Alert Notice 786: Monitoring of V338 Boo requested

July 12, 2022

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Drs. Kenneth Carrell (Angelo State University), Ron Wilhelm (University of Kentucky), and Horace Smith (Michigan State University) request AAVSO observers' assistance in monitoring the double-mode RR Lyr variable V338 Boo. Monitoring should begin now and continue at least through mid-October 2022.

Dr. Carrell writes: "We are requesting follow-up observations of this target to more fully characterize the long-term changes of this double-mode (RRd) RR Lyrae variable star. We published a paper last year (Carrell et al. 2021, ApJL, 916, 12) using Transiting Exoplanet Survey Satellite (TESS) data that showed unique behavior for this star. Because the window of observations from TESS was not long enough (only ∼54 days) we need a longer monitoring campaign to fully describe the long-term changes we are seeing.

"This target was shown by previous AAVSO observers to be a strange, or “anomalous”, RRd type. TESS observations (shown in Figure 1) were able to shed more light on this object, and in particular showed that there were major changes in the pulsation properties of the star over many tens of days.

"We need observations of max light for this star several cycles in a row with gaps of ∼10 days. As can be seen in Figure 1, the light curve of this star changes dramatically over tens of days, but the full period of these changes cannot be seen in TESS data. Therefore, to accurately determine the period of the longer term changes in this star, we plan to use AAVSO follow-up measurements in combination with the TESS results.


Figure 1. The TESS light curve of V338 Boo.

"The target should be observed with a CCD camera, preferably with a Johnson V filter. Consistency of the measured magnitudes is critical, and changes in max light will be several tenths of a magnitude, so photometric errors of a few hundredths of a magnitude or better are necessary.

"The preferred cadence of observations would be to image the star with a spacing of 1-3 minutes (maximum of 5 minutes) between exposures for as long as the star is visible each night, for several continuous nights. Gaps of roughly 10 days to 2 weeks between 3 (+/-) nights of observations over the course of 2 or 3 months should allow us to fully characterize the overall changes."

Coordinates (J2000.0): 15 46 26.14 +44 18 47.2  (from VSX page for V338 Boo)

Please submit observations to the AAVSO International Database using the name V338 BOO.

Figure 2 is the AAVSO finder chart for V338 Boo with a 20’ FOV. The table below lists the requested stars for comparison. Finder charts including these comparison stars may be created using the AAVSO Variable Star Plotter (VSP).

Comparison Star AUID Label V B-V Comments
000-BKT-600 130 12.971 +/- 0.018  0.631 +/- 0.033 PREFERRED
000-BKT-602 136 13.577 +/- 0.023 0.708 +/- 0.045 Secondary
000-BKT-603 140 14.031 +/- 0.024 0.576 +/- 0.035 Secondary
000-BKT-598 114 11.427 +/- 0.024 0.957 +/- 0.041  




Figure 2. AAVSO finder chart (20' field of view) with comparison stars for V338 Boo.

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using material provided by Dr. Carrell.


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