Proposal #77

Monitoring SU Lyn

SU Lyn is a pre-symbiotic star, composed of an M5.8III star that dominates in the red wavelengths, along with a WD companion. The red giant has a low amplitude quasi-periodic variation with a period of about 126 days. The WD star exhibits a hard X-ray spectrum and UV variation (Mukai et al. 2016). Contrary to most symbiotic stars, it does not exhibit strong optical emission lines, a defect thought to arise from a lack of hydrogen shell burning. As such, Mukai argues that the true population of symbiotic stars is underrepresented, and that more SU Lyn systems must exist.

Munari and the ANS group have been monitoring SU Lyn for about one year in the B and V passbands. However, working with only one color index, it is not easy to separate what is due to stellar semi-regular pulsations from the contribution by the accreting WD companion. The latter causes continuous changes in the fine details they see in their high resolution spectroscopic monitoring. In collaboration with the ANS group, I would like to request AAVSOnet time for the current season to monitor SU Lyn with observations that extend over the widest wavelength range, ideally u/U-BV-griz. Given the very red color of the star, observations in "u" or "U" bands should be carried out only if the corresponding filters are known *not* to leak in the far red. The u' filter on AAVSOnet telescopes is Astrodon's 2nd generation, and supposedly very well red-blocked. It is possible to observe SU Lyn at griz with BSM_NH and BSM_NM. A BSM system is necessary for the red wavelengths because the star is so bright there. At u, this source may be too faint for a BSM, but it may be doable with reasonable stacking. However, the griz monitoring should be no problem.

In addition, once a month, we would like to search for u/U band flickering using SRO or TMO61. These would be time-series observations taking three hours or more. The X-ray and ultraviolet satellite observations of SU Lyn by Munari clearly show hints of the flickering usually associated with accretion processes onto WDs. However, the observing session for each of these satellite observing runs has been too short to properly characterize the properties of the flickering, properties that would better constrain the origin in the accreting system of the flickering region. One single B band observation at the beginning, end and every hour of the observing run would be helpful to get control of the instantaneous color equation transformations. Even better, if technically feasible, would be continuously cycling between u/U and B filters for the all observing run (like uBuBuBuBuB...). The time length of the ideal exposure would be the shortest one that allows SU Lyn and at least a few field stars to be recorded with good S/N so that the search for flickering (expected in the 0.05/0.15 amplitude range and time scale of (a few) minutes) is not hampered by the noise in the measurements.