0Assigned ID: 984
----- Attempting Submission 1 (Fri Apr 10 22:49:05 GMT 2015) -----
HST Phase I Proposal 984 (APT_template_proposal1.apt) successfully submitted.
Receipt: # 984-1
----- Attempting Submission 2 (Fri Apr 10 23:05:25 GMT 2015) -----
HST Phase I Proposal 984 (APT_template_proposal1.apt) successfully submitted.
Receipt: # 984-20----- Attempting Submission 1 (Tue Jul 21 15:29:36 GMT 2015) -----The Suppression of Convection in Magnetic White DwarfsMagnetic white dwarfs account for ~10% of the overall remnant population, and contribute an even more significant fraction in the high-mass regime. An understanding of magnetic remnants is critical to derive a precise star formation history and initial mass function in the Galactic disk from the the local volume-complete sample of white dwarfs. The origin of magnetic white dwarfs remains elusive, and they could be remnants of magnetic main-sequence stars or the product of white dwarf mergers, the latter scenario providing constraints on the nature of type Ia supernovae. In the next decade, Gaia will provide precise distances, masses, and luminosities for all magnetic white dwarfs, allowing to trace their evolution path.
We request to test the prediction of our new 3D radiation magnetohydrodynamics simulations of magnetic white dwarf atmospheres that convective energy transfer is suppressed in the line forming regions for B > 5 kG. This result implies a major change in the manner magnetic white dwarfs should be modeled, from the predicted colors to the cooling rates. We propose to observe a cool magnetic white dwarf with B = 10 kG where we predict that convection is supressed at the surface, as well as two standard convective remnants with similar parameters. The radiative and convective solutions are very similar in the optical, but the UV slope will confirm whether convective energy transfer is suppressed. As a consequence, we request 3 HST orbits with COS/FUV to confirm the 3D predictions and set the theoretical framework for the 10,000 magnetic white dwarfs that Gaia will observe.We request to constrain the slope of the UV flux for the magnetic white dwarf WD 2105-820 (V = 13.60) to determine whether convection is suppressed in the photosphere. Figure 1 of Phase I demonstrates that a low resolution UV spectrum centered at 1800 A, with a coverage of delta lambda = 400 A, is sufficient to constrain the slope in the UV. While previous grids of 1D spectra had an uncertain convective solution due to the presence of free parameters in the 1D mixing-length model (Bergeron et al. 1995), our 3D spectra do not have this shortcoming. As a consequence, we expect an unambiguous difference between the convective and non-convective cases. We therefore require a signal-to-noise ratio of 25 at lambda = 1800 A. We computed a model spectrum for the UV range based on the effective temperature, surface gravity, and V-band magnitude from Gianninas et al. (2011), and fed it into the STIS and COS Exposure Time Calculators (ETCs). We found that the COS G140L/1105 grism yields the best signal, requiring 1 orbit to achieve the desired S/N.
Our spectroscopic COS setup will be able to detect metal pollution in WD 2105-820, which has a small observed abundance of [Ca/H] = -8.6 from high-resolution optical observations (Koester et al. 2005). Such small abundance produces a negligible feedback effect on the structure and will not hamper our science objectives, hence it remains the best target given its small and well characterized magnetic field. A full orbit to reach the maximum S/N is essential to identify possible metal absorption lines in the UV, which could impact the identification of the continuum. Our analysis procedure will consist in fitting iteratively both the requested HST COS observations and our already secured ground based optical observations (Gianninas et al. 2011). We note that the magnetic field is too weak to produce significant Zeeman splitting and hamper the Balmer line fitting, as illustrated by the good match between the published photometric and spectroscopic solutions (Gianninas et al. 2001, Giammichele et al. 2012).
We also request to observe two control targets that are not know to be magnetic and should have the convective solution from Figure 1 of phase I. We have scanned the White Dwarf Catalog (McCook & Sion 1999) and the local 20 pc sample (Giammichele et al. 2012) for targets, requesting that 9750 < Teff (K) < 11,000. We selected the brightest, single, and non-pulsating pure-hydrogen WDs in the range. The first object is WD 1544-377 with V = 13.0, Teff = 10,610 +/- 151 K and log g = 7.91 +/- 0.04 and the second target is WD 1310+583 with V = 14.1, Teff = 10,682 K +/- 156 and log g = 8.10 +/- 0.05. The atmospheric parameters are derived from precise optical observations (Giammichele et al. 2012). Their brightness is similar to that of our primary magnetic target, hence they also require one orbit with COS G140L/1105 to achieve the science goals. We note that our control targets have no spectropolarimetric observations to confirm that they have no magnetic fields below 10 kG. No WD in the Teff range of interest has a firm non-detection. Only the much fainter WD 2333-049 (V = 15.9) has an upper limit of 10 kG (Landstreet et al. 2012), which is not enough to rule out a radiative atmosphere. However, the probability that both our control targets harbor a kG-range field is estimated at <1% (Landstreet et al. 2012), and we will request ground based spectropolarimetric observations to reach an upper limit of 1 kG, which has been shown to be feasible at the brightness of our targets.
Our targets have never been observed in the UV, and while a few magnetic WDs have been observed with COS, they usually have Teff values large enough that convection is not predicted for both the magnetic and non-magnetic cases. We found one cool magnetic object (WD 2316+123 with Teff ~ 11,000 K), although it has a much stronger magnetic field of B ~ 45 MG with strong Zeeman line splitting in the optical, making it difficult to fit the object and predict an accurate UV flux. Furthermore, the COS data is limited to lambda < 1700 A, where Lyman- pseudo-molecular satellites are found. It is currently difficult to predict the shape of these satellites in the presence of a strong magnetic field. Nevertheless, we will use this archive data as a check of our results.
All our targets have high proper motions and we will acquire images in 2015 at Liverpool Telescope (WD1310+583) and Prompt Telescope (WD 2105-820 and WD 1544-374). Once we have reliable coordinates and proper motions, we don't need the ACQ/SEARCH, which saves exposure time.
The control target WD 1544-374 (HD 140901B) is a companion to the main-sequence star HD 140901 of type G6V and V = 6.01 with a seperation of 15 arcsec. The wide binary was first identified by Wegner 1973, MNRAS, 163, 381 and has been referenced as such in 50+ papers. Subsequent observations leave little doubt that it is a physical pair (see, e.g., Catalan et al. 2008, A&A, 477, 213). BOT does not give any warning with GALEX fluxes when no PM is applied, which should be the correct case since both bright objects in the field have common PM. Nervertheless, feeding ETC with a Kurucz Models G5V model (COS.sa.727761) confirms that the FUV flux of the companion is not a safety problem. Finally, observed GALEX FUV fluxes in the field do not trigger any safety warning, and only fluxes extrapolated to the NUV suggest problems (but they assume the wrong spectral types for the objects in the field).HOT STARS12falsetruefalsefalseSUPPORTEDSTARDANOCoordinates and proper motions from PPMXL. We will acquire an image at Prompt Telescope to confirm the proper motion.J2000J2000STARDANOProper motions from PPMXL (from G-star companion). Coordinates from the SIMBAD database. We will acquire an image at Prompt Telescope to confirm the proper motion.J2000J2000STARDANOCoordinates and proper motions from PPMXL. We will acquire an image at Liverpool Telescope to confirm the proper motion.J2000J2000COS/FUVSpectroscopicNoNoG140L1105COS/FUVSpectroscopicNoNoG140L1105COS/FUVSpectroscopicNoNoG140L1105The scheduling data was generated by CASM 23.2.CASM was run in Phase II mode.C:\Travail\APT Phase 2\OP-cache\proposal1_phase2_final-v01.xml(Tic-Visit
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