Using Fe II emission lines to determine dust properties in jets

1
Using Fe II emission lines to determine dust
properties in jets
Adam Ginsburg and Pat Hartigan
Abstract
Data and Methods
Methods (continued)

Data was acquired with a 4m telescope and long
slit spectrograph at Kitt Peak National
Observatory. The Image Reduction and Analysis
Facility, IRAF, was used to reduce the data,
which includes removing sky emission and CCD
flaws, and to extract the spectra. Apertures are
used to increase the signal to noise by averaging
over a few pixels, and eleven were used on HH34
(see figure 5). Data was measured by integrating
the flux under each emission line peak using an
IRAF task. A planar radiative
shock model1 with an added iron cooling module
was used to simulate conditions in the shock
region. A grid of shock models using different
initial conditions will be compared to the data
using a chi-squared goodness of fit test, and the
best fits will tell approximately what the
initial conditions and the expected iron to
sulfur ratios are. The presence of dust in
each aperture can be determined by comparing the
measured Fe II/S II ratio to the predicted
value. If there is a change in the measured
ratio not accounted for by shock model
predictions, dust must be forming or breaking
apart.
We present fluxes for optical forbidden
emission lines and hydrogen emission lines
ranging from 4800 to 10000Å in HH 34. When
compared to planar radiative shock models, our
ratios agree well with past estimates of a shock
speed of 25-33 km/s and ionization fraction 1
in the HH 34 jet. The ratio of Fe II l8619 to
S II l6716 6731 provides a good trace of
dust. Because the ratio remains approximately
constant along the jet, dust probably does not
form or dissipate along the jet. An accurate fit
using future runs of the shock model should allow
us to determine elemental abundances in the
region.
Introduction, or Who cares about dust?

• Shock regions around T-Tauri stars, which
  are young forming stars similar to what our solar
  system would have looked like 4.5 billion years
  ago, shine brightly in forbidden optical
  emission. Determining how dust operates will
  help in figuring out how polar jets work and may
  allow us to determine elemental abundances in
  star forming regions.
• A few points about dust, illustrated in Figure 3
  below
• Dust particles going through multiple shocks may
  be broken apart by spluttering, which occurs when
  an energetic proton hits the dust particle
• Dust may form in the cool post-shock regions
• There may be dust present that has been ejected
  from the accretion disk

Figure 7. (left) Shock models with varying
initial parameters (right) measured S II/Ha
ratios at each aperture. These plots determine
which model to use.

• Figure 1 (below) a T-Tauri star with an
  accretion disk. The accretion disk is the source
  of gas (and maybe dust) ejected to form
  Herbig-Haro objects
• Figure 2 (right) diagram of a Herbig-Haro object

Figure 8. (left) Fe II/S II vs. Shock Speed
(right) measured ratios at each aperture
Conclusions and Future Work
There is probably no dust formation or
destruction occurring in the HH 34 shock. For
the acceptable shock models, the measured iron to
sulfur ratio approximately matched the predicted
value and furthermore remained constant along the
jet. However, a strong conclusion awaits a few
thousand more runs of the shock code and
comparison using a chi-squared goodness of fit
test. The null result of this experiment is
actually fairly promising. If dust isnt being
formed or destroyed, it is possible to determine
how much is present from the iron depletion, and
then the abundance of iron in the region can be
determined. Once an interface for the
chi-squared test has been developed and the grid
of shock models has been completed, a comparative
study of abundances in different star forming
regions will be possible.
Figure 3 The path of a dust particle traveling
through a jet and multiple shock regions
Literature Cited
Hartigan, P., Raymond, J., Pierson, R. 2004, ApJ,
614, L69-71 Hartigan, P., Morse, J., Tumlinson,
J., Raymond, J., Heathcote, S. 1999, ApJ, 512,
901-915 Brown, Korgsen, Evenson 1998, ApJ,
509927-930 Morse et al., 1993, AJ vol 106 no 3
p 1139 Hartigan, P., Morse, J., Raymond, J.,
1994, ApJ, 436, 125-143
Figure 6. An energy level diagram showing why
Fe II 8617 and S II 6717,31 are being
compared they have similar energy levels and
therefore should act about the same in a given
environment
Figure 4 an image of HH 34 in red sulfur emission