When astronomers observe some white dwarf stars, there can sometimes be small amounts of hydrogen trapped in the stars' upper layers. The traditional view is that these tiny stellar husks suck up diffuse interstellar hydrogen gas, but now a team of researchers suggest something else is going on: comets are likely raining down into the white dwarfs' ancient atmospheres from exo-Oort clouds, seeding them with hydrogen.
ANALYSIS: White Dwarfs Are Eating ‘Earth-like' Planets for Dinner
White dwarfs form after sun-like stars extinguish the majority of hydrogen fuel in their fusion cores. This causes heavier and heavier elements to fuse, turning the star from a once-quiescent state into a violent, churning red giant. Eventually, through powerful stellar convulsions, the bloated red giant is ripped apart, leaving a small, dense white dwarf in its wake.
A white dwarf's structure is not maintained by the outward pressure supplied by fusion reactions to counter gravity, like our sun. The quantum pressure supplied by electron degeneracy is what prevents its gravity from causing it to collapse in on itself. This balance between forces creates a very dense stellar object that may have a mass comparable to the sun, but collapsed into a diameter comparable to the Earth. As such, white dwarfs can continue to shine for many billions of years.
When observing white dwarf spectra (i.e. the different wavelengths of light), astronomers have noted that many examples possess atmospheres that are "metal rich." In astronomy terms this means there are elements trapped in the upper layers of the white dwarf heavier than helium. Depending on the elements present, astronomers have interpreted these spectroscopic signatures as being asteroids or even planets that have survived the death of their sun-like stars, only to later become shredded by the white dwarf's intense tides. This shredded, dusty material rains down on the white dwarfs, leaving a spectroscopic signature of the ultimate death of planetary systems.
ANALYSIS: Sifting Through the Ashes For Shredded Planets
This area of white dwarf study has led to some fascinating observations of star systems that may resemble our solar system after the sun has run out of fuel in a few billion years time and turned into a white dwarf; the planets and asteroids near to our sun's corpse become ripped apart, enriching our sun's white dwarf with metals.
In new research accepted for publication in the journal Monthly Notices of the Royal Astronomical Society, astrophysicist Dimitri Veras of the University of Warwick and colleagues have now found a possible mechanism that could be lacing white dwarf atmospheres, not with metals, but with hydrogen.
"Here we explore the possibility that the gradual accretion of exo-Oort cloud comets, which are a rich source of hydrogen, contributes to the apparent increase of trace hydrogen with white dwarf cooling age," writes Veras.
The build-up of hydrogen in white dwarf atmospheres has been attributed to interstellar hydrogen being collected by white dwarfs as they age, but to account for the quantities being observed, there must be another source, argues Veras.
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Surrounding our solar system is hypothesized to be a region of space containing billions of icy bodies - the nuclei of comets. By calculating the trajectory of long-period comets that fall through the solar system, this region - known as the Oort cloud - is thought to extend around 1 light-year from the sun. Intermittently, possibly after a stellar close pass, these cometary nuclei are gravitationally knocked out of the Oort cloud and drop under the influence of the sun's gravity and careen through the inner solar system, often falling into the sun.
The presence of comets have been detected around other stars before, mainly through the detection of cometary dust around young stars. But Veras' team suggests that, through numerous computer models of cometary distributions surrounding white dwarf stars, the trace spectroscopic signal of hydrogen in some white dwarf atmospheres is caused by in-falling comets from those stars' exo-Oort clouds.
The researchers point out that only a relatively small number of white dwarfs with trace quantities of hydrogen have been studied, and most are located fairly 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 flybys are most frequent, possibly stirring up these exo-Oort clouds more often. Should spectroscopic signatures of white dwarfs further afield be measured, it will be interesting to see if this hydrogen enrichment via comets hypothesis applies.
This is yet another example of how fascinating white dwarf systems are - they may be the remnants of dead stars, but we still observe some fascinating dynamic behavior long after their planetary systems have been blended by their powerful tides.
Source: arXiv preprint service