The place of WELS (weak emission line stars) in the context of central stars of planetary nebulae.

G. R. Keller (P.I.), W.J. Maciel, R.D.D. Costa, P.J. Lago, P.F. Penteado

WELS are a class of central stars of planetary nebulae (CSPNe) with at least 101 objects, out of less than 500 CSPNe for which spectral information exist. Although numerous, their nature and homogeneity are unclear and their evolutionary status and relation with other classes of CSPNe, unknown. These stars have not been systematically analyzed by means of stellar atmosphere modeling and their abundance patterns are mostly unknown.


We are obtaining on period 95 high signal-to-noise VLT/UVES spectra spanning the regions 3260-4450 and 4580-6680A of 4 WELS CSPNe, for both ranges are needed to provide enough spectral diagnostics to uniquely constrain the many stellar and wind parameters such as T*, logg, wind velocity profile, terminal wind velocity, mass-loss rate, clumping (wind inhomogeneities), and  abundances. The selected wavelength range holds some of the best diagnostics of mass-loss and T* available in H-poor CSPNe spectra, as shown in Fig.1A, and will be modeled with our extensive CSPNe grids of synthetic spectra (Keller et al., 2011, 2012b), calculated with the non-LTE stellar atmosphere code CMFGEN (Hillier and Miller, 1998), which accounts for line-blanketing, clumping, spherically-symmetric, expanding atmospheres and was previously used in the modeling of several classes of CSPNe, including H-rich, [WC], [WC]-PG1159 and PG1159 stars (Keller et al., 2011, 2012a,b, 2014; Herald and Bianchi, 2004a,b, 2007, 2009, 2011; Herald et al., 2005). The synthetic spectra are computed at high resolution and the model atmospheres include many ionic species previously neglected (H, He, C, N, O, Ne, Si, P, S, Fe, Ar, Ni, Mg, Na, Co). Further models will be computed as needed to ensure a detailed analysis.

Fig. 1: A.Example of line diagnostics in the requested wavelength range, taken from our CMFGEN model grids for [WC] stars and expanded for stars with fainter winds. B. Grid model with S/N simulation.

High resolution is important for our science case to separate the narrow nebular emissions superimposed to stellar features and to allow detailed modeling of the complicated line profiles in order to constrain the wind velocity profile and terminal wind velocity, thus securing a reliable determination of all other stellar parameters. The narrow slit we require (0.8") allows for R=50000 and minimizes nebular contamination. The 7"(blue arm) and 10"(red arm) slit lengths and the 1" seeing limited spatial resolution will enable extraction of the nebular spectra next to the central stars, which will be used to further evaluate possible contamination of the stellar spectra and, as a parallel project, to study nebular gas kinematics in the central regions of the PNe.

The analysis of the CS is complicated by the fact that He II lines from transitions 4 - n, with n even, have wavelengths very close to those of H I lines, making H abundances dicult to constrain and entails the need for the modeling of several of these lines in order for their relative strengths to be determined. With the selected spectral range, we have access to the entire sequence of H Balmer transitions and several He II lines. Based on our simulations (e.g. Fig.1B), we found S/N=100 at 3350A to be sucient for our science requirements. The selected objects all have archival low resolution IUE UV spectra, which will be modeled simultaneously with the requested data and will provide absolute fluxes and reddening for the sample objects. The stellar parameters derived by our uniform analysis will allow us to place the WELS within the context of CSPNe population, according to abundance patterns, photospheric, and wind parameters and test possible evolutionary links with other classes of CSPNe and proposed evolutionary scenarios, besides providing constraintsfor post-AGB evolution and AGB intershell processes.

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