Michael M. Shara

The disruption and accretion of planetary material onto white dwarfs are expected to be inherently dynamic and stochastic, potentially driving variability in the accretion rate and therefore the shape and depth of the photospheric metal absorption lines. This paper presents an 18 yr optical spectroscopic monitoring campaign of five warm (11,000–23,000 K), polluted white dwarfs with sinking timescales of days–months, observed using Magellan/MIKE and SALT/the High Resolution Spectrograph to directly test this prediction. At four of the five systems, no statistically significant variability is detected over baselines of 15–18 yr corresponding to hundreds to thousands of diffusion timescales, with inferred accretion rates stable to within 15%–30% (1 σ ) showing remarkably stable accretion on decadal timescales. This implies that either the processes maintaining the accretion of the disrupted planetary material are stable on the same timescales or currently uncharacterized photospheric processes act to smooth observable abundance variations on these timescales. The one exception, WD 0106−328, shows statistically significant variability in the 4481 Å Mg ii doublet from the ground-based data. Yet no significant equivalent width or abundance changes are seen between two Hubble Space Telescope ultraviolet spectra taken in 2016 and 2025, despite probing a larger set of transitions. This may imply that the ground-based observations witnessed a stochastic excursion from a stable baseline accretion rate, rather than a sustained change in the bulk accretion rate.

We present 120 s cadence TESS observations of three superoutbursts of the SU UMa-type dwarf nova VW Hydri. Two events (SO2 in Sectors 87+88 and SO3 in Sector 93) exhibit a temporally pronounced precursor dip followed by a rapid rise into the main superoutburst plateau. This morphology, previously seen in Kepler light curves of V1504 Cyg and V344 Lyr, is a key prediction of the thermal–tidal instability (TTI) model when a normal (precursor) outburst expands the disk only marginally beyond the 3:1 resonance radius, allowing the tidal instability to grow slowly and produce a deep dip approaching quiescence before rapid amplification drives the main superoutburst. A sliding-window time–frequency analysis reveals superhump power already during the decline and near minimum light, with a smooth period evolution across the dip and stabilization after the system returns to the hot state, consistent with the growth and saturation of disk eccentricity at the 3:1 resonance. From the stabilized Stage A superhump periods, we infer a representative mass ratio q = 0.131 ± 0.002. Combined with either a typical SU UMa white-dwarf mass prior or the semiempirical donor sequence at an orbital period of 107 minutes, the implied component masses are M ₁ ≃ 0.6–1.0 M _⊙ and M ₂ ≃ 0.08–0.14 M _⊙ , ruling out a brown dwarf donor and establishing VW Hyi as a benchmark system for testing tidal instability models in low- q dwarf novae.

FQ Cir was an ordinary fast He/N classical nova, peaking at V = 10.9. The pre-eruption and post-eruption counterpart was at V = 14.0, making this the smallest known classical nova amplitude, 3.1 mag. The nova light and the counterpart coincide at 0 . ″ 034, and the counterpart is a rare hot/blue emission-line star with flickering, so the identification of the quiescent nova is certain. The counterpart is a weak Be main-sequence star, B1 V(n)(e). A coherent photometric period appears in all four TESS Sectors and in the AAVSO post-eruption light curve, as ellipsoidal modulation with an orbital period of 2.041738 days. The companion must have been spun up to a fast rotation, and like all Be stars, a decretion disk is exuded. With the constraints of the blackbody radius and the main sequence, the companion mass is 13.0 − 0.5 + 0.2 M _⊙ , with radius 6.2 ± 0.2 R _⊙ . This is the discovery of a cataclysmic variable with a high-mass companion, a new class that we call “high-mass cataclysmic variables.” The white dwarf mass is 1.25 ± 0.10 M _⊙ and must have an accretion disk that supplies fuel for the nova eruption. FQ Cir represents a new mode of accretion in interacting binaries, with Roche lobe overflow from the decretion disk feeding mass into the usual accretion disk around the white dwarf, for disk-to-disk accretion. From the mass budget of the binary, the primary star must have its initial mass be >7.7 M _⊙ , forming an ONe white dwarf, so FQ Cir can never become a Type Ia supernova.

Recurrent novae undergo thermonuclear-powered eruptions separated by less than 100 yr, enabled by subgiant or red giant donors transferring hydrogen-rich matter at very high rates onto their massive white dwarf companions. The most rapidly moving parts of envelopes ejected in successive recurrent nova events are predicted to overtake and collide with the slowest ejecta of the previous eruption, leading to the buildup of vast (∼10–100 pc) superremnants surrounding all recurrent novae, but only three examples are currently known. We report deep narrowband imaging and spectroscopy, which have revealed a ∼70 pc diameter shell surrounding the frequently recurring nova RS Ophiuchi. We estimate the superremnant mass to be ∼20–200 M _⊙, expanding at a few tens of km/s, with an age of order 50–100 kyr. Its extremely low surface brightness and large angular size help explain the hitherto surprising absence of nova superremnants. Our results support the prediction that all recurrent novae are surrounded by similar extended structures.

Recurrent novae undergo thermonuclear-powered eruptions separated by less than 100 years, enabled by subgiant or red giant donors transferring hydrogen-rich matter at very high rates onto their massive white dwarf companions. The most-rapidly moving parts of envelopes ejected in successive recurrent nova events are predicted to overtake and collide with the slowest ejecta of the previous eruption, leading to the buildup of vast (~ 10 - 100 parsec) super-remnants surrounding all recurrent novae; but only three examples are currently known. We report deep narrowband imaging and spectroscopy which has revealed a ~ 70-parsec-diameter shell surrounding the frequently recurring nova RS Ophiuchi. We estimate the super-remnant mass to be ~ 20 - 200 solar masses, expanding at a few tens of km/s, with an age of order 50-100 kyr. Its extremely low surface brightness and large angular size help explain the hitherto surprising absence of nova super-remnants. Our results support the prediction that ALL recurrent novae are surrounded by similar extended structures.

We used the Condor Array Telescope to obtain deep imaging observations through luminance broadband and He ii, [O iii], He i, Hα, [N ii], and [S ii] narrowband filters of an extended region of the M81 Group spanning ≈8 × 8 deg² on the sky centered near M81 and M82. Here, we report aspects of these observations that are specifically related to (1) a remarkable filament known as the “Ursa Major Arc” that stretches ≈30° across the sky roughly in the direction of Ursa Major, (2) a “giant shell of ionized gas” that stretches ≈0.8 deg across the sky located ≈0.6 deg northwest of M82, and (3) a remarkable network of ionized gaseous filaments revealed by the new Condor observations that appear to connect the arc, the shell, and various galaxies of the M81 Group and, by extension, the group itself. We measure flux ratios between the various ions to help to distinguish photoionized from shock-ionized gas, and we find that the flux ratios of the arc and shell are not indicative of shock ionization. This provides strong evidence against a previous interpretation of the arc as an interstellar shock produced by an unrecognized supernova. We suggest that all of these objects, including the arc, are associated with the M81 Group and are located at roughly the distance (≈3.6 Mpc) of M81, that the arc is an intergalactic filament, and that the objects are associated with the low-redshift cosmic web.

Stellar collisions have garnered renewed attention for their role in the formation of peculiar objects, such as blue stragglers, and their potential to explain transients with atypical observational and spectroscopic signatures. Among these, white dwarf–main sequence (WD–MS) collisions are particularly intriguing due to the diverse evolutionary pathways they can produce – such as peculiar red giants, novae, or sub-Chandrasekhar supernovae. We present 3D smoothed particle hydrodynamics (SPH) simulations of WD–MS collisions, exploring a range of mass ratios and impact parameters. We analyze the dynamics, energetics, gas morphology, and mass-loss from these interactions. Using a 34-isotope nuclear network, we further predict the nucleosynthesis products generated during these collisions. Our models suggest that at early times the ejecta have a bipolar structure and, along with the stellar remnant, may be enriched in isotopes such as $^{13}$C, $^{15}$N, and $^{17}$O. In the case of near head-on collisions, the ejecta may also show an overabundance of $^{7}$Li relative to solar values.

We used the Condor Array Telescope to obtain deep imaging observations through the luminance broadband and He ii 468.6 nm, [O iii] 500.7 nm, He i 587.5 nm, Hα, [N ii] 658.4 nm, and [S ii] 671.6 nm narrowband filters of an extended region comprising 13 “Condor fields” spanning ≈ 8 × 8 deg² on the sky centered near M81 and M82. Here we describe the acquisition and processing of these observations, which together constitute unique very deep imaging observations of a large portion of the M81 Group through a complement of broad- and narrowband filters. The images are characterized by an intricate web of faint, diffuse, continuum produced by starlight scattered from Galactic cirrus, and all prominent cirrus features identified in the broadband image can also be identified in the narrowband images. We subtracted the luminance image from the narrowband images to leave, more or less, only line emission in the difference images, and we masked regions of the resulting images around stars at an isophotal limit. The difference images exhibit extensive extended structures of ionized gas in the direction of the M81 Group, from known galaxies of the M81 Group, clouds of gas, filamentary structures, and apparent or possible bubbles or shells. Specifically, the difference images show a remarkable filament known as the “Ursa Major Arc;” a remarkable network of criss-crossed filaments between M81 and NGC 2976, some of which intersect and overlap the Ursa Major Arc; and details of a “giant shell of ionized gas.”