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The YSO Newsletter
This is the date of the first 'proper' online YSO meeting, time 20:00 UTC. Note there is a typo in the recent AAVSO communication where the date is given as the 12th. It is definitely the 21st! Please drop me a line or two if you would like to see anything specific included, otherwise you will have to listen to me all the time. At the small meeting held a couple of months back, everybody thought our newsletter could not be improved upon - so I'm looking forward to some contrary opinions this time!
AAVSO HQ should shortly be sending out a form to register for the meeting. Please do this, as it is helpful to have some idea of numbers.
As a member of the sequence team, we sometimes get lists of former comparison stars that have turned out to be variable, and we need to see if any replacements are needed. I like doing the research for this (I don't get out much) so was doubly pleased when one of the stars on the latest list was a bright UXOR. NSV 1664 varies from 9.9 to 11.3 visual and so is an easy target for small scopes. It was first suspected of variation in 1924, having been previously used as a comparison for the neighbouring Algol star RZ Tau, and was itself - like several other UXORs - thought to be a star of the same type.
Since it is quite close in the sky to T Tauri the two can be observed in quick succession. Furthermore it lies in the same field as the bright Cepheid SZ Tau - itself interesting as it was discovered by Schwarzschild (of 'radius' fame)
Eruptive accretion events are expected to play an important role in the mass buildup stage of individual star formation. FU Ori objects experience the most extreme eruptive outbursts, which raise the accretion rate of the disc and last for decades. A recent study focussing on the H-band region led to the inference that the mass distribution of FUors is consistent with inferred Solar neighborhood initial mass functions, suggesting all young stars undergo a period of FUor outbursts in their pre main-sequence evolution (though I would add a caveat that 'all young stars' might be better phrased as 'all young Solar-type stars').
Near-IR polarized images of the AB Aur disc at three epochs spanning 3.85 years, as well as Hα images leads to analysis of the dynamics of the entire disc and of the various structures, and to identify sources of Hα emission to derive constraints on their mass accretion rate. The near-IR survey showed that the disc globally follows Keplerian rotation, but reveals a departure from this behaviour at radii smaller than about 60au. Two bright spirals within the millimeter cavity have different dynamic trends, and there could be a link with the identified planet candidates. The non-Keplerian behaviour could also suggest that it could be related to interactions with multiple protoplanets orbiting out of the disc plane on elliptical orbits.
Furthermore, the orbital analysis of some compact sources suggests that their orbital planes are significantly inclined with respect to the disc plane by several tens of degrees. The variability of shadows further suggests that they are produced by optically thick regions located within ~60au. If one of these sources (designated f1) was a point source and the accretion remained constant for 1Myr, it would correspond to 5-20 Jupiter masses according to the magnetospheric accretion model or 6-10 Jupiter masses according to the boundary layer accretion model. All of this shows what a dynamic environment lives around AB Aur.
Most main sequence stars, unlike our Sun, belong to multiple systems with two or more stars. How and when these multiples come together and become bound is uncertain, since the earliest stages of star formation are difficult to resolve. Simulations of star cluster formation in Milky Way-like conditions, including all key physics and stellar feedback mechanisms, can provide an understanding as to how multiple systems form. It shows that around 70 - 80% of binaries are gravitationally bound from the moment the second star forms. Binaries evolve and accrete together, which will affect their planetary systems and chemical evolution. Half of the binaries are disrupted by the end of the star-formation epoch, such that about 40% of the final single stars belonged to a multiple at some point, with implications for the stellar initial mass function. Formation in multiples is the dominant mode of star formation, accounting for at least 57% of stars.
There is now an extended catalogue of 1193 Herbig-Haro (HH) objects, comprising 477 isolated HH objects and 716 HH knots, compiled through a meticulous review of the literature available through mid-2025. The catalogue provides comprehensive data for each entry, including celestial coordinates, distances, knot separations, exciting sources (with spectral types where available), object characteristics, and bibliographic references. The authors also performed a preliminary statistical analysis of key parameters such as distance, exciting source properties, morphology, and excitation state. By combining their extended catalogue with the two earlier HH object catalogues published by Hippel et al in 1988 and Reipurth in 2000, astronomers can access comprehensive information on all known HH objects, thereby facilitating research on star formation and stellar outflows.