Eclipsing White Dwarves

Graduate student at the University of California at Santa Barbara finds edge on transiting binary white dwarf system, with self-microlensing!

Ok, this is just way too cool...


i-32ecefa1b83b732cb8f2fbc93f9c9487-double-WD.jpg
artist conception - click for full res version

The modestly named NLTT 11748 is a dim helium white dwarf about 500 light years away.

Helium white dwarfs are the partially burned out cores of moderate mass stars, whose final stages of evolution as giants were interrupted, generally by a companion stars ripping off the giant envelope, leaving the partially burned core, typically at about 0.2 solar masses, composed primarily of helium, and with a radius of about 25,000 km - less than half the radius of Jupiter!

Such stars are not uncommon, but are faint and hard to find, and play an important role in stellar evolution of binary systems, and also are expected to be the progenitors of interesting sub-classes of novae, leading to significant contribution to nucleosynthesis channels for some of the heavier elements.

Graduate student Justin Steinfadt at UCSB working with Lars Bildsten and David Kaplan (KITP), and Steve Howell at NOAO, has been monitoring a number of white dwarfs, using the LCOGT Faulkes Telescope North, doing high cadence imaging to look for variability.

With the help of Avi Shporer of LCOGT, the variation was confirmed to be a transit!
There was a 3 minute long dip in the brightness of the optical primary, with an interval of 5.64 hours.

NLTT 11748 is an eclipsing binary with a transiting high mass white dwarf as the secondary!

The optical secondary, but dynamical primary, is about 0.7 solar masses, with a radius about that of the Earth, and is orbiting in an edge on orbit which allows it to partially eclipse the lower mass white dwarf. It is the fully burnt out core of a star that evolved through the giant phase, leaving a Carbon/Oxygen core (with maybe some Neon).

The high mass white dwarf is the star that ate the envelope of the low mass white dwarf progenitor. It is about 30 times fainter than the lower mass star, the two orbit each other at about 600 km/sec, or a respectable 0.2% of the speed of light.
The system is emitting gravitational radiation, but won't merge for another 5-10 billion years.

Even more funky, there is measurable self-lensing of the light from the lower mass white dwarf by the higher mass star.
Dave Kaplan has put together a very nice animation illustrating the effect.
As the higher mass, more compact star passes in front of the primary, its gravitational field deflects the light from the primary, distorting and magnifying its image.

Followup observations ought to provide strong calibration on the mass and radius of the objects, testing theories of formation and cooling of white dwarfs, and their atmospheres.
More precise knowledge of their masses will also help determine the final fate of the system when it merged in a few billion years and whether it might detonate in a thermonuclear explosion.

Steinfadt et al. discovery paper (arXiv) - to appear in ApJL

Really nice discovery.

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Now that is COOL!

Lucky grad student -- monitoring a bunch of white dwarfs for your thesis is normally the kind of project only an advisor could love (i.e. right thing to do scientifically, but tedious to execute with a non-zero probability of a null result). Remember all those poor folks doing high-z lyman alpha emitter searches back in the 80's and early 90's?

Might want to mention that first "photo" is an artist's conception.

Nice result! It makes me wonder, though, whether this "self-microlensing" has ever been seen in a neutron star-white dwarf binary. Certainly the lensing effects on the NS pulse arrival times are a standard tool for measuring companion masses, but I'm not sure anyone's seen the optical effects on the light from the WD. (For that matter, the double pulsar is "transiting" in the sense that the magnetosphere of one pulsar blocks the pulses from the second, but the NSs are so tiny they don't actually transit in front of each other. Shame, that, or we'd get a radius measurement...) If you did see a NS transit in front of a WD, would you be able to get a mass and radius measurement? I guess you'd probably need incredible signal-to-noise and a tight binary, since the transiting object is so tiny and the lensing effects so large.

really cool..
Setting aside feasibility, do you think the discovery of an NS transiting a WD would merit consideration for the Nobel Prize? :)
I don't think there other model-independent observational methods to determine the radii of NSs, are there?
Anyway, given a 10 km radius NS and 1 earth radius WD, and ignoring GR effects, the amount of the white dwarf surface blocked by an NS would be 2.5 parts-per-million. I'd be curious to see how lensing would affect the transit depth. Anyway, I'm sure it would be a tall order for observers!

i'm guessing that such an NS-WD pair would be easier to find by the WD eclipsing a pulsar. i don't even know how rare WD-pulsar binaries are, and then you are prob killed by the eclipse probability. Another interesting question is how lensing affects the eclipse probability :) It must enhance the probability a little...

pout: NS-WD binaries are pretty common really - there are tens known. In many of them the WD eclipses the NS, though usually it's gas streaming out from the WD rather than the WD body itself. But if my mental picture of the geometry is right, any system in which the WD passes in front of the NS also has a time when the NS passes in front of the WD. Unfortunately, almost all of these NS-WD binaries are extremely faint as optical stars - we found them because the NS is a pulsar, which we can detect from more than a kiloparsec, and at that distance WDs are very feeble optical sources. Nevertheless it's an interesting question - are there any such binaries close enough to observe the transit optically?

Unfortunately, there's a lot going on in these systems. For one thing, a radio pulsar will typically be putting out more energy than its companion in the form of a wind of mixed electrons and positrons, which heats the side of the companion facing it. And in most of these systems the companion is filling its Roche lobe, or nearly, so it's not even spherical. So inferring anything from the transit is going to be a real challenge.

Thanks for the info Anne! Interesting stuff.