A SPECTROSCOPIC AND PHOTOMETRIC STUDY OF THE PLANETARY NEBULAE Kn 61 AND Pa 5

Long-slit, high-resolution spectroscopic observations of the nebulae Kn 61 and Pa 5 were obtained at the OAN-SPM, México, on 2012 May 25, and are available in the San Pedro Mártir Kinematic Catalogue of Galactic Planetary Nebulae (López et al. 2012).

The data were obtained using the Manchester Echelle Spectrometer (MES-SPM; Meaburn et al. 2003) on the 2.1 m telescope in its f/7.5 configuration The MES-SPM is a long-slit, echelle spectrometer that has no cross-disperser; it isolates single orders using interference filters. For the present observations, we used two filters of 90 Å and 50 Å bandwidths to isolate the 87th and 114th orders containing the Hα+[N ii] 6584 Å, and [O iii] 5007 Å nebular emission lines, respectively. This instrument was equipped with a Marconi CCD detector with 2048 × 2048 square pixels, each 13.6 μm on a side. The detector was set to a binning of 3 × 3 in both the spatial and spectral directions.

We used a 150 μm slit of 19 and 547 projected width and length, respectively. The resultant spectral dispersion was approximately 0.26 Å pixel⁻¹ and 0.19 Å pixel⁻¹ at Hα and [O iii] 5007 Å, respectively (equivalent to ∼11.5 km s⁻¹). For Kn 61, we placed the slit at a position angle P.A. = 0° and through the central star of the nebula (see Figure 1). For Pa 5, we placed the slit along the bright elongated structure whose axis is oriented at P.A. = 50° (see Figure 2). A second exposure along the axis defined by the lobes was obtained, but it turned out of poor signal to noise due to thick clouds. Images of the slit projected on the sky were obtained to establish the exact position of the slit on the target. Wavelength calibration against the spectrum of a Th/Ar lamp was obtained after each science exposure, which yields an accuracy of ±1 km s⁻¹ when converted to radial velocity. Spectra presented in this work are corrected to heliocentric velocity (V_☉). The bi-dimensional Hα and [O iii] 5007 Å emission line spectra or position–velocity (P − V) arrays are shown in Figures 3 and 4 for Kn 61 and Pa 5, respectively.

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OAN-SPM. Medium-resolution spectroscopy was performed with the 2.1 m telescope at the OAN-SPM in 2012 and 2013. We used the Boller & Chivens (B&Ch) spectrograph using a 600 l mm⁻¹ grating with a dispersion of 1.17 Å pixel⁻¹. The spectra span the wavelength range from 4000 Å to 7000 Å. We also used a 1200 l mm⁻¹ grating with a dispersion of 0.60 Å pixel⁻¹ covering the wavelength range from 4560 Å to 5750 Å. A Marconi CCD with 2048 × 2048 pixels and no binning was used. In all observations the slit was oriented east–west and on the central star for each PN. The exposure time was 1200 s per spectrum. An He-Ne-Ar comparison lamp was obtained after every third spectra to ensure good wavelength calibration. For flux calibration, a standard star was observed at the end of the night, taken from the list of Landolt (1992) and Bohlin et al. (2001).

Figures 5 and 6 are mosaics of the stellar and nebular spectra for Kn 61 and Pa 5, respectively, taken from OAN-SPM. The left panels show medium-resolution spectra of the central star, while the right panels show the nebular spectra. Panels (a) and (b) were taken with the 600 l mm⁻¹ grating, while panels (c) and (d) were taken with the 1200 l mm⁻¹ grating. The 1200 l mm⁻¹ grating spectra shown in these figures were averaged over a single night of observation to improve the signal to noise. The spectra for the nebulae were extracted from the full length of the slit that covers the nebulae.

SAO-RAS. Medium-resolution spectroscopic data of Kn 61 were recently obtained at the Special Astrophysical Observatory 6 m telescope on 2013 September 12, using the SCORPIO spectrograph (Afanasiev & Moiseev 2005) installed at the prime focus of the SAO 6 m telescope of the Russian Academy of Sciences in the long-slit unit mode. In this mode, the slit dimensions are 6' long × 1'' wide. We used the CCD detector EEV42–40 and the grism VPHG 1200 g (1200 lines mm⁻¹ with a spectral dispersion of 0.88 Å pixel⁻¹). Three spectra were taken in the spectral range between 3900 Å and 5700 Å. The standard star (BD28d4211) was observed for flux calibration. The calibrated spectra are shown in Figure 7.

Figure 5 (left panels) shows the spectra from OAN-SPM of the central star of Kn 61. The spectrum has been binned ×4 in the spectral axis to improve signal to noise and help highlight photospheric lines. The stellar spectra show lines characteristic of a Group 3 = lg E (low-gravity central star) class of PG 1159 star (e.g., Parthasarathy et al. 1998), revealed by the absorption line at C iv 4647 Å and the double emission lines also from C iv 5801, 5812 Å. These lines are more clearly appreciated in the medium-resolution stellar spectrum of Kn 61 obtained at SAO-RAS (see Figure 7, left panel). The later spectrum for the central star indicates the presence of an emission line at 4713 Å which is likely a nebular contamination from either He i or [Ar iv], as it is sometimes observed in other PG 1159 stars (Parthasarathy et al. 1998). Additionally, the C iv 4647 Å absorption line is detected. The PG 1159 stars are considered to be very hot, hydrogen-deficient post-AGB stars. In a recent survey of such stars detected in the Data Release 10 of the Sloan Digital Sky Survey (Werner et al. 2014), seven stars are of PG1159 type with spectra similar to the central star of Kn 61. A particularly close resemblance is found between the spectra of the central star of Kn 61 and SDSS J075415.12+085232.18, a PG1159 star with 120,000 K, log g = 0.33, and C/He = 0.33, where the C iv 4647 Å has also been identified.

We have monitored the photometric behavior of the central star of Kn 61 for 2–3 hr each night for 16 consecutive nights. No strong amplitude or periodic variability beyond the natural data scatter on the order of 0.028 mag was detected on such short timescales. Therefore, the photometric data were averaged for each night and the resulting light curve covering 16 nights was analyzed for the presence of a periodic signal using the Period04 program. The latter employs a discrete Fourier transformation technique to calculate the power spectra and spectral window corresponding to the analyzed time series. This produced a statistically significant peak at a frequency of 0.176 days⁻¹ in the power spectrum (see Figure 8, left panel), which corresponds to a period of 5.7(±0.4) days. The semi-amplitude of variability is A ≈0.02 mag and is comparable with the root mean square (rms) of individual nightly light curves before averaging. Similar analysis on two field stars of comparable brightness proved that the periodic signal is authentic and far exceeds random noise. The significance level corresponding to 99% is indicated in Figure 8 by a horizontal dashed line. In addition, we frequency folded the power spectrum within the spectral window, which helps to sort out the alias periods produced by uneven time series. The result confirms a strong, single peak at the indicated frequency. The light curve folded with the determined period is presented in the right panel of Figure 8. The best-fit sin curve with the A = 0.0176 amplitude around the average magnitude m_R = 18.674 is also plotted. The phase zero is set arbitrarily to the moment when the object has an average brightness. The error bars in Figure 8 (right panel) reflect the rms of individual points around each night's average and they far exceed statistical errors of aperture photometry.

Long et al. (2013) analyzed the light curve of the central star of Kn 61 and found cyclical variability which might be periodic on timescales from two to six days. Our result does not contradict their non-conclusive findings. The object requires longer monitoring and more uniform observations to determine the origin of the variability.

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It is interesting to note, however, that for more than a couple of decades, binary nuclei in PNe have been suggested as being responsible for shaping non-spherical, axis-symmetric, or point-symmetric nebulae (e.g., Livio & Soker 1988) and as a source of their collimated outflows. Kn 61 does not display collimated outflows and it is close to spherical symmetry. However, if the central star of Kn 61 is a binary, its possible orbital period of ≈6 days is relatively long compared to other binary central stars of planetary nebulae (CSPN). The majority of close binary CSPN have usually periods of only a fraction of a day (De Marco 2009). A longer period means that the secondary is relatively distant, leading to reduced chance of interaction between stars and thus reduced influence on the nebular shape and collimation of an outflow.

It is also of interest to compare Kn 61 with the PNe NGC 7094 and A43 (Rauch et al. 2005). These PNe have a very similar morphological structure to Kn 61, i.e., they are both roughly spherical filamentary bubbles and in both cases their central stars are classified as hybrid-PG 1159 type (Solheim et al. 2007). They also display high expansion velocities, ⩾50 km s⁻¹. These stars are thought to have experienced a late thermal pulse while they were still on the AGB and hydrogen-shell burning was still on, yet they are able to keep small amounts of atmospheric hydrogen. We have drawn the images and spectra of these PNe from the San Pedro Mártir Kinematic Catalogue of Galactic Planetary Nebulae (López et al. 2012) and these are reproduced in Figure 9. Note the remarkable similarity with Kn 61 in both the images and the bi-dimensional line profiles. In these cases, the Hα line is bright, in contrast with Kn 61. The [N ii] lines are also absent in both cases, as in Kn 61 (see López et al. 2012). Pereyra et al. (2013) found from an analysis of 100 evolved PNe that those objects located at the point of maximum temperature in the evolutionary tracks, right before their luminosity starts dropping toward the white dwarf region, exhibit a lack of [N ii] emission. The lack of [N ii] is a consequence of the small fraction of nitrogen in the singly ionized state. This, in turn, follows from the high ionization levels of the nebula due to a very hot central star that photoionizes most of the N into doubly and triply ionized states. The relation between soap bubble shapes, i.e., highly filamentary spherical bubbles, and the PG 1159-type stars is puzzling and is noted here for the first time.

Østensen, et al. (2010), using low-resolution spectroscopy in the spectral range of 3500–5300 Å, found that the central star of Pa 5, J19195+4445, is a very hot star and they identify it as belonging to the PG 1159 on the basis of the presence of C iv and He ii close to 4686 Å. The spectra for the central star of Pa 5, shown in Figure 6 (panels (a) and (c)), reveal the presence of lines from the Pickering series He ii  4200, 4541, 4686, 5412 Å, Hβ, Hγ, and Ca ii 5889 Å, but we do not detect the C iv lines so we are unable to confirm the PG 1159 nature of this star.

In addition, Østensen et al. (2010), using the Kepler data, find a period of 1.12 days for the central star of Pa 5. They report a very small amplitude of only 0.05% in terms of the significance of the variability and consider the origin of the variability as unknown. Although we attempted to measure radial velocity and photometric variations over several nights, the period of this object being close to one day precludes observing different phases from a single ground-based telescope and we could not detect any orbital modulation.