2 Observations

 

 

Table 1: Source list. Observational details of the sources in this study.

Object	IRAS name	Obs.a	$\alpha$	$\delta$	TDTb	Sp./T	Obj. Type
		Mode	(J2000)	(J2000)		kK
NGC 40	00102+7214	01(3)	00 13 01.10	+72 31 19.09	30003803	WC	PN
IRAS 00210+6221	00210+6213	01(1)	00 23 51.20	+62 38 07.01	40401901		C-star
IRAS 01005+7910	01005+7910	01(2)	01 04 45.70	+79 26 47.00	68600302	OBe	post-AGB
HV Cas	01080+5327	01(1)	01 11 03.50	+53 43 40.30	62902503		C-star
RAFGL 190	01144+6658	01(2)	01 17 51.60	+67 13 53.90	68800128		C-star
R Scl $^{\dagger}$	01246-3248						C-star
-		01(2)	01 26 58.10	-32 32 34.91	37801213
-		01(2)	01 26 58.05	-32 32 34.19	37801443
IRAS Z02229+6208	Z02229+6208	01(1)	02 26 41.80	+62 21 22.00	44804704	G0	post-AGB
RAFGL 341	02293+5748	01(1)	02 33 00.16	+58 02 04.99	80002450		C-star
IRC+50 096	03229+4721	01(2)	03 26 29.80	+47 31 47.10	81002351		C-star
IRAS 03313+6058	03313+6058	01(1)	03 35 31.50	+61 08 51.00	62301907		C-star
U Cam	03374+6229	01(2)	03 41 48.16	+62 38 55.21	64001445		C-star
RAFGL 618	04395+3601	01(3)	04 42 53.30	+36 06 52.99	68800561	B0	PN
W Ori	05028+0106	01(3)	05 05 23.70	+01 10 39.22	85801604		C-star
IC 418	05251-1244	01(2)	05 27 28.31	-12 41 48.19	82901301	361	PN
V636 Mon	06226-0905	01(1)	06 25 01.60	-09 07 16.00	86706617		C-star
RAFGL 940	06238+0904	01(2)	06 26 37.30	+09 02 16.01	87102602		C-star
IRAS 06582+1507	06582+1507	01(2)	07 01 08.40	+15 03 40.00	71002102		C-star
HD 56126 $^{\dagger}$	07134+1005					F5	post-AGB
-		06	07 16 10.20	+09 59 48.01	71802201
-		06	07 16 10.30	+09 59 48.01	72201702
-		01(3)	07 16 10.20	+09 59 48.01	72201901
CW Leo	09451+1330	06	09 47 57.27	+13 16 42.82	19900101		C-star
NGC 3918	11478-5654	01(1)	11 50 18.91	-57 10 51.10	29900201		PN
RU Vir	12447+0425	01(2)	12 47 18.43	+04 08 41.89	24601053		C-star
IRAS 13416-6243	13416-6243	01(3)	13 45 07.61	-62 58 18.98	62803904		post-AGB
II Lup	15194-5115	06	15 23 04.91	-51 25 59.02	29700401		C-star
V Crb	15477+3943	06	15 49 31.21	+39 34 17.80	25502252		C-star
PN K 2-16 $^{\dagger}$	16416-2758					WC	PN
-		01(1)	16 44 49.10	-28 04 05.02	29302010
-		01(2)	16 44 49.10	-28 04 05.02	67501241
IRAS 16594-4656	16594-4656	01(1)	17 03 09.67	-47 00 27.90	45800441		post-AGB
NGC 6369	17262-2343	01(1)	17 29 20.80	-23 45 32.00	45601901	WC82	PN
IRC+20 326	17297+1747	01(1)	17 31 54.90	+17 45 20.02	81601210		C-star
CD-49 11554	17311-4924	01(2)	17 35 02.41	-49 26 22.31	10300636	BIIIe	post-AGB
PN HB 5	17447-2958	01(3)	17 47 56.11	-29 59 39.70	49400104		PN
RAFGL 5416	17534-3030	01(1)	17 56 36.90	-30 30 47.02	12102004		C-star
T Dra	17556+5813	01(2)	17 56 23.30	+58 13 06.38	34601702		C-star
RAFGL 2155	18240+2326	01(1)	18 26 05.69	+23 28 46.31	47100261		C-star
IRAS 18240-0244	18240-0244	01(1)	18 26 40.00	-02 42 56.99	14900804	WC	PN
IRC+00 365	18398-0220	01(2)	18 42 24.68	-02 17 25.19	49901342		C-star
RAFGL 2256	18464-0656	01(1)	18 49 10.35	-06 53 03.41	48300563		C-star
PN K 3-17	18538+0703	01(2)	18 56 18.05	+07 07 25.61	49900640		PN
IRC+10 401	19008+0726	01(1)	19 03 18.10	+07 30 43.99	87201221		C-star
IRAS 19068+0544	19068+0544	01(1)	19 09 15.40	+05 49 05.99	47901374		C-star
NGC 6790	19204+0124	01(1)	19 22 57.00	+01 30 46.51	13401107	703	PN
RAFGL 2392	19248+0658	01(1)	19 27 14.40	+07 04 09.98	85800120		C-star
NGC 6826	19434+5024	01(4)	19 44 48.20	+50 31 30.00	27200786	504	PN
IRAS 19454+2920	19454+2920	01(1)	19 47 24.25	+29 28 11.78	52601347		post-AGB
HD 187885	19500-1709	01(2)	19 52 52.59	-17 01 49.58	14400346	F2	post-AGB
RAFGL 2477	19548+3035	01(1)	19 56 48.26	+30 43 59.20	56100849		C-star
IRAS 19584+2652	19584+2652	01(1)	20 00 31.00	+27 00 37.01	52600868		C-star
IRAS 20000+3239	20000+3239	01(1)	20 01 59.50	+32 47 33.00	18500531	G8	post-AGB
V Cyg $^{\dagger}$	20396+4757						C-star
-		01(2)	20 41 18.20	+48 08 29.00	42100111
-		01(2)	20 41 18.20	+48 08 29.00	42300307

 

Table 1: continued.

Object	IRAS name	Obs.a	$\alpha$	$\delta$	TDTb	Sp./T	Obj. Type
		Mode	(J2000)	(J2000)		kK
NGC 7027 $^{\dagger}$						2005	PN
-		01(4)	21 07 01.71	+42 14 09.10	02401183
-		01(1)	21 07 01.70	+42 14 09.10	23001356
-		01(2)	21 07 01.70	+42 14 09.10	23001357
-		01(3)	21 07 01.70	+42 14 09.10	23001358
-		01(4)	21 07 01.63	+42 14 10.28	55800537
S Cep	21358+7823	01(1)	21 35 12.80	+78 37 28.20	56200926		C-star
RAFGL 2688		01(3)	21 02 18.80	+36 41 37.79	35102563	F5	post-AGB
RAFGL 2699	21027+5309	01(1)	21 04 14.70	+53 21 02.99	77800722		C-star
IC 5117	21306+4422	01(1)	21 32 30.83	+44 35 47.29	36701824	773	PN
RAFGL 5625	21318+5631	01(1)	21 33 22.30	+56 44 39.80	11101103		C-star
IRAS 21489+5301	21489+5301	01(1)	21 50 45.00	+53 15 28.01	15901205		C-star
SAO 34504	22272+5435	01(2)	22 29 10.31	+54 51 07.20	26302115	G5	post-AGB
IRAS 22303+5950	22303+5950	01(1)	22 32 12.80	+60 06 04.00	77900836		C-star
IRAS 22574+6609	22574+6609	01(2)	22 59 18.30	+66 25 49.01	39601910		post-AGB
RAFGL 3068	23166+1655	01(2)	23 19 12.48	+17 11 33.40	37900867		C-star
RAFGL 3099	23257+1038	01(1)	23 28 16.90	+10 54 40.00	78200523		C-star
IRAS 23304+6147	23304+6147	01(3)	23 32 44.94	+62 03 49.61	39601867	G2	post-AGB
IRAS 23321+6545	23321+6545	01(1)	23 34 22.53	+66 01 50.41	25500248		post-AGB
IRC+40 540	23320+4316	01(2)	23 34 27.86	+43 33 00.40	38201557		C-star
non detections
R For	02270-2619	01(1)	02 29 15.30	-26 05 56.18	82001817		C-star
SS Vir	12226+0102	01(1)	12 25 14.40	+00 46 10.20	21100138		C-star
Y CVn	12427+4542	01(2)	12 45 07.80	+45 26 24.90	16000926		C-star
RY Dra	12544+6615	01(3)	12 56 25.70	+65 59 39.01	54300203		C-star
C* 2178	14371-6233	01(1)	14 41 02.50	-62 45 54.00	43600471		C-star
V1079 Sco	17172-4020	01(1)	17 20 46.20	-40 23 18.10	46200776		C-star
T Lyr	18306+3657	06	18 32 19.99	+36 59 55.50	36100832		C-star
S Sct	18476-0758	01(2)	18 50 19.93	-07 54 26.39	16401849		C-star
V Aql	19017-0545	01(2)	19 04 24.07	-05 41 05.71	16402151		C-star
V460 Cyg	21399+3516	01(1)	21 42 01.10	+35 30 36.00	74500512		C-star
PQ Cep	21440+7324	01(1)	21 44 28.80	+73 38 03.01	42602373		C-star
TX Psc	23438+0312	06	23 46 23.57	+03 29 13.70	37501937		C-star

^a SWS observing mode used (see de Graauw et al. 1996). Numbers in brackets correspond to the scanning speed.
^b TDT number which uniquely identifies each ISO observation.
$^{\dagger}$ These spectra have been obtained by co-adding the separate SWS spectra also listed in the table, see text.
Effective temperatures from ¹Mendez et al. (1992), ²Perinotto (1991), ³Kaler & Jacoby (1991), ⁴Quigley & Bruhweiler (1995) and ⁵Latter et al. (2000).

Figure 1: The IRAS two-colour diagram for the sources studied in this sample. The triangles represent the C-stars, the squares are the post-AGB objects, the stars are the PNe and the diamonds are the C-stars without a "30'' $\mu$m feature detected. The dashed line represents the position of blackbodies of different temperatures. The dotted line sketches the evolution of a C-star with a detached, expanding and cooling circumstellar shell.

We present observations obtained with ISO of a sample of bright IR sources at different stages along the evolutionary track from C-star via post-AGB object to PN. The observations presented here consist of data obtained with the ISO/SWS using astronomical observing template 06 and 01 at various speeds. These observing modes produces observations from 2.3 to 45 $\mu$m with a resolving power ( $\lambda/\Delta\lambda$) ranging from 500 to 1500. The sample consists of all carbon-rich evolved objects in the ISO archive which exhibit a "30'' $\mu$m feature stronger than 8 Jy peak intensity and have been observed over the full 2.3-45.2 $\mu$m wavelength range of the ISO/SWS. This peak intensity and the typical noise level of SWS band 4 (29-45.2 $\mu$m) allows to extract a reasonably reliable feature strength and profile. The complete wavelength coverage is needed in order to provide a sufficient baseline to estimate the continuum. We have further completed the sample with all observed C-stars with an IRAS 25 $\mu$m flux over 13 Jy. These sources serve as a control group since we would expect to detect the "30'' $\mu$m feature based on this brightness, the typical noise levels and the typical feature over continuum level. These sources without the "30'' $\mu$m feature detected are listed separately in Table 1. It should be emphasised that the ISO archive does not contain a statistically representative sample of objects. The database of observations for the carbon stars provides a reasonable sampling over stellar properties (e.g. mass-loss rates or colour temperatures). However the post-AGB sample is heavily biased towards the "21'' $\mu$m objects; a peculiar type of C-rich post-AGB object. The sample of PNe contains a collection of either bright, well-known or well-studied objects without a proper statistical selection. It also contains a relatively large proportion of PNe with hydrogen-poor central stars. The total sample of 75 sources contains 48 C-stars, 14 post-AGB objects and 13 PNe. We have detected the "30'' $\mu$m feature in 36 out of 48 C-stars.

We present in Fig. 1 the IRAS two-colour diagram for the sources in our sample following van der Veen & Habing (1988). There are four sources in our sample without an entry in the IRAS point source catalogue. For these sources we have used ISO/SWS and LWS observations at 12, 25, 60 and 100 $\mu$m to calculate the IRAS colours. For IRAS Z02229, no measurements at 60 and 100 $\mu$m are available. In Fig. 1, the warmest sources are located in the lower left corner. These are the optically visible carbon stars with a low present-day mass-loss rate ( $\dot{M}\simeq10^{-8}{-}10^{-7}~M_{\odot}$). With increasing mass loss the stars become redder and move up and to the right. After the AGB, when the mass loss has terminated, the dust moves away from the star and cools; i.e., these sources move further to the top-right corner of the diagram. The C-stars located above the main group of C-stars have a clear 60 $\mu$m excess. This is evidence for an additional cool dust component. Some of these sources are known to have an extended or detached dust shell around them (Young et al. 1993). The empty region between the C-stars and the post-AGBs is physical. When the mass loss stops the star quickly loses its warmest dust and within a short time span (<1000 yr) the star moves to the right in the two-colour diagram. Notice how the sources without a detected "30'' $\mu$m feature cluster on the left of the diagram, i.e., among the warmest C-stars.

   
2.1 Data reduction

The SWS data were processed using SWS interactive analysis product; IA³ (see de Graauw et al. 1996) using calibration files and procedures equivalent with pipeline version 10.1. Further data processing consisted of extensive bad data removal primarily to remove the effects of cosmic ray hits and rebinning on a fixed resolution wavelength grid. If a source has been observed multiple times and these observations are of similar quality and of comparable flux-level these data are co-added after the pipeline reduction. These sources are indicated in Table 1 with a dagger ($^{\dagger}$). Since the features we discuss here are fully resolved in all observing modes, we combine the data obtained in all different modes to maximise the S/N. Although the wavelength coverage of the SWS instrument is well suited to study the profile of the "30'' $\mu$m feature, there are some important instrumental effects which hamper the unbiased extraction of the emission profiles. We discuss these below.

2.1.1 Splicing

Figure 2: Examples of the splicing of the SWS band 3D (19.5-27.5 $\mu$m) and 4 (28.9-45.2 $\mu$m) data. We show the data before (grey line) and after splicing (black line). All data have been scaled to form a continuous spectrum. As can be seen; after splicing, the slope of band 3D and band 4 match. We do not show the band 3E data. The sharp rise at 27 $\mu$m in R Scl and IRC+40 450 is an instrumental artifact (see text for details).

One complete SWS AOT01 spectrum is obtained in 12 different subbands. These subbands are observed through 3 different rectangular apertures which range in size from $14\arcsec\times20\arcsec$ at the shortest wavelengths to $20\arcsec\times33\arcsec$ at the longest wavelengths. All these data are independently flux calibrated and need to be combined to form one continuous spectrum for one source. We apply scaling factors to combine the different subbands to obtain the continuous spectra. The C-stars and post-AGB objects we present in this study all have a small angular extent even compared to the smallest aperture used. Therefore we don't expect large jumps to be present due to the differences between the apertures used. The angular extent of some PNe can be large compared to the sizes of the apertures. If there is a clear indication of flux jumps due to aperture changes we have not included the source in our sample.

2.1.2 Leakage

At wavelengths longer than between 26 and 27.5 $\mu$m the data of SWS subband 3D are affected by leakage adding flux from the 13 $\mu$m region. The sources used to derive the instrumental response function are all stellar sources without circumstellar material. These calibration sources are all very blue and emit much more flux at 13 $\mu$m relative to 26 $\mu$m than the cool, red sources we present in this study. Therefore these calibrators are more affected by the leakage than our sources. The instrumental response function derived in this way has been implicitly corrected for leakage for the blue sources. This resulted in fluxes in red sources to be systematically underestimated. More recent calibrations ($\ge$OLP 10.0), have been corrected for this effect. With the improved calibrations, the resulting slopes of the spectra beyond 26 $\mu$m have been checked and are in general agreement with the slope of subband 4.

2.1.3 The 27.0-27.5 and 27.5-29.0 $\mu$m region

At wavelengths longer than 27.0 $\mu$m the data of subband 3D show a sharp increase which is found throughout the complete database of ISO/SWS observations independent of source type. The data of subband 3E (27.5-29.0 $\mu$m) are generally unreliable both in shape and absolute flux level. These combined instrumental effects make it inherently difficult to interpret the 27-29 $\mu$m spectra. Any substructure detected solely in this region alone should be distrusted.

The instrumental effects between 27 and 29 $\mu$m and the fact that each of the subbands is independently flux calibrated make it necessary to devise a strategy for splicing the band 3D, 3E and 4 data. There is unfortunately no objective way to choose this strategy. We choose to assume minimal spectral structure between the end of subband 3D and the beginning of band 4, i.e. to splice the subband 3D-4 data in such a way that the matching slopes of 3D and 4 also match in flux level. Some examples are shown in Fig. 2. The observed discontinuities between subbands are relatively small (<20 per cent) and can be understood as the result of absolute flux calibration uncertainties alone.

   
2.2 Full spectra

Figure 3: Overview of the spectra of carbon stars exhibiting the "30'' $\mu$m feature. The spectra are ordered according to continuum temperature from high to low temperature, bottom to top, left to right. The dashed line marks $\lambda=26~ \mu$m.

Figure 4: Overview of the spectra of post-AGB objects (left panel) and PNe (right panel) exhibiting the "30'' $\mu$m feature. The spectra are ordered according to continuum temperature from high to low temperature, bottom to top. The dashed line marks $\lambda=26~ \mu$m. The spectrum of RAFGL 618 although warmer than NGC 3918 is shown at the top of the PNe for clarity.

The resultant spectra for the sources that exhibit a "30'' $\mu$m feature are shown in Figs. 3, 4. The SWS spectra of this large group of objects show a spectacular range in colour temperature, molecular absorption bands and solid state features. The C-stars have molecular absorption bands of C₂H₂ at 3.05, 7-8 and 14 $\mu$m, of HCN at 7 and 14 $\mu$m, CO at 4.7 $\mu$m and C₃ at 4.8-6 $\mu$m. The sharp absorption band at 14 $\mu$m is due to C₂H₂and HCN. There is an emission feature due to solid SiC at 11.4 $\mu$m. In the reddest C-stars, we find the SiC in absorption. We also find evidence for a weak depression in the 14-22 $\mu$m range in the reddest objects. This depression could be due to aliphatic chain molecules like those found in RAFGL 618 (Cernicharo et al. 2001).

The post-AGBs and PNe exhibit many, sometimes broad solid state emission features. In many sources we find emission due to polycyclic aromatic hydrocarbons in the 3-15 $\mu$m range. There is a broad plateau feature from 10-15 $\mu$m which may be due to hydrogenated amorphous carbon (Guillois et al. 1996; Kwok et al. 2001). Many post-AGBs and two PNe in the sample have a feature peaking at 20.1 $\mu$m, called the "21'' $\mu$m feature in the literature. Recently the carrier of this feature has been identified with TiC (von Helden et al. 2000). The feature at 23 $\mu$m found in IRAS 18240 and PN K3-17 is likely due to FeS (Hony et al. 2002). These absorption and emission features have to be taken into account when determining the profile of the "30'' $\mu$m feature or the shape of the underlying continuum.

Focusing on the "30'' $\mu$m feature we can see variations in the strength and shape of the band. The most marked difference is however a shift in the peak position going from 26 $\mu$m in some of the AGB stars to 38 $\mu$m in the PNe. The dashed line in Figs. 3 and 4 indicates $\lambda=26$$\mu$m. There are systematic changes in the appearance of the "30'' $\mu$m feature from the C-stars to the PNe. The feature in the C-stars almost exclusively peaks at 26 $\mu$m. There are some exceptions like R Scl. In the post-AGB sample, the feature is broader and in some sources the feature peaks long ward of 26 $\mu$m. In the PNe sample, there are no sources that peak at 26 $\mu$m. However, the appearance of a broad feature like the "30'' $\mu$m feature is sensitive to the shape of the underlying dust continuum, especially since we have a sample with such a wide range of continuum colour temperatures.