Planets around stellar remnants II

The morning of the second day was dedicated to planets orbiting white dwarfs (WDs). Matt Burleigh opened with an overview of recent attempts to detect substellar objects around WDs with direct imaging. So far there are few positive results, with the UKIDSS survey having turned up a few candidates, showing maybe a half of a per cent of white dwarfs host wide L/T dwarf companions, and a possible Y dwarf companion to GJ3483 in a very wide (~2500 AU) orbit. Meanwhile, constraints from the DODO survey show that <5% of WDs host companions heavier than 13 Jovian masses on orbits between about 10 and 60 AU, and less than a third host such companions heavier than 6 Jovian masses. A great number of such companions is perhaps not to be expected, given that they are not seen around many main sequence stars.

Hans Zinnecker then gave another negative result, from a small NICMOS survey of 7 Hyades WDs, none of them having companions heavier than 10 Jovian masses. WDs in cludters are very useful to study, since their total age can be determined with reasonable accuracy, and the age is needed to convert a brown dwarf flux into an actual mass.

JJ Holmes then talked about the use of the timing of WD oscillation modes to detect planets. Periodic signals that could arise from acceleration due to planets have been observed from several pulsating WDs. Among them is GD66 (aka V361 Aurigae), where a planet on a ~6 year orbit was announced in 2007. Since then, another “orbital” cycle has been observed, which would seem to strengthen the attribution of the signal to a planet rather than some quasi-periodic effect. However, when the timing variations of a separate mode were analysed, it was found to vary with the same period but exactly the opposite phase. Hence, the changes cannot be due to the acceleration of the whole star due to the planet, and the signals must be due to some as-yet unidentified asteroseismological mechanism. This underlines the importance of confirming “planetary” signals from timing analysis with independent measurements.

Wei Wang next presented evidence for the existence of a brown dwarf or low-mass red dwarf orbiting the young WD at the centre of the planetary nebula NGC 246. A radial velocity signal with a period of 4 days and amplitude of ~9 km/s was seen, which could be due to an object of mass at least 60 times Jovian. There also appears to be a an IR excess, indicative of hot dust or a companion radiating in the infrared. There was some criticism from the audience, since planetary nebulae are very confusing environments and contamination of the spectra could be a serious issue.

To end tuesday morning’s talks, Steven Parsons discussed the detection of companions by timing eclipsing WD binaries. He went through a similar list of caveats as mentioned by Richard Wade on Monday: timing variations may be due to angular momentum loss from winds, the Applegate mechanism, star spots, apsidal precession, and of course planets. Of the eclipsing binaries with proposed planets, NN Serpentis seems the most secure detection, particularly as newer data has vindicated the original ephemeris.

On Tuesday afternoon we were dedicated to a tour of the Observatory. This was somewhat rushed, since the megawatt RADAR transmitter was needed urgently for tracking a Near-Earth Object. While the health risks posed by 20W mobile phone towers are not credible, having a 4MW RADAR pulse aimed at you is probably a bad idea. So first we hurried to cross the narrow catwalk to the central antenna platform, suspended 450 feet over the main mirror:

The world's most handsome 'blog author prepares to cross the catwalk to the central antenna platform.

We set out…

We set out across the bridge.


Looking down from the central platform. You can see the rectangular waveguide heading back to the control room on the left.

The views from the top were pretty spectacular:

The view back down the bridge to the visitor/conference centre.

After going over the dish, we went under it. Rather than resting on the ground, the mirror is suspended about 4m above it, allowing its shape to be controlled to milimetric precision.

Underneath the dish. It feels like being in a really big athletics stadium.

From the very centre, you can look right up to the receiver structure:

This thing is 100m long.

The tour concluded with a brief look at the control centre, which was a series of rooms full of computer hardware. Not so impressive, after seeing the telescope itself!

The day ended with a drinks reception at the observatory pool, where Marc Kuchner chatted about his book Marketing for Scientists, aiming to help scientists to argue more forcefully for the value of science in society, and give practical careers advice for young scientists.