Two New Planetary Nebulae and an AGN in the
Galactic Plane

S. H. Beer and A. E. Vaughan, PASA, 16 (2), in press.

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The Planetary Nebulae

Each of the objects presented here displays easily resolved emission lines, and no detectable continuum. Spectra of the two PN are shown in Figures 4 and 5. The identified lines are H$_\beta $, along with the 5007 and 4959 Å [OIII] ``nebulium'' lines labelled as N1 and N2 respectively. In both objects the [OIII] lines are significantly stronger than the H$_\beta $ line. The objects are spatially unresolved as determined from their intensity profiles along the spectrograph slit. Upper limits on their angular sizes are given in Table 2.

Figure 4: [OIII] and H$_\beta $ lines IRAS 08418-4847.

$\textstyle \parbox[c]{0.5cm}{\rotatebox{90}{\footnotesize \axsize Flux $(\times 10^{-15}$erg cm$^{-2}$\ s$^{-1}$\ Hz$^{-1})$}}$$\textstyle \parbox{10cm}{\scalebox{0.8}{\includegraphics*{spec0088.eps}}}$
Wavelength (Å)

Figure 5: [OIII] and H$_\beta $ lines in IRAS 14132-5839.

$\textstyle \parbox[c]{0.5cm}{\rotatebox{90}{\footnotesize \axsize Flux $(\times 10^{-15}$erg cm$^{-2}$\ s$^{-1}$\ Hz$^{-1})$}}$$\textstyle \parbox{10cm}{\scalebox{0.8}{\includegraphics*{spec96.eps}}}$
Wavelength (Å)

PN are not the only objects which satisfy these properties. Other objects which may display emission-line spectra such as these are novae, compact emission-line galaxies, HII regions, and emission-line stars. Extragalactic objects are easily distinguishable by their larger redshifts, while galactic objects have smaller redshifts. The redshifts of IRAS 08418-4847 and 14132-5839 (Table 2) are typical of galactic objects. While it is not possible to completely rule out all of the other abovementioned object types, objects with the properties of IRAS 08418-4847 and 14132-5839 are regarded as being PN by the majority of authors (for example Cuisinier, Terzan, & Acker 1993). Compact HII regions are generally brighter than PN, and may usually be excluded by the presence of a continuum (Morgan & Good 1992). The six emission-line objects observed in this survey that were classed as HII regions all had some evidence of extended structure on the DSS images.

The level of excitation may be inferred from the ratio of the [OIII] lines to H$_\beta $. This ratio is higher on average for PN than for HII regions, since the central stars of PN generally have higher temperatures than in HII regions. The two PN (and the galaxy) of this survey all have a higher (N1+N2)/H$_\beta $ ratio than any of the six observed HII regions. In addition, both PN lie well inside the infrared colour box described by Pottasch et al. (1988), far away from the region where HII regions are usually found.

The excitation class p is calculated for each of the PN as described by Gurzadyan (1997), and shown in Table 2. The lower classes are based on the (N1+N2)/H$_\beta $ ratio, up to p = 4 for (N1+N2)/H$_\beta > 15$. For classes higher than p = 4, the 4686 Å line of HeII is used, however this line was not observed in either of the objects.

Milne & Aller (1975) derive a method for estimating the distances to planetary nebulae using the 5 GHz radio flux. The method is based on average values over a large number of planetary nebulae for the parameters y = 0.11,

$\varepsilon = 0.6,$ Te = 10,000 K,

$x^{\prime\prime} = 0.5$ and M = 0.16 M$_\odot$. Here y is the number abundance ratio of helium to hydrogen,

$x^{\prime\prime}$ is the fraction of HeIII to HeI atoms, and M is the nebular mass. The filling factor $\varepsilon$ measures the degree of divergence of the nebula from homogeneity. The distance is

\begin{displaymath} D = 6180 \: \theta^{-3/5}F_{\rm 5GHz}^{-1/5} \end{displaymath} (1)

where D is given in parsecs, $\theta$ is the angular diameter of the nebula in arcseconds, and F is given in janskys. Using angular sizes from Table 2 this method gives a lower limit of $D \ge 5300$ pc for IRAS 08418-4847 and $D \ge 7000$ pc for IRAS 14132-5839. Milne & Aller (1975) also include a method for estimating the expected flux in H$_\beta $ from the 5 GHz radio flux. The expected H$_\beta $ flux is given by

\begin{displaymath} \frac{10^{-26} F_{\rm 5 GHz}}{F({\rm H}_\beta)} = 3.05\! \ti... ...\{ 1+(1-x^{\prime\prime}) y + 3.7 x^{\prime\prime} y \Big\} , \end{displaymath} (2)

with t = Te/104 K, F(H$_\beta $) in erg cm-2 s-1, and $F_{\rm 5 GHz}$ in Jy as before. Using the average values for Te,

$x^{\prime\prime}$, and y quoted previously, this method gives a predicted H$_\beta $ flux of

3.1 x 10-11 erg cm-2 s-1 for IRAS 08418-4847 and

9.6 x 10-12 erg cm-2 s-1 for IRAS 14132-5839. Comparing the predicted and observed fluxes in H$_\beta $ gives a measure of the interstellar extinction between the source and the Earth. The extinction coefficient C, calculated as

\begin{displaymath} C = {\rm log_{10}}\left(\frac{F({\rm H}_\beta)\:{\rm predicted}} {F({\rm H}_\beta)\:{\rm observed}} \right), \end{displaymath} (3)

is shown in Table 2 for the two objects.

A second method of calculating distances to PN is quoted by several authors, for example Pottasch (1984). This method uses the H$_\beta $ flux alone and applies to PN with densities higher than 4 x 102 particles cm-3. From examining known PN data it is apparent that, for PN of high density, assuming a constant average mass (such as the M = 0.16 M$_\odot$ used above) gives a poor approximation. A better approximation is to take the mass as M = 102 M$_\odot$ cm

$^{-3} \, n_e^{-1}$, which has the convenient effect of removing the dependence on angular size seen in equation (1). Using this approximation, the distance is given by

\begin{displaymath} D = 3\,\Big\{F({\rm H}_\beta)\Big\}^{-1/2}, \end{displaymath} (4)

with D in kiloparsecs and F(H$_\beta $) in 10-11 erg cm-2 s-1. The H$_\beta $ value to be used is that corrected for interstellar extinction, and thus the values estimated from the radio flux are used, giving distances of 1700 pc and 3100 pc to IRAS 08418-4847 and 14132-5839 respectively.

Clearly the two methods used give a significantly different distance. This may be explained by the inherently low accuracy of these statistical distance methods and the unknown density of the nebulae. The densities may be low, and thus equation (4) would not apply. Alternatively, the density may be high, and thus the constant mass method of equation (1) gives a poor approximation.

A recent treatment of distances to PN is given by Schneider and Buckley (1996). Their method uses an empirically determined relationship between surface brightness and radius, in the form of a second degree polynomial fit to a selection of PN. The method is thus more reliable, and able to cope with a larger range of physical properties of PN. Specifically, the method gives better results for PN of both high and low density and surface brightness.

Using this method, the radius-surface brightness relationship is

\begin{displaymath} \textnormal{log} \: r = a \, ({\rm log} \: I_\nu)^2 + b \, ({\rm log} \: I_\nu) + c. \end{displaymath} (5)

Here r is the nebular radius in pc,

$I_\nu = F_{\rm 5 GHz}/(\pi \theta^2)$ is the surface brightness in mJy arcsec-2, and a = -0.0261, b = -0.299, and c = -1.116 are empirically determined constants. The distance D in kpc is then given by

\begin{displaymath} {\rm log} \: D = {\rm log} \: r - {\rm log} \: \theta + {\rm log} \: 206.265, \end{displaymath} (6)

with $\theta$ measured in arcseconds. This method gives a distance of $D \ge 3700$ pc for IRAS 08418-4847 and $D \ge 5300$ pc for IRAS 14132-5839. The nebular radii estimated by equation (5) are $r \le 0.050$ pc for IRAS 08418-4847 and $r \le 0.065$ pc for IRAS 14132-5839. The distance figures given by this recent method are seen to be in between those given by the earlier two methods.


Next Section: The Seyfert Galaxy
Title/Abstract Page: Two New Planetary Nebulae
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Contents Page: Volume 16, Number 2

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