. We show that the profile variations map the density structure of the ejecta and are entirely due to changes in the hardness of the incident spectral distribution of the white dwarf. No dynamical explanation is required. Conclusions . Nebular stage line intensity and profile variations permit a comprehensive view of the development of the central white dwarf without recourse to other wavelength intervals, such as X-rays or the far-UV.

Since it began in 2022, the outburst of AT 2023txn continues to unfold via multiple maxima and deep minima, bringing the system well below quiescence brightness. It has been monitored with B V R I g r i photometry, low- and high-resolution spectroscopy, and Swift XRT+UVOT observations. AT 2023txn has turned out to be a previously unknown symbiotic star, harboring an M6III red giant of great photometric stability during quiescence. A large and cold accretion disk around the white dwarf companion provided about 3/4 of the system brightness at blue wavelengths during quiescence; however, it was unable to power a significant emission-line spectrum, as implied by the 2004 observations from the IPHAS r i H α survey and Gaia DR3 B _P R _P spectrum. The outburst begun toward the end of 2021, passed through a first unnoticed maximum in March 2022, and was eventually announced as transient Gaia23cse ∼19 months later (in September 2023) when the object was peaking at a second and brighter maximum. Close to that epoch, we measured 1500 L _⊙ as the luminosity radiated by the outbursting component over the 2000–9000 Å interval, with an upper limit of < 1 L _⊙ to the X-ray luminosity over the 0.3−10 keV range. The emission-line spectrum has been characterized by persistent low ionization conditions, with FeII and Balmer series being the dominating species and varying their integrated flux in phase with the brightness evolution during the outburst. Three deep minima separated by ∼866 days brought the system Δ g ∼ 1.5 mag below the brightness in quiescence, but they have no counterpart prior to the start of the outburst season. Interpreting them as eclipses of the hot component by the red giant encounters severe difficulties; thus, alternative explanations have been considered within the framework of a massive accretion disk similar to T CrB transferring mass to the WD only episodically via a collapse, which affects the inner radii.

Context. Expectations for an imminent new outburst of the recurrent symbiotic nova T CrB are mounting, initiated by the discovery in 2015 of a new enhanced mass-transfer phase (SAP), which is reminiscent of the one preceding the last recorded outburst in 1946. Aims. We aim to derive a robust estimate of the most important parameters describing the physical nature of T CrB, trace the accretion history onto its white dwarf, and account for the unexpected delay in the occurrence of the new outburst. In particular, the SAP prior to 1946 was brighter and followed by a nova eruption within six months from its conclusion. This time the 2015–2023 SAP has been fainter and although two years have passed since the end of this phase, no new eruption has taken place. Methods. Between 2005–2025, the period covering the SAP and the preceding quiescence, we collected a massive amount of photometric and spectroscopic observations at optical wavelengths. We analyzed these data together with the abundant ultraviolet (UV) observations available in the archive of the Swift satellite. Results. Guided by the results of the orbital solution and, in particular, by the radiative modeling process we employed on the whole set of available data, we derived for T CrB a binary period of 227.5528 days, along with an inclination of 61° and masses of 1.35 M_⊙ and 0.93 M_⊙ for the white dwarf and the M3III companions, respectively, making the mass transfer dynamically stable. The red giant completely fills its Roche lobe and at V_rot sin i = 4.75 ± 0.26 km s⁻¹, it is rotating much more slowly than the 16 km s⁻¹ co-rotation value. The ∼20° azimuth of the hot spot, implied by the hump shaping the optical light curve in quiescence, fixes the outer radius of the disk to ∼58 R_⊙. This is the same as the canonical value expected from disk theory. In quiescence, the disk is cold and mostly neutral. The SAP was caused by an inside-out collapse of the disk, during which the mean accretion rate onto the WD was ∼28× larger than in quiescence. The SAP ended in late April 2023, but from May 2024, the mass flow has intensively resumed at disk inner radii, while the collapse wave reached the outer portions of the disk. The consequent revamp in the mass accretion could fill the gap inherited by the fainter 2015–2023 SAP and eventually lead the WD accreted shell to ignition.

(abridged) We aim to derive a robust estimate of the most important parameters describing the physical nature of T CrB, trace the accretion history onto its white dwarf, and account for the unexpected delay in the occurrence of the new outburst: the SAP prior to 1946 was brighter, and it was followed by the nova eruption within 6 months from its conclusion. This time the 2015-2023 SAP has been fainter and two years past its conclusion no new eruption has yet taken place. During 2005-2025, a period covering SAP and the preceding quiescence, we collected a massive amount of photometric and spectroscopic observations that we have analyzed together with Swift UVOT data. Guided by the results of the orbital solution and in particular by the radiative modeling to which we subjected the whole set of available data, we found for T CrB a binary period of 227.5528 days, an inclination of 61 deg, and masses of 1.35 Msun and 0.93 Msun for the white dwarf and the M3III companion, respectively, making mass transfer dynamically stable. The red giant fills completely its Roche lobe, and at Vrot sin(i)=4.75 +-0.26 km/s it is rotating much slower that the 16 km/s co-rotation value. The ~20 deg azimuth of the hot spot, implied by the hump shaping the optical light curve in quiescence, fixes the outer radius of the disk to 58 Rsun, the same as the canonical value expected from disk theory. In quiescence the disk is cold and mostly neutral. SAP has been caused by an inside-out collapse of the disk, during which the mean accretion rate onto the WD has been ~28x larger than in quiescence. SAP ended in April 2023, but from May 2024 mass-flow has intensively resumed at disk inner radii while the collapse wave reached the outer portions of the disk; the consequent revamp in mass accretion could fill the gap inherited by the fainter 2015-2023 SAP and eventually lead the WD accreted shell to ignition.