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[MATH] mK. Previously only lower [MATH] transitions of HC N have been reported (Henkel et al., 1985 ; Bujarrabal et al., 1994 ; Fukasaku et al., 1994 ; Woods et al., 2003 and to the best of our knowledge all these detections are new. Because of a gap in the 1.3 mm spectrum, the HC N (27–26) at 245.606 GHz was not detec... |
and ). Opposite to CS and C H lines, HC N lines have a brighter blue wing relative to the red one. This indicates that the molecular envelope is asymmetric along the line of sight, i.e., the chemical compositions and/or physical conditions in the red and blue sides are different. |
[MATH] There are 14 favorable CH CN transitions in the survey region. We detected the blended features of the (12 –11 and the (12 –11 ) transitions with [MATH] mK, which are the strongest two among the 14 CH CN transitions. |
Woods et al. ( 2003 failed to detect CH CN in this object and only estimated the intensity upper limit of the (6 –5 transition. This is therefore the first detection of CH CN in CIT 6 (catalog ) |
All the lines discovered in this survey have been detected in IRC+10216 (catalog ) (He et al., 2008 ; Cernicharo et al., 2000 . Compared to the spectrum of IRC+10216 (catalog ) in the same frequency range, the non-detected species in the spectrum of |
CIT 6 (catalog ) include H, C , C S, C S, H CO, SiC, SiN, PN, and metal containing molecules, all of which have only weak emission in the spectra of IRC+10216 (catalog ) , and thus are below our detection limit. A number of vibrationally excited species have been detected in IRC+10216 (catalog ) |
(see Cernicharo et al., 2000 , and the references therein) For CIT 6 (catalog ) , however, no vibrationally excited lines except those of HCN are strong enough to be detected. Moreover, we find no evidence for the presence of ionic species in CIT 6 (catalog ) |
3.2 Rotation diagram analysis and fractional abundances The standard “rotation-diagram” method was applied to calculate the excitation temperatures ( [MATH] and column densities ( [MATH] ) of the molecules observed in our spectra. From the equation of radiative transfer and assuming that the lines are optically thin, t... |
[EQUATION] [MATH] [MATH] , and [MATH] are the population, degeneracy, and excitation energy of the upper level, [MATH] is the integration of the source brightness temperature over the velocity, [MATH] is the line strength, |
[MATH] is the dipole moment, [MATH] is the line frequency, and [MATH] is the rotational partition function. If several transitions arising from levels covering a wide energy range are observed, [MATH] and [MATH] can be deduced using a straight-line fit to [MATH] versus [MATH] |
The rotation-diagram provides important tools for studies of excitation conditions. Departure from the linear relation can be caused by different excitation mechanisms or misidentification. For SiC , SiS, HC N, and C H, there are adequate numbers of detected transitions covering a wide range of excitation energy and th... |
[MATH] , where [MATH] is the antenna full beam at half-power ( [MATH] ″ and 30″ for the ARO 12 m and the SMT respectively) and [MATH] is the source diameter. |
[MATH] may be different for different species. We followed the assumption by Fukasaku et al. ( 1994 and took an common [MATH] of [MATH] for all the species. Since the source size is likely to vary from species to species (Lindqvist et al., 2000 , this assumption will introduce a [MATH] uncertainty in the derived column... |
of 40 K was assumed for the calculations of their column densities. Assuming that the molecular envelope is a spherical shell, the emission is optically thin, [MATH] is uniform throughout the envelope, mass loss rate and expansion velocity are constant during the formation of the envelope, and the molecular density fol... |
[EQUATION] where [MATH] is given in K km s -1 the full half power beam width [MATH] is in arc sec, the expansion velocity [MATH] is in km s -1 [MATH] |
is the distance in pc, [MATH] is the mass loss rate in M [MATH] [MATH] the line frequency in GHz, [MATH] is the statistical weight of the upper level, [MATH] is the Einstein coefficient for the transition, |
[MATH] is the energy of the lower level, and [MATH] with [MATH] and [MATH] the inner radius and outer radius of the shell. For the calculations, we adopted [MATH] pc (Cohen & Hitchon, 1996 |
For the determination of molecular abundances using eq. 2, we first derive the expansion velocity [MATH] =18 km s -1 from the profile of the CO (2–1) line. The mass loss rate of [MATH] [MATH] is obtained by applying eq. 2 of Winters et al. ( 2002 to the CO (2–1) line assuming [MATH] . These values of [MATH] and [MATH] ... |
The resultant abundances are given in Table. Combined with the uncertainties introduced by calibration, noise, baseline, and source size, we estimate that the errors of the column densities and abundances amount to a factor of [MATH] One should bear in mind that when the emission is optically thick the [MATH] and [MATH... |
Fukasaku et al. ( 1994 and Woods et al. ( 2003 are also given in Table. and no significant discrepancies are found. Discussion 4.1 Chemistry |
4.1.1 Oxygen-bearing molecules Similar to IRC+10216 (catalog ) the molecular envelope of CIT 6 is also characterized by a lack of oxygen-bearing compounds and abundance of carbon-bearing compounds. In IRC+10216 (catalog ) three O-bearing molecules are observed, CO, SiO and HCO |
(Cernicharo et al., 2000 whereas in CIT 6 (catalog ) , only two O-bearing molecules (CO and SiO) are detected. We find that the SiO (6–5)/ 13 CO (2–1) integrated intensity ratio in CIT 6 (catalog ) is 1.05, in excellent agreement with the value of 1.13 in IRC+10216 (catalog ) |
(He et al., 2008 In our discussion, 13 CO is taken as a reference molecule because the 13 CO (2–1) line is likely optically thin and the 12 C/ 13 C isotopic ratios are similar in CIT 6 (catalog ) and IRC+10216 (catalog ) (see Sect. 4.2). SiO has been commonly detected in C-rich envelopes (Schöier et al., 2006 , implyin... |
(Agúndez & Cernicharo, 2006 and/or non-equilibrium chemical processes (Cherchneff, 2006 HCO has a relatively low abundance in IRC+10216 (catalog ) |
(Glassgold, 1996 and is presumably below the current detection limit even if it might be present in CIT 6 (catalog ) 4.1.2 Carbon-bearing molecules |
We observed abundant carbon chains and radicals in CIT 6 (catalog ) , including CO, SiC , CN, HCN, CS, C H, C N, C H, HC N, and CH CN, all of which are linear. This characteristic feature is similar to those of IRC+10216 (catalog ) and |
TMC 1 (catalog ) (see Cernicharo et al., 2000 although these lines are much fainter in CIT 6 (catalog ) The most intriguing characteristic of CIT 6 (catalog ) |
is the strong CN emission. The integrated intensity ratio of the CN (2–1) group and the 13 CO (2–1) transition is 4.6, a factor of 2.2 larger than the value in |
IRC+10216 (catalog ) (He et al., 2008 . CN is mainly formed through the photodissociation of HCN, [EQUATION] According to He et al. ( 2008 , the H 13 CN (3–2)/ 13 CO (2–1) integrated intensity ratio in IRC+10216 (catalog ) is 4.5, a factor of 3.2 larger than that in CIT 6 (catalog ) Therefore, our observations provide ... |
CIT 6 (catalog ) The above discussion also suggests that about 30 [MATH] CN formed from HCN has been destroyed. On the other hand, CN can be reprocessed into HC N through the reaction |
[EQUATION] We do find that the HC N line intensities relative to the 13 CO (2–1) transition in CIT 6 (catalog ) are a factor of [MATH] larger than those in |
IRC+10216 (catalog ) , indicating efficient formation of HC N in CIT 6 (catalog ) We did not find evidence for the enhancement of the C N radical, suggesting that photodissociation of HC N into C N is insignificant in this object. |
CIT 6 (catalog ) shows strong C H emission. The H radical is dominantly produced through the photodissociation reaction [EQUATION] |
Our observations show that the H line intensities relative to the 13 CO (2–1) transition in CIT 6 (catalog ) are almost the same at those in IRC+10216 (catalog ) suggesting that there is no significant C H enhancement in CIT 6 (catalog ) compared to IRC+10216 (catalog ) This probably has an implication that C H is domi... |
(Wootten et al., 1980 . Our results yield a ratio of 4.2 for [MATH] (C H)/ [MATH] (HC N), which is in good agreement with those found in interstellar clouds by Wootten et al. ( 1980 |
and is consistent with the prediction of gas phase chemistry. On the other hand, as shown in Figs. and HC N lines show profiles that differ from those of the C H lines. This suggests that the [MATH] (C H)/ [MATH] (HC N) ratio is not a constant in the envelope. The chemistry structures in the red and blue sides of the s... |
H is positively detected in CIT 6 (catalog ) The C H radical can be formed via [EQUATION] Nejad & Millar ( 1987 suggested that ion-molecule reactions also play an important role for the production of C H, unlike those for HC N. Based on millimeter interferometer observation, |
Dayal & Bieging ( 1993 found that the photochemical model underestimates the C H abundance in IRC+10216 (catalog ) by a factor of 5. Our observations suggest that CIT 6 (catalog ) has a [MATH] (C H)/ [MATH] (HC N) abundance ratio of 3.1, which is lower than that in IRC+10216 (catalog ) |
(He et al., 2008 by a factor of [MATH] . On the other hand, H can be photodissociated into C H. However, as no enhancement of C H is found in CIT 6 (catalog ) , destruction of C H should be insignificant in this C-rich envelope. Therefore, the high radical abundance of C H in IRC+10216 (catalog ) |
still remains mystery. CH CN is a new finding for this C-rich envelope. This molecule is a symmetric top and has been widely used as diagnostics of excitation temperature. We detected only one weak CH CN line in CIT 6 (catalog ) , and thus cannot use it to derive an excitation temperature. CH CN can be produced via |
[EQUATION] followed by [EQUATION] The formation of CH CN may be very efficient in CIT 6 (catalog ) since strong HCN emission is detected. |
CIT 6 (catalog ) has a CH CN (12–11)/ 13 CO (2–1) integrated intensity ratio of 0.058, in good agreement with the value of 0.043 in IRC+10216 (catalog ) |
(He et al., 2008 Strong CS emission has been detected in our survey. According to Willacy & Cherchneff ( 1998 , CS can be rapidly destroyed by shocks which might occur after a star leaves the AGB stage and ejects material in a very fast wind |
(Herpin et al., 2002 . Therefore, the high abundance of CS in CIT 6 (catalog ) suggests that shocks are not important for the chemistry in this C-rich envelope. |
4.1.3 Silicon-bearing molecules Three refractory Si-bearing species (SiO, SiS, and SiC ) were detected in CIT 6 (catalog ) . Although other Si-bearing species were detected in |
IRC+10216 (catalog ) (Cernicharo et al., 2000 , emission from the other Si-bearing species is relatively faint and should be below the detection limit of our observations of CIT 6 (catalog ) |
We find that the abundances of SiO and SiC in CIT 6 (catalog ) are similar to those in IRC+10216 (catalog ) determined by He et al. ( 2008 . Although the situation may be complicated by optical depth effects, the HCN/SiO line intensity ratio has the potential to provide a useful tool to discriminate between C-rich and ... |
is 8.4, in good agreement the value of 9.7 in IRC+10216 (catalog ) (He et al., 2008 . There is no evidence showing that the HCN/SiO line intensity ratio has dependance on the mass loss rate of C-rich stars. |
González Delgado et al. ( 2003 and Schöier et al. ( 2006 found a correlation between the mass loss rate and the SiO abundance for AGB stars. This is described as freeze-out of SiO molecules onto dust grains. The similarity of the SiO abundances in CIT 6 (catalog ) and IRC+10216 (catalog ) |
suggests that the depletion of SiO onto dust grains might be insignificant for the two C-rich envelopes. Our observations show that the SiS abundance in CIT 6 (catalog ) |
is lower than that in IRC+10216 (catalog ) . The SiS (14–13)/SiO (6–5) intensity ratio in CIT 6 (catalog ) is 1.1, about half of that in IRC+10216 (catalog ) |
found by He et al. ( 2008 Schöier et al. ( 2007 did not find a strong correlation between the mass loss rate and the SiS abundance, suggesting that SiS molecules are less likely to be depleted onto dust grains than SiO molecules. Therefore, freeze-out should not be the reason for the depletion of SiS in CIT 6 (catalog ... |
has destroyed SiS, and silicon chemistry has been ongoing in this evolved C-star envelope. Interferometric observations of these Si-bearing molecules are obviously needed to verify the conjecture. |
4.2 Isotopic ratios Isotopic ratios of various elements provide substantial tests for nucleosynthesis of low- and intermediate-mass stars (LIMS). When a LIMS evolves into the AGB stage, the nucleosynthesized products synthesized through the CNO cycle inside the star are dredged up to the surface and then are ejected in... |
CIT 6 (catalog ) The results are listed in Table . The errors estimated from the measurement and calibration are given. For comparison, we also list the isotopic ratios for IRC+10216 (catalog ) |
(Cernicharo et al., 2000 and the Sun (Lodders, 2003 4.2.1 Carbon The 12 C/ 13 C abundance ratio is the most studied isotopic abundance in LIMS. Standard stellar models predict that the 12 C/ 13 C abundance ratio can be significantly increased during the nucleosynthesis and dredge-up processes in the AGB stage. However,... |
(e.g. Charbonnel & do Nascimento, 1998 Charbonnel ( 1995 proposed an extra mixing process to account for the low 12 C/ 13 C. In low-mass AGB stars, the nonstandard mixing called cool bottom processing may decrease the 12 C/ 13 C ratio to [MATH] |
(Sackmann & Boothroyd, 1999 ; Boothroyd & Sackmann, 1999 . For AGB stars more massive than [MATH] , the hot bottom burning may take place and induce 12 C/ 13 C to further decrease to its equilibrium value of [MATH] |
(Frost et al., 1998 . Current observations of the CO isotopologues in PNs (Balser et al., 2002 ; Josselin & Bachiller, 2003 suggest that the 12 C/ 13 C ratio is in the range of 2.2–40, supporting the theory including nonstandard mixing processes. |
Three 13 C-bearing species have been detected in this survey, including 13 CO, 13 CS, and H 13 CN. However, their main lines are likely optically thick. Therefore, the abundance ratios of CO, CS, and HCN and their isotopologues only provide lower limits of the 12 C/ 13 C ratio. Our results are in good agreement with th... |
Sopka et al. ( 1989 who derived the abundance ratios 12 CO/ 13 CO [MATH] and 12 CN/H 13 CN [MATH] for CIT 6 (catalog ) We have detected the rare isotopes, 12 34 S and |
13 32 S. If the 32 S/ 34 S abundance ratio were known, we could obtain the 12 C/ 13 C ratio using the two optically thin species. Cernicharo et al. ( 2000 |
found that the sulfur isotopic ratios in IRC+10216 (catalog ) are close to solar. Therefore, we reasonably assume that the 32 S/ 34 S ratio in CIT 6 (catalog ) |
is the solar value. It follows that we obtained the 12 C/ 13 C ratio of [MATH] in CIT 6 (catalog ) which is in perfect agreement with that found in |
IRC+10216 (catalog ) and is significantly lower than the solar value. The 12 C/ 13 C ratios found in these C-rich envelopes are also lower than the value of [MATH] in the Orion Bar proposed by Keene et al. ( 1998 using the 18 O/ 13 18 O abundance ratio. This is consistent with the hypothesis that nonstandard mixing pro... |
As shown in Table , different 12 C/ 13 C values are obtained for CIT 6 (catalog ) if different species are used for the calculations. If completely ascribing the 12 C/ 13 C discrepancies found for CIT 6 (catalog ) to the opacity effects of the main lines, we can estimate the optical depths of the CO (2–1), CS (3–2), an... |
4.2.2 Oxygen The nucleosynthesis and dredge-up processes in the AGB stage can lead to strong enrichment of 17 O relative to 16 O and 18 |
(see Busso, 2006 , for a recent review) Wannier & Sahai ( 1987 found that the 17 O/ 18 O ratios in C-rich envelopes are markedly higher than the terrestrial and interstellar values, but the |
16 O/ 18 O ratios are comparable to the solar value. We have detected C 17 O, allowing us to derive the 16 O/ 17 O ratio in CIT 6 (catalog ) The C 16 O/C 17 O abundance ratio give a lower limit of |
[MATH] . We may also use the optically thin species 13 16 O and 12 17 to derive the 16 O/ 17 O ratio. Assuming 12 C/ 13 [MATH] (see above), we obtain a |
16 O/ 17 O ratio of [MATH] Cernicharo et al. ( 2000 did not obtain the oxygen isotopic ratios. Kahane et al. ( 1992 calculated the 16 O/ 17 O ratio for a sample of C-rich envelopes. They found that 16 O/ 17 [MATH] |
and [MATH] for CIT 6 (catalog ) and IRC+10216 (catalog ) respectively. These values are in good agreement with our result, agree with each other, and are lower than the solar value by a factor of about three, which is consistent with predictions of stellar models. |
No 18 O-bearing species was detected. The non-detection of the C 18 O (2–1) transition at 219560 MHz seems to suggest that 17 O/ 18 [MATH] in CIT 6 (catalog ) . However, this result should be taken with some caution since the C 18 O line is very close to the edge of the spectrum. |
4.2.3 Silicon and sulfer The nucleosynthesis in LIMS is expected to hardly affect the elements in the 3rd row of the periodic table, such as Si and S. Cernicharo et al. ( 2000 indeed found that the Si and S isotopic ratios in IRC+10216 (catalog ) are compatible with the solar values. |
The silicon isotopes, 29 Si and 30 Si, have been detected through faint emission from 29 SiO, 30 SiO, and 29 SiC . Since the SiO and SiC lines are probably optically thick, the lower limits of the 28 Si/ 30 Si and 28 Si/ 29 Si ratios were derived. Given the low abundances of 29 Si and 30 Si, the 29 Si/ 30 Si ratio is n... |
The C 32 S/C 34 S ratio gives a lower limit of 32 S/ 34 S in CIT 6 (catalog ) . C 33 S was only marginally detected. The two optically thin species, 33 S and C 34 S, give a 33 S/ 34 S ratio of [MATH] . Consequently, we did not find a significant deviation of the S isotopic ratios in CIT 6 (catalog ) from those in |
IRC+10216 (catalog ) and in the Sun. 4.3 Is IRC+10216 unique? Given its brightness and abundant molecular emission, IRC+10216 (catalog ) |
is the most surveyed object and has frequently served as a standard reference for studying circumstellar chemistry. A commonly asked question is whether IRC+10216 (catalog ) can truly represent C-rich AGB stars. To investigate the problem, we systematically compare the spectra of CIT 6 (catalog ) with those of IRC+1021... |
We find that all lines detected in this work have also been observed in IRC+10216 (catalog ) and all the strong lines detected in IRC+10216 (catalog ) are seen in CIT 6 (catalog ) . In Fig. , we compare the intensity ratios of the lines detected in both objects. The results show that the line intensity ratios in the tw... |
CIT 6 (catalog ) is a more compact object and thus suffers from a larger beam dilution effect. If we correct for the beam dilution effect, the [MATH] (CIT 6)/ [MATH] (IRC+10216) ratios should increase by a factor of [MATH] , where |
[EQUATION] Assuming [MATH] and [MATH] (Fukasaku et al., 1994 and using [MATH] and 30 ′′ for the ARO 12m and the SMT respectively, we obtain the [MATH] values of 1.8 and 1.6 for the 12 m and SMT data, respectively. While recognizing that eq. 9 assumes uniform brightness temperature which is probably not realistic, the r... |
Fig. also shows that there are a few molecular species for which the [MATH] (CIT 6)/ [MATH] (IRC+10216) ratios depart from the average value. For the CN and HC N lines, the ratios are higher, whereas for HCN, SiS, and C H, the ratios are lower. As discussed in Sect. 4.1.2 , this partly reflects the chemical evolution i... |
CIT 6 (catalog ) . However, the cause of the abnormally strong H emission in IRC+10216 (catalog ) remains unknown. Far-IR spectra of CIT 6 (catalog ) and IRC+10216 (catalog ) have been obtained by the ISO Long Wavelength Spectrometer (LWS) |
(Schöier et al., 2002 ; Cernicharo et al., 1996 . The rotational transitions revealed by the ISO spectra can trace the inner regions of the circumstellar envelopes (Herpin et al., 2002 In Fig we compare the far-IR spectra of the two objects. The spectra were retrieved from the ISO archive. Inspection of the figure show... |
Fig. gives the continuum-subtracted ISO LWS spectra of the two C-rich envelopes. A number of lines from CO, HCN, H 13 CN, and vibrationally excited HCN have been identified by Cernicharo et al. ( 1996 in the spectrum of IRC+10216 (catalog ) . Fig. shows that the relative flux ratios of the lines detected in the two obj... |
IRC+10216 (catalog ) is a factor of about 20 times higher than that of CIT 6 (catalog ) , as shown in Fig . This probably suggests that IRC+10216 (catalog ) has a larger dust-to-molecular gas ratio. |
4.4 Abundance variations expected in later evolutionary stages Since CIT 6 (catalog ) has been proposed to be a highly evolved AGB star on the verge to become a Proto-PN (or PPN) (Schmidt et al., 2002 , it would be useful to compare the molecular abundances of CIT 6 (catalog ) with the corresponding molecular abundance... |
has abundant molecular emission. It is one of the brightest molecular sources in the sky and is an ideal target for investigating circumstellar chemistry (see e.g. Pardo et al., 2007 . All three objects are carbon rich, and are commonly assumed to belong to a common evolutionary sequence. Any systematic difference in m... |
Fig. gives the fractional abundances of the species in the three objects as a function of their photodissociation rates taken from the UMIST database for a temperature of 300 K. The abundances in IRC+10216 (catalog ) and CRL 618 (catalog ) |
are taken from Woods et al. ( 2003 and Pardo et al. ( 2007 , respectively. To facilitate the comparison, we follow Pardo et al. ( 2007 and give the molecular abundances relative to HC N. According to this figure, except for CN and HCN, the molecular abundances in CIT 6 (catalog ) are obviously closer to those in |
IRC+10216 (catalog ) than to CRL 618 (catalog ) CRL 618 (catalog ) has lower abundances of SiO and CS, suggesting that destruction by shocks and depletion onto dust grains may play an important role for the chemistry in this object. Compared to the other two objects, |
CRL 618 (catalog ) , having a B0 central star, is exposed in a stronger UV radiation field. However, we do not find a correlation between the abundance differences of these objects and the molecular photodissociation rates, suggesting that the destruction by photodissociation is not a factor affecting the chemistry in ... |
We should note that this study is limited to simple molecules in the gas phase. There is strong evidence from infrared spectroscopy that the solid-state phase chemistry is very active in the AGB-PPN evolutionary transition, with many aromatic and aliphatic compounds being formed in the circumstellar envelope (Kwok, 200... |
Conclusions The presence of rich molecular species around evolved stars provides an opportunity to study the evolution of chemistry in circumstellar envelopes, which have been widely suggested as one of the main sources of organic compounds in space. As part of our project of investigating circumstellar chemistry, this... |
We find that the line profiles for some molecular species have different shapes, suggesting that the chemical structure is asymmetric in the envelope. A comprehensive 3D photochemistry model is required to account for the line intensities and profiles in CIT 6 (catalog ) |
The excitation temperatures, column densities and abundances of the detected molecules are determined through rotation-diagram analysis. The spectra of CIT 6 (catalog ) are characterized by a large CN/HCN abundance ratio. Our results suggest that there is evidence for the photodissociation of HCN and SiS and the format... |
In order to investigate whether the molecular environment of IRC+10216 (catalog ) is intrinsically unique, we systematically compare its spectra with those of |
CIT 6 (catalog ) . According to the comparison, we find that the molecular species can be classified into three groups, a) for most of the species, the intensity ratios of individual lines in the two objects are in good agreement with each other; b) the emission from HC N and CN may be enhanced in |
CIT 6 (catalog ) c) the emission from SiS, HCN, and C H may be depleted in CIT 6 (catalog ) . The differences of the line-intensity ratios in the two objects are probably a consequence of chemical evolution with the exception of C H, for which a high abundance in IRC+10216 (catalog ) cannot be explained by photochemica... |
CIT 6 (catalog ) has a lower continuum-to-line ratio than IRC+10216 (catalog ) , suggesting that the latter might have a larger dust-to-molecular gas ratio. |
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