We have performed the first study of the variable star population of Ursa Major I (UMa I), an ultra-faint dwarf satellite recently discovered around the Milky Way (MW) by the Sloan Digital Sky Survey. Combining time series observations in the B and V bands from four different telescopes, we have identified seven RR Lyrae stars in UMa I, of which five are fundamental-mode (RRab) and two are first-overtone pulsators (RRc). Our V, B – V color-magnitude diagram of UMa I reaches V ~ 23?mag (at a signal-to-noise ratio of ~6) and shows features typical of a single old stellar population. The mean pulsation period of the RRab stars P ab = 0.628, σ = 0.071?days (or P ab = 0.599, σ = 0.032?days, if V4, the longest period and brightest variable, is discarded) and the position on the period-amplitude diagram suggest an Oosterhoff-intermediate classification for the galaxy. The RR Lyrae stars trace the galaxy horizontal branch (HB) at an average apparent magnitude of V(RR) = 20.43 ± 0.02?mag (average on six stars and discarding V4), giving in turn a distance modulus for UMa I of (m – M)0 = 19.94 ± 0.13?mag, distance d = ?kpc, in the scale where the distance modulus of the Large Magellanic Cloud is 18.5 ± 0.1?mag. Isodensity contours of UMa I red giants and HB stars (including the RR Lyrae stars identified in this study) show that the galaxy has an S-shaped structure, which is likely caused by the tidal interaction with the MW. Photometric metallicities were derived for six of the UMa I RR Lyrae stars from the parameters of the Fourier decomposition of the V-band light curves, leading to an average metal abundance of [Fe/H] = –2.29?dex (σ = 0.06?dex, average on six stars) on the Carretta et al. metallicity scale.
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机译：我们已经对Ursa Major I（UMa I）的变星种群进行了首次研究，Ulsa Major I是最近通过Sloan Digital Sky Survey在银河系（MW）附近发现的超微弱矮星。结合来自四个不同望远镜的B和V波段的时间序列观测，我们在UMa I中确定了七颗RR天琴星，其中五颗是基模（RRab），两颗是初泛脉冲星（RRc）。我们的UMa I的V，B – V色度图达到V〜23？mag（信噪比为〜6），并显示了单个旧恒星种群的典型特征。 RRab星的平均脉动周期为P ab = 0.628，σ= 0.071？days（或如果去除了最长周期和最亮变量V4，则P ab = 0.599，σ= 0.032？days）以及该周期的位置振幅图建议对银河系进行Oosterhoff中间分类。 RR天琴星以平均视在大小V（RR）= 20.43±0.02？mag（六颗星的平均值并丢弃V4）跟踪银河系水平分支（HB），从而得出UMa I的距离模数为（m – M）0 = 19.94±0.13？mag，距离d =？kpc，其中大麦哲伦星云的距离模数为18.5±0.1？mag。 UMa I红色巨星和HB星（包括本研究中确定的RR Lyrae星）的等渗线轮廓表明，星系具有S形结构，这可能是由于潮汐与MW相互作用引起的。从V波段光曲线的傅里叶分解参数得出了六颗UMa I RR天琴星的光度学金属，得出平均金属丰度为[Fe / H] = –2.29？dex（σ= 0.06？ dex，平均六颗星）。金属度标度。
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