The Large Far-ultraviolet Flux Deficiency of α Doradus (B8 IIIp Si)

The bright southern star α Dor (HD 29305, B8 IIIp Si, V = 3.28) has not been extensively studied. A query of SIMBAD ¹ yields only 139 references for this bright star. α Dor is listed in Renson's catalog of Chemically Peculiar stars as a A0p Si star (Renson & Manfroid 2009) which means that it has overabundances of silicon. In their volume limited survey of magnetic CP stars within 100 parsecs of the Sun, Sikora et al. (2019a, 2019a) provide determinations of the effective temperature, surface gravity, projected rotational velocity ${v}_{{\rm{e}}}\sin i$ and abundances of silicon, titanium, chromium and iron in α Dor (the abundances of other elements seem to be unknown in α Dor). They also provide a measurement of the dipole field strength, B_d  ≃ 349 Gauss, which remains a modest field in comparison to the other fields determined in their sample.

α Dor has been observed on 1988 February 11 with the International Ultraviolet Explorer in the frame of the USSBS program (US Bright Star Survey, PI: Dr. Cathy Imhoff). Two high resolution SWP (Short Wavelength Prime camera observing from 1200 up to 2000 Å) spectra have been obtained through the large aperture. They are separated by about 9 hr and one LWP spectrum (Long Wavelength Prime camera observing from 1800 up to 3200 Å) was also taken. Observations through the large aperture allowed to measure fluxes. These unpublished spectra were retrieved from the Mikulski Archive Space Telescopes. ² The normal late B-type giant ζ Draconis has been routinely observed with IUE in the frame of the PHCAL program (Photometric Calibration) as it is a bona-fide constant star (its flux remains constant over time). The seven high resolution SWP spectra of ζ Draconis obtained through the large aperture are also unpublished and were also retrieved from the MAST archive. In this study, ζ Draconis serves as a comparison star to α Dor because it has a similar effective temperature, surface gravity and projected rotational velocity but nearly normal i.e., solar chemical composition.

The purpose of this note is to compare the far-ultraviolet spectral energy distribution of α Dor to that of ζ Draconis. A number of strong Si ii lines are expected shortwards of 2000 Å, they should substantially lower the flux in α Dor compared to ζ Draconis. The two SWP spectra of α Dor separated by 9 hr are also compared to look for flux variations.

Several strong and broad absorption features are observed in the far-UV energy distributions of α Dor and ζ Draconis as can be seen in Figure 1. The mean SWP spectrum of ζ Draconis and SWP32901 for α Dor have been normalized to their respective fluxes at 1800 Å in order to compare them. These features can be identified by running a spectrum synthesis for the parameters of these stars. The following fundamental parameters: T_eff = 12,392 ± 250 K and $\mathrm{log}\ g$ = 3.94 ± 0.25 dex for α Dor and T_eff = 12,900 K and log = 3.90 for ζ Draconis are obtained by using the Strömgren photometry of α Dor and ζ Dra taken from Hauck & Mermilliod 1998 and running Napiwotzky's UVBYBETA code (Napiwotzki et al. 1993). Two model atmospheres were then computed for these parameters using ATLAS9 (Kurucz 1992) assuming local thermodynamical equilibrium, hydrostatic equilibrium and radiative equilibrium. Synthetic spectra were then conputed with SYNSPEC49 (Hubeny & Lanz 1992) using the linelist gfall08Oct17.dat retrieved fom B. Kurucz's site ³ (Kurucz 2018) for the abundances derived by Sikora et al. (2019a, 2019a) in order to identify the strongest features in the far-UV SED of α Dor.

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As can be seen in Figure 1, the flux is severely depressed in α Dor compared to ζ Draconis due to the much stronger lines identified below. Several of the strong absorption features can be explained by Si ii lines. The radial velocity corrected positions of the line centers and their identifications provided by the spectrum synthesis are as follows: λ₁ = 1260.35 Å (Si ii 1260.42 Å), λ₂ = 1264.82 Å (Si ii 1264.73 Å, Si ii 1265.00 Å), λ₃ = 1305.85 Å (Si ii 1305.59 Å), λ₄ = 1309.35 Å (Si ii 1309.28 Å, Mg ii 1309.44 Å, Si ii 1309.45 Å), λ₅ = 1335.76 Å (C ii 1335.66 Å, C ii 1335.71 Å), λ₆ = 1485.33 Å (P ii 1485.50 Å, Si ii 1485.50 Å), λ₇ = 1526.71 Å (Si ii 1526.71 Å), λ₈ = 1533.43 Å (Si ii 1534.02 Å).

A broad flux depression is also observed in the spectrum of α Dor from 1350 to 1450 Å. This structure is much weaker in the spectrum of ζ Dra. According to Lanz et al. (1996), the broad flux depresion observed around 1400 Å in Bp stars could be explained by autoionization lines of Si ii (resonances in the continuous cross-section of Si ii). Their Table 2 lists the approximate wavelengths of four autoionization lines in this region at 1374.0, 1384.0, 1389.0 and 1437.0 Å. On the smoothed SWP32901, these wavelengths do coincide with observed absorption features but they do not account for all the absorption features from 1350 up to 1450 Å.

The comparison of the two SWP spectra of α Dor taken 9 hr apart reveals flux variations of modest amplitude. SWP32901 has slightly more flux than SWP32900 at wavelengths shorter than 1320 Å and then less flux at wavelengths greater than 1500 Å. A null-wavelength region where both spectra coincide is observed from 1300 to 1500 Å. A 9 hr interval represents only 13% of the rotational period (P_rot = 2.943176 days) determined by Sikora et al. (2019a, 2019a) , it is therefore not surprising to observe only modest variations. Note that TESS observed α Dor for almost 27 days in Sector 22. The simple aperture photometry (SAP) from the 2 minutes cadence light curve of α Dor was retrieved from MAST (Mikulski Archive at Space Telescope. It reveals a clear single-wave structure with a period close to 2.9 days and a peak-to-peak amplitude of about 10 ppt. The most likely explanation of these variations is rotational modulation by chemical spots over the surface of α Dor.

The results of this investigation show that the far-UV energy distribution of α Dor is clearly deficient compared to that of the normal star ζ Draconis. This deficiency is partly explained by the strengthening of Si ii lines in α Dor and reinforces the classification of α Dor as a Bp Si star. However other currently unidentified lines are also important absorbers.The FUV flux of α Dor shows modest variability over a timescale of 9 hr which probably is caused by inhomogeneous distribution of silicon and other species over the stellar surface. Further work is necessary to determine which species (aside from silicon) are important absorbers in α Dor.