In space, cataclysmic events for stars happen all the time. Some explode as supernovae, some get torn apart by black holes, and some suffer other fates. But when it comes to planets, the stars turn the tables. Then the stars are the ones who wreak havoc.
accommodate red giant stars Planets that get too close are consumed and destroyed, and a new study takes a deeper look at the process of stellar engulfment.
Stars like our Sun will eventually become red giants. during Nuclear fusionthey convert mass into energy (E = mc2, right?) Over the course of their lives, they shed so much mass as energy that they eventually expand and turn red. For planets very close to these bulging spheres, it marks the end. They are finally overwhelmed and completely destroyed.
Much research has delved into the process of planetary engulfment, and a new study calculates that one in ten evolving stars in the Milky Way galaxy will swallow the planets of Jupiter.
The study is titled “Giant Planet Evolving Swallow”. Giant stars: Light Curves, Astrology of Venus and Viability. The first author is Christopher O’Connor. O’Connor is a PhD student in the Department of Astronomy at Cornell University. The study has not yet been peer-reviewed and is available at arXiv server.
The study focuses on two closely related types of evolved stars: red giant branch (RGB) and asymptotic giant branch (AGB) stars. The two are similar, and in fact, RGB stars can become AGB stars. The term evolving star is descriptive enough to cover both, and in this work the important thing is that RGB stars and AGB stars have left the main sequence.
When these evolving stars lose mass, they expand, and at this point, any planets in them Close to in danger. The star’s convective envelope swells and infects the planet. This creates clouds, causing the planet to rotate inward toward the star. Astronomers know this, and in this work, the authors examined the frequency of these events and how the stars respond.
They describe a sunlike star as one that has one to two solar masses. About 10% of these stars will swallow a planet between 1 and 10 times the size of Jupiter. For these mass relationships, the spiral would take between 10 and 100 years or between 100 and 1,000 orbits.
To determine these ranges and how the star responds, the researchers used an open-source astronomy software tool called MESA (Modules for Experiments in Stellar Astrophysics.) time,” they explain. MESA has revealed how different evolved stars respond to swallowing planets of different masses.
While many astrophysical events occur over thousands, millions, or even hundreds of millions of years, the engulfment of planets is a much faster process. But before a planet and a star touch, two things hold them together: stellar expansion and orbital decay. This is the first stage of engulfment, in which orbital friction causes the planet’s orbital decay. The authors explain that tidal friction “is likely due to turbulent dissipation in the star’s convective envelope.” At this point in the process, the drag from the corona and stellar winds are minimal.
Once a star and planet start to connect with each other, things change. Tidal friction takes a back seat to pulling forces. The authors call this stage “grazing”. “The ‘shepherd’ hydrodynamic interaction between star and planet is complex and three-dimensional,” they wrote. Intricacies in the grazing phase can include phenomena such as the ejection of matter from the star and X-ray and optical transients from impacts. But this study leaves these phenomena aside for the time being. They wrote: “We are focusing on the later ‘inspirational’ phase of immersion, when the planet is completely submerged in the mantle.
When the planet is in the catarrhal phase, it deposits heat into the star. The last part of this phase is called the late inspirational phase, and the heat added to the star during this phase is largely responsible for the star’s response. The mass of the planet is a determining factor in the amount of heat that is deposited.
Engulfing causes the stellar envelope to expand and contract, but not monotonously. The shell of a given mass can expand and contract many times during the event. The researchers say the planet can be envisioned as a local source of heat in the atmosphere, the source moving toward the center of the star. This motion, and other properties of the star, create various expansions and contractions.
This research agrees with previous research that showed planet-embracing results in optical and infrared bursts of luminosity. The strength and duration of these outbursts is largely determined by the mass of the planet and star, although other factors such as rotation can play a role. The researchers found that for all RGB stars, and for AGB stars that swallow planets up to five Jupiter masses, the star brightens dramatically in just a few years.
The researchers’ overall results show that for both types of evolved stars engulfing a planet on the low side of the scale, up to three Jupiter masses, changes in stellar structure are mild to moderate. The star’s brightness increases by up to 1 magnitude in just a few years. Brighter stars can experience a double peak.
For stars in later AGB stages, a sinking planet can cause significant disruption in the star’s outer layers. It can lead to the supersonic expansion of the outer layers of the star. In this case, the stars can look like luminous red supernovae (LRN) because they produce bright, red, dusty eruptions.
No matter the type of star, the mass of the planet, and how the star responds to being swallowed up, the fate of the planet is always the same: tidal disruption.
This study has limited application to our solar system. Our Sun will become a red giant in a few billion years, but unless something very disturbing happens before then, Jupiter is out of reach. Instead, the inner, rocky planets face being swallowed up.
This study is based on simulations rather than observations, but simulations can help astronomers pinpoint the real thing as it happens. Engulfments are transient events, and some current and future telescopes and observatories focus entirely on transient and time-domain astronomy. When the Vera Rubin Observatory comes online around August 2024, it will detect a slew of transient events, some of which will be progressing stars swallowing Jupiter’s mass. planets.
The results of this study can help figure it out.
Christopher E O’Connor et al., Giant Planets Swallowed by Evolving Giant Stars: Light Curves, Stellar Venus Science, Survivability, arXiv (2023). doi: 10.48550/arxiv.2304.09882
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