Presentation on theme: "EF Eri: Onset of Chromospheric Activity in the Sub-Stellar Secondary"— Presentation transcript:
1 EF Eri: Onset of Chromospheric Activity in the Sub-Stellar Secondary Steve B. Howell, NOAO/WIYN“Studying the starsis just likestudying the sun,only different.”
2 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 ( MG).The secondary has a mass from ~1.2 M-sun to ~0.05 M-sun.
3 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.
4 Polars have high/low states of brightness due to changes in mass accretion.Cause - stellar activity?Below is the long term (13.5years) light curve of the polarAM Her (Kafka et al 2006)During high states,polar SEDs are dominatedby flux from theaccretion onto thewhite dwarf - bright bluecontinuum plus strongemission lines. Thetwo stars are not visible.
5 The Polar EF EriWhite Dwarf Primary star is ~0.6 solar mass and has B=13.5 MGSub-stellar secondary starOrbital period = 81 minutesDistance = pcDiscovered in 1970’s as weak, soft X-ray source, id’ed as a blue variable starEntered low mass accretion state in 1996Entered high state, after 9 years, 10 Mar 06
6 Here is an example HIGH STATE Polar spectrum: EF Eri as it appears when ~3 magnitudes brighterthan its low state. Note the blue continuum and the strong H and He emission lines.
7 The Optical Spectrum during the LOW STATE: H emission faded quickly after Five years into the lowstate, EF Eri’s optical spectrum shows Zeeman split Balmerabsorption 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
8 The Secondary Star (?) in the low state - H and K band, phase-resolved spectroscopyshow no definite secondary star features but reveal cyclotron humpsdue to near zero accretion onto the magnetic pole(s).KeckNIRSPECGemini NIRI
9 EF Eri SED based on light curves SED consistent with 9500K WD + L6-like secondary starStars are high state SEDFilled dots are observedpoints and dotted line is a9500K white dwarf(BB) modelOpen squares are WDsubtracted SED and L6spectrum is shownNote J band is transitionregion WD/M2
10 Models of the Current Paradigm Evolving 10 million modelCVs, not differentiating bytype, we show the present-daypopulation of CVs in theMilky Way (assuming an ageof 10 Gyr).Secondary star mass scalesnicely with orbital period;but not equal to MS M-Rrelation (for P_orb >2.5 hr).Masses after the periodminimum are <0.06 M-sun.Howell et al., 2001
11 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
12 Keck II Low State spectroscopy of EF Eri (Jan 2006) SMARTS SP.Note emission lines from H, He, Na, Ca II as well asunderlying Zeeman split WD absorption
13 Velocities -> K amplitude = 270 km/sec, must be from M2 Lead to new ephemeris; M1=0.6Msun; M2=0.055Msun
14 Are the M2 emission lines due to irradiation? The Halpha emission does not go away when the back of thesecondary star is in view nor is its eq. width or line fluxsharply peaked near the front side of the secondary.Eq. WidthOrbital PhaseEach color is a separate orbit: Nov 2005, Dec 2005, Feb 2006
15 Stellar activity on other Polar secondary stars Observational evidence of stellar activityon the secondary of AM Her and VV Pupduring low states.AM HerWIYNVV PupVLTSatellite lines phase with secondarybut are produced in “WD facing” region.Kafka et al. (2006), Mason et al. (2006)
16 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 magneticcoupling, a phenomenon known in RS CVn stars (Walter 1983).RS CVn Model - Uchida & Sakurai (1985)KIII1000GFV100G
17 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 yearsHigh State starts -->SMARTS Halpha line measurements -Oct Feb 2006
18 EF Eri - SummaryEF 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 = 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
20 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 starsIR excess was foundA circumbinary disk?
21 Models of the Current Paradigm Evolving 10 million modelCVs, not differentiating bytype, the figure on the left isproduced. This modelrepresents the present-daypopulation of CVs in theMilky Way assuming an ageof 10 GYr.Major predictions:-- 90% of all CVs are shortorbital period (<2.5 hr)-- 70% of all CVs are past aminimum period (near 70 min)and evolving to longer periodsHowell et al., 2001
22 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 spectrumnear the He I lines