Proposal #674

Proposal: Semi-regular variables with ambiguous or multi-periodic behavior, studied via BVRcIc color–magnitude trajectories over multiple cycles.
In this proposal, I request observation time within AAVSOnet to perform weekly Johnson-Cousins BVRcIc photometric measurements of 10 semi-regular variable stellar targets.
Semi-regular variables are giants or supergiants of intermediate and late spectral types that show noticeable periodicity in their light variations, often accompanied by, or sometimes interrupted by, various irregularities. Periods lie in the range from 20 to >2000 days, while the shapes of the light curves are rather different and variable, and the amplitudes may be from several hundredths to several magnitudes (usually 1-2 mag. in V). In several cases, the simultaneous presence of two or more periods of light variation is observed.
While extensive single-band visual and CCD observations exist for many of these stars, systematic multicolor photometric studies remain comparatively scarce. I propose to conduct a homogeneous BVRcIc photometric analysis of a carefully selected sample of ten semi-regular variables with reported periods ranging from approximately 25 to 260 days.
Using sparsely sampled but long-baseline observations obtained with the AAVSOnet facilities, I will investigate long-term light curves, color–magnitude relations, and cycle-dependent color evolution. Particular attention will be given to the morphology and repeatability of color–magnitude trajectories, which shall provide diagnostic insight into the relative roles of temperature variation, atmospheric effects, and circumstellar extinction.
Sparse-cadence, multicolor CCD photometry is a powerful tool for probing the physical mechanisms governing semi-regular variability and for refining the characterization of semi-regular stars beyond period determination alone. The results should highlight the continuing potential for high-impact amateur contributions to the study of late-type stellar variability.
Cadence: 1x/week. Preferably, 3 frames/filter for each observing session.
Filters: B-V-Rc-Ic;
Suggested exposures (I leave it up to the AAVSOnet team to modify these suggestions if needed to achieve a desired SNR >200):
B filter: 10 seconds
V filter: 10 seconds
Rc filter: 10 seconds
Ic filter: 5 to 10 seconds
The resulting color-transformed data will be uploaded to the AID. Any publication(s) derived from this project shall be submitted to the JAAVSO.