# Presentation: 833601

## SEARCHING FOR HOT WHITE DWARFS

**J. Allyn Smith**
**Austin Peay State University**
- DES Calibration Meeting
- 13 June 2008
- Fermilab

## SEARCHING FOR HOT WHITE DWARFS

- What is a White Dwarf?
- Space Densities
- Atmospheric Models
- Search Techniques
- Surveys

## What is a White Dwarf?

- White Dwarf 101
- Nomenclature
- Spectra
- Existing Catalogs

## White Dwarf 101

- About 97% of all stars in the Universe will end their lives as white dwarfs.
- Progenitor stars are up to ~8Mʘ.
- The canonical WD is ~0.7Mʘ, but can range from ~0.25-1.41Mʘ.
- Radius is about 0.01Rʘ (size of the Earth).
- Most have a surface gravity of log(g)~8, but can range from log(g) ~7-9.

## White Dwarf 101

- Temps range from ~150,000--> ~3300K.
- A fast calculation:
- For a T=5800K WD (solar temp)‏
- R2T4 ==> 0.0001 Lʘ (MV ~ 14.4 at 10 pc)‏
- or ~19.4 at 100 pc.
- So, these stars are generally “faint” and nearby.

## White Dwarf 101

- The cores are generally C/O though other cores are known.  Crystallization is important in the cooler stars.
- Except for a thin non-degenerate layer of the atmosphere, the rest of the star is electron degenerate matter.
- Atmospheres are generally H or He, some with trace elements.
- It is the non-degenerate layer which we observe and which controls radiative loss rates.

## White Dwarf 101

- Determine physical properties via line fits in medium-high resolution spectra.  Wings of the lines are extremely sensitive to the surface gravity.  Depth and width of lines are sensitive to temperature and H layer thickness.

## Example

## Nomenclature

**Sion, Greenstein, Landstreet, Liebert, Shipman, Wegner 1983 ApJ, 269, 253**
- White dwarf spectral types all start with a “D” for “degenerate”.
**Primary and secondary features**
- A  H lines present; no He I or metal lines
- B  He I lines; no H or metal lines
- C  Continuous spectrum; no lines
- O  He II lines, accompanied by He I or H lines
- Z  Metal lines; no H or He I lines
- Q  Carbon lines present
- X  Unclear or unclassifiable spectrum
- #  is a temperature indicator = 50,400/Temp
**Secondary features only**
- P  Magnetic white dwarf with detectable polarization
- H  Magnetic white dwarf without detectable polarization
- E  Emission lines present
- V  Variable
- Examples: DA4 or DABZH

## Spectra

- DA, DAH, DAE
- DB
- DO
- DC
- DZ
- DQ

## A star

## DA

## DAH

## DAE

## DB

## DO

## DC

## DZ

## DQ

## DQ2

## Existing Catalogs/DB

**McCook-Sion White Dwarf Catalog ****http://heasarc.nasa.gov/W3Browse/all/mcksion.html**
**SDSS DR1 and DR4**
**    ****http://www.sdss.org/dr6/products/value_added/index.html **
**Jay Holberg -U. Az. - Under Construction**
**http://procyon.lpl.arizona.edu/WD/**

## Space Densities

- Liebert, Dahn, Monet 1988, ApJ, 332, 891
- 42 stars, 0.003/pc3
- Oswalt, Smith, Wood, Hintzen 1996, Nature, 382, 692
- 69 stars, ~0.007/pc3
- Smith, 1997, Ph.D. thesis
- 152 stars, 0.0045/pc3 , 4.65x10-13/yr/pc3 birthrate
- first volume based corrections for missing stars
- Harris, et al. 2006, AJ, 131, 571 (SDSS DR3)‏
- 6000 stars, 0.0046/pc3, 4x10-5/pc3 in the Halo
- First definitive Halo LF, solved contamination issues,
- Scale height determinations.

## WDLF

## Space Densities

- Jay Holberg will show a plot later of expected numbers by galactic latitude.

## SEARCHING FOR HOT WHITE DWARFS

- What is a White Dwarf?
- Space Densities
- Atmospheric Models
- Search Techniques
- Surveys

## Atmospheric Models

- Pierre Bergeron, University of Montreal
- Hubeny & Lanz 1995, ApJ, 439, 875

## Search Techniques

- Use of Reduced Proper Motion plots.
- Unique color-color space for the right filters.
- DDO-51 filter survey – centered on MgH band

## RPM

- Lepine 2005 AJ 130, 1247

## Color Space

- Fan 1999 AJ, 117, 2528

## Color Space

- SEGUE target selection, courtesy Brian Yanny

## Complications

- Hard to determine Bolometric Corrections from broad band photometry alone.
- Unresolved companions (substellar, dust shells...)‏
- could be as high as 10-15% (or more)‏
- Undetected He in atmosphere of a DA ... requires JHK photometry to sort out – more of a problem with cool Wds.
- magnetic fields