When stars get too close to supermassive black holes (SMBHs), they can be torn apart in a phenomenon called a Tidal Disruption Event (TDE). While most studies have focused on regular stars (main-sequence stars), this paper by Núria Navarro Navarro and Tsvi Piran investigates what happens when giant stars, which have dense cores surrounded by extended gaseous envelopes, undergo such events. Due to their structure, giant stars are often only partially disrupted, meaning parts of their envelopes are torn away, while their cores and inner layers remain intact. These partially disrupted stars evolve differently from normal stars, raising intriguing questions about their observable properties and long-term fates.

What Makes Giant Star Disruptions Unique?

A star gets disrupted when the gravitational pull from a black hole exceeds the star’s own gravity. For main-sequence stars, this leads to total disintegration at a specific distance from the black hole, called the tidal radius. However, giant stars have a core that’s roughly a million times denser than their outer envelope. This means the dense core remains intact even if the envelope is partially stripped away. The result is a Partial Tidal Disruption Event (PTDE). Complete disruptions of giant stars are rare and occur only near small black holes.

Giant stars, being larger, have a higher chance of experiencing PTDEs compared to smaller stars. Despite being fewer in number, their size increases the region around the black hole where they can be disrupted, making such events relatively common.

Modeling Partial Disruptions

Using a stellar evolution tool called MESA, the authors simulated how giant stars of varying masses behave after partial disruptions. They stripped away portions of the star’s outer envelope at different stages of its life and then tracked the evolution of the remaining stellar core.

Key findings from these simulations include:

1. Stripped stars rapidly return to their original giant-like size but with a slightly reduced mass.

2. Stars with minimal envelopes become dimmer, while those retaining more envelope mass stay as bright, if not brighter, than regular giants.

3. The long-term structure and evolution of partially disrupted stars closely resemble those of undisturbed giants, but their lifetimes may vary slightly depending on how much mass was stripped.

The Life of a Stripped Star

Once stripped, the giant star settles into a stable configuration. Its outer envelope re-expands, though it remains less massive than before. These remnants can continue their life cycles, burning helium in their cores like normal giants. However, their lifetimes may be shorter or longer depending on when the stripping occurred during their evolution. Interestingly, stripped stars may have higher luminosities than normal stars of the same mass. This happens because their cores, which dictate the star’s brightness, are relatively larger compared to the stripped envelope.

Successive Partial Disruptions

The study also explored what happens when a giant repeatedly passes close to a black hole, undergoing multiple partial disruptions. Each encounter strips away more of the star’s envelope, but the dense core always survives. Over time, the star becomes lighter and eventually settles as a "light giant," with a mass as low as 0.7 times that of the Sun. Despite losing mass, the star’s lifetime is not significantly altered since the disruption does not affect its core processes.

Observational Significance

Partially disrupted giant stars are challenging to identify. However, their unique traits, such as a combination of low total mass and unusually high brightness for their size, might make them detectable in certain regions, especially near galactic centers. These stars could serve as indirect evidence for past TDEs and the presence of black holes in such areas.

Conclusions

This study highlights the resilience of giant stars during tidal encounters with black holes. Their cores' extreme density prevents destruction, allowing them to survive successive disruptions and continue evolving. The remnants of these events could reveal insights into both stellar and galactic evolution. While detecting such stars in distant galaxies is challenging, observations in the Milky Way's center might provide opportunities to find them, offering clues about past black hole activity and star dynamics.

Source: Navarro