The SW Sextantis starsand the

evolution ofcataclysmic variables

Pablo Rodríguez Gil

5 January 2006

Summary

Introduction.

Cataclysmic variable structure and evolution.

The SW Sextantis stars: new insights on their accretion structure.

Towards a global understanding.

The HQS and SDSS and CV evolution.

Astrophysical context

Most of the stars are born in binary or multiple systems.

~50% of the binaries will interact at some point (Iben 1991).

Many exotic astrophysical objects (e.g. binary pulsars, black holes, LMXBs, symbiotics…) are descendent from binary systems.

Type Ia supernovae: accretion on to white dwarfs. Cosmological distance scale.

Compact binaries

They harbour a compact stellar remnant (i.e. WD, NS or BH).

Their evolution critically depends on the angular momentum loss rate (dJ/dt).

t Detached binary

Common envelope

AML; a decreases

Envelope ejection

AML: magnetic braking or gravitational radiation

Contact

Summary

Introduction.

Cataclysmic variable structure and evolution.

The SW Sextantis stars: new insights on their accretion structure.

Towards a global understanding.

The HQS and SDSS and CV evolution.

What is a cataclysmic variable?

Donor star

Bright spot

White dwarf

Gas stream

Accretion disc

~ Main sequence

Secondary star

Late type (K-M)

M2~ 0.05-1 M

White dwarf

Primary component

<M1> ~ 0.7 M

CVs have magnetic fields…

…accretion discs…

…or both!

The intermediate polar CVs

The height of the shock front depends on the cooling mechanisms in the column.

The emission spectrum of the column:

108 K

105 K

Bremsstrahlung: Hard X rays (kTbr ~ 30 keV)

Compton: UV and soft X rays (kTBB ~ 40 eV)

Cyclotron: IR-optical-UV (POLARISED)

Magnetic accretion

Orbital period distribution

Ritter & Kolb (2003): 496 systems

Gives observational input on the AML rate

Flashback – 1983: ‘Disrupted magnetic braking’

Paczynski & Sienkiewicz; Spruit & Ritter; Rappaport et al. (1983)

Two AML mechanisms:

Magnetic braking (stellar wind) & gravitational radiation

MB+GR GR

Standard theory predictions

- Paucity of the number of CVs in the range Porb=2-3 hr

The ‘period gap’

Standard theory predictions

- Minimum Porb of ~ 65 min

- Paucity of the number of CVs in the range Porb=2-3 hr

The minimum orbital period

~80 min!

Standard theory predictions

- Minimum Porb of ~ 65 min X

- Paucity of the number of CVs in the range Porb=2-3 hr

- Pile-up of systems at Pmin

Population syntheses: the minimum period

Kolb & Baraffe (1999)

The minimum orbital period

~80 min!

Standard theory predictions

- Minimum Porb of ~ 65 min X

- Paucity of the number of CVs in the range Porb=2-3 hr

- 99% of all CVs should have Porb < 2 hr

- Pile-up of systems at Pmin X

Orbital period distribution

191=38%250=51%

55=11%

Standard theory predictions

- Minimum Porb of ~ 65 min X

- Paucity of the number of CVs in the range Porb=2-3 hr

- Pile-up of systems at Pmin X

- 99% of all CVs should have Porb < 2 hr X

Standard theory predictions

- Minimum Porb of ~ 65 min X

- Paucity of the number of CVs in the range Porb=2-3 hr

- Pile-up of systems at Pmin X

- 99% of all CVs should have Porb < 2 hr X

- CV density ~

Observed ~ X

We are in deep trouble!

An alternative AML prescription

Verbunt & Zwaan (1981) vs. Sills et al. (2000).

Nevertheless, something is happening above the gap……………

Summary

Introduction.

Cataclysmic variable structure and evolution.

The SW Sextantis stars: new insights on their accretion structure.

Towards a global understanding.

The HQS and SDSS and CV evolution.

The SW Sextantis stars

- Unusual spectral features, inconsistent with a standardoptically thick, geometrically thin accretion disc.

- Extremely high mass accretion rates.

- ~50% of all CVs in the 3-4 hr strip are SW Sextantis stars.

- No place for such maverick systems in the standardevolution theory.

Trailed spectra. Pulsed S-wave (Rodríguez-Gil et al. 2001)

LS Pegasi

Pulse separation

~ 0.1 Porb

A likely magnetic nature

A likely magnetic nature

V533 Her (Rodríguez-Gil & Martínez-Pais 2002)

DW UMa (Smith et al., unpublished)

- Emission-line flaring also characteristic of IPs.

PEW = 33.5 2.2 min

A likely magnetic nature

LS Pegasi

Circularly polarised continuum.

P1 = 29.6 1.8 min

1/ P1 - 1/PEW = 1/Porb

(PEW synodic period)

Cyclotron radiation

B1 ~ 10 MG

A likely magnetic nature

LS Pegasi

(Rodríguez-Gil et al. 2001)

Strong HeII 4686 emission, typical of mCVs.

A likely magnetic nature

RX J1643.7+3402

(Martínez-Pais, de la Cruz & Rodríguez-Gil, submitted)

Discovery of circular polarisation.

P1 = 19.4 min

RX J1643.7+3402

(Martínez-Pais, de la Cruz & Rodríguez-Gil, submitted)

A likely magnetic nature

Variable HeII 4686 EW.

P =25.5 min (coherent for at least 15 cycles).

V1315 Aql

A likely magnetic nature

Light curve oscillations (QPOs).HS 0728+6738

A likely magnetic nature

Summary

Introduction.

Cataclysmic variable structure and evolution.

The SW Sextantis stars: new insights on their accretion structure.

Towards a global understanding.

The HQS and SDSS and CV evolution.

Magnetism and CV evolution

- Magnetic fields can play a crucial role in CV evolution.

- At least half the CVs in the 3-4 h range can be magnetic(only 3% of isolated WDs are!).

A comprehensive study of the SW Sextantis stars is therefore mandatory.

1) Masses involved.

2) Search for more systems.

3) Circular polarimetry.

Weighing the components

- SW Sextantis stars ocassionally fade. ’LOW STATES’.

- The absence of DN-type outbursts during the low statessupports a magnetic scenario (Hameury & Lasota 2002).

(Honeycutt & Kafka 2004)

Weighing the components

- Photometric monitoring campaigns in the North and South.

- ToO programmes at the VLT, and the WHT and NOT.

HS 0220+0603

Sp(2)=dM3-4 V

T1 > 25000 K

d ~ 0.7-1.0 kpc

VLT

Weighing the components

- First donor star radial velocity curve at the WHT (R = 19.4).

HS 0220+0603

K2 = 330 km/s

i = 83º

M2 = 0.3 M

M1 = 1.0 M

Weighing the components

- WD exceeds the average mass of 0.65 M.

- T1 is unusually high. DW UMa has ~ 50000 K.

- Secular heating of the WD. Why such a high transfer rate?

Fundamental to measure the physical parameters of a large sample of

systems!

A growing family

- Search programmes in both hemispheres.Linda Schmidtobreick (ESO), Boris Gänsicke (Warwick, UK).

- Targets: CVs in the 3-4 h orbital period range. Asymmetric line profiles with enhanced wings. Short-time scale photometric variability (QPOs). Presence of the HeII 4686 line and Bowen. Low states. ...

- Preliminary results in the south show great success.

A growing family

V380 Ophiuchi

NTT + WHT

P = 3.72 hr

A growing family

AH Mensae

NTT

P = 2.97 hr

Summary

Introduction.

Cataclysmic variable structure and evolution.

The SW Sextantis stars: new insights on their accretion structure.

Towards a global understanding.

The HQS and SDSS and CV evolution.

The HQS and the SDSS

- The current CV population is a mixed bag (novae, DNoutbursts, rapid variablility, blue colour, X rays).

- Need for an UNBIASED CV sample.

- The HQS and the SDSS CVs are spectroscopically selected.

The HQS: 53 new CVs, 35 Porb

SW Sextantis excess @ 215 min (20%)

The HQS and the SDSS

- The current CV population is a mixed bag (novae, DNoutbursts, rapid variablility, blue colour, X-rays).

- Need for an UNBIASED CV sample.

- The HQS and the SDSS CVs are spectroscopically selected.

- The SDSS (g < 21) have provided ~120 new CVs.

The SDSS: 120 new CVs, 45 Porb

The HQS and the SDSS

- The current CV population is a mixed bag (novae, DNoutbursts, rapid variablility, blue colour, X-rays).

- Need for an UNBIASED CV sample.

- The HQS and the SDSS CVs are spectroscopically selected.

- The SDSS (g < 21) have provided ~120 new CVs.

The period distribution of the SDSS CVs will serve as afundamental test to the standard theory.

Major revision is expected…

The SW Sextantis starsand the

evolution ofcataclysmic variables

Pablo Rodríguez Gil

5 January 2006