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PRODIGE -- envelope to disk with NOEMA VIII. Sulfur oxides trace a shock caused by a streamer in the inner envelope of a protostar

Using NOEMA observations within the PRODIGE program, this study identifies sulfur oxide emission peaks around the Class I protostar Per-emb 50 that reveal a low-velocity shock caused by an infalling streamer impacting the inner envelope, highlighting the significant role such streamers play in shaping the physical and chemical structures of forming stars and their disks.

Original authors: María Teresa Valdivia-Mena, Jaime E. Pineda, Caroline Gieser, Paola Caselli, Dominique M. Segura-Cox, Yuxin Lin, María José Maureira, Tien-Hao Hsieh, Laura A. Busch, Ana Lopez-Sepulcre, Laure Bouscass

Published 2026-03-19

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CC BY 4.0

Original authors: María Teresa Valdivia-Mena, Jaime E. Pineda, Caroline Gieser, Paola Caselli, Dominique M. Segura-Cox, Yuxin Lin, María José Maureira, Tien-Hao Hsieh, Laura A. Busch, Ana Lopez-Sepulcre, Laure Bouscasse, Dmitry Semenov, Asunción Fuente, Nichol Cunningham, Thomas Henning, Julián J. Miranzo-Pastor, Yu-Ru Chou, Roberto Neri, Izaskun Jimenez-Serra, Edwige Chapillon, Stephane Guilloteau, Felipe Alves, Mario Tafalla, Anne Dutrey, Riccardo Franceschi, Sierk van Terwisga, Kamber Schwarz

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A Cosmic Traffic Jam

Imagine a baby star (a protostar) growing up in a giant cloud of gas and dust. Usually, we think of this gas falling straight down onto the star like rain into a bucket. But in reality, the universe is messy. Sometimes, huge rivers of gas—called "streamers"—flow from far away and crash into the baby star's neighborhood.

This paper is about a specific baby star named Per-emb 50. Astronomers used a super-powerful telescope (NOEMA) to look at this star and discovered that a giant streamer of gas is currently crashing into the star's outer atmosphere (the "envelope"). This crash is creating a shockwave, kind of like a sonic boom or a car crash, which heats up the gas and changes its chemistry.

The Detective Work: Following the "Smoke"

How do we see a crash in space? We can't see the gas itself easily, but we can look for the "smoke" left behind by the crash. In space, Sulfur acts like that smoke.

The Clues: The astronomers looked for two specific sulfur molecules: SO and SO₂. These molecules are like "shock detectors." They only light up brightly when gas gets heated up suddenly by a violent collision.
The Map: When they looked at Per-emb 50, they didn't just see one bright spot. They saw two distinct bright spots (peaks) of sulfur light, both located to the southwest of the star.
- Peak 1: A bright spot about 180 times the distance from Earth to the Sun (180 AU) away.
- Peak 2: A slightly dimmer spot much further out, about 400 AU away.

The Mystery: What Caused the Crash?

The astronomers had to figure out what was making these sulfur spots glow.

1. The "Far" Crash (Peak 2)
This is the main discovery. The gas at Peak 2 is moving in a weird way. It's not falling straight down like normal; it's moving faster and in a different direction than the rest of the star's atmosphere.

The Analogy: Imagine a calm river (the star's envelope) flowing gently. Suddenly, a massive log (the streamer) comes rushing down a side channel and slams into the riverbank.
The Result: The water at the impact point gets turbulent, hot, and compressed. The astronomers believe Peak 2 is exactly that impact point. The streamer is hitting the star's outer atmosphere, creating a shockwave that heats the gas and makes the sulfur molecules glow.

2. The "Near" Crash (Peak 1)
This spot is closer to the star. It's a bit more confusing. It could be the same streamer hitting the very edge of the star's disk (where the planets will eventually form). Or, it could be the star's own outflow (a wind blowing away from the star) crashing into the falling gas.

The Analogy: This is like the splash zone right next to where the log hits the water, or perhaps a wave crashing against the shore. It's definitely a crash, but it's harder to tell exactly which "car" hit which "wall."

The Physics: Why Does This Matter?

The paper does some heavy math to prove this isn't just a random hot spot.

Speed Check: They measured the speed of the gas. The "crash" is happening at a moderate speed (about 3–4 km/s). It's fast enough to heat things up and create sulfur, but not so fast that it shatters dust grains (which would create different chemicals like Silicon Monoxide, which they didn't find).
Temperature: The gas is hot (around 46°C to 64°C in space terms, which is actually quite warm for deep space!).
Density: The gas at the crash site is very crowded (dense), just like you'd expect when two things smash together and squish the material.

The Takeaway: Why Should We Care?

This study tells us that star formation isn't a quiet, orderly process.

Streamers are Real: Material doesn't just fall from everywhere equally; it comes in focused "rivers" or streamers.
Chemical Recycling: When these streamers crash into the star's atmosphere, they create shockwaves. These shocks heat up the gas and change its chemical makeup.
Planet Building: Since planets form from this gas, the chemicals we find in our own solar system might have been "cooked" or altered by these streamer crashes billions of years ago.

In short: The astronomers found a cosmic "fender bender" between a river of gas and a baby star. This crash is heating up the neighborhood and changing the chemistry of the ingredients that will eventually become planets. It proves that the early stages of star formation are violent, dynamic, and full of surprises.

1. Problem and Scientific Context

The formation of protostars and their planet-forming disks involves complex interactions between infalling envelope material, outflows, and the central disk. Recent observations have identified "streamers"—filamentary structures transporting material from beyond the envelope directly toward the protostar. A critical open question is how these streamers interact with the inner envelope and disk: do they simply deliver pristine material, or do they induce shocks that chemically and physically reprocess the material?

Sulfur-bearing molecules, specifically SO and SO₂, are established tracers of shock fronts due to their sensitivity to temperature and density changes. While shocks at disk-envelope interfaces are known, the specific chemical and physical signatures of shocks caused by streamers impacting the inner envelope remain poorly understood. This study focuses on the Class I protostar Per-emb 50 (located in NGC 1333, ~298 pc), which is known to host a massive streamer, to investigate whether sulfur oxide emission traces a shock induced by this streamer.

2. Methodology

The authors utilized high-resolution observations from the NOrthern Extended Millimeter Array (NOEMA) as part of the large program PRODIGE (Protostars and Disks: Global Evolution).

Observations: They combined data from the C and D configurations (from 2019/2020) with new, high-resolution A-array observations (from January 2023). This combination provided a spatial resolution of approximately 0.8" × 0.3" (beam size), allowing for the resolution of structures previously unresolved in lower-resolution data.
Molecular Tracers: The study analyzed:
- SO₂: Three rotational transitions with upper energy levels ( $E_{up}$ ) ranging from 19 K to 94 K.
- SO: The $5_5 - 4_4$ transition.
- H₂CO and CO: Used to trace the streamer kinematics and outflow structures, respectively.
Data Analysis Techniques:
- Morphological Analysis: Integrated intensity maps and Position-Velocity (PV) diagrams were constructed to compare emission against an Infalling-Rotating Envelope (IRE) model (generated using the FERIA code).
- Kinematic Decomposition: Bayesian model selection (using Nested Sampling via pyspecnest) was applied to SO₂ spectra to disentangle multiple velocity components (narrow vs. wide Gaussian profiles) within each pixel.
- Physical Parameter Estimation:
  - LTE Analysis: The XCLASS code was used to derive rotational temperatures ( $T_{rot}$ ) and column densities ( $N$ ) assuming Local Thermodynamic Equilibrium.
  - Non-LTE Analysis: The RADEX code was employed to test line ratios against kinetic temperatures and densities, accounting for sub-thermal excitation.
- Shock Modeling: Derived abundance ratios (SO₂/SO) were compared with shock models (van Gelder et al. 2021) to estimate shock velocities.

3. Key Results

A. Morphology and Kinematics

Resolution of SO₂ Peaks: The previously unresolved SO₂ emission toward the southwest of Per-emb 50 was resolved into two distinct peaks:
- Peak 1: Located ~180 au from the protostar.
- Peak 2: Located ~400 au from the protostar.
Kinematic Separation:
- Peak 1 exhibits velocities consistent with the expected infalling-rotating envelope (IRE) model and is located near the base of the outflow.
- Peak 2 is significantly blueshifted (~1.4 km s⁻¹) relative to the IRE model prediction. Its velocity lies between the systemic velocity of the protostar and the velocity of the infalling streamer head (derived from H₂CO).
Component Separation: Bayesian analysis revealed that the emission consists of:
- A wide component ( $\sigma_v > 1.8$ km s⁻¹) concentrated near the protostar, tracing a rotating disk.
- A narrow component ( $\sigma_v \approx 0.7-0.8$ km s⁻¹) tracing the envelope and the two distinct peaks.

B. Physical Conditions

Temperatures and Densities:
- Peak 1: $T_{rot} \approx 64$ K, $N(\text{SO}_2) \approx 1.3 \times 10^{15}$ cm⁻².
- Peak 2: $T_{rot} \approx 46$ K, $N(\text{SO}_2) \approx 7 \times 10^{14}$ cm⁻².
- Non-LTE analysis for Peak 2 indicates a volume density of $n_{H_2} \geq 10^7$ cm⁻³ and a kinetic temperature of ~45–50 K, suggesting the gas is denser and warmer than the ambient envelope.
Abundance Ratios: The derived SO₂/SO abundance ratios (0.2–0.3) are consistent with low-velocity shocks ( $\sim 3-4$ km s⁻¹).

C. Shock Velocity Estimation

By modeling the 3D velocity vectors of the streamer (free-fall trajectory) and the envelope (IRE model), the authors calculated a relative velocity difference of ~2.6 km s⁻¹ at the location of Peak 2 (400 au) and ~4 km s⁻¹ at the centrifugal radius. These values align with the low-velocity shock regime inferred from the chemical abundance ratios.

4. Interpretation and Discussion

Origin of Peak 2 (The Streamer Shock): The authors propose that Peak 2 is caused by a shock front generated where the infalling streamer impacts the inner envelope.
- The system is viewed nearly edge-on ( $i \sim 70^\circ$ ). As the streamer approaches the disk, it strikes the inner envelope from the side, compressing and heating the gas.
- This compression explains the localized density increase, the temperature rise, and the blueshifted velocity (streamer motion) relative to the redshifted envelope rotation.
- The lack of SiO detection suggests the shock is not strong enough to sputter silicate grains, consistent with the low-velocity estimate.
Origin of Peak 1: This peak is likely a shock at the disk-envelope interface or potentially influenced by the streamer/outflow interaction. It follows the expected envelope kinematics but may be enhanced by asymmetries caused by the streamer.

5. Significance and Conclusions

Chemical Inheritance: The study demonstrates that streamers do not merely deliver material; they actively reprocess the chemical inventory of the inner envelope through shocks. This affects the "chemical inheritance" of forming planets, as shock-induced heating can alter molecular abundances (e.g., via thermal desorption of ices) before the material reaches the disk.
Physical Impact: Streamers can create shocked, compressed layers in the inner envelope, altering the physical structure of the star-forming region even before the material reaches the protoplanetary disk.
Methodological Advance: The use of high-resolution NOEMA A-array data combined with Bayesian kinematic decomposition successfully disentangled overlapping emission components (disk vs. envelope vs. streamer shock), a technique crucial for interpreting complex protostellar environments.

In summary, this paper provides direct observational evidence that streamers induce shocks in the inner envelopes of protostars, significantly influencing the physical and chemical conditions of the material that eventually feeds the protoplanetary disk.