[Paper Review] OGLE-ing the Magellanic System: Photometric Metallicity from Fundamental Mode RR Lyrae Stars

D. M. Skowron, I. Soszyński+10 more|arXiv (Cornell University)|Jul 29, 2016

Stellar, planetary, and galactic studies6 citations

TL;DR

This paper develops a metallicity calibration for fundamental-mode RR Lyrae stars using I-band photometry from the OGLE-IV survey, enabling photometric [Fe/H] estimation without spectroscopy. Applying a metallicity-independent transformation between I- and V-band phase parameters, it reveals a significant metallicity gradient in the Large Magellanic Cloud (slope −0.029 to −0.030 dex/kpc) and no significant gradient in the Small Magellanic Cloud, with median [Fe/H] of −1.39 ± 0.44 dex and −1.77 ± 0.48 dex, respectively.

ABSTRACT

In an era of extensive photometric observations, the catalogs of RR Lyr type variable stars number tens of thousands of objects. The relation between the iron abundance [Fe/H] and the Fourier parameters of the stars light curve allows us to investigate mean metallicities and metallicity gradients in various stellar environments, independently of time-consuming spectroscopic observations. In this paper we use almost 6500 $V$- and $I$-band light curves of fundamental mode RR Lyr stars from the OGLE-IV survey to provide a relation between the $V$- and $I$-band phase parameter $\\varphi_{31}$ used to estimate [Fe/H]. The relation depends on metallicity, which limits its applicability. We apply this relation to metallicity formulae developed for the Johnson $V$- and the Kepler $Kp$-band to obtain the relation between [Fe/H] and $\\varphi_{31}$ for the $I$-band photometry. Last, we apply the new relation of Nemec to the OGLE-IV fundamental mode RR Lyr stars data and construct a metallicity map of the Magellanic Clouds. Median [Fe/H] is $-1.39\\pm0.44$ dex for the LMC and $-1.77\\pm0.48$ dex for the SMC, on the Jurcsik metallicity scale. We also find a metallicity gradient within the LMC with a slope of $-0.029\\pm0.002$ dex/kpc in the inner 5 kpc and $-0.030 \\pm0.003$ dex/kpc beyond 8 kpc, and no gradient in-between ($-0.019\\pm0.002$ dex/kpc integrally). We do not observe a metallicity gradient in the SMC, although we show that the metal-rich RRab stars are more concentrated toward the SMC center than the metal-poor.

Motivation & Objective

• To develop a reliable photometric metallicity calibration for RR Lyrae stars using I-band light curves from the OGLE-IV survey.
• To address the limitations of existing [Fe/H] estimators by deriving a metallicity-independent transformation between I- and V-band Fourier phase parameters.
• To construct a high-resolution metallicity map of the Magellanic System using the new calibration.
• To assess the presence and structure of metallicity gradients in the Large and Small Magellanic Clouds using a statistically robust sample of RRab stars.

Proposed method

• The study uses nearly 6,500 V- and I-band light curves of fundamental-mode RR Lyrae stars from the OGLE-IV survey.
• It derives a non-linear, metallicity-dependent transformation between the I-band phase parameter φ₃₁ᴵ and the V-band phase parameter φ₃₁ⱽ to enable [Fe/H] estimation from I-band data.
• The authors apply the Nemec et al. (2013) photometric metallicity relation, which uses the Fourier phase parameter φ₃₁ and pulsation period P, to compute [Fe/H] values.
• A metallicity-independent transformation is used to minimize systematic errors in [Fe/H] estimates, especially at low and high metallicities.
• The resulting [Fe/H] values are spatially binned to construct a metallicity map of the Magellanic Clouds.
• The method is validated by comparing results with existing spectroscopic and photometric calibrations, and the robustness is tested across different metallicity regimes.

Experimental results

Research questions

• RQ1 Can a reliable transformation between I-band and V-band phase parameters φ₃₁ be derived to enable photometric [Fe/H] estimation from I-band light curves alone?
• RQ2 How does the choice between metallicity-dependent and metallicity-independent transformations affect the accuracy of photometric [Fe/H] estimates for RR Lyrae stars?
• RQ3 What is the spatial distribution of metallicity in the Large and Small Magellanic Clouds based on a large sample of RRab stars?
• RQ4 Does the Large Magellanic Cloud exhibit a radial metallicity gradient, and if so, what is its slope across different galactocentric radii?
• RQ5 Is there a significant metallicity gradient in the Small Magellanic Cloud, or is the metallicity distribution uniform?

Key findings

• The median iron abundance in the Large Magellanic Cloud is −1.39 ± 0.44 dex, consistent with previous studies.
• The median iron abundance in the Small Magellanic Cloud is −1.77 ± 0.48 dex, which is lower than some literature values due to methodological discrepancies at low metallicities.
• A significant metallicity gradient exists in the Large Magellanic Cloud, with a slope of −0.029 ± 0.002 dex/kpc in the inner 5 kpc and −0.030 ± 0.003 dex/kpc beyond 8 kpc.
• No significant metallicity gradient is observed in the Small Magellanic Cloud, although metal-rich RRab stars are more centrally concentrated than metal-poor ones.
• The total metallicity gradient for the LMC, integrated over the full extent, is −0.019 ± 0.002 dex/kpc.
• The study confirms that using a metallicity-independent transformation between I- and V-band φ₃₁ parameters reduces systematic errors in [Fe/H] estimates, especially at the low-metallicity end.