A complete spectral model of a ring nebula and its associated … · 2014. 1. 29. · Jonnathan...
1
A complete spectral model of a ring nebula and its associated central star Jonnathan Reyes-Pérez & Christophe Morisset Instituto de Astronomía, Universidad Nacional Autónoma de México, México, D. F. [email protected] RESULTS The stellar model The input values we constrained were the mass-loss rate Mdot, the terminal wind velocity Vinf, the stellar luminosity L, the radius of the star R* and the chemical composition. In the stellar winds it is expected that the recombination line luminosity is proportional to the mass-loss rate, therefore recombination lines as H were used to find a good value of Mdot. The value of the terminal velocity was constrained by fitting the blue edge of the satured C IV 1550 Å P-Cygni profile, the obtained value fits well other P-Cygni profiles as P V and S IV in the far UV spectrum of the star. When we consider a stellar atmosphere with spherical simmetry it does not make sense to assign a surface temperature to the star, therefore the temperature cannot be a global input value in the model, but it is possible to define a temperature value where = 2/3 and it is set in the model through the luminosity and the stellar radius at this optical depth. In this way, the L and R* values were changed to fit the line profiles corresponding to consecutive ionization degrees of the same element, as the He I and He II lines. Our best model values are shown in the Table 1 and in Figure 2 is shown the model in red over the observations. INTRODUCTION When we make a single study of these systems it is difficult to assign a value to the physical parameters only based on generical data, that is why we make a semiempirical methodology based in the selfconsistent modelisation of the star and the optical nebula associated. This methodology gives us the advantage to obtain coherent and more constrained physical values between all the components of the entire system than those resulting from a single study of each component. It is logic to proceed in this way because the nebula associated to the star is naturally an additional restriction to the stellar atmosphere model, as it offers information about the amount of ionizing photons. In this work we focus on the ring nebula NGC 6888. NGC 6888 is an emission nebula associated to the Wolf-Rayet star WR 136 (spectral type WN 6) and is a typical exemple of the ring nebula objects resulting from the continuous interaction between the strong stellar wind and the interstellar medium. Projected on the sky, this nebula is seen to have an almost elliptical shape with an optical axis-size of 12’ x 18'. The combination of its clumpy morphology and its nitrogen enhanced abundance suggest that the gaseous ejections coming from the WR star have contributed in an important way to the formation of this ring nebula. DISCUSION AND CONCLUSIONS The derived values as L, Mdot and Vinf from the stellar model of the star WR 136 agree with the values found by other authors, i.e. Hamman et al. (1994) and Crowther & Smith (1996), but the effective temperature is lower than the precedent values and we believe this is a more realistic value; this is because a star with a Teff higher than 50000 K produces a higher ionized nebula than it is observed, in particular, the emission of [O III] lines obtained by the pyCloudy models are overestimated leading to decrease even more the oxygen abundance to perform a good fit. In other hand, the combination of the temperature and mass-loss effects in the wind gas considered for this model can reproduce the line profiles coming from recombination process, therefore if we really are near a good value of Teff so it is the same for the mass-loss rate value. It is important because this value can add a good constraition for the hydrodynamical simulations. For exemple, recently Zhekov & Park (2010) conclude that it is necessary a value 10 times lower than expected in order to reproduce the X-ray flux of the emitting hot bubble, therefore once the Mdot can be well constrained, we can better understand about the heating and cooling processes inside the hot bubble. About the nebula NGC 6888, optical observations have been shown that a thin skin of gas, mainly emitting on [O III], is covering the H and the [N II] emission (Moore et al. 2000). Contrary to the presence of shocks as the only mechanism leading to such ionization structure, for this model we alternatively propose the presence of a gaseous component with very low density. In this way we are capable to reproduce the observed ionization structure when it is considered two gaseous components in order to obtain a higher ionization parameter beyond the H and [N II] emitting region of NGC 6888. We cannot immediatly rule out the importance of shock effects but given we can reproduce several emission features of the nebula only with photoionization assumptions in addition with some plasma diagnostics it leads us to suggest that maybe the photoionization heating is more important than the presence of shocks (Reyes-Pérez et al. in progress). This last idea was already established by Esteban & Vilchez (1992) combining the diagnostic diagrams of Sabbadin et al. (1977) with their observations on a zone near the one studied in the present work. The abundances derived from this model correspond with similar values in other works preceding this one (i.e Esteban & Vílchez 1992). It is remarked that in both the stellar atmosphere of WR 136 and the nebular gas there is an enhancement of nitrogen an carbon and a depletion of the oxygen content. This contibutes to the idea that the observed gas is enriched by the central star through the mass-lost driven by the stellar wind (a large discusion in Mesa-Delgado et al. in preparation). Finally, in this work we show the advantages when modelling simultaneously the stellar and nebular spectra of a Wolf-Rayet nebula system. In this way we shown a coherent stellar and nebular model that reproduces observations of the ring nebula NGC 6888. We tested whether a stellar atmosphere model of the WR 136 star provides an optimal description of the stellar observations and is able to satisfactory reproduce the nebular observations as the ionization degree. This allowed us to determine all the physical parameters of both the star and the nebula, including the chemical abundances. HOW TO DO A SELF-CONSISTENT MODEL FOR A NEBULA ASSOCIATED TO ONE STAR? From spectroscopical observations first we model the stellar atmosphere using the CMFGEN code (Hillier & Miller 1998). In this case for WR 136, we have analyzed a spectrum between 900 Å and 1200 Å taken from the FUSE satellite observational program C097. To extend the wavelenght range to 1800 Å the FUSE’s spectrum was combined with the individual high dispersion IUE spectra. In addition to the UV spectra, WR 136 was observed with the echelle spectrograph at the 2.1 m telescope at the National Astronomical Observatory (NAO) in San Pedro Mártir, Mexico. The so obtained spectrum ranges from 3000 Å to 7000 Å. Once the stellar model is done, this one is used as the ionization source input for the photionization model calculated with the pyCloudy 3D formalism (Morisset 2011). The main analysis of the physical conditions of the gas in NGC 6888 was based on the echelle optical observations carried out with the 2.1 m telescope at the NAO for three different slit positions, each one with a total exposure time of two hours. After the standar reduction with the astronomical software MIDAS resulted three high resolution spectra from 3700 Å to 7300 Å. Additionally was used echelle spectroscopy performed with the Subaru Telescope (Mesa- Delgado in progress.) (Figure 1.) Finally, if it is not possible to reproduce the ionization degree given by the lines present in the nebular spectra, the stellar model is recalculated in the way to obtain, for exemple, a hotter or colder effective temperature according with the exigence of the nebula. In this way an interational process is performed. Table 1. 1.28 P 1.07 S 1.26 Fe 2.08 Si 0.12 O 1.91 N 2.40 C 3.42 He Abundances Z/Zsun 50150 Teff[=2/3](K) 1550 Vinf (km/s) 2.86e+5 L(Lsun) 1.18e-5 Mdot(Msun/yr) Stellar CMFGEN model of WR 136 7.90 7.90 8.67 11.20 He C N Abundances from the model 12 + log (X/H) O 316.81 310 ± 180 ne[O II] /cm-3 *Q is a quality factor and it means how good is the fit. In general, a good line intensity fit gives a Q value between -2.0 and 2.0. 0.00 100 100 ± 1.0 H1_4861A 10263 9550 ± 810 Te[O III] K Plasma diagnostics 7.40 24.87 19.84 ± 0.60 Ar3_7136A 4.19 5.48 3.79 ± 0.35 S2_6731A 2.91 5.29 3.99 ± 0.41 S2_6716A 1.18 133.47 130.09 ±3.0 N2_6583A 0.51 281.32 274.34 ± 5.0 O3_5007A 2.67 30.74 24.02 ±2.30 O2_3729A 3.37 29.96 21.94 ± 2.10 O2_3726A 0.00 0.69 0.69: C2_4267A -0.95 23.72 25.99 ± 2.60 He1_5876A -2.23 4.09 4.41 ± 0.15 He1_5016A 0.93 290.86 285.00 ± 6.0 H1_6563A -1.07 47.62 50.13 ± 2.5 H1_4340A Q* Imod Iobs Line Table 2. The nebular model To reproduce the optical spectral signatures of NGC 6888 was built a photoionization model using the pyCloudy 3D formalism, this method enable us to create a pseudo 3D model of a symmetric photoionized region through the interpolation between individual Cloudy models, this has the advantage to compare directly the observed line intensities under almost the same observational conditions, i.e. it is possible to obtain the resulting total line emissivities by integrating the contribution of each individual Cloudy model over a region constrained by a mask that represents the slit position on the nebula where the observations were performed (Fig. 3). In this photoionization model was used, as ionization source, a stellar atmosphere model of the star WR 136 calculated with the CMFGEN code described above. We assume that NGC 6888 has an ellipsoidal shape and is made mainly of two gaseous components, one denser (450 cm-3) than the other one (1 cm-3), both with a constant density profile where the free parameters are the inner distance from the star following the respective ellipticity values and a filling factor. The required He, C, N ,O abundances to reproduce the line intensities are shown in Table 2. Additionally, taking in mind the very clumped structure of NGC 6888, was added to the model a weight value for the line emissivities, this value indicates from which component comes the major contribution to the total emission in the line of sight, once the abundances were fixed only this wieght value was changed to fit the observed fluxes. Wavelenght Å Flux normaized to continuum Wavelenght Å Flux (erg/cm2/s-1/Å) References - Crowther, P. A. & Smith, L. J. 1996, A&A, 305, 541. - Esteban, C. & Vílchez, J. M. 1992, ApJ, 390, 536. - Hamman, W.-R., Wessolowski, U. & Koesterke, L. 1994 A&A, 281, 184. - Hillier, D. J. & Miller, D. L. 1998, ApJ, 496, 407. - Mesa-Delgado et al. in preparation - Morisset, C. 2011. sites.google.com/site/cloudy3d - Reyes-Pérez et al. in progress. - Sabbadin, F., Minello, S. & Bianchini, A. 1977, A&A, 60, 147 - Zhekov, S. A. & Park, S. 2011, ApJ, 728, 135. Figure 1. The ring nebula NGC 6888. Black lines represent the slit positions where the observations were performed. Image from Daniel López IAC taken with the Isaac Newton Telescope. Figure 2. Model spectrum in red over observed spectra in black. Figure 3. Intensity maps of [O II], [O III], [N II] ions RGB image plotted with pyCloudy 3D. Squares represents the corresponding slits positions where the spectroscopical observations were performed.
Transcript of A complete spectral model of a ring nebula and its associated … · 2014. 1. 29. · Jonnathan...
A complete spectral model of a ring nebula and its associated central starJonnathan Reyes-Pérez & Christophe Morisset
Instituto de Astronomía, Universidad Nacional Autónoma de México, México, D. [email protected]
RESULTS
The stellar modelThe input values we constrained were the mass-loss rate Mdot, the terminal wind velocity Vinf, the stellar luminosity L, theradius of the star R* and the chemical composition. In the stellar winds it is expected that the recombination line luminosityis proportional to the mass-loss rate, therefore recombination lines as H were used to find a good value of Mdot. Thevalue of the terminal velocity was constrained by fitting the blue edge of the satured C IV 1550 Å P-Cygni profile, theobtained value fits well other P-Cygni profiles as P V and S IV in the far UV spectrum of the star. When we consider astellar atmosphere with spherical simmetry it does not make sense to assign a surface temperature to the star, therefore thetemperature cannot be a global input value in the model, but it is possible to define a temperature value where = 2/3 and itis set in the model through the luminosity and the stellar radius at this optical depth. In this way, the L and R* values werechanged to fit the line profiles corresponding to consecutive ionization degrees of the same element, as the He I and He II lines. Our best model values are shown in the Table 1 and in Figure 2 is shown the model in red over the observations.
INTRODUCTION
When we make a single study of these systems it isdifficult to assign a value to the physical parameters onlybased on generical data, that is why we make asemiempirical methodology based in the selfconsistentmodelisation of the star and the optical nebula associated.This methodology gives us the advantage to obtaincoherent and more constrained physical values betweenall the components of the entire system than thoseresulting from a single study of each component. It is logicto proceed in this way because the nebula associated tothe star is naturally an additional restriction to the stellaratmosphere model, as it offers information about theamount of ionizing photons. In this work we focus on thering nebula NGC 6888. NGC 6888 is an emission nebulaassociated to the Wolf-Rayet star WR 136 (spectral typeWN 6) and is a typical exemple of the ring nebula objectsresulting from the continuous interaction between thestrong stellar wind and the interstellar medium. Projectedon the sky, this nebula is seen to have an almost ellipticalshape with an optical axis-size of 12’ x 18'. Thecombination of its clumpy morphology and its nitrogenenhanced abundance suggest that the gaseous ejectionscoming from the WR star have contributed in animportant way to the formation of this ring nebula.
DISCUSION AND CONCLUSIONS
The derived values as L, Mdot and Vinf from the stellarmodel of the star WR 136 agree with the values found byother authors, i.e. Hamman et al. (1994) and Crowther &Smith (1996), but the effective temperature is lower thanthe precedent values and we believe this is a more realisticvalue; this is because a star with a Teff higher than 50000K produces a higher ionized nebula than it is observed, inparticular, the emission of [O III] lines obtained by thepyCloudy models are overestimated leading to decreaseeven more the oxygen abundance to perform a good fit.In other hand, the combination of the temperature andmass-loss effects in the wind gas considered for this modelcan reproduce the line profiles coming fromrecombination process, therefore if we really are near agood value of Teff so it is the same for the mass-loss ratevalue. It is important because this value can add a goodconstraition for the hydrodynamical simulations. Forexemple, recently Zhekov & Park (2010) conclude that itis necessary a value 10 times lower than expected in orderto reproduce the X-ray flux of the emitting hot bubble,therefore once the Mdot can be well constrained, we canbetter understand about the heating and cooling processesinside the hot bubble.
About the nebula NGC 6888, optical observations havebeen shown that a thin skin of gas, mainly emitting on [OIII], is covering the H and the [N II] emission (Moore etal. 2000). Contrary to the presence of shocks as the onlymechanism leading to such ionization structure, for thismodel we alternatively propose the presence of a gaseouscomponent with very low density. In this way we arecapable to reproduce the observed ionization structurewhen it is considered two gaseous components in order toobtain a higher ionization parameter beyond the H and[N II] emitting region of NGC 6888. We cannotimmediatly rule out the importance of shock effects butgiven we can reproduce several emission features of thenebula only with photoionization assumptions in additionwith some plasma diagnostics it leads us to suggest thatmaybe the photoionization heating is more importantthan the presence of shocks (Reyes-Pérez et al. inprogress). This last idea was already established byEsteban & Vilchez (1992) combining the diagnosticdiagrams of Sabbadin et al. (1977) with their observationson a zone near the one studied in the present work. Theabundances derived from this model correspond withsimilar values in other works preceding this one (i.eEsteban & Vílchez 1992). It is remarked that in both thestellar atmosphere of WR 136 and the nebular gas thereis an enhancement of nitrogen an carbon and a depletionof the oxygen content. This contibutes to the idea that theobserved gas is enriched by the central star through themass-lost driven by the stellar wind (a large discusion inMesa-Delgado et al. in preparation).
Finally, in this work we show the advantages whenmodelling simultaneously the stellar and nebular spectraof a Wolf-Rayet nebula system. In this way we shown acoherent stellar and nebular model that reproducesobservations of the ring nebula NGC 6888. We testedwhether a stellar atmosphere model of the WR 136 starprovides an optimal description of the stellar observationsand is able to satisfactory reproduce the nebularobservations as the ionization degree. This allowed us todetermine all the physical parameters of both the star andthe nebula, including the chemical abundances.
HOW TO DO A SELF-CONSISTENT MODEL FOR ANEBULA ASSOCIATED TO ONE STAR?
From spectroscopical observations first we model the stellaratmosphere using the CMFGEN code (Hillier & Miller1998). In this case for WR 136, we have analyzed a spectrumbetween 900 Å and 1200 Å taken from the FUSE satelliteobservational program C097. To extend the wavelenghtrange to 1800 Å the FUSE’s spectrum was combined withthe individual high dispersion IUE spectra. In addition to theUV spectra, WR 136 was observed with the echellespectrograph at the 2.1 m telescope at the NationalAstronomical Observatory (NAO) in San Pedro Mártir,Mexico. The so obtained spectrum ranges from 3000 Å to7000 Å.
Once the stellar model is done, this one is used as theionization source input for the photionization modelcalculated with the pyCloudy 3D formalism (Morisset 2011).The main analysis of the physical conditions of the gas inNGC 6888 was based on the echelle optical observationscarried out with the 2.1 m telescope at the NAO for threedifferent slit positions, each one with a total exposure time oftwo hours. After the standar reduction with the astronomicalsoftware MIDAS resulted three high resolution spectra from3700 Å to 7300 Å. Additionally was used echellespectroscopy performed with the Subaru Telescope (Mesa-Delgado in progress.) (Figure 1.)
Finally, if it is not possible to reproduce the ionization degreegiven by the lines present in the nebular spectra, the stellarmodel is recalculated in the way to obtain, for exemple, ahotter or colder effective temperature according with theexigence of the nebula. In this way an interational process isperformed.
Table 1.
1.28P
1.07S
1.26Fe
2.08Si
0.12O
1.91N
2.40C
3.42He
Abundances Z/Zsun
50150Teff[=2/3](K)
1550Vinf (km/s)
2.86e+5L(Lsun)
1.18e-5Mdot(Msun/yr)
Stellar CMFGEN model of WR 136
7.90
7.90
8.67
11.20He
C
N
Abundances from the model 12 + log (X/H)
O
316.81310 ± 180ne[O II] /cm-3
*Q is a quality factor and it means how good is the fit. In general, a good line intensityfit gives a Q value between -2.0 and 2.0.
0.00100100 ± 1.0H1_4861A
102639550 ± 810Te[O III] K
Plasma diagnostics
7.4024.8719.84 ± 0.60Ar3_7136A
4.195.483.79 ± 0.35S2_6731A
2.915.293.99 ± 0.41S2_6716A
1.18133.47130.09 ±3.0N2_6583A
0.51281.32274.34 ± 5.0O3_5007A
2.6730.7424.02 ±2.30O2_3729A
3.3729.9621.94 ± 2.10O2_3726A
0.000.690.69:C2_4267A
-0.9523.7225.99 ± 2.60He1_5876A
-2.234.094.41 ± 0.15He1_5016A
0.93290.86285.00 ± 6.0H1_6563A
-1.0747.6250.13 ± 2.5H1_4340A
Q*ImodIobsLine
Table 2.The nebular model
To reproduce the optical spectralsignatures of NGC 6888 was built aphotoionization model using thepyCloudy 3D formalism, this methodenable us to create a pseudo 3Dmodel of a symmetric photoionizedregion through the interpolationbetween individual Cloudy models,this has the advantage to comparedirectly the observed line intensitiesunder almost the same observationalconditions, i.e. it is possible to obtainthe resulting total line emissivities byintegrating the contribution of eachindividual Cloudy model over a regionconstrained by a mask that representsthe slit position on the nebula wherethe observations were performed (Fig.3). In this photoionization model wasused, as ionization source, a stellaratmosphere model of the star WR 136calculated with the CMFGEN codedescribed above.
We assume that NGC 6888 has anellipsoidal shape and is made mainlyof two gaseous components, onedenser (450 cm-3) than the other one(1 cm-3), both with a constant densityprofile where the free parameters arethe inner distance from the starfollowing the respective ellipticityvalues and a filling factor. Therequired He, C, N ,O abundances toreproduce the line intensities areshown in Table 2. Additionally, takingin mind the very clumped structure ofNGC 6888, was added to the model aweight value for the line emissivities,this value indicates from whichcomponent comes the majorcontribution to the total emission inthe line of sight, once the abundanceswere fixed only this wieght value waschanged to fit the observed fluxes.
Wavelenght Å
Flux
nor
mai
zed
to c
ontin
uum
Wavelenght Å
Flux
(erg
/cm
2/s-
1/Å
)
References
- Crowther, P. A. & Smith, L. J. 1996, A&A, 305, 541.
- Esteban, C. & Vílchez, J. M. 1992, ApJ, 390, 536.
- Hamman, W.-R., Wessolowski, U. & Koesterke, L.1994 A&A, 281, 184.
- Hillier, D. J. & Miller, D. L. 1998, ApJ, 496, 407.
- Mesa-Delgado et al. in preparation
- Morisset, C. 2011. sites.google.com/site/cloudy3d
- Reyes-Pérez et al. in progress.
- Sabbadin, F., Minello, S. & Bianchini, A. 1977, A&A,60, 147
- Zhekov, S. A. & Park, S. 2011, ApJ, 728, 135.
Figure 1. The ring nebula NGC 6888. Black lines represent the slit positionswhere the observations were performed. Image from Daniel López IAC takenwith the Isaac Newton Telescope.
Figure 2. Model spectrum in red over observed spectra in black.
Figure 3. Intensity maps of [O II], [O III], [N II] ions RGB image plotted with pyCloudy 3D. Squaresrepresents the corresponding slits positions where the spectroscopical observations were performed.
