SCIENCE CHINA Physics, Mechanics & Astronomy • Research Paper • March 2010 Vol.53 No.3: 579–585 doi: 10.1007/s11433-009-0228-5 Spectral analysis of two solar twins and the colors of the Sun ZHAO ZhengShi1,2, CHEN YuQin1, ZHAO JingKun1 & ZHAO Gang1* 1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China; 2 Graduate University of Chinese Academy of Sciences, Beijing 100049, China Received April 24, 2009; accepted June 8, 2009 High resolution (R~40,000) and high signal-to-noise ratio (>150) spectra of two solar twins, HD146233 and HD195034, are obtained with the Coude Echelle Spectrograph at the 2.16 m telescope of the National Astronomical Observatories of Chinese Academy of Sciences (Xinglong, China). Based on the detailed spectrum match, comparisons of chemical composition and chromospheric activity, HD146233 and HD195034 are confirmed that they are similar to the Sun except for lithium abundance, which is higher than the solar value. Moreover, among nine solar twin candidates (including HD146233 and HD195034) found in the previous works, we have picked out six good solar twin candidates based on newly-derived homogenous parameters, and collected their colors in the Johnson/Cousins, Tycho, 2MASS and StrÖmgren system from the literature. The average color are (B-V)⊙=0.644 mag, (V-Ic)⊙=0.707 mag, (BT-VT)⊙=0.725 mag, (J-H)⊙=0.288 mag, (H-K)⊙=0.066 mag, (v-y)⊙=1.028 mag, (v-b)⊙=0.619 mag, (u-v)⊙=0.954 mag and (b-y)⊙=0.409 mag, which represent the solar colors with higher precision than pre- vious works. Sun-colors, Star-atmospheres, abundances, fundamental parameters PACS: 96.60.Fs, 97.10.Ri, 97.10.Ex, 97.10.Tk, 97.10.-q As the star closest to us and the one which we know best, measurement ranged from 0.63 to 0.69 (Stebbins & Kron the Sun serves as important and fundamental reference in 1957 [1]; Gallouëtte 1964 [2]; Tüg & Schmidt-Kaler 1982 astronomy because accurate physical parameters (mass, [3]). The solar color (B-V)⊙ based on theoretic model was distance, luminosity etc.) of the Sun can be measured di- 0.67 (Bertelli et al. 1994 [4]; Bessell et al. 1998 [5]). One of rectly. At the same time, since the Sun is so bright and not a the indirect methods to infer the solar color was averaging point-like source, it is not easy to derive accurate colors the colors of sun-like or solar analog stars, which have very from direct measurement with a photometer, which is used similar properties to the Sun. This method dose not need for ordinary stars. The directly measured solar color (B-V)⊙ direct measurements of the Sun and does not depend on by Stebbins & Kron (1957) [1] was 0.63, but the value de- theoretical stellar models. The solar color inferred from this rived by Gallouëtte (1964) [2] was 0.68, which is much method ranged from 0.62 to 0.65 (Hardorp 1978 [6]; Wam- higher than the former. Then, the controversy about photo- stecker 1981 [7]; Cayrel de Strobel 1996 [8]; Sekiguchi & metric values of the Sun has been going on even today since Fukugita 2000 [9]; Soubiran & Triaud 2004 [10]; Ramírez different works derive different values. & Meléndez 2005 [11]; Holmberg et al. 2006 [12]; Pasquini While most of the work was based on indirect methods, et al. 2008 [13]). Apparently, the most important point in direct or partly direct methods were undertaken in few this method is to select stars that are really similar to the works so far. The solar color (B-V)⊙ obtained from direct Sun. In this sense, solar twin stars are the best choice be- cause they are (ideally) identical to the Sun in all physical parameters: mass, age, luminosity, chemical composition, *Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2010 phys.scichina.com www.springerlink.com 580 ZHAO ZhengShi, et al. Sci China Phys Mech Astron March (2010) Vol. 53 No. 3 temperature, surface gravity, photospheric velocity field, al. (1996) [21]. The color index (b-y) is taken from Olsen magnetic field, rotation velocity, and chromospheric activity (1993) [22] and [Fe/H] is derived from the strÖmgren m1 level, within the observational uncertainties, i.e. their spec- index using the calibrations of Schuster & Nissen (1989) tra are indistinguishable from the solar ones. However, this [23], which is in good agreement with [Fe/H] derived from kind of work had been done by solar analogs in general, not spectroscopic study shown in Table 1. Mass is estimated yet by solar twins. from the stellar positions on the Mv-Teff diagram by inter- Spectroscopic observation technology allows the identi- polating the evolutionary tracks from Girardi et al. (2000) fication of more solar twin stars. We thus collect solar twin [24]. Absolute magnitude is determined from magnitude candidates found in the previous spectroscopic studies, and and stellar parallax based on new the Hipparcos catalog estimate solar color from photometric values of these solar (van Leeuwen 2007 [19]). As shown in Table 2, masses of twins. The advantages of this work are twofold: (1) Pa- our sample stars range from 0.95 to 1.00 M⊙. Figure 1 rameters based on the high resolution spectroscopy have shows our sample stars in the log Teff -Mv diagram compared higher quality, and thus more reliable solar twin candidates to isochrones (Girardi et al. 2000 [24], Z = 0.019). can be selected. (2) Our selected samples are very similar to Although HD129357, HD142093 and HD143436 were the Sun in many aspects and thus their colors are closer to detected as good solar twin candidates by King et al. (2005) the Sun rather than sun-like or solar analog stars. In addition, [15], they are apparently not as close to the Sun as com- we confirm that HD146233 and HD195034 are good solar pared with other stars in our log Teff-Mv diagram (See Fig- twin candidates based on analysis of their high resolution ure 1). For example, the effective temperature and metallic- spectra. ity of HD142093 in Table 2 are 5911 K and −0.15 dex while solar values are 5777 K and 0.0 dex. HD129357 and 1 Samples of solar twins HD143436 are quite different from the Sun in absolute magnitude. In particular, HD143436 was mentioned by King et al. (2005) [15] as a star closest to the Sun among The search for solar twins is mainly based on photometric their samples. Its absolute magnitude is updated in this work and spectroscopic observation. Since the atmospheric pa- by using new parallax given in new Hipparcos catalog (van rameters derived from spectroscopy are more accurate than Leeuwen 2007 [19]) from Mv = 4.87 mag in King et al. those from photometric data, and accurate chemical compo- (2005) [15] to Mv = 5.01 mag. Since bolometric corrections sition values are necessary to identify solar twins, the spec- for our samples are similar to the solar value, the deviation troscopic observation plays a more important role in the of Mv indicates a different absolute bolometric magnitude search for solar twins. So far, only nine solar twins were Mbol (or luminosity) of these stars from the Sun. In a word, recognized in these searches: HD146233 (Porto de Mello & we consider that HD129357, HD142093 and HD143436 da Silva 1997 [14]; Soubiran & Triaud 2004 [10]), may not be good solar twins and they are eliminated from HD159222 (Soubiran & Triaud 2004 [10]), HD129357, our samples. HD138573, HD142093 and HD143436 (King et al. 2005 The positions of HD146233 and HD195034 are closest to [15]), HD98618 (Meléndez et al. 2006 [16]), HD195034 the Sun among the sample stars (shown in Figure 1). (Takeda et al. 2007 [17]) and HD101364 (Meléndez et al. HD146233 was selected by several works as the best solar 2007 [18]). Table 1 shows these solar twin candidates and twin candidate for almost a decade. It was included in cata- their atmosphere parameters collected from the literature. Different selection criteria in different works get difference candidates, and stellar atmospheric parameters derived in Table 1 Atmospheric parameters of solar twin candidates collected from different ways are different for the same stars. Thus, there the literature needs to be a consistent way to select reliable solar twins. Name Teff (K) Log g (dex) [Fe/H] (dex) Ref. HD98618 5843 4.45 +0.05 [16] 5726 4.40 +0.07 [17] 2 Stellar parameters 5704 4.42 −0.02 [18] HD101364 5782 4.38 +0.01 [18] HD129357 5749 4.16 −0.02 [15] Since the methods of determining stellar atmospheric pa- HD138573 5717 4.20 −0.03 [15] rameters are different, it is not suitable to compare the pa- 5743 4.50 −0.00 [17] rameters listed in Table 1 directly. Furthermore, Hipparcos HD142093 5710 4.20 −0.15 [15] parallax was updated recently by van Leeuwen (2007) [19]. HD143436 5768 4.28 −0.00 [15] Therefore, we decide to recalculate temperature and [Fe/H] HD146233 5789 4.49 +0.05 [14] 5817 4.45 +0.02 [16] of our samples in a consistent way as introduced by Chen et 5779 3.38 +0.04 [17] al. (2000) [20]. The results are given in Table 2. Specifi- 5834 4.45 +0.04 [18] cally, temperature is determined from the photometric color HD159222 5834 4.30 +0.06 [10] index (b-y) and [Fe/H] by using the calibration of Alonso et HD195034 5775 4.41 −0.01 [17] ZHAO ZhengShi, et al. Sci China Phys Mech Astron March (2010) Vol. 53 No. 3 581 tion, spectrum extraction and continuum fitting) is per- formed by using standard ESO/MIDAS routines.