An Unknown Object Is Emitting Immense Dust Clouds That Erratically Obscure A Star

The star TIC 400799224 in the crosshairs is actually two stars, one of which is being obscurred on a 20-day cycle, but we don't understand what could produce the erratic gas clouds required to explain the pattern observed. Image Credit: DECam Plane Survey (Schlafly et al. 2018)

Dramatic dips in the brightness of a stellar pair have been attributed to pulses of dust on an almost unimaginable scale, proving once again that the universe contains things stranger than we can imagine – until we find them. We don't know what is causing this process, but southern hemisphere residents in possession of medium-sized telescopes could help us find out.

The Kepler and TESS space telescopes' intended missions were to search for planets by seeking dips in stars' light as objects pass in front of them. They've succeeded to a spectacular degree, but also turned up some changes in brightness too large or irregular to be explained by any planet – most prominently KIC 8462852, famous for early speculation its light was being blocked by a “megastructure”.

The latest example comes from the pair of stars named TIC 400799224 until something catchier is found, both a little more massive than our Sun. In The Astronomical Journal, a team of professional and citizen scientists announce one of TIC 400799224's stars is undergoing drops in brightness on a 20-day cycle, but the dips' unpredictability indicates something very complex is going on.

TIC 400799224 first came to astronomers' attention when citizen scientists reported a large drop in its brightness. Professional follow-up revealed approximately a quarter of the total light was being obscured. However, three overlapping dips, rather than a single smooth fading, indicated what paper calls “a highly irregular occulting body”. Combining TESS's observations with archival data from other instruments reveals a 19.77-day cycle.

In binary systems, interactions between the stars are usually suspected in anything odd. However, the stars in TIC 400799224 are around 300 astronomical units apart – ten times the distance between the Sun and Neptune. At that distance, any effects would be too small and too slow to explain something like this.

Despite this large separation, TESS's image resolution is insufficient to determine which star is dipping, particularly since their brightness is similar. The 25 percent fall in total brightness observed means a loss of either 37 percent of the brighter star's light, or an unprecedented 75 percent of the fainter one.

The most puzzling aspect to this, the paper notes is that; “While the occultations appear to have a strict underlying period, they are erratic in shape, depth, and duration.” More often than not, no dip can be detected at all.

TIC 400799224 and KIC 8462852 are not the only examples of stars experiencing brightness dips too large to be explained by planets. KIC-1520 was discovered in 2012 to be on a 15.6-hour cycle, eventually explained as being from an enormous dust cloud created by a small and close-in planet disintegrating. Every time the planet and cloud passed between the star and ourselves, the brightness drops dramatically.  

Several other stars have since been explained the same way. However, all these examples are much fainter and less massive stars than TIC 400799224, and their dips far smaller and more regular. TIC 400799224 is emitting far more radiation that should disrupt nearby dust, making its existence even more remarkable. If Ceres was giving off this much dust it would dissolve entirely in 8,000 years – a blink of an eye for a star system like this.

The authors admit they cannot explain what forces would destroy an asteroid like that, among other mysteries. If you're keen to find out, however, you may be able to help. At magnitude 12.6 TIC 400799224 is bright enough to be visible in large backyard telescopes. Just as citizen scientists alerted astronomers to it, regular check-ups from amateurs could supplement professional data to track its changes, although it lies too far south to be seen from northern latitudes.

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