Parenting planets is a game-changer | #parenting

This guest post was written by Andreas Christ Sølvsten Jørgensen, a postdoctoral research fellow at the University of Birmingham. Previous to his position in Birmingham, Andreas earned his PhD at the Max Planck Institute for Astrophysics in 2019. He works within the field of theoretical stellar physics, primarily focusing on hydrodynamic phenomena, such as convective processes in the outermost layers of low-mass stars. He thus develops upon stellar structure and evolution models and compares model predictions with observational data.

Title: The solar abundance problem and eMSTOs in clusters

Authors: Richard Hoppe, Maria Bergemann, Bertram Bitsch, and Aldo Serenelli

First Author’s Institution: Max Planck Institute for Astronomy, Heidelberg (Germany)

Status: Open access on arXiv

Planets influence fates and lifecycles. While this might sound like astrological mumbo-jumbo, today’s authors show that the statement is demonstrably true … that is, when talking about the lifecycles of stars.

Over the last decades, astronomers have detected and characterised a multitude of planets around distant stars. Stars without planets seem to be the exception rather than the rule. Even single stars thus do not form in isolation but are rather embedded in protoplanetary (PP) discs during the pre-main sequence. The fact that PP discs are indeed ubiquitous is supported by direct observations from facilities such as ALMA. Today’s authors show how this notion can help us to understand stellar evolution better.

Why bother?

The current understanding of stellar structures and their evolution builds upon detailed numerical stellar models. One of the simplifying assumptions upon which these models rest is the premise that stars evolve in isolation. However, stars that are embedded in PP discs will accrete gas and dust from the disc or might even engulf entire planets. This accretion of material will change the chemical composition of the newly born star. 

At first glance, this statement might seem paradoxical. After all, the star and the associated PP disc are both parts of the same collapsing cloud. However, while the accreted gas primarily contains Hydrogen and Helium, the accreted dust is mainly composed of heavier elements. Any process that alters the accreted dust to gas ratio will thus have an impact on the stellar composition. For instance, for discs with low viscosity, dust grains grow large and quickly drift inwards, falling onto the host star.

Now, consider our Sun during its infancy surrounded by a PP disc. The outermost layers of such a young star are convective. In other words, material that falls onto the star is efficiently mixed into the envelope but does not reach the stellar centre. To understand why this is of any concern, we must take a closer look at how the chemical composition of the Sun is determined. It is based on spectroscopic measurements, and it is, therefore, based on the light that is emitted close to the solar photosphere. In other words, we have de facto constraints on the composition of the convective envelope. If you were to construct a model of the Sun, you would thus strive to recover the composition of the envelope. However, if you were to do so without accounting for composition changes that arise from PP disc accretion, your model would necessarily have to have a skewed initial composition profile. Otherwise, how would you get the right result, whilst leaving out essential phenomena? Now, remember that state-of-the-art stellar models do precisely that. They assume that stars form in isolation. So, you might suspect that standard solar models do not get the solar structure entirely right. You might even expect some tensions with data to arise. If so, you are spot on.

The devil is in the details

To interpret spectroscopic data from the Sun, astronomers compare measurements of solar spectral lines with the predictions of hydrodynamic simulations. These simulations have drastically improved over the last three decades. One result of these improvements has been a more accurate determination of the abundances of heavy elements in the solar atmosphere. As a result, the Sun is now determined to be more metal-poor than it was thought to be thirty years ago. However, stellar models that use the outdated constraints on the solar metallicity are better at recovering helioseismic constraints than models that employ the updated constraints from hydrodynamic simulations. Helioseismology denotes the study of solar oscillations and is a hugely successful field of research that builds upon stellar structure and evolution models. What do we believe in now? The spectroscopic predictions of sophisticated hydrodynamic simulations or seismic predictions from equally sophisticated stellar structure and evolution models? Needless to say that this question causes quite a few headaches within the field of stellar physics.

However, remember that stellar structure and evolution models assume that stars form in isolation. Today’s authors show that the dissonance between spectroscopic and seismic constraints can be overcome by taking accretion from PP discs into account. If the Sun accumulated metal-poor material, i.e. mainly gas, during its early evolution, spectroscopic data would naturally make it look more metal-poor than it initially was. In this scenario, the tension between helioseismic and spectroscopic constraints would disappear. While the employed accretion model is somewhat simplistic, and not all aspects have been fully explored, the mere suggestion that the presence of PP discs could resolve the tension is tantalising. After all, this tension between spectroscopic and seismic constraints has haunted stellar physics since 2005.

Reading clocks

After having addressed the tension between helioseismic and spectroscopic constraints on the solar composition, today’s authors take a look at another problem that has caused some headache in stellar physics. When you want to estimate the age of open clusters, you can do this based on the position of the individual stars in the Hertzsprung-Russell (HR) diagram using isochrones. It is a well-known problem that there will be some spread around even the best-fitting isochrone. More importantly, this spread cannot be accounted for. At the turnoff point, the spread between data and observations is known as the extended main-sequence turn-off (eMSTO). 

Isochrones are constructed based on stellar structure and evolution models. In other words, the eMSTO tells us that stellar structure and evolution models cannot probably account for the position of stars in the HR diagram. This is where PP discs come into the picture. Today’s authors demonstrate that the eMSTO is, once again, a consequence of assuming that stars evolve in isolation. The accretion of material from the PP disc changes the stellar mass, composition, and internal structure. Consequently, accretion affects stellar evolution tracks. Today’s authors show that the impact of accretion from PP discs can easily explain shifts of the isochrone of a few hundred Kelvin at the turnoff point. These shifts are compatible with the observed eMSTO. The left panel of Figure 1 illustrates this finding and includes different accretion scenarios. Not only does the composition of the accreted material affect the isochrones, but so does the time frame, during which accretion takes place. As illustrated in the right panel of Figure 1, the properties of the star, including the stellar composition, naturally likewise play a role.

Fig 1: Left panel: HR diagram with Gaia data from nearby open clusters. The solid red line shows a well-fitting isochrone. The blue shaded area depicts changes that would arise when including different disc accretion scenarios that span 1 Myr. The dotted lines indicate the shifts that arise from accretion histories spanning 3 Myr. This panel corresponds to Figure 4 in today’s paper. Right panel: Composition of the convective envelope as a function of time for different accretion histories. The composition of the envelope depends on the initial composition (Zini) of the star since the initial composition affects how quickly the convective envelope recedes. This panel corresponds to Figure 5 in today’s paper.

Today’s authors are thus able to kill two birds with one stone. They propose a method that improves solar models as well as our understanding of the age determination of clusters. The message is the following: To understand stellar evolution properly, we must understand the environment in which stars are born.

Astrobite edited by: Jamie Sullivan

Featured image credit: Today’s paper

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