Have you heard of LU Camelopardalis, QZ Serpentis, V1007 Herculis and BK Lyncis? No, they are not members of a boy band in ancient Rome. They are cataclysmic variables, binary stars so close that one star pulls material from its sibling. This causes the pair to vary greatly in brightness.
Can planets exist in this chaotic environment? Can we detect them? A new study says yes to both.
Cataclysmic variables (CVs) experience large increases in brightness. All stars vary in brightness to some degree, even our own sun. But the increase in brightness of CVs is much more pronounced than in stars like our Sun, and they happen irregularly.
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There are different types of cataclysmic variables: classic novae, dwarf novae, some supernovae, and others. All types share the same basic mechanics. A pair of stars orbit each other closely and one of the stars is more massive than the other. The most massive star is called the primary star, and it extracts gas from the lower-mass star, which astronomers call the donor star. The host star of a CV is a white dwarf, and the donor star is usually a red dwarf. Red dwarfs are cooler and less massive than white dwarfs. They have masses between 0.07 and 0.30 solar masses and a radius of about 20% of the sun. White dwarf primary stars have a typical mass of about 0.75 solar masses, but much smaller radii, about the same as Earth.
When the primary star extracts material from the donor star, the material forms an accretion disk around the primary star. The material in the accretion disk heats up, and this causes an increased luminosity. The increase can dominate the light of the star couple. If there is a faint third body, a planet, in the system, its gravity can affect the transfer of material from the donor to the primary star. These perturbations affect the brightness of the system, and this is the focus of the new study.
This is an image of the cataclysmic variable AE Aquarii, which is not part of this study. The smaller but more massive white dwarf is pulling material away from its main sequence companion, usually a red dwarf. Image Credit: By Casey Reed / NASA – Nasa – Pulsar-like White Dwarf Pulses, Public Domain,
The study authors show how chaotic environments around CVs can host planets and explain how astronomers can detect them. The study is “Testing the third-body hypothesis in the cataclysmic variables LU Camelopardalis, QZ Serpentis, V1007 Herculis and BK Lyncis.” It is published in the Monthly Notices of the Royal Astronomical Society (MNRAS). The main author is Dr. Carlos Chávez, from the Autonomous University of Nuevo León in Mexico.
Material attracted to the primary star clumps together in an accretion ring and heats up, creating greater luminosity. But the transfer of material to disk is not constant; rises and falls as the CV stars orbit each other. Chávez and his colleagues examined four cataclysmic variables in their study: LU Camelopardalis, QZ Serpentis, V1007 Herculis and BK Lyncis. All four CVs have very long photometric periods (VLPPs), which are periods of enhanced luminosity that do not match the orbital periods of the binary.
There is a point between both stars and the third body called the L1 point or Lagrangian point. It is a point of gravitational balance between the stars. The L1 point is dynamic and its position changes as the stars move. Lead author Chávez demonstrated in a previous paper that a third body, a planet, can cause oscillations at the L1 point.
As the L1 point changes, the amount of material dragged into the primary star (the mass transfer rate) changes. A change in the mass transfer rate creates a change in the luminosity of the entire three-body system.
By measuring the brightness changes of the four CVs, the researchers calculated the distances and masses of the systems’ potential third bodies based on the brightness changes of each system. Their calculations show that the variations have periods much longer than the orbital periods of the stars. According to the team, two of the four CVs they studied have “planet-like bodies” orbiting them.
“Our work has shown that a third body can perturb a cataclysmic variable in such a way that it can induce brightness changes in the system,” Dr. Chavez said in a press release. “These perturbations can explain both the very long periods that have been observed – between 42 and 265 days – and the amplitude of these brightness changes. Of the four systems we studied, our observations suggest that two of the four have objects of planetary mass in orbit around it”.
This is not the first time scientists have tackled curriculums and tried to find an explanation for variations in luminosity. In 2017, an independent team of researchers published a paper presenting the four curricula and their VLPPs. They suggested that the planets were the cause. But they said that “… the orbital plane of the third body would have to be greater than 39.2 degrees for this mechanism to be effective in disrupting the inner binary effectively.”
“Here we explore a new possibility, namely that secular perturbation by a low-eccentricity, low-inclination third object explains the VLPP and also the magnitude change observed in these four CVs,” Chávez and his coauthors write in their paper . They say that “…a third body in a planar near-circular orbit could produce perturbations to the central binary eccentricity.”
According to Chávez, his work represents a new way of detecting exoplanets. Planet hunters find most exoplanets using the transit system. When an exoplanet transits in front of its star, there is a detectable drop in starlight. Although effective (we’ve found thousands of planets this way), the transit method has limitations. It only works when things are well aligned. We have to look at it sideways, so to speak, or else the planet is not transiting the star from our point of view, and there is no drop in starlight.
But the method that Chavez and his colleagues developed does not depend on planetary transits. It is based on the intrinsic change in luminosity that is observable from different angles.
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