# Turbulent fluctuations and the excitation of Z Cam outbursts

## TL;DR

This paper investigates how stochastic turbulent viscosity fluctuations in accretion disks can trigger outbursts in Z Cam dwarf novae, highlighting their role near bifurcation points and complementing mass transfer variability.

## Contribution

It introduces the idea that turbulent viscosity fluctuations can induce outbursts in Z Cam systems, providing a new perspective beyond mass transfer variations.

## Key findings

- Turbulent disk fluctuations can trigger outbursts near bifurcation points.
- Models reproduce observed outburst dip pairs in Z Cam variables.
- Long trains of outbursts are difficult to generate with turbulence alone.

## Abstract

Z Cam variables are a subclass of dwarf nova that lie near a global bifurcation between outbursting ("limit cycle") and non-outbursting ("standstill") states. It is believed that variations in the secondary star's mass-injection rate instigate transitions between the two regimes. In this paper we explore an alternative trigger for these transitions: stochastic fluctuations in the disk's turbulent viscosity. We employ simple one-zone and global viscous models which, though inappropriate for detailed matching to observed light curves, clearly indicate that turbulent disk fluctuations induce outbursts when the system is sufficiently close to the global bifurcation point. While the models easily produce the observed "outburst dip" pairs exhibited by Z Cam and Nova-like variables, they struggle to generate long trains of outbursts. We conclude that mass transfer variability is the dominant physical process determining the overall Z Cam standstill outburst pattern, but that viscous stochasticity provides an additional ingredient explaining some of the secondary features observed.