On the white dwarf cooling sequence with extremely large telescopes^⋆

¹ Dipartimento di FisicaUniversita’ di Roma Tor Vergata via Della Ricerca Scientifica 1, 00133 Roma, Italy
e-mail: bono@roma2.infn.it
² INAF, Rome Astronomical Observatory, via Frascati 33, 00040 Monte Porzio Catone, Italy
³ Astrophysics Research Institute, Liverpool John Moores University, 12 Quays House, Birkenhead CH41 1LD, UK
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany

Received: 16 July 2012
Accepted: 26 October 2012

Abstract

We present new diagnostics of white dwarf (WD) cooling sequences and luminosity functions (LFs) in the near-infrared (NIR) bands that will exploit the sensitivity and resolution of future extremely large telescopes. The collision-induced absorption (CIA) of molecular hydrogen causes a clearly defined blue turn-off along the WD (WDBTO) cooling sequences and a bright secondary maximum in the WD LFs. These features are independent of age over a broad age range and are minimally affected by metal abundance. This means that the NIR magnitudes of the WDBTO are very promising distance indicators. The interplay between the cooling time of progressively more massive WDs and the onset of CIA causes a red turn-off along the WD (WDRTO) cooling sequences and a well defined faint peak in the WD LFs. These features are very sensitive to the cluster age, and indeed the K-band magnitude of the faint peak increases by 0.2–0.25 mag/Gyr for ages between 10 and 14 Gyr. On the other hand, the faint peak in the optical WD LF increases in the same age range by 0.17 (V band) and 0.15 (I band) mag/Gyr. Moreover, we also suggest to use the difference in magnitude between the main sequence turn-off and either the WDBTO (optical) or the WDRTO (NIR). This age diagnostic is also independent of distance and reddening. Once again the sensitivity in the K band (0.15−0.20 mag/Gyr) is on average a factor of two higher than in the optical bands (0.10 [V band], 0.07 [I band] mag/Gyr). Finally, we also outline the use of the new diagnostics to constrain the CO phase separation upon crystallization.