Free Access
Issue
A&A
Volume 541, May 2012
Article Number A98
Number of page(s) 10
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361/201118389
Published online 10 May 2012

© ESO, 2012

1. Introduction

The interaction between planetary nebulae (PNe) and the interstellar medium (ISM) is an important process whose effects allow us to gain a better understanding of the two components involved. Indeed, on one hand, it is a valuable tool for probing the ISM structure (mainly at small scale) and also for determining some ISM physical properties such as density, filling factor of coronal gas, and magnetic field effect. On the other hand, the PN-ISM interaction phenomenon is helpful for studying the evolution of old PNe and PNe halos. The process can also be used to predict the direction of motion of planetary nebula central stars (CS). Finally, the interaction is an essential key for understanding the “missing mass phenomenon” (i.e. the mass difference between the white dwarf and its parent star) because it traces the return of stellar material into the ISM, which is important for understanding the chemical evolution of the Galaxy.

Theoretical studies on the PN-ISM interaction process started in the late 60s and can be divided into four categories: (1) prediction and simple analytical calculations (Gurzadyan 1969; Smith 1976; Isaacman 1979; Borkowski et al. 1990); (2) numerical simulations (Soker et al. 1991); (3) analytical calculations of instabilities and the role of the interstellar magnetic field (IMF) (Soker & Dgani 1997, hereafter SD97; and Dgani & Soker 1998, hereafter DS98); (4) 2D and 3D hydrodynamic simulations (Villaver et al. 2002, 2003; Villaver 2004; Müller et al. 2004; Wareing et al. 2007).

From an observational point of view the detection of interacting planetary nebulae (IPNe) is made difficult by the low surface brightness generally associated with large, evolved and/or diluting nebulae. However, over the years some progress has been made (notably by improving the observing techniques), and in the following we list the most noticeable researches made up to date. Borkowski et al. (1990, hereafter BSS) brought the number of known IPNe to about 17 objects and listed 11 objects as possible IPNe by examining their morphology. CCD imaging campaigns were performed by Xilouris et al. (1996, hereafter X96) and Tweedy & Kwitter (1996, hereafter TK96), who investigated 9 and 19 large aging PNe, respectively. Guerrero et al. (1998) studied 15 multiple shell planetary nebulae whose morphologies strongly indicate an interaction with the ISM, and some other examples of interaction between PNe’s halos and the ISM have been shown by Corradi et al. (2003). Kerber and his colleagues combined deep CCD observations of IPN candidates with spectroscopic analysis as a powerful tool to study their physical conditions (Kerber et al. 1998, 2000a, 2002). More recently, Sabin et al. (2010) detected 21 new IPN candidates in the framework of the IPHAS survey, which goes deeper in terms of sensitivity than past surveys and therefore allows us to identify very faint nebulae. Finally, the observation of IPNe was extended to the mid-infrared by Ramos-Larios & Phillips (2009) and Ramos-Larios et al. (2011), who have reported observational evidence for interaction between some PNe halos and the ISM. All these observations are giving us a ground for a larger investigation.

Therefore this paper presents a statistical analysis of a large portion of IPNe known today. It will focus on their physical parameters, their preferential galactic orientation and their morphological classifications according to the criteria defined by Wareing et al. (2007). We established an interacting planetary nebulae database, which is presented in Sect. 2; the results are discussed in Sect. 3. Our concluding remarks are given in Sect. 4.

2. The database

2.1. The sample

The present sample has been established by collecting IPNe data from a large number of articles discussing the subject. A first selection was made dividing the sample into two groups: confirmed and possible IPNe based on the evidence for interaction. The criteria used to distinguish between both groups are: (1) objects displaying more than one sign of interaction (e.g. flux enhancement and drop of ionization level in the interaction region, filaments due to shock compression by ISM, displacement of central star and asymmetry of the outer region of the nebula) are considered as confirmed IPNe. This concerns most of the objects listed by BSS, TK96, XI96 in addition to those identified as highly reliable IPNe by Soker (1997); (2) objects presented by Ali et al. (2000) as interacting PNe due to their one-sided shape and/or defined by Soker (1997) as low-confidence IPNe and have no other sign of interaction, are considered as possible IPNe. In addition, we can also refer to some other examples for possible IPNe members such as the 21 new IPHAS PN-ISM candidates mentioned earlier (Sabin et al. 2010) and the many other candidates from the “Macquarie/AAO/Strasbourg” Hα Planetary Nebula Catalogue – MASH I (Parker et al. 2006) and MASH II (Miszalski et al. 2008). We introduce here the following examples from the MASH catalogs: PN G221.0-01.4, PN G249.8-02.7, PN G238.5+01.7, PN G224.3-03.4, PN G011.2-02.7, PN G315.4-08.4, and PN G307.2-05.4. These objects show a one-sided shape that could be related to an advanced stage of interaction owing to the presence of a bow shock structure.

A caveat, however, is the wrong classification of PNe as IPNe in the literature, either because the objects are simply not true PNe (but mimics, see Frew et al. 2010) or because their morphology does not derive from an ISM interaction process. PHL 932 (HII region, Frew et al. 2010), S181 (HII region, Cazzolato & Pineault 2003), A33 (no sign of interaction, Soker 1990), and Sp3 (wide binary, Soker 1990) are examples of such contaminants.

To better constrain our results we chose to ignore the possible IPNe and focused only on confirmed ones, and therefore 117 objects were selected. In each case, the common PN name, its Galactic coordinate, inclination angle and its associated error, distance, angular radius, linear radius, expansion velocity, Galactic height, dynamical age, and interaction classification of the sample are given in Table 1. The latter is established using the Wareing et al. (WZO in what follows) classification method (Wareing et al. 2007).

2.2. Distances and physical parameters

The accurate determination of the distance of planetary nebulae is necessary to derive essential parameters such as their size, mass, age, luminosity, and Galactic height. It is well known that the individual distances of PNe have better accuracy and are more reliable than statistical distances. Fortunately, roughly half of our sample (62 objects) has known individual distances. These distances are derived from different individual methods such as trigonometric, expansion, extinction, and gravity methods. A mean error on the distance of about 22% is estimated for this group. The mean distance is adopted for objects with more than one measurement.

However, we have to rely on statistical distance estimators for the remaining IPNe. Among the statistical scales is that of Phillips (2002). This scale was derived using the radio surface temperature-radius calibration of 66 nearby nebulae. With this method most of the PNe (radio) flux is then taken into account. Using a statistical scale based on radio fluxes appears to be a safer choice than a scale based on optical flux measurements (for example that using Hα flux, e.g. Frew & Parker 2006) because in our case the rather irregular morphology of the IPNe (particularly the evolved objects) would make the flux and then the distance estimation more difficult to determine accurately. The distances of 39 IPNe in our sample were therefore derived following this method. The mean error on the distance of this group is considered to be 33% (Phillips 2002). Among the objects left, nine PNe have their statistical distances taken from the literature. They are relying on an assumption of a constant physical property such as the ionized mass. This group also has the less accurate distances among our sample (leading to less accurate derived parameters such as the Galactic height). Finally, we were unable to find a distance for seven objects.

thumbnail Fig. 1

Two examples showing the measurement method of the inclination angle. A21 on the left, which displays stripes inside its main body and K2-2 on the right, showing a semi-circular bow-shock. The straight or curved black dashed line represents the interaction area, the white dot-dashed line represents the axis of interaction and the black solid line represents the axis parallel to Galactic plane. Both images are from the Digitized Sky Survey.

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The angular radius (θ) and expansion velocity (Vexp) parameters were also extracted from the literature (e.g. Acker et al. 1992; Weinberger 1989; Pottasch 1996). And for PNe with multiple measurements of expansion velocities, we adopted a mean value. Regarding the objects with unknown Vexp, we adopted a standard value Vexp = 20 km s-1 (Weinberger 1989). The linear radius (R) and dynamical age (Tdyn) were subsequently derived and listed in Table 1. Therefore in the latter we find the number, the name, and Galactic coordinates (l,b) of the PNe with the associated references in Cols. 1 to 5, respectively, the measured interaction angle (IA) and its error (e) are given in Cols. 6 and 7, the distances (D) and their measurement methods as well as the associated references are presented in Cols. 8 − 10, the angular radius (θ) and references are presented in Cols. 11 and 12, and the expansion velocity (Vexp) and references are given in Cols. 13 and 14. The four last entries are the height above the plane (Z), the radius (R), the dynamical time (Tdyn), and WZO stage in Cols. 15−18 respectively.

2.3. IPNe orientation

To derive accurate orientation and morphological classification of the sample under investigation, a cautious visual inspection was made involving imaging resources such as The Planetary Nebula Image Catalogue by Bruce Balick1, the morphological catalog of Northern Galactic Planetary Nebula by Manchado et al. (1996) and the Digitized Sky Survey archives. To deduce the orientation of the interacting (enhanced) region relative to the Galactic plane as simply and accurately as possible, we reproduced each IPN image in a new reference system, i.e., according to their Galactic coordinates using the Sky View Virtual Observatory2, where the X and Y axes of each image represent the parallel to the Galactic latitude and Galactic longitude, respectively (Fig. 1). The orientation of the enhanced region is determined by measuring its inclination angle (IA), which is ranging from 0° to 90° (Ali et al. 2000). This is defined by the angle between the axis of the interacting rim (defined as a straight line between the PN geometric center and the middle of the PN’s enhanced region, which we assume is the direction of propagation of the bow shock into the interstellar material and is represented by a white dot-dashed line in Fig. 1) and the axis parallel to the Galactic plane (represented by a black solid line in Fig. 1). When the two axes are perpendicular to each other (IA = 90°), the interaction area is parallel to the Galactic plane (see for example the case of IPN K2-2 in Fig. 1). Also, when the IA  ~  75°, the enhanced region will have only  ~15° margin before being fully parallel to the Galactic plane, and so on. A source of error in the measurement of the inclination angle is based on the quality of the image. When the interaction area is well-defined, the uncertainty will not exceed  ± 2°.

A similar work was compiled by Ali et al. (2000), and they showed that of an overall sample of 40 one-sided PNe, a third display an orientation roughly parallel to the Galactic plane.

3. Results and discussion

3.1. Galactic distribution of the sample

Generally, the galactic distribution of the sample (Fig. 2) does not show any preferential location for the occurrence of interaction between PNe and the ISM. However, we can point out that the majority of the objects are located close to the Galactic plane with a mean absolute Galactic latitude  ⟨ |b| ⟩   =  12° (Table 2).

thumbnail Fig. 2

Galactic distribution of the IPNe sample according to the WZO stages defined by Wareing et al. (2007): black diamond: WZO1, red square: WZO2, green circle: WZO3 and blue triangle: WZO4.

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Table 2

Characteristics of the four different stages of interaction.

Table 3

Consistency of the three IPNe classifications.

Figure 3 shows the distribution of IPNe according to their Galactic height. The associated error is mainly caused by the error on the distance. An estimated error range σz) of 22 − 42% was derived for the distribution.

Figure 3 and Table 1 also reveal that around 77% of the sample is located within the thin Galactic disk (z  <  400 pc, X96)3. Therefore, they are coexisting with the molecular, cold neutral and warm neutral interstellar media. The patchy nature of these media and the (sometime) large Galactic height range make any strong claim on the exact location of the IPNe difficult. However, we observe that roughly one-third of the sample shares the same area as molecular clouds (z  ≤  75 pc, Spitzer 1978) and cold neutral medium (z  ≤  100 pc, Heiles & Troland 2003).

This dense part of our galaxy makes the PNe-ISM interaction very probable and more noticeable in both media. The density number of the molecular medium is larger than 103 cm-3 and its temperature is  ~20 K, while the cold neutral medium has a density range from 20 to 60 cm-3 and a temperature of  ~100 K (Dopita & Sutherland 2003). Our result here gives higher probability for detecting IPNe toward the molecular and cold media, in contrast to BSS, who expected few interacting PNe because of the low filling factor of those environments.

thumbnail Fig. 3

Number of IPNe according to their Galactic heights above the plane.

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Planetary nebulae moving with low or moderate velocity through high-density media (e.g. HFG1 located at z  ~  40 pc with relative velocity  ~40 km s-1, Ali et al., in prep.) are expected to create isothermal shocks as they interact with the ISM. In this case the cooling time post shock becomes shorter than the flow time and the magnetic pressure overcomes the thermal pressure (SD97). Around 55% of our sample exists at z  ≤  200 pc. Figure 3 shows that  ~23% of the sample is located within the Galactic thick disk and Galactic halo. The chance of interactions of these objects is low. The high-velocity PNe moving through these low-density media (e.g. NGC 7094 located at z  ~  800 pc with relative velocity of  ~200 km s-1, Ali et al., in prep.) are expected to undergo adiabatic shocks when the cooling time post shock becomes longer than the flow time and the thermal pressure overcomes the magnetic pressure (SD97).

We derived a mean absolute Galactic height  ⟨ |z| ⟩   =  270 pc for our sample (Table 2), although we point out the uncertainty on the statistical distances of some objects. About 62% of the sample has a mean absolute latitude  ⟨ |b| ⟩  below 10° with a mean value of 4.5° and a mean absolute Galactic height of 151 pc, and the remaining 38% has  ⟨ |b| ⟩  above 10° with a mean value of 27.2° and a mean absolute Galactic height of 468 pc. Sn1 and JaFu2, which are globular cluster members, were not considered while deriving the mean values of bz, and consequently the other parameters.

3.2. The interacting nebulae classifications

Three different classifications were established for the PNe-ISM interaction process. TK96 introduced a short code describing the IPNe morphologies: B (bipolar PNe); F (uniformly filled); H (halo-interaction); S (thick shell) and C (crescent-shaped). Based on the age and strength of interaction, Rauch et al. (2000) characterized three different stages of interaction: young, middle-aged, and old. Wareing et al. (2007) introduced four evolutionary stages for IPNe through their triple-wind model. Hence during their WZO1 stage the PN is not affected and a faint bow shock may be observable due to the interaction between the AGB wind and the ISM (as discussed by Villaver et al. 2003). The WZO2 stage is characterized by a brightening of the PN shell in the direction of motion. During the WZO3 phase the central star shifts away from the geometric center and ultimately the PN appears completely disrupted with its central star located outside the PN in the so-called WZO4 stage. The classifications of TK96 and Rauch et al. (2000) are generally consistent with that of Wareing et al. (2007) as illustrated in Table 3.

In the present analysis, we chose to work with the scheme by Wareing et al. (2007). The results are presented in the final column of Table 1 and the average parameters derived for each stage are presented in Table 2. Hence, the mean absolute Galactic latitude  ⟨ |b| ⟩  does not seem to vary noticeably from one stage to another, although we can still point out that WZO4 IPNe seem to be located at higher latitudes on average than the rest of the sample.

Table 2 also reveals that the majority of IPNe are observed in association with the WZO2 and WZO3 stages, while the least observed members are associated with WZO4 stage. This result should be handled with care as it could reflect a detection bias rather than a real pattern. Indeed, the relatively low surface brightness associated with WZO4 IPNe combined with the high interstellar extinction close to Galactic plane, where the majority of these objects are located (see below), make them difficult to identify/detect (therefore surveys such as IPHAS and MASH are expected to contribute to solving this problem). In contrast, WZO2 and WZO3 IPNe are more likely to be detected via their bright rim.

In terms of evolutionary status, the results show that there is an obvious trend of increasing dynamical time (Tdyn) with the interaction evolution stages, which was to be expected. The earliest stage of interaction occurs at Tdyn  ~  8500 years, while the most advanced stage occurs at Tdyn  ~  67 600 years (Table 2). During the PN evolution, its size and ionized mass increase whilst its density decreases, especially after the PN passes the transition between the optically thick and the optically thin limit. So, we expect a strong correlation between the size and age of PNe. Our results given in Table 2 indeed confirm this trend (with a correlation factor r = 0.96) between R and Tdyn where the radius increases from WZO1 to WZO4 stage. In calculating the average radius of the WZO4 stage, we ignored the peculiar PG 1034+001 nebula that represents the largest known interacting nebula and has a radius  ~8 pc. The mean radius of the sample is on average five times the standard value of  ⟨ R ⟩  = 0.1 pc (Pottasch 1983). This would indicate that most of the IPNe in the sample are in their middle and advanced stage of evolution.

We also calculated the mean absolute height  ⟨ |z| ⟩  for each stage. The WZO2 stage shows the highest value, followed by the WZO1, WZO3, and finally the WZO4 stage, respectively, closer to the Galactic plane. There is a trend of decreasing  ⟨ |z| ⟩  with the advancement of interaction stages and consequently with PN age, except for WZO2. The correlation between  ⟨ |z| ⟩  and  ⟨ Tdyn ⟩  shows an inverse relation with a coefficient r =  −0.85 (Table 2). The two evolutionary younger stages of interaction tend to exist at higher Galactic height compared with the other two more advanced stages.

3.3. The Galactic orientation of IPNe

Studying the orientation of planetary nebulae is very common (e.g. Shain 1956; Grinin & Zvereva 1968; Akhundova & Seidov 1970; Melnick & Harwit 1975; Phillips 1997; Corradi et al. 1998; Weidmann & Díaz 2008). It provides information about mechanisms capable of explaining the various shapes of PNe such as bipolar and elliptical objects. For IPNe, Ali et al. (2000) investigated the spatial orientation and distribution of one-sided PNe. This time we investigate the orientation of IPNe with respect to the Milky Way based on a larger sample.

We successfully measured the inclination angle (IA, as defined before) for 112 objects in our sample (Table 1). We found that while  ~73% of the sample has IA  >  45°,  ~38% has IA  >  70° (Fig. 4). This result is consistent with that achieved by Ali et al. (2000), regardless of the differences between the two samples. Figure 4 shows the increase of IPNe frequency with the inclination angle.

thumbnail Fig. 4

Distribution of the measured inclination angles of the sample.

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The reason for this orientation is still under debate. Soker & Zucker (1997) suggested that the gas responsible for the long stripes in the outer region surrounding NGC 6894 has been ionized by its central star and could originate from the stripping of the object’s halo by the ISM. They speculate that the stripes were shaped by the interstellar magnetic field (IMF) due to their orientation, which coincides with that of the IMF lines in the Galaxy (Mathewson & Ford 1970). Disregarding the particular problem raised by the stripes’ origin (see below) and following Soker & Zucker (1997) on the correlation IMF/IPNe orientation for the total sample, our results deduced from Fig. 4 indicate that more than one-third of the sample (42 objects) has a direction of interaction roughly parallel to the Galactic plane with an orientation within 20° of it, so the interstellar magnetic field might play an important role in shaping the interacting regions of these PNe.

The only direct observational evidence for the role of IMF in shaping IPNe was given by Ransom et al. (2008). Using the estimate of the rotation measure and the electron density in the shell of S216 nebula, they derived a line-of-sight magnetic field in the interaction region of 5.0  ±  2.0 μG. Very recently, Ransom et al. (2010) showed the influence of the IMF in the interacting DHW5 by studying the Faraday rotation structure in the inner tail of the object. This particular subject will be discussed in a forthcoming article.

Table 4

List and properties of striped PNe in our sample.

Except for the IMF, few other mechanisms can explain the alignment with respect to the Galactic plane. Phillips (1997) quoted some of them to interpret the orientation of a sample of binary PNe parallel to the plane. Therefore the preferential orientation of binary systems, the orientation of the angular momentum vectors of molecular clouds parallel to the plane, or the temporal orientation of the axis of mass-loss ejection have been suggested. Unfortunately, none of these hypotheses, which could be applied to the IPNe, is supported by strong observational evidence. We also looked for kinematical studies that would give an indication on a possible preferential orientation on the dynamical motions of IPNe or even PNe (without “any” marked ISM interaction), i.e., indicating a strong velocity component perpendicular to the Galactic plane, but we found no exhaustive investigation. However, the recently released San Pedro Mártir Kinematic Catalogue of Galactic Planetary Nebulae (López et al. 2012), which contains detailed kinematical information of hundreds of PNe including IPNe, is likely to provide an answer to the role of kinematics in the alignment with the Galactic plane in the near future.

Finally, our results in Table 2 also show a strong correlation between the mean inclination angle  ⟨ IA ⟩  and  ⟨ |z| ⟩ ,  ⟨ R ⟩  and  ⟨ Tdyn ⟩ . The correlation coefficient between  ⟨ IA ⟩  and  ⟨ |z| ⟩  is  − 0.87. This indicates that the interaction tends to be parallel for objects close to the plane, and this tendency becomes less noticeable as we move away from it.

3.4. Striped PNe

Striped planetary nebulae are believed to provide observational evidence of the effect of the IMF in shaping IPNe. Indeed, Dgani & Soker (1998) stated that RT instability can fragment the halo of PNe and thereby allow the ISM to flow into their inner regions. They presented four criteria to define the striped PNe, and illustrated the different morphologies in their Fig. 3. However, the study of striped PNe and their relation with the IMF is hampered by the large uncertainties on the origin of the stripes themselves. Indeed, few kinematical investigations on those particular objects and at those particular locations have been realized. They would indicate a difference or an agreement in radial velocity between the stripes and the main nebula, which would lead to a different or common origin, respectively (e.g. the case of NGC 6751 where the most outer stripes in the northeast turned out to be an ISM pattern (Chu et al. 1991; and Clark et al. 2010). The size, height, inclination angle, and WZO classification of 19 striped PNe are listed in Table 4. In this list only two PNe have kinematics data related to their stripes: NGC 3242 (Meaburn et al. 2000), where the link between the filamentary system and the main PN is still unclear, and the well-studied Sh 2-188 (Rosado & Kwitter 1982; and Wareing et al. 2006), where the stripes are indeed related to the PN. However, and to quote Soker & Zucker (1997) in their analysis of the stripes in NGC 6894 and their relation with the IMF, it would be a “fortuitous coincidence” for all the structures to be located only at those particular location, at “ionization range” of those 19 IPNe and to have unrelated origins (e.g. ISM filaments). This is even more remarkable for IPNe showing curved stripes and a thin morphology. If we assume this, the mean inclination angle of the interaction areas (⟨IA⟩   ≃  70°  ±  8°) indicates that the striped PNe are mainly parallel to the Galactic plane. Based on their adopted distances we localize all of them in the thin Galactic disk, where the mean vertical height from the Galactic plane is about 100 pc. We also note that the small IA of some objects such as Ton 320 and A21, which are located at high latitudes, could reflect the local orientation of IMF lines in those areas where more precise observations are needed. The lack of kinematical information is a strong handicap and the kinematic catalog by López et al. (2012) would here again be a valuable tool to help solving this long-standing problem.

4. Conclusions

We discussed the classification and orientation of a large sample of IPNe. Overall, the results show that the majority of interaction regions are parallel to the Galactic plane and appear to be strongly correlated to the direction of the IMF lines. The effect of the IMF in shaping the interaction area is highlighted, although we do not discard the role and influence of other dynamical motions such as IPNe kinematics. We observed a similar pattern after studying 19 striped PNe, which would need deeper kinematic investigation to ascertain the link between the stripes and the main nebula. The main results of our statistical analysis are summarized as follows: (1) the distribution of IPNe shows non-preferential locations for interaction in the Milky Way, except for a concentration close to the Galactic plane; (2) the majority of IPNe are moving through the thin disk and have a high probability to follow isothermal shocks and be subject to magnetic pressure; (3) the number of IPNe coexisting with the molecular and cold neutral media are larger than previously expected in the literature; (4) the majority of the interaction regions have a tendency to be parallel to the Galactic plane, especially for the objects very low on the plane, which are in a more advanced stage of interaction; (5) the interaction is more likely observable in the mid stages of interaction, WZO2 and WZO3, rather than the early and advanced stages of interaction, WZO1 and WZO4; (6) the IPNe frequency tends to increase with the inclination angle (the majority of the IPNe having large IA) and IPNe frequency tends to decrease with the Galactic height (few IPNe are found at high latitudes).


3

The peculiar radial velocity (|ΔV|, the difference between the observed local standard of rest radial velocity, and the velocity determined from the rotation curve) was calculated for the objects with available radial velocity data. We found that  ⟨ |ΔV| ⟩   ~  36 km s-1 for IPNe with z  ≤  400 pc (45 objects) and  ⟨ |ΔV| ⟩   ~  71 km s-1 for IPNe with z  ≥  400 (12 objects). Quireza et al. (2007) and Maciel & Dutra (1992) show that objects belonging to the Galactic thin disk should have |ΔV|  ≤  60 km s-1. Therefore this result, which is applicable to half of our sample, can be used as another argument to show that the majority of IPNe with z  ≤  400 pc are indeed consistent with the Galactic thin disk.

Acknowledgments

It is a great pleasure for the authors to express their gratitude to the Deanship of Scientific Research at King Abdulaziz University, where this work has been carried out as a part of sponsored researches project (Project No: 3-75/430). L.S. is supported by PAPIIT-UNAM grant IN109509 (Mexico). The authors thank the referee W. Steffen for his valuable comments, which improved the paper. We are indebted to H. A. Ismail for fruitful discussions and help. This research has made use of the SkyView developed by NASA under the auspices of the HEASARC at the NASA/GSFC Astrophysics Science Division.

References

Online material

Table 1

The main parameters of the interacting planetary nebulae sample.

Notes. References for interaction: (1) Soker (1997); (2) Wareing et al. (2007); (3) Rauch & Kerber (2005); (4) Borkowski et al. (1990); (5) Tweedy & Kwitter (1996); (6) Dgani & Soker (1998); (7) Soker (1999); (8) Ramos-Larios et al. (2011); (9) Tweedy & Kwitter (1994a); (10) Rauch et al. (2004); (11) Jacoby (1981); (12) Kwitter et al. (1988); (13) Xilouris et al. (1996); (14) Pierce et al. (2004); (15) Kerber et al. (1997); (16) Kerber et al. (2000a); (17) Kerber (1998); (18) Ali & Pfleiderer (1999); (19) Soker & Hadar (2002); Zucker & Soker (1993); (21) Jacoby & Van de Steene (1995); (22) Frew et al. (2011); (23) Kerber et al. (1998); (24) Ali et al. (2000); (25) Rauch et al. (2000); (26) Bond & Livio (1990); (27) Corradi et al. (2003); (28) Guerrero et al. (1998); (29) Martin et al. (2002); (30) Villaver et al. (2003); (31) Kerber et al. (2000b); (32) Ramos-Larios & Phillips (2009); (33) Wareing (2010); (34) Muthu et al. (2000); (35) Rauch (1999); (36) Corradi & Schwarz (1995); (37) Weinberger & Aryal (2004); (38) Aryal et al. (2009); (39) Guerrero et al. (2003); (40) Borkowski (1993); (41) Clark et al. (2010); (42) Chu et al. (1991); (43) Soker & Zucker (1997); (44) Tweedy & Kwitter (1994b); (45) Tweedy & Napiwotzki (1994); (46) Tweedy et al. (1995).

References for IPNe distance: (1) Pottasch (1996); (2) Harris et al. (2007); (3) Pottasch & Acker (1998); (4) Acker et al. (1998); (5) Zhang (1993); (6) Sabbadin (1986); (7) Pottasch (1983); (8) Cazetta & Maciel (2001); Kaler & Feibelman (1985); (10) Kaler et al. (1985); (11) Napiwotzki (2001); (12) Napiwotzki (1999); (13) Saurer (1995); (14) Benedict et al. (2009); (15) Rauch et al. (1996); (16) Giammanco et al. (2011); (17) Cahn & Kaler (1971); (18) Ali & Basurah (2008); (19) Napiwotzki & Schonberner (1995); (20) Ali (2006); (21) Bensby & Lundstrom (2001); (22) Mellema (2004); (23) Gathier et al. (1986); (24) Jacoby et al. (1997); (25) Kerber et al. (1998); (26) Hartl & Weinberger (1987); (27) Tajitsu & Tamura (1998); (28) Weinberger & Sabbadin (1981); (29) Werner et al. (1991); (30) Dengel et al. (1980); (31) Cudworth & Peterson (1988); (32) Cudworth (1990); (33) Frew et al. (2010); (34) Rauch et al. (2004).

“Ind.” means individual distance, “Ph02” means distances calculated according to Phillips distance scale (2002) and “St.” means statistical distances from literature.

References for IPNe radius: (1) ESO-Strasburg Catalog; (2) Ishida & Weinberger (1987); (3) Kerber et al. (2000b); (4) Jacoby & Van de Steene (1995); (5) Jacoby et al. (1997); (6) Kohoutek (1962); (7) Kerber et al. (1998); (8) Kerber et al. (2000a); (9) Melmer & Weinberger (1990); (10) Vauclair et al. (1998); (11) Pierce et al. (2004); (12) Rauch et al. (2004); (13) Tweedy & Kwitter (1996); (14) Borkowski et al. (1990).

References for IPNe Expansion Velocity: (1) ESO-Strasburg Catalog; (2) Weinberger (1989); (3) Meaburn et al. (1998); (4) Rodríguez et al. (2001); (5) Pottasch (1996); (6) Guerrero et al. (1998); (7) Phillips (1984); (8) Sabbadin (1984); (9) Hippelein & Weinberger (1990).

All Tables

Table 2

Characteristics of the four different stages of interaction.

Table 3

Consistency of the three IPNe classifications.

Table 4

List and properties of striped PNe in our sample.

Table 1

The main parameters of the interacting planetary nebulae sample.

All Figures

thumbnail Fig. 1

Two examples showing the measurement method of the inclination angle. A21 on the left, which displays stripes inside its main body and K2-2 on the right, showing a semi-circular bow-shock. The straight or curved black dashed line represents the interaction area, the white dot-dashed line represents the axis of interaction and the black solid line represents the axis parallel to Galactic plane. Both images are from the Digitized Sky Survey.

Open with DEXTER
In the text
thumbnail Fig. 2

Galactic distribution of the IPNe sample according to the WZO stages defined by Wareing et al. (2007): black diamond: WZO1, red square: WZO2, green circle: WZO3 and blue triangle: WZO4.

Open with DEXTER
In the text
thumbnail Fig. 3

Number of IPNe according to their Galactic heights above the plane.

Open with DEXTER
In the text
thumbnail Fig. 4

Distribution of the measured inclination angles of the sample.

Open with DEXTER
In the text

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