Substellar Initial Mass Function of Trumpler 14

TL;DR

This study tackles whether the IMF, especially in the substellar regime, varies in a high-density, strong-FUV environment by analyzing Trumpler 14. It employs deep GeMS/GSAOI near-infrared imaging with two independent control-field decontaminations (Besançon model and a VISTA CF) and detailed completeness corrections to derive masses down to ~0.01 $M_\odot$. The inferred IMF shows a high-mass slope $\alpha \approx 1.72$ over $0.2$–$4.5\,M_\odot$ and a shallow low-mass slope $\alpha \approx 0.14$ (or $0.03$) for $0.01$–$0.2\,M_\odot$, with a characteristic mass $m_c \approx 0.20\,M_\odot$; the star-to-BD ratio in $0.03$–$1\,M_\odot$ is about 4.0. However, the flattening at the lowest masses is strongly influenced by incompleteness in the faint bin, and excluding that bin yields consistency with other regions. The results suggest the environment of Tr 14 may suppress formation of objects with $M<0.03\,M_\odot$, highlighting how high stellar density and intense FUV radiation can shape the substellar end of the IMF, while still maintaining typical star-to-BD ratios overall. This work informs the IMF universality debate by providing a detailed, environment-aware census of substellar objects in one of the most massive nearby star-forming regions.

Abstract

Young, massive stellar clusters offer a prime setting to explore brown dwarf (BD) formation under high densities and intense UV radiation. Trumpler 14 (Tr 14), a 1 Myr-old cluster located at a distance of 2.4 kpc, and particularly rich in O-type stars, is an ideal target for such a study. Our goal is to measure the initial mass function (IMF) in the young massive, high UV flux cluster. We present the deepest study to date of the IMF in Tr 14 based on GeMS/GSAOI imaging. We construct the IMF using both the Besancon Galactic model and an observational control field from VISTA for background correction. Completeness was assessed using artificial-star tests and applied to the IMF derivation. We estimate the IMF down to the 20% completeness limit found at 0.01 MSun. Using the control field-based IMF as our primary result, we find a slope of alpha=0.14+-0.19 for masses between 0.01-0.2 MSun, and alpha=1.72+-0.04 for 0.2-4.5 MSun. The low-mass slope is largely influenced by the incompleteness-affected lowest bin; excluding it brings our results into agreement with those from other young clusters. The resulting median for the star-to-BD ratio in the 0.03-1 MSun mass range is 4.0, with a 95% confidence interval of 2.8-5.8. Our analysis reveals that Tr 14 hosts a relatively flat substellar IMF, but this is strongly influenced by the lowest-mass bin, which may be affected by incompleteness. When that bin is excluded, the IMF becomes consistent with those of other regions. The star-to-BD ratio falls within the usually observed 3-6 range, indicating that brown dwarfs with masses above 0.03 MSun form with similar efficiency across environments. However, the relative lack of objects below this threshold suggests that the presence of an environment with both high stellar density and FUV flux may play a role in shaping the IMF by suppressing the formation of BDs at masses < 0.03 MSun.

Figures (11)

• Figure 1: Left panel shows an image of Tr 14 in the Carina Nebula taken from carina. The black square represents the $\sim$$1.4'\times1.4'$ area studied in this work. The right panel shows the $J$-band image of Tr 14 taken with GSAOI. North is up and east to the left.
• Figure 2: Photometric uncertainties of our catalogue as a function of magnitude. In all bands, two prominent sequences are visible, originating from the long- and short- exposure observations.
• Figure 3: Completeness of the photometry, obtained using the artificial star test. Different coloured lines represent corresponding photometric bands. The dashed grey line represents the 90% completeness limit, while the dotted grey line represents the 50% completeness limit and the solid grey line the 20% completeness limit. The thin lines show the completeness curves calculated at the 0.2 mag step, while the thick lines were smoothed using UnvariateSpline of scipy.interpolate sub-package.
• Figure 4: Left and central panel show the CMDs. The mean uncertainty is in the upper right part of the panels. The red solid line represents the 1 Myr isochrone combined from the PARSEC and BT-Settl models at 1 $M_\odot$ (bressan2012parsecchen2014improvingbaraffe2015new). The reddening vector of Av=3 mag is shown extinction_law. The dashed black line in the first two panels represents the 90% completeness limit, while the dotted black line represents the 50% completeness limit and the solid black line the 20% completeness limit. The right panel represents the colour- colour diagram (CCD). The solid red line is the isochrone as in the CMDs. The dashed red lines are parallel to extinction vectors, while the red crosses represent Av=0, 5 and 10 mag. The dashed-dotted magenta line represents the T-Tauri stars locus with the corresponding uncertainties as blue dashed lines meyer+1997, transformed to 2MASS system using carpenter_color_2001. The average uncertainty is in the lower-right corner in the right-most panel, in other panels it is up right. The starting points for the left and middle reddening vector are determined by the masses in the model (<0.5 $M_\odot$ and >=0.005 $M_\odot$, respectively) while the right reddening vector is from muzic2017, adapting the locus of Herbig AeBe stars from Hernandez_2005.
• Figure 5: CMDs for the Besançon model in the upper panels and VISTA CF in the lower panels. The black dots show all the objects in the line of sight of Tr 14 in all panels. The grey dots are the model foreground, while the red ones are the reddened model background. The amount of reddening is given above each corresponding panel. The orange dots represent the reddened CF. Again, the amount of reddening is given above each panel. Each extinction is chosen to decontaminate the most amount of background sources.