The Age of the Nearest Multi-Planet System YZ Cet (GJ 54.1) Determined from Stellar Rotation and X-ray Emission

ATel #10678; Scott G. Engle and Edward F. Guinan (Villanova University)
on 29 Aug 2017; 19:45 UT
Credential Certification: Edward Guinan (edward.guinan@villanova.edu)

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YZ Cet (GJ 54.1, M4.5 V; V = 12.07 mag; d = 3.74 pc) was recently discovered by Astudillo-Defru et al. 2017 (arXiv170803336) to host at least three close-in Earth-size exoplanets. The YZ Cet a,b,c system resembles the more distant TRAPPIST-1 multi-planet transiting system, except that planetary transits have yet to be observed for the YZ Cet system. However, at a distance of only 3.74 pc (12.2 ± 0.1 ly), YZ Cet a,b,c is the nearest multiple-planet system to be discovered thus far.

Jayasinghe et al. (ATel # 10643) recently found the rotation period of the red dwarf is P = 68.3 days, using ASAS-SN photometry, rather than P ~ 83 days as reported by Astudillo-Defru et al. High precision, time-series V-band CCD photometry of YZ Cet has been carried out from 2010 - 2016 with the 1.3-m Robotically Controlled Telescope (RCT) as part of our Living with a Red Dwarf Program (Engle and Guinan 2011 ASPC 451, 285; Guinan, Engle and Durbin 2016 ApJ 821, 81). Analysis of this photometry reveals a (rotation) period of P = 67.0 ± 0.8 days and a mean light amplitude of <V-amp> = 0.012 mag. This result confirms the period given previously in ATel #10643. These period determinations are also in accord with the period of P = 69.1 days given for YZ Cet in the survey by Suarez Mascareno et al. (2016 A&A, 595, 12). Depending on the star spot temperature adopted, the projected surface of the star covered with spots is ~3-5%.  

Utilizing our Rotation - Age relation for M3-5 V stars returns an age estimate of 4.0 ± 0.6 Gyr. This age is consistent with the star's coronal X-ray luminosity determined from the weighted mean of two ROSAT measures of log Lx = 27.1 ± 0.2 erg/s.  Using a recently calibrated Age - Lx relation for M3-5 V stars of log Age (Gyr) = 19.62 - 0.704 log (Lx) (Guinan, Engle and Durbin 2016 ApJ 821, 81) and the above, we determine an independent age estimate for YZ Cet of 3.5 ± 0.6 Gyr. Although the Lx-Age relation is reliable, this inferred age of YZ Cet is based on only two short X-ray measures.  And like other cool stars, YZ Cet probably also undergoes coronal X-ray variations arising from flares and magnetic activity cycles. The age estimate determined from the rotation period is more reliable as it is free of the potential effects of an activity cycle or flares on activity-age indicator such as Lx. Thus, we adopt the gyrochronological age estimate of 4.0 ± 0.6 Gyr for the star. The average of the rotational and X-ray activity derived age is 3.8 ± 0.5 Gyr. We note that other stellar age indicators such as space motions (U, V, W = -28, 0, -24 km/s), metal abundance ([Fe/H] = -0.26 ± 0.08), and log (R'HK) = -4.70 ± 0.21 (Astudillo-Defru et al. 2017) are consistent with these ages.

We also note that the X-ray luminosity of YZ Cet is similar to the mean Lx of the Sun (Ayres 2014 AJ 147, 59). However, for YZ Cet the hosted exoplanets are very close to the host star (a < 0.03 AU) and thus receive greater than 1000 times the X-ray radiation (even during quiescence) as compared to the average incident X-ray irradiance of the Earth-Sun. Such greatly enhanced X-ray irradiances could have deleterious consequences on the hosted planets of YZ Cet, leading to the loss of their atmospheres and water inventories.

This research is supported by the NSF and NASA through grants NSF/RUI-1009903, HST-GO-13020.001-A and Chandra Award GO2-13020X to Villanova University. We are very grateful for this support.