Julie Hollek and Chris Lindner. Background on HK II 17435-00532 Stellar Analysis in Reality ...
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Transcript of 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