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Some Like It Hot, Part 3: Ionizing stars of HII regions

Submitted by Kevin Jardine on 25 April, 2018 - 09:48
HII region star catalog detai
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.

Some Like It Hot, Part 2: The Humphreys catalog of luminous stars

Submitted by Kevin Jardine on 24 April, 2018 - 09:25
Humphreys catalog detail
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.

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 kevinjardine@gmail.com 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.

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