Proposal #582

In this proposal, I ask for observation time with the AAVSOnet to perform weekly photometric measurements of TU Tau (HD 38218, HIP 27135). This proposal is a suggestion from the Spectroscopy Discussion Group, which includes +20 professional and amateur astronomers. Dr. Richard O. Gray is the first author of the referenced paper cited below and PI of the TU Tau’s research project for which this proposal is submitted to the AAVSOnet as continuing data collection is needed.

TU Tau is a binary system consisting of a C-N carbon star primary and an A-type secondary, with a magnitude range between 7.7 - 8.8 V., classified as an SRB variability type object in VSX, with a mean periodicity of 200.2 days. Its J2000.0 coordinates are: 05 45 13.73, +24 25 12.4 (86.30721, +24.42011)

The VSX SRB variability type encompasses semi-regular late-type (M, C, S or Me, Ce, Se) giants with poorly defined periodicity (mean cycles in the range of 20 to 2300 days) or with alternating intervals of periodic and slow irregular changes, even with light constancy intervals. Every star of this type may usually be assigned a certain mean period (cycle), which is the value given in the catalog. In several cases, the simultaneous presence of two or more periods of light variation is observed.

Gray et al. (2023) reported TU Tau's new photometric and spectroscopic data which tracked the recent disappearance of the A-star secondary. The dimming of the A star was gradual and irregular, with one or more brief brightenings, implying the presence of non-homogeneities in the carbon star outflow. The authors presented evidence that the A star is actively accreting s-process enriched material from the carbon star and suggested that it will eventually evolve into a Barium giant. This is also an important system because the A-type star can serve as a probe of the outer atmosphere of the carbon star. It is the understanding of the group that TU Tau needs to be studied at higher resolution and over a longer time. For continued monitoring of this system, multi-filter photometry of TU Tau is desired while its spectroscopy is performed by several of the co-authors of Gray et al., 2023 paper. Although TU Tau has been (irregularly) observed over 6,185 times by 15 AAVSO contributors, this proposal aims to observe it continuously and methodically.

Target: TU Tau (HD 38218, HIP 27135).
Cadence: 1x/week.
Filters: B-V-Rc-Ic; at least 3 frames/filter for each observing session.
Suggested exposures (desired SNR >200):
B filter: 90 seconds
V filter: 45 seconds
Rc filter: 10 seconds
Ic filter: 5 to 10 seconds
The suggested exposures relate to the fact that the TU Tau fluxes in the B band are very different from those in the Rc and Ic bands. Depending on the instrument AAVSOnet will use, this might require some testing.

As for the comparison and check stars, it is worth mentioning that in the paper cited below, TYC 1866-2430-1 was the comparison star, and TYC 1866-990-1 was the check star. It might be necessary to see if those stars are in the field of the AAVSOnet telescope to be chosen.

The resulting color-transformed data will be uploaded to the AID.

Reference:
Gray et al., (2023). TU Tau B: The Peculiar “Eclipse” of a Possible Proto-barium Giant. The Astronomical Journal, 166: 161