All Figures

Figure 1: The upper panel shows the location of the targets on an optical red image of the LMC. The targets observed in and near the 30 Dor star-forming region are shown in the lower panel, which is an enlargement of the rectangle shown in the upper panel. Target details are listed in Table 1. SN 1987A was extensively observed by Vladilo et al. (1987), Vidal-Madjar et al. (1987) and Welty et al. (1999). Image courtesy of Karl Gordon, based on Bothun & Thompson (1988), Kennicutt et al. (1995) and Parker et al. (1998).

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In the text

	Figure 2: Normalised spectra of the 5780 and 5797 Å DIBs observed in the line of sight towards Sk -69 223, together with the low reddening standard Sk -70 120. For comparison the corresponding DIB spectra of HD 144217 (-type, blue) and HD 149757 (-type, red) are shown. The shift in wavelength is due to the receding velocity of the LMC.
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In the text

Figure 3: Normalised and median smoothed spectra centred at the 6203, 6284, 6379 and 6613 Å DIB spectral ranges observed towards Sk -69 223, Sk -69 243, Sk -67 2, Sk -68 135, Sk -67 5 and Sk -70 120. For comparison the corresponding DIB spectra for HD 149757 (red/grey) and HD 144217 (blue/black) are shown at the bottom of each panel. The 6284 Å DIB spectra have been corrected for telluric contamination as described in Sect. 2.1. The vertical dashed lines show the Milky Way ( left) and LMC ( right) rest wavelengths of the observed DIBs. The between the two lines corresponds to the  km s-1 between Milky Way and LMC components of the interstellar atomic lines. A) The sharp narrow features near 6375 Å in three of the spectra are due to a CCD defect in one of the two overlapping orders. B) The emission line at 6617 Å in Sk -68 135 is a stellar N II emission line (see e.g. Evans et al. 2004).

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In the text

Figure 4: The panels of each figure show, from top to bottom, the K I, Na I, Ca II, Ti II and H I apparent column density velocity profiles for the LMC lines of sight towards Sk -69 223 ( top figure) and Sk -69 243 ( bottom figure). The velocity profile is constructed according to the method of Jenkins (1996, Sect. 5#. The H I profile is extracted from the large scale H I (21 cm) survey of the LMC by Staveley-Smith et al. (2003) and Kim et al. (2003). Johansson et al. (1994) list intensities and velocities of CO, CS, HCO+ and HCN lines towards 30 Dor. On the top panel we display the central DIB velocities, including error bars, for some observed DIBs (see also Sects. 2 and 6.2). The Na I and Ca II profiles for Sk -69 243 are in good agreement with the spectra published by Vladilo et al. (1993).

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In the text

Figure 5: Apparent column density velocity profiles for K I, Na I, Ca II, Ti II and H I towards the LMC targets Sk -67 2 ( top panel) and SK -68 135 ( bottom panel). Construction of the profiles and the derivation of column densities as in Fig. 4. Although similar in reddening the column densities of the atomic species exhibit large differences. Sk -68 135 shows high column densities with respect to Sk -67 2, while its sightline produces slightly less reddening and weaker DIBs. On the top panel we display the central DIB velocities, including error bars, for some observed DIBs (see also Sects. 2 and 6.2).

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In the text

Figure 6: Apparent column density velocity profiles as in Fig. 4 for the two comparison targets Sk -67 5 ( top) and Sk -70 120 ( bottom). No DIBs were observed for these targets. A tentative weak 6203 feature is seen in spectra of Sk -67 5. Both sightlines have weak K I and Na I lines, corresponding to their low reddening. However, the Ca II and Ti II column densities are comparable to those of Sk -67 2. A notable result, not discussed in detail in this work, is the prominent intermediate velocity absorption in the 230-250 km s-1 range seen in Ca II but not in other atoms towards Sk -67 5.

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In the text

Figure 7: N(Na I)/N(H I) ( a) and N(Na I)/N(Ca II) ( b) at LMC velocities. For a detailed discussion of these ratios we refer to Sects. 5.2 and 5.3. We can distinguish between cold and warm components. A higher N(Na I)/N(H I) ratio indicates a colder environment. A depletion of 7.0 dex is typical for cold dense gas, and 10.0 dex corresponds to warm low density gas (e.g. Hobbs 1974a; Ferlet et al. 1985a,b; Hobbs 1976a). Ca III could be the dominant ionisation stage. A higher N(Na I)/N(Ca II) ratio means more depletion of Ca in cold dense clouds, a lower ratio points to a warm diffuse cloud. Note that both Na I and Ca II are trace ionisation stages.

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In the text

Figure 8: K I ( top) and H I ( bottom) column density versus the reddening for a variety of Galactic sightlines and the LMC lines of sight in this work. The top panel shows that the environments that are exposed to a (relatively) strong UV field lie below the average linear regression (based on the Galactic targets), and shielded regions lie above. In the bottom panel the diagonal lines represent different gas-to-dust ratios. The upper three lines show, from top to bottom, the gas to dust ratio N(H I)/ = 19.2, 14.3, and  cm-2 mag-1 for the LMC-2 (Gordon et al. 2003), the plotted LMC data (dotted, linear fit) and the average LMC regions (Gordon et al. 2003), respectively. The dotted line (N(H I)/  =  ) is a fit to the Galactic data and the dashed line the Galactic gas-to-dust ratio ( ) by Bohlin et al. (1978). This panel shows that the shielded regions, e.g. -type clouds, contain more dust, while the exposed (-type) edge dominated environments contain less dust, and the UV dominated (Orion) sightline even less. The higher gas-to-dust ratio of the LMC is a result of an, on average, stronger/harder UV field and a lower metallicity of the gas phase ISM.

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In the text

Figure 9: The 5780 Å ( top) and 5797 Å ( bottom) DIB equivalent width versus reddening. Values for the LMC DIBs are for Sk -69 223 from Tables 2, 3 and 5. Milky Way interstellar cloud lines of sight are included to illustrate different types of environment; (Galazutdinov et al. 2004), (Thorburn et al. 2003), (Herbig 1993), (Chlewicki et al. 1986), + (Kreowski et al. 1999). The 5797 and 5780 Å DIB strengths observed towards Sk -69 223 are consistent with the Galactic relationships (dashed lines).

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In the text

Figure 10: The 6284 Å ( top) and 6613 Å ( bottom) DIB equivalent width versus reddening. Values for the LMC DIBs are listed in Tables 2 and 3. Milky Way interstellar cloud lines of sight are included to illustrate different types of Galactic environment. The diffuse clouds towards HD 144217 and HD 149757 are typical examples of so-called  and  type clouds, respectively (see main text for details). The weak (W) sightlines are those that show relatively weak DIB features. The lines of sight towards the Orion stars () probe an H II region (for comparison to the 30 Dor region). No large deviations are found with respect to the Galactic trend.

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In the text

Figure 11: The 6613 and 6284 DIB strengths are plotted against four different ratios: gas-to-dust ratio N(H I)/, Na I/Ca II, Ca II/Ti II, and Na I/K I (see Sect. 5.3). Panel  a) seems to indicate that a lower gas-to-dust ratio (i.e. more like that of the MW) implies stronger DIBs. For high values of Na I/Ca II the DIBs are of weak/intermediate strength (panel  b)), and for values 1 the DIBs show maximum strength. There are two types of sightlines: either Ca II is undepleted (low ratio) or depleted (high ratio) from the gas phase. As calcium is released from dust grains through disruptive processes (e.g. shocks) this could imply that the diffuse clouds towards the 30 Dor targets are in a more quiescent environment than those towards Sk -67 2 and Sk -68 135. The 6284 and 6613 Å DIBs show a maximum strength for Ca II/Ti II 1-1.5 (panel  c)). From panel  d) we see that the 6284 and 6613 Å DIBs are strong only when the Na I/K I is high enough. 6284 Å DIB strength for Sk -69 202 from Vladilo et al. (1987), atomic lines from Welty et al. (1999).

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In the text

Figure 12: The 6284 Å ( top) and 6613 Å ( bottom) DIB strengths as a function of K I column density. The LMC data are obtained from Tables 2 and 4. Galactic lines of sight (from Fig. 10) are included for comparison. The two reddened targets Sk -69 223 and Sk -69 243 show DIBs that are strong with respect to the K I column density, while those for Sk -68 135 and Sk -67 2 are weak with respect to the amount of potassium when compared to the Galactic trend.

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In the text

	Figure 13: Apparent MW column density profiles of Ca II, Ti II, Na I and K I.
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In the text