YSO Bulletin
- February 2024 -

- Streamers -

It's all Kicking Off!

Multispectral studies of nearby forming stars provide insights into all classes of accreting systems. Objects which have magnetic fields, spin, and accrete produce jets and collimated outflows. Jets are seen in systems ranging from brown dwarf stars to supermassive black holes. Outflow speeds are typically a few times the escape speed from the launch region, which in the case of young stars is mesaured in hundres of metres per second. Since many YSOs are relatively nearby, we can see outflow evolution and measure proper motions on timescales of years. Because the shocks in YSO outflows emit in atoms, ions, and molecules in addition to the continuum, many physical properties such as temperatures, densities, and velocities can be measured. YSO outflows are a major source of feedback in the self-regulation of star formation. Some dense star-forming regions produce powerful explosions. The nearest massive star-forming region, Orion OMC1, powered a ~10⁴⁸ erg explosion about 550 years ago (that is when the light from the event would have reached the Solar System). The explosion was likely powered by an N-body interaction which resulted in the formation of a compact, AU-scale binary or resulted in a protostellar merger. The binary or merger remnant, the 15 solar mass object known as Src I was ejected from the core with a speed of ~10 km/s along with two other stars, a 10 M☉ object was ejected at 30 km/s and a 3 M☉ one at 55 km/s.

Recent Activity section

John Pickett reports a 1-mag fade of V339 Ori, from magnitude 13.1 to 14.1, which is the faintest point on the current lightcurve. The precise type of the star is uncertain.

VY Tau, the possible EXOR, had a minor eruption at the end of January to about magnitude 13½ visual

V730 Cep has definitely recovered from its record faint state and was actually at a bright maximum of 13.1 in Mid-January.

Streamers take 1

Recent observations of protostellar cores suggest that most of the material in the protostellar phase is accreted along streamers. Streamers in this context are defined as velocity-coherent funnels of denser material potentially connecting the large scale environment to the small scales of the forming accretion disc. A study on a core collapse within a turbulent filament found that overdensities are formed naturally by the initial turbulent velocity field inherited from the filament and subsequent gravitational collimation. This leads to streams which show a sheet-like morphology, with the same radial infall velocities as the low density material. As a main consequence of the turbulent initial condition, the mass accretion onto the disk does not follow the predictions for solid body rotation. Instead, most of the mass is funnelled by the overdensities to intermediate disk radii.

β Pictoris

For decades, the spectral variations of Beta Pictoris have been modelled as the result of the evaporation of exocomets close to the star, termed falling evaporating bodies (FEBs). Resonant perturbations by a giant planet have been proposed to explain the dynamical origin of these stargrazers. The disk is now known to harbour two giant planets, Beta Pic b and c, orbiting the star at 9.9 au and 2.7 au. While the former almost matches the planet formerly suspected, the discovery of the latter complicates the picture. A recent paper questioned the stability of the two-planet system, then investigated the dynamics of a disk of planetesimals orbiting the star with both planets, to check the validity of the FEB generation mechanism. They then focussed on regions where disk particles are able to reach high eccentricities thanks to resonant mechanisms.
The first result is that the system is dynamically stable. Simulations reveal that the whole region extending between 1.5 and 25 AU is unstable to planetary perturbations. However, a disk below 1.5 au survives, which appears to constitute an active source of FEBs via resonance with Beta Pic c, which acts as a distant perturber that helps sustain the whole process. They found that FEBs are likely to originate from a region much further in and related to resonance with Beta Pic c. That mechanism also appears to last longer, as new planetesimals are able to continuously enter the resonance zones and evolve towards the FEB state. Subsequently, the physical nature of the FEBs may differ from that previously thought, and presumably may not be icy.

DG Tau: streamers take 2

DG Tau is a nearby T Tauri star associated with a collimated jet, a circumstellar disk and a streamer a few hundred AU in length. The streamer connects to the disk at about 50 au from DG Tau. At this location SO emission is observed, likely due to the release of sulphur from dust grains caused by the shock of the impact of the accretion streamer onto the disk. We investigate the possibility that the DG Tau streamer was produced via cloudlet capture, considering a cloudlet initiating infall at 600 au from DG Tau with low angular momentum so that the centrifugal force is smaller than the gravitational force, even at 50 au. The elongation of the cloudlet into a streamer is caused by the tidal force when its initial velocity is much less than the free-fall velocity. The elongated cloudlet reaches the disk and forms a high density gas clump. The streamer should perturb the disk for several thousands of years after impact.