Empirical near-infrared colors for low-mass stars and brown dwarfs in the Orion Nebula Cluster

Context. Current atmospheric and evolutionary models for low-mass stars and brown dwarfs rely on approximate assumptions on the physics of the stellar structure and the atmospheric radiative transfer. This leads to biased theoretical predictions on the photospheric spectral energy distributions of these system, especially when applied to low surface gravity objects such as pre-main sequence (PMS) stars, and affects the derivation of stellar parameters from photometric data. Aims. Our main goal is to correct the biases present in the theoretical predictions for the near-IR photometry of low-mass PMS stars. Using empirical intrinsic IR colors, we assess the accuracy of current synthetic spectral libraries and evolutionary models. We investigate how the uncertainty in the intrinsic colors associated with different PMS models affect the derivation of the initial mass function of young clusters from near-IR photometry. Methods. We consider a sample of ~300 PMS stars in the Orion Nebula Cluster (age ≃ 1 Myr) with well measured luminosities, temperatures and photospheric JHKS photometry. This sample is used as a benchmark for testing both atmospheric and evolutionary theoretical models. Results. By analyzing the photospheric colors of our sample of young stars, we find that the synthetic JHKS photometry provided by theoretical spectral templates for late spectral types (>K6) are accurate at the level of ~0.2 mag, while colors are accurate at ≲ 0.1 mag. We tabulate the intrinsic photospheric colors, appropriate for the Orion Nebula Cluster, in the range K6-M8.5. They can be conveniently used as templates for the intrinsic colors of other young (age ≲ 5 Myr) stellar clusters. Conclusions. The theoretically-predicted JHKS magnitudes of young late type stars do not accurately reproduce the intrinsic ones of the Orion Nebula Cluster members. An empirical correction of the atmospheric templates can fix the discrepancies between expected and observed colors. Still, other biases in the evolutionary models prevent a more robust comparison between observations and theoretical absolute magnitudes. In particular, PMS evolutionary models seem to consistently underestimate the intrinsic near-infrared flux at the very late spectral types, and this may introduce spurious features in the low-mass end of the photometrically-determined initial mass function of young clusters