Laboratory formation of scaled astrophysical outflows

Abstract

Astrophysical systems exhibit a rich diversity of outflow morphologies, yet their mechanisms and existence conditions remain among the most persistent puzzles in the field. Here we present scaled laboratory experiments based on laser-driven plasma outflow into magnetized ambient gas, which mimic five basic astrophysical outflows regulated by interstellar medium, namely collimated jets, blocked jets, elliptical bubbles, as well as spherical winds and bubbles. Their morphologies and existence conditions are found to be uniquely determined by the external Alfvenic and sonic Mach numbers Me-a and Me-s, i.e. the relative strengths of the outflow ram pressure against the magnetic/thermal pressures in the interstellar medium, with transitions occurring at Me-a ~ 2 and 0.5, as well as Me-s ~ 1. These results are confirmed by magnetohydrodynamics simulations and should also be verifiable from existing and future astronomical observations. Our findings provide a quantitative framework for understanding astrophysical outflows.

Figures (12)

• Figure 1: Schematic of the experimental setup of laser-produced scaled astrophysical outflow.
• Figure 2: Laboratory observed morphologies of laser-driven scaled astrophysical outflow in ambient gas medium. The maps show two cases with (panel A), and without (panel B) the applied 7 T axial magnetic field $\bm{B}_{ext}$, observed at 15 ns after the laser irradiation. The ambient gas pressure $P_{amb}$ and the external Alfvénic Mach number $M_{e-a}$ in the columns i to v are $0.06 Pa$ and 0.014, $10 Pa$ and 0.18, $100 Pa$ and 0.57, $400 Pa$ and 1.14, $1000 Pa$ and 1.81, respectively. The corresponding experimental parameters are provided in \ref{['tab:S1']}. The edges of the bubbles/cavities are indicted by dashed red curves. The horizontal dashed black lines at $z$$\sim 4.8$ mm indicate the position of the current coils.
• Figure 3: Experimental measured bubble (or cavity) size and jet length versus the external Alfvénic Mach number $M_{e-a}$. Red triangles are for the combined cavity and jet lengths (longitudinal/axial). Blue circles and crossed green circles are for the longitudinal ($\parallel$) and transverse ($\perp$) sizes of the bubbles/cavities, respectively. Hollow squares are for the ratio $R_{r-B}$ of the ram to magnetic pressure as obtained from the MHD simulations, which exhibits a function of $R_{r-B}\sim (M_A/2)^{1/2}$ denoted by the black curve. One can see that in the left (collimated jet) and center (blocked jet) regions the jet length (distance between red triangles and blue circles at a given $M_{e-a}$) decreases as $M_{e-a}$ increases. Moreover, the longitudinal size of the bubble/cavity is larger than the transverse size, which exhibits as an elliptical bubble. The corresponding experimental parameters are provided in \ref{['tab:S1']} of the Supplementary Materials.
• Figure Extended Data Figure 1: Schematic of the experimental setup. (A) Nanosecond laser heats the planar silicon target surrounded by ambient gas and the current coils produce axial magnetic field. After passing through the plasma along target surface, a femtosecond-laser probe beam is split into two parts by a beam splitter. One part is for the optical interferometry. The other is for the dark-field schlieren, with a $\Phi$300 $\mu$m stop dot to block the zero-order light. The temporal profile of pulsed magnetic field, the ablating laser beam, and fs probe laser beam are shown in (B, C), measured by photodiode (DET10A2, THORLABS) and home-made Rogowski coil.
• Figure Extended Data Figure 2: Experimental images of laser-driven scaled astrophysical outflow observed at 15 ns after laser irradiation of the target. Line-integrated electron density distributions from optical interferometry (A and D), electron density maps from Abel inversion (B and E), dark-field schlieren fringes (C and F), for with (A to C) and without (D to F) the axial external magnetic field. The ambient gas pressures $P_{amb}$ and external Alfvénic Mach number $M_{e-a}$ in the rows i to viii are $<0.1$ Pa and $0.014$, $10$ Pa and $0.18$, $100$ Pa and $0.57$, $200$ Pa and $0.81$, $400$ Pa and $1.14$, $700$ Pa and $1.52$, $1000$ Pa and $1.81$, $1400$ Pa and $2.15$, respectively. The corresponding experimental parameters are given in \ref{['tab:S1']}. The edges of the bubbles/cavities are indicated by dashed red curves with jet at the front in columns A. The weak central linear patterns in columns B and E are numerical pseudo-data caused by the process of Abel inversion. The horizontal dashed lines at $z\sim4800\mu$ m indicate the edge of the coils in columns A to C.