[Paper Review] Evidence for a substellar secondary in the magnetic cataclysmic binary EF Eridani

K. Beuermann, P. J. Wheatley+3 more|arXiv (Cornell University)|Jan 11, 2000

Stellar, planetary, and galactic studies4 citations

TL;DR

This study presents evidence that the magnetic cataclysmic variable EF Eri hosts a substellar companion, either a brown dwarf or a transition object near the hydrogen-burning limit, based on the absence of detectable spectral features in its low-state optical spectrum. The white dwarf's unusually low effective temperature of 9500 ± 500 K implies a cooling age exceeding 10⁹ years, ruling out a warm brown dwarf and indicating the system has likely passed the period minimum in cataclysmic variable evolution.

ABSTRACT

Low-state spectrophotometry of the short-period polar EF Eridani (Porb=81 min) found the system at V=18.0 with no trace of the companion (Wheatley and Ramsay 1998). We show that the lack of such spectral features implies that the companion to the white dwarf in EF Eri has a spectral type later than M9 and is either a transition object at the brink of hydrogen burning or a brown dwarf. The optical low state spectrum indicates a temperature of the white dwarf of Teff=9500+-500K. This is one of the coldest white dwarfs in cataclysmic variables, implying a cooling age t_cool>=10e9 yrs or accretional heating at a rate as given by gravitational radiation. The large age of the system excludes a warm brown dwarf as companion. EF Eri has either just passed through the period minimum of cataclysmic variable stars or has started mass transfer from an old brown dwarf secondary.

Motivation & Objective

• To determine the nature of the secondary star in the magnetic cataclysmic variable EF Eri, which is undetected in optical spectra during low accretion states.
• To constrain the spectral type and mass of the unseen companion using the absence of flux features in the low-state spectrum.
• To assess the evolutionary state of EF Eri by comparing its orbital period (81 min) to the theoretical period minimum of 77 min, and to infer whether it has passed the period bounce.
• To evaluate whether the white dwarf's low temperature (9500 K) is consistent with cooling or accretional heating, and to infer implications for the companion's mass and evolutionary history.

Proposed method

• Low-state spectrophotometry of EF Eri was conducted using the ESO 3.6-m telescope at La Silla, Chile, during February 1997, with observations corrected for airmass and telluric absorption.
• Cyclotron emission was removed from the spectra using a difference spectrum between cyclotron maximum and minimum phases, isolating the non-magnetic continuum.
• The white dwarf's effective temperature was determined by fitting the observed continuum (λ > 5300 Å) with non-magnetic and magnetic model atmospheres, yielding T_eff = 9500 ± 500 K.
• The maximum possible flux contribution from a late-type M or L star was estimated using empirical calibrations of surface brightness, and the absence of such features constrained the companion's spectral type to later than M9.
• Theoretical models of white dwarf cooling and cataclysmic variable evolution were used to infer the companion's mass and evolutionary status, considering the system's age and orbital period.
• The accretion rate was estimated from the white dwarf's temperature and cooling age, and compared to gravitational radiation-driven accretion rates to test consistency with a high-mass white dwarf.

Experimental results

Research questions

• RQ1 What is the spectral type and mass of the unseen secondary star in the magnetic cataclysmic variable EF Eri, based on the absence of detectable optical flux in the low state?
• RQ2 Is the white dwarf in EF Eri old enough (cool enough) to rule out a warm brown dwarf companion, given its effective temperature of 9500 K?
• RQ3 Has EF Eri passed the period minimum in cataclysmic variable evolution, and what does its current orbital period (81 min) imply about the mass and nature of its secondary?
• RQ4 Can the observed temperature of the white dwarf be explained by cooling alone, or is it maintained by accretional heating, and what does this imply about the accretion rate and companion mass?
• RQ5 Does the discrepancy between the observed orbital period (81 min) and the theoretical minimum period (77 min) suggest the presence of an additional angular momentum loss mechanism or a hidden evolutionary pathway in CV systems?

Key findings

• The absence of detectable spectral features in the low-state spectrum of EF Eri implies that the secondary star has a spectral type later than M9, ruling out main-sequence M-type stars.
• The white dwarf in EF Eri has an effective temperature of 9500 ± 500 K, making it the coldest known white dwarf in a cataclysmic variable with a reliably determined temperature.
• The low temperature implies a cooling age of at least 10⁹ years, which rules out a warm brown dwarf companion, as such objects would have cooled to lower temperatures over that time.
• The system's orbital period of 81 minutes is 4 minutes longer than the observed minimum period of 77 minutes, indicating that EF Eri has likely passed the period minimum and is evolving to longer periods.
• The companion is most likely a brown dwarf (M₂ < 0.06 M☉) or a transition object near the hydrogen-burning limit (M₂ ≈ 0.075 M☉), based on evolutionary models and the age constraint.
• The accretion rate required to maintain the white dwarf's temperature is consistent with gravitational radiation-driven accretion, supporting a high white dwarf mass (M₁ ≥ 0.6 M☉) and a substellar companion.