Often the stars are too far for us to talk about their surroundings. But in the case of newborn stars surrounded by protoplanetary disks, there is a brilliant trick that astronomers can use to explore the structure of their dusty cradles.
A star can form from a molecular cloud of gas, which under certain conditions is destroyed by mutual attraction. This collapse forms a knot of dense material that can melt and produce the core of a young star. Over time, material will gather around this protostar, forming a swirl disk. Eventually, the planets will condense from this protoplanetary disk. In order to better understand how planets form in the solar system, astronomers study disks around other stars.
“Understanding protoplanetary disks can help us understand some of the secrets of exoplanets in star systems beyond our own,” said researcher Huan Meng, of the University of Arizona, Tucson. "We want to know how planets form, and therefore we find large planets called hot Jupiters, which are close to their stars." If the young star systems are far away from us, this makes it difficult for the optical structure of these disks to be visible.
However, studying the brightness variations of a star called YLW 16B, located about 400 light years from Earth, Maine and his colleagues were able to detect the reflected light of a star from the inner boundary of its protoplanetary disk, which allows for extremely accurate measurements of its location and structure.
This particular star is about the same mass as our Sun, but it is only 1 million years old (compared to 4, 6 billion years of our Sun, this star can be called a star embryo). This makes it an ideal candidate for understanding the physics of our Solar System until the planets began to form around the young Sun.
Using data from the NASA Spitzer space telescope, which observes the Universe in infrared light, as well as from ground-based observatories, astronomers have applied a technique called “photo-reverb” to study the starlight reflected from the inner edge of the protoplanetary disk.
It just so happened that the YLW 16B has variable and unpredictable radiation oscillations, so astronomers measure these oscillations of radiation and wait for the reflected light from the disk. Variations in the brightness of a star can be compared with a slight echo, which appears later. The time delay is used to calculate the distance of the star from the inner edge of the protoplanetary disk. For this star system, the gap between the star and the inner disk is about 0.08 AE, where 1 AE, or astronomical unit, is the average distance between the Sun and the Earth's orbit. As a better comparison, the inner edge is about one quarter of the distance between Mercury and Sun.
These observations helped to conclude that the disk was thick, providing an additional interesting hint about how much material the disk might contain.
Young stars are bright and have powerful stellar winds that "blow out" the insides of the protoplanetary disk, leaving a gap (as shown in the image above). Understanding how large this gap and its location from the star will help us improve the models of newborn star systems and, ultimately, gives an idea of how our solar system was formed 4, 6 billion years ago.