Cataclysmic Variables:
Christian Knigge
University of Southampton
Christian Knigge
P. Marenfeld and NOAO/AURA/NSF
10 Breakthroughs in 10 Years
University of Southampton
School of Physics & Astronoy
Outline
• Introduction
– Cataclysmic variables: a primer
987654
3 2 1
35 minutes
• XX
10 breakthroughs in 10
years
XXXXX
(a personal and hugely biased perspective...)
• Evolution
• Accretion
• Outflows
Links to Other Systems
(BH/NS LMXBs)
• The Role of UV Astronomy
• Summary
Christian Knigge
University of Southampton
School of Physics & Astronoy
Cataclysmic Variables: A Primer
The Physical Structure of CVs
• White dwarf primary
–
UV bright
• “Main-sequence” secondary
Red Dwarf
White Dwarf
• 75 mins < Porb < 6 hrs
• Roche-lobe overflow
• Accretion usually via a disk
― UV-bright
• Disk accretion is unstable if
below critical rate
Accretion Disk
• dwarf novae
• Mass transfer and evolution
driven by angular momentum loss
Credit: Rob Hynes
• Evolution is (initially) from long
to short periods
Christian Knigge
University of Southampton
School of Physics & Astronoy
Cataclysmic Variables: A Primer
The Orbital Period Distribution and the Standard Model of CV Evolution
Knigge 2006
•
Clear “Period Gap” between 2-3 hrs
•
Suggests a change in the dominant
angular momentum loss mechanism:
– Above the gap:
• Magnetic Braking
• Fast AML  High 𝑀
– Below the gap:
• Gravitational Radiation
• Slow AML  Low 𝑀
•
•
Christian Knigge
Minimum period at Pmin ≈ 80 min
–
donor transitions from MS  BD
–
beyond this, Porb increases again
This disrupted magnetic braking
scenario is the standard model for CV
evolution
University of Southampton
School of Physics & Astronoy
Breakthrough I: Evolution
Disrupted Angular Momentum Loss at the Period Gap
• Standard model prediction
–
Howell et al. 2001
The period gap is caused by a disruption in AML
when the donor becomes fully convective
• Magnetic braking drives high
M
above the gap
• Donor is slightly out of TE and thus oversized
• At Porb 3 hrs , donor becomes fully convective
• MB ceases (or is severely reduced)
•
M drops --> donor relaxes (shrinks) to TE radius
• Donor loses contact with RL
• CV evolves through gap as detached binary
• Residual AML (e.g. GR) shrinks orbit (and RL)
• Contact with donor re-established at Porb 2 hrs
•
Observational reality pre-2005
–
No direct empirical support for this picture (other than
the existence of the gap itself)
Christian Knigge
University of Southampton
School of Physics & Astronoy
Breakthrough I: Evolution
Disrupted Angular Momentum Loss at the Period Gap
Patterson et al. (2005),
Knigge (2006)
M-R relation based on
eclipsing and
“superhumping” CVs
M gap  0.2M
•
Donors are significantly larger than MS stars both above and below the gap
•
Clear discontinuity at M2 = 0.20 M☼, separating long- and short-period CVs!
– Direct evidence for disrupted angular momentum loss!
Christian Knigge
University of Southampton
School of Physics & Astronoy
Breakthrough II: Evolution
Reconstructing CV Evolution Empirically
•
We can even use the donor 𝑀2 − 𝑅2 relation to quantitatively reconstruct CV evolution
•
CV Donors are significantly larger than MS stars because they are bloated by mass loss
–
•
Higher 𝑀
→ Larger 𝑅2
So we can use the degree of donor bloating at given 𝑃𝑜𝑟𝑏 to infer 𝑀 𝑃𝑜𝑟𝑏
Knigge
Knigge,
(2006)
Baraffe &
Patterson (2011)
•
Above the gap: slightly reduced “standard” MB recipes work well
•
Below the gap: need enhanced AML, 𝐽 ≃
2.5 𝐽𝐺𝑅
 significant revision of the standard model!
Christian Knigge
University of Southampton
School of Physics & Astronoy
Breakthrough III: Evolution
Period Bouncers with Brown Dwarf Secondaries
• Standard model predictions
99% of CVs should be found below the period gap
–
A full 70% should be “period bouncers” with brown
dwarf secondaries
Observational reality pre-2006
–
Not a single definitive period bouncer
• Only ~10 candidates out of ~1000 CVs
–
No secondary with a well-established mass below the
H-burning limit
–
Is this a selection effect or model failure?
Christian Knigge
Howell et al. 2001
•
–
University of Southampton
School of Physics & Astronoy
Breakthrough III: Evolution
Period Bouncers with Brown Dwarf Secondaries
•
SDSS has yielded a deep new sample of
~200 CVs (Szkody et al. 2002-9)...
•
...including a sub-set of faint,
WD-dominated systems near Pmin
Littlefair et al. 2006, Science, 314, 1578
(Gaensicke et al. 2009; see later)
•
A few of these are eclipsing, allowing
precise system parameter determinations
•
At least 3 of these have M2 < 0.072 M☼
(Littlefair et al. 2006, 2008)
At least some post-period-minimum
systems with brown dwarf donors do
exist!
But one of them is very strange…
Christian Knigge
University of Southampton
School of Physics & Astronoy
Breakthrough III: Evolution
Period Bouncers with Brown Dwarf Secondaries
•
•
SDSS J1507 is one of the three eclipsing CVs with
sub-stellar donors…
… but its 𝑃𝑜𝑟𝑏 < 𝑃𝑚𝑖𝑛 for other CVs
•
Two ideas:
–
Littlefair Patterson
et al. (2007)
et al. (2008)
J1507 is young -- born with a sub-stellar donor
(Littlefair et al. 2007)
–
J1507 is a low metallicity halo CV
(Patterson et el. 2008)
 How can we test which is correct?
•
Uthas et al. (2011)
UV astronomy to the rescue!
–
•
Littlefair et al. (2007)
Stehle et al. (1999)
FUV spectroscopy shows that [Fe/H] = -1.2
SDSS J1507 is an eclipsing period bouncer in
the Galactic halo!
–
Rosetta stone for studying effects of metallicity
on accretion and evolution?
Christian Knigge
University of Southampton
School of Physics & Astronoy
Breakthrough IV: Evolution
The Period Spike at Pmin
• Standard model prediction
–
The number of CVs found in a particular Porb range is
inversely proportional to the speed with which they
evolve through it
NCV ( Porb )  | Porb |1
–
So there should be a spike at Pmin, in the period
distribution since
Porb (Pmin )  0
•
Observational reality pre-2009
–
No convincing spike anywhere near Pmin in the CV
Porb distribution
Barker & Kolb 2003
Christian Knigge
University of Southampton
School of Physics & Astronoy
Breakthrough IV: Evolution
The Period Spike at Pmin
•
Boris Gaensicke and collaborators have
obtained orbital periods for most of the new
SDSS CVs
•
The resulting period distribution does show
a spike at Pmin for the first time (Gaensicke
Gaensicke et al. 2009
Previously known CVs
et al. 2009)
CVs do in fact “bounce” at Pmin!
SDSS CVs
Christian Knigge
University of Southampton
School of Physics & Astronoy
Breakthrough V: Evolution
CVs in Globular Clusters
•
A typical GC should contain ~100 CVs purely based on its
stellar mass content
CV space density:
(e.g. Pretorius & Knigge 2007, 2011)
Effective volume of MW:
•
But bright X-ray binaries are overabundant in GCs by ~100x
(Clark 1975, Katz 1975)
– New dynamical formation channels are available in GCs
• tidal capture (Fabian, Pringle & Rees 1976)
•
White Dwarf
Expected # of CVs in MW:
Fraction of MW mass in GCs:
3- and 4-body interactions
# of GCs in MW:
•
Could CV numbers also be enhanced?
–
Theory says yes, but “only” by a factor of ~2
(di Stefano & Rappaport 1994, Davies 1995/7, Ivanova et al. 2006)
•
There should be hundreds of accreting WDs in GCs!
•
Important and useful:
–
–
–
→ expected # of CVs per GC:
3-body exchange encounter
Large samples of CVs at known distances
Drivers and tracers of GC dynamical evolution
Are GCs SN Ia factories? (Shara & Hurley 2006)
So where are they?
Christian Knigge
University of Southampton
School of Physics & Astronoy
Breakthrough V: Evolution
CVs in Globular Clusters
• Early searches typically found only a handful per GC
Shara et al. (1996)
(e.g. Shara et al. 1996, Bailyn et al. 1996, Cool et al. 1998)
– Are CVs not formed or maybe even destroyed in CVs?
• Significant implications for GC dynamics!
– Selection effects?
• Survey depth?
• Dwarf nova duty cycle?
Shara et al. (1996)
Difference
of the
Core of 47 Tuc
47Imaging
Tuc with
Chandra
47 Tuc with the ROSAT HRI
(Grindlay et
al. 2001;etHeinke
et al. 2005)
(Hasinger
al. 1994)
Pooley & Hut (2006)
• X-rays would be a great way to find CVs in GCs
– But this used to be really hard!
• Chandra has revolutionized the field
– Deep X-ray surveys typically find tens per cluster
– Numbers scale with collision rate
 dynamical formation matters!
GCs do harbour significant populations of
dynamically-formed CVs!
Christian Knigge
University of Southampton
School of Physics & Astronoy
Breakthrough V: Evolution
CVs in Globular Clusters
• UV astronomy has also played a key role
–
FUV (~1500A)
The core of 47 Tuc: U-band
Efficient way of finding new CVs and
confirming X-ray-selected candidates
(Knigge et al. 2002, Dieball et al. 2005, 2009, 2010,
Thomson et al. 2012)
–
Even slitless multi-object spectroscopic
identification/confirmation is possible!
• Still many key unsolved questions!
–
Are there enough CVs in GCs?
–
Are they different from field CVs?
–
Where are the double WDs?
–
Are there SN Ia progenitors?
Knigge et al (2002, 2003, 2008)
Christian Knigge
University of Southampton
School of Physics & Astronoy
Breakthroughs VI and VII: Accretion / Outflows
Outburst Hysteresis and Jets
•
Both CVs (dwarf novae) and XRBs (X-ray transients) exhibit outbursts
–
•
Thermal/viscous disk instability
Dwarf nova eruption (optical): SS Cyg
Wheatley et al (2003)
XRBs
–
GX339:
Gallo et
SS Cyg:
Koerding
etal.
al.(2004)
2008, Science
Outbursts trace a q-shape in the X-ray hardness vs intensity plane
(Fender, Belloni & Gallo 2004)
 hysteresis
•
–
Collimated (radio) jets are seen (almost only) in the hard state
–
Hard-soft transition produces a powerful jet ejection episode
CVs (pre-2008)
– No evidence for collimated jets in any CV
•
–
•
Gallo et al. 2004
Constraint on theories of jet formation (e.g. Livio 1999)?
No constraints on outburst hysteresis
X-ray transient outburst (X-ray): GX 339
Gallo et al (2004)
Elmar Koerding et al. (2008)
–
Do dwarf novae also execute a q-shaped outburst pattern?
•
–
Yes they do!
Best chance to see a powerful jet is during the “hard-to-soft”
transition during the rise to a dwarf nova outburst
•
Adapted from Fender,
Belloni & Gallo 2004
Discovery of the first CV radio jet!
Christian Knigge
University of Southampton
School of Physics & Astronoy
Breakthrough VIII: Accretion
Periodic Variability: Oscillations
•
Both XRBs and CVs often exhibit (quasi-)periodic
oscillations on short (~dynamical) time-scales
•
Origin is poorly understood, but intimately connected to
accretion/outflow processes in the innermost disk regions
•
Key result in XRBs (accreting NSs and BHs):
–
•
Warner & Woudt 2004
Psaltis, Belloni & van der Klis 1999
NS & BH
LMXBs
strong correlations between different types of oscillations,
especially LKO and HBO
CVs also exhibit two types of oscillations
26 CVs
–
•
Is there a direct connection to LMXBs?s
Yes! (Warner & Woudt
[2002...2010], Mauche [2003])
–
DNOs : QPOs in CVs ↔
–
Universality of accretion physics extends to periodic
variability
–
Models relying on ultra-strong gravity or B-fields are ruled out
Christian Knigge
LKOs : HBOs in LMXBs
DNOs in VW Hyi
Woudt & Warner (2002)
University of Southampton
School of Physics & Astronoy
Breakthrough IX: Accretion
Non-Periodic Variability: The RMS-Flux Relation
•
What about non-periodic accretion-induced variability
(“flickering”)?
•
Stochastic variability has been closely studied in XRBs
A Black
CV (Pretorius
Knigge
2007) 2001)
Hole XRB &
(Uttley
& McHardy
MV Lyr (Scaringi et al. 2011)
MV Lyr (Scaringi et al. 2011)
•
Key discovery: the “rms-flux relation” (Uttley & McHardy 2001)
–
•
Rules out “additive” models (e.g. shot-noise)
What about CVs?
–
Non-trivial to study:
variability
time-scales
are much longer,
AGN
(Vaughan et
al. 2011)
so need high-cadence, uninterrupted long-term light curves
An XRB (Churazov et al. 2003)
Neutron Star XRB (Uttley & McHardy 2001)
--> Kepler!
NGC 4051
(Seyfert
show 1)the
•
CVs also
•
Accretion-induced variability is universal!
–
rms-flux relation! (Scaringi et al. 2011)
Key properties shared by supermassive BHs, stellarmass BHs, NSs and WDs
Christian Knigge
University of Southampton
School of Physics & Astronoy
Breakthrough X: Evolution / Accretion / Outflows
Do all CVs go nova?
•
We all “know” that CVs burn accreted matter explosively
(Fujimoto, Iben, Starrfield, Shaviv, Shara, Townsley, Bildsten, Yaron...)
→ Nova Eruptions (typical recurrence time ~10,000 yrs)
•
Shara et al. 2007, Nature 446, 159
But all known novae were actually discovered as such
–
How can we establish the general link empirically ?
•
r
Ejected nova shells may be detectable for ~1000
yrs!
•
So Shara et al. (2007) searched for resolved nebulae
around ordinary CVs in the GALEX imaging archive....
•
...and disovered an ancient nova shell around the prototypical dwarf nova Z Cam
→ ordinary CVs do undergo nova eruptions!
•
Postscript: Chinese astronomers would have disagreed with the
classification of Z Cam as an “ordinary CV”...
Christian Knigge
University of Southampton
School of Physics & Astronoy
Summary
The last decade has seen several breakthroughs in our understanding of CVs, many of which were
made possible by ultraviolet observations
•
•
Evolution
–
The basic disrupted-angular-momentum-loss picture of CV evolution is correct !
–
We know how to reconstruct CV evolution from both primary and secondary properties
–
CVs do exist in significant numbers in GCs
–
CVs not discovered as novae can still have nova shells --> all CVs experience nova eruptions
Accretion, Outflows and Links to Other Systems
–
CV outbursts exhibit hysteresis (“turtlehead” diagram) – just like XRBs and AGN
–
CVs can drive radio jets – just like XRBs and AGN
–
Accretion-induced oscillations in CVs are… – just like those in XRBs
–
Stochastic variability in CVs follows an rms-flux relation – just like XRBs and AGN
The physics of disk accretion is universal
CVs provide excellent, nearby, bright accretion laboratories
Christian Knigge
University of Southampton
School of Physics & Astronoy