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A New Technique to Measure Y/ZA. A. R. Valcarce (UFRN)Main collaborators:J. R. de Medeiros (UFRN)M. Catelan (PUC)XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 2012

1OutlineIntroduction

Determination of YTheoretical models (PGPUC)MethodComparison

Application

SummaryXXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 20122OutlineIntroduction

Determination of YTheoretical models (PGPUC)MethodComparison

Application

SummaryXXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 20123IntroductionThe helium-metallicity relation (Y-Z relation) is the keystone to understand the formation and evolution of stars and all the objects related to them.

This relation reads:Y = Yp + Y/Z x Z

The importance of the Y-Z relation:One can know a free parameter (Y) and then assume that the differences between theory and observations are only associated to differences in ages, masses and/or other free parameters. Yp: primordial helium abundance Y/Z: helium-to-metal enrichment ratio XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 20124Effects of Y on Evolutionary TracksEvolutionary tracks are different if they have the same [Fe/H], [/Fe] and mass, but a different He abundance (Y).

Some effects include:Variations in luminosity (L), effective temperatures (Teff), and surface gravity (g).Faster evolution for higher Y.

XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 20125Effects of Y on Evolutionary TracksEvolutionary tracks are different if they have the same [Fe/H], [/Fe] and mass, but a different He abundance (Y).

Some effects include:Variations in luminosity (L), effective temperatures (Teff), and surface gravity (g).Faster evolution for higher Y.

The problem is that maybe Y Yp + Y/Z x Zas happens in some globular clusters. XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 20126Y Yp + Y/Z x ZThe CMD of some GCs show they are not simple stellar populations. In some cases implying Y Yp + Y/Z x Z .NGC 2808 (Piotto et al. 2007)

XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 20127OutlineIntroduction

Determination of YTheoretical models (PGPUC)MethodComparison

Application

SummaryXXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 20128Theoretical Models

For more information see:Valcarce, Catelan, & Sweigart (2012, ArXiv:astro-ph/1208.5127)PGPUC stellar evolutionary code: updated version of the code of Sweigart (1971 1998), that is a highly modified version of the code created by Schwarzschild & Hrm (1965).

Evolutionary Tracks:Grevesse & Sauval (1998) chemical composition.7 masses ( 0.5 M/M 1.1 )7 helium abundances ( 0.230 Y 0.370 )12 metallicities ( -2.00 [Fe/H] 0.75 )2 alpha-elements distributions ( [/Fe]=0.0, 0.3 )XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 20129Method for Determining Y

For a star with a given chemical composition only one evolutionary track reproduces Mbol and Teff at the same time.

XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201210Method for Determining Y

For a star with a given chemical composition only one evolutionary track reproduces Mbol and Teff at the same time.

However, if Y is unknown several evolutionary tracks with the same [Fe/H] pass through the same point.

XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201211Method for Determining Y

Another observable is required to solve this mathematical problem. For a star with a given chemical composition only one evolutionary track reproduces Mbol and Teff at the same time.

However, if Y is unknown several evolutionary tracks with the same [Fe/H] pass through the same point.

XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201212Since the stellar mass (M) is different for each Y at the same MbolTeff, the spectroscopic surface gravity (g) can be used to determine Y.

If Y is known, it is straightforward to determine the other stellar properties (Z, M, Age).

However, the precision in the measurement of g have to be really high to constrain them.

Method for Determining YXXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201213Method for Determining YSince the stellar mass (M) is different for each Y at the same MbolTeff, the spectroscopic surface gravity (g) can be used to determine Y.

If Y is known, it is straightforward to determine the other stellar properties (Z, M, Age).

However, the precision in the measurement of g have to be really high to constrain them.

log g = 4.53 0.06XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201214Because this method require 3 parameters (Mbol , Teff, and log g), we use the observational results listed in Casagrande et al. (2006).

Low-mass MS stars -2.0 [Fe/H] +0.4 with [Fe/H] 0.15 dex4400 Teff[K] 6400 with Teff 100 K4.1 log g 5.0 with log g 0.20 dex

Comparison: Observational DataXXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201215Because this method require 3 parameters (Mbol , Teff, and log g), we use the observational results listed in Casagrande et al. (2006).

Low-mass MS stars -2.0 [Fe/H] +0.4 with [Fe/H] 0.15 dex4400 Teff[K] 6400 with Teff 100 K4.1 log g 5.0 with log g 0.20 dex

Comparison: Observational Data

Yp

Casagrande et al. (2007)XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201216Comparison: Y/Z from nearby starsCasagrande et al. (2007) determined Y assuming all stars are 5 Gyr old.Y=0.245 + Y/Z x Z with Y/Z=2.0

Yp

If we assume all stars are 5 Gyr old, we also find helium abundances below the primordial value.XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201217Comparison: Y/Z from nearby starsCasagrande et al. (2007) determined Y assuming all stars are 5 Gyr old.Y=0.245 + Y/Z x Z with Y/Z=2.0

t < 13.5 Gyrt > 13.5 GyrInterpolatedExtrapolated

However, when we use our method (age is not constant) metal poor stars show more realistic Y values.YpXXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201218Comparison: Y/Z from nearby starsCasagrande et al. (2007) determined Y assuming all stars are 5 Gyr old.Y=0.245 + Y/Z x Z with Y/Z=2.0

t < 13.5 Gyrt > 13.5 GyrInterpolatedExtrapolated

Mass Limit 0.75 Mdue to the classic radius problem of low mass stars (e.g., Feiden & Chaboyer 2012).YpXXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201219OutlineIntroduction

Determination of YTheoretical models (PGPUC)MethodComparison

Application

SummaryXXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201220Application

Baumann et al. (2010)Baumann et al. (2010) study the lithium abundances in nearby stars with solar properties. -0.4 [Fe/H] +0.3 with [Fe/H] 0.025 dex5600 Teff[K] 6100 with Teff 40 K4.0 log g 4.6 with log g 0.06 dexThey determined stellar masses and ages using the theoretical Teff vs log g diagram together with Y2 isochrones (Y=0.23+0.20xZ, Yi et al. 2001).XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 2012We use our method (Y Yp +Y/Z x Z) to determine the fundamental properties of the stars of Baumann et al. (2010): Y, Z, M, and age.Application

XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 2012When a Y-Z relation is used instead of assuming an unknown Y, there are differences in masses and ages around 0.02 M and 2 Gyr.

TW: This work with Y Yp +Y/Z x Z

B10: Baumann et al. (2010) with Y = Yp +Y/Z x Z

ApplicationXXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201223

www2.astro.puc.cl/pgpuc/

XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 201224OutlineIntroduction

Determination of YTheoretical models (PGPUC)Method

Testing the MethodObservational DataY/Z from nearby stars

SummaryXXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 2012SummaryWe present a new method to determine the He abundance in nearby stars using Mbol, Teff, and g, that can be used to determine Y/Z.

However, this method has mass limit around 0.75 M.

We show that assuming all stars are 5 Gyr old is not a good approximation (specially for metal poor stars), inducing an error Y determination.

When a Y-Z relation is assumed instead of a variable Y value, there will be differences of |M|0.02 M and |Age|2 Gyr.

Finally, we present the PGPUC online database for theoretical models for a wide range of M, Y, and Z (and soon [/Fe]).

www2.astro.puc.cl/pgpuc/XXXVII SAB meeting guas de Lindia, Brazil, Oct 16th, 2012