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YSO Bulletin
- April 2021 -

- Two Pro-Am Campaigns -

SU Aur imaged

ESO VLT/SPHERE polarimetric observations of the nearby (~150 pc) YSO SU Aur resolving structure down to scales of ~7 AU. The disc shows an intricate spiral structure and a shadow lane, cast by an inner, misaligned disk component.
The observations suggest that SU Aur is undergoing late infall of material, which can explain the observed disk structures. SU Aur is the clearest observational example of this mechanism at work and demonstrates that late accretion events can still occur in the class II phase such that SU Aur is, thereby significantly affecting the evolution of circumstellar disks. Constraining the frequency of such events with additional observations will help determine whether this process is responsible for the spin-orbit misalignment in evolved exoplanet systems. Credit: Ginski et al. 2021, ESO.

Breaking up isn't hard to do

ALMA observations at 1.4 mm and 0.2” (750au) angular resolution of the Main core in the high-mass star forming region G31.41+0.31 have revealed a puzzling scenario: on the one hand, the continuum emission looks very homogeneous and the core appears to undergo solid-body rotation, suggesting a monolithic core stabilized by the magnetic field; on the other hand, rotation and infall speed up toward the core centre, where two massive embedded sources have been detected, pointing to an unstable core having undergone fragmentation.
The observations have clearly resolved the area into at least four sources within 1”, indicating that the core is not monolithic. The deconvolved radii of the dust emission of the sources, estimated at 3.5 mm, are 400-500au, and their masses range from 15 to 26 M☉
This starforming region near the Galaxy's centre was found, by another team of astronomers, to contain the spectral evidence of a sugar molecule necessary for the development of RNA.

New Dipper in Orion

V555 Ori is a T Tauri star, whose 1.5 mag brightening was published as a Gaia science alert in 2017. A Hungarian group ()including longtime YSO expert Mária Kun) carried out optical and near-infrared photometric, and optical spectroscopic observations to understand the light variations. The light curves show that V555 Ori was faint before 2017, entered a high state for about a year, and returned to the faint state by mid-2018. In addition to the long-term flux evolution, quasi-periodic brightness oscillations were also evident, with a period of about 5 days. At optical wavelengths both the long-term and short-term variations exhibited colourless changes, while in the near-infrared they were consistent with changing extinction.
These variations can be explained as the consequence of changing extinction. The object has a low accretion rate whose variation in itself would not be enough to reproduce the optical flux changes. This behaviour makes V555 Ori similar to the pre-main sequence star AA Tau, where the light changes are interpreted as periodic eclipses of the star by a rotating inner disc warp.
The brightness maximum of V555 Ori was a moderately obscured (AV=2.3 mag) state, while the extinction in the low state was AV=6.4 mag. While the Gaia alert hinted at an accretion burst, V555 Ori is a standard dipper, similar to the prototype AA Tau. However, unlike in AA Tau, the periodic behaviour was also detectable in the faint phase, implying that the inner disc warp remained stable in both the high and low states of the system.
V555 Ori is in the Orion Nebula region, close to the bright pair 42 and 45 Orionis, and although it was not part of the recent AAVSO campaign (which concentrated on different T-regions in Orion) observers with decent-sized equipment might like to consider it for this coming apparition (variation is around magnitude 16).

NGC1977NGC1977, in which V555 is located.