Tuesday, September 30, 2014

Hot Giant Planet that is Blacker than Coal

Gandolfi et al. (2014) report on the discovery of a half-Jupiter mass planet transiting an old Sun-like star every 2.7 days. This discovery combines data collected by NASA’s Kepler space telescope from 13 May 2009 to 11 May 2013 with spectroscopic follow-up observations performed with the FIES spectrograph at the Nordic Optical Telescope in La Palma, Spain. Photometric data from Kepler indicates how much starlight is blocked when the planet transits in front of its host star, allowing the size of the planet to be estimated. The FIES spectrograph measures the amount of gravitational tugging the planet has on its host star and provides the estimated mass of the planet.

Figure 1: Artists’ illustration of a hot-Jupiter orbiting a Sun-like star. Image credit: Haven Giguere & Nikku Madhusudhan.

Figure 2: Phase-folded transit light curve of KOI-183b showing the best fitting model and residuals. Gandolfi et al. (2014).

Figure 3: Radial velocity data from the FIES spectrograph with the median, 68th and 99th percentile limits. Gandolfi et al. (2014).

The planet, identified as KOI-183b, is estimated to have 0.595 ± 0.081 times the mass of Jupiter and 1.192 ± 0.052 times the radius of Jupiter. Given the mass and size of the planet, its bulk density is 0.459 ± 0.083 g/cm³. KOI-183b orbits its host star at a distance of only ~1/28th the Earth-Sun distance. As a result, KOI-183b is intensely heated and is classified as a hot-Jupiter. The radius of KOI-183b is consistent with theoretical models for heavily irradiated coreless gas-giant planets. Being so near to its host star, temperatures on KOI-183b can reach ~2000 K, hot enough to melt titanium metal.

Data from Kepler also indicates that KOI-183b periodically passes behind its host star in what is known as a secondary eclipse. The secondary eclipse signal has a depth of 14.2 ± 6.6 ppm. From the depth of its secondary eclipse signal, KOI-183b is estimated to have a very low Bond albedo of only 0.037 ± 0.019, making it one of the “darkest” gas-giant planets known so far. Basically, KOI-183b reflects only ~4 percent of the incoming radiation from its host star back into space. For comparison, that is darker than coal. Other hot-Jupiters with similarly low Bond albedos include TrES-2b and Kepler-77b.

Gandolfi et al. (2014), “KOI-183b: a half-Jupiter mass planet transiting a very old solar-like star”, arXiv:1409.8245 [astro-ph.EP]

Monday, September 29, 2014

A Highly Eccentric Brown Dwarf around a Giant Star

To date, ~10 brown dwarfs are known around giant stars (i.e. evolved stars). Brown dwarf are objects more massive than planets, but are not massive enough to count as full-fledged stars. M. I. Jones et al. (2014) report on the discovery of a brown dwarf on a highly eccentric orbit around the giant star HIP 97233. The brown dwarf, identified as HIP 97233 b, has an orbital period of 1058.8 days and a minimum mass of 20 times the mass of Jupiter.

With an orbital eccentricity of 0.61, HIP 97233 b is the brown dwarf with the most eccentric orbit known around a giant star. The mass and orbit of HIP 97233 b were both determined from the gravitational “tugging” it exerts on its host star which was observed in the form of a radial velocity signature (i.e. Doppler shifts in the star’s spectral lines).

Figure 1: Artist’s impression of a giant planet.

 Figure 2: Upper panel: Radial velocity curve for the host star of HIP 97233 b. Lower panel: Residuals from the best fit. M. I. Jones et al. (2014).

HIP 97233 b highly eccentric orbit takes it from as near as 1.0 AU to as far as 4.1 AU from its host star. M. I. Jones et al. (2014) estimate that the host star of HIP 97233 b has 1.84 ± 0.14 times the Sun’s mass and 5.20 ± 0.50 times the Sun’s radius. The host star of HIP 97233 b is considerably larger and more luminous than the Sun. At closest approach, the dayside of HIP 97233 b receives roughly 16 times the intensity of insolation as Earth receives from the Sun.

There are a number of ways through which an object like HIP 97233 b can form. Firstly, the host star of HIP 97233 b is much more massive than the Sun, enabling it to have a more massive protoplanetary disk which can allow massive planets and brown dwarfs to form more efficiently. Also, as the star evolves and swells in size, it begins to blow an enhanced stellar wind from which a giant planet can accrete a significant amount of mass and grow in mass till it reaches the brown dwarf mass regime.

The star’s high metallicity might also have enabled HIP 97233 b to form by core accretion, believed to be the main mechanism through which planets form. Finally, interaction with the protoplanetary disk before it was dissipated or with the star’s outer layers as it evolves to a giant star might have caused HIP 97233 b to migrate inward from beyond ~4 AU to where it currently is.

M. I. Jones et al. (2014), “A planetary system and a highly eccentric brown dwarf around the giant stars HIP 67851 and HIP 97233”, arXiv:1409.7429 [astro-ph.EP]