The Architecture of the GW Ori Young Triple-star System and Its Disk: Dynamical Masses, Mutual Inclinations, and Recurrent Eclipses

Abstract

We present spatially and spectrally resolved Atacama Large Millimeter/submillimeter Array (ALMA) observations of gas and dust orbiting the pre-main sequence hierarchical triple star system GW Ori. A forward-modeling of the 13 CO and C 18 O J=2-1 transitions permits a measurement of the total stellar mass in this system, 5.29 ± 0.09 M , and the circum-triple disk inclination, 137.6 ± 2.0• . Optical spectra spanning a 35 year period were used to derive new radial velocities and, coupled with a spectroscopic disentangling technique, revealed that the A and B components of GW Ori form a double-lined spectroscopic binary with a 241.50 ± 0.05 day period; a tertiary companion orbits that inner pair with a 4218 ± 50 day period. Combining the results from the ALMA data and the optical spectra with three epochs of astrometry in the literature, we constrain the individual stellar masses in the system (M A ≈ 2.7 M , M B ≈ 1.7 M , M C ≈ 0.9 M ) and find strong evidence that at least one (and likely both) stellar orbital planes are misaligned with the disk plane by as much as 45• . A V -band light curve spanning 30 years reveals several new ∼30 day eclipse events 0.1-0.7 mag in depth and a 0.2 mag sinusoidal oscillation that is clearly phased with the AB-C orbital period. Taken together, these features suggest that the A-B pair may be partially obscured by material in the inner disk as the pair approaches apoastron in the hierarchical orbit. Lastly, we conclude that stellar evolutionary models are consistent with our measurements of the masses and basic photospheric properties if the GW Ori system is ∼1 Myr old.

GW Ori: Interactions Between a Triple-star System and its Circumtriple Disk in Action

“…This resulted in an RMS noise level of ∼ 40 µJy beam -1 and an enhanced peak signal-to-noise ratio (SNR) of ∼ 157, compared with ∼ 63 µJy beam -1 and ∼ 90 before self-calibration, respectively. The integrated flux density of the disk (195 ± 20 mJy) is consistent with the result from previous ALMA observations (202 ± 20 mJy; Czekala et al 2017).…”

“…Our dust and gas observations alone cannot break the degeneracy in the mutual inclination between different parts in the disk due to the unknown on-sky projected orbital direction of the disk. Previous studies indicate that the on-sky projected gas motion is likely to be clockwise (Czekala et al 2017), same as the orbital motion of GW Ori C given by astrometric observations (Berger et al 2011). Given the inclination and longitude of ascending node of the AB-C binary orbit being 150 ± 7 and 282 ± 9 degrees (Czekala et al 2017), we assume that the entire disk has the same clockwise on-sky projected orbital direction.…”

“…Previous studies indicate that the on-sky projected gas motion is likely to be clockwise (Czekala et al 2017), same as the orbital motion of GW Ori C given by astrometric observations (Berger et al 2011). Given the inclination and longitude of ascending node of the AB-C binary orbit being 150 ± 7 and 282 ± 9 degrees (Czekala et al 2017), we assume that the entire disk has the same clockwise on-sky projected orbital direction. Following Fekel (1981), we find out that the binary-disk misalignments at 46 AU (the inner ring), 100 AU (a gap), 188 AU (the middle ring), and 338 AU (outer ring) are 11 ± 6, ∼28 2 , 35 ± 5, and 40 ± 5 degrees, respectively.…”

“…A tertiary component (GW Ori C) was detected by near-infrared interferometry at a projected distance of ∼ 8 AU (Berger et al 2011). The stellar masses have been constrained to be ∼ 2.7, 1.7, and 0.9 M , respectively (Czekala et al 2017). The system harbours a rare circumtriple disk, with dust extending to ∼ 400 AU, and gas extending to ∼ 1300 AU (Fang et al 2017).…”

Weighing stars from birth to death: mass determination methods across the HRD

“…Even for low S/N data (peak S/N per beam of 12), statistical uncertainties of M as low as 1% are consistently achieved. Analyses by Rosenfeld et al (2012) and Czekala et al (2015Czekala et al ( , 2016Czekala et al ( , 2017a have validated the systematic precision of the technique (\4%) by comparison to independently determined masses of spectroscopic binaries and extended the sample towards the lowest mass stars (Simon et al 2017).…”