Blogs

A face-on map of the Milky Way within 3kpc (about 10 thousand light years). The full revised map is is available as a PDF here.

I have revised the face-on map I published in my last blog post. As before, this is based on the Humphreys/Blaha data set of ionizing and highly luminous stars and a (now expanded) list of ionizing stars for HII regions. For this revision, I used the distance estimates for Gaia DR2 stars provided by Bailer-Jones and his colleagues at Germany's Max Planck Institute for Astronomy. These distances provide more accurate distance estimates for many stars than a simple reciprocal calculation, and allow distances to be estimated for a much larger set of stars.

In order to provide an additional check on the HII region distances, I have compared the Bailer-Jones distances for the ionizing stars with existing photometric and kinematic distance estimates for the associated HII regions. If a photometric or kinematic distance estimate provided an error range, I used that, or otherwise used a range of +/- 25% of the published value.

If an ionizing star had a Bailer-Jones distance outside the error range for any published distance estimate for the associated HII region, I considered it an outlier and filtered it out. I then computed a filtered distance for the HII region as the median distance for the filtered ionizing stars. If 50% or more of the ionizing stars for an HII region made it through the filtering process, I used the filtered distance value for the revised map. If less than 50% of the ionizing stars for an HII region made it through the filtering process, I excluded the HII region from the map.

For a small number of HII regions with very bright ionizing stars I used Hipparcos stars with err/plx < 0.2 when Gaia DR data was not available. I used the reciprocal method to compute the distance and then applied the same filtering process described above.

Fortunately, almost all the major HII regions within 3kpc made it through the filtering process. The ones that did not were mostly small regions with few known ionizing stars. I have added a table below listing the HII regions that had at least one ionizing star with a Bailer-Jones distance compatible with a previously published HII region distance. I provided links to the relevant distance papers in the last column of the table.

The full revised map is is available as a PDF here. You will need to zoom into about 200% in the PDF to read the labels for each HII region.

You can also see the list of ionizing stars I used for the HII regions.

A detail from a face-on map of the Milky Way within 3kpc (about 10 thousand light years). The full map is is available available as a PDF here.

Now that Gaia DR2 is available, it is possible to do an enormous amount of mapping and stellar analysis, much of which is announced on Twitter via the #GaiaDR2 hash tag.

I have used the two data sets I mentioned in my last two blog posts (The Humphreys/Blaha catalog of luminous stars and the expanded Avedisova catalog of ionizing stars for HII regions) to create a simple map of the Milky Way out to 3kpc (about ten thousand light years). It is available as a PDF here.

There is a lot of detail on the map, so you will need to zoom into 100% to see it. In general I used the Sharpless, Gum, Georgelin and RCW names for the HII regions, following Avedisova, which are not necessarily the most commonly used names today. At some point I will do a version with expanded labels.

Blue dots are OB and Wolf Rayet stars. Magenta dots are cooler stars. The yellow dot at the centre is the Sun. Red circles are HII regions.

I filtered the GaiaDR2 cross matches to only select stars with err/parallax < 0.2. This found roughly two-thirds of the stars in the data sets. If I spent more time on the cross matches I could probably find more.

This is very much a draft map. There are many possibilities for errors with this kind of map: errors in the ionizing stars for specific HII regions, bad cross matches, etc.

Also, a better map would be based on Gaia DR2 star density and not just a specific list of stars. And would be 3D. All of that is under development by myself and others. Expect to see some amazing Milky Way maps over the next few weeks and months!

A detail from the catalog of ionizing stars for HII regions. A full all sky image in galactic coordinates is available here.

In 1984, Avedisova and Kondratenko published an important paper Exciting stars and the distances of the diffuse nebulae [ADS link: 1984NInfo..56...59A] in which they listed several hundred ionizing stars for HII regions.

Although the paper is written in Russian, the tables are provided in Roman characters.

Over the last few years I have expanded the Avedisova ionizing star catalog to include data from about 90 other scientific papers as part of a project to determine the ionizing stars for the major visible HII regions. I am making this expanded catalog available today in preparation for the Gaia DR2 release (expected in just a few hours now!). It contains more than 500 stars.

You can download the ionizing star catalog as a zip here. The catalog includes a reference to the scientific paper for each star, a SIMBAD identifier, the HII region it ionizes and, crucially, a cross match identifier for 2MASS or Tycho-2 so that parallaxes can be retrieved via the Gaia DR2 cross match tables.

In addition, the zip includes a table giving basic parameters for each HII region, including the galactic longitude and latitude, and the radius of the region in the sky in arcminutes.

Interestingly, the Avedisova catalog of ionizing stars was finished in the same year (1984) as the Humphreys/Blaha catalog of luminous stars I mentioned in my last blog post. This may not be entirely a coincidence. In some ways these catalogs were a culmination of a period from about 1970 to 1985 when astronomers were intensely interested in mapping optically visible objects in the Milky Way. After 1985, interest moved to non-visual frequencies and more astrophysical topics like dark matter or the age of the universe.

It seems clear that the release of Gaia DR2 today is likely to spark a new period of interest in mapping the Milky Way, making the Avedisova and Humphreys/Blaha catalogs very relevant again.

If you are interested in learning more about luminous stars or HII regions, please read The Cloud Hunters and The Star Sweepers for more information.

A detail from the Humphreys catalog of luminous stars. A full all sky image in galactic coordinates is available here.

Roberta Humphreys first published her catalog of luminous stars in her highly cited 1978 paper, Studies of luminous stars in nearby galaxies. I. Supergiants and O stars in the Milky Way [ADS link: 1978ApJS...38..309H].

The catalog was further refined by Cindy Blaha and colleagues and divided into two sections on luminous stars in associations and field stars. There are about 5000 stars in total.

The catalog was completed in 1984 and eventually made available for download from several locations, including NASA and ADS servers. However, over time the catalog disappeared from these servers and for some reason does not appear to have been uploaded to Vizier.

You can click here to download a zip of the catalog files here, along with a SIMBAD cross match I did recently. The cross match includes ids for existing star catalogs like 2MASS, Tycho and UCAC4. The Gaia DR2 dataset will include best neighbour cross matches for these catalogs when it is released, so the cross match file can be used to look up parallaxes for these stars.

I was able to find cross matches for almost all the stars in the Humphreys catalog and will put up a simple face on map of the data set after Gaia DR2 is released. For now you can click here to download an image of the data in the sky with galactic coordinates. Blue dots are O, B and Wolf-Rayet stars. Red dots are cooler stars.

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.