The use of PNe precursors in the study of Diffuse Interstellar Bands

1. The use of PNe precursors in the study of Diffuse Interstellar Bands Pedro García-Lario¹, Ramon Luna² & M.A. Satorre² ¹ESA/ISO Data Centre. ESAC, Madrid, Spain ² E. Politécnica Superior de Alcoy, Spain In collaboration with: H. van Winckel, M. Reyniers (K.U. Leuven); O. Suárez (INTA/LAEFF); B. Foing, N. Boudin (ESA/ESTEC)

2. What are DIBs ? • Diffuse Interstellar Bands (DIBs) are bands of variable strength and width of still unknown origin which appear overimposed on the spectra of bright but heavily reddened stars • Discovered in the early 1900’s ! but still unknown origin (presumed interstellar because of their correlation with dust extinction) Adapted from P. Jenniskens From P. Jenniskens

3. What do we know about DIBs ? • More than 300 catalogued (McCall et al. 2002) from UV to near-infrared wavelengths (3600 -10200 Å) • The most studied ones: • 4430 Å, 5780, 5797 Å, 6284 Å • Many carriers proposed; none convincing • A major challenge for spectroscopists, astronomers, and physicists

4. What are their carrier(s) ? • Detection of substructures in the profiles of several DIBs indicates the molecular nature of some DIB carriers (e.g. 5797, 6379 and 6614 Å ) (Kerr et al. 1998)

5. What are their carrier(s) ? • Existence of `families’ of DIBs suggest not a unique carrier (Krelowski & Walker 1987) Families of DIBS: 1: 4430, 6180 2: 5780, 6196, 6203, 6269, 6284 3: 5797, 5850, 6376, (2200)

6. What are their carrier(s) ? • Interstellar origin supported by correlation with reddening found in galactic early-type stars, measured as E(B-V) (Herbig 1995) Prototypical star: HD 183143

7. What are their carrier(s) ? • They are ubiquitous; detected towards a wide variety of astronomical sources • Most promising hypothesis: large carbon-bearing molecules: • Long carbon chains? (Douglas 1977) • PAH cations? (Allamandola et al. 1998; Salama et al. 1999) • Fullerenes? (Foing & Ehrenfreund 1997)

8. What else can we do? • There are strong evidences that therelative strength of DIBsare correlated with the properties of the clouds in the line of sight • Environmental dependence of DIBs may reflect an interplay of ionization, recombination, dehydrogenation and destruction of chemically stable, carbonaceous species (Salama et al. 1996) • Investigations of DIBs in regions of different metallicity, chemical properties and UV radiation fieldmay allow us to constrain the physico-chemical properties of the (different) DIB carriers. • Difficult to probe the ISM along a given line of sight; usually this is a combination of many different clouds with inhomogeneous properties and complex morphologies

9. What about circumstellar DIBs? • Are there also Diffuse Circumstellar Bands (DCBs) ? • First suggested by Le Bertre & Lequeux (1993) • Circumstellar shells around low- and intermediate-mass evolved stars are a ‘natural’ environment where DB carriers may form. • They are among the most important contributors of gas and dust to the ISM • Dense outflows of cool C-rich AGB stars are the best candidates • Observational problems because of the presence of strong molecular bands in their optical spectra; difficult to model stellar continuum; complex photospheres • Thus… No attempt yet made for a systematic search for DCBs • Ways around to address the problems needed (IRC +10º216) unsuccessful

10. Search for DCBs in AGBs • Diffuse Interstellar Bands (DIBs) are 12.4’ 2’ IRC +10º 216, Kendall 2002

11. A way around: post-AGB stars • Diffuse Bands (DBs)may potentially be detected also towards post-AGB stars • Post-AGB stars show a wide range of spectral types (from M to B) in their way to become PNe • High galactic latitude helps! • For many of them we know the chemical composition of the dust grains (ISO, mm/submm, radio) • Some results on individual post-AGB stars look promising (Zacs et al. 1999, 2001; García-Lario et al. 1999; Klochkova et al. 2000) post-AGB

12. DBs in post-AGB stars (Zacs et al. 1999)

13. Not always so simple… Not the 5850 Å DB !

14. A systematic search for DBs • 9 of the strongest Diffuse Interstellar Bands (DIBs) were investigated in a sample of 33 post-AGB stars • Spectral types: B – G • A mixture of C-rich and O-rich stars (chemistry derived from ISO data in most cases; also from submm and/or radio observations) • Wide range of galactic latitudes and overall extinction • high-radial velocity stars were favoured (to help discrimination of ISM vs. CSE features) • Several runs using 5 telescopes at three different observatories • ESO/La Silla (ESO 1.52m/FEROS + ESO NTT/EMM) • ESO/Paranal (VLT/UVES) • Roque de los Muchachos, La Palma (TNG/SARG + WHT/UES) • Spectral resolution  50,000; most of the observations so far analyzed were initially taken for other purposes

15. A systematic search for DBs Recalculation of EW / E(B-V) dependence using a sample of 53 reddened stars ofearly spectral type (Thorburn et al. 2003) At 5780, 5797, 6196, 6284, 6379 and 6614 Å Original spectroscopic data R  38000

16. A systematic search for DBs EW = α· E(B-V) Recalculation of EW/E(B-V) dependence using 4 reddened early type stars (Jenniskens et al. 2003) At 5850, 6196 and 7224 Å Original spectroscopic data: R 20000 HD 183143

17. ISM vs. CS extinction Overall extinction = ISM contribution + CS contribution

18. A systematic search for DBs

19. A systematic search for DBs t t t

20. The DB at 6284 Å IRAS 19500-1709 E(B-V) = 0.37 IRAS 22023+5249 E(B-V) = 0.52

21. DB strength vs. E(B-V) No clear correlation between EW and E(B-V) in P-AGB stars Many stars show values well below the expectations Some DBs are not even detected in strongly reddened P-AGB stars

22. DB strength vs. E(B-V) In general, DCS PAGB stars show always the lower values Non-detections at high E(B-V) are only found in DCS PAGB stars Non-DCS PAGB stars show values which are in many cases consistent with the values expected for ISM DIBs

23. The DB at 6284 Å

24. The DB at 5780 Å

25. The DB at 5797 Å

26. The DB at 6614 Å

27. The DB at 7224 Å

28. Analysis of radial velocities Na D2 5896 Å

29. Analysis of radial velocities

30. Conclusions • The strength of 9 of the strongest DBs has been systematically searched and analysed in a sample of 33 PAGB stars. • They are found to be extremely weak as compared to the results obtained in other samples of reddened stars • The effect is more clearly observed in P-AGB stars dominated by circumstellar extinction • Our results suggest that DIBs are not formed (yet) in the circumstellar shells around PAGB stars • If connected with PAHs, as suggested in the literature, their carriers must form at a later stage as the result of their processing by the hard UV field in the ISM • Their identification as strongly ionized PAHs and/or radicals liberated from carbonaceous species as a consequence of photoevaporation of dust grains in the ISM would be consistent with our observations • Interesting to observe CSPNe with various dominant chemistries and a lot of internal extinction!