Artist's impression of white dwarf binary pair CSS 41177. Image: Andrew Taylor.
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Two stars have been discovered locked in a mutually-destructive embrace, a relationship that will end with both losing their individual identities as they spiral increasingly closer, eventually becoming a single hot body that is destined to quickly fizzle out.
No, we’re not talking about the cover of a Hollywood tabloid, these are two white dwarf stars 1,140 light-years away in the constellation Leo, and they are the second such pair of their kind ever to be discovered.
Astronomers at the University of Warwick in the UK have identified a binary pair of white dwarf stars named CSS 41177 that circle each other closely in an eclipsing orbit. What’s particularly unique about this pair is that both stars seem to have been stripped down to their helium layers – a feature that points at an unusually destructive history for both.
White dwarfs typically form from larger stars that have burned through their hydrogen and helium, leaving behind hot, dense cores composed of carbon and oxygen – after going through a bloated red giant phase, that is. But when stars are very close to each other, such as in the case of binary pairs, the expanding hydrogen shell from the larger one undergoing its red giant phase is stripped away by its smaller companion, which absorbs the material. Without the compression and heat from the hydrogen layer the first star cannot fuse its helium into heavier elements and is left as a helium white dwarf.
When the time comes for the smaller star to expand into a red giant, its outer layers are likewise torn away by the first star. But the first star cannot use that hydrogen, and so both are left as helium white dwarfs. The unused hydrogen is ultimately lost to the system.
It’s a case of a destructive codependent relationship on a stellar scale.
The white dwarf stars in CSS 41177 will eventually merge together in about a billion years, gaining enough mass in the process to begin fusing their combined helium, thus becoming a single star called a hot subdwarf. This period could last another 100 million years.
This discovery was made using data gathered from the Liverpool Telescope in the Canary Islands and the Gemini Telescope on Hawaii. The paper was accepted for publication in the Astrophysical Journal and is entitled A deeply eclipsing detached double helium white dwarf binary. (Authors: S. G. Parsons, T. R. Marsh, B. T. Gaensicke, A. J. Drake, D. Koester.)
The image above was created by Andrew Taylor, a.k.a. digital_drew. He specializes in starry-night landscapes as seen from speculative planets orbiting familiar stars in our galaxy and was kind enough to provide me with this custom binary pair image. Check out his photostream for more!
The white dwarf G29-38. Many stars, including our Sun, end their lives as white dwarfs. Determining the masses of white dwarf stars is key to the new technique of determining a star's age. Image Credit: NASA
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While supernovae are the most dramatic death of stars, 95% of stars will end their lives in a far more quiet fashion, first swelling up to a red giant (perhaps a few times for good measure) before slowly releasing their outer layers into a planetary nebula and fading away as a white dwarf. This is the fate of our own sun which will expand nearly to the orbit of Mars. Mercury, Venus, and Earth will be completely consumed. But what will happen to the rest of the planets in the system?
While many stories have suggested that as the star reaches the red giant phase, even before swallowing the Earth, the inner planets will become inhospitable while the habitable zone will expand to the outer planets, perhaps making the now frozen moons of Jupiter the ideal beach getaway. However, these situations routinely only consider planets with unchanging orbits. As the star loses mass, orbits will change. Those close in will experience drag due to the increased density of released gas. Those further out will be spared but will have orbits that slowly expand as the mass interior to their orbit is shed. Planets at different radii will feel the combination of these effects in different ways causing their orbits to change in ways unrelated to one another.
This general shaking up of the orbital system will result in the system becoming once again, dynamically “young”, with planets migrating and interacting much as they would when the system was first forming. The possible close interactions can potentially crash planets together, fling them out of the system, into looping elliptical orbits, or worse, into the star itself. But can evidence of these planets be found?
A recent review paper explores the possibility. Due to convection in the white dwarf, heavy elements are quickly dragged to lower layers of the star removing traces of elements other than hydrogen and helium in the spectra. Thus, should heavy elements be detected, it would be evidence of ongoing accretion either from the interstellar medium or from a source of circumstellar material. The author of the review lists two early examples of white dwarfs with atmospheres polluted in this respect: van Maanen 2 and G29-38. The spectra of both show strong absorption lines due to calcium while the latter has also had a dust disk detected around the star?
But is this dust disk a remnant of a planet? Not necessarily. Although the material could be larger objects, such as asteroids, smaller dust sized grains would be swept from the solar system due to radiation pressure from the star during the main sequence lifetime. Much like planets, the asteroids orbits would be perturbed and any passing too close to the star could be torn apart tidally and pollute the star as well, albeit on a much smaller scale than a digested planet. Also along these lines is the potential disruption of a potential Oort cloud. Some estimates have predicted that a planet similar to Jupiter may have it’s orbit expanded as much as a thousand times, which would likely scatter many into the star as well.
The key to sorting these sources out may again lie with spectroscopy. While asteroids and comets could certainly contribute to the pollution of the white dwarf, the strength of the spectral lines would be an indirect indicator of the averaged rate of absorption and should be higher for planets. Additionally, the ratio of various elements may help constrain where the consumed body formed in the system. Although astronomers have found numerous gaseous planets in tight orbits around their host stars, it is suspected that these formed further out where temperatures would allow for the gas to condense before being swept away. Objects formed closer in would likely be more rocky in nature and if consumed, their contribution to the spectra would be shifted towards heavier elements.
With the launch of the Spitzer telescope, dust disks indicative of interactions have been found around numerous white dwarfs and improving spectral observations have indicated that a significant number of systems appear polluted. “If one attributes all metal-polluted white dwarfs to rocky debris, then the fraction of terrestrial planetary systems that survive post-main sequence evolution (at least in part) is as high as 20% to 30%”. However, with consideration for other sources of pollution, the number drops to a few percent. Hopefully, as observations progress, astronomers will begin to discover more planets around stars between the main sequence and white dwarf region to better explore this phase of planetary evolution.
Herschel image of the carbon star CW Leonis. The arc visible to the left of the star is a bow showck, where the stellar wind encounters the interstellar medium. ESA/PACS/SPIRE/MESS Consortia
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There’s something strange going on around the red giant star CW Leonis (a.k.a. IRC+10216). Deep within the star’s carbon-rich veil, astronomers have detected water vapor where no water should be.
CW Leonis is similar in mass to the sun, but much older and much larger. It is the nearest red giant to the sun, and in its death throes it has hidden itself in a sooty, expanding cloud of carbon-rich dust. This shroud makes CW Leonis almost invisible to the naked eye, but at some infrared wavelengths it is the brightest object in the sky.
Water was originally discovered around CW Leonis in 2001 when the Submillimeter Wave Astronomy Satellite (SWAS) found the signature of water in the chilly outer reaches of the star’s dusty envelope at a temperature of only 61 K. This water was assumed to be evidence for vaporizing comets and other icy objects around the expanding star. New observations with the SPIRE and PACS spectrometers on the Herschel Space Observatory reveal that there’s something much more surprising going on.
“Thanks to Herschel’s superb sensitivity and spectral resolution, we were able to identify more than 60 lines of water, corresponding to a whole series of energetic levels of the molecule,” explains Leen Decin from the University of Leuven and leader of the study. The newly-detected spectral lines indicate that the water vapor is not all in the cold outer envelope of the star. Some of it is much closer to the star, where temperatures reach 1000 K.
No icy fragments could exist that close to the star, so Decin and colleagues had to come up with a new explanation for the presence of the hot water vapor. Hydrogen is abundant in the envelope of gas and dust surrounding carbon stars like CW Leonis, but the other building block of water, oxygen, is typically bound up in molecules like carbon monoxide (CO) and silicon monoxide (SiO). Ultraviolet light can split these molecules, releasing their stored oxygen, but red giant stars don’t make much UV light so it has to come from somewhere else.
An illustration of the chemical reactions caused by interstellar UV light interacting with molecules surrounding CW Leonis. ESA. Adapted from L. Decin et al. (2010)
The dusty envelopes around carbon stars are known to be clumpy, and that turns out to be the key to explaining the mysterious water vapor. The patchy structure of the shroud around CW Leonis lets UV light from interstellar space into the depths of the star’s envelope. “Well within the envelope, UV photons trigger a set of reactions that can produce the observed distribution of water, as well as other, very interesting molecules, such as ammonia (NH3),” says Decin. “This is the only mechanism that explains the full range of the water’s temperature.”
In the coming months, astronomers will test this hypothesis by using Herschel to search for evidence of water near other carbon stars.
Much like any living being, stars go through a natural cycle. This begins with birth, extends through a lifespan characterized by change and growth, and ends in death. Of course, we’re talking about stars here, and the way they’re born, live and die is completely different from any life form we are familiar with.
For one, the timescales are entirely different, lasting on the order of billions of years. Also, the changes they go through during their lifespan are entirely different too. And when they die, the consequences are, shall we say, much more visible? Let’s take a look at the life cycle of stars.
Molecular Clouds:
Stars start out as vast clouds of cold molecular gas. The gas cloud could be floating in a galaxy for millions of years, but then some event causes it to begin collapsing down under its own gravity. For example when galaxies collide, regions of cold gas are given the kick they need to start collapsing. It can also happen when the shockwave of a nearby supernova passes through a region.
As it collapses, the interstellar cloud breaks up into smaller and smaller pieces, and each one of these collapses inward on itself. Each of these pieces will become a star. As the cloud collapses, the gravitational energy causes it to heat up, and the conservation of momentum from all the individual particles causes it to spin.
Protostar:
As the stellar material pulls tighter and tighter together, it heats up pushing against further gravitational collapse. At this point, the object is known as a protostar. Surrounding the protostar is a circumstellar disk of additional material. Some of this continues to spiral inward, layering additional mass onto the star. The rest will remain in place and eventually form a planetary system.
Depending on the stars mass, the protostar phase of stellar evolution will be short compared to its overall life span. For those that have one Solar Mass (i.e the same mass as our Sun), it lasts about 1000,000 years.
T Tauri Star:
A T Tauri star begins when material stops falling onto the protostar, and it’s releasing a tremendous amount of energy. They are so-named because of the prototype star used to research this phase of solar evolution – T Tauri, a variable star located in the direction of the Hyades cluster, about 600 light years from Earth.
A T Tauri star may be bright, but this all comes its gravitational energy from the collapsing material. The central temperature of a T Tauri star isn’t enough to support fusion at its core. Even so, T Tauri stars can appear as bright as main sequence stars. The T Tauri phase lasts for about 100 million years, after which the star will enter the longest phase of its development – the Main Sequence phase.
Main Sequence:
Eventually, the core temperature of a star will reach the point that fusion its core can begin. This is the process that all stars go through as they convert protons of hydrogen, through several stages, into atoms of helium. This reaction is exothermic; it gives off more heat than it requires, and so the core of a main sequence star releases a tremendous amount of energy.
This energy starts out as gamma rays in the core of the star, but as it takes a long slow journey out of the star, it drops down in wavelength. All of this light pushes outward on the star, and counteracts the gravitational force pulling it inward. A star at this stage of life is held in balance – as long as its supplies of hydrogen fuel lasts.
The life cycle of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant on the right after billions of years. Credit: ESO/M. Kornmesser
And how long does it last? It depends on the mass of the star. The least massive stars, like red dwarfs with half the mass of the Sun, can sip away at their fuel for hundreds of billions and even trillions of years. Larger stars, like our Sun will typically sit in the main sequence phase for 10-15 billion years. The largest stars have the shortest lives, and can last a few billion, and even just a few million years.
Red Giant:
Over the course of its life, a star is converting hydrogen into helium at its core. This helium builds up and the hydrogen fuel runs out. When a star exhausts its fuel of hydrogen at its core, its internal nuclear reactions stop. Without this light pressure, the star begins to contract inward through gravity.
This process heats up a shell of hydrogen around the core which then ignites in fusion and causes the star to brighten up again, by a factor of 1,000-10,000. This causes the outer layers of the star to expand outward, increasing the size of the star many times. Our own Sun is expected to bloat out to a sphere that reaches all the way out to the orbit of the Earth.
The temperature and pressure at the core of the star will eventually reach the point that helium can be fused into carbon. Once a star reaches this point, it contracts down and is no longer a red giant. Stars much more massive than our Sun can continue on in this process, moving up the table of elements creating heavier and heavier atoms.
White Dwarf:
A star with the mass of our Sun doesn’t have the gravitational pressure to fuse carbon, so once it runs out of helium at its core, it’s effectively dead. The star will eject its outer layers into space, and then contract down, eventually becoming a white dwarf. This stellar remnant might start out hot, but it has no fusion reactions taking place inside it any more. It will cool down over hundreds of billions of years, eventually becoming the background temperature of the Universe.
