Stellar Encounters at the Galactic Centre

Abstract

Using methods of smoothed particle hydrodynamics (SPH) an N -body simulation was set-up to investigate stellar encounters between a 1 solar-mass main sequence (MS) star, modeled after the Sun, and a 0.6 solar-mass white dwarf (WD). This was done primarily to investigate the assumptions made by Mastrobuono-Battisti et al. (2021) in search of the impact of stellar encounters on stellar population in nuclear stellar clusters (NSCs). Four different possible outcomes of the encounter were identified and described in detail. The boundary between a merger taking place between the stars and no merger was located in velocity-periastron space. Properties, such as, change in specific orbital energy and mass of the WD were investigated for a set of select encounters sharing the same velocity of the WD to look for relations between different input parameters of the encounter. It was found that the assumptions about no mass loss and the location of the boundary of the merger and no-merger encounters between WDs and MS stars made by Mastrobuono-Battisti et al. (2021) were false. I show that in encounters, where either a physical collision or a merger between the two stars takes place, a substantial amount of unbound mass is produced, and that encounters with periastron values larger than 1.0 solar-radii can result in a merger, given that the kinetic energy of the WD is sufficiently small. The possible effects of these findings on the evolution of stellar population in NSCs were discussed. Less significant relations between the mass accreted by the WD, WDs final velocity, orbital period of the WD in lengthy merger encounters and periastron value of the encounter were also analysed and summarized.

Popular Abstract

Imagine a rogue star was on a collision path to cross through the Solar System. What are the possible outcomes of this meeting, and what parameters decide the severity of it? How would our Sun change, and how would its future be effected? These are some of the major questions the study of stellar encounters tries to answer. In this article I will sketch short answers to the above questions and sum up the study of stellar encounters, with focus on encounters between main sequence stars like the Sun and some of the oldest stars in the Universe - white dwarfs.

In most places in the Universe one should not be concerned about such an event taking place, as distances between star orbits are so large that the probability that they cross each others gravitational pull is very, very small. However, in the densest regions in the Universe, termed nuclear star clusters, the orbits are so closely packed that such events are a common occurrence. The best place for us to study these encounters is the centre of our own Milky Way galaxy. Despite the galactic centre being the closest nuclear star cluster to us, physical observations are still hard to perform, mostly due to two reasons: firstly, the galactic centre is majorly populated by many bright stars and dust making visual observations very hard to perform; secondly since we are studying astronomical events, the time scale accordingly is not suited for human observations. To by pass these problems, we invoke methods of computational physics by performing advanced computer simulations based on hydrodynamics models used to simulate, for example, a ball moving in a water tank.

Stellar encounters come in different verities, depending on the stars that are meeting. For each type of encounter the final result and the impact of the encounter on the stellar population in nuclear star cluster will depend on the type of stars involved. For encounters between Sun like stars and white dwarfs the final result can either be a merger between the two stars, or the white dwarf escaping the gravitational pull of the main sequence star. The outcome and the severity of it, in broad strokes, mainly depends on the rate at which the distance between the stars decreases and the distance of the closest approach between them. These two parameters will decide if the product of the encounter will be a star still capable of nuclear fusion in its core, a dim object destined to remain faint for the rest of its existence, or two separate objects which have undergone a traumatizing experience and must carry on with their lives to best of their abilities. In either of the outcomes, a lasting impact is left on the products of the encounter. In most cases the structure of the star is heavily altered, in some even irreversibly. This experience alters the future evolution of the star, disrupting it from its normal evolutionary path. Given a number of these encounters can take place in the nuclear star clusters, the normal evolutionary path of the star cluster could be altered.

In summary, a star passing so close to the Sun that it might alter its future is not something we should worry about. However, the study of stellar encounters allows us to better understand the evolution of stars exposed to such events, how their internal structure has changed, what parameters decide the severity of encounter and what relationships exist between them, and how big of a role in evolution of the stellar population in nuclear star clusters do stellar encounters play.

• author: Fisers, Roberts
• supervisor: Ross Church
• organization: Lund Observatory - Undergoing reorganization
• course: ASTK02 20212
• year: 2022
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Galaxy, encounter, SPH, NSC
• report number: 2022-EXA187
• language: English
• id: 9074260
• date added to LUP: 2022-09-06 09:27:43
• date last changed: 2022-09-06 09:27:43