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  • richardmitnick 4:46 pm on May 20, 2017 Permalink | Reply
    Tags: , , , , , part II, The stellar evolution conspiracy   

    From astrobites: “The stellar evolution conspiracy, part II” 

    Astrobites bloc

    Astrobites

    May 20, 2017
    Leonardo dos Santos

    Article: The influence of atomic diffusion in stellar ages and chemical tagging
    Authors: Aaron Dotter, Charlie Conroy, Phillip Cargile, Martin Asplund [see disclaimer below]
    First author’s institution: Harvard-Smithsonian Center for Astrophysics

    Status: Accepted for publication in the Astrophysical Journal, open access

    Because we know the Sun so well, we use it and other similar stars as templates to understand the physics of other objects in the universe. Thus the theory and modelling of stellar evolution play a key role on our understanding of the universe: from the habitability of exoplanets to the nature of dark matter, you name it! The problem is that it is extremely complicated and different authors use different treatments, which leads to some inconsistencies, especially for stars that diverge from the solar standard (see part I). With that in mind, let us ask ourselves again, this time with feeling: how much should we trust our understanding of stellar evolution?

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    Figure 1. Cutaway image show oddities in the speed of sound in the deep interior of the Sun. Credit: SOHO (ESA & NASA), MDI/SOI and VIRGO, A. Kosovichev, Stanford University

    ESA/NASA SOHO


    3
    SOHO SOI/MDI Results

    Layers of deceit

    Stars are sort of like onions (see Fig. 1): they have various concentric layers on the inside and can remotely make a fully-grown human teary-eyed. The Sun in particular has three main layers: the core, the radiative zone and the convection zone (Suk Sien Tie’s bite is a great read about this). In part I we briefly saw how convection is usually treated: It is mainly this process that mixes the material inside the Sun, changing the chemical composition in its surface. But, in today’s paper, the authors analyze the effects of another, usually overlooked process that changes the Sun: atomic diffusion.

    Diffusion happens mostly in the radiative zone of Sun-like stars: it is driven by gradients of pressure, temperature and concentration of chemicals. Even though it was theoretically proposed in the context of stars in 1917, to this date it has not become a standard ingredient in stellar evolution models. Aiming to patch this hole and test how diffusion changes things, the authors employ what I like to call the “cool kid” of stellar evolution codes: MESA (it has previously been featured on this bite, this one and others).

    Ain’t no easy way

    Do you recall that I said that stellar ages are the Achilles’ Heel of modern astrophysics? Well, this is where diffusion makes things even trickier (see Meredith’s bite for a summary on how stellar age estimation can be performed). It turns out that atomic diffusion is actually very important: it has the effect of extending the convective mixing into more deep regions, which changes the surface composition even further. This means that if we assume that Sun-like stars keep the same surface composition during most of their lives (which is something that I actually did in my undergrad research project), we would make systematic errors of up to 20% in their ages (see Fig. 2).

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    Figure 2. Differences in ages determined from the variable- and constant-metallicity assumptions, in billions of years. The majority of points are negative, which means that the constant metallicity assumption usually overestimate ages.

    In order to account for this evolution of chemical abundances (i.e., the surface composition), it is necessary to apply what the authors call the variable-metallicity approach to stellar age estimation. This method allows the abundances to evolve from the original bulk composition of the star, reflecting the effects of atomic diffusion.

    “Your sister looks nothing like you”

    As you can imagine, diffusion also has strong implications for chemical tagging (see Ingrid’s bite). We cannot lump them willy-nilly by their chemical composition anymore — we also have to consider the evolution of the abundances due to diffusion, which makes chemical tagging even more complicated than it already was. But this problem can be mitigated either by working with stars in the same evolutionary phase or focusing on abundance ratios instead of absolute ones (see Fig. 3).

    4
    With all that said, my hope is that, well, you haven’t lost hope on stellar evolution yet. Of course, the complications can be overwhelming, but it only means that there is still a lot of work to do. Let’s just keep tabs on the assumptions and approximations we make, shall we?

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 1:24 pm on April 30, 2017 Permalink | Reply
    Tags: , , , , MLT-mixing length theory, part I, Radiative diffusion, The life and death of stars, The stellar evolution conspiracy   

    From astrobites: “The stellar evolution conspiracy, part I” 

    Astrobites bloc

    Astrobites

    Apr 30, 2017
    Leonardo dos Santos

    Article: Confronting uncertainties in stellar physics II. Exploring differences in main-sequence stellar evolution tracks
    Authors: R. J. Stancliffe, L. Fossati, J.-C. Passy and F. R. N. Schneider
    First author’s institution: Argelander-Institut für Astronomie, University of Bonn
    1
    Status: Published in Astronomy & Astrophysics (February 2016), open access

    Practically all areas of research in astrophysics depend on how well we understand the life and death of stars. Habitability of exoplanets? Yes. Evolution of galaxies? Definitely. The nature of dark matter? Yup. The search for extraterrestrial life? You bet. This is such a crucial component of astrophysics that I decided to discuss the issue in more than one bite (the next one is coming soon). Stars are ubiquitous and drive countless phenomena in the universe. And that is why, at the end of every day, I always ask myself: how much should we trust our understanding of stellar evolution?

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    The Pleiades cluster in infrared. This well-known object is a laboratory for testing theories of stellar evolution and structure. Credit: NASA/JPL-Caltech/UCLA

    No need for alternative facts

    Now, I don’t want to sound like a conspiracy theorist or anything, but this is something that is keeping some of us awake at night. Let’s start with the Achilles’ Heel of modern astrophysics: ages of stars. Except for very special cases, stellar ages are particularly tricky to measure because stars change very little throughout their lifetimes. To complicate things further, small changes in the interior structure of a star can produce significant changes in its surface chemical composition. This is why we need our models to be very accurate so that we can have decent estimates of the physical properties of stars (notice that I said “decent”, and not “good”).

    There are many stellar evolution models out there and they are very similar, but it is not clear if any of them are even correct. For starters, it is practically impossible to compute stellar evolution from the first principles of physics, which is why we have to appeal to a series of simplifications and assumptions. Different authors apply different theoretical shortcuts, leading to the emergence of different models.

    Window-shopping stellar evolution models

    Suppose you observed a star identical to the Sun with the Gaia spacecraft and you want to estimate, say, its mass (see Meredith’s bite for a summary on how this estimation can be performed).

    ESA/GAIA satellite

    The authors of today’s paper found that, depending on which one of six available models is chosen, the mass of the star will be between 0.97 and 1.01 solar masses. That is actually a pretty good agreement, which means the models are consistent with each other (see Fig. 1). This is expected, because stellar evolution codes are usually calibrated to reproduce the Sun at its exact mass and age, which we know from other, more precise and accurate methods.

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    Figure 1. Evolutionary tracks of a star identical to the Sun (atmosphere temperature in the x-axis, luminosity in the y-axis). The curves are for different models, and the sets of symbols represent different ages. The square corresponds to the observational uncertainties of the Gaia satellite. Notice that all models fall well inside the observational uncertainties, which signals that they are consistent.

    The significant differences start to emerge when we work with stars that have masses and ages that depart from solar values. These are the regimes where our uncertainties about the approximations and assumptions may catch us off-guard. The authors observed that the six stellar evolution models of stars with 3 solar masses are particularly divergent after the main sequence phase (see Fig. 2).

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    Figure 2. Similar to Fig. 1, but for a star with 3 times the solar mass. Notice that the models are much more divergent in this case.

    How to mix a giant ball of plasma

    Another issue is that more recent developments in the theory of stellar structure, such as radiative diffusion (which we will discuss in part II), have an impact on the outcomes of models. When the authors tried to re-calibrate these changes with the Sun (using the openly available code MESA), they could not obtain a perfect global fit; it was either a good fit for the solar luminosity and temperature, or its chemical composition, but not all of them at the same time.

    Proposed by Erika Böhm-Vitense in 1958, one widely used approximation to model the convection of material in the atmospheres of stars is known as the mixing length theory (MLT). In a nutshell, the mixing length is the distance a convective cell traverses before dispersing itself. MLT has since been very successful in stellar evolution models, but it comes with a strong caveat: too many free parameters. That means that we observe a well known-star (e.g., the Sun) and calibrate these parameters so that the outcomes of models reproduce what we observe. Free parameters bother us because we don’t know to what extent they are applicable. An alternative to MLT that looks promising is the implementation of 3D hydrodynamical simulations of convection.

    In summary, it turns out that asking “what model should I choose?” is not that useful of a question; what we should actually ask is what are their assumptions and approximations. That way, we are able to analyze if the model is applicable or not to our research given its limitations. In the next part, we will discuss another development on stellar structure that is being heavily discussed by the community, and how it affects stellar age estimates and the search for cosmic siblings.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
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