Is 18 Scorpii a Solar Twin ?

     18 Sco:    

     RA = 16^h 15 ^m 37^s   DEC = -8° 22' 11''     

     Distance Modulus: (m - M); 14.0 ± 0.65 pc  m = +5.49, M = +4.76 

     Parallax  =  0.0713'';   Distance: 14.0 ± 0.8 pc   (5% error)

In 1963 Olin Wilson began a program of studying some 60 stars in an effort to find some with identical properties to that of the Sun (Solar analogues). In 1978, the 1st paper appeared on Solar analogues by J. Hardorp. The search for a Solar Twin, i.e., a star whose physical properties are similar to that of the Sun is important for several reasons: 1) Our Sun is very bright, thus detailed photometric comparisons with fainter objects is difficult, 2) studying Solar analogues aids astronomers in analysis of the Sun's evolution, since we can observe once and future snapshots in the evolutionary state at different times in the life of such stars, 3) Solar analogues are useful candidates in the search for extrasolar planets, 4) assists in the search for population effects in the Solar neighborhood, 5)  during the years 1645 - 1715 the 11 year Solar cycle vanished, and this same time period matched the onset of unusually severe winters in Europe.  Studying Solar twins is useful in predicting what lies ahead for our Sun.  While there are hundreds of solar-like stars, true solar twins must be nearly identical to the Sun over a broad range of physical and photometric parameters.

Some of the criteria used in searching for Solar Twins are:   

a)   Spectral Type  (Sun G2V)

b)   Mass

c)   B-V,  color index,  (B-V) = 0.59-0.69

    (Sun B-V) = 0.64

d)   T_eff K,  Sun = 5877 ºK

e)   M_bol

f)    Chemical composition

g)    Luminosity

h)    candidate star must possess a trigonometric parallax

The Sun is the only star in which we have accurate values for most of its physical parameters.  Only until a detailed analysis of the star 18 Scorpii, (HD 146233, HR 6060)  few stars were known to have a "cycle", similar to the well known 11 year Solar sunspot cycle.

A comparison of the physical parameters of 18 Sco and the Sun are given below.

                             Sun                     18 Sco

B - V                    0.648                     0.65

b - y                     0.409                    0.405

Spectral Type        G2V                     G2Va

Fe/H                     0.00                     0.05 ± 0.06            

T_eff ºK                  5,877                     5,789

Luminosity            1.00                      1.05 ± 1.06  

In a detailed study of 18 Sco in 1997, it was argued that this star is the best solar twin. From its optical spectrum it was determined that it resembled the Sun in all aspects except for a slightly higher than solar luminosity and age.  

In an analysis of 18 Scorpii by Jeffrey Hall and G. W. Lockwood of Lowell Observatory in the years 1995-2000, they made careful measurements of its Ca K lines. At the same time, they made measurements of our Sun's Ca K lines intensity also. The results showed 18 Sco to have a similar activity cycle slightly higher (14%) in energy output than in the Sun's chromosphere. See trhe Figure below taken From Hall and Lockwood's paper.          

 Figure 1. Graph of 18 Sco's intensity vs. Sun. Both 18 Sco's and the Sun's data was obtained with the same instrument with Lowell Observatory's 42" telescope. Diagram used from Hall & Lockwood 2000.

Coincidently, the rise and fall of 18 Sco's intensity seems to parallel that of the Sun's activity cycle. The appearance of a activity cycle on 18 Sco doesn't necessarily prove that this is the norm for stars. We have only been observing the Sun's activity cycle for only 400 of its 5 billion year life, so the existence of a solar cycle might be a freak event in the Sun's life, or it could be a real, regular cycle that stars go through during their lives. Evidence of such a cycle on 18 Sco provides additional proof that the laws of physics we know can be applied elsewhere in our galaxy.     

When Olin Wilson and his colleagues observed the 60 solar type stars at Mt. Wilson Observatory (not named after him) for 3 decades, some did not exhibit solar cycles. Perhaps these stars do have cycles, but he caught them in a period similar to the Sun between 1645 and 1715.

For more information, see the following references:

Cayrel de Strobel, G., 1996, Stars Resembling the Sun, Astronomy and Astrophysics Review, 7, pp.243-288.

Hall, J., Lockwood, G.W., 2000,  Evidence of a Pronounced Activity Cycle in the Solar Twin 18 Scorpii, Astrophysical Journal Letters, 545, L43-L45.

Hall, J., 2001, Solar and Stellar Activity Cycles, Lowell Observer, Issue 52.

Porto de Mello, G.F., 1997, HR 6060: The Closest Ever Solar Twin?, Astrophysical Journal Letters, 482, L89-L92.