Perhaps there is no brown dwarf desert? A study of sub-stellar companions with Gaia DR3

TL;DR

The paper investigates the presence of brown dwarf companions with periods under $5$ years around nearby Sun-like stars using Gaia DR3 astrometry, employing RUWE-based selection and a Bayesian single-companion inference framework to estimate companion mass and orbital period. It accounts for observational biases with a detection-probability model and finds an initial BD occurrence of about $2.41\%$ for $P<5$ years, while highlighting degeneracies that arise from the DR3 data in distinguishing single vs. multiple companions. Simulations show that epoch data, available in DR4, will be essential to break these degeneracies and accurately map the mass–period distribution of BD companions. The work projects a substantially larger BD yield for DR4 depending on the true BD mass function, indicating Gaia’s potential to robustly characterize the brown dwarf desert and inform formation theories.

Abstract

The brown dwarf desert describes a range of orbital periods (<5 years) in which fewer brown dwarf-mass companions have been observed around Sun-like stars, when compared to planets and low mass stellar companions. It is therefore theorised that brown dwarf companions are unlikely to form or remain in this period range. The Gaia space telescope is uniquely sensitive to companions in this period range, making it an ideal tool to conduct a survey of the brown dwarf desert. In this study, we use Bayesian inference to analyse data from nearby (<200pc) Sun-like stars in Gaia's DR3 catalogue, assuming single companions. From this, we identify 2673 systems (2.41% of the sample) with possible brown dwarf companions in this period range. Accounting for observational biases, we find that 10.4+0.8-0.6% of nearby Sun-like stars have astrometric errors consistent with a brown dwarf-mass companion with a period less than 5 years, significantly higher than previous studies which reported occurrence rates of <1%. However, we acknowledge the limitations of DR3 and are unable to make a definitive statement without epoch data. By simulating epoch data with multiple companions, we find that, while some of the data can be explained by multiple low-mass brown dwarf companions and high-mass planets (>10MJ), high-mass brown dwarfs (>50MJ) in this period range are comparatively rare. Finally, we used our studies of the brown dwarf distribution to predict the number of companions in the brown dwarf desert we can expect to discover in DR4.

Figures (14)

• Figure 1: Colour/magnitude distribution of sample (upper), and calculated mass distribution. Mass is taken from FLAME models.
• Figure 2: RUWE as a function of companion mass and age for two examples with estimated masses of 0.75 M$_{\odot}$ (a) and 1.25 M$_{\odot}$ (b). Dashed lines are RUWE from the Gaia catalogue.
• Figure 3: Average detection probability as a function of companion mass and orbital period. Probability of 1 means the companion is guaranteed to produce a RUWE of more than 1.25 regardless of eccentricity or orbital configuration. The dashed region marks companions with mass in the brown dwarf range and periods of less than 5 years.
• Figure 4: Posterior distributions of inferred mass and period of a companion to Gaia DR3 1985383408935925120.
• Figure 5: Top: Distribution of companion masses and periods. The colour bar is the number of detections per bin divided by the total number of sources (110,749). Bottom: Integrated period distributions for brown dwarf and low-mass stellar companions.