Phase-Resolved Infrared H- and K-Band Spectroscopy of EF Eridani

We present new phase-resolved H- and K-band spectroscopy of the ultrashort-period magnetic cataclysmic variable EF Eri in its current prolonged ``low'' state obtained using NIRI on Gemini North and NIRSPEC on Keck II. These new data show that the H-band spectrum of EF Eri appears to be dominated by cyclotron emission during the entire orbital cycle. The K-band spectrum of EF Eri is likewise dominated by cyclotron emission during most of an orbital period, but near binary phase 0.0 the secondary star spectrum may be visible. The lack of strong CO or CH<sub>4</sub> absorption features and the weakness of the water vapor features in this spectrum, however, suggests the possibility of peculiar abundances for carbon and/or oxygen. We have used the PHOENIX stellar atmosphere code to produce model brown dwarf spectra with nonsolar abundances of carbon, nitrogen, and oxygen, and achieved limited success in fitting the observed spectra. We conclude that strong, highly variable cyclotron emission is responsible for the photometric variation previously reported for EF Eri. The nature of this cyclotron emission is complex: the H-band spectra show that the dominant cyclotron harmonic at phase 0.5 peaks at 1.65 μm, but at phase 0.0 the harmonic peaks near 1.72 μm. At phase 0.5, there is another cyclotron feature present that peaks in between the H and K bands (near 1.93 μm), but at phase 0.0 no such feature is present. These data suggest that cyclotron emission from both poles is occurring. In the high state, the cyclotron emission has been modeled as coming from the pole that is oriented toward the secondary star. One interpretation for the phase 0.5 cyclotron emission is that it originates from the opposite pole. In its current ultralow state, EF Eri reveals no outward signs of accretion (such as H I emission) but continues to have a few, strong cyclotron features. Thus, EF Eri joins the small group of magnetic cataclysmic variables whose accretion rate is so low that they are in the ``bombardment scenario'' regime.

Partially based on observations obtained at La Silla (ESO proposal 69.D-0142[A]).