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. a. b. c.
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Figure 5. Aperture selection for HH 34. (a) is a Hubble picture with Ha in green and [S II] in red. The white box is the observed area. (b) shows the [S II] lines from the spectrum, aligned with (a). (c) shows the labeling of the 11 apertures and the relative strength of Ha and [S II]
Figure 4: The path of a dust particle traveling through a jet and multiple shock regions
Figure 3. 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 6. (top) A fully reduced CCD image of the spectrum. The horizontal line is the star’s blackbody spectrum
(bottom) The extracted spectrum from aperture 6 (knot F)
Using [Fe II] emission lines to determine dust properties in jets
Adam Ginsburg and Pat Hartigan
Data and Methods
Observations of HH 34 were taken with a long slit spectrograph over the wavelength range 4800 to 10000Å in order to determine a reddening correction, shock properties, and iron to sulfur line ratios along its jet. Comparing the Raymond-Cox shock model to observed lines gives good agreement with accepted values on general properties of the shock region. The iron to sulfur ratio was measured from the [Fe II] 8617 and [S II]6717,31 line ratio, which serves as a trace of dust along the jet. Because no significant change in that ratio is observed, dust is probably not formed or destroyed in HH 34. However, we may be able to use the data to determine elemental abundances in the region.
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 extracted from HH34 (see figure 5). Data was measured by integrating the flux under each emission line peak (see figure 6) using an IRAF task.
A planar radiative shock modelwith 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.
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 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 isn’t 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.
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