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Thread: T Tauri Stars and Core Hydrogen Fusion

  1. #1
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    T Tauri Stars and Core Hydrogen Fusion

    Most sources I have consulted assert that there is no core fusion taking place in T Tauri stars (eg Wikipedia). Once fusion ignition takes place, however, the star joins the main sequence.

    A number of other sources, however, say that the T Tauri phase begins when core ignition occurs and ends when core contraction is halted.

    Now, hydrogen fusion occurs at temperatures of about 7 x 106 K, but a star like the Sun surely had a much higher core temperature when it joined the main sequence. (Its current core temperature is 15.6 x 106K.)

    Does anyone know what the precise sequence and timing of events for a Sunlike star is?

  2. #2
    I can't say I know, or that there is a precise timeline. However, this is what I've come to know about the general timing of the life of our sun or a star like it.
    It starts with the giant molecular cloud. Near 20K or so.

    Shockwaves, passing body, or something jostles it. The GMC begins to fall in on itself.

    ~2 million years, the GMC becomes a stellar globule. 0.1 light-years in diameter. No light emitted, but heat is coming soon as the stellar globule continues to collapse.

    ~2.05 million years, the stellar globule now has a recognizably spherical shape, and a temperature of 10 kK (kilo Kelvins?). 10,000K. We now have a protostar.

    ~2.2 million years, we now have a stellar nebula, beginning to be flattened at the poles. Somewhere between 100 and 200 AU across.

    ~3 million years, this is what I know as the T Tauri phase. The protostar is still shrinking, with a core of around 5 MK, and surface of around 5000K. The star is just now developing a strong magnetic field as the contents are fairly well ionized. Angular momentum has it spinning maybe once every ten days. The extra strong magnetic field does all kinds of fun things to the planetary disc.

    Big, red, and angry. Then the outflow starts.

    Solar winds of 200 kps pour out. Due to magnetic fields and planetary disc deflection, the wind traveled perpendicular to the disc, called bipolar molecular outflow. This is what signals the beginning of the end of the T Tauri phase. The star has stopped gaining material for the most part and is now losing it.

    The heavy winds might last for only a stellar eyeblink - 10 thousand years or so. Shrinkage continues for another 50 million years. Maybe less.

    That's when the proton-proton chain reaction starts, which is more common than the CNO in stars around a single solar mass.

    Core temp is around 15MK, and this is the main sequence.
    This is a very very rough idea of the evolution of a solar mass star.

    I'd also be happy to share the details I've omitted if you'd like, since I'm not sure what you're looking for, specifically.

  3. #3
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    Thanks for the response. I'm pretty familiar with the evolution of a solar mass star from cradle to grave (though the timescales I learned are somewhat different than yours).

    You have core fusion (pp-chain) starting after the T Tauri phase, which is the bit I am fuzzy about.

    The Sun's core temperature today is 15.6 million K. But PP1 starts at about 7 million K. What's more, the Sun's luminosity a few billion years ago was only about 70% of what it is today, so its core temperature when it joined the main sequence was perhaps about 11 million K?

  4. #4
    The transition from T Tauri to main sequence means the star would go from 5MK to 15MK in about 50 million years or fewer. Core fusion would begin before hydrostatic equilibrium was achieved, so fusion begins in the T Tauri phase, but isn't "viable" as stellar support until gravity has exerted enough pressure to find that balance.

    So if you're wanting the exact temperature when the sun joined the main sequence, can the difference between 11MK and 15MK matter overmuch?

  5. #5
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    Quote Originally Posted by seanhogge View Post
    That's when the proton-proton chain reaction starts, which is more common than the CNO in stars around a single solar mass.
    A lot also depends on whether or not it was a primary, secondary, tertiary, etc. star (or combinationg thereof).

    What I mean by that is that the earliest stars were mostly just hydrogen. But during a hydrogen star's life, the hydrogen is converted to helium, and successively heavier elements.

    Thus, when supernova events happened long ago, they put out all sorts of elements, when later recondensed into a star (and planets) with heavier elements and the process began all over again. Fusion isn't just limited to hydrogen. Even lithium, beryllium, and boron can undergo fusion. And fusion is not also limited to just combining two atoms. Free neutrons can fuse with one element to form another, releasing energy. It's still fusion.

    In our own sun there are around a dozen different fusion processes, all of which contribute, in unequal amounts, to the sun's power output. I did a brief search, but couldn't find the link.

  6. #6
    It seems an elementary point, but one that is well remembered. My version is an extremely simplified timeline. I only hope it's not misleadingly so.

    Certainly, the presence of heavier elements in population I stars (such as our sun) do affect the mechanics of stellar evolution. Even the details of the comparatively simple proton-proton chain reaction are not as straightforward as the nomenclature might indicate.

    Fusion also occurs in brown dwarfs (deuterium and lithium), which further complicates the usage of fusion in any singular sense as defining main sequence or even stars at all.

  7. #7
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    Lightbulb T Tauri Stars

    T Tauri stars are pre-main sequence and do not yet engage in proton-proton fusion in the core.

    From Stellar Interiors: Physical Principles, Structure and Evolution; Hansen, Kawaler & Trimble; Springer 2004 (2nd edition), page 373:
    Application to Pre-Main Sequence Evolution
    A direct application of this discussion is to the evolution of pre-main sequence stars. If we suppose that they have no interior thermonuclear energy sources (although burning of deuterium, which may have been present in the protostar nebula, may play a role), then contraction from a protostellar cloud will eventually yield high luminosities at large radii, and large luminosities usually require convection. If accretion of matter onto the forming star can be neglected (and this is not really true), the object follows a path on the Hertzsprung-Russell diagram ... These paths are appropriately known as "Hayashi tracks" and are those taken by the T-Tauri stars ...

    Although I have a lot of stellar evolution & star formation resources, this is the only one I have that is definitive about the interior of a T-Tauri star. A real account of the "precise sequence and timing of events for a Sunlike star" can be excruciatingly complicated. We do know that the theory of stellar evolution does a good job with the observed properties of young stellar objects, including T-Tauri stars. But the more detail you want, the more you reveal chinks in the theoretical armor.

    Here is a short list of resources I recommend for the study of stellar evolution & star formation that might satisfy any level of curiosity, aside from the book already cited above.

    Count 851 pages. This is certainly the current, basic text book on the title subject. There is a lot about T-Tauri stars, but interestingly I could not find a definitive statement about P-P fusion. I am guessing that's one of those things we are all supposed to know. But it would have been nice to see anyway.

    The 2007 issue of the Annual Review of Astronomy and Astrophysics includes a trio of papers which certainly set the current standard of research on the topic. All 3 a freely accessible on the web through the arXiv link.

    That trio will take you through the collapse of the protostellar core of the molecular cloud, all the way to star formation. T-Tauri stars are low mass pre-main sequence stars, meaning they are never more than ~8 solar masses. They are extensively discussed by McKee & Ostriker, but almost entirely along the lines of their observed properties, as opposed to their interior structure. To see the interior structures described, aside from the book by Hansen, Kawaler & Trimble, you might have to go all the way back to the modern source of pre-main sequence evolution: Hayashi, 1961 & Hayashi, 1966, as well as the citing papers.

  8. #8
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    Thanks for all that, Tim. Unless proven otherwise, I'll take it that pp fusion does not occur in T Tauri stars.

    Meanwhile, those papers should keep me busy.

  9. #9
    Looking into the minimum temperature for PP-I reactions, it would appear to require 10MK at pressures generated by 1 M_sol. Which would mean I was stipulating it happened much sooner.

    Thanks for the links, Tim.

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