Proposal #170

Title: Imaging the EA/RS System V818 Tau; A collaborative Project

Member Name and Observer Code: Edward O. Wiley, WEY

Summary: V818 Tau (HD 27130) is a Hyades cluster EA/RS binary with G6V and K8V stars varying from 8.3-8.45 V-magnitude (VSX). McClure (1982: A. J., 254: 606-6115) analyzed the system in an effort to derive a distance to the Hyades Cluster based on photoelectric observations of two minima under what was described as less than ideal conditions. One of the comparison stars used is now known to be a 0.01 variable. Since 1982, estimates available in the Gaia catalogue have supplanted other distance measurements for many systems, including V818 Tau (46.66 ± 0.30 pc). Although it would be interesting to use this system to derive an independent distance measure, that is not the main thrust of the proposal. Rather, the thrust is model the system with new radial velocity data and to demonstrate the utility of the BSM system to gather critical data of EA/RS variables for use in modeling.

The current light elements are HJD = 2452500.0471 + E x 5.6092159 based on 5 minima (Kreiner, 2004 et seq., Acta Astronomica, 54: 207-210). The period does not appear to be changing. Two sets of improved radial velocity curves have been collected for the system (Dr. Bob Nelson, pers.com), affording an opportunity to better model the system and perhaps provide an independent estimate of its distance if a full light curve and be obtained. Radial velocity data are critical as the eclipses are not total. This proposal is directed toward this goal using both the BSM Network to capture minima and the facilities at my observatory to collect both data for observable minima and other parts of the light curve.

Collection and use of the data: These data will be used to fully characterize the light curve of V818 Tau using both the BSM resources (BSM_TX is suggested as suitable) and observations made with my 80mm refractor in BVIc at the TexaCan Observatory, Texas (30.98N, -98.94W). My observatory will be used to fill in as much of the light curve as possible by continuously monitoring the variable each clear night. Note that observatory obstructions prevent me from carrying out three of the four eclipses shown in Table 1, thus the need for collaboration with BSM. My local observations will be calibrated and transformed. The BSM_TX data comes calibrated; Differential photometry with transformation will be performed using both VPhot and LesvePhotometry (if the FOV contains all three comp stars and I can perform ensemble photometry). Both sets of data (local and BSM) will be made available to Dr, Bob Nelson for modeling (I am not qualified to model eclipsing binary systems but would like to learn, this affords an opportunity). It is unlikely that the Gaia distance to the Hyades Cluster will be challenged by any results obtained from this study (McClure’s original goal). If successful the project will result in a refined model of an interesting binary system, perhaps including characterization of the RS behavior, and demonstrate the utility of a small refractor in obtaining data from bright eclipsing binaries suitable for modeling. All data will be uploaded to AAVSO.

Note: If we can get started with the 24 December time series, I will immediately evaluate the eclipse data and share the results with Dr. Nelson to insure that the data are suitable.

Request images be sent to VPHOT: Yes

Check Variable Star Index: A search of databases (e.g., ASAS-SN) did not contribute additional data from which times of minima might be derived to refine the period.
Target(s) name verification to ensure name is listed. V0818 Tau (000-BCX-284)

Target Sky Coordinates: (J2000) 04:17:38.64 +16:56:52.3

Target V magnitude range: 8.30-8.45

Appropriate comparison stars with standard photometry: Chart X25685ND: 66 (000-BBG-758), 83 (000-BBG-762), 88 (000-BBR-498)
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Times for observations:
JD UT Date UT ToM UT Begin UT End ToM CST
2459208.669 25Dec2020 04:03 01:30 06:30 24Dec2020-- 22:03
2459219.888 5Jan2021 09:18 07:50 11:50 5Jan2021--03:18
2459236.715 21Jan2021 05:10 03:40 07:40 20Jan2021--23:10
2459264.761 19Feb2021 06:16 04:45 08:45 19Feb2021--00:16

Cadence: This is timing of minima for an eclipsing binary. I request 1-minute cadence with timing of 20 seconds for B, and Ic and 10 seconds for V. So: BVIc-delay 1 minute-BVIc-delay 1 minute- etc. Duration of the eclipse is approximately 2.6 hours, imaging beginning 2.5 hours before and after the onset of eclipse to catch and RS activity in insure the shoulders are captured in sufficient detail. In addition, I request nightly monitoring consisting of a single set of BVIc images taken near the meridian.
Filters needed in the session sequence: BVIc

Number of images to be taken in each filter: The number of images per session depends on overhead. The maximum number would be 30 images/hour/filter or about 375 imagers over the 4.5-hour time series
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Exposure time in seconds for each filter: 20 seconds for B and Ic, 10 seconds for V. This is based on preliminary imaging done with the 80mm refractor at my observatory (Fig. 1).

The BSM telescopes operated unguided. At this time, we typically limit exposures to no more than 80 seconds.
Start and finish date for the Project: 24 December 2020 – 19 February 2021: four imaging sessions.

[Image available on request]
Figure 1: Photometry for V 818 Tau, night of 6 December 2020. Data from the 80mm refractor and ASI 183mm sensor. Integration times 20s (B, Ic), 10s (V). Accuracy over the three filters, as determine by standard deviation: 0.015 (B), 0.028(V) and 0.031 (Ic). SNR varies between 300-600 depending on filter and airmass.