[SII] Observations and what
they can tell us
Snippets of possibly useful stuff
by Lesa Moore
31st October 2012
Forbidden lines

Lifetime in excited states



Lyman a: 10-8 s
[SII]: 3846 s and 1136 s
Forbidden lines appear in low-density gas:


for transitions that have long lifetimes in
excited states [SII], [NII], [OII], [OIII]
where there are too few collisions to de-excite
collisionally (< a few hundred thousand cc-3)
2
Spectra on the CCD
OI sky
emission
at 5577Å
OH sky
emission
bands
3
Wavelength-Calibrated Galaxy Spectrum
4
Most common emission lines

“A brief
introduction to
narrowbanding”

http://www.astros
urf.com/bregosky/A_glimpse_to_
the_night_sky/Arti
cles_files/what_is.
pdf
5
Targets



Emission
nebulae
Planetary
nebulae
Supernova
remnants
6
Exercise 3


http://web.williams.edu/astronomy/research/P
N/nebulae/exercise3.php
Ratio of intensities of [SII] lines


l6716 to l6731
can indicate gas density
In between, ratio
describes actual
gas density
Low density:
Ratio is statistical
weight (how
many electrons
the level can
hold)
High density:
Shorter lifetime
(6731) makes
stronger line
7
SNR in the Magellanic Clouds


http://www.mcsnr.org/about.aspx
SNR classification - meet at least two of
the following criteria:



Diffuse nonthermal radio emission
Diffuse thermal X-ray emission
High [SII]/Ha ratio (> 0.4) and/or high optical
expansion velocity
8
SNR in M31





http://aas.org/archives/BAAS/v25n2/aas182/abs
html/S219.html
High [SII]/Ha implies likely SNR
Ratio is not very high for many SNRs
Complementary test based on a comparison of
the Ha flux and the presence or absence of hot,
ionizing stars
Combined BVRI, Ha, [SII] data has revealed
~200 objects which are candidate SNRs
throughout M31 based on this test
9
Optical Imaging and Spectroscopic
Observation of the Galactic
Supernova Remnant G85.9-0.6



http://arxiv.org/pdf/0909.2959v1.pdf
Images taken with narrow-band interference filters H
and [SII] and their continuum.
[SII]/Ha ratio image is performed


[SII]ll6716/6731 average flux ratio from spectra


Ratio ~0.42, indicating that the remnant interacts with HII
regions.
electron density Ne is obtained to be 395 cm−3.
[OIII]/Hb ratio




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shock velocity has been estimated
pre-shock density of nc = 14 cm−3
explosion energy of E = 9.2 × 1050 ergs
interstellar extinction of E(B−V ) = 0.28
neutral hydrogen column density of N(HI) = 1.53 × 1021 cm−2
10
Observations
11
SNR IMAGES
12
The final image

The negative
if the
[SII]/Ha
image of
G85.9-0.6
(smoothed)
13
SNR SPECTRA
14
Measurements
15
Conclusions from image
16