Gazing at the Youngest Star From Chile

David Principe and Kenji Hamaguchi

As our guest blogger, we welcome Kenji Hamaguchi of the Center for Research and Exploration in Space Science & Technology and NASA’s Goddard Space Flight Center in Greenbelt, MD. He is part of the team that made the discovery described in our latest press release on an X-ray flare from a very young Sun-like star.

The observatory dome appeared over the hill. "Where is it standing?" I mumbled to myself. The Southern Astrophysical Research (SOAR) Telescope stands at the dead-end of a mountain ridge. The road just before the observatory has a bottleneck with steep slopes on both sides. The dome looked as if it were standing on the head of a spear. The telescope is a part of the Cerro Tololo Inter-American Observatory in Chile, along with the Gemini South observatory, and the Vera Rubin Observatory (formerly the LSST), which is now under construction. David Principe and I had come to the SOAR observatory in mid-December 2017, to take images of the extremely young star ("protostar") HOPS 383 at near-infrared wavelengths.

The observation we were undertaking was conducted in coordination with an observation of the same protostar with NASA’s Chandra X-ray Observatory. The Chandra observation aimed to find any sign of high-energy (X-ray) radiation from this newborn star. In contrast, the SOAR observation was designed to monitor the current status of an outburst from HOPS 383 that peaked in 2008. We planned to deepen the understanding of its nature by combining these two observing techniques.

Young stars are known to have many explosive events driven by magnetic fields on their surfaces, twinkling in X-rays like the flashing lights on Christmas trees. Interestingly, this activity is enhanced in younger stars and protostars, at least down to ages of about 100,000 years. Explosions around these very young stars can produce extremely hot gas, with temperatures of hundreds of million degrees Fahrenheit and a power that is thousands of times larger than those of even the most extreme solar flares. But, these same protostars are also surrounded by massive reservoirs – infalling cocoons and whirling disks – of very cold gas, with temperatures of –300 degrees Fahrenheit (–100 degrees Celsius) or so. Not so incidentally, the cold disks are the eventual birthplaces of planets. This hot and cold coexistence seems as bizarre as Tempura ice cream, a Japanese dessert. The crust is deep-fried, sizzling hot, but the ice cream inside is cold, unmelted. A key difference, of course, is that it is the inside of Tempura ice cream that’s cold, and the outside that's hot. (It's also yummy.)

Getting back to protostars, one crucial question is: At what time after the protostar's formation does this magnetically-derived, explosive, high-energy radiation start? Stars are born from cold interstellar gases, so such a mechanism must begin to work and heat gas at some early stage in its evolution. But Chandra had not detected X-ray emission from extremely young stars (classified as a “Class 0 protostar”). Perhaps the ubiquitous cold gas around these stars, within their cocoons and disks, could be too thick to allow X-ray emission from the nascent stellar core to escape.

In 2015, we had stumbled onto a paper reporting the first discovery of a Class 0 protostar undergoing a powerful outburst due to a sudden increase in its rate of accumulating mass, a process known as “accretion”. During a protostellar outburst, the accretion drastically accelerates, dumping a large amount of surrounding gas onto the star in a relatively short interval (astronomically speaking, that is; the durations of such "accretion-driven outbursts" are typically months to years). The protostar was HOPS 383. We knew from our studies of the outbursts from V1647 Ori, led by Joel Kastner, that young stars could increase their X-ray output by factors of tens or hundreds or more during mass accretion outbursts. So we proposed that, as a result of its sudden increase in its mass accretion rate, HOPS 383 could emit more X-rays, some of which might penetrate its thick veil of surrounding gas.

Thankfully, we were awarded a Chandra observation in 2017. The award of Chandra observing time came with a parallel award of near-infrared observing time with the SOAR telescope, which was vital: it provided the means to establish the status of the mass accretion outburst from HOPS 383 during the Chandra observation, which would take place nearly ten years after HOPS 383's sudden eruption. This was why David and I had flown to Chile.

Luckily, the weather was perfect on the observing date, with HOPS 383 in the constellation Orion's sword rising around dusk. Nicolas Grosso joined the observation remotely from France and the planned observation ended around 1 am. After that, we went star gazing outside of the observatory for a couple of hours. We saw the city lights of La Serena far away on the horizon, but the sky overhead was dark. It was the season of the Geminids meteor shower, and I saw dozens of shooting stars. Yes, it was a good sign.

The infrared images we obtained that night did not show the infrared nebula seen several years ago around HOPS 383. With this (negative) result, we had established that the major outburst had ended before the observation. But, we were still lucky. Chandra detected an X-ray flare from HOPS 383 that generated a grand total of 28 X-ray photons in 3.3 hours. At the (roughly) 1,400-light-year distance of HOPS 383, these precious few photons correspond to the power output of a gigantic X-ray flare from the Sun. We do not know if the X-ray flash was residual activity from the mass accretion outburst, or if the star quasi-regularly erupts with such magnetic X-ray flares. But we can surely say that there is intense magnetic activity lurking around, perhaps deep inside, this very young star. And the high-energy radiation given off as a result of its magnetism – which raises the temperatures and alters the chemical content within the protostar's surrounding cocoon and orbiting disk – likely plays an important role in the formation stages and eventual compositions of Earth-like planets.

The next morning, we took a ride to the airport in La Serena. When the airplane was ascending, I saw the SOAR observatory shining far away in the sunlight. The airplane flew down to Santiago alongside the Andes mountains. The mountain surfaces are beautifully colored in yellow or red with minerals. The connection to HOPS 383 was right there in front of me.

Finally, we really appreciate Jonathan (Jay) Elias, Patricio Ugarte, and Ximena Herreros for supporting our SOAR observation.

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