PRIMA Astrometry Object and Reference Selection Doc. Nr. VLT-PLA-AOS Astrometric Survey for Extra-Solar Planets with PRIMA J.Setiawan & R. Launhardt

Chapter 1. Introduction Scope of the document To provide a guideline for observers who want to use PRIMA for high-precision differential astrometric measurements on how to select and characterize their target and phase reference stars. Observers should be able to select and characterize their own science target and reference star pairs so that they are both suitable for the specific observing mode and ensure the achievement of the anticipated astrometric accuracy.

Chapter 2. High-precision differential astrometry with PRIMA - Interferometry with the VLTI (especially ATs) - PRIMA and its subsystems (what is PRIMA, star separator,metrology,FSU,DDLs) - High-precision differential astrometry with PRIMA (general description how it works, what is the difference to other astrometry programs) - The need for phase reference stars (how to define and select reference stars) - From the observables to astrometric data (short description about the data reduction and analysis: format, software, etc.)

Chapter 3. Requirements on PRIMA astrometric target and reference stars - Location (telescope site, declination limitations, u-v coverage, baseline, others) - Minimum brightness of target and reference stars (K magnitude brighter than 12 for target, 15? for reference stars) - Color difference between target and reference stars (OPD calibration uncertainty (and astrometric accuracy) as function of color difference) - Angular separation between target and reference stars (minimum separation due to star separator, maximum separation) - Number and relative location of reference stars (dependence of astrometric accuracy on the number of reference stars) Constrains set by the instrument, observing mode and atmosphere

- Parallax and proper motions of reference stars (preferred distant and bright reference stars, e.g. K giants) - Perspective acceleration (under study: Sabine) - Unseen companion (radial velocity survey of reference star candidates is needed) - Chromospheric activity and stellar pulsations (photocentric displacement by the starspots, nonradial pulsations, etc.) - Circumstellar disks, extended/resolved targets Astrometric stability of target and reference stars Chapter 3. Requirements on PRIMA astrometric target and reference stars

Chapter 4. Selection criteria for astrometric target and reference stars This section contains practical selection criteria (numbers and diagrams) based on the requirements and effects described in the previous section. - Top level selection criteria for astrometric targets (brightness, declination, magnitude and time scale of expected astrometric signal,...) - Top level selection criteria and availability of reference stars (diagrams, statistical probability to find one or more ref. stars, list of catalogs, etc.) - Target and goal-specific refined selection criteria anticipated astrometric accuracy vs. angular separation and number of available reference stars, astrometric noise as function activity levels,...

Chapter 5. Recommended selection strategy - Setting the goal, accuracy, and strategy of the anticipated astrometric observations (e.g. Properties of target stars --> which astrometric accuracy) - Defining the specific requirements and selection criteria (from top level criteria + calculation ) - Pre-selecting reference stars from existing data bases (catalogs: 2MASS, USNO, other publications ) - Defining strategy for preparatory observations (photometric and spectroscopic surveys ) - Final characterization and selection of target and reference stars pairs.