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Mapping TGAS - Part 1

Submitted by Kevin Jardine on 2 November, 2016 - 18:53

Mapping is all about providing physical context for data. On Earth, the question "Where am I?" is answered on a map by showing the user a set of hierarchical context, including

  • local streets
  • nearby buildings
  • elevation
  • parks and rivers
  • transportation systems

and so on.

A good map of the solar neighbourhood would have its own set of structures:

  • dust clouds
  • regions of ionized gas
  • hydrogen concentrations
  • star formation regions
  • supernova remnants

and so on.

I have placed dust clouds, some ionized gas and a few supernova remnants on the Tycho Galaxy interactive TGAS display.

However, the truth is that Gaia has already made this information outdated and much more accurate information will be available in a few years, especially once Gaia scientists publish a catalog containing the distances and spectral types for a billion stars. For example, by comparing a star's real spectral type with its colour index as seen from Earth, we can determine its reddening and therefore the dust that lies between Earth and that star. With reddening data for a billion stars, astronomers will be able to construct an incredibly detailed 3D map of dust and gas in the solar neighbourhood.

But for some things we don't have to wait.

In principle, the TGAS data in Gaia DR1 already allows us to produce detailed maps of the star formation regions within about 600 parsecs. The key is the location of the hot stars.

Hot stars tend to be young and young stars have not drifted far from the sites where they were born.

We can compute a star's temperature from its colour index. A star's colour index can be determined using the BT and VT magnitudes from the Tycho-2 catalog. Specifically,

Ci = 0.85*(BT - VT)

Usually a hot star is considered to be an O and B class star (Ci < 0) or even only O stars and B stars down to B3 (Ci < -0.18). However, we have to consider that many stars in the Tycho-2 catalog are reddened by dust, and so some hot stars might have a positive colour index.

Once we have the temperatures for the hot stars, we can create a temperature density function, T(x,y,z), that essentially tells us how close any point (x,y,z) in space is to hot stars (and how hot these are). It is this "hotness" function that will help us map the structure of star formation regions.

See Bouy, H., and J. Alves. "Cosmography of OB stars in the solar neighbourhood." Astronomy & Astrophysics 584 (2015): A26 for a similar approach using Hipparcos data.

So how can we create T(x,y,z) and how can we use it to map the solar neighbourhood once we have it? Check my next blog post for many more details.