Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

We report new observations of the infrared (IR) spectrum of low-current Th-Ar hollow cathode lamps with the 2 m Fourier transform spectrometer (FTS) at the National Institute of Standards and Technology. These observations establish more than 2400 lines that are suitable for use as wavelength standards in the range 900-4500 nm. The line list is used as input for a physical instrument model that provides the wavelength calibration for the Cryogenic High-Resolution IR Echelle Spectrometer (CRIRES), the European Southern Observatory's new high-resolution -->(R ~ 100,000) IR spectrograph at the Very Large Telescope. We have also observed the variation of the spectrum of Th-Ar lamps as a function of operating current. The results allow us to optimize the spectral output in terms of relative intensity and line density for operation on the telescope. Our results should be generally useful for wavelength calibration in near-IR astronomy, providing a high density of sharp, well-characterized emission lines with the ease and efficiency of operation of a commercial discharge lamp.

Context. In 1996, Sakurai's object (V4334 Sgr) suddenly brightened in the center of a faint Planetary Nebula (PN). This very rare event was interpreted as being the reignition of a hot white dwarf that caused a rapid evolution back to the cool giant phase. From 1998 on, a copious amount of dust has formed continuously, screening out the star that remained embedded in this expanding high optical-depth envelope. Aims. We present observations that we use to study the morphology of the circumstellar dust to investigate the hypothesis that Sakurai's Object is surrounded by a thick spherical envelope of dust.Methods. We acquired unprecedented, high angular-resolution, spectro-interferometric observations, with the mid-IR interferometer MIDI/VLTI, which resolved the dust envelope of Sakurai's object.Results. We report the discovery of a unexpectedly compact ( milliarcsec, AU assuming a distance of 3.5 kpc), highly inclined, dust disk. We used Monte Carlo radiative-transfer simulations of a stratified disk to constrain its geometric and physical parameters, although such a model is only a rough approximation of the rapidly evolving dust structure. Even though the fits are not fully satisfactory, some useful and robust constraints can be inferred. The disk inclination is estimated to be 75° ± 3° with a large scale height of 47 ± 7 AU. The dust mass of the disk is estimated to be . The major axis of the disk (132° ± 3°) is aligned with an asymmetry seen in the old PN which was re-investigated as part of this study. This implies that the mechanism responsible for shaping the dust envelope surrounding Sakurai's object was already at work when the old PN formed.

We report on an episode of extremely low precipitable water vapour (PWV) of approximately 0.1 mm with a duration of more than 12 h at European Southern Observatory's Paranal observatory [2635 m above sea level (asl)]. Such conditions are more commonly expected at sites at much higher altitude such as ALMA on the Chajnantor plateau (5000 m asl) or otherwise particularly dry sites such as locations in Antarctica. We provide a full account of the measurements of PWV and other relevant atmospheric parameters. An explanation of the observed conditions is given in terms of the prevailing meteorological pattern. Based on statistical evidence from measurements by VLT spectrographs (UVES and CRIRES) covering more than a decade, we find that PWV <0.2 mm can be expected on less than 1 per cent of the nights, while <0.5 mm is encountered on 6–7 nights per year (≈ 2 per cent). The scientific potential of using this small but significant fraction of observing time is illustrated in the context of service mode observing.

We report laboratory measurements to characterize the spectral output of platinum/chromium hollow cathode lamps containing neon carrier gas. The spectra were recorded photographically with the National Institute of Standards and Technology 10.7 m normal-incidence spectrograph. The lamps investigated are equivalent to the lamps used for wavelength calibration of the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. Wavelengths and intensities are given for more than 1200 lines in the wavelength range 1132-1827 Å. The uncertainty of the measured wavelengths is estimated to be ±0.0020 Å. During an aging test mimicking the operations on board STIS one Pt/Cr-Ne lamp has passed 1000 hr of operation and is still in excellent working condition. This suggests that such a lamp can be operated for significantly more than 10 years and that it will not be a limiting factor for the useful lifetime of STIS.