Astrometric follow-up observations of directly imaged sub-stellar companions to young stars and brown dwarfs★

Abstract

The formation of massive planetary or brown dwarf companions at large projected separations from their host star is not yet well understood. In order to put constraints on formation scenarios we search for signatures in the orbit dynamics of the systems. We are specifically interested in the eccentricities and inclinations since those parameters might tell us about the dynamic history of the systems and where to look for additional low-mass sub-stellar companions. For this purpose we utilized VLT/NACO to take several well calibrated high resolution images of 6 target systems and analyze them together with available literature data points of those systems as well as Hubble Space Telescope archival data. We used a statistical Least-Squares Monte-Carlo approach to constrain the orbit elements of all systems that showed significant differential motion of the primary star and companion. We show for the first time that the GQ Lup system shows significant change in both separation and position angle. Our analysis yields best fitting orbits for this system, which are eccentric (e between 0.21 and 0.69), but can not rule out circular orbits at high inclinations. Given our astrometry we discuss formation scenarios of the GQ Lup system. In addition, we detected an even fainter new companion candidate to GQ Lup, which is most likely a background object. We also updated the orbit constraints of the PZ Tel system, confirming that the companion is on a highly eccentric orbit with e>0.62. Finally we show with a high significance, that there is no orbital motion observed in the cases of the DH Tau, HD 203030 and 1RXS J160929.1-210524 systems and give the most precise relative astrometric measurement of the UScoCTIO 108 system to date.

An ALMA and MagAO Study of the Substellar Companion GQ Lup B∗

“…Figure 5 shows the astrometric monitoring in the last ∼20 years, and GQ Lup B's orbital motion is evident. Our results are consistent with the trends derived by Ginski et al (2014), r D~-1.4 mas yr −1 and ΔPA∼+0°.16 yr −1 . Table 1 also lists our astrometric measurements.…”

Alma Measurements of Circumstellar Material in the Gq Lup System

“…Indeed, the models of Ginski et al (2014) and Schwarz et al (2016) do not exclude low eccentricity orbits as would be expected for a planet-like formation within a larger protoplanetary disk. The morphology of the GQ Lup A disk points against a scattering origin for the companion.…”

“…The projected separation of the companion from the primary star is 0 7 (Ginski et al 2014), and recent work by Schwarz et al (2016) favors orbits with high eccentricity and semimajor axes 100-185 au. Near-infrared spectroscopy by Seifahrt et al (2007) showed Paβ line emission (equivalent width, EW=−3.83±0.12 Å), though subsequent observations by Lavigne et al (2009) give a limit an order of magnitude lower for the same line (EW=−0.46±0.08 Å), possibly pointing to time variability of Paβ and ongoing disk accretion.…”

“…Schwarz et al (2016) and Ginski et al (2014) (assuming a stellar mass of 0.7 M e ) propose three families of orbital solutions for GQ Lup b: (1) semimajor axis ∼100 au, i∼57°, eccentricity ∼0.15, (2) semimajor axis <185 au, 28°<i<63°, eccentricity 0.2 to 0.75, and (3) semimajor axis >300 au, 52°<i<63°, eccentricity >0.8. More specifically, they note that orbits with lower eccentricities between 0.1 and 0.4 have high inclinations between 48°and 63°.…”

The Young Substellar Companion ROXs 12 B: Near-infrared Spectrum, System Architecture, and Spin–Orbit Misalignment*

“…In principle, clues about their origin can be inferred from a comparative abundance analysis with their host stars (e.g., Konopacky et al 2013;Barman et al 2015), measurements of their occurrence rate over time, long-term monitoring of their orbits (e.g., Ginski et al 2014;Rameau et al 2016;Blunt et al 2017), the physical properties of their circumplanetary disks Kraus et al 2015;Stamatellos & Herczeg 2015), and constraints on their mass and semimajor axis distributions (Biller et al 2013;Brandt et al 2014;Reggiani et al 2016). Indeed, Bryan et al (2016) recently concluded that dynamical scattering is probably not the dominant origin of wide PMCs based on the lack of close-in scatterers, the low rate of close-in giant planets in the field, and early orbital constraints for wide PMCs.…”