Basic Information on Their Classification and Characterization
David R. Soderblom & Jeremy R. King
Space Telescope Science Institute
3700 San Martin Drive, Baltimore MD 21218
There are three classes of stars like the Sun that are defined by Cayrel de
Strobel (1996): Solar-type (or Solarlike) Stars, Solar Analogous Stars, and
Solar Twins. The defining physical properties of any star are its mass,
composition, and age, yet none of these is a fully or directly observable
quantity. Also, angular momentum and companionship are important influences
on the behavior of stars like the Sun and cannot be ignored.
We will review some of the observational limits on these fundamental
properties and discuss how recent observations allow samples of solar-type
stars to be defined and tabulated. Some of the questions raised are:
- What is a solar analog? What is a solar twin? Who cares?
- The Sun as a star: how would we judge it from afar?
- How well can we measure basic stellar properties relative to solar?
- Are the spectroscopic micro-differences we see significant, or just
variations within a common envelope?
- Is angular momentum a big deal or small potatoes?
- Do we understand the basic physics of stellar evolution?
1. Solar Analogs and Twins
1.1. The Definitions
Cayrel de Strobel (1996) has recently provided definitions for stars that
are similar to the Sun. These definitions are necessarily vague to some degree
because they depend at least as much on how well we can measure properties
as on how important that property is intrinsically. For example, a star's
mass is surely its single most important datum, yet the mass can only be
measured directly for stars in binaries, and for such stars we suspect
the propinquity of a stellar companion influences some of what we see.
Here we specify some working definitions for these
categories, based on the practical degree of observational accuracy.
1.1.1. Solar-like Stars (aka "Solar-Type''):
Solar-type stars are the least restricted category here; they are similar to
the Sun in mass and evolutionary state. This means that physically they have
broadly similar structure, and the presence of a convective envelope is
especially important, but without totally dominant convection, as is found
in M dwarfs, for example. Convection,
and its interaction with rotation, is critical for the occurrence of phenomena
we associate with the Sun, especially magnetically-related activity.
In practical terms, Cayrel de Strobel (1996) (and Neckel 1986a, b, c) define
solar-like as 0.48 to 0.80 in B-V. Our own working definition is about F8V
to K2V, or 0.50 to 1.00 in B-V. Such stars are reasonably bright, hence easy
to study, and can be identified readily. They appear to share the same physics
and phenomenology (see, e.g., Soderblom 1985).
1.1.2. Solar Analogs:
Solar analogs are that subset of the solar-type stars known to have detailed
properties "similar'' to the Sun's. To some extent this category is meant
to include stars that have the potential of being solar twins, but we do not
yet know enough about them. This is probably the most difficult
category to quantify, and the following are somewhat arbitrary, but are
based on our present ability to infer fundamental properties of stars
(see Bedding et al.  for reviews of these topics):
- Temperature within ~500 K of the Sun's. This is based on
first-order temperatures from photometry being good to roughly 100 K
(Saxner & Hammerback 1985).
- Metallicity within a factor of two of solar. Again, rough metallicity
values from, e.g., Strömgren photometry, are about this good
(Böhm-Vitense 1981; Doyle & Butler 1990).
- Within 1 magnitude of the Zero-Age Main Sequence (ZAMS). This
means almost any age is permitted as long as the star is still burning
- Any kinematics. The relationship between Galactic kinematics and age
is statistical at best, so the kinematic properties of individual stars
tell us nothing.
- No close companion.
This requirement is imposed because close companions (when the orbital
period is about 10 days or less) can force the rotation to be synchronous
with the orbit because of tidal effects (Mathieu & Mazeh 1988). This makes
the star spin much faster than it would if it were single, and observationally
the star appears extremely active no matter what the age. This requirement
also avoids any issue of mass transfer within a system.
1.1.3. Solar Twins:
Solar twins should be indistinguishable from the Sun as near as we can tell.
- Temperature within ~10 K. Such precision can now be achieved
- Metallicity within ~0.05 dex. Again, this is now possible
if observations and analysis of high quality are applied (see paper by
Nissen in Bedding et al. 1997).
- Age within ~1 Gyr, so that the evolutionary state is comparable.
- No known stellar companion, since the Sun has none.
1.2. Solar Twins: Who Cares?
Does it really matter if we can't find two stars that appear to be identical?
We don't expect to find a twin to ourselves, do we?
This use of the word "twin'' brings to mind human twins, and perhaps that
analogy is misleading. For example, consider some twins I know,
the Brown brothers, Jeremy and Zach. Both are Boy Scouts.
How do I tell them apart when they're both in uniform?
The answer, of course, is there are always differences if one looks closely
enough, yet Jeremy and Zach unambiguously fulfill any reasonable definition
of identical twins. In the case of biological twins, we have a clear notion
of twinness based on having identical DNA. Stars are different, of course,
and not just because we're dealing with physics instead of biology. In
seeking solar twins we are not seeking a common origin, but instead
the same outcome, which is a very different thing after billions of
years have elapsed, and considering that the stars we are comparing are
certain to have been born in very different places. This leads to:
- For a time one wore glasses, and the other didn't, but now
both now wear contacts.
- For a time, one was Star Scout and the other Life, now both are Life
- Freckles, maybe? Zits?
Axiom: All stars are different. If you can't tell them apart,
you just haven't looked closely enough yet.
Question: What does it mean for stars to be twins? How much may
they differ and still be twins for all practical purposes?
Postulate: "The same'' means that stars may vary, but they do so within
a common envelope. For example, the Sun is still the Sun when it's in a
Maunder minimum state or as it goes through its activity cycle, and so stars
may differ in some details yet remain within a family of characteristics.
The Sun defines the envelope.
Flawed reasoning 1: Solar "twins'' are preferred targets for planet
searches. We do not yet know enough to rule out any class of stars as
parents of planets. (But there are valid observational reasons for preferring
solar-type stars because they have narrow lines and are photometrically quiet;
Flawed reasoning 2: We can find a star that we can use as a
spectroscopic or spectrophotometric stand-in
for the Sun, without reservations or doubt. How can we ever know for sure
the proxy is identical?