<p>Please point us to the peer-reviewed reference! As far as I'm aware the problems that plague gyrochronology still persist. These are (in no particular order).</p>

<ol>
<li><p>A lack of calibrating clusters for stars with ages older than the Sun and for stars older than about 1 Gyr and mass much less than a solar mass.</p></li>
<li><p>It just doesn't work for stars younger than about 200 Myr at a solar mass and younger than 1 Gyr at half a solar mass, because rotation rates have not converged to a single-valued function of age.</p></li>
<li><p>You need rotation <em>periods</em> to use it. You cannot get around the unknown inclination angle so if you only have a projected equatorial velocity $v \sin i$, then this is considerably less useful. </p></li>
<li><p>Measuring the rotation periods for stars takes a lot of effort. For young stars they potentially have a large photometric modulation caused by starspots that can be measured through monitoring over timescales of days to weeks. For older stars like the Sun, the spot modulation is so small that you need something like Kepler in order to be able to measure it. It is not something that could be routinely done over huge swathes of the sky.</p></li>
<li><p>The precision is not that great. Aside from the calibration issue there is the problem that there is a dispersion in rotation rates at a given age that leads to an uncertainty in the estimated age for a given rotation rate. This amounts to at least 10-20% for stars younger than a Gyr but probably decreases for older stars. However, this is counterbalanced by the fact that older stars appear to suffer more differential rotation with latitude. That means if you measure the rotation period on one occasion it is quite possible it will be different the next time you measure it, if spots are at a different latitude. This would give rise to errors approaching 20% in a star like the Sun.  </p></li>
</ol>

<p>Work to overcome these issue will be slow and incremental. Asteroseismology is certainly helping for solar-type stars because for these stars, you can estimate the age in an independent (though model-dependent) way, which offers an alternative calibration route. The original Kepler project has also observed a number of older clusters and there are a few more which might prove to be decent calibrators in the K2 fields. However, my bet would be that the problems of calibration for K and M dwarfs will still be there even after all this data has come in.</p>

<p>You can find a discussion of the above and a comparison with other age estimation techniques in <a href="http://arxiv.org/abs/1404.7156" rel="nofollow">Jeffries (2014)</a>. Estimates of age based on metallicity would be <em>very</em> crude indeed, and subject to catastrophic errors. For instance, the metallicity of young, star-forming clusters in the solar vicinity is almost identical to the Sun. What you can <em>probably</em> say, is that if a star has a metallicity less than about 0.5 solar then it is <em>likely</em> to be older than 5 Gyr; anything less than 0.1 solar is <em>likely</em> to be older than 10 Gyr. But that's about it.</p>

<p>EDIT: So, the paper that triggered your question is <a href="http://arxiv.org/abs/1501.05651" rel="nofollow">Meibom et al. (2015)</a> "A spin-down clock for cool stars from observations of a 2.5-billion-year-old cluster", which talks about Kepler observations of the open cluster NGC 6819, in which the rotation periods have been found for 30 cool stars with masses between 0.85 and 1.4 solar masses. These data are indeed an extremely useful calibration set which fills in the gap for solar type stars between the ages of the Sun and the well-studies clusters like the Hyades and Praesepe below 1 Gyr.</p>

<p>However, all the problems I mentioned above still exist. The range of masses probed here is rather small, but what they are able to establish is that the 20 or so stars below about 1.2 solar masses have a dispersion around a mean period-colour relationship with a standard deviation of only 5%. This means that the <em>solar mass</em> stars have converged to a fairly tight relationship between rotation period, mass and age and that errors in measuring the periods are also relatively small. Because $t \propto P^{2}$, this leads to the
 claim that ages could be determined with a precision of 10% <em>at this age, if</em> the age-rotation-mass space were fully calibrated. Note though that these rotation periods were measured by Kepler; that the median <em>peak-to-peak</em> amplitude was only 4 milli-mag and that in most cases the period is a mean from several independent measurements of the same star. Such excellent data will not be forthcoming for very large samples of field stars anytime soon.</p>

CC BY-SA 3.0
2015-12-30T10:49:11.847Z
2016-01-01T18:46:21.833Z
2016-01-01T18:46:21.833Z
ProfRob
<p>I recently became aware of the work of the <a href="https://www.google.com/search?q=+Kepler+Cluster+Study" rel="nofollow">Kepler Cluster Study</a>. In January of this year it was announced that a long awaited method of finding ages of "cool" stars is now available. This is huge and I'm surprised that I just found out about it. </p>

<p>I wonder if there has been a little too much "extrapolation" and that more data is needed. I also wonder if they have gotten around the fact that it is vsini that is being measured and not the actual rotation velocity.</p>

<p>The bottom line is that I want to see the catalogs of updated ages based on this new information. Does anybody know of any such publications in the works? It will be interesting to compare results to ages based on metallicity.</p>

CC BY-SA 3.0
2015-12-29T17:16:52.520Z
"*I also wonder if they have gotten around the fact that it is vsini that is being measured and not the actual rotation velocity.*" I don't think that the viewing angle (vsini) matters. If you view the star's equator straight on or at almost tangential angle, doesn't matter for how frequently spots pass by in and out of sight. They do so every period of the star's rotation regardless of from what angle they are viewed.
LocalFluff
CC BY-SA 3.0
2016-01-01T02:50:53.600Z
@RobJeffries: Here http://www.nature.com/nature/journal/v517/n7536/abs/nature14118.html is the paper in Nature (I can't get full text.) It may have been sensationalized by press releases. Also, here www.cfa.harvard.edu/~smeibom/ is the website for Soren Meibom with several links where the Kepler Cluster Project is talked about.
Jack R. Woods
<stellar-evolution>
Gyrochronology?
Jack R. Woods
# Gyrochronology?

(Asked by: username_0 on 2015-12-29)

I recently became aware of the work of the [Kepler Cluster Study](https://www.google.com/search?q=+Kepler+Cluster+Study). In January of this year it was announced that a long awaited method of finding ages of "cool" stars is now available. This is huge and I'm surprised that I just found out about it.

I wonder if there has been a little too much "extrapolation" and that more data is needed. I also wonder if they have gotten around the fact that it is vsini that is being measured and not the actual rotation velocity.

The bottom line is that I want to see the catalogs of updated ages based on this new information. Does anybody know of any such publications in the works? It will be interesting to compare results to ages based on metallicity.

**Tags:** stellar-evolution
> **Comments:**
> **username_1**: "*I also wonder if they have gotten around the fact that it is vsini that is being measured and not the actual rotation velocity.*" I don't think that the viewing angle (vsini) matters. If you view the star's equator straight on or at almost tangential angle, doesn't matter for how frequently spots pass by in and out of sight. They do so every period of the star's rotation regardless of from what angle they are viewed.  
> **username_0**: @RobJeffries: Here http://www.nature.com/nature/journal/v517/n7536/abs/nature14118.html is the paper in Nature (I can't get full text.) It may have been sensationalized by press releases. Also, here www.cfa.harvard.edu/~smeibom/ is the website for Soren Meibom with several links where the Kepler Cluster Project is talked about.  

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## 1 answers

### Answer by username_2 on 2015-12-30. Score: 1 | Accepted answer

Please point us to the peer-reviewed reference! As far as I'm aware the problems that plague gyrochronology still persist. These are (in no particular order).

1. A lack of calibrating clusters for stars with ages older than the Sun and for stars older than about 1 Gyr and mass much less than a solar mass.
2. It just doesn't work for stars younger than about 200 Myr at a solar mass and younger than 1 Gyr at half a solar mass, because rotation rates have not converged to a single-valued function of age.
3. You need rotation *periods* to use it. You cannot get around the unknown inclination angle so if you only have a projected equatorial velocity $v \sin i$, then this is considerably less useful.
4. Measuring the rotation periods for stars takes a lot of effort. For young stars they potentially have a large photometric modulation caused by starspots that can be measured through monitoring over timescales of days to weeks. For older stars like the Sun, the spot modulation is so small that you need something like Kepler in order to be able to measure it. It is not something that could be routinely done over huge swathes of the sky.
5. The precision is not that great. Aside from the calibration issue there is the problem that there is a dispersion in rotation rates at a given age that leads to an uncertainty in the estimated age for a given rotation rate. This amounts to at least 10-20% for stars younger than a Gyr but probably decreases for older stars. However, this is counterbalanced by the fact that older stars appear to suffer more differential rotation with latitude. That means if you measure the rotation period on one occasion it is quite possible it will be different the next time you measure it, if spots are at a different latitude. This would give rise to errors approaching 20% in a star like the Sun.

Work to overcome these issue will be slow and incremental. Asteroseismology is certainly helping for solar-type stars because for these stars, you can estimate the age in an independent (though model-dependent) way, which offers an alternative calibration route. The original Kepler project has also observed a number of older clusters and there are a few more which might prove to be decent calibrators in the K2 fields. However, my bet would be that the problems of calibration for K and M dwarfs will still be there even after all this data has come in.

You can find a discussion of the above and a comparison with other age estimation techniques in [Jeffries (2014)](http://arxiv.org/abs/1404.7156). Estimates of age based on metallicity would be *very* crude indeed, and subject to catastrophic errors. For instance, the metallicity of young, star-forming clusters in the solar vicinity is almost identical to the Sun. What you can *probably* say, is that if a star has a metallicity less than about 0.5 solar then it is *likely* to be older than 5 Gyr; anything less than 0.1 solar is *likely* to be older than 10 Gyr. But that's about it.

EDIT: So, the paper that triggered your question is [Meibom et al. (2015)](http://arxiv.org/abs/1501.05651) "A spin-down clock for cool stars from observations of a 2.5-billion-year-old cluster", which talks about Kepler observations of the open cluster NGC 6819, in which the rotation periods have been found for 30 cool stars with masses between 0.85 and 1.4 solar masses. These data are indeed an extremely useful calibration set which fills in the gap for solar type stars between the ages of the Sun and the well-studies clusters like the Hyades and Praesepe below 1 Gyr.

However, all the problems I mentioned above still exist. The range of masses probed here is rather small, but what they are able to establish is that the 20 or so stars below about 1.2 solar masses have a dispersion around a mean period-colour relationship with a standard deviation of only 5%. This means that the *solar mass* stars have converged to a fairly tight relationship between rotation period, mass and age and that errors in measuring the periods are also relatively small. Because $t \propto P^{2}$, this leads to the
claim that ages could be determined with a precision of 10% *at this age, if* the age-rotation-mass space were fully calibrated. Note though that these rotation periods were measured by Kepler; that the median *peak-to-peak* amplitude was only 4 milli-mag and that in most cases the period is a mean from several independent measurements of the same star. Such excellent data will not be forthcoming for very large samples of field stars anytime soon.


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