Frequency stability of maser oscillators operated with cavity Q

The short term frequency stability of masers equipped with an external feedback loop to increase the cavity quality factor was studied. The frequency stability of a hydrogen and a rubidium maser were measured and compared with theoretical evaluation. It is shown that the frequency stability passes through an optimum when the cavity Q is varied. Long term fluctuations are discussed and the optimum mid term frequency stability achievably by small size active and passive H-masers is considered

Cool White Dwarfs Found in the UKIRT Infrared Deep Sky Survey

We present the results of a search for cool white dwarfs in the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky Survey (SDSS) to identify cool hydrogen-rich white dwarf candidates by their neutral optical colors and blue near-infrared colors, as well as faint Reduced Proper Motion magnitudes. Optical spectroscopy was obtained at Gemini Observatory, and showed the majority of the candidates to be newly identified cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants. Our initial search of 280 deg2 of sky resulted in seven new white dwarfs with effective temperature T_eff ~ 6000 K. The current followup of 1400 deg2 of sky has produced thirteen new white dwarfs. Model fits to the photometry show that seven of the newly identified white dwarfs have 4120 K <= T_eff <= 4480 K, and cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km/s <= v_tan <= 85 km/s and are likely to be thick disk 10-11 Gyr-old objects. The other half of the sample has 4610 K <= T_eff <= 5260 K, cooling ages between 4.3 Gyr and 6.9 Gyr, and 60 km/s <= v_tan <= 100 km/s. These are either thin disk remnants with unusually high velocities, or lower-mass remnants of thick disk or halo late-F or G stars.Comment: To appear in ApJ, accepted April 18 2011. 34 pages include 11 Figures and 5 Table

Pure-hydrogen 3D model atmospheres of cool white dwarfs

A sequence of pure-hydrogen CO5BOLD 3D model atmospheres of DA white dwarfs is presented for a surface gravity of log g = 8 and effective temperatures from 6000 to 13,000 K. We show that convective properties, such as flow velocities, characteristic granulation size and intensity contrast of the granulation patterns, change significantly over this range. We demonstrate that these 3D simulations are not sensitive to numerical parameters unlike the 1D structures that considerably depend on the mixing-length parameters. We conclude that 3D spectra can be used directly in the spectroscopic analyses of DA white dwarfs. We confirm the result of an earlier preliminary study that 3D model spectra provide a much better characterization of the mass distribution of white dwarfs and that shortcomings of the 1D mixing-length theory are responsible for the spurious high-log g determinations of cool white dwarfs. In particular, the 1D theory is unable to account for the cooling effect of the convective overshoot in the upper atmospheres.Comment: 14 pages, 17 figures, accepted for publication in Astronomy and Astrophysic

Spectroscopic analysis of DA white dwarfs with 3D model atmospheres

We present the first grid of mean three-dimensional (3D) spectra for pure-hydrogen (DA) white dwarfs based on 3D model atmospheres. We use CO5BOLD radiation-hydrodynamics 3D simulations instead of the mixing-length theory for the treatment of convection. The simulations cover the effective temperature range of 6000 < Teff (K) < 15,000 and the surface gravity range of 7 < log g < 9 where the large majority of DAs with a convective atmosphere are located. We rely on horizontally averaged 3D structures (over constant Rosseland optical depth) to compute spectra. It is demonstrated that our spectra can be smoothly connected to their 1D counterparts at higher and lower Teff where the 3D effects are small. Analytical functions are provided in order to convert spectroscopically determined 1D effective temperatures and surface gravities to 3D atmospheric parameters. We apply our improved models to well studied spectroscopic data sets from the Sloan Digital Sky Survey and the White Dwarf Catalog. We confirm that the so-called high-log g problem is not present when employing spectra and that the issue was caused by inaccuracies in the 1D mixing-length approach. The white dwarfs with a radiative and a convective atmosphere have derived mean masses that are the same within ~0.01 Msun, in much better agreement with our understanding of stellar evolution. Furthermore, the 3D atmospheric parameters are in better agreement with independent Teff and log g values from photometric and parallax measurements.Comment: 15 pages, 18 figures, 10 pages online appendix, accepted for publication in Astronomy and Astrophysic