We don’t have to worry too much about our Sun. It can burn our skin and can emit powerful doses of charged material – called solar storms – that can damage electrical systems. But the Sun is alone up there, making things simpler and more predictable.
Other stars are locked into relationships with each other as binary pairs. A new study has found a binary pair of stars that are so close together that they orbit every 51 minutes, the shortest orbit ever seen in a binary system. Their proximity to each other causes problems.
Stars this close together are called cataclysmic variables. In cataclysmic variables, the main star is a white dwarf. in this pair, the other star is a star similar to the Sun, but larger.
White dwarfs are tiny for stars, about the size of Earth, but incredibly dense. The white dwarf’s strong gravity pulls material away from its companion, the donor star. The material forms an accretion ring around the white dwarf. This process creates flashes of light at irregular or variable times as the disk heats up and material falls onto the white dwarf.
Stars in a cataclysmic variable (CV) must be close together for the white dwarf “vampire star” to draw material from the donor star.
Astronomers know of more than 1,000 CVs, and only a dozen of them have orbits shorter than 75 minutes. But the authors of this study found the closest orbit yet. This pair of stars takes only 51 minutes to complete one orbit. This is rare, and the binary pair is evidence of a missing link in astrophysics.
The study is “A Dense 0.1 Solar Mass Star in an Eclipsing Binary of 51-Minute Orbital Period” and was published in the journal Nature. The lead author is Kevin Burdge of the MIT Department of Physics. The stars in this study are about 3,000 light-years away in the direction of the constellation Hercules.
These stars are at the end of a long story. They have been partners for about 8 billion years, although they have aged differently. One is a white dwarf, the stellar remnant of a main-sequence star that has passed through the red giant phase and is now just an ultradense, unfused core of matter. Its companion is a Sun-like star on its way to becoming a red giant and eventually a white dwarf. But the existing white dwarf disrupts this path by slowly consuming it.
The largest donor star is about the same temperature as our Sun. But it has lost so much of its mass that it is tiny. only one-tenth the diameter of the Sun, or about the size of Jupiter.
“This one star looked like the Sun, but the Sun can’t fit into an orbit of less than eight hours – what’s going on here?” Burge said in a press release.
The white dwarf is even smaller. Its diameter is about 1.5 times that of Earth, while its dense matter means it is about 56 percent as massive as the Sun. A strange object.
Astronomers have discovered other eclipsing binaries, but none are as close. Not only are the pair very close to each other, but they overshadow each other from our field of view. This gave the researchers multiple opportunities to observe the eclipses and make precise measurements for both stars.
This binary pair is named ZTF J1813+4251. ZTF stands for Zwicky Transient Facility, a remarkable public-private partnership engaged in a visual study of the northern sky in search of transient phenomena such as variables. But the name is not so important. Instead, it’s the specific stage the couple is in that makes scientists sit up and take note.
The researchers found that the vampire star is removing hydrogen from the donor star and is now beginning to cannibalize the helium.
“This is a rare case where we caught one of these systems changing from hydrogen to helium accretion,” said lead author Burdge.
Observing a binary star switching from hydrogen to helium accretion is essential because the switch is a missing link in astrophysics. Astronomers are aware of a population of CVs called helium CVs, but there was no clear indication of how the stars in these CVs changed from hydrogen to helium.
Prior to this study, the evolution from hydrogen accretion to helium accretion in helium CVs was unclear. Astronomers had never observed a star making the transition.
But observations of ZTF J1813+4251 have changed that. Observations showed that the donor star is about the same temperature as the Sun but 100 times denser. This density means the star has a helium-rich composition, and the white dwarf companion is accreting helium rather than hydrogen.
Scientists predicted decades ago that binary stars could shrink until their orbits are extremely short and become cataclysmic variables. As the white dwarf consumes the Sun-like star’s hydrogen, the denser Helium is left behind. The Sun-like star burns up and a helium core is left behind. The heavy helium core is enough to keep the dead star in a tight orbit.
Repeated observations of stars eclipsing each other were just the beginning. With the more accurate data the researchers gathered, they ran more accurate simulations to see what could be done with the pair. These simulation results answered long-standing questions about cataclysmic variables and their shrinking trajectories.
Simulations show that in about 70 million years, the pair will get even closer until their orbital period is just 18 minutes. At that point, it will be a binary CV of helium. This transition is “… a previously missing link between binary helium CVs and hydrogen-rich CVs,” the authors write.
In the images below, the orange dashed line, red dashed line and blue dashed line represent different evolutionary trajectories depending on when the donor star started to lose mass from its WD companion in its lifetime. Orange is when it started at 97 percent of its main sequence lifetime, red is at 95 percent, and blue is at 94 percent. The black star in the red line is ZTF J1813+4251. (The purple line represents the evolutionary path for another possible transition CV called El Psc and is shown for comparison.)
The team’s simulation presented an evolutionary path for the binary star in (a). As the stars get closer, mass loss accelerates and the temperature of the donor star rises as it tries to keep up with the mass loss. The temperature then decreases as the last hydrogen fuses.
As the orbital period shrinks and the donor star loses mass, it expands and its temperature drops adiabatically due to the expansion. At that point, the binary star is a helium CV.
(b) shows how the binary star will reach a period of at least about 18 minutes in about 75 million years. After that, the pair will spend the next 300 million years drifting apart until their period is about 30 minutes. (Y-axis shows increments of 100 million years, unlabeled.)
c) shows the evolution of the donor star mass associated with the orbital period, reaching only a few centimeters of the solar mass as the traces evolve to longer orbital periods as helium CVs.
d) shows how the donor loses its hydrogen on its way to becoming a helium CV. The star loses all its hydrogen at about minimum orbital period.
Gravitational waves play a role in this study as well. Burdge’s specialty is astrophysical sources of gravitational and electromagnetic radiation. Gravitational waves were first measured in 2015, although predicted long before that, and are an important area of study in astronomy. “Gravitational waves allow us to study the universe in a whole new way,” Burdge said.
This binary pair should emit gravitational waves at such close proximity to each other. They must be very close together to emit the waves, but not too close. at a distance of about 10,000 km, they will merge and explode, ending the gravitational wave emissions.
“People have predicted that these objects would have to go into ultrafast orbits, and it’s been debated for a long time whether they could get close enough to emit detectable gravitational waves. This discovery puts that to rest,” Burdge said in a release. type.
Burdge and his colleagues worked hard to find this binary pair. They examined data from the ZTF, looking for variables that blinked repeatedly in less than an hour. This signals that the stars cross each other’s orbits and that the orbital period is short. First, they used an algorithm to search for ZTF data in over a billion stars. This algorithm produced about a million twinkling stars approximately every hour. Burge then examined this selection, looking for interesting signals.
Finally, it zeroed in on ZTF J1813+4251.
“This thing came up where I saw an eclipse happening every 51 minutes, and I said, OK, this is definitely a binary,” Burdge said.
“This is a special system,” Burdge said. “We are doubly fortunate to have found a system that answers a big open question and is one of the most beautifully known cataclysmic variables.”
This article was originally published by Universe Today. Read the original article.