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Evidence for an MHD disk wind via optical forbidden line spectro-astrometry

Spectro-astrometry is used to investigate the low velocity component (LVC) of the optical forbidden emission from the T Tauri stars RU Lupi and AS 205 N. Both stars also have high velocity forbidden emission (HVC) which is tracing a jet. For AS 205 N, analysis reveals a complicated outflow system. For RU Lupi, the [O I] 6300 and [S II] 6716, 6731 LV narrow component (NC) is offset along the same position angle (PA) as the HVC but with a different velocity gradient than the jet, in that displacement from the stellar position along the rotation axis is decreasing with increasing velocity. From the LVC NC PA and velocity gradient, it is inferred that the NC is tracing a wide angled MHD disk wind. A photoevaporative wind is ruled out. This is supported by a comparison with a previous spectro-astrometric study of the CO fundamental line. The decrease in offset with increasing velocity is interpreted as tracing an increase in the height of the wind with increasing disk radius. This is one of the first measurements of the spatial extent of the forbidden emission line LVC NC (~ 40 au, 8 au for RU~Lupi in the [S II] 6731 and [O I] 6300 lines) and the first direct confirmation that the LVC narrow component can trace an MHD disk wind.


INTRODUCTION
A crucial step in understanding how stars accrete their mass, as well as how disks evolve, is clarifying how the accreting disk gas loses angular momentum. Originally MHD disk winds were favored (Koenigl & Ruden 1993) but the prevailing view for several decades has been that MRI-induced turbulence (Balbus & Hawley 1991) leads to outward viscous transport of angular momentum (Armitage 2011), enabling disk material to flow radially inward. However, recent simulations find that non-ideal MHD effects suppress MRI over a large range of disk radii (∼ 1−30 au), restoring radially extended MHD disk winds as the prime means for extracting angular momentum and enabling accretion at the observed rates (Bai & Stone 2013;Gressel & Pessah 2015).
On the observational side, there has been renewed interest in identifying disk wind tracers and testing the emerging paradigm of disk evolution. See Ercolano & Pascucci (2017) for a recent review. Emission from optical forbidden lines has been a long-established tracer of flowing material from young stars (Edwards et al. 1987) and recent observations have helped clarify its origin. The so-called "high-velocity" component (HVC), emission blueshifted by > 30 km/s (Hartigan et al. 1995), is confirmed to be associated with collimated micro-jets (e.g., Hartigan & Morse 2007) and its [O I] λ6300 luminosity is found to be better correlated with accretion luminosity than with stellar luminosity or mass (Nisini et al. 2018). High-resolution (∆v < 10 km/s) spectroscopy revealed that the lower velocity component (LVC) can be often described as the combination of two Gaussian profiles (a "broad component," BC, and a "narrow component," NC; (Rigliaco et al. 2013;Simon et al. 2016;McGinnis et al. 2018). The LVC-BC blueshifts and large FWHMs point to an MHD disk wind origin (Simon et al. 2016) while the origin of the LVC-NC is less certain. However, two observables suggest that the NC most likely traces the same MHD wind, instead of a photoevaporative thermal wind (Weber et al. 2020): i) its peak centroids and FWHMs correlate with those of the BC (Banzatti et al. 2019) and ; ii) the observed [S II] λ6731 LVC luminosities are much weaker than predicted in photoevaporative winds (Pascucci et al. 2020). Clarifying whether the NC truly traces an MHD disk wind and measuring its vertical extent is critical to estimate wind mass loss rates (Fang et al. 2018) and directly test one of the main predictions of the disk wind theory, i.e., that wind mass loss rates are similar to mass accretion rates.
Here, we apply the spectro-astrometry (SA) technique to the optical forbidden lines from RU Lupi and AS 205 N, two well known late K-type stars with evidence for jets and winds (see Section 2.1 and Table 1 for further details). Both of our targets are among a handful of TTS with high accretion rates, that also show strong [O I] λ6300 with very broad high velocity emission that is blended with the low velocity emission, and a low velocity component that includes both a NC and a broad BC component, where the NC dominates over the BC. Moreover, in contrast to the majority of accreting TTS where the LVC is not detected in [S II] λ6731, our two targets show prominent LVC emission at [S II] λ6731 as well as [O I] λ6300. RU Lupi and AS 205 N have previously been investigated with SA (Takami et al. 2001(Takami et al. , 2003 and the results of these works are discussed in Sections 2.1 and 3. While the results presented here confirm the previous results, the much improved spectral resolution and positional uncertainty of our study, allowed new details to be revealed about the kinematics and spatial properties of the line regions under investigation. Furthermore, Takami et al. (2003) did not investigate the FELs of the AS 205 system but analysed several permitted lines (including Hα) to search for a binary signal.
SA is a powerful technique for recovering spatial information from spectra below the angular resolution limitations of the observations (Bailey 1998;Whelan & Garcia 2008;Cahill et al. 2019). Therefore, our goal is to use this technique to explore the origin of the low velocity forbidden emission in both stars. The technique is based on measuring the centroid of the spatial profile of an emission line region as a function of wavelength, and with respect to the centroid of the spatial profile of the continuum. Frequently, this has been done using Gaussian fitting to produce what is referred to as a position spectrum. For ground-based spectroscopy the accuracy to which this can be done is dependent on the signal to noise ratio (SNR) of the observation and the seeing, and sub-milliarcsecond precision has been achieved at near-infrared wavelengths with for example VLT/CRIRES (Pontoppidan et al. 2011). This study focuses on the [O I] λ5577, [O I] λ6300 and [S II] λ6731 lines and the work is organised as follows. In Section 2 the targets, observations and reduction of the data are described. In Section 3 the spectra and spectro-astrometric results are discussed. Sections 4 and 5 are devoted to the Discussion and Summary.

Targets
RU Lupi is a CTTS located in the Lupus Star forming region at d ≈ 140 pc. Takami et al. (2001) used spectroastrometry to investigate the Hα emission of RU Lupi, and found that there is a distinct spatial symmetry in the offset for blue and red shifted wings of the Hα line. This signal was attributed to a bipolar outflow driven by RU Lupi and the bipolar outflow theory was supported by their spectro-astrometric study of the [O I] λ6300 and [S II] λ6731 lines. The PA of the outflow was estimated at around 225 • from this work. Pontoppidan et al. (2011) measure a spectroastrometric signal in the CO fundamental line consistent with that of a wide-angle molecular wind. The DSHARP study measured the PA of the RU Lupi disk at ∼ 121 • (Huang et al. 2018), which is approximately perpendicular to the outflow direction estimated by Takami et al. (2001).
AS 205 is a triple system (AS 205 N, AS 205 Sa,b) (Kurtovic et al. 2018), located in the northern region of the Ophiuchus star-forming region at d ∼ 127 pc (Gaia Collaboration et al. 2018). It's primary star AS 205 N (also AS 205 A) is a CTTS and it is distinguished by its bright molecular emission (Salyk et al. 2008 (Kurtovic et al. 2018). Pontoppidan et al. (2011) found evidence for a molecular wind using spectro-astrometry in the CO fundamental line. Takami et al. (2003) carried out a spectro-astrometric study of the Hα emission from the AS 205 system. Analysis showed a displacement along a PA of ∼ 33 • (E of N) which is interpreted as a binary signature. The authors suggest that any difference between the PA of the displacement and the PA of AS 205 N and AS 205 S can be explained by the orbital motion of the system. Spectra of RU Lupi and AS 205 were collected with the UV-Visual Echelle Spectrograph (UVES) on the European Southern Observatory's Very Large Telescope (ESO VLT) in May and June 2012 (Dekker et al. 2000) through the ESO programme 089.C-0299 (PI. I. Pascucci). The wavelength range was approximately 5000Å to 7000Å and a spectral resolution of R = 40,000 was achieved. As the spectro-astrometric precision is given by the following equation

Observations and Data Reduction
where N p is the number of detected photons and the FWHM is the seeing, the exposure times were set, assuming typical seeing, to reach 2 mas precision. This goal was achieved and sub 2 mas precision was reached, in general, for the emission line regions. Additional details on the observations are given in Table 2. The slit width was set to 1 . With UVES it is possible to rotate the slit PA to any position on the sky. For both of our targets, spectra were obtained at four orthogonal slit PAs. The slit PAs were chosen so that spectra were obtained parallel and anti-parallel to the known accretion disk PA 1 and parallel and anti-parallel to any outflow emission. It was assumed that the PA of any outflow emission is perpendicular to the disk PA. The strategy of obtaining four Combining the perpendicular slit PAs to recover the 2D centroid at each wavelength(velocity). The continuum emission has been subtracted here to reveal the full displacement in the outflow. The black circle is the 3-σ uncertainty in the centroid, the black line is the best fit PA to the flows and the red arrow is the PA of the RU Lupi disk major axis.
perpendicular slit PAs is integral to the spectro-astrometric analysis. Firstly, the observations at anti-parallel slit PAs allow for spectro-astrometric artefacts to be ruled out (Whelan et al. 2015;Brannigan et al. 2006). Secondly, spatial offsets measured in spectra taken at perpendicular slit PAs can be combined to recover the on-sky PA of the extension in the emission line region. Further details on the observations are given in Table 2. The data were initially reduced using the ESO pipeline. Whelan et al. (2015) discuss how the re-binning of X-Shooter spectra by the ESO pipeline can introduce a false spectro-astrometric signal due to a spatial aliasing effect. A similar problem was found for some of the observations, when the 2D spectra produced by the UVES pipeline were analysed using SA. For this reason the spectra were also reduced using standard IRAF routines for the reduction of echelle spectra and it was this data that was used in the analysis and that is presented below. It was found by the authors that using IRAF for the data reduction reduced the spatial aliasing problem. The SA analysis was done using standard fitting routines. The continuum subtraction was done using the IRAF continuum routine. With this routine the continuum is fitted and removed from the spectrum row by row, where a row is along the dispersion direction (Whelan & Garcia 2008). For each row only the continuum emission is fitted by masking the emission line regions. The spectra were also also corrected for telluric and photospheric lines using the method outlined in Hartigan et al. (1995) and adopted in subsequent papers (e.g., Rigliaco et al. 2013;Banzatti et al. 2019).

RESULTS
The UVES spectra cover two oxygen forbidden lines, one at 6300Å, and the other at 5577Å, with the latter being the weaker of the two (e.g., Simon et al. 2016), and the sulphur forbidden line doublet at 6716Å, 6731Å. The oxygen profiles from both sources, obtained at a resolution similar to UVES, were previously analyzed in Fang et al. (2018); Banzatti et al. (2019). These works separated CTTS line profiles into distinct kinematic components by finding the minimum number of Gaussian profiles that reproduce the observed one. Components are identified as HVC, where the absolute value of the Gaussian velocity centroid is larger than 30 km s −1 , or LVC, with centroids within 30 km s −1 of the photospheric velocity. The LVC can be further decomposed into a BC or NC (FWHM larger or smaller than 40 kms −1 ), see e.g. Simon et al. (2016). In those studies both RU Lupi and AS 205 N were found to have quite broad HVC emission, requiring fits with more than one component, and their LVC showed both BC and NC.   Combining the perpendicular slit PAs to recover the 2D centroid at each wavelength(velocity). The continuum emission has been subtracted here to reveal the full offset in the outflow. The black circle is the 3-σ uncertainity in the centroid, the red arrow the disk PA, the magenta arrow the binary PA and the black arrow the best PA fit to the signals. These results are difficult to explain if the triple system is driving only one outflow.  Table 3. All velocities quoted here are in the stellocentric reference frame. The differences between the results of the kinematical fitting presented here, and those of previous studies, for the [O I] λ6300 line, are explained by variability in the HVC and this is discussed in detail in the appendix. -2.6, 42.9 As described in Section 2.1 the AS 205 system is a multiple system with three known members. The aim here was to investigate the emission from AS 205 N. However, for the 55 • and 235 • slit PAs AS 205 S a,b are also included in the UVES slit. Therefore, given a separation of 1. 3 between the N and S components, and a seeing of ∼ 0. 8, the emission from AS 205 S could also be evaluated at these PAs (although AS 205 S a,b are not resolved). For AS 205 S only the [O I] λ6300 line was detected, showing a weak LVC BC. The spectro-astrometric analysis for AS 205 S will be discussed in a separate paper. As the binary PA is ∼ 212 • and the slit width was 1 , there was no emission from AS 205 S in the 145 • and 325 • spectra.
Next, we will be presenting the spectro-astrometric results and the following points should be noted for all the results.
1. The method of Takami et al. (2001) where the position spectra for the anti-parallel slit positions were subtracted was followed. Takami et al. (2001) discuss how this technique can remove artefacts caused by for example, optical distortion introduced by the spectrograph, misalignment of the spectrum within the CCD columns, or imperfect flat fielding.
2. The 1-σ uncertainties in the centroid position given in Figures 1 and 3 are the average values for the whole spectral range analysed. The precision will increase at the emission line peaks due to an increase in the SNR.
3. The continuum contamination was removed using the continuum subtraction method outlined in Section 2.2. Position spectra before and after continuum subtraction are shown.
4. The spectro-astrometric analysis shown in Figures 2 and 4 does not provide any information on the width (disk radial extent) of the flow, as the spectro-astrometric centroid measures only the the position of the brightest part of the emission region. Any spread perpendicular to the jet PA (black line), is related to the accuracy to which the emission centroid is measured. Spectra taken perpendicular to the jet axis would be required to recover the flow width. be expected due to the lower critical density of the [S II] λ6731 line (∼ 2 × 10 4 cm −3 ) with respect to the [O I] λ6300 (∼ 2 × 10 6 cm −3 ). The PA of the extended emission can be recovered by combining the results after continuum subtraction for the perpendicular slit positions. This is shown in Figure 2 with a black line. The LVC NC, HVC1, and HVC2 are shown separately. It is clear that the HVC1 is not extended, and we suggest that it is tracing the outflow close to it launching point. An average PA of 229 • ± 10 • is measured for the extended emission in agreement with the spectro-astrometric analysis of Takami et al. (2001). These results reveal two important properties of the LVC NC. Firstly, the LVC NC is extended along the same PA as the HVC2 for both the [O I] λ6300 and [S II] λ6731 lines. This rules out its origin in the disk and connects it to the jet. Secondly, the velocity gradient for the NC emission is opposite to that of the HVC2: while there is a clear pattern of an increase in offset with velocity in the HVC2, the slower LVC NC emission is offset further from the star. The meaning of these features are discussed in Section 4.

AS 205: A multiple System
As described in Section 2.1 the AS 205 system is a multiple system with three known members. The aim here was to investigate the low velocity emission from AS 205 N. However, for the 55 • and 235 • slit PAs AS 205 S a,b are also included in the UVES slit. Therefore, the emission from AS 205 S could also be considered (keeping in mind that As 205 S a,b is not resolved). Considering the disk PA for AS 205 N of 114 • and the binary PA of 212 • , a complication with interpreting any spectro-astrometric signal becomes apparent. As any outflow would be expected to be launched approximately perpendicular to the accretion disk, and as the binary PA is also approximately perpendicular to the AS 205 N accretion disk, it will be difficult to distinguish a binary signal from an outflow signal. However, it would be reasonable to assume that any forbidden emission is originating in an outflow.
For AS 205 N both blue and red-shifted displacements are detected in the forbidden emission lines. Again the [O I] λ5577 emission region is not found to be displaced and the 1-σ uncertainty in the SA can again be taken as an upper limit on the extent of this emission region. For AS 205 N this is 2 au (after continuum subtraction) which equals 6 au when the inclination of the system is accounted for. Analysis of the [O I] λ6300 and [S II] λ6731 line regions reveals interesting results. As presented in Figure 3 the HVC3 and the HVC1 are displaced in the same direction while the HVC2 is displaced in the opposite direction. As HVC1 and HVC2 are both blue-shifted one would expect them to be extended in the same direction, if they are part of the same flow. The complex nature of the signals detected becomes clear in Figure 4 As noted, it is difficult to distinguish between a binary and outflow signal in this system. However, it is argued here that a likely explanation is that we are detecting an asymmetric jet (V blue ∼ -250 kms −1 and V red ∼ 140 kms −1 ) driven by AS 205 N, with the signals seen in HVC1, the [S II] λ6731 LVC, and the AS 205 S [O I] λ6300 emission being binary signals. The red-shifted jet emission is located beyond the shrouding effects of the disk. High angular resolution imaging in jet tracing lines could help to fully understand the outflow activity in the vicinity of the stars. We conclude that the nature of the LVC remains elusive until the flows can be disentangled and driving sources identified.

DISCUSSION
As outlined above, spectro-astrometry of visible spectra of RU Lupi and AS 205 N was previously presented by Takami et al. (2001Takami et al. ( , 2003. What is new about the analysis presented here is that the much improved spectral and spatial resolution (better seeing conditions) offered by observing with UVES, means that the different kinematical components which make up the forbidden emission can be easily distinguished and analysed separately. Furthermore, the improved spectral resolution allows the relationship between displacement and velocity to be probed at good resolution. In the case of AS 205 the improved seeing allows the N and S components to be separated. Another important development since the analysis by Takami et al. (2001Takami et al. ( , 2003 is the spectro-astrometric analysis of the CO emission from both stars by Pontoppidan et al. (2011), who conclude that the CO fundamental at ∼4.7 µm is tracing a disk wind.
While the kinematic analysis of the RU Lupi and AS 205 N forbidden line emission summarised in Table 3  . To further clarify how the velocities are distributed with distance for the LVC NCs, the spatial offsets along the outflow PAs, as a function of velocity, are plotted in Figure 6. In Figures 2 and 4 the offsets shown were not de-projected but they are in Figure 6. The plotted offset is therefore the height above the circumstellar disk (z).
Observations of TT jets forbidden emission line regions have clearly shown that there is an initial increase in the velocity of the jet with distance from the driving source and this behaviour was first recorded by Hirth et al. (1997). Decreases in velocity at individual knots in the jets are also observed highlighting the complicated knotty structure of the jets. Takami et al. (2001) note a similar behaviour in the Hα and FEL regions of RU Lupi. They point out that this would be an expected feature of magnetically driven flows and use this as supporting evidence for the origin of the extended Hα emission in the RU Lupi outflow. This jet-like behaviour is found here for the HVC2 component in RU Lupi, and we conclude that this is also found for all the AS 205 N components except the HVC1 component in The RU Lupi LVC NC is clearly showing something very different to what would be expected for a jet. Not only does it show a negative gradient in z but the gradient is much smoother and does not show evidence of regions of acceleration and deceleration as might be expected for shocked emission. In agreement with previous work, we find that a likely explanation of the velocity gradient is emission in an uncollimated MHD disk wind. The gradient seen in the RU Lupi NC was hinted at in the Takami et al. (2001) results. Takami et al. (2001) also investigated the velocity field of the RU Lupi [O I] λ6300 and [S II] λ6731 low velocity emission and Figure 8 here can be directly compared to their Figure 3. Based on their data in Figure 3 of their article, Takami et al. (2001) conclude that the [O I] λ6300 LVC does not show a gradient and argue that this is consistent with origin in a moderately collimated MHD disk wind. For the [S II] λ6731 line they note a slight positive gradient which they say is due to blending between the HVC and LVC. Therefore, they cannot draw any conclusions about the [S II] λ6731 LVC gradient. The spectra analysed in Takami et al. (2001) were taken with the RGO spectrograph on the AAT telescope at much poorer spectral resolution than what was achieved here and so while they see a hint of what we find here in the [O I] λ6300 line it is likely that the [S II] λ6731 LVC was not resolved from the HVC.

RU Lupi
AS 205 Figure 9. Comparison of the RU Lupi and AS 205 N spectro-astrometric results for the LVC NC to what was found for the CO by Pontoppidan et al. (2011). The CRIRES CO spectra and the UVES spectra were taken at different slit PAs, so the spectra closest in PA are compared. Also the CO results were not corrected for continuum subtraction and so what is compared is the offsets measured before continuum subtraction and shown in Figures 1 and 3. The vertical lines mark the velocities of the forbidden line NCs and HVC1s in both sources. They are shown dashed for the [SII]λ6731 line. RU Lupi: The shape of the offsets is similar between the two lines. The maximum offset is at the peak of the line and then offset gradually tails off as velocity increases. This shows that the LVC NC of the FELs and CO have the same origin. AS 205 N: Unlike what is found for RU Lupi there is no agreement between the FEL and CO results here emphasising that an MHD disk wind is not detected in this case.
The extent of the emission regions can also be deduced from Figure 8. While the displacements shown in Figures  1 to 5 were corrected for the continuum contamination they did not take the inclination of the systems into account. In Figure 8 the displacements along the measured jet PA are plotted and they are also de-projected. For RU Lupi both the LVC NC and HVC2 are displaced further in [S II] λ6731 than in [O I] λ6300, as expected. From Figure 8 the RU Lupi NC is displaced as far out as ∼ 8 au and ∼ 40 au in [O I] λ6300 and [S II] λ6731 respectively, giving one of the first true measurements of the spatial extent of the LVC. The error on the measurement is given by the precision of the technique. The 1-σ uncertainty is 0.6 au for [O I] λ6300 and 1 au for [S II] λ6731. Figure 7 compares the results from this work with the spectro-astrometric analysis of the CO emission from both stars presented by Pontoppidan et al. (2011). For both sources the HVC1 and LVC NC components are delineated by vertical lines with the [S II] λ6731 components represented by a dashed line. The same color scheme as shown in previous figures is followed. We find that the RU Lupi CO results agree with the FELs while there is no such agreement for AS 205 N. The comparison between the CO and FEL results for AS 205 N do show the signatures to be similarly shaped but then are along opposite PAs. Pontoppidan et al. (2011) model their CO spectro-astrometric results as being produced by a wide-angled wind and they also see the negative gradient detected here.
Since the RU Lup disk is at a low inclination angle, the line of sight velocity measures largely the velocity component along the disk rotation axis (z direction). A negative gradient in this velocity with z therefore implies a decreasing v z to observer 19 o Figure 10. Sketch illustrating how the spectro-astrometric signals presented in Figures 1 and 2 are tracing the outflows from RU Lupi. The coloured dots are the positional displacements shown in Figure 2 with the colors from yellow (0 km/s) to dark blue (-230 km/s) representing increasing velocities. The HVC2 is tracing the accelerating jet with velocities from -90 km/s to -230 km/s. The LVC NC is tracing a wide angled MHD disk wind with a velocity range of -40 km/s to 0 km/s. For the MHD wind component, the velocity of each point corresponds to flow streamline and therefore a disk radius r. The spectro-astrometric technique is recording the height of the emission above the disk z, at each velocity (or r). As the velocity is decreasing with increasing r and z is increasing with r, the spectro-astrometric results shown in Figure 2 for the NC, has a negative velocity gradient.
and for a wind this is consistent with a decreasing flow velocity with distance from the star and/or with a decrease in the collimation angle of the flow streamline which also results in a decrease in the v z component of the flow velocity. Both of these are predicted by MHD disk wind models. A similar fall off in velocity with disk radius has been observed in TT jets where an "onion-like" velocity structure with fast flowing gas enclosed by slower less collimated gas, has been observed (see Frank et al. (2014) and references within). This interpretation of the negative velocity gradient is also consistent with the conclusion of Fang et al. (2018) that while the BCs trace disk radii within 0.5 au and heights (∼ 0.15 au) close to the disk (not detected in the UVES data), the NCs trace gas which originates in a more extended wind at several au. Figure 10 is a sketch of how properties of the outflows from RU Lupi result in the spectro-astrometric offsets presented in Figures 1 and 2.
Our findings are very relevant to the ongoing debate about the origin of the LVC emission. Simon et al. (2016) found that the combination of blue-shifts in the centroid and large FWHM of the LVC BC pointed to a MHD disk wind origin while the origin of the NC was much less clear and a photoevaporative wind could not be ruled out, see also Ercolano & Owen (2016). As Banzatti et al. (2019) found that both the BC and NC kinematics were correlated, they further argued that they both are part of the same MHD wind and they put a constraint of 35 • on the wind opening angle. While, Weber et al. (2020) claimed that these same correlations can be achieved in an MHD inner wind (producing the HVC and BC) plus an X-ray driven photoevaporative wind (giving rise to the NC), the predicted [S II] λ6731 line luminosities are orders of magnitude larger than observed, see discussion in Pascucci et al. (2020), casting doubts on their proposed scenario.
We note that while a declining flow velocity with disk radius and poor collimation is also expected with photoevaporation, a photoevaporative flow for the origin of the RU Lupi NC is very unlikely. Firstly, the observed blue-shifts of ∼ 30 km/s at low z, are too high to be consistent with photoevaporative flows which, instead, predict shifts at thermal speeds of 5 kms −1 to 10 kms −1 (Ballabio et al. 2020). Secondly, the [OI]λ6300 emission can be traced down to a vertical height z of 2 au from the disk midplane 3 , see Figure 8, with the wind being launched relatively deep within the gravitational potential well of the star.
We conclude, from the analysis of the velocity distribution of the different components and the comparison with the CO that the RU Lupi NC is most likely tracing a MHD disk wind. Gangi et al. (2020) compare molecular Hydrogen emission at 2.12 µm with the [O I] λ6300 emission from a sample of TTSs and conclude that the H 2 and [O I] λ6300 LV NC traces the same wind but cannot distinguish between a photoevaporative and a MHD wind. We also note the recent ALMA observations detected a non-Keplerian envelope in 12 CO, extended from the RU Lupi disk, which Huang et al. (2020) comment could be the colder/radially extended counterpart of the inner MHD wind inferred in the CO fundamental line. The forbidden emission investigated here probes material closer to the fundamental CO emission, raising the interesting possibility of disk winds being present at nearly all disk radii. While it appears that the CO is tracing part of the blue-shifted outflow of AS 205 N the situation is uncertain for the FEL HVC1 and LVC NC. Salyk et al. (2014) also introduced some uncertainty as to whether the CO is tracing a disk wind in AS 205 N as they suggest that the CO spectro-astrometric signature could have been confused by the binary. Further investigation is need to disentangle the complicated outflow emission from the AS 205 system. As a wide-angled wind would enclose the fast collimated jet (Klaassen et al. 2013), it would be expected that the PA of the wind component would be found to be the same as the PA of the jet in AS 205, as seen for RU Lupi.

SUMMARY AND CONCLUSIONS
We carried out UVES spectro-astrometry in the different kinematic components of the [O I] λ6300 and [S II]λλ6716, 6731 from two accreting stars, RU Lupi and AS 205 N. For AS 205 N several outflow components were revealed indicating the presence of at least two outflows, which is not surprising given that it is a multiple system. Analysis of the relationship between velocity and offset for the different AS 205 N flow components generally showed them to be jet-like. For RU Lupi a clear jet component was detected (HVC2, V∼ −195 km s −1 ) which showed an increase in velocity with distance and evidence of a knotty structure. Another HVC component (HVC1, V ∼ −66 km s −1 ) shows no offset and we argue that it traces the base of the jet. The most significant finding was for the RU Lupi LVC NC which was found to be extended along the same PA as the jet (to a distance of ∼ 40 au in [S II] λ6731) but with a negative velocity gradient. This is opposite to the behaviour seen for jets. The evidence extracted from the spectroastrometric analysis for RU Lupi and the comparison with a previous spectro-astrometric study of CO emission points to an origin in an disk wind for the RU Lupi NC. This is the first clear evidence of spatially resolved emission in a MHD disk wind for a LVC NC in a FEL and one of the first measurements of the wind height. Furthermore, this work highlights that a kinematical study alone is not enough to clarify the origin of the different velocity components. From the kinematical analysis alone, the FEL regions of both stars seems similar, but spectro-astrometry tells a different story. This approach should now be extended to a greater sample of objects to see if the LVC NC routinely traces a disk wind or if indeed RU Lupi is a unique object.

ACKNOWLEDGMENTS
This work is based on data collected by UVES (089.C-0299) observations at the VLT on Cerro Paranal (Chile) which is operated by the European Southern Observatory (ESO). We thank Klaus Pontoppidan for providing the CO position spectra originally published in Pontoppidan et al. (2011) Fang et al. (2018). Both studies examined the same datasets, that is Keck/HIRES archival spectra taken in 2008. In Table  4 the results of the kinematical fitting in these works are compared to our results. Both works identify a LVC broad and narrow component and two blue-shifted HVCs for both sources, although for RU Lup the centroid and FWHM  Fang et al. (2018) and Banzatti et al. (2019) to our own kinematic decomposition (last column). for the BC and one HVC are significantly different (see Table 4) 4 " Banzatti et al. (2019) outline the changes in their approach to the kinematical fitting which accounts for these differences. The main difference between the results of this paper and these previous works is that no LVC BCs are identified. In Figure 11 we compare the normalised HIRES and UVES [O I] λ6300 line profiles with the UVES. Note that at the epoch of the UVES observations the high-velocity emission has changed in both sources. This, in combination with the complex [O I] λ6300 profiles, leads us to identify different Gaussian components than those published. Most notably, following the 30 km/s separation for the LVC and HVC (e.g., Simon et al. 2016), we do not identify the weaker and more blended LVC BC in the UVES spectra.