Near-IR spectrum of FUOr candidate NWISE-F J213723.5+665145

ATel #14035; Michael Connelley (U. of Hawaii), Bo Reipurth (U. of Hawaii)
on 21 Sep 2020; 22:11 UT
Credential Certification: Michael Connelley (msc@ifa.hawaii.edu)

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Introduction NWISE-F J213723.5+665145 is an object in a dark cloud in the Cepheus star forming region that appears to have undergone an outburst between 2011 and 2013. Lynne Hillenbrand has written an excellent summary of what is known about this object here: https://www.astro.caltech.edu/~lah/cephneb/. A new nebula created by this outburst was first reported in ATel #13832, and further described in ATel #13834 and ATel #13856.

Observations NWISE-F J213723.5+665145 was observed by the IRTF with SpeX on the night of July 11, 2020. For the guider imaging, we used a 9 point dither pattern with 1 minute exposures at H and K-band. Ten minutes of integration time at H-band did not detect the object. Our K-band image is consistent with the image shown by Hillenbrand in ATel 13856, and we observed the object to be at magnitude K=15.9. We also took a low resolution (R~150) prism mode spectrum of the object, using 7.5" beam switches between our guide boxes. We took 10 three minute long exposures for a total integration time of 30 minutes. Wavelength and flat field calibration was done using the SpeX calibration lamps, and telluric correction was done with a nearby A0 standard star. The spectrum has a peak S/N=20 at 2.3 microns

Discussion The spectrum shows a red and fairly featureless continuum from H to K-bands, with negligible signal short of 1.5 microns. Apparent spikes in the spectrum near 1.8-1.9 microns are likely noise as the atmosphere is very opaque in this region. Regions of strong telluric absorption are shown in gray. The apparent emission line near 2.05 microns is also likely false as there are no commonly observed emission lines in YSOs at that wavelength. We do not see any of the common YSO emission lines (e.g. [FeII] at 1.64 microns, H2 at 2.12 microns, Brackett gamma at 2.16 microns). There is a break in the continuum at 2.3 microns at the start of the CO band heads. CO does not seem to be in emission, and the break in the continuum near 2.3 microns suggests that CO may be in absorption.

The spectrum of the object, overlaid with a spectrum of FU Ori reddened by Av=50 magnitudes for comparison, can be seen here: http://irtfweb.ifa.hawaii.edu/~msc/look_here/outburst_spec.pdf

Based on this spectrum, and the light curve of this outbursting YSO, we believe that this object is most likely a FUor. The near-IR spectra of FUors lack emission lines and have strong CO absorption, and our spectrum of this source is consistent with other near-IR FUor spectra (Connelley & Reipurth, 2018). In fitting the observed spectrum with the spectrum of FU Ori and adding extinction, we find that the visual extinction is about 50 magnitudes. Based on the observed spectrum, and because this object has remained bright in the 7-9 years since the initial outburst, this is likely a FUor event seen through very high extinction.

Near-IR spectrum of NWISE-F J213723.5+665145