Investigating the Nested Structure of the Outflow from the Low Luminosity Protostar IRAS 16253-2429 using JWST and ALMA

TL;DR

The paper investigates how mass accretion, outflows, and disk evolution interlink in the very low-luminosity protostar IRAS 16253-2429 by combining JWST (NIRSpec IFU and MIRI MRS) and ALMA observations. It reveals a nested outflow architecture comprising a narrow, fast jet and a wider molecular wind traced by H$_2$ lines, together with a compact dust disk of radius about $15$ au and a protostellar outflow width $\leq 35$ au at the source. The H$_2$ excitation and velocity structure scale with $E_ ext{up}$, showing increasing speeds for higher energy levels and a opening-angle narrowing for higher-J transitions, implying a magnetohydrodynamic disk wind launching mechanism with slower material attributed to envelope gas accelerated along cavity walls by the wind. These results highlight a coherent, multi-component feedback process in a very low-luminosity protostar and provide crucial constraints on wind launching regions and angular-momentum transport relevant to early planet formation.

Abstract

Understanding the earliest stage of star and planet formation requires detailed observations to address the connection and interplay between the accretion, outflow and disk evolution. We present results from the observations of the low luminosity ($L_\mathrm{bol}\sim~0.2~L_\odot$) and mass (M$_*\sim$\,0.15~M$_\odot$) Class~0 protostar IRAS 16253$-$2429, conducted as part of the \textit{eDisk} ALMA large program and the JWST cycle-1 GO program \textit{IPA}. Observations reveal a wide hourglass-shaped continuum cavity traced in scattered light (at $\leq$~5~$μ$m), with a brighter, extended northern side. We detect 15 pure rotational H$_2$ transitions (E$_\mathrm{up}$:~1015--21411~K), revealing a wide-angle molecular outflow. {The outflow width (as traced in H$_2$~0-0~S(11)) at the protostellar location measures $\leq$35 au, slightly larger than the dust and Keplerian disk diameters ($\sim$30 au) but wider than the 20--23~au jet width in [Fe II].} {The opening angle narrows from 40--35\arcdeg{} for the low-J H$_2$ lines (up to S(5)) and the cold gas component (ALMA $^{12}$CO) to $\sim$28--19\arcdeg{} for the high-J H$_2$ lines (S(7)--S(11)).} Position-velocity diagrams of H$_2$ reveal higher velocities for higher E$_{up}$, ranging from ~12.5 km~s$^{-1}$ for H$_2$~0-0~S(1) and S(2) to ~28.5 km~s$^{-1}$ for H$_2$~0-0~S(5)~and~S(7) with respect to the mean flow velocity. The nested excitation and velocity structure of the collimated jet and wide angle wind suggest a magnetohydrodynamic wind as a likely launching mechanism, similar to the findings in other protostars and Class II sources. The lower velocity mm CO may be gas from the infalling envelope accelerated outwards by the wide angle wind along the cavity walls.

Figures (8)

• Figure 1: (a) The Spitzer IRAC three color (IRAC 3.6, 4.5 and 8 $\mu$m are red, green, and blue, respectively) image of the IRAS 16253$-$2429 field. The field of view is $\sim$ 0.8$^\prime$$\times$ 0.8$^\prime$. The dashed gray square outlines the $\sim$ 24$^{\prime\prime}$$\times$ 24$^{\prime\prime}$ field corresponding to the ALMA FOV in panel b. (b) Moment 1 (velocity) maps in the $^{12}$CO $J=2-1$ (robust =2) line from the ALMA eDisks observations. The dashed back square in the center outlines the $\sim$ 8$^{\prime\prime}$$\times$ 8$^{\prime\prime}$ field centered on the JWST observations. (c) A zoomed-in view of the NIRSpec and MIRI channel-1 Short (A) field with the red representing continuum emission at 4.17 $\mu$m, the green showing H$_2$ 0-0 S(11) at 4.18 $\mu$m and the blue depicting [Fe II] at 5.34 $\mu$m. A scale bar is given on the bottom left corner for each of the images.
• Figure 2: The MIRI MRS and NIRSpec IFU complete spectrum from 2.87 $\mu$m to 28 $\mu$m extracted from the ALMA continuum position with an aperture radius of 1$^{\prime\prime}$ centered at the 14 $\mu$m protostellar position. The offset plots show the zoomed-in view of some of the prominent molecular gas lines detected in IRAS 16253-2429.
• Figure 3: The continuum morphology of IRAS 16253$-$2429 as a function of wavelength from NIRSpec IFU and MIRI MRS. All images are cropped to the same spatial scale. The green marker is the MRS 14 $\mu$m~ position, while the ALMA 1.3~mm position is shown as the red marker. A scale bar corresponding to 100~au is shown in the bottom right corner, while the beam size is shown in the bottom left corner as the blue circle.
• Figure 4: (left) The observed 14 $\mu$m image, (center) the best-fit model and (right) the residual. All the three images are on the same spatial and intensity scale.
• Figure 5: The morphology of the H$_2$ 0-0 S lines from S(1) to S(15). The green marker is the MRS 14 $\mu$m~ position, while the ALMA 1.3~mm position is shown as the red marker. A scale bar corresponding to 100~au is shown in the bottom right corner (of the S(1) image), while the JWST PSF FWHM is shown in the bottom left corner as the blue circle. We also highlight the location of the spur in H$_2$ as a white arrow in the S(12) image. See Table \ref{['TableH$_2$']} for wavelength information for the H$_2$ transitions.