Time-resolved Spectroscopy of the Polar EU Cancri in the Open Cluster Messier 67

TL;DR

This study presents detailed spectroscopic and polarimetric observations of the magnetic cataclysmic variable EU Cnc in Messier 67, confirming its cluster membership and characterizing its magnetic and orbital properties.

Contribution

First detailed spectroscopic and polarimetric analysis of EU Cnc as a cluster member, constraining its magnetic field, mass, and evolutionary status.

Findings

EU Cnc has a magnetic field strength of 41 MG.

The system's orbital period matches previous photometric data.

The accretor mass is estimated to be around 0.83 M_sun.

Abstract

We present time-resolved spectroscopic and polarimetric observations of the AM Her system EU Cnc. EU Cnc is located near the core of the old open cluster Messier 67; new proper motion measurements indicate that EU Cnc is indeed a member of the star cluster, this system therefore is useful to constrain the formation and evolution of magnetic cataclysmic variables. The spectra exhibit two-component emission features with independent radial velocity variations as well as time-variable cyclotron emission indicating a magnetic field strength of 41 MG. The period of the radial velocity and cyclotron hump variations are consistent with the previously-known photometric period, and the spectroscopic flux variations are consistent in amplitude with previous photometric amplitude measurements. The secondary star is also detected in the spectrum. We also present polarimetric imaging measurements of EU Cnc that show a clear detection of polarization, and the degree of polarization drops below our detection threshold at phases when the cyclotron emission features are fading or not evident. The combined data are all consistent with the interpretation that EU Cnc is a low-state polar in the cluster Messier 67. The mass function of the system gives an estimate of the accretor mass of M_WD >= 0.68 M_sun with M_WD ~ 0.83 M_sun for an average inclination. We are thus able to place a lower limit on the progenitor mass of the accreting WD of >= 1.43 M_sun.