A deep NIR survey of very low-mass objects in the R CrA region

¹ Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura, Saitama, Saitama, Japan
² Faculty of Education, Saitama University, 255 Shimo-Okubo, Sakura, Saitama, Saitama, Japan
³ Graduate School of Education, Saitama University, 255 Shimo-Okubo, Sakura, Saitama, Saitama, Japan

^★ Corresponding authors: s15pp223@gmail.com; yummy@mail.saitama-u.ac.jp

Received: 29 March 2024
Accepted: 26 March 2025

Abstract

Aims. Our aim is to identify the population of very low-mass object (VLMO) candidates in the R CrA region and reveal their formation dependence in the local environments.

Methods. We performed a deep near-infrared (NIR) photometric observation of the R CrA region by UKIRT/WFCAM. Class I and II candidates showing NIR excess were selected from their observed colors. We derived the photometric mass of each candidate with an age assumption of 1 Myr. We compared the derived mass of identified VLMO candidates to the dust column density at their position.

Results. The 10σ limiting magnitudes were 20.7, 19.6, and 19.2 mag in the J-, H-, and K-band, respectively, and we detected 2922 JHK sources in all three bands with an S/N greater than ten in the K-band. Fifteen Class I and 207 Class II candidates with NIR excess were selected from a [J-H]/[H-K] color-color diagram. Six low-mass stars, five brown dwarfs, and 196 planetary-mass object candidates were identified from the J-band luminosity of Class II candidates with the age assumption of 1 Myr using the evolutionary models. The derived initial mass function (IMF) does not appear to decrease in the brown dwarf and planetary-mass regime, even when taking into account the background star and galactic contamination. From comparison between the spatial distributions of Class I and II candidates and dust column densities derived from the Herschel observation, we found that all the low-mass star and brown dwarf candidates are located in the region where the dust column densities are higher than 2.5 × 10²¹/cm², while planetary-mass object candidates are independent of their local dust densities. Our results suggest that the formations of vey low-mass stars and very low-mass objects may be dependent on the local cloud properties.