Julie Hollek and Chris Lindner

 Background on HK II  Stellar Analysis in Reality  Methodology  Results  Future 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 indicator  Observed 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

 Demand no trend between excitation potential and abundance  Gives temperature

Stellar Analysis  Demand 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] = 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 profile  Example: 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 Tracks  Used model (Girardi et al. 2000) for star of Z = 0.01 and M = 0.8 Msun  Models 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 atmospheres  Using 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 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 narrower  As 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) = 3.00 Teff = 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