Disk fraction among free-floating planetary-mass objects in Upper Scorpius

TL;DR

The paper addresses how circumstellar disks persist around free-floating planetary-mass objects in Upper Scorpius and what their disk fractions imply about formation. It deploys unWISE W1–W2 colors and a Bayesian outlier detection framework to identify IR excesses and derive mass-resolved disk fractions across ages 5 and 10 Myr, extending sensitivity down to ~6–$M_{\rm Jup}$. The analysis reveals disk fractions rising toward lower masses, exceeding ~30% near the substellar–planetary boundary ($\sim$13 $M_{\rm Jup}$), with hints of a flattening at 25–45 $M_{\rm Jup}$; USC disks are generally less common than in younger regions, consistent with disk dispersal over time. These results constrain formation scenarios for FFPs and brown dwarfs and motivate deeper follow-up with JWST and ALMA to elucidate disk masses, sizes, and evolution.

Abstract

Free-floating planetary-mass objects (FFPs) have been detected through direct imaging in several young, nearby star-forming regions. The properties of circumstellar disks around these objects may provide a valuable probe into their origin but are currently limited by the small sample sizes explored. We aim to perform a statistical study of the occurrence of circumstellar disks down to the planetary-mass regime. We performed a systematic survey of disks among the population identified in the 5-10 Myr-old Upper Scorpius association (USC), restricted to members outside the younger, embedded Ophiuchus region and with estimated masses below 105 M_Jup. We took advantage of unWISE photometry to search for mid-infrared excesses in the WISE (W1-W2) color. We implemented a Bayesian outlier detection method, which models the photospheric sequence and computes excess probabilities for each object, enabling a statistically sound estimation of disk fractions. We explored disk fractions across an unprecedentedly fine mass grid, reaching down to objects as low as ~6 M_Jup assuming 5 Myr or ~8 M_Jup assuming 10 Myr, thus extending the previous lower boundary of disk fraction studies. Depending on the age, our sample includes between 17 and 40 FFPs. We confirm that the disk fraction steadily rises with decreasing mass and exceeds 30% near the substellar-to-planetary mass boundary at ~13 M_Jup. We find hints of a possible flattening in this trend around 25-45 M_Jup, potentially signaling a transition in the dominant formation processes. This shift in trend should be considered with caution and needs to be confirmed with more sensitive observations. Our results are consistent with the gradual dispersal of disks over time, as disk fractions in Upper Scorpius appear systematically lower than those in younger regions.

Figures (13)

• Figure 1: Spatial distribution of USC and Oph members from MiretRoig2022a. Objects with estimated masses below 13 M$_{\rm Jup}$ and between 13 -- 105 M$_{\rm Jup}$ (for an assumed age of 5 Myr) are represented by red circles and blue triangles, respectively. The dashed white line indicates the extinction boundary used in this study, as defined by the polygon in Sect. \ref{['sect:area']}. Background image: Planck 857 GHz Planck.
• Figure 2: Source density as a function of magnitude in the unWISE/W1 and unWISE/W2 bands. Solid lines represent the kernel density estimates of the histograms, while vertical dashed lines mark the turning points used to define the completeness limits of the unWISE data.
• Figure 3: W1--W2 IR color as a function of mass for USC members, assuming ages of 5 ( Top) and 10 Myr ( Bottom). The solid black line represents the best-fit relation, while the blue-shaded region corresponds to the 3$\sigma$ confidence interval of the fit parameters (i.e., slope and intercept). The color scale represents the inferred excess probability, as indicated by the color bar. Red circles highlight sources with an excess probability greater than 0.5. Objects identified as outliers ($P_{\mathrm{excess}}>0.5$) below the fitted photospheric sequence are marked with black crosses, as they are excluded from the subsequent analysis. Background shading is used to delineate mass bins used in Figures \ref{['fig:violin_fractions_woWbl']} and \ref{['fig:fractions_lit']}.
• Figure 4: W1--W2 IR color as a function of mass for USC members in the 0--25 M$_{\rm Jup}$ mass range, assuming ages of 5 and 10 Myr respectively. This zoomed-in view focuses on the low-mass regime to better visualize the linear fits and the associated excess probabilities in this domain, shown in Fig. \ref{['fig:MCMC_plot_woWbl']} and discussed in Sect. \ref{['sect:MCMC']}. See the caption in Fig. \ref{['fig:MCMC_plot_woWbl']} for a description.
• Figure 5: Comparison of W2 excess detections between this study and LuhmanEsplin2020 using Venn diagrams, for assumed ages of 5 Myr ( Left) and 10 Myr ( Right). In each case, the central intersection shows the number of sources identified as exhibiting excess in both studies, while the left and right circle arcs show those detected only in one study but not the other. The orange-hatched region represents sources without excess detection in either work.