Detection of the Central Star of the Planetary Nebula NGC 6302

Abstract

NGC 6302 is one of the highest ionization planetary nebulae known and shows emission from species with ionization potential >300eV. The temperature of the central star must be >200,000K to photoionize the nebula, and has been suggested to be up to ~ 400,000K. On account of the dense dust and molecular disc, the central star has not convincingly been directly imaged until now. NGC 6302 was imaged in six narrow band filters by Wide Field Camera 3 on HST as part of the Servicing Mission 4 Early Release Observations. The central star is directly detected for the first time, and is situated at the nebula centre on the foreground side of the tilted equatorial disc. The magnitudes of the central star have been reliably measured in two filters(F469N and F673N). Assuming a hot black body, the reddening has been measured from the (4688-6766\AA) colour and a value of c=3.1, A_v=6.6 mag determined. A G-K main sequence binary companion can be excluded. The position of the star on the HR diagram suggests a fairly massive PN central star of about 0.64,M_sun close to the white dwarf cooling track. A fit to the evolutionary tracks for (T,L,t)=(200,000K, 2000L_sun, 2200yr), where t is the nebular age, is obtained; however the luminosity and temperature remain uncertain. The model tracks predict that the star is rapidly evolving, and fading at a rate of almost 1 % per year. Future observations could test this prediction. Comment: 13 pages, 5 figures, submitted to ApJ Letters on 25.09.2009 accepted on 19.10.2009

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arXiv:0909.5143v2 [astro-ph.SR] 21 Oct 2009
Detection of the Central Star of the Planetary Nebula NGC 6302
C. Szyszka1,2, J. R. Walsh3, Albert A. Zijlstra2, Y. G. Tsamis4,5
cszyszka@eso.org
ABSTRACT
NGC 6302 is one of the highest ionization planetary nebulae known and shows
emission from species with ionization potential >300eV. The temperature of the
central star must be > 200000K to photoionize the nebula, and has been sug-
gested to be up to ∼ 400000K. On account of the dense dust and molecular disc,
the central star has not convincingly been directly imaged until now. NGC 6302
was imaged in six narrow band filters by Wide Field Camera 3 on HST as part of
the Servicing Mission 4 Early Release Observations. The central star is directly
detected for the first time, and is situated at the nebula centre on the foreground
side of the tilted equatorial disc. The magnitudes of the central star have been
reliably measured in two filters (F469N and F673N). Assuming a hot black body,
the reddening has been measured from the (4688-6766˚ A) colour and a value of
c=3.1, Av=6.6 mag determined. A G-K main sequence binary companion can be
excluded. The position of the star on the HR diagram suggests a fairly massive
PN central star of about 0.64M⊙close to the white dwarf cooling track. A fit
to the evolutionary tracks for (T,L,t)=(200000K, 2000L⊙, 2200yr), where t is
the nebular age, is obtained; however the luminosity and temperature remain
uncertain. The model tracks predict that the star is rapidly evolving, and fading
at a rate of almost 1% per year. Future observations could test this prediction.
Subject headings: planetary nebulae: individual (NGC 6302) — stars: AGB and
post-AGB — stars: imaging
1European Southern Observatory, Karl-Schwarzschild Strasse 2, D-85748 Garching, Germany
2Jodrell Bank Centre for Astrophysics, School of Physics & Astronomy, University of Manchester, Oxford
Road, Manchester M13 9PL, UK
3Space Telescope European Coordinating Facility, European Southern Observatory, Karl-Schwarzschild
Strasse 2, D-85748 Garching, Germany
4Instituto de Astrofis´ ıca de Andaluc´ ıoa (CSIC), Apartado 3004, 18080 Granada, Spain
5Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA,
UK

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1.Introduction
Planetary nebulae (PN) trace one of the fastest phases of stellar evolution. They form
when an Asymptotic Giant Branch (AGB) star ejects its envelope in a massive wind, leaving
only the remnant degenerate C/O core. After the ejection, the star heats up rapidly, reaching
temperatures over 100000K, within typically a few thousand years. The hydrogen burning
ceases at this point, and the star quickly fades by a large factor, before entering the white
dwarf cooling track. The hot star ionizes the previously ejected nebula: the bright ionized
nebula combined with the very hot, optically faint star, can make the stellar emission difficult
to detect. This is made worse if circumstellar extinction hides the star from direct view in
the optical and ultra-violet domain.
The central star of NGC 6302 has proven to be one of the most elusive of all PN cen-
tral stars. In spite of many attempts, it has not been detected so far. NGC 6302 shows a
strongly bipolar morphology, with filamentary emission extending over 7′. Bipolar morpholo-
gies are thought to be associated with circumstellar discs and torii (Balick et al. 1987; Icke
2003), which are commonly seen in post-AGB nebulae (Si´ odmiak et al. 2008). NGC 6302
provides an extreme case for such a massive, optically opaque torus (Matsuura et al. 2005;
Peretto et al. 2007; Dinh-V-Trung et al. 2008).
The central star of NGC 6302 is believed to be extremely hot, as indicated by the opti-
cal spectrum (e.g. Tsamis et al. 2003) and observation of infrared coronal lines of highly
ionized elements, of which the highest observed is Si+8with an ionization potential of
303eV (Casassus et al. 2000). To produce such high ionization species various authors have
suggested very hot stars: Ashley & Hyland (1988) proposed a black body of 430000 K;
Pottasch et al. (1996) 380000K; and Casassus et al. (2000) 250000K. Groves et al. (2002),
based on the observed scattered continuum, estimated the temperature of the central source
to be 150000K. Most recently, Wright et al. (2007), based on 3D photo-ionization modeling,
proposed a hydrogen deficient central star with a surface temperature of 220000K.
These stellar temperatures are similar to those of NGC 7027, another very bright, com-
pact PN at a similar distance. But whilst the star of NGC 7027 has been unambiguously
identified in images (e.g. Wolff et al. 2000), the star of NGC 6302 has remained hidden even in
previous HST and ground-based images. The only possible detection is from Matsuura et al.
(2005), who located a compact source emitting at 3.94 µm, possibly associated with the
[Si IX] emission line. It is not known whether the star of NGC 6302 is fundamentally dif-
ferent from NGC 7027 (higher temperature or lower luminosity). We here present new HST
observations with the newly-installed WFC3 instrument, which for the first time directly
reveal the central star, and allow us to address these questions.

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2. Observations
NGC 6302 was observed as part of the HST Early Release Observations (ERO) to
demonstrate the scientific capabilities of the newly installed Wide Field Camera 3 (WFC3)
on board the Hubble Space Telescope (HST). At optical wavelengths, the UVIS channel
has two 2k×4k CCDs, with a spatial scale of 39 mas/pixel and a field of view of 162 arcsec.
Imaging programme 11504 (PI: K. Noll) was executed on 2009 July 27 using 9 orbits. Table 1
lists the observations in six narrow band filters that isolate emission lines from ionization
potentials of 10eV ([S II] 6716 & 6731˚ A) to >54eV (He II 4686˚ A).
3.Reduction and Results
The pipeline-reduced long-exposure images for all the filter bands (i.e. FLT files) were
combined using multidrizzle (Fruchter et al. 2009) rebinning the output to 0.040′′/pixel. The
images in different filters were matched using field stars in common. The alignment was
verified by inspecting star positions in the final images. Comparison of the multidrizzled
images with those in the public ERO release showed good agreement.
Figure 1 shows the F673N filter image of the central arcminute of NGC 6302 and the
2.4′′×2.4′′area around the central star, for all the six HST WFC3 filter images in the ERO
release. This star is well detected on the F469N and F673N filters at the position α =
17h13m44.39s,δ = −37◦06′12.′′93 (J2000) and marginally detected on the F502N, F656N
and F658N images. From the filter parameters summarized in Table 1 and the emission line
spectrum of NGC 6302, as taken for example from Groves et al. (2002), the only emission
line contribution to the F469N filter is He II 4686˚ A. The F673N filter contains weak lines
of He I and C IV, in addition to the [S II]6730˚ A line; the [S II]6716˚ A line is just at the half
power point. All the other filters contain strong emission lines. The ease of detection of the
star in the F673N filter is accounted for by the lower emission line contribution to the band,
producing a much higher star/background contrast. This also applies to the F469N filter,
where the He II line is<
and of the Hα line in F656N.
∼10% of the strength of the [O III]5007˚ A line (in the F502N filter)
Photometry was then performed on the star at the centre of NGC 6302 in the six com-
bined multidrizzled images. Robustness of the results depends on accurate determination
of the high and structured background. We applied three different approaches. First, us-
ing a small aperture (2 pixels radius) and a 3 pixel wide ’sky’ (in reality nebula) annulus.
Second, a surface fit to the local background, excluding a small annulus around the cen-
tral star, was made in the same way for all images: this was subtracted from the image,

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and aperture photometry was performed on these images. As a third approach, Sextractor
(Bertin & Arnouts 1996) photometry was performed. For the F469N and F4673N filter im-
ages the results showed good consistency, but for the other images there is a larger scatter of
typically ±0.3mag. In Table 1, the 2.0 pixel radius photometry is listed, where an aperture
correction was applied by performing photometry of uncrowded stars in the periphery of the
nebula with increasing aperture size.
Figure 2 shows the photometric points (in AB magnitudes) plotted as a function of the
wavelength. At the position of each point, a horizontal bar shows the width of the respective
filter. Also plotted on the X-axis (in green) is a compressed log spectrum of NGC 6302 taken
from Groves et al. (2002), applying a logarithmic extinction at Hβ of c = 1.2. A nebular
continuum (computed for Te= 15000K and He/H=0.18) was added to this emission line
spectrum and the result converted to WFC3 filter photometry using the STSDAS.synphot
package. This allows comparison of the stellar photometry with the colours expected from
the nebula spectrum, the latter shown as red diamonds on Figure 2 (with arbitrary absolute
flux scaling tied to the F656N filter measurement of the stellar magnitude). From this
comparison it is clear that the photometric points for the F469N and F673N filters are
inconsistent with nebular emission. Although there appears to be a feature in the F373N,
F502N, F656N and possibly F658N, filters, at the position of the star in the F469N and
F673N images (see Figure 1), the heavy nebula contribution prevented reliable measurement
of the stellar magnitudes and the results correspond more closely to that of (inadequately
subtracted) nebula emission. The stellar magnitude is marginally measurable on the F502N
image. Even though the stellar flux is higher in the F373N band, the high extinction prevents
reliable detection in this filter.
Taking the two photometric points at 4690 and 6766˚ A (Table 1) as reliable detections
of a putative central star of NGC 6302, the (4690–6766˚ A) colour was compared with that
of a star of a given black body temperature subjected to line-of-sight extinction. Given the
very high temperature estimates for the central star, a very large value of extinction must be
applied to produce the observed (4690–6766˚ A) colour of 2.1 mag. Two extinction laws were
used to determine the extinction - the standard Galactic law of Seaton (1979) with the ratio
of total to selective extinction of R = 3.2 and, on the basis of the good agreement of the
observed and theoretical nebula continuum determined by Groves et al. (2002), the Whitford
law (as tabulated by Kaler 1976), for the three estimates of the stellar temperature, using a
black body. For such high temperatures, a black body should be a good approximation to
the optical colours. Table 2 lists the values of c determined in these three cases. The values
of c can be directly converted to AV by multiplying by 2.18 and are listed in Table 2.
For such high temperatures there is not sufficient sensitivity to determine the tempera-

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ture of the central star, and the extinction only changes by 0.02 for black body temperatures
from 150000 to 400000K. From the unreddened flux, taking the F673N magnitude as the
most reliable, assuming a black body, and the distance of 1.17kpc (Meaburn et al. 2008),
the derived stellar luminosity can be determined and is listed in Table 2.
4. Discussion
The HST point spread function and the excellent optical and CCD quality of WFC3
UVIS have allowed the central star of NGC 6302 to be unambiguously detected for the first
time. The star is situated in the centre of the large-scale nebular outflows, as expected,
and lies on the eastern edge of the thick dust lane. This location is also the expected one
given that the eastern lobe is tilted to the foreground (Meaburn et al. 2008). However the
central star is not at the expected position at the centre of the innermost torus as shown
by the radio data (Matsuura et al. 2005). No optical emission counterpart was found at
the position of the infrared source of Matsuura et al. (2005), which is approximately 2.′′6
north from the proposed central star. Nor was a point source detected at the position of the
6cm radio continuum emission peak reported by Matsuura et al. (2005) (about 2.′′2 from the
star); only an upper limit of 23.8 (AB mag.) was measured for the F673N magnitude of a
source at this position by aperture photometry. The (4688-6766˚ A) colour at this radio peak
position resembles nebular emission. Thus if a star was present at this position, it would
suffer much higher extinction than indicated by the Matsuura et al. (2005) extinction maps.
The very high extinction towards the central star derives mostly from the circumstellar
torus. From the ratio map of Hα to radio flux, Matsuura et al. (2005) derive AHα= 5–7 for
the central region, corresponding to AV = 6–8. Field stars indicate a foreground extinction
AV = 1.1 (Matsuura et al. 2005), leaving the remainder as circumstellar extinction. The
Brα image of Matsuura et al. (2005) shows that the peak extinction is considerably higher
than derived from Hα, possibly as high as AV = 30: Hα cannot be detected from such
regions. The region of highest extinction is offset from the centre, consistent with the fact
that the torus is seen with a small inclination of 18◦(Meaburn et al. 2008; Peretto et al.
2007; Dinh-V-Trung et al. 2008).
The distance to NGC 6302 determined by Meaburn et al. (2008) as 1.17kpc from ex-
pansion proper motions of filaments agrees fairly well with an assumed distance of 0.91kpc
from Kemper et al. (2002) and 1.0kpc from Matsuura et al. (2005). Older values include
Rodriguez et al. (1985) who argue for a distance of 1.7kpc, with a possible association to
the star formation region NGC 6334. Distances much larger than 1kpc lead to very high
masses for the shell and torus (e.g. Matsuura et al. 2005).