A search for diffuse bands in the circumstellar envelopes of post-AGB stars
R. Luna, N.L.J. Cox, M.A. Satorre, D. A. Garcia-Hernandez, O. Suarez, P. Garcia-Lario
aa r X i v : . [ a s t r o - ph ] N ov Astronomy&Astrophysicsmanuscript no. 5282luna c (cid:13)
ESO 2018November 2, 2018
A search for diffuse bands in the circumstellarenvelopes of post-AGB stars ⋆ R. Luna , N.L.J. Cox , M.A. Satorre , D. A. Garc´ıa Hern´andez , O. Su´arez , and P. Garc´ıa Lario Laboratorio de Astrof´ısica Experimental. Escuela Polit´ecnica Superior de Alcoy, Universidad Polit´ecnica de Valencia, Plaza de Ferr´andiz yCarbonell, E-03801 Alcoy, Alicante, Spain Herschel Science Centre. Research and Scientific Support Department of ESA, European Space Astronomy Centre, P.O. Box 78, 28691,Villanueva de la Ca˜nada, Madrid, Spain W. J. McDonald Observatory. The University of Texas at Austin. 1 University Station, C1400. Austin, TX 78712-0259, USA LUAN, Universit´e de Nice Sophia Antipolis, Parc Valrose, 06108 Nice cedex 2, FranceReceived 27 March 2006; accepted 6 November 2007
ABSTRACT
In this work we present the results of a systematic search for di ff use bands (DBs, hereafter) in the circumstellar envelopes of a carefully selectedsample of post-AGB stars. We concentrated on the analysis of 9 of the DBs most commonly found in the interstellar medium. The strength ofthese features is determined using high resolution optical spectroscopy and the results obtained are compared with literature data on field starsa ff ected only by interstellar reddening. Based on the weak features observed in the subsample of post-AGB stars dominated by circumstellarreddening we conclude that the carrier(s) of these DBs must not be present in the circumstellar environment of these sources, or at least notunder the excitation conditions in which DBs are formed. The conclusion is applicable to all the post-AGB stars studied, irrespective of thedominant chemistry or the spectral type of the star considered. A detailed radial velocity analysis of the features observed in individual sourcesconfirms this result, as the Doppler shifts measured are found to be consistent with an interstellar origin. Key words.
Stars: AGB and post-AGB – ISM: dust, extinction – ISM: lines and bands
1. Introduction
The di ff use interstellar bands (DIBs) are absorption features,showing a broad range of widths and strengths, which ap-pear over-imposed on the spectra of bright stars whose linesof sight probe (extra)galactic di ff use to dense interstellarclouds. Currently, more than 200 DIBs have been identifiedand catalogued in the spectral range from 3600 to 10200 Å(Jenniskens & D´esert 1994; Cox et al. 2005), the most studiedones being those found at 4430, 5780, 5797 and 6284 Å. Sincetheir discovery (Heger 1922), they have been associated to theinterstellar medium (ISM), because their strengths show a posi-tive relationship with the observed extinction (Merrill 1936) aswell as to the neutral sodium column density (Herbig 1993).Many carriers have been proposed, however, no unambigu-ous identification has yet been made and it is debated whetherthey arise from the dust or the gas component of the ISM (seereviews by Herbig 1995 and Sarre 2006). There is increas-ing observational evidence that the DIB carriers constitute aset of carbonaceous gas phase molecules as evidenced from Send o ff print requests to : R. Luna, e-mail: [email protected] ⋆ Based on observations collected at the European SouthernObservatory (Chile) and at the Spanish Observatorio del Roque delos Muchachos of the Instituto de Astrof´ısica de Canarias. substructures resembling rotational contours in some bands(Sarre et al. 1995, Ehrenfreund & Foing 1996). In particular,photo-UV-resistant organic molecules, such as carbon chains(Douglas 1977), PAHs (Salama et al. 1999; Allamandola et al.1999), fullerenes (Foing & Ehrenfreund 1994; Iglesias-Groth2007) and / or buckyonions (Iglesias-Groth 2004) are promis-ing candidates. The local interstellar environmental conditionsset the balance of local formation and destruction of the carri-ers as well as their level of ionization and hydrogenation. Theinterstellar radiation field is one of the most important factorsin this (Ruiterkamp et al. 2005).There is a possible link between the DIB carriers and thecarriers of the unidentified (aromatic) infrared bands (UIBs),the so-called PAH-DIB hypothesis (Crawford et al. 1985;Leger & D’Hendecourt 1985; Van der Zwet & Allamandola1985).Thus, although PAHs are thought to reside and to be pro-cessed (ionisation, dehydrogenation, destruction) in the di ff useISM, this does not exclude the scenario that these molecules(or their parent species) are produced elsewhere. Since circum-stellar shells are sources of replenishment of the ISM, it hasbeen argued that DIBs (and / or parent structures) may have acircumstellar origin, either in dense stellar winds or circum- R. Luna et al.: A search for di ff use bands in post-AGB stars stellar shells, thus somehow contravening the name they wereinitially given. The suspected connection between DIB carriersand some carbon-rich compounds can be investigated attend-ing to the usually known chemistry and physical properties ofthese circumstellar shells.Observationally, the detection of di ff use bands (DBs, here-after) around evolved stars is hampered by the fact that most ofthem are mass-losing stars, usually strongly variable, and sur-rounded by very cool extended atmospheres where moleculesare the dominant source of opacity. These stars are very di ffi cultto model and DBs are hardly detected (in absorption) againstthe forest of features attributed to molecular transitions whichappear over-imposed on the stellar continuum. This has ham-pered the systematic search for DBs in evolved stars in the past.Furthermore, if detected, it is di ffi cult to determine whether theDBs are originating from the interstellar or the circumstellarenvironment, or even both.In face of these di ffi culties Snow & Wallerstein (1972)and Snow (1973) searched for circumstellar di ff use bands (at4430, 5780, 5797 and 6614 Å) in 26 stars with suspected cir-cumstellar dust shells / envelopes but found no evidence fortheir presence. Several other authors have since searched forand commented on the presence of di ff use bands and theirinterstellar or circumstellar nature, in spectra observed to-wards planetary nebulae (NGC 6210, NGC 7027, IC 351 andAFGL 2688 by Prichet & Grillmair 1984; IRAS 21282 + + +
30 3639, CPD-56 8032, Hen 104), a carbon richRV Tauri star (AC Her), wolf-rayet stars (WR137, WR140)and post-AGB stars (HR 4049, HD213985), revisiting severalsources studied in the past ( e.g.
CS 776, NCG 7027, HR 4049,IRAS 21282 + ff erent state of ionization / hydrogenation than in the ISM. Strong DBs were detectedtoward carbon-rich sources that do not show PAH emission,as well as toward most of their oxygen-rich and nitrogen-richsources in the sample, although in all cases the observed DBscould be attributed to the interstellar material in the lines ofsight. For unexplained reasons enhanced DBs were detectedtoward WN stars and LBVs. Exceptionally, narrow emissionfeatures possibly related to DIBs as well have been observedtoward the Red Rectangle (Scarrott et al. 1992), although theiridentification and nature remains controversial.A largely unexplored alternative exists. This concerns theso-called post-AGB stars that are in a short-lived transitionphase between the Asymptotic Giant Branch (AGB) and thePlanetary Nebula (PN) stage, evolving very rapidly in the H-R diagram while they are still surrounded by the remnant of the AGB circumstellar shell. Post-AGB stars show all possi-ble spectral types from M to B in what probably representsan evolutionary sequence of increasing e ff ective temperaturein their way to become PNe (Garc´ıa-Lario et al. 1997b). Thismeans that in these stars we should easily be able to detectDBs formed in the remnant AGB shell over-imposed on theintermediate or early-type spectrum of the central star with-out the confusion originating from the presence of molecularbands in AGB stars. Interestingly, while many of these DBsare common to those observed in the ISM, the relative ratiosare sometimes found to be very di ff erent (Garc´ıa-Lario et al.1999). These circumstellar DBs could form and survive forsome time under conditions which might be substantially dif-ferent to those found in the ISM in terms of density, UV radi-ation field, etc. and could hold the key to understand and solvethis long-standing problem. Another advantage is the fact thatthe chemical composition of the gas and dust in these shellscan easily be determined from observations in the optical, in-frared, mm / sub-mm or radio wavelengths. In addition, post-AGB stars are located in many cases at relative high galacticlatitudes, and are as such a ff ected only by little interstellar red-dening. This facilitates the attribution of a circumstellar originto the features observed.The potential formation of DBs around post-AGB stars has,however, been explored so far only occasionally for a lim-ited number of sources. Nevertheless, the presence of strongDBs has been reported in the optical spectra of a few post-AGB stars (Le Bertre & Lequeux 1993; Garc´ıa-Lario et al.1999; Zacs et al. 1999a, 2001; Klochkova et al. 1999, 2000;Kendall et al. 2002) and some carbon rich (barium) stars(Zacs et al. 2003). In several cases tentative claims have beenput forward of DBs detected at radial velocities coincidingwith the photospheric absorption lines or shell / envelope expan-sion velocity (IRAS 04296 + + . The goal is to per-form a detailed analysis of the di ff erential properties observedin the DBs associated to post-AGB stars in comparison with thestandard DBs observed towards reddened, early-type field stars(where these bands are expected to be essentially of interstellarorigin).To perform this task we have studied the intensity of 9 ofthe strongest absorption features identified as DBs in the spec-tral range 4000–10000 Å using high resolution optical spec-troscopy. The comparison of the properties observed in carbon-rich and oxygen-rich post-AGB stars is used to test the carbon- The sample also includes three very young planetary nebulae,which are here also considered post-AGB stars in a broad sense.. Luna et al.: A search for di ff use bands in post-AGB stars 3 Table 1.
Observation log.
Name Other Names Observing Date Observatory Telescope Instrument Spectral range (Å) ResolutionIRAS 01005 + − + − + − + − − + − − + − − − − − − − − − − − − + − − + − − + − + +
34 1 11.09.03 La Palma TNG (3.58 m) SARG 4960 − + − − + − + +
34 26 17.08.96 La Palma WHT (4.2 m) UES 5300 − + +
52 24 14.07.01 La Palma WHT (4.2 m) UES 4300 − + +
42 4388 24.08.94 La Palma WHT (4.2 m) UES 4440 − + − + − Fig. 1.
Removal of telluric lines at 6284, 6993 and 7224 Å.A few examples are shown before (left) and after correction(right). In the middle panel we show the stellar spectrum ofHD 172324 (B9Ib), one of the sample stars, which was used astelluric divisor (see text). rich nature of the DB carrier(s) and determine which of the DBsdetected are most probably of circumstellar origin (if any).In Section 2 we describe the observations made and the datareduction process. The strategy followed in our analysis is pre-sented in Section 3. The main results are discussed in Section 4as a function of various observational parameters. Finally, theconclusions are presented in Section 5.
2. Observations and Data Reduction
The high-resolution Echelle spectroscopic data analysed in thispaper were taken using a wide variety of instruments andtelescopes over the period 1993-2003. Originally, these ob-servations were carried out for chemical abundance analysispurposes and they correspond to observations performed us-ing the Utrecht Echelle Spectrograph (UES) at the WilliamHerschel Telescope (WHT 4.2m) and the High ResolutionSpectrograph (SARG) at the Telescopio Nazionale Galileo(TNG 3.58m), both in the Spanish Observatorio del Roquede los Muchachos (La Palma, Spain); the UV-Visual EchelleSpectrograph (UVES) installed at the Very Large Telescope-U2 (VLT 8m) in Paranal Observatory (Chile); and the ESOMulti-Mode Instrument (EMMI) at the New TechnologyTelescope (NTT 3.5m) and the Fiber-fed Extended Range
R. Luna et al.: A search for di ff use bands in post-AGB stars Table 2.
Main characteristics of the post-AGB stars selected for the analysis
IRAS Name E ( B − V ) Ref. Chemistry Ref. Sp.Type Ref. GLON. GLAT.01005 + ± + + ± + + ± − + ± − ± /
07f (14) 215.21 − + ± − ± − + ± + ± + ± − ± + ± + ± − ± + G5? (34) 359.84 + ± + ± + ± − ± + G (11) 17.02 + ± + + ± + ± − + ± + ± + + ± − + ± + + ± − ± − + ± + + ± − + ± − + ± − + ± − + ± + (1) Arellano et al. (2001) (13) Gauba & Parthasarathy (2004) (25) Klochkova et al. (2002) (37) Reddy et al. (1999)(2) Arkhipova et al. (2000) (14) Heap & Augensen (1987) (26) Kwok et al. (1995) (38) Reed & Vance (1996)(3) Bakker et al. (1997) (15) Hrivnak (1995) (27) Kwok et al. (1999) (39) Reyniers et al. (2004)(4) Bujarrabal et al. (1992) (16) Hrivnak & Kwok (1991) (28) Lewis (2000) (40) Su´arez et al. (2006)(5) Desmurs et al. (2002) (17) Hrivnak & Kwok (1999) (29) Maas et al. (2003) (41) te Lintel Hekkert et al. (1991)(6) Fernie (1983) (18) Hrivnak & Reddy (2003) (30) Malfait et al. (1998) (42) Torres-Peimbert et al. (1980)(7) Fujii et al. (2002) (19) Hrivnak et al. (1989) (31) Meixner et al. (1999) (43) Turner & Drilling (1984)(8) Garc´ıa-Lario et al. (1997a) (20) Hrivnak et al. (1999) (32) Oudmaijer et al. (1992) (44) van de Steene & van Hoof (2003)(9) Garc´ıa-Lario et al. (1999) (21) Hrivnak et al. (2000) (33) Parthasarathy et al. (2000a) (45) van der Veen et al. (1989)(10) Garc´ıa-Lario (priv.comm.) (22) Hu et al. (1993a) (34) Parthasarathy et al. (2000b) (46) van Winckel (1997)(11) Gauba et al. (2003) (23) Hu et al. (1993b) (35) Pottasch et al. (2004) (47) van Winckel & Reyniers (2000)(12) Gauba & Parthasarathy (2003) (24) Hu et al. (1994) (36) Reddy & Parthasarathy (1996) (48) Volk & Kwok (1989) Optical Spectrograph (FEROS) at the ESO 1.52m telescope inLa Silla Observatory (Chile).The spectra obtained cover a wide wavelength range (usu-ally from 4000 to 10000 Å) with a resolving power in the range50000 − ∼
30 min, leading to a signal-to-noise of 20 −
200 over the spectral range considered.The two-dimensional spectra were reduced following thestandard procedure for echelle spectroscopy using IRAF astro-nomical routines. The process includes: identification of badpixels, bias determination and scattered light subtraction, flat- field correction, order extraction and wavelength calibration.For the DBs at 6284, 6993 and 7224 Å, strongly a ff ected byterrestrial features, the spectrum of HD 172324 (B9Ib), one ofthe sample stars very little a ff ected by extinction, was used asdivisor to remove the telluric absorption lines (see Figure 1). Note that the initial strategy was to use the spectrum of a hot,rapidly rotating star observed on the same night for this purpose, but itwas found that the latter showed faint but detectable DBs in its spec-trum which did not allow us to perform this correction properly. Incontrast, the 6284, 6993 and 7224 Å DBs were found to be totallyabsent in HD 172324.. Luna et al.: A search for di ff use bands in post-AGB stars 5 Fig. 2.
Equivalent width measurements taken from the litera-ture for field stars, plotted as a function of E ( B − V ). Table 3gives slopes EW / E ( B − V ) and correlations r for the linear fits.
3. Selection of the sample
In order to make a systematic study of the presence of DCBs inpost-AGB stars, we carefully selected a sample of 33 sourcesfrom the literature trying to cover as much as possible a widevariety of observational properties such as the chemistry ofthe circumstellar envelope (carbon-rich and oxygen-rich) or thespectral type of the central stars . Priority was given to sourceslocated at high galactic latitudes showing a strong colour ex-cess E ( B − V ) (as these sources are most likely dominated bycircumstellar extinction) and to those for which a high radialvelocity has been reported in the literature (since this may laterfacilitate the identification of spectral features of circumstellarorigin).The main objective is to understand whether systematic dif-ferences are detected which depend on one (or more) of theabove observational parameters.The list of stars selected for analysis, most of themIRAS sources belonging to the GLMP catalogue of post-AGBstars (Garc´ıa-Lario et al. 1997a; Su´arez 2004), is displayed inTable 1, where a summary description of the observations made Stars with spectral types later than G- were discarded for the anal-ysis, as their continuum is dominated by the presence of molecularbands, which makes the identification of DBs a very di ffi cult task. Fig. 3.
Sample spectra showing the region around the DB at6284 Å, before (left panel) and after (right panel) removalof the telluric lines using as the telluric divisor HD 172324.Dotted lines indicate the continuum level adopted in each case.is presented. In Table 2, additional information is given on thesources included in our observing programme. This includesthe colour excess E ( B − V ), dominant chemistry (carbon-richor oxygen-rich), spectral type and galactic coordinates (GLON,GLAT), as well as the bibliographic references from where thisinformation was extracted.The spectral regions corresponding to 9 di ff erent DBswhich are among the strongest ones reported in the lit-erature have been investigated in detail for each of thesources included in our sample. Table 3 lists the accuratewavelengths ( λ ) corresponding to each of these features,taken from Galazutdinov et al. (2000), as well as their cen-tral depth A C and sensitivity to the extinction, measured as EW / E ( B − V ), observed toward the star HD 183143 (B7 I; E ( B − V ) = and . Other well known DBs at 4430and 6177 Å are even stronger than the selected ones, but theyhave been discarded for study because of the di ffi culty to detecttheir extremely broad (and relatively shallow) profiles (FWHM >
17 Å; A c < .
1) in our high resolution spectra.
R. Luna et al.: A search for di ff use bands in post-AGB stars Table 3.
Main characteristics of the selected DBs (cen-tral depth A C and normalised equivalent width EW / E ( B − V )), as measured towards the prototype star HD 183143(Herbig 1995) (cols. 3 and 4). The reference wavelengths aretaken from Galazutdinov et al. (2000) (col. 2). The equivalentwidth per extinction unit (this work) derived from publisheddata from Jenniskens & D´esert (1994), Weselak et al. (2001),Thorburn et al. (2003) and Megier et al. (2005) is given foreach DIB in col. 5, with corresponding correlation coe ffi cientsin col. 6. HD 183143 this workDB λ A c EW / E ( B − V ) EW / E ( B − V ) r(Å) (Å) (Å / mag) (Å / mag)5780 5780.37 0.32 0.63 0.46 0.745797 5796.96 0.20 0.19 0.17 0.735850 5849.80 0.069 0.06 0.061 0.756196 6195.96 0.084 0.06 0.053 0.826284 6283.85 0.32 1.5 0.90 0.696379 6379.29 0.10 0.096 0.088 0.626614 6613.56 0.24 0.29 0.21 0.806993 6993.18 0.14 0.14 0.12 0.957224 7224.00 0.21 0.29 0.25 0.99 A c = λ ) / F(continuum)
4. Discussion
Although it is generally accepted that there is a tight correlationbetween the equivalent width of DBs and the value of E ( B − V )in field stars dominated by interstellar reddening, the availableresults in the literature generally cover only the stronger DBs( λλ = / or based on high resolution spectroscopyare scarce. Prior to derive any conclusion on the existence (ornot) of a similar correlation between DB strength and extinc-tion in our sample of post-AGB stars it is, thus, necessary toestablish this dependency for each of the 9 DBs observed to-ward field stars.For this purpose, we have re-derived ourselves these cor-relation parameters using a large number of early-type starstaken from Weselak et al. (2001); Thorburn et al. (2003) andMegier et al. (2005), for which accurate DB strength measure-ments are available covering a wide range of extinction values.Weselak et al.’s sample contains 41 stars observed at R ∼ ∼ ∼ ∼ ff ected only by interstellar extinction.Figure 2 shows all the equivalent width measurements used inour analysis, plotted as a function of the interstellar extinction,measured as E ( B − V ).For each DB under analysis we have applied a linear fit tothe data available. We have also imposed the condition EW = E ( B − V ) =
0, i.e.: EW = a · E ( B − V ), where a is a con-stant that represents the equivalent width per extinction unity.In practice this is equivalent to assume that there is a direct linkbetween the DB carrier(s) and the material which is responsiblefor the extinction observed in the ISM. The fits obtained rep-resent the DB strength expected as a function of the colour ex-cess for any given source in which interstellar reddening is thedominant contributor to the overall extinction. These are repre-sented by solid lines in Figure 2. The slopes ( EW / E ( B − V )) andcorrelation coe ffi cients r of the linear fits are given in Table 3.As we can see, a reasonable correlation between equiva-lent width and E ( B − V ) is always found, although the disper-sion is in some cases considerable. The new results obtainedare in agreement as well with those derived for the prototypestar HD 183143 by (Herbig 1995) although the 5780 and 6284DIBs are significantly stronger toward the latter, probably dueto local environmental conditions. These results provide confi-dence to proceed with the study of the post-AGB stars in oursample, based on the assumption that the above values can betaken as references for the subsequent analysis. In Table 5 (Online only) we show the equivalent width of eachof the 9 DBs considered in our analysis for every post-AGB starin the sample, as determined from the available high resolutionspectra. Note that three of these bands are strongly a ff ected bytelluric contamination, namely those centred at 6284, 6993 and7224 Å. For these features, a careful removal of the telluriccomponent was performed prior to the determination of their EW (see Figures 1 and 3; Sect. 2). For the other DBs, the mea-surements were performed directly on the normalised spectra.We note here that in none of the spectra we found evidence forDB features in emission.The resulting values are plotted in Figure 4 as a function ofthe colour excess E ( B − V ). The values of E ( B − V ) which wereused to produce this figure are directly taken from the literature(see Table 2) or estimated from the available information onspectral type and photometry by comparing the observed B andV magnitudes with the intrinsic B − V colours expected for starsof the same spectral type and luminosity class I . A luminosity class I is adopted because this is the class corre-sponding to low-gravity stars, but note that post-AGB stars only looklike super-giants, but they are not genuine population I massive super-giant stars (Fitzgerald 1970). Values quoted by di ff erent authors aregenerally in good agreement, and the discrepancies found, when rele-vant, are reflected in the associated errors provided in Table 2.. Luna et al.: A search for di ff use bands in post-AGB stars 7 Fig. 4.
Equivalent width (in Å) of the 9 DBs selected for analysis as a function of E ( B − V ) for the post-AGB stars in the sample.Solid lines correspond to the fits derived in this paper for field stars dominated by interstellar extinction (see also Sect. 4.1). Theinverted triangles represent upper limits.Figure 4 shows the overall results obtained for the 9 DBsstudied. In general we find that the equivalent width of theobserved features seems to be still correlated with the valueof E ( B − V ). However, in contrast to the results obtained forthe field stars (shown in Figure 2), this correlation is now veryweak in some cases and we identify a much larger number ofoutliers.Usually, for a given extinction E ( B − V ), the measuredequivalent widths in post-AGB stars are well below the ex-pected values. Only a subset of sources follow exactly the samebehaviour observed in field stars. We interpret this result as the consequence of the absence (or at least the under-abundance)of the DB carriers in the circumstellar envelopes of most ofthese post-AGB stars, but further analysis is needed to confirmthat there is no other alternative explanation. In order to determine whether our preliminary hypothesis isconsistent with the measurements here presented, it is neces-sary to take into account that in general, the overall extinctionobserved towards a given source in the sky is the result of the
R. Luna et al.: A search for di ff use bands in post-AGB stars Table 4.
Line-of-sight properties for the observed post-AGB stars. DCS = dominated by circumstellar reddening; HV = highradial velocity; CS1 designated DCS based on lat / long vs reddening, excluding those that are discarded based on interstellar (IS)reddening estimate. CS2 indicates DCS found by estimating the upper limit to the IS reddening. Principal extinction estimatesare given with rescaling of disk / spiral component (col. 7). No-rescaling estimates are given (col. 8) in those cases for which theextinction estimate is significantly higher than with use of rescaling. The angular scale of COBE data is 0.35 ◦ × ◦ and thisdust extinction model therefore only gives mean extinction estimates. Erroneous rescaling factors can arise for directions towardstrong extra-galactic sources such as M31, M33, SMC and LMC as well as toward peculiar galactic regions such as Orion and theRho Ophiuchus complex. Also, lines of sight corresponding to arm tangents may have large systematic errors. Distance estimates(references in col. 4) and corresponding model extinctions (converted to E ( B − V )) are given when available in Cols. 3 & 5). Themaximum model reddening (with and without re-scaling) and the corresponding distance in the target direction are given in cols.7, 8 and 6, respectively. Col. 9 gives the resulting lower limit for the circumstellar reddening. The final column (10) indicateswhen the target is dominated by circumstellar reddening (CS1 or CS2) and / or is a high velocity target (HV). NAME E ( B − V ) d Ref. E ( B − V )IS d max E ( B − V )IS E ( B − V )-CS DCS / HVobserved kpc kpc scaling no scaling min01005 + ± + ± > + ± + ± ± + ± ± + ± / / CS208005-2356 0.7 ± / ± ± ± ± ± ± ± <
18 (2) 9 (0.9) 5 1.817245-3951 1.0 ± ± ± / HV17436 + ± ± / / HV18062 + ± / HVHD 172324 0.03 ± + ± + ± + ± ± > + ± > > + ± + ± + ± + ± + ± = combined e ff ect of the contribution coming from the ISM andof the internal extinction produced in the circumstellar shell.Making this distinction is generally not important in field stars,since for them the latter contribution is negligible. However,for the evolved stars in our sample the situation is completely di ff erent as, in many cases, the observed reddening is almostexclusively of circumstellar origin.Disentangling interstellar versus circumstellar extinctionfor a given source is a very di ffi cult task, if we need to relyonly on the available observations. The only option we have isto use a statistical approach to estimate whether the observed . Luna et al.: A search for di ff use bands in post-AGB stars 9 Fig. 5. E ( B − V ) versus Galactic latitude distribution of thepost-AGB stars in the sample (circles) and for reference starstaken from Guarinos (1988a,b, 1997) (small squares). Post-AGB stars dominated by circumstellar extinction (DCS-type)are indicated by filled circles and they are labeled with theirIRAS name.extinction corresponds preferentially to one or another compo-nent.For this we have represented in Figure 5 the E ( B − V ) ver-sus galactic latitude distribution of the post-AGB stars in oursample and compared this distribution with that shown by fieldstars taken from the catalogue of Guarinos (Guarinos 1988a,b,1997). This catalogue, also used for the study of DB strengths,contains observations of 270 early-type field stars homoge-neously distributed along the Galactic Plane (but excluding theGalactic Bulge), located at a variety of galactic latitudes and forwhich the value of E ( B − V ) has previously been determined.Figure 5 shows clearly that a subsample of post-AGB starsare clear outliers in this plot. This indicates that the redden-ing excess in these stars must be circumstellar in origin. Othersources, however, show a relatively small reddening fully com-patible with the values observed in field stars located at thesame galactic latitude.Based on this analysis, we have divided our sample of post-AGB stars in two groups according to whether the overall ex-tinction observed is more likely to be dominated by the circum-stellar contribution (DCS-type stars; filled circles in Figure 5;CS1 in Table 4) or just consistent with the interstellar extinc-tion expected according to its Galactic Latitude (rest of stars;indicated by open circles in Figure 5).Note that the above classification is very rough and that itjust considers a star as belonging to the DCS group if it showsa relative large reddening excess with respect to the nominalvalue expected from its galactic location. Stars in which thereis only a moderate (although possibly significant) contributionfrom the circumstellar shell to the observed extinction mayhave escaped detection. This means that the group of stars notclassified in the DCS group may still contain sources in whichthe circumstellar contribution to the observed reddening is not negligible, and vice versa, those classified DCS may still con-tain a significant interstellar dust contribution.In order to estimate an upper limit to the contribution of theinterstellar reddening to the total reddening we use the Galactic3D-extinction model map by Drimmel et al. (2003) that givesthe mean visual extinction as a function of sky (galactic) co-ordinates and distance. The extinction has a projected resolu-tion of 0.35 ◦ × ◦ (this is set by the COBE map which isused to re-scale the extinction in order for the model to repro-duce correct far-infrared flux). It is evident that any small scalestructure (including the circumstellar contribution of the tar-get star) is washed out in these estimates, which neverthelessgive us information on the global spatial distribution of dust inspecific directions. We take distance estimates from literaturewhere possible (column 3 in Table 4) and / or extract the max-imum extinction (column 7) and the corresponding distance(column 6) for a particular line-of-sight. For high latitudes theextinction versus distance curve flattens rapidly, within a fewkpc. These (upper limit) estimates for the interstellar visual ex-tinction, converted to reddening by dividing by the canonicalvalue for R V = E ( B − V ) CS ≥ . + Following the above criteria, we find that for 17 (out of 33) starsin the sample a significant fraction of the observed reddening isdue to the presence of circumstellar dust (i.e. DCS-type), whilefor the remaining targets the colour excesses are expected to bepredominantly due to interstellar dust.A comparison of the DB strengths measured in stars be-longing to each of the two groups considered above with thosefound in reference field stars is presented in Figure 6, where wecan see that there is a general trend for the stars with a dominantcircumstellar extinction contribution (DCS-type) to show muchweaker DB strengths compared to the rest of post-AGB stars inthe sample. In the most extreme cases, there are stars in thisgroup a ff ected by a large overall extinction in which surpris- ff use bands in post-AGB stars ingly some DBs are completely absent, and only upper limitsto their equivalent width can be reported. In contrast, we finda significant number of sources among the rest of post-AGBstars in the sample showing DB strengths fully in agreementwith those observed in field stars.For many cases DBs are observed towards the DCS-typestars (Figs. 7 to 15; left panels). In order to assess the circum-stellar contribution the observed EW can be corrected by sub-tracting the expected DIB EW found by applying the estimatedinterstellar reddening (Table 4) to the respective EW / E ( B − V )for field stars (Table 3) as well as subtracting the IS redden-ing contribution from the total observed reddening. The intro-duced uncertainties are quite large due to the scatter on the de-rived linear relationships (see above, Section 4.1). In particularIRAS 02229 + EW / E ( B − V ) (which we assume to be zero) fromthe observed EW . It shows then that all stars coincide neatlywith the average Galactic relation. This is most noticeable forIRAS 17086-403 and IRAS 17395-0841 which have estimatedCS contributions of 0.3 and 0.2 mag, respectively).In principle, this result supports our initial interpretationthat the DB carrier(s) may not be present in the circumstellarenvelopes of post-AGB stars. However, strong variations fromsource to source are still visible in both groups of stars, whichmay be related to other observational properties of the shellsnot yet considered (Significant scatter is also observed for thesample of field stars; Sect. 4.1). Indeed, the results obtainedsuggest that some of the DB carriers could be completely ab-sent in some of these envelopes while not in others. In order to explore whether other environmental conditions,like the dominant chemistry in the shell or the spectral type ofthe central star could also play a role in the di ff erences observedbetween individual stars in the sample, we have further dividedthe two groups defined above in another four subgroups as afunction of whether the chemistry of the shell is carbon-rich oroxygen-rich, or the spectra of the central star is of early-type(B-A) or of intermediate-type (F-G).We do this because the dominant chemistry of the shell cancompletely determine the formation of specific compounds inthe circumstellar shell. In oxygen-rich shells we expect to findaluminum oxides, amorphous or crystalline (fayalite, enstatite,forsterite, etc) silicates, water ice and other main constituentsof oxygen-rich dust grains. In carbon-rich stars, instead, we canfind carbon-based constituents, like chains or rings of carbon,graphite, hydrogenated amorphous carbon grains, fullerenes,nanodiamonds or PAHs. Note that among the subgroup of early-type stars we have alsoincluded the few sources in Table 2 which are classified as planetarynebulae.
Fig. 6.
Equivalent width of the 9 DBs selected for analysis as afunction of E ( B − V ) for the post-AGB stars in the sample inwhich filled circles represent the subsample of post-AGB starsdominated by circumstellar extinction (DCS-type) and open tri-angles the rest of stars in the sample. Solid lines again corre-spond to the fits derived in this paper for field stars dominatedby interstellar extinction. The arrows indicate upper limits.If a DB carrier had their origin in a compound or constituentrelated to only one of the above chemistries, we would expectto observe di ff erences in strength from source to source as afunction of their particular chemical composition.On the other hand, it is also well known that the UV radi-ation field plays a crucial role in the processing of the circum-stellar dust grains, not only immediately after they are formed,while they are still part of the shell, but also later when theyare released to the ISM. Dust grains in the circumstellar en-velopes of post-AGB stars are exposed to increasing doses ofUV radiation due to the increasing e ff ective temperature of thecentral star during its fast evolution towards the planetary neb-ula stage. First, when the central star is still showing late tointermediate spectral type, the UV radiation can be neglected,both as a consequence of the low e ff ective temperature of thecentral star and because the higher density in the envelope dur-ing the early post-AGB stage would e ff ectively protect (at least . Luna et al.: A search for di ff use bands in post-AGB stars 11 Fig. 7.
Equivalent width of the 6284 Å band vs. E ( B − V ) forthe group of stars dominated by circumstellar extinction (leftpanel; DCS type) and for the rest of stars in the sample (rightpanel) with di ff erent symbols indicating the dominant chem-istry and spectral type of the observed stars. The solid line rep-resents the behaviour observed in field stars dominated by in-terstellar extinction, as deduced from the data shown in Figure2.temporarily) circumstellar dust grains from the energetic UVphotons coming from the ISM. These conditions favour theformation of large dust grains which can survive in this lessaggressive environment. Later in the post-AGB evolution thecentral stars become early-type and they start producing a con-siderable number of UV photons which may lead to an e ffi cientprocessing of the dust grains in the shell, which in turn is lessprotected and more vulnerable also to the UV radiation fieldcoming from the ISM. The combined e ff ect of the UV photonscoming from the central star and from the ambient ISM is ex-pected to accelerate the processing of the dust grains, leadingto new species like molecules, radicals (more or less complex)and other byproducts resulting from the partial or total evap-oration of the grains, which will eventually be released to theISM. Indeed, these byproducts could be the actual carriers ofthe DIBs commonly observed in the ISM.If DB carriers are only related to the byproducts of the de-composition of these large circumstellar grains, we would ex-pect to observe a deficit in DB strengths in post-AGB stars,only while the central stars are still showing a relatively lowe ff ective temperature.Both e ff ects can be combined, and it may also happen thatthe DB carriers are related to the byproducts of only a particularclass of grains associated to a given dominant chemistry. Inthis case, we will be able to detect significant di ff erences fromsource to source, both as a function of the spectral type of thecentral star as well as of the dominant chemistry in the shell.In the next section we will analyse the influence of these en-vironmental conditions (dominant chemistry and spectral type)on the observed results for each of the 9 DBs under study inour sample of post-AGB stars. The 6284 Å band is not only the strongest ( EW / E ( B − V ) = .
90 Å / mag in the ISM) but also the broadest band included inour analysis. As such, it is relatively easy to measure, in spite ofthe contamination by telluric lines already shown in Figure 3,which must be carefully removed.In Figure 7 we show the results obtained as a function of thedominant chemistry and of the spectral type of the central starfor each of the two main subgroups identified in our sample.The strength of the 6284 Å band as a function of E ( B − V )for the group of stars dominated by circumstellar extinction ispresented in the left panel, while the results obtained for the restof stars in our sample is shown in the right panel. As we cansee, it is obvious that the post-AGB stars belonging to the DCSgroup show DB strengths systematically below those observedin the ISM (represented by the solid line).Actually, in some cases this band is so weak thanwe can only determine an upper limit for its equivalentwidth. This is the case for IRAS 17436 + + E ( B − V ). This star is a well-known bipolar proto-planetary nebula which seems to be a ff ected by a high internalextinction.For IRAS 06530-0213 the band strength is typical for thetotal observed reddening being due to interstellar dust. On theother hand, the interstellar and circumstellar reddening con-tributions are estimated to be 1.3 and 0.4 mag, respectively(Sect. 4.3). Though this could indicate the presence of circum-stellar DBs it should be noted that this line of sight lies in thegalactic plane (GLAT = − This DB has a EW / E ( B − V ) = .
46 Å / mag in the ISM, so itis the second most intense after the 6284 Å band. In this par-ticular case, it is important to remark that the spectral regioncorresponding to this band can be contaminated by the pres- ff use bands in post-AGB stars Fig. 8.
Same as Figure 7, for the 5780 Å band.
Fig. 9.
Same as Figure 7, for the 7224 Å band.ence of photospheric lines, which makes the evaluation of theband strength very di ffi cult in stars with intermediate and latespectral types.Note that to distinguish weak features from weak stellarlines or telluric contaminations is not always a simple task andmakes it necessary to use detailed stellar models (to subtractthe atmospheric features) and high resolution spectroscopy (toproperly remove undesired contaminations), as the only way toderive the accurate strength of the band, which is beyond thescope of this work.In Figure 8 we show the equivalent width of the 5780 Åband vs. E ( B − V ) for each of the two main subgroups in whichwe have divided the sample. In the group of stars dominated bycircumstellar extinction (left panel) we observe again strengthssignificantly weaker than those expected in stars for which theextinction is mainly of interstellar origin, represented by thesolid line.The non-detection of this DB in IRAS 05341 + E ( B − V ) CS ≈ Fig. 10.
Same as Figure 7, for the 6614 Å band.
Fig. 11.
Same as Figure 7, for the 5797 Å band.rier of this band is completely absent at least in the envelopesof carbon-rich stars with intermediate spectral types. A similarconclusion can be derived for oxygen rich stars with interme-diate spectral types from the very weak strength observed inIRAS 18025-3906 (oxygen-rich; B-type).Unfortunately, the spectrum available forIRAS 16594-4656 does not cover the spectral range cor-responding to this band, so we cannot extend the aboveconclusion to carbon-rich stars with earlier spectral typesbased on our data.As in the case of the 6284 Å band, we can also observe inthe right panel of Figure 8 that the rest of stars in the samplenot identified as dominated by circumstellar extinction show aposition in the diagram which is, overall, in better agreementwith the results obtained for field stars in which the extinctionis mainly of interstellar origin. . Luna et al.: A search for di ff use bands in post-AGB stars 13 Fig. 12.
Same as Figure 7, for the 6993 Å band.
Fig. 13.
Same as Figure 7, for the 6379 Å band.
This DB is not usually analysed in the literature because it isstrongly a ff ected by telluric contamination. As in the case of the6284 Å band, we have carefully eliminated this contributionby dividing the normalised spectrum by the spectrum of theunreddened target HD 172324 (see Sect. 2).In Figure 9 we show the equivalent width of the 7224 Åband vs. E ( B − V ) for each of the two main subgroups iden-tified in our sample. As for the two previous features, we findstrengths which are much weaker than those measured in fieldstars in the subgroup formed by the stars in which the circum-stellar contribution to the overall extinction is dominant (DCS;left panel). This again suggests that this band is not formed inthe circumstellar envelope of post-AGB stars.Again, the measured intensity of the 7224 Å band inIRAS 16594-4656 is rather weak and, once more, we find sev-eral non-detections: IRAS 05113 + + Fig. 14.
Same as Figure 7, for the 5850 Å band.
Fig. 15.
Same as Figure 7, for the 6196 Å band.neither detected the 6284 Å band. The non-detection of DBsin oxygen-rich envelopes around post-AGB stars indicates thatthe DB carriers are probably not generated in oxygen-richenvironments. The absence of the band in the specific caseof IRAS 18062 + ff ected only by inter-stellar extinction. The exception is IRAS 17086-2403 (carbonrich star) for which we detect no DBs. In Figure 10 we show the results of our analysis applied thistime to the 6614 Å band.In the left panel we show the equivalent widths measuredin the subgroup of post-AGB stars dominated by circumstel- ff use bands in post-AGB stars lar extinction. As for the other bands, we see strengths whichare systematically weaker than in the reference stars dominatedby interstellar extinction, represented by the solid line in thediagram. The only non-detection in this case corresponds toIRAS 18062 + ff ecting this sourceis the result of a quite similar contribution from the ISM andfrom the circumstellar material. The 6614 Å DB detected to-ward IRAS 06530-0213 is unusually strong for DBs in the DCSgroup and even with respect to the Galactic relationship. Asmentioned earlier for the 6284 Å band toward the same target,this could point, for this particular source, towards the presenceof circumstellar DBs or, perhaps more likely, an underestima-tion of the interstellar reddening.For the rest of stars (right panel), as usual, we find thatmost of them are located in the region of the diagram cor-responding to the field stars dominated by interstellar ex-tinction. In this case, we would like to remark only theslightly discrepant position occupied by the oxygen-rich, F-type star IRAS 17245-3951 not yet previously identified asoutlier in the above discussion. Again, the carbon rich starIRAS 17086–2403 shows very weak DBs. This DB has been included in numerous studies in the literaturebecause of its proximity to the nearby 5780 Å band. This hasallowed a comparative analysis of their relative intensities indi ff erent astrophysical environments.The 5797 Å band has a lower sensitivity to the extinction EW / E ( B − V ) = .
17 Å / mag when it has been measured in theISM, compared to the previous DBs. Similar to the adjacent5780 Å band, it is necessary to take into account in our analysisthe possible contamination due to the presence of atmosphericstellar lines in this spectral range in stars of intermediate andlate spectral types, as it can a ff ect our measurements.In Figure 11 we show the equivalent width of this band vs. E ( B − V ) as it has been measured from the available spectra foreach subgroup of stars in which we have divided the sample.For stars in the DCS group (left panel), all post-AGB stars arefound to show DB strengths which are considerably weakerthan in the field stars, consistently with the results found in theother bands analysed so far.Among the non-detections, we emphasizeIRAS 05341 + E ( B − V ) CS = Fig. 16.
The complex profile of the Na i D line, as observed inIRAS 04296 + This band is also among the ones not usually analysed in theliterature, likely because of the presence of telluric lines inthe spectral range adjacent to this band but also because ofthe intrinsic weakness of this DB, for which EW / E ( B − V ) = .
12 Å / mag in the ISM.In Figure 12 we show the results of our analysis for this DBfor each of the subgroups in which we have divided the sample,again as a function of the dominant chemistry and the spectraltype of the central star.The results obtained are once more consistent with previousanalysis performed for other DBs. We find a better agreementwith the values obtained in reference stars dominated by inter-stellar extinction for the sources in the right panel, although inthis case the e ff ect is not so evident as in the previous analysisdue to the larger errors associated to the measurements.Consistent results, although more sensitive to measurementerrors, are obtained when the features at 6379, 5850 and 6196 Åare analysed (see Figures 13, 14 and 15). An additional way to check whether our conclusions are con-sistent with the available observational data is to analyse theDoppler velocities associated to the DBs detected in our stars.The overall strategy consists of comparing these Dopplervelocities with the radial velocities associated either to the at-mospheric stellar absorptions or to the nebular and recombina-tion emission lines sometimes detected in our spectra. In gen-eral, atmospheric and nebular lines are expected to match eachother within the errors unless the central star is part of a binarysystem or the nebular shell shows a complex morphology.If the DBs detected are formed in the circumstellar en-velopes of these stars, we should measure Doppler velocities inthese bands consistent with the characteristic radial velocitiesderived from the absorption and / or emission lines identified inthe stellar spectra. . Luna et al.: A search for di ff use bands in post-AGB stars 15 P NOSiFe
HeIHalpha
DBsStellarNebularKI (abs)KI (em)NaI (em)NaI (abs) [NII]6584[NII]6583[SII]6716[SII]6730 PP DIB5780DIB5850DIB61966DIB6284DIB6379DIB6614DIB6993DIB7224 P DIB5797
DCSDCS DCSDCS HV DCS
Fig. 17.
Velocities of the DBs and the stellar and interstellarabsorption lines and the nebular emission line components foreach target are plotted per panel. The y-axis is in arbitraryunits. The x-axis is the LSR velocity in km s − , with big tick-marks separated by 50 km s − . Note that the width of the pan-els are identical, i.e. 200 km s − , but the central velocity ofeach is shifted to show all lines for each target. Error bars are ≤
10 km s − for stellar lines, ≤
20 km s − for DBs and ≤ − for the sodium and potassium components. Targets dominatedby circumstellar reddening are labeled ‘DCS’ and high radialvelocity targets are labeled ‘HV’ The stellar and DB veloc-ity components can be directly compared to those of neutralsodium and potassium in either emission (upward arrow) orabsorption (downward arrow) plotted at the top of each panel.In Table 6 (Online only) we give the radial velocities (inkm s − ), measured with respect to the Local Standard of Rest(LSR), associated to several atoms and ions, as derived fromvarious atmospheric stellar absorptions and nebular emissionlines identified in the stars of our sample. In addition, we alsodisplay the measurements made in H α at 6563 Å and in the He i line at 6678 Å. In the case of the atmospheric absorption linesshown in Table 6 the average velocity derived from several linemeasurements corresponding to various ions of the elementsC, N, O, Si and Fe is presented. For the nebular lines, we haveonly considered the forbidden lines of [N ii ] and [S ii ], foundaround H α . The typical uncertainties are of the order of 5 − − .In addition, in Table 7 (Online only) we present the ve-locities derived from the analysis of the Na i D (5889.95 and5895.92 Å) doublet and of the K i (7698.97 Å) line, which arein most cases also well detected in our spectra (uncertainties DIB5850DIB61966DIB6284DIB6379DIB6614DIB6993DIB7224 P DBsStellarNebularKI (abs)KI (em)NaI (em)NaI (abs) P HeI CNOSiFe [NII][NII][SII][SII] P Halpha
DIB5797DIB5797DIB5780
DCS HVDCS/HV DCS/HVDCS/HVDCS/HVDCS/HV
Fig. 17. (continued).are ∼ − ). These lines, like the DBs that we wantto analyse, usually originate in the ISM, but they can also formin the circumstellar shell. In this case, the circumstellar compo-nent usually appears in emission over the interstellar absorption(see Figure 16). In general, these lines show very complex ab-sorption profiles as they reflect the di ff erent velocities of theclouds located along the line of sight. In some of our stars thecircumstellar component may contribute significantly to the ob-served profile and can be used as a further test to identify theorigin of analogue velocity components which may be presentin our favourite DB.Table 8 (Online only) shows the radial velocities associatedto the DBs observed in the stars of our sample, which can thenbe compared to the velocities provided in Tables 6 and 7.It is important to take into account that deriving velocitiesfor DBs is in many cases a complicated task, especially if thefeatures under analysis are weak in strength. In general, theDoppler shift measurements are determined by assuming thatthe absorption peak is a good approximation to the centre ofthe feature. We estimate that, on average, the errors in Table 8may be a ff ected by errors of the order of 10 −
20 km s − .Comparing Tables 6 and 7 we observe that in most casesthere is at least one velocity component associated to thesodium doublet or the potassium line, either in emission or inabsorption, that can be interpreted as having a stellar or circum-stellar origin. The circumstellar nature of these lines is easyto determine when they are found in emission. The radial ve-locities measured in this case are usually coincident with thesystemic velocity of the post-AGB star. The few cases found inwhich our measurements do not support this statement are indi- ff use bands in post-AGB stars P PP
DIB5780DIB5797DIB5850DIB61966DIB6284DIB6379DIB6614DIB6993DIB7224CNOSiFe[NII]6584[NII]6583[SII]6716[SII]6730HalphaHeI
DBsStellarNebularKI (abs)NaI (em)NaI (abs)
HD 172324 19114+0002 19200+3457 19386+0155 19500-1709 20000+323920462+3416 22023+5249 22223+4327 23304+6147*22272+5435
DCSHVHV HV
Fig. 17. (continued).cated with an asterisk in Table 7 and Fig. 17. They correspondto very complex Na i D line profiles in which the circumstellaremission appears over-imposed to the interstellar absorption.Figure 17 shows, for ease of comparison, the velocities ofthe DBs and the (inter-)stellar absorption and emission com-ponents for each target. For the majority of the targets thesegraphs show consistent velocities for the stellar lines. DB ve-locities are also consistent with each other. For several casesthe nebular (emission) lines are significantly shifted with re-spect to the atmospheric lines ( e.g.
IRAS 17245-3951) due tobinarity of the system and / or a complex wind structure. Thesestellar and DB velocity components can be directly comparedto those of neutral sodium and potassium in the respective line-of-sight.If DB carriers are present in the circumstellar envelopes ofsome of the post-AGB stars in our sample, we would expect tofind as well matches between the velocities shown in Table 8and those in Table 6 (see Figure 18), especially for those starsin which we have detected circumstellar Na i in emission be-longing to the DCS group. Remarkably, in not any case wefind values consistent with the velocities associated to the DBswhich cannot be explained as a natural consequence of inter-stellar clouds with a similar velocity present in the line of sight.The inconsistency between velocities is more obvious if wehave a look at those stars showing very high radial velocities(HV in Table 4 and Fig. 17). Several of the HV targets haveradial velocities larger than 100 km s − . Such large velocitydi ff erences are comparable to those measured for successfullydetected extra-galactic DBs (Ehrenfreund et al 2002; Cox et al.2007). Fig. 18.
The 5850 Å band observed in the high radial veloc-ity ( ∼
100 km s − ) sample star IRAS 19114 + ff erence expected between the interstellar (solidline) and circumstellar (dashed line) DBs for the HV targetIRAS 19114 + ff erences between interstellar and circumstel-lar lines and that show significant circumstellar reddening ( e.g. IRAS 17086-2403, IRAS 17423-1755, IRAS 18025-3906 andIRAS 18062 + . Luna et al.: A search for di ff use bands in post-AGB stars 17 the presence of (weak) circumstellar DBs separated from theinterstellar DBs. Our current spectra are of insu ffi cient qualityto search for these weak features next to the observed DIBs.Note that both CS and IS DBs could coexist. And, if separatedby more than their FWHM ( ∼ −
60 km s − for narrow DBs)the central velocity of the IS and (possibly) CS DB would notbe a ff ected by each other.
5. Conclusions
The equivalent widths of 9 DBs commonly found in the ISMhave been determined for a representative sample of galac-tic post-AGB stars displaying a wide variety of observationalproperties. We present here the results of our extensive surveyto look for DBs in envelopes of evolved stars.We have carefully disentangled the observed extinction byassessing the expected interstellar extinction for each of the ob-served targets. This allowed us to select a sub-sample of targetswhose line of sight reddenings are dominated ( > E ( B − V ) observed in field stars onlyin those sources showing little circumstellar contribution tothe overall reddening. In contrast, DBs are weak or absent insources dominated by circumstellar reddening, irrespective ofthe dominant chemistry and spectral type of the central star, al-though our conclusions should be taken with caution due to therelatively small sample size.The results obtained suggest that the carrier(s) of the DBsdo not form or at least they are not “available” to produce anydetectable spectral feature during the post-AGB phase. The car-riers, if present in the circumstellar envelope of these stars arenot found under the environmental conditions needed to excitethe transitions which we identify as DBs in the ISM.The radial velocity analysis of the features observed in in-dividual sources confirm this result, as the Doppler shifts mea-sured are always found to be consistent with an interstellar ori-gin for the bands observed.DB carriers may be carbonaceous species or radicals at-tached to large organic molecules, trapped in lattice or morecomplex structures, or constituents of the mantle of circumstel-lar dust grains which are liberated to the ISM only after strongUV irradiation (either UV photons from the central star or fromthe more energetic interstellar UV field).In this sense, the identification of the carriers as stronglyionised PAHs and / or radicals liberated from carbonaceousspecies as a consequence of photo-evaporation of dust grainsin the ISM looks tempting and would be consistent with ourobservations.However, we do not find any evidence of the carbonaceousnature of the carrier(s) in our sample stars, something generallyaccepted in the literature, nor any correlation with the presenceof PAHs in the mid-infrared spectrum of these sources, as it hasbeen claimed by several authors in the past.If DBs are connected with PAHs or with any other carbona-ceous species such as the ones suggested in the introduction ofthis paper, their carrier(s) must form at a later stage, probablyunder di ff erent excitation conditions, once the envelope of the post-AGB star is totally diluted in the interstellar medium as aresult of the expansion of the shell. Acknowledgements.
Many of the spectra used in the analysis herepresented were kindly provided by Hans van Winckel and MaartenReyniers, working at the Katholieke Universiteit Leuven, Belgium.The authors are also grateful to Bernard Foing and Nathalie Boudin,who participated in the early stage of this project and with whomwe had very fruitful discussions. We sincerely thank the refereesfor their helpful and constructive comments. This work was par-tially funded by grants AYA2003–09499 and AYA2004–05382 of theSpanish Ministerio de Ciencia y Tecnolog´ıa.
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Online Material . L un ae t a l . : A s ea r c h f o r d i ff u s e b a nd s i npo s t - AG B s t a r s , O n li n e M a t e r i a l p2 Table 5.
Equivalent width measurements (in Å) corresponding to the 9 DBs analysed in our sample of post-AGB stars. EW (Å)IRAS Name 5780 5797 5850 6196 6284 6379 6614 6993 722401005 + ± ± ≤ ± ± ± ± ± ≤ + ± ± ± ± ± ± + ± ≤ ± ± ± + ± ≤ ± ≤ − ± ± ± ± ± ± ± ≤ ≤ + ≤ ≤ ± ± ± − ± ± ± + ± ± ± ≤ ≤ ± ≤ − ≤ ± ± ≤ ≤ ≤ ≤ − ± ≤ ± ± ± ± ± − ± ± ± ± ± ± ± ± − ± ± ± ± ± ≤ ± ≤ − ± ± ± ± ± ± ± − ± ≤ ± ≤ ≤ ≤ ≤ ± ≤ ≤ − ± ± ± ± ± ± ± − ± ± ± ± ± ± − ≤ ± ± ± ± ± − ± ± ≤ ± ± ± ± ≤ ± − ± ± ≤ ± ≤ ± ≤ + ≤ ± ± ≤ ± − ± ± ≤ ≤ ≤ ± ± ± + ± ≤ ≤ ≤ ≤ ≤ ± ≤ ± ± ≤ ≤ ≤ ≤ ≤ ≤ + ± ≤ ± ± ± ± ± ± + ≤ + ± ± ± ± ± ± − ± ± ≤ ± ± ± ± ± + ± ± ± ± ± + ± ± ± ± ± ± ± ± + ± ± ± ± ± ± ± ± ± + ± ≤ ± ± + ≤ ≤ + ± ± ± ± ?: contamination by atmospheric stellar lines or poor S / N; —: spectral range not covered .. Luna et al.: A search for di ff use bands in post-AGB stars , Online Material p 3
Table 6.
Radial velocity measurements (in km s − ) associated to several atmospheric stellar and nebular lines detected in thepost-AGB stars of our sample. H α He i C N O Si Fe [N ii ] [N ii ] [S ii ] [S ii ]IRAS Name 6563 6678 6548 6583 6716 673001005 + −
34 e −
76 P − − − −
38 — — — − − + + −
11 a — − − − − + − − − − − −
10 — — — —05251 − + − + − − − − − − − − − − −
17 — — − − − − −
18 — − − ∗ − ∗
14 P −
83 a − − − − −
77 20 20 − − − − + −
55 a −
56 a − − − − −
50 — — — —18025 − −
104 a −
89 a − − − − −
101 — — — —18062 + −
72 e −
117 a − − − − −
115 — — — —19114 + + − − + − + + −
68 e −
44 P − − − −
75 — − − − − + −
136 e −
114 e − − − − − − − − − + −
24 e −
37 a − − − − −
28 — — — —22272 + −
28 a −
38 a −
35 — − − −
26 — — — —23304 + −
15 a — — − −
15 — — — — — —
P: P-Cygni profile; a: absorption; e: emission; ∗ : di ff erent atmospheric and nebular velocities . Luna et al.: A search for di ff use bands in post-AGB stars , Online Material p 4
Table 7.
Radial velocity measurements (in km s − ) associated to the Na i D (5889.95 and 5895.92 Å) and K i (7698.97 Å) linesdetected in the post-AGB stars of our sample. K i i i + ∗ n.d. 1, − − − − − − − + −
2, 25 n.d. −
2, 2404296 + ∗ − −
18, 58 21 −
17, 6, 56 21 −
16, 6, 5605113 + ∗ n.d. −
17, 2 − −
17, 6 − −
18, 505251 − −
11 n.d. 5, − + − + − ∗ n.d. 24, −
8, 48 − −
47, 19 − −
52, 1708143 − − −
42, 15, 39 n.d. −
41, 16, 4312175 − − − −
6, 31 32 − −
5, 3216594 − ∗ n.d. − −
41, 5 14 −
29, 6 13 −
29, 817086 − ∗ n.d. 12 − − − −
29, 3 n.d. −
29, 317150 − − −
1, 27 − −
57 19 − − −
31 19 − − − − − − − + −
50 n.d. − −
19 n.d. − − − − −
73 n.d. − −
34, 8 n.d. − −
31, 1018062 + −
28, 7 n.d. −
28, 819114 + + + − − + −
2, 21 n.d. 2, 2120462 + − − − −
6, 19 n.d. − −
7, 1722023 + −
23 27, − −
46, 2 27, − −
46, 422223 + − − −
43 n.d. − −
18, 0 n.d. − −
18, 022272 + −
41 n.d. − − − − + ∗ n.d. − − −
29 24 − − − − − − ∗ : emission over absorption; —: spectral range not covered; n.d.: non detected . Luna et al.: A search for di ff use bands in post-AGB stars , Online Material p 5
Table 8.
Radial velocity measurements (in km s − ) associated to the DBs detected in the post-AGB stars of our sample. Di ff use bandIRAS Name 5780 5797 5850 6196 6284 6379 6614 6993 722401005 + −
16 29 ∗ — − − − −
16 —02229 + − ∗ — 33 52 22 — 24 —04296 + + − − −
14 0 — —05341 + − + − −
14 2 — — — 5 — —08005 − − − − ∗
19 17 28 5 812175 − − − − − − − − − − − − − − − − − − −
10 17 — 8 0 3 3 — − − + − − ∗ — — — − ∗
24 19 —18062 + − − ∗ — — — — — — —19114 + − + + − − ∗ — 4 6 3 35 ∗ — 920000 + + − − ∗ −
14 5 − − − + − − ∗ − −
17 2 2 0 − + + + − −
26 5 − ∗ : likely contaminated measurement . Luna et al.: A search for di ff use bands in post-AGB stars , Online Material p 6
List of Objects ‘NGC 6210’ on page 2‘NGC 7027’ on page 2‘IC 351’ on page 2‘AFGL 2688’ on page 2‘IRAS 21282 + + +
30 3639’ on page 2‘CPD-56 8032’ on page 2‘AC Her’ on page 2‘WR137’ on page 2‘WR140’ on page 2‘HR 4049’ on page 2‘HD213985’ on page 2‘NCG 7027’ on page 2‘HR 4049’ on page 2‘IRAS 21282 + + + + + + + + + + + + + +
34 1’ on page 3‘IRAS 19386 + + + +
34 26’ on page 3‘IRAS 22023 + +
52 24’ on page 3‘IRAS 22223 + +
42 4388’ on page 3‘IRAS 22272 + + + + + + + + + + + + + + ++