We present results from an ongoing survey of the neutron-capture elements Se and Kr in planetary nebulae (PNe). These elements are created in the s-process during the thermally-pulsing AGB phase, and a PN may display heightened Kr and Se abundances if third dredge-up (TDU) was effective in the progenitor star. We have observed 27 PNe in the K band, and detected [Kr III] 2.199 and/or [Se IV] 2.287 mm in 13 of them. The detection rate of nearly 50% indicates that these lines are observable in a significant fraction of the Galactic PN population. These emission lines are clearly affected by the level of nebular excitation, in that [Kr III] detections are more prevalent in PNe of low excitation class while [Se IV] is more often detected in mid- to high-excitation nebulae. In addition, we show that the strength of [Se IV] 2.287 mm increases with the strength of the He II 2.189 mm. We derive elemental abundances of Se and Kr by using the ionization correction factors Se3+/Se ≈ Ar++/Ar and Kr++/Kr ≈ S++/S, respectively, and find a wide range of Kr and Se abundances, from approximately solar to overabundant by a factor of ~5–10. This implies that the PN progenitors in our sample have a broad spectrum of dredge-up histories, from those that experienced no TDU to those in which it was quite efficient. We explore possible correlations between enrichment of neutron-capture elements and other nebular properties, such as C/O and N/O, but do not find significant relations (likely due to the limited sample size reported here; our completed sample will consist of ~100 objects).
In thermally-pulsing asymptotic giant branch (AGB) stars, the reaction 13C(a,n)16O releases neutrons which are captured by iron-peak elements. These species undergo subsequent neutron captures and b-decays as they are converted into heavier elements (the “s-process”), including isotopes of krypton and selenium. The processed material may be transported to the surface of the star during third dredge-up (TDU), which is exemplified by the descent of the convective envelope beyond the (inactive) H-burning shell. This region is enhanced in He-burning products as well as s-processed material, so that TDU also conveys carbon-rich material to the stellar envelope (see, e.g. Busso et al. 1999). Indeed, it has been found that enhancements in n-capture elements correlate with the C abundance in AGB (e.g. Smith & Lambert 1990; Abia et al. 2002) and post-AGB stars (Reyniers 2002). Therefore, when star expels its envelope to form a PN, the ejecta may be enriched in carbon and s-process products if TDU occurred.
Carbon abundances are notoriously difficult to measure reliably in PNe (Rola & Stasinska 1994)), and therefore the fraction of PN progenitors which have undergone TDU is not well known. Alternatively, s-process products can be used to assess the degree of TDU and enrichment. In fact, the initial abundances of n-capture elements are so low that even modest enhancements result in a substantial increase in their abundances. We show that the emission lines [Kr III] 2.199 and [Se IV] 2.287 mm, first identified by Dinerstein (2001), are detectable in a significant fraction (~50%) of PNe. Since neither Se nor Kr are observed to be depleted in the diffuse interstellar medium (Cardelli et al. 1993; Cartledge, Meyer, & Lauroesch 2003), the derived abundances represent the true elemental abundances. Therefore, these two lines may be of paramount importance in deciphering the evolutionary and dredge-up history of PN progenitors.
We recently (as of Jan. 2003) began our survey of PNe in the K band, using the CoolSpec spectrometer (Lester et al. 2002) on the 2.7m Harlan J. Smith telescope at McDonald Observatory. We have observed 27 PNe in the spectral region 2.14–2.30 mm thus far, at a resolution ~500. Figure 1 shows examples of the spectra obtained, with detected lines identified; the tick marks below the M 1-11 spectrum denote the location of H2 transitions. As can be seen, [Kr III] and [Se IV] are well separated from other spectral features, with the exception of the H2 3-2 S(3) and S(2) transitions at 2.201 and 2.287 mm, respectively. Only five objects in our sample display H2, and in those cases we assume the maximum contribution of the H2 3-2 lines (fluorescent excitation; model 14 of Black & van Dishoeck 1987) in determining the presence of the n-capture lines. Overall, we have detected one or both [Kr III] and [Se IV] in 13 of the 27 PNe observed (48%). Adding 40 K band PN spectra from the literature, of which 20 exhibit one or both n-capture lines, the total fraction of detections is 33/67 (~50%).
As expected, the presence and strength of [Kr III] and [Se IV] in the PN spectra show marked excitation effects. All objects which display [Se IV] also exhibit He II, while [Kr III] is not detected (without [Se IV]) in any nebula with He II emission. The strength of the [Se IV] line is correlated with the strength of He II, as shown in Figure 2. We illustrate the excitation effects further in Figure 3, where we plot the fraction of detections against excitation class (here, we make use of the 40 spectra from the literature. Note that [Kr III] detections are prevalent in lower excitation nebulae than [Se IV].
We have derived the ionic abundances of Kr ++ and Se3+ in the low density limit, where the emissivity of a transition is proportional to the collision strength. This is a good approximation for [Kr III] 2.199 mm, which has a critical density of 2.1×107 cm-3 (Biémont & Hanson 1986; Schöning 1997). The atomic data are unknown for the [Se IV] transition, and thus the Se abundances are modulo the effective collision strength. In order to calculate the elemental abundances of Kr and Se, we used an ionization correction factor based upon similar ionization potential range: Kr++/Kr ≈ S++/S and Se3+/Se ≈ Ar++/Ar. The abundances derived are displayed in Table 1 (only for objects in which we detected [Kr III] or [Se IV]), which also lists references for S and Ar data from the literature.
Note that some of the objects have nearly solar Kr abundances (e.g. M 1-5), while others are clearly enriched (e.g. NGC 7026). Therefore, we can distinguish a wide range of enrichment factors, from ~1 to ~5–10. This indicates that we are detecting these lines both in objects which have undergone TDU and those that have not (or at least the enrichments are very mild).
It is of interest to examine whether the Kr and Se enhancements correlate with other nebular properties. In Figure 4, we plot [Se/H] against C/O. Unfortunately, few of our targets have carbon abundances determined from collisionally excited lines (e.g. in the UV), to which we restrict the C/O ratios used. There does not seem to be a significant correlation, but this could change once our sample size increases. Figure 5 shows [Kr/H] versus N/O, both with and without [Kr/H] upper limits included. Both appear to be scatter plots, but we stress that we have detected [Kr III] in only eight PNe thus far! Therefore, it appears that the sample size must grow before such plots take on any significance.
Fig. 4 (Left) – Upper limits are shown by downward pointing arrows.
Fig. 5 (Below) – [Kr/H] vs. N/O with (left) and without (right) PNe with [Kr/H] upper limits
As a final note, we mention the cross-section of the PN population observed. Certainly, we are restricted to compact, bright PNe in order to detect [Kr III] or [Se IV], and therefore cannot study evolved (large) objects with this project. However, we will attempt to include several interesting classes of objects – for example, Peimbert types I–III PNe (type IV are generally too faint) and those with [WC] central stars. Too few PNe with [WC] central stars have been detected in Se or Kr (only one object) to draw any conclusions. Figure 6 shows the number of PNe of each Peimbert type observed (including literature data), as well as the fraction detected in Kr or Se. The abundances have been derived for only one type I, six type II, and one type III PNe so far, thereby disallowing a meaningful discussion of variations with Peimbert type. We simply note that the type I (M 1-17) is clearly enriched in Kr and Se, while only two of the six type II PNe with known Kr abundances are enriched. The sole type III PN (M 1-12) detected in [Kr III] in our sample is not enriched. This may indicate a dividing line in mass, below which TDU does not occur. More observations are needed to confirm this claim.
Fig. 1 – Tick marks below the M 1-11 spectrum indicate H2 transitions
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Fig. 2 – Total number of PNe observed in each class are denoted at the top of each bar.
Table 1 – Abundances are given in the form [X/H] = log(X/H) – log(X/H)סּ
Fig. 6 – Total number of each PN type observed is given at the top of each bar. Type V refers to Galactic bulge objects.