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Register a new userHAT-P-26b: A Neptune-Mass Exoplanet with a Well Constrained Heavy Element Abundance
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A correlation between giant-planet mass and atmospheric heavy elemental abundance was first noted in the past century from observations of planets in our own Solar System, and has served as a cornerstone of planet formation theory. Using data from the Hubble and Spitzer Space Telescopes from 0.5 to 5 microns, we conducted a detailed atmospheric study of the transiting Neptune-mass exoplanet HAT-P-26b. We detected prominent H2O absorption bands with a maximum base-to-peak amplitude of 525ppm in the transmission spectrum. Using the water abundance as a proxy for metallicity, we measured HAT-P-26bs atmospheric heavy element content [4.8 (-4.0 +21.5) times solar]. This likely indicates that HAT-P-26bs atmosphere is primordial and obtained its gaseous envelope late in its disk lifetime, with little contamination from metal-rich planetesimals.
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We report the discovery of HAT-P-26b, a transiting extrasolar planet orbiting the moderately bright V=11.744 K1 dwarf star GSC 0320-01027, with a period P = 4.234516 +- 0.000015 d, transit epoch Tc = 2455304.65122 +- 0.00035 (BJD), and transit duration 0.1023 +- 0.0010 d. The host star has a mass of 0.82 +- 0.03 Msun, radius of 0.79 + 0.10 - 0.04 Rsun, effective temperature 5079 +- 88 K, and metallicity [Fe/H] = -0.04 +- 0.08. The planetary companion has a mass of 0.059 +- 0.007 MJ, and radius of 0.565 + 0.072 - 0.032 RJ yielding a mean density of 0.40 +- 0.10 g cm-3. HAT-P-26b is the fourth Neptune-mass transiting planet discovered to date. It has a mass that is comparable to those of Neptune and Uranus, and slightly smaller than those of the other transiting Super-Neptunes, but a radius that is ~65% larger than those of Neptune and Uranus, and also larger than those of the other transiting Super-Neptunes. HAT-P-26b is consistent with theoretical models of an irradiated Neptune-mass planet with a 10 Mearth heavy element core that comprises >~ 50% of its mass with the remainder contained in a significant hydrogen-helium envelope, though the exact composition is uncertain as there are significant differences between various theoretical models at the Neptune-mass regime. The equatorial declination of the star makes it easily accessible to both Northern and Southern ground-based facilities for follow-up observations.
We investigate possible pathways for the formation of the low density Neptune-mass planet HAT-P-26b. We use two formation different models based on pebbles and planetesimals accretion, and includes gas accretion, disk migration and simple photoevaporation. The models tracks the atmospheric oxygen abundance, in addition to the orbital period, and mass of the forming planets, that we compare to HAT-P-26b. We find that pebbles accretion can explain this planet more naturally than planetesimals accretion that fails completely unless we artificially enhance the disk metallicity significantly. Pebble accretion models can reproduce HAT-P-26b with either a high initial core mass and low amount of envelope enrichment through core erosion or pebbles dissolution, or the opposite, with both scenarios being possible. Assuming a low envelope enrichment factor as expected from convection theory and comparable to the values we can infer from the D/H measurements in Uranus and Neptune, our most probable formation pathway for HAT-P-26b is through pebble accretion starting around 10 AU early in the disks lifetime.
From its discovery, the low density transiting Neptune HAT-P-26b showed a 2.1-sigma detection drift in its spectroscopic data, while photometric data showed a weak curvature in the timing residuals that required further follow-up observations to be confirmed. To investigate this suspected variability, we observed 11 primary transits of HAT-P-26b between March, 2015 and July, 2018. For this, we used the 2.15 meter Jorge Sahade Telescope placed in San Juan, Argentina, and the 1.2 meter STELLA and the 2.5 meter Nordic Optical Telescope, both located in the Canary Islands, Spain. To add upon valuable information on the transmission spectrum of HAT-P-26b, we focused our observations in the R-band only. To contrast the observed timing variability with possible stellar activity, we carried out a photometric follow-up of the host star along three years. We carried out a global fit to the data and determined the individual mid-transit times focusing specifically on the light curves that showed complete transit coverage. Using bibliographic data corresponding to both ground and space-based facilities, plus our new characterized mid-transit times derived from parts-per-thousand precise photometry, we observed indications of transit timing variations in the system, with an amplitude of ~4 minutes and a periodicity of ~270 epochs. The photometric and spectroscopic follow-up observations of this system will be continued in order to rule out any aliasing effects caused by poor sampling and the long-term periodicity.
We report the discovery of HAT-P-14b, a fairly massive transiting extrasolar planet orbiting the moderately bright star GSC 3086-00152 (V = 9.98), with a period of P = 4.627669 +/- 0.000005 days. The transit is close to grazing (impact parameter 0.891 +0.007/-0.008) and has a duration of 0.0912 +/- 0.0017 days, with a reference epoch of mid transit of Tc = 2454875.28938 +/- 0.00047 (BJD). The orbit is slightly eccentric (e = 0.107 +/- 0.013), and the orientation is such that occultations are unlikely to occur. The host star is a slightly evolved mid-F dwarf with a mass of 1.386 +/- 0.045 M(Sun), a radius of 1.468 +/- 0.054 R(Sun) effective temperature 6600 +/- 90 K, and a slightly metal-rich composition corresponding to [Fe/H] = +0.11 +/- 0.08. The planet has a mass of 2.232 +/- 0.059 M(Jup) and a radius of 1.150 +/- 0.052 R(Jup), implying a mean density of 1.82 +/- 0.24 g/cm3. Its radius is well reproduced by theoretical models for the 1.3 Gyr age of the system if the planet has a heavy-element fraction of about 50 M(Earth) (7% of its total mass). The brightness, near-grazing orientation, and other properties of HAT-P-14 make it a favorable transiting system to look for changes in the orbital elements or transit timing variations induced by a possible second planet, and also to place meaningful constraints on the presence of sub-Earth mass or Earth mass exomoons, by monitoring it for transit duration variations.
We present the first comprehensive look at the $0.35-5$ $mu$m transmission spectrum of the warm ($sim 800$ K) Neptune HAT-P-11b derived from thirteen individual transits observed using the Hubble and Spitzer Space Telescopes. Along with the previously published molecular absorption feature in the $1.1-1.7$ $mu$m bandpass, we detect a distinct absorption feature at 1.15 $mu$m and a weak feature at 0.95 $mu$m, indicating the presence of water and/or methane with a combined significance of 4.4 $sigma$. We find that this planets nearly flat optical transmission spectrum and attenuated near-infrared molecular absorption features are best-matched by models incorporating a high-altitude cloud layer. Atmospheric retrievals using the combined $0.35-1.7$ $mu$m HST transmission spectrum yield strong constraints on atmospheric cloud-top pressure and metallicity, but we are unable to match the relatively shallow Spitzer transit depths without under-predicting the strength of the near-infrared molecular absorption bands. HAT-P-11bs HST transmission spectrum is well-matched by predictions from our microphysical cloud models. Both forward models and retrievals indicate that HAT-P-11b most likely has a relatively low atmospheric metallicity ($<4.6 ; Z_{odot}$ and $<86 ; Z_{odot}$ at the $2 sigma$ and $3 sigma$ levels respectively), in contrast to the expected trend based on the solar system planets. Our work also demonstrates that the wide wavelength coverage provided by the addition of the HST STIS data is critical for making these inferences.
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