Since late-2015 I have been actively hunting for unknown Planetary Nebula (PN) candidates using the publically available images online. In this section, I discuss my method of hunting, demonstrated by various examples of known Planetary Nebulae, and Planetary Nebula candidates!
In this section I make comparisons to many “Non-PN” objects. More detail on those objects can be found in the Planetary Nebula Mimics rubric.
Digitalized Sky Survey (DSS) Plates
In DSS (Digitalized Sky Survey) one should hunt for well defined nebulae with typical PNe-like morphologies. The image just below shows some common recurring structures.
Examples of Planetary Nebulae showing typical PN-like morphologies in DSS2 (Red+Blue) images: Circular/annular, bipolar, compact quasi-stellar, and elliptical. Image credit: DSS2/Aladin Lite.
Often PNe can appear more contrasted in some parts more than others, likely due to the nebula interacting with Interstellar Matter (ISM). This can cause PNe to appear “asymetric” or even irregular. For example, circular and elliptical PNe might appear like “arcs” instead of showing a complete “annular” structure.
Two likely well evolved PNe as seen in DSS2 (Red+Blue) images, their morphology is clearly asymetric. Images from Aladin Lite.
Another common morphology are “stellar PNe”, but these are difficult to find in DSS as they (by definition) blend well with the background star field. These usually display particular colors when seen in SDSS and Pan-STARRS1 images (see below).
I also make sure the candidate is fainter in DSS2-IR and 2MASS images, in comparison to the Optical DSS plates. This is a common characteristic of PNe.
Planetary Nebula Pa 1 as seen in DSS plates and 2MASS. Despite being obvious in the DSS Red and Blue plates, it’s barely detectable in the DSS2-IR image and 2MASS. Images are from the SuperCosmos Sky Survey and Aladin Lite.
Unfortunately, many objects can mimic the morphology of PNe, especially in DSS images. These are commonly Galaxies and other kinds of nebulae (cometary globules for example). The imposters might even appear faint or absent in DSS2-IR images. Luckily SDSS and Pan-STARRS1 data can be used distinguish between the two.
Stellar planetary nebulae are also a problem when it comes to DSS, as they won’t necessarily be able to distinguish themselves from other stars. The only major characteristic will be their poor appearance in DSS2-IR plates, but Bluer stars also reply to this criteria.
Furthermore, the DSS2-IR plates are not always a reliable source for magnitude comparisons, as many objects can appear alot fainter in these images without that actually being the case! A second check is possible though via Pan-STARRS1 data (see further below).
Sloan Digital Sky Survey (SDSS)
Combined color images of the SDSS can be used to distinguish between true PN candidates (and other mimics) as PNe display very particular colors in these images (very faint in the i and z filters, in comparison to the optical-UV ones). In Planetary Nebulae will generally have a blue or green appearance. Hence, hunting the SDSS images can easily reveal some obvious Planetary Nebulae, as seen in the image below.
Examples of planetary nebulae candidates that show typical colours of Planetary Nebulae in SDSS data. These are typical PN candidates one can expect to find when hunting the SDSS data. Image credit: SDSS Aladin Lite.
This technique applies very well to “stellar” PNe too. These objects might appear as green, turquoise or oddly blue stars in SDSS images. Stellar Planetary Nebulae are impossible to spot in DSS, as they don’t display any particular colour in these images.
Some examples of “stellar” Planetary Nebulae in SDSS images. Notice their interesting green – blue tint, typical of Planetary Nebulae. Images from Aladin Lite.
Below are some examples of Nebulae whose colours in SDSS are not typical of planetary Nebulae, despite showing a typical PN-like Morphology in DSS. Instead of appearing anywhere between Gree-Blue, they have a Brown-green, or brown-grey tints.
Examples of Nebulae that appear appear to have the morphology of Planetary Nebulae in DSS, but that display Non-typical PN-colours in SDSS. LDû 17 appears to be a galaxy, while Pre 36 might be a possible Cometary globule. Image credit: DSS2 and SDSS AladinLite.
Pan-STARRS1 is similar to the SDSS, as it’s composed of a numerous filters, hence its able to better distinguish the colorimetry of objects, Planetary nebulae included. Planetary Nebulae typically appear best in the g and r filters, while sometimes too in y. Combining the g, r and i filters one hence get the resulsts seen below.
Examples of Planetary Nebula candidates that display tymical PN-like colours in Pan-STARRS1 images. These images are a combination of the g (color-coded blue), r (cc red), and i (cc red).
Pan-STARRS1 g, r and i can be assimilated to the DSS Blue, Red and Ir filters, hence Pan-STARRS1 can be a good tool to confirm the appearance of an object in DSS2, knowing that the DSS2-IR plates are occassionally rather unreliable, as can be seen in the case of Mo 2 (below).
Mo 2 as seen in Pan-STARRS1 and DSS filters. Notice how the object is hardly detectable in DSS2-IR, while it’s very promininent in Pan-STARRS1 – i. Image credit: The SuperCosmos Sky Survey and The Pan-STARRS1 Science Consortium.
Below are objects with colours atypical of Planetary Nebulae. The two examples presented below display colours typical of Galaxies rather than Planetary Nebulae.
Examples of Nebulosity that appear appear to have the morphology of Planetary Nebulae in DSS, but that display Non-typical PN-colours in Pan-STARRS1. Both Ra 63 and LDû Object 1 appear to possibly be galaxies. Image credit: DSS2 AladinLite and The Pan-STARRS1 Science Consortium.
IPHAS and SHS
Knowing that Planetary Nebulae typically display Halpha emissions, it is hence useful to study potential PN candidates using the online IPHAS and SHS data, to see if they appear better in these images. In fact, many Planetary Nebulae will be too faint to appear in DSS, SDSS and Pan-STARRS1 images, but might appear obvious (or at least faintly) in IPHAS and/or SHS.
Examples of Planetary Nebula candidates appearing essentially in the Halpha images. It’s only because of these images that Pre 27 was able to be kept as a candidate. In fact these are the only optical images showing this small nebula! Image credit: SHS.
However, Halpha emissions are not a fundamental property of Planetary Nebulae, in fact, some seem to lack Halpha emissions entirely! This is notably the case of Pre 30, which seems to appear the at the same brightness in both SHS short Red and SHS Halpha images.
In these images I hunt for any objects with ususually strong emissions in WISE’s W4 (22 micron) band. PNe tend to appear very bright in this band (as well as in W3 sometimes) in comparison to the W1 and W2 WISE bands. For this I use Aladin Lite’s combined (W1 +W2 + W4) AllWISE images.
Individual WISE band frames showing Planetary Nebula candidate Mul 3 (marked by the blue circles). The object appears essentially in the W3 and W4 bands. There are only faint traces of it in W2 and W1. This is typical of PNe. Images from WISE/IRSA.
An advantage of the (All)WISE images is that it can reveal optically very faint PNe that might not be detectable in DSS and/or SDSS images (except for possibly Narrow-band images). This is the case of PN candidates Pre 12 and PrKn 1.
Planetary Nebula candidate Pre 12, a small diffuse nebula visible only in SHS halpha images, with a PN-like signature in the WISE W4 band. Images are from Aladin Lite and SuperCosmos Halpha Sky Survey.
Planetary nebula candidate PrKn 1, a circular nebula mainly apparent in OIII exposures, images, with an evident PN-like signature in the WISE W4 band. Images from Aladin Lite and Las Campanas Observatory (LCO) and George Jacoby.
However, like for the DSS images, WISE images show many PN mimics, most notably OH/IR stars, Young Stellar Objects (YSOs), Active Galactic Nuclei (AGNs) and Proto-Planetary Nebulae. One way YSOs and OH/IR stars *often* differ from PNe is the presence of high-amplitude variability (by a few magnitudes). This is the case of Mul-IR 92 (see below).
Discovery images showing the highly variable nature of Mul-IR 92. The object varied by 4 magnitudes between both images! Image credit: Trygve Prestgard and the DSS Red/DSS Plate Finder.
Mul-IR 92 appears to be a highly variable object, varying by ~4 magnitudes in the Red band. Such strong variations are not in favor of a Planetary Nebula, despite having a rather favorable WISE signal (see below). The object is now also currently listed as a possible YSO candidate by AAVSO.
Mul-IR 92 as seen in the different WISE filters. Notice how the object best in the W3 and W4 band, typical of Planetary Nebula. Image credit: IRSA WISE.
Like in WISE, Planetary Nebula often appear very well in Spitzer data, often displaying a particular “green tint” in the combined images available on Aladin Lite images. The high-resolution of these images also allow one to detect some of the objects nebulosity alot better (see below), especially if it only has a very faint optical counterpart. For example, Kan 5 is extremely faint in optical images (currently only visible in SHS Halpha images!), however, it’s a very obvious circular nebula as seen in Spitzer.
Planetary Nebula candidates Mo 1 and Kan 5 as seen in combined Spitzer images from Aladin lite. Notice their obvious nebulous nature in these images, and their typical “green” appearance. Image credit: Spitzer Aladin Lite.
New found Planetary Nebula candidates are sent to Agnès Acker (France) and Pascal le Dû (France). I report any candidate that responds at least partially to the above criteria. It is also possible to report discoveries (any Deep sky objects) on the DeepSkyHunters (DSH) Yahoo forum.
If your object is kept as a Planetary Nebula candidate, it will recieve an inofficial designation based on your surname. It will then eventually recieve an official designation (based on the object’s galactic coordinates) and an entry in the Hong Kong/AAO/Strasbourg H-alpha planetary nebula (HASH) database.
What about other nebulae?
Often one can stumble upon a nebulous object that responds poorly to the criteriae above (if at all!). In fact, such objects should be reported to Agnès Acker and Pascal Le Dû too, as they keep track of such discoveries too in a second list. These objects are given a distinct designation and will not be included in the HASH database. Below are some examples of such objects kept by Agnès and Pascal.
Examples of different objects kept by Agnès Acker and Pascal Le Dû that repsond poorly to the criteriae of Planetary Nebulae. These objects are assigned a sperate desgination than that of PN candidates. Hu Object 1 and Pre Object 5 are likely cometary globules, Mul Object 1 is a possible Supernova Remnant and LDû Object 1 is likely a galaxy. Image credit: DSS2, SDSS and AllWISE AladinLite, and the Pan-STARRS1 Science consortium.
Hu Object 1 and Pre Object 5 show a typical morphology of a cometary globule, and both appear simlarly prominent in DSS2-IR images as in the optical DSS plates (see below). Both also display colours of globules in Pan-STARRS1 images, with the latter also being supported by the SDSS colours in the case of Pre Object 5.
Hu Object 1 as seen in different DSS filters. Notice how it appears very prominently in the Infrared plate, similarly to the optical ones. This is very atypical of Planetary nebula, not to mention that the object has a very cometary morphology, typical of globules. Image credit: DSS Plate Finder.
Mul Object 1 has an atypical WISE signal of a Planetary Nebula, while also having a morphology similar to many Supernova Remnants.
LDû Object 1 is an object with obvious colours of a galaxy in Pan-STARRS1 imagery, and a WISE signal also typical of a galaxy, appearing essentially in the W1 and W2 wise bands.
Appearance of LDû Object 1 as see in different WISE filters. Notice how it essentially appears in the W1 and W2 bands, typical of a galaxy. Image credit: WISE IRSA.