Discovery of an Accretion Streamer and a Slow Wide-angle Outflow around FU Orionis

We present Atacama Large Millimeter/submillimeter Array 12-m, 7-m, and Total Power Array observations of the FU Orionis outbursting system, covering spatial scales ranging from 160 to 25,000 au. The high-resolution interferometric data reveal an elongated 12CO(2–1) feature previously observed at lower resolution in 12CO(3–2). Kinematic modeling indicates that this feature can be interpreted as an accretion streamer feeding the binary system. The mass infall rate provided by the streamer is significantly lower than the typical stellar accretion rates (even in quiescent states), suggesting that this streamer alone is not massive enough to sustain the enhanced accretion rates characteristic of the outbursting class prototype. The observed streamer may not be directly linked to the current outburst, but rather a remnant of a previous, more massive streamer that may have contributed enough to the disk mass to render it unstable and trigger the FU Orionis outburst. The new data detect, for the first time, a vast, slow-moving carbon monoxide molecular outflow emerging from this object. To accurately assess the outflow properties (mass, momentum, and kinetic energy), we employ 13CO(2–1) data to correct for optical depth effects. The analysis indicates that the outflow corresponds to swept-up material not associated with the current outburst, similar to the slow molecular outflows observed around other FUor and Class I protostellar objects.


INTRODUCTION
The flaring of FU Orionis in 1936 (Hoffleit 1939) marked the discovery of a new phase in low-mass star formation.FU Ori increased its brightness by over 5 magnitudes within a year (Wachmann 1954).Herbig (1966) noted that the brightening could not be explained in terms of a nova event or by sudden changes in extinction, and that it was most likely due to 'some phenomenon of early stellar evolution.' Hartmann & Kenyon (1985) argued that the increased luminosity could be explained by a rapid onset of accretion from a rotating accretion disk (i.e. an accretion outburst).In the accretion disk model, the energy released by the accreting gas heats up the disk up to several thousand K, reproducing the increase in luminosity, observed spectral energy distribution (SED), and peculiar spectral properties such as the apparent wavelengthdependent stellar spectral type which can be explained by absorption from different regions of superheated disk atmosphere (Hartmann & Kenyon 1996;Hartmann et al. 2016).
For decades FU Ori was the only known star of its class, but the progressive discovery of more outbursting sources suggested that these are not isolated cases, but a common, yet short-lived, phase in star formation.Newly discovered outbursting stars showed diverse outburst properties forcing the definition of sub-classes: FUors (named after FU Ori) have high-amplitude (> 5 mag), long-lived (years to decades) optical bursts (Herbig 1966), whereas EXors (misnamed after EX Lupi) have low amplitude (2-4 mag) and shorter (days to months) periods of enhanced accretion (e.g.Audard et al. 2014, for a review).The advent of systematic photometric surveys increased the discovery rate of new eruptive objects (e.g.Kun et al. 2014).Milky Way surveys indicate that 3-6 % of Class I protostars show eruptive behavior in a 4-year span and that eruptive events are more common in Class I objects than in Class II (Contreras Peña et al. 2017).
As new sources are discovered, the heterogeneous nature of their characteristics presents a significant challenge in terms of classification and understanding.Fischer et al. (2022) discusses the difficulties encountered in distinguishing between various types of outbursts, such as FU Orionislike events and EX Lupi-like events.These challenges arise due to the overlapping characteristics exhibited by these phenomena and the limitations imposed by the available observational data.Recent advancements in time-dependent, multiwavelength, and spectroscopic analyses suggest a continuum of diverse phenomena rather than distinct and separable classes.This emphasizes the need for comprehensive and multifaceted approaches to understand the complexity of these outburst events fully.
Accretion outbursts are now believed to be central to the formation of low-mass stars.The well-known Luminosity Problem (Kenyon et al. 1990) arises from the fact that the measured luminosities of low-mass protostars are smaller than expected by the quiescent (non-outbursting) mean accretion rates (10 −6 M ⊙ yr −1 ).The problem is mitigated if the accretion rates are variable, with stars accreting an important fraction of their mass during several bursts of enhanced accretion during the Class I phase (Evans et al. 2009;Dunham et al. 2010) while spending most of their time in lower quiescent accretion rates than predicted (10 −8 M ⊙ yr −1 ).Submillimeter monitoring of star-forming regions indicates that episodic accretion may occur even earlier, at the Class 0 stage (Safron et al. 2015;Johnstone et al. 2018).
The physical mechanisms responsible for the outburst remain a matter of debate.A funneling mechanism in which matter from an envelope is accumulated in the accretion disk and released episodically could explain the outbursts (Zhu et al. 2009).Thermal instabilities can also be responsible for the piling up and sudden release of inner disk material (Bell & Lin 1994).Additionally, the formation and fragmentation of spiral waves, driven by gravitational instabilities, have been suggested to trigger these accretion outbursts (see Kratter & Lodato 2016, for a review).Observations of spiral structures due to gravitational instabilities, such as those seen in the massive disk of Elias 2-27 (Pérez et al. 2016) and the recently observed fragmenting spirals around the V960 Mon system after its FUor-like outburst in 2014 (Weber et al. 2023), support this theory.
Despite their significance in our understanding of lowmass star formation, the exact causes of these outbursts remain elusive.The dense star-forming environments of these young circumstellar disks offer another possible explanation.It is proposed that the sudden outbursts could be linked to interactions within binary systems or encounters with passing stars, which are quite common in these crowded regions (Bonnell & Bastien 1992;Reipurth & Aspin 2004).Additionally, the multiplicity of stars and the substantial crosssectional area of these disks can foster interactions with passing stars (star-disk flybys; Pfalzner 2008;Cuello et al. 2019) as well as disk-disk interactions which can excite the disks turning them unstable (Muñoz et al. 2015).
Various FUor-like protostars have been identified as binary companions, including notable cases such as L1551 IRS5, RNO 1B/C, AR 6A/B, HBC 494 (Pueyo et al. 2012;Nogueira et al. 2023, and references therein).The archetypical FU Ori system, FU Orionis itself, is a known binary composed of two stars separated by 0.5 ′′ where the northern component (FU Ori N hereafter) is the optically visible star driving the outburst.Wang et al. (2004) discovered the southern binary component, which according to a spectroscopical analysis by Beck & Aspin (2012), is the more massive of the two (hence FU Ori S is the primary member).Early ALMA Observations of FU Ori at 0.5 ′′ resolution resolved the binary system in continuum and revealed complex kinematical features (Hales et al. 2015).High-resolution ALMA observations of FU Ori at 0.04 ′′ resolution by Pérez et al. (2020) resolved each individual disk, reporting very compact sizes possibly truncated by binary interactions, and kinematical signatures of Keplerian rotation around the outbursting source (FU Ori N).These discoveries contribute to the ongoing debate on whether the enhanced accretion can be attributed to close companion interactions.Another perspective posits perturbation by sub-stellar companions (Clarke et al. 2005).
In terms of the evolutionary stage, FUor objects seem to resemble Class I protostars more than Class II objects.Many FUors have Class I type SEDs (e.g., Gramajo et al. 2014) as well as dust silicate absorption features, and therefore have been associated with Class I protostars (Quanz et al. 2006).Most FUors are surrounded by reflection nebulae, remnants from their parent cores, some of which are seen to brighten up during the outburst (Herbig 1966;Goodrich 1987;Aspin & Reipurth 2003).FU Ori itself, located near the Lynds Bright Nebula 878 (LBN 878), has a fan-shaped reflection nebula that appeared next to the star after the flare, which was not present before (Wachmann 1954).Figure 1 a shows large-scale view of the FU Ori environment, as seen in optical wavelengths.The optical composite image is courtesy of as-trophotographer Jim Thommes1 , also known for the discovery of the Thommes Nebula, a reflection nebula that appeared after the outburst of FUor-type object V900 Mon (Thommes et al. 2011).
The enormous energy released during the outburst can power winds and jets which in turn may help disperse the primordial core, a process that will eventually regulate the final stellar mass (Shu et al. 1987).FUors power prominent winds measured via the presence of P Cygni profiles in some specific lines (Connelley & Reipurth 2018), from which the inferred mass loss rates correspond to 10% of mass accretion rate, thus several orders of magnitude larger than those of classical T Tauri stars (Calvet 2004).The observed profiles can be well reproduced by the standard magnetocentrifugally driven wind model with the addition of an inner disk wind (Milliner et al. 2019).
On the other hand, EXors seem to show little outflowing activity.Outflows have been reported around the ambiguously classified FUor/EXor sources V1647 Ori and GM Cha (Cieza et al. 2018;Principe et al. 2018;Hales et al. 2020), and only tentatively around EX Lup (Hales et al. 2018;Rigliaco et al. 2020).Although the sample size is small, the differences in outflow activity between FUors and EXors suggest that the two types of objects represent different evolutionary stages, with EXors more evolved than FUors.
There is also observational evidence that most FUor sources are surrounded by large envelopes that are still transferring material onto the disk (Kóspál et al. 2017a;Fehér et al. 2017).It has been proposed that the imbalance between mass transferred from the envelope to the disk and the accretion of disk mass onto the star may trigger the disk instability responsible for the outburst (Bell & Lin 1994;Kóspál et al. 2017b).Studying the larger-scale structure of eruptive sources is thus crucial for understanding the nature of the FUor/EXor outbursts.
Asymmetric, non-keplerian gaseous structures are now routinely discovered by ALMA.Kinematic modeling of these structures reveals that they can be interpreted as anisotropic infall of material from the surrounding medium into the circumstellar disk.These structures end up following elongated patterns for which they have been named 'accretion streamers' (see Pineda et al. 2023, for a review).Accretion streamers have now been detected around many Class 0 and Class I sources and are relevant for improving our understanding of low-mass star formation.For instance, their discovery directly challenges the traditional model of axisymmetric collapse of protostellar cores (Shu 1977).The role accretion streamers play in episodic accretion processes still needs to be demonstrated; to our knowledge, no accretion streamers have been discovered around eruptive stars yet.
In this work, we present ALMA observations of the surroundings of FU Ori at angular scales ranging from 160 to 25000 au at 1.3mm continuum and CO isotopologes.Section 2 provides details on the ALMA observations and data reduction, while Section 3 presents the primary observational findings and their analysis.These results are subsequently discussed in Section 4, and Section 5 offers a summary of the study's findings.Throughout the paper, we assume a distance to FU Ori of 408 ± 3 pc (Gaia DR3; Bailer-Jones et al. 2021).

OBSERVATIONS AND DATA REDUCTION
Observations of FU Ori (project code 2017.1.00015.S, PI: J. Williams) were carried out between November 2017 and August 2018 using the ALMA 12-m, 7-m, and Total Power (TP) arrays in order to obtain spatial coverage from 0.4 ′′ to up to a map size of 1 ′ (160 to 25000 au).The log of the observations, calibrators used, weather conditions, and array characteristics are shown in Table 1.The full map required a 27-pointing mosaic for the 12-m array and a 7-pointing mosaic for the 7-m.The total antenna number for each 12-m, 7-m, and TP arrays were 45, 11, and 3, respectively.
All data were calibrated by the ALMA staff using the ALMA Science Pipeline (version 40896 of Pipeline-CASA51-P2-B) in CASA 5.1.12(McMullin et al. 2007;CASA Team et al. 2022).The calibration process includes offline Water Vapor Radiometer (WVR) calibration, system temperature correction, as well as bandpass, phase, and amplitude calibrations.Online flagging and nominal flagging such as shadowed antennas and band edges were applied during calibration.Continuum subtraction in the visibility domain was performed on the interferometric data prior to imaging.
Imaging of the continuum and molecular emission lines was performed using the TCLEAN task in CASA 6.1.The TP observations were converted into visibilities using the task TP2VIS (Koda et al. 2019), and then input into TCLEAN along with the interferometric data.
The 12-m array data was self-calibrated using the standalone version of the automated self-calibration module (Tobin et al., in prep.).The self-calibration process was performed separately for each observation, with one iteration of phase-only self-calibration.The improvements in signalto-noise (S/N) after self-calibration ranged between 10-20%.The self-calibration tables derived from the continuum data were applied to the FDM spectral windows before imaging the CO lines.The continuum and spectral line data were imaged using the TCLEAN task interactively in CASA (using a Hogbom deconvolver, creating manually-defined CLEAN masks, and using natural weighting to maximize sensitivity).Continuum subtraction was performed before imaging each molecular line using the task UVCONTSUB.During CLEANing of the spectral lines, the spectral axis was binned on-the-fly to 0.25 km s −1 to increase the S/N.The resulting spatial resolution for the combined 12m+7m+TP dataset corresponds to a synthesized beam size of 0.4 ′′ × 0.3 ′′ (PA = 41.6 • ).
Integrated intensity (moment 0) and intensity-weighted mean velocity (moment 1) maps for the spectral line data were produced with CASA task IMMOMENTS.The moment 0 images were produced by integrating the signal in channels with emission above 3σ.There is a strong cloud contamination close to the system's velocity (11.75 km s −1 ; Hales et al. 2015), particularly strong in the 7m+TP data; thus, for display purposes, we created separate moment 0 maps for the red-shifted and blue-shifted portions of the observed large-scale emission.

RESULTS
We detect 12 CO, 13 CO, and C 18 O in a variety of angular scales.The source large-scale structure mapped by the 7-m+TP arrays dominates the final image, overwhelming the structures traced by the 12-m array.For this reason, we also produced continuum and spectral line cubes using the 12-m data only.This approach allows for a more detailed examination of the specific characteristics and features contained in each dataset.
Figure 1 (inset) shows the integrated intensity (moment 0) map for 12 CO, and Figure 2 shows the integrated intensity maps for all three CO isotopologues at the spatial scales traced by the 7-m+TP arrays.The angular resolution of the combined 12+7m+TP image is 0.4 ′′ ×0.3 ′′ .However, the maps in Figure 2 have been smoothed to 2.5 ′′ to enhance the sensitivity to large-scale structures (the nominal resolution of 7m-only data is 4.9 ′′ whereas the angular resolution of the 12m-only data is 0.2 ′′ ). Figure 3 displays the 12 CO and 13 CO moment 1 maps obtained by imaging the 12m-array data only.C 18 O was not detected in the 12-m array data.

Wide, slow-moving outflows
The ALMA observations reveal the presence of largescale, wide-angle bipolar outflows for the first time around the class prototype FU Ori.The blue-shifted portion of the 12 CO emission traces the northeastern part of the well-known reflection nebula.The 13 CO emission partially coincides with the 12 CO emission (Figure 2), whereas the C 18 O is more compact.The 12 CO and 13 CO emission have an extension of approximately 20000 au.The line-of-sight velocity of the outflows is low, typically around ∼1-2 km s −1 .The wideangle, slow-moving bipolar outflow traced by 12 CO emission is reminiscent of the emission detected towards other FUor objects: V883 Ori (Ruíz-Rodríguez et al. 2017a), HBC 494 (Ruíz-Rodríguez et al. 2017a), V2775 Ori (Zurlo et al. 2017), V1647 Ori (Principe et al. 2018), V900 Mon (Takami et al. 2019), and GM Cha (Hales et al. 2020) as well as in low-mass Class I young stellar objects (e.g.Arce & Sargent 2006;González-Ruilova C. et al. in prep.).This slow outflow is also comparable to those seen towards First Hydrostatic Core candidates (e.g.Pineda et al. 2011;Maureira et al. 2020).

Streamer in FU Ori
Figure 3 shows the zoomed-in view of the 12 CO(2-1) and 13 CO(2-1) integrated velocity maps (moment 1) of FU Ori, as revealed by the 12-m array data only.The intermediatescale emission traced by the 12-m Array shows complex kinematics.The 12 CO(2-1) map is similar to the Cycle 0 12 CO(3-2) map presented in Hales et al. (2015), yet significantly more extended.The elongated feature extending south of the central objects at velocities 10-11 km s −1 , also observed in the Cycle 0 12 CO(3-2) channel maps, is now confirmed to be real by our deeper, higher-resolution 12 CO(2-1) data.Linear structures have been observed in scattered light around FUor type objects (Liu et al. 2018;Takami et al. 2018), however, this is the first time such a structure is reported in molecular line emission.Furthermore, the feature we observe in 12 CO does not coincide with any of the linear features reported in the near-IR.The elongated feature has neither connection to the larger-scale molecular outflow nor to the inner disk rotation (Pérez et al. 2020), and is more similar to accretion streamers recently reported around young stellar objects (Pineda et al. 2020;Alves et al. 2019;Valdivia-Mena et al. 2022;Gupta et al. 2023;Pineda et al. 2023).

Outflow properties
Swept-up cavities in the envelope of young stellar objects play a significant role in understanding the complex processes occurring during their early stages of formation (Arce et al. 2007, and references therein).These cavities are created by the powerful outflows and jets emitted from protostars, which interact with their surrounding molecular clouds.As the outflows propagate through the dense material, they sweep away and evacuate cavities, shaping the surrounding environment.These cavities serve as valuable indicators of the energetic feedback mechanisms associated with star formation.By studying the size, shape, and kinematics of these swept-out cavities, researchers can gain insights into the dynamics of protostellar outflows, the interaction between the outflows and their surroundings, and the impact of these processes on the overall evolution of young stellar objects.
Assuming both the 12 CO and 13 CO lines trace the bipolar cavity, we use the 12 CO and 13 CO emissions to derive estimates of the mass of the outflow and its kinematic properties in the traditional manner (e.g.; Cabrit & Bertout 1990;Dunham et al. 2014).Following the process described in Ruíz-Rodríguez et al. (2017a), we estimate the outflow quantities from the blue-and red-shifted emissions separately.The optically-thin tracer 13 CO is used to correct for the optical depth effects in 12 CO by computing the ratio of the brightness temperatures of each line from all the channels with detections above 5σ.To apply the correction factor to data with only 12 CO detection, we extrapolate values from a parabola fitted to the weighted mean values.Figure 4 shows the best fit parabola (solid green line).The derived outflow properties are shown in Table 2.

Streamer Model
To ascertain the infalling nature of the observed streamer, we fitted 12 CO(2-1) streamer emission (see Figure 3) with analytical equations of infalling trajectories given by Mendoza et al. (2009).For this fitting, we use TIPSY (Trajectory of Infalling Particles in Streamers around Young stars; Gupta et al. 2024) which simultaneously fits the morphology and velocity profile of the streamer observations.TIPSY requires prior estimation of the mass of the central object and systemic line-of-sight velocity of the center of mass, which were taken to be 1.8 M ⊙ and 11.75 km s −1 , respectively, since the masses of FU Ori N and FU Ori S are 0.6 and 1.2 M ⊙ respectively.(Pérez et al. 2020;Beck & Aspin 2012).Pérez et al. (2020) inferred the disk dust masses for FU Ori N and FU Ori S to be 22 M ⊕ and 8 M ⊕ , respectively.Assuming the typical gas-to-dust ratio of 100, the total masses of the disks are expected to be ∼ 7 M jupiter (or ∼ 6.6 × 10 −3 M ⊙ ) and ∼ 3 M jupiter (or ∼ 2.8 × 10 −3 M ⊙ ), respectively, which are negligible for the streamer model calculations.
The fitting results suggest that the morphology and the velocity profile of observed streamer emission can be wellrepresented as a trail of infalling gas, as shown in Figure 5.Using the distribution of trajectories which can fit the ob-   The red solid dots depict the weighted mean values that were not employed during the fitting since the emission is optically thin in those channels and, therefore, no correction is needed.
To represent the data, a green solid line is used to illustrate the bestfit second-order polynomial.
served streamer, infalling timescale, of the streamer was estimated to be 8537 ± 3317 year.Here the infalling timescale is defined as the time taken for infalling material to reach the closest point to the protostars in their trajectory, starting from the farthest point in the observed streamer.The uncertainties were computed as the standard deviation of infalling timescales, as estimated for the trajectories that could fit the data reasonably well (see Sec. 2.3. in Gupta et al. 2024).The uncertainty on our trajectory solution is mainly due to lack of curvature in the projected streamer morphology, which is useful in constraining gas velocities along the plane of the sky.Note that the theoretical trajectories used for the fitting assume that the only force acting on the particle is gravity from a point source.Although FU Ori is a binary system, this assumption is still valid at the length scale of the streamer which is roughly six times the binary separation.The streamer was also observed in 12 CO(3-2) emission (Hales et al. 2015), which allows us to simultaneously estimate temperature and column density of the gas in the streamer.We estimated the brightness temperature of infalling gas to be 11.3K and 7.1 K in 12 CO(2-1) and (3-2) emission, respectively.Using the radiative transfer code, RADEX, brightness temperatures were found to correspond to the gas temperature of ∼ 25 K and gas column density of ∼ 2.5 × 10 16 cm −2 .This is also in agreement with the non-detection of the streamer in 13 CO(2-1) emission.Assuming the typical CO to H 2 ratio of 10 −4 , the mass of gas in the streamer is then estimated to be ∼ 5.6 × 10 −7 M ⊙ .Although we use the same boundaries to select streamer emission in 2-1 and 3-2 emission maps, the resulting ratios may be affected by the different angular scales traced by the two datasets (which differ by a factor of 2 in terms of angular resolution and maximum recoverable scale), thus adding extra uncertainty to the derived quantities.
Although these estimates do not include parts of streamers beyond the primary beams of the interferometric observations, they can still be used to estimate mass infall rates as Ṁinf = M streamer /T inf , where M streamer is observed streamer mass and T inf infall time for the observed streamer.We find the streamer to be feeding the FU Ori system at a rate of ∼ 6.6 × 10 −11 M ⊙ yr −1 .Note that this value should be treated as an order of magnitude estimate, because of uncertainties on the streamer mass.A more reliable mass infall rate estimation will require modeling the total reservoir of bound gas and ideally, in a more optically thin line emission like 13 CO(2-1), which was not detected in our streamer observations.

Molecular Outflow
Our new ALMA observations detect a wide-angle outflow emerging from the FU Ori binary system for the first time.The northern component of the binary system, FU Ori N, is the outbursting source.Thus it is natural to assume that FU Ori N was the source driving the outflows.Prior searches for molecular outflows around FUors, mainly using singledish telescopes (Evans et al. 1994), reported outflowing material from many FUors but failed to detect flows emerging from the FUor class prototype.These non-detections instigated the belief that there were no molecular outflows around the FU Ori system.Our discovery ends the mystery by clearly demonstrating the presence of a molecular outflow from FU Ori itself.
The main characteristics of the outflow discovered around FU Ori are its wide angle and low velocity.The blue-shifted lobe extends approximately 10000 au in each direction.We compute the dynamical timescale of the outflow by dividing the extension of the blue-shifted lobe by the maximum gas velocity (not considering correction for the system's inclination).We obtain a Kinematic Age of 3800 yr, which is significantly larger than the typical duration of FUor outbursts (< 10 2 yr).This implies that the current outburst is not responsible for the observed outflows, similar to what it was concluded for V883 Ori (Ruíz-Rodríguez et al. 2017b).
The blue-shifted lobe has a projected opening angle of ∼140-150   2006) reported a trend in which outflow opening angles increase with the age of the protostars.A possible explanation for the widening of outflows with age is that in the early stages, the main observed outflow structures are highly collimated, fast-moving jets that begin to pierce the envelope.As most of the envelope material along the jet axis is cleared out, the prevailing structure in the molecular outflow will be primarily shaped by the gas carried along by the wide-angle portion of the wind.Alternatively, the parabolic morphology of FU Ori's wide opening molecular outflow is consistent with the shell disk wind model (Lee et al. 2000;Arce et al. 2007;de Valon et al. 2022).
In this context, the FU Ori system is more evolved than other Class I protostars that are still heavily embedded in their surrounding envelope and have narrower cavity opening angles.This interpretation is consistent with the fact that FU Ori is optically visible because most of the surrounding material has been accreted or swept up by mass dissipation processes (e.g.primordial high-velocity jets, disk winds, disk+star accretion).The 12 CO and 13 CO outflows detected by ALMA coincide with the wide-angle conical reflection nebula seen in optical wavelengths (Figure 1), suggesting that the dissipative processes acting in FU Ori effectively disperse gas and dust from the primordial envelope.Nevertheless, there is still a significant amount of cloud/envelope material, evidenced by the substantial contamination in systemic velocities (Figures 6-10) and the centrally peaked C 18 O emission traced by the 7m+TP array observations (Figure 2), likely associated with the remnant of a primordial envelope.
The projected velocity of the outflows is small, with a characteristic velocity of ∼1-2 km s −1 .Using relative velocities of 1 km s −1 and a distance from the center of 10000 au, we find the outflow material to be unbound.The low outflow velocity of the 12 CO and 13 CO outflows is much smaller than the typical 10-40 km s −1 observed around other FUors (Ruíz-Rodríguez et al. 2017b, and references therein) but is similar to that of V883 Ori (0.65 km s −1 Ruíz-Rodríguez et al. 2017b).Pérez et al. (2020) estimated an inclination 37 • degrees for the dust disk around FU Ori N. Thus, assuming that the outflow's direction coincides with the disk's angular momentum, we discard that the low velocity observed is due to inclination effects.Ruíz-Rodríguez et al. (2017b) proposed that in the case of V883 Ori, the low outflow velocities can be explained if the outflow we see is very old, a remnant of older fast-moving outflows that have now ceased or emerging from a quiescent disk without the creation of high-velocity components, similar to what we observe in the FU Ori system.FU Ori is known to have deep, broad absorption P Cygni profiles that can be described by disk wind models capable of sustaining mass-loss rates of the order of 10% the accretion rate (i.e.mass loss due to wind of ∼ 10 −5 to 10 −6 M ⊙ yr −1 ; Milliner et al. 2019).
The properties of the outflow listed in Table 2 can be readily compared with those of V883 Ori and HBC 494, as they were derived using the same method.The estimated mass for the blue-shifted lobe is two orders of magnitude larger than the red-shifted lobe.We cannot confirm whether this difference is real or possibly caused by the absorption of cloud/envelope material in the line of sight.Nevertheless, the mass estimation of the blue-shifted lobe is comparable to that of V883 Ori and one order of magnitude larger than that of HBC 494.The mass loss, momentum, energy, and luminosity are also similar to those of V883 Ori yet still consistent with the values detected around other young protostars (e.g.Arce & Sargent 2006).

Accretion Streamer
As discussed in Section 3.4, the 12 CO streamer (Figure 5) seems to be feeding mass to the protostellar system at a rate of ∼ 6.6 × 10 −11 M ⊙ yr −1 (or ∼ 0.07 M jupiter Myr −1 ).As mentioned in Section 3.4, the disk masses for FU Ori N and FU Ori S are ∼ 6.6 × 10 −3 M ⊙ and ∼ 2.8 × 10 −3 M ⊙ , respectively.This would suggest that the observed streamer will require ∼ 100 Myr to replenish disks masses, which is at least an order of magnitude greater than the typical disk lifetimes.
The estimated mass infall rate in the streamer around FU Ori is lower than those detected in streamers around other Class I sources (∼ 10 −6 M ⊙ yr −1 , e.g., Valdivia-Mena et al. 2022;Pineda et al. 2023).The streamer was not detected in C 18 O, consistent with it not being as massive as the ones detected around other Class I protostars.There is also no clear detection of streamer in 13 CO emission, although there is still some hint of stream-like emission in some 13 CO channel (channels 11-11.25 km s −1 , see Figure 10), just adjacent to 12 CO streamer emission (mostly in channels 10.25-10.75km s −1 , see Figure 9).Therefore, some of the 12 CO streamer emission may be hidden due to high optical depth effects.This may suggest that we are underestimating streamer mass, although not by orders of magnitude as we do not detect the streamer in 13 CO or C 18 O emission.
The streamer needs to be more massive to sustain FU Ori's outburst accretion rates (by several orders of magnitude).The estimated streamer mass infall rate is not even sufficiently massive to sustain quiescent stellar accretion rates.Despite not being massive enough to sustain the outburst accretion rate, the streamer can deliver material to the disk and trigger disk instabilities, which can further lead to accretion.Anisotropic infall, cloudlet capture events, inhomogeneous delivery of material, and building-up of material around dust traps can all lead to the disk instabilities that could trigger accretion outbursts (Vorobyov & Basu 2010;Dullemond et al. 2019;Kuznetsova et al. 2022;Hanawa et al. 2024).It still needs to be determined if the low observed mass infall rate is enough to trigger disk instabilities, for which detailed modeling would be required.
It is also possible that the streamer we observe now is not directly connected to the current outburst but instead is a remnant of a more massive streamer that could have played a role in feeding mass to the disk in the past, but now has already relinquished most of its mass into the protostellar system.In this scenario, the streamer we are observing is the smoking gun for previous, vigorous episodes of cloud accretion into the disk that may have led the disk to become unstable and trigger FU Ori's outburst.
There may also be more than one streamer interacting with the binary system.For instance, the high-resolution ALMA observations of FU Ori from Pérez et al. (2020) detected CO emission that delineates the position of the brightest feature in scattered light: a prominent arc to the northeast.This arc could represent a signpost of active accretion into the system or could be light reflected from material at the base of the outflow (Zurlo et al. in preparation), similar to features recently reported in other outbursting systems (Weber et al. 2023).

CONCLUSIONS
The current study presents ALMA observations of the FU Ori outbursting system covering spatial scales from 160 to 25,000 astronomical units.
The remarkable sensitivity to a wide range of spatial scales unveil a wide, slow-moving molecular outflow.To obtain the first determination of the outflow properties, including mass, momentum, and kinetic energy, we have employed 13 CO(2-1) data to correct for optical depth effects.
A thorough analysis of the higher-resolution interferometric data reveals intricate gas kinematics, likely resulting from a confluence of ongoing physical processes, including accretion, rotation, ejection, and spreading of material due to binary interaction The new observations corroborate the existence of an elongated 12 CO feature that can be modeled consistently as an accretion streamer.The streamer analysis suggests that the stream cannot sustain the enhanced accretion rate.Nevertheless, the observed streamer may be feeding the disk, helping to produce conditions of instability that can give rise to the outburst.Follow-up hydro-dynamical modeling is required to ascertain the plausibility of this scenario.Alternatively, the observed streamer might not be directly tied to the current outburst but instead be a leftover from a prior, more massive streamer that previously contributed to the disk mass, rendering the disk unstable and triggering the FU Ori outburst.These results demonstrate the value of multi-scale interferometric observations to enhance our understanding of the FU Ori outbursting system and provide new insights into the complex interplay of physical mechanisms governing the behavior of FUor-type and the many kinds of outbursting stars.

Figure 1 .
Figure1.Left: Optical RGB composite image of LBN 878 (the red and brown nebula dominating the field) obtained by astro-photographer Jim Thommes.FU Ori (and its reflection nebula) is the bright object located at the center of the image.Inset shows the integrated intensity 12 CO(2-1) maps as traced by the ALMA observations (see Figure2for details).Redshifted and blueshifted 12 CO integrated intensity maps of FU Ori are plotted over optical emission (color scale).The blue-shifted moment 0 map (blue contours) was constructed including emission from 8.0 to 11.5 km s −1 while the red-shifted integrated emission (red contours) includes the emission between 12.7 and 17.5 km s −1 .The contour levels are 3, 5, and 8σ.

Figure 2 .
Figure 2. Integrated intensity in FU Ori maps for 12 CO, 13 CO and C 18 O.The blue-shifted moment 0 maps (blue contours) were constructed including emission from 8.0 to 11.5 km s −1 while the red-shifted images (red contours) include the 12.7 to 17.5 km s −1 .Grayscale shows the optical image from Figure 1.The white cross at the center of the image shows the position of FU ori N. The angular resolution of the images (2.5 ′′ ×2.5 ′′ ) is shown as a black circle in the lower left.

Figure 3 .
Figure3.Zoomed-in view of FU Ori in 12 CO and 13 CO emission generated imaging the 12m-array data only.Both maps show the velocity centroids (moment 1) in jet color scale ranging from 9.5 to 13.5 km s −1 (centered on the systemic velocity of 11.75 km s −1 ).The moment zero maps are shown in contours ranging from 3 times the zeroeth moment RMS to the peak.Contours range from 39 to 707 mJy beam −1 km s −1 , and 26 to 78 mJy beam −1 km s −1 , for 12 CO and 13 CO, respectively.The white cross shows the position of the continuum maximum which we take as a proxy for the position of FU Ori N. The ellipse on the lower left represents the synthesized beam.The blue and red arrows show the expected direction of the large-scale outflow assuming it is perpendicular to the disk PA of 133.6 • measured from the long baseline data inPérez et al. (2020).

Figure 4 .
Figure4.Relationship between the brightness temperatures of 12 CO (T12) and 13 CO (T13) presented as a function of velocity relative to the systemic velocity.The plotted data includes representing the weighted mean values (blue solid dots), accompanied by error bars indicating the corresponding weighted standard deviations in each channel.The red solid dots depict the weighted mean values that were not employed during the fitting since the emission is optically thin in those channels and, therefore, no correction is needed.To represent the data, a green solid line is used to illustrate the bestfit second-order polynomial.

Figure 5 .
Figure 5.The best fit infalling trajectory from TIPSY (green curved line) overplotted on the 12 CO integrated intensity map (left panel) and position-velocity (PV) diagram along the streamer (right panel).In the PV diagram, the outermost contours represent the noise level of the data and each subsequent contour denote an increment by a factor of two.The red rectangles in both panels represent the initial R.A., Decl., and VLSR limits given to TIPSY to extract the sub-cube of the streamer emission.A clustering algorithm does the rest of fine-tuning of the extraction (see Gupta et al. 2024, for details).et al. 2020; Arce & Sargent 2006).These outflows are wider than those of other FUor observed with ALMA such as V346 Nor, Haro 5a IRS, V1647 Ori, V2775 Ori and V900 Mon (Kóspál et al. 2017b; Kospal 2018; Principe et al. 2018; Zurlo et al. 2017; Takami et al. 2019).Arce & Sargent (2006) reported a trend in which outflow opening angles increase with the age of the protostars.A possible explanation for the widening of outflows with age is that in the early stages, the main observed outflow structures are highly collimated, fast-moving jets that begin to pierce the envelope.As most of the envelope material along the jet axis is cleared out, the prevailing structure in the molecular outflow will be primarily shaped by the gas carried along by the wide-angle portion of the wind.Alternatively, the parabolic morphology of FU Ori's wide opening molecular outflow is consistent with the shell disk wind model(Lee et al. 2000;Arce et al. 2007;de Valon et al. 2022).In this context, the FU Ori system is more evolved than other Class I protostars that are still heavily embedded in their surrounding envelope and have narrower cavity opening angles.This interpretation is consistent with the fact that FU Ori is optically visible because most of the surrounding material has been accreted or swept up by mass dissipation processes (e.g.primordial high-velocity jets, disk winds, disk+star accretion).The 12 CO and 13 CO outflows detected by ALMA coincide with the wide-angle conical reflection nebula seen in optical wavelengths (Figure1), suggesting that the dissipative processes acting in FU Ori effectively disperse gas and dust from the primordial envelope.Nevertheless, there is still a significant amount of cloud/envelope material,

Figure 6 .
Figure6. 12 CO channel maps towards FU Ori (7m+TP, smoothed to 2.5" resolution).The velocity of the channels is shown in the Local Standard of Rest (LSR) frame, centered at the rest frequency of 12 CO(2-1).The data has been binned to a velocity resolution of 0.25 km s −1 .Contours of the 1.3mm continuum emission at 0.3" resolution are overlaid (contour levels are 0.5, 1, and 1.8 mJy beam −1 ).

Figure 7 .
Figure 7. Same as Figure 6 but for 13 CO.

Table 1 .
Summary of ALMA Observations