Bu Abby Van Selous (COM’24)
When she was a graduate student, Associate Professor of Astronomy Catherine Espaillat studied a young protostar called SZ Chamaeleontis — or SZ Cha — and the protoplanetary disk of dust and gas that surrounded it.
Protostars, such as SZ Cha, have the potential to form planets when they are surrounded by protoplanetary disks, so long as the disk doesn’t dissipate. However, by the time a protostar reaches 10 million years old, its disk has usually disappeared, along with the materials needed to form a planet.
Espaillat first started studying SZ Cha, which is 2-million-years-old, in 2008 as a graduate student at the University of Michigan, using data from the Spitzer Space Telescope, which was launched in 2003 to study space with an ultra-sensitive infrared telescope but is now retired, to calculate the lifetime of the protoplanetary disks of 50 protostars.
In her research, Espaillat measured mid-infrared neon emission lines, which trace gas in protoplanetary disks and are an excellent indicator of what materials are present, to examine the states of the disks.
The ratio of the neon emission lines in the Spitzer data told Espaillat that the dissipation rate around SZ Cha was very slow, indicating that the protoplanetary disk would be around for a long time and could potentially form a planet. Out of the 50 protoplanetary disks observed, the disk around SZ Cha was the only protostar with this indicator.
Espaillat wanted to determine how and when protoplanetary disks dissipated, so 15 years later she decided to return to SZ Cha. Now, her new results are making her question whether SZ Cha is a good model for learning about our solar system.
“Once again, the universe is showing us that none of its methods are as simple as we might like to make them,” said Espaillat, whose research will be published in Astrophysical Journal Letters. “We need to rethink, re-observe, and gather more information.”
In April, when Espaillat received data collected by the James Webb Space Telescope to continue her research, she saw a notable change in SZ Cha’s data. In just 15 years — not much time at all when a 2-million-year-old star is considered young — SZ Cha’s neon lines became less elevated and more like the other protoplanetary disks she studied.
Seeing such a significant change in the neon lines within that short period was noteworthy. It was the first time scientists had discovered that young planetary objects could still be very variable in their neon emission, Espaillat said.
“We did the due diligence to make sure the data quality was good,” Espaillat said. “And then we started just thinking, ‘What could possibly be the answer?’”
Espaillat and her team theorized that when the James Webb data was collected, there may have been a wind that shielded the protoplanetary disk from ultraviolet light radiation. When the Spitzer data was collected, however, it was likely that wind wasn’t present to prevent ultraviolet radiation from reaching the protoplanetary disk. The variability in the data suggests that dissipation rates and the materials in protoplanetary disks can change over the disk’s lifetime due to the wind, Espaillat said.
Espaillat will observe SZ Cha again in a few months with data collected from the James Webb telescope, along with data from the Hubble telescope, the Chandra X telescope, and a ground based telescope. With the help of the additional telescopes, Espaillat should be able to get a better idea of what caused the change in SZ Cha’s protoplanetary disk and neon emission lines and what this suggests about protoplanetary disk variability, she said.