Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656

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Table 2.

Parameters derived for MWC 656 from composite spectroscopic analysis, assuming the observed UV features originate from a hot luminous companion.

	Be star	Stripped star
T_* (kK)	21 ± 1	$85_{- 5}^{+ 10}$ $Mathematical equation: $ 85^{+10}_{-5} $$
T_2/3 (kK)	20.8 ± 1	$82 . 7_{- 5}^{+ 10}$ $Mathematical equation: $ 82.7^{+10}_{-5} $$
log g_* (cm s⁻²)	3.4 ± 0.1	5.3 ± 0.3
log g_2/3 (cm s⁻²)	3.38 ± 0.1	5.26 ± 0.3
log g_true^† (cm s⁻²)	3.65 ± 0.1	5.3 ± 0.3
log L (L_⊙)	3.9 ± 0.1	4.0 ± 0.2
R_* (R_⊙)	6.8 ± 1.0	$0 . 51_{- 0.14}^{+ 0.16}$ $Mathematical equation: $ 0.51 ^{+0.16}_{-0.14} $$
log Ṁ (M_⊙yr⁻¹)	−8.5 ± 0.2	−7.5 ± 0.2
v_∞ (km s⁻¹)	800 (fixed)	400 ± 100
v sin i (km s⁻¹)	300 ± 50	10 (fixed)
v_mac (km s⁻¹)	150 ± 30	10 (fixed)
X_H (mass fr.)	0.737	0.2 (fixed)
X_He (mass fr.)	0.26	0.79 (fixed)
M_spec (M_⊙)	7.4 ± 2.7	$1 . 9_{- 1.6}^{+ 1.7}$ $Mathematical equation: $ 1.9 ^{+1.7}_{-1.6} $$
log Q_H (s⁻¹)	46	48
log Q_He II (s⁻¹)	−	45

E_B − V (mag)	0.22 ± 0.02
R_V	2.45 ± 0.05

Notes. log Q_H, log Q_He II denote the inferred emission rates of H- and He II-ionising photons.

^†

log(g_* + (v sin i)²/R_*).

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