Two More Bow Shocks and Off-Center Halpha Nebulae Associated with Nova-like Cataclysmic Variables

TL;DR

This paper reports the discovery of bow-shock nebulae and off-center Halpha nebulae around five nova-like cataclysmic variables, revealing their interactions with interstellar gas and demonstrating the effectiveness of long-exposure small-telescope observations.

Contribution

It presents new observations of bow shocks and nebulae around multiple CVs, linking these features to their motion and environment, and suggests a common formation mechanism involving interstellar gas encounters.

Findings

Bow shocks align with proper motions of CVs.

Nebulae are likely formed by stars' wakes in interstellar gas.

All five CVs are nova-like variables with similar nebular features.

Abstract

We report discoveries of bow-shock nebulae, seen in Halpha and [O III] 5007, around two cataclysmic variables (CVs): LS Pegasi and ASASSN-V J205457.73+515731.9 (hereafter ASASJ2054). Additionally, both stars lie near the edges of faint extended Halpha-emitting nebulae. The orientations of the bow shocks are consistent with the directions of the objects' proper motions. The properties of LS Peg and ASASJ2054, and of their nebulae, are remarkably similar to those of SY Cancri, which we described in a recent paper; SY Cnc is a CV likewise associated with a bow shock and an off-center Halpha nebula. These objects join V341 Arae and BZ Camelopardalis, CVs that are also accompanied by similar nebulae. All five stars belong to the nova-like variable (NLV) subclass of CVs, characterized by luminous optically thick accretion disks that launch fast winds into the surrounding space. We suggest that the bow shocks and nebulae result from chance encounters of the NLVs with interstellar gas clouds, with the stars leaving in their wakes Stromgren zones that are recombining after being photoionized by the CVs' ultraviolet and X-ray radiation. Our discoveries illustrate the power of small telescopes equipped with modern instrumentation, and used to accumulate extremely long exposure times, for the detection of very low-surface-brightness nebulae.