Alert Notice 859: Photometry and spectroscopy requested for bright binaries being observed with CHARA - Revised

Note: The Alert Notice text below has been revised from the original version issued April 18, 2024.  - Elizabeth O. Waagen, April 19, 2024

Observers are also reminded that it is essential to use the comparison and check stars indicated for the photometry target. If they are too far apart to fit into the same field of view as the target, use PEP-style observing, that is, alternate images of variable, comp, and check.

April 22, 2024                                            REVISED VERSION

AAVSO Forum threads (scroll to the bottom of a thread for latest posts):
- Campaigns and Observation Reports: https://www.aavso.org/chara-bright-binaries-photometry-spectroscopy-2024

Please subscribe to this thread if you are participating in the campaign so you can be updated by the astronomers and by HQ. Join in the discussion or ask questions there!

Juraj Jonák and Dr. Denis Mourard (Observatoire de la Cˆote d’Azur) have requested AAVSO observers' assistance in obtaining photometry and spectroscopy of several bright detached binary variable stars in support of their study of these targets.

INTRODUCTION

Jonák writes: "With the recent installment of the CHARA/SPICA interferometer, located at Mt. Wilson, CA, USA we aim to observe a large sample of bright detached binaries in order to determine their visual trajectories. The goal is to use a robust combined model including interferometry, spectroscopy and/or photometry, which will open access to a wide range of direct mass determination and fundamental ingredients of the stellar evolution models.

"Fundamental stellar parameters are the primary data required for an in-depth under- standing of stellar evolution, interiors, and environments...Spectroscopic binary systems provide direct information on stellar mass and structure from the mutual interaction via radial velocity of orbital motion. However, this method is highly dependent on the inclination of the orbit, which cannot be determined purely from spectroscopy...This dependence can be alleviated by combination with observations of eclipses...In fact, the method of simultaneous modelling of spectroscopy and photometry has been a common method since the 1970s. A different approach is utilising interferometric measurements. Although these don’t necessarily require viewing the system edge-on, they demand other constraints on the system...Naturally, the observations are not mutually exclusive, and in fact, using all of them can improve our understanding of the system.

"The SPICA team has 5 nights per month allocated for observing with the CHARA/SPICA instrument from now through July 2024. However, as our goal is to study the entire orbit, simultaneous observations are not required or preferred."

OBSERVER INSTRUCTIONS

Observations are requested now through July 2024. Updates to observer instructions will be made via the forum above, so please be sure to subscribe to it.

Targets requiring Photometry and Spectroscopy

Photometry
Photometric observations are requested for the 4 eclipsing binaries (Table 1) during their respective primary and secondary minima. CCD/CMOS and PEP observations are preferred.
Filters: To determine the orbital period, measurements in one standard filter (Johnson or Cousins, preferably Johnson V) are sufficient. Observations in multiple filters are welcome as they will give further constrains on the stellar parameters.
Cadence: For eclipses, requirements are a time series of at least 10 points and additional 5 points outside the eclipse to establish the magnitude level. To properly model the stars, three complete primary minima observations are needed, and, if present in the system, two secondary minima should suffice. We want to establish the light curve over several orbits, with focus on the eclipses.

Notes to CCD/CMOS observers: For targets brighter than 5th magnitude, please stack images to a minimum of 10 seconds to reduce scintillation. For all targets, be careful not to saturate. PEP-style photometry will be required because of the location of the comparison and check stars; it is essential to use these comp/check stars so datasets from different observers will be homogeneous.

Spectroscopy
As it is necessary to be able to determine the radial velocity of the two components, spectral resolution of at least 10,000 is required and preferably in the vicinity of a strong line (Hα or Hβ). Resolution below 9,000 will not be usable for this campaign.
Cadence: As the plan is to reconstruct the orbit, 20 spectra covering the entire orbital period are necessary. Non-uniform spacing of observations is requested to prevent aliasing; do not observe at intervals which are multiples of the period.

Table 1. Photometry and spectroscopy targets