Age Spreads and Systematics in Lambda Orionis with Gaia DR2 and the SPOTS Tracks

In this paper we investigate the robustness of age measurements, age spreads, and stellar models in young pre-main-sequence stars. For this effort, we study a young cluster, lambda Orionis, within the Orion star-forming complex. We use Gaia data to derive a sample of 357 targets with spectroscopic temperatures from spectral types or from the automated spectroscopic pipeline in APOGEE Net. After accounting for systematic offsets between the spectral type and APOGEE temperature systems, the derived properties of stars on both systems are consistent. The complex interstellar medium, with variable local extinction, motivates a star-by-star dereddening approach. We use a spectral energy distribution fitting method calibrated on open clusters for the Class III stars. For the Class II population, we use a Gaia G-RP dereddening method, minimizing systematics from disks, accretion, and other physics associated with youth. The cluster age is systematically different in models incorporating the structural impact of starspots or magnetic fields than in nonmagnetic models. Our mean ages range from 2-3 Myr (nonmagnetic models) to 3.9 +/- 0.2 Myr in the SPOTS model (f = 0.34). We find that star-by-star dereddening methods distinguishing between pre-main-sequence classes provide a smaller age spread than techniques using a uniform extinction, and we infer a minimum age spread of 0.19 dex and a typical age spread of 0.35 dex after modeling age distributions convolved with observed errors. This suggests that the lambda Ori cluster may have a long star formation timescale and that spotted stellar models significantly change age estimates for young clusters.

Automated summary

In this paper, we investigate the robustness of age measurements, age spreads, and stellar models in young pre-main-sequence stars. By studying the lambda Orionis cluster within the Orion star-forming complex, using Gaia data and star-by-star dereddening methods, we find that incorporating the structural impact of starspots or magnetic fields significantly changes the cluster age. We also conclude that star-by-star dereddening methods provide a smaller age spread compared to techniques using uniform extinction, indicating a potentially long star formation timescale for the lambda Orionis cluster.