FU Orionis, a young binary star in the Orion constellation, has long intrigued astronomers due to its dramatic brightness variations. Recently, observations using the Hubble Space Telescope‘s COS and STIS instruments have provided unprecedented insights into the star’s accretion processes. These findings not only enhance our understanding of FU Orionis itself but also shed light on the behavior of similar young stellar objects.
Over the decades, FU Orionis has been recognized as a prototype for a class of variable young stars characterized by significant brightness outbursts. Previously, astronomers understood that these outbursts were linked to sudden increases in mass accretion from the circumstellar disk onto the star. However, the exact mechanisms governing the interaction between the disk and the star remained elusive.
What Did Hubble Discover About the Accretion Boundary?
The Hubble observations revealed that the accretion boundary between FU Orionis and its disk is much hotter than previously estimated.
“The Hubble data indicates a much hotter impact region than models have previously predicted,”
stated lead author Adolfo Carvalho. This discovery suggests that the material from the disk impacts the star’s surface at temperatures reaching 16,000 kelvins, nearly three times the Sun’s surface temperature.
How Does This Change Previous Models?
The previously accepted viscous disk accretion model needs revision to accommodate the higher temperatures observed.
“In FU Ori, the temperature is 16,000 kelvins… It challenges and encourages us to think of how such a jump in temperature can be explained,”
Carvalho explained. The revised model now accounts for a hot shock produced when the rapidly infalling material collides with the star, emitting significant ultraviolet light.
What Are the Implications for Planet Formation?
These intense ultraviolet emissions could influence the chemical composition and evolution of forming planets around such stars. Carvalho noted,
“Outbursts from an FU Ori object should influence what kind of chemicals the planet will ultimately inherit.”
Additionally, planets forming close to the star might be adversely affected, potentially leading to their inward migration or destruction due to the extreme conditions.
The latest findings position FU Orionis as a critical case study for understanding the dynamics of young, accreting stars. By pinpointing the high-temperature shock regions, researchers can refine models of stellar growth and disk interaction. This enhanced understanding may also inform studies of planet formation in similar environments, highlighting the delicate balance between stellar activity and planetary development.
