Multiplicity of young isolated planetary mass objects in Taurus and Upper Scorpius

TL;DR

This work targets the multiplicity of free-floating planetary-mass objects in the Taurus and Upper Scorpius regions using high-spatial-resolution imaging from HST WFC3/UVIS and VLT ERIS/NIX. Analyzing 77 targets in the mass range $6-66\,M_{\rm Jup}$, the study identifies one robust Taurus companion at $\sim18$ au with a near-equal mass ratio and places tight limits on additional companions, while USco shows no binaries in this mass range, yielding an upper limit of $\le 1.2\%$. By combining with previous surveys, the authors find a significant regional difference in multiplicity below $\sim 30-50\,M_{\rm Jup}$, with Taurus exhibiting higher binary fractions than USco, a trend that can be explained by warmer cloud environments in USco increasing the Jeans mass and fragmentation scale. The results imply that environmental conditions during star formation influence the production and survival of very low-mass binaries, highlighting the need for spectroscopic confirmations and deeper imaging to map the full separation and mass-ratio distributions across diverse star-forming environments.

Abstract

Free-floating planetary mass objects--worlds that roam interstellar space untethered to a parent star--challenge conventional notions of planetary formation and migration, but also of star and brown dwarf formation. We focus on the multiplicity among free-floating planets. By virtue of their low binding energy (compared to other objects formed in these environments), these low-mass substellar binaries represent a most sensitive probe of the mechanisms at play during the star formation process. We use the HST and its WFC3 and the VLT and its ERIS AO facility to search for visual companions among a sample of 77 objects members of the USco and Taurus young nearby associations with estimated masses in the range between approximately 6-66 M$_{\rm Jup}$. We report the discovery of one companion candidate around a Taurus member with a separation of 111.9$\pm$0.4~mas, or $\sim$18~au assuming a distance of 160~pc, with an estimated primary mass in the range between 3--6~M$_{\rm Jup}$and a secondary mass between 2.6--5.2~M$_{\rm Jup}$ depending on the assumed age. This corresponds to an overall binary fraction of 1.8$^{+2.6}_{-1.3}$\% among low-mass brown dwarfs and free-floating planetary mass objects over the separation range $\ge$7~au. Despite the limitations of small-number statistics and variations in spatial resolution and sensitivity, our results, combined with previous high-spatial-resolution surveys, suggest a notable difference in the multiplicity properties of objects below $\sim$30--50~M$_{\rm Jup}$ between USco and Taurus. In Taurus, a binary fraction of $5.6^{+3.2}_{-2.3}$\% is found for objects with masses below 30M$_{\rm Jup}$, and of $7.8^{+3.0}_{-2.4}$\% for objects with masses below 50M$_{\rm Jup}$, whereas no binary were found among 80 objects over the matching luminosity range in USco, corresponding to an upper limit of $\le$1.2\%.

Figures (11)

• Figure 1: ( i, i-z) diagram of Taurus members (left) identified by Esplin2019 and USco members (right) from MiretRoig2022, represented as blue dots. The HST targets are over-plotted as red dots and the VLT targets are orange squares. The Chabrier2023 isochrones at 3 Myr (Taurus) and 5 and 10 Myr (USco) and 140 pc are represented by green dashed lines and the corresponding masses are indicated on the right vertical axis. The 5 and 10 Myr isochrones for USco overlap almost perfectly and are represented as one. A reddening vector A$_{\rm V}$=3 mag is also represented, and the planetary mass limit of 13 M$_{\rm Jup}$ is indicated. The new binary candidate identified in Taurus is over-plotted as a black open circle.
• Figure 2: Positions of the USco targets on a color photograph showing the clouds and nebulae. HST targets are represented with red dots and VLT targets with orange squares. Background photograph credit: Mario Cogo.
• Figure 3: Positions of the Taurus targets on a color photograph showing the main Taurus molecular clouds. HST targets are represented with red dots and the new binary candidate is indicated with a white cross. Background photograph credit: Chris McGrew
• Figure 4: Detection limits in the HST F814W (top) and F850LP (bottom) images.
• Figure 5: HST F814W (left) and F850LP (right) images of the companion identified around J042705.86+261520.3 through direct inspection. A square root stretch is used. North is up and east is left.