YSO Bulletin
- February 2025 -

- Those Titillating Triple T Tauris! -

GW Orionis

This bright YSO in Ori T1 (the star formation region around λ Orionis) gets the 'my favourite object' treatment by Prof. Stefan Kraus of Exeter University here in the UK, which is quite a centre of YSO studies. He describes it as a pre-main-sequence triple with a warped disk and a torn-apart ring and is a hint - if any were needed - that star formation is a very violent process! For instance, he cites the phenomenon of disc tearing that might occur in the discs around multiple stars whose orbital angular momentum vectors are misaligned with respect to the rotation axis of the disc. All 3 stars are slightly more massive than the Sun while remaining T Tau stars. The A and B stars show a 242-day orbital period while the more eccentric, and inclined C orbits A and B in about 11 years, while both components (A/B and C) have their own circumstellar discs. Three large dust rings were detected using ALMA and some idea of the size of the system can be got from the scale in the graphic.

It can also be seen that the inner ring is eccentric to the stars themselves - all in all a peculiar setup. GW Ori shows significant, but not huge, optical variation.

TW Hydrae

Matter flowing through a star's accretion disc is channelled onto the stellar surface by the stellar magnetic field. This is thought to be strong enough to truncate the disk close to the corotation radius, at which the disk rotates at the same rate as the star. Studies of YSOs show that hydrogen emission (a well known tracer of accretion activity) mostly comes from a region a few milliarcseconds across, usually located within the dust sublimation radius. Its origin could be the stellar magnetosphere, a rotating wind or a disk. In the case of T Tauri sources, their larger magnetospheres should make them easier to resolve than with more massive stars.
A recent study used long-baseline interferometric observations that spatially resolve the inner disk of TW Hydrae, finding that the NIR hydrogen emission comes from a region approximately 3.5 stellar radii across. This region is within the continuum dusty disk emitting region (7 stellar radii across), indicating that the hydrogen emission originates in the accretion columns (funnel flows of matter accreting onto the star), as expected in magnetospheric accretion models, rather than in a wind emitted at much larger distance.

T Tau itself

A joint 2020 US-French analysis set out to reveal the nature of the enigmatic T Tauri triple star system. Multiwaveband observations show morphologies of disk material and outflow kinematics. A dark lane of obscuring material is seen in silhouette in several emission lines and dust evidence near the position of T Tau Sa+Sb, revealing the circumbinary ring around T Tau S. The flux variability of T Tau S is linked in part to the binary orbit; T Tau Sb brightens near orbital apastron as it emerges from behind circumbinary material. Outflow diagnostics confirm that T Tau N powers the blue-shifted western outflow, and the T Tau S binary drives the NW-SE flow.
Of course at the moment T Tau is going through a faint phase which is being well-followed. Again we have a very active system; analysis of the southern outflow shows periodic arcs ejected from the T Tau system. Correlation of these arc locations suggests that launch of the last four southern outflow ejections is contemporaneous with, and perhaps triggered by, the T Tau Sa+Sb binary periastron passage. The study revealed a geometry of the T Tau triple that has the southern components foreground to T Tau N, obscured by a circumbinary ring, with mis-aligned disks and interacting outflows. Particularly, a wind from T Tauri Sa that is perpendicular to its circumstellar disk might interact with the circumbinary material. T Tauri is an important laboratory to understand early dynamical processes in young multiple systems.

HQ Tau

This is a 1.9M☉ classical T Tauri star (CTTS). These objects host a strong magnetic field, and both magnetospheric accretion and ejection processes develop as the young star interacts with its disk. Variability of the system was investigated by a multinational team using Kepler-K2 and complementary light curves, and from a high-resolution spectropolarimetric time series.
The Kepler-K2 light curve exhibits a sinusoidal period of 2.424 d, which they ascribe to the rotational period of the star. The radial velocity of the system shows the same periodicity, as expected from the modulation of the photospheric line profiles by surface spots. A similar period is found in several emission lines due to the appearance of inverse P Cygni components, indicative of accretion funnel flows.
Signatures of outflows are also seen in the line profiles, some being periodic, others transient. Additionally, they report HQ Tau to be a spectroscopic binary candidate whose orbit remains to be determined.