Planets around evolved stellar systems

Tom Marsh

Department of Physics, University of Warwick

Tom Marsh, Department of Physics, University of Warwick Slide 1 / 35

Outline

1. White dwarfs

2. White dwarf binary stars

3. ULTRACAM

4. Eclipsing white dwarfs

5. Timing measurements

6. A role for variable star observers?

7. Conclusions

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White Dwarfs

The most common end-point ofstellar evolution; ∼ 95% of allstars > 1M� become whitedwarfs.

A typical white dwarf iscomparable in size to Earth, butwith a mass similar to the Sun,they have mean densitiesρ ∼ 1 tonne per cc.

There are around 1000 millionwhite dwarfs in our Galaxy.

NGC 6543 (Cat’s Eye), with nascentwhite dwarf (HST)

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White dwarfs & Type Ia Supernovae

Exploding white dwarfs arethought to make the Type Iasupernovae (2011 Physics NobelPrize → “dark energy”):

1. Add mass to a white dwarf

2. As mass nears 1.4M�, thewhite dwarf shrinks andcompresses.

3. Carbon/Oxygen materialundergoes runaway fusion. . .

Khoklov et al (1997)

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White dwarfs & Type Ia Supernovae

Exploding white dwarfs arethought to make the Type Iasupernovae (2011 Physics NobelPrize → “dark energy”):

1. Add mass to a white dwarf

2. As mass nears 1.4M�, thewhite dwarf shrinks andcompresses.

3. Carbon/Oxygen materialundergoes runaway fusion. . .

Khoklov et al (1997)

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Tycho’s supernova was a Type Ia

Remnant of an exploded whitedwarf in X-rays (Tycho’s SN of1572, now about 30 light-years

across).

Light-echo from Tycho’s SN showsthat it was a Type Ia

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White Dwarfs in Binary Stars

Modern astronomical surveysare producing large numbers ofwhite dwarfs in binary systems.

So many, that even theexclusive class of eclipsingsystems is growing rapidly −→

Number of eclipsers known vs time

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CVs – accreting white dwarfs

Cataclysmic variables are along-lived phase of evolutionin which mass transfer isdriven by weak angularmomentum loss.

They give us the chance tomeasure the build-up orotherwise of mass on whitedwarfs.

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Detached WD+MS systems

Simple systems withspectra which are acombination of whitedwarfs and low-massmain-sequence stars.

Can be detected throughsimultaneous flux excessin ultraviolet and redfilters.

Around 2000 now known,mostly from the SloanDigital Sky Survey.

Pyrzas et al (2009), spectra of 4 eclipsingWD/dM systems.

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ULTRACAM

White dwarfs are small, withorbital speeds at their surfacesof ∼ 4000 km s−1.

⇒ They must be observed fastto resolve variations, typically1 to 30 sec.

In May 2002 we commissioneda new high-speed camera tofacilitate observations of whitedwarf binary stars. ULTRACAM mounted on the 4.2m

WHT in La Palma

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ULTRACAM

To reduce light-losses due tofilters, in ULTRACAM we splitthe light up into three bands,UV, green and red, beforeimaging.

The detectors used are “frametransfer” CCDs in which oneexposure is taken while theprevious one is being read out.Can take 100s offrames/second with highefficiency.

Timestamps are taken fromthe GPS. ULTRACAM design

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ULTRACAM

ULTRACAM and twoastronomers

ULTRACAM at the 8.2m VLT,Cerro Paranal, Chile

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White dwarf / main-sequence eclipsers

Exquisite measurementsare possible on largetelescopes (the VLT inthis case).

[Magnitude: V = 17.0,eclipse length: 10 mins,exposure time: 1.5 sec.]

Parsons et al (2010a), GK Vir, a hot WD+ 0.1M� M-dwarf eclipser.

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White dwarf + very low mass starVery dim companions areoutshone by the whitedwarf at all wavelengths.

Rise in between eclipsescaused by heating of coolstar by the white dwarf.

The dip at the top of thelightcurve occurs as thewhite dwarf transits theheated face.

TW = 35, 300K,

MW = 0.51M�,

MC = 0.09M�.

Parsons et al (2011), CSS 03170,P = 94 mins

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NN Serpentis

In the best cases,complete solution ofMW , RW , MR , RR and iis possible. e.g NN Ser

MW = 0.535± 0.013M�

RW = 0.0211± 0.002R�

MR = 0.111± 0.004M�

RR = 0.149± 0.002R�

i = 89.6◦ ± 0.2.

Parsons et al (2010b), NN Ser.

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Testing “Equations of State”

These are good enough to testmodels of both white dwarfs andvery low mass stars (and,potentially, brown dwarfs),themselves dependent upon thebehaviour of high density matter.

Parsons et al (2010b)

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Timing studies

One motivation for high-speedwork is to study the angularmomentum loss that drivesbinary evolution.

A steady rate of period changeP alters the times of eclipse by aquadratic function of time

∆t =1

2

P

Pt2.

In practice, rather erraticvariations always seem to be therule. −→

Period changes in WD/dM systemQS Vir, O’Donoghue et al (2003)

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Applegate’s Mechanism

Applegate (1992) suggestedthat such variations are driven byvariations in the shape of the MSstar driven by solar type cycles.

No angular momentum is lost inthe process; Applegate’smechanism is not a driver ofevolution.

Applegate’s mechanism doesrequire energy however.

Variations in shape alter thegravitational attraction between the

stars

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Violating Applegate

We have found that some systemsshow too large a period change forthe MS star to have supplied theenergy needed.

Here the eclipses in QS Vir areseen to arrive ∼ 200 sec earlierthan expected from O’Donoghueet al.’s data.

It seems certain that there is a“third body” orbiting the binary,probably a brown dwarf

Last 3 O’Donoghue

times

ULTRACAM

ULTRACAM

QS Vir, Parsons et al (2010)

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“Third bodies” from timing

Unseen object

Unseen object

different time Eclipse arrival time

delayed or advanced

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Planets!

NN Ser is the best measuredsystem to date.

Its timing variation in NN Sercan be fit to within very smallerrors with two planets,Mc sin i = 6.9MJ ,Md sin i = 2.2MJ in 15.5 and7.7 yr orbits.

Two planet fit to NN Ser times,Beuermann et al (2010)

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Planets around a CV

Here, Mc sin i = 6.3MJ ,Md sin i = 7.7MJ , with periodsof 16 and 5.3 years.

System here is accreting and somay be subject tovariability-induced scatter.

In optimum cases such asNN Ser, timing is sensitive toplanets of a few Earth masses inlong period orbits. Two planet fit to UZ For times,

Potter et al (2011)

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Liverpool Telescope Project

Even well-measuredsystems can be hard topin down, andwell-spaced sampling isneeded.

⇒ Start using roboticLiverpool Telescope datato extend coverage.

Some possible orbits of NN Ser given thedata in hand.

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First LT results (started Feb 2011)

Upper panel: an eclipse of NN Ser;lower panel: QS Vir times.

More than 30 eclipses of differentsystems measured.

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All systems show timing anomalies . . .

RR Caelum RXJ2130.6+4710

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Are the “planets” really there?

Orbital stability canplace strongconstraints uponmultiple-planetsystems.

In NN Ser, theplanets seems to beclose to a 2:1resonance.

New data has significantly shrunk theallowable parameter space, but is still

consistent with the 2:1 resonance.

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Perhaps not always . . .

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Test with double eclipsing white dwarfs

Pairs of eclipsing of white dwarfsshould be much less vulnerable tostellar-induced timing noise thanwhite dwarf/main-sequence binaries.

Although the chances are low, thefirst such system, NLTT 11748, wasdiscovered last year.

0.2M� + 0.7M� pair of whitedwarfs in a 5.6-hour, at 0.1◦ frombeing exactly edge-on.

The eclipses are total but only 6%and 3% deep.

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A second eclipsing DWD

This year we discovered a second with 40% and 10% deep eclipsesusing Liverpool Telescope and Gemini data.

Remarkably 4 are now known. If these systems show planets, it willbe a firm indication of their reality. Watch this space!

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Origin of the circum-binary planets

• Systems like NN Ser are only a solar radius or so apart, withorbital periods of ∼ 10 hours or less.

• However prior to the formation of the white dwarf they were∼ 1.5 AU apart, raising problems with stability of the planets.

• Two possibilities:

1. The planets predated the white dwarf and spiralled in as thewhite dwarf lost its envelope

2. The planets formed out of the material lost by the white dwarf.

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Calling Variable Star Observers

• In several cases we are limitedby coverage with large gaps.

• Some systems have variationsof order a minute or more.

• There are a few systems –V471 Tau, QS Vir, DE CVn,RXJ2130.6+4710 and (forAustralians) RR Cae – withinreach of small telescopes.

Please contact me attom.marsh@warwick.ac.uk if you areinterested.

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. . . and a couple of longshots . . .

White dwarfs could be totally eclipsed by any planets they host.Two interesting possibilities:

• G24-9 aka V1412 Aql: V = 15.75 white dwarf with tworeports of ∼ 2 mag. eclipses. Arno Landolt requested anAAVSO campaign in Feb 2009. No reports of success, butvery interesting to continue this.

• EG 131: V = 12.3, a white dwarf but reported as varying by±0.35 mag.

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V471 Tau with ULTRACAM

V471 Tau (V = 9.8) isbright, but lacks nearbycomparison stars(ULTRACAM FOV = 5to 6’).

The eclipse is muchstronger in the UV thanin red light, so withULTRACAM we can useV471 Tau as its owncomparison!

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V471 Tau with ULTRACAM

V471 Tau (V = 9.8) isbright, but lacks nearbycomparison stars(ULTRACAM FOV = 5to 6’).

The eclipse is muchstronger in the UV thanin red light, so withULTRACAM we can useV471 Tau as its owncomparison!

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V471 Tau with ULTRACAM

V471 Tau (V = 9.8) isbright, but lacks nearbycomparison stars(ULTRACAM FOV = 5to 6’).

The eclipse is muchstronger in the UV thanin red light, so withULTRACAM we can useV471 Tau as its owncomparison!

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Conclusions

• Large surveys are hugely increasing the number of eclipsingwhite dwarf binaries enabling precision parameter studies.

• These require high time resolution and large apertures toexploit fully.

• The same studies give precise eclipse times.

• Almost all eclipsing detached systems show timing anomaliesthat can be interpreted in terms of planets. It remains to beseen whether this will stand the test of time.

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