IPHAS extinction distances to Planetary Nebulae

IPHAS extinction distances to Planetary Nebulae

C. Giammanco Instituto de Astrofísica de Canarias (IAC), C/ Vía Láctea s/n, 38200 La Laguna, Spain
corrado@iac.es Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
   S. E. Sale Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, U.K.    R. L. M. Corradi Instituto de Astrofísica de Canarias (IAC), C/ Vía Láctea s/n, 38200 La Laguna, Spain
corrado@iac.es Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
   M. J. Barlow Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK    K. Viironen Instituto de Astrofísica de Canarias (IAC), C/ Vía Láctea s/n, 38200 La Laguna, Spain
corrado@iac.es Centro Astronómico Hispano Alemán, Calar Alto, C/Jesús Durbán Remón 2-2, E-04004 Almeria, Spain Centro de Estudios de Física del Cosmos de Aragón (CEFCA), C/General Pizarro 1-1, E-44001 Teruel, Spain
   L. Sabin Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 877, 22800 Ensenada, B.C., México    M. Santander-García Isaac Newton Group of Telescopes, Ap. de Correos 321, 38700 Sta. Cruz de la Palma, Spain Instituto de Astrofísica de Canarias (IAC), C/ Vía Láctea s/n, 38200 La Laguna, Spain
corrado@iac.es Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
   D. J. Frew Department of Physics, Macquarie University, NSW 2109, Australia    R. Greimel Institut für Physik, Karl-Franzens Universität Graz, Universitätsplatz 5, 8010 Graz, Austria    B. Miszalski    S. Phillipps Astrophysics Group, Department of Physics, Bristol University, Tyndall Avenue, Bristol, BS8 1TL, U.K.    A. A. Zijlstra Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, M13 9PL, Manchester, U.K.    A. Mampaso Instituto de Astrofísica de Canarias (IAC), C/ Vía Láctea s/n, 38200 La Laguna, Spain
corrado@iac.es Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
   J. E. Drew Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, U.K. Centre for Astrophysics Research, STRI, University of Hertfordshire, College Lane Campus, Hatfield, AL10 9AB, U.K.    Q. A. Parker Department of Physics, Macquarie University, NSW 2109, Australia Anglo-Australian Observatory, PO Box 296, Epping, NSW 1710, Australia    R. Napiwotzki. Centre for Astrophysics Research, STRI, University of Hertfordshire, College Lane Campus, Hatfield, AL10 9AB, U.K.
Received September 15, 1996; accepted March 16, 1997
Key Words.:
Planetary nebulae: general, individual distances – Methods: data analysis.
Abstract

Context:

Aims:The determination of reliable distances to Planetary Nebulae (PNe) is one of the major limitations in the study of this class of objects in the Galaxy. The availability of new photometric surveys such as IPHAS covering large portions of the sky gives us the opportunity to apply the “extinction method” to determine distances of a large number of objects.

Methods:The technique is applied to a sample of 137 PNe located between -5 and 5 degrees in Galactic latitude, and between 29.52 and 215.49 degrees in longitude. The characteristics of the distance-extinction method and the main sources of errors are carefully discussed.

Results:The data on the extinction of the PNe available in the literature, complemented by new observations, allow us to determine extinction distances for 70 PNe. A comparison with statistical distance scales from different authors is presented.

Conclusions:

1 Introduction

Distances to Galactic planetary nebulae (PNe) present a severe and longstanding problem. Obtaining an accurate distance scale for PNe will allow us to compute the total number of PNe in the Galaxy, which has important implications for the Galactic ultraviolet radiation field, the total processed mass returned to the interstellar medium, and more generally to our understanding of the chemical evolution of the Galaxy.

A method that is a priori independent of assumptions about the physical or geometrical properties of the nebulae is the extinction method. Assuming that the interstellar extinction to a certain nebula can be determined, if one is able to build up the extinction-distance relation using field stars around the line of sight to the nebula, the relation can be used to infer the distance to the PN.

The application of this method is not new. Lutz (1973) measured the distance to 6 PNe using some 10 field stars per object. Later, the method was applied by Acker (1978) who provided reliable distance values for 11 PNe and a rough estimation for 34 other ones, and more recently by Gathier et al. (1986) who measured the distance to 12 PNe using some 50 stars per PN. In the last paper, a comprehensive discussion about different aspects of this method can be found. The number of stars for determining distances was increased by Pollacco & Ramsay (1992) who, using a colour analysis of the field stars, were able to reach an accurate spectral classification for stars later than F5 type. As late-type stars constitute the most numerous objects in all galactic-plane directions, they allow an extensive application of this method to determine distances to PNe in the Galaxy.

The availability of the IPHAS H survey of the Northern Galactic Plane (Drew et al. 2005) and its coming extension to the South (VPHAS+) opens new doors for the application of the method. IPHAS allows us to determine extinction-distance curves using a large number of field stars, typically several hundred in areas as small as around each line of sight. The technique is presented and discussed by Sale et al. (2009). In this paper, we will present its application to Galactic PNe. We determine the distance to 70 PNe included in the ESO/Strasbourg catalogue (Acker et al. 1992), and compare the distances obtained with those obtained by other authors using different methods. This shows how IPHAS and its successor surveys potentially provide a new and powerful tool to obtain distances to a large number of Galactic PNe.

2 The data

IPHAS is a wide-field, CCD, H survey of the Northern Galactic Plane (Drew et al., 2005), carried out at the 2.5m Isaac Newton Telescope on La Palma, Spain. Imaging is performed also in the r and i bands down to r (10). The high quality photometry and characteristics of the survey permit the spectral classification of main-sequence stars to be determined, based on the H line strength. The availability of reliably calibrated (r - H) colours permit the spectral classification of most stars, while the (r- i) colour provides their extinctions, allowing distances to be estimated from the r measurements. The limiting magnitude in r permits extinctions to be measured to distances of up kpc.

IPHAS covers the Galactic latitude range between -5 and 5 deg, and longitude range between 29.52 and 215.49. In this region of the sky there are 190 known PNe according to the ESO/Strasbourg catalogue (Acker et al., 1992). We were able to extract a reddening for 137 of them from different sources; for 27 PNe we present the first determinations.

2.1 PN extinctions extracted from the literature

The ESO/Strasbourg catalog provides the line ratio for most of the PNe considered here. These data were measured from spectra obtained using two different telescopes: the 1.52m ESO telescope for the Southern nebulae, and the 1.93m OHP telescope for the Northern ones. Further details about the instruments (photographic plate or CCD) and spectra can be found in Acker et al. (1992, SECGPN), Acker & Stenholm (1987), and Stenholm & Acker (1987). Later, Tylenda et al. (1992) used these SECGPN line intensities to obtain the value of the extinction constant roughly determined for about 900 PNe. In this paper the visual extinction was obtained from the line ratios by applying the formulae:

(1)
(2)

which come from Fitzpatrick (2004). Here we assumed R=3.1, in order to be consistent with the method by which the IPHAS extinction curves are built (see section 3).

Other compilations from which was retrieved are Cahn et al. (1992) and in few cases Stasińska et al. (1992). The latter paper compared the optical and radio extinction determinations, using the same “SECGPN” line fluxes as Tylenda et al. (1992), and adding some other measurements, for a total of 130 PNe. For individual nebulae, these listings where supplemented with data from other papers, where available. In all cases, when in a paper the assumed reddening law is reported, we normalize the published extinction using Eqs. (1) and (2). Specifically, this is the case for data in Cahn et al. (1992) and Stasińska et al. (1992). For most of the nebulae the only available extinction coefficient comes from the Strasbourg catalog, while for a subset there are multiple determinations. For a small number of nebulae there is a very extensive literature; and in those cases we selected what we believe is the most accurate determination of after a careful analysis of bibliographic sources. In addition, for some nebulae we also present new optical determinations.

A general rule in case of multiple determinations and in the absence of other information (the quality of one measurement with respect to another) is to choose the smallest . The main reason is that slit spectra taken at non-negligible zenith distances are often affected by differential atmospheric dispersion which removes blue light leading to an overestimate of the ’s. Extinction measurements obtained via radio data are free from this effect, but these give extinctions systematically lower than those determined from the Balmer decrement (Stasińska et al., 1992, Ruffle et al. 2004). In order to deal with a homogeneous set of measurements, we will only consider in this paper obtained via optical measurements.

Finally, in order to illustrate the differences between extinction determinations by different authors, we show in Fig. 1 the values obtained by Cahn et al. (1992) versus those computed from fluxes in the ESO/Strasbourg catalog. The associated scatter is , which shows how critical the choice of the extinction values can be. We note that data from the ESO/Strasbourg catalog are not all of the same quality, those for the Northern nebulae () observed at the OHP being generally of lower quality than those of (mainly Southern) nebulae observed at ESO.

Figure 1: measurements by by Cahn et al. (1992) plotted against data in Acker et al. (1992). The dispersion is around 0.4 in .

2.2 New extinction determinations

26 PNe considered in this paper (see Tabs. 1 and 3) were observed at the 2.5m Isaac Newton Telescope at the Observatorio del Roque de los Muchachos, from August 23 to 30, 1997. The IDS spectrograph was used, together with its R300V grating, which gives a reciprocal dispersion of 3.31 Å per pixel of the 1kx1k Tek3 detector, and a spectral coverage from 3700 to 6900 Å. The slit width was 1.5 arcsec projected on the sky, providing a spectral resolution of 6.7 Å. Total exposure times on each nebula varied from a few seconds for the brightest target (NGC 7027) up to one hour for the faintest ones; exposures were split in order to have both the bright and faint nebular lines well exposed without saturation. Several spectrophotometric standards were observed during each night for relative flux calibration. Data reduction was performed using the package onedspec in IRAF.

An additional two PNe (PC 20 and Sa 3-151) were observed with the Dual Beam Spectrograph (DBS, Rodgers et a.1988) on May 10 and 14 2008 by B. Miszalski at the Australian National University 2.3-m telescope. The exposure times were 300 s and 150 s, respectively to achieve a SNR of about 20 in the peak intensity of . The 1200B and 1200R gratings were used with a slit width of 2″(positioned at deg) to give wavelength coverage windows of 4030-5050 Å and 6245-7250 Å at a resolution of 1.6 Å (FWHM). Data reduction was performed using IRAF and a number of flux standards were observed each night to derive the spectrophotometric response across the separate blue and red spectrograph arms.

3 IPHAS extinction-reddening curves for the PNe

Extinction-distance relationships for the lines of sight toward the sample of 136 Galactic PNe in the IPHAS area have been computed with the algorithm MEAD, as described by Sale et al. (2009). MEAD exploits a feature of the IPHAS colour-colour plane, whereby it is possible to simultaneously accurately determine the spectral type of a star and its reddening, avoiding the serious degeneracies that exist in other filter systems and with only a slight dependence on a Galactic model.

Typically, several thousand A to K4 stars in a box of sides 10, centred on the PNe, have been employed in the computation of each relationship. Distances to each star are obtained from their apparent magnitude, the MEAD extinction , and the absolute magnitude of the star derived from the MEAD spectral type. The photometric errors are propagated to give an error on the estimated distance to each star. The stars were then binned by distance, with each bin being at least 100 pc deep, containing at least 8 stars and having a total signal to noise ratio in the bin of at least 130. In a small departure from the method described in Sale et al. (2009), the prior probabilities of each luminosity class are determined on a sightline by sightline basis. MEAD returns monochromatic extinctions, these have then been converted to using the R=3.1 reddening law of Fitzpatrick (1999).

4 The inversion of Distance-Extinction curves

The extinction-distance curves obtained are in general linear for a significant range of distances, until they generally flatten out and reach an asymptotic value once the line of sight leaves the dust layer within the Galactic Plane (Fig. 2). Even in the cases where they show a more complex shape, it is generally possible to select a linear region around the point in which we are interested, e.g. the measured for the nebula.

We assume that the measured are distributed around a true value following a Gaussian law whose dispersion is . If so, a least square fit to the selected part of the curve allows us to determine all the needed parameters. Specifically, around a point, for each , the probability-distribution to measure a given A is P(). The next step is to invert the probability distribution, i.e. to obtain the probability to find a distance for a given A: P().

We note that:

(3)

and using the previous assumptions of linearity and Gaussian distribution for P(), the new distribution for P() is itself Gaussian. We call the central value of this distribution and the dispersion. It follows that

(4)

and

(5)
Figure 2: Extinction law within around the line of sight toward the NGC 6894. The measured extinction of the nebula is also indicated by the horizontal line, and the two vertical lines limit the interval in which the law is linear and which we have used to find the corresponding distance to the PN.

In Fig. 2, we show as an example the well defined extinction curve along the line of sight towards NGC 6894. For this nebula, the measured extinction value is , or (data from Ciardullo et al., 1999). The limits for the selected linear range are 800 and 2300 pc (sensitivity range of the method). The estimated distance is pc and the formal standard deviation is pc. We stress that the formal is only part of the total uncertainty, whose major contribution is due to the error in measurement; other uncertainties we will consider in the following section.

Figure 3: Distribution of the values of the slope mag/pc obtained from a linear fit of the interstellar extinction curves. We analyzed more than 100 lines of sight corresponding to the know PNe in the IPHAS survey. Almost 80% of the values are distributed between 1.3 and 8

In order to simplify the discussion in the next section about the uncertainty introduced by the inaccuracies in , we will fix representative values the parameters and . This is done by analysing almost two hundred lines of sight. We found a representative mean value for the parameter of 6.53 mag/pc (Fig.3), and a typical error in the fitting process of around . A test distance of 2500 pc is also assumed. This is in the middle of the range over which the extinction-distance method is sensitive for the low Galactic latitude coverage that characterizes the IPHAS survey. This range was calculated by taking the mean of the lower and upper limits of the linear interval corresponding to each curve. For the adopted distance, the typical error for the fitting process is around 250 pc. If so, we can express the relative uncertainty for the distance as:

(6)
(7)

where is the total uncertainty and is the contribution of . From the previous formulae for =0.16 (=0.077) we obtain 10% and 14%. Finally we note that the previous formulae are exact in the case of purely statistical errors, otherwise they represent a lower limit to the true uncertainty.

5 On the errors

In section 2 we saw how large the difference can be between the values determined for the same nebula by different authors. The origin of these differences could be systematic errors due to the instruments or in the reduction processes. Unfortunately it is impossible to quantify a priori this effect. However, in the determination of the interstellar reddening and hence of the extinction-distance there are other sources of uncertainty that we discuss below.

5.1 Sources of error relevant for all objects

In order to determine the visual extinction to a PN, it is common to use the ratio. However, often the literature data quote only the extinction coefficient . In some cases, the adopted extinction law is also quoted, in such a way that we can convert the c’s to a common scale, that we adopt to be the law given in Fitzpatrick (2004), for R=3.1, for a theoretical ratio =2.86.

The first uncertainty that we consider is that associated with the theoretical ratio as a function of the electron temperature, for case B. Using Osterbrock (2005) for this ratio we find that if the electron temperature varies between 8000 and 20000 K, the largest error that we would make by adopting a ratio of 2.86 is 0.04 dex in , or 0.09 mag in visual extinction. This implies an uncertainty of up to 11% in the distance determination.

The second error source that we consider is the adopted extinction law. Several authors use extinction laws different from that adopted in this paper. For example, Stasińska et al. (1992) used the Seaton (1979) law assuming R=3.2, Cahn et al. (1992) used the Whitford (1958) law (R=3.2), while the Howarth (1983, R=3.1) law was used by other authors. The maximum difference between our adopted law occurs when the Cardelli et al. (1989) law is used. We conservatively adopt this difference when the extinction law is not quoted in a paper from which the for a specific PN is adopted. This results in a difference of 15% in , or = 0.3 (for A=2), and a contribution to the error on the distance of about 20%.

Another common hypothesis is that the characteristics of the interstellar dust are constant for all lines of sight, and correspond to , the mean value of the Milk Way. Fitzpatrick (1999) and Fitzpatrick & Massa (2007) state that for optical wavelengths a single value of R is a good approximation, but can vary for different lines of sight. Several authors report through the galactic bulge (Byun 1996, Udalski 2003, Ruffle et al. 2005) and towards the halo (Larson and Whittet, 2005). Stasinska et al. (1992), based on the study of a sample of PNe, suggest that for the Galactic Plane a more appropriate value is . The parameter could be instead significantly larger if the line of sight crosses a dense dust cloud (due to the generally larger size of the grains). However, it would be unlikely to be able to detect the blue part of the spectrum of PNe (and thus their flux) along such highly extinguished directions. In general, the value for a particular nebula can in principle be determined if the number of Balmer lines measured with precision is large enough.

Therefore, we cannot exclude variations of for our IPHAS sample. Having adopted as a common value, we estimate the error that we make if the true extinction ratio were 2.1 or 5.5 (maximum and minimum values from Fitzpatrick, 1999 and Fitzpatrick & Massa 2007). For the case , the Fitzpatrick law gives while we use . This results in an overestimate of by some 21% , or = 0.4 (for A=2). However, in this case the corresponding distance extinction curve is also overestimated by about 15% (section 3). This means that when determining distances these errors partially compensate and the corresponding overestimate of is of about 6% , or about 12% for a distance of 2500 pc. For , we underestimate by some 33% , but the corresponding extinction curve is also underestimated by 25% , so that a final compensated error is 8% in , or 14% for the distance.

5.2 Error coming from the object’s properties

An additional uncertainty resides in the hypothesis that the interstellar medium is responsible for all the reddening measured for a PN from the Balmer decrement, in other words, that the PNe are free of internal or circumnebular absorbing dust (for a review see Barlow, 1983). This hypothesis is in general supported by the study of Köppen (1977) which, assuming that in a planetary nebula the dust and gas components are well mixed, finds that the nebular dust optical depth is very small (), for a dust/gas ratio around . However this could be too simple a picture, and in this original work there were also some PNe (6 out of a total of 21) with evidence for associated extinction. The effect is probably negligible for standard, elliptical PNe (Barlow, 1983), but could be more significant for particular nebular morphologies, such as that of bipolar PNe from massive progenitors (e.g. Corradi & Schwarz 1995) which can have massive neutral equatorial envelopes. There is some evidence in the literature that this might be the case.

The most studied example of the latter type is NGC 7027. Osterbrock (1974) estimated a maximum internal absoption of 0.6 mag. Woodward et al. (1992) suggested that the obscuring dust lies in a shell or disc external to the ionized gas, and recently Bieging et al. (2008) found variations of from 0.8 to 2.4. A variation of A has been measured also for NGC 650-1 between the SW and NE limits of the central emission bar by Ramos–Larios et al. (2008). Other type-I bipolar nebulae, such as Sh 2-71, K 3-94, K 4-55 and M 1-75, show some evidence for (generaly modest) extinction variations through their structures (Bohigas 1994, 2001). Extreme variations are reached for NGC 6302 and NGC 6537 (Matsuura et al. 2005a, 2005b).

There are also a few indications of associated extinction for non-bipolar morphologies111Note the possible morphological misclassification of a fraction of PNe due to orientation effects: see Manchado (2004). In this context an interesting object is NGC 6741, for which Sabbadin et al. (2005) find a circumnebular neutral halo generated during a recombination phase following the fading of the central star. This halo was estimated to be responsible for 10-20% of the measured total . The Helix nebula (NGC 7293) seems to be a very complex object where dust, ionized and molecular gas cohabit (Speck et al. 2002). Dense knots in a PN could also evolve in absorbing filaments, as reported by O’Dell et al. (2003) for the bipolar PN IC 4406.

For NGC 6781, Mavromatakis et al. (2001) provide a 2D Balmer decrement map, finding variations of ranging from 5.6 to 7. Though smaller, similar structures are also visible in a similar map obtained for Menzel 1 by Monteiro et al. (2005).

Summarizing, in some cases care has to be taken when determining the interstellar extinction, especially for particular geometries (bipolar). However, due to the small number of studies it is difficult to estimate the percentage of objects belonging to these categories whose interstellar extinction could be wrongly estimated. In any case the number of bipolar PNe is 15% of the total number of planetary nebulae (Corradi & Schwarz, 1995), and from this sub-sample significant unrecognized errors are probable only for distant, spatially unresolved objects. We can conclude that only a small fraction of our total sample will be affected by this kind of uncertainty in the estimate.

Concluding, at present the major contribution to the uncertainty on the distances is due to the ’s error measurements. For two nebulae, NGC 6842 and NGC 7048, our own measurements allows us to estimate these errors. Combining it to the statistical fitting error leads to a final uncertainty on the distance of about 35%.

6 Distance determinations for the NGC sample

PNG Name distance [pc]
Stras. Cahn Stas. this paper Others dC dVst dMa this paper
033.8-02.6 NGC 6741 1.96 1.96 1.93 1.73 2047 2260 1700 2680 110
041.8-02.9 NGC 6781 2.09 1.96 2.16 0.11 1.78 699 840 900 1000
045.7-04.5 NGC 6804 1.85 1.85 0.11 1.58 1709 1660 1600 800
046.4-04.1 NGC 6803 0.96 1.62 1.60 1.14 2987 3190 2500 950 130
060.8-03.6 NGC 6853 0.36 0.55 262 400 400 800
065.9+00.5 NGC 6842 0.96 2.04 2.08 0.15 1366 1700 2700 950
069.4-02.6 NGC 6894 1.80 1.85 0.11 1.44 1653 2000 1500 1000 100
074.5+02.1 NGC 6881 3.27 3.62 4.12 2473 3950 1700 n.d.
084.9-03.4 NGC 7027 2.53 2.97 0.46 273 630 700 n.d.
088.7-01.6 NGC 7048 0.65 1.53 1.20 0.13 1.27 1598 2220 1200 1000–2000
089.0+00.3 NGC 7026 1.40 1.34 2.10 0.11 1.69 1902 1940 900 1500
107.8+02.3 NGC 7354 2.94 3.62 3.99 0.08 3.23 1271 1230 800 1000 150
Table 1: Distances inferred for the NGC sample. Columns labelled as “Stras”, “Cahn” and “Stas” are referred to the Strasbourg catalog, Cahn et al. (1992) and Stasińska et al. (1992), as well as “dC” and “dVst” are referred to Cahn et al. (1992) and van de Steene & Zijlstra (1994), respectively, while “dMa” is referred to Maciel (1984). “Other” references are given in the text. When significantly different extinction estimates are available, the values reported for the distances are obtained using the absorption values in bold face. Errors are the statistical ones coming from the linear fit on the distance extinction curve, except for NGC6842 and NGC 7048, where we have also included the errors on the extinction measurement.
Figure 4: In the figure we present the extinction distance curves for the first 6 nebulae reported in Table 1. The abscissae are in pc, the ordinates, in visual magnitudes and the horizontal lines represent the visual extinction corresponding to each nebula, as discussed in the text. The short dash-long dash lines represent the 1- error associated to our A measurement presented in Table 1.
Figure 5: Extinction-Distance curves for the second group of six nebulae presented in Table 1. For all graphics the abscissae are in pc, the ordinates, in visual magnitudes. The horizontal lines represent the visual extinctions corresponding to the nebulae considered in the text. The short dash-long dash lines represent the 1- error associated to our A measurement presented in Table 1.

In order to minimize the uncertainties we will infer the distances for a sample of very well known PNe in the IPHAS area, shown in Table 1.

The first PN is NGC 6741 (PNG 033.8-02.6, Fig. 4) which, as discussed in the previous section, was found to have non-negligible internal extinction by Sabbadin et al. (2005). We use the interstellar value given in the same paper to determine its distance. The reported error on the distance comes from the statistical . This also applies to the other PNe in this section, except for NGC 6842 and NGC 7048 as discussed later.

For NGC 6781 (PNG 041.8-02.9, Fig. 4), adopting all the reported values of , from the minimum value inferred by Mavromatakis et al. (2001) to the maximum value obtained in this paper, we get a distance smaller than 1000 pc. We can not be more precise since the curve is not determined within this range. However, the result is in line with that of Schwarz and Monteiro (2006), who found a distance of 750 pc using 3D photoionization modeling.

For NGC 6804 (PNG 045.7-04.5, Fig. 4), we use the determination of obtained by Ciardullo et al. (1999), using Hubble images. The graph has been transfered to the Fitzpatrick law (), which is our adopted law for the interstellar extinction. In this case, the inferred distance is pc, while from our measurement of and the corresponding error we estimate pc. This is a factor of 2 smaller than the statistical distances reported by Cahn et al. (1992) and van de Steene & Zijlstra (1994). The error on the statistical scales is typically 30% (1), so that these values do not exlude the much shorter (and with a smaller uncertainty) distance found here. In favour of the smaller distance, we also note that the risk of the extinction method is a tendency to over-estimate distances. Moreover, this is a bipolar nebula for which it may will be possible that the measured extinction is significantly bigger than the interstellar component due to absorbing structure associated with the PN.

In our method, the only circumstance in which we can underestimate the distance is if the parameter . This can occur when the line of sight intersects a dense molecular cloud (a rare event). The map of dense molecular clouds in the Galaxy given by Hartmann & Thaddeus (2001) does not show any clouds at these coordinates. So we recommend the use of our estimated range pc, which is also near the estimate of 870 pc by Frew (private communication, Frew et al. 2010 in preparation), based on a relationship between size and surface brightness for PNe.

For NGC 6803 (PNG 046.4-04.1, Fig. 4), using the extinction value determined by Peimbert & Torres-Peimbert (1987) (=0.53, ), we infer a distance of 950 pc, again less than half the previously reported estimates. Even using the much higher values of presented by Cahn et al. (1992) we obtain a smaller distances ( pc) than those previously reported.

For the nearby nebula NGC 6853 (PNG 060.8-03.6, Fig. 4), there exists a parallax measurement (Benedict et al., 2003) yielding a distance of pc and a visual extinction coefficient tabulated in column labelled “Others” of Tab. 1. In this case the nebula is outside of the sensitivity range of our method and we can give only an upper limit to the distance.

For NGC 6842 (PNG 065.9+00.5, Fig. 4), there is little information in the literature. From our own measurement obtained at the 2.5m Isaac Newton Telescope on La Palma, we determine an extinction measurement that is clearly higher than that inferred by adopting line fluxes from the Strasbourg catalog. Observations imply a distance of 2700 pc. The error on the distance has been calculated taking into account the fitting = 749 pc, the error on the extinction measurement 0.15, and the slope of the curve, pc/mag.

NGC 6881 (PNG 074.5+02.1, Fig. 5) is a quadrupolar young nebula (Guerrero & Manchado, 1998) suggested by Sabbadin et al. (2005) to have been produced by a massive progenitor and presently in a similar recombination phase to NGC 6741. If so, it is possible that overestimates the interstellar extinction and thus the distance using the extinction method, which would then be greater than pc using the very high absorption reported in column “Others” (Kaler & Kwitter, 1987), as well as the value obtained from the Strasbourg catalog. For this reason we do not quote a distance in Tab. 1.

NGC 6894 (PNG 069.4-02.6, Fig. 5) and NGC 7048 (PNG 088.7-01.6, Fig. 5) are morphologically simpler. The of NGC 6894 is taken from Ciardullo et al. (1999). The extinction coefficient determinations for NGC 7048 are more contradictory. By adopting the lowest one, we found that this is a nearby nebula (1000 pc). However, any distance closer than 1000 pc makes the central star absolute magnitude so faint that using a standard evolutionary track gives an age of yrs for the PN, so it is likely more distant. Assuming instead the obtained by our measurement, which agrees with the value in Sabbadin et al. (1987) (), we found a distance between 1000 and 2000 pc.

NGC 7027 (PNG 084.9-03.4, Fig. 5) is the clearest case presenting evidence of strong internal absorption, as discussed in section 5. Given the difficulty in estimating its exact amount as a function of the position at which is measured, no extinction distance is given in Tab. 1 for this nebula.

NGC 7026 (PNG 089.0+00.3, Fig. 5) is a bipolar nebula with a low extinction coefficient. Using any of the extinction values in Tab. 1, we found a distance pc.

Finally, Benetti et al. (2003) propose NGC 7354 (PNG 107.8+02.3, Fig. 5) as a probable recombining nebula in its first phase, as derived from a detailed study of NGC 6818. Using the Feibelman (2000) extinction data and extrapolating the extinction-distance curve to low distances, we found a possible distance of about 1000 pc.

7 Determination of distances for whole sample

In this section distances are presented for 64 other planetary nebulae in the area of sky covered by IPHAS. For a further 29 objects only a lower limit to the distance can be obtained. For 17 PNe we found an upper limit, as given in Table 3. We adopt a distance upper limit when the measured extinction to the nebula is smaller than the first point on the corresponding distance-extinction curve. A distance lower limit is defined when the visual extinction to the nebula lies on the plateau of the interstellar extinction curve. For multiple determinations of the source of the adopted value is reported in bold-face. For upper and lower limits, if no indication is given it means that the same conclusion holds for all tabulated ’s. The errors on the distances given in Table 3 are the fitting errors .

For the 14 PNe listed in Tab. 2, we could not estimate the distances because the measured extinction was too far above the interstellar extinction plateau. According to the discussion of section 2.1 and the result presented in Fig. 1, we assume that we know with the uncertainty =0.41, which is 0.88 mag in , for the data extracted from Acker et al. (1992).

So we consider all PNe with a visual extinction more than 0.88 magnitude higher than the plateau value to live “above” the plateau. Considering these nebulae above the plateau is a zero order hypothesis, because the measured extinctions are not a homogeneous set of measurements and using a common sigma is not rigorous. However, the PNe listed in Table 2 form a sample for future detailed studies, to understand whether the discrepancy with the reddening-distance curve is due to measurement errors, or if the nebulae have peculiar physical conditions that affect the determination of (e.g. very high densities which introduce optical thickness effects), whether they are misclassified objects (symbiotic stars, nebulae around young stars,etc.), or whether there is indeed a non negligible amount of reddening associated with the PNe themselves and not of interstellar origin.

The inferred distances are compared in Fig. 6 with those of Cahn et al. (1992), based on a modified Shklovsky-Daub method, of van de Steene & Zijlstra (1994), based on a correlation between radio continuum brightness temperature and radius, and of Maciel (1984), based on a mass–radius relationship established from selected electron densities and distances. It is interesting to note that the first two methods seem to provide larger distances than our extinction method, while the latter seems to provide shorter distances. The spread in these graphs is 4.0 kpc (Cahn et al. 1992, 39 objects), 6.6 kpc (van de Steene & Zijlstra 1994, 23 objects), and 2.5 kpc (Maciel, 1984). It therefore seems that the distances found by Maciel better agree with ours than those from the other authors. However, the small number of objects considered (16 PNe, nine of which are at an estimated distance lower than 2 kpc), prevents any conclusion to be drawn with the present sample.

Comparison with other distance scales would be very valuable to better understand the problem. A detailed comparison with distance determined via the relation between size and surface brightness (Frew et al., 2006; Frew, 2008) will be presented in a future paper, when the global photometric calibration of the IPHAS survey becomes available.

Figure 6: Comparison between the reddening distance and the statistical determination.The straight line is the 1:1 locus, the statistical distance for the nebulae over the line is bigger than that inferred by the extinction. Those PNe for which the difference between the two inferred distances is larger than 4 kpc are labeled in the figure.
PNG Name Plateau Reference
032.0-03.0 K3-18 5.56 4.5 1
032.9-02.8 K3-19 4.49 3 1
048.1+01.1 K3-29 6.31 3.5 2
051.0+02.8 WhMe1 7.60 5 1
054.4-02.5 M1-72 4.80 2.5 1, 3
055.2+02.8 He2-432 6.32 4 1
059.9+02.0 K3-39 5.78 3.8 1
064.9-02.1 K3-53 6.25 3.8 1
069.2+02.8 K3-49 4.76 3.4 1
072.1+00.1 K3-57 4.56 3 1
094.5-00.8 K3-83 4.72 3 1
104.1+01.0 Bl2-1 5.76 4 1
110.1+01.9 PM1-339 7.35 6 4
112.5+03.7 K3-88 5 4 1
Table 2: List of PNe with a reported visual extinction more than 1 mag larger than the plateau of the corresponding distance-extinction curve. The principal source for the / ratio is the ESO/Strasbourg catalog marked as 1 in the column “Reference”, while the other sources are: 2) Stasińska et al. 1992, 3) Cahn et al. 1992, 4) Manchado et al. 1989. Note: K3-18 is a possible symbiotic star.

8 Conclusions

In this paper we have discussed the extinction-distance method for determining distances to PNe, motivated by the opportunity provided by the IPHAS survey with its wide application in the Galaxy.

The technique is presented and the derivation of the distance and its error is put on a formal mathematical basis. We find a mean value of the slope mag/pc for the galactic extinction distance curve, valid for the analyzed region of the sky.

The numerical and physical reasons that can affect the measurement of the interstellar extinction toward a PN are then discussed. All these, which add to the unavoidable measurement errors the effects of an incomplete knowledge of the physics of interstellar dust through the Galaxy and inside PNe, have to be taken into account every time the method is applied.

In spite of these uncertainties which will need to be addressed in future, we have computed extinction distances for a sample of 70 PNe, which is the largest sample of Galactic PNe to which the technique has been applied. In the best conditions, when the extinction measurements of the PNe are of good quality and the lines of sight are well behaved in terms of their associated distance-extinction maps, the errors in the PN distances can be as good as 20%, which is a very promising result for a wider application of the method. Unfortunately this is not the common situation for the current data set. As previously mentioned, the uncertainty on the visual extinction for the whole set of nebulae is , which implies much larger errors in the distances. For a subsample of the PNe considered, extinction is more precisely measured, and for the two cases for which we could determine the associated error, an uncertainty of 35% on the distance is estimated. This can be taken as a figure to be associated with PNe for which good spectroscopic data exist. A main message of this study is that a careful determination of the nebular extinction is mandatory to limit the errors in a way that reliable distances can be obtained for individual objects. In this respect, our plan is to improve the extinction measurements for a significant sample of Galactic PNe.

We have then compared these new extinction distances with other statistical methods. Some of the methods considered (Cahn et al. 1992, and van de Steene & Zijlstra 1994) provide distances that are generally larger than our determinations, and in general the dispersion is quite high. This emphasizes the large 1- dispersion in the statistical distances, and the dangers in relying on the determination of a statistical distance to a PN without considering the particular characteristic of a nebula.

Concluding, the analysis in this paper demonstrates the increased potential of the extinction method gained with the availability of surveys like IPHAS, which provide precise photometric data in large areas of sky. This opens the possibility of making a significant step forward in the calibration of the PN distance scale in the Galaxy, and in our understanding of important issues such as the properties of the dust distribution within the Galactic disc, and the possible importance of dust associated to PNe.

Acknowledgements.
CG, RLMC, AM, and MSG acknowledge funding from the Spanish AYA2007-66804 grant.

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PNG
Name distance [pc] key
Stras. Cahn Stas. this p. Other Cahn Vst. Ma. THIS
(1) (2) (3) (4) (5) PAPER
031.7+01.7 PC20 2.51 3.04 1100
032.5-03.2 K3-20 1.88 1100
032.7-02.0 M1-66 1.39 1.44 3439 5000 3000
033.2-01.9 Sa3-151 1.52 3500 300
035.9-01.1 Sh2-71 1.65 1.38 1.18 997 1000
038.7-03.3 M1-69 1.32 3300 500
040.4-03.1 K3-30 1.67 1.66 6291 6550 3000
041.2-00.6 HaTr14 0.56 1000
041.8+04.4 K3-15 1.28 6000
043.0-03.0 M4-14 1.42 6692 7570 1600 2600 300
043.1+03.8 M1-65 1.24 1.23 6536 6280 3200 900
043.3+02.2 PM1-276 1.74 1350 100 1
045.4-02.7 Vy2-2 1.94 1.71 2159 2300 170
046.3-03.1 PB9 1.81 4661 3990 3800 450
047.1+04.1 K3-21 1.15 10500 2700 366
047.1-04.2 A62 2.07 0.18 494 1100 900 2
048.0-02.3 PB10 2.01 4685 3830 2000
048.5+04.2 K4-16 1.51 1.50 15127 5000
048.7+01.9 He2-429 2.12 1.87 3383 1500 600 3
049.4+02.4 He2-428 1.23 1.14 1000 100 7
050.1+03.3 M1-67 1.07 682 1000 800 100
051.0+03.0 He2-430 2.22 2.21 3972 3000
051.0-04.5 PC22 0.58 4000 500
051.9-03.8 M1-73 0.88 0.95 4669 4400 2900 500 4
052.2-04.0 M1-74 0.94 1.01 0.98 4118 8640 5300 1000
052.5-02.9 Me1-1 0.05 0.72 0.46 0.58 4618 1000 5
052.9+02.7 K3-31 2.32 3941 6880 800 700
052.9-02.7 K3-41 1.13 20464 28690 3200 500
053.3+03.0 A59 1.77 1.61 1412 1800 1000
053.8-03.0 A63 1.12 1.09 2700 8000
055.1-01.8 K3-43 1.66 1500 2800
055.3+02.7 He1-1 2.06 2500 1000
055.5-00.5 M1-71 1.82 1.80 2.06 2900 400 6
055.6+02.1 He1-2 1.86 3000
056.0+02.0 K3-35 2.01 3975 6580 1000
057.9-01.5 He2-447 2.18 2874 2000
058.9+01.3 K3-40 1.72 7068 6490 3000
059.0+04.6 K3-34 0.35 5700 1000
059.0-01.7 He1-3 1.67 1000 200
059.4+02.3 K3-37 1.77 7148 7910 2400 400
060.5-00.3 K3-45 0.84 1900 1000
060.5+01.8 He2-440 2.94 3962 4000
061.3+03.6 He2-437 1.45 0.88 1100 300 7
062.4-00.2 M2-48 2.02 1.35 6970 1600 900 200 8
067.9-00.2 K3-52 1.49 2459 7200 2300 200
068.6+01.1 He1-4 1.77 1.44 5000
068.7+01.9 K4-41 1.50 7929 7820 4000
068.7+03.0 PC23 1.53 6680 7740 4000
068.8-00.0 M1-75 2.11 3190 3100 4000
069.2+03.8 K3-46 0.69 3200 1600 100
069.6-03.9 K3-58 1.34 5635 5700 3000
071.6-02.3 M3-35 2.07 2.41 1760 4840 2150 150
075.6+04.3 Anon.20h02 0.01 0.85 7000 1300
076.3+01.1 A69 4.35 4173 300 4000
076.4+01.8 KjPn3 0.93 2000 100
077.5+03.7 KjPn1 1.54 5500
077.7+03.1 KjPn2 1.52 3700 400
078.9+00.7 Sd1 0.11 0.11 1200 9
084.2-04.2 K3-80 1.18 900 100
084.9+04.4 A71 1.06 722 900 2800 400
088.7+04.6 K3-78 0.35 7831 7290 1200
089.3-02.2 M1-77 1.23 5496 4880 700 2000 250
089.8-00.6 Sh1-89 0.99 0.82 1941 1350 200
091.6-04.8 K3-84 0.51 5200 900
093.3-00.9 K3-82 0.96 2675 3250 900 130
093.3-02.4 M1-79 0.13 0.78 0.77 2627 3610 1000 4700 800
093.5+01.4 M1-78 3.02 2.65 700 3100 2200
095.1-02.0 M2-49 1.17 2870 6140 2300
095.2+00.7 K3-62 2.54 2270 4050 2500
096.3+02.3 K3-61 1.32 7334 6510 1100 1000 200
097.6-02.4 M2-50 0.95 0.63 0.61 10028 9280 1500 2700 500
098.1+02.4 K3-63 1.18 0.99 5780 1700 1400 130
098.2+04.9 K3-60 2.27 1.90 2.59 3887 6210 2000 200
102.9-02.3 A79 1.33 0.81 0.36 1784 1300 1000 160 7
103.7+00.4 M2-52 1.72 1.44 4411 1500 4000 360
104.4-01.6 M2-53 0.52 1.26 1.06 0.73 3737 4940 900 1900 400
107.4-00.6 K4-57 1.32 4000 1000
107.4-02.6 K3-87 1.14 9203 9680 4400 800
107.7-02.2 M1-80 0.52 5496 4880 1600 1500 200
111.8-02.8 Hb12 1.35 0.60 1000 11
112.5-00.1 KjPn8 0.67 0.42 1400 100 17
114.0-04.6 A82 0.75 1868 2000 3150 500
119.3+00.3 BV5-1 1.05 1.31 1.01 0.82 3000 400 12
122.1-04.9 A2 0.76 0.56 0.34 0.80 3929 3000 1500 150
126.3+02.9 K3-90 0.83 0.97 5759 5600 1000
128.0-04.1 Simeiz22 0.34 1000 13
130.2+01.3 IC1747 1.48 0.74 1.03 2937 2420 1300 200
130.4+03.1 K3-92 1.96 1.30 1.23 6806 3100 900
131.5+02.6 A3 1.23 1.22 1.33 1.23 2560 5100 500 14
132.4+04.7 K3-93 1.35 1.94 1000
136.1+04.9 A6 1.39 1.39 958 2240 1000
138.8+02.8 IC289 1.03 1.09 1.16 1434 1480 8000
142.1+03.4 K3-94 0.91 0.52 6507 7740 1000 15
147.4-02.3 M1-4 2.47 1.84 1.62 2988 3390 3300 350
147.8+04.1 M2-2 1.34 1.50 1.50 4356 3880 2000
151.4+00.5 K3-64 0.73 1800 200
153.7-01.4 K3-65 1.83 10512 3700 300
160.5-00.5 We1-2 1.44 2500 180 16
163.1-00.8 We1-3 0.70 2700 300 16
167.0-00.9 A8 1.77 0.68 1633 5500 1000 2
170.7+04.6 K3-69 1.34 7946 17850 6000
178.3-02.5 K3-68 0.80 5945 2200 240 16
181.5+00.9 Pu1 1.03 7000 800
184.0-02.1 M1-5 1.39 1.38 2922 4940 4000
184.6+00.6 K3-70 1.55 12117 6000
184.8+04.4 K3-71 1.14 2500 1000
194.2+02.5 J900 0.56 0.64 0.63 2756 3270 4300 650
197.8-03.3 A14 1.18 1.52 0.85 3317 5400 800 10
201.9-04.6 We1-4 0.85 3000 1100 15
204.8-03.5 K3-72 1.06 5119 4000 15
211.2-03.5 M1-6 1.84 1.17 1.83 2648 4290 2000 160
Table 3: Continued

key: 1) van de Steene et al. 1996; 2) Phillips et al. 2005; 3) Girard et al. 2007; 4) Wesson et al. 2005; 5) Shen et al. 2004; 6) Wright et al. 2005; 7) Rodriguéz et al. 2001; 8) López-Martín et al. 2002; 9) Kazarian et al. 1998; 10) Bohigas 2003; 11) Rudy et al. 1993; 12) Bohigas 2008; 13) Kwitter & Jacoby 1989; 14) Kaler 1983; 15) Bohigas 2001; 16) Kaler et al. 1990; 17) Gonçalves et al. 2009.

Note: K3-87 is a possible symbiotic.

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