In this blog post I describe the discovery of three astronomical objects that were discovered by amateur astronomers, accidentally! The three featured objects in this article are of very different nature. Indeed, one being an asterism, and the other two being a nebula and a variable star!
Calvet 1 – Asterism
During the spring of 2000, amateur astronomer Cyril Calvet (France) decided to scan the rich star fields located within Cygnus constellation using his monture, including an Achromatic lens. During this particular night, Cyril was hoping to observe some of the various catagloued asterisms located within this region of this sky. Among the asterisms he observed, he came across a vast (30′ long) alignment of stars (between V= +8 mag and +11 mag) in the shape of an inversed interrogation mark (see figure 1). Based on the object’s bright and evident nature, he assumed it was an already catalogued asterism. He made a sketch the object and added it to his catalogue of interesting targets. Since then, he and fellow observers had been continuing to observe it repetively during star parties, still assuming it was a known asterism.
Figure 1: Calvet 1 as imaged by its discoverer. Notice the interesting alignment of this group of stars, making it appear similar to an inversed question mark. (c) Cyril Calvet
It wasn’t until 2006, when Xavier Galliegue (France) independently spotted the asterism, that action was taken in order to identify the asterism. Indeed, after looking through Cyril’s comet sketches, Xavier scanned the sky in Cygnus and eventually stubmled upon the asterism, not realizing it corresponded to Cyril’s object found back in 2000. Once the identification was made, Cyril decided to contact experienced asterism discoverer Alexandre Renou (France) for his help. To his surprise, Alexandre could find no mention of this asterism! Philipp Teutch (Austria) confirmed the star group to be uncatalogued, and hence deicded to add it to the Deep Sky Hunters (DSH) database, under the designation DSH J2105.6+4639, or Calvet 1! Due to the morphology of the asterism, Calvet 1 has often been referred to as the “Question Mark”.
Figure 2: Sketch of Calvet 1 made by Cyril in 2006. (c) Cyril Calvet
In images from the Digitalized Sky Survey (see figure 3), the asterism more difficult to distinguish from the background star field. However, once the asterism is spotted, it is difficult to see anything else! 🙂
Note that the most west-maying stars of this asterism are a group of three blue stars, between mag +8 and +10. It is perhaps possible that the two brighter stars are part of a true binary system (based on Gaia DR2 data astrometry), but it is difficult to be sure. Using Gaia DR2, Bruno Alessi (Brazil) found that Calvet 1 could be an open cluster. Note also that Calvet 1 is located only a few degrees north east of the NGC 7000, or the “North America Nebula”!
Figure 3: A coloured DSS extract showing Calvet 1. At first glance, the asterism is difficult to spot as it blends easily with the back ground stars in these images. However, when comparing this image with figure 2, it is easily noticeable. Image credit: Skymap.org.
Cyril is an experienced astrophotographer, regularly publishing images of all types of celestial objects, including comets, nebulae, star clusters. More information can be found on his blog: http://astronomiecyrille.over-blog.com/
Mul 1 – Planetary Nebula candidate, Possible Strömgren Sphere
Inspired by an image of the “Necklace nebula” (PN G054.2-03.4) captured by the Hubble Space Telescope (HST), amateur astronomer Lionel Mulato (France) decided to image the same object using his own equipment. He started with an Halpha exposure of PN G054.2-03.4, in November of 2013. In the resulting exposure he quickly noticed a faint and diffuse 2′ sized nebula, only 0.5° distant from PN G054.2-03.4. Despite the excitement, Lionel first reasoned that the nebula could be an artefact. However, this was ruled out when he was able to recover the object in plates from the Digitalized Sky Survey!
Figure 4: Extract of the original Halpha + OIII + OIII (HOO) Wide_field image enabling the discovery of Mul 1. Mul 1 is the red smudge to the left, while the Necklace nebula appears in the upper right quadrant. (c) Lionel Mulato.
Now that Lionel knew the nebula was real, he decided to image the object using an [OIII] filter, to test appearance at that wavelength. Interestingly, the object was completely absent in those (see figure 4 and 5)! As Lionel was unable to identify the object in any online catalogue, he contacted Pascal Le Dû (France), who then suggested he should contact Agnès Acker (France) as it could possibly be considered a planetary nebula candidate. Agnès confirmed the nebula to be unknown, and suggested that it might possibly be a Strömgren sphere rather than Planetary nebula, due to the lack of any obvious OIII signal in his images. Despite this, Agnès designated the nebula as Mul 1 and listed it as a possible Planetary Nebula candidate in the French Database of Planetary Nebula discoveries. Wether the nebula is a Strömgren sphere or a Planetary Nebula, no central star of Mul 1 has been discovered, so far.
Figure 5: Crop of the original discovery image centered on Mul 1. The object’s red colour is due to its strong Halpha emissions (while being absent in OIII). (c) Lionel Mulato
Mul 1 is also clearly detectable in Halpha images from the INT/WFC Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS), see figure 6. In fact, the nebula was independently spotted by Sabin et al. (2014) using the IPHAS Halpha exposures. Their team classified the nebula as interstellar matter, and designated the object as IPHASX J194422.5+165605. Mul 1 is also apparent in Mid-Infrared data from the WISE satellite, similarly to many Planetary Nebulae.
Figure 6: Mul 1 as seen in an Halpha image extract from IPHAS. Image credit: INT/WFC Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS).
Lionel is an experienced astrophotographer, and discoverer of several planetary nebula candidates. He is perhaps most famous for his work on Wolf-Rayet stars. More information on his work can be found on his website: https://sites.google.com/site/lionelmulato/home
IRAS 18322-1921 – Mira Variable Star
On June 19th, 2012, amateur astronomer Amar Sharma (India) decided to photograph Pluto to test its appearance in unfiltered images aqcuired using his new CCD camera (model: SBIG ST8XME). In order to spot Pluto in his images, he compared them with archival Digitalized Sky Survey plates (Red filter), as Pluto is absent in those, at that location. Coincidentally, he noticed a bright transient in his image (CV= +11.5 – 12 mag) that he first mistakened for Pluto. However, once he compared his images with one he took a couple of days later (June 21st, 2012), he realized that what he had found was not Pluto. Indeed, Pluto was in fact 4′ south of this object!
Figure 7: The two unfiltered CCD images taken by Amar Sharma, clearly showing IRAS 18322-1921 (red crosshairs) and the motion of Pluto against the background starfield (green crosshairs). It was these images, along with a Digitalized Sky Survey plate which enabled the discovery of the variability of IRAS 18322-1921. The bright star to the lower-right is HIP 91135 (c) Amar Sharma.
While Pluto had clearly moved during the time both Amar’s photographs were taken, the transient he first spotted had not (see figure 7). It remained just as bright in both his images. Amar hence concluded that he had stumbled upon a deep sky transient, possibly an unknown Nova. He was unable to find any mention of it on the American Association of Variable Star Observers (AAVSO) websites, neither did he find any match within the General Catalogue of Variable Stars (GCVS).
Figure 8: Image extracts demonstrating the variable nature of IRAS 18322-1921. By comparing an image Amar himself (far right), with one of the Digitalized Sky Survey (far left), Amar discovered the star’s high amplitude variability. (c) Amar Sharma, DSS Plate Finder and the SuperCosmos Halpha Sky Survey.
Amar soon reached out to amateur and professional astronomers worldwide for opinions and follow-up observations in regards to his potential Nova discovery. Alan Hale (USA) visually estimated the transient to be about V= +13.5 mag on June 26th, 2012. Amar also contacted Dan Green, of the The Central Bureau for Astronomical Telegrams, (CBAT), which eventually lead to a mention of the object on the Transient Objects Confirmation Page (TOCP).
The Himalayan Chandra Telescope (HCT) telescope obtained a spectrum by June 27th, in which G. C. Anupama interpeted the spectrum as that of a possibe a Red dwarf star. This suggested Amar’s transient to be a flare star, undergoing an eruption. She added however that the faible presence of Hydrogen Balmer lines (Halpha emissions, for instance) in the spectra could be in favour of a Mira star. The latter classification was confirmed by Dan Green and Brian Skiff (CBAT) based on the ASAS-3 light curve of the object (see figure 2). They also confirmed the variable star to be unreported. The star has been designated IRAS 18322-1921, and is the first modern-time astronomical discovery made in India!Figure 9: The light curve which enabled to confirm the Long Period Variable (LPV) nature of IRAS 18322-1921. Image credit; The All Sky Automated Survey III (ASAS-3).
Indeed, the data indicated that the star displayed periodic high-amplitude variations in brightness, with a period of 270 days. The data also indicated a maximum visual magnitude of V= +12.6 mag, and a minimum below V= +14.5 mag (see figure 2). Indeed, the object appeared between R= +16 mag and R= +17 mag on the DSS Red plate, probably meaning that the DSS plate shows the star near its minimum brightness.
Figure 10: ASAS-SN light curve of IRAS 18322-1921. Image credit: Jayasinghe et al. 2018.
In 2018, the results from an automatic search for variable stars performed by the ASAS-SN team was published, in which IRAS 18322-1921 had been detected too. ASAS-SN data suggests that the star has a pulsation period of 280 days, rather than 270.
I wish to thank Amar Sharma and Cyril Calvet for the information they provided on IRAS 18322-1921 and Calvet 1, respetively.
Cyril Calvet, Nouvel astérisme dans le Cygne, Available at: http://www.astreschevelus.fr/articles.php?lng=fr&pg=67
Lionel Mulato, Découverte d’un nouvel objet inconnu : Mul 1, Available at: https://sites.google.com/site/lionelmulato/Imagerie-CCD/mu1
Amercan Association of Variable Star Observers (AAVSO) and The Variable Star Index (VSX).
Private communication with Amar Sharma and Cyrille Calvet.