A&A 367, L1-L4 (2001)
J. Cami1,2 - I. Yamamura3
1 - Astronomical Institute "Anton Pannekoek'', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam,The Netherlands
2 - SRON-Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
3 - The Institute of Space and Astronautical Science (ISAS), Yoshino-dai 3-1-1, Sagamihara, Kanagawa 229-8510, Japani
Received 30 November 2000 / Accepted 22 December 2000
We report the discovery in the ISO/SWS spectrum of the post-AGB star HR 4049 of emission bands due to 17O and 18O isotopes locked up in CO2 molecules. It is the first time these isotopomers are detected outside the solar system. Isotopic ratios derived in the optically thin limit are as low as 16O/17O and 16O/ . These values are at least one order of magnitude lower than any previously determined isotopic ratio in any type of evolved star.
Key words: stars: AGB and post-AGB - stars: circumstellar matter -
stars: individual: HR 4049 -
stars: abundances - stars: late-type - infrared: stars
It is well known that the spectra of several O-rich AGB stars exhibit
a few strong CO2 bands in this wavelength range, most often seen in
emission (Cami et al. 1997,1998; Ryde et al. 1998; Cami et al. 2000; Markwick & Millar 2000).
In the spectrum of HR 4049, all previously observed bands are
present and in emission; it is obvious from first inspection however
that the CO2 spectrum of HR 4049 is much richer in lines than any
other object observed so far (see Fig. 1).
|Figure 1: Parts of the ISO/SWS spectrum of HR 4049 (top curve) between 13-17 m showing the strongest CO2 emission bands. The spectrum is continuum subtracted and subsequently scaled to the peak flux in each plot window. Error bars on the individual datapoints are indicated. For comparison we also show the spectrum of EP Aqr, rebinned to the same resolution as the spectrum of HR 4049. An LTE model spectrum is then shown for each CO2 isotopomer, with model parameters T=600 K, N=1019 cm-2. All model spectra are scaled to the peak flux in each plot window and should therefore only be used for identification purposes. The features observed at 14.0 and 14.31 m are close to the expected positions for the two strongest HCN bands in this wavelength range, as observed in carbon stars. However, LTE models of these bands show that the peak positions of both features are off by at least 0.02 m indicating that HCN is not the major constituent of these features|
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When comparing to the spectra of O-rich AGB stars like EP Aqr (Cami et al. 2000), the hot bands observed in the spectrum of HR 4049 are unusually strong compared to the 14.98 m band and much broader, indicating a contribution from other species.
In addition to the bands due to the 12C16O2 and 13C16O2 isotopomers there seem to be other bands present in the 13-17 m region in the ISO/SWS spectrum of HR 4049. Our attention was
caught by a conspicuous peak on the red side of the 14.98 m band. This peak is present in two independent scans of this spectral
region, and we verified its presence also in the previously published
spectrum (AOT 1 speed 1, see Beintema et al. 1996),
confirming that this feature is real.
This band peaks at 15.08 m which we identify with the
bending mode of the 16O12C18O isotopomer.
The bandheads of the
0110 and 1000
of this isotopomer should be at 16.75 and 14.21 m respectively. The first one is probably present in the spectrum but
coincides with the 0310
0220 band of 12C16O2;
the second one can be clearly seen as an emission feature. The 1110
0220 band of this isotopomer (at 13.81 m) is
At 15.04 m there might be emission from the fundamental bending mode of the 16O12C17O isotopomer. The 0200
0110 and 1000
0110 bands of this isotopomer
should be at 16.46 and 14.06 m; the former is clearly present in
the spectrum of HR 4049 as an isolated emission feature while the
latter does not exhibit a strong emission but might well be hidden in
the broad shoulders from the corresponding 12C16O2 and 13C16O2 bands. The 1110
0220 band of this isotopomer
should be at 13.66 m and might be a small wobble in the
Given the relative strength of the fundamental 12C16O2 and 13C16O2 bands, and the relative strength of the 12C16O2 and 16O12C18O and 16O12C17O bands, one might also expect to see (a) band(s) of the 16O13C18O and 16O13C17O isotopomers. 16O13C18O has its fundamental bending mode at 15.54 m and might be the bump at the red side of the 13C16O2 fundamental. Although the observed peak wavelength seems to be significantly shifted to the blue, we note that for this isotopomer the HITEMP database used for identification of the bands includes only one resonant hot band which is indeed blueshifted with respect to the fundamental. Moreover the observed blueshift for this isotopomer is of the same order as the observed blueshift of the 12C16O2 isotopomer when comparing to the fundamental. We therefore believe that the observed shift is due to high gas temperatures combined with the incompleteness of the HITEMP database. The HITEMP database furthermore only lists 2 more bands in this wavelength range for this particular isotopomer, the 0200 0110 band at 16.62 m and the 0200 0110 band at 14.31 m. The first one is probably blended with the corresponding bands of the other isotopomers; the second one can be clearly seen as an isolated emission feature. The fundamental for the 16O13C17O isotopomer seems not to be present at the right wavelength (15.49 m), but also here we would expect the band to be blueshifted and therefore be blended with the 13C16O2 fundamental. The HITEMP database again lists only two more bands, the 0200 0110 band at 16.40 m and the 0200 0110 band at 14.10 m. The first one might be a small wobble; a contribution from the second seems to be present in the spectrum in a more or less isolated region.
Table 1 gives a summary of the expected peak positions and the observed peak fluxes for the four strongest bands for each isotopomer.
The detection of O-rich molecules in an environment where all the dust
seems to be C-rich is a surprise in itself, and will be discussed in
detail in Paper I. The detection of 17O and 18O isotopes is
even more of a surprise considering predicted and observed isotopic
ratios for various types of evolved stars (see
|Figure 2: Oxygen isotopic ratios for HR 4049 derived from the CO2 bands in the optically thin limit compared to measured isotopic ratios for RGB and AGB stars. Data are from: K & M Stars (Harris & Lambert 1984; Harris et al. 1988), MS & S Stars (Harris et al. 1985b; Smith & Lambert 1990), Carbon Stars (Harris et al. 1987), Ba Stars (Harris et al. 1985a)|
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From the measured line intensities one can estimate isotopic ratios
assuming that the lines are optically thin and formed in LTE. When using
the line intensities of the 12C16O2 and 16O12C17O bands for
instance, one finds
As can be seen from Fig. 2, the relative circumbinary 17O and 18O abundances derived from the CO2 bands in HR 4049 are two orders of magnitude higher than the surface abundances measured for the bulk of the evolved stars. Although we cannot entirely rule out the possibility that 16O depletion or 17O or 18O enrichment occurred in the circumbinary disk as a consequence of fractionation, this process seems unlikely given its low efficiency or the low temperatures at which it generally occurs. We therefore believe that the observed abundances reflect the abundances in the stellar wind (currently or in the past) of HR 4049. This conclusion is strengthened by an evolutionary link between HR 4049 and the Ba star HD 101013. As the mass lost by HR 4049 is not only stored in the circumbinary disk but also partly transferred to the (currently undetected) companion, the surface abundances of this companion would be the same as those found in the circumbinary disk if no mixing occurs. When mixing would occur, the relative 17O and 18O abundances at the surface of the companion star would both decrease, and follow a track in Fig. 2 which goes up and to the right, exactly toward the Ba star HD 101013. As the companion to HR 4049 will probably only be observable at the time it becomes a red giant, and given the time it would take the companion to reach this phase, there is plenty of time for mixing of this material and reach abundances similar to those of HD 101013 at the time it becomes an observable Ba star. Indeed, if mixing occurred in HD 101013 between the time where its companion dumped enriched material onto the surface and present day, the relative 17O and 18O abundances must have been higher in the past.
It is important to realize that HR 4049 has most likely gone through a phase of case C Roche lobe overflow. Most scenarios for case C Roche lobe overflow assume highly non-conservative mass transfer, where most of the material lost is stored in a circumbinary disk or lost from the system rather than being transferred to the companion star. If we assume that the observed CO2 is only formed from material lost during this phase, and furthermore that highly enriched material dumped onto the companion is diluted with 0.6 (the total mass of the companion, see Bakker et al. 1998) of material having solar abundances, it would show the abundances observed in HD 101013 if it would have accreted no more than 0.06 ; the remaining mass lost by the primary would then have to be dumped in the circumbinary disk. As a typical AGB envelope mass is 0.3-0.5 this implies that almost the entire envelope must exhibit the extreme 17O and 18O enrichments we observe in the CO2 bands.
The relative enrichment of the oxygen isotopes measured here cannot be explained with current nucleosynthesis models for single stars. Some models might be stretched to explain either the 17O or 18O overabundance, but to our knowledge there is no model that can explain the simultaneous enrichment of both isotopes to the extent observed in HR 4049 (M. Lugaro, T. Blöcker, J. Lattanzio, private communication). It is likely that the binary nature is an important element for explaining these abundances.
We appreciate encouraging discussions with Rens Waters. J.C. acknowledges support from an NWO Pionier grant.