Are binary stars with only one visible sibling common enough to contribute noticeable to dark matter? Single-lined spectroscopic binaries is a binary system where only one star can be detected.
Are they common enough to contribute noticeable to dark matter?
The luminosity of normal stars is a strongly increasing function of mass. e.g. $L \propto M^3$. If another star is &quot;hidden&quot; in a binary system, then it is of lower mass. So the amount of hidden mass is less than what is seen.
Of course this can (and is) be accounted for when estimating the mass present in luminous matter because we know typical binary frequencies and properties. The luminous matter is only about 1% of that required to explain the properties of the universe as a whole and less than 10% of the mass required to explain the dynamics of ours and other galaxies, so even a factor of 2 is not going to solve the problem.
Further, from big bang nucleoynthesis calculations and estimates of the primordial abundances of helium and deuterium, we know the missing mass is not &quot;baryonic&quot;. It is not normal matter made if protons and neutrons.
So hidden binary companions are not an important contributor to the dark matter problem.
You ask for a quantitative answer. Well, a typical binary frequency for low-mass stars is 30-40% and they have a flat mass ratio distribution (i.e. the binary companions have a uniform distribution of mass). Given that stars make up less than 1% of the critical density of the universe then even if one ignored the &quot;binary problem&quot; (which astronomers don't), then the additional mass in hidden binary system is less than 0.15-0.2% of the critical density.
This is to be compared with the ~30% that needs to be there to satisfy the dark matter problem (and as I mentioned, the vast majority of that cannot be in the form of normal matter like binary star companions in any case).
To be even more specific. Hidden binary companions make no contribution to dark matter at all, since they are luminous. What they change is the average mass/luminosity ratios of stars that must be used in order to estimate a mass from a luminosity. The values that are used by astronomers to estimate the amount of luminous matter attempt to take account of a binary population. Are the effects of binarity noticeable? Yes of course we have many means to show that stars have a less massive companion. Are they noticeable on large scales - no. The precision with which the dark matter content of an individual galaxy is estimated (e.g. from a rotation curve) will have uncertainties of at least 10% and the contribution of hidden mass in stellar binary systems will be tiny compared with that.