The born-again planetary nebula A78: an X-ray twin of A30
J.A. Toalá, M.A. Guerrero, H. Todt, W.-R. Hamann, Y.-H. Chu, R.A. Gruendl, D. Schönberner, L.M. Oskinova, R.A. Marquez-Lugo, X. Fang, G. Ramos-Larios
aa r X i v : . [ a s t r o - ph . S R ] N ov Draft version July 16, 2018
Preprint typeset using L A TEX style emulateapj v. 5/2/11
THE BORN-AGAIN PLANETARY NEBULA A78: AN X-RAY TWIN OF A30
J.A. Toal´a , M.A. Guerrero , H. Todt , W.-R. Hamann , Y.-H. Chu , † , R.A. Gruendl , D. Sch¨onberner ,L.M. Oskinova , R.A. Marquez-Lugo , X. Fang , and G. Ramos-Larios Instituto de Astrof´ısica de Andaluc´ıa IAA-CSIC, Glorieta de la Astronom´ıa s/n, 18008 Granada, Spain; [email protected] Institut f¨ur Physik und Astronomie, Universit¨at Potsdam, Karl-Liebknecht-Str. 24/25, D-14476 Potsdam, Germany Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL 61801, USA Leibniz-Institut F¨ur Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany and Instituto de Astronom´ıa y Meteorolog´ıa, Av. Vallarta No. 2602, Col. Arcos Vallarta, 44130 Guadalajara, Mexico
Draft version July 16, 2018
ABSTRACTWe present the
XMM-Newton discovery of X-ray emission from the planetary nebula (PN) A78, thesecond born-again PN detected in X-rays apart from A30. These two PNe share similar spectral andmorphological characteristics: They harbor diffuse soft X-ray emission associated with the interactionbetween the H-poor ejecta and the current fast stellar wind, and a point-like source at the positionof the central star (CSPN). We present the spectral analysis of the CSPN, using for the first time aNLTE code for expanding atmospheres which takes line blanketing into account for the UV and opticalspectra. The wind abundances are used for the X-ray spectral analysis of the CSPN and the diffuseemission. The X-ray emission from the CSPN in A78 can be modeled by a single C vi emission line,while the X-ray emission from its diffuse component is better described by an optically thin plasmaemission model with temperature kT =0.088 keV ( T ≈ × K). We estimate X-ray luminositiesin the 0.2–2.0 keV energy band of L X , CSPN =(1.2 ± × erg s − and L X , DIFF =(9.2 ± × erg s − for the CSPN and diffuse components, respectively. Subject headings: planetary nebulae: general – planetary nebulae: individual (A78) – stars: winds,outflows – X-rays: ISM INTRODUCTION
Born-again planetary nebulae (PNe) are thought tohave experienced a very late thermal pulse (VLTP) whenthe central star (CSPN) was on the white dwarf (WD)track. The VLTP event occurs when the thermonuclearburning of hydrogen in the stellar envelope has builtup a shell of helium with the critical mass to igniteits fusion into carbon and oxygen (Herwig et al. 1999;Lawlor & MacDonald 2006; Miller Bertolami & Althaus2006; Miller Bertolami et al. 2006). As the WD envelopeis shallow, the increase of pressure from this last heliumshell flash leads to the ejection of newly processed ma-terial inside the old PN, leaving the stellar core intact.As the stellar envelope expands, its effective tempera-ture decreases and the star goes back to the asymptoticgiant branch (AGB) region in the HR diagram. Thesubsequent stellar evolution is fast and will return thestar back to the post-AGB track in the HR diagram(e.g. Miller Bertolami et al. 2006): the envelope of thestar contracts, its effective temperature and ionizing pho-ton flux increase, and a new fast stellar wind develops.This canonical model, however, has notable difficultiesto reproduce the relatively low C/O ratio and high neonabundances found in born-again PNe (e.g., Wesson et al.2003, 2008). Alternative scenarios, invoking the possibleevolution through a binary system or a nova event im-mediately after the late helium shell flash, have been dis-cussed by Lau et al. (2011), although none of them arecompletely satisfactory.The born-again phenomenon is rare, with A30, A58 † Now at the Institute of Astronomy and Astrophysics, AcademiaSinica (ASIAA), Taipei 10617, Taiwan (Nova Aql 1919), A78, and the Sakurai’s object (V 4334Sgr) being the most studied objects of this class. ThesePNe harbor complex physical processes: the hydrogen-poor material ejected by the star during the born-againevent will be photoevaporated by the ionizing photon fluxfrom the CSPN and swept up by the current fast stellarwind (see Guerrero et al. 2012; Fang et al. 2014). Theseobjects evolve very fast after the VLTP, thus, they pro-vide a rare opportunity to study such complex phenom-ena and their real-time evolution (e.g., Evans et al. 2006;Hinkle et al. 2008; Clayton et al. 2013; Hinkle & Joyce2014).Among these PNe, A30 and A78 share similar charac-teristics suggesting that they are at a comparably latestage after the born-again event. They exhibit large( ∼ ′ in size) limb-brightened outer shells that surroundan ensemble of H-poor clumps that are prominent in[O iii ] narrow band images (Jacoby 1979, see Figure 1).The outer hydrogen-rich shells are ellipsoidal in shapeand expand at ∼
40 km s − , while the H-deficient knotsdetected in [O iii ] have velocities up to 200 km s − (Meaburn & Lopez 1996; Meaburn et al. 1998). Thecentral parts in A30 and A78 were imaged by the HubbleSpace Telescope ( HST ; Borkowski et al. 1993; Fang et al.2014). The detailed
HST
WFPC2 [O iii ] images revealedcometary structures distributed on an equatorial plane(a few arcsec from the star) and polar features, which,in the case of A78, are more diffuse and are located at ∼ ′′ from its CSPN (see Figure 1-right panel).X-ray emission has been detected within a num-ber of PNe, including A30 (e.g., Guerrero et al. 2012;Kastner et al. 2012; Ruiz et al. 2013; Freeman et al.2014, and references therein). This born-again PN has Toal´a et al. WN
30" He II 30" [O III]
Fig. 1.— He ii (left) and [O iii ] (center) narrow-band images of A78 as obtained at the Nordic Optical Telescope (see Section 3for details), and (right) HST
WFPC2 F502N image of the central part of A78. The equatorial plane and the bipolar flows are markedwith a dashed-line ellipse and arrows, respectively. been studied with
ROSAT (PSPC and HRI),
Chan-dra , and
XMM-Newton
X-ray satellites (Chu & Ho 1995;Chu et al. 1997; Guerrero et al. 2012). Its X-ray emis-sion originates from the CSPN, but there is also diffuseemission spatially coincident with the cloverleaf-shapedH-poor structure detected in [O iii ]. The X-ray emissionfrom both the CSPN and the diffuse extended emissionare extremely soft.The X-ray properties of A30 and the similarities ofthis nebula to A78 motivated us to obtain
XMM-Newton observations, as the only previous X-ray observations ofA78 by
Einstein yielded a rather insensitive upper limit(Tarafdar & Apparao 1988). In this paper we present theanalysis of new
XMM-Newton observations that revealthe existence of hot gas associated with the H-poor knotsinside the eye-shaped inner shell of A78, and also a point-like source of X-ray emission at its CSPN. The outline ofthis paper is as follows. The stellar wind properties andabundances of A78 CSPN are derived in §
2. The
XMM-Newton observations are presented in §
3, and the spatialdistribution and spectral properties of the X-ray emissionin § §
5, respectively. A discussion is presented in § § NLTE ANALYSIS OF THE CENTRAL STAR
We analyzed the optical and UV spectra of the CSPNof A78 using the most recent version of the Pots-dam Wolf-Rayet (PoWR) model atmosphere . ThePoWR solves the NLTE radiative transfer problem ina spherical expanding atmosphere simultaneously withthe statistical equilibrium equations and at the sametime accounts for energy conservation. Iron-groupline blanketing is treated by means of the superlevelapproach (Gr¨afener et al. 2002), and a wind clump-ing in first-order approximation is taken into account(Hamann & Gr¨afener 2004). We did not calculate hy-drodynamically consistent models, but assumed a veloc-ity field following a β -law with β = 1. We also performedtests with different β -laws, e.g., β = 0 . β = 2, and adouble- β law, but we found the impact of different β -lawsto be much smaller than the change of other parameters,such as effective temperature and mass-loss rate. Our computations applied here include complex atomic mod-els for helium, carbon, nitrogen, oxygen, neon, fluorine,hydrogen, and the iron-group elements.The synthetic spectrum was corrected for interstel-lar extinction due to dust by the reddening law ofCardelli et al. (1989), as well as for interstellar line ab-sorption for the Lyman series in the UV range.The UV spectrum of the CSPN of A78 has beenobserved by the Far Ultraviolet Spectroscopic Explorer(FUSE) and the
International Ultraviolet Explorer (IUE) satellites. Data from these observations have been re-trieved from MAST, the Mikulski Archive for Space Tele-scopes at the Space Telescope Science Institute . Weused the FUSE observation with ID e1180101000 (PI:J. Kruk) obtained on 2004 November 11 in the spectralrange 916-1190 ˚A for a total exposure time of 58 ks. The
IUE observations in the spectral range 1150-1975 ˚A con-sists of two datasets with IDs SWP19879 and SWP19906,both taken with the large aperture at high dispersionwith total exposure times of 25.5 ks and 25.2 ks, respec-tively. Only low dispersion
IUE observations were avail-able for the spectral range 2000-3300 ˚A. The data setwith ID LWP23314LL obtained through the large aper-ture for a total exposure time of 600 s was used. Op-tical spectra of R ≈ vi vs. O v wind lines. The best fit to these lines was ob-tained with an effective temperature of T ∗ = 117 ± L = 6000 L ⊙ and M = 0 . M ⊙ (see e.g. Sch¨onberner et al. 2005; STScI is operated by the Association of Universities for Re-search in Astronomy, Inc., under NASA contract NAS5-26555. he born-again planetary nebula A78: an X-ray twin of A30 3
TABLE 1Parameters of the CSPN of A78
Parameter Value Comment T eff (kK) a L/L ⊙ ) 3.78 Adopted M ∗ ( M ⊙ ) 0.6 Adopted R ∗ ( R ⊙ ) b R ∗ ∝ L / R τ =2 / ( R ⊙ ) 0.19 v ∞ (km s − ) 3100 D (clumping factor) 10 Adoptedlog ˙ M ( M ⊙ yr − ) -7.8 ˙ M ∝ D − / L / d (kpc) 1.40 d ∝ L / E B − V (mag) 0.12Abundances (mass fraction)He 0.55C 0.30N 0.015O 0.10Ne 0.04F 1 . × − Fe-group 1.4 × − T eff is defined as the effective temperature at the radius R ∗ . b The stellar radius R ∗ refers to, by definition, the point where theradial Rosseland optical depth is 20. H e II - OV ’ - OV s ’ - s H e II - L y γ N e V II N o r m a li ze d f l ux H e II - L y β ( g e o c . ) OV I - , s NV - OV D - P o - k m / s - k m / s - k m / s F V I D - P o F e V II F e V II L y α ( g e o c o r on a l ) NV - s OV I - C I V - OV - C I V - s L y α ( g e o c o r on a l ) NV - s OV I - C I V - OV - C I V - s λ / A o N o r m a li ze d f l ux Fig. 2.—
Details of the normalized UV spectrum of A78. Observations (blue thin line) from
FUSE (upper panels) and
IUE (lowerpanel) are shown together with the synthetic spectrum of our model (dashed lines) with parameters as in Table 1. The model spectrumwas convolved with a Gaussian with 0 . Top-left panel:
A model with 4% Ne (reddashed) compared to a model with only solar Ne abundance (green dashed).
Top-middle panel : Best-fit model with v ∞ = 3100 km s − plusdepth dependent microturbulence (red dashed). Top-right panel : Best-fit model to the Fe vii vi FUSE spectrum indicate different Doppler shifts with respect to the F vi line. Bottom panel : Our best-fit model (reddashed) vs.
IUE observation.
Toal´a et al. O I V D - P O O V I - C I V - O I V ’ F O - ’ D O V I P O - s S H e II - C I V - H e II - H e II - H e II - OV H e II - C I V - H e II - O V I - NV - H e II - C I V - C I V - N V - s C I V - H e II - C I V - H e II - N V - [ O III] P - D i . s . N o r m a li ze d f l ux [ O III] P - D i . s . C I V s S - P O O V P O - s S O V I - H e II - C I V - O V D - P O C I V s S - P O N a I i . s . OV I - OVNV - OV H e II - λ / A o N o r m a li ze d f l ux Fig. 3.—
Optical spectrum of the central star of A78. Normalized observation (blue) vs. synthetic spectrum of our best-fit model (reddashed). The model spectrum was convolved with a Gaussian with 2 ˚A FWHM to match the resolution of the observation, inferred fromthe interstellar Na i doublet. H e II - H β H e II - H γ λ / A o H e II - H δ N o r m a li ze d f l ux Fig. 4.—
Details of the optical spectrum of A78: Observation (blue thin solid lines) vs. models without hydrogen (red thick dashed),with 10% H (green thick dotted), and with 30% H (black thin dashed).
J H K S U B V R IFUSE IUE WISE MSXGALEX 13.5 13.4 13.311.8613.0413.2513.013.13 A C D EW1 W2 W3 W4FUV NUV -16-14-12 3.0 3.5 4.0 4.5 5.0log λ [A o ] l og f λ [ e r g s - c m - A o - ] Fig. 5.—
Spectral energy distribution (SED) of the central star of A78 from the UV to the infrared range. Observations (blue) arephotometric measurements in the indicated bands and calibrated
IUE and
FUSE
UV spectra. The theoretical SED derived from our stellarmodel with the parameters compiled in Table 1 is also shown. he born-again planetary nebula A78: an X-ray twin of A30 5Miller Bertolami & Althaus 2007). The inferred valueof ˙ M = 1 . × − M ⊙ yr − is about half of the valuefrom Werner & Koesterke (1992) and Leuenhagen et al.(1993), also because we account for clumping anduse a different luminosity (see Table 1 for scaling re-lations). Indeed, the mass-loss rate determined byKoesterke & Werner (1998) is the same as ours, ifrescaled to the values of distance and stellar luminosityused in our analysis.The blue edge of the P Cygni profiles was used to esti-mate a terminal wind velocity of about 3100 ± v D = 50 km s − in the photosphereup to v D = 300 km s − in the outer wind was taken intoaccount and allows to fit the widths of the O vi and theC iv resonance lines simultaneously (see Figure 2).The strong Ne vii line at 973.33 ˚A (Herald et al. 2005)observed in the UV spectrum (see Figure 2) can only bereproduced by models with a supersolar Ne abundance.Similarly, the strength of the N v lines (Figures 2 and 3)implies a supersolar nitrogen abundance of 1.5% by mass.To reproduce the observed strength of the F vi × the solar valueis needed, similar to what was found by Werner et al.(2005) for the same object as well as for other H-deficientpost-AGB stars. They also mentioned an asymmetry ofthis line meaning that the line is partly formed in thewind, as reproduced by our wind models (see Figure 2).Initial solar abundances of the iron group resulted inFe vii lines more intense than those observed in the FUSE spectrum (see Figure 2-top right panel). Accord-ingly, the abundance of the iron group elements had tobe reduced down to 1/10 of the solar value to obtain aconsistent fit of the Fe vii lines. Our iron estimate is incontrast to that found by Werner et al. (2011), who sug-gested that the strong Fe viii viii line profile, much broader than the predictionfrom their static NLTE models, called for an analysisusing expanding atmosphere models as that performedhere. Following the interpretation of the subsolar Fe/Niratio reported in Sakurai’s object (Asplund et al. 1999),the iron deficiency in A78 can be explained by the con-version of iron into heavier elements by s-process neutroncaptures.We also tried to constrain the hydrogen abundance.The best fit to the Balmer lines is obtained by modelswithout hydrogen. However, at the given resolution andS/N of our optical observation, a hydrogen abundancebelow 10% by mass would escape detection (Figure 4).The absolute flux of the model is diluted by the dis-tance to the central star, which we consider to be a freeparameter. We obtain a consistent fit of the spectralenergy distribution from the far UV to the near infraredrange (see Figure 5) for a reddening of E B − V = 0 .
12 magand a distance of d = 1 . E B − V = 0 .
15 mag and thedistance of d = 1 . OBSERVATIONS
XMM-Newton observed A 78 on 2013 June 3 (Observa-tion ID 0721150101, PI: M.A. Guerrero) using the Euro-pean Photon Imaging Cameras (EPIC) and ReflectiveGrating Spectrographs (RGS). The observations wereperformed in the Full Frame Mode with the thin opti-cal filter for a total exposure time of 59.4 ks. The datawere reprocessed with the
XMM-Newton
Science Analy-sis Software (SAS) 13.5 with the most up-to-date
XMM-Newton calibration files available on the Current Calibra-tion File as of 2014 January 7. The net exposure timeswere 59.4, 59.1, 59.1, 59.4, and 59.4 ks for the EPIC-pn, EPIC-MOS1, and EPIC-MOS2, RGS1, and RGS2,respectively. After processing the effective times werereduced to 23.9, 43.6, 42.5, 59.1, and 59.0 ks for theEPIC-pn, MOS1, MOS2, RGS1, and RGS2 respectively.To help study the distribution of the X-ray emission,we obtained He ii and [O iii ] narrow-band images of A78on 2014 July 19 using the Andalusian Faint Object Spec-trograph and Camera (ALFOSC) at the Nordic OpticalTelescope (NOT). The central wavelengths and band-passes (FWHM) of the filters are 4687 ˚A and 43 ˚A forHe ii , and 5010 ˚A and 35 ˚A for [O iii ], respectively. Theimages have total exposure times of 1800 s each. The av-erage seeing during the observation was ∼ . ′′
7. The finalprocessed images are shown in the left and middle panelsof Figure 1. SPATIAL DISTRIBUTION OF THE X-RAY EMISSION
For direct comparison with the
XMM-Newton obser-vations of A30, we created EPIC images of A78 in fourdifferent energy bands: soft 190-275 eV, medium 275–450 eV, hard 450–600 eV, and total 190–600 eV. Indi-vidual EPIC-pn, EPIC-MOS1, and EPIC-MOS2 imageswere extracted, merged together, and corrected for expo-sure maps. The final smoothed exposure-corrected im-ages of the four energy bands are show in Figure 6.Figure 6 reveals a bright source associated with theCSPN and diffuse X-ray emission within A78. Both thepoint-like source and diffuse emission seem to fade awayat energies >
450 eV. Other point-like sources are presentin the panels shown in Figure 6, in particular the point-like X-ray source detected in all energy bands ∼ ′′ northof the CSPN of A78. The optical counterpart of this X-ray source is detected faintly in the optical images inFigure 1 with coordinates (R.A,Dec.)=(21 h m s ,+31 ◦ ′ ′′ ). No counterpart is identified in the NEDand SIMBAD databases. This source is most likely abackground source, as it does not have any morphologicalcorrelation with A78.In Figure 7 we compare the spatial extent of the X-ray emission with the optical H α and [O iii ] images pre-sented in Figure 1. The diffuse X-ray emission in A78does not fill the elliptical outer shell, but it seems tobe bounded by the [O iii ] bright eye-shaped shell, as itwas also the case for the distribution of diffuse X-raysin A30. Furthermore, there is a local peak in the dif-fuse X-ray emission that seems to be associated with aH-poor clump toward the SW direction from the CSPN.In a similar manner, A30 also presents a maxima in theX-ray emission associated with the H-poor clumps, sug-gesting that the two born-again PNe may have similarorigins of X-ray emission. Toal´a et al. Fig. 6.—
Exposure-corrected
XMM-Newton
EPIC images of A78 in different bands. The images have a pixel size 1 ′′ . The images arecentered at the central star in A78. Other point-like sources are presented on the images. The black lower contours correspond to 1, 3, 5,and 10 σ over the background level, while the white upper contours represent 20% and 60% of the peak intensity. To better assess the extent and intensity of the diffuseX-ray emission, we have used the SAS task eradial toextract a radial profile of the X-ray emission centered onthe CSPN of A78 and compare it to the theoretical point-spread function (PSF) of the observation. This compar-ison is shown in Figure 8, where the PSF scaled to theradial profile fits nicely the emission from the CSPN untila radius of 12 ′′ . An excess of diffuse emission is detectedabove the PSF profile between 17–35 ′′ . To estimate thecontribution of the CSPN emission to the diffuse emissioncomponent, we have integrated the radial profile emissionobtained with eradial for distances smaller than 34 ′′ , andcomputed the percentage to the total emission from the CSPN using the PSF model. The contribution of theCSPN to the total emission is 99% for radial distances < ′′ , 79% for distances < ′′ , and only 24% for dis-tances between 17 ′′ and 34 ′′ . SPECTRAL PROPERTIES OF THE X-RAY EMISSION
To study the global spectral properties of the X-rayemission from A78, we have extracted the EPIC-pn,EPIC-MOS, and combined RGS1+RGS2 spectra shownin Figure 9. The EPIC spectra have been extracted froman elliptical region that encloses the whole emission fromA78. These spectra (Figure 9-top) are very soft and re-semble those presented by Guerrero et al. (2012) for A30.The EPIC-pn spectrum peaks at 0.3-0.4 keV with a rapidhe born-again planetary nebula A78: an X-ray twin of A30 7
Fig. 7.—
Composite color picture of the
XMM-Newton
EPIC190-600 eV (blue) and NOT ALFOSC [O iii ] (green) and He ii (red)images of A78. To emphasize the comparison between the spatialdistributions of the X-ray-emitting gas and nebular component inA78, X-ray contours of the same energy band have been overplot-ted. The emission to the north of A78 corresponds to a point-likeX-ray source in the field of view of the observations (see text).
10 20 30 40Radius [arcsec]0.000.050.100.150.200.250.300.350.40 I [ c o un t s a r c s e c − ] DiffuseCSPN
Fig. 8.—
EPIC-pn radial profile of the X-ray emission from A78extracted using eradial . The dashed line is the fitted PSF to theradial profile. decay at energies greater than 0.5 keV. This spectrumshows evidence for an emission line at ∼ vii triplet. The count rates for the EPIC-pn, EPIC-MOS1, and EPIC-MOS2 in the 0.2–2.0 keVenergy range are 18.2, 2.4, and 2.1 counts ks − , respec-tively.The combined RGS1+RGS2 background-subtractedspectrum of A78 can help us identify the emission de-tected around 0.3-0.4 keV in the EPIC-pn camera. Fig- I [ c o un t s s − k e V − ] EPIC-pnEPIC-MOS1EPIC-MOS2
Energy [keV] -3 0.0 3 ∆ I / σ
20 22 24 26 28 30 32 34 36
Wavelength [ ◦ A] −5 F l u x [ c m − s − ◦ A − ] C V I L y α . ◦ A Fig. 9.—
XMM-Newton
EPIC (top) and combined RGS1+RGS2(bottom) background-subtracted spectra of A78. The best-fitmodels to the EPIC-pn and MOS cameras are shown with solidlines in the top panel (see § vi emission line at33.7 ˚A(=0.37 keV). ure 9-bottom panel shows that this is mostly due to theC vi emission line at 33.7 ˚A (=0.37 keV). However, be-cause of the low MOS and RGS count rates, we willmainly focus on the spectral analysis from the spectrumextracted from the EPIC-pn camera for further discus-sion.We have extracted separately EPIC-pn spectra for theCSPN and for the diffuse X-ray emission. The spectrumfrom the CSPN was extracted using a circular apertureof radius 12 ′′ centered at the position of the star witha background extracted from regions with no contribu-tion of diffuse emission. The spectrum from the diffuseemission was extracted using an elliptical aperture thatcovered the extension of the [O iii ] filamentary shell witha minor axis of 34 ′′ to avoid contamination from thepoint-like source to the North. According to §
4, a cir-cular region of radius 17 ′′ centered at the position ofthe CSPN was excised to reduce the contamination fromthe CSPN. The resultant background-subtracted EPIC-pn spectra of the CSPN and that of the diffuse compo-nent, shown in Figure 10, have net count rates of 8.8 and8.0 counts ks − , respectively. Toal´a et al.Figure 10 shows subtle differences in the spectralshapes from the CSPN and diffuse emission. For exam-ple, the spectrum from the diffuse component (Figure 10-right) shows the spectral line at ∼ ∼ vi line. Spectral analysis
The spectral analysis of the X-ray emission from A78was performed using XSPEC v.12.7.0 (Arnaud 1996).The spectral fits include one or two of the followingcomponents: (1) an emission line at ∼ apec optically thin plasma emission model withabundances as those listed in Table 1. The estimates ofthe luminosity and electron density assume a distance of1.4 kpc (see § E B − V = 0 .
15 mag, which is in agreementwith the UV absorption towards the CSPN derived in §
2, but these measurements do not help disentanglethe relative contributions of interstellar and circumstel-lar absorptions. To determine these contributions, wehave used long-slit intermediate-dispersion optical spec-troscopic observations (Fang et al., in preparation) of theouter ellipsoidal shell to derive the interstellar extinctiontowards the outer nebular regions. Our measurements in-dicate a negligible extinction, E B − V = 0 .
014 mag, thusproving that most of the absorption towards the cen-tral regions of A78 has a circumstellar origin, i.e., as inA30. The absorption would be produced by the dust inthe central regions of A78. Since the IR emission of thedust (Kimeswenger et al. 1998; Phillips & Ramos-Larios2007) shares the same spatial distribution of the H-poorknots, the chemical composition of the dust is expectedto be similar to that of the H-poor knots. Given thelarge metal-to-hydrogen ratio of this absorbing material,a relatively small hydrogen column density, N H ≈ × cm − , will contain similar amounts of carbon and oxygenas the interstellar hydrogen column density required toproduce the observed absorption.As a first inspection, the spectra of the three EPICcameras were fit simultaneously using an apec compo-nent and an emission line at 0.37 keV. This gives a goodquality fit ( χ =72.6/77=1.06) with a plasma tempera-ture of kT =0.072 keV and ∆ E =26 eV . The model iscompared to the observed spectra in the 0.2–1.5 keVenergy range in Figure 9- top , and the plasma temper-ature ( kT ), fluxes ( f ), emission line central energy ( E ),and normalization factors ( A ) of the best-fit modelsare listed in Table 2. The absorbed flux and intrin-sic luminosity in the 0.2–2.0 keV energy range of thismodel are f TOT =(2.2 ± × − erg cm − s − and L TOT , X =(2.1 ± × erg s − .Another emission model consisting of an apec compo-nent and two emission lines at 0.37 keV and 0.58 keVwas tried in an attempt to reproduce the emission ex- It is worth noting that, in the cases in which an emission lineis used to model the X-ray emission, its line width is a few tens ofeV, which means that the line is not resolved by EPIC-pn. cess at ∼ apec component, resulted in a poorer fit, χ /dof =70.26/45=1.56, for a similarly low plasma tem-perature of kT =0.086 keV.We next proceeded to perform spectral fits of the X-ray emission from the CSPN and from the diffuse com-ponent separately. The different emission models usedto describe these components are described in the nextsections. We want to emphasize that the spectrum ofthe diffuse emission is contaminated by emission fromthe CSPN. This is not the case for the spectrum of theCSPN, which corresponds mostly to emission from theCSPN. X-ray emission from the CSPN
The X-ray emission from the central star in A78 wasfirst modeled by an apec plasma model. This resultedin a fit with reduced χ greater than two and plasmaemission model of kT =0.071 keV. The absorbed flux inthe 0.2–2.0 keV energy range is f X =(1.08 ± × − erg cm − s − .A second fit was performed using only the contributionof an emission line around ∼ χ /dof=1.30. The correspondingabsorbed flux is (1.12 ± × − erg cm − s − , verysimilar to the flux level derived for the single apec modeldescribed above. The intrinsic X-ray luminosity of thismodel is (9.9 ± × erg s − .A third model including the combination of an apec model and an emission line at 0.37 keV model was alsoattempted, but this model does not improve the previousfits ( χ /dof=0.65). Indeed, XSPEC cannot constrain thetemperature of the apec component.The three resultant models are compared with thebackground-subtracted spectrum in the 0.2-1.5 keV en-ergy range in Figure 10-left. The parameters of the best-fit models are summarized in Table 2. The diffuse and extended X-ray emission
Similarly, the diffuse X-ray emission in A78 wasfirst fit using an apec optically thin plasma model.The fit has a reduced χ of 1.40, for a plasma tem-perature of kT =0.086 keV and an absorbed flux of(9.70 ± × − erg cm − s − .The second fit adopted a single emission line at ∼ χ /dof of 1.38, for an emission line with energy0.362 keV and ∆ E =36 eV. The corresponding absorbedflux is (8.8 ± × − erg cm − s − .The third model used a combination of an apec plasmamodel and an emission line at 0.37 keV. This modelgives a reduced χ of 1.32, but the plasma tempera-ture is basically unconstrained and the line width im-plies it is unresolved. Still, the resultant absorbed flux,(1.0 ± × − erg cm − s − , is consistent with thoseimplied by the other two models.One final spectral fit was attempted taking into ac-count that the CSPN contributes significantly to theemission registered in this region. This contribution was The degrees of freedom, dof, is equal to the number of spec-tral channels which are used into the fit minus the number of freeparameters in the adopted model. he born-again planetary nebula A78: an X-ray twin of A30 9 I [ c o un t s s − k e V − ] APECLineAPEC+lineCSPN
Energy [keV] -3 0.0 3 ∆ I / σ I [ c o un t s s − k e V − ] APECLineAPEC+lineDiffuse emission
Energy [keV] -3 0.0 3 ∆ I / σ Fig. 10.—
EPIC-pn background-subtracted spectra of the CSPN (left) and diffuse emission (right) of A78. Different line colors representthe different fits described in Table 2. The dotted histogram in the right panel illustrates the contribution of the CSPN to the diffuseemission.
TABLE 2Best-fit models for the X-ray emission in A78 * Region Model kT A E ∆ E A f X χ /dof(keV) (cm − ) (keV) (keV) (cm − ) (erg cm − s − )A78 line+apec 0.072 +0 . − . × − × − × − × − +0 . − . × − . . . . . . . . . 1.1 × − +0 . − . × − × − × − × − +0 . − . × − × − × − +0 . − . × − . . . . . . . . . 9.7 × − +0 . − . × − × − × − × − × − × − × − *** diffuse+line 0.088 +0 . − . × − × − × − × − * All models were computed assuming an absorbing column density of N H = 2 × cm − . ** The normalization parameter A is defined as A =1 × − R n e n H dV/ πd , where d is the distance, n e is the electron number density,and V the volume, all in cgs units. *** The line component is for the spillover CSPN emission. estimated to be ≈ apec model. The best-fit parameters of this model arelisted in the last row of Table 2. The quality of thefit is similar to the previous ones, but the temperatureof the plasma is constrained to kT =0.088 keV. Its cor-responding flux is (9.6 ± × − erg cm − s − , butonly (7.4 ± × − erg cm − s − corresponds to thediffuse X-ray emission. The intrinsic X-ray luminosity ofthe net diffuse emission is (6.8 ± × erg s − .The best-fit models are compared to the background-subtracted spectrum from the diffuse emission in the0.2–1.5 keV in Figure 10-right, and the parameters listedin Table 2. DISCUSSION
The present
XMM-Newton observations have discov-ered very soft X-ray emission in the born-again PN A78.These X-ray observations reveal the existence of a point-like source associated with the CSPN and a source ofextended X-ray emission within A78. The spatial distri-bution of the diffuse X-ray emission does not fill the ellip-tical outer shell; instead, this emission can be associatedwith the H-poor knots and is enclosed by the filamentarycavity detected in [O iii ] narrow band images.The best-fit model for the CSPN of A78 seems tobe that including only a line emission at ∼ ′′ , is f X , CSPN =(1.32 ± × − erg cm − s − which corresponds to an intrinsic X-ray luminosity of L X , CSPN =(1.2 ± × erg s − .0 Toal´a et al.The X-ray emission from the diffuse component inA78 is better explained by a thermal plasma withtemperature of kT =0.088 keV ( T ≈ × K). Itscorresponding total flux, after subtracting the contri-bution from the CSPN and adding that of regionswith aperture radius < ′′ , is f X , DIFF =(1.0 ± × − erg cm − s − , which corresponds to an intrinsic luminos-ity of L X , DIFF =(9.2 ± × erg s − . The normaliza-tion parameter for the apec component ( A =5.20 × − cm − ) has been used to estimate an electron density ofthe diffuse X-ray-emitting gas for a distance of 1.4 kpc as n e =0.002 ( ǫ/ . / cm − , with ǫ as the gas-filling factor. Comparison with A30
The four bona fide born-again PNe, A30, A58, A78,and Sakurai’s object, represent different stage of thesame evolutionary path. A30 and A78 are very simi-lar in many ways. The morphology and spectral simi-larities between A30 and A78 and their central stars areremarkable. Their optical narrow band images show sim-ilar limb-brightened outer nebulae which correspond tothe expected shell in the canonical formation of a PN(Kwok et al. 1978; Balick 1987), with estimated dynam-ical ages, τ dyn5 , of 12,500 and 10,700 yr for A30 andA78, respectively. The processed H-poor material isthought to have been ejected around a thousand yearsago (e.g., Guerrero et al. 2012; Fang et al. 2014), whichmeans that the stellar wind velocity must have increasedvery rapidly within this time-lapse in both cases. The in-teraction of this stellar wind with the material ejected inthe born-again event is responsible for the shaping of thecloverleaf and eye-shaped H-poor clumpy distributions inA30 and A78, respectively (see Fang et al. 2014).The X-ray properties of A30 and A78 are very muchalike. The diffuse X-ray emission can be modeled by anoptically thin plasma model with similarly low temper-atures for both PNe, besides the different relative im-portance of an emission line at ∼ vii in their spectra. The origin of this hot plasmamay be due to pockets of shocked and thermalized stel-lar wind, as the current fast wind from the CSPNe( V ∞ . − ) interacts with the processed ma-terial from the born-again ejection. The plasma tem-perature from an adiabatic shocked stellar wind can bedetermined by kT = 3 µm H V ∞ /
16, where µ is the meanparticle mass (Dyson & Williams 1997). Therefore, forthe stellar winds of A30 and A78, the temperature ex-pected from the shocked material would be T ∼ × K,in sharp contrast with the observed temperatures. Thisdiscrepancy is found in all PNe in which diffuse X-rayemission is detected (see Ruiz et al. 2013, and referencestherein) and it is always argued that thermal conduc-tion is able to reduce the temperature of the shockedstellar wind and to increase its density to observable val-ues (Soker 1994). Even though one-dimension radiative-hydrodynamic models on the formation of hot bubblesin PNe as those presented by Steffen et al. (2008) andSteffen et al. (2012) are able to explain this discrepancy,they are not tailored to the specific evolution of a star The dynamical age can be estimated as τ dyn = R/v exp , where R and v exp are the radius and velocity of the outer optical shell,respectively. that experiences a VLTP and creates a born-again PN.The fact that the diffuse X-ray emission of A30 and A78is confined within filamentary and clumpy H-poor shellsis a clear indication that a variety of physical processesare taking place to reduce its temperature. These pro-cesses may include mass-loading and photoevaporationfrom the H-poor clumps (Meaburn & Redman 2003),which mix the material with the thermalized shockedwind. The realization of numerical simulations on theformation of born-again PNe including these complex in-teractions and accounting for their singular abundancesis most needed to understand the puzzling X-ray emis-sion in these objects (Toal´a & Arthur in prep.).The X-ray emission from the point sources at theCSPNe of A30 and A78 is dominated by the C vi line.As discussed by Guerrero et al. (2012), the origin of theX-ray emission associated with the CSPNe of born-againPNe is inconclusive. Several mechanism are capable ofproducing this X-ray emission (e.g., charge transfer reac-tions from highly ionized species of carbon, oxygen, andnitrogen), but the present observations cannot provide adefinite answer.It is worth mentioning here that, given the large opac-ity of this material (see figure 11 in Guerrero et al. 2012),the C vi On the origin of the hydrogen-poor material
The origin of the newly processed hydrogen-poor ma-terial inside the old PN is still a matter of debate(Lau et al. 2011). The Ne abundance of the ejecta canhold important clues. The Ne abundances of A30 andA58 (Wesson et al. 2003, 2008) seem to point to a novaeruption on an O-Ne-Mg WD (Lau et al. 2011). Thishypothesis was invoked by Maness et al. (2003) to inter-pret the high Ne abundances in the X-ray-emitting gasin BD+30 ◦ ∼ ix and Ne x emissionlines. To test this, we have compared the observed X-rayspectrum with models with Ne abundance enhanced by5, 10, and 20 times the value reported in §
2, where themid values in this range implies Ne abundances by masssimilarly high to those reported in A30 and A58. Thesechanges in the abundances do not produce noticeable ef-fects in the synthetic spectrum, revealing a fundamen-tal insensitivity of the X-ray spectrum of A78 to the Neabundance due to its low plasma temperature. SUMMARY AND CONCLUSIONS
We report the
XMM-Newton discovery of X-ray emis-sion in A78, making it the second born-again PN de-tected in X-rays, besides A30 Guerrero et al. (2012). TheX-ray data of A78 have been analyzed in conjunctionwith narrow-band optical images of the nebula to deter-mine the spatial distribution of the hot gas. Multiwave-length spectral observations of its CSPN have also beenanalyzed using a NLTE code for expanding atmosphereshe born-again planetary nebula A78: an X-ray twin of A30 11to assess its stellar parameters and wind properties. Inparticular we find: • The spatial distribution and spectral properties ofthe X-ray emission detected towards A78, and thechemical abundances and stellar wind parametersof its CSPN are very similar to those reported forA30 by Guerrero et al. (2012). • The X-ray emission from A78 consists of a point-like source and diffuse emission. The point-likesource is coincident with the position of the CSPN.The distribution of diffuse X-ray emission does notfill the outer nebular shell, instead it traces theH-poor clumps and eye-shaped cavity detected in[O iii ] narrow band images. An apparent maximumin the diffuse X-ray emission is detected at the lo-cation of one H-poor clump toward the southwest. • The X-ray emission from A78 is very soft. Most ofthe X-ray emission has energies lower than 0.5 keV,pointing at the C vi ∼ vii triplet. • The analysis of the optical and UV spectra of theCSPN of A78 helped us to constrain its abundancesand stellar wind parameters. These have been usedfor the analysis of the X-ray spectra of A78. • The best-fit model for the diffuse X-ray emissionresulted in a plasma temperature of kT =0.088 keV( T ≈ × K) with an estimated absorbed flux of f X , DIFF =1.0 × − erg cm − s − . The estimatedX-ray luminosity is L X , DIFF =9.2 × erg s − . Avariety of processes may have played significantroles in lowering the plasma temperature (e.g., mix-ing, ablation and photoevaporation) of the diffuseX-ray emission. • The X-ray spectra of A78 cannot be used to con-strain the Ne abundance of the hot plasma due toits low temperature. • The main X-ray spectral feature in the CSPN inA78 is the C vi emission line, as revealed by theEPIC and RGS spectra. Its estimated flux in the0.2–2.0 keV energy band is f X , CSPN =1.32 × − erg cm − s − which corresponds to a luminosity of L X , CSPN =1.2 × erg s − . The physical mecha-nism for the production of the emission associatedwith the CSPN of A78 is elusive, as it is also thecase for the CSPN of A30.We would like to thank the referee, Orsola De Marco,for her valuable comments and suggestions. We alsothank K. Werner for fruitful discussion. J.A.T. acknowl-edges support by the CSIC JAE-Pre student grant 2011-00189. 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