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Astronomy & Astrophysics manuscript no.
(will be inserted by hand later)
The kinematic relationship between disk and jet in the DG Tauri
system
Leonardo Testi 1 , Francesca Bacciotti 1 , Anneila I. Sargent 2 , Thomas P. Ray 3 , Jochen Eisloel 4
1 Osservatorio Astrosico di Arcetri, INAF, Largo E.Fermi 5, I-50125 Firenze, Italy
2 California Institute of Technology, MS 105-24, Pasadena, CA 91125, USA
3 Dublin Institute for Advanced Studies, 5 Merrion Square Dublin 2, Ireland
4 Thuringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany
Received ...; accepted ...
Abstract. We present high angular resolution millimeter wavelength continuum and 13 CO(2{1) observations of
the circumstellar disk surrounding the TTauri star DG Tauri. We show that the velocity pattern in the inner
regions of the disk is consistent with Keplerian rotation about a central 0.67 M star. The disk rotation is also
consistent with the toroidal velocity pattern in the initial channel of the optical jet, as inferred from HST spectra
of the rst de-projected 100 AU from the source. Our observations support the tight relationship between disk
and jet kinematics postulated by the popular magneto-centrifugal models for jet formation and collimation.
Key words. Circumstellar matter - jets and out ows - Stars: formation - Stars:individual: DG Tauri
1. Introduction
The interplay between accretion and ejection of matter is
believed to be a crucial element in the formation of stars.
In particular, stellar jets may contribute substantially to
the removal of excess angular momentum from the sys-
tem, thereby allowing the central star to accrete to its -
nal mass (e.g. Eisloel et al. 2000, Konigl & Pudritz 2000,
Shu et al. 2000). Most of the proposed models invoke the
simultaneous action of magnetic and centrifugal forces in a
rotating star/disk system threaded to open magnetic eld
lines. Even if widely accepted, these models have not yet
been tested observationally on the launching scale (a few
AU from the star), although this may be possible with
the coming generation of interferometers. On much larger
scales, there is some evidence for the requisite relationship
between the jet and envelope kinematics in the HH 212
protostellar system (Davis et al. 2000). Hints of rotation
are seen in the H 2 jet knots at 210 3 to 10 4 AU from
the powering source; the sense of rotation is the same as
that of the attened envelope detected in NH 3 VLA ob-
servations (Wiseman et al. 2001). Although encouraging,
these measurements probe regions too far from the central
source to allow detailed comparison between the disk and
jet kinematics.
Protostellar disk/jet systems are too embedded to
probe the jet close to the launching region with current
techniques which rely on optical and near infrared obser-
Send oprint requests to: lt@arcetri.astro.it
vations. Moreover, the kinematics of disk/envelope sys-
tems may encompass both rotational and infall motions,
hampering tests of disk-jet interaction models. Optically
visible TTauri stars, which have associated disks but little
remnant envelopes, are much more suitable candidates for
such studies.
The optical jet from the TTauri star DGTauri has
been extensively studied at high resolution in recent years
and displays properties that are in general agreement with
magneto-centrifugal models for jet-launching (Dougados
et al. 2000; Bacciotti et al. 2000; 2002). The latter studies
showed that the ow appears to have an onion-like kine-
matic structure, with the faster and more collimated ow
continuously bracketed in a wider and slower one. The ow
becomes gradually denser and more excited from the edges
toward the axis. The mass loss rate in the ow is about
one tenth of the estimated mass accretion rate through the
disk (Bacciotti et al. 2000). Even more interestingly, for
the spatially resolved ow component at moderate veloc-
ity (peaked at 70 km s 1 ) systematic osets in the radial
velocity of the lines have been found in pairs of slits sym-
metrically located with respect to the jet axis (Bacciotti et
al. 2002). If these results are interpreted as rotation, then
the jet is rotating clockwise (looking toward the source)
with average toroidal velocities of about 10{15km s 1 , in
the region probed by the observations (i.e. 10-50 AU from
both the star and jet axis). All of these properties, includ-
ing the implied velocities and angular momentum uxes
are in the range predicted by the models, assuming a cen-

2 Testi et al.: The DG Tauri disk-jet system
tral star mass of 0.67 M (Hartigan et al. 1995). The kine-
matic properties of the material surrounding the star are
of crucial importance in further establishing if the models
apply.
DGTauri is known to be surrounded by a circumstel-
lar disk (Beckwith et al. 1990; Kitamura et al. 1996a;
Dutrey et al. 1996). Previous interferometric observations
of the molecular component of the system (Sargent &
Beckwith 1994; Kitamura et al. 1996b, hereafter KKS)
could not identify a clear signature for rotation around
the central object. These relatively low-resolution (4{5 00 ),
13 CO(1{0) observations could not disentangle the kine-
matics of the circumstellar disk from the out ow and the
outer envelope velocity elds. In fact, the environment of
the star on large scales appears to be dominated by out-
ow motions, possibly due to the interaction between the
outer regions of the disk and the stellar wind (KKS). In
contrast, the inner portion of the disk, closer to the jet
launching region, is expected to display a Keplerian rota-
tion pattern. We have carried out new, higher-resolution
millimeter wavelength observations of the 13 CO(2{1) tran-
sition toward the DG Tauri system with the aim of dis-
tinguishing the velocity eld close to the star and ascer-
taining if it is consistent with that expected for Keplerian
rotation and to check that the disk and jet rotate in the
same sense.
2. Observations and results
Millimeter wavelength interferometric observations of the
DG Tauri system was performed using the Owens Valley
Radio Observatory (OVRO) mm-array located near Big
Pine, California, between Oct 1999 and Dec 2001. The
six 10.4 meter dishes were deployed in congurations that
provided baselines from 15 to 240 m. Continuum obser-
vations centered at 220 and 108 GHz used an analog
correlator with a total bandwidth of 2 GHz. The digital
correlator was congured to observe the 13 CO(2{1) tran-
sition with 0.125 MHz resolution over an 8 MHz band
(0.17 and 11 km s 1 , respectively). Frequent observations
of 0528+134 were used to perform phase and gain calibra-
tion. The passband calibration was obtained by observ-
ing 3C273, 3C454.3 and/or 3C84. The ux density scale
was derived by observing Neptune and/or Uranus, and
the calibration uncertainty is expected to be  20%. All
calibration and data editing used the MMA software pack-
age (Scoville et al. 1993). Calibrated (u; v) data were then
loaded into the AIPS and/or GILDAS packages for imag-
ing, deconvolution and analysis. Continuum maps and line
cubes were produced using natural weighting of the (u; v)
data, and smoothed to a spectral resolution of 0.5 km s 1 ,
unless specically noted. The synthesized beam full width
at half maximum is 1: 00 71: 00 4. Continuum subtraction was
performed on the dirty images before deconvolution using
channels at the edge of the band.
Fig. 1. OVRO 13 CO(2{1) integrated intensity map (contours)
overlaid on the 2.7 mm continuum image (greyscale).
2.1. Continuum maps
We detect unresolved continuum emission from DG Tauri
at both 1.3 and 2.7 mm. The peak position is
the same at both wavelengths, (2000)=04 h 27 m 04. s 66
ф(2000)=26 ф 06 0 16: 00 3, in agreement with previous mea-
surements (e.g. Kitamura et al. 1996a; KKS). The to-
tal ux density is 215 mJy at 222 GHz and 55 mJy at
108 GHz. Within calibration uncertainties, the 3 mm value
agrees with earlier interferometer measurements (KKS;
Dutrey et al. 1996; Looney et al. 2000) but the 1.3 mm
value is a factor of two lower than the single dish ux
(Beckwith et al. 1990), probably because of spatial lter-
ing by the interferometer. If we assume optically thin emis-
sion from dust grains at T40 K and a dust opacity co-
eфcient k  =k 230GHz  (=230 GHz)  , with k 230GHz =0.01
(Hildebrand 1983, including a gas to dust ratio of 100 by
mass), and  0.5 (Beckwith & Sargent 1991), our mea-
surements imply a total mass of 0.04 M , consistent
with previous estimates.
2.2. Line maps
In Figure 1 we show the 13 CO(2{1) integrated intensity
map overlaid on the 2.7 mm continuum image. Our obser-
vations are sensitive only to the compact features of the
emission and resolve out most of the extended core emis-
sion seen in the KKS maps. In spite of the ltering by the
interferometer, most of the 13 CO(2{1) emission is not con-
centrated in the inner regions of the system close to the
position of the optical star. The velocity pattern exhibited
by this extended gaseous component has been interpreted
by KKS as due to an expanding disk-like structure, pos-
sibly the outer edge of the disk that is being dispersed by
the stellar wind. Our higher angular resolution map is in
good agreement with their interpretation. In this paper
however, we will focus on the inner regions of the disk.
In Figure 2 we show 13 CO(2{1) channel maps
(0.5 km s 1 resolution) of the central 12 00 region centered

Testi et al.: The DG Tauri disk-jet system 3
Fig. 2. 13 CO(2{1) channel maps. Top panels: blue wing; bottom panels: red wing. Each panel is labelled with the appropriate
velocity (vLSR in km s 1 ). The cross marks the position of the 1.3 mm continuum peak. Contour levels start at 3 and are
spaced by 1 = 60 mJy/beam. The central velocity of the system is assumed to be 6.0 km s 1 (rightmost, isolated panel).
on the continuum peak position (marked by a cross). The
core of the line (v LSR =5.0-7.0 km s 1 ) is dominated by the
poorly imaged extended structure discussed above. Note
that the central velocity channels may also be aected
by self-absorption due to cold foreground gas (Schuster
et al. 1993). By contrast, the higher velocity wings, cor-
responding to the leftmost three blue and red channels,
display compact emission arising from the inner disk. The
maps show that the emission peak in the blue channels
is shifted toward the south-east with respect to the con-
tinuum peak, while in the red channels it is shifted to
the north-west. In order to emphasize this velocity gra-
dient, in Fig. 3 we show the wing emission integrated
over the ranges vLSR =3.5{4.5 km s 1 (blue wing) and 7.5{
8.5 km s 1 (red wing). These maps were obtained with a
robust weighting of the (u; v) data and the resulting angu-
lar resolution is 1: 00 41: 00 0. The red and blue wing emission
peaks on opposite sides of the continuum, which is as-
sumed to trace the stellar position (see also KKS), and is
aligned along a line approximately perpendicular to the
observed direction of the optical jet (p.a.226 ф , marked
with a thick line in Fig. 3).
3. Discussion
The prime goal of this study was to investigate the velocity
pattern in the inner regions of the DG Tauri disk and to
relate it to the velocity pattern detected at the base of the
optically visible jet by Bacciotti et al. (2002). The channel
and wing maps of Figs. 2 and 3 indeed show a velocity
gradient across the inner regions of the circumstellar disk.
If interpreted as rotation within a disk the axis of which
coincides with the jet axis, the direction of the gradient is
consistent with the sense of rotation inferred for the jet.
Due to the contamination of the poorly imaged exter-
nal regions of the disk, and self-absorption, it is not pos-
sible to study the kinematics of the inner disk using the
line core within 1 km s 1 from the systemic velocity, as-
sumed to be 5.8 km s 1 (KKS). In particular, there can be
Fig. 3. 13 CO(2{1) wing maps (contours) overlaid on the
1.3 mm continuum image (greyscale). Blue and red wing are
shown as solid and dashed contours, respectively. Contour lev-
els start at 3 and are spaced by 1, dotted lines show negative
contours. The thick solid line indicates the direction and extent
of the initial segment of the optical jet studied by Bacciotti et
al. (2002).
no detailed comparison of the observed velocity patterns
with Keplerian rotation models such as those undertaken
by Koerner et al. (1993), Guilloteau & Dutrey (1994) and
Mannings et al. (1997). Nevertheless, in Fig. 4 position-
velocity diagrams along directions parallel and perpendic-
ular to the jet suggest that, along the disk major axis
(
? ), higher absolute velocity (with respect to the sys-
temic velocity) peaks are located closer to the star, while
the lower absolute velocities peak further away. Moreover,
blue velocities systematically peak to the south-east and
red velocities peak to the north-west of the stellar position.
This behaviour is qualitatively consistent with Keplerian
rotation in the inner regions of the disk. A more quantita-
tive comparison with the expected line-of-sight velocities
for a disk surrounding DG Tauri is shown in Fig. 4, bot-

4 Testi et al.: The DG Tauri disk-jet system
tom panel. The theoretical curves in the gure were com-
puted for a central star with mass M ? =0.67 M (Hartigan
et al. 1995), and an inclination from the line of sight of
38 ф (Eisloel & Mundt 1998); these are the parameters
adopted by Bacciotti et al. (2002) to check the rotational
hypothesis for the jet. The dotted lines include an uncer-
tainty in these parameters of 0.25 M and 15 ф . The
observed velocity pattern in the inner regions of the disk is
in excellent agreement with the model predictions. A more
detailed comparison, including the complete derivation of
the disk rotation from molecular line observations, will re-
quire higher angular resolution and more sensitive obser-
vations of optically thinner transitions, such as C 18 O(2{1),
which are possibly less aected by the external regions of
the disk/envelope.
Summarizing our results, we have shown for the rst
time that the disk kinematics in a young TTauri system
are in qualitative and quantitative agreement with the ve-
locity pattern at the base of the jet. In other words, the
simultaneous and kinematically consistent rotation of disk
and jet postulated by the popular magneto-centrifugal
models has been observationally inferred for the rst time
for the region within 200 AU from the central source.
Acknowledgements. The OVRO mm array is supported by
NSF grant AST-99-81546, research on young stars and disks
is also supported by the Norris Planetary Origins Project and
NASA Origins of Solar Systems program (grant NAG5{9530).
We thank the referee, Chris Davis, for comments that improved
the presentation of our results.
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