Paper Title: Black Hole Survival Guide: Searching for Stars in the Galactic Center That Endure Partial Tidal Disruption
Authors: Rewa Clark Bush, Samantha C. Wu, Rosa Wallace Everson, Ricardo Yarza, Ariadna Murguia-Berthier, Enrico Ramirez-Ruiz
First Author’s Institution: Department of Astronomy and Astrophysics, University of California, Santa Cruz, California, USA
Status: submitted to the Astrophysical Journal Letters [open access]
This guest post was written by Isabella Oaks, a second-year undergraduate at Texas A&M University majoring in physics with minors in astrophysics, mathematics, and secondary education. Isabella mainly conducts research in physics education but will be beginning her journey into astronomy research by studying supermassive black holes this summer.
A tidal disruption event (TDE) is when a star gets too close to a Supermassive Black Hole (SMBH) and essentially gets ripped apart due to the tidal force, creating a hot accretion disk that emits lots of radiation, allowing the event to be something that is observed. The remains of these stars will be the brightest in the first 100 thousand years after the disruption event occurs, appearing brighter and more luminous than stars that did not experience a TDE. However, this event is limited: after about 10 million – 100 million years, the majority of TDEs will lose this luminosity, making it difficult to find stars that encounter these events, and even harder to study.
Despite this, the paper today mainly focuses on partial TDEs. A partial TDE occurs when a star is able to survive this spaghettification, leaving behind a remnant in the form of another star. After their TDE encounter, remnants are altered compared to their original state, losing mass based on the strength of the encounter with the SMBH, with radius, luminosity, and temperature increasing for this brief period. Additionally, these encounters are limited and fast, with astronomers estimating that one remnant is present in the Milky Way at any given time. These partial TDEs are predicted to occur more often than full TDEs; however, these remnants fade more rapidly, after about 100 thousand years, and will look like a star on the main sequence. These partial TDEs can be viewed observationally. After partial TDEs occur, the stars will cool down over a period of 1 million -10 million years before beginning hydrogen fusion in their cores, reaching equilibrium consistent with the main sequence. From here, they will join the main sequence again, which can make it difficult to tell which stars actually experience TDEs and which do not.
Today’s authors focused on these partial TDEs and where they are, relating them to G Objects. Because this event occurs at SMBHs, they are predicted to be plentiful in the Galactic Center of the Milky Way, which is where these G Objects are located. These objects move and behave like stars, however, they are surrounded by lots of dust, making them appear to look like clouds. Additional characteristics of these objects are large radii and luminosity. While these objects can be observed, there is no explanation for them as of now.
This paper and others predict that these G Objects, based on their characteristics, are young, partial TDEs based on similarities in luminosity, radii, and location. But, is this actually the case?
Today’s authors estimate the TDE rate to decide if they occur often enough to suggest that these G Objects are, in fact, young partial TDEs accounting for some or all of the population of G Objects observed. This rate, as well as other characteristic observations, predicts that the Galactic Center is going to have some TDE remnants that are bright enough to match the luminosity of the dimmest G Objects observed (see Figure 1), but will not account for the entire population.
The focus of the paper is to analyze young TDEs based on luminosity, but what about older TDEs? How can they be identified, especially since most TDEs are going to be older? Well, this paper discusses exactly that, combining kinematics and spectroscopy to analyse stars, and discuss which are possible candidates for being an older TDE remnant. For kinematics, stars that have large velocities are analyzed. When in the Galactic Center, these remnants can have either an elliptical orbit or a more hyperbolic or parabolic orbit. Having an elliptical orbit will cause the remnant to stay in the galactic center, while having a different orbit shape will cause an exchange of energy and mass between the SMBH and the remnant, effectively flinging it out of the Galactic Center. Once this has occurred, the remaining star will travel at a high-speed trajectory. Studying stars with high speeds and a trajectory that looks like coming out from the Galactic Center can give a hint as to whether or not it is a remnant. To further analyze these stars, spectroscopy can be used to analyze the elements present in the potential remnant. Stars with higher counts of helium or nitrogen can indicate whether or not a star is a survivor of a TDE, as the mixing from the event can effectively bring these heavier elements to the surface of the star from its core. Looking at both of these things can allow astronomers to determine whether or not a star is a survivor of one of these TDEs.
For the future, the authors discuss the building of the Vera C. Rubin Observatory Legacy Survey of Space and Time, and how this can be used to bring more study to TDEs, using it as an opportunity to view gravitationally intense interactions from a different perspective.
Astrobite edited by Brandon Pries
Featured image credit: NASA/CXC/M. Weiss
