# Presentation: 818465

## Calibrating Large Photometric Surveys Using Hot White Dwarfs 

- Calibrating Large Photometric Surveys Using Hot White Dwarfs

- Calibrating Large Photometric Surveys Using Hot White Dwarfs

**Jay B. Holberg**
**University of Arizona**

## DA White Dwarfs and Synthetic Photometry

**I.  DA White Dwarfs Placed on the HST Photometric Scale**
***Holberg & Bergeron (2006) AJ, 132, 1221***
**II.  DA White Dwarf  Distances**
***Holberg, Bergeron & Gianninas (2008) AJ, 135, 1239***

## Topics

**DA White Dwarfs as Absolute Flux Standards**
**(Synthetic Photometry)**

**II.    Estimating the Projected Density of White Dwarfs on the Sky**

## Synthetic Photometry 

*f**** **= Observed flux at the top of the Earth’s Atmosphere*
*H****(**T**eff**, log g) = Eddington Flux at the Stellar Surface*
*R = Stellar Radius  (Mass-Radius Relation)*
*D = Stellar Distance*

## Spectral Fitting for Teff and log g

**Spectral Fitting for T****eff**** and log g**

**Sirius B**

## Absolute Magnitudes

**Stellar flux integrated over band-pass**

**Synthetic Magnitude for band-pass S**

**The constant C****s**** can be defined with respect to Vega**
**For DA white dwarfs C****s**** can be defined so that M****s**** is the absolute magnitude for band pass S as function of T****eff ****and log g.   Note this requires knowledge of the relative response of the band-pass.**

## Example 

## Example

**GD 140 (WD 1134+300)**
**T****eff ****= 21,276 K & log g = 8.545 **

| Abs Mag. | MU | MB | MV | MR | MI | MJ | MK | MH |
| --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Synth | 10.464 | 11.477 | 11.545 | 11.661 | 11.778 | 12.080 | 12.151 | 12.233 |
| Obs | U | B | V | R | I | J | K | H |
|  | 11.377 | 12.400 | 12.493 | 12.603 | 12.723 | 12.993 | 13.105 | 13.183 |
| Obs-Syn | 0.913 | 0.923 | 0.948 | 0.942 | 0.945 | 0.913 | 0.945 | 0.950 |

**Ave (O-S) = 0.934**

*** http://www.astro.umontreal/~bergeron/CoolingModels**

## Distances

**Distance Modulus**

**Distance (pc)**

**GD 140  (UBVRI+JHK)   D = 15.38 ****± 0.25 pc**

**GD 140  (ugriz)                 D = 15.44 ****± 0.25 pc**

**GD 140  1/********  (*****Hipparcos )*****       D = 15.32 ****± 0.63 pc**

## Correlation of ‘Photometric Parallaxes with Trigonometric Parallaxes

**Correlation of ‘Photometric Parallaxes with Trigonometric Parallaxes**

## Estimating the Areal Density of Hot White Dwarfs

- No. of Hot WDs/sq. deg.

***b******II****** : Galactic Latitude***
***h : Galactic Scale Height of  WDs ***
***n******0****** : Local Space Density of  WDs******1***
***f  :  Fraction of  Hot WDs***
**1 ****Holberg, et al. (2008b) AJ, 135, 1225**

## Harris et al. (2007) Luminosity Function

**T****eff**** > 12,000 K**

## n0 = 4.8x10-3 WD’s pc-3  Holberg et al. (2008a)

**n****0**** = 4.8x10****-3**** WD’s pc****-3****  Holberg et al. (2008a)**
**f  = 0.2  Holberg et al. (2008b)**
**h = 250 pc  Harris et al. (2007)**

## Summary

** ****Calibrations based on DA white dwarfs have a **_**Strong Physical Basis**_
**  ****They are applicable from the UV (and X-ray) into the Near IR**
**  ****They are widely distributed in magnitude and over the sky**
**  ****Photometric calibrations based on white dwarfs should be easy to ****	****compare between different large surveys and different filters.**

-   Other Considerations
-  In general white dwarfs are blue objects
-  They may not be sufficiently densely distributed  over the sky so that 	there is a suitable standard in every field.