4 Effective temperature

It is well established that the Balmer lines are good temperature indicators for $T_{\rm eff}$  $\leq$ 8000-8250 ${\rm K}$  because of their small gravity and metallicity dependence (Solano & Fernley 1997). As the spectral orders are not wide enough to fully embrace the wings of the observed Balmer lines (H ${\rm _\delta}$, H ${\rm _\gamma}$, H ${\rm _\beta}$), an alternative method based on the variations in intensity at two different wavelengths was used instead. These wavelengths, free of contamination of metallic lines, were selected in such a way that they lie in the region of the Balmer line profile where the dependency with temperature is maximum. Prior to this, the observed Balmer lines were shifted to the laboratory values to correct for radial velocity displacements. Effective temperatures were then calculated by comparing the observed intensity ratio with the intensity ratio measured in a grid of synthetic Balmer profiles previously convolved with the corresponding rotational and instrumental profiles.

Balmer lines are known to be very sensitive to convection as it can alter the temperature structure where the lines are formed. In this work, the turbulent convection model developed by Canuto et al. (1996) and implemented in the ATLAS9 code by Kupka (1999) has been used. This convection model has been found to reproduce adequately the temperature of standard stars whereas the standard mixing-length theory models with overshooting, originally implemented in ATLAS9, are clearly discrepant (Gardiner et al. 1999; Smalley & Kupka 1997). The effective temperatures are given in Table 2. An error of $\Delta$ $T_{\rm eff}$$=\pm200$ ${\rm K}$  has been assumed. Some concern may also exist in the use of Balmer lines as temperature indicators due to the existence of peculiar Balmer profiles (weak, narrow core typical of late A-type dwarfs but with strong wings proper to early A-type dwarfs) in some $\lambda$ Bootis stars (Gray 1988). Based on this criterion, $\lambda$ Bootis stars have been traditionally classified in two groups: normal hydrogen-lines (NHL) and peculiar hydrogen-line profiles (PHL). Iliev & Barzova (1993) suggested that PHL profiles could be fitted by using two theoretical profiles with different effective temperatures, one for the core and other for the wings. Faraggiana & Bonifacio (1999) pointed out that this duplicity can be explained in terms of a binary system although some of the stars catalogued as PHL objects (e.g., HD 142703, HD 142994, HD 204041) do not show in their spectra any sign of the presence of companion. Moreover, this scenario would not explain why PHL profiles are more frequent at lower temperatures as proposed by Iliev & Barzova (1993). To ensure the reliability of the temperatures calculated using Balmer lines, they have been compared with those calculated using the photometric calibration by Moon & Dworetsky (1985) finding no systematic differences (Table 2).

 

 

Table 3: Stars with surface gravities derived from Hipparcos parallaxes. Ages have been derived using Main Sequence (Claret 1992) and Pre-Main Sequence (Palla 1993) evolutionary tracks.

Identification	Parallax	Bolometric	$M_{\rm bol}$	log $(L/L_{\odot}$)	log ( $M/M_{\odot}$)	log t
		Correction				PMS	MS
HD 68758	$6.80 \pm 0.62$	-0.32	0.37	1.74	0.38	6.21	8.63
HD 75654	$12.82 \pm 0.58$	-0.15	1.77	1.18	0.27	6.92	8.97
HD 107233	$12.32 \pm 0.81$	-0.17	2.65	0.82	0.20	7.17	8.93
HD 111005	$5.75 \pm 1.01$	-0.15	1.62	1.23	0.28	6.84	8.61
HD 142703	$18.89 \pm 0.78$	-0.25	2.26	0.98	0.22	7.00	9.13
HD 156954	$12.18 \pm 0.94$	-0.09	3.03	0.67	0.17	7.30	8.50
HD 168740	$14.03 \pm 0.69$	-0.20	1.66	1.22	0.23	6.90	9.32
HD 204041	$11.46 \pm 0.99$	-0.32	1.43	1.31	0.29	6.90	8.91
HD 210111	$12.70 \pm 0.89$	-0.25	1.65	1.22	0.28	6.80	8.31