Over the course of 2018, amateur astronomers have continued scrutinizing the online survey images in search for previously unknown nebulae, in particular Planetary Nebulae (PNe). Many of these discoveries (if not most) were made possible due to the high-resolution DECam Plane Survey (DECaPS) imagery, which was only made available to the public this year. While many of these discoveries have been kept as Planetary Nebula candidates, others are more likely to be Reflection Nebulae, globules or HII regions. Furthermore, some even appear to not be nebulae at all. Indeed, galaxies and highly active Young Stellar Objects (YSOs) are also likely included in the mix!
In this post I feature only a fraction of the numerous objects that were just recently added to the French database of PN candidates (maintained by Pascal Le Dû, France). All the active YSOs have been added to AAVSO’s Variable Star Index (VSX). The database also includes a sublist dedicated specifically to other kinds of nebulae, rather than PNe.
Note: This blog post discusses and characterizes the appearance of Planetary Nebula candidates (and PN mimics) in online survey images (such as the SuperCosmos Halpha Survey images, WISE and Spitzer). Hence, I recommend reading the “Hunting for Planetary Nebulae guide” of this website, for better understanding of this article! 🙂 https://skyhuntblog.wordpress.com/planetary-nebulae/personal-method-and-discoveries/
Small And/Or Compact PN Candidates
These are examples of Planetary Nebula candidates with small apparent dimensions. None of the below examples objects exceed 25″ in diameter. The largest one among this selection is Mo 16, measuring about 20″ across. Many of these candidates are likely to be true PNe, but only spectroscopy can confirm their true nature.
Mo 16 is an interesting Bipolar Planetary Nebula candidate discovered by Sankalp Mohan (India) while searching Aladin Lite’s coloured Spitzer images. Indeed, he states “While observing an area of 22.1′ Fov in Spitzer, I noticed a small fuzzy-greenish patch. As I zoomed-in, I realized something about that patch which made my heart melt: A Bipolar morphology, similar to Mo 1 and Mo 3!” (see figure 1). Planetary Nebulae often appear to have a recurrent green tint in Aladin Lite’s colour-coded Spitzer images. Sankalp continues by saying “I took a deep breathe, copied the coordinates [of the nebula] and searched for it in Simbad. Fortunately the search came up negative, so I decided to […] send it to Mr. Pascal Le Dû for further analysis.” Figure 1: Planetary Nebula candidates Mo 1 and Mo 16 as seen in colour-coded Mid-IR images from Spitzer. Note the obvious “green” tint, typical of PNe. One can also distinguish a possible bipolar morphology in both these objects (a bit more obvious in the case of Mo 16). Image credit: IRAC/Spitzer Aladin Lite.
Mo 16 also displays a Mid-IR WISE signature typical of most PNe (essentially emitting in the WISE W4 band) and appears faintly in the SHS Halpha plate, while being absent in all other optical survey images (likely due to Halpha emissions, which are typical of PNe). Mo 16 is a strongly obcured object, as it is hidden behind the optically opaque dust making up the Barnard 58 dark nebula.
Tan 5 (discovered by Hanjie Tan, China) is similar to Mo 16 as it seems to be a highly obscured bipolar PN (see figure 4). Furthermore, Tan 5 is practically invisible in optical survey images, but it does however emit well in Halpha (according to the SHS plates). Dana Patchick (USA) notes that Tan 5 is only located 18.7′ NW of Hadar (Beta Centauri).
Mur 1 is Gabriel Murawski’s (Poland) first PN candidate discovery. He spotted it in early July 2018 using DECaPS and WISE imagery. The object appears to have the typical Mid-IR signature of a PN in WISE (see figure 2), and displays an annular morphology in DECaPS (see figure 4). However, based on its colours in DECaPS, it could be possible that Mur 1 is a distant active galaxy rather than a true PN. Figure 2: Individual WISE filter extracts centered on Mur 1. Notice how it appears much brighter in the W3 and W4 filters versus W1 and W2. This is a very typical characteristic of PNe. Image credit: WISE/IRSA.
Pre 42 is a small elliptical PN candidate I first spotted in DECaPS imagery. The object also seems to emit significantly in Halpha, according to plates from the SuperCosmos Halpha Sky Survey (SHS, see figure 3). Pre 42 appears only very faintly in WISE. Pre 48 is possibly an object similar to Pre 42, except that it displays a much more typical PN-like WISE signature and a bluer optical counterpart (as well as being a bit more compact, see figure 4).
Figure 3: Pre 42 as seen in the SuperCosmos Halpha Sky Survey (SHS) plates. Note how the nebula appears very clearly on the Halpha plate (very typical of PNe, which often are accompanied by Halpha emissions). Image credit: UKST/SHS.
SuFe 1 is another similar nebula, but appears to have a more defined spherical morphology. SuFe 1 was discovered independently by Guoyou Sun (China) and Laurent Ferrero (France). The nebula displays the typical WISE mid-IR signature, as well as an optical-NIR colorimetry similar to many (if not most) Planetary Nebulae (green colour in Pan-STARRS1 g+r+i images, as seen in figure 4).
Figure 4: Six recently discovered Planetary Nebula candidates found in online survey images. Notice the obvious difference in colours and morphology. The Mo 16 and Tan 5 are both possibly bipolar and highly reddened due to interstellar dust. Mur 1 appears annular, and might be a galaxy. Pre 42, Pre 48 and SuFe 1 all display typical optical colours. Image credit: DECaPS/Aladin Lite and the Pan-STARRS1 Science Consortium.
Large PN Candidates And Other Similar Nebulae
The nebulae listed here have much larger dimensions than the ones listed previously. Indeed, most have diameters superior to 1′. Despite most of these nebulae sharing many similarities to PNe (especially in the Mid-IR), many are likely to be reflection nebulae and/or associated to star forming regions.
Both Mo 10 and PreMo Object 1 (discovered by Sankalp Mohan and myself) possibly appear to be reflection nebulae. While Mo 10 is associated with a single star, PreMo Object 1 appears to be associated with an entire (young) Open Cluster (Prestgard 115, see figure 6). PreMo Object 1 is also likely linked to the possible HII region WISE G254.160-02.889, which displays a strong WISE signal similar to many Planetary Nebulae (making PreMo Object 1 a rather interesting object). Mo 10 is more difficult to rule out as a PN. Indeed, its blue colour in DECaPS, its central star and PN-like WISE signal are very much characteristics one would expect from a PN. Only a spectra can determine its true nature.Figure 5: Individual WISE filter extracts centered on Mo 13 and Mo 18. Notice how both nebulae much brighter in the W3 and W4 filters versus W1 and W2. This is a very typical characteristic of PNe. Image credit: WISE/IRSA.
It seems possible that Mo 18 could be a reflection nebula too. Indeed, its brightness may have been significantly enhanced due to the presence of a central white star, which could potentially be the reason that Mo 18 is detectable in optical images at all! Mo Object 4 is an obvious dark nebula/globule, as one can clearly be seen in figure 6. Mo 13 appears to possibly be globule/dark nebula based on its DECaPS colours. However, its WISE signal is similar to many PNe (see figure 5), hence it cannot be ruled out as a PN. Mur Object 1 (another recent discovery by Gabriel Murawski) could potentially be an HII region, or nebulosity associated with a YSO (perhaps similar to IRAS 17079-4032 and/or Fe Object 1, see figure 10).
Figure 6: DECaPS image extracs of Mo 10, Mo 13, Mo 18, Mo object 4 and PreMo Object 1. While the former four objects are classified as possible PN candidates, it is also possible that they may actually be reflection nebulae and/or globules. Mo Object 4 is a beautiful dark nebula/globule. Mur Object 1 and is an interesting nebula that might be an HII region or something associated to YSO. Image credit: DECaPS Aladin Lite.
Young Stellar Objects and associated nebulae
IRAS 17079-4032 is a very interesting variable nebula in Scorpius, displaying a beautiful cometary morphology in DECaPS (see figure 10). Indeed, as Sankalp first comically told me over Facebook when spotting this object: “A Comet, Journey of a Star into Black Hole XD”. At the time, we didn’t realize that the object was uncatalogued. It wasn’t until I was composing this blog post that I realized the nebula had no actual reference! Furthemore, I discovered that it was a highly variable in nature (see figure 6). Indeed, the nebula is likely a reflection nebula (similar to Hind’s and McNeil’s variable nebula) surrounding a highly active YSO. IRAS 17079-4032 is located only 4′ west of Mo 16, which is how Sankalp Mohan first spotted it.
Figure 7: DSS Plate Comparison demonstrating the highly variable nature of IRAS 17079-4032. Image credit: DSS Plate Finder.
Fe Object 1 was discovered by Laurent Ferrero (France) in SDSS images. It is possible that it is an object similar to IRAS 17079-4032 and Mur Object 1. Indeed, The nebula seems to stem from a single star (likely a YSO). In the case of Fe Object 1, the nebula has a rather annular morphology (see figure 10), giving the visual impression of bipolar jets streaming from the core (proto-)star. Unlike IRAS 17079-4032 however, no obvious variability could be detected, even in ASAS-SN and NSVS data!
Figure 8: DSS Plate Comparison demonstrating the highly variable nature of 2MASS J14553751-5728023. Image credit: DSS Plate Finder.
Unlike the two previous examples, both 2MASS J14553751-5728023 and IRAS 18044-2420 are not associated with any obvious nebulae. The former was first spotted by Dana Patchick (USA) due to its Mid-IR WISE signature similar to many PNe. At first glance it ressembles a stellar PN (see figure 10). However, when studying archive images from the Digitalized Sky Survey (DSS), one could clearly detect obvious variability in its brightness, typical of an active YSO (see figure 8).
Figure 9: Individual WISE filter extracts centered on IRAS 18044-2420. Notice how it appears much brighter in the W3 and W4 filters versus W1 and W2. This is a very typical characteristic of PNe. Image credit: WISE/IRSA.
IRAS 18044-2420 appears to be a similar object to 2MASS J14553751-5728023. I first reported it in December, 2017 as a possible stellar PN candidate (based on its PN-like WISE signal and PN-like colours in Pan-STARRS1, see figure 9 and 10). It wasn’t until August, 2018 that I realized it displayed a high-amplitude variable nature, similar to a 2MASS J14553751-5728023.
Figure 10: 2MASS J14553751-5728023, IRAS 17079-4032, IRAS 18044-2420, Fe Object 1, as seen in DECaPS, Pan-STARRS1 and SDSS respectively. Note the cometary morphology of IRAS 17079-4032 and the annular nature of Fe Object 1. Image credit: SDSS DECaPS Aladin Lite and the Pan-STARRS1 Science Consortium.
I wish to thank Sankalp Mohan for sharing details on his discovery of Mo 16 and IRAS 17079-4032, Dana Patchick for his help searching the HASH database, and (last but not least) Pascal Le Dû for his continous analysis of our discoveries.