Don Winget The Department of Astrophysical Astronomy Context of McDonald White Dwarf Observatory Texas Cosmology Stars Center The University of Texas Karen & Don Winget Scale and Dept. of Content Astronomy of and McDonald The Universe Observatory University of Texas

The Otto Struve Telescope at McDonald Observatory photo by Frank W. Cianciolo, Jr. “One of the first tasks to be undertaken by the staff of the McDonald Observatory will be to investigate further the mysteries of the white dwarfs.” -- May 5th, 1939 White Dwarf Stars: Eddington’s “Impossible” Star

•1844: Bessel notices “wobble” in Sirius’ position

•1862: Alvan Clark directly observes a faint companion

Sirius B White Dwarf Stars: Eddington’s “Impossible” Star •Dark companion is hot and compact, roughly the size of Earth and the mass of the Sun

•Interior, even if made of the smallest atoms, must be ionized “ … to cool the star must expand and do work against gravity ….” Eddington.

•Heisenberg uncertainty principle and Pauli exclusion principle to the rescue – Fowler 1926

•Chandrasekhar limit (1931)

•Mestel (1952) Theory: develops understanding of decoupled mechanical and thermal properties •Macroscopic demonstration of QM

•Endpoint of evolution for most stars

•Homogeneous in mass and surface composition

•Simple internal structure and composition; evolution is just cooling hot pre-white dwarf model

cool white dwarf model

EoS: Extreme Physics

Fontaine, Brassard & Bergeron (2001) White Dwarf Flavors Not too long ago, in a galaxy close at hand … only two major classes of white dwarfs existed.

Hydrogen-dominated Helium-dominated atmospheres atmospheres

DA DO DB DBA DZ DC (cool) DQ …

Cosmochronology •Constrain Age of Universe •Measure Age and History of Components of the Galaxy –Thin disk –Open clusters –Thick disk –Halo –Globular clusters

NGC 6397

NGC 6397 with HST AC Luminosity Function for NGC 6397 proper motion screened WD sample

Crystallization Visualization by M.H. Montgomery Spectral Line-Broadening in White Dwarf Photospheres PI: Winget

The GalacticFig. Disk:14.— The(left observedpanel space): The densityobserv ofed whitespace dwarfsdensity (points),of white with dwarfs (points), with thetheoretical the “white the theoreticaldwarfwhiteluminosity dwarf luminosityfunction” functionas afunction (WDLF) asof aintrinsic function luminosityof intrinsic . Thecurves aretheoretical modelsassuming luminosity. The curves are theoretical models assuming a given age for star formationa ingi ourven Galaxy.age for star formation in our Galaxy. (right panel): The cooling time for a white dwarf to reach a −4.5 −4.3 given luminosity (solid curve: L = 10 L⊙; dashed curve: L = 10 L⊙) as a function of its mass.

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Fig. 15.— Hertzsprung-Russell diagram showing the locations of pulsating stars. The regions labeled DAV, DBV, and DQV denote the pulsating WDs. Our group was the first to find pulsations in the carbon- dominated DQVs (Montgomery et al. 2008).

– 24 – Dancing with the White Dwarf Stars: White Dwarfs in Binaries

Discovery: Extremely Low Mass (ELM) White Dwarf Stars • All are binaries • Gravities more than 100 times lower than normal mass white dwarf stars

Properties discovered at McDonald with the 2.1m (Otto Struve) telescope

• Relativistic beaming

• Eclipsing system

• Pulsators! Rate of Orbital Decay Measured:

Mean DA Mass From GR How do we solve this problem?

=> How do we solve this problem?

=> WD (surface)

(courtesy of Greg Rochau) Asteroseismology: Calibrating Uncertainties in Cosmochronoloy and Exploring Extreme Physics • Pulsating white dwarf stars have largest number of identified modes except for Sun. Periods of 100-1000s, amplitudes from .01% up to 30% • WET => revolution in asteroseismology – Resolves and identifies modes – Demonstrates extreme stability of some modes • HDAV stars most stable known pulsators (yet we don’t know why!) Complex Pulsators Probe interior physics

Asteroseismology: Using normal modes of pulsating WDs to study extreme physics and time itself

Simple Pulsator

=> Most stable clock Surface Brightness Variations 100-1,000 s l = 2 modes l = 3 modes Quantum numbers Nonradial Gravity Modes: g-modes l,m,n Most commonly observed modes are l =1 l = 1 modes

Massive Cool Pulsators are Crystallized!

A New Carbon WD (Hot DQ) Varaible: A New Kind of (Pulsating?) WD The first new class of pulsating white dwarf found in the last 25 years?

DQV The Pulsational H-R Diagram

log g ~ 5-6 log g ~ 4.2-4.4 log g ~ 8 DQV The pulse shape looks different from single-mode pulsators, but is disconcertingly close to AM CVn.

DAV

DBV AM CVn

Also, the star appears to have a large magnetic field of about 500,000 Gauss (one million times Earth’s field) Unique opportunities from the newly discovered Pulsating ELM Pulsators?

• Boundary conditions for comparing with stellar evolution codes • Accurate comparison points for lab measurements at low electron densities • A chance to study core He EoS

• Time dependent nuclear burning 3 of the 8 New ELM Pulsators: ELMVs A closer look at the ELMV strip Candidates from SDSS DR12: 8-12 variables so far 0 1 2 3 4 (hours) SDSSJ1131-0742; What the heck is this thing? SDSS: ~30,000 new WDs

Apache Point Obs. 2.5 m telescope 640 fibers per plates 3 800 – 9 200 Å Resolution: 1 800 S/N ~ 4 (g=20.2)

Photometry: 5 bands Spectroscopy: candidates

BUT: Biased sample! CNS 10 July 2014

White Dwarfs in HETDEX and parallel mode

. ~10000 White Dwarfs . Magnitude limited sample . For V=22, S/N=3 . Main (dark energy) and Parallel (other) surveys

A powerful survey! White dwarf science goals from ~ 10 000 WDs in Main and Parallel Survey:

. Correct for the selection effects of SDSS . Statistics to unravel evolutionary history and origin of all the types of WDs . Improved WD Cosmochronolgy . Improved Asteroseismology and, with it, improved understanding of physics under extreme conditions White Dwarf Surface Plasma Conditions:  -> Temperature -> Mass Cosmochronology  Type Ia Supernovae

Asteroseismology

Illustration: David A. Hardy, PPARC Image: FORS, 8.2-m VLT Antu, ESO Nuclear Fusion Intergalactic Distances Dark Matter

EOS Dark Energy

Image: NASA / A. Fruchter / STScI

Illustration: Harvard-Smithsonian Center for Astrophysics/Travis Metcalfe, Ruth Bazinet Graphic: NASA / WMAP Thanks!