The interstellar medium towards the Ara OB1 region

Abstract

We present high resolution (R∼4 km s⁻¹) absorption measurements of the interstellar NaI and CaII lines measured towards 14 early-type stars of distance 123 pc–1650 pc, located in the direction of the Ara OB1 stellar cluster. The line profiles can broadly be split into four distinct groupings of absorption component velocity, and we have attempted to identify an origin and distance to each of these interstellar features. For gas with absorption covering the velocity range −10 km s⁻¹ < V _helio<+10 km s⁻¹, we can identify the absorbing medium with local gas belonging to the Lupus-Norma interstellar cavity located between 100 and 485 pc in this galactic direction. Gas with velocities spanning the range −20 km s⁻¹<V _helio<+20 km s⁻¹ is detected towards stars with distances of 570–800 pc. We identify a wide-spread interstellar feature at V _helio∼−15 km s⁻¹ with the expanding HI shell called GSH 337+00–05, which is now placed at a distance of ∼530 pc.

Gas with velocities in the range −45 km s⁻¹<V _helio<−25 km s⁻¹ is only detected towards stars with distances >1 kpc, which places this absorbing gas at a similar distance to that of the Ara OB1 stellar cluster. The similarity between the radial velocity of the stellar cluster and that of the absorbing gas suggests that the cluster stars may be presently embedded within, and traveling through, this interstellar cloud complex. Absorption at velocities <−50 km s⁻¹ is seen towards only two stars, HD 154873 and HD 150958, both of which have distances >1300 pc. Although the NaI/CaII for this component is consistent with the presence of interstellar shocks, we believe that it is unlikely that this highly negative velocity gas is physically associated with the Ara OB1 cluster itself, and its origin may be linked to the more distant (and larger) Sco OB1 cluster.