Local Interstellar Matter : The Apex Cloud Priscilla

Several nearby individual low column density interstellar cloudlets have been identified based on kinematical features evident in high-resolution Ca observations near the Sun. One of these cloudlets, the ‘Apex Cloud” (AC), is within 5 pc of the Sun in the solar apex direction. The question of which interstellar cloud will constitute the next galactic environment of the Sun in principle can be determined from cloudlet velocities. The velocity of the ISM towards the nearest star α Cen is consistent with the projected AC velocity in this direction. However, because of measurement uncertainties the α Cen cloud velocity is also consistent with the projected velocities of the the “G” cloud (GC) and the cloud inside of the solar system (Local Interstellar Cloud, LIC). The high GC column density towards α Oph compared to α Aql suggests part of the GC might be behind α Aql, indicating that the AC cloud is closer to the Sun than the GC (since both are seen in α Aql), which favors the AC as the next cloud to be encoutered by the Sun. However, the very weak feature at the AC velocity towards 36 Oph suggests that the AC cloud must be patchy. If the AC cloud is adjacent to the LIC, the velocity difference between these two clouds is about the LIC isothermal sound velocity, suggesting a shock will form where the clouds collide. The Sun moves ∼7 pc per million years with respect to the AC, and should enter it winin ∼200,000 years. A component group at the LIC velocity in the first galactic quadrant, at the LIC velocity, may sample the tangential region of the Centaurus-Lupus shell of the Loop I superbubble. Subject headings: ISM: clouds, structure