0Assigned ID: 465
----- Attempting Submission 1 (Fri Apr 11 13:25:06 GMT 2014) -----I changed Visit 2 to r-band (F625W). I left Visit 3 as i-band (ACS/F775W).-1233858090----- Attempting Submission 1 (Tue Jul 22 16:38:29 GMT 2014) -----
----- Attempting Submission 2 (Mon Aug 11 15:40:17 GMT 2014) -----
----- Attempting Submission 3 (Wed Aug 12 22:40:18 GMT 2015) -----A 1.05Msun Companion to PSR J2222-0137: The Coolest Known White Dwarf?The recycled pulsar PSR J2222-0137 is one of the closest known neutron stars, with a parallax distance of 267+/-1 pc. Using radio Shapiro delay measurements, we determine a companion mass (1.05+/-0.06 Msun) consistent with either a low-mass neutron star or a high-mass white dwarf (WD). However, the orbital eccentricity is too low to be the product of two supernovae and we are forced to conclude that the companion is a WD. And yet, despite deep optical and near-infrared searches with SOAR and the Keck telescopes we have not discovered the optical counterpart of the system and can limit its effective temperature to <3,000 K, a limit that is robust to distance, mass, and atmosphere uncertainties. This would make the companion to PSR J2222-0137 the coolest WD ever observed. For the implied age to be consistent with the age of the Milky Way requires the WD to have already crystallized and entered the faster Debye-cooling regime.
We wish to identify the companion of PSR J2222 and measure its spectral-energy distribution. Atmosphere models in this regime are extremely poorly tested, with significant uncertainties in the equation-of-state. Cooling ages for these models are even less well-known, as the impacts of state changes, sedimentation, and chemical processes are not precisely known. Here we request 3 bands of imaging with HST (WFC3/UVIS for g, r, and ACS/WFC for i) designed to detect the companion under the most pessimistic scenario. This will measure the effective temperature and atmosphere, and constrain the radius. These models will then help calibrate the ages of the globular clusters, halo WDs, and millisecond pulsars.We propose to identify the companion of PSR J2222 and to begin to measure its spectral-energy distribution. This will help us measure the effective temperature and atmosphere, and constrain the radius/internal composition. If collisionally-induced absorption (CIA) is important in the at- mosphere (i.e., there is some non-negligible amount of H), then the source will be considerably bluer than a blackbody, and so despite the cold temperatures we should focus on the optical regime rather than the near-infrared. We find that ACS/WFC/F625W (r-band) is the most sensitive band for most atmospheric compositions. Based on our faintest available models (considering a range of helium-to-hydrogen ratios for M = 1.15Msun, which is 2sigma above our best-fit value), we predict r < 27.7. We can reach this limit at S/N of 8 using 5 orbits of HST (total exposure time of 244 min, split into 4 dithered exposures per orbit).
We will complement the r-band search (effectively the deepest for most atmosphere models) with F475W (g) and F775W (i) observations using ACS/WFC (6 orbits each, giving g = 28 and i = 27.5 at 8sigma). These data will give us some wavelength coverage, without moving too far from the optimal window where these ultra-cool WDs peak. We require three bands as there are three primary unknowns: Teff, radius, and atmospheric composition. These don't reach quite the same radius limits, but in the event of a solid detection in one band the other bands will likely have at least 5sigma detections that can help determine Teff and the atmospheric composition. Again, we use 4 dithered exposures per orbit.
We will also use PSF photometry to improve the S/N. Our request is for 15 orbits total. If possible, we request that the different bands be scheduled in different visibility periods (separated by > 10 months) so that we can confirm any counterpart through a proper motion measurement (mu = 45 arcsec/yr, or 1 WFC3 pixel/year, which should be detectable if the S/N is high enough) but this is not a requirement.
To be conservative, we will first take the r-band observations and examine them before moving to the other bands. If our expectations are completely wrong we will use the remaining orbits to go deeper in r-band rather than do the other bands.COOL STARS12falsefalsefalsefalseSUPPORTEDSTARDAJ2000J2000WFC3/UVISImagingNoNoF625WWFC3/UVISImagingNoNoF475WACS/WFCImagingNoNoF775WPOS-TARGDITHER0.173LINE0.112falsefalse81.785423.884WFC3-UVIS-DITHER-BOX1falsePOS-TARGDITHER0.265LINE0.187falsefalse69.05420.67ACS-WFC-DITHER-BOX2falseTiming link suitability includes constraints ([02 After 01 by 180.0 to 365.0 days] [03 After 01 by 180.0 to 365.0 days])The scheduling data was generated by CASM 23.2.CASM was run in Phase II mode.141824392600066.9018740810385(Tic-Visit
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