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2 November, 2018

Time Resolved Spectroscopy of Dust and Gas from Extrasolar Planetesimals

Most Milky Way stars are orbited by planets, and more than 97% are expected to end their lives as white dwarfs. The evolution of host stars after the main sequence will change the properties of the original planetary systems. While planets closer than approximately 5 astronomical units will most likely not survive the post-main-sequence lifetime of their parent star, any planet further away will survive.

Some planets in previously stable orbits around a star undergoing mass loss will become dynamically unstable, and may get sufficiently close to the white dwarf that tidal forces will be capable of disrupting them. The indirect evidence of such disrupting events is the presence of debris discs around at least 4% of white dwarfs, and metal lines in the spectra of around 25-50 % of white dwarfs.

So far the most compelling confirmation of a remnant planetary system has been the detection of asymmetric transits in the light curve of the white dwarf WD 1145+017, caused by material from possibly disintegrating planetesimal orbiting near the Roche limit.

WD 1145+017 also shows an infra-red excess characteristic for a dust disc, and metal photospheric lines in its spectrum. In addition, a circumstellar gas disc was detected in the system.

Transits of WD 1145+017 showed several distinct periods in the range of 4.5-4.9 h and were variable in depth. The system evolved quickly, with the overall dip activity increasing from 2014 until mid-2017, with only a small activity decrease in mid-2016.

An international team of astronomers led by Marie Karjalainen (Isaac Newton Group of Telescopes, Spain), used the QUCAM CCDs mounted on ISIS at the William Herschel Telescope to obtain time resolved spectroscopy of dust and gas from the extrasolar planetesimals orbiting WD 1145+017.

The QUCAM detectors are electron-multiplying frame-transfer CCDs, capable of performing high-speed or faint-target spectroscopy, with minimal dead-time and essentially zero readout noise.

Observations were taken both in the blue arm, covering 3800-4025 Å, and in the red arm, covering 7000-7430 Å. When comparing the transits in both arms, observations show significant colour difference between the in- and out-of-transit data of the order of 0.05-0.1 mag (deeper in the red arm). This 'bluing' is surprising and not usual in dusty environments, which typically show reddening.

For each transit T1-T7 the upper panels show light curves of WD 1145+017 observed during the night of 2016 January 7 with the ISIS/QUCAM2 red arm (red points) and ISIS/QUCAM3 blue arm (blue triangles). The lower panels for each transit show the difference of the normalized flux between the red and blue arms (black solid line). For all transits except T6 there is a colour difference of the order of 0.05-0.1 mag, with transits being deeper in the red arm. Transit T6 is most likely not a real transit event, but is a result of imperfections of the differential photometry when thin clouds were passing by. Credit: M. Karjalainen. Large format: [ PNG ].

Analysing individual spectra showed that spectral lines in the blue are significantly shallower during transits than during out-of-transit. For the circumstellar lines it also appears that during transits the reduction in absorption is larger on the red side of the spectral profiles.

For the three main detected lines in the blue arm and the HeI line in the red arm, top panels show average of all out-of-transit (green solid line) and all except transit T6 in-transit spectra (black dashed line). Also shown is the in-transit spectrum during transit T2 (red dash-dotted line), which was the deepest transit during observations. Middle (bottom) panels show a difference between average out-of-transit and in-transit spectrum (spectrum during transit T2) (black solid line). The black dashed horizontal lines in the two lower panels for each line indicate the 3 sigma significance level. Results show that spectral lines in the blue arm are significantly shallower during transits than during out-of-transit. Credit: M. Karjalainen. Large format: [ PNG ].

The new results confirm previous findings showing the u'-band excess and a decrease in line absorption during transits. Both can be explained by an opaque body blocking a fraction of the gas disc causing the absorption, implying that the absorbing gas is between the white dwarf and the transiting objects.

The new results also demonstrate the capability of QUCAM CCDs at the William Herschel Telescope to perform high-quality time resolved spectroscopy of relatively faint targets (r = 17.3 mag).

More information:

Marie Karjalainen, Ernst J W de Mooij, Raine Karjalainen, Neale P Gibson, 2018,"Time-resolved spectroscopy of dust and gas from extrasolar planetesimals orbiting WD 1145+017", MNRAS, 482, 999 [ MNRAS | ADS ]

Fast and faint-object spectroscopy with the QUCAMs web pages.

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Last modified: 16 November 2018