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

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 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, -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, 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 , 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, , photospheric velocity field, al. (1996) [21]. The (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]. is determined from magnitude candidates found in the previous spectroscopic studies, and and 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 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. Then, correction for the shift, measured from a set of unblended lines (~20) with intermediate strength, is ap- plied before the normalization. The normalized stellar spectra are compared directly with the solar spectrum. As shown in Figure 2, the spectra of HD195034 and HD146233 are very similar to the Sun.

4 Chemical composition

Although the spectra in most sections of these two stars are similar to the Sun, there are still some differences in par- ticular spectral lines, like the Li I 6708 Å line. It is interest- Figure 1 Positions of our samples on Mv-log(Teff) diagram. ing to compare chemical composition element by element based on detailed abundance analysis. Taking advantage of Table 2 Parameters for our samples derived in this work the Platform for High-Resolution Spectra Procession and Abundance Analysis written by Zhao et al. (2007) [27], the Name Teff (K) [Fe/H] (dex) Mv (mag) Mass (M/M⊙) abundances of 17 elements, Li, O, S, Mg, Al, Si, K, Ca, Sc, HD 98618 5690 0.05 4.65 0.95 Ti, V, Cr, Mn, Fe, Ni, Cu and Ba, are derived for HD 101364 5685 −0.04 4.59 0.95 HD146233 and HD195034, as well as for the Sun. This HD 129357 5827 −0.02 4.49 1.00 platform is based on Kurucz stellar atmosphere model [28], HD 138573 5676 −0.03 4.79 0.95 and abontest8 program to calculate abundances. Within the HD 142093 5911 −0.15 4.83 1.00 HD 143436 5788 −0.04 5.01 1.00 measurement precision, [Fe/H] of HD146233 and HD 146233 5757 0.03 4.79 0.98 HD195034, 0.09 and −0.02 dex, is in good agreement with HD 159222 5765 0.05 4.65 0.97 the [Fe/H] shown in Table 2. The absolute abundance of HD 195034 5695 0.05 4.85 0.96 lithium in these two stars is appreciably larger than solar values by 0.6–0.8 dex, which has been discovered in the previous works (Meléndez et al. 2006 [16], Takeda et al. log of suspected variables by Kukarkin et al. (1981) [25] but 2007 [17] and Meléndez et al. 2007 [18]). it was identified by Adelman (2001) [26] as one of 681 Porto de Mello & da Silva (1997) [14] derived abun- photometrically stable stars. Takeda et al. (2007) [17] found dances for elements, C, Na, Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Co, that HD195034 is a solar twin similar to HD146233 among Ni, Cu, Zn, Sr, Y, Zr, Ba, Ce, Pr, Nd, Sm, Eu and Gd in solar analog stars. We have obtained high resolution spectra HD146233 and found that differences in abundances were of these two stars and checked if their compositions are less than 0.08 dex as compared with the Sun. In Meléndez similar to the Sun. For other stars, we assume that their et al. (2006) [16], we can know the difference in [O/Fe] chemical compositions from the literature are reliable and between HD146233 and the Sun was 0.05 dex. Meléndez et they are similar to the Sun. al. (2007) [18] derived Li, C, O, Na, Mg, Al, Si, S, K, Ca, Ti, V, Cr, Fe, Co, Ni, Cu and Zn abundances for HD146233 and concluded that the abundance pattern for these elements 3 Spectral comparison showed no difference within the error except for lithium. Takeda et al. (2007) [17] had shown abundances of ele- Spectra of HD146233 and HD195034 were obtained by ments Si, Ti, V, Co and Ni for HD146233 and HD195034, using the Coude Echelle Spectrograph equipped with 1024× and their abundances are also consistent with solar ones. We 1024 CCD, at the 2.16 m telescope of the National Astro- have derived [X/Fe] for 15 elements for HD146233 and nomical Observatory (Xinglong, China) on November 20, HD195034, which are shown in Figure 3, where the solar 2007 and June 15, 2008. The spectra cover the region values are presented by dotted lines. Unfortunately, we 5500–8800 Å with a resolving power of R ~ 37000 and their could not get the Cu abundance for HD195034 because the signal to noise ratios are 173 and 210/pixel, respectively. Cu I line of this star had been polluted by a cosmic ray. The Meanwhile, lunar spectra, Sun01 and Sun02, are also ob- derived abundance pattern for the elements in HD195034 tained using the same instruments as stars and their signal to reveals a perfectly solar pattern within 0.05 dex. HD146233 noise ratios are 540 and 370/pixel. also shows a good match within 0.1 dex. In particular, we The data reduction (bias subtraction, order determination, derive abundances of these elements, O, S, Mg, Al, K, Ca, wavelength calibration, flat reduction, background subtrac- Sc, Cr, Mn and Ba for HD195034, which was not obtained 582 ZHAO ZhengShi, et al. Sci China Phys Mech Astron March (2010) Vol. 53 No. 3

Figure 2 The comparison of the spectra of HD146233 and HD195034 with the Sun.

is in good match with the Sun. Some differences in both

parts of Hα line for HD146233 and the Sun are due to pollu- tion of telluric lines. We notice that the spectrum of HD146233 is affected more seriously by telluric lines than HD195034 and the Sun because it was observed under quite humid weather condition (in summer) and took quite a long exposure time (40 min). With the spectrum of a rapidly ro- tating B star, which was observed on the same night as HD146233, it is possible to find telluric lines (marked by symbol ‘×’) on the spectrum of HD146233. It is seen in Figure 4 that real spectral lines (marked by symbol ‘○ ’) of HD146233 are in good agreement with solar ones. As shown in Figure 5, HD195034 and HD146233 are identical to the Sun in Ca II triplet lines except for the Ca II 8542 Å line of HD146233, which is polluted by a cosmic ray. In Figure 3 [X/Fe] for HD146233 and HD195034. conclusion, these two stars are similar to the Sun in terms of chromospheric activity. In Table 3 we show S index, log R′HK and Prot for these two stars, as well as those for the Sun, before. We also recognize that differences in Li may not from Wright et al. (2004) [29]. Actually, the S index has indicate their dissimilarity from Teff because abundances of both chromospheric and photospheric contributions. log O and S elements, which are also sensitive to T , show a eff R′HK value is part of the chromospheric contribution. Rota- general consistency with the Sun within attainable precision. tion periods Prot, is derived from the R′HK value by using Since Li-diversity will not affect flux distribution, these two empirical fits. These values also show the similarity of these stars could still be used to determine solar colors. two stars to the Sun in terms of chromospheric activity. Ta- keda and Tajitsu (2009) [30] compared Ca II H and K lines of HD146233 and HD195034 with the solar spectrum. As 5 Chromospheric activity shown in Figure 6 of their paper, there is no appreciable difference in the Ca II H and K lines. The spectral type, Figures 4 and 5 show comparisons of the H α line and Ca II collected from SIMBAD, shows that HD195034 is quite triplet, which are sensitive to chromospheric activity, of different from the Sun and HD146233. The Teff (about HD146233 and HD195034 with the solar spectrum. As 5700 K) derived from this work and Takeda et al. (2007) shown in Figure 4, the Hα line of HD146233 and HD195034 [17], together with the relation between spectral type and ZHAO ZhengShi, et al. Sci China Phys Mech Astron March (2010) Vol. 53 No. 3 583

Figure 4 The comparison of H α line for HD195034 and HD146233 with the Sun.

Figure 5 The comparison of Ca II triplet for HD195034 and HD146233 with the Sun.

Table 3 The chromospheric activity parameters of HD146233, HD195034 and the Sun Parameters Sun HD146233 HD195034 Spec.type G2V G2V G5 S 0.166 0.169 0.166

log R′HK −4.96 −4.95 −4.96 Prot (d) 25 24 23 584 ZHAO ZhengShi, et al. Sci China Phys Mech Astron March (2010) Vol. 53 No. 3

Teff given in ref. [31], indicates that the spectral type of samples. They included 9 stars for BVIc, 30 stars for JHK, HD195034 should be G2, not G5. 51 stars for BT, VT and uvby photometry. Although their samples were somewhat larger than ours, our results show smaller standard deviation based on only solar twins, whose 6 The derived solar colors parameters are closer to the Sun, rather than solar analogs in

their work. Our (B-V)⊙ and (b-y)⊙ are in good agreement The photometric data of six solar twins are collected from with their estimates, whereas others show deviations of 0.01 the literature as shown in Table 4. For these stars, UBV to 0.03 mag. We have updated the solar colors with photometry was taken from Mermilliod et al. (1997) [32]; (V-Ic)⊙=0.707 mag, (BT-VT)⊙=0.725 mag, (J-H)⊙=0.288

(V-Ic) data were collected from the Hipparcos catalog; JHK mag, (H-K)⊙=0.066 mag, (v-y)⊙=1.028 mag, (v-b)⊙=0.619 photometry was available from the 2MASS survey; BTVT mag and (u-v)⊙=0.954 mag. photometry was available from Tycho-2 catalog (Hog et al.

2000 [33]); uvby data had been selected from Olsen (1993) 7 Conclusions [22] and Hauck et al. (1998) [34]. We have only selected the photometric data marked by ‘AAA’ level in 2MASS in With homogenous stellar parameters, we select six good solar order to assure the precision of data when we collected JHK twins from nine solar twin candidates (including HD146233 photometry. Thus, the JHK photometry of HD146233, and HD195034) found in previous works. Meanwhile, using which is marked by ‘DCD’ level, is excluded while calcu- the high resolution spectra of HD146233 and HD195034, lating the solar colors. chemical composition and chromospheric activity are com- As the fundamental parameters (Teff, mass and luminos- pared with the solar ones. Except for higher lithium abundance, ity etc.) and chemical composition of six solar twin candi- they are very close to the Sun in the HR diagram and have dates are similar to the Sun, they can be used to derive the similar atmospheric parameters, stellar abundances, mass, age solar colors. The colors for the Sun are estimated by aver- and chromospheric activity. Moreover, solar colors in the aging their color indices and they are shown in the first Johnson/Cousins, Tycho, 2MASS and StrÖmgren systems are column of Table 5. We find that solar colors had been de- derived in this work. This work not only renews the solar col- rived by Holmberg et al. (2006) [12] with a similar method ors of (V-Ic)⊙, (BT-VT)⊙, (J-H)⊙, (H-K)⊙, (v-y)⊙, (v-b)⊙ and

(in Table 5, Col. 2), but they used solar analog stars as their (u-v)⊙ but also improves the precision of solar colors.

Table 4 Photometric values of solar twin candidates

Mag.& color index HD 98618 HD101364 HD138573 HD146233 HD159222 HD195034 V 7.658 8.673 7.232 5.503 6.528 7.090 (B-V) 0.640 0.647 0.656 0.652 0.639 0.642

(V-Ic) 0.710 0.710 0.720 0.690 0.700 0.710

(BT-VT) 0.713 0.714 0.766 0.722 0.726 0.718 (J-H) 0.306 0.275 0.285 − 0.266 0.269 (H-K) 0.081 0.044 0.080 − 0.078 0.053 (v-y) 1.020 1.033 1.045 1.021 1.028 1.020 (v-b) 0.609 0.623 0.633 0.616 0.622 0.612 (u-v) 0.956 0.936 0.955 0.961 0.986 0.931 (b-y) 0.411 0.410 0.412 0.405 0.406 0.408

Table 5 Solar colors derived by this work and by Holmberg et al. (2006) [12]

Color index Solar this work Solar Holmberg | SolarΔ |

(B-V) ⊙ 0.644±0.007 0.642±0.016 0.002

(V-Ic) ⊙ 0.707±0.009 0.688±0.014 0.019

(BT-VT) ⊙ 0.725±0.017 0.708±0.030 0.017

(J-H) ⊙ 0.288±0.021 0.258±0.035 0.030

(H-K⊙) 0.066±0.015 0.096±0.038 0.030

(b-y) ⊙ 0.409±0.002 0.403±0.013 0.006

(v-y) ⊙ 1.028±0.010 1.011±0.035 0.017

(v-b) ⊙ 0.619±0.018 0.609±0.023 0.010

(u-v) ⊙ 0.954±0.018 0.979±0.064 0.025

ZHAO ZhengShi, et al. Sci China Phys Mech Astron March (2010) Vol. 53 No. 3 585

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