A Multi-color Optical Survey of the Orion Nebula Cluster. II. the H-R diagram

TL;DR

This paper presents a detailed analysis of the Orion Nebula Cluster's stellar population using multi-band optical data, refining models for intrinsic colors, and deriving a new H-R diagram and IMF with updated age and luminosity estimates.

Contribution

It introduces a new method for self-consistently deriving reddening and accretion excess, and provides an improved H-R diagram and IMF for the ONC based on refined models and recent distance measurements.

Findings

Higher luminosity for late-type stars in the H-R diagram

Lower luminosity for early-type stars in the H-R diagram

Age distribution peaks at 2-3 Myr, older than previous estimates

Abstract

We present a new analysis of the stellar population of the Orion Nebula Cluster (ONC) based on multi-band optical photometry and spectroscopy. We study the color-color diagrams in BVI, plus a narrow-band filter centered at 6200A, finding evidences that intrinsic color scales valid for main-sequence dwarfs are incompatible with the ONC, while a better agreement is found employing synthetic intrinsic colors obtained constraining the typical lower surface gravity of young stars. We refine these model colors even further, empirically, by comparison with a sample of ONC stars with no accretion and no extinction. We consider the stars with known spectral types from the literature, and add 65 newly classified stars from slit spectroscopy and 182 M-type from narrow-band photometry; in this way we isolate a sample of about 1000 stars with known spectral type. We introduce a new method to self-consistently derive reddening and accretion excess from the location of each star in the BVI color-color diagram. This enables us to accurately determine the extinction of the ONC members. We adopt a lower distance for the ONC than previously assumed, based on recent parallax measurements. With a careful choice also of the spectral type-temperature transformation, we produce the new H-R diagram of the ONC population, more populated than previous works. With respect to previous works, we find higher luminosity for late-type stars and a lower luminosity for early types. We determine the age distribution of the population, peaking at 2-3 Myr, a higher age than previously estimated. We study the distribution of the members in the mass-age plane, and find that taking into account selection effects due to incompleteness removes an apparent correlation between mass and age. We derive the IMF for low- and intermediate-mass members of the ONC, which turns out to be model-dependent, and shows a turn-over at ~<0.2Msun.