The selection of the sample objects had to be done with special consideration for the instruments used to observe them, and the use of the ISO satellite had in this respect the most serious impact on our choice of objects. The three main selection criteria for most of the objects in the sample were that they are bright enough to be detectable with ISO, that the geometry is relatively simple, and that the objects are small enough to fit into the ISO-SWS apertures.

Consequently, many of the objects in the sample are bright, highly excited compact blobs (HEBs), of which several have been discovered in the Magellanic Clouds in the last two decades (e.g. Heydari-Malayeri & Testor 1985). These HEBs are usually only a few arcseconds across, can have a rather large dust content (N88A, Kurt et al. 1999) and are small enough to be ionized by only a few massive stars (N81, Heydari-Malayeri et al. 1999c). These HEBs are thought to be the first step in the evolution from an ultracompact H II region towards more extended structures.

The objects observed in this study are given in Table 1 together with the ISO-SWS/LWS pointing coordinates and observation dates. The distribution of the sample H II regions across the Magellanic Clouds can be seen in Fig. 1. A short description of the sample objects, highlighting some of their features, is given below.

The giant H II region 30 Doradus (N157A) is the most conspicuous object in the Large Magellanic Cloud. Extending about 15 $\arcmin$ across the sky, the object is the closest extragalactic giant H II Region. This makes 30 Dor very interesting and the nebula has therefore been the object of many different studies. The morphology of 30 Dor is very complex, with filaments, bubbles and shells. A thorough kinematic study by Chu & Kennicutt (1994) revealed the presence of networks of high-velocity expanding shells in the inner parts of 30 Doradus. Hot bubbles inside the nebula make 30 Dor a strong diffuse X-ray emitter (e.g. Wang 1999).

Spectrophotometric studies of the region have been performed by e.g. Rosa & Mathis (1987) and Mathis et al. (1985). The spectra, taken at various locations, not only in the inner core of 30 Dor but also in the more diffuse outer parts, have shown the nebula to be chemically homogeneous. The ionization structure of the inner part of 30 Doradus has been investigated using narrow-band WFPC2 images in the H $\alpha$ , [O III] 5007 Å and the [S II] (6717+6731) Å lines (Scowen et al. 1998).

$\begin{figure} \par\includegraphics[width=14.3cm,clip]{H3246F2.PS}\end{figure}$

Figure 2: The LMC objects 30 Doradus ( upper left), N157B ( upper right), N159-5 ( middle left), N160A ( middle right), N11A ( lower left) and N4A ( lower right) in H $\alpha$ . Shown are the ISO-SWS and ISO-LWS pointings in these objects. The SWS aperture plotted is 20 $\arcsec\times$ 33 $\arcsec$ (band 4). The star 1797 in 30 Doradus is from Parker (1993). The stars 52 and 87 in N157B are from Shield & Testor (1992). The white circle shows the position of the X-ray source in N157B. North is up and East is left.

Studies of the stellar content of 30 Doradus have shown the nebula to be incredibly rich in massive stars (e.g. Parker & Garmany 1993; Hunter et al. 1995). Within the massive star cluster R136 at the core of 30 Doradus, about 39 O3 stars have been found, of which some are supergiants (Massey & Hunter 1998). At least five distinct epochs of consecutive star formation have been identified in 30 Doradus (Walborn & Blades 1997), of which the youngest is still ongoing. The detection of water maser activity near R136 betrays the presence of recently formed stars (Whiteoak et al. 1983; Van Loon & Zijlstra 2001).

In this work, the pointings in 30 Doradus were chosen in a roughly semi-circular pattern centered on the massive central cluster R 136 (see Fig. 2, upper left).

$\begin{figure} \par\includegraphics[width=14.3cm,clip]{H3246F3.PS}\end{figure}$

Figure 3: The LMC objects N79A ( upper left) and N83B ( upper right) and the SMC objects N81 ( middle left), N88A ( middle right) and N66 ( lower right) in H $\alpha$ . Shown are the ISO-SWS and ISO-LWS pointings in these objects. The SWS aperture plotted is 20 $\arcsec\times$ 33 $\arcsec$ (band 4). North is up and East is left.

Table 2: The log of the optical observations made in December 1995 and December 1996 at the ESO 1.52 meter telescope.
Observing date Detector Grating Spectral Range Slit width Spatial res. Spectral res.

13/16 Dec. 1995 Ford Aerospace 2048L Grating #2 2880 Å-10 550 Å 1 $\farcs$ 7 0 $\farcs$ 81 pix^-1 3.8 Å pix^-1

15 $\mu$ m pixels 300 gr mm^-1

2/5 Dec. 1996 Loral/Lesser #39 Grating #5 3455 Å-11 150 Å 1 $\farcs$ 7 0 $\farcs$ 81 pix^-1 3.8 Å pix^-1

15 $\mu$ m pixels 900 gr mm^-1

Grating #25 3977 Å-9780 Å 2.8 Å pix^-1

400 gr mm^-1

2.2 LMC: N4A

The object N4A is located a few arcminutes southwest from N11, and is the brighter of the two parts making up N4. The N4 region has been studied in detail by Heydari-Malayeri & Lecavalier des Etangs (1994). Their study of the morphology of N4A shows the presence of a bright compact structure embedded in fainter nebulosity extending towards the west (see Fig. 2, lower right). Associated with N4A is a molecular complex of which a sheet is located in front of the H II region. In a recent study of N4 with ISOCAM, the presence of emission from Polycyclic Aromatic Hydrocarbons (PAHs) and Very Small Grains (VSGs) was detected (Contursi et al. 1998).

2.3 LMC: N11A

Being the second largest H II region in the LMC after 30 Doradus, N11 lies quite isolated at about 4 $^{\rm o}$ from the LMC bar. The region consists of several parts (N11 A-L, Henize 1956), of which N11B is the most prominent. Many rich stellar associations are located in the complex, like LH9 and LH10 (Lucke & Hodge 1970). The sample object N11A (Fig. 2, lower left), a blob of $\sim$ 12 $\arcsec$ in diameter, is located 1 $\arcmin$ east from N11B. The blob is highly excited, showing an [O III]/H $\beta$ ratio in excess of 4 (Heydari-Malayeri & Testor 1983, present work), and is not heavily extincted. A study by Rosado et al. (1996) shows no evidence of disturbed kinematics inside N11A. An HST study of N11A revealed the presence of a tight cluster of stars in the center of the object (Heydari-Malayeri et al. 2001b).

2.4 LMC: N79A and N83B

The H II regions N79 and N83 in the LMC (Fig. 3, upper left and right, respectively) both lie in the western part of the bar. With only the occasional appearance in larger samples for chemical abundance studies (e.g. Pagel et al. 1978), not much attention has been paid to either of these objects. A study of N83B by Heydari-Malayeri et al. (1990) has shown the presence of a very compact blob at the eastern part of the nebula (N83B-1). The morphological study of N83B has been extended using HST (Heydari-Malayeri et al. 2001a), resulting in two new compact blobs being added to the collection (N83B-2 and N83B-3). Both N83B-1 and N83B-2 are clearly visible in the upper right panel of Fig. 3.

2.5 LMC: N157B

The object N157B, lying 6 $\arcmin$ south-west from 30 Doradus, is an interesting mix of an H II region and a supernova remnant (SNR). The complex was first recognized to contain an SNR through its Crab-like radio spectrum (Le Marne 1968; Mills et al. 1978), which was later confirmed by others (e.g. Dickel et al. 1994). The discovery of a bright X-ray source at this position by Long & Helfand (1979) established the nature of the object. An optical counterpart in [S II] emission showing signs of shock enhancement was detected by Danziger et al. (1981). It was on this optical counterpart that ISO was pointed (see Fig. 2, upper right). The discovery of an X-ray pulsar with a period of only 16 ms by Marshall et al. (1998) has further strengthened the picture of N157B as a Crab-like SNR.

2.6 LMC: N159-5 and N160A

The H II regions N159 and N160, together with N158, form a nice chain of H II regions extending southwards from 30 Doradus. The regions lie approximately 30 $\arcmin$ south from 30 Dor and are very complex. The N160 region is subdivided into two pairs of H II nebulosity, N160 B/C and N160 A/D. The two HEBs observed in this work (N160A1 and A2) are embedded within the component N160A, and clearly stand out in H $\alpha$ images (see Fig. 2, center right). The H II region N159 roughly consists of two parts, a triangular shaped part (N159A) and a very complex H II structure at the east arching all the way to the north. The object N159-5 is located 6 $\arcmin$ north-east of N159A (see Fig. 2, center left). High-resolution HST images of the blob show it to have a "butterfly'' like morphology (Heydari-Malayeri et al. 1999b). The HEBs in N160A and N159-5 have about the same angular size, 6 $\arcsec$ for N159-5 and $\sim$ 4 $\arcsec$ for N160A1/A2. Both N159-5 and N160A1 are very highly excited, showing a very high [O III]/H $\beta$ ratio of about 8, with N160A2 only slightly less. Locally, the extinction seen towards the objects is very high, with $A_{\rm H\beta}=1.7$ mag for N160A1 and as much as 2.9 mag for N159-5. Much of this extinction is internal, due to the presence of dust in the objects. In a recent HST study of N160A, the two blobs have been resolved and their stellar content determined (Heydari-Malayeri et al. 2002). The CO content of N159 and N160 has been studied by Johansson et al. (1998) as part of the ESO-SEST key programme.

2.7 SMC: N66

The SMC object N66 (NGC 346) is a giant H II region containing about 40 O stars (Massey et al. 1989). As can be seen in the lower right panel of Fig. 3, the region has two distinct components. The first component is a semi-circular nebula showing a wealth of filaments and ridges, while the second component, lying somewhat northeast at the center of the circle, is formed by an isolated H II region known as N66A.

The region has been studied extensively in many wavelength regimes. In the optical, spectroscopic work has been done by e.g. Dufour & Harlow (1977) and Pagel et al. (1978). In a study by Taisheng et al. (1991), a SNR and SNR candidate have been discovered in N66 by using H $\alpha$ images and radio data at four frequencies. More recently, the nebula has been observed in the mid-infrared with ISOCAM, complemented by CO(2-1) and H₂ data (Contursi et al. 2000; Rubio et al. 2000). In their work, a detailed study is given of the photodissociation region associated with N66. The presence of PAHs has also been detected. In this work, the ISO satellite was pointed at a position halfway the two main components of N66 (see Fig. 3, lower right).

2.8 SMC: N81 and N88A

The compact SMC H II regions N81 and N88 have been studied by e.g. Testor & Pakull (1985) and Heydari-Malayeri et al. (1988). The large scale morphology of the two objects is reasonably simple. In H $\alpha$ , N81 shows up as a round object embedded, slightly off-center, in diffuse emission (Fig. 3, center left). The object N88 is composed of four main components forming a roughly triangular shape. The dominant component N88A is a very compact blob of about 4 $\arcsec$ in diameter (Fig. 3, center right).

In recent times, both objects have been revisited by Heydari-Malayeri et al. (1999a,c) and Kurt et al. (1999) using the HST. The HST study of N81 revealed a small cluster of stars inside the core of the nebula, with the main exciting sources being two deeply embedded stars only 0 $\farcs$ 27 apart. For N88A, the main exciting star could not be identified, which is due to the uncommonly high extinction. Both N81 and N88A show complex structures inside their cores, with dark lanes and bright and dark globules.

2 Sample objects

2.1 LMC: 30 Doradus