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Discussion on candidates for the M stars with 11.2 μm SiC feature
New Astronomy 9 (2003) 17–26 www.elsevier.com/locate/newast
Discussion on candidates for the M stars with 11.2 lm SiC feature P.S. Chen *, Y.F. Gao, H.G. Shan, X.H. Wang National Astronomical Observatories/Yunnan Observatory, CAS, Kunming 650011, China Received 13 March 2003; received in revised form 12 June 2003; accepted 12 June 2003 Communicated by G.F. Gilmore
Abstract The oxygen-rich stars with SiC emission features are unexpected according to the evolutionary theory in the late stage of stellar evolution. After the careful re-examination of such samples reported previously and searching for new samples, seven stars are found to be the candidates of the M stars with 11.2 lm SiC emission feature. However, with the IRAS low-resolution spectrum, the IRAS two colour diagram and other information, it is seen that no strong candidates for oxygen-rich AGB stars with carbon-rich circumstellar dust could be found so far. Ó 2003 Elsevier B.V. All rights reserved. PACS: 97.10.Fy; 97.10.Tk; 97.10.Zr; 97.10.Jg Keywords: Stars: late-type; Stars: chemically peculiar; Infrared: stars
1. Introduction It has been assumed that the spectral classes of M–S–C are one of the evolutionary sequences for stars on the asymptotic giant branch (AGB) phase (e.g., Iben and Renzini, 1983; Chen and Kwok, 1993). Those AGB stars begin as oxygen-rich (C=O < 1) stars with the spectral type of M and they often show the features of the oxygen-rich material in the infrared such as silicate features in emission or in absorption at 10 and/or 18 lm. As a
*
Corresponding author. E-mail address: [email protected] (P.S. Chen).
result of carbon dredge-up from the interior they may evolve through the stage of the spectral type of S (C=O 1) to carbon-rich (C=O > 1) ones. The discovery that many visual carbon stars have large excesses in the far infrared (Threnson et al., 1987) has led to the theory that those visual carbon stars represent an evolutionary stage after a severe mass-losing episode as M stars (e.g. Willems and de Jong, 1988; Chan and Kwok, 1988). However, Schroder et al. (1999) pointed out that there is no convincing evidence that the star was oxygen-rich at this stage, especially, for the lower-mass AGB stars (say, about 1.2 solar mass). Another possibility is that these lower-mass AGB stars suffer much more from the effects of thermal pulses,
1384-1076/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1384-1076(03)00088-5
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which may cause brief sharp enhancements in the mass-loss rate for these stars, leading to detached shells of fossil mass. In the early stages for the stars in the AGB phase, the carbon is tied up in CO molecules with oxygen but there is not enough carbon available for the formation of other carbon-rich material. As Chan and Kwok (1988) suggested, a plausible scenario for some stars after this stage is as follows: when the remnant oxygenrich circumstellar envelope begins to expand into the interstellar medium, the star gradually becomes a visual carbon star. After more dredge-up, more carbon atoms have accumulated in the photosphere to form carbon-based grains, such as amorphous carbon and silicon carbide (SiC), etc., the star then creates a carbon-rich circumstellar dust shell that makes the star itself invisible and then the star becomes a so-called infrared carbon star that often shows the silicon carbide (SiC) emission feature at 11.2 lm. On the other hand, as van Loon et al. (1999) pointed out, the scenario above may be suitable for some carbon stars, but it is not yet clear whether this is true for all carbon stars. Because there are some indications that carbon star formation may take place whilst the AGB star is optically invisible, i.e., a lull in the mass-loss rate on birth of the carbon star is not certain to happen. After the IRAS mission, the IRAS low-resolution spectrum (LRS) has become the most important tool in the infrared for discriminating between carbon stars and M stars. As mentioned above the silicate features at 10 and 18 lm are indicators of oxygen-rich stars (M stars) and the silicon carbide (SiC) featured at 11.2 lm is an indication of carbon-rich stars (Olnon and Raimond, 1986; Kwok et al., 1997). However, there are some exceptions. One case is the carbon stars with silicate dust shells, now known as silicate carbon stars (Little-Marenin, 1986; Willems and de Jong, 1986). The number of silicate carbon stars has now increased to 25, which includes two carbon-rich objects, recently discovered by the ISO SWS observations, to have the crystalline silicate features at the wavelength longer than 15 lm (Groenewegen, 1994; Kwok et al., 1997; Waters et al., 1998; Chen et al., 1999 and Molster et al., 1999, 2001). Another case may be the oxygen-rich
M stars with 11.2 lm SiC dust (Papoular, 1988; Skinner et al., 1990; Groenewegen, 1994; Sylvester, 1999). Both groups of stars mentioned above are unpredicted according to the known evolutionary theory in the late stage of stellar evolution. The silicate carbon stars have received much attention so far, although their nature is still not very clear. Little-Marenin (1986) proposed a binary model consisting of a carbon-rich giant and an oxygenrich giant to explain the silicate carbon star phenomenon. However, this idea is now thought to be unlikely because no evidence of oxygen-rich giants in silicate carbon stars have been found (Lambert et al., 1990; Engels and Leinert, 1994). Willems and de Jong (1986) and Chan and Kwok (1991) suggested that silicate carbon stars are the transition objects between the oxygen-rich stars and the carbon-rich stars and the oxygen-rich matter is the remnant of a previous mass-loss phase. This transition model has also been criticized from the point of view of the evolutionary timescale (e.g., Lloyd Evans, 1990). The most widely accepted model now to explain the silicate carbon star phenomenon is another binary model consisting of a carbon star and a main-sequence star with a disk that may surround the companion or the whole system (Lloyd Evans, 1990; Kahane et al., 1998; Yamamura et al., 2000). In contrast, the situation about the M stars with 11.2 lm SiC feature is unclear now and no models so far could be successfully employed to try to explain the phenomenon. One of the reasons is that we have no convincing sample of such stars now. However, from an evolutionary standpoint and from the view of dust formation, the M stars with 11.2 lm SiC feature would be as interesting as the silicate carbon stars. In this paper, we try to re-examine the M stars with SiC feature reported in previous literature and to find more candidates to try to establish their actual nature.
2. About M stars with SiC feature in the literature Papoular (1988) first reported 15 M stars, which are classified as 4n indicative of SiC emission, in the IRAS low-resolution spectrum (LRS) Atlas
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(Olnon and Raimond, 1986). He also picked out nine objects with the LRS classification 1n (indicative of featureless spectra) and believed to show the weak SiC feature. Skinner et al. (1990) listed 11 M stars with SiC dust on the basis of their LRS class of 4n. However, it is well documented that in some cases the automated LRS classification in the LRS Atlas gives unreliable results, especially, including the case for which LRS spectra with weak silicate emission were misclassified as 4n objects (e.g., Little-Marenin et al., 1987; Walker and Cohen, 1988; Chan and Kwok, 1990; Volk et al., 1991; Groenewegen et al., 1992; Volk, 1993). Fortunately, Kwok et al. (1997) re-sorted out the raw database of the IRAS LRS spectra with the letter classification scheme proposed by Volk and Cohen (1989) and published a new IRAS LRS database in which the source number reaches 11,224. This is more than twice that in the previous IRAS LRS Atlas. It was expected that in this new LRS database many sources of interest could be found and erroneous identifications appearing in the automated classification in the LRS Atlas could be corrected. In fact, in the new LRS database of 11,224 spectra, sources are grouped into 10 classes according to their spectral characteristics. Among them, the sources in class C are considered as having SiC emission feature at 11.2 lm. Based on the spectral types compiled by Bidelman (1980) for the IRC survey (Neugebauer and Leighton, 1969, hereafter IRC) and the LRS number classification used in the LRS Atlas, Groenewegen (1994) proposed 39 M and MS stars with SiC dust, which include all sources in the papers of Papoular (1988) and Skinner et al. (1990). After using the new LRS classification defined by Volk and Cohen (1989), and careful cross-identification and spectral inspection, Groenewegen pointed out that among these 39 sources, only two, IRAS 175442951 ¼ IRC-30340 ¼ V1717 Sgr and IRAS 180932107 ¼ IRC-20445, have the letter classification of C indicative of the 11.2 lm SiC feature. Moreover, he also noted that IRAS 18093-2107 shows the amorphous silicate feature in emission at 18 lm and doubted that the emission between 9 and 13 lm is due to SiC. Also, he referred to Lee (1970) for the spectrum of the optical counterpart of this source as M4I, i.e. a red supergiant. However, it is
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well known that red supergiants never have carbon-rich dust, as the C/O ratio in their atmospheres does not exceed unity. Recently Sylvester (1999) made the spectral observations in 7.5–13.3 lm by using the infrared spectrometer CGS3 attached to UKIRT for 18 evolved stars with unusual dust shells. He declared that although the LRS letter classification of IRAS 18093-2107 ¼ IRC-20445 is C and its feature would resemble SiC emission, the feature of this star around 10–13 lm could likely be a broad silicate feature and/or UIR feature which is often seen in red supergiants. In addition, Groenewegen (1994) already admitted that the classification of IRAS 17544-2951 as an oxygen-rich object with SiC is uncertain, as the identification of the IRC object with the IRAS source is ambiguous and there is uncertainty about the spectral classification of the IRC object. Interestingly, by using the scheme proposed by Volk and Cohen (1989), Sloan and Price (1998) classified IRAS 17544-2951 as having silicate emission, so its chemical classification is still doubted. Therefore, the conclusions of studies by Groenewegen (1994) and Sylvester (1999) are actually negative in relation to the presence of oxygenrich stars having the SiC feature. In addition, Sloan and Price (1998) presented the new infrared spectral classifications for some 600 optically identified oxygen-rich AGB stars and red supergiants. Among these stars they declared that IRAS 06224+1701 ¼ GN Ori and IRAS 16118-4439 ¼ RU Nor have the 11.2 lm SiC feature. Therefore, from previous literature, it seems that four objects: IRAS 06224+1701 ¼ GN Ori, IRAS 16118-4439 ¼ RU Nor, IRAS 17544-2951 ¼ V1717 Sgr and IRAS 18093-2107 ¼ IRC-20445 might be the candidates of the M stars with SiC dust shells.
3. Working sample: search for new M stars with SiC feature According to the new LRS classification defined by Volk and Cohen (1989), Kwok et al. (1997) resorted the raw database of the IRAS LRS spectra that includes all those in the LRS Atlas and in Volk and Cohen (1989), and published a new IRAS LRS database including 11,224 sources. On
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the other hand, very recently Alksnis et al. (2001, hereafter CGCS3) published the third edition of the galactic carbon star catalog: General Catalog of Galactic Carbon Stars by C.B. Stephenson. In this catalog, the number of carbon stars is increased to 6891. From this paper, two databases are used for our study. In the first step, all the objects in the LRS group C (indicative of the 11.2 lm SiC feature) from Kwok et al. (1997) are extracted as our working sample. There are some 700 sources in the group C in total. Cross-identification between those IRAS sources and the carbon stars from the CGCS3 has been made according to the method proposed by Chen (1996). The main points of this method are as follows: there is a positional error ellipse for each IRAS source in the IRAS Point Source Catalog (1988, hereafter PSC) and this error ellipse has the reliability of over 95% (1988, PSC); on the other hand, in the CGCS3 the positional error for each star is given so that if the position of a CGCS3 star is located in a certain error ellipse of an IRAS source or the positional error ranges of the two sources are overlapping, the association is basically confirmed, otherwise there is no such association between the CGCS3 star and the IRAS source. After the cross-identification between the IRAS sources and the carbon stars in the CGCS3, all carbon stars has been removed from the working sample. Thus, only those some 90 sources that are not identified as carbon stars by the CGCS3 remain. The next step in the present study concerned the databases of the IRC survey with the spectral classification from Bidelman (1980), the GCVS5 (Kholopov et al., 1985-1995), the SAO catalog, the Simbad database and some references used to check spectral types for these 90 objects. In consequence, nine objects in the group C of the LRS have been listed with spectral type M in, at least, one of the databases above. In addition, the distances between each one of those nine objects and the nearest carbon star from the CGCS3 are checked. For all cases, except for IRAS 075383928, such distances are larger than 60 arc min, and only for IRAS 07538-3928, the distance from the nearest carbon star, CGCS 1966, is as far as about 10 arc min. It means that not only those carbon stars, but also CGCS 1966, are far enough
from the IRAS sources to be not responsible for the related IRAS LRS spectra. These nine objects then form our final sample and they are listed in Table 1. It is noted that the two sources proposed by Groenewegen (1994) are included, but the two sources suggested by Sloan and Price are not (which will be explained in Section 4.1). The structure of Table 1 is as follows in column: 1. IRAS name; 2. Common name from the GCVS5 or SAO; 3. IRC number; 4. Spectral type, for which the origin is shown; 5. LRS classification from Kwok et al. (1997) and the LRS Atlas (if any); 6–7. Position in the epoch of 1950 from the HST Guide Star Catalog (1989, hereafter GSC) or the US Naval Observatory Catalog (1997, hereafter USNO); 8. Magnitudes in V or B from the GSC or USNO; 9–11. 12, 25 and 60 lm fluxes in Jy from the IRAS PSC; 12. Note.
4. Data analysis 4.1. Re-identification First of all, according to the criteria proposed by Chen (1996) the GSC and USNO catalogs are used together with the IRAS PSC to find possible counterparts or confusions in the GSC or USNO for sources in Table 1. It is found that besides AC Cet (with its counterpart in the GSC), there is another very evolved star with 20.2 mag. in B and 11.9 mag. in R also in the positional error ellipse of IRAS 00084-1851. It is not sure whether the SiC emission is from this star or from AC Cet. A similar situation is seen for IRAS 17544-2951. Besides V1717 Sgr ¼ SAO 185989, there is another evolved star with 19.1 mag. in B and 16.4 mag. in R in the positional error ellipse of IRAS 17544-2951. Therefore, these two objects are suspected to be ones we are concerned with and in the following they will be excluded from our discussion. In addition, in the positional error ellipse of IRAS 13011-5604, besides AF Cen, there is a star with 16.3 mag. in B and
? Y Y Y Y Y ? Y Y 1.302 2.990 0.903 0.642 1.104 – 12.16 – 2.201 7.748 21.41 5.134 3.336 7.089 5.912 40.21 10.11 4.017 16.27 52.34 17.62 8.668 16.55 17.66 73.07 24.76 7.755 7.8 (14.7) 7.0 (10.4) 11.4 8.0 12.4 (9.0) 9.4 000825.23–185101.7 075350.93–392822.4 084345.29–103846.6 091726.21–682414.5 130106.89–560409.5 163631.99–481238.4 175426.92–295147.5 180921.13-210718.5 230803.84–605812.4 C(14) C(15) C(18) C(15) C(15) C(44) C(41) C(42) C a; b
GCVS5. Houk and Smith-Moore (1988). c Jones (1972). d Simbad. e SAO. f Bidelman (1980). g Houk (1978). h Humphreya (1970). b
a
)20003 – )10206 – – – )30340 )20445 +60388 AC Cet – SAO154616 – AF Cen SAO227026 V1717 Sgr – GU Cep 00084-1851 07538-3928 08437-1038 09174-6824 13011-5604 16365-4812 17544-2951 18093-2107 23080+6058
IRC Name IRAS
Table 1 Candidates for the M type stars with SiC feature
c;f
M5III , M3III M..d Mbe , M3f Med M7a Mbe , M3II:g Ce(M0:e)a , K5e ,
LRS Spectrum type
R.A (1950) Dec.
V(B)
F12
F25
F60
Note
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16.5 mag. in R. However from its B and R magnitudes, clearly it is not a late type star and may not be responsible for the IRAS fluxes. As a consequence, the SiC emission is likely to be from AF Cen. Besides the sources discussed above, all of the other sources are also checked with the GSC and/or the USNO, and only one counterpart is in the positional error ellipse of the related IRAS source for each one, which means that the association between the IRAS source and M star is unique. Finally, among nine sources in Table 1, except IRAS 00084-1851 and IRAS 17544-2951, there are seven most likely to be candidates of M stars with SiC features. In the following we will restrict our discussion to those seven sources. Moreover, it should be mentioned that Sloan and Price (1998) considered IRAS 16118-4439 ¼ RU Nor to have SiC emission. However in the LRS classification from Kwok et al. (1997), its LRS is in the group F indicative of featureless spectrum so that it is not discussed here. For IRAS 06224+1701, Sloan and Price (1998) identified it as GN Ori, however, GN Ori is far away from the IRAS position (the separation is about 2.5 arc min). On the other hand, another evolved star identified by us as CGCS 1261, which Jura and Kleinmann (1990) considered as a very dusty carbon star, is in the positional error ellipse of the IRAS source. Therefore the SiC emission should come from this carbon star but not from GN Ori which should be rejected. We have also checked the released 2MASS PSC data for all sources here, but no data in the 2MASS are available so far, because either the sources (including IRAS 07538-3928, IRAS 084371038 and IRAS 18093-2107) are too bright to be observed by 2MASS or 2MASS data for the sources (including IRAS 09174-6824, IRAS 130115604, IRAS 16365-4812 and IRAS 23080+6058) are not released as yet. 4.2. IRAS LRS spectra In order to clearly show the spectral features, the LRS spectra from Kwok et al. (1997) for these seven sources are illustrated in Fig. 1. Although all the LRS of those seven objects are classified as C indicative of 11.2 lm SiC emission according to Kwok et al. (1997), the extensive inspections on
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Fig. 1. IRAS LRS spectra for the candidates of the M type stars with SiC feature.
these spectra are still needed. From the inspections above it can be summarized for those sources that there are some common properties, i.e., weak emission features starting at about 10 lm and ending at around 13 lm are interpreted as possibly due to SiC. Amorphous silicate emission, on the other hand, generally peaks around a wavelength of 9.7–10 lm. The detailed discussions about the LRS spectra of those seven samples will be presented in Section 5. 4.3. IRAS colour–colour diagrams Taking the IRAS fluxes from Table 1 and defining two infrared colours, [12]–[25] and [25]–[60]
the same as suggested by van der Veen and Habing (1988) these sources can be plotted on the standard IRAS two-colour diagram (van der Veen and Habing, 1988) as shown in Fig. 2. It is obvious that, except for IRAS 23080+6058, all sources are located in the region I or II indicative of oxygenrich non-variable stars or oxygen-rich variable stars with a little mass loss, i.e., have rather blue [12]–[25] colours. Although IRAS 16365-4812 and IRAS 18093-2107 are not included in the [25]–[60] versus [12]–[25] diagram because of 60 lm upper limits, it is worth pointing out that the [12]–[25] colours of these two objects are with [12]– [25] ¼ )1.2 and )1.0, respectively, equally blue as the others. It is noted that the blue [12]–[25]
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Fig. 1. (continued)
colours of the objects virtually exclude the interpretation of the IRAS LRS spectra as showing silicate absorption just shortward of 10 lm , as someone might suggest for objects like IRAS 13011-5604, IRAS 16365-4812, IRAS 23080+6058 and (perhaps self-absorbed) IRAS 18093-2107. Moreover, the blue colour suggests a small mid-IR excess and hence the photospheric contribution to the IRAS LRS may be important, especially around 8–10 lm where there is the fundamental band of SiO molecules at 7.6 lm (see, e.g., Aringer et al., 1997). In addition, IRAS 23080+6058 is located in the region of VIb indicative of variable stars with relatively hot dust close to the star and
relatively cold dust at large distances; some of the objects have proven to be oxygen-rich (see van der Veen and Habing, 1988). The locations of sources here in Fig. 2 imply not much infrared excesses and low mass loss rate from the central stars that are in accord with the weak features of these sources in the LRS spectra shown in Fig. 1.
5. Discussion (1) According to the evolutionary theory, in the late stage of stellar evolution, oxygen-rich stars with the 11.2 lm SiC emission are unexpected. The
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Fig. 2. IRAS colour–colour diagram for the candidates of the M type stars with SiC feature.
oxygen-rich stars that have C=O < 1 should not present such a carbon-rich composition, SiC at 11.2 lm, because CO should be formed in their circumstellar envelopes first with no extra carbon available to be used to form SiC (Iben and Renzini, 1983). Skinner et al. (1990) suggested that these sources could be the stars for which the C/O ratio is not far from unity. If Skinner et al. (1990) are right and oxygen-rich stars could exhibit SiC emission feature when their C/O ratio is close to unity, then these stars would show S or MS-type spectra instead of genuine M-type spectra. In addition, Groenewegen (1994) argued that a larger photospheric C/O ratio does not necessarily imply a higher chance that SiC is formed. However, these authors did not give any clear explanations for the presence of oxygen-rich stars with the 11.2 lm SiC emission. Furthermore, Sylvester (1999) suggested that the wide feature around 10–13 lm for IRC20445 ¼ IRAS 18093-2107 would resemble SiC emission, but could also be identified as silicate emission superposed with the unidentified infrared (UIR) feature at 11.3 lm. Therefore the phenomenon of the oxygen-rich stars with SiC emission is now further from our understanding. One may easily think that the samples in this paper may be in a binary system that consists of a carbon-rich star and an M type star like that proposed by Little-Marenin (1986) for silicate carbon stars. However, this binary system is an extremely low possibility of occurrence from the
point of view of the stellar evolution. Moreover, such stars must have large excesses in the far infrared region, which are not observed in the current samples. Another possibility to understand the presence of the samples in this paper is that the features around 11 lm may not be due to the SiC emission. Before the ISO mission, we knew that the features around 11.3 lm (including features at about 7.7 and 8.7 lm) may also be attributed to the PAH molecules (see, e.g., Cohen et al., 1985). It is noted that in the new IRAS LRS database (Kwok et al., 1997) all sources are classified into 10 groups in which there is a group P indicative of the PAH feature sources. However, none of the sources in this paper were classified into group P, but were all classified into group C by these authors. If it is the case, the features around 11 lm for samples here is not due to the PAH molecules. Fortunately, when the ISO SWS spectra became available, one of the most exciting discoveries was the finding of clear evidence of crystalline silicate features around 11.2, 13.8, 16.3 , 19.5 and 21.5 lm in the IRAS LRS region (see, e.g., Jager et al., 1998; Kraemer et al., 2002). Furthermore, the features around 11.2, 16.3 and 19.5 lm are quite strong (Jager et al., 1998). If we re-examine the LRS spectra presented in Fig. 1, it is found that some traces of the features in other wavelengths may exist, although most of the spectra in Fig. 1 have rather poor signal-to-noise ratio. For instance, for IRAS 16365-4812, there are not only emission feature around 11 lm, but also some little bumps around 16.5, 19.5 and 21.5 lm in the relatively good signal-to-noise ratio spectrum. If this is the case, the features may be considered as due to the crystalline silicates which are not surprising for the oxygen-rich objects. Moreover, Molster et al. (2002) pointed out that the crystalline silicates need high temperatures (>900 K) to form. From Table 1 it is seen that most of the sources have the rather earlier sub-types of the spectra that imply a rather high temperature environment to meet the condition of the formation of crystalline silicates. This fact is supported by the interpretation of the emission features as possibly due to crystalline silicates rather than SiC. It is a pity that ISO did not observe all sources here. Furthermore,
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much more is known about the shape of the emission features of various kinds of oxygen-rich dust. For instance, the peak wavelength and breadth of the emission feature depend on the degree of crystallinity, the dust temperature and dust particle size. In general, aluminium oxides are thought to be amongst the first to condense into dust grains, and give rise to emission features peaking longward of 10 m, as do large grains of a few microns in size (see, e.g., Speck et al., 2000). It is also noted that, as mentioned in Section 4, photospheric contributions to the IRAS LRS spectra should be considered, especially around 8–10 lm, where there is the fundamental band of SiO molecular features in absorption at 7.6 lm. (2) From Table 1 it can be seen that besides IRAS 18093-2107, which is discussed in Section 2, IRAS 23080+6058 is also a red supergiant and one would therefore not expect to see any carbonrich dust around it. In fact, Jura and Kleinmann (1990) included this object in their study of red supergiants. From Table 1 it is seen that IRAS 16365-4812 is also a possible red supergiant. Moreover, on the basis of a combination of nearIR and mid-IR colours, Guglielmo et al. (1993) did not classify IRAS 16365-4812 as carbon stars, which may mean that its photosphere and the emission caused by the circumstellar envelope are oxygen-rich. It should be emphasized that although the main interest of this work is in the area of AGB evolution, as mentioned in Section 1, some of the candidates, as described here, are more massive red supergiants, for which the carbon-rich features never existed in their circumstellar envelopes. Another source which should be discussed here is IRAS 07538-3928. Haikala et al. (1994) detected SiO maser emission at 86 GHz (v ¼ 1, J ¼ 2 1). This means that the zone in between the stellar photosphere and the dust envelope is oxygen-rich and that the dust envelope is being replenished with oxygen-rich material which will almost certainly condense to form oxygen-rich dust, because the locations of SiO maser emissions are just above the stellar photosphere within twice to four times the stellar diameter (Habing, 1996). (3) Finally, inspections on the LRS spectra shown in Fig. 1 have been made extensively.
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According to suggestions from van Loon (2003) together with discussions above, it can be primarily concluded for the seven candidates as shown in Fig. 1 that: IRAS 07538-3928: SiO maser emission, silicates and/or aluminium oxides; IRAS 08437-1038: featureless/photospheric; IRAS 09174-6824: too noisy, consistent with featureless/photospheric; IRAS 13011-5604: broad/red silicate emission, possibly at 18 lm too, possible contribution from photospheric SiO absorption; IRAS 16365-4812: possible red supergiant, crystalline silicates, possible contribution from photospheric SiO absorption; IRAS 18093-2107: red supergiant, broad/red silicate emission; IRAS 23080+6058: red supergiant, noisy, broad/red silicate or aluminium oxides emission, possible contribution from photospheric SiO absorption.
6. Summary From the study of the presently available material it appears that there are no strong candidates for oxygen-rich AGB stars with carbon-rich circumstellar dust. The few weak candidates for such objects are consistent with the presence of silicates and/or aluminium oxides and/or photospheric SiO absorption. Nevertheless, the conclusion above should be proven in some way, because of the rather poor signal-to-noise ratio in the LRS spectra presented. In particular, spectral observations with much higher resolution in the infrared should be taken to verify these features for the samples here in the future.
Acknowledgements We thank Dr. van Loon for his helpful advice and suggestions. This work is supported by grants from the National Natural Science Foundation of China (No. 10073018) and the Natural Science Foundation of Yunnan Province
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(No. 2002A0021Q). This work has made use of the NASAs ADS database and the Simbad database.
References Alksnis, A., Balklavs, A., Dzervitis, U., et al., 2001. Baltic Astronomy 10, 1 (CGCS3). Aringer, B., Jorgensen, U.G., Langhoff, S.R., 1997. A&A 323, 202. Bidelman, W.P., 1980. Pub. Warner & Swasey Obs. 2, 185. Chan, S.J., Kwok, S., 1988. ApJ 334, 362. Chan, S.J., Kwok, S., 1990. A&A 237, 354. Chan, S.J., Kwok, S., 1991. ApJ 383, 837. Chen, P.S., Kwok, S., 1993. ApJ 416, 769. Chen, P.S., 1996. ChA&A 20, 169. Chen, P.S., Wang, X.H., Wang, F., 1999. ChA&A 23, 371. Cohen, M., Tielens, A.G.G.M., Allamandola, L.J., 1985. ApJ 299, L93. Engels, D., Leinert, Ch., 1994. A&A 282, 858. Groenewegen, M.A.T., de Jong, T., van der Bliek, N.S., et al., 1992. A&A 253, 150. Groenewegen, M.A.T., 1994. A&A 290, 207. Guglielmo, F., Epchtein, N., Le Bertre, T., et al., 1993. A&AS 99, 31. Habing, H.J., 1996. A&AR 7, 97. Haikala, L.K., Nyman, L.-A., Forsstroem, V., 1994. A&AS 103, 107. Houk, N., Smith-Moore, M., 1988. Michigan Spectral Survey, 4. Houk, N., 1978. Michigan Spectral Survey, 2. Humphreya, R.M., 1970. AJ 75, 602. HST Guide Star Catalog, 1989. Baltimore: STScI, ADC CD-ROM, Selected Astronomical Catalogs. p. 1 (GSC). Iben, I., Renzini, A., 1983. ARA&A 21, 271. IRAS Point Source Catalog, Version 2, 1988. Joint IRAS Science Working Group, Washington, DC (PSC). Jager, C., Molster, F.J., Dorschner, J., et al., 1998. A&A 339, 904. Jones, O.H.P., 1972. ApJ 178, 467. Jura, M., Kleinmann, S.G., 1990. ApJ 364, 663. Kahane, C., Barnbaum, C., Uchida, K., 1998. ApJ 500, 466.
Kholopov, P.N. et al., 1985–1995. General Catalogue of Variable Stars, fifth ed. ADC CD-ROM, Selected Astronomical Catalogs. p. 4 (GCVS5). Kraemer, K.E., Sloan, G.C., Price, S.D., et al., 2002. ApJS 140, 389. Kwok, S., Volk, K., Bidelman, W.P., 1997. ApJS 112, 557. Lambert, D.L., Hinkle, K.H., Smith, V.V., 1990. AJ 99, 1612. Lee, T.A., 1970. ApJ 162, 217. Little-Marenin, I.R., 1986. ApJ 307, L15. Little-Marenin, I.R., Ramsay, M.E., Stephenson, C.B., et al., 1987. AJ 93, 663. Lloyd Evans, T., 1990. MNRAS 243, 336. Molster, F.J., Yamamura, I., Waters, L.B.F.M., et al., 1999. Nature 401, 563. Molster, F.J., Yamamura, I., Waters, L.B.F.M., et al., 2001. A&A 366, 923. Molster, F.J., Waters, L.B.F.M., Tielens, A.G.G.M., et al., 2002. A&A 382, 222. Neugebauer, G., Leighton, R.B., 1969. NASA SP-3047 (IRC). Olnon, F.M., Raimond, E., 1986. A&AS 65, 607 (LRS Atlas). Papoular, R., 1988. A&A 204, 138. Schroder, K.-P., Winters, J.M., Sedlmayr, E., 1999. A&A 349, 898. Skinner, C.J., Griffin, I., Whitmore, B., 1990. MNRAS 243, 78. Sloan, G.C., Price, S.D., 1998. ApJS 119, 141. Speck, A.K., Barlow, M.J., Sylvester, R.J., et al., 2000. A&AS 146, 437. Sylvester, R.J., 1999. MNRAS 309, 180. Threnson, H.A., Latter, W.B., Black, J.H., et al., 1987. ApJ 322, 770. US Naval Observatory Catalog, 1997. CD-ROM (USNO). van der Veen, W.E.C.J., Habing, H.J., 1988. A&A 194, 125. van Loon, J.Th., Groenewegen, M.A.T., de Koter, A., et al., 1999. A&A 351, 559. van Loon, J.Th., 2003. private communication. Volk, K., Cohen, M., 1989. AJ 98, 932. Volk, K., Kwok, S., Stencel, R.E., et al., 1991. ApJS 77, 607. Volk, K., 1993. ASP Conf. Ser. 41, 63. Walker, H., Cohen, M., 1988. AJ 95, 1801. Waters, L.B.F.M., Cami, J., de Jong, T., et al., 1998. Nature 391, 868. Willems, F.J., de Jong, T., 1986. ApJ 309, L39. Willems, F.J., de Jong, T., 1988. A&A 196, 173. Yamamura, I., Dominik, C., de Jong, T., et al., 2000. A&A 363, 629.
Discussion on candidates for the M stars with 11.2 lm SiC feature P.S. Chen *, Y.F. Gao, H.G. Shan, X.H. Wang National Astronomical Observatories/Yunnan Observatory, CAS, Kunming 650011, China Received 13 March 2003; received in revised form 12 June 2003; accepted 12 June 2003 Communicated by G.F. Gilmore
Abstract The oxygen-rich stars with SiC emission features are unexpected according to the evolutionary theory in the late stage of stellar evolution. After the careful re-examination of such samples reported previously and searching for new samples, seven stars are found to be the candidates of the M stars with 11.2 lm SiC emission feature. However, with the IRAS low-resolution spectrum, the IRAS two colour diagram and other information, it is seen that no strong candidates for oxygen-rich AGB stars with carbon-rich circumstellar dust could be found so far. Ó 2003 Elsevier B.V. All rights reserved. PACS: 97.10.Fy; 97.10.Tk; 97.10.Zr; 97.10.Jg Keywords: Stars: late-type; Stars: chemically peculiar; Infrared: stars
1. Introduction It has been assumed that the spectral classes of M–S–C are one of the evolutionary sequences for stars on the asymptotic giant branch (AGB) phase (e.g., Iben and Renzini, 1983; Chen and Kwok, 1993). Those AGB stars begin as oxygen-rich (C=O < 1) stars with the spectral type of M and they often show the features of the oxygen-rich material in the infrared such as silicate features in emission or in absorption at 10 and/or 18 lm. As a
*
Corresponding author. E-mail address: [email protected] (P.S. Chen).
result of carbon dredge-up from the interior they may evolve through the stage of the spectral type of S (C=O 1) to carbon-rich (C=O > 1) ones. The discovery that many visual carbon stars have large excesses in the far infrared (Threnson et al., 1987) has led to the theory that those visual carbon stars represent an evolutionary stage after a severe mass-losing episode as M stars (e.g. Willems and de Jong, 1988; Chan and Kwok, 1988). However, Schroder et al. (1999) pointed out that there is no convincing evidence that the star was oxygen-rich at this stage, especially, for the lower-mass AGB stars (say, about 1.2 solar mass). Another possibility is that these lower-mass AGB stars suffer much more from the effects of thermal pulses,
1384-1076/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1384-1076(03)00088-5
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which may cause brief sharp enhancements in the mass-loss rate for these stars, leading to detached shells of fossil mass. In the early stages for the stars in the AGB phase, the carbon is tied up in CO molecules with oxygen but there is not enough carbon available for the formation of other carbon-rich material. As Chan and Kwok (1988) suggested, a plausible scenario for some stars after this stage is as follows: when the remnant oxygenrich circumstellar envelope begins to expand into the interstellar medium, the star gradually becomes a visual carbon star. After more dredge-up, more carbon atoms have accumulated in the photosphere to form carbon-based grains, such as amorphous carbon and silicon carbide (SiC), etc., the star then creates a carbon-rich circumstellar dust shell that makes the star itself invisible and then the star becomes a so-called infrared carbon star that often shows the silicon carbide (SiC) emission feature at 11.2 lm. On the other hand, as van Loon et al. (1999) pointed out, the scenario above may be suitable for some carbon stars, but it is not yet clear whether this is true for all carbon stars. Because there are some indications that carbon star formation may take place whilst the AGB star is optically invisible, i.e., a lull in the mass-loss rate on birth of the carbon star is not certain to happen. After the IRAS mission, the IRAS low-resolution spectrum (LRS) has become the most important tool in the infrared for discriminating between carbon stars and M stars. As mentioned above the silicate features at 10 and 18 lm are indicators of oxygen-rich stars (M stars) and the silicon carbide (SiC) featured at 11.2 lm is an indication of carbon-rich stars (Olnon and Raimond, 1986; Kwok et al., 1997). However, there are some exceptions. One case is the carbon stars with silicate dust shells, now known as silicate carbon stars (Little-Marenin, 1986; Willems and de Jong, 1986). The number of silicate carbon stars has now increased to 25, which includes two carbon-rich objects, recently discovered by the ISO SWS observations, to have the crystalline silicate features at the wavelength longer than 15 lm (Groenewegen, 1994; Kwok et al., 1997; Waters et al., 1998; Chen et al., 1999 and Molster et al., 1999, 2001). Another case may be the oxygen-rich
M stars with 11.2 lm SiC dust (Papoular, 1988; Skinner et al., 1990; Groenewegen, 1994; Sylvester, 1999). Both groups of stars mentioned above are unpredicted according to the known evolutionary theory in the late stage of stellar evolution. The silicate carbon stars have received much attention so far, although their nature is still not very clear. Little-Marenin (1986) proposed a binary model consisting of a carbon-rich giant and an oxygenrich giant to explain the silicate carbon star phenomenon. However, this idea is now thought to be unlikely because no evidence of oxygen-rich giants in silicate carbon stars have been found (Lambert et al., 1990; Engels and Leinert, 1994). Willems and de Jong (1986) and Chan and Kwok (1991) suggested that silicate carbon stars are the transition objects between the oxygen-rich stars and the carbon-rich stars and the oxygen-rich matter is the remnant of a previous mass-loss phase. This transition model has also been criticized from the point of view of the evolutionary timescale (e.g., Lloyd Evans, 1990). The most widely accepted model now to explain the silicate carbon star phenomenon is another binary model consisting of a carbon star and a main-sequence star with a disk that may surround the companion or the whole system (Lloyd Evans, 1990; Kahane et al., 1998; Yamamura et al., 2000). In contrast, the situation about the M stars with 11.2 lm SiC feature is unclear now and no models so far could be successfully employed to try to explain the phenomenon. One of the reasons is that we have no convincing sample of such stars now. However, from an evolutionary standpoint and from the view of dust formation, the M stars with 11.2 lm SiC feature would be as interesting as the silicate carbon stars. In this paper, we try to re-examine the M stars with SiC feature reported in previous literature and to find more candidates to try to establish their actual nature.
2. About M stars with SiC feature in the literature Papoular (1988) first reported 15 M stars, which are classified as 4n indicative of SiC emission, in the IRAS low-resolution spectrum (LRS) Atlas
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(Olnon and Raimond, 1986). He also picked out nine objects with the LRS classification 1n (indicative of featureless spectra) and believed to show the weak SiC feature. Skinner et al. (1990) listed 11 M stars with SiC dust on the basis of their LRS class of 4n. However, it is well documented that in some cases the automated LRS classification in the LRS Atlas gives unreliable results, especially, including the case for which LRS spectra with weak silicate emission were misclassified as 4n objects (e.g., Little-Marenin et al., 1987; Walker and Cohen, 1988; Chan and Kwok, 1990; Volk et al., 1991; Groenewegen et al., 1992; Volk, 1993). Fortunately, Kwok et al. (1997) re-sorted out the raw database of the IRAS LRS spectra with the letter classification scheme proposed by Volk and Cohen (1989) and published a new IRAS LRS database in which the source number reaches 11,224. This is more than twice that in the previous IRAS LRS Atlas. It was expected that in this new LRS database many sources of interest could be found and erroneous identifications appearing in the automated classification in the LRS Atlas could be corrected. In fact, in the new LRS database of 11,224 spectra, sources are grouped into 10 classes according to their spectral characteristics. Among them, the sources in class C are considered as having SiC emission feature at 11.2 lm. Based on the spectral types compiled by Bidelman (1980) for the IRC survey (Neugebauer and Leighton, 1969, hereafter IRC) and the LRS number classification used in the LRS Atlas, Groenewegen (1994) proposed 39 M and MS stars with SiC dust, which include all sources in the papers of Papoular (1988) and Skinner et al. (1990). After using the new LRS classification defined by Volk and Cohen (1989), and careful cross-identification and spectral inspection, Groenewegen pointed out that among these 39 sources, only two, IRAS 175442951 ¼ IRC-30340 ¼ V1717 Sgr and IRAS 180932107 ¼ IRC-20445, have the letter classification of C indicative of the 11.2 lm SiC feature. Moreover, he also noted that IRAS 18093-2107 shows the amorphous silicate feature in emission at 18 lm and doubted that the emission between 9 and 13 lm is due to SiC. Also, he referred to Lee (1970) for the spectrum of the optical counterpart of this source as M4I, i.e. a red supergiant. However, it is
19
well known that red supergiants never have carbon-rich dust, as the C/O ratio in their atmospheres does not exceed unity. Recently Sylvester (1999) made the spectral observations in 7.5–13.3 lm by using the infrared spectrometer CGS3 attached to UKIRT for 18 evolved stars with unusual dust shells. He declared that although the LRS letter classification of IRAS 18093-2107 ¼ IRC-20445 is C and its feature would resemble SiC emission, the feature of this star around 10–13 lm could likely be a broad silicate feature and/or UIR feature which is often seen in red supergiants. In addition, Groenewegen (1994) already admitted that the classification of IRAS 17544-2951 as an oxygen-rich object with SiC is uncertain, as the identification of the IRC object with the IRAS source is ambiguous and there is uncertainty about the spectral classification of the IRC object. Interestingly, by using the scheme proposed by Volk and Cohen (1989), Sloan and Price (1998) classified IRAS 17544-2951 as having silicate emission, so its chemical classification is still doubted. Therefore, the conclusions of studies by Groenewegen (1994) and Sylvester (1999) are actually negative in relation to the presence of oxygenrich stars having the SiC feature. In addition, Sloan and Price (1998) presented the new infrared spectral classifications for some 600 optically identified oxygen-rich AGB stars and red supergiants. Among these stars they declared that IRAS 06224+1701 ¼ GN Ori and IRAS 16118-4439 ¼ RU Nor have the 11.2 lm SiC feature. Therefore, from previous literature, it seems that four objects: IRAS 06224+1701 ¼ GN Ori, IRAS 16118-4439 ¼ RU Nor, IRAS 17544-2951 ¼ V1717 Sgr and IRAS 18093-2107 ¼ IRC-20445 might be the candidates of the M stars with SiC dust shells.
3. Working sample: search for new M stars with SiC feature According to the new LRS classification defined by Volk and Cohen (1989), Kwok et al. (1997) resorted the raw database of the IRAS LRS spectra that includes all those in the LRS Atlas and in Volk and Cohen (1989), and published a new IRAS LRS database including 11,224 sources. On
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the other hand, very recently Alksnis et al. (2001, hereafter CGCS3) published the third edition of the galactic carbon star catalog: General Catalog of Galactic Carbon Stars by C.B. Stephenson. In this catalog, the number of carbon stars is increased to 6891. From this paper, two databases are used for our study. In the first step, all the objects in the LRS group C (indicative of the 11.2 lm SiC feature) from Kwok et al. (1997) are extracted as our working sample. There are some 700 sources in the group C in total. Cross-identification between those IRAS sources and the carbon stars from the CGCS3 has been made according to the method proposed by Chen (1996). The main points of this method are as follows: there is a positional error ellipse for each IRAS source in the IRAS Point Source Catalog (1988, hereafter PSC) and this error ellipse has the reliability of over 95% (1988, PSC); on the other hand, in the CGCS3 the positional error for each star is given so that if the position of a CGCS3 star is located in a certain error ellipse of an IRAS source or the positional error ranges of the two sources are overlapping, the association is basically confirmed, otherwise there is no such association between the CGCS3 star and the IRAS source. After the cross-identification between the IRAS sources and the carbon stars in the CGCS3, all carbon stars has been removed from the working sample. Thus, only those some 90 sources that are not identified as carbon stars by the CGCS3 remain. The next step in the present study concerned the databases of the IRC survey with the spectral classification from Bidelman (1980), the GCVS5 (Kholopov et al., 1985-1995), the SAO catalog, the Simbad database and some references used to check spectral types for these 90 objects. In consequence, nine objects in the group C of the LRS have been listed with spectral type M in, at least, one of the databases above. In addition, the distances between each one of those nine objects and the nearest carbon star from the CGCS3 are checked. For all cases, except for IRAS 075383928, such distances are larger than 60 arc min, and only for IRAS 07538-3928, the distance from the nearest carbon star, CGCS 1966, is as far as about 10 arc min. It means that not only those carbon stars, but also CGCS 1966, are far enough
from the IRAS sources to be not responsible for the related IRAS LRS spectra. These nine objects then form our final sample and they are listed in Table 1. It is noted that the two sources proposed by Groenewegen (1994) are included, but the two sources suggested by Sloan and Price are not (which will be explained in Section 4.1). The structure of Table 1 is as follows in column: 1. IRAS name; 2. Common name from the GCVS5 or SAO; 3. IRC number; 4. Spectral type, for which the origin is shown; 5. LRS classification from Kwok et al. (1997) and the LRS Atlas (if any); 6–7. Position in the epoch of 1950 from the HST Guide Star Catalog (1989, hereafter GSC) or the US Naval Observatory Catalog (1997, hereafter USNO); 8. Magnitudes in V or B from the GSC or USNO; 9–11. 12, 25 and 60 lm fluxes in Jy from the IRAS PSC; 12. Note.
4. Data analysis 4.1. Re-identification First of all, according to the criteria proposed by Chen (1996) the GSC and USNO catalogs are used together with the IRAS PSC to find possible counterparts or confusions in the GSC or USNO for sources in Table 1. It is found that besides AC Cet (with its counterpart in the GSC), there is another very evolved star with 20.2 mag. in B and 11.9 mag. in R also in the positional error ellipse of IRAS 00084-1851. It is not sure whether the SiC emission is from this star or from AC Cet. A similar situation is seen for IRAS 17544-2951. Besides V1717 Sgr ¼ SAO 185989, there is another evolved star with 19.1 mag. in B and 16.4 mag. in R in the positional error ellipse of IRAS 17544-2951. Therefore, these two objects are suspected to be ones we are concerned with and in the following they will be excluded from our discussion. In addition, in the positional error ellipse of IRAS 13011-5604, besides AF Cen, there is a star with 16.3 mag. in B and
? Y Y Y Y Y ? Y Y 1.302 2.990 0.903 0.642 1.104 – 12.16 – 2.201 7.748 21.41 5.134 3.336 7.089 5.912 40.21 10.11 4.017 16.27 52.34 17.62 8.668 16.55 17.66 73.07 24.76 7.755 7.8 (14.7) 7.0 (10.4) 11.4 8.0 12.4 (9.0) 9.4 000825.23–185101.7 075350.93–392822.4 084345.29–103846.6 091726.21–682414.5 130106.89–560409.5 163631.99–481238.4 175426.92–295147.5 180921.13-210718.5 230803.84–605812.4 C(14) C(15) C(18) C(15) C(15) C(44) C(41) C(42) C a; b
GCVS5. Houk and Smith-Moore (1988). c Jones (1972). d Simbad. e SAO. f Bidelman (1980). g Houk (1978). h Humphreya (1970). b
a
)20003 – )10206 – – – )30340 )20445 +60388 AC Cet – SAO154616 – AF Cen SAO227026 V1717 Sgr – GU Cep 00084-1851 07538-3928 08437-1038 09174-6824 13011-5604 16365-4812 17544-2951 18093-2107 23080+6058
IRC Name IRAS
Table 1 Candidates for the M type stars with SiC feature
c;f
M5III , M3III M..d Mbe , M3f Med M7a Mbe , M3II:g Ce(M0:e)a , K5e ,
LRS Spectrum type
R.A (1950) Dec.
V(B)
F12
F25
F60
Note
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16.5 mag. in R. However from its B and R magnitudes, clearly it is not a late type star and may not be responsible for the IRAS fluxes. As a consequence, the SiC emission is likely to be from AF Cen. Besides the sources discussed above, all of the other sources are also checked with the GSC and/or the USNO, and only one counterpart is in the positional error ellipse of the related IRAS source for each one, which means that the association between the IRAS source and M star is unique. Finally, among nine sources in Table 1, except IRAS 00084-1851 and IRAS 17544-2951, there are seven most likely to be candidates of M stars with SiC features. In the following we will restrict our discussion to those seven sources. Moreover, it should be mentioned that Sloan and Price (1998) considered IRAS 16118-4439 ¼ RU Nor to have SiC emission. However in the LRS classification from Kwok et al. (1997), its LRS is in the group F indicative of featureless spectrum so that it is not discussed here. For IRAS 06224+1701, Sloan and Price (1998) identified it as GN Ori, however, GN Ori is far away from the IRAS position (the separation is about 2.5 arc min). On the other hand, another evolved star identified by us as CGCS 1261, which Jura and Kleinmann (1990) considered as a very dusty carbon star, is in the positional error ellipse of the IRAS source. Therefore the SiC emission should come from this carbon star but not from GN Ori which should be rejected. We have also checked the released 2MASS PSC data for all sources here, but no data in the 2MASS are available so far, because either the sources (including IRAS 07538-3928, IRAS 084371038 and IRAS 18093-2107) are too bright to be observed by 2MASS or 2MASS data for the sources (including IRAS 09174-6824, IRAS 130115604, IRAS 16365-4812 and IRAS 23080+6058) are not released as yet. 4.2. IRAS LRS spectra In order to clearly show the spectral features, the LRS spectra from Kwok et al. (1997) for these seven sources are illustrated in Fig. 1. Although all the LRS of those seven objects are classified as C indicative of 11.2 lm SiC emission according to Kwok et al. (1997), the extensive inspections on
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Fig. 1. IRAS LRS spectra for the candidates of the M type stars with SiC feature.
these spectra are still needed. From the inspections above it can be summarized for those sources that there are some common properties, i.e., weak emission features starting at about 10 lm and ending at around 13 lm are interpreted as possibly due to SiC. Amorphous silicate emission, on the other hand, generally peaks around a wavelength of 9.7–10 lm. The detailed discussions about the LRS spectra of those seven samples will be presented in Section 5. 4.3. IRAS colour–colour diagrams Taking the IRAS fluxes from Table 1 and defining two infrared colours, [12]–[25] and [25]–[60]
the same as suggested by van der Veen and Habing (1988) these sources can be plotted on the standard IRAS two-colour diagram (van der Veen and Habing, 1988) as shown in Fig. 2. It is obvious that, except for IRAS 23080+6058, all sources are located in the region I or II indicative of oxygenrich non-variable stars or oxygen-rich variable stars with a little mass loss, i.e., have rather blue [12]–[25] colours. Although IRAS 16365-4812 and IRAS 18093-2107 are not included in the [25]–[60] versus [12]–[25] diagram because of 60 lm upper limits, it is worth pointing out that the [12]–[25] colours of these two objects are with [12]– [25] ¼ )1.2 and )1.0, respectively, equally blue as the others. It is noted that the blue [12]–[25]
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23
Fig. 1. (continued)
colours of the objects virtually exclude the interpretation of the IRAS LRS spectra as showing silicate absorption just shortward of 10 lm , as someone might suggest for objects like IRAS 13011-5604, IRAS 16365-4812, IRAS 23080+6058 and (perhaps self-absorbed) IRAS 18093-2107. Moreover, the blue colour suggests a small mid-IR excess and hence the photospheric contribution to the IRAS LRS may be important, especially around 8–10 lm where there is the fundamental band of SiO molecules at 7.6 lm (see, e.g., Aringer et al., 1997). In addition, IRAS 23080+6058 is located in the region of VIb indicative of variable stars with relatively hot dust close to the star and
relatively cold dust at large distances; some of the objects have proven to be oxygen-rich (see van der Veen and Habing, 1988). The locations of sources here in Fig. 2 imply not much infrared excesses and low mass loss rate from the central stars that are in accord with the weak features of these sources in the LRS spectra shown in Fig. 1.
5. Discussion (1) According to the evolutionary theory, in the late stage of stellar evolution, oxygen-rich stars with the 11.2 lm SiC emission are unexpected. The
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Fig. 2. IRAS colour–colour diagram for the candidates of the M type stars with SiC feature.
oxygen-rich stars that have C=O < 1 should not present such a carbon-rich composition, SiC at 11.2 lm, because CO should be formed in their circumstellar envelopes first with no extra carbon available to be used to form SiC (Iben and Renzini, 1983). Skinner et al. (1990) suggested that these sources could be the stars for which the C/O ratio is not far from unity. If Skinner et al. (1990) are right and oxygen-rich stars could exhibit SiC emission feature when their C/O ratio is close to unity, then these stars would show S or MS-type spectra instead of genuine M-type spectra. In addition, Groenewegen (1994) argued that a larger photospheric C/O ratio does not necessarily imply a higher chance that SiC is formed. However, these authors did not give any clear explanations for the presence of oxygen-rich stars with the 11.2 lm SiC emission. Furthermore, Sylvester (1999) suggested that the wide feature around 10–13 lm for IRC20445 ¼ IRAS 18093-2107 would resemble SiC emission, but could also be identified as silicate emission superposed with the unidentified infrared (UIR) feature at 11.3 lm. Therefore the phenomenon of the oxygen-rich stars with SiC emission is now further from our understanding. One may easily think that the samples in this paper may be in a binary system that consists of a carbon-rich star and an M type star like that proposed by Little-Marenin (1986) for silicate carbon stars. However, this binary system is an extremely low possibility of occurrence from the
point of view of the stellar evolution. Moreover, such stars must have large excesses in the far infrared region, which are not observed in the current samples. Another possibility to understand the presence of the samples in this paper is that the features around 11 lm may not be due to the SiC emission. Before the ISO mission, we knew that the features around 11.3 lm (including features at about 7.7 and 8.7 lm) may also be attributed to the PAH molecules (see, e.g., Cohen et al., 1985). It is noted that in the new IRAS LRS database (Kwok et al., 1997) all sources are classified into 10 groups in which there is a group P indicative of the PAH feature sources. However, none of the sources in this paper were classified into group P, but were all classified into group C by these authors. If it is the case, the features around 11 lm for samples here is not due to the PAH molecules. Fortunately, when the ISO SWS spectra became available, one of the most exciting discoveries was the finding of clear evidence of crystalline silicate features around 11.2, 13.8, 16.3 , 19.5 and 21.5 lm in the IRAS LRS region (see, e.g., Jager et al., 1998; Kraemer et al., 2002). Furthermore, the features around 11.2, 16.3 and 19.5 lm are quite strong (Jager et al., 1998). If we re-examine the LRS spectra presented in Fig. 1, it is found that some traces of the features in other wavelengths may exist, although most of the spectra in Fig. 1 have rather poor signal-to-noise ratio. For instance, for IRAS 16365-4812, there are not only emission feature around 11 lm, but also some little bumps around 16.5, 19.5 and 21.5 lm in the relatively good signal-to-noise ratio spectrum. If this is the case, the features may be considered as due to the crystalline silicates which are not surprising for the oxygen-rich objects. Moreover, Molster et al. (2002) pointed out that the crystalline silicates need high temperatures (>900 K) to form. From Table 1 it is seen that most of the sources have the rather earlier sub-types of the spectra that imply a rather high temperature environment to meet the condition of the formation of crystalline silicates. This fact is supported by the interpretation of the emission features as possibly due to crystalline silicates rather than SiC. It is a pity that ISO did not observe all sources here. Furthermore,
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much more is known about the shape of the emission features of various kinds of oxygen-rich dust. For instance, the peak wavelength and breadth of the emission feature depend on the degree of crystallinity, the dust temperature and dust particle size. In general, aluminium oxides are thought to be amongst the first to condense into dust grains, and give rise to emission features peaking longward of 10 m, as do large grains of a few microns in size (see, e.g., Speck et al., 2000). It is also noted that, as mentioned in Section 4, photospheric contributions to the IRAS LRS spectra should be considered, especially around 8–10 lm, where there is the fundamental band of SiO molecular features in absorption at 7.6 lm. (2) From Table 1 it can be seen that besides IRAS 18093-2107, which is discussed in Section 2, IRAS 23080+6058 is also a red supergiant and one would therefore not expect to see any carbonrich dust around it. In fact, Jura and Kleinmann (1990) included this object in their study of red supergiants. From Table 1 it is seen that IRAS 16365-4812 is also a possible red supergiant. Moreover, on the basis of a combination of nearIR and mid-IR colours, Guglielmo et al. (1993) did not classify IRAS 16365-4812 as carbon stars, which may mean that its photosphere and the emission caused by the circumstellar envelope are oxygen-rich. It should be emphasized that although the main interest of this work is in the area of AGB evolution, as mentioned in Section 1, some of the candidates, as described here, are more massive red supergiants, for which the carbon-rich features never existed in their circumstellar envelopes. Another source which should be discussed here is IRAS 07538-3928. Haikala et al. (1994) detected SiO maser emission at 86 GHz (v ¼ 1, J ¼ 2 1). This means that the zone in between the stellar photosphere and the dust envelope is oxygen-rich and that the dust envelope is being replenished with oxygen-rich material which will almost certainly condense to form oxygen-rich dust, because the locations of SiO maser emissions are just above the stellar photosphere within twice to four times the stellar diameter (Habing, 1996). (3) Finally, inspections on the LRS spectra shown in Fig. 1 have been made extensively.
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According to suggestions from van Loon (2003) together with discussions above, it can be primarily concluded for the seven candidates as shown in Fig. 1 that: IRAS 07538-3928: SiO maser emission, silicates and/or aluminium oxides; IRAS 08437-1038: featureless/photospheric; IRAS 09174-6824: too noisy, consistent with featureless/photospheric; IRAS 13011-5604: broad/red silicate emission, possibly at 18 lm too, possible contribution from photospheric SiO absorption; IRAS 16365-4812: possible red supergiant, crystalline silicates, possible contribution from photospheric SiO absorption; IRAS 18093-2107: red supergiant, broad/red silicate emission; IRAS 23080+6058: red supergiant, noisy, broad/red silicate or aluminium oxides emission, possible contribution from photospheric SiO absorption.
6. Summary From the study of the presently available material it appears that there are no strong candidates for oxygen-rich AGB stars with carbon-rich circumstellar dust. The few weak candidates for such objects are consistent with the presence of silicates and/or aluminium oxides and/or photospheric SiO absorption. Nevertheless, the conclusion above should be proven in some way, because of the rather poor signal-to-noise ratio in the LRS spectra presented. In particular, spectral observations with much higher resolution in the infrared should be taken to verify these features for the samples here in the future.
Acknowledgements We thank Dr. van Loon for his helpful advice and suggestions. This work is supported by grants from the National Natural Science Foundation of China (No. 10073018) and the Natural Science Foundation of Yunnan Province
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(No. 2002A0021Q). This work has made use of the NASAs ADS database and the Simbad database.
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