A&A 490, 715-724 (2008)
DOI: 10.1051/0004-6361:200810721
S. R. Pottasch1 - J. Bernard-Salas2
1 - Kapteyn Astronomical Institute, PO Box 800, 9700 Av.
Groningen, The Netherlands
2 - Center for Radiophysics and Space
Research, Cornell University, Ithaca, NY 14853, USA
Received 31 July 2008 / Accepted 12 September 2008
Abstract
The spectra of the planetary nebulae NGC 3242 and
NGC 6369 are reanalysed using spectral measurements made in the
mid-infrared with the Spitzer Space Telescope and the Infrared
Space Observatory (ISO). The aim is to determine the chemical composition
of these objects. We also make use of International Ultraviolet
Explorer (IUE) and ground based spectra. These elliptical PNe are
interesting because they are well-studied, nearby, bright objects and
therefore allow a reasonably complete comparison of this type of nebulae.
Abundances determined from the mid-infrared lines, which are
insensitive to electron temperature, are used as the basis for the
determination of the composition, which are found to differ somewhat
from earlier results. The abundances found, especially the low value
of helium and oxygen, indicate that the central star was originally
of rather low mass. The abundance of phosphorus has been
determined for the first time in NGC 3242. The electron temperature
in both of these nebulae is roughly constant unlike NGC 6302 and
NGC 2392 where a strong temperature gradient is found. The
temperature of the central star is discussed for both nebulae. Finally
a comparison of the element abundances in these nebulae with the solar
abundance is made. The low abundance of Fe and P is noted and it is
suggested that these elements are an important constituent of the nebular
dust.
Key words: ISM: abundances - ISM: planetary nebulae: individual: NGC 3242, NGC 6369 - infrared: ISM
NGC 3242 (PK 261.0+32.0) is a bright planetary nebula with a
low radial velocity and is located at a rather high galactic latitude.
The nebula has a
bright inner ellipsoidal shell of about
.
This is
surrounded by an almost spherical halo of
with
considerably lower emission. Both the inner and outer regions contain
little structure. The nebula is located about 32 degrees above the
galactic plane and has little or no extinction. Because of its
brightness it is clear that it is a nearby nebula. An expansion
distance is known for this nebula to be about 0.5 kpc (Terzian
1997; Mellema 2004).
The nebula has a rather bright central star (V=12.43) which has been
studied by several authors. Pauldrach et al. (2004) have
compared the stellar spectrum with a model atmosphere and conclude
that the star has an effective temperature
000 K which
is in between the hydrogen Zanstra temperature of about
59 000 K, and
somewhat less than the ionized helium Zanstra temperature which is
close to 91 000 K (Gathier & Pottasch 1988). Tinkler &
Lamers (2002) prefer to assign a higher effective
temperature of
K. to the star based on the high
ionized helium Zanstra temperature. We shall discuss the stellar
temperature later in the paper.
NGC 6369 (PK 002.4+05.8) is located in the direction of the galactic
bulge but it is undoubtably a nearby PN for two reasons. First, it is
a rather large nebula, with a diameter of about 32
.
Second, it
is the third brightest PN in the sky, at least at radio frequencies.
It cannot however be very close because the extinction is quite high
and its radial velocity is also high (-101 km s-1). The nebula can be
described as a bright ring with an outer diameter of 32.4
and
an inner diameter of about 13
.
There is faint emission both to
the east and the west of the main structure.
The central star is clearly visible. It is classified as
spectral type WC4 and has a magnitude V=15.91 (Gathier &
Pottasch 1988). Its hydrogen Zanstra temperature is about
70 000 K and ionized helium Zanstra temperature is 106 000 K
(Monteiro et al. 2004). Judging from these temperatures it
would appear that the central star of NGC 6369 is somewhat hotter than
that of NGC 3242. Yet as we shall see, the HeII lines are considerably
stronger in NGC 3242. This will be discussed further in Sect. 6 on the
basis of the excitation of other elements found in the nebulae.
The purpose of this paper is to study the element abundances in these
nebula with the help of mid-infrared spectra, in the hope that the
chemical abundances will shed some light on the evolution of these
nebulae. In recent years abundances in NGC 3242 have been studied by
Tsamis et al. (2003), by Barker (1985), by Henry
et al. (2000), by Aller & Czyzak (1979) and by Krabbe
& Copetti (2006). For NGC 6369 abundances have been studied
by Aller & Keyes (1987), Pena et al. (2001) and
Monteiro et al. (2004). All of these groups use optical
nebular spectra (taken by themselves) and in the case of NGC 3242
ultraviolet IUE spectra were also used. For NGC 6369
ultraviolet IUE spectra were taken but because of the very high
extinction in the direction of this nebula they are underexposed and
unusable. Barker (1985) has measurements of NGC 3242 taken
at five different positions in the nebula. He uses low dispersion
IUE measurements taken with the small aperture (3
diameter)
which are very noisy. Henry et al. (2000) use low dispersion
IUE measurements taken with the large aperture
(
).
We have measured the spectrum of both NGC 3242 and NGC 6369 in the mid-infrared with the IRS spectrograph of the Spitzer Space Telescope (Werner et al. 2004) and with the ISO spectrograph. The use of the mid-infrared spectrum permits a more accurate determination of the abundances. The reasons for this have been discussed in earlier studies (e.g. see Pottasch & Beintema 1999; Pottasch et al. 2000, 2001; Bernard Salas et al. 2001), and can be summarized as follows:
Observations of both NGC 3242 and NGC 6369 were made using the
Infrared Spectrograph (IRS, Houck et al. 2004) on board the
Spitzer Space Telescope with AORkeys of 16463360 and 4905216
respectively. NGC 6369 was observed in staring mode using the
SH module (9.5-19.5 m,
). In this observing mode the
spectrum is taken in two positions at 1/3 and 2/3 of the length of the
slit which are referred as the nod positions. The reduction in this
case started from the droop images which are equivalent to the
most commonly used Basic Calibrated Data (bcd) images but lack
stray-cross removal and flatfield. NGC 3242 was observed in cluster
mode at 3 positions, one centered at the target and two for background
of which only the closer sky position relative the target was used,
with the SL, SH, and LH modules (5.4-37
m). In cluster mode the
observations are taken in the center of the slit, as opposed to the
nod positions. Because the stars used for calibration were observed at
the nod positions in this case the reduction started from the bcd images. The data were processed using the s15.3 version of the
pipeline and using a script version of Smart (Higdon et al.
2004). The tool irsclean was used to remove rogue
pixels. The different cycles for a given module were combined to
enhance the S/N. At this point the background images were subtracted
to remove the sky contribution in NGC 3242. We note that since we are
interested in line fluxes the removal of background is irrelevant for
our analysis except for aiding in the removal of any rogue pixel that
may have been left out by the irsclean tool (i.e. those with low
flux). Then the resulting HR images were extracted using full aperture
measurements and the SL module in NGC 3242 using a fix column
extraction (10 pixels).
The great advantage of the IRS spectra compared to the ISO SWS spectra is the very high sensitivity of the IRS. Otherwise the two instruments are comparable. The IRS high resolution spectra have a spectral resolution of about 600, which is a factor of between 2 and 5 less than the resolution of the ISO SWS spectra.
The mid-infrared measurements are made with several different diaphragm sizes. Because most of the diaphragms are smaller than the size of the nebulae we are presently studying, we first discuss how the different spectra are placed on a common scale.
The spectra made with the IRS are taken with three different
diaphragms: the IRS high resolution instrument (spectral resolution
of about 600) measures in two spectral ranges with two different
modules: the short high module (SH) measures from 9.9 m to
19.6
m and the long high module (LH) from 18.7
m to
37.2
m. The SH has a diaphragm size of
,
while the LH is
.
If the nebulae are
uniformly illuminating then the ratio of the intensities would
simply be the ratio of the areas measured by the two diaphragms.
Since this is probably not so (because of low intensity holes in the
nebulae), we may use the ratio of the continuum intensity in the
region of wavelength overlap at 19
m. There are other ways of
determining this ratio which will be discussed presently. The SL
(low resolution) spectra are made with a long slit which is 4
wide and extends over the entire nebula.
The ISO diaphragms are somewhat larger. The SWS measurements below
12 m are made with a diaphragm
;
between 12
m and 27
m it is somewhat larger
(
)
and above 27
m it is
.
The ISO LWS spectra
(which cover a spectral region from 45
m to almost 200
m)
are taken with a diaphragm which has a diameter of about 80
and covers the entire nebula.
The IRS measurement of NGC 3242 was centered at RA(2000)
1024
46.11
and Dec(2000)
-18$^$38
32.6
.
This is almost the same as the value
measured by Kerber et al. (2003) of RA(2000)
10
24
46.138
and Dec(2000)
-18$^$38
32.26
,
which is presumably the coordinate of
the central star. Thus the IRS measurement was well centered on the
nebula and both the LH and SH diaphragms measured the inner,
brighter nebula. Since the LH diaphragm is larger, more of the
nebula is seen in this diaphragm and therefore a correction must be
made to bring the two measurements to the same scale. This was done
first by making use of the fact that the two spectrographs had a
small wavelength region in common at about 19
m. To make the
continuum emission at this wavelength equal, the SH emission had to
be increased by 3.96. The ratio can also be determined by using the
ratio of lines of a given ion which are observed both with the SH
and LH diaphragms and at the same time are also insensitive to the
electron temperature and density. Such a pair of lines is from
[Cl IV] at 11.76 and 20.31
m, which give about the same
factor between the SH and LH intensities. Normally the
[Ne III] lines could also be used to obtain this ratio but
this was not used in this case because the intensity of the
[Ne III] line at 36.01
m is badly determined.
The measured emission line intensities for NGC 3242 are given in
Table 1, after correcting the SH measurements by the factor 3.96 in
the column labeled ``intensity''. The fluxes were measured using the
Gaussian line-fitting routine.
The last column gives the ratio of the intensity to H
where
the H
is found from the strongest hydrogen line(s) measured
in the IRS spectrum. It has been assumed that the ratio of the sum
of the two hydrogen lines (n=7-6 and n=11-8) at 12.372
m to
H
has a value of
,
which is given by Hummer
& Storey (1987) for an electron temperature of 12 500 K.
No correction for extinction is made since it is very small. Other
hydrogen lines listed in the table can be used as well but the line
at 11.306
m is blended with the He II transition
n=18-14. All the lines predicted a value of H
through the
SH diaphragm of
erg cm-2 s-1 which translates
to a value of
erg cm-2 s-1 in the
LH diaphragm. The [Ar III] line at 8.992
m was measured with
the low resolution spectrograph (SL) using a diaphragm of
.
The intensity has been normalized so that the other
four lines measured in SL have the same intensities as these lines
in SH.
Table 1:
IRS spectrum of NGC 3242. The measured line intensity is
given in Col. 3. The last column gives the ratio of the line
intensity to H(=100).
There are no ISO SWS measurements of NGC 3242 because it was not
often visible to the satellite. There is however one ISO LWS
measurement and the resulting line intensities are listed in the last
three lines of Table 1 (taken from Liu et al. 2001).
Because the LWS had a diaphragm of almost 80
,
the intensities
given are for the entire nebula. To obtain the ratio of the
intensities to H
(the last column) the intensities were divided
by the value of H
obtained from the 6 cm radio continuum
emission given in the next section.
The IRS measurement of NGC 6369 was centered at RA(2000)
1729
20.78
and Dec(2000)
-23$^$45
32.3
.
This is almost the same as the value
measured by Kerber et al. (2003) of RA(2000)
17
29
20.443
and Dec(2000)
-23$^$45
34.2
,
which again is presumably the coordinate
of the central star. The IRS measurement was again well centered on
the nebula and the SH diaphragm measured the inner, brighter nebula
and part of the less bright hole. We do not have measurements made with
the LH diaphragm. We do however have ISO measurements of this nebula.
They were centered at almost exactly the same position as the
IRS measurements. The IRS measurements are listed in Table 2 which is
arranged in a similar way as Table 1. The intensities listed in
Col. 3 are those measured with the SH diaphragm. The ratio of the
intensity to H
shown in the last column is found in the
following way. The H
used is derived from the hydrogen lines
listed in the table which are measured through the same diaphragm.
They are interpreted in terms of H
using the theoretical ratios
given by Hummer & Storey (1987) for an electron temperature
of 12 500 K (see Sect. 4). Furthermore because the extinction is
high in this nebula a correction for this is also made in this column.
The correction is small in the infrared.
Table 2:
IRS spectrum of NGC 6369. The measured line intensity is
given in Col. 3. The last column gives the ratio of the line
intensity to H(=100).
The ISO measurements of NGC 6369 are shown in Table 3. As can be
seen from the table, the wavelength covered is considerably large
than the IRS measurements but because the sensitivity is lower only
the stronger lines are seen. The intensities measured are given in
Col. 3 of the table and are accurate to about 20%. Because the
extinction, which is given in the next section, is large, the
intensities corrected for extinction are given in Col. 4. In the last
column the ratio of the corrected intensity to the value of H
which would have been measured through the same diaphragm (and
corrected for extinction). A few words must be said about the values
of H
used because it varies with the diaphragm used. The
value of H
used below 12
m is found from the Brackett
and
lines. Using an electron temperature of
T=12 500 K this predicts a value of
H
erg cm-2 s-1 in this region. For the spectral regions
above 12
m we have increased the H
flux by the ratio of
the diaphragm size; thus from 12
m to 27
m we find
erg cm-2 s-1 and above this wavelength (and
less than 40
m) we use
erg cm-2 s-1.
This is the correct procedure as long as the nebula is larger than
the largest diaphragm used and the emission is uniform. This may be
checked by comparing the continuum emission ratios measured at the
transition wavelengths. This gives a consistent result although the
continuum is rather noisy, especially at 12
m. Another manner of
check is to note that the ratio of the [Ne III] lines
15.5/36.0, which has only a very small dependence on electron
temperature and density, has its predicted theoretical value. For
the wavelength region above 50
m the diaphragm is large enough
to include the entire nebula; therefore The value of H
used
is that found from the 6 cm flux density:
erg cm-2 s-1 (see the following section).
The values of I/H
found from the IRS and the ISO measurements
can be compared for four lines in common to the two instruments (see
Tables 2 and 3). In three of the four cases agreement is within 10%. Only for the [S IV] line is the agreement less good. We
suggest that the difficulty lies with the IRS measurement because
this part of the spectrum is badly calibrated. The line measured in
different orders differs by about 34%. In the discussion below the
ISO value will be used for this line and the IRS values for the
other lines.
Table 3:
ISO spectrum of NGC 6369. The measured line intensity is
given in Col. 3 and is corrected for extinction in Col. 4. The last column
gives the ratio of the line intensity to H(=100).
The extinction may be found both by a comparison of radio emission with
H flux and by a comparison of observed and
theoretical Balmer decrement, both of which we will discuss.
The 6 cm flux density for both nebulae has been measured by
Griffith et al. (1994) in the Parkes-MIT-NRAO survey. Their
reduction gave two values: one using a Fixed width fit and the other
a General width fit. For NGC 6369 they find 2041 and 1893 mJy and
for NGC 3242 760 and 731 mJy. In addition NGC 6369 was measured by
Milne & Aller (1975) who find a value of 2002 mJy for this
object. We will use a value of 2000 mJy for NGC 6369 which, using
values of
and helium abundance determined below together with
the equation quoted in Pottasch (1984), implies a total
H
flux of
erg cm-2 s-1 for this
object. Similarly a value of 745 mJy for NGC 3242 implies a total
H
flux of
erg cm-2 s-1 for this
object.
The measured value of the integrated H
flux for NGC 6369 and
NGC 3242 is
and
erg cm-2 s-1 respectively (see Acker et al.
1992). Using the value of H
from the radio
measurements leads to a value of C=2.116 or
EB-V=1.444. For NGC 3242 these values are C=0.12
EB-V=0.083.
The extinction determined from the Balmer decrement differs somewhat
according to different authors. For NGC 6369 Monteiro et al. (2004) find C=2.17, Aller & Keyes (1987)
give C=2.23 and Pena et al. (2001) find
C=1.9. For NGC 3242
Henry et al. (2000) give C=0, Aller & Czyzak (1979)
give C=0.14, and Barker (1985) finds C=0.15. These values
are in rough agreement with those found from the radio/H
method. Since this method is the most accurate determination, this
value will be used when necessary in this paper.
The visual spectrum has been measured by several authors for each of
the nebulae. We list here the results of three of the most recent
high resolution spectra for each nebula. The line intensities
reported have been corrected by each author for a value of
extinction determined by them to obtain a theoretically correct
Balmer decrement. The result are listed in Tables 4 and 5, where the
last column lists the average value which we have used. No attempt
has been made to use a common extinction correction because then the
Balmer decrement will be incorrect. The value of extinction C which
the individual authors found is listed at the bottom of the table.
All authors estimate that the strongest lines have a 10% error, the
intermediate strength lines (about 5% of H)
have about 20%
error and the weakest lines have about 30% error. The average
intensities have about the same error.
The measured optical line fluxes of NGC 3242 are shown in Table 4 and that of NGC 6369 in Table 5.
Table 4: Visual spectrum of NGC 3242.
Table 5: Visual spectrum of NGC 6369.
Only a few IUE observations of NGC 6369 were made. All of them were
underexposed in spite of the fact that the nebula is so bright. The
very large extinction reduces the ultraviolet emission in this case
to such an extent that the measurements are not useful. By contrast
there are 91 low resolution IUE observations of NGC 3242 as
well as six high resolution observations of this nebula. Seventy of
the low resolution measurements were taken with the large aperture
(
)
with varying exposure times, and 12 were
taken with a small aperture (3
diameter). We find that the
small aperture measurements are too noisy and we do not use them.
The large aperture measurements do not cover the entire nebula. We
have used two of the highest S/N observations made with long
exposure times both in the short and the long wavelength regions.
These are SWP15495 and SWP16418 for the short wavelength region and
LWR11973 and LWR 12678 for the long wavelength region. We have
checked that the strongest lines are not saturated by measuring
these same lines on spectra with shorter exposure times: SWP15495,
SWP 16419 and LWR12679. In addition two high resolution large
aperture measurements are used: SWP03643 and LWR03206. All of these
measurements are centered within 6
of the center of the
nebula and thus are representative of the nebula as a whole. The
results are shown in Table 6. The uncertainties are about the same
as given by Henry et al. (2000): about 10% for the
strongest lines and about 25% for the weaker lines. From the high
resolution spectra we find the C III ratio
1906/1909 =
and the Ne IV ratio
2422/2425 =
.
The extinction correction was made by assuming a theoretical ratio
for the He II line ratio 1640/
4686 Å at
T=11 500 K and an
of 104 cm-3. The ratio of
1640 to H
can then be found using the
4686/H
ratio in Table 4. A further correction for
extinction relative to
1640 Å is then made using the
reddening curve of Fluks et al. (1994) but because of the
small extinction this is never more than 20%, which indicates the
uncertainties of the UV intensities above the errors of measurement
given above. The results are shown in the last two columns of Table 6.
Table 6: IUE spectrum of NGC 3242.
The method of analysis is as follows. First the electron density and temperature as a function of the ionization potential are determined. The ionic abundances are then determined, using density and temperature appropriate for the ion under consideration. Then the element abundances are found for those elements in which a sufficient number of ionic abundances have been derived.
The ions used to determine
for NGC 3242 are listed
in the first column of Table 7; those for NGC 6369 are listed in
Table 8. The ionization potential required to reach this stage of
ionization, and the wavelengths of the lines used, are given in
Cols. 2 and 3 of the tables. Note that the wavelength units are
Å when 4 figures are given and microns when 3 are shown. The
observed ratio of the lines is given in the fourth column; the
corresponding
is given in the fifth column. The
temperature used is discussed in the following section, but is
unimportant since these line ratios are essentially determined by the
density. The density from the C III] lines in NGC 3242 is
uncertain because the two measurements used differed substantially..
There is no indication that the electron density varies with ionization potential in a systematic way in either of the nebulae. The electron density appears to be about 2500 cm-3 in NGC 3242 and slightly higher (about 2800 cm-3) in NGC 6369. The error is about 20% in NGC 3242 and slightly higher in NGC 6369. We will use these densities in further discussion of the abundances, but any value between 1500 cm-3 and 3000 cm-3 will give the same values of abundance.
Table 7: Electron density indicators in NGC 3242.
Table 8: Electron density indicators in NGC 6369.
A number of ions have lines originating from energy levels far
enough apart that their ratio is sensitive to the electron
temperature. These are listed in Tables 9 and 10 for each of the
nebulae. These tables are arranged similarly to the previous tables.
No temperature gradient is seen in either of the nebulae. Both
nebulae have very similar electron temperatures: the temperature of
NGC 3242 is about 11 500 K while that of NGC 6369 seems to be
slightly lower: T=11 000 K. The temperature for NGC 3242 is better
determined because ultraviolet measurements are available. Although
a weak line is seen at 14.3 m, no temperature can be obtained
from Ne V because no good measurement of the ultraviolet line
at
3425 has been made.
Table 9: Electron temperature indicators in NGC 3242.
Table 10: Electron temperature indicators in NGC 6369.
The ion abundances have been determined using the following equation:
The results are given in Tables 11 and 12, where the first column
lists the ion concerned, the second column the line used for the
abundance determination, and the third column gives the intensity of
the line used relative to H.
The fourth column gives the
value of the ionic abundance assuming the ion is formed at T=11 500 K for NGC 3242 and T=11 000 K for NGC 6396, while the fifth
column gives the ionization correction factor (ICF), which has been
determined empirically. Notice that the ICF is close to unity for
all elements listed in the table except for Fe and P.
The error of measurement of the IRS intensities as can be seen in Table 1 is usually small, often not more than 5-7%. In the few cases when the error is large this has either been indicated with a ``:'' or by not using the line. The correction for adjusting the SH to LH intensity scales and the diaphragm size is also small, about 10%. This includes the assumption that the unmeasured parts of the nebula have the same composition as the measured parts. This has been checked in the optical region by Barker (1985) and can be seen to be a reasonable approximation in the ultraviolet region by comparing the IUE spectra obtained in different parts of the nebula. The uncertainty of the collisional strengths introduces an error of 10-15% so that the total error for the ions of neon, sulfur and argon determined with the IRS measurements is less than 20%. This will also be true of the abundances of these elements because the ICF for these elements is close to unity. The error for the nitrogen and oxygen abundances is somewhat higher because the visual and ultraviolet measurements are less certain. In addition the temperature is more important for these ions and the total errors may be twice as large. The element abundances are given in the last column. The carbon recombination line abundance for NGC 3242 is given at the end of Table 11. It is somewhat more than a factor of 3 higher than the value obtained from the collisional transition. This difference is found in other PNe as well but is not yet understood. No recombination line has been measured in NGC 6369.
The helium abundance was derived using the theoretical work of
Benjamin et al. (1999) and Porter et al. (2005) For recombination of singly ionized helium,
most weight is given to the 5875 Å line, because the
theoretical determination of this line is the most reliable.
Table 11:
Ionic concentrations and chemical abundances in NGC 3242.
Wavelengths in Angstrom for all values of
above 1000, otherwise
in
m.
Table 12:
Ionic concentrations and chemical abundances in NGC 6369.
Wavelengths in Angstrom for all values of
above 1000, otherwise
in
m.
Tables 13 and 14 show a comparison of our abundances with three of the most important determinations in the past 20 years for each nebulae. For NGC 3242 reasonable agreement is found. Good agreement is found for oxygen; this is because the same electron temperature is used for the most important oxygen ion. For the other elements the agreement is less good. For nitrogen a somewhat higher abundance is found which is probably due to the fact that the most important ionization stages have not been measured earlier. Nitrogen is somewhat higher than in the Sun. A C/O ratio substantially lower than unity and close to the solar value is found but the error is about 30%. Sulfur is in good agreement with earlier determinations but is a factor of 4 lower than the solar value. This is a general phenomenon in PNe and has been discussed earlier (e.g. Pottasch & Bernard-Salas 2006; Bernard-Salas et al. 2008). Phosphorus, which has never been measured before in this nebula and is slightly uncertain because only a single ionization stage has been measured, is considerably lower than solar. Chlorine also seems rather strongly depleted compared to the solar abundance. The solar abundance listed in the table is taken from Asplund et al. (2005). Note that for solar sulfur and chlorine more weight has been given to the abundance determination in meteorites since this determination is more accurate than for the Sun itself. Neon and argon abundances are taken from the references given in Pottasch & Bernard-Salas (2006) and differ substantially with those given by Asplund et al. (2005). We point out that the solar abundances have changed greatly in the past 10 years and it is uncertain that there will be no further changes in the near future (see discussion by Bernard-Salas et al. 2008).
Table 13: Comparison of abundances in NGC 3242.
For NGC 6369 the agreement is somewhat less good. The reason for this is that the comparison determinations use only the visible spectra since no ultraviolet measurements have been made. For nitrogen only the N+ abundance can be measured and a large correction for higher ionization stages must be made. Besides this the very high extinction correction which must be made in this nebula leads to larger uncertainties in line intensities. The advantage in using the infrared measurements is quite clear.
Table 14: Comparison of abundances in NGC 6369.
For both nebulae there is a general agreement with the (more uncertain)
earlier values. Helium, oxygen and nitrogen abundances are essentially
solar. This is probably true of neon as well. On the other hand,
sulfur, argon, chlorine and phosphorus are all significantly lower
than solar by a factor of 3 to 4, both in NGC 6369 and NGC 3242.
This is also remarkably similar to most elliptical PNe which have been
studied in the infrared and which are at approximately the same
galactocentric distance. The abundances of these nebulae are given in
Table 15. These nebulae not only have similar morphology, but are
also all at rather high galactic latitude. The last five PNe listed
all have galactic latitudes between 10
and 20
,
which is much
higher than the average value. Part of this difference occurs because
these are all relatively nearby nebulae. But the main reason for the
high galactic latitude is probably that these nebulae are formed from
stars which initially had a relatively low mass.
Table 15: Comparison of abundances in several elliptical PNe.
In considering all the nebulae in Table 15 the following is noted.
The low helium abundance indicates that no helium has been produced
which implies that the second dredge-up and hot-bottom burning have
not taken place. For some PNe the carbon abundance is somewhat higher
than oxygen suggesting that the third dredge-up has taken place in these
nebulae. Following the models of Karakas (2003) for the case
Z=0.008 (which is close to the solar abundance) the increase in the
carbon abundance will occur at a stellar mass of about 1.7 M.
The lower mass models for which the carbon has not yet increased show
an increased nitrogen abundance, sometimes by a factor of two or three.
When the increased carbon is found the nitrogen is predicted to have
its original (solar) value. This seems to be exactly what is observed.
Of the nebulae listed in Table 15 we therefore predict that NGC 3242,
NGC 7662 and NGC 2022 have descended from stars with a mass of
between 0.8 M
and 1.5 M
and NGC 6826, NGC 2392 and
IC 418 are descendents of stars from between 1.6 M
and
1.9 M
.
Above this mass Karakas predicts an increased neon abundance
by more than a factor of two, which is not seen. NGC 6369 is harder to
classify since no carbon has yet been measured. From the low nitrogen abundance
we tentatively conclude that this nebula is probably in the second (high mass)
group.
Notice that for all the nebulae listed in Table 15 the elements S, Ar, Cl, P and Fe are underabundant compared to the Sun. There are several possible reasons for this. First the solar abundances may be wrong. Secondly our determinations of the abundances may be wrong. Thirdly the elements may be tied up in the dust grains in the nebula. We do not believe that our determination of the abundances can be so badly wrong. The worst case is P because only a single ionization stage is observed and a reasonably large correction for the missing stages of ionization has been made. It seems unlikely that this has been so poorly done. For S, Ar and Cl the solar abundance is somewhat uncertain and could be wrong. This is certainly the case for Ar for which a coronal value must be used. Furthermore the difference between the nebular value and the solar value is only a factor of two to three for these 3 elements. For S and Cl it is not possible to say whether the difference is due to wrong solar values or that the elements are in taken up in the dust. Ar is a noble gas and it is unlikely that it can be present in dust. For P and Fe the difference with the solar value is so large that these elements must be in the form of dust. Since iron has been known to be depleted in all PNe in which it has been observed, there has been some speculation in the literature as to whether iron molecules have been observed in the infrared continuum. Hony et al. (2002) claim to have detected FeS in two PNe. These authors also note that iron sulfides are abundantly found in meteoritic material. Iron could also be in the form of FeO. But iron molecules are easily destroyed. Iron is highly refractory and should easily stick to carbonaceous dust grains which are present in PNe and are responsible for their IR continuum. Measuring the iron content of the dust grains is very difficult and has not yet been done. But it is significant that about 1% of the nebular mass must be contained in dust containing this iron molecule assuming that the original composition was solar. Notice that the spectrum of dust does not show any hydrocarbon (PAH) emission bands in the oxygen-rich nebulae listed in Table 15. Very weak PAH features are seen in the carbon-rich nebula IC 418 and in NGC 6369.
As discussed in the introduction, the spectrum of the central star
of NGC 3242 has been studied by several authors. Pauldrach et al.
(2004) have fitted model atmospheres to the measured
ultraviolet stellar spectrum and Kudritzki et al. (1997) have
studied the optical spectrum. Pauldrach et al. (2004)
determined the effective stellar temperature from the FeIV/FeV
ionization balance to be 75 000 K while Kudritzki et al.
(1997) using the ionization equilibrium of HeI and HeII also
obtained a value of 75 000 K. Neither of these authors has
studied the spectrum of the central star of NGC 6369. This star is
of the Wolf-Rayet type and was originally classified as WC4 (see
Acker et al. 1992). More recently it is classified as WO3
(Acker & Neiner 2003). The stellar temperature
associated with this spectral type is usually about 85 000 K to
90 000 K, which is often determined by the Zanstra temperature
(Acker & Neiner 2003). This at first sight it would
appear that the central star of NGC 6369 has the higher temperature
of the two. The Zanstra temperatures give a slightly different
picture. The hydrogen Zanstra temperature
(H) = 69 000 K for NGC 6369 and 57 000 K for
NGC 3242 while the ionized helium Zanstra temperature is 71 000 K
for NGC 6369 and 90 000 K for NGC 3242. Since the ionized helium
Zanstra temperature is often taken as representative of the actual
temperature this would imply that NGC 3242 is the hotter star. This
is a direct result of the much stronger He II line in the
latter nebula. This is also reflected in the ionic distribution of
the other elements. For example, 10% of the oxygen is in the form
of O+3 in NGC 3242 while only 1% of the oxygen is in this
ionization stage in NGC 6369.
The stellar temperature can also be estimated from the energy
balance temperature of the central star. This method makes use of the fact
that the average excess energy per ionizing photon (hence the temperature)
can be found from the ratio of collisionally excited lines to H.
For NGC 3242 this can be found from the sum of the values given in
Tables 1, 4, and 6. Unobserved lines must be estimated usually from predicted
ratios of observed lines. For NGC 6369 it is the sum of values given in
Tables 3 and 5 again corrected for unobserved lines. For this latter nebula
no ultraviolet measurements are available. For NGC 3242 the
ultraviolet measurements are about 20% of the total and because the
spectra of the two nebulae are so similar we will assume this to be
the same in NGC 6369. This value of the ratio of the summation
I/H
is about 25 or 26 in both cases. This could be slightly
higher if there are important unmeasured lines. To convert this
value to a stellar temperature it is not possible to use a simple equation
because the result, at least for higher temperatures, depends on the amount
of helium present in the nebula, its ionization state, the opacity in
the nebula and ultraviolet spectral distribution of the exciting star. To
obtain a temperature the formulation of Preite-Martinez
& Pottasch (1983) is used, assuming blackbody radiation from
the central star. This leads to a value for the energy balance
temperature (
(EB) = 70 000 K in both cases. If a model
atmosphere had been used instead of a blackbody, the energy balance
temperature could be lower. When the three values of temperature
are compared for NGC 6369 they are consistent and we adopt a
temperature T=70 000 K for the central star of NGC 6369. Because of
the higher ionization state in NGC 3242, the temperature of the
central star of this nebula must be somewhat higher since it cannot
be explained by a lower density. A temperature T=80 000 K is used because
of the higher ionized helium Zanstra temperature.
If the distance to these nebulae is known the radius and luminosity
of the central star may be found. For NGC 3242 an expansion
distance has been measured by Hajian et al. (1995) and
corrected by Mellema (2004) to a somewhat uncertain value
d=550 pc.
The distance to NGC 6369 has only been determined stastisically and thus is
quite uncertain; we shall use a value of d=1 kpc which is an average value of the statistical distances listed in
Acker et al. (1992). The nebula could be somewhat closer
because it is the third brightest PNe after correction for
extinction. The visual magnitudes for the central stars are
and 15.91 for NGC 3242 and NGC 6369 respectively. When
corrected for the extinction found in Sec. 3 these values are
and 11.42, which leads to radii of 0.141 R
and
0.336 R
respectively. Combined with the above values of
central star temperature, the luminosities of these stars are
730 L
and 2430 L
for NGC 3242 and NGC 6369. These
values of luminosity are considerably lower than found in the recent
literature. For NGC 3242 Pauldrach et al. (2004) find a value
of 3200 L
while Kudritzki et al. (1997) find
10 000 L
.
For NGC 6369 Monteiro et al. (2004) find
8100 L
,
but in this case much of the difference is due to
the larger distance used by these authors.
The nebular abundances of nine elements have been determined for the PN NGC 3242; abundances of the same elements, except for carbon, have been found for NGC 6369. Both nebulae are classified as having an elliptical shape. The abundances found for both nebulae are very similar. For helium, oxygen and neon they are essentially the same as in the Sun. This is consistent with the expectation that these PNe are rather local. This is because the gradient in the stellar abundances as a function of distance from the galactic center (e.g. see Pottasch & Bernard-Salas 2006) would lead to the expectation that a different abundance prevails when the position of these PNe is non-local.
The state of evolution of these nebulae is considered in conjunction
with five other local PNe with elliptical shape. A comparison is made
between the observed abundances and those expected using the stellar
evolution models calculated by Karakas (2003). This
comparison leads to the conclusion that NGC 3242 must be a descendent
of a low mass star, probably between 1 M and 1.5 M
.
NGC 6369 probably has a slightly higher initial mass, but this conclusion
is less certain because the abundance of carbon is unknown.
The stellar temperature and luminosity is considered with the help of
the Zanstra method, the Energy Balance method and the observed nebular
excitation. For NGC 3242 we find
000 K and
L=730 L
and for NGC 6369
K and
L=2400 L
.
The temperatures are in quite good agreement with what is given in the
literature. The luminosities, which are distance dependent, are
considerably lower than given by the theories of Kudritzki et al.
(1997) or Pauldrach et al. (2004). This has already been
discussed by Napiwotzki (2006) in a general way and by
Surendiranath & Pottasch (2008) for the case of NGC 6826.
This is evidence that all of the nebulae listed in Table 15 have
central stars of quite low luminosity, probably between
500 L
and 2000 L
.
The abundances of the other elements, S, Cl, P and Fe are all lower than solar. For the first two of these elements the difference with respect to the sun is a factor of two to three. In this case we cannot distinguish between the possibility that this is caused by a poor determination of the solar abundance or by the possibility that important amounts of these elements have condensed in the form of dust. For the remaining elements, P and Fe, the underabundance with respect to the Sun is larger; it is likely that both of these elements have formed molecules which have condensed in the form of dust. This is probably true for all the PNe listed in Table 15.
Acknowledgements
This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work was provided by NASA through Contract Number 1257184 issued by JPL/Caltech. We acknowledge that the IRS data on NGC 6369 has been taken from one of the GTO programs of Dale Cruikshank.