----- Submission 1 (Sun May 08 23:19:53 GMT 2005) ----- Phase II Proposal 10593 (10593) successfully submitted. ----- Submission 2 (Sun May 08 23:20:40 GMT 2005) ----- Assigned ID: 26 ----- Submission 1 (Tue Jan 18 01:35:25 GMT 2005) ----- 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, there is 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. We show that the binary should be easily resolved in the UV using ACS/HRC, thus providing the first direct detection of the companion. In the present proposal we request one HST orbit in order to make this detection and measure the separation. We show that this initial detection, combined with the HST/FGS parallax (see below), will provide a mass accurate to 0.9 Msun. Only HST's combination of high spatial resolution and UV sensitivity can achieve this result. We plan to continue the program in future cycles, leading to rapid refinement of the dynamical mass measurement of the Cepheid. The parallax is a key ingredient in the mass determination. In an ongoing multi-year program (GO-9888, GO-10113), we are using the FGS to improve significantly upon the Hipparcos parallax of Polaris. The ACS imaging proposed here will thus provide extremely valuable astrophysical information from a very modest additional investment of observing time.

The observations with ACS/HRC will consist of a series of 0.5-sec exposures in F220W, with dithering to improve the astrometric accuracy. The aim is to resolve the close companion of Polaris, and measure the separation and position angle. The target will be positioned in the HRC field of view so as to also show the wide companion, Polaris B, which will serve as a check on the position angle measurement. COOL STARS 12 false 0.0 false false false false false SUPPORTED STAR F3-F9 CEPHEID 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 HRC 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 ACS/HRC Imaging No No F220W POS-TARG DITHER 1.9 LINE false true 4 5.85 ACS-HRC-DITHER-LINE 1 false /var/tmp/OPVisit7487.xml 1115592586007 (Tic-Visit :proposal-id 10593 :visit-id "01" :proposal-version -1 :trans-version "20050323" :date "08-May-2005 18:49:43.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 809664 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10593 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10593 :visit-id "01" :orbit-number 1 :pcs-time 367 :si-vis-utilization 3150 :al-overhead-at-end 0 :internal-dur-at-end 260 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3670 :pcs-gap 0 ) true 1 0