Active longitudes and starspot evolution of the young rapidly rotating star USNO-B1.0 1388-0463685 discovered in the Yunnan-Hong Kong survey

We have discovered a young, rapidly rotating active star USNO-B1.0 1388-0463685 with a rotational period of 0.508750 +/- 0.000153 d in the Yunnan-Hong Kong wide-field photometric survey. Based on the chromospheric activity identified in the dedicated spectroscopic observations, the brightness variation of this star is attributed to cool spots on its photosphere. We derived the spot maps from 14 light curves, which demonstrate that the strong spot tends to appear in the same hemisphere, and the light curve amplitude varies with a period of about 2.4 yr. Low-resolution spectroscopic observations revealed the chromospheric activity of the star through the indicators H alpha, Na i D1 and D2, He i D3, and Ca ii H & K lines. Moreover, there was a clear anticorrelation between the equivalent width variation of the H alpha, Na i D1 and D2 lines and the simultaneous light curve, implying the photosphere and chromosphere connection. Using the spectral subtraction technique, the spectral type of the star was determined as K0-2V and its projected rotational velocity v sin i is 107.492 +/- 7.388 km s(-1). Both the lithium equivalent width of 254.0 +/- 43.3 m angstrom and its location on the Hertzsprung-Russell diagram indicate that this star should be in the stage from late pre-main sequence to zero-age main sequence.