September 19, 2016

Hidden interstellar cloud could explain unusual Tabby Star dimming

KIC 8462852, aka Tabby’s star, was spotted by NASA’s Kepler space telescope, which recently spent four years carefully watching the same patch of sky and looking for any stars that dipped in brightness at regular intervals. These dips, which can be as large as 1 per cent, happen when an exoplanet crosses in front of a star. But Tabby’s star dimmed randomly, and by as much as 20 per cent, leaving astronomers dumbfounded – and leading to the speculation that an alien megastructure was responsible for the signal.

Now researchers find that when the small dips occur, there is also a slight jump in the position of the light, leading them to conclude that it is actually a different star along the line of sight that is dimming. Tabby’s star is still responsible for the largest dips in light, however. If more than one star is dimming, say Makarov and Goldin, thinks there is an interstellar source for the dimming of two stars.

Jason Wright, who first noted the Tabby star dimming, has lately come to a similar conclusion. The latest evidence shows that not only does Tabby’s star dim sporadically, but it also appears to have been gradually fading for a century. Wright thinks that only an interstellar cloud passing in front of the star could explain such long-term dimming.

But, he concedes, “we don’t know of any such cloud along the line of sight”, and observing one directly will be tricky. Instead, Makarov thinks our best chance is to wait a decade or so until the blended, second star has moved away from Tabby’s star and we can catch the intruder red-handed.

Arxiv - Photometric and astrometric vagaries of the enigma star KIC 8462852

Researchers apply a PCA-based pre-whitening method to the entire collection of main Kepler mission long-cadence data for KIC 8462852 spanning four years. This technique removes the correlated variations of instrumental origin in both the detected light curves and astrometry, resolving intrinsic changes in flux and image position of less than 100 ppm and 1 mas, respectively. Beside the major dips in the light curve during mission quarters 8 and 16, when the flux dropped by up to 20%, we confirm multiple smaller dips across the time span of observation with amplitudes ranging from 0.1% to 7%. A variation of flux with a period of 0.88 d and a half-amplitude of approximately 90 ppm is confirmed in the PCA-cleaned data. We find that the phase of the wave is steady over the entire 15-month interval. We confidently detect a weak variability-induced motion (VIM) effect in the cleaned astrometric trajectories, when the moment-based centroids shift synchronously with the flux dips by up to 0.0008 pixels on the detector. The inconsistent magnitude and direction of VIM effects within the same quarter point at more than one source of photometric variability in the blended image. The 0.88 d periodicity comes from a different source, not from the target star KIC 8462852. We discuss a possible interpretation of the bizarre properties of the source as a swarm of interstellar junk (comets and planetoids) crossing the line of sight to the star and its optical companions at approximately 7 mas per year.

SOURCES- Arxiv, New Scientist

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