EF Eri: Onset of Chromospheric Activity in the Sub-Stellar Secondary Steve B. Howell, NOAO/WIYN “Studying the stars is just like studying the sun, only different.”

Cataclysmic Variables CVs are close, interacting binaries containing a white dwarf primary, a low-mass, secondary star, and often an accretion disk. CVs have P(orb)=12 hours to ~70 minutes. The white dwarf has a mass of 0.4 to 1.4 M-sun and can be non-magnetic or (~10-20%) magnetic (10-250 MG). The secondary has a mass from ~1.2 M-sun to ~0.05 M-sun.

Types of Cataclysmic Variable If the white dwarf has no (weak, 1-8MG) magnetic field --> dwarf nova, classical nova, nova-like (IP). These binaries contain an accretion disk. If the white dwarf has a ~10 to 250MG field --> Polar or AM Herculis type. These contain no accretion disk.

Polars have high/low states of brightness due to changes in mass accretion. Cause - stellar activity? Below is the long term (13.5 years) light curve of the polar AM Her (Kafka et al 2006) During high states, polar SEDs are dominated by flux from the accretion onto the white dwarf - bright blue continuum plus strong emission lines. The two stars are not visible.

The Polar EF Eri White Dwarf Primary star is ~0.6 solar mass and has B=13.5 MG Sub-stellar secondary star Orbital period = 81 minutes Distance = 45-90 pc Discovered in 1970’s as weak, soft X-ray source, id’ed as a blue variable star Entered low mass accretion state in 1996 Entered high state, after 9 years, 10 Mar 06

Here is an example HIGH STATE Polar spectrum: EF Eri as it appears when ~3 magnitudes brighter than its low state. Note the blue continuum and the strong H and He emission lines.

The Optical Spectrum during the LOW STATE: H emission faded quickly after 1997. Five years into the low state, EF Eri’s optical spectrum shows Zeeman split Balmer absorption lines caused by the WD B field and NO emission lines. No secondary star features are detected. Note non-BB WD shape. Separation gives B=13.8MG

The Secondary Star (?) in the low state - H and K band, phase-resolved spectroscopy show no definite secondary star features but reveal cyclotron humps due to near zero accretion onto the magnetic pole(s). Keck NIRSPEC Gemini NIRI

EF Eri SED based on light curves SED consistent with 9500K WD + L6-like secondary star Stars are high state SED Filled dots are observed points and dotted line is a 9500K white dwarf (BB) model Open squares are WD subtracted SED and L6 spectrum is shown Note J band is transition region WD/M2

Models of the Current Paradigm Evolving 10 million model CVs, not differentiating by type, we show the present-day population of CVs in the Milky Way (assuming an age of 10 Gyr). Secondary star mass scales nicely with orbital period; but not equal to MS M-R relation (for P_orb >2.5 hr). Masses after the period minimum are <0.06 M-sun. Howell et al., 2001

SMARTS (1.6-m) spectroscopy of V=18.6 EF Eri 2003-2006 Starting ~Oct 2004, weak Halpha emission was present, ~7 years after start of low state

Keck II Low State spectroscopy of EF Eri (Jan 2006) SMARTS SP. Note emission lines from H, He, Na, Ca II as well as underlying Zeeman split WD absorption

Velocities -> K amplitude = 270 km/sec, must be from M2 Lead to new ephemeris; M1=0.6Msun; M2=0.055Msun

Are the M2 emission lines due to irradiation? The Halpha emission does not go away when the back of the secondary star is in view nor is its eq. width or line flux sharply peaked near the front side of the secondary. Eq. Width Orbital Phase Each color is a separate orbit: Nov 2005, Dec 2005, Feb 2006

Stellar activity on other Polar secondary stars Observational evidence of stellar activity on the secondary of AM Her and VV Pup during low states. AM Her WIYN VV Pup VLT Satellite lines phase with secondary but are produced in “WD facing” region. Kafka et al. (2006), Mason et al. (2006)

The emission lines of H, He, Na, and Ca II in all 3 secondary stars are stronger toward the WD facing side of the secondary, but not absent at any phase. Is this concentration due to magnetic coupling, a phenomenon known in RS CVn stars (Walter 1983). RS CVn Model - Uchida & Sakurai (1985) KIII 1000G FV 100G

Does stellar activity cause Polar high/low states? >AM Her’s secondary star seems to be “on” all the time in low states >VV Pup’s observed to be 100% on/off during successive low states <-- No emission first ~7 years High State starts --> SMARTS Halpha line measurements - Oct 2004 - Feb 2006

EF Eri - Summary EF Eri has just recovered from a nine year low state - the longest known for any polar. Secondary star line emission started ~7 years in, 1.5 years before new high state. RV solution yields secondary star mass = 0.055 Msun (fairly insensitive to M1) Emission lines not irradiation produced, seem to be chromospheric activity on the sub-stellar secondary. The binary may contain a circumbinary dust disk

END

New, (confusing) EF Eri observations SPITZER - IRAC observations of EF Eri (and 3 other short-period polars) were undertaken in an attempt to detect the brown dwarf-like secondary stars IR excess was found A circumbinary disk?

Models of the Current Paradigm Evolving 10 million model CVs, not differentiating by type, the figure on the left is produced. This model represents the present-day population of CVs in the Milky Way assuming an age of 10 GYr. Major predictions: -- 90% of all CVs are short orbital period (<2.5 hr) -- 70% of all CVs are past a minimum period (near 70 min) and evolving to longer periods Howell et al., 2001

2) He I triplet (5876A) to singlet (6678A) line ratio provides a diagnostic. The lines themselves set T>= ~20,000K. Three mechanisms populate these states: a) recombination after photoionization (~3); b) collisional excitation from the ground state (~45); c) singlet only population via resonance scattering. If choice (b) high density, if choice (a) low density, (not choice c). EF Eri ratio =3.3, close to the ratio of statistical weights (3), a value consistent with a low density, i.e., a chromosphere/corona. Keck II EF Eri spectrum near the He I lines