YSO Bulletin
- September 2022 -

- The YSO Circular -

...Hopefully!

At present the AAVSO issues regular circulars for various variable types (the CV circular, LPV circular for example) which members can sign up for to follow their favourite stars to see how they are doing. So I have asked HQ if they can produce one for YSOs as well. As yet (Aug 29) I have had no reply, but I see no reason why we shouldn't have one.

Organics found again

Recent astrochemical models and experiments have explained that complex organic molecules (COMs; molecules composed of six or more atoms) are produced on the dust grain mantles in cold and dense gas in prestellar cores. However, the detailed chemical processes and the roles of physical conditions on chemistry are still far from understood. To address these questions, a Korean team investigated twelve high-mass star-forming regions using ALMA. They are associated with CH₃OH masers, indicative of undergoing active accretion.
The team found 28 hot cores with COMs emission among 68 continuum peaks at 1.3 mm and specified 10 hot cores also associated with similar masering. Up to 19 COMs were identified including oxygen- and nitrogen-bearing molecules and their isotopologues in cores. The derived abundances show a good agreement with those from other low- and high-mass star-forming regions, implying that the COMs chemistry is predominantly set by the ice chemistry in the prestellar core stage. One clear trend is that the COMs detection rate grows steeply with the gas column density, which can be attributed to the efficient formation of COMs in dense cores.
Finally, the results suggest that the enhanced abundances of several molecules in those hot cores could be originated by active accretion as well as the differing physical conditions of cores.

New YSO Catalogue

Active star forming regions are excellent laboratories for studying the origins and evolution of YSO clustering. The W40 - Serpens South (SS) region is such a region, and a catalogue has been compiled of IR point sources in it, based on deep NIR observations with 2MASS, UKIDSS, and the Spitzer catalog. 832 YSOs were found, and 15, 135, 647, and 35 found to be deeply embedded sources, Class I, Class II YSOs, and transition disk sources, respectively. In general, these YSOs are well correlated with the filamentary structures of molecular clouds, especially the deeply embedded sources and Class I's. The W40 central region is dominated by Class II YSOs, but in the SS region, half of the YSOs are Class I. This implies a mixture of YSO distributions composed of both primordial and somewhat evolved YSOs in this star forming region.

ALMA and Organics again!

With the advent of ALMA, it is now possible to observationally constrain how disks form around deeply embedded protostars. In particular, the recent ALMA C₃H₂ line observations of the nearby protostar L1527 in the Taurus SFR have been interpreted as evidence for the so-called "centrifugal barrier," where the protostellar envelope infall is gradually decelerated to a stop by the centrifugal force in a region of super-Keplerian rotation (rotation rate independent of an adjoining rotating object - MP).

To test the concept of centrifugal barrier, which was originally based on angular momentum conserving-collapse of a rotating test particle around a fixed point mass, a simulation by Jones et al of protostellar disk formation including a minimum set of ingredients: self-gravity, rotation, and a prescribed viscosity that enables the disk to accrete. They found that a super-Keplerian region can indeed exist when the viscosity is relatively large but, unlike the classic picture of a centrifugal barrier, the infalling envelope material is not decelerated solely by the centrifugal force. The region has more specific angular momentum than its surrounding envelope material, which points to an origin in outward angular momentum transport in the disk rather than the spin-up of the envelope material envisioned in the classic picture, as it falls closer to the centre in order to conserve angular momentum.
For smaller viscosities, the super-Keplerian rotation is weaker or non-existing, and they conclude that, despite the existence of super-Keplerian rotation in some parameter regime, the classic picture of centrifugal barrier is not supported by their simulations.
L1527 is less than half a million years old, possibly much less.

Westerhout 40, part of the study on the left.