Origin of blue straggler stars in the Milky Way halo

Abstract

Blue straggler stars (BSSs) are exotic stellar objects that appear to be younger than the age of the population they come from suggests. They are observed as the extension of the main sequence stars, beyond the turn-off point. They are present in all types of stellar populations, such as open clusters, globular clusters, dwarf galaxies, and even in the open field. It is believed that they form mainly through binary star evolution. In this work, we are particularly interested in BSSs in the field of the Galactic halo as this is where the ancient dwarf galaxies, that were accreted by the Milky Way, deposited their debris. Studying the BSSs in this part of the Milky Way allows us to not only test the theories of binary star evolution, but it also gives us an insight into some of the oldest stellar populations, which probe the early history of our galaxy. We investigate how these BSSs form, how the theoretical formation channels vary between the accreted (formed ex-situ) and non-accreted (formed in-situ) samples, what the chemical abundance patterns are in the samples, and what that tells us about the populations they come from.

Large datasets are needed to identify these rare stars and we use data from the spectroscopic surveys APOGEE and GALAH, cross-matched with Gaia. The datasets provide the necessary kinematic and chemical information about stars, which allows us to distinguish between stars that formed in the Milky Way and those that formed outside our galaxy, but were eventually accreted into the halo. Since BSSs are thought to form primarily through mass transfer in binary systems, we use the Binary Star Evolution code (BSE) to model possible formation channels for stars in the final sample.

After applying our selection criteria, we end up with a small sample of possible BSSs, and study their chemical abundances and radial velocities. We use some chemical abundances (e.g. barium) to constrain the possible formation scenarios. We generate a large sample of model BSSs using BSE, look at their formation channels, and make synthetic radial velocity measurements, which we compare with the observed values. Lastly, we create detailed theoretical models of formation for two stars in the final sample, which have the radial velocity curve measured.

We find that the vast majority of the stars in the final sample show radial velocity variations, which is consistent with our expectation that they form through binary star evolution. We show that the formation channel of a BSS correlates with its currently observed orbital period and mass. The modeling results show that mainly mass-transfer types B (mass transfer from a red giant), C (mass transfer from an asymptotic giant branch star), and D (accretion of mass from stellar winds) are able to reproduce the observed stars. We conclude that the accreted and non-accreted samples are not inconsistent with being the same. We are largely limited by the small size of the accreted sample of stars. The models also predict higher variation in radial velocity than is observed. Eccentric orbits of BSSs might be able to explain this offset, but our current understanding of binary star evolution cannot easily explain the higher eccentricity in post-mass-transfer systems. This proves to be the case even when modeling the two individual systems, which we are able to recreate accurately through type B or C mass transfer, except for the observed eccentricity.

Popular Abstract

We study our galaxy, the Milky Way, through observations of stars in the night sky. They tell us about the structure, composition, and kinematics of different components of our galaxy. Some stars form binaries, which are systems consisting of two gravitationally bound stars that orbit each other. They are great laboratories for testing the current theories of stellar evolution.

Stars of different masses evolve at different rates, which can lead to some interesting interactions. The more massive a star is, the quicker it burns all of its fuel and the shorter it lives. In close binary systems, as the more massive star evolves and expands, its outer layers will not be gravitationally bound to it anymore and they get transferred onto the companion star. As a result, the companion significantly grows in size and becomes a star that we observe as a blue straggler. They are blue because they are massive and hot, and hotter stars emit more energetic blue light. On the other hand, cooler stars appear redder. Blue and hot stars must have formed recently since they live \say{only} for millions of years, which is quite short on cosmic timescales. However, these young-looking stars have been observed in some of the oldest stellar populations, which are billions of years old. Their presence in these populations is therefore quite puzzling.

The Milky Way is a spiral galaxy consisting of a disk surrounded by a so-called Galactic halo. In this project, we are particularly interested in the halo as it contains some of the oldest stellar populations. During the early history of the Milky Way, there were a lot of interactions with other galaxies. Some dwarf galaxies were gravitationally pulled towards the Milky Way, ripped apart, dissolved, and mixed with other stars. The stars coming from some of these ancient dwarf galaxies are hard to find since they are spread across a large area of the night sky. Luckily, we can find them by studying their motion and chemical composition. Finding blue stragglers in the halo is even more challenging, but they can give us an insight into what these ancient dwarf galaxies looked like.

In this project, we use data from spectroscopic surveys. These are designed to gather and analyze spectra for as many stars as possible, making them great for our science goals as we are looking for very rare objects. These datasets consist of millions of stars, so we need to make reasonable cuts to narrow down the selection. Spectroscopic surveys provide information about the chemical composition of stars, which is useful for our analysis. Stars synthesize different elements during different evolutionary stages and it helps us to better constrain the possible formation mechanisms. When a star transfers its mass onto the blue straggler, it leaves an imprint in its chemical composition, which we can observe in the spectrum.

We study the chemical abundances of blue stragglers that made it into our final sample. We find that the vast majority of these stars are in binary systems, confirming our expectation that they form mainly via mass transfer in binaries. We use computer simulations to model binary systems and investigate how some of the blue straggler stars could have formed. We compare the stars coming from accreted (stars formed outside the Milky Way) and non-accreted (stars formed in the Milky Way) populations, and we do not see major differences in the formation mechanisms of blue stragglers.

• author: Brcek, Adam
• supervisor: Diane Feuillet; Ross Church
• organization: Astrophysics; Department of Physics
• course: ASTM32 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Blue Straggler, Galactic Halo, Spectroscopic Surveys, Binary Star Evolution, Mass Transfer
• report number: 2023-EXA208
• language: English
• id: 9125577
• date added to LUP: 2023-06-16 15:28:42
• date last changed: 2023-08-30 15:10:13