Grantecan spectroscopic observations and confirmations of planetary nebulae candidates in the Northern Galactic Plane Free

ABSTRACT

We present Gran Telescopio Canarias (GTC) spectroscopic confirmations of 55 faint Planetary Nebulae (PNe) candidates discovered largely in the INT Photometric H α Survey (IPHAS) of the Northern Galactic Plane by our pro-am collaboration. We confirm 46 of them as ‘True’ (T), 4 as ‘Likely’ (L), and 5 as ‘Possible’ (P) PNe and including 5 new PNe central star (CSPN) discoveries. This was from observations of 62 new candidates yielding a maximum PN discovery success rate of 89 per cent. The sensitivity and longer wavelength coverage of IPHAS allows PNe to be found in regions of greater extinction and at these lower Galactic latitudes, including PNe in a more advanced evolutionary state and at larger distances compared to previously known Galactic PNe. We use a holistic set of observed characteristics and optical emission-line diagnostics to confirm candidates. Plasma properties have been determined in a self-consistent way using pyneb. This work is facilitated by the functionality of our powerful, multiwavelength database ‘HASH’ (Hong Kong, Australian Astronomical Observatory, Strasbourg Observatory H-alpha Planetary Nebula catalogue) that federates known imaging, spectroscopy, and other pertinent data for all Galactic T, L, P PNe, and the significant numbers of mimics. Reddenings, corrected radial velocities, and PNe electron density and temperature estimates are provided for these new PNe wherever possible.

1 INTRODUCTION

Planetary Nebulae (PNe) are the expanding shells of ionized gas ejected from low- to intermediate-mass stars (∼1–8 M_⊙) towards the end of their lives. They are powerful astrophysical tools for studying late-stage stellar evolution (Kwitter & Henry 2022) and plasma physics (Hajduk et al. 2021), as well as investigating the chemical evolution of the whole Galaxy (Dopita et al. 1997; Maciel & Costa 2003). Due to their very bright emission lines, they can be observed to great distances across our own Galaxy and detected and traced in other nearby galaxies in the Local Group and beyond.

Since Charles Messier observed the first PN (Dumbell Nebula in Vulpecula) in 1764, over 3800 of these beautiful objects have now been discovered in our Galaxy (Parker, Bojičić & Frew 2016). Before the advent of the SuperCOSMOS Hα Survey (SHS; Parker et al. 2005) and INT Photometric H α Survey (IPHAS; Drew et al. 2005), narrow-band H α surveys of the Southern and Northern Galactic Planes, respectively, the vast majority of the previously known PNe (∼1500) were compiled from over 200 yr of observations from a wide variety of telescopes and spectrographs into the Strasbourg-ESO Catalogue and its supplement (Acker et al. 1992; Acker, Marcout & Ochsenbein 1996), and the largely overlapping but independent compendium of Kohoutek (2001). However, these new H α surveys led to a more than doubling of known Galactic PNe as reported in Parker et al. (2006) and Miszalski et al. (2008) for the Southern Galactic plane and Sabin et al. (2014) for the Northern Galactic plane. These ‘new’ PNe were not simply more of the same but are generally fainter, more evolved/extended, more obscured and also, in many cases, more compact compared to the previous catalogues. Other notable numbers of new PNe discoveries have also come from the Deep Sky Hunters ‘amateur’ consortium and more recently from a dedicated group of French amateurs, e.g. Acker et al. (2012). This is via painstaking, fresh analysis of the now online broad and narrow-band Digital Sky Survey plates, e.g. Jacoby et al. (2010) and Kronberger et al. (2006, 2012, 2016). This includes our own recent professional-amateur (pro-am) collaboration (Le Dû et al., in press) that reports the discovery of 210 PNe. Many of these new discoveries are from outside the narrow Galactic latitude confines of the Milky-Way H α surveys (ie ∼±10 degrees for the SHS and ±5 degrees for IPHAS). We are indeed currently in a new golden age of PNe discovery in our own Galaxy where pro-am collaborations are playing an increasingly important role with their access to bespoke, dedicated facilities and time.

2 OBSERVATIONS

A total of 62 new PN candidates were found by careful scrutiny of the IPHAS survey data and other data for more ‘out of plane’ candidates by our combined pro-am team. A total of 55 subsequently newly confirmed PNe were carefully examined across the available HASH (Parker et al. 2016) imagery compiled as part of the process of ingesting any new PN. These data also provided all the independently discovered PN central stars (CSPN) by looking for faint blue stars at/near the geometric centres of these new PNe. Even for large angular size PNe, the CSPN is nearly always located at its exact centre (if visible in the available surveys) and is usually the only really blue star in the vicinity, giving confidence in its veracity. In Fig. 1, we give example images of one of the newly confirmed PNe ‘Ju 1’ (HASH ID 4408) that is 4 arcmin in diameter and its independently found CSPN. The left-hand panel is a H α quotient image clearly showing the slightly oval PNe with the CSPN indicated by a red half-cross. The right-hand panel gives a 2 × 2 arcmin RGB broad-band image created from the SuperCOSMOS Sky Survey (SSS; Hambly et al. 2001) i-band, r-band, and B_j band deep photographic data where the faint blue CSPN is clearly evident.

All candidates were observed as part of a long-term ‘filler’ program with the OSIRIS long-slit spectrograph on the Spanish 10.4 m GTC on La Palma in the Canary Islands. Such projects make decent use of time not suitable for the most challenging observing programs. This list is comprised of mainly large, very low surface brightness candidates not previously observed spectroscopically. It was supplemented by a further 16 PNe identified previously but that needed deeper spectroscopy for firmer confirmation. This provides a total of 78 GTC spectroscopic observations reported here. Of the 16 previously observed PNe candidates, one has been demoted as a result of these new observations from a ‘T’(True) to a ‘P’ (probable), IPHASX J023538.6+633823 has been upgraded from ‘P’ to ‘T’; IPHASX J055242.8+262116 from ‘L’ (Likely) to T and one, ‘Pa30’, is now known as a supernova remnant (SNR) of the historical Chinese guest star SN 1181 AD (Ritter et al. 2021). The other 12 remain as ‘T’ PNe.

The observations were performed between 2016 March 1 and 2018 May 17, under the programs GTC4-16AMEX, GTC12-17AMEX, GTC8-17BMEX, and GTC11-18AMEX. Due to the constraints inherent to the filler mode, the observations were conducted under different seeing (up to 2 arcsec), moon phase (dark to bright) and sky transparency (photometric to non-photometric) conditions. Such conditions are no serious impediment to the successful spectroscopic follow-up of even very low surface brightness PNe that emit most of their light in narrow emission lines.

The spectroscopic data were obtained using two 2048 × 4096 Marconi CCD44-82 detectors with a pixel size of 15 μm/pix. The plate scale was 0.254 arcsec per pixel with 2 × 2 binning adopted. We used the R1000B grating that provides long spectral coverage over the entire optical band from 3630 to 7500 Å, a dispersion of 2.12 Å/pix and a spectral resolution of 2.15 Å. Such a configuration allows detection of all significant optical emission lines from ionized plasmas, indispensable for the diagnostic identification and analysis of different kinds of nebulae, including PNe. The concurrent observation of spectrophotometric standard stars allowed flux calibration, necessary to determine the logarithmic extinction (cHβ) from the corrected Balmer decrement. The chosen spectrograph setup also provides a resolving power sufficient to separate nearby diagnostic emission lines such as  [S ii] 6716 Å and  [S ii] 6731 Å used here for electron density N_e estimates from the [S ii] 6716/6731 Å line ratio (see later). The slit width was typically set to 0.8 arcsec (despite the variable seeing) to retain decent spectral resolution and the total exposure time ranged from 2000 to 3600 s depending on the surface brightness of the candidate nebulae.

In Table 1, we present summary data for the 55 discovered PN presented here and the 16 re-observations. The table lists, in order, the IAU PNG designation, PN usual name, HASH ID number, positional information (RA/Dec. J2000 and Galactic latitude and longitude), PN status as T, L, P, whether there is a CSPN detected, the angular diameter in H α in arcsec, the PN morphological classification following the ’ERBIAS sparm’ scheme outlined in (Parker et al. 2006), the heliocentric radial velocity in km s⁻¹, slit position angle (0 for East to West, positive East towards South, negative East towards North), and exposure time.

3 DATA REDUCTION

Standard CCD spectrograph reduction steps were employed. These included combination of individual frames with cosmic ray rejection, bias subtraction, illumination correction, flat-fielding, and wavelength calibration via standard calibration arc lamps. We used the iraf-based GTCMOS pipeline (Gómez-González, Mayya & Rosa-González 2016) built for the purpose. The resulting 2D images were then visually inspected to identify suitable sky and nebula extraction areas. This is given the extended nature of most of our PN candidates across the slit and in order to avoid contamination by stars. Any clearly identified CSPN, if falling on the slit, were extracted separately. The sky-subtracted, 1D spectra of the PNe candidates were then co-added if necessary and subsequently flux calibrated to produce the final spectrum used for evaluation. An example flux-calibrated PN spectrum of Ju-1, first confirmed from these GTC data, is shown in Fig. 2 and displays the most common PN emission lines as marked.

4 DATA ANALYSIS

The first task was to use the observed spectral signatures and emission line ratios in an holistic combination with all other available imagery (including for object morphology and environment), object measurements, and separate observations available conveniently in HASH, to make a decision on the true nature of the observed nebulae. This is following the robust precepts we have previously established, e.g. Frew & Parker (2010) and Parker (2022).

Of the 62 new PN candidates observed for this program, we confirm 55, with 46 as True, 4 as Likely and 5 as Possible PNe. This gives a maximum confirmation rate of 89 per cent with the remaining 7 rejected candidates being identified as a mixture of the various kinds of the usual PNe mimics (such as H ii regions, parts of SNRs and objects of unknown nature, but not PNe). These 7 rejected PNe and their GTC spectra can be examined, if required, via their unique HASH ID’s as follows: 8241, 8248, 8448, 11583, 31955, 31956, and 31957.

4.1 Major axis diameters

A histogram of the major axis diameters in arcseconds (") measured from the IPHAS H α imagery for the 55 new PNe confirmed here are presented in Fig. 3. Only 38 per cent of the sample are less than 50" in size with only 3 less than 10" across while 34 (62 per cent) have diameters >50". The average diameter for the sample is 111" with σ = 107" confirming the wide angular size range but generally more extended nature of this sample. For comparison, the average angular diameter for all 2695 True PN in HASH, as in 2022 September, is ∼56".

4.2 Spectral analysis

For the 55 new, confirmed PNe and for the 16 re-observed, spectral analysis was undertaken after first applying the standard heliocentric velocity correction in kms⁻¹. The spectral resolution employed is insufficient to determine useful kinematic information from the PNe, such as the PN shell’s expansion velocity, but systemic velocities are reasonably well determined with typical velocity error of ±15 kms⁻¹. We then fit all available emission lines above the background noise level in each spectrum with Gaussian profiles (recall there is little continuum in PNe spectra). Uncertainties were estimated by adding noise (the standard deviation of the spectrum around the emission lines) to the fitted profiles and re-fit 100 times. Using the python package pyneb developed by V. Luridiana, C. Morisset, and R.A. Shaw (Luridiana, Morisset & Shaw 2015), we first calculated the reddening and corrected for it. We also simultaneously calculated nebula electron densities and, where possible, electron temperatures (given the principle diagnostic emission lines for the latter can be very weak). The emission line ratios are all used to self consistently determine such electron densities and temperatures as these variables are not independent but weakly correlated. This has been typically ignored in the past when determining electron densities for PNe by assuming a T_e of 10 000 K.

To estimate the uncertainties of these physical characteristics, we added 500 Monte Carlo mock observations with the uncertainties from the line intensities to each spectrum. From this work, because of the limited S/N in many cases due to very low surface brightness and despite use of a 10-m class telescope, only 27 objects (⁠|$\sim 39{{\ \rm per\ cent}}$|⁠, measured line intensities shown in Table 2) out of combined total of 70 T, L, P PNe yielded both electron temperatures and densities from the diagnostic emission line ratios (this is from 55 new confirmations and 16 re-observations with 15 of these being confirmed). These pyneb results are based on the usual [S ii] 6717/6731 Å and [O ii] 3737/3729 Å ratios traditionally used for plasma electron density estimates but accounting for electron temperature and then the usual [O iii] 4363/(5007+4959) Å or [N ii] 5755/(6548 + 6583) Å ratios for electron temperature, while simultaneously accounting for density variations. Out of these 27 objects, 7 yielded the electron temperatures and densities from both sets of line ratios used to estimate these characteristics. The comparison of those 7 independently estimated values shows good agreement as can be seen for all the values given in Table 3.

In Fig. 4, we present the traditional ‘BPT’ diagnostic diagram (often used to separate different kinds of nebula source) (Baldwin, Phillips & Terlevich 1981) of all T, L, P PNe in this study following Frew & Parker (2010). In this particular example, we plot emission line flux ratios of (H α)/[N ii] versus log (H α)/[S ii], where [N ii] refers to the sum of the fluxes of the two [N ii] at 6548 and 6584 Å, and [S ii] refers to the equivalent sum of the fluxes of the two [S ii] lines at 6717 and 6731 Å. Nearly all points lie within the normal PNe range as indicated by the two black line tracks – see Fig. 4 of (Frew & Parker 2010).

In Table 3, we present, where possible, estimates of the interstellar reddening E(B-V) and PN electron temperature and density determined from the measured flux calibrated emission lines as produced by pyneb. Of the total PNe sample, only 27 spectra had sufficiently high S/N to enable these measurements from detected lines.

4.3 Central stars and Gaia eDR3 distances

The identification of the CSPNe is often difficult. They are generally faint and can be in a crowded field. While automated searches for CSPNe in GAIA have been developed (e.g. Chornay & Walton 2021; González-Santamaría et al. 2021), Parker, Xiang & Ritter (2022) have shown that purely automated procedures can lead to miss-identifications. In fact, 4 CSPNe reported in González-Santamaría et al. (2021) for PNGs 037.6-04.7, 059.2+01.0, 105.7+02.2, and 149.1+08.7 are likely wrong, as well as 1 CSPN reported in Chornay & Walton (2021; PNG 037.6-04.7). We therefore carefully inspected each PN for possible CSPN making use of the imagery conveniently provided by the HASH database. For 36 new and 10 already known PN candidates (including Pa 30 which turned out to be a SNR and has subsequently been left out of the CSPNe analysis), a likely CSPN could be identified. Out of those 45 CSPNe, 41 could be identified in the GAIA DR3, with 38 (33 new, 5 already known PNe) having distances determined in Bailer-Jones et al. (2021). Only 1 CSPN has a radial velocity determined in GAIA DR3 (Gaia Collaboration 2016, 2022). The Bailer-Jones et al. distances, GAIA DR3 radial velocities (where available), and the physical sizes (calculated from the geometric distances and HASH angular diameters) are shown in Table 4. A histogram of the physical sizes of the 38 PNe with distances is shown in Fig. 5.

5 CONCLUSIONS

We have spectroscopically confirmed 46 true, 4 likely, and 5 possible usually low surface brightness, generally large angular size PNe in the Northern Galactic plane for a total of 55 new PNe. These have now been incorporated into HASH together with their usually determined characteristics such as accurate position(s), morphology, and angular size and presence of any CSPN. This PNe sample was effectively too difficult to confirm spectroscopically on smaller aperture telescopes. We have further refined the confidence and spectral characteristics for 16 previously observed PNe candidates, where the existing spectroscopy was too poor, and updated their HASH entries accordingly (with one being rejected as a PN). Seven new candidates observed were rejected as PNe based on our GTC spectroscopy. They remain in HASH as part of our large catalogue of various mimics. Where the PNe spectral S/N permits we have also calculated electron temperatures and densities from either the [N ii] / [S ii] or the [O iii] / [S ii] line ratios for 27 objects. We have also included estimates of interstellar reddening E(B-V) towards these PNe. We have found 37 of the 55 newly confirmed PNe to have credible blue CSPN identified from deep multiwavelength images where PanSTARRS (Chambers et al. 2016) and Sloan Digital Sky Survey (SDSS; Gunn et al. 1998) imagery often play the key role. These CSPN co-ordinates have been added to HASH and cross-checked against Gaia EDR3. This work has added about |$\sim 2{{\ \rm per\ cent}}$| to the current total of confirmed Galactic PNe in HASH of over 3848 T, L, and P entries.

SUPPORTING INFORMATION

APPENDIX A: PNE IMAGES AND SPECTRA.

Please note: Oxford University Press is not responsible for the content or functionality of any supporting materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the article.

ACKNOWLEDGEMENTS

QAP thanks the Hong Kong Research Grants Council for General Research Fund research support under grants 17326116 and 17300417. AR thanks HKU for the provision of a postdoctoral fellowship. LS acknowledges support by Universidad Nacional Autónoma de México (UNAM) Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica (PAPIIT) project IN110122 (Mexico). This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

DATA AVAILABILITY

The data underlying this article are available in the article itself and in its associated online material freely accessible from the HASH database found here: http://hashpn.space by simply entering the unique HASH ID number for each source as provided.

REFERENCES