0 Assigned ID: 182 ----- Attempting Submission 1 (Thu Apr 07 02:16:50 GMT 2016) ----- 0 ----- Attempting Submission 1 (Wed Jun 29 18:42:39 GMT 2016) ----- Connecting Variability and Metals in White Dwarfs The Kepler and K2 missions have revealed that about half of the observed white dwarfs with sufficient signal-to-noise ratio light curves have low-level photometric variations at hour to day timescales. Potential explanations for the observed variability include the relativistic beaming effect, ellipsodial variations, eclipses, and reflection off of giant planets in close orbits. However, these are all rare events. Roughly 10% of white dwarfs are magnetic, and magnetic fields can explain part of this puzzle. However, the high incidence (50%) of variability is currently unexplained. HST COS spectroscopy of nearby white dwarfs show that about half of them have metals on their surface. Hence, we propose that the observed variability is due to the rotation of the star coupled with an inhomogeneous surface distribution of accreted metals. We have recently discovered an ideal system to test this hypothesis. J1529 is an apparently non-magnetic white dwarf that shows 5.9% photometric dips in the optical every 38 min. We propose to obtain COS TIME-TAG spectroscopy of J1529 over 4 orbits to search for surface abundance differences throughout the orbit and look for the flux redistribution effect in the optical. These observations will confirm or rule out the idea that inhomogeneous metal accretion on white dwarfs can explain the high incidence of variability. We predict that the LSST will identify ~100,000 variable white dwarfs. Hence, understanding the source of variability in white dwarfs has implications for the current and future transient surveys. We will use COS/TIME-TAG mode to obtain G230L spectra (with the nominal wavelength setting of 2635 Angstrom) of a white dwarf that shows photometric dips every 38 minutes. We think that the source of these dips is surface inhomogeneities of accreted metals. Our goals is to get a S/N=20 spectrum to search for metal lines within and outside of the photometric dips. We divide each orbit into 2 sub-exposures using FP-POS 1,2,3, and 4 for the orbits 1,2,3, and 4, respectively. We will obtain simultaneous ground-based, optical photometry from McDonald and APO telescopes. Hence, we will need to know the HST scheduled dates as soon as possible so that we can plan for the ground-based observations. Stellar Physics 12 false true false false false false SUPPORTED STAR DA NO J2000 J2000 u=17.77 AB mag COS/NUV Spectroscopic No No G230L 2635 /home/kilic/APT_24.2/OP-cache/cycle24-v01.xml The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 14736 :visit-id "01" :proposal-version -1 :trans-version "20160601" :date "29-Jun-2016 13:38:11.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "32" :si-bright-earth-limb-angle 20.0 :data-volume 556768 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14736 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14736 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2896 :al-overhead-at-end 0 :internal-dur-at-end 95 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14736 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3007 :al-overhead-at-end 0 :internal-dur-at-end 105 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14736 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3007 :al-overhead-at-end 0 :internal-dur-at-end 105 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14736 :visit-id "01" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3007 :al-overhead-at-end 0 :internal-dur-at-end 398 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) true 4 0