The accretion-ejection connection in the asymmetric Th 28 jet revealed by MUSE-NFM

TL;DR

The study investigates the accretion–ejection connection in the asymmetric Th 28 jet by using VLT/MUSE-NFM AO observations to map the inner jet (<6'') with ~0.12'' resolution and to perform optical FEL diagnostics. By combining BE-type line ratios and additional [Oii]-based diagnostics, the authors derive electron density, ionisation, temperature, and shock speeds near the jet base, enabling robust estimates of mass accretion and outflow rates. They detect multiple new knots and find that knot ejections occur on 3–6 year timescales, with mass outflow rates in both lobes near the base being similar and tracking changes in the accretion rate (dot M_acc ≈ 2.11 × 10^{-7} M⊙ yr^{-1}), yielding a jet efficiency around 0.1. The results support magnetohydrodynamic jet-launch models and reveal notable accretion–ejection variability, making Th 28 a strong target for ongoing high-resolution monitoring to further elucidate the launching mechanism and intrinsic jet asymmetry.

Abstract

Mass loss through stellar jets is closely tied to the process of accretion through the disk. Understanding phenomena such as episodic ejections and outflow asymmetries can thus shed light on the mechanism of jet launching and its connection to both mass accretion and the evolution of the protoplanetary disk. We use new VLT/MUSE Narrow Field Mode observations of the Classical T Tauri Star Th 28 to map the jet structures within 6'' of the source at an effective angular resolution of 0.''12, provided by the combination of the AO correction and image deconvolution. The emission line profiles and flux ratios are investigated and diagnostic analysis of the optical forbidden emission lines (FELs) is used to estimate the electron density, ionisation fraction, electron temperature and shock velocities in both jet lobes within 200 au of the star. The mass outflow rates in each lobe are obtained using the derived total densities and FEL luminosities and compared with the mass accretion rate. We identify several new knots in both jet lobes which have been ejected in the previous 10 years on a timescale of 3-6 years, which is significantly more frequent than previously estimated. In both lobes we find comparable mass outflow rates close to the jet base. Th 28 has undergone a significant rise in mass accretion rate between 2014 and 2023, which may be linked to the most recently ejected knot pair detected in each side of the jet. The red-shifted jet mass outflow rate shows a similar increase of a factor 2, indicating that the ratio of mass outflow to accretion remains constant. A moderately lower mass outflow rate is found in the faster blue-shifted lobe, supporting the possibility that momentum ejection is conserved on each side of the jet. The frequent knot ejections indicate that this source is a good target for further monitoring to study the accretion-ejection connection.

Figures (26)

• Figure 1: The VLT/MUSE Narrow-Field Mode view of Th 28, showing the jet in H$\alpha$ emission before continuum subtraction. A black cross marks the source position (the peak of the continuum emission). The two fields of view A and B are highlighted, centred on the red-shifted (western) and blue-shifted (eastern) jet lobes, respectively.
• Figure 2: Spectro-images of the blue-shifted jet channels (-140 km s$^{-1}$ to +90 km s$^{-1}$) in Field B, before and after deconvolution. Contours are smoothed using a Gaussian filter (kernel $\sigma$ = 1.5) and start at 3-$\sigma$ of the background rms level pre-deconvolution, approximately 10 $\times$ 10$^{-20}$ ergs s$^{-1}$ cm$^{-2}$. The levels increase as a factor of $\sqrt{2.5}$. We note that the right-hand side of the image is distorted by the edge of the detector FoV. Additional emission lines are shown in Fig. \ref{['fig:decon_ims_app']}.
• Figure 3: Spectro-images of the red-shifted jet channels (-90 km s$^{-1}$ to +140 km s$^{-1}$) in Field A, before and after deconvolution. Contours are as in Fig. \ref{['fig:decon_blue']}, with starting levels of approximately 8 $\times$ 10$^{-20}$ ergs s$^{-1}$ cm$^{-2}$ and increasing as a factor of $\sqrt{2}$. The left edge of the image is at the edge of the detector FoV. Additional emission lines are shown in Fig. \ref{['fig:decon_ims_app']}.
• Figure 4: Selected flux profiles along the jet axis in the full mosaic (without deconvolution). Top panel: The H$\alpha$ profile (grey) without scattered emission subtraction, showing the components of the fitted model. The line emission at the source peak is fitted with a Moffat function (black dashed line) and knot peaks are fitted with Gaussian profiles (coloured dashed lines). Lower panels: Flux profiles in three key FELs after scattered emission subtraction (see Appendix \ref{['section:scattered_sub']}). The tentative and detected knot centres are marked with vertical blue lines, showing their correspondence to the features in the FEL profiles.
• Figure 5: Deconvolved jet widths and opening angles $\alpha$ fitted across the full red-shifted jet velocity range; widths are fitted at +05 from the source and at the knot positions. Over-plotted is the average jet width obtained for this lobe from HST-STIS optical data Coffey2004Coffey2007.