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A Deep Precursor-Dip-Main Superoutburst Sequence in VW Hydri Observed with TESS: High-Cadence Constraints on the Thermal-Tidal Instability Model

Bisi Bernard Ogunwale, Shara Michael, Avi Shporer, Dafne Guetta, Lev Tal-Or

TL;DR

VW Hyi is a prototypical SU UMa dwarf nova; the paper uses 120 s cadence TESS photometry to resolve a deep precursor-dip sequence predicted by the thermal-tidal instability (TTI) model and to measure Stage-A superhumps for a dynamical mass-ratio constraint. They find $q ≈ 0.131$ and component masses $M_1 ≈ 0.6$–$1.0$ Msun and $M_2 ≈ 0.08$–$0.14$ Msun, ruling out a brown-dwarf donor and confirming VW Hyi as a benchmark for tidal-instability tests in low-q DNe. The disk reaches the 3:1 resonance during outburst, with observed superhump evolution showing growth during the dip and smooth period evolution consistent with the growth and saturation of disk eccentricity. The study demonstrates how high-cadence space-based photometry can tightly constrain TTI physics and disk dynamics, motivating future multiwavelength campaigns to map inner-outer disk coupling.

Abstract

We present 120s 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 pronounced, 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 $q = 0.131 \pm 0.002$. Combined with either a typical SU UMa white-dwarf mass prior or the semi-empirical donor sequence at an orbital period of 107~min, the implied component masses are $M_1 \simeq 0.6$--$1.0\,M_\odot$ and $M_2 \simeq 0.08$--$0.14\,M_\odot$, ruling out a brown-dwarf donor and establishing VW~Hyi as a benchmark system for testing tidal-instability models in low-$q$ dwarf novae.

A Deep Precursor-Dip-Main Superoutburst Sequence in VW Hydri Observed with TESS: High-Cadence Constraints on the Thermal-Tidal Instability Model

TL;DR

VW Hyi is a prototypical SU UMa dwarf nova; the paper uses 120 s cadence TESS photometry to resolve a deep precursor-dip sequence predicted by the thermal-tidal instability (TTI) model and to measure Stage-A superhumps for a dynamical mass-ratio constraint. They find and component masses – Msun and – Msun, ruling out a brown-dwarf donor and confirming VW Hyi as a benchmark for tidal-instability tests in low-q DNe. The disk reaches the 3:1 resonance during outburst, with observed superhump evolution showing growth during the dip and smooth period evolution consistent with the growth and saturation of disk eccentricity. The study demonstrates how high-cadence space-based photometry can tightly constrain TTI physics and disk dynamics, motivating future multiwavelength campaigns to map inner-outer disk coupling.

Abstract

We present 120s 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 pronounced, 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 . Combined with either a typical SU UMa white-dwarf mass prior or the semi-empirical donor sequence at an orbital period of 107~min, the implied component masses are -- and --, ruling out a brown-dwarf donor and establishing VW~Hyi as a benchmark system for testing tidal-instability models in low- dwarf novae.
Paper Structure (14 sections, 14 equations, 8 figures, 2 tables)

This paper contains 14 sections, 14 equations, 8 figures, 2 tables.

Table of Contents

  1. Introduction
  2. TESS Observations
  3. Results
  4. Superhump Detection and Superoutburst Characterization
  5. Stage-A Superhump Periods and the Mass Ratio of VW Hyi
  6. Disk Radius and Precession Period
  7. Outburst Duration and TESS-band Energetics
  8. Masses of the White Dwarf and Donor Stars
  9. Discussion: Comparisons and Broader Implications
  10. An unusually deep and long-lived precursor dip
  11. Superhump growth through the dip and into the plateau
  12. Energetics
  13. Are We Observing the Thermal--Tidal Instability?
  14. Conclusion

Figures (8)

  • Figure 1: TESS Primary Mission (Year 1) Light curve.TESS SPOC Simple Aperture Photometry (SAP) raw flux light curves of VW Hyi across Sectors 1--13. Each panel shows the raw flux as a function of time in TESS BJD (BTJD = BJD $-$ 2457000).
  • Figure 2: TESS First Extended Mission (Year 3, Cycle 3) light curve.TESS SPOC SAP light curves of VW Hyi covering Sectors 27 and 29--37.
  • Figure 3: TESS Second Extended Mission (Year 5, Cycle 5) light curve.TESS SPOC SAP light curves of VW Hyi across Sectors 61--69.
  • Figure 4: TESS Third Extended Mission (Year 7, Cycle 7) light curve.TESS SPOC SAP light curves of VW Hyi across Sectors 87--96.
  • Figure 5: Three superoutbursts of VW Hyi recorded by TESS, showing the precursor, main plateau, and decay phases. Green and red markers indicate the start and end of the superoutburst episode.
  • ...and 3 more figures