Ionization Sources of the Local Interstellar Clouds: Two B-stars, Three White Dwarfs, and the Local Hot Bubble

TL;DR

This work addresses how extreme-ultraviolet (EUV) photoionization from nearby sources shapes the Local Interstellar Cloud (LIC). It combines updated β CMa parameters from non-LTE atmospheres with EUV attenuation calculations using $N_{HI}=(1.9±0.1)×10^{18}$ cm$^{-2}$ to derive H I and He I ionization rates at the LIC boundary, then solves coupled ionization balance for H and He in a constant-density cloud irradiated by five EUV sources plus Local Hot Bubble (LHB) emission. The results yield surface ionization fractions $x≈0.28$ and $y≈0.50$ with $n_H≈0.30$ cm$^{-3}$ and $n_e≈0.33$ cm$^{-3}$, and show that LHB emission can provide substantial ionizing flux ($Φ_H≈7–9×10^{3}$ cm$^{-2}$ s$^{-1}$ for solar abundances, or $2–4×10^{3}$ cm$^{-2}$ s$^{-1}$ with metal depletion), while past SN-driven cavity conditions and a hot wake further influence the current LIC state. The paper also highlights that β CMa and ε CMa were within ~10 pc of the Sun ~4.4 Myr ago, implying ionization histories with 100–200× flux enhancements and non-equilibrium ionization developing as clouds traverse the local tunnel. Overall, the study constrains LIC ionization structure and demonstrates the interplay between stellar EUV sources and hot-gas emission in shaping the local interstellar environment.

Abstract

The dominant sources of photoionizing radiation in the extreme ultraviolet (EUV) incident on the exterior of the local interstellar clouds include two nearby early B-type stars, $ε$ CMa ($124\pm2$ pc) and $β$ CMa ($151\pm5$ pc), three hot dwarfs, and the local hot bubble (LHB). Line emission (170-912A) from highly ionized metals (Fe, Ne, Mg) in million-degree LHB plasma may be responsible for the elevated ionization fractions of helium ($n_{\rm HeII}/n_{\rm He} \approx 0.4$) compared to hydrogen ($n_{\rm HII} / n_{\rm H} \approx 0.2$) in the local clouds. We update the stellar parameters and ionizing flux for $β$ CMa, after correcting the EUV spectra for intervening HI column density, $N_{\rm HI} = 1.9\pm0.1\times10^{18}~{\rm cm}^{-2}$, and its hotter effective temperature, $T_{\rm eff} \approx 25,000$K vs. 21,000K for $ε$ CMa. These two stars produce a combined H-ionizing photon flux $Φ_{\rm H} \approx 6800\pm1400$ cm$^{-2}$ s$^{-1}$ at the external surface of the local clouds. The hot bubble could produce comparable fluxes, $Φ_{\rm H} =$ 2000-9000 cm$^{-2}$ s$^{-1}$, depending on the amount of metal depletion into dust grains that survive sputtering. The radial velocities and proper motions of $β$ CMa and $ε$ CMa indicate that both stars passed within $10\pm1$ pc of the Sun $4.4\pm0.1$ Myr ago, with 100-200 times higher local ionizing fluxes. At that time, the local clouds were likely farther from the Sun, owing to their transverse motion. Over the last few Myr, EUV radiation from these two stars left a wake of highly ionized gas in a hot, low-density cavity produced by past supernova explosions in the Sco-Cen OB association and connected with the LHB.

Figures (6)

• Figure 1: The location of $\beta$ CMa on the Hertzsprung-Russell diagram is shown for our derived parameters, $\log (L/L_{\odot}) = 4.41\pm0.06$ and $T_{\rm eff} = 25,180\pm1120$ K, based on new radius $R = 8.44\pm0.56~R_{\odot}$ and parallax distance $d = 151\pm5$ pc. The evolutionary tracks are from L. Brott et al. (2011) with Milky Way metallicities and initial masses labeled from 7--30 $M_{\odot}$. The location of $\epsilon$ CMa (J. M. Shull et al. 2025) is shown for comparison.
• Figure 2: Far-UV and EUV spectra for $\beta$ CMa from a model atmosphere computed with the non-LTE line-blanketed code WM-basic for effective temperature $T_{\rm eff} = 25,000$ K and surface gravity $\log g = 3.70$. (Left) Flux distribution $\log F_{\lambda}$ from 500--1200 Å, showing the Lyman limit decrement at 912 Å. (Right) Flux distribution from 228 Å to 550 Å. The absence of an edge at the He1 ionization limit (504 Å) is a result of non-LTE effects from backwarming of the upper atmosphere from a wind in early B-type stars.
• Figure 3: Locations of the five stars (yellow circles) that dominate the ionization of the local clouds, shown in Galactic coordinates ($\ell$, $b$) centered on (0, 0). Locations are plotted on the All-sky New Horizons Alice Ly$\alpha$ map (G. R. Gladstone et al. 2025) taken at 57 AU from the Sun. Wavy lines indicate the outlines of four of the important local interstellar clouds (LIC in red; Aql in green; Blue in blue; and G in tan). The map also indicates locations of the Sun (right edge), north and south ecliptic poles (NEP, SEP), notable stars and galaxies, and the "upstream and downstream" directions of flow of interstellar H1 through the solar system and interplanetary medium (IPM). Black dots show the $\sim90,000$ stars in the M. A. Velez et al. (2024) catalog of potential sources of far-UV emission.
• Figure 4: Model of constant density cloud ($n_{\rm H} = 0.2$ cm$^{-3}$, $T = 7000$ K) using ionizing fluxes from all five stellar sources (two B-stars, three white dwarfs) but not the hot bubble. Fluxes are attenuated by mean column densities $N_{\rm HI} = 10^{18}~{\rm cm}^{-2}$ and $N_{\rm HeI} = 10^{17}~{\rm cm}^{-2}$. Spectra and ionization fractions are shown at various depths into the cloud, with distance intervals from step-0 (external surface) to step-20 (entering heliosphere) and step length $\Delta L = 5\times10^{17}$ cm. (Top) Attenuated spectra with depth into the local cloud, with the bottom spectrum (black) showing the attenuated flux seen at Earth. (Bottom) Ionization fractions of H and He with depth.
• Figure 5: Modeled spectral distribution of the flux, $\log F_{\lambda}$ (in erg cm$^{-2}~{\rm s}^{-1}~{\rm \AA}^{-1}$), of ionizing (EUV) photons produced in the Local Hot Bubble, calculated using the Chianti code for plasma at three temperatures. We assume a constant electron density $n_e = 0.004~{\rm cm}^{-3}$, with EUV emissivity integrated out to a bubble radius $R = 85$ pc from the Sun. The emissivities of Fe and Ne ions decline with temperature over the range $\log T = 5.9, 6.0, 6.1$, with color-coded fluxes (blue, orange, green) as labeled in box. Prominent EUV emission lines are noted. (Top panel) Fluxes with solar metal abundances including [Fe/H] $= 6.46 \pm 0.04$ (M. Asplund et al. 2021). (Bottom panel) Fluxes with refractory elements (Fe, Mg, Si) reduced in abundance by a factor of 5, owing to depletion into dust grains. Neon is not depleted, and its lines remain strong. The underlying continuum is from bremsstrahlung and radiative recombination.