Monday 5 May 2014

The debris disk around HD 4796 A.

The term debris disk is applied to any ring of sub-planetary sized objects around a star. In our Solar System both the Main Asteroid Belt and the Kuiper Belt would qualify as debris disks, though in practice neither of these belts would be detectable around another star with current technology. However younger star systems often have much more detectable debris disks, containing extensive fields of dust from frequent collisions from asteroid or comet sized bodies (such collisions probably petered out when our Solar System was about 40 million years old). To date over three dozen such debris disks have been discovered around young stars, and are taken as signs of planetary formation occurring or having occurred in the very recent past, and planets have been subsequently discovered in a number of systems with debris disks.

HD 4796 A is an A0 type white dwarf star with a mass 2.2-2.4 times the mass of our Sun around 237 light years from Earth in the constellation of Centaurus, which is thought to be 8-10 million years old. This star has been known to have a debris disk since 1998. This is viewed at an inclination of 75.88˚ when viewed from Earth, and is brighter in the northwest than in the southeast. In centre of the ring was found to be slightly offset from the star. It has been suggested that this asymmetry may be due to the ring receiving a variation in stellar radiation around its circumference, or possibly due the presence of a M type red dwarf companion star. The ring is apparently unusually narrow, with an inner edge 76.4 AU from the star (i.e. 76.4 times as far from the star as the Earth is from out Sun, and an outer edge that appears to become rapidly more tenuous as it gets further from the star. It has been suggested that the dust is being created by constant collisions between small planetesimals within a narrow ring, and the tailing off on the outer edge is caused by the dust being blown away by the pressure of radiation from the star.

In a paper published in the journal Astronomy & Astrophysics on 29 April 2014, and on the arXiv database at Cornell University Library on 25 April 2014, a team of scientists led by Zahed Wahhaj of the European Southern Observatory describe the results of a new study of HD 4796 A using the Near Infrared Coronagraphic Imager on the Gemini-South 8.1 meter Telescope, and discuss the results of this study.

Hahaj et al. observed HD 4796 A on 14 January 2009 and 6 & 7 April 2012, at a total of five different wavelengths (different materials radiate and reflect light at different wavelengths – giving them distinctive colours – as well as using this to understand the physical properties of observed objects, astronomers can use observations at different wavelengths to filter out background interference), thereby producing a better model of the system than had previously been the case.

A JHKs false-color image of the HR 4796 A ring, showing its relative reflectivity, as estimated in section 4.4. North is up and east is left (the directions east and west are reversed in astronomical images as the observer is looking up). The J, H and Ks-bands are coloured blue, green and red, respectively. The unsaturated star is normalized to one in all the bands and appears white in the image. The ring appears yellow because it reflects light more efficiently in the H and Ks-bands than in the J-band. The noise-dominated regions with 0.2′′ to 0.5′′ separation from the star are not shown. Regions away from the ring are coloured white and given the median intensity of the three bands. Wahaj et al. (2014).

Based upon these observations Wahaj et al. were able to construct a better model of the HD 4796 A system. This confirmed that the ring is offset compared to the star, and also showed the ring to be narrower and more confined than previously thought, restricted to between 71.7 and 86.9 AU from the star.

Wahaj et al. conclude that the most likely explanation for these observations is an undetected planet in the HD 4796 A system. Planets are often used to explain sharp edges to debris disks, but this does not happen simply by the planet hovering up material gravitationally, but rather by the action of tidal forces. 

Objects orbiting at certain distances from larger bodies are in stable areas and free from the tidal influences of the larger body. However objects close to these distances are subject to much stronger tidal forces, either locking them into the stable point or pushing them away from it. Some of these stable points are found at orbital resonances with the larger object; thus a small body in a 2:1 or 3:2 resonance with a large body is in a stable orbit (an object in a 2:1 resonance with another completes 2 orbits for every one of the other’s), but one in a 2:1.05 or 2.95:3 resonance will be ejected from the system by tidal forces. Since getting into an exact resonance with a large body with a slightly eccentric orbit is in practice very hard to do, bodies are effectively herded away from these resonances, and trapped into safe zone between them. In our Solar System there are several instances of this, for example the Kuiper Belt has sharply delaminated inner and outer limits, which coincide with the 2:1 and 3:2 orbital resonances of Neptune.

Wahaj et al. calculate that a planet orbiting at a distance of 54.7 AU from HD 4796 A would produce a 2:1 orbital resonance at 71.7 AU from the star and a 3:2 orbital resonance at 86.9 AU from the star, and therefore conclude that the presence of such a planet is the best explanation for the observed properties of the debris disk. In order to produce the observed properties the planet would have to have a mass of less than 1.6 times that of Jupiter (and could potentially be as small as three Earth masses), considerably smaller than would have been detected directly with the instruments that have been directed at the system so far. The offset observed in the orbit of the debris disk almost certainly reflects the eccentricity of the planet’s orbit.

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