Heating the Local Interstellar Cloud

The Local Interstellar Cloud (LIC) is the region of the interstellar medium (ISM) that surrounds and helps to shape the heliosphere. The LIC is part of a collection of nearby low density warm clouds known as the Complex of Local Interstellar Clouds (CLIC), all of which exist inside the hot Local Bubble. Observations of interstellar neutral He atoms flowing into the heliosphere by the IBEX mission and Voyager have constrained the temperature of the LIC to be roughly 7500 K. This temperature is consistent with that derived from absorption line measurements toward nearby stars. Such observations also indicate that the LIC is partially ionized with elemental abundances consistent with a moderate level of depletion onto dust grains as might occur for low density ISM that has been subject to a shock that partially destroyed the dust. The temperature of the cloud is not unusual for the warm ionized medium in the ISM, but it is less ionized than typical. We discuss the various processes that may be important for heating the LIC. We show that the only viable heat source for the ongoing heating of the CLIC is photoionization. Equilibrium models of the ionization and heating of the LIC allow for solutions that match the observations, but the likely origins of the local interstellar medium suggest that the situation is more complex. We propose an evolution scenario in which the clouds were formerly cold and were heated by shocks to reach their current warm state. We present new magneto-hydrodynamical calculations of the evolution of the local ISM. Multiple supernova models can match the parameters of the Local Bubble and heat the cold clouds as desired, though the many constraints on the clouds and bubble have yet to be fully satisfied by the models.