Determining the true mass of radial-velocity exoplanets with Gaia

Abstract

Mass is one of the most important parameters for determining the true nature of an astronomical object. Yet, many published exoplanets lack a measurement of their true mass, in particular those detected as a result of radial-velocity (RV) variations of their host star. For those examples, only the minimum mass, or m sin i, is known, owing to the insensitivity of RVs to the inclination of the detected orbit compared to the plane of the sky. The mass that is given in databases is generally that of an assumed edge-on system (∼90 •), but many other inclinations are possible, even extreme values closer to 0 • (face-on). In such a case, the mass of the published object could be strongly underestimated by up to two orders of magnitude. In the present study, we use GASTON, a recently developed tool taking advantage of the voluminous Gaia astrometric database to constrain the inclination and true mass of several hundreds of published exoplanet candidates. We find nine exoplanet candidates in the stellar or brown dwarf (BD) domain, among which six were never characterized. We show that 30 Ari B b, HD 141937 b, HD 148427 b, HD 6718 b, HIP 65891 b, and HD 16760 b have masses larger than 13.5 M J at 3σ. We also confirm the planetary nature of 27 exoplanets, including HD 10180 c, d and g. Studying the orbital periods, eccentricities, and host-star metallicities in the BD domain, we found distributions with respect to true masses consistent with other publications. The distribution of orbital periods shows of a void of BD detections below ∼100 d, while eccentricity and metallicity distributions agree with a transition between BDs similar to planets and BDs similar to stars in the range 40-50 M J .

Speckle Imaging Characterization of Radial Velocity Exoplanet Systems

“…For the former, pixels that subtend many arcseconds (as in the case of TESS) can hide stellar companions leading to the inference of erroneous stellar and planetary properties, including radius and density (e.g., Furlan & Howell 2017Ziegler et al 2020). For the latter, the unknown orbital inclination of objects causing long-term accelerations can allow stellar companions to contaminate exoplanet catalogs if an edge-on geometry is assumed (e.g., Kiefer et al 2021). Therefore, direct imaging serves as a practical false-positive checking practice as well.…”

The SOPHIE search for northern extrasolar planets. XVII. A wealth of new objects: Six cool Jupiters, three brown dwarfs, and 16 low-mass binary stars

“…Recent studies have shown that information from the first Gaia Data Release (DR1) can constrain the mass of RV-detected nontransiting companions (Kiefer et al 2019;Mugrauer & Michel 2020). The GASTON code uses the astrometric excess noise, as published in DR1, to determine the amplitude of the astrometric motion of the host star of a known RV-detected companion (Kiefer 2019;Kiefer et al 2019Kiefer et al , 2021. This measures, or at least constrains from below, the inclination of the orbit, and thus resolves inclination ambiguity on the true mass.…”

“…Starting from the RV-derived parameters of the companion, the parallax of the star and an estimation of its mass as priors, GASTON uses a MCMC algorithm to explore the space of astrometric orbits compatible with the measured astrometric excess noise, hereafter ε DR1 , and hence to constrain the possible inclinations. It accounts for the systematics in the Gaia DR1 catalog and distinguishes between stars belonging to the TGAS (Tycho-Gaia Astrometric Solution) and the secondary subset of the DR1 catalog (Lindegren et al 2016;Kiefer et al 2021). For the targets in the TGAS dataset, the inputs to the 5-parameter fit include a 24-yr older Tycho-2 or HIPPARCOS-2 astrometric point in addition to 14 months of Gaia measurements, leading to an improved proper motion accuracy.…”

“…DR1 only published positions (and magnitudes) for those stars and we used their Gaia DR2 (Gaia Collaboration 2018) absolute parallax as a replacement for the missing DR1 parallax, with a conservatively allocated 10% relative uncertainty. Lastly, GASTON accounts for the effect of the companion's light on the position of the photocenter, by modeling the luminosity of both components from their mass (Appendix A in Kiefer et al 2021). Here we additionally impose that the secondary contributes no more than 10% of the total luminosity since the systems we study are single-lined spectroscopic binaries.…”

“…The RV technique can, in most cases, only determine the minimum mass of the companion (m sin i , where m is the true mass and i the orbital inclination of the planet). As mass is what distinguishes giant planets from BDs, and from SCs, the inclination ambiguity must be lifted to asserting the true nature of a substellar companion (e.g., Díaz et al 2012;Curiel et al 2020;Kiefer et al 2021). As just one example, HD 5388 b was first announced as a likely gas giant (Santos et al 2010) but turned out to be a BD companion when Sahlmann et al (2011a) detected its astrometric signature in the HIPPARCOS measurements of HD 5388.…”