0Assigned ID: 3157
----- Attempting Submission 1 (Fri Mar 25 08:07:02 GMT 2022) -----
HST Phase I Proposal 3157 successfully submitted.
Receipt: # 3157-1
----- Attempting Submission 2 (Fri Mar 25 08:30:54 GMT 2022) -----
----- Attempting Submission 3 (Sun Mar 27 15:34:14 GMT 2022) -----For the best data quality, it is generally required to use all four FP-POS positions when observing at a given COS cenwave. See the COS Instrument Handbook for exceptions that may apply to observations with G130M/1291 or G160M.
For the best data quality, it is generally required to use all four FP-POS positions when observing at a given COS cenwave. See the COS Instrument Handbook for exceptions that may apply to observations with G130M/1291 or G160M.The errors concerning FP-POS are due to spreading the 4 FP-POS positions between 2 visits.
Updated visit 4 NUM-POS to 5-314642474----- Attempting Submission 1 (Thu Jul 07 18:38:32 GMT 2022) -----
----- Attempting Submission 2 (Mon Sep 26 15:03:13 GMT 2022) -----The four FP-POS positions are spread out over 2 visits.The Origins and Evolution of Helium Atmosphere White DwarfsAims: The proposed observations will conduct the first UV investigation helium atmosphere white dwarfs
found within the "DB gap", and will determine their physical parameters.
Context: The origin and evolution of helium atmosphere white dwarfs (DBs) is poorly constrained.
White dwarfs are structurally simple; they consist of an electron degenerate core surrounded
by an insulating blanket of hydrogen (DA) and/or helium (DB). Their evolution is dominated by
secular cooling. The DB gap is a temperature range between 45,000 and 30,000 K where mostly
DA white dwarfs are found. There is no readily apparent reason why this should be, so theories of white
dwarf formation and evolution have been forced to scenarios that explain this gap. The Sloan
Digital Sky Survey has revealed a population of DB white dwarfs that lie within this gap.
Little is known about these objects besides their optical temperatures. The fundamental physical properties
and chemical abundances of these objects can be used to test competing theories of white dwarf formation and
evolution, leading to a better understanding of the white dwarf population and the final stages of stellar evolution.
Methods: The proposal presents justification for COS observations of 2 DB white dwarfs with temperatures
between 45,000 and 30,000 K. The observations will determine accurate effective temperatures, surface
gravities and chemical abundances for each.The proposed observations will conduct the first UV investigation of helium atmosphere white dwarfs found within the "DB gap", and will determine their physical parameters. The scientific goal is to constrain theories of the origins and evolution of white dwarf stars.
We have two targets: SDSSJ170727.11+203153.25 and SDSSJ141127.09+035429.70. These targets were chosen from the sample of DB stars in Genest-Beaulieu & Bergeron (2019). SDSSJ170727 has a temperature of 44000 K and is near the blue
edge of the DB gap. SDSSJ141127, with a temperature of 35600 K, is near the red edge of the DB gap.
The observations are relatively straight forward. We have selected the gratings and central wavelengths to detect photospheric lines of hydrogen, carbon, oxygen, silicon and other elements in the spectra of the two targets.
White dwarf stars are known for their nearly pure photospheres of either hydrogen or helium, at least at optical wavelengths. The UV wavelength range, however, contains numerous sensistive transitions that are important
for determining trace abundances of the elements of interest. Given the limits on Lyman alpha observations with G130M, we have decided to iinclude a hydrogen line at 1025 Angstroms. The G130M grating with a central
wavelength of 1096 Angstroms will access HI 1025 Angstroms, CII 1037 Angstroms, CIII 1175 Angstroms, CI 1993 Angstroms, and Si II at 1193 Angstroms. G130M with a central wavelength of 1222 Angstroms will cover SiII 1260,
1264 Angstroms, OII 1305 Angstroms, and CII 1335 Angstroms,
The limitations of the response of G160M at certain wavelengths were not as significant to our science as first anticipated, so we have included this grating. G160M with a central wavelength of 1589 Angstroms will cover
HeII 1640 as well as SiII lines at 1533 Angstroms.
Measuring the strengths of photospheric lines from different transitions such as CII and CIII can be an important diagnostic for temperature. A second method to determine temperature is these hot objects is to measure the slope of
the UV continuum. G140L combined with the G130M and G160M grating observations will result in a detailed measurement of the
UV flux. between 916 nad 2148 Angstroms. The temperature determined from these measurements will confirm the effective temperatures of the two targets.
We have use the COS Spectroscopy Target Acquisiton and COS Spectroscopy Exposure Time SImulators to determine our exposure times. Our targets do not exceed the brightness limits for COS, and are isolated with good coordinates.
For target acquistion, we determined the time required to reach a signal to noise of at least 40. White dwarf stars are faint and can have high proper motions. GAIA has reduced this issue by determining many proper motions.
However, to be safe, we will acquire our targets using ACQ/SEARCH, ACQ/PEAKXD, and ACQ/PEAKD. For our science observations, we have determined exposure times to achieve a S/N of 15-20 at a wavelngth of 1150 Angstroms for the G130M
grating, and a S/N of 15-20 at 1500 Angstroms for the G160M and G140L gratings.
The science observations will be done in TIME-TAG mode. This will enable us to search for any variability. Neither star is known to be a large amplitude variable. However, SDSSJ141127 is close to the suspected temperature for
the initiation of gmode pulsations in helium atmosphere whtie dwarfs. The amplitudes of these possible pulsations is very small ( less than 1 ppt).Dr. Judith Provencal has experience in HST data reduction for COS, the principal instrument used in the proposed observations. Dr. Provencal specializes in helium atmosphere white dwarfs
and hasmultiple papers published in the last several years on pulsating white dwarfs and data analysis from previosus HST observing campaigns. Dr. Provencal will serve as team
leader in data analysis and interpretation. Dr. Scot Kleinman and Dr. Atsuko Nitta are experts in white dwarf data reduction and analysis and will assist in data reduction. Dr. Michael
Montgomery has expertise in computational simulations and white dwarf modeling which will be invaluable in
interpreting the proposed data.Stellar Physics and Stellar Types6falsefalsetruefalsefalsefalsefalseSUPPORTEDSTARDBNOThis object was generated by the targetselector and retrieved from the SIMBAD database.J2000J2000G=18.403STARDBNOThis object was generated by the targetselector and retrieved from the SIMBAD database.J2000J2000u=16.7, g=16.95COS/FUVSpectroscopicNoNoG130M1222COS/FUVSpectroscopicNoNoG130M1222COS/FUVSpectroscopicNoNoG140L1280COS/FUVSpectroscopicNoNoG130M1291COS/FUVSpectroscopicNoNoG130M1055COS/FUVSpectroscopicNoNoG140L1280The scheduling data was generated by CASM 30.3.CASM was run in Phase II mode.(Tic-Visit
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