American Association of Variable Star Observers (AAVSO)
Thu, 11/19/2020 - 01:48

NSV 18024 is a UG star in Vela with a range of 10.0 -14.1V. I added this star to my list over a year ago. The variable star plotter showed the object combined with an 8.13V mag. star. I observed this star for a while not knowing what to expect and on January this year, I detected an outburst visually at 13.4 and clearly separate from the 8.13V mag. compassion 7" away. Visually a very blue object.

I followed the object fading over a number of nights to a minimum of around 16.6 of which I have constantly observed. Now for the confusing part.  The Gaia entry shows that the 14th mag. minimum value is a mean from 27 observations. Images obtained by J. Espinoza through clouds with the SMARTS 1.3m telescope at Cerro Tololo on 2008 Apr 25.064 show that HD 74771 has a blue (B-V ~ 0) companion at V ~ 11, located 6".8 WNW of the 8th-mag HD star. Spectra obtained during the 2008 April outburst confirmed strong, broad emission lines in the blue companion.

The DSS image shows it lost in the 8th mag. star. Sebastion found the star clearly visible in an ultraviolet image of the SkyMapper survey, so after speaking with Sebastion we both agree on the position and no other bright object nearby.

So why are there many observations recorded at mag. 14.0 is a mystery for now. Maybe a UGZ in standstill around 2015? Outbursts recorded at 10.0 mag. points to a UGSU? We need to determine its type and the lack of survey data due to the presence of the nearby star complicates things. This object deserves many more observations. It would be good if someone was able to get a V image of the object at minimum if that is at all possible.