Orion is the nearest and brightest massive stars forming region, a “stellar nursery” that has become our astrophysical experimentation laboratory. It is so close that we can take images of the entire region and, at the same time, study details of it. In this article we will focus on how ultraviolet radiation from stars influences the interstellar clouds of gas and dust that surround them.
Interstellar clouds are areas of space “among the stars” formed by gas and dust, regions monstrously larger than the clouds of the sky in which, in some lumps “chosen” by gravity, matter can condense and collapse into stars. In particular, the great cloud of Orion is a tremendously active region of the sky. Within it stands out the Trapezium Cluster, a group of massive and very energetic stars surrounded by gases that can be seen from the ground even with small amateur optical telescopes.
Compared to our Sun, massive stars (more than eight solar masses) are huge and have shorter lives because they consume the “fuel” of their core very quickly. They are so energetic that powerful winds emanate from them that “shake” the whole environment, and also emit a lot of “sterilizing” light, especially in the ultraviolet (UV) range of the electromagnetic spectrum.
Massive stars grow so fast that there is no time for the cloud of molecular gas and dust that spawns them to disappear (as with the birth of lower-mass stars like our Sun), so that they destroy (photoevaporate and/or photoerode) the cloud that gave birth to them: they are like children who devour their parents.
For researchers it is very important to determine the impact these massive stars have on the progenitor clouds and also their impact on the whole galaxy, since their birth and existence determine the properties and future of the entire interstellar environment.
The main characters
To talk about this research we need to present the main characters, and the first is the aforementioned area of Trapezium, dominated by relatively young and massive stars (up to 30 times the mass of the Sun for the brightest star of Trapezium) and located on the sword of the “hunter” of the Orion’s constellation. The entire region is surrounded by the Orion Nebula, formed by a very hot gas that has been ionized by UV radiation emitted by these stars.
On the other hand we have the molecular cloud that lies just behind the Trapezium and the nebula. In this cloud of molecular gas and dust hundreds of protostars are being “incubated”, colder objects that are not yet “adult” stars but are in the process of formation (you can watch this video to have an idea of the distribution of matter in this area). While we need optical telescopes to capture visible light emitted by hot gas from the nebula, the only way to “cross” the region and see the molecular cloud is to observe it in infrared and radio waves.
And in the outer skin of that molecular cloud, very interesting things are happening. For example, we know that UV photons emitted by Trapezium stars are beginning to “burn” the cloud – starting with the skin – with mechanisms that researchers know well (e.g. ionization of atoms). This causes a bright flash of Orion’s skin in the range of the far infrared. Something like an interstellar “tan”.
We know our third character thanks to the data obtained with the HIFI instrument aboard the Herschel Space Telescope: we have been able to see the “skin of Orion burned” because it emits in the line of ionized carbon (C+), a line that traces how the molecular cloud is being photoevaporated.
This C+ emission line, the brightest of the interstellar medium (we’ll call it the “superline”), is a fundamental tool for plotting how UV radiation destroys molecular clouds. It also gives us clues about the rate of stellar formation, a critical parameter in astrophysics to know fundamental details about our universe (how many stars are formed and at what rate?).
In addition, this emission cools interstellar neutral gas: the thermal agitation of the gas becomes, mainly, radiation emitted in the C+ line that escapes from the cloud and cools the medium. Emission is difficult to observe from the ground, so it is necessary to use space satellites or telescopes embarked on stratospheric aircraft to study it. In fact, the team that carried out this research work obtained ten hours of observation with the Herschel Space Telescope, managing to extract from the data and the maps information about the kinematics of the gas in the skin of Orion, thus revealing its three-dimensional structure and then elaborating this impressive video.
Far beyond time and space
The information we extract from this work doesn’t end here. We have a superline that tells us about how clouds are photoevaporated and how many stars are born in a certain environment of stellar formation, such as the Orion region. But what if it was able to tell us about much further areas?
By the redshift effect, which causes light emitted in a range to move to ever longer wavelengths (due to the expansion of the universe), the C+ line emitted from very far galaxies (when the universe was much younger) comes to us in the range of millimeter and submillimeter radio telescopes that astrophysicists build at high altitudes (as is the case with ALMA -Atacama Large Millimeter/submillimeter Array – installed in Chile’s Atacama desert, more than 5,000 meters high).
That means that, if we used to need ten hours with a satellite to observe regions of the Milky Way like Orion, now, in a matter of minutes, with radio telescopes like ALMA, composed of dozens of antennae, we can get the same information from very distant objects (young galaxies) thanks to that redshift of light.
But not only can ALMA offer great advances in detailed study of what happens in the “skin of Orion”. Members of the NANOCOSMOS team  participate in a project that has obtained time from the Impact Legacy Program to map the entire Orion region into C+ using the “upGREAT” instrument aboard SOFIA (Stratospheric Observatory for Infrared Astronomy). A NASA telescope “flying” at a height of about 14 km (about 4 km above commercial flights, fasten your seat belts!).
These are 54 hours of flights and observations (usually the programmes granted with SOFIA are one hour or less) that will be carried out over the next two years in order to map a region 20 times the one presented in this study. Astronomers, climbed on an airplane, will work from the stratosphere to learn more about Orion’s (less mysterious) skin in order to understand the mechanisms that produce the emission of C+ and then be able to understand more accurately the emission that ALMA observes from the primitive universe.
Study what we have around to understand what we observe far away. All thanks to Orion’s skin.
 The scientific team that obtained time with SOFIA is led by A. Tielens (Leiden) and includes three members of the NANOCOSMOS project (J. Goicoechea, O. Berné and J. Cernicharo). This project will allow the use of the C+ line to be established as a stellar formation rate indicator, to measure the mass of molecular clouds that cannot be measured with CO (the so-called “CO-dark” gas), and to determine semi-empirically the efficiency of photoelectric heating in PAHs (polycyclic aromatic hydrocarbons) and interstellar powder grains.
Image 2: The Orion Nebula (with annotations). Color composite image of the Orion Nebula (M42) taken in visible light with the Hubble Space Telescope (Robberto et al., 2013). The molecular cloud of Orion, where new protostars develop, lies behind the ionized nebula. Black contours show the emission of C+ in the far infrared detected with Herschel-HIFI, tracing the illuminated skin of the cloud (Goicoechea et al., 2015).
Image 3: Image of the [CII] 158μm emission captured by Herschel, with annotations indicating the location of the best known regions of the Orion cloud. Credits: Goicoechea et al., 2015.
Video 1: This video shows ionized carbon emission at different gas speeds. Thanks to the “high spectral resolution” technique, gas movements can be distinguished in detail. This video is analogous to an Orion “scanner” in which, first, the peripheral regions of the Orion Nebula (especially atomic and ionized gas seen in images of visible light) are detected and finishes penetrating into the molecular cloud and dust hidden behind the visible nebula (with gas speeds above 8 km/s). The skin of the cloud (illuminated by UV radiation from the stars of Trapezium) can be seen in the gas that moves at speeds between 8 and 10 km/s. Credits: Goicoechea et al., 2015.
Vídeo 2: Spectacular 3D video of the Hubble Space Telescope inside the Orion Nebula. These stars are in a dramatic landscape of gas and dust reminiscent of the Grand Canyon. The Orion Nebula is an illustrated book about the massive formation of young stars. Credits: NASA, ESA, G. Bacon and the Science Visualization Team (STScI) https://hubblesite.org/contents/media/videos/2006/01/513-Video.html?news=true
Original video and more information: http://hubblesite.org/newscenter/archive/releases/2006/01/video/c/
Originally published in Spanish on the Naukas website: La piel de Orión (2015/12/10).