AAVSO Alert Notice 785 announces a stellar variability observing campaign on V452 Vul (= HD 189733, host of exoplanet HD 189733b). Please see the notice for details and observing instructions.
There are threads for this campaign under the following forums:
Please subscribe to these threads if you are participating in the campaign so you can be updated. Join in the discussion or ask questions there!
Many thanks, and Good observing,
Elizabeth O. Waagen, AAVSO HQ
The alert notice indicated the need for monitoring of this star. Besides being an exoplanet host, it is also a modestly-active K dwarf. Any activity outside the planet transits is from spottedness and due to the rotation of the star. The contrast of the spots against the immaculate photosphere declines as you go to longer wavelengths. In 2015 and 2016 I got series in V over each season that confirmed the published rotation period; the full amplitude was only 0.02 - 0.03 mag. The period near 12 days is different than the orbital period of the planet. If you have a dataset spanning at least a few weeks with visits on a good fraction of those nights, try fitting periods between 10 and 15 days to see what you get. If your precision and night-to-night consistency are good enough, the rotation period should be present in the data!
I Have a newbee question ; it asked visual observation of V452 VUL, theoritical magnitude is 7.67+-0.03, but in the comparison chart (my N° is X28067OX) there is no comparison star below 8.1. How do you find around 7.7magnitude in results ?
The simple answer is that there aren't suitable comp stars in a 1.0 degree FOV close to V 7.7. For example in the Tycho2 catalog in a 1.0 degree FOV of the target and a V mag range of 6.5 to 8.5, there are a total of 5 stars excluding the target. Only one of these is brighter than the target and it is much too blue to be used as a comp. Of the 4 stars that are fainter than the comp, one is too blue. There are two 8.1 V stars and one of those is near the edge of the FOV and we try to avoid having stars with duplicate labels. The third star is an 8.4 magnitude star and it is about 26 arcmin separation from the target to the NW which puts it near the edge of your FOV. at B-V 0.343 it is also fairly blue compared to the target at B-V 0.931 based on Tycho2 data. The source, SRO(18), actually used, for the comps in VSP has several fewer stars in the V range 6.5 - 8.5 but has good photometry for the full J-C filter set. Which is important for detailed study of this star's variability.
Another issue is that even though Alert Notice 785 encourages any type of observation, Brian Skiff's comments about the nature of this star's variability from causes other then EP transit, describe variability that would probably not be detected by visual observation. Full amplitude (max-min) of 0.02 to 0.03 in V is darned hard to notice and essentially impossible to quantify by by eye. Therefore criteria for comp selection are oriented toward observation by camera/PEP.
If you read Alert Notice 785 carefully the following paragraph give clues that this target is better suited to Camera/PEP observation:
"All types of observations - CCD, DSLR, PEP, visual - are welcome. All filters and cadences are welcome. As Dr. Zellem states: "We are happy to take any variable star measurements that we can get! But the Hubble/WFC3 observations will be taking place in the near-IR (~1.1-1.7 um), so ideally filters closer to this wavelength range would be great. However, as stated above, we are happy to take all the data we can get, regardless of the filter and/or camera."
I hope this clarifies the reasoning behind the selection of comp stars for this target.
Brad Walter, WBY
AAVSO sequence team member