JWST imaging of omega Centauri I. Luminosity and mass functions of its main-sequence populations

This paper presents the first study of the most massive globular cluster in the Milky Way, omega Centauri (or ω Cen, also known as NGC 5139), employing recently acquired JWST deep images. By combining these data with archival Hubble Space Telescope (HST) images, we derived proper motions for a significant portion of the JWST field. Our analysis of the colour-magnitude diagram (CMD) reveals two prominent sequences extending from a magnitude of m_F322W2 ∼ 17.5 to the bottom of the main sequence (MS). These sequences correspond to the two main stellar populations of omega Centauri: the bMS (He-rich) and rMS (He-normal) populations. The two sequences intersect at the MS knee (m_F322W2 ∼ 19.5) and change positions for fainter magnitudes, with the bMS luminosity function (LF) ending at least ∼0.5 magnitudes brighter than the rMS LF. Our comparison with theoretical isochrones shows that the colour spread in the CMD is primarily driven by variations in the helium abundance above the MS knee, while below the MS knee the broader colour distribution is mainly influenced by variations in oxygen and carbon abundances, in combination with metallicity differences. We find that a single-population broken power-law mass function (MF) provides the best fit to the data. The MF exhibits a break around 0.2 M, with a steep slope above the break and a flatter slope below it. Finally, we identified a third group of stars (named gMS) along the MS located between the two primary ones and conducted a detailed analysis of the LFs and MFs for these three stellar populations. The LFs of these sequences show similar trends, with the rMS being the most populated and the bMS the least. The MFs display distinct power-law slopes: the rMS is well fitted by a single power law, while the gMS and the bMS are characterised by MFs steeper than that of the rMS for masses larger than 0.2 M and flatter MFs for smaller masses. The flattening around ∼0.2 M for the gMS and the bMS might be a real feature of the MFs of these populations or due to uncertainties in the adopted mass-luminosity relationship. The variation in the slope of the MFs of the gMS and bMS contributes to the steepening (flattening) of the combined MF for masses higher (lower) than 0.2 M .