New study reveals one in ten evolved stars in Milky Way will engulf Jupiter-mass planets

When it comes to space, stars often experience cataclysmic events like exploding as supernovae or getting torn apart by black holes. However, planets also have a way of causing destruction, but in this case, it's the stars that suffer the consequences.

As stars age, they eventually become red giants and expand due to the conversion of mass into energy through nuclear fusion. Planets that orbit too closely to these giants are ultimately destroyed by being consumed.

A new study has explored this process of stellar engulfment and estimates that around one in ten evolved stars in the Milky Way will swallow Jupiter-mass planets. Titled “Giant planet engulfment by evolved giant stars: light curves, asteroseismology, and survivability,” the study is authored by Christopher O'Connor, a Ph.D. student at Cornell University's Department of Astronomy. The research has not yet undergone peer-review and is currently available on the arXiv server.

The study focuses on two related types of evolved stars: red giant branch (RGB) and asymptotic giant branch (AGB) stars. Although they differ slightly, the critical factor in this study is that both types have left the main sequence.

As stars evolve and lose mass, they undergo expansion, putting planets in close proximity at risk. The star's enlarged convective envelope can capture and trap nearby planets, leading to a known as drag, which causes the planet to spiral inward toward the star. Astronomers have studied the frequency of these events and how stars respond to them.

In this research, the authors focused on sun-like stars, which are stars with masses ranging from 1 to 2 times that of our Sun. They found that approximately 10% of these stars will engulf a planet with masses between 1 to 10 times that of Jupiter. The process of inward spiraling, known as in-spiral, takes between 10 and 100 years or between 100 and 1000 orbits, depending on the mass relationships involved.

To investigate these ranges and understand stellar responses, the researchers utilized an open-source astronomy software tool called MESA (Modules for Experiments in Stellar Astrophysics). By employing MESA, they could track the stellar response to energy deposition while simultaneously simulating the of the planetary orbit. This allowed them to observe how different evolved stars reacted when engulfing planets with varying masses.

While many astrophysical events occur over extensive timescales spanning thousands, millions, or even hundreds of millions of years, the process of planetary engulfment is relatively rapid. However, before the star and planet come into contact, two factors contribute to their convergence: stellar expansion and orbital decay. During this initial phase, the planet's orbit decays due to tidal friction, primarily caused by turbulent dissipation in the star's convective envelope. At this stage, drag forces from the stellar corona and stellar wind have minimal impact.

Once the star and planet begin to interact, the dynamics change. Tidal friction becomes less significant compared to drag forces, leading to what the authors describe as the “grazing” phase. This phase involves complex three-dimensional hydrodynamic interactions between the star and the planet. It can entail phenomena like matter expulsion from the star and the occurrence of optical and X-ray transients triggered by shocks. However, the current study focuses specifically on the subsequent “inspiral” phase, which occurs when the planet is fully immersed in the stellar envelope.

This figure from the paper shows heat deposited in stars in the later inspiral phase. The RGBs and AGBs in the legend are modelled host stars with different masses. The x-axis shows planetary mass, and the y-axis shows the amount of heat deposited. Clearly, the more massive the planet, the more heat is deposited. Credit: O'Connor et al. 2023

During the inspiral phase, a planet transfers heat to the star, particularly during the late inspiral phase, which plays a significant role in the star's response. The amount of heat deposited depends on the mass of the planet.

As the planet is engulfed, the star's envelope undergoes expansion and contraction, although not in a consistent manner. Different sections of the star's mass shell can expand and contract multiple times throughout the event. The planet can be envisioned as a localized heat source within the shell, moving toward the center of the star. These movements, combined with other stellar properties, lead to diverse expansion and contraction patterns.

This study supports previous research indicating that planet engulfment results in bursts of optical and infrared luminosity. The intensity and duration of these bursts primarily depend on the masses of the planet and the star, although factors such as stellar rotation can also influence them. The researchers discovered that for all red giant branch (RGB) stars and asymptotic giant branch (AGB) stars engulfing planets weighing up to five Jupiter masses, the star's brightness significantly increases within a few years.

Overall, the results indicate that when evolved stars engulf planets on the lower end of the mass range (up to three Jupiter masses), the structural changes in the star are mild to moderate. The star's brightness can rise by up to one magnitude within a few years. In the case of brighter stars, a double peak in brightness may occur.

For stars in advanced stages of the AGB phase, the engulfed planet can cause significant disruption in the outer layers of the star. This disturbance can trigger supersonic expansion, leading to the star resembling Luminous Red Novae (LRN) and producing bright, red, dusty eruptions.

This figure from the study shows the changes in radius and magnitude for one of the host stars modelled in the study. The top panel shows how a star can expand and contract multiple times during engulfment. The bottom panel shows how the star's magnitude changes. Credit: O'Connor et al. 2023

Irrespective of the star type, the planet's mass, and the star's response, the ultimate fate of an engulfed planet is always tidal disruption.

It is important to note that this study has limited direct relevance to our own solar system. While our Sun will evolve into a red giant in several billion years, it is unlikely that Jupiter will be engulfed unless a highly disruptive event occurs before then. Instead, it is the inner rocky planets that may face the prospect of being engulfed.

The study relies on simulations rather than direct observations. However, these simulations can assist astronomers in recognizing real engulfment events when they occur. Engulfments are transient phenomena, and several existing and future telescopes and observatories are specifically designed to focus on transients and time-domain astronomy. For instance, the upcoming Vera Rubin Observatory, expected to become operational by August 2024, will detect numerous transient events, including cases where evolved stars engulf Jupiter-mass planets.

The findings of this study can serve as valuable guidance in identifying such events and studying them in greater detail.

Source: Universe Today

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