Spectral analysis of the binary nucleus of the planetary nebula Hen 2-428 - first results

TL;DR

This paper presents the first detailed spectral analysis of the planetary nebula Hen 2-428's binary core, providing key parameters that help evaluate its potential as a type Ia supernova progenitor.

Contribution

It offers the first quantitative spectral analysis of Hen 2-428's central stars using advanced non-LTE models, informing future studies on its supernova progenitor status.

Findings

Derived effective temperatures and surface gravities of the central stars.

Estimated helium abundances in the binary system.

Provides constraints for future evolutionary and supernova progenitor models.

Abstract

Identifying progenitor systems for the double-degenerate scenario is crucial to check the reliability of type Ia super-novae as cosmological standard candles. Santander-Garcia et al. (2015) claimed that Hen 2-428 has a double-degenerate core whose combined mass significantly exceeds the Chandrasekhar limit. Together with the short orbital period (4.2 hours), the authors concluded that the system should merge within a Hubble time triggering a type Ia supernova event. Garcia-Berro et al. (2016) explored alternative scenarios to explain the observational evidence, as the high mass conclusion is highly unlikely within predictions from stellar evolution theory. They conclude that the evidence supporting the supernova progenitor status of the system is premature. Here we present the first quantitative spectral analysis of Hen 2-428 which allows us to derive the effective temperatures, surface gravities and helium abundance of the two CSPNe based on state-of-the-art, non-LTE model atmospheres. These results provide constrains for further studies of this particularly interesting system.

Full text

1 Introduction

Type Ia supernovae (SN Ia) are the most important standard candles to measure the largest structures of our universe. Although there is a general consensus that only the thermonuclear explosion of a white dwarf can explain the observed features of those events, the nature of their progenitor systems still remains elusive. One of the progenitor scenarios discussed is the merger of two white dwarfs with a total mass exceeding the Chandrasekhar mass limit, the double-degenerate model (Iben and Tutukov,, 1984; Webbink,, 1984). Another progenitor scenario is the single-degenerate model, in which a white dwarf accretes material from a non-degenerate companion and explodes when it reaches the mass limit (Whelan and Iben,, 1973).

Santander-Garcia et al., (2015) discovered that the central star of the planetary nebula (CSPN) Hen 2-428 is a close binary with an orbital period as short as 4.2 hours based on sinusoidal variations in its light curve. Furthermore, they detected a time-dependent splitting of the He II $\lambda\,5411$ Å line, which they identified as a signature for a double-lined binary system consisting of two pre-white dwarfs with equal masses of 0.88 $M_{\odot}$. In this case, the system would merge within 700 million years and since its total mass would exceed the Chandrasekhar limit, it would be one of the best SN Ia progenitor candidates known.

This scenario has since been challenged by Garcia-Berro et al., (2016). The most important criticism is the strong mismatch between the luminosities and radii of both white dwarf components as derived by Santander-Garcia et al., (2015) with the predictions from stellar evolution models (Blöcker and Schönberner,, 1991). Furthermore, the evolutionary times for post-AGB stars with masses as high as 0.88 $M_{\odot}$  to join the white dwarf cooling curve are predicted to be of the order of hundreds of years or less (Miller Bertolami,, 2016). Therefore, it should be almost impossible to observe two such objects in exactly the same stage.

Garcia-Berro et al., (2016) also perform a photometric analysis, but used evolutionary tracks of Renedo et al., (2010) to constrain the stellar temperatures. They showed that the light curves of Hen 2-428 may also be fitted well by assuming two CSPNe with lower masses (i.e., masses of 0.47 $M_{\odot}$  and 0.48 $M_{\odot}$) in an overcontact binary. Thus, Garcia-Berro et al., (2016) conclude that the claim that Hen 2-428 provides observational evidence for the double degenerate scenario for SN Ia is premature.

Given the potential importance of Hen 2-428 as a unique laboratory to study the double degenerate merger scenario, it is highly desirable to resolve this debate. Here, we provide, for the first time, a spectroscopic analysis of the system in order to constrain the effective temperatures, surface gravities and helium abundance through state of the art non-LTE atmospheric models.

2 Observations

We downloaded the VLT/FORS2 spectra of Hen 2-428 from the ESO archive (ProgIDs 085.D-0629(A) and 089.D-0453(A)). The observations were performed at different phases of the orbital period using the 1200G grism (resolving power $R=$ $\leavevmode\nobreak\ 1605$). The spectra were reduced using standard IRAF procedures.

A FORS2/1200G spectrum of Hen 2-428 is shown in Fig. 1. We identified photospheric absorption lines of H I, He I and He II, as well as nebular emission lines of H I, He I, N II, O II, $[$O III$]$, $[$Cl III$]$, $[$Ar III$]$, $[$Ar IV$]$, $[$Fe II$]$, and $[$Fe III$]$, and a diffuse interstellar band (DIB) at 4430 Å. The H I and He I lines are clearly contaminated by nebula emission. The He II absorption features are double lined and differ between spectra, but the cause of this variability is disputed. Santander-Garcia et al., (2015) explained this as a super-composition of two photospheric absorption lines, one from each star, varying due to orbital motion. However, Garcia-Berro et al., (2016) speculated that the variability in the He II $\lambda\,5411$ Å line could be interpreted as a narrow He II emission line with varying strength that originates from the compact planetary nebula superimposed onto a broad absorption line from the central stars. On the other hand, the variability of the He II $\lambda\,4686$ Å line cannot be explained by the presence of a nebular emission line. Thus, we conclude that all four He II features are purely photospheric, double lined, and the variability is due to the orbital motion. Therefore, these lines are suitable to perform the spectral analysis on.

3 Spectral Analysis

For the spectral analysis we employed the Tübingen Model Atmospheres Package (TMAP, Werner et al., (2003), Werner et al., (2012), Rauch and Deetjen, (2003)) to produce state-of-the-art non-LTE model atmospheres. Initially, our models contained only hydrogen and helium. The model grid spans from $T_{\mathrm{eff}}$  $=$ 30.0 - 70.0 kK (2.5 kK steps), $\log g=4.25-6.00$ (0.25 steps), and covered three helium abundances ($\log\,(He/H)=-2,-1,0$, number fractions).

To constrain the parameters of the system we used the XSPEC software (Shafer et al.,, 1991; Arnaud et al.,, 2017). XSPEC is a chi square minimisation code, originally designed for X-ray spectra, but which has been adapted to work on optical data (Dobbie et al.,, 2004). XSPEC determines the best fit model for the input parameters, which in this case are the effective temperatures, surface gravities, helium abundances, and radial velocities. First the radial velocity of each component was found, and then these were fixed whilst deriving $T_{\mathrm{eff}}$, $\log g$ and $\log\,(He/H)$ simultaneously. The flux contribution from each star is accounted for in XSPEC by adding our model grid to itself, thus allowing us to treat each of the stars' parameters separately and, therefore, replicating the binarity of the system.

Our preliminary results give \mbox{T_{\mathrm{eff}}}=53.0\pm 6.9\,\mathrm{kK}, $\log g=5.25\pm 0.07$ and a helium mass fraction of 0.27 $\pm$ 0.05 for the first star. For the second star we obtained \mbox{T_{\mathrm{eff}}}=49.0\pm 6.9\,\mathrm{kK}, $\log g=4.91\pm 0.08$ and a helium mass fraction of 0.24 $\pm$ 0.05. The reduced chi squared value was 0.65 for these parameters. The results are displayed visually in Fig. 2, where grey lines show the observed spectrum from one of the 2012 observations, and the red line is our best-fit TMAP model.

Whilst He II $\lambda\lambda\,4200,4542,5412$ Å are reproduced nicely, the line cores of He II $\lambda\,4686$ Å appear too deep compared to the observation. This is a known problem when fitting the spectra of CSPNe and other hot stars with pure hydrogen and helium models only. It has been shown that He II $\lambda\,4686$ Å is particularly susceptible to metal line blanketing (Reindl et al.,, 2014; Latour et al.,, 2015). Therefore, as a next step, we will also include metals in our TMAP models. Rodriguez et al., (2001) measured the nebula abundances of O, N, S, and Ar of Hen 2-428, which will provide an estimate of photospheric metal abundances of the stars.

Our derived effective temperatures (\mbox{T_{\mathrm{eff}}}=53.0\,\pm\,6.9\,\mathrm{kK}, $\log g=5.25\,\pm\,0.07$ and \mbox{T_{\mathrm{eff}}}=49.0\,\pm\,6.9\,\mathrm{kK}, $\log g=4.91\,\pm\,0.08$) contradict the results from Santander-Garcia et al., (2015) who present a photometric model with stellar temperatures of 32.4 $\pm$ 5.2 kK and 30.9 $\pm$ 5.2 kK. In our synthetic spectra the He II $\lambda\,4686$ Å line is present even at our lowest temperature, 30 kK. However, we stress that the synthetic spectra show that temperatures above 33 kK are required to produce the He II $\lambda\lambda\,4200,4542,5412$ Å lines at the highest helium abundances of our grid, $\log\,(He/H)=0$. This is also true at lower abundances, such as $\log\,(He/H)=-1$ which is close to the value derived for the CSPNe of Hen 2-428 (Rodriguez et al.,, 2001). For this helium abundance temperatures exceeding $T_{\mathrm{eff}}$ $\approx 40$ kK are needed to reproduce He II $\lambda\lambda\,4200,4542,5412$ Å at their observed strengths. Even higher temperatures are required to produce these lines if the gravity is higher than our derived value.

As already noted by Garcia-Berro et al., (2016), the upper limit placed by Santander-Garcia et al., (2015) on the effective temperatures of $T_{\mathrm{eff}}$ $\leq 40$ kK of the two CSPNe of Hen 2-428 is not valid. Santander-Garcia et al., (2015) established this upper limit based on the absence of a He II $\lambda\,5412$ Å emission line, but there are many PNe whose central stars have effective temperatures larger than 40 kK and do not have He II emission lines. In particular, Garcia-Berro et al., (2016) showed that the light curves of Hen 2-428 can be fitted equally well with models that assume 40-45 kK, which is consistent with our results.

4 Future Work

We have carried out the first quantitative spectral analysis of Hen 2-428 and constrained the effective temperatures, surface gravities and helium abundance of the two CSPNe based on the VLT/FORS2 spectra. We will extend our spectral analysis to include the OSIRIS spectra taken at the Gran Telescopio Canarias to place stronger constrains on the atmospheric parameters of the CSPNe. This is the first step towards a more comprehensive analysis of this exceptional system. In a next step, we envisage a more sophisticated radial velocity analysis by measuring the radial velocities of the two components using our synthetic spectra, in contrast to the Gaussian fitting of Santander-Garcia et al., (2015). In particular, we will take into account all four He II lines for these measurements instead of only He II $\lambda\,5412$ Å used by Santander-Garcia et al., (2015). The derived radial velocities will provide more reliable dynamical masses, which are the key point in understanding the true nature of the system. The dynamical masses, as well as the effective temperatures derived by our spectral analysis, will then serve as steady base for the light curve models and allow a sound conclusion on the true nature of the CSPNe of Hen 2-428.

Acknowledgements

This work is based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 085.D-0629(A) and 089.D-0453(A). NF is supported by an STFC Studentship. NR was supported by a Leverhulme Trust Research Project Grant. NR is supported by a research fellowship of the Royal Commission for the Exhibition of 1851. The TMAD service (http://astro-uni-tuebingen.de/~TMAD) used to compile atomic data for this work was constructed as part of the activities of the German Astrophysical Virtual Observatory. This research used the SPECTRE and ALICE High Performance Computing Facilities at the University of Leicester.