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The use of PNe precursors in the study of Diffuse Interstellar Bands. Pedro García-Lario ¹ , Ramon Luna ² & M.A. Satorre ². ¹ ESA/I SO Data Centre . ESAC, Madrid, Spain ² E. Politécnica Superior de Alcoy, Spain. In collaboration with : H. van Winckel , M. Reyniers (K.U. Leuven);

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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)

what are dibs
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

what do we know about dibs
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
what are their carrier s
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)

what are their carrier s1
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)

what are their carrier s2
What are their carrier(s) ?
  • Interstellar origin supported by correlation with reddening found in galactic early-type stars, measured as


(Herbig 1995)

Prototypical star:

HD 183143

what are their carrier s3
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)
what else can we do
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
what about circumstellar dibs
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
search for dcbs in agbs
Search for DCBs in AGBs
  • Diffuse Interstellar Bands (DIBs) are



IRC +10º 216, Kendall 2002

a way around post agb stars
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)


dbs in post agb stars
DBs in post-AGB stars

(Zacs et al. 1999)

not always so simple
Not always so simple…

Not the 5850 Å DB !

a systematic search for dbs
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
a systematic search for dbs1
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

a systematic search for dbs2
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

ism vs cs extinction
ISM vs. CS extinction

Overall extinction = ISM contribution + CS contribution

the db at 6284
The DB at 6284 Å

IRAS 19500-1709

E(B-V) = 0.37

IRAS 22023+5249

E(B-V) = 0.52

db strength vs e b v
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

db strength vs e b v1
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

  • 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!