Abstract
Protostellar outflows and jets are one of the first signposts of a new star being born. We discuss here the revolution brought by the James Webb Space Telescope (JWST) to the studies of these phenomena with an emphasis on the synergies with other state-of-the-art observatories. Atomic and ionized content can be revealed with JWST NIRSpec and MIRI instruments at unprecedented detail. In concert with submillimeter observations, the complete chemical content of the jets can be studied. Young disks are also objects of active research, as the nascent location for planets. JWST reveals warm molecular gas in the warm inner disks elucidating the contents of gas that feeds the young planetary systems.
1. Introduction
The formation of stars is a complex interplay of different physical processes. Infalling gas and dust fuel the formation of the protoplanetary disk. At the same time, supersonic jets and wide-angle winds are ejected from the system. Those processes are most pronounced in the earliest stages of star formation, in the protostellar stage (up to 500,000 years since the beginning of the star-formation process). Their accretion is the most vigorous, and outflow activity is at its peak. However, this stage is also very well hidden in the dense molecular cloud from which the protostars gain material. Near- and mid-infrared (IR) wavelength range unlocked with the James Webb Space Telescope (JWST) is therefore essential, as it probes hot gas while at the same time offering to peer through the layers of dust, blocking light at shorter wavelengths. Since the beginning of JWST science observations, several works on protostellar sources have been published. Here, we present studies of emission lines from the outflows and jets coming from the compact regions close to the protostar and hot molecular gas from the young, freshly formed disk. They shed light on physical conditions and processes relevant at the onset of planet formation in the first stages of the star formation process.
1. Jets and outflows from the youngest protostars
Figure 1: Three-colour image of HH211 outflow with NIRCam instrument on JWST, F335M (blue), F460M (green), and F470N (red) filter (Ray et al., 2023). Credit: ESA/Webb, NASA, CSA, Tom Ray (Dublin).
Apart from being one of the first signposts of ongoing star formation, protostellar jets, and outflows are also important in extracting the angular momentum from the system, allowing the accretion to continue. Jet activity is also variable in nature, showing periodic knots (or bullets), which are explained by the variable nature of the accretion process. Therefore, they carry a fossil record of the accretion history of the system (Vorobyov et al., 2018). Finally, jets and winds are being launched from the inner regions of the protostellar system, in the inner few au. Therefore, they potentially carry information on conditions and composition of the inner disk, which are otherwise difficult to reveal in the youngest systems. Decades of multiwavelength studies of protostellar jets increased our understanding of their importance and the physical conditions that they experience. ALMA and single-dish telescopes revealed the molecular content of protostellar jets (Tafalla et al., 2010; Tychoniec, et al., 2019; Podio et al., 2021). However, the atomic and ionized components are much harder to study in young, embedded systems. Those tracers are present in the optical and infrared regime where young protostars are heavily embedded in their natal envelopes. Therefore, they are best studied in the mid-infrared wavelength regime where extinctions are lower than at the optical range. Only with JWST, launched in December 2021, are studies of this component at sufficient spatial resolution and sensitivity possible. Below, we discuss key discoveries in this area.
One of the key questions is how jets and outflows evolve across protostellar lifetimes. JWST pushed the limits of those studies, discovering an ionized jet in a protostar in a quiescent accretion state. IRAS 16253 is a protostar with bolometric luminosity of 0.2 L⊙. Both JWST instruments with capabilities of integrated field unit (IFU) – MIRI and NIRSpec, detected and mapped ionized iron [Fe II] launched at velocities of 170 km s−1, within the program Investigating Protostellar Accretion (IPA) – an open time JWST Cycle 1 program (PI: Tom Megeath) aiming to study protostars across the bolometric luminosity parameter space. Narang et al. (2024), studied the shock conditions in this jet and quantified the jet ejection rate to be on the order of 10−10 M⊙ yr−1. With a corresponding study of accretion rate based on OH emission, Watson et al. (in prep.) quantified the amount of material accreted by a protostar at 10−9 M⊙ yr−1. This is very low compared to typical accretion rates of Class 0 protostars in the range of 10−6 – 10−7 M⊙ yr−1 (Yen et al., 2017). Despite this quiescent accretion status, the protostar launches a jet nonetheless. This is an important piece of information: even the most quiescent protostars can propel a fast, ionized jet.
Interestingly, the IRAS 16253 outflow shows no traceable molecular emission, neither with JWST nor at longer wavelengths with ALMA. The contrasting example is another Class 0 object, HH211, in Perseus. This protostar is known for its knotted, high-velocity jet in CO and SiO, observed with sub-mm facilities (Lee et al., 2007). JWST images taken with NIRCam revealed a new, spectacular face of this iconic flow (Ray et al., 2023, Fig. 1). It appears that the jet, in this case, is mostly molecular, not showing a prominent ionized component. This suggests that shock velocities in this flow are very low. These observations also allowed us to shed new light on a more than decade-long mystery. With the aid of the NIRSpec instrument, the extended emission observed with Spitzer at 4.5 μm, called ‘green fuzzies,’ are now confidently attributed to the carbon monoxide (CO) rovibrational band.
More outflows from young stars are being observed as part of the CORINOS open-time program (PI: Yao-Lun Yang), with a first example of IRAS 15398, showing extended emission from [Fe II], [Ne II], and compact emission from [S I] (Yang et al. 2022, Okoda et al. in prep.). The largest sample of protostars in the first Cycles of JWST is observed with a guaranteed time observation program of the MIRI European Consortium JOYS (JWST Observations of Young protoStars; PI: Ewine van Dishoeck). First results on the high-mass protostar IRAS23385 reveal that such more massive sources share similar characteristics in jet and outflows tracers as the low-mass protostars (Beuther et al., 2023; Gieser et al., 2023).
Figure 2: Continuum emission at 5.4 μm, integrated emission of H2 0-0 S(7) line at 5.51 μm in colorscale and [Fe II] line at 5.34 μm in color scale (Tychoniec et al., 2024).
An exciting example from the JOYS sample is a more evolved protostar TMC1, which shows the potential of the MIRI instrument to resolve action in close binary systems and promises studies in outflow time evolution. Tychoniec et al. (2024), show that the two companions propel strikingly different outflows: TMC1-E shows prominent H2 emission, indicative of disk wind, while TMC1-W reveals a collimated and ionized jet (Fig. 2). With the aid of ALMA, able to trace colder, infalling gas, the difference in the outflow mechanism can be attributed to different accretion processes – TMC1-E dominated by inflow from the envelope to the disk, fueling the disk wind, and TMC1-W powering the jet that is related to the disk-to-star accretion process. The accretion can be traced with MIRI, covering several hydrogen recombination lines. This capability will enable studies of accretion in very embedded sources (Beuther et al., 2023).
TMC1 is also an excellent showcase of the sensitivity of the MIRI instrument, enabling detailed studies of the chemical composition of protostellar jets. Refractory elements like carbon, nickel, and cobalt are detected in TMC1-W. Neon and argon – noble gases with high ionization potential prominently detected in TMC1-E – suggest that the chemical content of jets can dramatically vary from source to source (even at the same evolutionary stage), either due to different launching radius or chemical composition at the launching point. Mapping jet tracers along the axis will be particularly powerful in revealing shock conditions along the flow and is being done for several targets (HH211, Caratti o Garatti et al. in prep.; BHR71, Tychoniec et al. in prep., L1448-mm, Navarro et al. in prep.).
2. Embedded disks
Seeds of planets are likely formed in the embedded disks around young protostars (Tychoniec et al., 2020). Knowing the conditions in those disks is therefore important to constrain the initial chemical and physical conditions of planet formation. They are vastly different from their older counterparts: warmer, more compact, and showing very few dust structures (van ’tHoff et al., 2020; Maury et al., 2019; Ohashi et al., 2023). ALMA provided a wealth of information on the structure and chemical content of those young disks. However, the innermost regions remain elusive because warm gas there is not well probed by ALMA cold gas tracers. Infrared wavelengths are crucial to uncover those inner few au regimes, most importantly, as those are expected regions where planet formation kicks off.
Here we present key results on the molecular lines from the embedded disks with JWST. JWST/MIRI delivered the first detection of the 7.3 μm SO2 ro-vibrational transition in a protostar (van Gelder et al., 2024). While sulfur-bearing species are typically associated with shocked gas, suggesting an accretion shock from the envelope onto the disk, in the case of IRAS 2A, the emission from a warm disk heated by the central protostar is most likely. Importantly, infrared emission lines show clear signs of radiative pumping. Warm CO and H2O gas has been detected in IRAS 15398 (Yang et al. 2022, Salyk et al. in press.), possibly coming from the inner regions of the disk although a more extended outflow or wind origin is not excluded. In a high-mass protostar example Francis et al., 2024 detected large reservoirs of hot organic gas (CO2, HCN, C2H2). However, the temperatures and emitting areas indicate an extended warm disk’s surface instead of the inner disks. In the case of CO, only high-energy transitions can be detected with MIRI. It is much better sampled in the NIRSpec wavelength range. In work of Rubinstein et al. (2023), IPA sources are studied with the CO emission lines in NIRSpec. So far, the picture emerging from the young disk observations with JWST is very different to that from Class II disks, which are very rich in water and small organic molecules (Grant et al., 2023).
3. Concluding remarks
Studies of the first stages of star and planet formation are experiencing an impressive boost with JWST observations. Unlocking the complete atomic and ionized content of protostellar jets opens studies of the physics and chemistry of shocks around protostars. Powerful synergies between observatories at different wavelengths, especially ALMA, will lead to a better understanding of accretion and ejection processes. At the same time, observing the evolutionary changes of the inner disk gas content from younger to older disks delivers insights on the onset of planet formation.
Acknowledgements
This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The following National and International Funding Agencies funded and supported the MIRI development: NASA; ESA; Belgian Science Policy Office (BELSPO); Centre Nationale d’Etudes Spatiales (CNES); Danish National Space Centre; Deutsches Zentrum fur Luft- und Raumfahrt (DLR); Enterprise Ireland; Ministerio De Economiá y Competividad; Netherlands Research School for Astronomy (NOVA); Netherlands Organisation for Scientific Research (NWO); Science and Technology Facilities Council; Swiss Space Office; Swedish National Space Agency; and UK Space Agency.
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