ABSTRACT
We identify [Rb iv] 1.5973 and [Cd iv] 1.7204
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1. INTRODUCTION
Planetary nebulae (PNe) mark the transition of asymptotic giant branch (AGB) stars to white dwarfs in low- and intermediate-mass (1–8 M⊙) star evolution. Their compositions bear the signatures of nucleosynthesis and mixing events during the AGB. In particular, He, C, N, and elements formed by slow neutron(n)-capture nucleosynthesis (the s-process; atomic number Z > 30) can be enriched in PNe (Busso et al. 1999; Herwig 2005; Karakas & Lattanzio 2014).
The low cosmic abundances of n-capture elements (Asplund et al. 2009) rendered them elusive to detection in astrophysical nebulae until Péquignot & Baluteau (1994) identified emission lines of several trans-iron species in the optical spectrum of the PN NGC 7027. However, the deep, high-resolution spectra needed to unambiguously identify optical n-capture element lines (Sharpee et al. 2007; Otsuka et al. 2011; García-Rojas et al. 2015) restrict such studies to relatively bright PNe.
The near-infrared (NIR) spectral region has proven more fruitful for studies of s-process enrichments in a large number of PNe. Dinerstein (2001) identified two K band emission lines in PNe as [Kr iii] 2.1986 and [Se iv] 2.2864
In this letter, we identify [Rb iv] 1.5973, [Cd iv] 1.7204, and [Ge vi] 2.1930
The detection of Rb and Cd are of particular note. Rb enrichments are sensitive to the s-process neutron density and hence the neutron source (
2. OBSERVATIONS AND ANALYSIS
The spectra were obtained with the Immersion GRating INfrared Spectrometer (IGRINS) on the 2.7 m Harlan J. Smith Telescope at McDonald Observatory. IGRINS provides complete simultaneous coverage of the H and K bands (1.45–2.45
The data were processed with the IGRINS Pipeline Package5 written by J.-J. Lee, after removal of cosmic rays. We used ThAr arc lamps for wavelength calibration, with a small correction from OH sky lines. Barycentric and systemic velocity shifts were removed using nebular lines with precisely known wavelengths. A0V standards were observed at similar times and airmasses as the targets for relative flux calibrations and telluric corrections. See K. F. Kaplan et al. (2016, in preparation) for further details.
For NGC 7027, the flux-calibrated 2D data were transformed to position–velocity space with 1 km s−1 wide pixels before extraction of line fluxes. The position–velocity diagrams for the trans-iron element lines are shown in Figure 1, along with the apertures used for flux extraction. Line fluxes were measured by summing all pixels within each aperture and subtracting the median value of pixels within 100 km s−1 of the line but outside the aperture. Since IC 5117 is essentially a point source, its spectrum was optimally extracted in 1D, transformed to velocity space, and line fluxes found by summing the flux above background for pixels within ±50 km s−1 of the line center.
Figure 1. Left panels: 2D profiles of n-capture element lines detected in NGC 7027, with apertures used for flux extraction. Right panels: 1D profiles of n-capture transitions in IC 5117, including the non-detection of [Ge vi]. Gray shading indicates 1-
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Standard image High-resolution image3. IDENTIFICATION OF [
] 1.5973, [
] 1.7204, AND [
]
We searched the IGRINS spectra of NGC 7027 and IC 5117 for n-capture element emission lines with wavelengths computed from energy levels in the NIST Atomic Spectra Database6
(Kramida et al. 2014). We detected features at 1.5973 and 1.7203
We used the Atomic Line List v2.05b187 to search for alternate identifications within ±10 Å of the detected features. We considered permitted features of ions in the first three rows of the Periodic Table, and forbidden transitions with upper-level excitation energies less than 10 eV. No molecular transitions match the wavelengths of these features, and telluric features can be readily distinguished from nebular lines at our spectral resolution.
For the possible alternate identifications for [Rb iv] 1.5973
Cd is predicted to be highly enriched by s-process nucleosynthesis models (e.g., Cristallo et al. 2015). Cd3+ has a 4d9 ground configuration with two levels separated by 5812.6 cm−1 (Joshi & van Kleef 1977), corresponding to a wavelength of 1.7204
Finally, we detect a line at 2.1935
4. ABUNDANCE ANALYSIS
Table 1 lists fluxes for the n-capture element lines detected in the IGRINS spectra of NGC 7027 and IC 5117. Error bars to the line fluxes include ∼10% uncertainties in the continuum placement in addition to statistical errors. The [Kr iii] and [Se iv] fluxes are in excellent agreement with previous findings (Sterling & Dinerstein 2008).
Table 1. Fluxes and Abundances of Neutron-capture Elements in NGC 7027 and IC 5117
Line | Flux | Ionic | Elemental | ||
---|---|---|---|---|---|
Ratio | Ratio | Abund. Xi+/H+ | ICFa | Abund. (X/H) | [X/H] |
NGC 7027 | |||||
[Rb iv] 1.5973/H i 1.5885 | (5.72 ± 0.57)E–02 | (6.05 ± 0.61)E–10 | 1.88 | (1.14 ± 0.55)E–09 | 0.54 ± 0.20 |
[Rb iv] 5759.55/H |
(1.68 ± 0.43)E–04 | (7.70 ± 1.78)E–10 | 1.88 | (1.45 ± 0.70)E–09 | 0.64 ± 0.20 |
[Cd iv] 1.7204/H i 1.6811 | (5.14 ± 0.54)E–03 | (5.74 ± 0.60)E–11 | 1.88 | (1.08 ± 0.81)E–10 | 0.32 ± 0.30 |
[Ge vi] 2.1930/H i 2.1655 | (5.61 ± 0.56)E–03 | (2.77 ± 0.28)E–10 | 6.52 | (1.81 ± 2.58)E–09 | −0.32 ± 0.50 |
[Kr iii] 2.1980/H i 2.1655 | (2.95 ± 0.30)E–02 | (2.28 ± 0.23)E–09 | 5.09 | (1.16 ± 0.26)E–08 | 0.81 ± 0.10 |
[Se iv] 2.2858/H i 2.1655 | (7.87 ± 0.79)E–02 | (1.32 ± 0.13)E–09 | 3.57 | (4.72 ± 1.06)E–09 | 0.34 ± 0.10 |
IC 5117 | |||||
[Rb iv] 1.5973/H i 1.5885 | (5.40 ± 0.64)E–02 | (7.61 ± 0.84)E–10 | 1.25 | (9.52 ± 4.54)E–10 | 0.46 ± 0.20 |
[Cd iv] 1.7204/H i 1.6811 | (1.12 ± 0.12)E–02 | (1.37 ± 0.15)E–10 | 1.25 | (1.71 ± 1.29)E–10 | 0.52 ± 0.30 |
[Kr iii] 2.1980/H i 2.1655 | (2.21 ± 0.23)E–02 | (1.89 ± 0.20)E–09 | 3.78 | (7.13 ± 1.61)E–09 | 0.60 ± 0.10 |
[Se iv] 2.2858/H i 2.1655 | (1.31 ± 0.13)E–01 | (2.39 ± 0.24)E–09 | 1.73 | (4.14 ± 0.92)E–09 | 0.28 ± 0.10 |
Notes. Neutron-capture element line fluxes relative to nearby H i lines are given, as are ionic abundances, ionization correction factors, and elemental abundances relative to solar (Asplund et al. 2009). Vacuum wavelengths in
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4.1. Ionic and Elemental Abundances
We compute ionic abundances (Table 1) relative to H+ using nearby H i lines, assuming an electron temperature Te = 12,600 ± 500 K and density ne = 52,300 cm−3 for NGC 7027 (Zhang et al. 2005), and Te = 11,800 ± 300 K and ne = 89,000 cm−3 for IC 5117 (Hyung et al. 2001). Error bars for the ionic abundances include uncertainties in ne (assumed to be 20%), Te, and line fluxes; the latter dominates the uncertainties due to the weak dependence of these transitions on temperature and density. We utilize the transition probabilities of Biémont & Hansen (1986) for [Kr iii], Biémont & Hansen (1987) for [Se iv] and [Ge vi], and the collision strengths of (Schöning 1997) and K. Butler (2007, private communication) for [Kr iii] and [Se iv] respectively. We have computed effective collision strengths and transition probabilities for [Rb iv], as described in the
Our [Rb iv] collision strengths can be tested by comparing the strength of the 1.5973
To derive elemental abundances (Table 1), unobserved ions must be accounted for via "ionization correction factors" (ICFs). Reliable ICF prescriptions require accurate photoionization cross sections and recombination rate coefficients, but these data are currently unknown for Rb, Cd, and Ge ions. We therefore adopt ICFs based on similarities in IP ranges: ICF(Rb, Cd) = Rb/Rb3+ = Cd/Cd3+ = O/O2+ and ICF(Ge) = Ge/Ge5+ = Ne/Ne4+. In the absence of reliable atomic data, it is difficult to quantify the effects of uncertainties in the adopted ICFs and the collision strengths for [Ge vi] and [Cd iv] on the abundance determinations. Based on the uncertainties in the Se and Kr abundances (0.1 dex), we roughly estimate uncertainties of 0.2 dex for Rb, 0.3 dex for Cd, and 0.5 dex for Ge (whose ICF is large and very uncertain).
The O2+, Ne4+, Kr2+, and Se3+ ionic fractions required to estimate ICFs were extracted from Cloudy models of NGC 7027 and IC 5117 (Sterling et al. 2015). Our derived Kr and Se abundances agree with the empirical values found by Sterling et al. (2015) to within 15%. The Rb and Cd abundances are larger than solar (Asplund et al. 2009) in both PNe by amounts (0.3–0.6 dex) similar to those of Se and Kr. The nominally subsolar Ge abundance in NGC 7027 is not a firm result due to the large and uncertain ICF caused by the fact that Ge5+ is a minority species. Depletion into dust may also be a factor for this moderately refractory element (Lodders 2003).
Depletion is unlikely to affect Cd due to its low condensation temperature (652 K; Lodders 2003) and mild depletion in the diffuse interstellar medium (Sofia et al. 1999). The situation is less clear for Rb, which has a condensation temperature of 800 K. Other alkali elements such as Na and K are depleted by factors of 2–4 in PNe (Pottasch et al. 2009; García-Rojas et al. 2015), including NGC 7027 (Bernard Salas et al. 2001). But Na and K have higher condensation temperatures (950–1000 K) than Rb. We conclude that Rb may be depleted, but probably by less than a factor of two (the typical depletion of Na in PNe).
4.2. Comparison With Models
Including optical Xe detections (Hyung et al. 2001; Sharpee et al. 2007), 5–6 n-capture elements have been detected in NGC 7027 and IC 5117. This allows for a systematic comparison of the observed enrichment patterns with theoretical predictions. We compare our results with models from the FRUITY database (Cristallo et al. 2011, 2015)8 , which samples initial stellar masses in the range 1.3–6.0 M⊙ and metallicities Z = 0.001–0.020. We select the models with Z = 0.006, which corresponds to [Fe/H] = −0.37 for Z⊙ = 0.014. This is consistent with the [Zn/H] values—a proxy for [Fe/H] (Dinerstein & Geballe 2001)—for NGC 7027 and IC 5117: [Zn/H] = −0.44 ± 0.08 and −0.31 ± 0.10, respectively (Smith et al. 2014).
In Figure 2 we plot predicted FRUITY final envelope abundances (Cristallo et al. 2015) for AGB stars of initial mass 1.5–3.0 M⊙ and metallicity Z = 0.006. The overall n-capture enrichments first increase with mass, peaking at 2.0–2.5 M⊙, then decrease. We also plot the derived n-capture element abundances for NGC 7027 and IC 5117 (Ge is not shown due to its uncertain abundance). We adopt [Xe/Ar] = 0.92 for NGC 7027 (Sharpee et al. 2007). For IC 5117, we use the transition probabilities of Biémont et al. (1995) and collision strengths of Schöning & Butler (1998) to compute Xe3+/H+ = 1.03 × 10−9 from the [Xe iv]
Figure 2. Comparison of empirical and theoretical s-process enrichments of n-capture elements in NGC 7027 (red circles) and IC 5117 (blue triangles). Data points for NGC 7027 are offset horizontally so that their error bars can be distinguished from those of IC 5117. Theoretical predictions [X/Fe] are from FRUITY models (Cristallo et al. 2011, 2015) with metallicity Z = 0.006 ([Fe/H] = −0.37).
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Standard image High-resolution imageThe theoretical curves represent [X/Fe], while the measured abundances are given as [X/H]. To convert the latter to [X/Fe] one would have to add 0.37 to [X/H] for each data point, which would raise the measured abundances above the predicted values. However, the absolute abundances at the end of the thermally pulsing AGB vary substantially among different sets of models for the same mass and metallicity, due to different treatments of mass loss, mixing at convective interfaces, and the number of thermal pulses before envelope ejection (Karakas & Lattanzio 2014). Therefore the measured abundances should be compared with the relative enrichments in Figure 2.
The relative Se, Rb, and Xe enrichments are consistent with an initial mass of 2.0–2.5 M⊙ for NGC 7027, in agreement with estimates of ∼2.5M⊙ based on its central star and nebular masses (Zijlstra et al. 2008; Santander-García et al. 2012). The Se and Rb abundances of IC 5117 suggest a lower initial mass, as expected based on its central star mass (Hyung et al. 2001). Interestingly, the measured Kr abundances are higher than predicted for both objects, as is the Xe abundance for IC 5117. The reason for these discrepancies is unclear, especially since the Kr abundances are well-constrained given the detection of multiple Kr ions in each object (e.g., Sterling et al. 2015). The derived abundance for Cd is not in agreement with theoretical predictions, most likely due to inaccuracies in our adopted [Cd iv] collision strength and ICF.
Ratios between enrichments of elements in the first ("light," or ls) and second ("heavy," or hs) s-process peaks are less sensitive to uncertainties in the physics of AGB models than absolute abundances. However, indices such as [hs/ls] used for studying s-process nucleosynthesis in stars are based on elements that cannot be measured reliably in nebulae, for reasons including depletion into dust. In nebulae, the ls peak is represented by Ge through Rb, while only Cd and Xe (and perhaps Ba; Péquignot & Baluteau 1994) have been detected among the elements beyond this peak. As we extend the sample of observed nebulae to include a wider range of initial stellar mass and metallicity, we plan to test the dependence of s-process enrichments on these parameters, and compare measured enrichments with different sets of nucleosynthesis models.
5. SUMMARY
We identify NIR [Rb iv], [Cd iv], and [Ge vi] emission lines in the spectra of the PNe NGC 7027 and IC 5117. The identification of Rb is important due to the sensitivity of its enrichment to the s-process neutron density and hence progenitor mass. Since Cd lies beyond the first ("light") s-process peak, its abundance relative to light n-capture elements is sensitive to the time-averaged neutron flux experienced by Fe nuclei during the s-process.
We derive ionic abundances using newly-computed effective collision strengths for [Rb iv] (see the
N.C.S. and M.A.B. acknowledge support from the NSF through award AST-1412928, and HLD from AST-0708425. This work used the Immersion Grating Infrared Spectrograph (IGRINS) developed by the University of Texas at Austin and the Korea Astronomy and Space Science Institute (KASI) with the financial support of NSF grant AST-1229522, the University of Texas at Austin, and the Korean GMT Project of KASI. This work has made use of NASA's Astrophysics Data System, the Atomic Line List v2.05B18, and the FRUITY Database of nucleosynthetic yields from AGB stars.
APPENDIX: [
] EFFECTIVE COLLISION STRENGTHS
We compute radiative rates (A-values) for dipole forbidden transitions among the 3P, 1D, and 1S levels of the 4s24p4 configuration of Rb3+ using the code AUTOSTRUCTURE (Badnell 1986, 2011). We allow for orbital relaxation and configuration mixing by including the configurations 4s4p5, 4s24p34d, 4s4p44d, 4s24p34f, 4s24p35s, 4s24p35p, and 4s24p35d. This expansion yields level energies in good agreement with experimental values from NIST (Kramida et al. 2014), but gives A-values that are a factor of two lower than those of Biémont & Hansen (1986). To produce more accurate A-values, it was necessary to include the configurations 3d94s24p5, 3d94s24p44d, and 3d94s24p45s.
The orbital wave functions were optimized by minimizing the energies of the eight lowest LS terms, and were fine-tuned by means of term energy corrections. Energy levels were then shifted to match experimental values, and A-values (Table 2) were computed. The predicted energy levels agree with experimental values to within 10% up to about 0.55 Ryd, and better for higher energy terms.
Table 2. Transition Probabilities and Effective Collision Strengths for the [Rb iv] 4s24p4 Ground Configuration
Lower | Upper | Aul | Effective Collision Strengths | ||||
---|---|---|---|---|---|---|---|
Level | Level | (s−1) | 5000 K | 7500 K | 10000 K | 15000 K | 20000 K |
3P2 | 3P1 | 5.374 | 1.908 | 2.105 | 2.314 | 2.704 | 3.025 |
3P2 | 3P0 | 0.000 | 0.6836 | 0.7090 | 0.7521 | 0.8487 | 0.9373 |
3P2 | 1D2 | 5.324 | 3.508 | 3.417 | 3.343 | 3.368 | 3.481 |
3P2 | 1S0 | 0.000 | 0.6057 | 0.6709 | 0.6982 | 0.7024 | 0.6856 |
3P1 | 3P0 | 1.720E–2 | 0.6613 | 0.7092 | 0.7674 | 0.8692 | 0.9443 |
3P1 | 1D2 | 0.4775 | 2.062 | 2.000 | 1.977 | 2.006 | 2.071 |
3P1 | 1S0 | 32.51 | 0.3921 | 0.4287 | 0.4468 | 0.4542 | 0.4471 |
3P0 | 1D2 | 9.635E–5 | 0.7158 | 0.7034 | 0.6978 | 0.7037 | 0.7181 |
3P0 | 1S0 | 0.000 | 0.1499 | 0.1669 | 0.1796 | 0.1938 | 0.2000 |
1D2 | 1S0 | 5.907 | 1.293 | 1.322 | 1.346 | 1.403 | 1.470 |
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Collision strengths for Rb3+ were computed with the BPRM+ICFT method, using a suite of parallel Breit-Pauli R-matrix programs (Mitnik et al. 2001, 2003; Badnell et al. 2004). We used the wave functions from the AUTOSTRUCTURE calculation, retaining configuration interaction from all 10 configurations and the lowest 23 LS terms from the 4s24p4, 4s4p5 and 4s24p34d configurations. The calculations explicitly include partial waves from states with L ≤ 9 and multiplicities 2, 4, and 6. The final collision strengths (Table 2) were produced with an energy resolution of 9 × 10−4 Ryd up to an energy three times that of the highest threshold. Full details of these calculations will be given in a forthcoming paper.
Footnotes
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This paper includes data taken at The McDonald Observatory of The University of Texas at Austin.
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In this paper we use vacuum wavelengths for NIR lines, and air wavelengths for optical lines.
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