Appearance of a New Nebula in the Constellation Puppis after an Outburst in 2015 around WISEA J075915.26-310844.6

Pre-main sequence stars, such as FU Orionis or EX Lupi variables can have intense outbursts of several magnitudes in optical/near-IR, with FU Orionis types having long outbursts of years or decades and EX Lupi types having more frequent outbursts lasting months or years (Reipurth 1990; Principe et al. 2018). These outbursts are interpreted as short and intense mass acrretion events, but the underlying mechanism is not well understood (Principe et al. 2018). FU Orionis variables have a spectrum similar to G-K supergiants in the optical and K-M supergiants in the near-IR (Kenyon et al. 2000). After the outburst the central object can be surrounded by a reflection nebula, such as around FU Orionis (Wachmann 1954) or around V1647 Ori (McNeil's Nebula McNeil et al. (2004)).

I searched for brown dwarfs for the Backyard Worlds project (Meisner et al. 2020), using CatWISE and WiseView, which plays a time-resolved animation of the unWISE images (Caselden et al. 2018; Meisner et al. 2019; Eisenhardt et al. 2020). WiseView is crucial to confirm the movement of high proper motion objects in my search, but it can also be used to confirm changes in brightness. To find the faintest objects I applied low snr-filters for CatWISE, which leads to the discovery of new objects, but it is required to search in a sea of false-positives. Common false-positives are variable objects, which are sometimes confused as proper motion objects in CatWISE.

In my search and at the coordinates R.A.: 1198136128, decl.: −311457397 (WISEA J075915.26-310844.6) I found a point-source increasing significantly in brightness. After looking at archived DECam images I found a nebula around this object. I was not able to find any nebula in DSS2, so I looked for deeper exposures. Neither VST, nor PanSTARRS images did show the nebula in pre-outburst images. The nebula is located about 088 distant from χ Puppis in south–southeast direction and has a size that is similar to the size of McNeil's Nebula.

The central source was identified with machine-learning (artificial neural network ANN, see Vioque et al. 2020) and a probability of 81.4% as a pre-main sequence star, and as a variable based on Gaia DR2 uncertainties in the G-band (Vioque et al. 2020). VPHAS+ DR2 shows pre-outburst (2012) AB magnitudes of 22.5 (g), 20.0 (r), 18.8 (i) (Drew et al. 2014), the pre-outburst (2013) PanSTARRS DR1 AB magnitude is 19.2 (i) (Chambers et al. 2016) and DECaPS DR1 shows post-outburst (2017) magnitudes (mean_mag) of 20.4 (g), 17.2 (r) (Schlafly et al. 2018). The pre-outburst i-band magnitudes seem to vary with <0.5 mag. The pre-post g, r magnitudes show an increase in brightness of 2–3 magnitudes.

The mid-infrared NEOWISE-R (Mainzer et al. 2014) single-exposure brightness measurements were searched within 2'' of the above position in IRSA and filtered with w2snr > 40. The resulting light curve can be seen in Figure 1. At pre-outburst brightness (2014 November 06) the source has magnitudes of about W1 ∼ 11.6 and W2 ∼ 9.4. AllWISE shows fainter magnitudes of W1 = 12.1 and W2 = 10.3 (Cutri et al. 2014). At 2015 April 23 the brightness increases significantly to magnitudes of W1 ∼ 6.7 and W2 ∼ 4.9, until reaching a maximum at around 2016 November 4 with W1 ∼ 3.6 and W2 ∼ 1.5. This maximum is still present at 2019.8 based on WiseView.

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The nebula surrounding the central source is shown in Figure 1. The nebula is located close to the galactic plane (b ∼ −079) and has a size of about 1' × 1'. Here I show only i-band images in a gray-scale to be able to compare to pre-outburst i-band images, but there are also images of the nebula from DECam in the g-, r-, z- and Y-band from 2017 available at the Astro Data Lab³ . The images were taken as part of the DECaPS survey (Schlafly et al. 2018). The colored image can also be seen in DECaPS images of the legacy viewer⁴ . The nebula and the central source have a red appearance and both are likely affected by dust clouds in front of them. The nebula has an overall triangular shape. Closer to the central source the nebula shows a more complex structure with two cavities toward east and west. After inspecting a DECam gri-image I found that the southern part of the nebula is redder than the northern part. The i- and z-band images from 2018 show a nebula that is fainter and in the g- and r-band images from 2018 I could not find any trace of the nebula.

Pre-outburst images from ESA Herschel SPIRE (Griffin et al. 2010) show a bright elongated object at the position of the central source, with the brightest part roughly overlapping with the central object. Nearby possible star-forming regions in the SPIRE images are toward the east: a bubble around CD-30 5422; and toward the north: a nebulosity around the open cluster [FSR2007] 1336 (Froebrich et al. 2007). Both are about 03–04 distant.

Here I report the 2015 outburst of WISEA J075915.26-310844.6 in a possible FU Orionis/EX Lupi event. The outburst is long (∼4.4 years) and is accompanied by the appearance of a nebula, which should be a reflection nebula. Based on non-detection of the nebula in PanSTARRS images and the NEOWISE light curve, the outburst happened between 2015 February 2 and 2015 April 23. The outburst has an amplitude of about 8 mag in W2 and can be compared with a recently discovered FU Orionis event in archived mid-infrared data (Lucas et al. 2020). It is desirable to acquire a spectrum of the central source. Additional near-IR images are also desired to confirm a persistent presence of the nebula.

The figure was created with SAO Image DS9, Microsoft Excel, WiseView and Adobe Photoshop Elements.

Facilities: CDS - , IRSA - , NEOWISE - , Herschel - , ESO:VST - , PS1 - , Blanco. -