EF Eri: Its White Dwarf Primary and L Dwarf Secondary

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Transcript EF Eri: Its White Dwarf Primary and L Dwarf Secondary

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).
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)
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
Eq. Width
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.
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
VV Pup
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)
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
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
• 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.
Howell et al., 2001
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
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