[Paper Review] The VLT-FLAMES Tarantula Survey XVI. The optical+NIR extinction laws in 30 Doradus and the photometric determination of the effective temperatures of OB stars

J. Maíz Apellániz, C. J. Evans+12 more|arXiv (Cornell University)|Feb 13, 2014

Stellar, planetary, and galactic studies50 references58 citations

TL;DR

This paper presents a new family of optical and near-infrared (NIR) extinction laws tailored for the 30 Doradus region, derived from VLT-FLAMES and HST/WFC3 data. By applying these laws to OB stars, it demonstrates that effective temperatures can be determined photometrically with moderate uncertainties and minimal bias up to E(4405-5495) ~ 1.5 mag, offering a robust alternative to traditional CCM laws with improved accuracy in the optical and NIR.

ABSTRACT

Context: The commonly used extinction laws of Cardelli et al. (1989) have limitations that, among other issues, hamper the determination of the effective temperatures of O and early B stars from optical+NIR photometry. Aims: We aim to develop a new family of extinction laws for 30 Doradus, check their general applicability within that region and elsewhere, and apply them to test the feasibility of using optical+NIR photometry to determine the effective temperature of OB stars. Methods: We use spectroscopy and NIR photometry from the VLT-FLAMES Tarantula Survey and optical photometry from HST/WFC3 of 30 Doradus and we analyze them with the software code CHORIZOS using different assumptions such as the family of extinction laws. Results: We derive a new family of optical+NIR extinction laws for 30 Doradus and confirm its applicability to extinguished Galactic O-type systems. We conclude that by using the new extinction laws it is possible to measure the effective temperatures of OB stars with moderate uncertainties and only a small bias, at least up to E(4405-5495) ~ 1.5 mag.

Motivation & Objective

• To develop a new family of extinction laws for the 30 Doradus region that improve upon the limitations of the widely used CCM (1989) laws.
• To test the applicability of these new laws to extinguished Galactic O-type stars and assess their general validity beyond 30 Doradus.
• To evaluate the feasibility of deriving effective temperatures (T_eff) of OB stars using only optical and NIR photometry, corrected with the new extinction laws.
• To address key shortcomings of the CCM laws, including incorrect parameterization of extinction (e.g., R_V vs. R_5495), functional form in the optical, and NIR behavior.
• To provide a practical, calibrated tool for extinction correction in high-precision photometric surveys, such as Gaia and LSST, where accurate T_eff determination is critical.

Proposed method

• Utilized high-precision spectroscopy and NIR photometry from the VLT-FLAMES Tarantula Survey (VFTS) and optical photometry from HST/WFC3 for 30 Doradus.
• Applied the CHORIZOS software to analyze the data under various extinction law assumptions, fitting observed magnitudes to synthetic SEDs.
• Derived a new family of extinction laws by fitting the observed color excesses (E(4405-5495)) and R_5495 values across the optical and NIR, using a monochromatic parameterization.
• Validated the new laws by comparing predicted photometry with observed data, ensuring consistency across different filter systems (Johnson-Cousins, Strömgren, WFC3).
• Used a normalization scheme to emphasize differences in extinction curves across wavelengths, particularly in the optical and NUV ranges.
• Tested the robustness of the new laws by comparing them with CCM laws and assessing discrepancies at different R_5495 values and extinction levels.

Experimental results

Research questions

• RQ1 Can a new family of extinction laws be derived for 30 Doradus that better describe the observed extinction in the optical and NIR than the CCM (1989) laws?
• RQ2 To what extent are the new extinction laws applicable to other regions, particularly to Galactic O-type stars with known spectroscopic parameters?
• RQ3 Can effective temperatures of OB stars be reliably determined from optical and NIR photometry alone when corrected with the new extinction laws?
• RQ4 How do the new extinction laws compare with CCM in terms of functional form, parameterization (especially R_5495 vs. R_V), and behavior in the NIR?
• RQ5 What is the maximum extinction level (E(4405-5495)) for which the photometric T_eff determination remains accurate and unbiased using the new laws?

Key findings

• A new family of optical and NIR extinction laws was successfully derived for 30 Doradus, showing improved accuracy over the CCM (1989) laws, especially in parameterization and functional form.
• The new laws are validated as applicable to extinguished Galactic O-type stars, confirming their broader relevance beyond the Tarantula Nebula.
• Photometric determination of effective temperatures for OB stars using the new laws yields uncertainties of only a few percent and a small systematic bias, even at moderate extinctions.
• The method enables reliable T_eff estimation up to E(4405-5495) ~ 1.5 mag, with the potential for extension to higher extinctions with further data.
• The study confirms that the CCM parameterization using R_V is fundamentally flawed, and that monochromatic parameters like R_5495 are essential for accurate extinction correction.
• The NIR extinction law used here (power law from Rieke & Lebofsky 1985) is shown to be adequate for low to intermediate extinctions (≤2.0 mag), though sightline-dependent variations may exist.