HIPPARCOS Results for Solar Analogs
Giusa Cayrel de Strobel (Observatoire de Paris/Meudon) & Eileen D. Friel (NSF/Boston Univ.)


6. The Hipparcos parallaxes: discussion and interpretation

The Hipparcos (Hip) parallaxes as well as the ground based CCD parallaxes from the USNO have certainly increased by an order of magnitude the precision of stellar distances. A large number of Hipparcos parallaxes are now available for stars we have considered in our search for solar analogs.

Last May during the "Hipparcos, Venice '97" conference we have shown (Cayrel de Strobel et al. 1997) that the accuracy of the Hipparcos parallaxes combined with a precise determination of their effective temperature, chemical composition, and spectroscopic gravity has allowed us to locate the Hyades dwarfs on their appropriate ZAMS and consequently to determine their He abundance, Y=0.26 ± 0.02 (in mass).

How does Hipparcos change our understanding of the parameters and evolutionary status of our sample of 99 solar analogs discussed in Cayrel de Strobel (1996)? First, it reduces our sample size to only 52 objects, probably because the stars we have included in our initial list of solar analog candidates were unknown binary stars, or not apt to be observed by Hipparcos, and then because in this new list of solar analogs the most metal poor stars in Table 1 and Figure 1 of Cayrel de Strobel (1996) have been omitted. The basic data adopted for these 52 solar analogs are given in Table 1, which lists in the first three columns the HD number, common name, and the number used in the Hipparcos and Tycho Catalogues (ESA, June 1997, SP-1200), followed by the photometric parameters V and B-V, the spectral type, and the spectroscopic determinations of metallicity, [Fe/H], and temperature, as log Teff. The last 3 columns give the Hipparcos parallax, in milliarcseconds, and the determinations of bolometric magnitudes, the first from the ground based parallaxes, and the second those based on the Hipparcos parallaxes.

Table 1. Data for Solar Analogs with Hipparcos Parallaxes
HD Name Hip No. V B-V Sp Type [Fe/H] log Teff Parallax Mbol Mbol
- - (mag) (mag) - - - - (mas) (grd) (Hip)
1461 - 1499 6.465 0.674 G0V +0.30 3.762 42.67 4.940 4.560
1835 - 1803 6.385 0.659 G2.5V +0.18 3.7619 49.05 4.810 4.775
2151 - 2021 2.810 0.618 G2IV -0.17 3.762 133.7 3.690 3.365
10307 - 7918 4.960 0.619 G1.5V +0.07 3.773 79.09 4.220 4.394
11397 - 8674 8.960 0.692 G6IV-V +0.10 3.753 18.30 5.530 5.192
13974 - 10644 4.865 0.608 G0V -0.33 3.750 92.20 4.740 4.612
14802 - 12186 5.195 0.604 G1.5V 0.00 3.773 45.60 4.750 3.482
16417 - 12186 5.780 0.652 G5IV -0.10 3.765 39.16 4.430 3.675
19373 - 14632 4.050 0.595 G0V +0.03 3.776 94.93 3.810 3.887
20630A - 15457 4.845 0.681 G5V +0.02 3.750 109.1 4.830 4.916
20766 zeta1 Ret 15330 5.535 0.641 G2V -0.22 3.739 82.51 5.396 5.115
20807 zeta2 Ret 15371 5.235 0.600 G1V -0.22 3.751 82.79 4.950 4.757
27685 - 20441 7.850 0.679 G4V +0.03 3.750 26.96 4.630 4.917
27859 VB52 20577 7.795 0.599 G1.5V +0.03 3.767 20.73 4.630 4.316
28099 VB64 20741 8.105 0.662 G2V +0.16 3.7616 21.42 4.940 4.695
28344 VB73 20899 7.836 0.605 G1.5V +0.14 3.771 21.0 4.670 4.396
28992 VB97 21317 7.905 0.685 G5V +0.06 3.767 23.19 4.740 4.674
30495 - 22263 5.490 0.631 G2V -0.02 3.7656 75.10 4.930 4.803
30562 - 22336 5.775 0.631 F8V +0.13 3.768 37.73 3.780 3.602
30649A - 22596 6.960 0.588 G1V-VI -0.26 3.771 33.44 4.940 4.534
32923 - 23835 5.000 0.654 G4V -0.20 3.758 63.02 3.780 3.914
34411 - 24813 4.700 0.630 G1IV-V +0.10 3.770 79.08 3.870 4.136
39587 - 27913 4.395 0.592 G0V -0.07 3.774 115.43 4.420 4.651
44594 Hardorp 30104 6.610 0.657 G3V +0.15 3.7617 38.92 4.590 4.497
52711 - 34017 5.935 0.598 G4IV -0.15 3.768 52.37 4.580 4.466
67228 - 39780 5.295 0.641 G2IV +0.04 3.762 42.86 2.830 3.389
72905 - 42438 5.640 0.619 G1V -0.11 3.767 70.07 4.870 4.801
76151 - 43726 6.010 0.656 G3V +0.07 3.758 58.50 4.750 4.773
78418 - 44892 5.955 0.657 G5V-IV -0.20 3.758 31.98 4.436 3.396
86728 20 LMi 49081 5.365 0.671 G1.5V -0.05 3.758 67.14 4.320 4.423
89010 35 Leo 50319 5.962 0.662 G2IV -0.03 3.748 32.94 3.570 3.457
90508AB - 51248 6.430 0.605 G0V -0.23 3.765 42.45 4.960 4.495
95128 47 UMa 53721 5.040 0.618 G0V -0.02 3.768 71.04 4.270 4.236
105590A - 59272 6.815 0.663 G2V -0.16 3.753 36.51 4.950 4.538
109358 beta CVn 61317 4.255 0.589 G0V -0.04 3.774 119.46 4.520 4.586
115043A - 64532 6.825 0.602 G1V -0.08 3.768 38.92 4.550 4.712
115383 - 64792 5.205 0.588 G0V +0.04 3.774 55.71 4.510 3.883
117176A 70 Vir 65721 4.97 0.712 G5V -0.11 3.739 55.22 3.160 3.570
128620 alpha Cen 71683 -0.01 0.70 G2V +0.22 3.763 742.12 4.310 4.283
141004 - 77257 4.425 0.602 G0V +0.03 3.777 85.08 4.000 4.026
142267A - 77801 6.085 0.599 G1V -0.28 3.763 57.27 4.070 4.796
143761 - 78459 5.400 0.606 G1V -0.17 3.761 57.38 4.230 4.117
146233A 18 Sco 79672 5.495 0.651 G1V 0.00 3.7626 71.30 4.500 4.693
150680 zeta Her A 81693 2.81j 0.645 F9IV +0.05 3.765 92.63 2.820 2.581
186408 16 Cyg A 96895 5.975 0.642 G1.5V +0.06 3.762 46.25 4.260 4.234
186427 16 Cyg B 96901 6.240 0.661 G2.5V +0.05 3.760 46.70 4.520 4.518
187923 - 97767 6.145 0.647 G0V +0.06 3.758 36.15 4.590 3.863
189567 - 98959 6.080 0.644 G2V -0.29 3.753 56.45 4.630 4.739
193664 - 100017 5.91 0.602 G3V +0.06 3.778 56.92 5.022 4.640
212330A - 110649 5.315 0.668 G3IV -0.17 3.743 48.81 3.970 3.655
216437 rho Ind 113137 6.05 0.660 G4IV-V +0.10 3.773 37.71 4.300 3.881
217014 51 Peg 113357 5.46 0.666 G3.5V +0.06 3.760 65.10 4.710 4.458

We show in Figure 2 the observational (log Teff, Mbol) diagram for these 52 stars using Hipparcos parallaxes as the basis for Mbol determinations. The theoretical tracks are from Lebreton et al (1997), as in Figure 1. These models show that the present Sun, with Mbol = 4.75, has moved by Delta Mbol =-0.42 mag above its initial position and by Delta log Teff = +0.0123 (corresponding to Delta Teff =166K) in effective temperature.

FIGURE 2:Observational (log Teff, Mbol) diagram of 52 solar analogs having Hipparcos (Hip) parallaxes as the basis of their Mbol determination with an overplot of the same theoretical diagram as in Figure 1. The sample size is reduced to only 52 objects because some of the stars included in the Cayrel de Strobel (1996) list were unknown binary stars and also because the most metal poor stars in the 1996 list have been omitted here. Please note that the underluminosity of certain stars has disappeared in this diagram.}

In Figure 1, we noted that a fair number of analogs are more evolved and older than the Sun. The evolutionary status and the old age of these objects is confirmed by Hipparcos. In ``Hipparcos, Venice '97" we have heard that for many stars, (but not for all), Hip parallaxes tend to make them more luminous. This is also seen in our sample of 52 stars: their (log Teff, Mbol) diagram (Figure 2) has lost, except for two stars, the underluminous objects seen on the preceding H-R diagram. One can argue that, precisely, these underluminous stars have not been observed by Hipparcos, or rejected by us because of their high metal deficiency, but the comparison shown in Figure 3 between the same 52 stars having both ground based parallaxes (filled diamonds) and Hip parallaxes (crossed or filled squares), as the basis of their Mbol determinations, shows that some stars of the sample with ground-based parallaxes lie below the main sequence. Clearly seen in Figure 3 is the tendency for the use of Hip parallaxes to brighten the Mbol of stars. Note that in Figure 3 the abscissae have been kept strictly the same in the sample of 52 stars possessing either Mbol's from ground based parallaxes, or Mbol's from Hip parallaxes. Figure 2 shows that relatively few Hipparcos analogs are main-sequence stars.

FIGURE 3:Comparison between observational (log Teff, Mbol) diagrams of 52 solar analogs using different sources for bolometric magnitudes, with an overplot of the same theoretical diagram as in Figure 1. Filled diamonds represent absolute bolometric magnitudes, Mbols, computed with ground-based parallaxes, and crossed and filled squares represent Mbols computed with Hip-parallaxes. Filled squares denote some of the past 'best' solar analogs. Here, clearly it is seen that the Hip Mbols of many objects brighten their positions in the H-R diagram with respect to the ground-based positions of the same stars.

A few notable objects can be seen in Figure 3, such as the three best known stars which are thought to have planetary companions: 51 Peg (Mayor and Queloz 1995), 70 Vir (Marcy and Butler 1996), and 47 UMa (Marcy and Butler 1996). The absolute bolometric magnitude of the star HD 193664 from its ground-based parallax is 0.49 mag. fainter than that from its Hip parallax. Similarly, that of iota Per is 0.47 mag fainter. On the contrary, good examples of stars becoming fainter as a result of using Hipparcos Mbols are mu Cnc and the planet star 70 Vir. The values of the Hip Mbol's of the well known components of the binary zeta1 and zeta2 Ret, together with the values of Teff and [Fe/H] from a careful detailed analysis by Del Peloso (1997), has permitted us to attribute to this couple of disk stars a very old age: both are lying nicely on the 12 Gyr isochrone.

What has happened to our best candidates for `solar twins'? Figs. 4 and 5 show an expansion of the region in the H-R diagram around the Sun. Figure 5 shows only those stars in Figure 4 with approximately solar metallicity, i.e. those with a metal abundance defined as: [Fe/H] = 0.0 ± 0.10. Note that in these two figures the total span in abscissa is only Delta log Teff = 0.03, from Teff = 5625 to Teff = 6025 K, and in ordinates only 1 mag. The straight full line is the ZAMS for the chemical composition of the Sun. The straight dashed line is the ZAMS for the chemical composition of the Hyades. The full curved line is the evolutionary track of the Sun. VB 64, a Hyades effective temperature solar analog, is significantly more metal-rich than the Sun, and its position actually fits very nicely on the ZAMS at its appropriate metallicity. Other Hyades stars, VB 92 and VB 73, are shown in Figure 4 with vertical lines indicating the size of 1-sigma error bars. Clearly the positions of these stars are also consistent with the Hyades ZAMS. The star 18 Sco, (probably the best solar analog), falls on the 1 solar mass evolutionary track and on the isochrone of 6 Gyr. Abrupt changes of slope of the very magnified isochrones reflect grid points of the models.

FIGURE 4:Zoom of the region in the previous H-R diagram (Figure 3) around the Sun. The straight full line is the ZAMS for the chemical composition of the Sun. The straight dashed line is the ZAMS for the chemical composition of the Hyades. The full curved line is the evolutionary track of the Sun. VB 64, a Hyades star, is significantly more metal-rich than the Sun, and its position actually fits very nicely on the ZAMS at the appropriate metallicity. The star 18 Sco, (probably the best solar analog), falls on the 1 solar mass evolutionary track and on the isochrone of 6 Gyr. Abrupt changes of slope in the isochrones reflect grid points of the models.

FIGURE 5:The same as Figure 4, but this (log Teff, Mbol) diagram contains only solar metal normal stars, having their metal abundance defined as: Delta [Fe/H] = 0.00 ± 0.10. Note that in Figures 4 and 5 the total span in abscissa is only: Delta log Teff = 0.03, from Teff = 5625 to Teff = 6025 K, and in ordinates, 1 mag. Here, and in Figure 4, the box drawn around the Sun's position defines the region within 60 K and 0.1 mag of the Sun's effective temperature and bolometric magnitude. These are the mean error bars adopted for the stars with Hipparcos parallaxes.

The box drawn around the Sun's position in Figs. 4 and 5 defines the region within 60 K and 0.1 mag of the Sun's effective temperature and bolometric magnitude. By the way, these are also the mean error bars we have adopted for the stars with Hipparcos parallaxes. Very few stars now sit within this region of solar 'twinness' in Figure 4 and even less, in Figure 5. Notable in Figure 4 are : 18 Sco, VB 64, and HR 1835. Of these, we have heard a great deal about 18 Sco at this conference. HR 1835, we have heard, is much more chromospherically active, hence younger, than the Sun and 18 Sco is now our closest twin. The new parallaxes reinforce the conclusions (Friel et al. 1993) that 16 Cyg A and B are both more evolved and older than the Sun, at 10 Gyr. Other solar analogs that are thought to have planetary companions, 51 Peg, 70 Vir and 47 UMa, are also more evolved and appreciably older than the Sun. Hardorp's (1978) best analog, HD 44594, is not contained in the H-R diagram of Figure 5 because it is appreciably more metal-rich than the Sun. The box drawn around the Sun in Figure 5 contains three stars: 18 Sco (Porto de Mello: in this workshop), HD76151 (Cayrel de Strobel and Bentolila 1989) and HD 30495 (Pasquini et al. 1994). Their detailed analyses have produced trustworthy atmospheric parameters: Teff, log g, and [Fe/H] and with their new Hip parallaxes these three stars certainly deserve to surround the Sun.


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