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TGAS, 2MASS and the "Fingers of God"

Submitted by Kevin Jardine on 4 July, 2017 - 00:38
Side view
Density isosurfaces of major OB star concentrations in the solar neighbourhood . Oriented so that the direction to the galactic centre is at the top.

Gaia DR1, which includes the TGAS parallax data set, provides no colours for its stars. Colour data is essential in order to produce maps of the hot OB stars, which mark the young star formation regions in our galaxy.

Colours will be provided in the next Gaia release, but for now, the second version of my solar neighbourhood map takes its colour information from the Tycho-2 catalog, which makes sense because TGAS is itself based on stars from that catalog.

However, I noticed that many professional astronomers publishing on Gaia were using colours from the near-infrared 2MASS catalog instead. Eric Mamajek has put up an invaluable colour reference table, which converts colours from many catalogs into star temperatures and spectral types. Mamajek's table includes the 2MASS colours.

Using the Tycho-2 colours with my usual filters (err/plx < 0.2, absolute magnitude brighter than 1.5) extracts 3400 OB stars from TGAS. However, using the 2MASS colours extracts 5800 OB stars - more than a 70% increase!

I suspect 2MASS colours are more accurate because near infrared colours are less likely to be distorted by dust than visual light colours.

I am now working on a third version of my map that combines the 2MASS-derived hot star colours with the latest solar neighbourhood dust map. I expect the map will be done before the end of the month. However, I already have some interesting results.

Perhaps the most important result is a structure analysis of the density isosurfaces for the hot OB stars. What I've done is compute a density isosurface for each integer percentage from 10% to 99% as well as generate statistics and a list of contained stars for each connected region inside each isosurface.

There are many isosurfaces and thousands of connected regions within them, but not all are equally important.

If we start with the low density 10% isosurface and watch what happens to each connected region within it as the density increases, we see that each region fragments into smaller and denser regions. The lower density regions contain a nested sequence of higher density regions, much like Russian matryoshka dolls.

Most of the time, the fragments are small. But occasionally a large fragment breaks off. The enclosure diagram below shows each fragment with 20 or more stars and the less dense enclosing region that it fragments from. Hovering over a circle should show the name I have given to the region.

There are many interesting features visible in this enclosure chart. I'll look at some of these in a future post. For now, I want to show what happens when I map these large fragments. The map at the beginning of this article shows the fragments denser than 12% with at least 20 stars. Other than Orion X, which is from the

Bouy, H., and J. Alves. "Cosmography of OB stars in the solar neighbourhood." Astronomy & Astrophysics 584 (2015): A26.

article, the others are named after a contained bright star, cluster or association. The prominent Wishing Well region is named after the common name for its largest cluster, NGC 3532.

The map looks very promising, but it has one disappointing feature. Many of the elongated structures are oriented so that they point towards the Sun.

You can see this here:

Elongated structures like these are a common artefact in Milky Way maps and are sometimes jokingly called "fingers of God" because of the way the galaxy appears to point at our minor G2 class main sequence star. Often the cause is dust, which blocks our view in some places but has gaps that allow us to see in certain directions for a long distance. It is possible that dust is a cause here too, but it also seems likely that the problem is due to errors in parallax measurement.

A poster displayed last week at an European astronomy conference in Prague by Larreina, Alves, Bouy, observes that "Due to errors in parallax many [TGAS] structures appear elongated towards the Sun".

We can hope that with better calibration and more observations, these "fingers of God" will be eliminated or at least significantly reduced in Gaia DR2, due out in April 2018.