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Register a new userLarge Adaptive Optics Survey for Substellar Objects (LASSO) Around Young, Nearby, Low-mass Stars with Robo-AO
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We present results from the Large Adaptive optics Survey for Substellar Objects (LASSO), where the goal is to directly image new substellar companions (<70 M$_{Jup}$) at wide orbital separations ($gtrsim$50 AU) around young ($lesssim$300 Myrs), nearby (<100 pc), low-mass ($approx$0.1-0.8 M$_{odot}$) stars. We report on 427 young stars imaged in the visible (i) and near-infrared (J or H) simultaneously with Robo-AO on the Kitt Peak 2.1-m telescope and later the Maunakea University of Hawaii 2.2-m telescope. To undertake the observations, we commissioned a new infrared camera for Robo-AO that uses a low-noise high-speed SAPHIRA avalanche photodiode detector. We detected 121 companion candidates around 111 stars, of which 62 companions are physically associated based on Gaia DR2 parallaxes and proper motions, another 45 require follow-up observations to confirm physical association, and 14 are background objects. The companion separations range from 2-1101 AU and reach contrast ratios of 7.7 magnitudes in the near infrared compared to the primary. The majority of confirmed and pending candidates are stellar companions, with ~5 being potentially substellar and requiring follow-up observations for confirmation. We also detected a 43$pm$9 M$_{Jup}$ and an 81$pm$5 M$_{Jup}$ companion that were previously reported. We found 34 of our targets have acceleration measurements detected using Hipparcos-Gaia proper motions. Of those, 58$^{+12}_{-14}$% of the 12 stars with imaged companion candidates have significant accelerations ($chi^2 >11.8$), while only 23$^{+11}_{-6}$% of the remaining 22 stars with no detected companion have significant accelerations. The significance of the acceleration decreases with increasing companion separation. These young accelerating low-mass stars with companions will eventually yield dynamical masses with future orbit monitoring.
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The abundance of brown dwarfs (BDs) in young clusters is a diagnostic of star formation theory. Here we revisit the issue of determining the substellar initial mass function (IMF), based on a comparison between NGC1333 and IC348, two clusters in the Perseus star-forming region. We derive their mass distributions for a range of model isochrones, varying distances, extinction laws and ages, with comprehensive assessments of the uncertainties. We find that the choice of isochrone and other parameters have significant effects on the results, thus we caution against comparing IMFs obtained using different approaches. For NGC1333, we find that the star/BD ratio R is between 1.9 and 2.4, for all plausible scenarios, consistent with our previous work. For IC348, R is between 2.9 and 4.0, suggesting that previous studies have overestimated this value. Thus, the star forming process generates about 2.5-5 substellar objects per 10 stars. The derived star/BD ratios correspond to a slope of the power-law mass function of alpha=0.7-1.0 for the 0.03-1.0Msol mass range. The median mass in these clusters - the typical stellar mass - is between 0.13-0.30Msol. Assuming that NGC1333 is at a shorter distance than IC348, we find a significant difference in the cumulative distribution of masses between the two clusters, resulting from an overabundance of very low mass objects in NGC1333. Gaia astrometry will constrain the cluster distances better and will lead to a more definitive conclusion. Furthermore, ratio R is somewhat larger in IC348 compared with NGC1333, although this difference is still within the margins of error. Our results indicate that environments with higher object density may produce a larger fraction of very low mass objects, in line with predictions for brown dwarf formation through gravitational fragmentation of filaments falling into a cluster potential.
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