0Assigned ID: 672
----- Attempting Submission 1 (Fri Apr 08 18:40:09 GMT 2016) -----
HST Phase I Proposal 672 (elcvn.apt) successfully submitted.
Receipt: # 672-1This is a first draft of GO-14778.
I assumed buffer time = exposure time.
The coordinates are well known so I suggest using ACQ/IMAGE for acquisition.1574642633----- Attempting Submission 1 (Tue Jun 28 19:50:50 GMT 2016) -----Hiding in Plain Sight: The Low Mass Helium Star Companion of EL CVnBinary stars with orbital periods of a decade or less are destined to
interact during their evolution. The mass donor star among intermediate
binaries may be stripped of its envelope by mass transfer to reveal its
helium core. In cases that avoid merger, the low mass helium star will
remain in a binary orbit but be lost in the glare of the mass gainer star.
Thanks to photometric time series from Kepler and WASP, we now know of
27 such systems that are oriented to produce mutual eclipses. Although
the helium star companions are too small and faint in the optical band
for spectroscopic detection, they contribute a larger fraction of the
total flux in the ultraviolet. HST/COS measurements of one long period
system, KOI-81, successfully detected the helium star's spectrum in the
far-ultraviolet, leading to estimates of its mass and temperature.
Here we propose to obtain new HST/COS FUV spectra of the prototype of
this class of evolved binaries, EL CVn, and to determine the mass and
physical properties of a star that barely escaped a merger.We propose to obtain COS spectra of the FUV spectrum of EL CVn
using the G160M grating to cover most of the spectrum from
1386 to 1774 Angstroms with a resolving power of 22,000.
In order to meet our radial velocity measurement criteria,
we need to obtain a S/N per resolution element of 25 or better.
This S/N level can be achieved with multiple exposures.
We created model spectra from the UVBLUE grid for both
component stars using temperatures from Maxted et al. (2014).
We scaled the flux of the hot component by (R2/R1)^2, and
then formed the sum of the two spectra. The summed spectrum
was reduced for interstellar extinction using E(B-V)=0.065
and then normalized to the observed GALEX FUV flux in
the vicinity of 1528 Angstroms.
We used the COS ETC with the summed model spectrum
to estimate that a single S/N = 12.3 spectrum can be
obtained with an exposure time of 153 s. Following the example
in the Instrument Handbook, we would obtain one exposure at
each of four FP-POS spectral offsets and two CENWAVE postions
to increase the net S/N to 35. By making four visits at different orbital
phases, the final S/N in the reconstructed spectra will be about 69.
Note that we expect to observe radial velocity semiamplitudes of
29 and 267 km/s for the orbits of the primary and white dwarf,
respectively (Maxted et al. 2014), which should be readily measured.
All these exposures will be made in TIME-TAG mode. There are no
bright limit problems associated with this target.
We estimate that there are 55 minutes of visibility per orbit
for the position of EL CVn. In one orbit, we require 8 minutes for
acquisition and a COS ACQ/IMAGE centering on the aperture
(ICRS astrometry is good enough for this direct acquisition).
This leaves time for eight science exposures of 153 sec each
(with preparation and readout times included).
Thus, we will need only one orbit for each visit to obtain
a combined spectrum with the requisite S/N.
Note that the binary orbital period (19 hours) is much longer
than the HST orbit, so any orbital related changes over
the duration of the exposures is small.
In order to determine the orbital elements for each star from
the Doppler shifts, we will require four visits, with two each
around the times of highest and lowest radial velocity excursions.
Because the orbital period is so short, there are many
opportunities during Cycle 24 to obtain the spectra during the
orbital phases required.Stellar Physics12falsefalsefalsefalsefalsefalseSUPPORTEDSTARA0-A3 V-IVNOThis object was generated by the targetselector and retrieved from the SIMBAD database.HD116608HIP65380J2000J2000F(1528 Angstroms) = 1.9e-13 erg/s/cm^2/Angstrom (GALEX FUV)COS/FUVSpectroscopicNoNoG160M1600COS/FUVSpectroscopicNoNoG160M1600COS/FUVSpectroscopicNoNoG160M1600COS/FUVSpectroscopicNoNoG160M1600/nfs/morgan4/gies/TeX/NASA/HST_EL_CVn/OP-cache/elcvn-v01.xmlPhase windows are not calculated when the gap between schedulable intervals is less than 1 days, assuming no scheduling restrictions for constraint Phase 0.110/0.260.The scheduling data was generated by CASM 24.0.CASM was run in Phase II mode.(Tic-Visit
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