The stellar to sub-stellar masses transition in 47 Tuc

TL;DR

This paper tackles the stellar-to-sub-stellar transition in the globular cluster 47 Tuc using JWST low-main-sequence photometry. It employs ATON-based stellar evolution models with two chemical compositions corresponding to the cluster’s 1G and 2G populations, incorporating non-grey atmospheric boundary conditions and realistic EOS to 12 Gyr. The authors find the stellar-to-substellar transition occurs at $M \approx 0.074\,M_\odot$ for 1G and $M \approx 0.071\,M_\odot$ for 2G, and show that both generations follow a Kroupa-like mass function down to about $0.22\,M_\odot$, with indications of a brown dwarf population at the very low-mass end. The work demonstrates the power of combining non-grey atmosphere modeling with JWST data to distinguish multiple populations and constrain the low-mass end of the mass function in globular clusters, bearing on the cluster's formation history and substellar demographics.

Abstract

Context: The study of the Globular Cluster 47 Tuc offers the opportunity to shed new light on the debated issue on the presence of multiple populations in Globular Clusters, as recent results from HST photometry and high-resolution spectroscopy outlined star-to-star differences in the surface chemical composition. Aims: The goal of the present investigation is the interpretation of recent JWST data of the low main sequence of 47 Tuc, in order to explore the stellar to sub-stellar transition, to derive the mass distribution of the individual sources and to disentangle stars from different populations. Methods: Stellar evolution modelling of low-mass stars of metallicity [Fe/H]=-0.78 and oxygen content [O/Fe]=+0.4 and [O/Fe]=0 is used to simulate the evolution of the first and the second generation of the cluster. The comparison between the calculated sequences with the data points is used to characterize the individual objects, to split the different stellar components and to infer the current mass function of the cluster. Results: The first generation of 47 Tuc harbours 45 % of the overall population of the cluster, the remaining 55 % making up the second generation. The transition from the stellar to the sub-stellar domain is found at $0.074 M_{\odot}$ and $0.07 M_{\odot}$ for the first and second generations, respectively. The mass function of both the stellar generations are consistent with a Kroupa-like profile down to $0.22 M_\odot$.

Figures (8)

• Figure 1: Full points represent the position on the HR diagram of grey (squares) and non-grey (triangles) stellar models with mass in the $\rm 0.07 - 0.6~M_\odot$ range, at the age of $\rm 12~Gyr$. The colour highlights the same mass of the two sequences. For grey stellar models of mass $\rm 0.07~M_\odot$ and $\rm 0.08~M_\odot$, and for the non-grey $\rm 0.07~M_\odot$ model, the evolution in the age range from 8 to 12 Gyr is displayed, the points indicating the position at $\rm 12~Gyr$.
• Figure 2: Time variation of the luminosity of stellar models calculated with the physical and chemical input described in section \ref{['mod']}. Stellar models representing the 1G and the 2G of the cluster are shown respectively in the top and bottom panel. The dashed green line indicates the age of $\rm 12~Gyr$. The tracks in green highlight the time evolution of the minimum mass supported by proton-proton chain luminosity at the age of 12 Gyr, namely $\rm 0.074~M_\odot$ (top panel, 1G) and $\rm 0.070~M_\odot$ (bottom panel, 2G). The tracks of the stars of mass $\rm 0.2~M_\odot$ (dotted lines), $\rm 0.1~M_\odot$ (dotted-dashed) and $\rm 0.08~M_\odot$ (dashed) are highlighted.
• Figure 3: Thermodynamic structure of the stellar models of different mass at the age of $\rm 12~Gyr$, in the density (g cm$^{-3}$) - temperature (K) plane. The boundaries of the partial helium and partial hydrogen ionization regions are shown, as well as the region in which corrections to the ideal gas EOS become important, limited at high density by the pressure ionization boundary. Top panel refers to 1G models, bottom panel refers to 2G. The same structures highlighted in Fig.2 are highlighted here. Below $\rm 0.08~M_\odot$ the mass step is $\rm 0.001~M_\odot$. This allows to see how fast in mass steps is the transition between stars and brown dwarfs.
• Figure 4: $\rm m_{F322W2}$ versus $\rm (m_{F115W}-m_{F322W2})$ colour-magnitude diagram of 47 Tuc stars from milone23 and MA24. Cyan dots indicate all sources observed for 47 Tuc, yellow dots indicate confirmed cluster members, according to the criteria described in section \ref{['2gen']} and grey dots indicate SMC members. The blue and the red lines indicate $\rm 12~Gyr$ isochrones for the 1G and 2G of the cluster, respectively. Some specific masses are labelled along the isochrones. The masses $\rm 0.5, 0.4, 0.3, 0.25, 0.2, 0.15, 0.12, 0.1, 0.09, 0.085, 0.08~M_\odot$, are shown. Below $\rm 0.08~M_\odot$ the models are spaced by $\rm 0.001~M_\odot$ down to $\rm 0.06~M_\odot$ for the 1G and to $\rm 0.058~M_\odot$ for the 2G. Finally, the upper dashed green line represents the saturation limit ($\rm m_{F322W2}\simeq18.5$), while the lower diagonal green line indicates the limit where proper motions are available ($\rm m_{F115W}=26.1$ as in MA24, properly converted in $\rm m_{F322W2}$).
• Figure 5: Colour magnitude data in the $\rm (m_{F150W2}-m_{F322W2}, m_{F322W2})$ plane (left, grey dots), are compared with the theoretical models. Contrary to the previous diagram, the agreement is unsatisfactory. These are the data on which the luminosity function has been derived. The LF is shown in the right side of the figure (grey dots), together with the theoretical luminosity function obtained by assuming a flat mass function below $\rm 0.3~M_\odot$, by normalizing the mass luminosity derivative of the models to the data. The figure shows that the observational LF would be reproduced if the transition masses were more luminous that computed by about 1.5 mag.