Applying the accretion-diffusion model to a sample of DAZ without

IR excess

Jean Dupuis (Canadian Space Agency), Pierre Chayer (STSCI) and Vincent

Hénault-Brunet (University of Edinburgh)

17th European White Dwarf WorkshopTübingen, Germany, August 16-20, 2010

Should we worry about the effect of radiative levitation during accretion on DAZ?

A significant fraction of DA white dwarfs have metal lines at a level generally well below solar abundances.

Accretion from dusty disks likely explanation for metals in cooler DAZ.

Several DAZ have low level metallicity without IR excess and may be sufficiently hot for radiative levitation support (T

eff

below 25,000K).

As abundance measurements are used to infer accretion rates, it is important to quantify the effect of radiative acceleration.

Ultraviolet +Optical Spectra:FUSE, HST, IUE

NLTE model atmosphere analysis:TLUSTY, SYNSPEC

Time-Dependent Accretion simulations including radiative acceleration

Accretion rate determination

The process of measuring accretion rates from UV observations to accretion/diffusion simulations.

DA that do show metals: an indication of ongoing accretion?

Carbon generally not supported but seen in two objects.

Detection of Aluminum mostly in agreement with grad.

WD1337+705 metal lines stronger than expected from grad

.

DA that do not show metals: where have the metals gone?

Silicon is predicted in several cases but notdetected with significant upper limits.

A possible scenario for the origin of metals in mid-rangeeffective temperatures DA white dwarfs.

1) Weak accretion and/or pure radiative levitation.

2) Accretion.

3) Metals lost during early WD cooling and not yet replenished by accretion.

Does it matter?

Yes for supported elements such as Si and Al for which inferred accretion rates can differ by up to a factor 2-3.

Not so much for heavier elements such as Ca; it will affect relative abundances (ex: Si/Ca, C/Ca) .

For accurate determination of relative abundances, grad

not

entirely negligible, even for relatively cool DA (ex: LB3303).

Si abundance profiles in EGGR 46(Teff=25239K, log g=7.94)

C abundance profiles in EGGR 102(Teff=20413K, log g=7.92)

Si abundance profiles in EGGR 102(Teff=20413K, log g=7.92)

Al abundance profiles in EGGR 102(Teff=20413K, log g=7.92)

Ca abundance profiles in EGGR 102(Teff=20413K, log g=7.92)

Si abundance profiles in LB 3303(Teff=15579K, log g=8.04)

C abundance profiles in CD-38°10980(Teff=24276K, log g=8.07)

Al abundance profiles in CD-38°10980(Teff=24276K, log g=8.07)

Si abundance profiles in CD-38°10980(Teff=24276K, log g=8.07)

Al abundance profiles in Wolf 1346(Teff=19918K, log g=7.90)

Si abundance profiles in Wolf 1346(Teff=19918K, log g=7.90)

Radiative accelerations in EGGR 102(Teff=20413K, log g=7.92)

Ultraviolet spectroscopy reveals stellar metal lines in DA.

We are primarily interested by a sample of white dwarfs observed by the FUSE satellite (T

eff < 25,000K).

Abundances measurements are performed using TLUSTY and SYNSPEC.

We have computed radiative accelerations in models with values we have adopted for the atmospheric parameters and with the formalism described by Chayer, Fontaine, and Wesemael (1992).