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# Evidence for an MHD disk wind via optical forbidden line spectro-astrometry
111Based on data collected by UVES (089.C-0299(A), 089.C-0299(B), P.I. I.
Pascucci) observations at the VLT on Cerro Paranal (Chile) which is operated
by the European Southern Observatory (ESO).

Whelan, E.T  Maynooth University Department of Experimental Physics, National
University of Ireland Maynooth, Maynooth, Co. Kildare, Ireland  Pascucci, I
Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721,
USA  Gorti, U  SETI Institute/NASA Ames Research Center, Mail Stop 245-3,
Moffett Field, CA 94035-1000, USA  Edwards, S  Five College Astronomy
Department, Smith College, Northampton, MA 01063, USA  Alexander, R.D  School
of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH, UK
Sterzik, M.F  European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748,
Garching, Germany  Melo, C  European Southern Observatory, Casilla 19001,
Santiago 19, Chile

###### Abstract

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] $\lambda$6300 and [SII]$\lambda\lambda$6716,6731 LV narrow component
(NC) is offset along the same position angle (PA) as the HVC but with a
different velocity gradient than the jet, in that displacement from the
stellar position along the rotation axis is decreasing with increasing
velocity. From the LVC NC PA and velocity gradient, it is inferred that the NC
is tracing a wide angled MHD disk wind. A photoevaporative wind is ruled out.
This is supported by a comparison with a previous spectro-astrometric study of
the CO fundamental line. The decrease in offset with increasing velocity is
interpreted as tracing an increase in the height of the wind with increasing
disk radius. This is one of the first measurements of the spatial extent of
the forbidden emission line LVC NC ($\sim$ 40 au, 8 au for RU Lupi in the [S
II] $\lambda$6731 and [O I] $\lambda$6300 lines) and the first direct
confirmation that the LVC narrow component can trace an MHD disk wind.

accretion, accretion disks – ISM: jets and outflows – protoplanetary disks

##  1 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 ($\sim 1-30$ 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] $\lambda$6300
luminosity is found to be better correlated with accretion luminosity than
with stellar luminosity or mass (Nisini et al., 2018). High-resolution
($\Delta 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] $\lambda$6731
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] $\lambda$6300 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] $\lambda$6731, our two targets show prominent LVC
emission at [S II] $\lambda$6731 as well as [O I] $\lambda$6300\. RU Lupi and
AS 205 N have previously been investigated with SA (Takami et al., 2001, 2003)
and the results of these works are discussed in Sections 2.1 and 3. While the
results presented here confirm the previous results, the much improved
spectral resolution and positional uncertainty of our study, allowed new
details to be revealed about the kinematics and spatial properties of the line
regions under investigation. Furthermore, Takami et al. (2003) did not
investigate the FELs of the AS 205 system but analysed several permitted lines
(including H$\alpha$) 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] $\lambda$5577, [O I] $\lambda$6300 and [S II]
$\lambda$6731 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.

Figure 1: RU Lupi: Spectro-astrometric analysis of the [O I]$\lambda$6300, and
[S II]$\lambda$6731 lines. Top: The continuum subtracted line emission,
normalised to the line peak. The fitted kinematic components are over-plotted
with with the LVC-NC shown in blue, and the HVCs in green. Bottom: The
position spectra with the ranges of the LVC-NC and HVC shown as the colored
shaded regions. The continuum subtracted offsets are over-plotted in black.
Figure 2: RU Lupi: Combining the perpendicular slit PAs to recover the 2D
centroid at each wavelength(velocity). The continuum emission has been
subtracted here to reveal the full displacement in the outflow. The black
circle is the 3-$\sigma$ 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.

##  2 Targets, Observations and Data Reduction

###  2.1 Targets

RU Lupi is a CTTS located in the Lupus Star forming region at d $\approx$ 140
pc. Takami et al. (2001) used spectro-astrometry to investigate the H$\alpha$
emission of RU Lupi, and found that there is a distinct spatial symmetry in
the offset for blue and red shifted wings of the H$\alpha$ 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]
$\lambda$6300 and [S II] $\lambda$6731 lines. The PA of the outflow was
estimated at around 225∘ from this work. Pontoppidan et al. (2011) measure a
spectro-astrometric 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 $\sim$ 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
$\sim$ 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). AS 205 N and AS 205 S have been detected at a
projected separation of 1$\farcs$3 and a PA of $\sim$ 212∘ (McCabe et al.,
2006). The DSHARP study measures disk PAs of 114∘ $\pm$12∘ and 110∘ $\pm$2∘
for AS 205 N and AS 205 S respectively (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 H$\alpha$ emission from the AS 205 system.
Analysis showed a displacement along a PA of $\sim$ 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.

Figure 3: AS 205: Spectro-astrometric analysis of the [O I]$\lambda$6300, and
[S II]$\lambda$6731 lines. Top: The continuum subtracted line emission,
normalised to the line peak. The fitted kinematic components are over-plotted
with with the LVC-NC shown in blue and the HVCs in green. Bottom: The position
spectra with the ranges of the LVC-NC and HVCs shown as the colored shaded
regions. The continuum subtracted offsets are over-plotted in black. Figure 4:
AS 205 N: Combining the perpendicular slit PAs to recover the 2D centroid at
each wavelength(velocity). The continuum emission has been subtracted here to
reveal the full offset in the outflow. The black circle is the 3-$\sigma$
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.

###  2.2 Observations and Data Reduction

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
5000 Å to 7000 Å and a spectral resolution of R = 40,000 was achieved. As the
spectro-astrometric precision is given by the following equation

$\sigma=\frac{FWHM}{2.35\sqrt{N_{p}}}$ (1)

where Np 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 PA222Note that
the disks PAs were subsequently updated by the DSHARP study. This is the
reason for the difference between the disk PAs given in Table 1 and the slit
PAs in Table 2. 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 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).

Table 1: Spectro-astrometric sample: properties relevant to this study

Object | SpTy | Log($\dot{M}_{\rm acc}$) | Disk | PA**This is the position angle of the disk major axis. The most recent values are given here and therefore in some cases they differ from the slit PAs chosen to be parallel to the disk PAs (see section 2.3 and Table 4.) | Disk Inc | Wind | References
---|---|---|---|---|---|---|---
|  | [M⊙/yr] | Type | [∘] | [∘] | Diagnostic |
RU Lup | K7 | -7.77 | full | 121 | 19 | CO, [O I] | Hu18, C11,P11,F18
AS 205 N | K5 | -6.58 | full | 114 | 20 | CO, [O I] | K18, F18,P11
AS 205 S | K7 | - | - | 110 | 66 | - | K18

Note. — References stand for: C11 = Curran et al. (2011); F18 = Fang et al.
(2018); Hu18 = Huang et al. (2018); P11 = Pontoppidan et al. (2011); K18=
Kurtovic et al. (2018)

Table 2: Observing log  Object | RA (J2000) | Dec (J2000) | Date | Slit PA | Exposure Times | Seeing
---|---|---|---|---|---|---
| (J2000) | (J2000) | (dd-mm-yyyy) | (∘) | (ncycle$\times$s) | (arcsec)
RU Lupi | 15:56:42.3 | -37:49:15.5 | 29-06-2012 | 80 | 5 $\times$ 300 | 1.3
|  |  | 28/29-06-2012 | 170 | 10 $\times$ 300 | 1.2
|  |  | 29-06-2012 | 260 | 5 $\times$ 300 | 1.0
|  |  | 28/29-06-2012 | 350 | 10 $\times$ 300 | 1.4
AS 205 | 16:11:31.4 | -18:38:28.3 | 02-05-2012 | 55 | 6 $\times$ 300 | 0.77
|  |  | 02-05-2012 | 145 | 6 $\times$ 300 | 0.76
|  |  | 02-05-2012 | 235 | 6 $\times$ 300 | 0.8
|  |  | 02-05-2012 | 325 | 6 $\times$ 300 | 0.68

##  3 Results

The UVES spectra cover two oxygen forbidden lines, one at 6300 Å, and the
other at 5577 Å, with the latter being the weaker of the two (e.g., Simon et
al. 2016), and the sulphur forbidden line doublet at 6716 Å, 6731 Å. 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.

We follow a similar approach here. Based on this approach, both [O I] lines of
RU Lupi and AS 205 N have an LVC and two (RU Lupi) or three (AS 205 N) HVC
components in the [O I] $\lambda$6300 transition. For both stars only a LVC NC
is identified. A similar decomposition scheme is applied to the [S II] lines
(see Table 3)333Where the [O I] $\lambda$6300 and [S II] $\lambda$6731 lines
are detected the same profiles are seen in the [O I] $\lambda$ 6363 and [S
II]$\lambda$ 6716 lines, albeit at a lower SNR.. The fits to the [O I]
$\lambda$6300 and [S II] $\lambda$6731 lines are shown in Figures 1 and 3 and
the kinematic results presented in 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] $\lambda$6300 line, are explained by variability in the HVC and this is
discussed in detail in the appendix.

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$\farcs$3 between the N and S
components, and a seeing of $\sim$ 0$\farcs$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] $\lambda$6300 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 $\sim$ 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. 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. 2.

The 1-$\sigma$ 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. 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. 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.

Table 3: Classification of the [O I] $\lambda$5577, [O I] $\lambda$6300, and [S II] $\lambda$6731 lines. Object | Line | LVC NC | LVC BC | HVC Blue | HVC Red
---|---|---|---|---|---
|  | Vrad, FWHM (km s-1) | Vrad, FWHM (km s-1) | Vrad, FWHM (km s-1) | Vrad, FWHM (km s-1)
RU Lup | [O I] $\lambda$6300 | -13, 18.2 |  | -66.11, 149.9 (HVC1) |
|  |  |  | -197.5, 84.5 (HVC2) |
| [S II] $\lambda$6731 | -23.9, 26.6 |  | -100.1, 157.1 (HVC1) |
|  |  |  | -194.71, 59.5 (HVC2) |
AS 205 N | [O I] $\lambda$6300 | -2.35, 16.8 |  | -30.3, 88.2 (HVC1) | 139.5, 13.7 (HVC3)
|  |  |  | -226.5, 76.4 (HVC2) |
| [S II] $\lambda$6731 | -27.3, 16.3 |  | -62.3, 39.8 (HVC1) | 119.8, 16.1 (HVC3)
|  |  |  | -259.2 , 35.6 (HVC2) |
AS 205 S | [O I] $\lambda$6300 |  | -2.6, 42.9 |  |

###  3.1 RU Lupi: a vertically extended LVC-NC

For RU Lupi only the blue-shifted outflow is detected in forbidden emission,
and vertical offsets from the star-disk plane are measured in the [O I]
$\lambda$6300 and [S II]$\lambda$6731, $\lambda$6716 lines (see Figure 1 where
the [O I] $\lambda$6300 and [S II] $\lambda$6731 results are shown). No
spectro-astrometric signal is measured in the highest critical density ($\sim
10^{8}$ cm-3) [O I] $\lambda$5577 line. The 1-$\sigma$ uncertainty in the SA
can be taken as an upper limit on the extent of the [O I] $\lambda$5577
emission region. For RU Lupi this is 3 au (after continuum subtraction) which
equals 9 au when the inclination of the system is accounted for. For both the
[O I] and [S II] lines it is found that the LVC NC and the HVC2 are displaced
from the stellar position while the HVC1 is coincident with the stellar
position (within the uncertainty of the centroid fitting). Note that the [S
II] $\lambda$6731 emission is more extended than the [O I] $\lambda$6300
emission, for both the LVC and HVC2, as would be expected due to the lower
critical density of the [S II] $\lambda$6731 line ($\sim 2\times 10^{4}$ cm-3)
with respect to the [O I] $\lambda$6300 ($\sim 2\times 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∘ $\pm$ 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] $\lambda$6300 and [S II] $\lambda$6731 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.

Figure 5: AS 205 S: Spectro-astrometric analysis of the [O I]$\lambda$6300
emission. Top: The continuum subtracted line emission, normalised to the line
peak. The fitted kinematic component is over-plotted. Bottom: The position
spectrum with the range of the fitted LVC-BC shown as the colored shaded
region. The continuum subtracted offsets are over-plotted in black. Figure 6:
Continuum subtracted position velocity diagram of the [O I] $\lambda$6300 and
[S II] $\lambda$6731 emission from AS 205 for the 55∘ slit PA. Both AS 205 N
and AS 205 S were included in the slit and the position of AS 205 S is marked
with the green dashed line.

###  3.2 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] $\lambda$5577 emission region is not
found to be displaced and the 1-$\sigma$ 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]
$\lambda$6300 and [S II] $\lambda$6731 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, where the continuum subtracted
results for perpendicular slits are combined to recover the PA of the signals.
The four identified components are shown separately. The magenta line here
marks the PA of AS 205 S with respect to AS 205 N. The red line gives the PA
of the AS 205 N disk major axis. In [O I] $\lambda$6300 HVC2, HVC3 and HVC1,
while offset in different directions, all lie along similar PA. The same
directions are seen for the three HVCs in [S II] $\lambda$6731\. Also note
that the PAs of the extended components are very close to the binary PA and
are also close to being perpendicular to the disk PA.

The analysis of the AS 205 S [O I] $\lambda$6300 line is presented in Figure
5. No [S II] $\lambda$6731 emission was detected for AS 205 S. No offset is
measured for the [O I] $\lambda$6300 before continuum subtraction but there is
a signal at the velocity of the red-shifted emission detected for AS 205 N and
in the direction of AS 205 N. After continuum subtraction this red-shifted
feature is located at $\sim$ 100 au from AS 205 S, in agreement with its
location with respect to AS 205 N and the separation of $\sim$ 163 au between
the two stars. Also the AS 205 S [O I] $\lambda$6300 emission line shifts in
the direction of AS 205 N after continuum subtraction. In Figure 6, position
velocity diagrams of the [O I] $\lambda$6300 and [S II] $\lambda$6731 emission
line regions are presented offering a further picture of the system. The
location of the red-shifted emission feature between the two sources is clear.

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 (Vblue $\sim$ -250 kms-1 and Vred $\sim$
140 kms-1) driven by AS 205 N, with the signals seen in HVC1, the [S II]
$\lambda$6731 LVC, and the AS 205 S [O I] $\lambda$6300 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.

Figure 7: Comparing the spatial properties of the different kinematical
components. For the blue-shifted components the HVCs are shown in green to
represent their larger velocity. The black stars show the stellar positions.
The agreement in the PA of the different RU Lupi components is clear while in
the case of AS 205 N the [S II] $\lambda$6731 NC and HVC1 do not agree with
the idea of there being one outflow, as they are clearly in the direction of
the red-shifted flow.  Figure 8: De-projected spatial offset along outflow PA,
as a function of radial velocity for the RU Lupi and AS 205 N HVCs and HVCs
and LVC NCs. These results were extracted from Figures 2 and 4 are the average
of the results for the two sets of perpendicular slit PAs. RU Lupi: While the
jet shows a general trend of an initial increase in velocity with distance
(typical of TT jets) a negative gradient is seen in the LVC NC emission. AS
205 N: For all components except the [O I] $\lambda$6300 HVC1 an initial
increase in displacement with velocity is seen. This could be due to mixing
between the HVC1 and HVC2, which are displaced in opposite directions. Figure
9: Comparison of the RU Lupi and AS 205 N spectro-astrometric results for the
LVC NC to what was found for the CO by Pontoppidan et al. (2011). The CRIRES
CO spectra and the UVES spectra were taken at different slit PAs, so the
spectra closest in PA are compared. Also the CO results were not corrected for
continuum subtraction and so what is compared is the offsets measured before
continuum subtraction and shown in Figures 1 and 3. The vertical lines mark
the velocities of the forbidden line NCs and HVC1s in both sources. They are
shown dashed for the [SII]$\lambda$6731 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.

##  4 Discussion

As outlined above, spectro-astrometry of visible spectra of RU Lupi and AS 205
N was previously presented by Takami et al. (2001, 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. (2001, 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 $\sim$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 could suggest a similar spatial origin (within
each component), the spectro-astrometric analysis presented in Figures 2 and 4
clearly points to a different origin. The important information to be taken
from these figures are the PAs of the extended emission and the relationship
between velocity and displacement. From Figure 2, it is clear that the HVC2
(tracing the jet) and the LVC NC have the same PA but for AS 205 N (Figure 4)
the fastest component assumed to trace the jet (HVC2) and the displacement
seen in the [S II] $\lambda$6731 LVC NC are in opposite directions. The
agreement in the PA of the different RU Lupi components is further highlighted
in Figure 5. Here the different components are over-plotted without the
kinematical information given in Figures 2 and 4. The NCs are shown with
filled blue symbols and the HVCs with unfilled green and red (AS 205 N)
symbols. Green is chosen for the blue-shifted HVCs to reflect the higher
velocities with respect to the NCs. The agreement in PA for RU Lupi is
striking while for AS 205 N the [S II] $\lambda$6731 NC and HVC1 are not
straightforward to explain.

The distribution of the offsets with velocity also offer a clue to the origin
of the emission. For the RU Lupi HVC2 there is a clear initial increase in
velocity with displacement along the disk rotation axis, as would be expected
for a collimated jet, while for the LVC NC velocity decreases with increasing
vertical offset. This negative velocity gradient is seen in both [O I]
$\lambda$6300 and [S II] $\lambda$6731 emission lines. Due to the complicated
nature of the signals seen in AS 205 N the velocity distribution is less clear
in Figure 4, although the HVC2 in [O I] $\lambda$6300 and the LVC NC in [S II]
$\lambda$6731 do show a positive gradient (an increase in offset with
velocity), while the HVC1 shows a negative gradient in the [O I]
$\lambda$6300\. 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 H$\alpha$ 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 H$\alpha$ 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 [O I]
$\lambda$6300.

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]
$\lambda$6300 and [S II] $\lambda$6731 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] $\lambda$6300 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] $\lambda$6731 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]
$\lambda$6731 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] $\lambda$6300 line it is likely that the [S II]
$\lambda$6731 LVC was not resolved from the HVC.

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.

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] $\lambda$6731 than in [O I] $\lambda$6300, as expected. From
Figure 8 the RU Lupi NC is displaced as far out as $\sim$ 8 au and $\sim$ 40
au in [O I] $\lambda$6300 and [S II] $\lambda$6731 respectively, giving one of
the first true measurements of the spatial extent of the LVC. The error on the
measurement is given by the precision of the technique. The 1-$\sigma$
uncertainty is 0.6 au for [O I] $\lambda$6300 and 1 au for [S II]
$\lambda$6731.

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] $\lambda$6731 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 $v_{z}$ 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 ($\sim$ 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] $\lambda$6731 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 $\sim 30$ 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]$\lambda$6300
emission can be traced down to a vertical height z of $\lesssim$2 au from the
disk midplane444The [O I] $\lambda$6300 NC emission extends back closer to the
star than the [S II] $\lambda$6731 emission but this is due to the lower
critical density of the [S II] $\lambda$6731 line, 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 $\mu$m with the [O I] $\lambda$6300 emission from a sample of
TTSs and conclude that the H2 and [O I] $\lambda$6300 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
${}^{12}\mathrm{CO}$, 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.

##  5 Summary and Conclusions

We carried out UVES spectro-astrometry in the different kinematic components
of the [O I] $\lambda$6300 and [S II]$\lambda\lambda$6716, 6731 from two
accreting stars, RU Lupi and AS 205 N. For AS 205 N several outflow components
were revealed indicating the presence of at least two outflows, which is not
surprising given that it is a multiple system. Analysis of the relationship
between velocity and offset for the different AS 205 N flow components
generally showed them to be jet-like. For RU Lupi a clear jet component was
detected (HVC2, V$\sim$ $-195$ km s-1) which showed an increase in velocity
with distance and evidence of a knotty structure. Another HVC component (HVC1,
V $\sim$ $-66$ 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 $\sim$ 40
au in [S II] $\lambda$6731) but with a negative velocity gradient. This is
opposite to the behaviour seen for jets. The evidence extracted from the
spectro-astrometric 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.

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). Emma Whelan would
like to acknowledge support from The Maynooth University Seed Fund 2019. I.P.,
U. G., and S.E. acknowledge support from a Collaborative NSF Astronomy &
Astrophysics Research grant (ID: 1715022, ID:1713780, and ID:1714229). R.D.A.
acknowledges funding from the European Research Council (ERC) under the
European Union’s Horizon 2020 research and innovation programme (grant
agreement No 681601).

##  Appendix A [O I] $\lambda$6300 Variability

The [O I] $\lambda$6300 emission in RU Lupi and AS 205 N were previously
investigated in Banzatti et al. (2019) and 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
for the BC and one HVC are significantly different (see Table 4)555Note that
Banzatti et al. (2019) report in their tables only the most blueshifted HVC
but complete fits are available here: http://distantearths.com/Ilaria/disk-
winds/” 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]
$\lambda$6300 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] $\lambda$6300 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.

Table 4: Comparison of the [O I] $\lambda$6300 decomposition from Fang et al. (2018) and Banzatti et al. (2019) to our own kinematic decomposition (last column). | Fang 2018 | Banzatti 2019 | UVES
---|---|---|---
| Vrad, FWHM (km s-1) | Vrad, FWHM (km s-1) | Vrad, FWHM (km s-1)
RU Lup |  |  |
| LVC NC (-10.9, 13.9) | LVC NC (-12, 15) | LVC NC (-13.0, 18.2)
| LVC BC (-10.1, 176.4) | LVC BC (-36, 44) | not identified
| HVC (-32.3, 40.5) | HVC (-60, 292) | HVC (-66.0, 150.0)
| HVC (-149.4, 109.9) | HVC (-148, 80) | HVC (-197.5, 59.5)
AS 205 N |  |  |
| LVC NC (-0.7, 14.5) | LVC NC (-0.4, 14.5) | LVC NC(-2.3, 16.8)
| LVC BC (-13.5, 54.5) | LVC BC (-14.4, 54) | not identified
| HVC(-85.4, 225.5) | not identified | HVC (-30.3, 88.2)
| HVC (-221.9, 56.3) | HVC (-223, 58) | HVC (-226.5, 76.4)
| not identified | not identified | HVC (139.5, 13.7)
Figure 11: A comparison of the [O I] $\lambda$6300 line profile in the HIRES
(green) and UVES (black) spectra. In both cases the high velocity emission has
decreased and this is most striking for RU Lupi.

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