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Diffuse Bands Don't Originate in Circumstellar EnvelopesWHT+UES
The origin of the diffuse interstellar bands is a long-standing observational problem that now can see some light. A systematic survey of post-AGB stars, conducted by a team of Spanish and Dutch astronomers, has demonstrated for the first time that the carriers of the diffuse bands cannot be present in circumstellar envelopes. The result has been highlighted in Astronomy & Astrophysics journal this week and it indeed places the origin of the diffuse bands in the interstellar medium.
The diffuse interstellar bands are absorption features, showing a broad range of widths and strengths, which appear over-imposed on the spectra of bright stars whose lines of sight probe (extra)galactic diffuse-to-dense interstellar clouds. Currently, more than 200 bands have been identified and catalogued in the spectral range from 3600 to 10200 Å. Since their discovery, they have been associated to the interstellar medium, most probably to carbonaceous compounds, but no unambiguous identification has yet been proved.
Since circumstellar shells are sources of replenishment of the interstellar medium, it has been argued that the diffuse interstellar bands may have a circumstellar origin, either in dense stellar winds or circumstellar shells, thus somehow contravening the name they were initially given.
Observationally, the detection of diffuse bands around evolved stars is hampered by the fact that most of them are losing mass and are usually strongly variable and surrounded by very cool extended atmospheres where molecules are the dominant source of opacity. These stars are very difficult to model, and diffuse bands are hardly detected against the forest of features attributed to molecular transitions that appear superimposed on the stellar continuum. This has hampered any systematic search for diffuse bands in evolved stars in the past. Furthermore, if detected, it is difficult to determine whether the diffuse bands are originating from the interstellar or the circumstellar environment, or even both.
Post-AGB stars are in a short-lived transition phase between the asymptotic giant branch (AGB) and the planetary nebula (PN) stages, evolving while they are still surrounded by the remnant of the AGB circumstellar shell. As this evolution takes place, they become hotter very rapidly and the diffuse bands formed in the remnant AGB shell can easily be detected superimposed on the spectrum of the central star, which no longer shows molecular bands in the optical spectrum. In addition, post-AGB stars are located in many cases at relative high galactic latitudes and are as such only affected by low interstellar reddening. This facilitates the attribution of a circumstellar origin to the features observed.
Using the above way to proceed, the astronomers decided to conduct, for the first time, a systematic survey to detect the circumstellar diffuse bands in a carefully selected sample of post-AGB stars, including stars of various spectral types and chemical composition. For the analysis, they used some of the most efficient high-resolution spectrographs in the world, among them, the Utrecht Echelle Spectrograph (UES) on the William Herschel Telescope, from 1993 to 2003. In some cases, they reutilised data previously obtained for a different scientific purpose.
The results showed that the carriers of the diffuse bands do not form, or at least they are not available to produce any detectable spectral feature during the post-AGB phase. The carriers, if present in the circumstellar envelope of these stars, are not found under the environmental conditions needed to excite the transitions which can be identified as diffuse bands in the interstellar medium. The radial velocity analysis of the features observed confirmed this result, as the Doppler shifts measured are always found to be consistent with an interstellar origin for the bands observed.
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