julie hollek and chris lindner. background on hk ii 17435-00532 stellar analysis in reality ...

Julie Hollek and Chris Lindner

Background on HK II 17435-00532Stellar Analysis in RealityMethodologyResultsFuture Work

Overview

Part of the HK Objective Prism Survey (Beers, Preston, Shectman 1985) Looked for low metallicity stars Used Ca II H and Ca II K lines as a metallicity

indicatorObserved as part of Chemical

Abundances of Stars in the Halo (CASH) Project Characterize the abundance pattern of the

galactic halo R~15,000 S/N ~50/1

Spectroscopically determined parameters Measure equivalent widths of known lines▪ e.g. Fe I, Fe II

Demand all abundances are the same from all lines

Stellar Analysis

Stellar AnalysisDemand no trend between

excitation potential and abundance Gives temperature

Stellar AnalysisDemand no trend between

equivalent width and abundance Gives “correct” microturbulence

Stellar Analysis

Demand ionization balance to determine the gravity

For example, demand the same abundance for Fe I and Fe II to determine correct value for surface gravity

Stellar Parameters: Teff =5200 K

log g =2.15 [Fe/H] = -2.25 ξ = 2.0

Carbon, r+s -process, and lithium enhanced

Most metal-poor Li enhanced star known to date

Li burns at 2.5x10^6 K Should be heavily depleted by the giant stage

Li enhancement calls for some mechanism to produce more Li Extrinsic▪ Binary companion

Intrinsic▪ Cameron-Fowler Beryllium Transport

Mechanism▪ Thermohaline Mixing

Motivation

By determining the stellar evolutionary state of this star, we can determine its enhancement mechanism.

Motivation

Changes in stellar parameters result in radically different line profiles

Result from Voigt profileExample: change in Teff of 200 K of Li

region

Motivation

Observations of low metallicity candidates

Change Teff and log g according to stellar evolutionary models (Girardi et al. 2000)

Track how the Teff and log g change the line profile of a specific region in the spectrum

Try to “match” observations

Project Outline

Stellar Evolution TracksUsed model (Girardi et al. 2000) for

star of Z = 0.01 and M = 0.8 MsunModels given as time steps with

changes in luminosity, gravity, and effective temperature expressed

Stellar Analysis

With approximate values for the stellar parameters of Teff, log g, [Fe/H], and ξ, we can create model atmospheres in a program such as TLUSTY or using Kurucz grid point models.

We then input these stellar atmosphere models into a spectral synthesis program, like SYNSPEC or MOOG to model specific spectral features

SYNSPEC

TLUSTY

Kurucz Models

LTE model atmospheresUsing statistical opacity distribution

function (ODF) of ~10^6 lines Monte-Carlo-like sampling of frequency points

(Dreizler)Convection is available, though not used

HK II 17435-00532 is a low metallicity star, without the opacity source required for convection

Abundance Analysis

MOOG Performs spectral

synthesis Requires model

atmosphere, line list, and observed spectrum

Results

As log g decreases, the lines get narrowerAs Teff decreases, the lines get stronger

(deeper for a given abundance)

Discussion of results

Teff = 5900 K log(g) = 4.75Teff = 5200 K log(g) = 3.00Teff = 4400 K log(g) = 1.00

Only low resolution observations exist

These abundances are assumed to be constant over the lifetime of the star Probably change

Stellar evolution tracks aren't exactly correct for the star

Caveats

More observations for HK II are in the HET queue

Detailed abundance analysis in the works Pb C12/C13 ratio

Future Work