KID 03644542 unscheduled dimming

American Association of Variable Star Observers (AAVSO)
Mon, 07/06/2020 - 15:05

I observed KID 03644542 about 12 hours ago and it was noticably dimmer than the comparison star 84

This has only been the case when it is eclipsing?

However it is not supposed to be due for an eclipse until August 16

This is only one observation and it was visual, however KID 03644542 was unequivocally dimmer than the 8.4 magnitude comparison star. I am now presented with whether KID 03644542 is dimming when it is not supposed to, or the comparison star has become brighter than it is supposed to be or I made some error like looking at the wrong stars. I will of course look at it again now that I have noticed this. Thoughts?

American Association of Variable Star Observers (AAVSO)

Good catch!  And good question.

I would say definitely observe it again (several nights), ASAP!  It would be great if you can get it tonight.

It is an interesting star, because it is an eccentric binary with the secondary eclipse greatly displaced from 50% phase.  It is at phase 0.820, i.e. 0.180 cycles before a primary.  Thus a secondary eclipse will occur 0.180 x 119.68 = 21.54 days before a primary.  A primary is expected at 245 9078.22 (from VSX, which only lists primary eclipses), less 21.54 d means a secondary is expected at 245 9056.68 = July 26 at 0419 UT.  So, that is closer to your observation, but still quite a ways off.

Another reason to observe it again, is because all the other observations were by one other observer, on two back-to-back nights, using different technology than you.  Your significantly different observation raises a flag, but as you acknowledge, there could be some other "systematic" difference.  

I would be suspicious of my own observation (I am a CCD observer), in such a situation, and seeking verification by further observations.

Gary Billings


American Association of Variable Star Observers (AAVSO)
Kepler and Hipparcos data

I checked the elements with more Kepler data and they were basically okay. Also there is an eclipse in Hipparcos data from 30 years ago so the period is well-constrained. The ephemeris changed slightly to July 26 at 04.03 UT for Min II but that doesn't affect your observation.
The light curve doesn't show anything unusual at other phases.


American Association of Variable Star Observers (AAVSO)
My R-band value from UT 2020 Jul 07 is brighter than catalog

I measured this star this morning, UT 2020 Jul 07 08:35, using a CCD and R-band filter.  Using the two stars marked "98" and "99" in the AAVSO chart, and their UCAC4 r-band magnitudes, my measurements indicate that KID has an r-band magnitude of about 8.6.  That is about 0.4 mag _brighter_ than the UCAC4 magnitude for this star, which is 9.1.

The comparison star "84" is close to the edge of my field of view, in a region where vignetting is strong, so I don't put much weight in the following statement, but ... KID appears about 0.15 mag fainter than star "84" in my R-band images.

I'm not sure whether these measurements support the claim of a dimming, or not. 

American Association of Variable Star Observers (AAVSO)
84 comp star is actually 81 (1" double)

Hi Michael,
I see that you mixed R and r band magnitudes in the same post and they are not the same, I guess your R magnitudes are Rc while the r magnitudes are Sloan.
But you also mention "UCAC4 r-band magnitudes" and there is no such thing. The UCAC4 bandpass is between V and R. The r magnitude you quote (9.08) comes from APASS (the UCAC4 VizieR entry displays APASS data in B, V, r and i) and it is saturated, see that V is also 9.05. So the actual r-band magnitude is around 8.2.

About the 84 comp star, I checked and unfortunately its magnitude is wrong. This is surely the cause of the original report by John. It is a 1" double with a combined magnitude of V= 8.10. I will update its values in VSD.

Thanks for your input, mistery solved!

American Association of Variable Star Observers (AAVSO)
KID 03644542

It has been very stormy by me recently, but I finally was able to take a second look. I can confirm that this inconsistency is likely a result of the comp stars former magnitude being incorrect. I will edit my prior observation for posterities ease of accuracy. 

I compared it to the further away 78 and 90 comparison stars in addition to the formerly 84 but now 81 star, HD 183078, and found KID 03644542 to be much closer to the expected magnitude of 8.3. I am still finding the star dimmer than what it is supposed to be right now, however, I am well within the range of error based on my method. The only way I could be sure is by taking many more observations over time. 



the formerly 84, but now 81 comparison star in question:

Thank you all very much for your help.