Astron. Astrophys. 355, 1080-1086 (2000)

2. Observations

2.1. Longitudinal magnetic field measurements

The longitudinal magnetic field represents the disc-integrated line-of-sight component of the magnetic field. Observations of the longitudinal fields of HD 12288 and HD 14437 were obtained at the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO) and at the University of Western Ontario Elginfield Observatory (UWO).

2.1.1. SAO measurements

Thirteen longitudinal field measurements of HD 12288 and 28 measurements of HD 14437 were obtained at the SAO, using the 6 metre telescope and main stellar spectrograph equipped with an achromatic circular polarisation analyser. SAO magnetic data obtained before JD 2445901 were recorded on Kodak IIaO photographic plates, with a reciprocal linear dispersion of . Those obtained after JD 2450000 were measured from CCD spectra, recorded using the spectroscopic CCD with a reciprocal linear dispersion of . The longitudinal field was inferred from measurements of the relative displacement of spectral lines in the left and right circular polarisation spectra. Quoted uncertainties are at the 1 level, and were computed from the distribution of the displacement measurements. The SAO equipment and observing technique are described by Glagolevski et al. (1986) and Borisenko et al. (1991).

In addition to the spectropolarimetric measurements, two measurements of HD 14437 obtained on JD 2449555 and 2449556 were acquired using the SAO photoelectric Balmer line magnetometer. This instrument is similar in principle to the UWO photoelectric polarimeter described below.

2.1.2. UWO measurements

Seven longitudinal field measurements of HD 12288 and 6 measurements of HD 14437 were obtained at UWO. The UWO photoelectric polarimeter was used as a Balmer line Zeeman analyser at the UWO 1.2 metre telescope. The polarimeter measures the fractional circular polarisation in the wings of H, isolated using narrowband interference filters, at Å from line centre. It has been shown (Landstreet 1982) that the polarisation thus measured is linearly related to the mean longitudinal magnetic field. For these observations, mean conversion factors  G and  G per percent circular polarisation were found respectively for HD 12288 and HD 14437 from H line scans. Uncertainties associated with the UWO measurements were determined from photon counting statistics, and are at the 1 level. A more detailed description of the instrument and observing technique is given by Landstreet (1980).

As will become apparent, the different measurement techniques described above can result in systematic differences between the longitudinal field as measured using metallic lines (denoted ) and that measured using Balmer lines (denoted ). We will retain these different symbols to help reinforce that the two methods do in fact measure somewhat different quantities, and we should therefore not expect them to be fully consistent.

2.2. Mean magnetic field modulus measurements

Respectively 28 and 32 spectroscopic observations of HD 12288 and HD 14437 were obtained by Mathys et al. (1997) and Mathys et al. (1999) in the context of a larger study of magnetic fields in Ap stars. From these spectra measurements of the mean magnetic field modulus were obtained. The field modulus, which represents the disc integrated strength of the magnetic field, was inferred from fits to the magnetically split components of the Fe  II Zeeman doublet. The relevant observing, reduction and analysis procedures are described in detail by Mathys et al. (1997).

For HD 12288 the mean value of is 7978 G, with a minimum measured value of 7439 G and a maximum value of 8496 G, while for HD 14437 the mean value of is 7665 G, with a minimum measured value of 7024 G and a maximum value of 8219 G. For HD 14437 Mathys et al. (1997) estimate the uncertainty of individual measurements to be about 200 G. While Mathys et al. do not estimate the uncertainty of individual measurements for HD 12288, they do provide the rms deviations (120 G) about the best-fit curve. We provisionally adopt this value as the 1 uncertainty of individual measurements.

2.3. HIPPARCOS photometry

Respectively 164 and 100 photometric measurements of HD 12288 and HD 14437 were obtained in the context of the HIPPARCOS mission, in the HIPPARCOS photometric system described in Volume 1 of The HIPPARCOS and Tycho Catalogues (ESA 1997). The measured magnitude (in the HIPPARCOS photometric system) of HD 12288 is 7.779 with an rms scatter of 0.016 mag, while that of HD 14437 is 7.258 with an rms scatter of 0.013 mag. The photometry is available in Volume 17 of the HIPPARCOS Catalogue, CD-ROM no. 3 (ESA 1997). In the following analysis a single spurious observation of HD 14437 (obtained on JD 2448844.510) and two spurious observations of HD 12288 (obtained on JD 2448024.534 and JD 2448313.639) have been removed.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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