1. Christian Knigge University of Southampton School of Physics & Astronoy P. Marenfeld and NOAO/AURA/NSF Christian Knigge University of Southampton Low-Mass and Substellar Donors in Interacting Compact Binary Systems Low-Mass and Substellar Donors in Cataclysmic Variables

2. Christian Knigge University of Southampton School of Physics & Astronoy Outline Introduction –Cataclysmic Variables: A Primer The Donor-Driven Evolution of CVs –The stellar response to mass loss –The origin of the “period gap” How can we learn from each other? –Example 1: Magnetic Braking –Example 2: Star spots & differential rotation –Example 3: The effect of irradiation of (sub)stellar & planetary atmospheres Summary

3. Christian Knigge University of Southampton School of Physics & Astronoy Cataclysmic Variables: A Primer The Physical Structure of CVs White Dwarf Accretion Disk Red Dwarf White dwarf primary Main-sequence (ish) secondary Roche-lobe overflow Accretion usually via a disk 75 mins < P orb < 12 hrs Mass transfer & evolution driven by angular momentum losses Evolution is (initially) from long to short periods Credit: Rob Hynes

4. Christian Knigge University of Southampton School of Physics & Astronoy Cataclysmic Variables: A Primer The Orbital Period Distribution and the Standard Model of CV Evolution 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 –Below the gap: Gravitational Radiation Slow AML  Low Minimum period at P min ≈ 80 min –donor transitions from MS  BD –beyond this, P orb increases again So CV donors evolve along the MS! This is the “standard model” of CV Evolution Knigge 2006 Howell, Nelson & Rappaport 2003

5. Christian Knigge University of Southampton School of Physics & Astronoy Should donors be on the main sequence? The stellar response to mass loss In CVs,, so the donor cannot shrink quite fast enough to keep up with the rate at which mass is removed from its surface The secondary is therefore driven slightly out of thermal equilibrium, and becomes somewhat oversized for its mass Stehle, Ritter & Kolb 1996 Does any of this actually matter? Yes: this slight difference is key to our understanding of CV evolution!

6. Christian Knigge University of Southampton School of Physics & Astronoy Thought to be due to a sudden reduction of magnetic-braking-driven AML when the donor becomes fully convective This reduces and increases Donor responds by relaxing closer to its equilibrium radius This causes loss of contact and cessation of mass transfer on a time-scale of Orbit still continues to shrink (via GR), while donor continues to relax Ultimately, Roche lobe catches up and mass transfer restarts at bottom edge All of this only works if the donor is significantly bloated above the gap Is there any observational evidence for all this? The Donor-Driven Evolution of CVs The Origin of The Period Gap

7. Christian Knigge University of Southampton School of Physics & Astronoy Patterson et al. (2005), Knigge (2006), Knigge et al (2011) Donors are significantly larger than isolated MS stars both above and below the gap Clear discontinuity at M 2 = 0.20 M ☼, separating long- and short-period CVs! –Direct evidence for disrupted angular momentum loss! –Change in MB efficiency across fully-convective boundary! M-R relation based on eclipsing and “superhumping” CVs The Donor-Driven Evolution of CVs The Empirical Donor Mass-Radius Relationship

8. Christian Knigge University of Southampton School of Physics & Astronoy How Can We Learn From Each Other? Example 1: Magnetic Braking All of CV evolution is driven by angular momentum losses Magnetic braking is critical in this respect –Basic physics is straightforward The donor drives a weak wind that co-rotates with donor’s B-field out to the Alfven radius This spins down the donor and ultimately drains AM from the orbit –Magnetic braking is almost certainly dominant above the period gap –It is usually assumed to stop when donor becomes fully convective, but some residual MB may also operate below the gap Certainly implied by observations of activity in single/detached fully convective stars So how well do we understand magnetic braking?

9. Christian Knigge University of Southampton School of Physics & Astronoy Knigge, Baraffe & Patterson 2011 Matt et al. 2015 How Can We Learn From Each Other? Example 1: Magnetic Braking

10. Christian Knigge University of Southampton School of Physics & Astronoy Inverting the Donor Mass-Radius Relation A Magnetic Braking Recipe based on CV Donors Main Results Above the gap, a slightly suppressed RVJ MB recipe works well -- is this consistent with MB in single stars??? Below the gap, need roughly ≈2.5xGR! -- MB in fully convective / sub-stellar objects? Knigge, Baraffe & Patterson 2011

11. Christian Knigge University of Southampton School of Physics & Astronoy How Can We Learn From Each Other? Example 2: Star Spots and Differential Rotation Smith, Dunford & Watson 2012 Main Results Significant spot coverage Spots appear preferentially - Near L 1 point  Irradiation? - At high latitudes  Polar spots Exception: V426 Oph? Same pattern in the donor of a neutron star X-ray binary Multi-epoch observations  DR AE Aqr: ~265 day lap time  imperfect synchronization Cen X-4 (NS LMXB) Low Sta te

12. Christian Knigge University of Southampton School of Physics & Astronoy How Can We Learn From Each Other? Example 3: Strongly Irradiated Substellar & Planetary Atmospheres Hernandez Santisteban, Knigge et al. 2015, in prep Reprocessing Efficiency = 1 – Bond Albedo Average donor temperature Teff ~ 2000 K  SpT ~ L1 Clear irradiation signatures –Photometric: reflection effect –Spectroscopic: water bands Inferred Bond albedo ≈ 0  reprocessing efficiency ≈ 1 –In line with models of low- mass stars and brown dwarfs! Day/night temperature differences of ~200 K

13. Christian Knigge Summary University of Southampton School of Physics & Astronoy CV donors are fantastic laboratories for stellar physics – They are synchronised, so we know their (fast!) rotation rates –They are Roche-lobe-filling, so we know their shape and can use this to infer their star spot patterns –They lose mass, so we can use them to study the stellar response to mass loss –Their evolution is driven by magnetic braking, so we can use them to test/infer MB recipes –They evolve across the fully convective boundary, so we can test for changes in stellar properties and activity across the boundary –They evolve across the Hydrogen-burning limit, so we can directly observe the transition from the stellar to the sub-stellar regime –Their primaries are nearby and often luminous, so we can use them to study the effect of irradiation on (sub)stellar and planetary atmospheres