Discovery of a Fast-expanding Shell in the Inside-out Born-again Planetary Nebula HuBi 1 through High-dispersion Integral Field Spectroscopy

Multi-epoch images of HuBi 1 in the [N ii] λ6584 emission line were obtained at the 2.5 m Nordic Optical Telescope (NOT) at the Observatorio del Roque de los Muchachos (ORM) in La Palma, Spain, using the ALhambra Faint Object Spectrograph and Camera (ALFOSC) and the Observatorio de Sierra Nevada (OSN) [N ii] narrowband filter E16 (λ_c  = 6583 Å, Δλ = 13 Å). Two 600 s exposures were obtained on 2008 September 2 using the EEV 2K × 2K CCD camera with a plate scale of 0184 pix⁻¹ (Guerrero et al. 2018), and three 900 s exposures on 2017 May 27 and 2020 July 27 using the E2V 231-42 2K × 2K CCD with a plate scale of 0211 pix⁻¹. In the latest run, three 900 s exposures were obtained through the OSN Hα filter H01 (λ_c  = 6565 Å, Δλ = 13 Å) and five 30 s exposures in $r^{\prime}$-SDSS. In all cases, a dithering of a few arcseconds was applied between individual exposures to improve the quality of the final image.

The individual exposures were bias-subtracted and flat-fielded using twilight sky frames, and then aligned and combined to remove cosmic rays using standard IRAF routines. The spatial resolution of the images, as derived from stars in the field of view (FoV), was 065 in 2008 and 075 in 2017 and 2020. A color-composite image of HuBi 1 using the images obtained in 2020 is presented in Figure 1.

IFS observations were obtained on 2020 August 6 with the Multi-Espectrógrafo en GTC de Alta Resolución para Astronomía (MEGARA; Gil de Paz et al. 2018) at the Gran Telescopio de Canarias (GTC) of the ORM. The integral field unit (IFU) mode covering an FoV of 125 × 113 with 567 hexagonal spaxels of maximal diameter 062 was used. The High-resolution Volume-Phased Holographic (VPH) grism VPH665-HR used during the observations covers the 6405.6–6797.1 Å wavelength range and provides a spectral dispersion of 0.098 Å pix⁻¹ and a resolution R of 18,700 (i.e., ≃16 km s⁻¹). Three 900 s exposures were obtained to facilitate cosmic-ray removal.

The MEGARA raw data were reduced following the Data Reduction Cookbook (Universidad Complutense de Madrid; Pascual et al. 2019) using the megaradrp v0.10.1 pipeline released on 2019 June 29. This pipeline applies sky and bias subtraction, flat-field correction, wavelength calibration, and spectra tracing and extraction. The sky subtraction is done using 56 ancillary fibers located ≈20 from the center of the IFU. The regularization grid task megararss2cube ⁹ was used to produce a final 52 × 58 × 4300 data cube with 02 square spaxels. The spatial resolution of the data after processing was ≃10 as derived from the FWHM of stars in the FoV. The flux calibration was performed using observations of the spectrophotometric standard HR 7596 obtained immediately after those of HuBi 1.

An inspection of the 2008 and 2020 [N ii] images of HuBi 1 is suggestive of the angular expansion of its innermost structure. The profile cross-correlation (PCC) and quantified magnification (QM) methods of minimization of residuals between images of different epochs described by Santamaría et al. (2019) provide a reliable method to determine quantitatively its angular expansion rate (Guerrero et al. 2020). In preparation of these analyses, the [N ii] images of the three different epochs were carefully aligned, and the pixel scale of the 2017 and 2020 images were matched to that of the 2008 image using a sample of well-detected field stars distributed uniformly around HuBi 1. Point-spread functions (PSFs) of these stars were then compared and the images smoothed using a 2D Gaussian filter to make the PSF of all images similar with a final FWHM of 08.

For the PCC method, spatial profiles along four different directions (PA = 0°, 45°, 90°, and 135°) were extracted from the 2008 and 2020 images using a 2 pixel-wide (≃037) rectangular aperture. The emission from the CSPN in the 2008 image, not present in the 2020 image, was modeled and removed using the averaged PSF of stars in the FoV. The comparison of the 2020 (black line) and 2008 (red line) spatial profiles in Figure 2 (left) confirms the angular expansion, with a noticeable excess of emission in the difference profile (blue line). The spatial profile of the 2008 image was progressively shifted outward to simulate its angular expansion and subtracted from that of 2020. The residuals of the differences of these two profiles at the location of the inner structure decrease until they reach a minimum value and then increase again. The normalized residuals are plotted in Figure 2 (middle) together with a quadratic least-squares fit. The best fit implies an expansion rate 00119 yr⁻¹, which results in the residuals shown as a cyan line in Figure 2 (left).

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For the QM method, we note that the expansion rate derived above implies magnification factors of a few percent that would artificially broaden the inner shell of HuBi 1. To avoid those effects, the 2008 and 2020 images were transformed into polar coordinates and the 2020 image shifted along the radial coordinate to simulate an angular expansion and subtracted from the 2008 image (Figure 2, right). The values of the dispersion in the difference maps in a box with radius 10 ≤ r ≤ 30 that excises the emission from the CSPN are normalized and plotted in the middle panel of Figure 2. As in the previous case, the dispersion decreases until it reaches a minimum value and then increases again, with a quadratic least-squares fit implying an expansion rate 00129 yr⁻¹. This corresponds to a shift ≃0153 between the 2008 and 2020 images, close to the central polar image difference in Figure 2 (right).

The QM method was also applied to the pair of [N ii] images obtained in 2017 and 2020. These are closer in time, but they are not affected by the emission from the CSPN. In this case, the quadratic least-squares fit to the dispersion corresponds to an expansion rate 00098 yr⁻¹ (Figure 2, middle).

The three estimates of the angular expansion rate of the inner shell of HuBi 1 derived above are rather consistent (Figure 2, middle). Hereafter we will adopt an averaged angular expansion rate of 00115 ± 00016 yr⁻¹, implying a velocity on the plane of the sky (55 ± 8) × d km s⁻¹, where d is the distance in kpc. Adopting a distance of 5.3 ± 1.3 kpc derived using the method described by Frew et al. (2016) with improved values for the nebular radius and Hα flux (Guerrero et al. 2018), this expansion velocity is 290 ± 80 km s⁻¹.

The velocity expansion along the line of sight of the [N ii]-bright inner regions of HuBi 1 can be investigated using the MEGARA IFS observations. The [N ii] λ6584 emission line profile of the whole region shown in Figure 3 unveils broad wings indicative of a fast velocity component with a full-width at zero intensity (FWZI) ≈ 500 km s⁻¹. The blue (approaching) wing of this profile is noticeably brighter than the red (receding) one.

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The spatial distribution of this fast component is uniquely revealed by MEGARA's tomographic capabilities provided by the nine velocity channels shown in Figure 4 according to the velocity ranges defined in the [N ii] line profile in Figure 3, with four channels 50 km s⁻¹ in width mapping the blue component and another four the red component, and one 100 km s⁻¹ in width channel mapping the emission at the systemic velocity. The blue component has a compact appearance, with the emission peaking in all velocity channels at an angular distance ≈03 northeast (PA ≃ 50°) of the nebula center. On the other hand, the red component shows an arrowhead shape in the two channels closer to the systemic velocity, with its emission peaking at ≈10 toward the southwest of the center of the nebula. In the next two "reddest" channels, the emission looks more alike that of the blue component, with the emission peak close to the nebular center. The emission is extended, with a spatial extent that decreases along with the difference with the systemic velocity both for the blue and red components.

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As for the emission at systemic velocities (0 km s⁻¹ < v_hel < +100 km s⁻¹), it can be described by a donut-shaped morphology, similar to the [N ii] image in Figure 1, as the emission from this velocity channel dominates the total emission (Figure 3). The emission in this channel shows two peaks ≃19 apart along PA ≈ 50°.

The MEGARA IFS observations can be used to extract position-velocity (PV) maps along any PA of interest. In particular, the preferential direction along PA 50° and the orthogonal direction at PA 140° have been selected to extract PV maps from pseudo-slits in the MEGARA data cube (Figure 5). The broad wings from the fast velocity component are present in these PV maps, which confirm that the blue wing is brighter than the red one. In addition, these PV maps reveal a notable difference between the blue and red wings: the blue component of the outflow (−200 km s⁻¹ ≤ V_hel ≤ 0 km s⁻¹) has a Gaussian profile along the spatial direction, but the red component has diminished emission at offsets ≃0'' in the velocity range 100 km s⁻¹ ≤ V_hel ≤ 200 km s⁻¹. This is consistent with the spatial distribution of the emission shown in panels (1)–(4) and (6)–(9) in Figure 4.

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