Discovery of Raman-scattered He ii λ6545 in the Planetary Nebulae NGC 6886 and NGC 6881

Low- and intermediate-mass stars (0.8 − 8 M_⊙) lose a significant fraction of their mass through slow stellar winds in the asymptotic giant branch (AGB) stage, which plays a crucial role in the chemical enrichment of the interstellar medium (Kwok 2005; Höfner & Olofsson 2018). The hot core part evolves into a white dwarf with a mass less than the Chandrasekhar limit (M_WD < 1.4 M_⊙), and the ejected material forms a planetary nebula (PN). A young PN is an ionization-bounded system that contains both ionized and neutral regions (Webster et al. 1988; Dinerstein 1991; Taylor et al. 1990; Kastner et al. 1996; Huggins et al. 2005; Guzman-Ramirez et al. 2018). Because young PNe have recently entered into the PN stage, they are ideal objects to investigate the mass-loss history in the late stage of stellar evolution.

The existence of an H i component in PNe is known, but still not well understood. One possibility for the existence of an H i component is left over from the previous AGB stage (Glassgold & Huggins 1983). Another possibility is that the atomic hydrogen component is formed by the photodissociation of H₂. H i provides important information about the outer region of the circumstellar envelope (CSE) and the interaction between the CSE and interstellar medium (Matthews et al. 2013). Although atomic components are difficult to investigate due to severe confusion from the Galactic emission, successful H i 21 cm observations have been carried out, resulting in successful detections of H i regions with mass  ∼ 0.01 M_⊙ in a number of young PNe (Taylor et al. 1990; Gussie & Taylor 1995).

Another useful tool to probe the thick neutral region is Raman-scattered emission line features formed via the inelastic scattering of far-UV photons by hydrogen atoms. When a far-UV photon with energy near Lyβ is incident on a hydrogen atom, an optical photon near Hα can be emitted if the hydrogen atom deexcites into the 2s state instead of the ground state. Raman scattering with atomic hydrogen as an astrophysical tool was first suggested by Schmid (1989). He identified the broad emission features at 6830 Å and 7088 Å in symbiotic stars as Raman-scattered features of O vi λλ1032 and 1038 (Allen 1980; Akras et al. 2019). Symbiotic stars are wide binary systems consisting of an accreting white dwarf and a mass-losing giant. They provide an ideal place to observe Raman scattering because of the copious amount of far-UV photons emanating from the vicinity of the white dwarf. These UV photons are incident on a thick neutral hydrogen region formed around the evolved giant. Nussbaumer et al. (1989) proposed that photons from He ii can be Raman-scattered with hydrogen atoms producing emission lines that can be observed. These Raman-scattered He ii features have been detected in the symbiotic stars RR Telescopii, V1016 Cygni, HM Sagittae, and V835 Centauri (van Groningen 1993; Lee et al. 2001; Birriel 2004).

Strong He ii emitters are found among young PNe having a central star still sufficiently hot to ionize He ii with a thick atomic hydrogen component ejected in the previous AGB stage. Raman-scattered He ii λ6545 was reported to be found in the young PNe NGC 7027, IC 5117, and NGC 6790 (Péquignot et al. 2003; Lee et al. 2006; Kang et al. 2009). The detection of Raman-scattered He ii λ4851 arising from Raman scattering of He ii λ972 was reported in the PNe NGC 7027, NGC 6302, NGC 6886, and IC 5117 (Péquignot et al. 1997; Groves et al. 2002; Péquignot et al. 2003; Lee et al. 2006).

In this Letter, we report our first detection of Raman-scattered He ii λ6545 in the two young PNe NGC 6886 and NGC 6881.

Raman scattering of atomic hydrogen with incident far-UV radiation near the Lyman n → 1 (n ≥ 3) series may result in the emission of an optical photon near the Balmer n → 2 series. The wavelength of Raman-scattered radiation (λ_o) is related to that of the incident radiation (λ_i) by energy conservation:

$$\begin{eqnarray}&&{\lambda }_{{\rm{o}}}^{-1}={\lambda }_{{\rm{i}}}^{-1}-{\lambda }_{\mathrm{Ly}\alpha }^{-1},\end{eqnarray} \tag{ 1 }$$

where λ_Lyα is the wavelength of Lyα . He ii and H i are single-electron systems, sharing the same electronic level structure. The level spacing of He ii is larger than that of H i by a factor slightly exceeding 4 augmented by the fact that the two-body reduced mass of He ii is larger by a factor  ∼3m_e/4m_p, where m_e and m_p are the electron and proton masses. This leads to the far-UV He ii (2n → 2) line being systematically blueshifted by an amount  ∼−120 km s⁻¹ compared to the H i (n → 1) Lyman line (Lee et al. 2001). In particular, He ii λ1025 (n = 4 → 2) is Raman-scattered to form an optical feature at 6545 Å according to Equation (1). In addition, Raman scattering He ii λ972 (n = 6 → 2) may form an optical feature at 4851 Å.

Differentiation of Equation (1) yields the relation between the line widths (Δλ_o and Δλ_i):

$$\begin{eqnarray}&&\displaystyle \frac{{\rm{\Delta }}{\lambda }_{o}}{{\lambda }_{o}}=\left(\displaystyle \frac{{\lambda }_{o}}{{\lambda }_{i}}\right)\displaystyle \frac{{\rm{\Delta }}{\lambda }_{i}}{{\lambda }_{i}},\end{eqnarray} \tag{ 2 }$$

giving rise to the formation of broad Raman-scattered features by a factor λ_o/λ_i. In the case of Raman scattering near Lyβ, this factor is ∼6.4. This means that a far-UV line with a line width Δv = 30 km s⁻¹ may form a Raman feature with a significantly broadened width of  ∼190 km s⁻¹.

Cross sections for Rayleigh and Raman scattering are computed using time-dependent second-order perturbation theory (e.g., Nussbaumer et al. 1989; Lee et al. 2006). In particular, the cross sections for Rayleigh and Raman scattering of He ii λ1025 are  ∼5.7 × 10⁻²¹ cm² and  ∼7.4 × 10⁻²² cm², respectively. Raman He ii at 6545 Å becomes an excellent probe of H i regions characterized by the H i column density N_HI in excess of 10²⁰ cm⁻².

3.1. Observation

3.2. Spectra of NGC 6886 and NGC 6881

While there were several marginal detections of the Raman-scattered He ii features, here we present the two most clear detections. These sources were NGC 6886 and NGC 6881. The left and right panels of Figure 1 show the BOES spectra of NGC 6886 and NGC 6881, respectively. The upper and lower panels show the short- and long-exposure spectra, respectively. Total integrated times for the long exposure were 2400 and 3300 s for NGC 6886 and NGC 6881, respectively. Strong emission lines including Hα, He ii λ6560, and [N ii] λλ 6548 and 6583 are found in the upper panels.

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In the lower panels, two weak emission lines, He ii λ6527 and C ii λ6578, clearly appear. There are broad emission features seen in both sources (denoted with ovals) that are blended with [N ii] λ6548. These are the Raman-scattered He ii lines at 6545 Å. Despite the [N ii] λ6583 line being theoretically 3 times stronger than [N ii] λ6548, the absence of a broad emission feature around the [N ii] λ6583 line means we are confident in our identification of the Raman-scattered He ii lines. Furthermore, case B recombination theory predicts that the peak value of He ii λ6527 is comparable to that of the Raman-scattered He ii λ6545, which can be seen in Figure 1 (Storey & Hummer 1995; Lee et al. 2006).

The profiles of strong lines Hα and He ii λ6560 shown in the upper panels are fairly symmetric. However, it is apparent that the Raman He ii features exhibit an extended red tail. Although severe blending with [N ii] λ6548 hinders quantitative analyses, as a first approximation to the line profiles, we applied a single Gaussian fitting to each of the Hα, He ii, and Raman He ii λ6545 lines. The results are presented in Table 1. Here, λ_c is the observed center wavelength of each emission line, and f₀ is the peak value of line flux. The FWHM in velocity space is represented by v_G.

Table 1.  Single Gaussian Parameters for Hα, He ii Emission Lines, and Raman He ii λ6545 Feature of NGC 6886 and NGC 6881 Spectra

PN	Line	λc	f0	vG
		(Å)	(erg s−1cm−2Å−1)	(km s−1)
NGC	Hα	6562.51	1.47 × 10−11	50
6886	He ii λ6560	6559.84	4.29 × 10−13	36
	He ii λ6527	6526.78	1.96 × 10−14	36
	Raman He ii	6548.65	7.31 × 10−15	412
NGC	Hα	6562.23	3.18 × 10−12	34
6881	He iiλ6560	6559.57	9.04 × 10−14	29
	He ii λ6527	6526.63	4.02 × 10−15	29
	Raman He ii	6547.93	2.05 × 10−15	379

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The line center of Raman He ii λ6545 is expected to be found at 6544.47 Å for NGC 6886 and at 6542.76 Å for NGC 6881 based on the values of λ_c of He ii λ6560 and the atomic physical relation given by Equation (1). However, considerable redward deviations of  ∼+190 km s⁻¹ and  ∼+240 km s⁻¹ are found for NGC 6886 and NGC 6881, respectively. Considering the line-broadening effect of Raman scattering described by Equation (2), these deviations correspond to receding motions of the H i region with  ∼30 km s⁻¹ and  ∼40 km s⁻¹ with respect to the central far-UV He ii emission region in NGC 6886 and NGC 6881, respectively.

The He ii emission lines are observed to be narrower than Hα with v_G ∼ 36 km s⁻¹ for NGC 6886 and 29 km s⁻¹ for NGC 6881, respectively. If the Raman conversion efficiency is independent of wavelength, then we may expect that the Raman He ii λ 6545 features have the line widths of v_G ∼ 200 km s⁻¹, which is much smaller than the values  ∼400 km s⁻¹ shown in Table 1. However, the sharply increasing cross section across the He ii λ1025 line implies that the Raman conversion efficiency increases toward the red part. This leads to the resultant line profiles of Raman He ii that are significantly enhanced in the red part with additional line broadening (Choi et al. 2020).

Choi et al. (2020) performed an extensive study of line formation of Raman He ii in an expanding H i region with a constant speed by carrying out grid-based Monte Carlo simulations. They showed that complicated behavior manifests itself as center shifts, significant distortions of the line profile, and the appearance of a secondary peak when the H i region expands with a typical speed of  ∼30 km s⁻¹.

The same grid-based Monte Carlo code "STaRS² " is used to investigate the distribution and kinematics of the neutral hydrogen regions in NGC 6886 and NGC 6881. Figure 2 shows a schematic illustration of the scattering geometry adopted in this work. The scattering region is a partial spherical shell surrounding the central He ii region. The covering factor CF is defined as ${CF}=\tfrac{\theta }{\pi }$, where θ is the opening angle of the partial spherical shell. The H i region is assumed to be of uniform density expanding with a single speed ${v}_{\exp }$.

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Far-UV He ii λ1025 photons generated in the central He ii region are incident on the H i region. The neutral region is optically thick to Rayleigh-scattered UV photons, and optically thin to Raman-scattered optical photons. Each far-UV photon is traced until escape from the neutral region through multiple Rayleigh scattering, or after Raman scattering. The incident far-UV He ii λ1025 is assumed to be described by a single Gaussian function determined in Section 3.2.

A simulation model is set with the following free parameters: the covering factor CF of the scattering region, H i column density N_HI, and expanding speed ${v}_{\exp }$. The incident He ii λ1025 flux is deduced from the observed He ii λ6560 line flux and using case B recombination theory (Storey & Hummer 1995). We refer to Hyung et al. (1995) and Pottasch & Surendiranath (2005) for NGC 6886 and also Kaler et al. (1987) for NGC 6881 for the values of T_e = 1.25 × 10⁴ K and n_e = 10⁴ cm⁻³.

In Figure 3, we present our best-fit results for NGC 6886 and NGC 6881 in the middle panels. The gray lines represent the observational data and the black lines show our best-fit simulation results. Despite severe blending of the Raman-scattered He ii features with strong [N ii] λ6548 lines, the broad features with a red-tail structure are conspicuous in both PNe. These red-tailed features are attributed to the expansion of H i region.

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In the case of NGC 6886, the best-fit model parameters are CF = 0.3, N_HI = 5 × 10²⁰ cm⁻², and ${v}_{\exp }=25\,\mathrm{km}\ {{\rm{s}}}^{-1}$ (CF30-N5E20-V25). Similarly for NGC 6881, the best-fit model is found with parameters of CF = 0.6, N_HI = 3 × 10²⁰ cm⁻², and ${v}_{\exp }=30\,\mathrm{km}\ {{\rm{s}}}^{-1}$ (CF60-N3E20-V30). The line flux of Raman He ii is mainly determined by the product of the covering factor CF and H i column density N_HI, resulting in degeneracy in the modeling of Raman He ii. However, it appears that the fit to Raman He ii λ6545 becomes better with a somewhat lower covering factor CF < 0.5 for NGC 6886 and a higher value CF > 0.5 for NGC 6881, respectively.

In Figure 3, the models with a lower expansion speed (top panels) and a higher speed (bottom panels) show significantly poorer fit to the observed data, providing valid ranges of ${v}_{\exp }=25\pm 5\,\mathrm{km}\,{{\rm{s}}}^{-1}$ for NGC 6886 and ${v}_{\exp }=30\pm 10\,\mathrm{km}\,{{\rm{s}}}^{-1}$ for NGC 6881, respectively. However, the insufficient data quality and severe blending with [N ii] prevent us from putting strong constraints on the model parameters.

We report our successful detection of Raman-scattered He ii at 6545 Å in two young planetary nebulae NGC 6886 and NGC 6881. Gaussian model fitting is applied to find that the Hα and He ii λ6560 lines are symmetric, whereas the Raman-scattered He ii features appear redshifted from their expected atomic line center and exhibit an extended red-tail structure indicative of the expanding H i regions with respect to the hot central emission region. We perform Monte Carlo simulations using the grid-based 3D code "STaRS" to obtain best-fitting profiles.

In the case of NGC 6886, Péquignot et al. (2003) mentioned their detection of Raman-scattered He ii at 4851 Å blueward of Hβ. Our best-fit models suggest that the column density of atomic hydrogen region N_HI ∼ 5 × 10²⁰ cm⁻² with its covering factor CF ∼ 0.3. We also measure its expanding speed ${v}_{\exp }$ to be 25 km s⁻¹. A similar result was reported by Taylor et al. (1990).

On the other hand, it is the first direct detection of an H i component in NGC 6881. NGC 6881 is an interesting object exhibiting highly collimated quadrupolar lobes and a multiple ring structure of the ionized region that is attributed to a precessing jet (Guerrero & Manchado 1998; Kwok & Su 2005). Furthermore, Ramos-Larios et al. (2008) argued that its ionized quadrupolar lobes and H₂ bipolar lobes show different morphologies and collimation degrees, indicating that this system underwent multiple bipolar ejections with varying mechanisms. We estimate the H i column density N_HI ∼ 3 × 10²⁰ cm⁻² and the expansion speed ${v}_{\exp }=30\,\mathrm{km}\,{{\rm{s}}}^{-1}$ assuming a covering factor of CF ∼ 0.6.

The total H i mass M_HI based on our model is given by

$$\begin{eqnarray}&&{M}_{\mathrm{HI}}\simeq 1.4\times {10}^{-4}\,\left(\displaystyle \frac{{N}_{\mathrm{HI}}}{{10}^{20}\,{\mathrm{cm}}^{-2}}\right){\left(\displaystyle \frac{{R}_{\mathrm{out}}}{{10}^{3}\mathrm{au}}\right)}^{2}{CF}\,{M}_{\odot },\end{eqnarray} \tag{ 3 }$$

where we set R_out to 2 times R_in. The distance to NGC 6886 is about 2.6 kpc, and the angular size of its central ionized region as measured from its optical image is roughly 5'' (Pottasch & Surendiranath 2005; assumed to be R_in), corresponding to a linear size of 13,000 au. This leads to an estimate of M_HI ∼ 0.03 M_⊙. The distance to NGC 6881 is  ∼ 2.5 kpc (Cahn et al. 1992) with an angular size of its central ionized region being  ∼ 5'' (Kwok & Su 2005), corresponding to a linear size of 13,000 au and therefore M_HI ∼ 0.04 M_⊙. Setting R_out to be twice R_in is somewhat arbitrary, and of course if we set R_out to be 4 times R_in, then the H i mass estimate will increase by 4 times.

Raman-scattered He ii features can be formed near the hydrogen Balmer series at 6545 Å, 4851 Å, and 4332 Å derived from Raman scattering of far-UV He ii λ1025, λ972, and λ949, respectively. The integrated line analyses of Raman He ii features are an appropriate probe of H i regions with a column density of N_HI ∼ 10²⁰⁻²³ cm⁻². Deep spectroscopy is required to detect Raman He ii λλ4851 and 4332. The detection of these lines will allow us to carry out better studies of the kinematics and distribution of thick neutral hydrogen regions in PNe.

It is notable that both Raman He ii features and C ii λ6578 in emission are detected in NGC 7027, IC 5117, and NGC 6790, similar to our two targets NGC 6886 and NGC 6881 (Keyes et al. 1990; Lee et al. 2006; Kang et al. 2009). Furthermore, these objects are also strong polycyclic aromatic hydrocarbon (PAH) emitters (Smith & McLean 2008; Ohsawa et al. 2016) indicating that these PNe possess carbon-enriched environments from the previous AGB stage as a result of dredge-up processes. However, due to small number statistics it is too early to conclude that Raman-scattered He ii features are found in only carbon-enriched PNe. A definite conclusion should wait for a more systematic survey of PNe with Raman-scattered He ii.

The authors are grateful to the staff of the BOAO and Seulgi Kim for her help in the data acquisition of NGC 6886. We also thank Jeff Hodgson for his constructive comments on the manuscript. This research was supported by the Korea Astronomy and Space Science Institute under the R&D program (Project No. 2018-1-860-00) supervised by the Ministry of Science, ICT, and Future Planning. This work was also supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT: No. NRF-2018R1D1A1B07043944).