Mixed chemistry phenomenon during late stages of stellar evolution
Baltic Astronomy, vol. 16, xxx–xxx, 2007.
MIXED CHEMISTRY PHENOMENON DURING LATE STAGES
OF STELLAR EVOLUTION
R. Szczerba, M.R. Schmidt, M. Pulecka1
1 Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń,
Poland
Received 2006 October 15; revised —
Abstract. We discuss phenomenon of simultaneous presence of O- and C-
based material in surroundings of evolutionary advanced stars. We concentrate
on silicate carbon stars and present observations that directly confirm the binary
model scenario for them. We discuss also class of C-stars with OH emission
detected, to which some [WR] planetary nebulae do belong.
Key words: stars: Asymptotic Giant Branch, carbon stars, chemical com-
position, planetary nebulae, stars: individual (V778 Cyg, IRAS 04496−6859,
IRAS 06238+0904, M 2−43)
1. INTRODUCTION
During Asymptotic Giant Branch (AGB) phase of evolution stars with ini-
tial masses between 0.8 and 8M⊙ lose a significant amount of their initial mass
by ejecting the matter into interstellar space with rates between 10−7 and 10−4
M⊙ yr
−1. The chemistry in the formed circumstellar envelopes is determined by
the photospheric C/O ratio and is O-based for n(O)>n(C) (usually less evolution-
ary advanced stars) and C-based when carbon abundance exceeds that of oxygen
(evolved stars which experienced thermal pulses and dredged-up carbon to the
surface). This dichotomy is a consequence of CO molecule (very stable) formation
which is so efficient that less abundant element (C or O) is mostly used. There-
fore, the detection of co-existence of O-rich and C-rich material in surroundings
of evolved stars was (and still is) surprising and attracts a significant attention.
Hereafter, we call this phenomenon a mixed chemistry phenomenon.
Already, due to the IRAS observations it was realized that there is a group
of carbon stars which show typical for O-rich environment the 9.7 and 18µm
amorphous silicate features (Little-Marenin 1986, Willems & de Jong 1986). The
Infrared Space Observatory (ISO) observations (Yamamura et al., 2000) showed
that 9.7µm feature in one of such objects (V778 Cyg) is very stable and did not
change during the last 15 years (the time spanned between IRAS and ISO obser-
vations). This put a very strong constraint on a model and evolutionary status
of this class of objects with most likely explanation being a long-lived reservoir
of O-rich material located inside or around a binary system. In this review we
discuss MERLIN interferometer observations of V778 Cyg which proved existence
of such reservoir (disk) around companion of C-rich star. We note that the recent
Spitzer Space Telescope (SST) data showed that the first extra-galactic silicate
http://arxiv.org/abs/0704.0557v1
2 R. Szczerba, M.R. Schmidt, M. Pulecka
carbon star (IRAS 04496−6859, Trams et al. 1999) is in fact a normal carbon star
and do not show the 9.7µm dust emission (see Speck et al. 2006).
There is another group of carbon stars suspected to have mixed chemistry.
Namely, carbon stars with OH maser emission. Lewis (1992) listed a group of stars
with SiC emission seen in the IRAS Low Resolution Spectra (LRS) and OH maser
emission detected. While most of these sources appeared to have wrong LRS clas-
sification the 3 C-stars with OH maser emission remained and Chen et al. (2001)
added 6 more sources to this class. However, this class of mixed chemistry sources
did not attract a significant attention (except of [WR] planetary nebulae – see
below), since OH emission is not well spatially resolved and this group of sources
may be result of spatial coincidence between OH maser emission from interstellar
medium and location of C-star. For example, Szczerba et al. (2002) presented
observational evidence that IRAS 05373−0810 (C-star with OH maser emission)
is a genuine carbon star and that OH maser and SiO thermal emission detected
toward this star is not coming from its envelope, but from molecular clouds. Here
we discuss a case of another C-star with OH maser emission (IRAS 06238+0904)
toward which we have detected, using IRAM radiotelescope, the SiO thermal emis-
sion coming from its envelope. Here, we present arguments that shock and Photon
Dominated Region (PDR) chemistry allow to form a significant amount of SiO in
C-rich environment.
One of the most important achievements of the ISO mission was detection of
crystalline silicates. Surprisingly, crystalline silicates were detected also in [WR]
planetary nebulae, which have H-poor and C-rich central stars of WR-type1. [WR]
planetary nebulae show at the same time presence of Polycyclic Aromatic Hydro-
carbons (PAHs) and crystalline silicates (Waters et al. 1998, Cohen et al. 1999).
Scenarios proposed to explain simultaneous presence of PAHs and crystalline sil-
icates include: destruction of fossil comets orbiting the star, ejection of matter
before star become C-rich, formation of stable O-rich disk or torus around com-
panion or system at some point of binary evolution. Hajduk, Szczerba & Gesicki
(this Proceedings) present an attempt to determine spatial location of PAHs and
crystalline silicates inside the [WR] planetary nebula M 2-43, by means of the
radiative transfer modelling of ISO spectrum. They concluded that crystalline
silicates have to be located at significant distance from the central star to avoid
their emission at about 10 µm. We note also an attempt to find precursors of [WR]
planetary nebulae (C-rich stars with C- and O-rich material in their circumstellar
shells) among proto-planetary nebulae (Szczerba et al. 2003). The authors have
argued that formation of crystalline silicates is necessary before proto-planetary
nebula phase, while post-AGB star may be still O-rich and change to C-rich one
during the fatal thermal pulse. They indicated five proto-planetary nebulae as a
possible precursors of [WR] planetary nebulae, including famous Red Rectangle,
other C-rich source with crystalline silicates (IRAS 16279-4757), as well as three
O-rich sources which show presence of crystalline silicates in their ISO spectra:
AC Her, IRAS 18095+2704 and IRAS 19244+1115.
In this review we will not cover such cases as: NGC 6302 – O-rich planetary
nebula which show presence of crystalline silicates as well as PAHs (e.g. Kemper
et al. 2002); HD 233517 – an evolved O-rich red giant with orbiting polycyclic
1Note, that Zijlstra et al. (1991) detected OH maser emission from [WR] planetary nebula
IRAS 07027−7934. Therefore, at least this [WR] planetary nebula belongs also to the discussed
above class of C-stars with OH maser emission.
Mixed chemistry phenomenon 3
aromatic hydrocarbons (Jura et al. 2006); IRAS 09425−6040 – a carbon-rich AGB
star with the highest abundance of crystalline silicates detected up to now (Molster
et al. 2001); IRC +10216 – a well known C-rich AGB star with water and OH
maser lines detected (Melnick et al. 2001, Ford et al. 2003); and possibly some
other spectacular sources which we, not intentionally, have overlooked.
2. V778 CYG A SILICATE CARBON STAR
To test the hypotheses related to the mixed chemistry phenomenon observed in
silicate carbon stars, we observed water masers towards V778Cyg at high angular
resolution. Details of our observations and data analysis are presented by Szczerba
et al. (2006), so here we repeat only some of the most important points and
findings.
The observations were taken on 2001 October 12/13 under good weather con-
ditions, using five telescopes of MERLIN. The longest MERLIN baseline of 217 km
gave a fringe spacing of 12mas at 22.235080GHz. The bandwidth was 2MHz with
256 spectral channels per baseline providing a channel separation of 0.105km s−1.
The continuum calibrator sources were observed in 16MHz band with 16 channels.
The data were obtained in left and right circular polarisation and the velocities
were measured with respect to the local standard of rest.
We used the phase referencing method; 4min scans on V778Cyg were inter-
leaved by 2min scans on the source 2021+614 (at 3.◦8 from the target) over 11.5 h.
The flux density of 2021+614 at K band of 1.48 Jy was derived from observation of
4C39.25. At the epoch of observation the flux density of 3C39.25 was 7.5±0.3Jy
(Terasranta 2002, private communication). This source was also used for bandpass
calibration.
After initial calibration with the MERLIN software the data were processed
using the AIPS package. In order to derive phase and amplitude corrections for
atmospheric and instrumental effects the phase reference source was mapped and
self-calibrated. These corrections were applied to the target visibility data. The
absolute position of the brightest feature at −15.1 km s−1 was determined. The
phase solutions for this feature were obtained with self-calibration method and
were then applied for the all channels. The target was mapped and cleaned using
a 12mas circular restoring beam. The map noise of ∼27mJy beam−1 for I Stokes
parameter in a line-free channel was close to the predicted thermal noise level.
In order to determine the position and the brightness of the maser components
two dimensional Gaussian components were fitted to the emission in channel maps.
The position uncertainty of this fitting depends on the signal to noise ratio in the
channel map and is lower than 1mas for about 80% of the maser components to-
wards V778Cyg. The absolute position of the phase reference source is known with
an accuracy of ∼3mas. The highest uncertainties in the absolute position of maser
spots are due to tropospheric effects and errors in the telescope positions. The
first effects, estimated by observing the phase rate on the point source 3C39.25,
introduce the position error of ∼9mas, whereas uncertainties in telescope positions
of 1−2 cm cause an error of spot positions of ∼10mas. In order to check the posi-
tion accuracy of maser spots we applied a reverse phase referencing scheme. The
emission of 15 channels around the reference feature at −15.1 km s−1 was averaged
and mapped. The map obtained was used as a model to self-calibrate the raw tar-
get data then these target solutions were applied to the raw data of 2021+614.
4 R. Szczerba, M.R. Schmidt, M. Pulecka
The position of the reference source was shifted by ∼2mas with respect to the
catalogue position. This indicates excellent phase connection when referencing
2021+614 to the set of the brightest maser spots. The above discussed factors im-
ply the absolute position accuracy of the maser source to be of order of ∼25mas.
Fig. 1. The absolute positions of the H2O 22GHz maser components towards
V778Cyg relative to the reference feature at −15.1 km s−1 (RA(J2000) = 20h
36m07.s3833, DE(J2000) = 60◦05′26.′′024). The symbols indicate the ranges of
component velocities in km s−1. The size of each symbol is proportional to the
logarithm of peak brightness of the corresponding component.
The maser emission brigther than 150mJ beam−1 (∼ 5σ) was found in 51 spectral
channels. In these channels single and unresolved component only was detected.
The overall structure of the H2O maser is shown in Fig. 1. The maser com-
ponents form a distorted ”S” like shape structure along a direction of position
angle of about −10◦. There is a clear velocity gradient along this structure with
weak south components blueshifted with respect to the brightest north compo-
nents. The angular extend of maser emisson is 18.5mas. The axis of alonga-
tion of the maser structure is fairly perpendicular to the line towards the op-
tical position of V778Cyg measured by Tycho2 (see Fig. 2). Angular separa-
tion between the optical star and the maser reference component is 0.192±0.′′048.
Mixed chemistry phenomenon 5
200 150 100 50 0 -50
Relative RA [mas]
Tycho-2 
MERLIN 
Fig. 2. Comparison of th optical position of V778 Cyg as determined in the
Tycho-2 catalogue with the radio position of the H2O 22 GHz maser components
as obtained from the MERLIN measurements. The epochs of optical and radio
observations differ by about 10 yrs.
Szczerba et al. (2006) have argued that such separation cannot be explained by
proper motion and instead provide direct observational evidence for the binary
system model of Yamamura et al. (2000). They suggested that the observed wa-
ter maser components can be interpreted as an almost edge-on warped Keplerian
disk located around a companion object and tilted by no more 20◦ relative to the
orbital plane. More detailed model of disk around companion in V778 Cyg system
is presented by Babkovskaia et al. (2006). Finally, note that recently Ohnaka
et al. (2006) reported indirect detection of disk around another silicate carbon
star (IRAS 08002−38003). They argued that oxygen-rich material is stored in
circumbinary disk surrounding the carbon-rich primary star and its putative low-
luminosity companion. These two findings may suggest that there are two different
kinds of silicate carbon stars: with circumbinary disk and disk around companion
only.
3. IRAS 06238+0904 - AN OH MASER C-STAR OR GENUINE CARBON STAR?
Genuine carbon stars are formed during evolution on AGB. The star on that
stage posses extended circumstellar envelope (CSE). In its inner part (near the
photosphere) physical conditions (T∼2500 K, ρ ∼ 1014 cm−3) make the material
mainly molecular, with composition determined by the local thermodynamic equi-
librium (LTE). In carbon CSE (C/O>1) after CO formation there is almost no
oxygen left. However silicon monoxide (SiO) is observed in carbon stars. Recent
observations (Schöier et al. 2006) show relatively high SiO fractional abundances
(1× 10−7− 5× 10−5), while LTE models give ∼ 5× 10−8 (Millar 2004). Therefore
6 R. Szczerba, M.R. Schmidt, M. Pulecka
the non-equilibrium processes should be considered in modelling of circumstellar
chemistry.
In this review we focus on IRAS 06238+0904 – an OH maser C-star (see Chen
et al. 2001). We first built model of carbon circumstellar envelope (CSE) and
then computed radiative transfer in molecular rotational lines of HCN J=1-0, CS
J=3-2, CS J=5-4 and SiO J=3-2, detected by us with the IRAM radiotelescope.
Spectral energy distribution (SED) for IRAS 06238+0904 was modelled by
means of the code and method described in Szczerba et al. (1997). The best fit (see
Fig. 3) is obtained for the star’s effective temperature T∗=2500 [K], luminosity to
distance ratio L/d2=6500 [L⊙kpc
−2], mass loss rate Ṁ = 2×10−5 [M⊙ yr
−1], dust
temperature at the inner boundary Tdust(R
in )=900 [K], amorphous carbon (AC)
and silicon carbide (SiC) to gas ratios: ρ(AC)/ρgas=0.001, ρ(SiC)/ρgas=0.00019.
Fig. 3. Spectral energy distribution for IRAS 06238+0904. See text for details
concerning assumed and estimated parameters.
The chemical model is computed with the network based on RATE99 database
Le Teuff et al. (2000) composed of 343 species made of 10 elements. The gas tem-
perature profile is approximated by the power law function r−1.8 established by
iterations from the best fits to CS lines. We assume solar gas composition with
modifications of carbon (C/O=1.5) and sulfur ǫ(S)=6.71 abundances. As initial
concentrations we put LTE values of 23 important species, where SiO number den-
sity is equal 1×10−8 [cm−3] The effect of dust formation is included by reduction
of Si and C by amount locked up in SiC and amorphous carbon grains according
to dusty model. This results in decrease of silicon abundance to ǫ(Si)=7.39 and
decrease of C to O ratio to 1.3.
Mixed chemistry phenomenon 7
The radiative transfer is computed in Sobolev approximation with molecular
data taken from the Leiden database (Schöier et al. 2005). Only interstellar
radiation is taken into account as an important source of UV photons. Level
populations of investigated molecules were computed for the assumed temperature
and molecular densities resulting from chemical model. The half-width main beam
(HPBW) for SiO rotational transition J=3-2 (v=130 GHz) is equal to 18.9′′, 16.7′′
for CS(3-2), 10.0′′ for CS(5-4) and 28.9′′ for HCN(1-0) transition, in case of the
IRAM telescope observations. The synthetic profile was computed for assumed
distance to IRAS 06238+0904 being equal 2.3 kpc.
The observed and obtained molecular line profiles of SiO(3-2), CS(3-2) and
CS(5-4) are shown in 3 panels of Fig. 4. During line profiles modelling we included
simple treatment of CO self-shielding based on Mamon et al. (1988). This process
has considerable influence on all molecules, and is especially important for SiO.
As one can see in Fig. 4, when self-shielding is not included (solid line) we can
explain observed spectrum solely by the PDR chemistry. Around 1×1016 [cm]
we observed considerable reproduction of SiO. On the other hand, inclusion of
CO self-shielding prevents formation of SiO (dashed line). Partial reproduction in
PDR is still present. In both cases the exchange reaction OH + Si → SiO + H is
a main process responsible for formation of silicon monoxide. Exchanges between
atomic oxygen and SiH, SiC, and HCSi molecules (O + SiH → SiO + H, O +
HCSi → SiO + CH, and O + SiC → SiO + C) are also important.
Simulation of the shock passage (see Willacy & Cherchneff 1998) enlarge initial
abundance of SiO, in comparison to the LTE value, for about one order. Profile
obtain with abundance of this molecule increased by factor of 10 is shown as
dashed-dotted line in Fig. 4.
Fig. 4. Observed and modelled molecular rotational lines without (solid line)
and with (dashed line) CO self-shielding. Dashed-dotted line in the left panel
show results when the intial LTE abundance of SiO is increased ten times due to
the shock passage.
Therefore, we can conclude that IRAS 06238+0904 is a genuine C-star and
no assumption of mixed chemistry is necessary. Chemical reactions considered in
network can reproduce O-bearing SiO molecule in C-rich environment if no CO
self-shielding is considered. In presence of CO self-shielding the computed SiO
8 R. Szczerba, M.R. Schmidt, M. Pulecka
emission is too low. This may be improved, however, if we consider the effect of
shock passage which can increase the initial SiO abundance by order of magnitude
as predicted by Willacy & Cherchneff (1998).
ACKNOWLEDGMENTS. This work has been supported by grants 2.P03D.017.25
and 1.P03D.010.29 of the Polish State Committee for Scientific Research.
REFERENCES
Babkovskaia N., Poutanen J., Richards A. M. S., Szczerba R. 2006, MNRAS, 370,
Chen P. S., Szczerba R., Kwok S, Volk K. 2001, AA, 368, 1006
Cohen M., Barlow M. J., Sylvester R. J. et al. 1999, ApJ, 513, L135
Ford K. E. S., Neufeld D. A., Goldsmith P. F., Melnick G. J. 2003, ApJ, 589, 430
Jura M., Bohac C. J., Sargent B. et al. 2006, ApJ, 637, L45
Lewis B. M., 1992, ApJ, 396, 251
Le Teuff Y. H., Millar T. J., Markwick A. J. 2000, A&AS, 146, 157
Little-Marenin I. 1986, AA, 307, L15
Mamon G. A., Glassgold A. E., Huggins P. J., 1988, ApJ, 328
Melnick G. J., Neufeld D. A., Ford K. E. S. et al. 2001, Nature, 412, 160
Millar T. J. 2004, in AGB stars, eds. H. J. Habing, H. Olofsson, 247
Molster F. J., Yamamura I., Waters L. B. F. M. et al. 2001, AA, 366, 923
Ohnaka K., Driebe T., Hoffman K.-H. et al. 2006, AA, 445, 1015
Schöier F., Olofsson H., Lundgren A. 2006, AA, 454, 247
Schöier F., van der Tak F., van Dishoeck E., Black J. H. 2005, AA, 432, 369
Speck A., Cami J., Markwick-Kemper C. et al., 2006, ApJ, 650, 892
Szczerba R., Chen P. S., Szymczak M., Omont A. 2002, AA, 381, 491
Szczerba R., Omont A., Volk K. et al. 1997, AA.,317,859
Szczerba R. Stasińska G., Siódmiak N., Górny S. K. 2003, in Exploiting the ISO
Data Archive. Infrared Astronomy in the Internet Age, eds. C. Gry, S. Peschke,
J. Matagne, P. Garcia-Lario, R. Lorente, A. Salama, ESA SP-511, 149
Szczerba R., Szymczak M., Babkovskaia N. et al. 2006, AA, 452, 561
Trams N. R., van Loon J. Th., Zijlstra A. A. et al. 1999, AA, 344, L17
Waters L. B. F. M., Beintema D. A., Zijlstra A. A. 1998, AA, 331, L61
Willacy K., Cherchneff I., 1998, AA, 330, 676
Willems F. J., de Jong T. 1986, ApJ, 309, L39
Yamamura I., Dominik C., de Jong T. 2000, AA, 363, 629
Zijlstra A. A., Gaylard M. J., Te Lintel Hekkert P. et al. 1991, AA, 243, L9
