Contents of: VI/111/./abstract/JVDHULST_ISM_LGR1.abs

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SCIENTIFIC ABSTRACT

   Spectral studies of the interstellar medium (ISM) in the galaxies in
the Local Group, will enable us to probe the physical conditions and
composition of the ISM in and around star forming regions.
Fine-structure lines of several species in the infrared give us a unique
opportunity to determine temperature, density, chemical composition and
energy balance of the gas, while solid state features and transitions of
molecular hydrogen directly measure the amount of dust and state of the
molecular gas in the vicinity of the HII regions.

   The strength of the major cooling lines of [CII] and [OI] will
constrain the different models for photo dissociation; especially the
[CII] emission arising from photo-dissociated gas at the molecular cloud
surfaces is very useful for probing the warm, dense molecular cloud
surfaces around star forming regions.  Away from star forming regions,
the [CII] line emission can be used to probe the energy balance in the
cooler ISM.

   Solid state emission features of dust (due to PAHs, such as the 11.3
mu feature), and the quadrupole transitions of radiatively excited,
hot H_2, will be used to measure the dust properties and physical
state of the molecular gas surrounding the star forming regions.  These,
combined with the information from the cooling lines of [CII] and [OI],
and the fine structure lines from the HII regions themselves will enable
us to build self consistent models describing the physical conditions in
and around star forming regions, including the dissociation and heating
of the molecular clouds and dust.  To get a large range in physical
properties of the surrounding molecular material we will select both
known HII regions and a few embedded star formation regions found in the
IRAS survey of the Magellanic Clouds.

   To obtain information about the cool molecular gas observations of
the 17 mu line of H_2 are planned in a companion SOT proposal by
R.  Laureijs et al.  The 17 mu line of cool H_2 will be faint
and will be tried in two specific lines of sight.  Photo dissociation
region (PDR) models for the LMC HII regions, indicate that the line may
be stronger than in Galactic HII regions because of the lower dust
content and hence the greater penetration of the UV radiation field into
the molecular clouds.  A search for other molecular lines will be made
along the same line of sight.

   The selected objects are the brightest HII regions in the LMC, SMC
and M33.  The LMC and SMC regions have been selected in coordination
with members of the other ISO consortia (ISOCAM, ISOPHOT, LWS ans SOT)
to ensure maximum overlap with proposed observations for the other ISO
instruments.  The regions in the Magellanic Clouds fall in three
categories: diffuse, extended HII regions such as 30 Doradus, compact
and high excitation regions such as N88 and N81 and a couple of hot and
cool molecular clouds (LIRS 36, LIRS 49 and SMC-B1#1).  The seven HII
regions in M33 have been chosen to ensure a wide range in galactocentric
radius and hence metallicity and physical conditions.

OBSERVATION SUMMARY

   The main instruments used for this spectroscopic study of
extragalactic HII regions and the ISM in nearby galaxies will be the
SWS, the LWS and ISOPHOT-S.  The SWS and LWS are to be used in grating
scanning mode.  The total observing time for this program is around 40
hours.

   The first observations will be complete grating scans of 30 Doradus
and NGC 604.  Quick reduction of these spectra will show whether the
choice of lines and estimated line strengths of the proposed study are
correct.  These results will determine the detailed observing strategy.
The selected lines for the other HII region study are given below (table
3) and will be observed in individual grating settings in the case of
the SWS.  The lines are spread out in such a way over the short
wavelength and over the long wavelength section of the SWS that most
lines can be observed simultaneously thus improving the observing
efficience greatly.  The LWS will be used in full grating scan mode as
this is not less efficient than having about 6 separate grating
settings.  Coverage of the HII regions in the Magellanic Clouds with LWS
is provided by a complementary LWS proposal by P.  Cox et al.

   At each position not covered by other proposals we will take an
ISOPHOT-S exposure to get a low resolution spectrum between 2.5 and 12
mu for measuring broad spectral features such as those from PAH's.
The efficiency of ISOPHOT-S is such that a very short exposure of 2
minutes provides sufficient signal to noise.

  Similarly we will take short PHOT exposures at 25, 60, 100 and 160
mu (PHOT-P2 in P25, PHOT-C1 in C60 and C100 and PHOT-C2 in C160) and
a short CAM exposure on those positions not covered by other proposals.
Exposures of 2 - 4 minutes each are sufficient and add little extra to
the total observing time for this program.  These will provide
information about the distribution of emission in the field of view and
about the IR spectrum (i.e.  dust temperature) of the HII regions under
study.




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