The First Fluorine Abundance Determinations in Extragalactic AGB Carbon Stars
Fluorine (F) abundances (or upper limits) are derived in six extragalactic AGB carbon stars from the HF(1-0) R9 line at 2.3358 m in high resolution spectra. The stars belong to the Local Group galaxies LMC, SMC and Carina dwarf spheroidal, spanning more than a factor 50 in metallicity. This is the first study to probe the behaviour of F with metallicity in intrinsic extragalactic C-rich AGB stars. Fluorine could be measured only in four of the target stars, showing a wide range in F-enhancements. Our F abundance measurements together with those recently derived in Galactic AGB carbon stars show a correlation with the observed carbon and element enhancements. The observed correlations however, display a different dependence on the stellar metallicity with respect to theoretical predictions in low mass, low metallicity AGB models. We briefly discuss the possible reasons for this discrepancy. If our findings are confirmed in a larger number of metal-poor AGBs, the issue of F production in AGB stars will need to be revisited.
Comparisons between abundances of some key elements determined in stars in different evolutionary stages with nucleosynthetic stellar models is a fundamental tool for our understanding of stellar interiors. One element that can add new insight into this is fluorine, since this element is very sensitive to nuclear reactions involving proton and/or alpha captures. Actually, the origin of this light element is still uncertain although much observational progress has been made in recent years in cool K, M and Asymptotic Giant Branch (AGB) stars (e.g. Jorissen et al. 1992, hereafter JSL; Cunha et al. 2003-08; Smith et al. 2005; Uttenthaler et al. 2008), and in post-AGB stars and planetary nebulae (Werner et al. 2005; Otsuka et al. 2008). From these observational studies two conclusions are derived: i) AGB stars constitute the only evidence of stellar F production and, ii) the inferred evolution of the F abundance in the Galaxy, when compared with chemical evolution models (Renda et al. 2004), requires the contribution of the additional sources proposed so far: gravitational supernovae (Woosley et al. 1990) and Wolf-Rayet stars (Palacios et al. 2005).
Evidence of F production in AGB stars was found by JSL two decades ago: F enhancements up to a factor 30 solar and a correlation of F with the C/O ratio was first reported in Galactic (O-rich & C-rich) AGB stars of solar metallicity. Since the C/O ratio is expected to increase in the envelope during the AGB phase as a consequence of the third dredge-up (TDU) episodes, this was interpreted as evidence of F production. The F enhancements found by JSL, however, could not be accounted by detailed models for AGB stars at that epoch (Mowlavi et al. 1998), or by more recent ones (Lugaro et al. 2004) that included the impact of the variation of some nuclear reactions with uncertain rates affecting F. Nonetheless, a recent re-analysis of the JSL’s stars by Abia et al. (2009-10; hereafter Paper I and II) using improved line lists and model atmospheres found that the F abundances reported in JSL have been overestimated because of a possible lack of proper accounting for C-bearing molecule blends. This reanalysis reported F abundances by dex lower on average. The new F abundances and the observed correlation between F and element enhancements in solar metallicity C-stars, are now fully accounted by the current nucleosynthetic modelling of low mass AGB stars (Cristallo et al. 2009-11).
At low metallicities the situation is less
clear. Theoretically, the F production in AGB stars is
expected to increase when decreasing metallicity. AGB stars synthesise F via
NC ONF and
where the neutrons are provided by CO and the protons mainly by
NC. Thus, the production of F
basically depends on the availability of C in the He-rich
also on the amount of C available in the
ashes of the H-burning shell. The former is weakly dependent on the
stellar metallicity but the latter scales with the CNO
abundances in the envelope, which, in turn, depends on the (primary)
C dredged up during TDU episodes. As a consequence, the resulting
fluorine can be roughly considered a primary element. Because of this
primary nature, larger [F/Fe]
Here we present for the first time F abundance measurements in metal-poor AGB carbon stars in stellar systems other than the Galaxy: the SMC, LMC and Carina dSph galaxies. These C-stars are significantly more metal poor than the Galactic counterparts so far analysed providing valuable information on how F is produced in AGB stars as a function of metallicity.
2 Observations and Analysis
The stars were chosen from previous optical high-resolution spectroscopic studies of extragalactic AGB C-stars (de Laverny et al. 2006; Abia et al. 2008). In addition to the stars analysed there (BMB B30 belonging to the SMC, and ALW-C6 and ALW-C7 to Carina dSph), we added two stars in the LMC (TRM88 and MSX663) and a new target in the SMC (GM780). Details about the characteristics of BMB B30, ALW-C6 and ALW-C7 can be found in the above works (the other stars are described below). The stars were observed in classical mode with the 8.1 m Gemini-South telescope and the Phoenix spectrograph (Hinkle et al. 1998) at a resolving power R; and centred at 23350 Å,to include the HF(1-0) R9 line. In Paper I it was shown that this line is the most reliable for F abundance determinations in cool stars. A detailed description of the Phoenix observations and the corresponding data reduction can be found in Smith et al. (2002). Table 1 shows the exposure times for each object, and the signal-to-noise ratios reached in the final spectrum.
Two target stars are peculiar: MSX663 is a long-period variable classified as a S star by Cioni et al. (2001) (C-rich but with C/O). Zijlstra et al. (2006) indicated that this object may be a symbiotic star. Surprisingly, our spectrum of this object shows no spectral lines in the 2.3 m region; even the vibration-rotation CO lines, usually strong, are absent. This might be compatible with this object being a supergiant rather than an AGB star. In any case, we could not identify the R9 line and thus, it was discarded for analysis. GM780 is also a rare object. Lagadec et al. (2007) noted that the CH bands, commonly observed in C-stars, were absent. Also, the colour of this object (2.6), is quite red, and dust should be present around it. These authors concluded that GM780 has a C/O ratio considerably lower than that in any other star in their sample. Our high-resolution spectrum confirms this finding; all the features of CN and C molecules in the 2.3 m region appear very weak. Also, the high excitation CO lines look broader and affected by line doubling. Actually, a much larger macroturbulence value than typical one was required to fit the line profiles in this star. We do not know the reason of this, but the effect of dust or/and stellar pulsation might be at play (see e.g. Nowotny et al. 2011). In fact, our synthetic fit to its spectrum was not satisfactory.
The classical method of spectral synthesis was used in the
analysis. Theoretical LTE spectra were computed in spherical geometry
and convolved with Gaussian functions to mimic the corresponding instrumental
profile adding a macroturbulence velocity typically of km/s
(a 10 km/s value was used for GM780). For more details on the method of analysis,
and the adopted molecular and atomic line lists used see Paper I.
We adopted the atmospheric parameters derived in de Laverny et
al. (2006) and Abia et al. (2008) for the stars BMB B30, ALW-C6 and
ALW-C7, and followed the same procedure to derive the stellar
parameters in the remaining stars (see these works for details). Accordingly,
we estimate a T K for GM780 (note the red colour of this star),
and 3400 K for TRM88. Since our grid of C-rich atmosphere models (Gustafsson et al. 2008)
does not include a T as low as 2000 K, we used a model with such a T from
the C-rich models grid by Pavlenko & Yakovina (2010).
A gravity of log g and a km/s were adopted
for all the stars (see e.g. Lambert et al. 1986). The analysis of
BMB B30, ALW-C6 and ALW-C7 resulted in different C/O ratios
than those previously derived from optical spectra.
These differences were, nevertheless, within the uncertainties
Figure 1 shows examples of synthetic fits in the R9 line region for the stars TRM88 and ALW-C7. The blend to the left wing of the HF line has a significant contribution of CO. This feature allowed to derive the O/O ratio (see Table 1) in the most metallic stars of the sample. In the most metal-poor objects (stars in Carina) however, this blend is insensitive to variations of the O/O ratio. For some of the targets, Na abundances were also derived from the Na I line at Å. This line is, however, blended with molecular features, thus its detection depends on the current Na abundance in the star (see Table 1). For each element (and the O/O ratio), the uncertainty was calculated by determining individually the sensitivities of the derived abundance to the adopted T, gravity, C and O abundances, microturbulence, and metallicity. We then sum in quadrature the resulting uncertainties associated to each parameter. The synthetic fit to the HF and the Na I lines is particularly sensitive to the T adopted, the other parameters affecting at a lower degree (in particular gravity and metallicity, see Paper I). The resulting formal uncertainty is dex for F, dex for Na, and a factor for the O/O ratio.
3 Results and Discussion
Table 1 summarises the main results. We find a wide range in [F/Fe] ratios in metal-poor extragalactic AGB C-stars. Fluorine is enhanced in all except one star, confirming that this element is produced during the AGB phase. Figure 2 provides further evidence of this. The similarity in the run of F with C strongly support that the synthesis of F is tied to the production of C during the Thermal Pulsing (TP) AGB phase. Also Figure 2 qualitatively shows that F enhancement increases for decreasing stellar metallicity (although the star B30 seems to deviate from this trend). In Paper II we showed that at solar metallicity, the F and C correlation can be reproduced by current TP-AGB models (Cristallo et al. 2009, continuous lines in Figure 2). Models and observations, however, now seem to disagree in metal-poor AGB stars (dashed lines); theoretical models tend to overproduce C for a given F enhancement. Indeed, the two stars in Carina should be along the model computed with (right dashed line), while the star B30 along the model line with . This discrepancy, which has been already noted at solar Z (e.g. Abia et al. 2002), becomes more evident at lower metallicity as Figure 2 shows, since in current AGB models the efficiency of the TDU increases at low metallicity and thus, the amount of C dredged-up into the envelope. Note that this problem could be also an observational bias: intrinsic AGB C-stars with high C/O ratios should exist, as high values of C/O have been observed both in post-AGB stars and in extrinsic C-rich objects originated by mass transfer from an AGB star. However, in cool C-stars an excess of carbon is immediately translated into a copious production of C-rich dust and into a high opacity of the circumstellar environment, to the point of hiding the photosphere at visual wavelengths. Most of the C might be trapped into grains. These dusty C-stars should show large infrared excess. Curiously enough, the two stars in our sample (Table 1) with high C/O ratios do not show this as deduced from their available infrared photometry, while the star GM780, with large infrared excess (see Section 2) has the typical C/O ratio observed in C-stars.
Figure 3 provides another piece of information on the synthesis of F in AGB stars. It
shows the observed relation between F and the average
Now the question is how to account for these apparent low F enhancements?
In the recent years, some degree of extramixing processes have been invoked to explain
the O/O/O ratios found in grains of AGB
origin, and the low C/C ratios in C-stars (Busso et al. 2010).
Extramixing has been proposed by Denissenkov et al. (2006)
as a possible solution
In summary, from the comparison between the derived F abundances and the current models we conclude that the F synthesis in metal-poor AGB stars is probably not as large as expected or some physical mechanism, not currently considered in the models, efficiently destroys it. Obviously, additional F abundance measurements in metal-poor AGB stars would be extremely important to enlighten this problem. The origin of this element still remains unknown.
|Star||Exp. time (s)||S/N||T||[Fe/H]||C/N
||log CO)||O/O||log (F)||log (Na)||[F/Fe]||[Na/Fe]||[s/Fe]
- slugcomment: Based on observations made at GEMINI-S, NOAO Proposal ID 2010B-0183.
- We adopt the usual notation with [x/y]log log and log log .
- In Section 3 we show that the [F/Fe] ratio achieved in the envelope in the C-rich AGB phase is almost independent on the initial F content in the star.
- The derived C/O ratios have an additional uncertainty since the O abundance cannot be determined independently of the C abundance. This is because theoretical spectra are almost insensitive to a large variation of the O abundance provided that the difference log CO) is kept constant. The estimated error in the C and O abundances is dex. See de Laverny et al. (2006) and Paper I for details.
- Obtained as the mean value of the [Sr, Zr, Y, Ba, La, Nd, Sm/Fe] ratios.
- Na can be also produced during the TP-AGB phase, see Cristallo et al. (2009).
- There are other explanations more widely accepted, see e.g. Marino et al. (2008).
- N abundances are scaled to the stellar metallicity.
- The star LMC MSX663 shows no spectral lines in the 2.3350 m region and was discarded for abundance analysis (see text).
- The average s-element enhancements aren taken from de Laverny et al. (2006) for BMB B30, and Abia et al. (2008) for the stars in Carina.
- The star LMC MSX663 shows no spectral lines in the 2.3350 m region and was discarded for abundance analysis (see text).
- footnotetext: The amostpheric parameters adopted for this star and the derived abundances have to be considered as very uncertain (see text).
- The adopted solar abundances are from Asplund et al. (2009).
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