We establish correlations between equivalent widths of eight diffuse interstellar bands (DIBs), and examine their correlations with atomic hydrogen, molecular hydrogen, and E B?V . The DIBs are centered at ?? 5780.5, 6204.5, 6283.8, 6196.0, 6613.6, 5705.1, 5797.1, and 5487.7, in decreasing order of Pearson's correlation coefficient with N(H) (here defined as the column density of neutral hydrogen), ranging from 0.96 to 0.82. We find the equivalent width (EW) of ?5780.5 is better correlated with column densities of H than with E B?V or H2, confirming earlier results based on smaller data sets. We show that the same is true for six of the seven other DIBs presented here. Despite this similarity, the eight strong DIBs chosen are not correlated well enough with each other to suggest they come from the same carrier. We further conclude that these eight DIBs are more likely to be associated with H than with H2, and hence are not preferentially located in the densest, most UV shielded parts of interstellar clouds. We suggest that they arise from different molecules found in diffuse H regions with very little H2 (molecular fraction f < 0.01). Of the 133 stars with available data in our study, there are three with significantly weaker ?5780.5 than our mean H-?5780.5 relationship, all of which are in regions of high radiation fields, as previously noted by Herbig. The correlations will be useful in deriving interstellar parameters when direct methods are not available. For instance, with care, the value of N(H) can be derived from W ?(5780.5).

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STUDIES OF DIFFUSE INTERSTELLAR BANDS V. PAIRWISE CORRELATIONS OF EIGHT STRONG DIBs AND NEUTRAL HYDROGEN, MOLECULAR HYDROGEN, AND COLOR EXCESS

Aims: We study the effects of local environmental conditions affecting the diffuse interstellar band (DIB) carriers within the Upper Scorpius subgroup of the Sco OB2 association. The aim is to reveal how the still unidentified DIB carriers respond to different physical conditions prevailing in interstellar clouds, in order to shed light on the origin of the DIB carriers. Methods: We obtained optical spectra with FEROS on the ESO 1.52 m telescope at La Silla, Chile, and measured the equivalent widths of five DIBs (at 5780, 5797, 6196, 6379, and 6613 A) as well as those of absorption lines of di-atomic molecules (CH, CH+, CN) and atoms (K i, Ca i) towards 89 targets in the direction of Upper Scorpius. We construct a simple radiative transfer and chemical network model of the diffuse interstellar medium (ISM) sheet in front of Upp Sco to infer the effective radiation field. Results: By measuring the DIB and molecular spectrum of diffuse clouds towards 89 sightlines in the Upper Scorpius region, we have obtained a valuable statistical dataset that provides information on the physical conditions that influence the band strengths of the DIBs. Both the interstellar radiation field strength, IUV, and the molecular hydrogen fraction, fH2, have been derived for 55 sightlines probing the Upp Sco ISM. We discuss the relations between DIB strengths, CH and CH+ line strengths, E(B-V), IUV, and fH2. The ratio between the 5780 and 5797 A DIBs reveals a (spatial) dependence on the local environment in terms of cloud density and exposure to the interstellar radiation field, reflecting the molecular nature of these DIB carriers. Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO program 63.H-0456).Tables 1, 2, and 5, and Appendices are available in electronic form at http://www.aanda.org

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Diffuse interstellar bands in Upper Scorpius: probing variations in the DIB spectrum due to changing environmental conditions

We present spectroscopic VLT/UVES observations of two emerging supernovae, the Type Ia SN 2001el and the Type II SN 2003hn, in the spiral galaxy NGC 1448. Our high resolution and high signal-to-noise spectra display atomic lines of Ca  ,N a ,T i and K  in the host galaxy. In the line of sight towards SN 2001el, we also detect over a dozen diffuse interstel- lar bands (DIBs) within NGC 1448. These DIBs have strengths comparable to low reddening galactic lines of sight, albeit with some variations. In particular, a good match is found with the line of sight towards the σ type diffuse cloud (HD 144217). The DIBs towards SN 2003hn are significantly weaker, and this line of sight has also lower sodium column density. The DIB central velocities show that the DIBs towards SN 2001el are closely related to the strongest interstellar Ca  and Na  components, indicating that the DIBs are preferentially produced in the same cloud. The ratio of the λ 5797 and λ 5780 DIB strengths (r ∼ 0.14) suggests a rather strong UV field in the DIB environment towards SN 2001el. We also note that the extinction estimates obtained from the sodium lines using multiple line fitting agree with reddening estimates based on the colors of the Type Ia SN 2001el.

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Diffuse Interstellar Bands in NGC 1448

We report high-resolution spectroscopy of the moderately reddened ( -->AV = 3) early-type star Cernis 52 located in a region of the Perseus molecular cloud complex with anomalous microwave emission. In addition to the presence of the most common diffuse interstellar bands (DIBs) we detect two new interstellar or circumstellar bands coincident to within 0.01% in wavelength with the two strongest bands of the naphthalene cation (C10H -->8+) as measured in gas-phase laboratory spectroscopy at low temperatures and find marginal evidence for the third strongest band. Assuming these features are caused by the naphthalene cation, from the measured intensity and available oscillator strengths we find that 0.008% of the carbon in the cloud could be in the form of this molecule. We expect hydrogen additions to cause hydronaphthalene cations to be abundant in the cloud and to contribute via electric dipole radiation to the anomalous microwave emission. The identification of new interstellar features consistent with transitions of the simplest polycyclic aromatic hydrocarbon adds support to the hypothesis that this type of molecules are the carriers of both diffuse interstellar bands and anomalous microwave emission.

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Evidence for the Naphthalene Cation in a Region of the Interstellar Medium with Anomalous Microwave Emission

We present spectroscopic VLT/UVES observations of two emerging supernovae, the Type Ia SN 2001el and the Type II SN 2003hn, in the spiral galaxy NGC 1448. Our high resolution and high signal-to-noise spectra display atomic lines of Ca II, Na I, Ti II and K I in the host galaxy. In the line of sight towards SN 2001el, we also detect over a dozen diffuse interstellar bands (DIBs) within NGC 1448. These DIBs have strengths comparable to low reddening galactic lines of sight, albeit with some variations. In particular, a good match is found with the line of sight towards the sigma type diffuse cloud (HD 144217). The DIBs towards SN 2003hn are significantly weaker, and this line of sight has also lower sodium column density. The DIB central velocities show that the DIBs towards SN 2001el are closely related to the strongest interstellar Ca II and Na I components, indicating that the DIBs are preferentially produced in the same cloud. The ratio of the 5797 and 5780 DIB strengths (r~0.14) suggests a rather high UV field in the DIB environment towards SN 2001el. We also note that the extinction estimates obtained from the sodium lines using multiple line fitting agree with reddening estimates based on the colors of the Type Ia SN 2001el.

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This paper reviews briefly the history of observations of diffuse interstellar bands (DIBs) — features covering the spectral range from the far UV to near IR — in individual sightlines, and describes their relations to other spectroscopic features originating in the same clouds. Four different kinds of inter- and circumstellar clouds are briefly characterized. DIB intensity ratios are not constant between ISM clouds, thus several carriers are responsible for the whole DIB spectrum. Several recent surveys have indicated the existence of hundreds of very weak interstellar features — the patterns of which are clearly related to the well-known, strong DIBs. These patterns may facilitate the identification of the DIB carriers, which most likely are big molecules or very small dust grains. In single cloud cases, the DIB intensity ratios clearly split into 3–4 groups, most probably characterizing different cloud environments. The ISMs of single OB stellar associations probably are quite homogeneous — the DIB spectra of all members of such aggregates are similar. The DIB intensity ratios apparently correlate with the shapes of vacuum UV extinction curves, as well as with absorption line strengths of simple interstellar molecules when individual clouds are considered. The patterns of the recently discovered weak DIBs as well as the strong DIB ratios suggest that more than one cloud and of different optical parameters exist along most of individual sightlines. The Be stars, characterized by the emission in the HeI D 3 line, are apparently immersed in circumstellar shells that do not contain the DIB carriers. The carriers are apparently absent or extremely underabundant also in high velocity clouds.

C. Sneden

Papers

Diffuse Interstellar Bands in Individual Sightlines