Summary of the Goal
The information here was assembled to support long-slit spectroscopy observations of SNR E0102 in the SMC. The goal is to acquire data to allow mapping out the velocity structure of O III emission in X-ray-relevant regions of the remnant, e.g. see Figure 1 of X-ray Ne X emission. In particular the bright SE arc (red and 'green' shifted) and the central bright spoke (blue shifted) are the main targets and can be obtained in the same long slit observation(s).
Instrument/Observation Parameters: Spectral
The O III line pair at 5007 and 4959 AA is the main spectral feature to measure, see Figure 2 below. Velocity components of the optical emission have been seen in the -2500 km/s to +4000 km/s range [1] --- hence a range from 4918 to 5074 AA is needed to capture this velocity range for the pair. A resolution of 8 AA (~ 500 km/s) or better is desired. A slit width of 3 arc seconds or less is desired. A slit length of 1 arc minute or longer is desired to extend across the remnant's diameter.
An exposure time of ~1000 seconds was used by Tuohy and Dopita, see Figure 2 below, "in 1981 June using the RGO spectrograph and the image photon counting system (IPCS) on the Anglo-Australian Telscope" with a slit width of 2 arc seconds. While their spectrum clearly shows the O III emission at 5007 AA, the goal of determining or detangling multiple velocity components in a single spatial cell (e.g., 2"x2" say) requires better signal-to-noise ratio than seen here. For example the weak emission bump on the long wavelength side of the 5007 line in this spectrum, ~5030 AA, may be valid red-shifted emission that I'd like to measure. Note that in general the red-shifted emission is much weaker than the blue-shifted - perhaps due to absorption by some component of the remnant itself. For these reasons, exposure(s) several to 10 times deeper than the Tuohy and Dopita exposures are desired; given modern instruments the actual exposure time may still be of order 1000 seconds even for this improved S/N ?
Other lines of secondary interest that may be simultaneously acquired are O II at 3727 and Ne III at 3869 (and 3968) AA with similar velocity range.
Instrument/Observation Parameters: Spatial
A recent Chandra intensity color coded image of E0102 is shown in Figure 3. In this Figure two points are indicated and define the nominal aimpoint and position angle of the spectrometry slit. Specifically the Chandra-reported coordinate of the central circled point is:
central-ish circled point 01:04:02.075 alpha "FK5"(J2000)
-72:01:55.55 delta
and the position angle of the lines shown
is 114.4 degrees CCW from North (=90+24.4); some
convenient similar PA would be fine, e.g., 110 or 120 degrees.
Multiple adjacent parallel slit obervations could be made by offsetting in Declination by, say, +/- 1.5x the slit width, or greater. (Note that 1.1x the slit width in Dec causes the PA=114 slits to exactly abutt.) A range in Dec of +/- 10 arc seconds covers the SE arc and central spoke.
Optical Finding Charts
OK, so what does this field look like in the optical band? Figures 4, 5, and 6 give progressively zoomed in images of the target sky region. Note that the final of these, Figure 6, is in the light of O III and was made in 1981. Perhaps the best criterion for placement checking is that the slit go through the SE arc as indicated...
References
[1] Tuohy and Dopita, ApJ 268:L11-L15, 1983 May 1.
[2] Tuohy and Dopita, ApJ 248:L105-L108, 1981 Sep 15.
Figures
Figure 1: Velocity-colored image of E0102 emission in Ne X
X-ray line, from Flanagan et al. 2004.
Red represents velocities in the range 450 to 2250 km/s,
green in the range -450 to -1350 km/s and blue -1350 to -2250 km/s.
(Emssion in the range -450 to +450 km/s is not shown but is generally
between the red and green regions shown.)
A similar map is expected for Oxygen emission, hence the
desire for O III velocity measurements.
Figure 2: E0102 Spectrum example from Tuohy and Dopita, 1983.
(Wavelength scale is from 3500 AA to ~6200 AA with
ticks spaced by 200 AA.)
Figure 3: E0102 in X-rays: color-coded intensity
Chandra image.
The color cells in this image are ~ 1 arc second square.
Two reference locations are shown circled and the nominal
slit orientation connecting them is shown as well by the two
parallel lines.
Figure 4: Aladin image of the region, ~14 arc min square.
The purple cross-hair is at the location of the desired center of
the spectrometer slit. Red and blue squares show the fields of the
next two figures. Two bright stars which point to a useful fiducial
star are indicated with the green circle and line.
Figure 5: POSS2 Blue field
This 5 arc minute square region around E0102 is the red square in the
above figure. This image is from the "POSS 2 Blue" data.
The blue square shown here gives
the ~ 1.7 arc minute field of the region shown in Figure 6 below and
indicates the useful fiducial stars.
Figure 6: Image in O III from Tuohy and Dopita, 1981.
This image in O III was made using interference filters. The two fiducial
stars in the previous image are indicated with green lines. The desired
slit center and roll angle are indicated in yellow.
Note that this image is from 1981 and so some slight motion of stars or
evolution in the shape of the E0102 OIII emission may have taken place.
