Saturday, May 9, 2015

Metal-Rich Sun-Like Star Hosting a Dense Hot-Jupiter

HATS-4b is a dense gas giant plane in a close-in orbit around a metal-rich Sun-like star. A star is mostly made up of hydrogen and helium. In astrophysics, the term “metal” refers to all elements heavier than hydrogen and helium. Consequently, the metallicity of a star refers to the proportion of its matter that is comprised of elements other than hydrogen and helium. The host star of HATS-4b has a very high metallicity, well over twice the Sun’s metallicity. The star is very rich in elements heavier than hydrogen and helium. In fact, its metallicity is beyond the 99th percentile of all planet hosting stars known to date.

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

Transit observations together with follow-up radial velocity observations indicate that HATS-4b has 1.02 times the diameter and 1.32 times the mass of Jupiter. This gives HATS-4b a relatively high mean density of 1.55 g/cm³. For comparison, Jupiter has a mean density of 1.33 g/cm³. Transit observations (i.e. measuring the fraction of starlight the planet blocks when it passes in front of its host star) allow the planet’s size to be determined, while radial velocity observations (i.e. measuring the amount of gravitational tugging the planet exerts on its host star) allow the planet’s mass to be determined.

Among planets with masses between one to two times the mass of Jupiter, HATS-4b is one of the densest known. Its high density suggests that it has a heavy element content of around 75 times the mass of Earth. For comparison, gas giant planets around stars with similar metallicities as the Sun tend to have only 10 to 15 Earth-masses of heavy elements. The high heavy element content of HATS-4b is consistant with the correlation that gas giant planets around metal-rich stars tend to have higher amounts of heavy elements. HATS-4b orbits its host star at a distance of 0.036 AU, with an orbital period of 2.5 days. The intense stellar irradiation HATS-4b receives due to its proximity to its host star heats its dayside up to an estimated temperature of about 1,300 K. Therefore, HATS-4b is termed a “hot-Jupiter”.

Figure 2: Transit light curve of HATS-4b. The solid line shows the best-fit transit model. Jordan et al. (2014).

Jordan et al. (2014), “HATS-4b: A Dense Hot Jupiter Transiting a Super Metal-rich G star”, The Astronomical Journal 148 29