Faint southern spectrophotometric standard stars

Abstract

Context. The advent of the Extremely Large Telescope (ELT) will increase the collecting area by more than an order of magnitude compared to the individual Unit Telescopes of the Very Large Telescope (VLT). Fainter spectrophotometric standard stars than those currently available in the V = 11 to 13 mag (K = 12 to 14 mag) range are required for spectroscopic observations with instruments such as the Multi-AO Imaging Camera for Deep Observations (MICADO) on the ELT, notably in the near-infrared wavelength regime. Aims. We identify suitable spectrophotometric standard stars among white dwarfs with hydrogen atmospheres (DA white dwarfs) in the magnitude range K =14 to 16 mag and provide reference data based on stellar model atmospheres. Methods. We observed 24 candidate DA white dwarfs with the X-shooter instrument on the VLT, covering the wavelength range 300 nm to 2480 nm in three arms. We took care to include stars at latitudes below and above -25 degrees to allow observations for all wind directions at the location of the ELT. The spectra were analysed using model fluxes from 3D pure-hydrogen local thermodynamic equilibrium model atmospheres and multi-band photometry. From the sample of observed targets, we selected 14 reliable flux calibrators. For these targets, the residuals from the match between the model best-fit models and the observed spectra across the full wavelength range are < 3%, with the exception of the UV regions affected by the ozone Huggins bands (300 nm - 340 nm) and regions contaminated by telluric lines. Results. We have identified and fully characterised 14 DA white dwarfs that can be used as spectrophotometric standard stars for the MICADO instrument as well as any other future instrument with similar requirements in the brightness range, K = 14 to 16 mag (Vegamag), and provide reference fluxes