Complex morphology and kinematics at the heart of the very low luminosity object IRAM 04191+1522

Abstract: The formation of the majority of brown dwarfs (BDs) remains uncertain. They may form in molecular cloud cores in a process akin to low mass star formation, or via fragmentation in circumstellar discs. Studying the youngest, most embedded sources is crucial for distinguishing these scenarios. We investigate molecular gas morphology and kinematics around one young & embedded very low luminosity object (VeLLO), IRAM 04191+1522, utilising archival ALMA observations of 13CO, C18O, and SO. We trace gas on scales of a few 10s to 100s of au around the source to search for outflowing and/or infalling structures. The red and blueshifted 13CO (3-2) emission show distinct morphologies and kinematics. The blueshifted emission to the north-west may trace shocked material oriented differently from the previously reported approx. 0.1 pc CO outflow. Redshifted emission mainly to the south-east and south-west may trace the base of an outflow cavity. The position angle of this cavity suggests the presence of a second outflow, which supports the possible binary nature of this VeLLO. The C18O (2-1) emission is highly complex, comprising structures at different spatial scales and distances from the source. These may trace a mix of molecular outflow, outflow cavity, and disc emission. SO 65-54 reveals evidence for anticlockwise rotation around the central source, together with a northern structure of uncertain origin. We have identified a complex set of 13CO (3-2) and C18O (2-1) structures alongside evidence of a new outflow cavity at a distinct position angle from previously detected outflows. This supports the scenario that IRAM 04191+1522 is a binary system. The northern SO gas structure remains unexplained. Higher spectral resolution observations at intermediate scales are needed to characterise these substructures, their connection to larger scale structures, and to determine this system's final fate.

• The paper uses high-resolution ALMA observations of 13CO, C18O, and SO to map gas within tens to hundreds of au around the embedded VeLLO IRAM 04191.
• The data reveal a second outflow cavity near position angle 234°, nested cavity structures, compact rotating gas, and a possible 42-au accretion streamer, supporting the system’s binary interpretation.
• The findings favor star-like formation for this proto-brown-dwarf candidate but cannot determine its final stellar or substellar fate because of projection effects, spatial filtering, and unresolved kinematics.

Context and motivation

IRAM 04191+1522 (hereafter IRAM04191) is a young ($\sim$10$^4$ yr), highly embedded Class 0 system in Taurus at $\sim$140 pc, with a protostellar mass of 0.05 $M_\odot$, an envelope mass of 0.5 $M_\odot$, and an internal luminosity of $L_\mathrm{int} = 0.08 \pm 0.04$ $L_\odot$, placing it firmly in the VeLLO class. It was discovered through a 0.1 pc CO (2–1) outflow (2608.17593), an extent comparable to outflows driven by ordinary Class 0 protostars — a fact that sits uneasily with its low luminosity. Dunham et al. showed that driving this outflow would require $L_\mathrm{int} \geq 1$ $L_\odot$, leading to the hypothesis that IRAM04191 undergoes episodic accretion; Anderl et al.'s chemical modelling of N$_2$H$^4$0 and C$^4$1O profiles supports a past burst with luminosity $^4$2150 times the present value, implying the object could reach 0.2–0.25 $^4$3 by the end of the Class 0 phase. Most recently, Huélamo et al. resolved the system into two components separated by $^4$480 mas (11 au) with a dynamical mass of 50 ± 40 $^4$5, making it a proto-brown-dwarf binary candidate surrounded by a Keplerian circumbinary disc.

The paper under discussion presents archival ALMA Band 6/7 observations of $^4$6CO (3–2), C$^4$7O (2–1), and SO 6$^4$8–5$^4$9 at angular resolutions of 0$\sim$017–0$\sim$139, probing gas on scales of tens to hundreds of au — smaller than any previous molecular-line study of this source. The motivation is diagnostic: distinguishing between star-like formation (core collapse, scaled-down low-mass star formation) and planet-like formation (disc fragmentation and ejection) for brown dwarfs requires characterising accretion and ejection signatures in the youngest embedded substellar candidates.

Results: $\sim$2CO (3–2)

The blueshifted emission (4.57–6.08 km s$\sim$3) shows a ring-like filamentary structure at $\sim$46″ north-west of the source plus a compact oblong structure within 1″. The ring spatially coincides with a curved arc in archival Spitzer/IRAC imaging dominated by the 4.5 μm filter, which contains bright H$\sim$5 shock lines; the authors therefore interpret the ring as shocked gas, plausibly related to but directionally distinct from the previously reported large-scale outflow at PA = 30°.

The redshifted emission (6.96–8.07 km s$\sim$6) has a strikingly different morphology: sporadic emission extending to $\sim$710″ in the north-west, and a connected structure east and south-west of the source curving down to about −3$\sim$85. The velocity field is chaotic, with velocities near the source reaching $\sim$97.8 km s$M_\odot$0 east and $M_\odot$17.4 km s$M_\odot$2 west, and the highest velocity dispersions confined to the innermost 2″. Line profiles extracted within 0$M_\odot$32 are double-peaked for all three tracers, which may indicate rotating disc or envelope gas, though the authors caution that the profiles likely result from a superposition of multiple components rather than a clean disc signature.

Results: C$M_\odot$4O (2–1)

The C$M_\odot$5O emission resolves into several components across two velocity ranges. In the blueshifted channels (5.7–6.3 km s$M_\odot$6), components appear along the known outflow PA (~30°) and along a north-west/south-east axis of total projected length %%%%37$^4$38%%%%3. In the redshifted channels (6.9–7.4 km s$M_\odot$9), a knotty south-western component extends to $M_\odot$02″ (the knots attributed partly to interferometric filtering of extended emission).

Two results carry particular weight. First, the redshifted C$M_\odot$1O morphology fits inside the redshifted $M_\odot$2CO structure in the south-east, suggesting nested cavity gas with C$M_\odot$3O tracing the denser base of the outflow cavity — consistent with recent reports of nested atomic/molecular outflow morphologies in other YSOs. Second, PV diagrams along PAs of 54° and 125° reveal distinct kinematics: the south-western component shows a positive velocity gradient (from $M_\odot$47.1 km s$M_\odot$5 at −0$M_\odot$68 to $M_\odot$77.4 km s$M_\odot$8 at −2$M_\odot$94), indicative of accelerating outflowing gas, while the south-eastern component shows constant-velocity emission consistent with an outflow cavity wall, alongside a tentative Keplerian-like pattern whose confirmation exceeds the data's resolution.

The appendix strengthens the outflow-cavity interpretation quantitatively: cuts through the PV diagrams show velocity shifts of 0.45 km s$L_\mathrm{int} = 0.08 \pm 0.04$0 in $L_\mathrm{int} = 0.08 \pm 0.04$1CO and 0.25 km s$L_\mathrm{int} = 0.08 \pm 0.04$2 in C$L_\mathrm{int} = 0.08 \pm 0.04$3O between slits at different offsets, both exceeding the respective spectral resolutions (0.221 and 0.083 km s$L_\mathrm{int} = 0.08 \pm 0.04$4). The implied cavity lies at PA $L_\mathrm{int} = 0.08 \pm 0.04$5234° — a position angle not previously associated with any jet or outflow in this system (reported PAs are 18°, 20°, and 30°). The existence of a second outflow cavity at an independent PA naturally implies a second outflow, which the authors argue reinforces the binary nature of IRAM04191 proposed by Huélamo et al.

Results: SO 6$L_\mathrm{int} = 0.08 \pm 0.04$6–5$L_\mathrm{int} = 0.08 \pm 0.04$7

The SO emission is compact (<0$L_\mathrm{int} = 0.08 \pm 0.04$84), comprising an east-west structure slightly south of the source and a blob %%%%59$\sim$60%%%%28 to the north-east. The east-west structure shows blueshifted emission in the east and redshifted emission in the west — an anti-clockwise rotation pattern matching that seen in the C$L_\odot$1O disc. The authors suggest this flattened SO structure may be part of the circumstellar/circumbinary disc that has not yet fully collapsed into Keplerian rotation, plausible given the system's estimated age of 0.8–2 × 10$L_\odot$2 yr.

The northern SO structure is more enigmatic. Its PV diagram shows emission at approximately the systemic velocity extending only %%%%63$\sim$64%%%%3 (42 au) from the source, hinting that the material may be gravitationally bound. If confirmed as an accretion streamer with higher spectral-resolution observations in tracers such as HCO$L_\odot$5, it could explain the episodic accretion inferred for this system; however, the map size and spectral resolution of the current data preclude determining its origin.

The fate of IRAM04191

The paper weighs these findings against expectations for brown dwarf versus stellar formation. Scaled-down star-like formation predicts shorter, slower outflows and lower accretion rates; indeed, proto-BD outflows such as those of ρ-Oph (10–12.5 au, ~−40 km s$L_\odot$6) and M1701117 fit this picture, and the only known proto-BD streamer spans 2000–3000 au. IRAM04191 exhibits characteristics of both regimes: a 0.1 pc CO outflow typical of low-mass protostars, yet small-scale structures (the 42 au SO feature, the compact disc) consistent with a substellar object. Notably, Palau et al. showed that both the infall mass and the 3.6 cm radio jet of IRAM04191 follow the relations established for protostars, supporting a star-like formation pathway. Nevertheless, the authors explicitly decline to conclude the object's final fate, since simultaneous infall and outflow signatures complicate the mass-assembly picture and the connection between large- and small-scale structures remains unestablished.

Limitations and open questions

Several limitations constrain the interpretations. All velocities and distances are projected quantities, minimised by the assumed inclination of 50–60°. Interferometric spatial filtering prevents mass estimates for the outflowing gas at the scales probed here, and the maximum recoverable scales (1.9″ and 3.8″) mean structures beyond a few arcseconds are unrecovered — the knotty C$L_\odot$7O morphology is itself partly a filtering artefact. The apparent rise in velocity dispersion toward the source in $L_\odot$8CO (from $L_\odot$90.05 to 0.30 km s$L_\mathrm{int} \geq 1$0) is at the level of the channel width and cannot be confirmed as real. The Keplerian-like feature in the C$L_\mathrm{int} \geq 1$1O PV diagram lacks the resolution for definitive identification. Two observational needs follow directly: intermediate-spatial-scale observations (few 1000s of au) bridging these data and the single-dish literature, and a high-cadence monitoring campaign to test whether IRAM04191 undergoes EXor- or FUor-like outbursts, as suggested for the VeLLO IRAS 15398−3359.

Conclusion

This work demonstrates that the inner few hundred au of IRAM04191 host a far more complex gas architecture than previously recognised: shocked gas in a ring-like configuration, evidence for a second outflow cavity at PA $L_\mathrm{int} \geq 1$2234° distinct from all reported jets and outflows, nested molecular emission consistent with a dense cavity base, anti-clockwise rotating gas traced by SO that may represent an uncollapsed portion of the disc, and a gravitationally bound northern structure of unknown origin. The new cavity detection independently supports the binary interpretation of the system. While the accumulated evidence — scaled-down protostellar outflow relations, a Keplerian circumbinary disc, and now multi-directional ejection — favours star-like brown dwarf formation, whether IRAM04191 ultimately becomes a stellar or substellar system remains undetermined pending the intermediate-scale and time-domain observations the authors identify.