Mid Infrared Spectroscopic Standards: Observations and Models

We present high precision N-band spectroscopy of a subsample of ESO mid infrared (MIR) calibration standard stars. The spectra were taken with the TIMMI2 instrument at the ESO/La Silla observatory. The low resolution spectra (R ≈ 160) were analyzed with an optimal extraction procedure specifically designed for TIMMI2 spectroscopy. The wavelength calibration was done using atmospheric lines. Our analysis package is based on IDL and will be made available to the community. We compare the spectro-photometric flux of the 1d-spectra to the spectral irradiance calibration models by Cohen et al. (1999). This allows us to demonstrate that the derived absolute calibration of the spectra is accurate to better than 5% in most cases. Our subsample of MIR calibration standards presented here shall become the primary calibrators of the VISIR instrument. 1 Observations Here is an overview on the basic parameters of the observations: Telescope: ESO 3.6m at La Silla, Chile Instrument: Thermal Infrared Multimode Instrument 2 (TIMMI2) Date of observation: 3 nights in January 2003 Type of observation: Spectroscopy Wavelength range: 7.56 14.04 μm (N-band) Resolution: 0.02 μm/pixel (λ/∆λ = 160) Slit size: 1. 2× 70 Seeing: 0. 7 1. 5 Airmass: ≈ 1.1 (exception: HD 37160 at 1.4) 2 Sources & Data Reduction The spectra of the eight MIR standard stars listed in Table 1 were reduced and analyzed using IDL routines specifically designed for this purpose (see Siebenmorgen et al. (2004) or this issue for more details). The wavelength calibration was done using the ozone atmospheric feature. An extinction correction method for TIMMI2 spectra is presented by Schutz & Sterzik (this issue) but has not been applied yet. ESO Symposia: High Resolution Infrared Spectroscopy in Astronomy, pp. 96–99, 2005. © Springer-Verlag Berlin Heidelberg 2005 Mid Infrared Spectroscopic Standards 97 Table 1. HD number Spectral type Variable Other Model star? designations spectrum by (1) 29291 G8lll no H 32887 K4lll yes Lep C 37160 G8lll-lV no C 81420 K5lll no C 108903 M3.5V yes γ Crux C 110458 K0lll no C 123139 K0lllb no C 133774 K5lll no C (1) Model spectra taken from Cohen et al. (1999) (C) and P. Hammersely (H) 3 Questions There are two main questions to be answered within the course of this work: 1. How well do the measured spectra correspond to the model N-band spectra by Cohen et al. (1999)? 2. Are the results influenced by the choice of calibration source? 4 Results In the following, the results of two out of the three observation nights are presented separately. Each standard star was calibrated with every appropriate other standard star, i.e. one measured at similar airmass and at the same night. When inspecting Fig. 1, it becomes apparent that HD108903 is the only standard star which shows significant deviations from the model spectrum. It is also the only M-type star of the sample. In general it can be stated that the atmospheric ozone absorption feature could not be eliminated in all spectra during the data reduction. Fig. 2 shows that HD 37160 is the only star showing a significant absorption feature at ≈ 12.5 μm. There are two possible reasons for that: • HD 37160 has a real feature at 12.5 μm. • While HD29291 and all the stars from the previous night were observed at an airmass of 1.1, HD 37160 is the only star observed at an airmass of 1.4. Consequently, it could only be calibrated by stars observed at a lower airmass. Thus, it is very likely that this absorption feature is just the residual of the well-known atmospheric absorption line at 12.55 μm which could not be totally eliminated by the reduction routine. This unwanted effect is due to the different airmasses of the science observation and the calibration measurement. 98 Katrin Kampgen & Ralf Siebenmorgen 8 9 10 11 12 13 wavelength (μm) 50 100 150 200 flu x (J y) HD123139 +20 Jy +50 Jy +80 Jy +100 Jy HD133774 HD81420 HD110458 HD108903 HD32887 8 9 10 11 12 13 wavelength (μm) 0 10 20 30 40 flu x (J y) HD110458 +4 Jy +9 Jy +14 Jy +19 Jy HD133774 HD123139 HD81420 HD108903 HD32887 8 9 10 11 12 13 wavelength (μm) 10 20 30 40