Annual parallax measurement of a Mira variable star BX Cam with VERA

Abstract

We report results of astrometric VLBI observations toward a Mira variable star BX~Cam using the VLBI array "VERA". The observations were performed from February 2012 to November 2014. Obtained parallax is 1.73$\pm$0.03 mas corresponding to a distance of 0.58$\pm$0.01 kpc. Parallax of this source was also reported in Gaia DR2 as 4.13$\pm$0.25 mas, and there is a 240 \% difference between these two measurements. Astrometric results from our VLBI observations show that we exactly traced angular motions of the seven maser spots in BX~Cam. We calculated stellar luminosities using both parallaxes, and obtained luminosities of $L_{\ast }^{\mathrm{VERA}}=4950\pm 170L_{\odot }$ and $L_{\ast }^{\mathrm{Gaia}}=870\pm 110L_{\odot }$. Deduced luminosities also support a validity of the parallax that we determined with VERA. Evaluating the two parallaxes, we concluded that the parallax of 1.73$\pm$0.03 mas from the VERA observations is correct for BX~Cam. We obtained a systemic motion of BX~Cam as (${\mu }_{\alpha }\cos {\delta }^{\mathrm{sys}}$, ${\mu }_{\delta }^{\mathrm{sys}}$) $=$ (13.48$\pm$0.14, $-$34.30$\pm$0.18) mas\,yr${}^{-1}$. A total of 73 H${}_2$O maser spots detected from our VLBI observations show a spatial distribution of 30 au $\times$ 80 au with a strong elongation along north-south direction. They show outflows with a three-dimensional velocity of 14.79$\pm$1.40 km\,s${}^{-1}$. From a comparison between time variations of $V$-band magnitudes and H${}_2$O maser, we found that variation of the H${}_2$O maser is relevant to that seen in $V$-band even though the H${}_2$O maser does not recover its maximum flux in each cycle.

10 pages, 6 figures, 5 tables, Accepted by PASJ