Some Like It Hot, Part 1: Gaia and the ionizing stars

Submitted by Kevin Jardine on 23 April, 2018 - 10:14
Movie poster
Movie poster

Some Like It Hot is a classic comedy written and directed by Billy Wilder, starring Marilyn Monroe, Jack Lemmon and Tony Curtis.

I'm borrowing the title for a series of blog posts starting this week, the week of the Gaia DR2 release. With more than 1 billion parallax estimates extending throughout a quarter of the Milky Way disk and far into the halo, Gaia DR2 is a momentous event in the history of astronomy. For the first time we will be able to map a large portion of our galaxy in amazing detail.

There are two approaches to producing maps using Gaia DR2. One is a full 3D approach visualizing the dataset using star density meshes. The second is to map favourite lists of stars.

I am working on both types of maps. For this week, the second approach is faster and easier.

I have two favourite lists, both of very hot stars. The first was produced by Roberta Humphreys in the 1970s and extended in the 1980s by her graduate student Cynthia Blaha. It lists about 5000 extremely luminous stars. Most are ionizing stars: they are so hot that they rip apart any hydrogen atoms in their vicinity.

The second is a smaller list of about 500 stars, all ionizing stars that are known to be associated with HII regions: large regions of ionized hydrogen gas.

Ionizing stars are one of the main markers of the spiral arms. By mapping these data sets, we can get some insight into the local spiral structure.

Moreover, both of these data sets are self-labelling. The Humphreys and Blaha data set lists OB associations for about half their stars. The HII region data set contains the ionizing stars for many famous nebulae, including the Orion, Lagoon, and Carina regions among many others. We can map the positions of these nebulae if we can map their ionizing stars.

So if we map these data sets, we should get an instant face-on map of our region of the galaxy. Over the next few days I'll describe these data sets in more detail and after the Gaia release, I'll show the resulting face-on maps.

As I said, a more ambitious project would be a full 3D map based on star density. I'll be working on that as well but it will take more time to produce.

Next steps on TGAS mapping

Submitted by Kevin Jardine on 22 July, 2017 - 13:17
Enclosure diagram
A detail of the 40% density isosurface from the solar neighbourhood map.

Now that version 3 of the solar neighbourhood map is out, I am thinking about next steps.

Gaia DR2, expected in April 2018, will allow for more detailed, more accurate and much larger maps, but in the nine months or so we are waiting for it, there are quite a few things that I could do. Here are some possibilities. I would welcome your suggestions or feedback as well.

  • Create a 15-20 minute "Welcome to the Neighbourhood" video on Youtube including 3D animations made from the map meshes.
  • Design a board or video game played on a real map of the solar neighbourhood.
  • Create a larger map. I pushed the data about as far as I dared within reasonable error limits but there is a paper that uses models of star distribution to relax the error conditions somewhat. If I extended the map from its current radius of 650 pc to 1000 pc, I could include some larger star associations in the direction of Cepheus.

You can email me at or DM me at @galaxy_map on Twitter.

The structure of the solar neighbourhood

Submitted by Kevin Jardine on 21 July, 2017 - 00:45
Enclosure diagram
An enclosure diagram showing the 419 OB star density peaks with 5 or more stars in the solar neighbourhood (within 650 pc or 2100 ly).

Sometimes less is more. The above image is an enclosure diagram showing the star density peaks in the solar neighbourhood and how they are contained within each other.

It was constructed by computing the OB star density isosurfaces for each integer value from 10% to 99% and maintaining a list of stars contained by each connected subregion.

An enclosure diagram lacks position or shape data, but reveals the star distribution and structure in a clear way. The circle size represents the number of stars in the region and the colour intensity the density.

Using the large version of the enclosure diagram here you can hover over each component to see its region label, name, and the number of stars it contains.

The names are based on clusters, associations or the brightest star contained by the region.

You can see that the solar neighbourhood contains four major dense OB star concentrations: Scorpius OB2, Vela OB2, the Orion Belt (Orion OB1) and the Perseus / Taurus dark cloud concentration that includes the Pleiades and the Perseus OB3 association. Less dense but still large concentrations include the three northern regions (ASCC 123, Cepheus OB6, and the Sulafat highway) as well as the Wishing Well region named after its core Wishing Well cluster (NGC 3532).

In April 2018, Gaia DR2 will be released with distances to more than a billion stars. Density isosurfaces and enclosure diagrams will have key roles to play in mapping this dataset and identifying the major regions within it.

Version 3 of the solar neighbourhood map

Submitted by Kevin Jardine on 20 July, 2017 - 01:34
Orion region
A detail of the 18% isosurface around Orion taken from the third version of the solar neighbourhood map. Oriented so that the direction to the galactic centre is at the top.

As I mentioned in my previous blog post, I have created a new version (v. 3) of the solar neighbourhood map. This uses colours taken from the 2MASS catalog and has simpler controls (dust is always turned on and the views always show a 70% bright star isosurface). You can select several different hot star isosurface densities as well as three different label schemes.

The brightest stars now have labels if you select the isosurface or stars label options.

The new dust overlay is taken from figure 3 (top) in this preprint:

Three-dimensional mapping of the local interstellar medium with composite data,
Capitanio, Letizia; Lallement, Rosine; Vergely, Jean Luc; Elyajouri, Meriem; Monreal-Ibero, Ana
eprint arXiv:1706.07711

You can read detailed documentation by clicking on the Help link at the upper right of the map system, which can be found here.

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.


Subscribe to Galaxy Map RSS