New NIR light-curve templates for classical Cepheids^⋆

¹ Dipartimento di FisicaUniversità di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy
e-mail: laura.inno@roma2.infn.it
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany
³ Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁴ INAF–OAR, via Frascati 33, 00040 Monte Porzio Catone, Rome, Italy
⁵ Observatories of the Carnegie Institution of Washington 813 Santa Barbara Street, Pasadena 91101, Los Angeles, USA
⁶ INAF–Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy
⁷ Departamento de Astronomia, Universidad de Concepciòn, Casilla 160-C Concepciòn, Chile
⁸ Koninklijke Sterrenwacht van van België, Ringlaan 3, 1180 Brussel, Belgium
⁹ Astronomical Institute, Graduate School of Science, Tohoku University, 6-3 Aramaki, Aoba-ku, Sendai, 980-8578 Miyagi, Japan
¹⁰ South African Astronomical Observatory, PO Box 9, Observatory 7935 Cap Town, South Africa
¹¹ Department of Physics and Astronomy, N283 ESC, Brigham Young University, Provo, UT 84601, USA
¹² Astronomical Institute Anton Pannekoek, Science Park 904, PO Box 94249, 1090 GE Amsterdam, The Netherlands
¹³ Department of Astrophysics, Nagoya University, Furo-cho, Chikusa-ku, 464-8602 Aichi, Nagoya, Japan
¹⁴ Department of Physics and Astronomy, Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, 890-0065 Kagoshima, Japan
¹⁵ Institute of Astronomy, School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, 181-0015 Tokyo, Japan
¹⁶ INAF–Oss. Astr. di Trieste, via Tiepolo 11, 34131 Trieste, Italy
¹⁷ Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland

Received: 13 June 2014
Accepted: 12 October 2014

Abstract

Aims. We present new near-infrared (NIR) light-curve templates for fundamental (FU, J, H, K_S) and first overtone (FO, J) classical Cepheids. The new templates together with period-luminosity and period-Wesenheit (PW) relations provide Cepheid distances from single-epoch observations with a precision only limited by the intrinsic accuracy of the method adopted.

Methods. The templates rely on a very large set of Galactic and Magellanic Cloud Cepheids (FU, ~600; FO, ~200) with well-sampled NIR (IRSF data set) and optical (V, I; OGLE data set) light-curves. To properly trace the change in the shape of the light-curve as a function of pulsation period, we split the sample of calibrating Cepheids into ten different period bins. The templates for the first time cover FO Cepheids and the short-period range of FU Cepheids (P ≤ 5 days). Moreover, the phase zero-point is anchored to the phase of the mean magnitude along the rising branch. The new approach has several advantages in sampling the light-curve of bump Cepheids when compared with the canonical phase of maximum light. We also provide new empirical estimates of the NIR-to-optical amplitude ratios for FU and FO Cepheids. We perform detailed analytical fits using seventh-order Fourier series and multi-Gaussian periodic functions. The latter are characterized by fewer free parameters (nine vs. fifteen).

Results. The mean NIR magnitudes based on the new templates are up to 80% more accurate than single-epoch NIR measurements and up to 50% more accurate than the mean magnitudes based on previous NIR templates, with typical associated uncertainties ranging from 0.015 mag (J band) to 0.019 mag (K_S band). Moreover, we find that errors on individual distance estimates for Small Magellanic Cloud Cepheids derived from NIR PW relations are essentially reduced to the intrinsic scatter of the adopted relations.

Conclusions. Thus, the new templates are the ultimate tool for estimating precise Cepheid distances from NIR single-epoch observations, which can be safely adopted for future interesting applications, including deriving the 3D structure of the Magellanic Clouds.