Determination of [α/fe] and its Application to SEGUE F/G Stars. Young Sun Lee

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1 Determination of [α/fe] and its Application to SEGUE F/G Stars Young Sun Lee Research Group Meeting on June 16, 2010

2 Outline Introduction Why [α/fe]? Determination of [α/fe] Validation of estimate of [α/fe] Application to SEGUE F/G Stars Summary

3 Why [α/fe]? Alpha elements (O, Mg, Si, Ca, Ti) - formed in massive stars (> 8 Msun) and ejected by SNe II into ISM => alpha enhanced stars formed - a few million time scale => rapid star formation in massive systems Large fraction of Fe peak elements (Ni, Co, etc) - produced in SNe Ia => low α stars formed - a few Gyr time scale => slow star formation Good signatures for tracing star formation and chemical enrichment history; easily separate different populations Note : [α/fe] = <[Mg/Fe] + [Si/Fe] + [Ca/Fe] + [Ti/Fe]>

4 [α/fe] in The Galaxy and dsphs Geisler et al. 2007

5 SEGUE I, II SEGUE I -Use existing SDSS hardware and software -Photometric and spectroscopic survey, R ~ ,500 sq. deg. over b < 35 and South Galactic sky plate pairs of 45 / 135 minute exposures for stars with 14.5 < g < 20, 240,000 stars - Wavelength coverage : 3800 ~ 9100 Å - July 2006 ~ mid July 2008 SEGUE II - Dedicated stellar spectroscopic survey - About 210 plates => 130,000 stars

6 Grid of Synthetic Spectra Automated way needed due to a large number of stars => use grid of synthetic spectra Grid of synthetic spectra Teff = 4000 ~ 8000 K ( 250 K), log g = 1.0 ~ 5.0 ( 0.2 dex) [Fe/H] = -4.0 ~ +0.4 ( 0.2 dex) => -4.5 ~ +1.0 [α/fe] = -0.1 ~ 0.6 ( 0.1 dex) λ = Å

7 Wavelength Region Teff= 5500K, log g = 4.4, [Fe/H] = -1.4, [α/fe] = 0.0 Teff= 5500K, log g = 4.4, [Fe/H] = -1.4, [α/fe] = 0.4 Teff= 5500K, log g = 3.4, [Fe/H] = -1.4, [α/fe] = 0.4

8 χ2 Minimization χ2 minimization with synthetic templates - Calculate χ2 in Å region by χ2 = O S 2 / 2 = f(teff, log g, [Fe/H], [α/fe]) - Teff fixed, change log g, [Fe/H], and [α/fe] to minimize χ2

9 Validations of [α/fe] Determination Compare with 425 ELODIE Stellar Library (Moultaka et al. 2004) - assembled abundances of the alpha elements from the literature and took averages Compare with 91of HET (Carlo Allende Prieto) and ESI (David Lai) spectra for which we have SEGUE spectra - ESI spectra are mostly metal-poor giants, so good simple to check on α-measurement at the metal-poor end

10 Checks on Determination of [α/fe]

11 Noise Injection Experiments Checks on effects of S/N on α-measurment Inject Gaussian noise on the ELODIE spectra Emulate the SEGUE noise and inject it on the SEGUE spectra with the HET data Noise injection test sigma < 0.1 down to S/N = 20 S/N Full ELODIE Δ([α/Fe]) σ([α/fe]) S/N Full HET Δ([α/Fe]) σ([α/fe])

12 Covariance BWT log g and [α/fe] Mg b/h is the dominant feature in the wavelength region ( Å) => gravity sensitive => so possible to have a giant as a low-α dwarf? Check on this with the HET and ESI data - includes some metal-poor giants

13 Validation with High Resolution Data

14 Application to SEGUE F/G Stars Sample - SEGUE F/G type stars in the color range of 0.2 < g-r < Among 70,000, about 48,000 selected with well determined stellar parameters, including [α/fe], from the SSPP, radial velocities, distances, and proper motions - computed U, V, W, Vφ, Rapo, Rperi, Zmax, and eccentricity - selected ~ 20,000 stars with log g > 4.2, S/N > 30, Z < 2 kpc, 7 < R/kpc < 10, [Fe/H] < -1.4, and Vφ > 50 km/s

15 Thin and Thick Disk Stars Further division of the sample - high alpha ([α/fe] > 0.3) : thick disk stars - low alpha ([α/fe] < 0.2) : thin disk stars => investigate general properties of the separated two populations; hence possibly formation of the disks Radial migration models - thick disk naturally emerged by radial movement of stars - claim that metal-poor thin disk stars came from the outer disk, while the metal-rich stars from the inner disk - predict the low metallicity thin disk stars will have higher rotational velocity and larger mean orbital radius

16 Distribution of [Fe/H] and Vφ

17 Rmean, Vφ, and e in [Fe/H] and [α/fe]

18 Distribution of Eccentricity Our sample Sales et al. (2009)

19 Summary [α/fe] determination is as good as ~ 0.1 dex at S/N = 20 in wide ranges of log g and [Fe/H]. Larger rotational velocity and larger mean orbital radius among the metal-poor thin disk stars indicate that the stellar migration may play a role in the formation of the disks. => Distribution of eccentricity supports this idea.

20 UVW Velocities

21 Cylindrical Coordinates

22 Classification : Abundances VS Kinematics