YSO Bulletin
- January 2024 -

- A Real Enigma! -

BP Psc - YSO or No?

Emails have been flying all over the world, from Argentina to Siberia via Cambridge trying to find out exactly what this object is. It's not faint and obscure (12th mag visual) and its lightcurve looks like a double-mode pulsator of type SRd, which is how it appears in VSX. These stars usually have well-defined variations and are of spectral types G and K, as this is. The discovery paper gives it an alias of StHa 202 (i.e., a star showing Hα emission) as it had not yet been given a variable star name; and is worthy to note that paper explicitly defines it as a T Tauri star and says that its variation is not obviously periodic, in passing mentioning the high Lithium abundance as well. BP Psc also shows a very high Infra-red excess, possesses a circumstellar accretion disc and this, coupled with the presence of a bipolar outflow would suggest a YSO straight away. On the other hand, the regular variation and its position (57° below the galactic plane) are less suggestive of this. The latest paper definitely comes down on the YSO side but, even as a YSO fan, I remain to be convinced. Like V1117 Her, not only is it a long way from any starforming region but its earlier years were spent even farther away. So... it looks like a duck, quacks like a duck - but might be a goose.

Another Cygnus FUOR

A recent paper describes the outburst of a star associated with the filamentary dark nebula B 168, itself part of the Cocoon nebula starforming complex. Images show that over a period of 12 years the forming star increased its brightness by 2 magnitudes (2010-2022) and it is known to be a binary, which will add weight to the theory that FUOR outbursts may be related to binarity. The authors reveal that the luminosity of the deeply-embedded outbursting component is estimated to be about 0.23 L☉ before the outburst and some 0.95 L☉ during the outburst, which is 1 to 2 orders of magnitude fainter than bonafide FU Ori outbursts.
They go on to interpret this eruption as an FU Ori-type outburst, although the possibility of brightening following an extinction episode cannot be ruled out.

Planet-forming review

Over the past decade, advancement of observational capabilities such as ALMA, alongside theoretical innovations like pebble accretion, have reshaped our understanding of planet formation and the physics of protoplanetary disks. Despite this progress, mysteries persist along the winding path of micrometer-sized dust, from the interstellar medium, through transport and growth in the protoplanetary disk, to becoming gravitationally bound bodies. A recent review outlines our current knowledge of dust evolution in circumstellar disks, yielding the following insights:
• Theoretical and laboratory studies have accurately predicted the growth of dust particles to sizes that are susceptible to accumulation through transport processes like radial drift and settling.
• Critical uncertainties in that process remain the level of turbulence, the threshold collision velocities at which dust growth stalls, and the evolution of dust porosity.
• Substructures are widespread. Dust traps appear to be solving several long-standing issues in planet formation models, and they are observationally consistent with being sites of active planetesimal formation.
• In some instances, planets have been identified as the causes behind substructures. This underlines the need to study earlier stages of disks to understand how planets can form so rapidly.
In the future, better probes of the physical conditions in optically thick regions, including densities, turbulence strength, kinematics, and particle properties will be essential for unraveling the physical processes at play.

Bright YSOs studied

A recent study looked at 9 stars with protoplanetary discs, 2 of which will be familiar to AAVSO observers: AB Aur and CQ Tau. Two of these sources (V892 Tau, CQ Tau) have multi-epoch observations that reveal significant variability. CQ Tau and AB Aur have CO line profiles with centrally-peaked components that are similar to line profiles that have been interpreted as evidence of molecular gas arising from a wide-angle disk wind. Alternatively, emission from a circumplanetary disk (CPD) could also account for this component.