Thursday, 1 May 2014

Discovering new debris disks in old Hubble images.

The term ‘debris disk’ is used to refer to any ring of sub-planetary objects about a star; in our Solar System both the Main Asteroid Belt and the Kuiper Belt count as debris disks. In practice, asteroids and comets around other stars are not detectable with current technology, but large volumes of dust are, and since such volumes of dust around a star are expected to have a short lifetime before dispersing (about 10 000 years, in stellar systems with lifetimes of billions of years), where these are found it is assumed that they are being topped up from some source, probably constant collisions within an extensive field of asteroids or comets. Particularly large dusty debris disks are thought to be a sign of planetary formation in young star systems (under 40 million years old), where large protoplanetary disks may have dispersed, but smaller disks associated with planetary formation may still be detected.

In a paper published on the arXiv database at Cornell University Library on 22 April and accepted for publication in the Astrophysics Journal Letters, a team of scientists led by Rémi Soummer of the Space Telescope Science Institute, describe a new search for  detecting debris disks in Hubble Space Telescope images, the Archival Legacy Investigation of Circumstellar Environments (ALICE) project, which uses the Karhunen-Loeve Image Projection (KLIP) algorithm (better software) to analyse old images for debris disks that may not have previously been discovered.

Soummer et al. first used their method to look for the debris disk around HD 181327, a 12 million-year-old F-type yellow-white dwarf star 165 light years from the Earth surrounded by a disk of icy granular material (with grain sizes up to ~1 mm) at a distance of about 90 AU (i.e. 90 times as far from the star as Earth is from the Sun). This disk had previously been detected by the Hubble and Herschel Space Telescopes, so detecting it in old Hubble images where it had not previously been visible, using the KLIP algorithm software should confirm that this method was valid.

Improved residual starlight subtraction using the KLIP algorithm in the ALICE pipeline for the well-known debris disk around HD 181327. The left image was produced using conventional subtraction techniques, while the right image is obtained using KLIP and a library of reference PSFs from the LAPLACE Archive (Legacy Archive PSF Library And Circumstellar Environments Archive, a repository of old Hubble images). KLIP significantly improves the subtraction within 1”5 of the star, as evidenced by the lower residuals within the HD 181327 ring. The newly detected disks presented in this paper lie within this range of angular separation, which explains why they were not seen previously in the NICMOS images. At an angular separation of 1 arcsec, KLIP improves the coronagraphic image contrast by a factor of ~50 over classical PSF subtraction, based on average results from over 7 different coronagraphic images of stars without known circumstellar disks. The reduced apparent surface brightness with KLIP (algorithm throughput) can be calibrated with forward modelling. Soummer et al. (2014).

The team then turned their attention to the debris disk around HD 202917, a 10-40 million year old G-type yellow dwarf star (the same type of star as our Sun) 140 light years from Earth, which is a member of the Tucana-Horologium Association (a group of young stars with a common trajectory presumed to have a common origin). This disk had previously been detected by the Hubble Space Telescope Advanced Camera for Surveys (ACS) Guaranteed Time Observer team in 2007, but Soummer et al. tried to detect the disk in images from 1999, where it had not previously been visible, using the KLIP algorithm software, which would provide further confirmation that this method was valid.

Scattered-light image in F110W (1.1 micron) of the HD 202917 from an image found in the NICMOS coronagraphic archive after processing with KLIP. The surface brightness shown in mJy/arcsec² corresponding to lower limit estimates mainly because of algorithm throughput. HD 202917's disk was previously resolved in HST/ACS coronagraphic images. Soummer et al. (2014).

The debris disk of HD 202917 appears to reach 107 AU from the star (i.e. 107 times the average distance at which the Earth orbits the Sun). It is tilted at an angle of about 70˚ from the line of sight, as seen from our Solar System, and is apparently asymmetric, with the northwest side being significantly brighter and more extended than the southeast side, suggesting a highly perturbed disk. It is apparently made up of two types of component grains, one with a temperature of 75 K (-198˚C) and the other with a temperature of 289 K (16˚C).

Next Soummer et al. turned their attention to HD 30447, a member of the Columba Moving Group (which also contains the exoplanet host star HR 8799). HD 30447 is an F-type yellow-white dwarf star  261 light years from Earth, which is thought to be between 10 and 40 million years old.

Scattered-light images in F110W (1.1 micron) of the debris disk around HD 30447 found in the NICMOS coronagraphic archive after processingwith KLIP. The surface brightness shown in mJy/arcsec² correspond to lower limit estimates mainly because of algorithm throughput. The disk appears nearly edge-on. Soummer et al. (2014).

Soummer et al. found a debris disk around HD 30447, which was almost edge on in the Hubble images. The disk appears to extend from 60-200 AU about the star, and is roughly twice as bright in the northeast as in the southwest. This again appears to be made up of component dust grains at two temperatures, this time 55 K and 101 K (-218˚C and -172˚C).

Next Soummer et al. looked at HD 35841, another member of the Columba Moving Group. HD 35841 is an F-type yellow-white dwarf star 313 light years from Earth, which is also thought to be between 10 and 40 million years old.

Scattered-light images in F110W (1.1 micron) of the HD 35841 debris disk found in the NICMOS coronagraphic archive after processing with KLIP. The surface brightness shown in mJy/arcsec² corresponds to lower limit estimates mainly because of algorithm throughput. The disk appears nearly edge-on. Soummer et al. (2014).

Soummer et al. were able to detect a debris disk around HD 35841, which was again almost edge on in the Hubble images. This disk was quite faint compared to other disks observed, and appeared to be made up of dust grains with a temperature of 68 K (-205˚C). The disk extended out to 144 AU from the star.

Soummer et al. then turned their attention to HD 141943, a magnetically active, rapidly rotating G-type star 218 light years from the Earth, that is thought to be between 17 and 32 million years old, and is considered a good analogy for the young Sun during its planetary formation stage.

Scattered-light images in F110W (1.1 micron) of the debris disk around HD 141943 found in the NICMOS coronagraphic archive after processing with KLIP. The surface brightness shown in mJy/arcsec² corresponds to lower limit estimates mainly because of algorithm throughput. The disk appears nearly edge-on. Soummer et al. (2014).

Soummer et al. were able to detect a debris disk around HD 141943, seen almost edge on at an inclination of 85˚. This disk extends from 47 AU to 167 AU from the star, and is brightest on the southeast side, but otherwise it appears to be the most symmetrical and flattest of the disks discovered. It is comprised of dust particles at two temperatures, 60 K and 202 K (-213˚C and -71˚C). This debris disk is roughly consistent with predictions as to what the Kuiper belt would have looked like when the Solar System was about 10 million years old, although it is several times more massive.

Finally Soummer et al. looked at HD 191089, a member of the β Pictoris Moving Group, which also contains the well documented β Pictoris planetary system (which was first detected due to an extensive dust ring). HD 191089 is a young F-type yellow-white dwarf star 170 light years from the Earth. The system is thought to be between 8 and 20 million years old.

Scattered-light images in F110W (1.1 micron) of the HD 191089 disk found in the NICMOS coronagraphic archive after processing with KLIP. The surface brightness shown in mJy/arcsec² correspond to lower limit estimates mainly because of algorithm throughput. Soummer et al. (2014).

Soummer et al. detected a disk around HD 191089 moderately inclined to the line of sight (about 30˚). The disk is strongly asymmetrical, and its centre is offset from the star by several tenths of an arcsecond. The disk appears to have two components, with grains at temperatures of 53 K and 101 K (-220˚C and -172˚C).

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