Infrared Imaging of the New Nebula in Cepheus and Photometry of the Likely YSO Outburst

ATel #13856; Lynne A. Hillenbrand (Caltech); James E. Lyke (W. M. Keck Observatory); Mathias Straube, Andy Sischka, Michael Wenge, and Oliver Schneider (Sternfreunde Ostwestfalen-Lippe e. V, Germany); Manfred Mrotzek (Germany); Matthew Hankins, Kishalay De (Caltech)
on 3 Jul 2020; 07:00 UT
Credential Certification: Lynne Hillenbrand (lah@astro.caltech.edu)

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In Nov 2015, amateur astronomers MS, AS, MW, OS, and MM alerted LAH to their finding of a new nebula near IRAS 21363+6638 in Cepheus. Discovery was made based on optical images taken over 2013-2015 (observing dates 27 Sep, 28 Sep, 24 Oct 2013; 3 Sep 2014; 20 Aug, 21 Aug 2015). Whereas the object was not visible in 2012 images by E. Ivanov, it had appeared by 27 Sep 2013. Link below shows image stack combining 51 hours of 0.35m Newtonian Astrograph data.

On 30 Dec 2015, infrared images were obtained with MOSFIRE on the 10m Keck-I telescope. A 9-point dither pattern was used with single exposures of 8.73s (J-band), 4.36s (H-band), 1.45s (Ks-band). The nebula was also apparent in the near-infrared, extending over 1'. Link below shows infrared color image.

We were motivated to re-examine our data following the report of Stecklum (ATel #13834) regarding mid-infrared brightening of a point source near the same nebula. In 2020 the optical nebula was independently noticed and reported by Borisov (ATel #13832) while searching for comets.

In our images, one of several red point sources appears coincident with the reported mid-infrared position. We derive photometric measurements J > 20.5 mag, H = 18.9 +/- 0.2 mag, and Ks = 16.36 +/- 0.04 mag. Calibration was established based on comparison with up to 14 2MASS sources within 90", the number depending on band and hence faintness vs saturation effects. We are unaware of previous deep imaging to which our detection might be compared. The position of the outburst is refined to 21:37:23.44 +66:51:44.81 (J2000.).

The infrared time domain survey Palomar Gattini-IR (De et al. 2020) provides nightly point source sensitivity to J<14.1. A deep image stack over JD 2458437.6-2458805.7 of total exposure 3240s revealed nebulosity at the source location that extends north and then west, corresponding to the new nebula. Due to low spatial resolution (FWHM~16"), any emission from a point source is likely confused.

There is also no point source detection in the optical time domain surveys PTF (Law 2009) or ZTF (Bellm et al. 2019), which together span 2013-2020. However, the optical nebulosity pattern is confirmed.

The relationship between the large-scale nebular brightening detected in optical, near-infrared, and mid-infrared images, and the outbursting embedded young star, is not yet clear. Optical nebulosity appears in two locations, one just north-northwest of the point source, which is relatively compact, and one ~30" west, which is more diffuse. The near-infrared nebula is continuous, with its brightest and reddest part located between the two optically brightest nebular positions, with some faint correspondence in the optical amateur and PanSTARRS images. The optically brightest parts of the nebula have the bluest infrared color. In the mid-infrared, the brightest nebular component seems consistent with the eastern (smaller, fainter) optical nebula.

Extinction and the orientation of the presumed envelope and outflow cavity associated with the new NEOWISE source are important factors that still need to be estimated.

Regarding overall environment, the region is unstudied beyond having designation as a dark cloud and a dusty molecular core. There are a number of sources at <90" with good Gaia parallaxes, some of which are variable or infrared excess sources and likely young stars. Taking their median suggests a distance of ~900 pc, which is suggested as the distance of the outbursting source as well.

Due to faintness, establishing whether NWISE-F J213723.5+665145 is a true "FUOr Event", as promoted by Stecklum, will be difficult. It should be considered an interesting source for follow-up with faint-object near-infrared spectrographs. Narrow-band imaging of the region is also encouraged.

** images and further information **