HD 1: The number-one star in the sky

✍ Scribed by K.G. Strassmeier; M. Weber; T. Granzer; T.H. Dall

• Publisher: John Wiley and Sons
• Year: 2010
• Tongue: English
• Weight: 325 KB
• Volume: 331
• Category: Article
• ISSN: 0004-6337
• DOI: 10.1002/asna.201011356

✦ Synopsis

We present the first ever study of the bright star HD 1. The star was chosen arbitrarily just because of its outstanding Henry Draper number. Surprisingly, almost nothing is known about this bright 7. m 4 star. Our observations were performed as part of the commissioning of the robotic telescope facility STELLA and its fiber-fed high-resolution optical echelle spectrograph SES in the years 2007-2010. We found long-term radial velocity variations with a full amplitude of 9 km s -1 with an average velocity of -29.8 km s -1 and suggest the star to be a hitherto unknown single-lined spectroscopic binary. A preliminary orbit with a period of 6.2 years (2279±69 days) and an eccentricity of 0.50±0.01 is given. Its rms uncertainty is just 73 m s -1 . HD 1 appears to be a G9-K0 giant of luminosity class IIIa with T eff = 4850±100 K, log g = 2.0±0.2, L ≈ 155 L , a mass of 3.0±0.3 M , a radius of 17.7 R , and an age of ≈350 Myr. A relative abundance analysis led to a metallicity of [Fe/H] = -0.12 ± 0.09. The α-element silicon may indicate an overabundance of +0.13 though. The low strengths of some s-process lines and a lower limit for the 12 C/ 13 C isotope ratio of ≥16 indicate that HD 1 is on the first ascend of the RGB. The absorption spectral lines appear rotationally broadened with a v sin i of 5.5±1.2 km s -1 but no chromospheric activity is evident. We also present photometric monitoring BV (RI)C data taken in parallel with STELLA. The star is likely a small-amplitude (<10 mmag) photometric variable although no periodicity was found.