Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994mnras.267..904s&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Vol. 267, NO. 4/APR15, P. 904, 1994
Astronomy and Astrophysics
Astronomy
27
Scientific paper
A method has been developed to correct for the effects of telluric absorption on CCD spectroscopy in the visible and near-infrared, and is applied specifically to near-infrared spectroscopy of extragalactic H II regions. The technique involves calculating the detailed telluric spectrum in the vicinity of emission or absorption lines using the data base of molecular absorption lines HITRAN92 (1992). A comparison of calculated absorption bands with those measured in observations of metal- poor `telluric reference' stars enables the quantity of water vapour to be self-consistently set. With the reasonable assumption that the same telluric band strengths occur in object data collected at similar times and locations in the sky, telluric effects can be determined and removed line by line. It is shown that, if a nebular emission line is narrow enough, it can be massively depleted by chance coincidence with a single telluric line. A popular technique for `correction' of telluric absorption in astronomical CCD spectroscopy is to divide the absorption- affected object spectrum by the continuum-normalized spectrum of a `telluric reference' star. This is shown to be an adequate approach only for sources with velocity dispersions greater than about 40 km s^-1^ for data where the telluric lines themselves are not cleanly resolved. For many objects of interest, however, such as the great majority of extragalactic H II regions, velocity dispersions are commonly less than 25 km s^-1^, so a more careful treatment is necessary to effect an accurate correction. As an example, the method is applied to the compact H II region SMC-N9.
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