The Behavior of Bp Si Stars in the far-UV—Paper III: HD 112413

Renson & Manfroid (2009) have classified HD 112413 (V = 2.88) as a SiEuCr late B-type, i.e., a B-type magnetic chemically peculiar star, whose atmosphere harbours overabundance of silicon, chromium and europium compared to normal mid B-type stars. Using Napiwotzki et al. (1993) UVBYBETA code, the following fundamental parameters of HD 112413 can be derived: T_eff = 11,600 ± 250 K and $\mathrm{log}\ g$ = 4.00 ± 0.25 dex. HD 112413 is actually the prototype of the class of Bp Si variables. It is a known spectroscopic (Pyper 1969) and magnetic variable (Borra & Landstreet 1977). Using Stokes IQUV Doppler imaging, Silvester et al. (2014) have produced an abundance map of HD 112413. The purpose of this series of Notes is to perform a systematic study of the far-ultraviolet energy distribution of Bp stars using all spectra collected with the Short Wavelength Pattern camera through the large aperture which guarantees to measure absolute fluxes. In this third Note, I report on the variability of the far-ultraviolet spectral energy distribution of HD 112413 over the observations carried out by the IUE. A search for HD 112413 in the Mikulski Archive for Space Telescopes ¹ reveals that this star has also been observed with TESS ² (Ricker et al. 2015). The TESS lightcurve of HD 112413 has been studied by Shultz et al. (2022) who derived an improved rotational period of 5.463 days. It is interesting to revisit this IUE data now that the rotational period is better known.

HD 112413 has been observed 58 times with IUE in the frame of programs MVDSL (PI: Dr. D.S. Leckrone), APDRP (PI: Dr. R.J. Panek) and AVHEB (Dr. E. Bohm-Vitense). Twenty-three spectra have been taken with the SWP camera (Short Wavelength Prime camera observing from 1200 up to 2000 Å) through the large aperture (eleven at high resolution and twelve at low resolution). The other spectra were acquired with the LWR/LWP camera (Long Wavelength Redundant/Prime Camera observing from 1900 up to 3300 Å) or with the SWP camera through the small aperture.

The high and low resolution large aperture SWP spectra were obtained from 1978 March to 1986 March. These spectra which are calibrated into absolute fluxes can be compared. In this note, I am only using the high resolution spectra which all have a similar exposure time of 60 s. The 11 high resolution spectra have been degraded to the low resolution of IUE to highlight flux variations and coadded into a mean spectrum. ³

The comparison of the rebinned high resolution spectra to the coadded rebinned spectrum reveals that the flux from 1200 up to 2000 Å varies by more than the 5% IUE photometric reproducibility. At maximum FUV flux, which corresponds to SWP15828, the flux in the pseudo-continuum window at 1691 Å is about 10% larger than at minimum FUV flux which corresponds to SWP15882. The amplitude of the variations increases toward shorter wavelengths reaching 25% at 1358 Å. These two spectra are compared in Figure 1. The FUV variations are partly due to strong absorption lines from 1300 to 1600 Å and autoionization lines of Si ii. The FUV continuum which is also shaped by the continuous absorption of Si ii probably varies too. Interestingly, the FUV variations occur in antiphase with the measurements of the optical brightness with the Fine Error Sensor on board IUE which collected 2184 counts in Fast Underlap mode at FUV minmum and 2079 counts at FUV maximum. This reflects a redistribution of the FUV flux toward the optical range which keeps the integrated flux constant as the star rotates.

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The conclusion of this short study is that the FUV flux of HD 112413 significantly varied in the available spectra of HD 112413 collected over 7 yr. Large variations of the line and continuous opacity of Si ii occur in the range 1400 up to 1600 Å in about 3 days and 5 hr, i.e., 59% of the rotational period. These variations occur in antiphase with the variations of the optical brightness monitored with the Fine Error Sensor which probably is a signature of the redistribution of the FUV flux toward the optical range. This also confirms that horizontal abundance gradients of silicon and iron-peak elements exist over the surface of HD 112413 as already shown by Silvester et al. (2014).