2 Observational parameters for V652 Her and BX Cir

Since the discovery of V652 Her as a short period variable by Landolt (1975), several observations have been carried out to determine its stellar properties with better accuracy. Initially, it was identified as a radially pulsating star by Hill et al. (1981). A more precise measurement of its radius was obtained by Lynas-Gray et al. (1984) by including ultraviolet spectroscopy. Subsequent studies of the light curves and spectrum of V652 Her are summarized in Tables 1 and 2, and Fig. 1. In particular, Table 1 shows the recent observed parameters (Jeffery et al. 2001) measured (a) by visual magnitude and (b) by ultraviolet spectrophotometry, although the unrealistically low mass obtained by the first method, causes us to choose (b) parameters, where systematic errors are probably smaller.

A second pulsating helium star, BX Cir, was discovered by Drilling (1980) and, as V652 Her, was predicted to pulsate due to its location in the instability region (Saio 1995). Kilkenny & Koen (1995) measured a period of 0.1066 days. An atmospheric analysis by Drilling et al. (1998) reported the dimensions and abundances shown in Tables 1 and  2. Subsequent analysis by Woolf & Jeffery (2000) measured BX Cir's radial velocity, angular radius and stellar mass, providing the results also shown in Table 1.

Figure 1: H-R diagram showing the location of V652 Her and BX Cir according to references given in Table. 1. Stellar luminosity was calculated from the provided effective temperature and radius. The boundaries of the region of pulsational instability due to the iron-group bump mechanism for 0.5 $M_{\odot }$ (solid line) and 0.7 $M_{\odot }$ (dotted line) for metallicity Z=0.01 are also shown.

   

Table 1: Observational parameters for V652 Her and BX Cir. For V652 Her, right column shows the dimensions provided by recent observations, where two different methods were applied to determine the effective temperature (see text). Left column shows the dimensions determined by previous observations. Luminosity is calculated from the observed radius and temperature.

	V652 Her		BX Cir
	previous	recent	recent
$\Pi$/d	$0.108^{{\rm (1)}}$		$0.1066^{{\rm (4)}}$
M/$M_{\odot }$	${0.7^{{\rm +0.4}}_{{\rm -0.3}}}^{{\rm (2)}}$	$a~0.17\pm 0.05^{{\rm (3)}}$	$0.42\pm0.12^{{\rm (5)}}$
		$b~0.59\pm0.18^{{\rm (3)}}$
$T_{\rm eff}$/kK	$23.45\pm0.13^{\rm (2)}$	$a~22.93\pm0.01^{\rm (3)}$	$23.30\pm0.7^{\rm (6)}$
		$b~20.95\pm0.07^{\rm (3)}$
R/$R_{\odot}$	$1.98\pm0.21^{\rm (2)}$	$a~1.26\pm0.00^{\rm (3)}$	$2.31\pm0.10^{\rm (5)}$
		$b~2.31\pm0.02^{\rm (3)}$
log L/$L_{\odot}$	$3.03\pm0.21$	$a~2.59\pm0.00^{\rm (3)}$	$3.15\pm 0.09$
		$b~2.96\pm0.01^{\rm (3)}$
log g/cm s-2	$3.7\pm0.2^{\rm (2)}$	$3.46\pm0.05^{\rm (3)}$	$3.35\pm0.1^{\rm (6)}$

$^{{\rm 1}}$ Landolt (1975), $^{{\rm 2}}$ Lynas-Gray et al. (1984), $^{{\rm 3}}$ Jeffery et al. (2001),
$^{{\rm 4}}$ Kilkenny et al. (1999), $^{{\rm 5}}$ Woolf & Jeffery (2000), $^{{\rm 6}}$ Drilling et al. (1998).

 

 

Table 2: Photometric abundances for V652 Her and BX Cir. Comparisons are also given for the Sun. Note the excess of nitrogen reported for V 652 and the excess of carbon for BX Cir.

	Sun	V652 Her(3)	BX Cir(6)
$n_{\rm H}$	0.90976(7)	0.01159	0.00015
$n_{\rm He}$	0.08891(7)	0.98626	0.99590
$n_{\rm N}$	0.00010(7)	0.00139	0.00052
$n_{\rm C}$	0.00035(8)	$5.55\times 10^{-5}$	0.00301
$n_{\rm Fe}$	$2.75\times 10^{-5}$(9)	$3.12\times 10^{-5}$	$9.51\times 10^{-6}$
$n_{\rm Z}$	0.00133(7)	0.00216	0.00394

$^{{\rm 3}}$ Jeffery et al. (2001), $^{{\rm 6}}$ Drilling (1998), $^{{\rm 7}}$ Anders & Grevesse (1989),
$^{{\rm 8}}$ Stürenburg & Holweger (1990), $^{{\rm 9}}$ Holweger et al. (1990).