Substellar population of the young massive cluster RCW 36 in Vela

TL;DR

This study delivers the first, comprehensive IMF census for the young massive cluster RCW 36 in the Vela Molecular Ridge by combining new GLAO-assisted HAWK-I infrared imaging with archival data and Gaia DR3 kinematics. A novel DeNeb-based photometric pipeline enables deeper, nebula-clean photometry, leading to a distance of $954 \pm 40$ pc and an IMF well described by a broken power law with $\alpha_{\text{high}}=1.62 \pm 0.03$ for $0.20-20\,M_{\odot}$ and $\alpha_{\text{low}}=0.46 \pm 0.14$ for $0.03-0.20\,M_{\odot}$, breaking at $0.2\,M_{\odot}$. The star-to-BD ratio is $\sim 2$–$5$, and there is evidence for mass segregation, likely primordial, with a shallower inner IMF slope ($\alpha_{\text{inner}}\approx1.50$) compared to the outer region. RCW 36 shares common IMF characteristics with other Galactic YMCs, including a high-mass slope shallower than Salpeter and a broadly consistent star-BD ratio, underscoring both environmental regularities and ongoing debates about IMF universality.

Abstract

The initial mass function (IMF) is a cornerstone of star formation studies, yet its universality remains debated. We investigate the IMF in the young massive cluster RCW 36, located in the Vela Molecular Ridge and comparable to the Orion Nebula Cluster in stellar density. Our goal is to build the most complete census of RCW 36 and derive its first IMF and star-to-brown-dwarf (BD) ratio. We combine new GLAO observations from HAWK-I/VLT with archival data (2MASS, SOFI/NTT) and Gaia DR3 kinematics. Photometric accuracy and source extraction were improved using \textsc{DeNeb}, a deep-learning algorithm that removes complex nebular emission. Membership probabilities were assigned via color-magnitude diagram comparisons with a control field, and stellar masses were estimated using model isochrones. We find a revised distance of $954\pm40\,$pc and determine the IMF down to $\sim0.03\,M_{\odot}$, described by a broken power law ($dN/dM\propto M^{-α}$) with $α=1.62\pm0.03$ for $0.20$-$20\,M_{\odot}$ and $α=0.46\pm0.14$ for $0.03$-$0.20\,M_{\odot}$. The star-BD ratio is $2$-$5$, consistent with other Galactic clusters. Lastly, through a study of the differences in the IMF within and outside $0.2\,$pc and the cumulative mass distributions for low-mass and intermediate to high-mass sources, we also detected signs of possible mass segregation within RCW 36, which should be primordial. RCW 36 shares many characteristics with other young massive clusters, such as a shallower than Salpeter high-mass slope and the possibility of mass segregation. The flatter lower-mass regime of the IMF is similar to most Galactic clusters. The star-BD ratio is also in line with the observed values in other clusters, independent of their inherent properties.

Figures (13)

• Figure 1: Left: $Herschel$ PACS 70$\mu$m image of the region in Vela associated with the cluster RCW 36. The black rectangle marks the area under study. Right: $JHK_s$ bands colour mosaic obtained with HAWK-I/VLT. The position of the four detectors is marked.
• Figure 2: Left: Original HAWK-I J-band image of RCW 36. Centre: HAWK-I J-band image with only the nebular component. Right: HAWK-I J-band image with only the point sources (hereby called denebulised). The white contours trace the regions used in the present analysis; the different contour in chip 3 is due to the artefact described in Section \ref{['sec:observations_and_data_reduction']}. The dotted lines refer to the coordinate grid lines of constant right ascension or declination.
• Figure 3: Photometric uncertainties associated with the magnitude data extracted from both the original HAWK-I data (in black) and the denebulised HAWK-I data (in orange) for the J-band (top), H band (centre) and $K_s$ band (bottom). Green stars are measurements from SOFI and purple stars are measurements from 2MASS.
• Figure 4: Colour–magnitude diagrams of $J$, $J-H$ (left) and $H$, $H- Ks$ (middle), and $J-H$, $H-Ks$ colour–colour diagram (right) of the sources detected toward RCW 36 (black dots) and the control field (grey dots). The orange line shows the 1 Myr isochrone shifted to the distance of 950 pc, while the cyan solid line in the right-most panels represents the 5 Gyr isochrone used to estimate the extinction of the control field. The dotted red lines in the CCDs show the reddening vectors, and the red crosses along the reddening lines mark A$_V$ = 0 - 40 mag, in step of 5 mag. The black dotted lines mark the average 90% completeness limit of the photometry. The solid black line represents the T Tauri loci from Meyer+1997. The letter A refers to the region of giants and dwarfs, the letter B refers to the T Tauri region and finally the C region refers to the Herbig AeBe region Hernandez+2005. The upper panels show original control field photometry, and the lower ones contain control field photometry reddened to match the distribution of the extinction in the cluster field (see Section \ref{['sec:ext_diff']}).
• Figure 5: Distribution of the visual extinction towards RCW 36 (black) and the control field (red). The smooth lines represent Gaussian KDEs of the two distributions.