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Above: the NGTS light curve of the ULAS J2249-0112 rocket. The blue dots correspond to the NGTS photometry and the best adapted model is superimposed in red. Bottom: Residuals of the model fit. Credit: Jackman et al., 2019.
European astronomers have spotted a giant white-light flare on the ultracool L dwarf designated ULAS J224940.13-011236.9. The newly detected torch is one of the largest rashes ever seen in an ultra-cold dwarf. The discovery is detailed in an article published Feb. 3 on the pre-print server arXiv.org.
Dwarves are variable in many ways, including periodic modulation due to clouds, radio emission due to auroras or the presence of white-light flares. In general, white light emission occurs during reconnection events in the stellar magnetic field, resulting in lower chromosphere warming.
Although bursts of white light are regularly reported on different types of stars, they are rarely seen on dwarves. Notably, such events observed on the dwarves have some of the biggest changes in brightness ever recorded. However, it is difficult to find new white light torches on dwarves, as this requires long-term observations, especially high-speed, wide-field observations targeting very pale objects.
A team of astronomers led by James A. G. Jackman of the University of Warwick, UK, has conducted such observations with the help of the Next Generation Transit Survey (NGTS). The observation campaign, which aimed to search for stellar eruptions on low-mass stars, detected a white super-light emitted by ULAS J224940.13-011236.9 (or ULAS J2249-0112) on August 13, 2017 The object on which this flare is an ultracool L2.5 dwarf whose effective temperature is 1,930 degrees K, about 10 times lower than our sun's, located about 248 light-years from Earth.
"In this letter, we present the detection of a super-white light of the dwarf L2.5 ULAS J224940.13-011236.9, which was observed at high speed during the Next Generation Transit Survey (NGTS)", have writes the researchers in the paper. NGTS's observations show that the flare on ULAS J2249-0112 lasted about nine and a half minutes, with a rise time of 12 seconds. The torch had an amplitude of -10 mag, a brightness of 530 octillions of erg / s and an energy of 2.3 decillions of erg.
Astronomers note that the newly observed flare is the second largest flare detected in a dwarf L and the second to be spotted from the ground. In addition, ULAS J2249-0112 proves to be the coolest star ever showing a white light flare and the sixth dwarf L to have demonstrated one activity at a time.
"With a spectral type of L2.5, we think that ULAS J2249-0112 is the coolest star to date to show a stray light to white light," scientists noted.
According to the authors, the detection of a rocket of this size on ULAS J2249-0112 illustrates the importance of high-speed observations with NGTS for the study of the largest stellar rockets of the coldest stars.
"This behavior would not be visible in the TESS [NASA’s Transiting Exoplanet Survey Satellite] Full-frame images at 30-minute intervals, plus full-sky readings, monitor large areas of the sky every night. This shows how NGTS is in an ideal position to probe the behavior of stray light on the latest spectral types, "the researchers concluded.
Explore further:
Giant rocket detected on a star M before the main sequence
More information:
James A. G. Jackman et al. Detection of a giant white light flare on a L2.5 dwarf with the Next Generation Transit Survey. arXiv: 1902.00900 [astro-ph.SR]. arxiv.org/abs/1902.00900
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