Msini without the sini

In the “wish I’d read it a year ago” file here’s Yiting Li et al., “Precise Masses and Orbits for Nine Radial Velocity Exoplanets”.

Our first 1990s look at extrasolar planets caught nearby systems with a radial-velocity wobble. Their masses were not M but M times the sine of inclination (i); they were biased to Hot Jupiters. After a few years we saw some of these transit their stars so we knew their inclination to, like, 87-90°. As we kept looking we found more Msini’s at longer periods. The longer the periods – so semimajors – were, the less likely we’d be lucky enough to spot a transit; we need the inclination from other means. At least the long periods rule out signals from stellar rotation, on account no G mainseq star rotates once every Earth year or more.

Now we’ve been watching some of these for long enough that the periods are set to swing the star along, facewise. Here the Hipparchos is being tested against Gaia EDR3. That gives us astrometry good for 25 years – thereby (sine-of-)i. I imagine it also ensures we’re not mistaking multiple resonant planets for one big eccentric planet (we’ll get to this…).

Yiting Li’s crew have picked nine nearby (high-parallax) systems whose stars don’t clutter the readings with flares and general instability. These are G stars plus old K0 HD 87883. Some are in binaries so their total perturbations must be accounted; HD 196067/8 is called out as a possible Lidov-Kozai unto '7b. One mitzvah is that the team recalculated the stellar masses and Msini both based on the updated data. Gotta start somewhere! They are all Jovians on up, with HD 87883 b having the smallest Msini as recalculated 1.54 M_j.

Of these HD 81040 turns out edge-on, possibly a transit opportunity after all although I’d not bet on “0.9%”. Several more are face-on so their masses are much greater than Msini would suggest. Like that HD 87883 b: whose real mass turns out 9.7 M_j. As a result “superJovians” HD 106515 Ab and HD 221420 are herein reclassified brown dwarfs, with HD 196067 b at 12.5 M_j borderline.

Gaia EDR3 isn’t DR3 so the team did some number-magick around the edges, leaving to the future fine-tuning some base assumptions. Also although the “sini degeneracy” is no more, there remain “bimodal” models, such that i might be one or the other. To constrain this the team suggest another telescope-sweep at four of the radial-velocity targets: HD 87883, 171238, 98649, 196067. HD 221420 having a 10 AU 0.16-eccentricity 20.6 M_j brown dwarf stands to be Webb’d. Although at 6.6 Gya at this insolation the dwarf might not be warm enough anymore.

Semimajors run 2-10 AU. Nobody is suggesting, beneath them, Hot Jupiters nor, for that matter, those deep blue Sudarsky III’s. Terrestrials are possible downbelow; lookin’ to cooler HD 29021 L=0.66 or HD 87883 L=0.34. But. The eight planet-planets are all ridiculously eccentric (0.35+, on the 0-to-1 scale) such as to sweep the HZ of its material and I’ve not heard that a Hilda or a Lagrange-Trojan will work.

The exception is HD 221420’s dwarf out at a Saturn orbit. By inverse-square, HD 221420 b gravitationally would approximate a 5.15 M_j at 5 AU. The star’s luminosity is 3.85 for a 1.28 mass so ~2 AU for a reasonable HZ. Jupiter as of its first gigayear had, I believe, cleared 1-5 AU of everything larger than Mars and didn’t bring in anything larger than Ceres. HD 221420 might have accreted an Earth-mass in its 3:2 Hilda 18.467 year period… but that’s going to be, er, cold. Same goes for HD 98649’s 9.7 M_j at 6 AU against near-solar 0.97 L especially considering 0.85 e (wut). HD 171238 is L=0.627 with a 8.8 M_j at 2.52 AU… e=0.36.

So, er. Exomoons? HD 29021 b 2.3 AU from its L=0.66 star is claimed “just outside the habitable zone” but from those data I can read only “outside the HZ”, unless we count Hilda as a bound-object, which at e=0.453 I doubt exists. The true Sudarsky II here orbits HD 106252 L=1.33 which (10 M_j) planet is out at 2.655 AU. e=0.48 with summer every 4.2 Earth years.

So the dataset offers poor pickin’s for worldbuilders. It works better as a snapshot of systems-with-jovian.