It’s been a while since I’ve discussed the subgiant mass issue, but there’s a nifty new paper out that gives me occasion to revisit it (original controversy, our response, another mention here). The basic outline is that there is a mild controversy over the masses of subgiant stars — stars transitioning between their long Main Sequence lives and their final, giant phase before they run out of nuclear fuel. John Johnson studied subgiants that were between 1.5-2 solar masses back when they were on the Main Sequence (“Retired A Stars”) to show that they are more likely to have giant planets detected around them than stars of the Sun’s mass or lower. Jamie Lloyd at Cornell thinks these are really mostly just solar-mass stars, because there shouldn’t be very many retired A stars (it’s a short phase of a rare kind of star). After refereeing the discussion between my friends for a while, I finally sided with John.
Since then, Luan Ghezzi and John have a nice paper where they look at all of the subgiants with known masses and find that there is no reason to doubt their mass estimates.
But what we’ve really wanted to have is a transiting planet around a subgiant. Transiting planets provide a way to estimate the density of a star completely independently of other methods, which, combined with even a rough radius estimate, would allow a subgiant’s mass to be independently estimated.
The problems are many: subgiants don’t have a lot of close-in planets, they’re big, so the transiting planets have very small depths, and they’re big, so the transits are looooooong.
Well, the KELT team has found one, and around a bright subgiant! Josh Pepper leads the team that just announced KELT-11 b, an inflated 0.2 Jupiter mass giant orbiting a V=8 star. KELT is the “Kilodegree Extremely Little Telescope” array, designed to scan huge swaths of sky for transiting planets. Its architects include Thomas Beatty of Penn State, Jason Eastman now at Harvard, and Scott Gaudi at Ohio State, to name a few of those I’ve worked closely with.
KELT-11 is brightest star known to host a transiting planet in the southern hemisphere! The paper has a ton of authors because a big team of people worked really hard to get this star characterized. I’m involved because MINERVA contributed some photometry (fourth curve in figure 2) to try to nail this thing down.
Because it’s a subgiant, the signal is very weak, and very long. The transit is eight hours long, which means that virtually no single telescope on Earth can get the whole transit in one night (we don’t have any south pole photometers!). As a result, it’s challenging to remove systematics from ground-based observing: you never see both ingress and egress with the same data set.
A precise mass from the transit alone will still need to wait for space-based data that can get the entire transit in one go, but in the paper the team uses several independent methods to estimate the mass, including their best estimate from the light curve. In section 3.8, four different estimates are used and all four find a mass > 1.3 solar masses, with the right answer probably around 1.4. This further confirms the methodologies John uses to estimate masses, giving more evidence to the assertion that the subgiants in his sample really are “Retired A Stars”.
It’s a great paper about a superlative planetary system ripe for extensive followup. You can find it on the arXiv now here:
KELT-11b: A Highly Inflated Sub-Saturn Exoplanet Transiting the V=8 Subgiant HD 93396
We report the discovery of a transiting exoplanet, KELT-11b, orbiting the bright (V=8.0) subgiant HD 93396. A global analysis of the system shows that the host star is an evolved subgiant star with Teff=5370±51 K, M∗=1.438+0.061−0.052M⊙, R∗=2.72+0.21−0.17R⊙, log g∗=3.727+0.040−0.046, and [Fe/H]=0.180±0.075. The planet is a low-mass gas giant in a P=4.736529±0.00006 day orbit, with MP=0.195±0.018MJ, RP=1.37+0.15−0.12RJ, ρP=0.093+0.028−0.024 g cm−3, surface gravity log gP=2.407+0.080−0.086, and equilibrium temperature Teq=1712+51−46 K. KELT-11 is the brightest known transiting exoplanet host in the southern hemisphere by more than a magnitude, and is the 6th brightest transit host to date. The planet is one of the most inflated planets known, with an exceptionally large atmospheric scale height (2763 km), and an associated size of the expected atmospheric transmission signal of 5.6%. These attributes make the KELT-11 system a valuable target for follow-up and atmospheric characterization, and it promises to become one of the benchmark systems for the study of inflated exoplanets.