A New Sub-Period-Minimum CV with Partial Hydrogen Depletion and Evidence of Spiral Disk Structure

TL;DR

This paper reports on a unique short-period dwarf nova with low hydrogen content, providing evidence of spiral disk structures and suggesting it as a progenitor of AM CVn systems, based on spectroscopic and photometric analysis.

Contribution

It introduces a new sub-period-minimum CV with partial hydrogen depletion and evidence of spiral disk structure, linking it to AM CVn progenitors.

Findings

Short orbital period of 55 minutes below the CV minimum.

Spectral features indicating low hydrogen abundance (~0.1).

Detection of spiral disk structure via Doppler tomography.

Abstract

We present time-resolved spectroscopy and photometry of CSS 120422:111127+571239 (= SBS1108+574), a recently discovered SU UMa-type dwarf nova whose 55-minute orbital period is well below the CV period minimum of ~78 minutes. In contrast with most other known CVs, its spectrum features He I emission of comparable strength to the Balmer lines, implying a hydrogen abundance less than 0.1 of long period CVs---but still at least 10 times higher than than in AM CVn stars. Together, the short orbital period and remarkable helium-to-hydrogen ratio suggest that mass transfer in "CSS 1204" began near the end of the donor star's main-sequence lifetime, meaning that the system is probably an AM CVn progenitor as described by Podsiadlowski, Han, and Rappaport (2003). Moreover, a Doppler tomogram of the Halpha line reveals two distinct regions of enhanced emission. While one is the result of the stream-disk impact, the other is probably attributable to spiral disk structure generated when material in the outer disk achieves a 2:1 orbital resonance with respect to the donor.