The potential which infrared spectroscopic techniques provide for studying interstellar process is demonstrated. Several examples are given. The data were obtained at UKIRT, using its frequency-chopped Fabry-Perot spectrometer and its seven-channel cooled grating spectrometer

Geballe, T. R.

English

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ORIGINAL PAGE 18
OF POORQu .rry N87-15107
SOME RECENT INFRARED SPECTROSCOPY OF INTERSTELLAR PROCESSES
T. R. Geballe
U. K. Infrared Telescope; Hilo, Hawaii
I. Introduction
The intent of this paper is to demonstrate the potential which infrared
spectroscopic techniques provide for studying interstellar processes. Several
examples are given. The data were obtained at UKIRT, using its frequency-chopped
Fabry-Perot spectrometer and its seven-channel cooled grating spectrometer.
II. Shocked Molecular Hydrogen
The emission from the V=I-0 S(1) line of H 2 has been mapped over a large
portion of the supernova remnant IC443 by Burton et al. (1986, in preparation). A
section of this map is shown in Fig. i. The emission is clumped along a ridge where
the expanding shell of the SNR has interacted with a molecular cloud. The clumps
may be density enhancements in the
molecular cloud or be the result 22"28'
of fragmentation caused by the
shock. The observed intensity
ratios of the 2pm lines imply that =
c22"25'
the H 2 is collisionally excited
and that the extinction to the
shocked gas is 2 mag at 2zm. The" _
total luminosity from H 2 lines is 22"22'
about i000 L0, making IC443 one of
the most luminous galactic H 2 line
sources. The mass of hot, shocked
gas is _0.2 M 0.
I1)
,,,,,,  92/, , , , , , , , ,.
6hlSmO0 i 6hll_140' 6hll,'120' 6hlil'O01
Right ascension (1950)
Fig. 1
Figure 2 shows a 3-4_m spectrum of Pk 1 in Orion (Wade and Geballe, 1986, in
preparation). Numerous H 2 lines are easily seen. The most highly excited of these
is the 0-0 S(17), whose upper level energy (T = _5,500 K) is the highest yet
detected from H 2 in a shocked region. The even higher excitation S(21) line at
3.37_m may also be detected. Excitation temperatures derived from pairs of highly
excited lines in this spectrum are typically 2700 K, considerably higher than those
determined from the strong, but lower excitation 2pm lines. This result is expected
for H 2 undergoing post-shock cooling. Accurately determined intensity ratios and
velocity profiles of high and low excitation lines may yield important information
concerning the shock physics and/or the presence of multiple shocks.
i,300.
i, iO0 o _ + o ,,Ii.ooo, _ _ ._°
Fig. 2 )090 m ; ' Z J
_ o.eooq _ _ _ oA '_ 1
Orion _ _ _ o , o,, o.7o_ , , , _ oll , [
., ,ti till 1t
' ,,<Joo_,_ _.l'oo_._,o_,j;t> _,_'®_._oo_.<_oo
II. Molecules in Grain Mantles
Ice band spectra at resolving powers of
i, 300.
i, lO0.
1, iO0.
i,O00.
lo.goo<
"., o,_o<x
O. 40_
O, 300_
v
o
m
ui i
X;
iill %
I
_._ _,7_ _._
m!cron_
o
o o, 7
o o
i00 have been in existence for many
years. The combination of observations from Mauna Kea and higher spectral resolution
allow a significant increase in information. Figure 3 shows the ice band of GL2136
129
https://ntrs.nasa.gov/search.jsp?R=19870005674 2020-03-20T12:28:09+00:00Z
(Greenberg et al.; 1986, in preparation), obtained at a resolving power of N350.
For the most part the profile is smooth, but several distinctive structures are
seen. These are: a narrow maximum in optical depth at 3.09pm, a shoulder at 3.25_m,
and (possibly) a shallow feature at 3.4pm. All have been seen in spectra of other
sources. No sharp features have been detected on the long wavelength wing.
0._
i"
• - :o
....
1.
GL 2136 j-"'_'"
0_°
.o,"
... ....:2,4 ...
J .........i_ ....., .......i_-"'"'i.........i ......r..-.-...,......i ......, ......,'_mm"mr""q_mm
3.0 3.4 3.6
mlc_onm
5
C
E 4
E 3
2
.<
u. 1
0
W33A
...r J'+_
• --',,._
-I I r I , I , I , I
4.60 4.70 4.80 4.90 5.00
Wavelength (microns)
Fig. 3 Fig. 4
The infrared source W33A is perhaps the most important astronomical resource
for the study of grain mantles. Its 4.5-5.0_m spectrum (Geballe et al., 1985,
Astron. Ap., 146, L6) contains three strong absorptions (Fig. 4). The narrowest of
these, at 4.67pm, due to solid CO, has been detected in many obscured sources. The
others, at 4.62pm and 4.90pm, have not been clearly detected elsewhere. Work by the
Laboratory Astrophysics group (Leiden Univ.) indicates that their carriers are,
respectively, unidentified C_ N - and sulfur - bearing molecules which are produced
on the mantles by UV-photolysis.
III. Unidentified Emission Features
Recent spectroscopy of the well-known 3pm unidentified emission features has
resulted in I) accurate characterizations of their spectral shapes, 2) proof of the
existence of source-to-source variations in their relative strengths, and 3) the
discovery of several new emission features. Figures 5 and 6 are a good example of
the range of phenomena which has been found. The planetary nebula BD +30 3639
exhibits a strong 3.3pm feature, a weak or absent 3.40_m peak, and a "plateau" of
emission extending to 3.6_m (Geballe et al., 1985, Ap. J., 292, 500). In contrast
the source IRAS 21282+5050 has, in addition to a strong 3.3pm feature and plateau,
a strong 3.40pm feature and additional peaks at 3.46, 3.51, and 3.56_/m (de Muizon
et al., 1986, Ap. J. Letters, in press). The variety and wavelength range of the
3_m features appears to be consistent with their being emitted by PAHs (some with
molecular subgroups attached), but specific identifications have not yet been made.
_ 2
Z '
z_
0 I . i I I I "
3.1 3.2 33 3.4 3.5 3.6
Wavelength (microns) r. t'ee 3.zoe 3.3'ee 3.4'ee 3. s'ee 3. s'eo 3.Tee
microns
Fig. 5 Fig. 6
130


Some recent infrared spectroscopy of interstellar processes

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