Astron. Astrophys. 331, 639-650 (1998)

3. Atmospheric parameters

3.1. Spectroscopic indicators

The Balmer lines dominate the spectrum; next in intensity are the Ca II K line and Mg II    4481. The atoms and ions identified in the visual spectrum include H I, Mg I, Mg II, Al I, Si II, Ca I, Ca II, Sc II, Ti II, Cr I, Cr II, Mn I, Fe I, Fe II, Ni II, Sr II and Ba II. An identification of V II remains doubtful, and that of Zr II even more so; at best the stronger lines of V II multiplet 33 and Zr II multiplet 41 (Moore 1945 ) coincide with only very faint stellar features. Owing to significant rotational broadening, almost all metal lines are shallower than 10%. The FWHM of the best isolated lines, i.e. Fe II    4508, Ti II    4501, Ti II    4572, and Mg II    4481, implies that computed profiles should be blurred by a rotational broadening corresponding to  = 115   5 km s^-1. This error estimate includes only the uncertainty in the continuum level and the internal rms scatter between results from different lines.

Houk (1978 ) classified TV Pic as A2 V, but it is well known that the spectra of a significant fraction of A dwarfs show discordant spectral types according to the classification criteria adopted: Balmer, Ca II K or metallic lines. These ambiguities may be present not only in TV Pic but also in the A-type stars that we adopted to make a relative assessment of TV Pic: o Peg, a well-studied sharp-lined A1 V star (Maestre & Deutsch 1961 , Conti & Strom 1968a , Adelman 1973 ) and the A-type stars in Conti & Strom (1968b ). Comparisons show that the metallic spectrum of TV Pic corresponds to a lower temperature than A2 V. Relative to o Peg and other early-A stars, lines of neutral atoms are more prominent than those of the ions: e.g. Cr I    4254.3 is stronger than Cr II    4242.2, as in mid-A stars, while the opposite is true in A0-A2 V stars. The Fe I lines are more numerous than in o Peg.

The most prominent difference between TV Pic and o Peg is in the Ca I lines and the Ca II K line, whose strengths grow rapidly through the A-type spectral range. Ca I    4226.7 in TV Pic is stronger than Fe II    4233.2; it is markedly stronger than Fe II    4178.8, whereas in any A0-A2 V star the Ca I line is of no more than the same strength as that Fe II line but 1.3-1.7 times greater in mid-A stars. The Ca I lines of multiplet 4 (Moore 1945 ) in the  4420-4460 region are clearly present. Ca II K reaches a depth of about 70%, but the stronger (and sharper) component appears to be superposed on a weak broad component (Fig. 3). We return to the interpretation of that profile in Sect.  4. In conclusion, the appearance of the metal lines in the spectrum points to a mid-A spectral type rather than the A2 V classification of Houk (1978 ).

Sr II in TV Pic is clearly weaker than in o Peg; the  4215.5 line is as faint as Fe I    4202.0, as it is in most A stars (Conti & Strom 1968b ). However, Conti & Strom (1968a ) assign a mild Am character to o Peg on the grounds that this Sr II line is anomalous relative to the nearby Sc II    4246.8 line, but this is not echoed in TV Pic. TV Pic appears to show a so-called 'normal' A-type spectrum. None of the identified atoms or ions shows lines whose strength deviates conspicuously from 'normal'.

3.2. Photometric information

3.2.1. Walraven photometry

The colour indices of TV Pic (Table 2) lead to the reddening-free indices [ ] = 0.407 and [ ] = 0.173. [ ] = 0.078 is not useful in this spectral range, since the Kurucz models, on which the theoretical grids are based, do not give reliable predictions (see e.g. Brand and Wouterloot 1988 ).

Table 2. Strömgren and Walraven photometry of TV Pic (average over the orbital period) and of the comparison stars. The Johnson magnitude is derived from the y and V filters

There are two photometric solutions; one (that TV Pic is a highly reddened A0 V star) is incompatible with the spectroscopic evidence. The colours of the system indicate an unreddened star with  = 8180 K and  = 3.98. In the grid of Straizys & Kuriliene (1981 ) these parameters correspond to spectral type A6 IV . If we anticipate the estimated spectral type of the secondary and its contribution to the total light (early-F, 9 times fainter than the primary; see Sect. 4), we deduce that the colours of the primary must be near to  0.050,  0.196,  0.449,  0.138. This leads to an improved photometric solution:  = 8310 K,  = 3.99, no reddening, and spectral type A5 IV .

3.2.2. Strömgren photometry

The literature contains numerous calibrations of Strömgren photometry, including the -index, in terms of atmospheric parameters. Most rely on Kurucz model fluxes (cf. Smalley & Dworetsky 1993 ). Napiwotzki et al. (1993 ) recently intercompared them and found the calibration of Moon & Dworetsky (1985 ) to be the most reliable.

According to Moon's (1985 ) code, the mean colour indices of TV Pic (Table 2) give  = 8150 K,  = 3.86 and  = 0.035. If the secondary does indeed have an F2-F5 type spectrum, the improved colours for the primary become  0.067,  0.170,  1.038 and correspond to an unreddened star with  = 8310 K and  = 3.90. The metallicity indicator  = 0.031 points to an older Population I star.

3.3. Temperature and gravity of the primary

The synthesis code SPECTRUM (Gray & Corbally 1994 ), which depends on Kurucz's (1979) model atmospheres, was used to estimate and . A straightforward fit to the observed spectrum reveals a time-dependent asymmetry in the Balmer lines, indicating a small but probably non-negligible contribution from the secondary spectrum. A theoretical secondary spectrum, modelled according to the luminosity ratio and radial-velocity shifts deduced in Sect. 4, was therefore first subtracted from the observed spectra, and the residues (intended to represent the spectra of the primary alone) were compared with a suitable grid of SPECTRUM models. Simultaneous matching to the observed Balmer lines, the Ca II K line and the global metallic spectrum yielded the spectroscopic parameters  = 8400 K and  = 3.7. Fig. 2 shows the fit to the primary's Balmer lines and to its Ca II K line. Spectroscopy and photometry indicate thus a somewhat different gravity, while the temperature estimates are compatible within the uncertainties. The main discrepancy lies in the fact that the observed Ca line is too narrow for the photometrically suggested gravity; the weight given to this line also pushed the spectroscopic temperature estimate to a marginally higher value. Noting that the Ca II K line shows a peculiar profile in a number of stars, it seems reasonable to give the line lower weight and to adopt at present  8300 K and  3.8, which is more closely compatible with the observed colours and with the other spectral lines. Dynamical arguments in Sect.   5will provide independent information on the gravity.

Fig. 2. Model fit (  = 8400 K,  = 3.7) to the Ca II K, and line from which the contribution of the secondary was eliminated. The observed spectrum is shown as a full line, the model as a dashed line

© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998
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