UKIRT Newsletter : Issue 3 : Research : H2 Proper Motions
Proper Motions and Variability of H2 Knots in the HH 111/121
Protostellar Outflows
Chris Davis & Kristen Koppin
Joint Astronomy Centre, Hilo
WHY MEASURE H2 PROPER MOTIONS IN PROTOSTELLAR OUTFLOWS?
Kinematic studies of protostellar jets at near-IR wavelengths are of considerable
interest. Primarily, they allow us to confirm that H2 features
in Herbig-Haro (HH) flows correspond to their optical counterparts. They
also constrain models of molecular flows and molecular bow shocks, and
let us study and compare the often optically "invisible" counterflow. They
also allow us to compare the kinematic properties of some of the youngest,
most deeply embedded outflows with their more evolved, optical counterparts.
Based on observations taken over a 3.2 year time span with IRCAM3, we
have measured the proper motions and variability of the H2 knots
in the HH 111 and HH 121 outflows. Driven by a low mass protostar, HH 111
is notable for being one of the longest and best collimated Herbig-Haro
flows known to date. At optical wavelengths the flow comprises a series
of compact bow shocks, beautifully illustrated in recent Hubble Space Telescope
(HST) images. Near-IR images (Fig. 1a) reveal a knotty counterjet (extending
eastward; knots ZO-ZF) and a new, optically invisible bipolar outflow,
designated HH 121 (knots B, C and X) that is orientated almost perpendicular
to the HH 111 system and powered by a star very near to the HH 111 source.
 |
FIGURE 1 : (a) Narrow band 2.122 micron IRCAM image
of the HH 111 and 121 (knots B, C and X) outflows. (b) Proper motion vectors,
illustrating the projected motions over 100 years, are marked on the brighter
knots. |
OBSERVATIONS & RESULTS
Narrow-band images of the HH 111/121 region were obtained at UKIRT at three
epochs (17 December 1994, 14 December 1997 and 27 February 1998). The reference
image from 1994, shown in Fig.1a, was taken under conditions of superb
seeing (< 0.5"). This high resolution image reveals a number of faint
features and sub-knots that have not previously been observed. In particular
new knots in the counter-jet (ZO-ZF) and features between knots L and P
were detected (Fig.2). Indeed, some of the features labelled in Fig.2 have
only before been seen by HST. Their optical/near-IR morphologies are strikingly
similar. We were also pleasantly surprised when we detected the extended,
low excitation bow shock wings of the HH knot P, here labelled P1 and P2
(and likewise possibly H2 in the wings of knot O).
 |
FIGURE 2 : H2 2.122 micron (+ continuum)
image of part of the HH111 jet, illustrating the fainter emission features
between knots L and P. The outline of the optical bow shock knot P, observed
by HST, is drawn in - note the H2 emission detected in
the low-excitation wings, P1 and P2. |
Analysis of the three data sets ('94, '97 and '98) involved the application
of a series of linear transformations, including translation, rotation
and rescaling. The transformation coordinates were obtained by comparing
a number of stars' positions common to the images from the three epochs.
We then compared the positions of the best-defined and generally brightest
knots in the data from epoch to epoch by cross-correlating their surface
brightness distributions. The resulting proper motions are displayed as
tangential velocity vectors in Fig.1b. Our attempts to measure accurate
velocities was to some extent hampered by the morphological and brightness
variations evident in some features. The associated post-shock molecular
cooling times must be very short – comparable to the time-scales of our
observations. The measured proper motions are, nevertheless, very high;
in the range 200-400 km/s. Moreover, where H2 knots have optical
counterparts (e.g. F-L and V), the measured tangential velocities are very
similar.
HIGH PROPER MOTIONS THOUGH LOW SHOCK VELOCITIES
The proper motions of the H2 features in HH 111 and HH 121 are
high, far in excess of the dissociation speed limit (25-50 km/s) for H2
in molecular shocks. So how can we account for the presence of H2
emission in the knots in these outflows? Could the knots in each flow be
the result of flow variability? If, for example, the flow velocity at the
point of injection (the source) slowly increases with time, then a sequence
of internal working surfaces bounded by a leading bow shock and a trailing
reverse shock will develop along the length of the jet. Both shocks could
then possess high proper motions (high pattern speeds) though at the same
time low shock velocities relative to the jet speed, quite possibly below
the H2 dissociation speed limit. H2 could then be
excited (rather than dissociated) in either shock.
Alternatively, the knots could be caused by Kelvin-Helmholtz (KH) instabilites
along the beam of the jet. Numerical simulations predict the growth of
instabilites that are bounded by radiative biconical shocks, some of which
can show a bow-like shape, much like those observed in HH 111. Moreover,
these discrete, regularly spaced knots and bow shocks will possess high
proper motions, with values ranging from a few tenths of the flow speed
to values comparable to the flow speed itself. The KH scenario is therefore
a viable alternative to the variable flow model discussed above for at
least some of the knots in the HH 111 and HH 121 outflows. To distinguish
between the two models we await higher resolution H2 images
to better define the location of the molecular emission relative to the
bow shocks observed by HST in [SII] and H-alpha emission, and more accurate
proper motion measurements from future epoch observations, such as from
UFTI at UKIRT.
|
