When astronomers observe some past dwarfs, they sometimes notice a small amount of hydrogen trapped in the upper layers of a star. It is believed that this tiny stellar husk is the result of absorption of interstellar hydrogen gas, but now the team of researchers suggests that this is due to something else: most likely, comets slide down into the ancient atmosphere of white dwarfs from the Oort cloud.
White dwarfs form after the stars of the solar type consume most of their hydrogen fuel. This causes the star to move from a quieter state to a more cruel one - a red giant. In the end, after the powerful stellar convulsions, the red giant explodes, leaving a dense white dwarf in its place.
The structure of the white dwarf is not supported by external pressure, the source of which is a thermonuclear reaction, but due to the quantum pressure created by the remaining electrons, which interfere with its own gravity. This balance between forces creates a very dense stellar object, which can have a mass comparable to the Sun, but have the diameter of the Earth. Thus, white dwarfs can continue to shine for many billions of years.
When observing the spectrum of white dwarfs, astronomers note that many of them have atmospheres rich in various metals. From the point of view of astronomy, this means that there are trapped elements in the upper layers of the white dwarf heavier than helium. Depending on the elements present, astronomers interpret these astronomical imprints as the result of the destruction of asteroids or even planets that survived the death of their own star. This shredded, dusty material, in the form of rain, is showered on white dwarfs, leaving spectroscopic imprints of the death of planetary systems. This explored area of the white dwarf leads scientists to some fascinating observations of star systems that resemble our solar system, or rather, it will look like in a few billion years when our sun runs out of fuel and it turns into a white dwarf. Planets and asteroids located next to the corpse of our Sun will be torn apart, thereby enriching the white dwarf of our Sun with metals.
In a new study, accepted for publication in the Monthly Notices of the Royal Astronomical Society, astrophysicist Dmitry Veras and his colleagues at the University of Warwick found a possible mechanism that links the atmosphere of a white dwarf with not hydrogen, but metals.
"We are exploring the possibility that the gradual accretion of the Oort comets, which are a rich source of hydrogen, by the comets, contributes to an obvious increase in the volume of hydrogen in the atmosphere of a white dwarf," writes Veras.
"It used to be believed that the buildup of hydrogen in a white dwarf atmosphere is the result of interstellar hydrogen collected by a white dwarf, but for the observed values, there must be another source," says Veras.
Our solar system has a region of space containing billions of ice bodies - comets. This region, known as the Oort cloud, is located at a distance of one light year. Periodically, with a close star passage, the gravitational calm of comets is broken and knocks them out of the Oort cloud. Under the force of gravity of the Sun, comets begin their journey to the inner part of the solar system. The presence of comets has been detected around other stars, primarily due to the detection of comet dust around young stars. But the Veras team, using computer simulation, shows that the spectroscopic trace of hydrogen in the atmosphere surrounding the white dwarf stars is due to falling comets from the Oort ex-cloud.
The researchers note that only a small number of white dwarfs with traces of the presence of hydrogen were studied and most of them are located quite close to the Sun and in the direction of the galactic bulge (the center of the Milky Way). It is in this region, where galactic tides and stellar winds are strongest, perhaps a stronger influence on the Oort exo-clouds.
This is another example of how fascinating white dwarf systems are - they are remnants of dead stars, but we still see some interesting dynamic behavior.