Evidence of Enhanced Ionization in Protostellar Envelopes

Abstract

Ionization is a major driver of both physical and chemical evolution in protostellar systems. Recent observations reveal substantial chemical processing in protoplanetary disks by the time the surrounding envelope has cleared. Thus, physical conditions during the preceeding phase, when an infalling envelope of material is still present, are crucial for determining the extent of chemical processing at early stages. We used observations of H13CO+ and C18O from the Northern Extended Millimeter Array (NOEMA) and IRAM 30m telescope to constrain the ionization rate in the envelopes of three Class 0 protostars: NGC-1333 IRAS4A, L1448-C, and L1157. We find ionization rates in the range zeta = 1e-16 - 1e-13 s$^{-1}$ , several orders of magnitude above the ionization rate of zeta = 6e-17 s$^{-1}$ in the diffuse interstellar medium. This supports the idea that ionization driven chemistry is more efficient at earlier stages (< 1e5 years) of protostellar evolution.

Figures (6)

• Figure 1: Combined NOEMA+IRAM 30m observations of $\mathrm{H^{13}CO^+}$ 1-0 (background), and C$^{18}$O 2-1 (contours, convolved to the resolution of the $\mathrm{H^{13}CO^+}$) envelope emission toward three protostars: NGC 1333-IRAS4A (left), L1157 (center), and L1448-C (right). Contours start at $3\sigma$ and have $6\sigma$ spacing for IRAS4A and $3\sigma$ spacing for the other sources. The grey dashed lines are perpendicular to the small scale outflows. The grey ellipse in the bottom left corner of each panel show the synthesized beam of the $\mathrm{H^{13}CO^+}$ 1-0.
• Figure 2: $\mathrm{H^{13}CO^+}$ 1-0 (blue) and C$^{18}$O 2-1 (black, convolved to the spatial resolution of $\mathrm{H^{13}CO^+}$) spectra toward NGC 1333-IRAS4A. The purple line shows the best Gaussian fit to the C$^{18}$O line correcting for absorption. The spectra in each panel are offset by half $\theta_{maj}$ for the $\mathrm{H^{13}CO^+}$ observations.
• Figure 3: $\mathrm{H^{13}CO^+}$ 1-0 (blue) and C$^{18}$O 2-1 (black, convolved to the spatial resolution of $\mathrm{H^{13}CO^+}$) spectra toward L1448-C. The spectra in each panel are offset by half $\theta_{maj}$ for the $\mathrm{H^{13}CO^+}$ observations.
• Figure 4: $\mathrm{H^{13}CO^+}$ 1-0 (blue) and C$^{18}$O 2-1 (black, convolved to the spatial resolution of $\mathrm{H^{13}CO^+}$) spectra toward L1157. The spectra in each panel are offset by half $\theta_{maj}$ for the $\mathrm{H^{13}CO^+}$ observations.
• Figure 5: Maps of the derived column densities for CO (top) and HCO$^+$ (bottom) based on our observations of C$^{18}$O and $\mathrm{H^{13}CO^+}$ respectively. Contours indicate the isotopologue emission from 3 to 21 times the RMS in 5 equally spaced steps.