Leicester University Physics Department

Current Research

Imaging Planets around White Dwarfs; First Results

A near-IR spectroscopic search for very-low-mass cool companions around notable DA white dwarfs

The DODO Survey: Imaging Planets Around White Dwarfs

WD0137-349: Survival of a brown dwarf after engulfment by a red giant star.

Imaging Planets around White Dwarfs; First Results

In 2001 we began a project to image planets and brown dwarfs orbiting around nearby white dwarfs. Our technique is to obtain two deep near infrared images (J ~ 24) of white dwarfs to search for faint cool companions. The images are separated by = 1 yr, allowing the identification of companions by virtue of common proper motion with the white dwarf. To date (mid-2003), twenty-four white dwarfs have been observed once ("1st epoch") and one has been observed twice ("2nd-epoch").

Stellar evolution considerably increases chances of finding planets because white dwarfs are up to 10,000 times fainter than the stars they evolve from, and planetary orbital radii increase by a factor of three due to evolutionary mass loss in the star.

The proper motion of the 2nd-epoch white dwarf has been succesfully detected, and we have also found some objects which appear to have compatible proper motions. This suggests that they could be physically associated with the white dwarf, and their luminosities indicate very low mass (7-10 Mjupiter), but further observations and analysis are required before we can confidently state that these bodies are low mass companions.

A near-IR spectroscopic search for very-low-mass cool companions to notable DA white dwarfs.

There are many known examples of white dwarfs in binary systems with early-mid M dwarf companions, but few with late-M and cooler companions. We have undertaken a detailed near-IR spectroscopic analysis of eight notable whit

The DODO Survey: Imaging Planets Around White Dwarfs

The aim of the Degenerate objects around Degenerate objects (DODO) survey is to directly image low mass common proper motion companions in wide orbits around nearby white dwarfs. Data is obtained using the twin 8 metre Gemini North and Gemini South telescopes in Hawaii and Chile respectively, and the ESO VLT 8m in Chile. The survey consists of measurements of over 40 nearby white dwarfs, all with measurable common proper motions.

As an example, the diagram below shows the results from our search for planets around one of the nearest white dwarfs to Earth, van Maanen's star (WD0046+051). We have measured the motion of all objects detected in two images, centred on van Maanen's star, from December 2004 and August 2005. The left plot shows the direction and magnitude, multiplied by 20, of each object in the field between the two epochs. As indicated by the large arrow, the only believable movement in this field is by the white dwarf itself. Everything else is simple very distant, background stars and galaxies. The right plot clearly shows the proper motion of the white dwarf in milli-arcseconds, and the large circle enclosing all objects that we deem to be not moving. The scatter of arbitrary "motions" of these objects merely indicates the errors in our measurements.

But our non-detection of planets and brown dwarfs around van Maanen's star is still interesting. The results show with a 90% probability that van Maanen's star has no companions with a mass greater than 9 Jupiters, corresponding to a temperature of just ~330K (60 degrees C!), within an orbital range from 6 to 235AU (one AU is the distance between the Earth and the Sun).

Proper motion measurements for faint objects in the vicinity of van Maanen's star (WD0046+051), where the white dwarf is indicated by the large arrow in the left hand diagram, and lies in the lower right corner of the right hand diagram. ( The DODO Survey: Imaging Planets around White Dwarfs;Hogan, Burleigh and Clarke 2006 , 15th European workshop on white dwarfs)

WD0137-349: Survival of a brown dwarf after engulfment by a red giant star.

We have recently discovered the first example of an amazing binary system consisting of an Earth-sized white dwarf and a brown dwarf orbiting very close to each other. This important result was reported in the August 3rd edition of Nature.

WD0137-349 was first noticed to have a companion from radial velocity measurements made with high resolution spectra. The observed infra-red flux distribution (see diagram below) is consistent with the white dwarf having an old brown dwarf companion with a mass of just 0.055 solar masses (or 55 times the mass of Jupiter; note that its radius is actually very similar to Jupiter!). The two objects, separated by less than 2/3 of the radius of the Sun or only a few thousandths of the distance between the Earth and the Sun, rotate around each other in about 2 hours. The brown dwarf moves on its orbit at the amazing speed of 800,000 km/h!

The two stars were not so close in their past. Only when the solar-like star was a red giant, did the separation between the two objects diminish drastically. During this time, the giant engulfed its companion. The latter, feeling a large drag, spiralled in towards the core of the giant. The envelope of the giant was finally ejected, leaving a binary system in which the companion is in a close orbit around a white dwarf. (See http://www.star.le.ac.uk/~mbu/browndwarf.html for letter published in Nature).

Infra-red flux distribution of WD0137-349