YSO Bulletin
- September 2026 -

YSOs on the BBC!

Listening to Radio 4 (BBC) I caught a mention of a 'baby planet' being discovered optically. This turned out to be a 'hot jupiter' in an eccentric, highly-inclined orbit - which isn't I know a YSO but is a YP instead! 51 Eridani which is its parent star is a member of the famous β Pictoris moving group of young (though not baby) stars. 51 Eri is a triple system consisting of the main star plus a distant binary, though the three share the same proper motion. The main star, like β Pic itself, possesses a debris disc which may in fact shroud a second planet. Clearly the planet-making process is still going on around this star, which is slightly more massive and hotter than the Sun.
In Wikipedia you can see an actual animation of part of the new planet's orbit. Methane, water and ammonia have been detected in the atmosphere.

HD 142527

This 8th-magnitude UXOR in Lupus has been studied for some time because of its interesting circumstellar environment, and now a more detailed picture of its inner regions has emerged, as it is surrounded by a transitional disc with a large dust cavity, spiral structures, and an accreting low-mass companion (possibly two). Despite extensive observations, the system's inner regions remain poorly understood, particularly regarding their influence on disc morphology and planet formation. This study aims to investigate the inner region of HD 142527 (< 50 au) with high detection sensitivity to search for undetected components and explore their potential role affecting the disc's structure and evolution.
They revisited the known companion HD 142527 B and update its photometry, astrometry and accretion rate estimates. There is also a new candidate companion at about 14 AU, although it may also be a disc feature or a young gas-giant planet or even a brown dwarf with a mass of 15-50 Jupiter Masses.
They also report the discovery of a tightly wound Hα spiral feature in the inner disc, reconstructed for the first time by high contrast imaging. The spiral implies varying accretion dynamically linked to the known companion B and possibly to the feature above, suggesting ongoing interactions that influence the disc's structure. These findings provide new insights into the complex interactions within the HD 142527 system, highlighting the role of multiple companions in driving disc asymmetries and facilitating planet formation.

An Old Friend restudied

A standby of many of us interested in YSO's, RR Tauri, is the subject of a Russian study under the heading of Veiling of Photosphere Lines in the Spectra of UX ORI Stars at Deep Light Minima and the authors mention the observational signs that can be used to classify stars as UX Ori; which include the presence of deep Algol-like minima, a bluing effect, and an increase in the degree of linear polarization in the minima. These minima are caused by the shielding of stars by opaque fragments of protoplanetary discs tilted at a small angle to the line of sight.
There are two possible different sources of spectral variability. One of them is typical for many young stars. In this case, it is due to variations in temperature and density in the emitting region and along the line of sight. Observations show that such variations can be sufficiently strong, and thus lead to strong variability of spectral lines. Another mechanism of spectral variability is characteristic only of UX Ori type stars. It is caused by the coronagraphic effect created by dust components of the protoplanetary disc, which obscure the star from the observer and cause deep minima. By shielding the star from the observer, the opaque fragments of the disc also shield that part of the gas shell in front of the star that is responsible for the absorption component of the line profile, which can in fact disappear completely.

A new EXOR?

Gaia 21bja is a very young star of magnitude 14 in the Upper Scorpius starforming region that has been found to exhibit brightness surges on a roughly 900-day cycle, which suggests it is a member of the class of eruptive YSOs known as EX Lupi objects or 'EXORs'. Its outbursts last for up to 2 years and have an amplitude of almost 2 magnitudes which makes it worth following on a continuous basis.
As an object it is significantly smaller, cooler, and less luminous than the Sun, though the accretion luminosity and mass accretion rate increased by a factor of 5.5−6 during a recently-observed burst.