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Register a new userMapping atomic and diffuse interstellar band absorption across the Magellanic Clouds and the Milky Way
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Mandy Bailey
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Diffuse interstellar bands (DIBs) trace warm neutral and weakly-ionized diffuse interstellar medium (ISM). Here we present a dedicated, high signal-to-noise spectroscopic study of two of the strongest DIBs, at 5780 and 5797 AA, in optical spectra of 666 early-type stars in the Small and Large Magellanic Clouds, along with measurements of the atomic Na,{sc i},D and Ca,{sc ii},K lines. The resulting maps show for the first time the distribution of DIB carriers across large swathes of galaxies, as well as the foreground Milky Way ISM. We confirm the association of the 5797 AA DIB with neutral gas, and the 5780 AA DIB with more translucent gas, generally tracing the star-forming regions within the Magellanic Clouds. Likewise, the Na,{sc i},D line traces the denser ISM whereas the Ca,{sc ii},K line traces the more diffuse, warmer gas. The Ca,{sc ii},K line has an additional component at $sim200$--220 km s$^{-1}$ seen towards both Magellanic Clouds; this may be associated with a pan-Magellanic halo. Both the atomic lines and DIBs show sub-pc-scale structure in the Galactic foreground absorption; the 5780 and 5797 AA DIBs show very little correlation on these small scales, as do the Ca,{sc ii},K and Na,{sc i},D lines. This suggests that good correlations between the 5780 and 5797 AA DIBs, or between Ca,{sc ii},K and Na,{sc i},D, arise from the superposition of multiple interstellar structures. Similarity in behaviour between DIBs and Na,{sc i} in the SMC, LMC and Milky Way suggests the abundance of DIB carriers scales in proportion to metallicity.
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Ionized carbon is the main gas-phase reservoir of carbon in the neutral diffuse interstellar medium and its 158 micron fine structure transition [CII] is the most important cooling line of the diffuse interstellar medium (ISM). We combine [CII] absorption and emission spectroscopy to gain an improved understanding of physical conditions in the different phases of the ISM. We present high resolution [CII] spectra obtained with the Herschel/HIFI instrument towards bright dust continuum sources regions in the Galactic plane, probing simultaneously the diffuse gas along the line of sight and the background high-mass star forming regions. These data are complemented by observations of the 492 and 809 GHz fine structure lines of atomic carbon and by medium spectral resolution spectral maps of the fine structure lines of atomic oxygen at 63 and 145 microns with Herschel/PACS. We show that the presence of foreground absorption may completely cancel the emission from the background source in medium spectral resolution data and that high spectral resolution spectra are needed to interpret the [CII] and [OI] emission and the [CII]/FIR ratio. This phenomenon may explain part of the [CII]/FIR deficit seen in external luminous infrared galaxies. The C+ and C excitation in the diffuse gas is consistent with a median pressure of 5900 Kcm-3 for a mean TK ~100 K. The knowledge of the gas density allows us to determine the filling factor of the absorbing gas along the selected lines of sight: the median value is 2.4 %, in good agreement with the CNM properties. The mean excitation temperature is used to derive the average cooling due to C+ in the Galactic plane : 9.5 x 10^{-26} erg/s/H. Along the observed lines of sight, the gas phase carbon abundance does not exhibit a strong gradient as a function of Galacto-centric radius and has a weighted average of C/H = 1.5 +/- 0.4 x 10^{-4}.
Discovered over 30 years ago, the B[e] phenomenon has not yet revealed all its puzzles. New objects that exhibit it are being discovered in the Milky Way, and properties of known objects are being constrained. We review recent findings about objects of this class and their subgroups as well as discuss new results from studies of the objects with yet unknown nature. In the Magellanic Clouds, the population of such objects has been restricted to supergiants. We present new candidates with apparently lower luminosities found in the LMC.
We study the behavior of eight diffuse interstellar bands (DIBs) in different interstellar environments, as characterized by the fraction of hydrogen in molecular form [$f$(H$_2$)], with comparisons to the corresponding behavior of various known atomic and molecular species. The equivalent widths of the five normal DIBs ($lambdalambda$5780.5, 5797.1, 6196.0, 6283.8, and 6613.6), normalized to $E(B-V)$, show a Lambda-shaped behavior: they increase at low $f$(H$_2$), peak at $f$(H$_2$) ~ 0.3, and then decrease. The similarly normalized column densities of Ca, Ca$^+$, Ti$^+$, and CH$^+$ also decline for $f$(H$_2$) > 0.3. In contrast, the normalized column densities of Na, K, CH, CN, and CO increase monotonically with $f$(H$_2$), and the trends exhibited by the three C$_2$ DIBs ($lambdalambda$4726.8, 4963.9, and 4984.8) lie between those two general behaviors. These trends with $f$(H$_2$) are accompanied by cosmic scatter, the dispersion at any given $f$(H$_2$) being significantly larger than the individual errors of measurement. The Lambda-shaped trends suggest the balance between creation and destruction of the DIB carriers differs dramatically between diffuse atomic and diffuse molecular clouds; additional processes besides ionization and shielding are needed to explain those observed trends. Except for several special cases, the highest $W$(5780)/$W$(5797) ratios, characterizing the so-called sigma-zeta effect, occur only at $f$(H$_2$) < 0.2. We propose a sequence of DIBs based on trends in their pair-wise strength ratios with increasing $f$(H$_2$). In order of increasing environmental density, we find the $lambda$6283.8 and $lambda$5780.5 DIBs, the $lambda$6196.0 DIB, the $lambda$6613.6 DIB, the $lambda$5797.1 DIB, and the C$_2$ DIBs.
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