The Shape of Long Outbursts in U Gem Type Dwarf Novae from AAVSO Data

TL;DR

This study analyzes AAVSO data to identify the shape of long outbursts in U Gem type dwarf novae, discovering embedded precursors that suggest a more universal mechanism behind long outbursts beyond existing models.

Contribution

First detection of embedded precursors in long outbursts of dwarf novae above the period gap, challenging current theoretical models.

Findings

Embedded precursors found in two of three long outbursts.

First evidence of such precursors in systems above the period gap.

Supports a unified view of dwarf nova outbursts across different systems.

Abstract

We search the American Association of Variable Star Observers (AAVSO) archives of the two best studied dwarf novae in an attempt to find light curves for long outbursts that are extremely well-characterized. The systems are U Gem and SS Cyg. Our goal is to search for embedded precursors such as those that have been found recently in the high fidelity Kepler data for superoutbursts of some members of the SU UMa subclass of dwarf novae. For the vast majority of AAVSO data, the combination of low data cadence and large errors associated with individual measurements precludes one from making any strong statement about the shape of the long outbursts. However, for a small number of outbursts, extensive long term monitoring with digital photometry yields high fidelity light curves. We report the discovery of embedded precursors in two of three candidate long outbursts. This is the first time that such embedded precursors have been found in dwarf novae above the period gap, and reinforces van Paradijs' finding that long outbursts in dwarf novae above the period gap and superoutbursts in systems below the period gap constitute a unified class. The thermal-tidal instability to account for superoutbursts in the SU UMa stars predicts embedded precursors only for short orbital period dwarf novae, therefore the presence of embedded precursors in long orbital period systems - U Gem and SS Cyg - argues for a more general mechanism to explain long outbursts.