A massive protostar, still cradled within its tightly packed star-forming cloud, recently roared to life, shining 70 times brighter than before and warming the surrounding dust of its stellar nursery. This sudden outburst was likely triggered by an avalanche of star-forming gas crashing onto the surface of the star.
A team of astronomers made this discovery by comparing new observations from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile with earlier observations from the Submillimeter Array (SMA) in Hawaii.
"We were amazingly fortunate to detect this spectacular transformation of a seemingly ordinary protostar," said Todd Hunter, an astronomer at the National Radio Astronomy Observatory (NRAO) in Charlottesville, Va., and lead author on a paper published in the Astrophysical Journal. "By studying this object with both ALMA and the SMA, we could see that something dramatic had taken place, completely changing a stellar nursery over a surprisingly short period of time."
This abrupt transformation, which continues to this day, supports the theory that young stars can undergo intense growth spurts, brightening very quickly and reshaping their surroundings.
In 2008, before the era of ALMA, Hunter and his colleagues used the SMA to observe a small but active portion of the Cat's Paw Nebula, a massive star-forming region located about 5,500 light-years from Earth in the direction of the southern constellation Scorpius. This nebula is similar in many respects to its northern cousin, the Orion Nebula, which also is brimming with young stars, star clusters, and dense cores of gas that are on the verge of becoming stars. The Cat's Paw Nebula, however, is forming stars at a faster rate.
The SMA observations revealed what appeared to be a typical protocluster: a dense region of dust and gas harboring several still-growing stars.
Young stars form in these tightly packed regions when a pockets of gas becomes so dense that they begin to collapse under their own gravity. Over time, disks of dust and gas form around these nascent stars. The disks help funnel material onto a protostar's surface, helping it grow.
This process, however, may not be entirely slow and steady. Astronomers now believe that young stars can experience rapid growth spurts, during which they can become nearly 100 times brighter than normal and outshine their neighbors. These events also rapidly heat the surrounding material, making it stand out from the surrounding nebula, especially at millimeter wavelengths.
New observations taken in 2015 and 2016 with ALMA reveal that the same protocluster observed by the SMA, dubbed NGC 6334I, changed dramatically sometime in the past few years. It now is glowing many times more brightly, especially in millimeter-wavelength light. It also had taken on an entirely different shape, growing to nearly twice its original size. These changes are clear indications that a protostar is in the throes of a tremendous growth spurt.
The astronomers speculate that leading up to his outburst, an uncommonly large clump of material was drawn into the star's accretion disk, creating a logjam of dust and gas. Once enough material accumulated, the logjam burst, releasing an avalanche of gas onto the growing star. The researchers believe this extreme accretion event increased the star’s luminosity nearly 100 fold, heating its surroundings and causing the surrounding dust to "glow" brightly. Though similar events have been observed in the infrared, this is the first time that such an event has been identified at millimeter wavelengths.
To ensure that the differences between the observations were not the result of differences in the telescopes or even a simple data-processing error, Hunter and his colleagues reprocessed the ALMA data to have it more closely match the capabilities of the SMA. By digitally subtracting the original SMA data from the ALMA data, the astronomers confirmed there was a significant change to the protocluster.
"Once we made sure we were comparing the two sets of observations on an even playing field, we knew that we were witnessing a very special time in the growth of a star," said Crystal Brogan, also with the NRAO and co-author on the paper.
Further confirmation of this event came from complementary data taken by the Hartebeesthoek Radio Astronomy Observatory in South Africa. This single-dish observatory was monitoring the radio signals from masers in the same region. Masers are the naturally occurring cosmic radio equivalent of lasers and can be powered by a variety of energetic processes throughout the universe, including outbursts from rapidly growing stars.
The data from the Hartebeesthoek observatory revealed an abrupt and dramatic spike in maser emission from this region in early 2015, only a few months before the first ALMA observation. Such a spike is precisely what astronomers would expect to see if there were a protostar undergoing a major growth spurt.
"These observations add evidence to the theory that star formation is punctuated by a sequence of dynamic events that build up a star, rather than a smooth continuous growth," concluded Hunter. "It also tells us that it is important to monitor young stars at radio and millimeter wavelengths, because these wavelengths allow us to peer into the youngest, most deeply embedded star-forming regions. Catching such events at the earliest stage may reveal new phenomena of the star-formation process."
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.