EF Eri: Onset of Chromospheric Activity in the Sub-Stellar Secondary

Steve B. Howell, NOAO/WIYNSteve B. Howell, NOAO/WIYN

““Studying the stars Studying the stars is just like is just like

studying the sun, studying the sun, only different.”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 statesPolars have high/low statesof brightness due to of brightness due to

changes in mass accretion.changes in mass accretion.Cause - stellar activity?Cause - stellar activity?

Below is the long term (13.5Below is the long term (13.5years) light curve of the polaryears) light curve of the polarAM Her (Kafka et al 2006)AM Her (Kafka et al 2006)

During high states,During high states,polar SEDs are dominatedpolar SEDs are dominated

by flux from the by flux from the accretion onto theaccretion onto the

white dwarf - bright blue white dwarf - bright blue continuum plus strong continuum plus strong

emission lines. The emission lines. The two stars are not visible.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:Here is an example HIGH STATE Polar spectrum:EF Eri as it appears when ~3 magnitudes brighterEF Eri as it appears when ~3 magnitudes brighter

than its low state. Note the blue continuum and the than its low state. Note the blue continuum and the strong H and He emission lines.strong H and He emission lines.

The Optical Spectrum during the LOW STATE:The Optical Spectrum during the LOW STATE:

H emission faded quickly after 1997. Five years into the low H emission faded quickly after 1997. Five years into the low state, EF Eri’s optical spectrum shows Zeeman split Balmerstate, EF Eri’s optical spectrum shows Zeeman split Balmerabsorption lines caused by the WD B field and NO emission lines.absorption lines caused by the WD B field and NO emission lines.No secondary star features are detected. Note non-BB WD shape.No secondary star features are detected. Note non-BB WD shape.

Separation gives B=13.8MGSeparation gives B=13.8MG

The Secondary Star (?) in the low state -The Secondary Star (?) in the low state -H and K band, phase-resolved spectroscopyH and K band, phase-resolved spectroscopy

show no show no definitedefinite secondary star features but reveal cyclotron humps secondary star features but reveal cyclotron humpsdue to near zero accretion onto the magnetic pole(s).due to near zero accretion onto the magnetic pole(s).

Gemini NIRIGemini NIRI Keck Keck NIRSPECNIRSPEC

EF Eri SED based on light curvesEF Eri SED based on light curvesSED consistent with 9500K WD + L6-like secondary starSED consistent with 9500K WD + L6-like secondary star

Stars are high state SEDStars are high state SED

Filled dots are observedFilled dots are observedpoints and dotted line is apoints and dotted line is a

9500K white dwarf 9500K white dwarf (BB) model(BB) model

Open squares are WD Open squares are WD subtracted SED and L6subtracted SED and L6

spectrum is shownspectrum is shownNote J band is transitionNote J band is transition

region WD/M2region WD/M2

Models of the Current Paradigm

Howell et al., 2001

Evolving 10 million modelCVs, not differentiating bytype, we show the present-day population of CVs in the Milky Way (assuming an ageof 10 Gyr).

Secondary star mass scalesnicely 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.

SMARTS (1.6-m) spectroscopy of V=18.6 EF Eri 2003-2006SMARTS (1.6-m) spectroscopy of V=18.6 EF Eri 2003-2006

Starting ~Oct 2004, weak Halpha emission was present,Starting ~Oct 2004, weak Halpha emission was present,~7 years after start of low state~7 years after start of low state

Keck II Low State spectroscopy of EF Eri (Jan 2006)Keck II Low State spectroscopy of EF Eri (Jan 2006)

Note emission lines from H, He, Na, Ca II as well asNote emission lines from H, He, Na, Ca II as well asunderlying Zeeman split WD absorptionunderlying Zeeman split WD absorption

SMARTS SP.SMARTS SP.

Velocities -> K amplitude = 270 km/sec, must be from M2Velocities -> K amplitude = 270 km/sec, must be from M2Lead to new ephemeris; M1=0.6Msun; M2=0.055MsunLead to new ephemeris; M1=0.6Msun; M2=0.055Msun

Each color is a separate orbit: Nov 2005, Dec 2005, Feb 2006Each color is a separate orbit: Nov 2005, Dec 2005, Feb 2006

Eq.

Wid

thE

q. W

idth

Orbital PhaseOrbital Phase

Are the M2 emission lines due to irradiation?Are the M2 emission lines due to irradiation?The Halpha emission does not go away when the back of theThe Halpha emission does not go away when the back of the

secondary star is in view nor is its eq. width or line fluxsecondary star is in view nor is its eq. width or line fluxsharply peaked near the front side of the secondary.sharply peaked near the front side of the secondary.

Stellar activity on other Polar secondary starsStellar activity on other Polar secondary stars

Observational evidence of stellar activity Observational evidence of stellar activity on the secondary of AM Her and VV Pupon the secondary of AM Her and VV Pup

during low states.during low states.

AM HerAM HerWIYNWIYN

VV PupVV PupVLTVLT Satellite lines phase with secondarySatellite lines phase with secondary

but are produced in “WD facing” region.but are produced in “WD facing” region.Kafka et al. (2006), Mason et al. (2006)Kafka et al. (2006), Mason et al. (2006)

The emission lines of H, He, Na, and Ca II in all 3 secondary stars 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,are stronger toward the WD facing side of the secondary,

but not absent at any phase. Is this concentration due to magnetic but not absent at any phase. Is this concentration due to magnetic coupling, a phenomenon known in RS CVn stars (Walter 1983). coupling, a phenomenon known in RS CVn stars (Walter 1983).

KIIIKIII1000G1000G

FVFV100G100G

RS CVn Model - Uchida & Sakurai (1985)RS CVn Model - Uchida & Sakurai (1985)

SMARTS Halpha line measurements -SMARTS Halpha line measurements -

Oct 2004 - Feb 2006Oct 2004 - Feb 2006

High State starts -->High State starts -->

Does stellar activity cause Polar high/low states?Does stellar activity cause Polar high/low states?>AM Her’s secondary star seems to be “on” all the time in 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>VV Pup’s observed to be 100% on/off during successive low states

<-- No emission first ~7 years<-- No emission first ~7 years

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

• 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?

New, (confusing) EF Eri observations

Models of the Current Paradigm

Howell et al., 2001

Evolving 10 million modelCVs, not differentiating bytype, the figure on the left is produced. This model represents the present-day population of CVs in the Milky Way assuming an ageof 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

2) He I triplet (5876A) to singlet (6678A) line ratio provides a 2) He I triplet (5876A) to singlet (6678A) line ratio provides a diagnostic. The lines themselves set T>= ~20,000K. diagnostic. The lines themselves set T>= ~20,000K.

Three mechanisms populate these states: Three mechanisms populate these states: a) recombination after photoionization (~3); a) recombination after photoionization (~3);

b) collisional excitation from the ground state (~45); b) collisional excitation from the ground state (~45); c) singlet only population via resonance scattering.c) singlet only population via resonance scattering.

If choice (b) high density, if choice (a) low density, (not choice c).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), 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. a value consistent with a low density, i.e., a chromosphere/corona.

Keck II EF Eri spectrumKeck II EF Eri spectrumnear the He I linesnear the He I lines