GASP XVII. HI imaging of the jellyfish galaxy JO206: gas stripping and enhanced star formation.

We present VLA HI observations of JO206, a prototypical ram-pressurestripped galaxy in the GASP sample. This massive galaxy (M$_{\ast} =$ 8.5 $\times$ 10$^{10}$ M$_{\odot}$) is located at a redshift of $z =$ 0.0513, near the centre of the low-mass galaxy cluster, IIZw108 ($\sigma \sim575$ km/s). JO206 is characterised by a long tail($\geq$90 kpc) of ionised gas stripped away by ram-pressure. We find a similarly long HI tail in the same direction as the ionised gas tail and measure a total HI mass of $3.2 \times 10^{9}$ M$_{\odot}$. This is about half the expected HI mass given the stellar mass and surface density of JO206. A total of $1.8 \times 10^{9}$ M$_{\odot}$ (60%) of the detected HI is in the gas stripped tail. An analysis of the star formationrate shows that the galaxy is forming more stars compared to galaxies with the same stellar and HI mass. On average we find a HI gas depletiontime of $\sim$0.5 Gyr which is about four times shorter than that of "normal" spiral galaxies. We performed a spatially resolved analysis of the relation between star formationrate density and gas density in the disc and tail of the galaxy at the resolution of our HI data. The star formationefficiency of the disc is about 10 times higher than that of the tail at fixed HI surface densities. Both the inner and outer parts of JO206 show an enhanced star formationcompared to regions of similar HI surface density in field galaxies. The enhanced star formationis due to ram-pressurestripping during the galaxy's first infallinto the cluster.