The amount of ultraviolet irradiation and ablation experienced by a planet depends strongly on the temperature of its host star. Of the thousands of extrasolar planets now known, only six have been found that transit hot, A-type stars (with temperatures of 7,300-10,000 kelvin), and no planets are known to transit the even hotter B-type stars. For example, WASP-33 is an A-type star with a temperature of about 7,430 kelvin, which hosts the hottest known transiting planet, WASP-33b (ref. 1); the planet is itself as hot as a red dwarf star of type M (ref. 2). WASP-33b displays a large heat differential between its dayside and nightside, and is highly inflated-traits that have been linked to high insolation. However, even at the temperature of its dayside, its atmosphere probably resembles the molecule-dominated atmospheres of other planets and, given the level of ultraviolet irradiation it experiences, its atmosphere is unlikely to be substantially ablated over the lifetime of its star. Here we report observations of the bright star HD 195689 (also known as KELT-9), which reveal a close-in (orbital period of about 1.48 days) transiting giant planet, KELT-9b. At approximately 10,170 kelvin, the host star is at the dividing line between stars of type A and B, and we measure the dayside temperature of KELT-9b to be about 4,600 kelvin. This is as hot as stars of stellar type K4 (ref. 5). The molecules in K stars are entirely dissociated, and so the primary sources of opacity in the dayside atmosphere of KELT-9b are probably atomic metals. Furthermore, KELT-9b receives 700 times more extreme-ultraviolet radiation (that is, with wavelengths shorter than 91.2 nanometres) than WASP-33b, leading to a predicted range of mass-loss rates that could leave the planet largely stripped of its envelope during the main-sequence lifetime of the host star.

We report the first detection of the 205 µm 3 P 1 → 3 P 0 [NII] line from a ground-based observatory using a direct detection spectrometer. The line was detected from the Carina star formation region using the South Pole Imaging Fabry-Perot Interferometer (SPIFI) on the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) at South Pole. The [NII] 205 µm line strength indicates a low-density (n ∼ 32 cm −3 ) ionized medium, similar to the low-density ionized halo reported previously in its [OIII] 52 and 88 µm line emission. When compared with the ISO [CII] observations of this region, we find that 27% of the [CII] line emission arises from this low-density ionized gas, but the large majority (∼ 73%) of the observed [CII] line emission arises from the neutral interstellar medium. This result supports and underpins prior conclusions that most of the observed [CII] 158 µm line emission from Galactic and extragalactic sources arises from the warm, dense photodissociated surfaces of molecular clouds. The detection of the [NII] line demonstrates the utility of Antarctic sites for THz spectroscopy.

We report the detection of 158 μm [C ii] fine-structure line emission from MIPS J142824.0+352619, a hyperluminous (L IR ∼ 10 13 L ) starburst galaxy at z = 1.3. The line is bright, corresponding to a fraction−3 of the far-IR (FIR) continuum. The [C ii], CO, and FIR continuum emission may be modeled as arising from photodissociation regions (PDRs) that have a characteristic gas density of n ∼ 10 4.2 cm −3 , and that are illuminated by a far-UV radiation field ∼10 3.2 times more intense than the local interstellar radiation field. The mass in these PDRs accounts for approximately half of the molecular gas mass in this galaxy. The L [C ii] /L FIR ratio is higher than observed in local ultraluminous infrared galaxies or in the few high-redshift QSOs detected in [C ii], but theand L CO /L FIR ratios are similar to the values seen in nearby starburst galaxies. This suggests that MIPS J142824.0+352619 is a scaled-up version of a starburst nucleus, with the burst extended over several kiloparsecs.

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