Looking for trans ethyl methyl ether in Orion KL
When the Big Bad Wolf threatened the Three Little Pigs with blowing and blowing until destroying their houses, they challenged him by saying that each would build his house of a different material: straw, wood and brick. Obviously, it doesn’t take long to build a thatched or wooden house as a brick house (so the story criticized the vagrancy of two of the piglets). The wolf managed to blow down the houses of straw and wood (imagine the lung power of the canis lupus), but not the brick one, where the three tEMErarious piglets ended up scalding the fierce wolf. (My inner child wondered if that brick house, made so hastily, was not going to be a terrible quality one…).
The key factor, in this case, was time.
In Astrochemistry we also handle that variable (as in the entire universe) to determine the chemistry of gas in the interstellar medium. How long does it take for changes to be made to the chemistry of a given environment? What conditions of temperature, pressure, or other parameters, are needed?
In this particular work, we talk about the tentative detection [1] of a molecular species: by identifying a large number of lines of rotation of the molecule, a team of researchers, led by Belén Tercero (ICMM-CSIC), has presented in this paper the tentative detection, in Orion KL, of trans ethyl methyl ether (t-CH3CH2OCH3, from now on, tEME). In addition, in order to try to restrict the type of chemical processes that occur in this source, they also carried out the search for gauche-trans-n-propanol (Gt-n-CH3CH2CH2OH, an isomer of tEME, which we will call, for short, Gt-n-propanol).
But we’ve put a lot of technicality in at once… what does all this mean? Let’s go in parts.
First, what are rotation lines?
Molecules have different energy levels: electronic, vibrational and rotational. Because the energy is quantized, we can know what kind of transition has taken place when a molecular species is excited or deexcited (i.e. when its energy levels rise or drop).
Within a particular electronic state, the molecule can reach different types of vibrational states (those produced by the vibration of the atoms that make up the molecule) and, in turn, within the same vibrational state, the molecules rotate in space around their bonds.
These rotation changes can be detected with radio telescopes in the millimetric and submillimetric wave domain (the less energetic range of the electromagnetic spectrum), resulting in spectra loaded with lines to “translate”.
Thanks to the analysis of the data provided by the IRAM 30m radio telescope and the ALMA interferometer, lines of both species (tEME and Gt-n-propanol) have been identified, even being able to obtain maps with their spatial distribution [2].
Thousands of lines
A few years ago an exhaustive study of the Orion KL region was carried out with the IRAM 30m radio telescope. The result showed more than 15,400 spectral lines of which some 11,000 were identified and attributed to 50 molecules (199 isotopologues and different vibrational modes). To date, there have been several jobs based on this data.
As a result of fruitful collaboration between astrophysicists and laboratory molecular spectroscopy experts, 3,000 previously unidentified lines were assigned. Three molecular species and 16 isotopologues and vibrationally excited states of molecules abundant in Orion, never before detected in space, were identified.
With the same data set, now, a research team, led by Belén Tercero (ICMM-CSIC), has published the detection of another new molecule in space, tEME. In addition, several unidentified lines in this data have been provisionally identified as belonging to Gt-n-propanol (a tEME isomer).
Spatial Distribution
With ALMA data, maps of the spatial distribution of oxygen-carrying saturated organic species containing methyl, ethyl and propyl groups have been carried out, estimating the abundance ratios of related species and the upper limits of column densities of undetected ethers [3].
As for its provenance, while the tEME comes mainly from the “Compact ridge” area of Orion, the Gt-n-propanol appears in a hot core of southern Orion. Until now it was thought that the “Compact ridge” area was the main host of all oxygen-carrying organic saturated species in Orion, but recent studies (including the one at hand) show other regions within Orion KL where these complex oxygen-rich molecules are significantly more abundant than in “Compact ridge”. This result suggests a chemical complexity not yet well characterized, related to the processes that create and segregate these species in the region.
The abundance and spatial distribution of these molecules suggest important processes that would take place in the gas phase that occurs after the evaporation of the mantle that would cover the dust grains in the warmest areas of the region.
To summarize, by combining IRAM 30m and ALMA data, we can provide a solid starting point for the definitive identification of tEME in the interstellar medium.
Fierce Wolf
The formation of complex molecules in space is a mystery to unravel. Although, for starters, we should differentiate the term “complex molecule” on Earth and in space. Of course, given the hostile conditions in the interstellar environment and in environments such as Orion KL, combining molecules and forming more complex species is an achievement. Hence species that on earth can be common, in space are called “complex”.
Gradually we discover that dust grains, protective “bubbles” created by pressure, temperature and jets of material, and other phenomena that take place in space, generate environments that promote these changes.
As fierce as Orion KL is, there seem to be places where these chemical combinations make their way and, as much as it “blows”, they will continue to stand, tEMErarious, facing the hostilities, combining and surprising us over time.
Notes:
[1] In Astrochemistry we usually talk about tentative detection when we have almost all the keys to confirm the presence of a molecule in a certain environment but we lack a piece of the puzzle (in some cases, more than one). In this case, we talk about tentative detection because certain species that have a very abundant and complex pattern of rotational lines must be identified over a very wide range of frequencies to ensure detection. This work shows that in the frequency range studied there is no missing piece of the puzzle.
[2] Maps of CH3OCOH, CH3CH2OCOH, CH3OCH3, CH3OH, and CH3CH2OH are also provided to compare the distribution of these oxygen-carrying saturated organic species containing methyl and ethyl groups in this region. The work also includes abundance quotients of related species and higher limits to the abundances of undetected ethers. An abundance ratio of N(CH3OCH3)/N(tEME) ≥to 150 is derived in Orion’s “Compact ridge”.
[3] Column density is the amount of material contained in an imaginary cylinder (usually with a cross-section area of 1 cm2) between an observer and an astronomical object. (Oxford-Complutense Astronomy Dictionary, Ian Ridpath, 1999, Editorial Complutense). The derived column densities for these species at the location of their emission peaks are ≤(4.0±0.8)×1015 cm−2 and ≤(1.0±0.2)×1015 cm−2 for tEME and Gt-n-propanol, respectively. The rotational temperature is ∼100 K for both molecules.
Link to the paper: Searching for Trans Ethyl Methyl Ether in Orion KL
Image: «Methoxyethane-3D-balls», by Ben Mills and Jynto – Derived from File:Ethanol-alternative-3D-balls.png. Available under public domain license via Wikimedia Commons.
Originally published in Spanish on the Naukas website: ¿Quién tEME al Orión feroz? (2016/01/25).