Spectrophotometry: Stellar Temperature and gravity ...cemw10/talks/Bessell_Ringberg.pdf4000 4800...

Ringberg May 2010 Slide 1

Spectrophotometry

Spectrophotometry:Stellar Temperature and gravity

Determination

Mike BessellRSAA

The Australian National University


Spectrophotometry

Outline

• Synthetic spectra grids

• Spectrophotometric atlasesSpectrophotometric standards

• Te calibrationsIR flux method

Spectrophotometric fitsComparison with other Te determinations

• Summary


Spectrophotometry

Synthetic spectra• Munari, Sordo, Castelli, Zwitter 2005, A&A 442,1127 http://archives.pd.astro.it/2500-10500/

3500K – 47500K, Kurucz-Castelli models, ODF http://wwwuser.oat.ts.astro.it/castelli/spectra.html

Higher resolution spectra. Generated using line-by-line.Some molecules missing eg CaHExtra C-rich models and some lower Z models also.

• Hauschildt, Allard, Baron, Aufdenberg, Schweitzer 2003 GAIASpectroscopy.. ASP 298, 2003 1000K-10000K GAIA- Phoenix13http://www.hs.uni-hamburg.de/EN/For/ThA/phoenix/index.htmlhttp://perso.ens-lyon.fr/france.allard/


Spectrophotometry

Synthetic spectra• MARCS spectra K. Eriksson, B. Gustafsson, B. Plez (Montpellier) and U.G.

Jorgensen (Copenhagen) have completed the last Uppsala grid ofplane-parallel and spherical LTE model atmospheres for late-typestars (Te < 8500 K). (The next step will be hydrodynamical 3-dimensional model atmospheres in non-LTE.)

http://www.marcs.astro.uu.se/2500K – 8500K, opacity sampled models and fluxesDetailed molecular opacities for cool stars


Spectrophotometry

New high precision spectral atlases• MILES - http://miles.iac.es Sanchez-Blasquez et al. 2006 MNRAS, 371, 703985 stars spanning a large range in atmospheric parameters. The spectra

cover the range 3525-7500 A at 2.3 A (FWHM) spectral resolution.

• The HST/STIS Next Generation Spectral Libraryhttp://archive.stsci.edu/prepds/stisngsl/index.html

374 (600) high S/N spectra (R~1000) of Hipparcos stars covering a widerange in abundance, effective temperature, and luminosity distributedover four metallicity groups:

• very low [Fe/H] < −1.5, low −1.5 < [Fe/H] < −0.5• near-solar −0.3 < [Fe/H] < +0.1, super-solar +0.2 < [Fe/H]• Wavelength range 2000A – 10200A

STIS spectra of low metallicity G dwarfs make excellent spectro-photometric standards!


Spectrophotometry

Effective temperature calibrations

• IR flux method - Blackwell & Shallis 1977Alonso, Arribas, Martinez-Rodgers 94,95,96,98,99Ramirez, Melendez 2005Casagrande, Ramirez, Melendez, Bessell, Asplund

2010 A&A 512, 54• Excellent agreement with direct measurements for

the mostly near-solar composition stars• Excellent agreement with 1D model atmosphere

red colors for solar composition stars• Least model-dependent method of estimating Teff• Good color-temperature calibrations provided


Spectrophotometry


Spectrophotometry

IRFM based color-temperature relationcompared with synthetic colors

Teff vs. (V - IC) and (V - K)relations for dwarf (top) andgiant (bottom) stars of [Fe/H]= +0.0 (solid lines) and -2.0(dotted lines) according toRamirez, Melendez 2005(thick black lines), Bessell(thin cyan lines), andHoudashelt et al. (2000;magenta lines).


Spectrophotometry

Observational spectrophotometry

For some time now I have been using the 2.3m DBSand WiFeS with John Norris and Norbert Christlieb tofollow-up prospective metal-poor stars from the HESsurvey. With the availability of the Heap-Lindlercorrected Gregg STIS spectra I have now producedrelative-absolute spectrophotometry of all of ourcandidates for which Te and log g were not known.

In addition, the claim by Ramirez &Melendez in 2005that the Te of the very low metallicity halo dwarfswere systematically hotter by more than 300K thanhitherto assumed needed critical examination.


Spectrophotometry

WiFeS spectra

0

1 105

2 105

3 105

4 105

4000 4800 5600 6400 7200 8000 8800

HD160617 120sec

Wavelength


Spectrophotometry

Castelli-Kurucz spectra used

Fitted observed fluxes with Castelli fluxes.Used the Vandenberg halo isochrones to guide thedeterminination of Te, log g and metallicitycombinations for halo stars.

If the initial Te, log g lay well away from the locus andSchlegel maps indicated reddening likely present,fluxes were dereddened thus increasing Te and log g.

Some indication that systematically low values (~0.2)for log g are determined with the α=1.25 spectra.


Spectrophotometry

Halo isochrone


Spectrophotometry

Observed gravity of Hipparcos subdwarfs


Spectrophotometry

HD140283 - MILES

Blue - 5750 3.75Red - 5800 3.4

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

1.20

3500 4000 4500 5000 5500 6000 6500 7000 7500

HD140283

Fla

mb

da

Wavelength Angstroms

old


Spectrophotometry

HD 19445 MILES

Blue - 5875 4.5Red - 6125 4.5

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

1.20

3500 4000 4500 5000 5500 6000 6500 7000 7500

HD19445

Fla

mbda


old


Spectrophotometry

BD +3 740 DBS

Blue - 6250 4.0Red - 6500 4.0

0.00

0.20

0.40

0.60

0.80

1.00

1.20

4000 4800 5600 6400 7200 8000 8800

BD +3 740F

lam

bda


old


Spectrophotometry

HE0130-2303 DBS

Blue - 6750 4.5Red - 6437 3.75

0.00

0.20

0.40

0.60

0.80

1.00

1.20

4000 4800 5600 6400 7200 8000 8800

HE0130-2303

Te fl

ux -

oth

er


old


Spectrophotometry

Automatic flux fitting program• Recently an automatic fitting program was written in

Python by Simon Murphy.• Over the wavelength range examined 3500A - (6200)

7200A, there is significant degeneracy in the fitted Teand log g for different reddening.

• Within the observational uncertainties the fits aresimilarly good.

• Initial results are very encouraging but it needstweaking to take into account isochrone gravities andpossible systematically different model atmosphereeffects

• Castelli higher resolution grids need to be extendedfully to lower metallicities.


Spectrophotometry

HD27561 MILES

E(B-V)=0.0

E(B-V)=0.02


Spectrophotometry

HE 2035-5820 C-richDBS

CD -38 245DBS

E(B-V)=0.0

E(B-V)=0.0


Spectrophotometry

HE0147-1500DBS

HE0107-5240DBS

E(B-V)=0.0

E(B-V)=0.0


Spectrophotometry

Flux fitted temperature comparisons

-600

-400

-200

0

200

400

600

4000450050005500600065007000

dFAGdRM giantdRMdwarfdNissen dwarfdAsplunddRNB dwarf

dT

e flu

x -

oth

er

Te flux


Spectrophotometry

Assumptions and uncertainties in 1Datmospheres

• Castelli ATLAS model atmospheres arecomputed with α = 1.25 as it gives goodagreement with the solar flux.

• Smalley & Kupka (1997) and Heiter et al (2002)suggest that α = 0.5 gives improved Stromgrencolors for stars between 6000K and 7500K.

• It was suggested that the 1D temperaturestructure of these stars is better determined byusing a less efficient convection treatment.Reflection of the higher temperature in upflowsassociated with the granulations in actual stars.


Spectrophotometry

Implicit spectral differences Heiter et al 2002


Spectrophotometry

Calibration of αMLT for solar like starsSteffen & Ludwig 1999 ASP 173, 217 No unique α for all observables

Balmer lines

B-V v V-R colors


Spectrophotometry

6000K 4.00 -2.0: a=0.5 and 1.25

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

4000 4800 5600 6400 7200 8000 8800

blue: !=0.5

black: !=1.25

Wavelength


Spectrophotometryα=0.5 spectra (blue) fitted with α=1.25 spectra

5250 4.0 -2.0α=0.5

5500 3.5 -2.0α=0.5

α=1.25 green

α=1.25 green


Spectrophotometryα=0.5 spectra (blue) fitted with α=1.25 spectra

6000 4.0 -2.0α=0.5

6500 4.0 -2.0α=0.5


Spectrophotometry

Possible convective treatment effect on Te


SpectrophotometrySummary

• IR flux method Teff good agreement with modelatmosphere 1D red colors for near-solar metallicityand halo stars.

• Synthetic spectra in general good agreement withempirical spectra – improvements needed for Mstars.

• Need to critically compare the spectrophotometricallyderived Te and log g for the 65 MILES stars and 5UMP stars with IRFM Casagrande et al derivedvalues to better assess model atmospheres Te andlog g.


SpectrophotometrySummary

• Spectrophotometry is a useful and robust method forderiving Te and log g for GK dwarfs. There aresystematic differences of ~50K and 0.1-0.2 in log gfor many FG stars because of the treatment ofconvection in 1D atmospheres.

• Interstellar reddening remains an importantconsideration when fitting colors and fluxes.

• Use hydrogen line profiles as a reddening freetemperature indicator - Barklem

• Reexamine effect of αMLT on synthetic spectra.Reconsider blue and violet calibrations.

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