Planetary Nebulae (PNe) can be difficult to distinguish from many other types of objects, including in the optical domain. However, in the case of compact objects, multi-epoch photometry may prove to be helpful in order to distinguish them from variable “PN-mimics” (e.g. proto-stars or symbiotic stars), and may also allow to provide a more accurate re-classification of impostors (thanks to the morphology of light curves, if available), especially when optical spectra lack. In this post I discuss the cases of DeGaPe 71 and PN G326.9+08.2. These two objects were catalogued as PN candidates based on optical data (narrow-band imagery and spectra, respectively), but however display colours and luminosity variations typically associated with cataclysmic variable stars.
DeGaPe 71 – A Nova-like Star?
Only recently added to the French Planetary Nebula (PN.net) database, DeGaPe 71 is a star-like object that displays peculiar blue “fluorescent” colours in SHO ([SII]+Hα+[OIII]) images obtained by the Atacama Photographic Observatory (APO) team (see figure 1). The “SHO” colours suggest that the object might display [OIII] and Hα emission lines (especially the former), typical of planetary nebulae (PNe). Indeed, DeGaPe 71 resembles both KnDeGaPe 1 and DeGaPe 64 (also discovered by the APO team), two objects that are rather likely to be true PNe (see figure 2). More information regarding the APO team and their discoveries can be found here and here.
Figure 1: APO SHO image extract centered DeGaPe 71 (MGAB-V202), an object classified as a potential Planetary Nebula by the French PN database based on its appearance in this colour-combined image. (c) APO team.
Interestingly, unlike KnDeGaPe 1 and DeGaPe 64 (and compact PNe in general), DeGaPe 71 does not display any obvious Mid-IR emissions, nor does it appear to display obvious Hα emissions (based on the comparison between the SHS Short red and Hα plates, see figure 5).
Figure 2: APO/SHO discovery images of DeGaPe 64 (left) and KnDeGaPe 1 (right). Note the similar blue colours, typical of planetary nebulae and highly similar to DeGaPe 71. Note that KnDeGaPe 1 was also independently discovered by amateur astronomer Matthias Kronberger. (c) APO team.
However, despite its PN designation, amateur astronomer Gabriel Murawski was the first to note the object’s interesting nature. Though, rather than by its possible PN-like emissions, Gabriel found that the object displayed colours (highly blue: B-V= 0.07 mag) and high amplitude variations (more than a couple of magnitudes) typical of cataclysmic variable (CV) stars. He reported his findings to the AAVSO Variable Star Index (VSX) in July of 2019, and it has since been designated MGAB-V202; a potential Nova-like VY Sculptoris (NL/VY) star candidate. These objects are a class of CVs that undergo strong fadings rather than outbursts (e.g. Novae or Dwarf Novae). The highly blue nature these stars (as well as most CVs) are caused by the fact that they contain a hot and luminous white dwarf (WD) star component.
Figure 3: Digitalized Sky Survey (DSS) Red plate comparison showing the highly variable nature of DeGaPe 71/MGAB-V202. Image credit: DSS Plate Finder.
At the time of discovery, the available ASAS-SN data displayed only low-amplitude variations, though archival Digitalized Sky Survey (DSS) plates showed the object to have varied significantly in the past decades (see figure 3). However, since the object’s discovery, more recent data shows a strong fading event, typical of a NL/VY stars (see figure 4). It appears that the event started around the start of December, 2019, and might be ongoing. Perhaps this fading event may be sufficient to confirm the NL/VY classification of MGAB-V202?
Figure 4: ASAS-SN light curve (last updated on March 24th, 2020) showing a high-amplitude (g ~2 mag), long-lasting fading event that appears to have lasted more than 100 days. Image credit: ASAS-SN.
As a result of the object’s high amplitude variations and highly blue continuum, it can be difficult to know if DeGaPe 71 intrinsically displays [OIII] and/or Hα emissions, or if the observed APO/SHO colours are artefacts from the above two properties (especially knowing that the individual APO/SHO filters may have been taken months apart). As mentioned previously, based on the comparison of SHS Short Red and Hα plates (see figure 5), there does not appear to be obvious signs of Hα emissions (at least at first glance, to the naked-eye), however these plates were taken over one year apart, thus the same problem applies here too….
Figure 5: SHS plate comparison: Short Red (r) plate (left) and Hα (right), showing no obvious difference in brightness. This might suggest that the object does not display obvious Hα emissions, or it may be an artefact due to its highly variable nature. Image credit: SuperCosmos Halpha Sky Survey (SHS).
PN G326.9+08.2 – Nova Remnant?
PN G326.9+08.2 is a compact blue source (BP-RP= 0.55, Gaia DR2) in Lupus that was catalogued as a probable PN candidate based on the appearance of PN-like emission lines in MASH-II spectra (Miszalski et al., 2008) measured in 2007 (spectra are not publically available). However, similarly to DeGaPe 71, PN G326.9+08.2 does not display any obvious Mid-IR excess, typically observed in compact PNe. In addition, the coloured DSS2 images from Aladin Lite show the object to appear bright and extremely red (see figure 5), appearing much brighter than the reported Gaia DR2 magnitudes (G= +18.3 mag).
Figure 6: Colour-combined DSS2 (Red + Blue plate) showing PN G326.9+08.2/USNO-B1.0 0423-0551514. Notice the object’s unusually red colour (much brighter than in the blue band). The colours are not a result of the object’s intrinsic colours, but however result from the combination between a blue plate (measured during its quiescent phase) and a red plate (measured during outburst). Notice the weak saturation “spikes”. This is the image that lead to the discovery of its high amplitude nature.
As can be seen when comparing archival DSS plates, it is obvious that the unusually red colour in figure 6 is the result of the object’s highly variable nature rather than its intrinsic colours (see figure 7). Indeed, the coloured DSS2 images from Aladin Lite consist of a DSS Blue plate taken in the 1970s or 80s, combined with Red plate taken on June 1993 in which the object appears very bright (R= +9.8 mag). No other eruptions were detected in ASAS-3 or ASAS-SN data, however the object appears brighter in the 1997 DSS plates compared to the those taken in 1992 (see figure 7).
Figure 7: DSS Red plates showing the variability of PN G326.9+08.2/USNO-B1.0 0423-0551514. The object appears extremely bright (R= +9.8 mag) on the plate taken from June 1993. The object also appears relatively bright in the plate taken on April 1997 compared to that of 1992. Image credit: DSS Plate Finder.
Once the variability was first reported (official name AAVSO/VSX name: USNO-B1.0 0423-0551514), it was initially classified as a possible dwarf nova (UG type CV), with thoughts that it could be of the WZ-Sagittae subtype (UGWZs). These are UGs that have rare eruptions (up to decade intervals), but that can brighten up to 7 – 8 magnitudes (visual) during outburst. Indeed, this would explain the lack of outbursts detected in ASAS-SN and ASAS-3, as well as the high amplitude of the event (R~ 8 mag). Note that the amplitude is very similar to V1838 Aql a UGWZ that last had an outburst in May/June 2013 (CR = 8.5 mag – r’ = 18.5 mag).
Figure 8: ASAS-SN light curves showing the brightness of Nova Musca 2018 and Nova Lupi 2018 since their outbursts. Note the very progressive fading of both these events, especially the former. The “dip” in brightness at HJD 2458200 (Nova Musca) is due to dust temporarily obstructing good portions of the optical light. Image credit: ASAS-SN.
However, based on the large distance of the object (~ 15.000 ly according to Gaia DR2 astrometry), Taichi Kato suggested that the eruption from 1993 was possibly an overlooked nova rather than a UG. Indeed, novae are generally much less recurrent than UGWZs, which would also be coherent with the lack of outbursts detected in ASAS-SN and ASAS-3. In addition, novae tend to take years before they fully reach their pre-outburst (minimum) magnitude (see figure 8), which would explain the object’s brightness in the 1997 DSS red plates, compared to the ones from 1992. This may equally explain the PN-like optical spectrum obtained by the MASH-II survey, as well as the apparently strong Hα in the SHS plates (see figure 9).
Figure 9: SHS plates centered on PN G326.9+08.2/USNO-B1.0 0423-0551514. The images were taken in 2000 and are indicative of strong Hα emissions, at least at the time. Image credit: SuperCosmos Halpha Sky Survey.
Indeed, over the course of a nova outburst, the optical properties are dominated by the ejected matter (i.e. gas and dust) from the eruption (the progenitor remains obscured by this material). At the initial stages, the heat of the material gives it the appearance of a “star”. However, as time progresses (weeks or months?), the hot gas and dust (remnant) continues to expand and cool, in addition to being ionized by the central WD. This eventually gives it the appearance of a PN (and/or Wolf-Rayet star), most notably due to [OIII] and Hα emissions, as well as in regards to its continuum. This is referred to as the “nebular phase”. A better and more detailed explanation can be found here. A current example of a nova undergoing its “nebular phase” is most notably Nova Musca 2018 (V0357 Mus). Its discoverer, Rob Kaufman, imaged the object in late-February and found that it displayed “turqouise” colours in his optical images (see figure 10), giving it a similar appearance to many compact PNe. More information on this nova can be found in an old blog post I wrote around the time of the outburst.
Figure 10: Nova Musca 2018 as imaged by Rob Kaufman on February 26th, 2020. The turquoise colours of this object are due to the surrounding (now ionized) gas and dust ejected from the 2018 Nova eruption (rather than intrinsic to the progenitor itself). This gives it a similar appearance compact PNe. The main difference is however that the origin of the dust/gas in PNe are due to the progressive loss of outer layers from ancient red giant stars, rather than “brief” cataclysmic eruptions. (c) Rob Kaufman.
Consequently, in regards to PN G326.9+08.2, it is possible that the PN-like MASH-II spectra from 2007 are signatures of the remnants ejected from an undiscovered nova outburst that occured in June of 1993. Note however that the MASH team mentions the emissions-lines to be relatively weak (i.e. low S/N). This is interesting knowing the Hα response in the SHS Hα plate appears relativy strong (see figure 9). While the quality of the spectra may possibly be due to the observing conditions(?), it could perhaps suggest that the emissions may have faded over time (note that the SHS plates were taken in 2000), maybe as a result of the expansion and dissolution of the remnant? (I am quite unsure regarding the latter statements, hence please correct me if my reasoning is wrong! 🙂 )