What lies immediately outside of the heliosphere in the very local interstellar medium (VLISM): morphology of the Local Interstellar Cloud, its hydrogen hole, Stromgren Shells, and 60Fe accretion

Abstract

<p> We describe the very local interstellar medium (VLISM)<br>immediately outside of the outer heliosphere. The VLISM consists <br>of four partially ionized clouds - the Local Interstellar Cloud (LIC), <br>G cloud, Blue cloud, and Aql cloud that are in contact with the outer <br>heliosphere, and ionized gas produced by extreme-UV radiation <br>primarily from the star Epsilon CMa. We construct the <br>three-dimensional shape of the LIC based on interstellar line <br>absorption along 62 sightlines and show that in the direction of <br>Epsilon CMa, Beta CMa, and Sirius B the neutral hydrogen column <br>density from the center of the LIC looking outward is a minimum. <br>We call this region the ``hydrogen hole''. In this direction, the <br>presence of Blue cloud absorption and the absence of LIC absorption <br>can be simply explained by the Blue cloud lying just outside of the <br>heliosphere. We propose that the outer edge of the Blue cloud is a <br>Str\"omgren shell driven toward the heliosphere by high pressures in <br>the H II region. The outer edges of other clouds facing Epsilon CMa <br>are likely also Stromgren shells. Unlike previous models, the LIC<br>surrounds less than half of the heliosphere.</p><p>We find that the vectors of neutral and ionized helium flowing<br>through the heliosphere are inconsistent with the mean LIC flow <br>vector and describe several possible explanations. The ionization<br>of nearby intercloud gas is consistent with photo-ionization by <br>Epsilon CMa and hot white dwarfs without requiring additional <br>sources of ionization or million degree plasma. In the upwind <br>direction, the heliosphere is passing through an environment of <br>several LISM clouds, which may explain the recent influx of <br>interstellar grains containing 60Fe from supernova ejecta measured <br>in Antarctica snow. The Sun will leave the outer partof the LIC <br>in less than 1900 years, perhaps this year, to either enter the <br>partially ionized G cloud or a highly ionized intercloud layer. <br>The heliosphere will change in either scenario. An instrumented <br>deep space probe sending back in situ plasma and magnetic field <br>measurements from 500-1,000 AU is needed to understand the <br>heliosphere environment rather than integrated data along the <br>sightlines to stars.  </p>