It is now winter here in the northern hemisphere, and we’re expecting snow on Friday in Washington, DC. But fear of snow doesn’t stop us from going observing…and observing we have been! Jonathan Gagne returned from his last trip to the telescope with spectra of nine of our brown dwarf candidates—and seven of them are now bonafide brown dwarfs. That brings the total number of brown dwarfs discovered by the project to eight, when you add in our first confirmed brown dwarf from earlier this year. Thanks to Ellie and Eileen for helping with the observations, which they performed using the Folded-port InfraRed Echellette spectrometer prism on the Magellan telescope! And a big congratulations to Sam Goodman, Les Hamlet, Guillaume Colin and Dan Caselden for submitting these candidates that are now confirmed!
The spectral types of the new brown dwarfs are: T0,T2.8, T5, T6, T6.5, and two T8s. (WISEA 1101+5400, which we discovered earlier, is a T5.5). Curious how we tell the spectral type from the spectrum? We make plots like this one, below. The black curve is one of the new spectra from Magellan, and the colored lines are other brown dwarfs with known spectral types, ranging from T4 to T8. Which one do you think matches best?
Yes, it’s a T8, and it’s currently the coolest brown dwarf we have found with Backyard Worlds: Planet 9, with a temperature of around 750 Kelvin (about 890 degrees Farenheit). The other two objects turned out to be cool subdwarfs, a kind of star that is poor in iron and other metals, suggesting it formed before the most recent generations of stars enriched the galaxy with these metals. Those are poorly understood and interesting in their own right.
This new batch of confirmed brown dwarfs contained a few surprises for use. Three of them are strangely bright in the K band; we’re not sure how to interpret that yet. Also, one of the brown dwarfs initially seemed like it might be a member of the AB Doradus moving group, based on its proper motion. Its spectrum looks very similar to that of GU Psc b, a planetary-mass T dwarf in that AB moving group. But Jonathan took a higher resolution spectrum of it, and the new spectrum showed that our brown dwarf wasn’t in the moving group after all. Close call!
This new batch is still just the beginning for our follow-up program. First, we have half a night on the TripleSpec instrument on the ARC 3.5 meter at the Apache Point Observatory on January 6, thanks to a proposal led by Katelyn Allers. Then, we have two nights using the ARCoIRIS spectrograph on the Victor Blanco 4 meter telescope coming up on March 1+2 thanks to a proposal by Jackie Faherty. By the way, our sister citizen science project, Disk Detective, also won observing time on ARCoIRIS, for April 1+2 to follow up debris disk candidates, so we might do some trading between the two projects. And we have two more proposals for observing time still pending—and a list of now 337 brown dwarf candidates to follow up this winter and beyond.
So stay tuned—and have a super holiday, wherever your backyard may be! (And yes indeed, “brown dwarfs” is not spelled like “dwarves”.)
–Marc
Backyard Worlds: Planet 9 