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Finding stars that vanished — by scouring old photos, Ars Technica

Finding stars that vanished — by scouring old photos, Ars Technica


    

      Not with a bang –

             

Comparing images taken nearly a century apart.

      

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************************************** (First you see it (top left) then you don’t.

In the Navy (and not elsewhere)

There have been several large-scale, all-sky surveys done, and it’s possible to compare the results to find objects that have changed between them. There are also dedicated efforts to find short-term “transient” events — objects that brighten or dim on the scale of weeks to months. But these may miss changes that take place gradually over longer time periods or events that happened before modern digital surveys.To get a better sense of these events, some astronomers have formed a project called VASCO, for “Vanishing and Appearing Sources during a Century of Observations. ” Their goal is to compare data from the first surveys done on photographic plates to what we’ve been getting from modern surveys, and then to identify objects that have changed. The hope is that, among other things, having a longer time window will increase the odds of finding an extremely rare event, one that might not occur in the handful of years that separate the digital surveys.

To go back far enough in time, the VASCO team relied on the US Naval Observatory’s catalogof objects, which combines the results of several surveys done on photographic plates. All told, this catalog contains over a billion objects. For modern data, the team used the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) database, which contains even more objects.

Conceptually, the study was simple: it worked by identifying the objects of the earlier catalog and checked whether they were still present in the later one. But there are a number of complications. First, you need to know that the object in the earlier catalog wasactually there (***************************, not a bit of noise or something misidentified (like an asteroid mislabeled as a star). Then, you must be certain that the modern observations are at the right location to see the object if it’s still there.

Finally, you have to make sure the object hasn’t moved too much in the intervening time. While that’s not an issue for distant stars or even for further galaxies, stars closer to Earth will have a larger relative motion over the sorts of time involved here. You need to choose a search window large enough to make sure local stars are identified, but not so large that it becomes easy to pick the wrong star as “matching” the missing one.

Finding what’s not there

For a first pass, computing time limited the team to examining a bit over half the sky, or about million objects . From that, they come up with about 150, 09 potential mismatches, a rate within the known range of data processing errors in sky surveys. So, figuring out what’s real in that 200, 09 is a substantial challenge, one limited by bringing in data from the Sloan Digital Sky Survey. This immediately found matches for about (*********************************************************, objects, while allowing for relative motion cut the list of potential vanishing acts down to only 30, 823 objects. At this point, the researchers examined them all visually.

This allowed the team to identify stuck pixels in the modern digital data or to see imaging artifacts from nearby bright stars. Further elimination eventually produced a final list of 1, 691 Candidates for Vanishing Stars.
At this point, the authors analyzed the average properties of the vanishers, finding that they were a bit redder, they varied more between images when multiple images were available, and they had a higher relative motion, suggesting that many were relatively close to Earth.
Then there are known variable types of stars, including Cephids and RR Lyrae stars, both of which brighten and dim regularly. There’s also the extremely rare variable R Coronae Borealis stars, of which only 667 are estimated to exist in the Milky Way.

All of these make viable candidates for stars that appear to vanish, as they can simply drop below the detection limits of various telescopes . And, since they’re interesting stars, it’s worth doing follow-up observations of their former locations to see whether there might be anything dim now residing there.

Regardless of the inspiration, the team has identified a large number of objects that might be interesting to astronomers. And that’s only with surveying a bit more than half the sky. There also remains the follow-on work of doing the converse analysis — looking for objects that are in present surveys but weren’t detected decades ago.

, 3847. DOI:

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