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16 October, 2018
An Alternative Technique for Detecting Forming Exoplanets Around Young Stars
The large number of detected exoplanets around evolved stars sharply contrasts with the lack of detections of forming planets in protoplanetary disks around young stars, mainly because of the observational difficulties.
One example of the latter is the young, almost solar-mass star LkCa 15, which is located at around 520 light years in the Taurus-Auriga star-forming region.
On the one hand, a team of astronomers reported that LkCa 15 hosts 3 infrared bright planets in its protoplanetary disk. One of them, LkCa 15 b, was also detected in bright H-alpha emission and it is claimed to be the first exoplanet caught in the process of formation. This discovery was made using a technique called "near-IR non-redundant masking" using the Large Binocular Telescope, and from simultaneous differential imaging using the Magellan Adaptive Optics System on the Clay telescope.
On the other hand, a more recent work using SPHERE on the Very Large Telescope suggests that the infrared bright planets are not such, but instead persistent structures of the inner protoplanetary disk that surrounds LkCa 15. However, the bright H-alpha emission of LkCa 15 b remains unexplained.
Earlier this year, an international team of astronomers led by Ignacio Mendigutía (Centro de Astrobiología, Spain), decided to use the ISIS spectrograph on the William Herschel Telescope (WHT) to study the nature of LkCa15 b, by means of a technique called spectro-astrometry. This allowed them to derive not only the intensity spectrum around the H-alpha emission, but also the so called photocentre spectrum and the full width half maximum (FWHM) spectrum.
The photocentre spectrum tells us about the position in the sky where the brightness (as a function of wavelength) comes from. If an H-alpha-bright planet is surrounding LkCa 15, the bulk of the H-alpha emission should come from a specific point in between the star and the planet, whereas the brightness at other wavelengths, where the planet is faint, should only come from the star.
Therefore the photocentre spectrum should reveal a displacement precisely at the H-alpha wavelength when the spectrograph is oriented in the direction of the planet, and no displacement at all when the spectrograph is oriented in the perpendicular direction.
The FWHM spectrum is a measurement of the size of the emitting source. If a planet is surrounding LkCa 15, the FWHM should also increase a bit when the spectrograph is oriented in the direction of the planet, although such signature is so weak that we would not be able to detect it with the instrumentation used. If the spectrograph is oriented in the direction perpendicular to the position of the planet, then no FWHM displacement should be observed at all.
Surprisingly, the new observations revealed no photocentre signature at either orientation of the spectrograph, but a FWHM signature at both orientations (see the accompanying figure). These observations can be explained by the existence of a roughly symmetric H-alpha emitting source, that is larger that the central star, and may be related to an outflow or a disk wind, usually observed in young stars. Interestingly, the size of the H-alpha extended region inferred from the observations and models is similar to the orbit size initially attributed to a forming planet.
Spectro-astrometric results of LkCa 15 taken with ISIS on the WHT. From top to bottom, H-alpha intensity, photocentre, and FWHM spectra. The left panels refer to data taken when the spectrograph was oriented towards the reported position of the planet, and the right panels to the perpendicular orientation. The dotted lines show the original data and the solid lines after cleaning them to better observe possible signatures. The accuracy obtained is indicated with the horizontal dashed lines: less than one milli-arcsecond! The lack of photocentre signature at any spectrograph orientation and the similar FWHM signatures at both orientations is not compatible with the presence of a forming planet but with extended and roughly symmetric emission. The red lines show the result of the model for such extended source, reasonably fitting the observations. Credit: Ignacio Mendigutía. Large format: [ PNG ].
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