Pressure Equilibrium Between the Local Interstellar Clouds and the Local Hot Bubble

Abstract

Three recent results related to the heliosphere and the local interstellar medium (ISM) have provided an improved insight into the distribution and conditions of material in the solar neighborhood. These are the measurement of the magnetic field outside of the heliosphere by Voyager 1, the improved mapping of the three-dimensional structure of neutral material surrounding the Local Cavity using extensive ISM absorption line and reddening data, and a sounding rocket flight which observed the heliospheric helium focusing cone in X-rays and provided a robust estimate of the contribution of solar wind charge exchange emission to the ROSAT All-Sky Survey 1/4 keV band data. Combining these disparate results, we show that the thermal pressure of the plasma in the Local Hot Bubble (LHB) is P/k = 10,700 cm −3 K. If the LHB is relatively free of a global magnetic field, it can easily be in pressure (thermal plus magnetic field) equilibrium with the local interstellar clouds, eliminating a long-standing discrepancy in models of the local ISM.

Hubble Space Telescope Constraints on the Winds and Astrospheres of Red Giant Stars*

“…The astrospheric absorption features are therefore potentially useful as ISM pressure diagnostics. The ISM pressure that we are assuming in the γ Eri model (P/k B = 5000 cm −3 K) is consistent with measurements of pressures of neutral clouds within the LB (Jenkins 2002), but it may be a little low based on recent assessments of hot plasma within the LB, which suggest P/k B = 10, 700 cm −3 K (Snowden et al 2014). Thus, it is possible that an underestimate of the ISM pressure could be part of the explanation for the underestimate of astrospheric absorption.…”

“…We verify that the stars are within the LB using the LB maps of Lallement et al (2003). Most of the volume of the LB is believed to be very hot, ionized plasma, which produces much of the soft X-ray background emission (Snowden et al 1998(Snowden et al , 2014. The flow vector of the LB material is uncertain, but a plausible assumption is that it is roughly at rest relative to the Local Standard of Rest (LSR) (e.g., Dehnen & Binney 1998).…”

Discovery of a Very Hot Phase of the Milky Way Circumgalactic Medium with Non-solar Abundance Ratios

“…Nonetheless, we investigated whether dense structures such as supernovae remnants (SNR) or superbubbles made by expanding and merging SNR are responsible for the observed hot gas phase: a) There is no evidence of an individual SNR along our sightline, but the local hot bubble (LHB) is present. With a density of ≈ 3.9 ± 0.4 × 10 −3 cm −3 and path length of ≈ 100 pc (Snowden et al 2014), the LHB column density is 1.4×10 18 cm −2 , smaller than the column density of the hot gas by order(s) of magnitude. Thus the LHB contribution to the observed column density is negligible.…”

Studying the Interstellar Medium and the Inner Region of NPS/Loop 1 With Shadow Observations Toward Mbm36

“…Assuming that the SB has a uniform density equal to the LHB density (n e =4.7×10 −3 cm −3 from Snowden et al 2014) and the EM computed with the fit, we estimate that the path through the SB along the MBM36 line of sight is 450 180 50 -+ pc. This path corresponds to a spherical bubble of radius r≈150 pc, with the center located at a distance d≈280 pc away from the solar system (due to the linear relations involved in the computation, distances, and radii have errors +10%, −25%).…”