arXiv subjects

Ulisse Munari

Publications and source records attributed to Ulisse Munari.

At least 19 recordsLinked to original sources

arXiv2026

Ionized Nebulae Around Two New Symbiotic Stars: GR Cygni and [D75] 141

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 arcmin) and StDr 4 (diameter 2.5 arcmin). StDr Objet 14 is found to be a low-excitation nebula, bright only in Ha+[N II], and showing a unique double-ring "bull's-eye" morphology. StDr 4 exhibits a bow-shock structure, bright in Halpha and [S II], embedded in an extended emission nebula. Both central stars, respectively GR Cygni 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] 3727 A emission line is a knot displaced from the star by about 1.6 arcsec, 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.

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arXiv2026

The quiescent states of V745 Sco and V3890 Sgr: VLT/X-shooter and Swift/XRT+UVOT observations

Symbiotic recurrent novae are symbiotic binaries in which multiple thermonuclear outbursts have been observed. Only four such systems are currently known in the Galaxy: T CrB, RS Oph, V745 Sco, and V3890 Sgr. They provide convenient laboratories for studying accretion and binary interaction in symbiotic novae during quiescence. We characterise the quiescent states of the less-studied systems V745 Sco and V3890 Sgr and compare their properties with those of T CrB and RS Oph. We analyse VLT/X-shooter spectra together with Swift/XRT and UVOT observations, supplemented by ground-based optical photometry and archival data. We determine the spectral types of the red-giant donors and compare the spectral energy distributions of the systems with those of single bulge giants of the same spectral type to investigate the properties of the accretion. We classify the donor stars in V745 Sco and V3890 Sgr as M7III and M6.5III, respectively. We derive reddenings and distances consistent with both systems belonging to the Galactic bulge. Both objects exhibit a significant ultraviolet excess relative to normal late-type giants, indicating ongoing accretion during quiescence. From this excess we estimate mass transfer rates of the order of $\sim 5 \times 10^{-8}\ M_{\odot}\ \mathrm{yr}^{-1}$, broadly consistent with the observed recurrence timescales. These results suggest that recurrent nova eruptions in V745 Sco and V3890 Sgr can be sustained by long-term accretion at approximately the present rate, without requiring an episode of enhanced accretion similar to that observed in T CrB.

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arXiv2026

Probing Physical Conditions in Classical and Symbiotic Novae with the Square Kilometre Array Observatory

Cataclysmic variables and symbiotic stars are interacting binary systems in which a hot white dwarf (WD) orbits a companion main-sequence or red giant star, respectively. Accumulation of hydrogen-rich material on the WD surface may trigger re-ignition of thermonuclear reactions that, under degenerate conditions, lead to an explosive ejection of the accreted layer mixed with WD material. These explosions, known as classical novae, provide opportunities to study key astrophysical processes such as binary evolution, accretion, ionisation of circumstellar material, mass ejection, jet formation, and thermonuclear burning. Radio emission in these systems arises from both thermal and non-thermal processes, which manifest differently in classical and symbiotic novae. $γ$-ray emission has also been detected in several cases, and recent progress, driven by coordinated radio and multiwavelength observations, has greatly advanced our understanding of both types of novae. Multi-frequency, multi-epoch, and multi-scale interferometric observations are powerful probes of the evolving physical conditions following thermonuclear explosions, revealing information from both ionised and relativistic particle populations. The SKAO, particularly its SKA-Mid component, will enable regular monitoring of Galactic novae, multiple times per year for classical novae and every few years for symbiotic systems. It will explore a wide range of conditions, including companion types, WD masses, accretion regimes, and surrounding environments. The VLBI capabilities of the SKAO will target compact shocked regions, while its exceptional sensitivity will also allow characterisation of emission during quiescent phases of the binaries.

astro-ph.HE↗

arXiv2026

Pre-nova Observations of T CrB: A view from the CHARA Array

T CrB 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 CHARA Array observations obtained 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 $0.70\pm0.04$ mas in H-band and $0.72\pm0.07$ mas in K-band. Adopting a distance of $914^{+24}_{-22}$ pc, the stellar radius is $69\pm5~R_{\odot}$ in H-band and $71\pm8~R_{\odot}$ in K-band. This is consistent with filling a Roche lobe volume radius of $71~R_{\odot}$ inferred from published orbital solutions. These measurements provide a pre-eruption angular diameter and support a Roche lobe filling donor.

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arXiv2026

The hybrid Nova Vul 2024 (=V615 Vul)

We present daily optical, absolute-fluxed spectroscopy of the heavily reddened Nova Vul 2024 (=V615 Vul), obtained in both low- and high-resolution echelle modes from discovery on 29 July 2024 through the nebular phase. Refined astrometry confirms the positional consistency with a potential progenitor. Reliable estimates of the decline time (t3=10.7 +/- 0.5 d) and reddening (E(B-V)=1.6 +/- 0.1) enabled the application of the Maximum Magnitude - Rate of Decline relation, yielding a distance of 5.0 +/- 1.0 kpc in excellent agreement with 3D Galactic extinction maps. Around maximum light, the nova displayed a Fe II-type spectrum with very broad emission lines (FWZI 5800 km s-1) and high-velocity P-Cygni absorptions. Shortly after t3, coincident with the onset of hard X-ray emission, it showed pronounced photometric oscillations driven mainly by continuum variations. At the same time, He/N features developed alongside Fe II lines, identifying V615 Vul as a rare hybrid nova. During the nebular phase, the ionization level increased up to [Fe VII] and likely [Fe X]. The ejecta show no evidence of neon overabundance and expand ballistically, as indicated by the constant line widths, stable profiles, and characteristic castellation of the high-resolution emission lines.

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arXiv2026

A Detailed Study of the Physical and Orbital Characteristics, and Eclipse Timing Variations of the Post Common Envelope Binary DD CrB

We present an in-depth analysis of the eclipsing binary DD CrB, composed of a B-type subdwarf primary and an M-type main-sequence secondary, with the main goal of investigating its eclipse timing variations (ETVs). Our new multi-color photometric observations, radial velocity measurements, and precise eclipse timings from TESS allow us to constrain the system parameters. The Romer delay between primary and secondary minima yields a mass ratio of $q = 0.299 \pm 0.009$, enabling robust simultaneous modeling of the light and radial velocity curves with {\sc phoebe} 2.17. By fixing the albedo of the secondary to its maximum physically plausible value (A$_2 = 1.0$), despite the degeneracy between albedo, surface temperature, and radius, we obtained a satisfactory fit, resulting in a significantly lower temperature ($T_2 \sim 2360$ K) and a radius ($R_2 \sim 0.16$ R$_\odot$) in agreement with literature values. Using the total mass of the components and the orbital size derived from this modeling, we interpret the ETVs and find them best explained by a Jupiter-mass tertiary companion on a $\sim13$-year orbit in all competing models, while the eccentric (e $\sim0.46$) models perform better in terms of fit statistics.

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arXiv2025

TESS photometry of the nova eruption in V606 Vul: asymmetric photosphere and multiple ejections?

Lightcurves of many classical novae deviate from the canonical "fast rise - smooth decline" pattern and display complex variability behavior. We present the first TESS-space-photometry-based investigation of this phenomenon. We use Sector 41 full-frame images to extract a lightcurve of the slow Galactic nova V606 Vul that erupted nine days prior to the start of the TESS observations. The lightcurve covers the first of two major peaks of V606 Vul that was reached 19 days after the start of the eruption. The nova reached its brightest visual magnitude V=9.9 in its second peak 64 days after the eruption onset, following the completion of Sector 41 observations. To increase the confidence level of the extracted lightcurve, we performed the analysis using four different codes implementing the aperture photometry (Lightkurve, VaST) and image subtraction (TESSreduce, tequila_shots) and find good agreement between them. We performed ground-based photometric and spectroscopic monitoring to complement the TESS data. The TESS lightcurve reveals two features: periodic variations (0.12771 d, 0.01 mag average peak-to-peak amplitude) that disappeared when the source was within 1 mag of peak optical brightness and a series of isolated mini-flares (with peak-to-peak amplitudes of up to 0.5 mag) appearing at seemingly random times. We interpret the periodic variations as the result of azimuthal asymmetry of the photosphere engulfing the nova-hosting binary that was distorted by and rotating with the binary. Whereas we use spectra to associate the two major peaks in the nova lightcurve with distinct episodes of mass ejection, the origin of mini-flares remains elusive.

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arXiv2024

Symbiotic novae

(Invited Review) According to modern definition, a symbiotic nova is an otherwise normal nova (i.e. powered by explosive thermonuclear burning) that erupts within a symbiotic star, which is a binary where a WD accretes from a cool giant companion. Guided primarily by the very well observed eruptions of RS Oph in 2006 and 2021, and that of V407 Cyg in 2010, we investigate the main multi-wavelength properties of symbiotic novae and their relation to classical novae, and propose a 3D model structure that identifies the emitting source location for hard and supersoft X-rays, radio syncrothron and thermal, permitted and forbidden emission lines. Very few symbiotic novae are known in the Galaxy, and we compile a revised catalog based on firm astrometric identification. The exciting prospect of an imminent new outburst of T CrB is also discussed.

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arXiv2024

The symbiotic star V694 Mon has transitioned from the accreting-only state to the steady burning phase

We propose that the drastic photometric and spectroscopic changes affecting the symbiotic star V694 Mon since 2018, are due to its transition from the accreting-only state to steady hydrogen-burning on the surface of the white dwarf, closely mimicking the pattern followed by V4368 Sgr. The phase of peak optical brightness and weakest emission lines has probably been reached in early 2024. The high-velocity absorptions powered by jet-ejection and the wild flickering, which dominated the century-long quiescence, should not reappear as long has nuclear burning will hold (time-scale of decades). The 3500 Lsun burning luminosity suggests a mass of 0.60 Msun for the WD.

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arXiv2023

The secondary maximum of T CrB caused by irradiation of the red giant by a cooling white dwarf

Both the 1866 and 1946 outbursts of the recurrent symbiotic nova T CrB have displayed a mysterious secondary maximum peaking in brightness ~5 months past the primary one. Common to all previous modeling attempts was the rejection of plain irradiation of the red giant (RG), on the basis that the secondary maximum of T CrB would have been out of phase with the transit at superior conjunction of the RG. Implicit to this line of reasoning is the assumption of a constant temperature for the white dwarf (WD) irradiating the red giant. I show by radiative modeling that irradiation of the RG by a cooling WD nicely reproduces the photometric evolution of the secondary maximum, both in terms of brightness and color, removes the phasing offset, and provides a straightforward explanation that will be easy to test at the next and imminent outburst.

astro-ph.SR↗

arXiv2023

The GALAH survey: New diffuse interstellar bands found in residuals of 872,000 stellar spectra

We use more than 872,000 mid-to-high resolution (R $\sim$ 20,000) spectra of stars from the GALAH survey to discern the spectra of diffuse interstellar bands (DIBs). We use four windows with the wavelength range from 4718 to 4903, 5649 to 5873, 6481 to 6739, and 7590 to 7890 Å, giving a total coverage of 967 Å. We produce $\sim$400,000 spectra of interstellar medium (ISM) absorption features and correct them for radial velocities of the DIB clouds. Ultimately, we combine the 33,115 best ISM spectra into six reddening bins with a range of $0.1 \,\mathrm{mag} < E\mathrm{(B-V)} < 0.7\, \mathrm{mag}$. A total of 183 absorption features in these spectra qualify as DIBs, their fitted model parameters are summarized in a detailed catalogue. From these, 64 are not reported in the literature, among these 17 are certain, 14 are probable and 33 are possible. We find that the broad DIBs can be fitted with a multitude of narrower DIBs. Finally, we create a synthetic DIB spectrum at unit reddening which should allow us to narrow down the possible carriers of DIBs and explore the composition of the ISM and ultimately better model dust and star formation as well as to correct Galactic and extragalactic observations. The majority of certain DIBs show a significant excess of equivalent width when compared to reddening. We explain this with observed lines of sight penetrating more uniform DIB clouds compared to clumpy dust clouds.

astro-ph.GA↗

arXiv2022

V5856 Sagittarii/2016: Broad Multi-Epoch Spectral Coverage of a Sustained High Luminosity Nova

Nova V5856 Sagittarii is unique for having remained more than nine magnitudes above its pre-outburst brightness for more than six years. Extensive visible and IR spectra from the time of outburst to the present epoch reveal separate emitting regions with distinct spectral characteristics. Permitted emission lines have both broad and narrow components, whereas the forbidden line profiles are almost entirely broad. The permitted line components frequently display P Cygni profiles indicating high optical depth, whereas the broad components do not show detectable absorption. The densities and velocities deduced from the spectra, including differences in the O I 7773 and 8446 lines, are not consistent with an on-going wind. Instead, the prolonged high luminosity and spectral characteristics are indicative of a post-outburst common envelope that enshrouds the binary, and is likely the primary source of the visible and IR emission.

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arXiv2022

Flickering returns as RS Oph reestablishes quiescent conditions following its 2021 nova outburst

RS Oph has persistently displayed flickering at optical wavelengths when observed away from its repeating nova outbursts. During the 2006 eruption the flickering disappeared, and this repeated during the recent 2021 event. We have been monitoring RS Oph looking for the reappearance of flickering at B-band following the 2021 outburst. The flickering was still absent (sigma(B)<0.002 mag) on day +210 (counted from nova optical maximum), appeared at sigma(B)=0.008 mag on day +224, and raised to sigma(B)=0.029 mag on day +250. On following dates the amplitude remained large, although fluctuating. The recovery of B-band quiescence brightness by RS Oph begun around day +225 and was completed by day +260. The parallel patterns followed by the rise in system brightness and the reappearance of flickering confirm the central role played in RS Oph by the return to pre-outburst conditions of the accretion disk and the refilling by the RG wind of the immediate circumstellar space.

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arXiv2022

The 2021 outburst of RS Oph: a pictorial atlas of the spectroscopic evolution. II. From day 19 to 102 (solar conjunction)

We present the second part (days 19 to 102) of our high-resolution Echelle atlas of the spectral evolution of RS Oph during the 2021 nova outburst. Some quick conclusions that can be drawn are: (1) the spectra evolution has progressed smoothly, suggesting that both the expansion of the ejecta through the pre-existing RG wind and the photo-ionization from the central source did not encountered sudden discontinuities prior to switch-off of the nuclear burning; (2) the 2021 spectral evolution is similar to that of the 2006 eruption, with a mirror-appearance of the triple-peaked line profiles (blue peak brighter than red in 2006, the opposite in 2021); (3) the evolution of the triple-peaked profiles progressed similarly through different excitation and ionization degrees, suggesting an origin in a bipolar outflow nested to an equatorial torus; (4) the time-behavior of coronal emission lines appears quite smooth, both in terms of radiated flux as well as evolution of the triple-peaked profiles, with a plateau at maximum lasting up to about day 87; (5) there seems to exist within RS Oph two distinct types of ejecta: one fast moving and producing Gaussian-like line profiles (primarely [NII] and [OIII]), that keep expanding at FWHM=1000 km/s with no sign of ongoing deceleration during the two months leading up to Solar conjunction, and slower moving ejecta giving rise to the triple-peaked line profiles (HeI, HeII, coronal lines, and also Balmer lines at later times), for which the separation in velocity of blue and red peaks is still shrinking at the time of Solar conjunction when it reaches an average of 330 km/s; (6) the "symbiotic band" at 6825 Ang, due to Raman scattering of OVI 1032 by neutral hydrogen, appears simultaneously with the coronal lines: its very presence indicates that a sizeable part of the wind of the red giant has remained neutral during the development of coronal lines.

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arXiv2021

Optical and near-infrared spectroscopy of Nova V2891 Cygni: evidence for shock-induced dust formation

We present multi-epoch optical and near-infrared observations of the highly reddened, \pion{Fe}{ii} class slow nova V2891 Cygni. The observations span 15 months since its discovery. The initial rapid brightening from quiescence, and the presence of a $\sim$35 day long pre-maximum halt, is well documented. The evidence that the current outburst of V2891 Cyg has undergone several distinct episodes of mass ejection is seen through time-varying P Cygni profiles of the O\,{\sc i} 7773\,$Å$ line. A highlight is the occurrence of a dust formation event centred around $\sim$+273d, which coincides with a phase of coronal line emission. The dust mass is found to be $\sim0.83-1.25 \times 10^{-10} M_{\odot}$. There is strong evidence to suggest that the coronal lines are created by shock heating rather than by photoionization. The simultaneous occurrence of the dust and coronal lines (with varying velocity shifts) supports the possibility that dust formation is shock-induced. Such a route for dust formation has not previously been seen in a nova, although the mechanism has been proposed for dust formation in some core-collapse supernovae. Analysis of the coronal lines indicates a gas mass and temperature of 8.35--8.42$\times10^{-7}$ M$_\odot$ and $\sim(4.8-9.1)\times10^{5}$~K respectively, and an overabundance of aluminium and silicon. A Case B analysis of the hydrogen lines yields a mass of the ionized gas of ($8.60\pm1.73)\times10^{-5}$ M$_{\odot}$. The reddening and distance to the nova are estimated to be $E(B-V)$ = 2.21$\pm$0.15 and $d$ = 5.50 kpc respectively.

astro-ph.SR↗

arXiv2021

The GALAH Survey: Improving our understanding of confirmed and candidate planetary systems with large stellar surveys

Pioneering photometric, astrometric, and spectroscopic surveys are helping exoplanetary scientists better constrain the fundamental properties of stars within our galaxy, and the planets these stars host. In this study, we use the third data release from the stellar spectroscopic GALAH Survey, coupled with astrometric data of eDR3 from the \textit{Gaia} satellite, and other data from NASA's Exoplanet Archive, to refine our understanding of 279 confirmed and candidate exoplanet host stars and their exoplanets. This homogenously analysed data set comprises 105 confirmed exoplanets, along with 146 K2 candidates, 95 TESS Objects of Interest (TOIs) and 52 Community TOIs (CTOIs). Our analysis significantly shifts several previously (unknown) planet parameters while decreasing the uncertainties for others; Our radius estimates suggest that 35 planet candidates are more likely brown dwarfs or stellar companions due to their new radius values. We are able to refine the radii and masses of WASP-47 e, K2-106 b, and CoRoT-7 b to their most precise values yet, to less than 2.3\% and 8.5\% respectively. We also use stellar rotational values from GALAH to show that most planet candidates will have mass measurements that will be tough to obtain with current ground-based spectrographs. With GALAH's chemical abundances, we show through chemo-kinematics that there are five planet-hosts that are associated with the galaxy's thick disc, including NGTS-4, K2-183 and K2-337. Finally, we show there is no statistical difference between the chemical properties of hot Neptune and hot rocky exoplanet hosts, with the possibility that short-period rocky worlds might be the remnant cores of hotter, gaseous worlds.

astro-ph.EP↗

arXiv2021

Non-local thermodynamic equilibrium spectral analysis of five hot, hydrogen-deficient pre-white dwarfs

Hot, compact, hydrogen-deficient pre-white dwarfs (pre-WDs) with effective temperatures of Teff > 70,000 K and a surface gravity of 5.0 < log g < 7.0 are rather rare objects despite recent and ongoing surveys. It is believed that they are the outcome of either single star evolution (late helium-shell flash or late helium-core flash) or binary star evolution (double WD merger). Their study is interesting because the surface elemental abundances reflect the physics of thermonuclear flashes and merger events. Spectroscopically they are divided in three different classes, namely PG1159, O(He), or He-sdO. We present a spectroscopic analysis of five such stars that turned out to have atmospheric parameters in the range Teff = 70,000-80,000 K and log g = 5.2-6.3. The three investigated He-sdOs have a relatively high hydrogen mass fraction (10%) that is unexplained by both single (He core flash) and binary evolution (He-WD merger) scenarios. The O(He) star JL9 is probably a binary helium-WD merger, but its hydrogen content (6%) is also at odds with merger models. We found that RL 104 is the 'coolest' (Teff = 80,000 K) member of the PG1159 class in a pre-WD stage. Its optical spectrum is remarkable because it exhibits C IV lines involving Rydberg states with principal quantum numbers up to n = 22. Its rather low mass (0.48 +0.03/-0.02 Msun) is difficult to reconcile with the common evolutionary scenario for PG1159 stars due to it being the outcome of a (very) late He-shell flash. The same mass-problem faces a merger model of a close He-sdO plus CO WD binary that predicts PG1159-like abundances. Perhaps RL 104 originates from a very late He-shell flash in a CO/He WD formed by a merger of two low-mass He-WDs.

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arXiv2020

Fermi-LAT Observations of V549 Vel 2017: a Sub-Luminous Gamma-Ray Nova?

We report on the Fermi-LAT detection (with ~5.7 sigma significance) as well as the multi-wavelength analysis of the 2017 nova eruption V549 Vel. Unlike the recent shock-powered novae ASASSN-16ma and V906 Car, the optical and gamma-ray light curves of V549 Vel show no correlation, likely implying relatively weak shocks in the eruption. Gaia detected a candidate progenitor of V549 Vel and found a parallax measurement of 1.91+/-0.39 mas, equivalent to a mode distance of d=560 pc (90% credible interval of 380-1050 pc). The progenitor was also observed by the 2MASS and WISE surveys. When adopting the Gaia distance, the spectral energy distribution of the progenitor is close to that of a G-type star. Swift/XRT detected the supersoft X-ray emission of the nova (kT=30-40 keV) since day 236, and the inferred blackbody size is comparable to that of other novae assuming d=560 pc (i.e., R_bb~5*10^8 cm). However, there is also an unknown astrometric excess noise of 3.2 mas found in the Gaia data, and the inferred distance becomes controversial. If the Gaia distance is accurate, the gamma-ray luminosity of V549 Vel will be as low as L_gamma~4*10^33 erg/s, making it the least luminous gamma-ray nova known so far. This may imply that the shock properties responsible for the gamma-ray emission in V549 Vel are different from those of the more luminous events. If the nova is located farther away, it is likely a symbiotic system with a giant companion as the observed progenitor.

astro-ph.HE↗