The pure rotational S(2) line of molecular hydrogen at 12.28 microns was looked for in 44 positions in the Orion moleular cloud with 6 in. beams and 35 km/s spectral resolution; it was detected in 27 positions. Emission was observed over a velocity range of + or - 100 km/s. The lines are approximately symmetric, and have full widths at half maximum ranging from 100 km/s down to the resolution limit. The distribution of intensities and line shapes is largely consistent with that seen in the 2 micron hydrogen transitions. However, unexpectedly complex line profiles and point-to-point variations in linear shapes appear, particularly in the region near IRc9

Beck, S. C.

Bloemhof, E. E.

Lacy, J. H.

Serabyn, E.

Smith, H. A.

Tokunaga, A. T.

Townes, C. H.

English

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n S =663
High Spectral and Spatial Resolution Observations of the 12.28 Wn
Emission from H2 in the Orion Molecular Cloud
S.C. Beck l , E.E. Bloemho* 1 , E. Serabyn l , and C.H. Townesl
_
	
	
Department of Physics
University of California, Berkeley
A.T. Tokunaga2
Institute for Astronomy
University of Hawaii, Honolulu
J.H. Lacy 
Department of Physics
California Institute of Technology, Pasadena
and
H.A. Smith 
E.O. Hulburt Center for Space Research, Washington, D.C.
Naval Research Laboratory
(NASA-CU-164889) dIG6 SPICIhAL AND SFATIAL
	 Ntil-1C9uT
RESOLUTICN UitSERVATICNS Uk THE 12.26 MICRON
EMISSION F&UM H2 IN ThE CHICN MOLECULAS
CLOUL (Caiiforaia Uuiv.) 14 P HC A02 /NF A01	 Uacias
CSCL OJA G3/89 .39303
1 Visiting observer at the Infrared Telescope Facility which is operated
by the University of Hawaii under contract from the National Aeronautics
and Space Administration.
2Staff Astronomer at the Infrared Telescope Facility.
POPS 4,P
2.
ABSTRACT
The pure rotational S(2) tine of molecular hydrogen at
12.28 um has been looked for in 44 positions in the Orion Molecular
Cloud with 6" beams and 35 km s -1 spectral resolution; it was
detected in 27 positions. Emission has been observed over a velocity
range of #100 km s -1 . The lines ire approximately symmetric,
and have full widths at half maximum ranging from 100 km s -1 down
to the resolution limit. The distribution of intensities and line
shapes is largc'y consistent with that seen in the 2 Wn
hydrogen transi-0 ons. However, unexpectedly complex line profiles
and point-to-point variations in line shapes appear, particularly
in the region near IRc9.
ILF
3
I. INTRODUCTION
The pure rotational S(2) line of molecular hydrogen at 12.28 um,
first observed in the Orion Molecular Cloud by Beck, Lacy and Geballe
(1979, hereafter Paper I), provides information on a cooler component
of gas than that associated with the 2-3 um vibrational-rotational
lines. Since the 12 um line is much less affected by scattering and
obscuration than the 2-3 um lines, observations with high spatial and
spectral resolution should give less ambiguous data on the kinematics
of the emi tting gas. The observations of Paper I were made from Las
Campanas Observatory, Chile. During these observations the atmospheric
H2O lines close to the H2 line decreased the signal and limited possible
conclusions about the line shapes; it was clear that a drier atmosphere
was necessary for higher quality data. We report here the results of
observations of the 12.28 um hydrogen emission made at the NASA Infrared
Telescope Facility on Mauna Kea, Hawaii, in February and November-Decem!,er,
1980, and January, 1981. The atmospheric H2O absorption at 814.51 cm 1
was less than 10% deep throughout these observations, as compared to
30-50t deep for the measurements reported in Paper I.
The 12.28 um S(2) line has now been measured with spatial coverage
and spectral resolution comparable to the 2 um observations of Nadeau
and Geballe (1979), Nadeau, Geballe, and Neugebauer (1981, hereafter
NGN), and Scoville et al (1981). These more complete observations are
descrited here, with measured spectra displayed and briefly discussed.
Detailed comparison of the S(2) line with the 2 um H2 and other molecular
lines such as CO will be reported later.
C
^. I
^i
WMRFM
4.
II. OBSERVATIONS
The observations were made using the same Fabry-Perot and grating
spectrometer described in Paper I with a 6" beam and spectral resolution
of 0.10 CIO, or 35 km s-1 . The si gnal-to-noise ratio did not increase
appreciably near the H2O absorption, and the minimum detectable line
intensity (3v in a resolution element in 45 minutes) was 5x10-4
ergs s-1 cm-2 2 sr 1. The flux was'calibrated from blackbody measurements;
systematic and random uncertainties in the flux calibration are estimated
C
to be about ±10%. The wavelength scale was calibrated by reference to an
absorption cell of NH3 , giving an uncertainty in the calibration of
±4 km s -1 . The rest frequency of the v=0-0, S(2) line was taken to be
814.452±.005 cm 1 , as in Paper I.2
Jennings (1981, private communication ) has measured the S(2) line in
the laboratory to be at 814.4270±.005 cm- 1 , but he found the NH3 lines
to be shifted as well, so the velocities in Figure 1 would not change
if his results were used.
There was evidence for emission in 20 new positions. However, the
12.28 um line in Orion is quite weak, and several of these cases were
weak enough that they need confirmation. Our discussion of the dynamics
will be limited to the strongest lines, whose shapes are most accurately
determined. The observing procedure was designed to minimize false signals
such as varying instrumental offsets. A wobbling secondary mirror was
used to chop in a north-south direction ,etween positions separated by
90-120". The spectra were measured in scans lasting 20 seconds and
recorded in 256 channels. The spectra were co-added and the sum recorded
every 32 scans. The source was switched between signal and reference beams
after the 8th and 24th scans to cancel any beam imbalance. After every
set of 32 scans the blackbody source and the NH 3 cell were measured to
S.
assure that no sudden change had occurred in the system, and after two
sets a Ori vbs observed to check tracking and system .stability. Each
spectrum presented contains 128 scans or roughly 45 minutes of integration,
during which time system performance was checked against four measurements
of the blackbody and NH 3 source, and two of a Ori.
Comparison of the Mauna Kea data with those From Chile indicates
that the known source of poor signal-to-noise ratios, substantial
atmospheric absorption, has been largely removed. From examinations
of the subsets of 32 scans : appears that beam imbalance was not affecting
the baselines or the apparent noise level; the stronger line shapes
reproduce well from observation to observation. Position 1 has been
observed three times over a period of 22 months and shows no changes
in shape or intensity which would not be attributed to baseline problems
or variations in the atmosphere. A potentially large source of error in
these spectra is the tracking, which can introduce as much as 2" shift
in 10 minutes. For some complex line shapes (section IIIb), this may be
significant.
Figure 2 shows the 44 positions in Orion where the S(2) line has been
looked for. Positions 1-11 were observed from Las Campanas in February-
March, 1979, 12-16 from the IRTF on Mauna Kea in February, 1980, 17-32
from the IRTF in November-December, 1980, and 33-44 from the IRTF in
January, 1981. In addition, positions 1 and 16 were observed from the
IRTF in both February and November, 1980. and position 30 was observed
in both November and January.
Figure 1 displays the spectra of the 20 new positions where H 2 was
detected. These spectra were divided by an atmospheric transmission
function obtained by chopping between the sky and a blackbody, with sky
^y
temaerature assumed the same as that of the telescope. The November
observations of positions 1 and 16 and both observations of position 30
are given.
III. DISCUSSION
a) Line Intensities and Extinctions
The observed line intensities may be found from Figure 1. In our
narrow bandpass only the very brightest continuum sources are detectable.
Continuum emission can be seen clearly only in positions 38 and 41, very
c
close to IRc2, and in 17, which is on IRc2. In positions 25 and 44 it
is not clear whether there is continuum or bread low-intensity emission.
Since we could not detect continuum on position 3 (Paper I), the second
possbility is more likely.
The extinction to the region of H2 emission is not well known at
this time. Estimates range from 12 magnitudes of visual extinction
(Scoville et al. 1981) to 40 mag (Beckwith et al. 1979). If we apply
Becklin et al .'s 1968 work on the extinction to the galactic center,
the extinction at 12.28 um would be 1.5±.5 mag if A`,=40 mag, or 0.45
±.2 mag if Av=12 mag. This would cause all intensities in Table 1 to
be multiplied by factors of 4 or 1.5, respectively. Since the transition
is optically thin, the column density of molecules in the v=0, J=4 state
is given by N(0,4)= 4AFi1 , where I is the intensity and A=2.7640-9 s-1
(Dalgarno and Wright, 1972). N(0,4) derived from Table 1 would be of
order 1020 cm 2 with no extinction correction, and four times that if
Av=40 is used.
From the 2 um observations one can derive total column densities for
2000 K component of the H 2 . The column density in the v=0, J=4
s areater than that of the hot gas by factors which range from 5,
1­7r
if Av=40 mag is used, to 20 viith Nq'12 mg. Most of the 12 um line	
i
emission must therefore come from gas too coot to emit at 2 um. This 	 j
observation is in qualitative agreement with shock excitation models
(e.g. Hollenbach and Shull, 1977) which predict a gradual cooling from
2000 K to ti100 K behind a shock.
b) Velocity Structure
In Figure 2, the line widths are shown schematically superimposed
on Beckwith et al.'s (1978) map of the 2 wo v-1-0, S(1) emission. In
C
several positions the line shape is too complex to be fitted by a simple
Gaussian profile. We will discuss briefly the most striking features of
the 12 um lines, their breadth, symmetry, and peculiar shapes, and the
pattern of lines around IRc9 (IRS2).
i) Width of the Lines
Observations of the 2 um lines with high spectral resolution showed
them to be quite broad, with FWHM as great as 85 km s-1 (NGN). It is
difficult to reconcile such velocities with the commonly accepted shock
excitation model, because H2 dissociates at shock velocities >25 km s-1
(Hollenbach and McKee, 1980). The spectra of Figure 1 confirm the tentative
results of Paper I that the 12 um lines are at least as wide as the
near-infrared lines and that the breadths must be due to motion of the
gas rather than scattering as was suggested by Draine, Roberge and
Dalgarno 1979. In some places, for example at position 1, the 12 um
line is somewhat broader than the 2 um line measured at the same place.
The slightly higher resolution of the 2 um observations (20 km 0 vs.
35 km s-1 ) is not enough to explain this effect. It is likely that the
apparent increase in width is due to the greatly lessened extinction at
12 um, as will be discussed in the next section.
6.
ii) Symmetry of the Lines
There is an obvious and important difference between the 12 and
2 um line shapes. The most noticeable feature of the 2 um lines is
their asymmetry. Many have strongly enhanced blue sides, or suppressed
red sides. The extent of the asymmetry varies from place to place,
with positions on the very periphery of the region appearing essentially
symmetric. There is riot convincing evidence for systematic asymmetry in
the 12 um lines. The probable explanation for this is that the near
C
infrared lines have part of the red-shifted emission suppressed by
extinction within the emitting region (cf. NGN 1981). Such extinction
could account for the observed differences if the gas is expanding from
a center in the molecular cloud and there is obscuring material within
the region of expansion. The red-shifted hydrogen would then be on the
far side of the cloud and more heavily obscureQ. Consequently, emission
on the red side of the 2 um lines would be attenuated and the 12 um lines
would be broader and more symmetric. Absorption extending over the line
centers could also cause an apparent blue-shift of peaks of the 2 um
lines relative to these at 12 um, as is seen in some positions near the
center of the region.
J the intrinsic line shapes at 12 and 2 um are assumed to be the
same, and the apparent differences are due entirely to extinction, the
amount of extinction across the emitting region can be estimated. It is
not clear how well-founded such an assumption would be; the 2 um lines
show considerab',e structure on the blue sides which may or may not be
present at 12 um. However, if the simple assumption is made that the
2 um lines are intrinsically symmetric, at position 1, for example,
there must be ti1.25 magnitudes of extinction between the gas moving at
9.
+50 km s' 1
 and -50 kra's-1 . This would correspond to 12 magnitudes of
visual extinction. The similarity of Scoville eat el.i': (1961).value of
Av=12 mag for the total extinction is a coincidence only, as ,the value
obtained here is the relative-extinction between two regions-of H2
emission; presumably the 12-40 magnitudes previously reported from 2 um
measurements alone may lie in front of the emitting region.
iii) Complex Line Shapes'
In Figure 1 there are several spectra which clearly cannot be produced
C
by simple turbulent gas motions alone. One is position 30 (IRc9), where
there is a second peak in the line at a blue-shift of about 60 km s-1
relative to the main peak. When the position was reobserved five weeks
after the first spectrum, the blue peak appeared stronger compared to the
main peak. The change in the spectrum is presumably not due to intrinsic
variability, but rather to pointing errors, since apparently the blue
component is very localized and hence the intensity quite dependent on
pointing position. The other most interesting line shape is at position
33, where there is a pedestal structure, symmetric about zero velocity.
This position shows emission over a full 1100 km s -1 , with a central
spike no wider than the instrumental resolution. Finally, the extremely
broad lines at positions 21 and 14 may have a pedestal shape as well.
iv) The region of IRc9
The gas and maser motions in the OMC have been carefully studied by
several workers, including Genzel et al. (1981). Knapp et al. (1981),
and Solomon, Huguenin, and Scoville (1981). All agree that the dynamics,
specificall y the high velocity motions, are dominated by flow expanding
from the 8N-KL-IRc2 region, although they disagree on the exact source.
In the 12 ym line, an unexpectedly complex pattern of line shapes occurs
`, 7
within 15" of Mg. 40" north of the supposed dynamical center.
At the position of IRc9 there is a double-peaked line. Close to
it are the narrowest and broadest lines observed. In this rWon the
line shapes change greatly from point to point, which does not occur
anywhere also in Orion. Within 10" the line breadths go from 100 km s 1
to <35 km s-1 . In a sequence of slightly overlapping beams extending
south to north (positions 33;"13, 39 9 16j 9 the line shapes change from a
Adestal, to two broad red lines, to an unresolved line at +9 km s-1.
C
The emission disappears completely in position 8 and then reappears further
north. There may be two components, one broad and one narrow, both of
which occur in these spectra. -There may in fact be three: a narrow one
which looks like position 16, a blue-shifted, and a red-shifted component,
each with half-widths of about 75 km s-1 . However the line shapes are
interpreted, it is clear that there are peculiar motions in the area of
IRc9. This must be reconciled with the large bo 4y of evidence that th3
primary source of the gas flow is considerably further south. IRc9 may
have affected its immediate locality in a way that makes the flow appear
differently, or it may be responsible itself for some high-velocity motions.
The paucity of information on IRc9 makes difficult even speculation on
such questions. Further analysis of the 12 um data and comparison with
observations at other wavelengths, of H2 and of other molecules. may help
clarify the gas motions in Orion, which now appear even more complicated
than they have in the past.
This work was supported in part by NASA grant NGL 05-003-272 and NSF
grant AST 78-24453. A.T.T. acknowledges the support of NASA contract NASW
3159. We wish to thank the staff of the NASA Infrared Telescope Facility
,11.
for their, assistance. We are also grateful to-! Om. C. McKee,, 0. Hollenbach
and T.R. Geballe for very helpful discussion. S.C.B. thanks The Institute
of Theoretical Physics at the University of California at Santa Barbera
for its hospitality.
P
1
I jt^
1
F
is
REFOENCES
i	 I
BecK,.S.C., Lacy, J.H., and Geball'e. T.R. 1979 9
 Ap. J. Lett.,
Z3__ 44 9 L213.
Becklin. E.E., Matthews, K., Neugebauer. G. and Willner, S.P. 1978,
Ap. J., 220 9 831.	 ..
Beckwith, S., tersson, S.J., Neugebauer, G. and Becklin, E.E. 1978,
Ap. J., 223, 464.
Beckwith, S., Persson, S.E., and Neugebauer, G. 1979, Ap. J., U, 436.
Dalgarno, A. and Wright, E.L. 1972, Ap. J. Lett., 174, L49.
Draine, B.T., Roberge, W. and Dalgarno, A. 1979, Bull. of the AAS, 11, 689.
Genzel, R., Reid,M.J., Moran, J.M. and Downes, D. 1981, Ap. J.. ,M, 884.
Holle.sbach, D.J. and Shull, J.M. 1977, Ap. J., 216. 419.
Hollenbach, D.J. and McKee. C.F. 1980, Ap. J.,241, L47.
Jennings, D.E. 1981, private communication.
Knapp, G.R., Phillips, T G., Huggins, P.J. and Redman, R.O. 1981.
preprint.
Nadeau. D. and Geballe, T.R. 1979, Ap. J. Lett., 230, L169.
Nadeau, D.. Geballe, T. R. and Neugebauer, G. 1981. preprint.
Scoville, N.Z.. Kleinman. S.G.. Hall, D.N.B. and Ridgway, S.T.
1981. preprint.
Solomon;, P.M., Huguenin. G.R. and Scoville. N.Z. 1981, Ap. J. Lett.,
245. L19.
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9
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499 	 489	 479	 469	 459	 448
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Fig. 2
13.
FIGURE CAPTIONS	 , '
Figure,1. Spectra of the H2 lines detected in the OMC from Mauna Kea.;
The numbers of each spectrum refer to the beam position (see
Figure 2 or the Table for position location), All velocities
are with respect to the LSR. c is 1.740_
3
 er$/(sec sr cm2 an 1)
' 	 in a 35 km sec-1
 resolution element. Positioni30a is the
November, 1980, measurement and 30b the January, 1981, result.
There is a noticeable continuum only at position 17 which is
on IRc2. At positions 25 and 44 there is possible continuum
emission, but is more likely that the lines are very broad.
Figure 2. Locations where the S(2) line was looked for. The contour is
the outermost contour from Beckwith et al .'s (1978) map of the
v= 1-0, S(1) emission made with a 13" beam. The circles are
the beam size for the present measurements, the top number in
each beam is the position number and the bottom, the FWHM.
The FNHM is meant only as an indication of the overall trends,
since many lines have complex shapes. Where there is no bottom
number the line was not detected. For comments on positions
21, 30, and 33 see the text. Position 10 is located on the
Becklin-Neugebauer object, position 17 on IRc2, and position
30 on IRc9.
^r
S.Q. Beck, E.E. Bloemhof, E. Serabyn and C.H. Townes: Department of
Physics, University of California, Berkeley, CA 94720
J.N. Lacy: Department of Physics, California Institute of Technology,
Mail Code 320-47, Pasadena, CA 91125
H.A. Smith: 4139.5, Naval Research Laboratory, E.O:"Hulburt Center for
Space Research, Washington, DC 20375
i
A.T. Tokunaga: Institute for AstrononW, University of Hawaii, 2680 Woodlawn
Drive, Honolulu, HI 96822


High spectral and spatial resolution observations of the 12.28 micron emission from H2 in the Orion molecular cloud

High spectral and spatial resolution observations of the 12.28 micron emission from H2 in the Orion molecular cloud

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