An array of nano-scale protrusions, called the nipple array, is found on the body surface of various invertebrates, and this structure is believed to decrease light reflectance and water wettability on the surface in the terrestrial environment. However, its potential functions have not been well studied in aquatic environments. Clavelina spp. are colonial ascidians that have the nipple array on their integumentary matrix (i.e. tunic). We examined the physical properties on the surface of the tunic of C. cyclus and C. obesa, such as hardness, wettability and refractive indices, to evaluate the functional importance of this structure. The tunic cuticle of both species was covered with the nipple array, and the cuticle of C. cyclus was bent into folds forming parallel plications. The Clavelina tunic was very soft and had high bubble- and oil-repellency. The refractive-index deviation between the tunic and seawater was 0.07–0.095 for 589-nm light (D-line). Rigorous coupled wave analysis (RCWA) showed that the nipple array slightly reduced reflectance on the surface and the parallel plications reduced the reflectance still more. The nanoimprinted plates imitating the parallel plications have higher bubble repellency and lower reflectance than the flat plates. These findings support the functional importance of the plications as well as the nipple array.

In this proceeding paper, we introduce the recent results of Galactic maser astrometry by mainly focusing on those obtained with Japanese VLBI array VERA. So far we have obtained parallaxes for 86 sources including preliminary results, and combination with the data obtained with VLBA/BeSSeL provides astrometric results for 159 sources. With these most updated results we conduct preliminary determinations of Galactic fundamental parameters, obtaining R0 = 8.16 ± 0.26 kpc and Θ0 = 237 ± 8 km/s. We also derive the rotation curve of the Milky Way Galaxy and confirm the previous results that the rotation curve is fairly flat between 5 kpc and 16 kpc, while a remarkable deviation is seen toward the Galactic center region. In addition to the results on the Galactic structure, we also present brief overviews on other science topics related to masers conducted with VERA, and also discuss the future prospect of the project.

We have conducted astrometric observations toward a 22 GHz water maser source associated with the Sgr B2 complex in the Galactic center region with VERA (VLBI exploration of Radio Astrometry). We measured a trigonometric parallax and absolute proper motion of the Sgr B2 complex with respect to an extra-galactic source by observing the water maser source at 10 epochs from 2014 to 2017. The measured distance was 7.52+3.01−1.67 kpc for the Sgr B2M region.

We also succeeded to measure internal motions of maser spots in Sgr B2M, and N region. The number of spots which we could measure the internal motions is about 400. The distribution of the maser spots shows that the maser spots are associated with envelope of HII region seen in radio continuum image obtained with VLA and ALMA. We discuss relative motions between Sgr B2M, and N by using the internal motion.

We have detected [C I] 3P1–3P0 emissions in the gaseous debris disks of 49 Ceti and β Pictoris with the 10 m telescope of the Atacama Submillimeter Telescope Experiment, which is the first detection of such emissions. The line profiles of [C I] are found to resemble those of CO(J=3–2) observed with the same telescope and the Atacama Large Millimeter/submillimeter Array. This result suggests that atomic carbon (C) coexists with CO in the debris disks, and is likely formed by the photodissociation of CO. Assuming an optically thin [C I] emission with the excitation temperature ranging from 30 to 100 K, the column density of C is evaluated to be (2.2 ± 0.2) × 1017 and (2.5 ± 0.7) × 1016 cm−2 for 49 Ceti and β Pictoris, respectively. The C/CO column density ratio is thus derived to be 54 ± 19 and 69 ± 42 for 49 Ceti and β Pictoris, respectively. These ratios are higher than those of molecular clouds and diffuse clouds by an order of magnitude. The unusually high ratios of C to CO are likely attributed to a lack of H2 molecules needed to reproduce CO molecules efficiently from C. This result implies a small number of H2 molecules in the gas disk; i.e., there is an appreciable contribution of secondary gas from dust grains.

The Galactic centre region shows outstanding non-circular motion unlike the Galactic disk. As scenarios describing this non-circular motion, resonance orbits formed by the Galactic bar potential or expanding motion by past activity of the central BH are proposed. However, these both scenarios are based on line-of-sight velocities of molecular clouds in this region, and such one-dimension velocity information is insufficient to separate these scenarios.

To reveal dynamics of the Galactic centre region, we conducted astromertic observations of 22 GHz water maser sources toward the Galactic centre direction. We conducted astrometric observations toward water maser source associated with Sgr D HII region. As a result, we succeeded to measure the parallax and proper motion of the maser source. The measured distance was 2.36(+0.58/-0.39) kpc. This result clearly indicates that this source is not associated with the Galactic centre, but located on the Galactic disk.

The salp Thalia rhomboides is distributed in the shallow depths during the day, and is thus at increased risk of predation by visual predators and from the damaging effects of ultraviolet (UV) radiation compared with species distributed in deeper layers in the daytime. The integument (tunic) of T. rhomboides may have adaptive optical properties, but the absorption spectra of the unfixed tunic demonstrate that the tunic transmits UV as well as visible light, indicating that the tunic is not an effective barrier against UV radiation. Ultrastructural observation revealed that the surface of the tunic cuticle is covered in a nipple array consisting of hemispherical protuberances approximately 40 and 30 nm in diameter in solitary and aggregate zooids, respectively. Simulation of light reflection of a nipple array using rigorous coupled wave analysis (RCWA) indicated that the reflection is slightly lower for the nipple array than for a flat surface at high angles of incidence (θ > 80°). This result supports the idea that the nipple array serves to make the salp less visible. The simulation also indicated that the height and distribution of the salp nipple array do not have an optimal structure for causing an antireflection effect. A mechanical restriction might exist on the structures, and the nipple array could also serve another function. The size and distribution of nipples may be controlled by the need to meet the complex requirements of multiple essential functions.