Effects of Non-Solar Abundance Ratios on Star Spectra:

Comparison of Observations and Models.

Overview:-Importance of element abundancesNew measurementsComparisons with models

Spectra and line strengthsInterpretation, SNIa timescales and improvements.

Anne Sansom

Andre Milone (INPE), Alex Vazdekis (IAC) + extended MILES team.

Importance of element abundances• Affects stellar atmosphere models, structure, spectra.• Stars summed to give simple stellar populations (SSPs).• SSPs used to interpret star cluster & galaxy spectra:

– age, chemistry, IMF, duration of SF (SNII/SNIa), SFH etc.• Response functions R = changes of Lick line strengths to element abundance

changes from theoretical star spectra of Korn et al. (2005).

Example applications:- Thomas, Johansson, Maraston (2010, 2011)Used R of Korn et al. (2005).

Derived Ages, and 6 elements [X/Fe] for X=C,N,O,Mg,Ca,Ti (if [Fe/H]>-1 dex. Investigated chemical patterns in globular clusters.

Annibali et al. (2011)Used R of R Korn et al. (2005).

Derived Ages, & [/Fe] for dwarf and giant galaxies. Find strong morphology – [/Fe] relation (=ETG dwarf, = bright ETG).

Comparison between spheroidsGreen=E,S0 Red=S bulges Blue=LLE,E

(Sansom & Northeast 2008)

Trends of abundance patterns with galaxy and age.

Importance of element abundances[/Fe] ratios used as a clock for SF timescales and extents:-

– SNII provide rapid enrichment (,Fe rare elements) (108>t>3106 yrs)– SNIa provide extended enrichment over time (Fe) (prompt+delayed, t>108

yrs?)– IMS provide C,N,O enrichment (t>108 yrs)

Uncertainties:-

E.g. Tout 2005; Hashisu et al. 2008; Claeys et al. 2011Enrichment from SNIa - Uncertain progenitors (SD,DD,H,He,MCh,SubMCh...?) and timescale (delay time distributions).

Importance:-

We need to be able to accurately measure [α/Fe] ratios in stellar populations to be able to interpret them accurately.

New spectral measurements

Magnesium sensitive features at 5183 Å and 5528 Å - Measured [Mg/Fe] for 752 stars in MILES stellar library. (HR & MR results).[Mg/Fe] as a proxy for [/Fe].

(Milone, Sansom & Sanchez-Blazquez 2011

Fig 3).

See also the Poster by Milone et al. on abundance ratio measurements.

Example star:

[Mg/Fe] proxy for [/Fe](Milone et al. 2011: [Mg/Fe] catalogue)

HR Mg5183 Mg5548 Both

Model and Observed spectral changes: Varying [/Fe]

MILES spectral range

Recent models by Coelho et al. 2007

Observations :-MILES stellar library (Sanchez-Blazquez et al. 2006 – Teff, Log(g), [Fe/H]) Extended to include [Mg/Fe] (Milone et al. 2011) & [O/H] via Bensby et al. 2004, 2010.

Cassisi et al. 2004 models.

Ratios of enhanced /solar spectra Comparison of observations and theory

Dwarf stars

Theoretical (Coelho et al. 2007)

Empirical (MILES library 2006)

Ratio =

Dwarf stars: (Teff=5500K, Log(g)=4.0, [Z/H]=0.0) Giant stars: (Teff=4500K, Log(g)=2.0, [Z/H]=0.0) Find: Excess flux in blue region of enhanced spectra. Differences between obs. & theory. E.g. CaHK, Mg3835, CNO3862 (Serven et al. 2005).

0.04.0

FeFe

CNO3862

CN Mgb

00

Theoretical (Coelho et al. 2007)

Empirical (MILES library 2006)

Giant stars

Element response functions• Base model SSPs (empirical or theoretical spectra)

– Spectral line strengths (optical) versus Age & [Fe/H]– Lick standard – Worthey et al 2004, 2007

• Abundance pattern (theoretical spectra of stars)– Differential corrections. Approximate (weak lines)

– R=Response fn (Xi2Xi)

Xi=element i

– Correct from base star (I0) to new star [Fe/H], then [/Fe].– Response functions by Korn et al. 2005 (K05) widely used.

• Plot ratios of new/base indices for stars with the same Teff, Log(g).

][)( iXILn

3.0/

13.00 exp

iXn

inew iRII

Testing K05 Response functions: Fe-sensitive indicesObservations versus models

Key: = 1:1 line (agreement) = Cool dwarf stars (CD) = Turnoff stars (TO) = Cool giant stars (CG)Open symbols = low [Fe/H] <-0.4Error bars are plotted on base stars.

Similarly good agreement for other Fe sensitive features (Fe4383, Ca4455, Fe4531, C24668, Fe5015, Fe5270, Fe5335, Fe5406) plus weaker features (Fe5709, Fe5782).

Model ratio

Testing K05 Response functions: Fe-sensitive indices

Obs. versus models

Key: = 1:1 line (agreement) = Cool dwarf stars (CD) = Turnoff stars (TO) = Cool giant stars (CG)Open symbols = low [Fe/H] <-0.4Error bars are plotted on base stars.

Testing K05 Response functions: H-Balmer indicesObservations versus models

Key: = 1:1 line (agreement) = Cool dwarf stars (CD) = Turnoff stars (TO) = Cool giant stars (CG)Open symbols = low [Fe/H] <-0.4Error bars are plotted on base stars.

Testing K05 Response functions: -element indicesObservations versus models

Key: = 1:1 line (agreement) = Cool dwarf stars (CD) = Turnoff stars (TO) = Cool giant stars (CG)Open symbols = low [Fe/H] <-0.4Error bars are plotted on base stars.

Results: Fe sensitive features well modelled by K05 response functions (R[X/H]). H Balmer features – models differ from observations:

– H insensitive to [/Fe] in CG & TO .– H, K05 underestimates R for TO , over for CD & CG .

Mg, Ca, CN sensitive features – larger scatter (>errors). Caution for extrapolation (e.g. Mg1 TO ).

Differences between empirical and theoretical [/Fe] effects on spectra. Particularly important in the blue (<4500 Å). Empirical library will help to calibrate spectral responses to abundances. New observations underway to improve coverage for [/Fe] range.

U

BV

R

Blue excess for [/Fe]=0.4

Summary and InterpretationElement response functions R:-

• Fe indices – work well• H-Balmer – systematics – need revisiting• Mg, Ca, CN indices – large scatter but expected trends

Empirical stellar libraries:-• Vital for testing spectral dependence on abundance patterns • MILES library now has [Mg/Fe] measurements• Will improve -element measurements in populations• UV/Blue part most sensitive to abundance pattern.

Future:- H-Balmer indices – side bands influence on [/Fe] dependence. New features in the blue can now be explored. SNIa contributions need better understanding (level, progenitors, timing) for [/Fe] interpretation of extended SFHs.