Small Scale Structure in the ISM towards IC 2391 and NGC 6475

We describe UVES spectroscopic observations towards early-type stars located in the nearby open clusters IC 2391 (D=175 pc) and NGC 6475/M 7 (D=301 pc), with resolution ˜80,000 and S/N ratio per pixel of ˜140 to ˜360. The aim is to investigate the small-scale structure variations within parts of the local ISM and determine how they are dependent on element and ionisation stage observed, thus providing information on cloud parameters such as structure and sizes. The data used are taken from on-line versions of the Paranal Observatory Project (POP: http://www.eso.org/uvespop). A total of 25 early-type stars (A and B-type) are present in our sample towards IC 2391 with 23 towards NGC 6475/M 7, and enable us to probe differences in column densities on scales from ˜0.07--7.3-pc and ˜0.05---4.9-pc (in the respective clusters) for the optical transitions detected (Ti II (3383Å), Ca II (3933Å), Na I (5889, 5895Å) and K I (7698Å)). Towards NGC 6475 the Ca II column density is found to be constant to ˜0.15 dex over scales of ˜0.4--4-pc. A much greater scatter in the measurements for Na I D is observed with differences of up to ˜0.45 dex in sightlines separated by ˜4 pc. In the future work we will perform the same analysis for Ti II and K I towards both clusters.