<p>Very much a case of semantics and what exactly you mean by a T Tauri star.</p>
<p>If a requirement is for the star to have a significant disc, then no, a fair fraction of young stars, particularly those of low mass, have lost their discs before they reach the end of the Hayashi track. The exponential decay timescale for losing a disc is around 3 Myr. The timescale to get to the bottom of the Hayashi track can be 10-100 Myr for stars of 0.5-0.1 solar masses.</p>
<p>If the definition encompasses both &quot;classical&quot; T Tauri stars and &quot;weak-lined&quot; T Tauri stars (those without an active accretion disc), then yes, all young stars on the Hayashi track are T-Tauri stars.</p>
<p>Edit: I realised I hadn't completed the answer. It is also true that many higher mass T-Tauri stars (more than 0.5 solar masses) have left the Hayashi track and still have discs whilst they are on their radiative Henyey tracks.</p>

CC BY-SA 4.0
2022-11-21T21:07:51.507Z
2022-11-21T21:49:15.463Z
2022-11-21T21:49:15.463Z
ProfRob
<p>Reading the definitions of <a href="https://en.wikipedia.org/wiki/T_Tauri_star" rel="noreferrer">T Tauri stars</a> and the <a href="https://en.wikipedia.org/wiki/Hayashi_track" rel="noreferrer">Hayashi track</a> one can gather that:</p>
<blockquote>
<p>T Tauri stars are pre-main-sequence stars in the process of contracting to the main sequence along the Hayashi track</p>
</blockquote>
<p>and that:</p>
<blockquote>
<p>After a protostar ends its phase of rapid contraction and becomes a T Tauri star, it is extremely luminous. The star continues to contract, but much more slowly. While slowly contracting, the star follows the Hayashi track</p>
</blockquote>
<p>Can we then say that <strong>all</strong> stars on the Hayashi track are T Tauri stars? And moreover, are <strong>all</strong> T Tauris on the Hayashi track?</p>

CC BY-SA 4.0
2022-11-21T21:12:15.517Z
To clarify, is your question "Are all T-tauri stars on the Hayashi Track?"  or   "Are all stars on the Hayashi Track T-tauri stars"  (or are you asking both questions?)
James K
CC BY-SA 4.0
2022-11-21T21:20:09.097Z
Now that you mention it, probably both!
Gabriel
CC BY-SA 4.0
2022-11-22T23:24:27.883Z
somewhat T-Tauri related [History, significance and "drama" (if any) of T-Tauri stars, especially the early bits?](https://astronomy.stackexchange.com/q/49179/7982) and [Why would radio astronomers choose ¹³CO and C¹⁸O spectral lines instead of the most isotopically common combination?](https://astronomy.stackexchange.com/q/47375/7982)
uhoh
<star><stellar-evolution><t-tauri-stars>
Are all T Tauris on the Hayashi track?
Gabriel
# Are all T Tauris on the Hayashi track?

(Asked by: username_0 on 2022-11-21)

Reading the definitions of [T Tauri stars](https://en.wikipedia.org/wiki/T_Tauri_star) and the [Hayashi track](https://en.wikipedia.org/wiki/Hayashi_track) one can gather that:

> T Tauri stars are pre-main-sequence stars in the process of contracting to the main sequence along the Hayashi track

and that:

> After a protostar ends its phase of rapid contraction and becomes a T Tauri star, it is extremely luminous. The star continues to contract, but much more slowly. While slowly contracting, the star follows the Hayashi track

Can we then say that **all** stars on the Hayashi track are T Tauri stars? And moreover, are **all** T Tauris on the Hayashi track?

**Tags:** star, stellar-evolution, t-tauri-stars
> **Comments:**
> **username_1**: To clarify, is your question "Are all T-tauri stars on the Hayashi Track?"  or   "Are all stars on the Hayashi Track T-tauri stars"  (or are you asking both questions?)  
> **username_0**: Now that you mention it, probably both!  
> **username_2**: somewhat T-Tauri related [History, significance and "drama" (if any) of T-Tauri stars, especially the early bits?](https://astronomy.stackexchange.com/q/49179/7982) and [Why would radio astronomers choose ¹³CO and C¹⁸O spectral lines instead of the most isotopically common combination?](https://astronomy.stackexchange.com/q/47375/7982)  

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## 1 answers

### Answer by username_3 on 2022-11-21. Score: 9 | Accepted answer

Very much a case of semantics and what exactly you mean by a T Tauri star.

If a requirement is for the star to have a significant disc, then no, a fair fraction of young stars, particularly those of low mass, have lost their discs before they reach the end of the Hayashi track. The exponential decay timescale for losing a disc is around 3 Myr. The timescale to get to the bottom of the Hayashi track can be 10-100 Myr for stars of 0.5-0.1 solar masses.

If the definition encompasses both "classical" T Tauri stars and "weak-lined" T Tauri stars (those without an active accretion disc), then yes, all young stars on the Hayashi track are T-Tauri stars.

Edit: I realised I hadn't completed the answer. It is also true that many higher mass T-Tauri stars (more than 0.5 solar masses) have left the Hayashi track and still have discs whilst they are on their radiative Henyey tracks.


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