The stellar population of h and χ persei: Cluster properties, membership, and the intrinsic colors and temperatures of stars

Abstract

From photometric observations of ∼47,000 stars and spectroscopy of ∼11,000 stars, we describe the first extensive study of the stellar population of the famous Double Cluster, h and χ Persei, down to subsolar masses. By analyzing optical spectra and optical/infrared photometry, we constrain the distance moduli (dM), reddening (E(B - V)), and ages for h Persei, χ Persei, and the low-density halo population surrounding both cluster cores. With the exception of mass and spatial distribution, the clusters are nearly identical in every measurable way. Both clusters have E(B - V) ∼0.52-0.55 and dM = 11.8-11.85; the halo population, while more poorly constrained, likely has identical properties. As determined from the main-sequence turnoff, the luminosity of M supergiants, and pre-main-sequence isochrones, ages for h Persei, χ Persei, and the halo population all converge on 14Myr, thus showing a stunning agreement between estimates based on entirely different physics. From these data, we establish the first spectroscopic and photometric membership lists of cluster stars down to early/mid M dwarfs. At minimum, there are ∼5000 members within 10′ of the cluster centers, while the entire h and χ Persei region has at least ∼13,000 and as many as 20,000 members. The Double Cluster contains ≈8400 M ⊙ of stars within 10′ of the cluster centers. We estimate a total mass of at least 20,000 M ⊙. We conclude our study by outlining outstanding questions regarding the past and present properties of h and χ Persei. From comparing recent work, we compile a list of intrinsic colors and derive a new effective temperature scale for O-M dwarfs, giants, and supergiants. © 2010. The American Astronomical Society. All rights reserved.