Bok Globules and Herbig Haro Objects

ResearchBlogging.orgThe Milky Way Project science team are currently busy laying what we hope is the final hand on our first publication. In this paper, we’ll describe the project and why we decided to take the citizen science approach for the task of identifying bubble structures in the Galaxy. We will also present our first results from the hundreds of thousands of classifications we’ve logged on the site, and how our new bubble catalog might be useful for further studies of star formation and the interstellar medium. As we’re big fans of open data sharing, the paper will of course be made publicly available via Arxiv.

I spotted a bunch of interesting questions on the Milky Way Project Talk forums recently and wanted to take some time to jot down a few answers. Here the first one (or in fact, two).

User Ken Koester asks:

  1. Is the resolution of these images such that we ought to be able to detect Herbig Haro objects?
  2. Bok globules are pretty cold; do they still show as black in these images?

Herbig-Haro objects were first discovered in the early 1950s by, not surprisingly, astronomers George Herbig and Guillermo Haro, who spotted optical nebulosity in active star forming regions like Orion. Decades of further observations in the optical, infrared and radio have since established that these “nebulae” are bright knotty streams or jets, sometimes with a very marked bipolar and narrow shape, streaming out from newly forming stars. This type of powerful outflow of material is a typical feature of star formation at all different masses.

The optical radiation seen from these Herbig-Haro flows arises when material in the outflow powers into the quiescent medium surrounding the new star, causing shocks in the gas. Such shocks are also commonly seen in the infrared. In the Spitzer bands of observation, shocks are particularly prominent in the 4.5 μm channel – the blue channel in our Milky Way Project images. Outflows come in variety of size, and of course their apparent size depends on how far away they are. But the resolution of Spitzer is certainly sufficiently high to spot them.

If you want to know more about these objects, there are two excellent review articles from Annual Reviews of Astronomy & Astrophysics on the topic:

Schwartz, R. (1983). Herbig-Haro Objects Annual Review of Astronomy and Astrophysics, 21 (1), 209-237 DOI: 10.1146/annurev.aa.21.090183.001233 [ADS]

Reipurth, B., & Bally, J. (2001). Herbig-Haro Flows: Probes of Early Stellar Evolution, Annual Review of Astronomy and Astrophysics, 39 (1), 403-455 DOI: 10.1146/annurev.astro.39.1.403 [ADS]

Bok globules are very dense and compact cloudlets that are forming new stars in their interiors. They appear black in optical and infrared images, as they’re too cold to emit any radiation shortward of around 100 μm. To study what’s happening on the inside, we have to observe at those wavelengths and beyond, which are covered by e.g. the Herschel Space Observatory and millimeter telescopes such as IRAM or ALMA. They should be visible in our images, as they appear dark in all our colour channels. But not many are as distinct-looking as the ones we see in beautiful Hubble images like this one, so they can be very hard to spot.

The Spitzer image below of a giant Herbig-Haro flow inside a Bok globule towards the constellation of Vela, at a distance of 350 pc (1140 lightyrs), combines light at 3.6 µm (blue), 4.5 and 5.8 µm (green) and 8.0 µm (red). The colours used are a little different than in our Milky Way Project images, which use 4.5/8/24 µm respectively. The image below looks at an area on the sky of 10.2 x 6.5 arcminutes, which is just slightly closer in than the highest zoom level of the MWP images (18 x 9 arcmin). Compared to other H-H objects, HH46/47 is really enormous, so other outflows are likely to appear much smaller.

Spitzer’s view of a giant Herbig Haro flow, HH46/47, inside a Bok globule (Image: NASA/JPL-Caltech/A. Noriega-Crespo (SSC/Caltech), Digital Sky Survey).

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