Expected Number of Multiple Star Systems as a Function of Galactic Latitude

Sky-projected multiple star systems, including doubles and triples, are a major contaminent to any automated extended source (re: galaxy) detector. Unresolved and nearly unresolved multiples are very difficult to discriminate from faint (but fuzzy) galaxies. It is therefore desirable to understand what the degree of contamination is from these objects -- clearly it depends on where the telescope is pointing with respect to our Galaxy.

Using a starcount model designed for the near-infrared bands (Jarrett 1992; also, see below for link), we estimate the number of double and triple star systems as a function of stellar density or galactic latitude (holding the longitude fixed at 90 degrees). A multiple system is defined to be at least two stars located (by chance) within 5 arcseconds of their peaks, with a flux differential no greater than a factor of 10. Stars that lie beyond 5 arcseconds should be easily resolved by 2MASS (unless they are very bright) and thus in principle should not a contaminent problem to the 2MASS extended source processor (GALWORKS). The flux differential limit (10) is chosen to eliminate multiples in which the secondary (s) is too faint compared to the primary to amount to any appreciable change in the system flux (less than 10%), as well minimal affect to the radial profile of the primary. These limits, spatial and flux, are not exact; consequently, the results we present here should be viewed as simple estimates to the expected number of multiple stars.

• Jarrett Star Count Model

In Figure 1, we show the total (cummulative) star counts as a function of galactic latitude and the fraction (%) of the total stars that comprise a multiple system.

• Fig 1: Stellar number density as a function of galactic latitude and the expected fraction of double and triple stars.
  Upper panel: Stellar density vs. galactic latitude for two cases: K <= 13.5 and K <= 14.0.
  Lower panel: For the case in which K <= 13.5, the fractional (%) number of multiples vs. galactic latitude.

Another way of looking at the results is to consider how many multiples we expect in one 2MASS scan -- spanning one square degree of sky. In Figure 2, we show the expected number of multiples per scan or per square degree.

• Fig 2: Stellar number density as a function of galactic latitude and the expected fraction of double and triple stars per sq. degree.
  Upper panel: Stellar density vs. galactic latitude for two cases: K <= 13.5 and K <= 14.0.
  Lower two panel2: For the case in which K <= 13.5, the expected number of multiples in one square degree vs. galactic latitude. Both panels are the same except for a scale change to show the effect of triples at low galactic latitude.

It is clear from Figures 1 & 2 that multiple stars (re: doubles) become a problem only when the stellar density is greater than 1000 stars per sq. degree with K brighter than 13.5, corresponding to a galactic latitude of < 20 degrees. Triple star systems are very rare, appreciable numbers occur only for densities greater than 5000 stars per sq. degree with K brighter than 13.5, corresponding to a galactic latitude of < 5 degrees. Even then, triples stars comprise only a few % of the total number of multiples (dominated by double stars).

For galaxy - multiple star discrimination, parameter thresholds should be adjusted for densities greater than 1000 stars per sq. degree (K < 13.5), with the triple star "killer" discriminent parameter thresholds strongly tied to the stellar density when the target field lies within 5 degrees of the galactic plane -- corresponding to >5000 stars per sq. degree (K < 13.5).