Bowshocks driven by the pole-on molecular jet of outbursting protostar SVS 13

Abstract

Outflows play a key role in the star and planet formation processes. Some outflows show discrete clumps of cold molecular gas moving at extremely high velocities (EHVs) of $\sim$100 km s$^{-1}$, known as ''molecular bullets'', that are likely closely associated with their primary driving agent. Here we present ALMA CO(J=3-2) observations of a bright EHV molecular bullet that reveal its morphology in detail down to scales of 30 au and its kinematic structure across the entire intermediate velocity range ($\sim$30-100 km s$^{-1}$). These provide important new insights into how outflows transfer mass and momentum to the surrounding medium. The observed channel maps display several sequences of ring-like features whose velocity increases and size decreases with projected distance from the driving source, each sequence tracing a thin, bow-shaped shell culminating on-axis in a bright EHV head. The shape, kinematics, and mass of each shell all agree remarkably well with the simplest textbook models of momentum-conserving bowshocks produced by a time-variable EHV jet. The dynamical timescale between consecutive shells is of a few decades, with the latest ejection event coinciding with the protostar optical/IR outburst observed in $\sim$1990. The very strong evidence for bowshock-driven entrainment induced by jet variability revealed by this work suggests that accretion bursts, and therefore variations in the disk snowlines, should occur on decade timescales, which could substantially impact grain growth and planet formation.

Figures (39)

• Figure 1: ALMA images and position-velocity diagrams of the blueshifted Bullet 1. Primary beam corrected ALMA images of the velocity-integrated intensity, peak intensity of the spectral cube, mean velocity, and position-velocity diagrams of the observed CO($J$=3-2) emission. The higher (lower) angular resolution results are shown in the top (bottom) panels. Synthesized beams are shown at the bottom left corners of the images. Plus signs indicate the positions of the protostars, VLA 4A (west) and VLA 4B (east), of the SVS 13 binary anglada2000anglada2004diaz-rodriguez2022. The H$_2$ arcuate features hodapp2014 are plotted as black and white arcs, taking into account their observed proper motion and the $\sim 0.07"$ offset between the radio and optical/infrared positions of the star diaz-rodriguez2022gaia2023. All velocities are relative to VLA 4B, ranging from $-$9.3 to $-$102.7 (top panels) and $-$126.4 km s$^{-1}$ (bottom panels). These ranges exclude the CO disk emission but not contamination (stronger toward VLA 4A) from disk emission of other molecular transitions, with higher rest frequencies, falling within the frequency range of the blueshifted CO emission of the bullet. Note also that the low-resolution images include emission from the CO clumps 1 and 2 (see Extended Data Fig. 1 and Supplementary Table 1), near the southeast corner. The position-velocity diagrams have been obtained along the longitudinal axis of the bullet (PA = 160$^\circ$, indicated by the black line in the panels of the third column), with origin in VLA 4B. Dashed lines in these diagrams indicate the emission of the families of rings II, III, and VI (defined in Fig. \ref{['fig:ellipses']}). Contours are: 3, 6, 10, 16, and 25 times $0.05$ (top) and $0.61$ (bottom) Jy beam$^{-1}$ km s$^{-1}$ (first and third columns); 3, 6, 10, and 16 times $0.011$ (top) and $0.066$ (bottom) Jy beam$^{-1}$ (second column); 5, 10, 20, 35, 60 and 90 times 0.0025 (top) and 0.008 (bottom) Jy beam$^{-1}$ (fourth column). See Methods for further details.
• Figure 2: Observed CO($J$=3-2) spectral channel images at high angular resolution. A sample of high angular resolution channel maps of the CO($J$=3-2) emission observed by ALMA, which illustrate the ringed kinematic structure in Bullet 1. The synthesized beam is $0.173" \times 0.091"$ (PA = $-2.2^\circ$), and is plotted as an ellipse in the bottom left corner of the first image. The full set of observed channel maps is shown in Supplementary Fig. 1 and Supplementary Video 1. The positions of the two protostars of the SVS 13 binary are indicated by plus signs. Note that several velocity channels are contaminated by emission from different molecular transitions, with higher rest frequency than CO, coming from the disks associated with VLA 4A and VLA 4B diaz-rodriguez2022. This disk emission is stronger toward VLA 4A. The LOS velocity relative to VLA 4B is shown in the top left corner of each image. The velocity width of each of the channels shown in the figure is 0.53 km s$^{-1}$, which corresponds to five native channels. The r.m.s noise of the images is 2.6 mJy beam$^{-1}$. The images have not been corrected by the primary beam response. The H$_2$ arcuate features hodapp2014 are plotted in the first column as white arcs. Lower angular resolution channel maps, covering a wider range of velocities, are shown in Extended Data Fig. 3, Supplementary Fig. 2 and Supplementary Video 2.
• Figure 3: Elliptical fits to the ringed emission of the CO($J$=3-2) channel maps. Top: Color-coded plots of elliptical fits (see Methods) to the observed CO emission. Rings are represented by the ellipses that best-fit their emission, and filled emission regions by shaded areas, with colors indicating the LOS velocity relative to VLA 4B of the channel map. Their centers are indicated by color dots (rings) and triangles (filled emission). The rings tend to group in sequences that we call "families", with their centers well aligned, their velocity increasing (in absolute value) and their size decreasing with distance from the stars (indicated with black plus signs), and ending in a region of filled emission (the "head"). Separate plots for each family are shown in the right panels. Linear regression fits (plotted as black lines for families I to III) have been performed (for families IV to VI their origin is set at the position of VLA 4B). The [FeII] microjet hodapp2014 from VLA 4B is represented by a green segment. Black and white arcs represent the H$_2$ arc-like features hodapp2014. Offsets are relative to the position of VLA 4B. Bottom: Plots of the mean radius and the LOS velocity of the rings, as a function of the projected distance of their centers to VLA 4B. Error bars represent the estimated uncertainties, adopted as the deviations with respect to the fitted value that cause the intensity of the fitted ring to decrease by 1$\times$r.m.s of the channel map (see Methods). The plots illustrate the trends (except for the heads) for the radius to decrease and the velocity to increase with distance, with the different families clearly differentiated in the plots. The gray line in the left panel traces the suggested outer edge of the families, forming an angle of $\sim$$44^\circ$ from the axis ($\sim$$22^\circ$ after deprojection by an inclination angle of $25^\circ$).
• Figure 4: Decomposition of an approaching bowshock into velocity channel images. The $x'y'$ plane is the plane of the sky, the $z'$ axis is the LOS, and the $z$ axis is the symmetry axis of the bowshock, oriented at an inclination angle $i$ with respect to the LOS. Left: Side view illustrating the velocity gradient of the blueshifted bowshock, where the magnitude of the velocities increases as we approach the bowshock head. Black arrows represent the velocity field of the bowshock shell, while colored arrows represent their projection onto the LOS. The points in the bowshock with LOS velocities $v_{\rm ch1}$, $v_{\rm ch2}$, and $v_{\rm ch3}$ are also shown in colors. Right: Projection onto the plane of the sky of the points in the bowshock with LOS velocities $v_{\rm ch1}$, $v_{\rm ch2}$, and $v_{\rm ch3}$. Each colored curve corresponds to the emission observed in a channel map. Note that the LOS velocities ($v_{\rm ch}$) are defined as positive when the motion is away from the observer (redshifted), but in the coordinate system used, positive $v_{z'}$ velocities are directed toward the observer (blueshifted); therefore, $v_{\rm ch}=-v_{z'}$.
• Figure 5: Bowshock model fitting to the elliptical fits of the rings. Panels correspond to families II, III and VI, respectively, from top to bottom. The bowshock model fitting was performed through a least-square minimization to the data (see Methods). The best-fit model parameters are presented in Extended Data Table 1. First column: Shape and velocity field (black arrows) of the bowshock model, where $z$ is the symmetry axis (whose origin is VLA 4B) and $r$ is the cylindrical radius. The main parameters of the model, the characteristic scale ($L_0$), the velocity at which the jet material is initially ejected sideways by the working surface ($v_0$), and the velocity of the internal working surface along the $z$-axis ($v_{\rm jet}$), are listed in the top right corner of the panels. The fitted inclination angles are $i=20$-$25^\circ$, and the fitted velocity of the medium into which the jet is traveling is $v_{\rm amb}\simeq0$ km s$^{-1}$ (see Methods and Extended Data Table 1). Second column: Observed (dots indicate rings and triangles indicate filled emission of the head) and bowshock model (continuum line) mean radius of the elliptical rings in channel maps as a function of the projected distance to VLA 4B. Velocities are indicated in a color scale. The reduced chi-square of the fitting is shown on the right top corner. Third column: Observed (red dots) and model (blue line) position-velocity diagram for the rings (excluding the heads). Fourth column: Observed (red dots) and model (blue line) elongation factor of the rings ($f$), taken as the ratio of the ring axes along the longitudinal and transverse directions. Error bars indicate the estimated uncertainties of the data points as defined in Fig. \ref{['fig:ellipses']}, empty dots indicate that the axial ratio uncertainty could not be obtained (see "Ellipse fitting" in Methods). All velocities are LOS velocities relative to the velocity of VLA 4B.