Tuesday, November 12, 2013

Heat Redistribution on a Strongly Irradiated Brown Dwarf

KELT-1b, a brown dwarf with 27 times the mass of Jupiter, circles around an F-type star in a close-in 1.2-day orbit. The tight orbit places KELT-1b in a highly irradiated environment, where the incident radiation it receives from its parent star is 5,800 times more intense than what Earth gets from the Sun. Although the radiation environment of KELT-1b is similar to that for hot Jupiters, KELT-1b is different due to it large mass which places it in the brown dwarf regime. With several Jupiter masses packed into a volume that is only slightly larger than Jupiter’s, the surface gravity on KELT-1b is a whopping 115 times the surface gravity on Earth. In a way, KELT-1b can be perceived as a “hot Jupiter” with a very high surface gravity.

Artist’s Impression of a hot Jupiter. Credit: NASA.

Observations of KELT-1b using the Spitzer space telescope show that the amount of heat redistribution from its day side to its night side is very low. This is because KELT-1b quickly radiates the energy it receives from its parent star back into space before it is transported to the night side. As a consequence, KELT-1b has a very hot day night and a much cooler night side. The day side is estimated to have temperatures as high as 3,100 K. As a brown dwarf, KELT-1b is unusual due to the huge amount of insolation it receives from its parent star. If KELT-1b were an isolated brown dwarf, it would have a temperature of about 700 K.

The day side of KELT-1b is so hot that it is above the ~2,000 K condensation temperature of titanium oxide (TiO). This can cause a day-night cold trap for TiO since the night side of KELT-1b is cool enough for TiO to condense and settle out of the atmosphere. In fact, the lack of a strong TiO signal indicates that a day-night cold trap may exist in KELT-1b’s atmosphere. Because gaseous TiO is a strong absorber of optical radiation, its presence in an atmosphere can cause a temperature inversion (i.e. temperature increases with altitude). Therefore, the depletion of TiO due to a day-night cold trap inhibits the presence of a temperature inversion.

KELT-1b was discovered using the using the Kilodegree Extremely Little Telescope (KELT) in southern Arizona. KELT is a small telescope optimized for imaging bright stars. The telescope images of tens of thousands of stars every night in an attempt to detect planets that happen to pass in front of the star that they are orbiting. The discovery of KELT-1b was announced in a paper published in June 2012.

- Beatty et al. (2013), “Spitzer and z' Secondary Eclipse Observations of the Highly Irradiated Transiting Brown Dwarf KELT-1b”, arXiv:1310.7585 [astro-ph.EP]
- Siverd et al. (2012), “KELT-1b: A Strongly Irradiated, Highly Inflated, Short Period, 27 Jupiter-mass Companion Transiting a mid-F Star”, arXiv:1206.1635 [astro-ph.EP]