FQ Circini: An Ordinary Nova with a High-mass B1 V(n)(e) Companion Whose Decretion Disk Transfers Mass to the White Dwarf via Roche-Lobe Overflow

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 the smallest known classical nova amplitude of 3.1 mag. The nova light and the counterpart coincide to 0.034 arc-seconds, 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 {\it TESS} Sectors and in the AAVSO post-eruption light curve, as ellipsoidal modulation with orbital period 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$^{+0.2}{-0.5}$ $M{\odot}$, with radius 6.2$\pm$0.2 $R_{\odot}$. 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$\pm$0.10 $M_{\odot}$ 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 $>$7.6 $M_{\odot}$, forming an ONe white dwarf, so FQ Cir can never become a Type Ia supernova.

💡 Research Summary

The authors present a comprehensive observational study of the 2022 nova FQ Circini (FQ Cir) and argue that it represents the first confirmed example of a “high‑mass cataclysmic variable” (HMCV), a new class of interacting binary in which a massive, early‑type companion feeds a white dwarf (WD) via a novel disk‑to‑disk accretion channel. The nova was discovered on 2022 June 25 at an unfiltered magnitude of ≈10.7, peaked at V = 10.9, and faded extremely rapidly (t₂ ≈ 2 d, t₃ ≤ 11 d). Because the quiescent counterpart is a bright B‑type star (V ≈ 14.0), the overall eruption amplitude is only 3.1 mag—the smallest ever recorded for a classical nova. Precise astrometry (coincidence within 0.034″) and the presence of flickering and emission lines confirm that the B‑star is the true nova progenitor.

Long‑term photometry from DASCH (1894–1989), ASAS, ASAS‑SN, APASS, AAVSO, and four TESS sectors (S12, 38, 39, 65) reveals several key features: (1) a secular dimming of ~0.5 mag over ~8 yr, (2) month‑scale flares of 0.1–0.2 mag, and (3) a coherent 2.041738‑day modulation present in all TESS data and in the post‑eruption AAVSO light curve. The modulation is interpreted as ellipsoidal variability, fixing the orbital period at 2.041738 d.

Spectroscopy during eruption shows a fast He/N nova with broad Balmer, He I, and O I emission; H α has a full width at zero intensity of 10 000–14 600 km s⁻¹ and an FWHM of ≈2600 km s⁻¹, confirming a normal classical nova rather than a peculiar transient. The quiescent spectrum identifies the companion as a weak Be star, spectral type B1 V(n)(e). The “(n)” denotes rapid rotation, while “(e)” indicates emission lines and a circumstellar decretion disk.

Using the observed blackbody radius, the B‑star’s color and luminosity, the authors derive a companion mass of 13.0 +0.2/‑0.5 M☉ and a radius of 6.2 ± 0.2 R☉. The WD mass, inferred from the nova decline rate and ejecta velocity, is 1.25 ± 0.10 M☉. The mass ratio (q ≈ 0.1) is far larger than in any known cataclysmic variable, placing the system in a regime more akin to high‑mass X‑ray binaries (HMXBs).

The central novelty is the proposed accretion geometry: the rapidly rotating Be star sustains a decretion disk that extends to the Roche‑lobe surface. Material from this disk overflows the Roche lobe and feeds the WD’s accretion disk—a “disk‑to‑disk” mass transfer. This differs from the classic CV picture where the donor star’s photosphere directly overflows the Roche lobe. The authors argue that this configuration can sustain the high accretion rate needed for the observed fast nova, while also explaining the modest eruption amplitude (the bright B‑star dominates the system’s light).

Evolutionary considerations suggest the primary star’s initial mass exceeded 7.6 M☉, implying the WD is an ONe (oxygen‑neon) composition rather than a CO white dwarf. Consequently, the system cannot evolve into a Type Ia supernova, despite the massive WD. The paper also references V Sge as an “intermediate‑mass cataclysmic variable” (IMCV) and notes a contemporaneous study (Chamoli et al. 2025) that identifies a similar Be‑WD binary in M31, reinforcing the plausibility of this new class.

Critical appraisal highlights several uncertainties: the mass and radius estimates rely on a simple blackbody model that may not fully account for the Be star’s emission‑line contribution and disk veiling; the lack of radial‑velocity curves prevents a direct dynamical mass measurement; and the geometry of the decretion disk relative to the Roche lobe remains inferred rather than directly observed. High‑resolution spectroscopy, interferometric imaging of the disk, and long‑baseline monitoring of the orbital motion are recommended to confirm the disk‑to‑disk transfer scenario.

In summary, FQ Cir is presented as the first confirmed high‑mass cataclysmic variable, featuring a 13 M☉ Be companion, a 1.25 M☉ ONe white dwarf, a 2.04‑day orbit, and a novel mass‑transfer mode. This discovery bridges the gap between classical cataclysmic variables and high‑mass X‑ray binaries, opening a new avenue for studying binary evolution, mass transfer physics, and nova mechanisms in systems with massive, early‑type donors.