Cepheid variables are of central importance in Galactic and extragalactic astronomy. They are the primary standard candles for measuring extragalactic distances, and they provide critical tests of stellar-evolution theory. Surprisingly, however, until now there was not a single Cepheid with a purely dynamical measurement of its mass. Polaris (alpha UMi) is the nearest and brightest of all Cepheids. It offers the unique opportunity to measure the dynamical mass of a Cepheid, because it is in a binary system for which a single-lined spectroscopic orbit is already available. In Cycle 14, we resolved the system in the UV using ACS/HRC, thus providing the first direct detection of the companion, as well as a first approximation to the dynamical mass. In the present proposal we request one HST orbit per year for the next 3 Cycles, in order to refine the visual orbit. Combined with the HST/FGS parallax (see below), this program will provide an accurate mass for the Cepheid (the error should be about 0.5 Msun by Cycle 17), and the only one based purely on dynamical information. Only HST's combination of high spatial resolution and UV sensitivity can achieve this result. The parallax is a key ingredient in the mass determination. In an ongoing multi-year program (GO-9888, GO-10113, GO-10482), we are using the FGS to improve significantly upon the Hipparcos parallax of Polaris. The continued ACS/HRC imaging proposed here (updated to WFC3 following SM4) will thus provide extremely valuable astrophysical information from a very modest additional investment of observing time.

The observations, originally planned for ACS/HRC, will be made with WFC3/UVIS. We will use the FQ232N quad filter, which suppresses the light of Polaris Aa so that it will not saturate. We will take a series of 4-point dithered exposures. One set will be on Polaris A, 0.5 sec each, and a second set also on Polaris A at 0.7 sec each. One extra 0.7-sec exposure is also made, to fill the orbit. According to the WFC3 ETC, even the 0.7 sec exposures should not saturate. We will then move the single star Polaris B to the same chip position and take a 4-point dither set of 45-sec exposures, for the purpose of providing a PSF reference image. We will fill the orbit by taking one more 45-sec exposure on Polaris B. COOL STARS 12 false 0.0 0.0 false false false false false SUPPORTED STAR CEPHEID F3-F9 PULSATING VARIABLE Coordinates are from Hipparcos, but are adjusted so as to place the center of the field of view one-third of the way between Polaris A and Polaris B (which are separated by 18"). This will place both stars within the WFC3 quad filter field of view, as desired, with Polaris A about 6" from the center. ALPHA-UMI J2000 02 32 0.0000 +89 16 0.00 J2000 STAR F3-F9 These coordinates will place Polaris B at the same location in the FOV as Polaris A, as desired. ALPHA-UMI-B J2000 00 00 0.0000 +00 00 0.00 J2000 ACS/HRC Imaging No No F220W POS-TARG DITHER 0.173 LINE 0.112 false true 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. ORIENT is chosen to place Polaris Ab at 45 +/- 20 deg relative to the diffraction spikes of Polaris Aa. BETWEEN dates added to assure observation is completed by middle of Cycle 17, to maintain proper cadence for this long-term monitoring program. (Tic-Visit :proposal-id 11783 :visit-id "01" :proposal-version -1 :trans-version "20090130" :date "05-Jul-2009 18:49:12.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 986304 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11783 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11783 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3249 :al-overhead-at-end 167 :internal-dur-at-end 500 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3670 :pcs-gap 0 ) false 1 0