STARFORGE: the effects of protostellar outflows on the IMF

ABSTRACT The initial mass function (IMF) of stars is a key quantity affecting almost every field of astrophysics, yet it remains unclear what physical mechanisms determine it. We present the first runs of the STAR FORmation in Gaseous Environments project, using a new numerical framework to follow the formation of individual stars in giant molecular clouds (GMCs) using the gizmo code. Our suite includes runs with increasingly complex physics, starting with isothermal ideal magnetohydrodynamics (MHD) and then adding non-isothermal thermodynamics and protostellar outflows. We show that without protostellar outflows the resulting stellar masses are an order of magnitude too high, similar to the result in the base isothermal MHD run. Outflows disrupt the accretion flow around the protostar, allowing gas to fragment and additional stars to form, thereby lowering the mean stellar mass to a value similar to that observed. The effect of jets upon global cloud evolution is most pronounced for lower mass GMCs and dense clumps, so while jets can disrupt low-mass clouds, they are unable to regulate star formation in massive GMCs, as they would turn an order unity fraction of the mass into stars before unbinding the cloud. Jets are also unable to stop the runaway accretion of massive stars, which could ultimately lead to the formation of stars with masses ${\gt}500\, \mathrm{M}_{\rm \odot }$. Although we find that the mass scale set by jets is insensitive to most cloud parameters (i.e. surface density, virial parameter), it is strongly dependent on the momentum loading of the jets (which is poorly constrained by observations) as well as the temperature of the parent cloud, which predicts slightly larger IMF variations than observed. We conclude that protostellar jets play a vital role in setting the mass scale of stars, but additional physics are necessary to reproduce the observed IMF.

Failed and delayed protostellar outflows with high-mass accretion rates

ABSTRACT The evolution of protostellar outflows is investigated under different mass accretion rates in the range ∼10−5–$10^{-2}\, {\rm M}_\odot$ yr−1 with 3D magnetohydrodynamic simulations. A powerful outflow always appears in strongly magnetized clouds with $B_0 \gtrsim B_{\rm 0, cr}\, =10^{-4} (M_{\rm cl}/100\, {\rm M}_\odot)$ G, where Mcl is the cloud mass. When a cloud has a weaker magnetic field, the outflow does not evolve promptly with a high-mass accretion rate. In some cases with moderate magnetic fields B0 slightly smaller than B0, cr, the outflow growth is suppressed or delayed until the infalling envelope dissipates and the ram pressure around the protostellar system is significantly reduced. In such an environment, the outflow begins to grow and reaches a large distance only during the late accretion phase. On the other hand, the protostellar outflow fails to evolve and is finally collapsed by the strong ram pressure when a massive (≳ 100 M⊙) initial cloud is weakly magnetized with B0 ≲ 100 μG. The failed outflow creates a toroidal structure that is supported by magnetic pressure and encloses the protostar and disc system. Our results indicate that high-mass stars form only in strongly magnetized clouds, if all high-mass protostars possess a clear outflow. If we would observe either very weak or no outflow around evolved protostars, it means that strong magnetic fields are not necessarily required for high-mass star formation. In any case, we can constrain the high-mass star formation process from observations of outflows.

Probing the hidden atomic gas in Class I jets with SOFIA

Context. We present SOFIA/FIFI-LS observations of five prototypical, low-mass Class I outflows (HH111, SVS13, HH26, HH34, HH30) in the far-infrared [O I]63μm and [O I]145μm transitions. Aims. Spectroscopic [O I]63μm,145μm maps enable us to study the spatial extent of warm, low-excitation atomic gas within outflows driven by Class I protostars. These [O I] maps may potentially allow us to measure the mass-loss rates (Ṁjet) of this warm component of the atomic jet. Methods. A fundamental tracer of warm (i.e. T ~ 500–1500 K), low-excitation atomic gas is the [O I]63μm emission line, which is predicted to be the main coolant of dense dissociative J-type shocks caused by decelerated wind or jet shocks associated with protostellar outflows. Under these conditions, the [O I]63μm line can be directly connected to the instantaneous mass ejection rate. Thus, by utilising spectroscopic [O I]63μm maps, we wish to determine the atomic mass flux rate Ṁjet ejected from our target outflows. Results. Strong [O I]63μm emission is detected at the driving sources HH111IRS, HH34IRS, SVS13, as well as at the bow shock region, HH7. The detection of the [O I]63μm line at HH26A and HH8/HH10 can be attributed to jet deflection regions. The far-infrared counterpart of the optical jet is detected in [O I]63μm only for HH111, but not for HH34. We interpret the [O I]63μm emission at HH111IRS, HH34IRS, and SVS13 to be coming primarily from a decelerated wind shock, whereas multiple internal shocks within the HH111 jet may cause most of the [O I]63μm emission seen there. At HH30, no [O I]63μm,145μm was detected. The [O I]145μm line detection is at noise level almost everywhere in our obtained maps. The observed outflow rates of our Class I sample are to the order of Ṁjet ~ 10−6M⊙ yr−1, if proper shock conditions prevail. Independent calculations connecting the [O I]63μm line luminosity and observable jet parameters with the mass -loss rate are consistent with the applied shock model and lead to similar mass-loss rates. We discuss applicability and caveats of both methods. Conclusions. High-quality spectroscopic [O I]63μm maps of protostellar outflows at the jet driving source potentially allow a clear determination of the mass ejection rate.

Protostellar Outflows at the EarliesT Stages (POETS)

Context. 22 GHz water masers are the most intense and widespread masers in star-forming regions. They are commonly associated with protostellar winds and jets emerging from low- and high-mass young stellar objects (YSO). Aims. We wish to perform for the first time a statistical study of the location and motion of individual water maser cloudlets, characterized by typical sizes that are within a few au, with respect to the weak radio thermal emission from YSOs. Methods. For this purpose, we have been carrying out the Protostellar Outflows at the EarliesT Stages survey of a sample (38) of high-mass YSOs. The 22 GHz water maser positions and three-dimensional (3D) velocities were determined through multi-epoch Very Long Baseline Array observations with accuracies of a few milliarcsec (mas) and a few km s−1, respectively. The position of the ionized core of the protostellar wind, marking the YSO, was determined through sensitive radio continuum, multi-frequency Jansky Very Large Array observations with a typical error of ≈20 mas. Results. The statistic of the separation of the water masers from the radio continuum shows that 84% of the masers are found within 1000 au from the YSO and 45% of them are within 200 au. Therefore, we can conclude that the 22 GHz water masers are a reliable proxy for locating the position of the YSO. The distribution of maser luminosity is strongly peaked towards low values, indicating that about half of the maser population is still undetected with the current Very Long Baseline Interferometry detection thresholds of 50–100 mJy beam−1. Next-generation, sensitive (at the nJy level) radio interferometers will have the capability to exploit these weak masers for an improved sampling of the velocity and magnetic fields around the YSOs. The average direction of the water maser proper motions provides a statistically-significant estimate for the orientation of the jet emitted by the YSO: 55% of the maser proper motions are directed on the sky within an angle of 30° from the jet axis. Finally, we show that our measurements of 3D maser velocities statistically support models in which water maser emission arises from planar shocks with propagation direction close to the plane of the sky.

Protostellar Outflows at the EarliesT Stages (POETS)

Context. Although recent observations and theoretical simulations have pointed out that accretion disks and jets can be essential for the formation of stars with a mass of up to at least 20 M⊙, the processes regulating mass accretion and ejection are still uncertain. Aims. The goal of the Protostellar Outflows at the EarliesT Stages (POETS) survey is to image the disk-outflow interface on scales of 10–100 au in a statistically significant sample (36) of luminous young stellar objects (YSO), targeting both the molecular and ionized components of the outflows. Methods. The outflow kinematics is studied at milliarcsecond scales through very long baseline interferometry (VLBI) observations of the 22 GHz water masers, which are ideal test particles to measure the three-dimensional (3D) motion of shocks owing to the interaction of winds and jets with ambient gas. We employed the Jansky Very Large Array (JVLA) at 6, 13, and 22 GHz in the A- and B-Array configurations to determine the spatial structure and the spectral index of the radio continuum emission, and address its nature. Results. In about half of the targets, the water masers observed at separation ≤1000 au from the YSOs trace either or both of these kinematic structures: (1) a spatially elongated distribution oriented at close angle with the direction of collimation of the maser proper motions (PM), and (2) a linear local standard of rest (LSR) velocity (VLSR) gradient across the YSO position. The kinematic structure (1) is readily interpreted in terms of a protostellar jet, as confirmed in some targets via the comparison with independent observations of the YSO jets, in thermal (continuum and line) emissions, reported in the literature. The kinematic structure (2) is interpreted in terms of a disk-wind (DW) seen almost edge-on on the basis of several pieces of evidence: first, it is invariably directed perpendicular to the YSO jet; second, it agrees in orientation and polarity with the VLSR gradient in thermal emissions (when reported in the literature) identifying the YSO disk at scales of ≤1000 au; third, the PMs of the masers delineating the VLSR gradients hint at flow motions at a speed of 10–20 km s−1 directed at large angles with the disk midplane. In the remaining targets, the maser PMs are not collimated but rather tend to align along two almost perpendicular directions. To explain this peculiar PM distribution, and in light of the observational bias strongly favoring masers moving close to the plane of sky, we propose that, in these sources, the maser emission could originate in DW-jet systems slightly inclined (≤30°) with respect to edge-on. Magneto-centrifugally driven DWs could in general account for the observed velocity patterns of water masers.