White Dwarfs in the Solar Neighborhood

John P. Subasavage
2007 Publications of the Astronomical Society of the Pacific  
The study of white dwarfs (WDs) provides insight into understanding WD formation rates, evolution, and space density. As a population, a complete volume-limited sample is necessary to provide unbiased statistics that can then be applied to the Galaxy as a whole. Individually, nearby WDs are the easiest to characterize, thus providing critical test points for scrutinizing and modifying WD models. In addition, they serve as excellent candidates for astrometric planetary searches because the
more » ... etric signature is greater than for an identical, more distant WD system. However, their intrinsic faintness has allowed some to escape detection. The aim of this dissertation is to identify nearby WDs, accurately characterize them using a variety of observational techniques, and target a subset of potentially interesting WDs for follow-up analyses. WDs are usually identified based on color, as companions to brighter primaries, or by proper motions. Arguably, the most unambiguous method of identifying new WDs is by their proper motions through the use of reduced proper-motion diagrams. After evaluating all previous southern hemisphere proper-motion catalogs and selecting viable candidates, we embarked on our own southern hemisphere proper-motion survey, the SuperCOSMOS-RECONS (SCR) survey (Hambly et al. 2004 ). By utilizing four photographic plates ( , R ESO , R 59F , I IVN ) con-B J tained in the SuperCOSMOS Sky Survey (SSS), we completed a trawl for new proper-motion objects with yr Ϫ1 . m p 0.4 -10.0 A total of 299 new systems were discovered, as well as seven new common proper-motion companions to known proper-motion stars (Subasavage et al. 2005a (Subasavage et al. , 2005b . Of the new systems, 148 have yr Ϫ1 and are additions to the classic Luyten m ≥ 0.5 Half-Second (LHS) Catalogue (Luyten 1979). These constitute an 8% increase in the sample of all stellar systems with m ≥ yr Ϫ1 in the southern sky. A number of interesting objects 0.5 were discovered during the survey, including the 24th nearest star system, SCR 1845Ϫ6357, an M dwarf with a brown dwarf companion (Hambly et al. 2004; Biller et al. 2006; Henry et al. 2006 ). An additional 42 SCR red dwarf systems are estimated to be within 25 pc, the horizon of the Catalog of Nearby Stars (CNS) and the NStars Database. The SCR survey produced 18 WD candidates that were confirmed after a series of spectroscopic observations. In addition, 38 new WD discoveries were spectroscopically confirmed that were first identified as high proper-motion objects by other proper-motion surveys. Johnson-Kron-Cousins photometry was obtained V R I J KC KC for most of the 56 new systems using the 0.9 m telescope at Cerro Tololo Inter-American Observatory (CTIO). Near-infrared photometry was extracted from the Two Micron All-JHK S Sky Survey (2MASS) Database and combined with the optical photometry in order to model the physical characteristics and obtain distance estimates via spectral energy distribution (SED) fitting. Effective temperatures were derived using model atmospheres relevant for pure hydrogen, pure helium, and mixed hydrogen and helium, as well as helium-rich that include metals (i.e., calcium and carbon). The spectral lines were used to classify objects into the appropriate model bins. In cases in which no spectral lines were present (i.e., DC WDs), the SEDs were fit to both pure hydrogen and pure helium models and the better fit was adopted. By assuming a for each log g p 8.0 star, an absolute magnitude was estimated and compared to the apparent magnitude, yielding an distance estimate. A subset of 33 new WD discoveries, including photometry and modeled physical parameters, was published by Subasavage et al. (2007) , while the remaining 23 systems will be contained in a future publication. An independent distance estimate was also obtained by deriving an empirical color-relation for several colors based M V on WDs with known distances taken from the Bergeron et al. (2001) sample. Although not as reliable as the distance estimates determined from SED fitting, the empirical relations were useful to target nearby WD candidates for trigonometric parallax determinations. Any object whose distance estimate was within 25 pc was targeted for a trigonometric parallax via our parallax program, the Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI; Jao et al. 2005) . As of 2007 August, there are 62 WD systems on CTIOPI at various stages of completion, of which 23 are newly confirmed WDs. A subset of 53 systems has enough data for at least a preliminary parallax (24 are definitive). Of those 53 systems, nine are previously known WDs within 10 pc that we are monitoring for perturbations from unseen companions as part of the Astrometric Search for Planets Encircling Nearby Stars (ASPENS), and an additional 29 have first-time measured
doi:10.1086/523600 fatcat:2ppsopipcbfulmhag4ukpkza2u