The Evolved Main-Sequence Channel: HST and LBT observations of CSS120422:111127+571239

TL;DR

This study investigates the evolved main-sequence channel for AM CVn systems through detailed UV and optical spectroscopy of CSS120422 and V418 Ser, revealing unique chemical signatures and disk structures that inform binary evolution models.

Contribution

It provides new spectroscopic observations of CSS120422 and V418 Ser, confirming their status as evolved main-sequence systems and highlighting their distinct chemical compositions.

Findings

Detection of SiIV, NV, and HeII in UV spectrum

Absence of CIV indicating unusual nitrogen/carbon ratio

Evidence of spiral structure in the accretion disk

Abstract

The "evolved main-sequence" channel is thought to contribute significantly to the population of AM CVn type systems in the Galaxy, and also to the number of cataclysmic variables detected below the period minimum for hydrogen rich systems. CSS120422:J111127+571239 was discovered by the Catalina Sky Survey in April 2012. Its period was found to be 56 minutes, well below the minimum, and the optical spectrum is clearly depleted in hydrogen relative to helium, but still has two orders of magnitude more hydrogen than AM CVn stars. Doppler tomography of the H$\alpha$ line hinted at a spiral structure existing in the disk. Here we present spectroscopy of CSS120422:J111127+571239 using the COS FUV instrument on the Hubble Space Telescope and using the MODS spectrograph on the Large Binocular Telescope. The UV spectrum shows SiIV, NV and HeII, but no detectable CIV. The anomalous nitrogen/carbon ratio is seen in a small number of other CVs and confirms a unique binary evolution. We also present and compare the optical spectrum of V418 Ser and advocate that it is also an evolved main-sequence system.