The Sun Rotates Normally for its Age
A modern version of the classic "Skumanich plot" - log(age/yr) for stars within 5% of the mass of the Sun vs. log(rotation period/days). The sample stars come from young star clusters (<1 Gyr) and nearby Sun-like field stars (>1 Gyr).
There is still a wide range of rotation periods for solar-mass stars at ages <100 Myr, so I have not included the ~50 Myr-old IC 2391 and IC2602 clusters in the fits.
Without the Sun and IC 2391 & 2602 clusters (covering ages 0.12-6.8 Gyr), the unweighted fit is:
log(P/day) = (1.404+-0.024) + (0.524+-0.025)*(log(t/yr) - log(t_Sun/yr))
For the age of the Sun (log(age/yr)=9.6597), the predicted rotation period is 25.4+-1.4 day.
With the Sun, but without IC 2391 & 2602:
log(P/day) = (1.406+-0.019) + (0.526+-0.022)*(log(t/yr) - log(t_Sun/yr))
Predicted solar rotation period 25.5+-1.1 day.
Hence, inclusion or exclusion of the Sun has negligible impact on the best power-law fit.
Conclusions:
1) The Sun is no longer critical to pin down the Skumanich rotation curve for Sun-like stars.
2) The power-law index of the rotation spin-down (P ~ age^n) appears to be constrained to <5%.
3) The Sun appears to be a normal rotator for its age, with its average Ca H & K rotation period (26.1 day) within ~3% of that predicted (25.4+-1.4 day) to a fit to rotation-age data for clusters with isochronal and/or Li-depletion ages and field stars with asteroseismically-constrained ages.
Obviously, more stars will be added to this curve in the near future (most likely older field stars with asteroseismically-constrained ages and rotation periods from Kepler).
Comments on data (inverse age order):
16 Cyg B: age 6.8+-0.4 Gyr from Metcalf et al. (2012), rotation period 31.5+-6.5 days from Henry et al. (2000).
Alpha Cen (mean): stars A and B have masses 10% higher and lower than that of the Sun - I have averaged their rotation rates (adopting 34.9 day, interpolating rotation periods from Hallam et al. (1991; A) and Dumusque et al. (2012; B). I've adopted mean age of 6+-1 Gyr based on several studies (Thevenin+ 2002, Thoul+ 2003, Eggenberger+2004, Miglio+ 2005, Yildiz+ 2007, Bazot+ 2012).
Sun: I've adopted mean rotational period of 26.09 days based on multi-season Ca II H&K monitoring (Donahue+ 1996). This is a latitudinal average for active regions that are rotating at different latltudes - roughly ~24 days near equator to ~34 days near the poles. I've adopted the Sun's age to be 4568+-1 Myr based on age-dating of the oldest meteorite inclusions (Bouvier+2010).
18 Sco: "Solar twin" with age 3.66+0.44-0.50 Gyr (Li+2012), rotation period 22.7+-0.5 day (Petit+2008), and mass 1.02+-0.03 Msun (Bazot+2012).
NGC 6811: At cluster age 1.1+-0.2 Gyr (Janes+2011), I use a solar composition MESA stellar evolutionary track to estimate that the Sun would have had Teff = 5676K and (B-V)=0.684. At this color, based on the distribution of points in Fig. 4 of Meibom+2011, I estimate a solar-mass star would have period ~10.8 day (+-1 day).
Praesepe: I adopt age of 650+-18 Myr based on Bell+2014. Using MESA evolutionary track, I estimate 1 Msun star would have Teff=5662K and J-Ks=0.376. Using cluster rotation locus from Delorme+2011, this translates to P = 7.8 day (rms scatter +-0.46 day).
Hyades: I adopt cluster age 625+-50 Myr (Perryman+1998). Delorme+2011 survey of Hyades rotation data concludes that 1 Msun star would have P = 8.5 day (rms scatter +-0.45 day).
M37: I adopt age 550+-30 Myr from Hartman+2008. Using solar composition MESA evolutionary track, I estimate 1 Msun star would have Teff=5659K and (B-V)o=0.69. Cluster reddening is E(B-V)=0.23+-0.04 mag (Hartman+2008). Based on Hartman+2009 cluster rotation locus, I estimate 1 Msun star would have P = 6.1 day (rms scatter +-1.1 day).
Coma Ber: At cluster age 500+-100 Myr (Dobbie+2009), I use a solar composition MESA stellar evolutionary track to estimate that the Sun would've had Teff=5657K and color J-Ks=0.377. Using Collier-Cameron et al.'s (2009) locus, I estimate a period of 8.1+-0.2 day.
M11: I adopt age 250 Myr (Beaver+2013), but with +-50 Myr uncertainty based on scatter in previous published estimates. At this age, 1 Msun star of solar composition (MESA track) would have Teff=5649K (B-V)o=0.693. From Fig. 8 of Messina+2010, I estimate P = 6.5 day.
M34: I adopt cluster age 225+-25 Myr (Jones & Prosser 1996). At this age, using solar composition MESA stellar evolutionary track, I estimate Sun would have Teff=5649K and B-V=0.693. Zooming in on distribution of rotation periods between 0.6 < (B-V)o < 0.8 in Meibom+2011, I estimate the locus to be P = 4.812 + 1.234*((B-V)o - 0.693) with rms = +-0.58 day. Hence P = 4.81 day.
M35: I adopt age 150+-50 Myr based on discussions in Kalirai+2003 and Meibom+2009. At this age, a solar-mass star of solar composition (MESA evolutionary track) would have Teff=5646K (B-V)o=0.694. Based on rotation data in Meibom+2009, it appears that star of this color would have P ~ 4.5 day with rms scatter ~1 day.
M50: I adopt age of 130+-10 Myr from Kalirai+2003, and period 4.59 day (rms +-1.15 day) based on data of Irwin+2009.
Pleiades: I adopt age 120+-20 Myr based on review by Soderblom (2009). Based on data in Hartman+2010, I estimate I-sequence 1 Msun stars to have P ~ 4.36 day.
IC 2391: I adopt Li-depletion age of 50+-5 Myr from Barrado y Navascues+2004. Based on solar composition MESA track, I estimate a 1 Msun star would have Teff=5630K at this age. Based on distribution of rotation periods in Marsden+2009, I see an "arc" of slow-rotating stars which may be forerunners of the >100 Myr "I-sequence" stars. At the Teff appropriate for a 1 Msun star, it appears a typical rotation period would be 2.5 days (although including the fast rotators also, the predicted period would be ~1.2 days!).
IC 2602: I adopt Li-depletion age of 46+-6 Myr from Dobbie+2010. Using solar composition MESA stellar evolutionary track, a 1 Msun star would have Teff=5630K. Near locus of slower rotators (forerunners to "normal" I-sequence stars), it appears that typical 1 Msun star would have P ~ 2.9 day (scatter +-0.6 days). However including fast rotators, the median rate is ~2.0+-1.7 day.
A more complete bibliography with links to the studies referenced in the text will be forthcoming, however I thought it useful to post this plot in response to some discussions at the Cool Stars 18 meeting in Flagstaff.
