We had already spoken before about the presence of smelly gases in astrophysical environments. Now, a team of researchers has discovered new molecular species with nitrogen in a circumstellar envelope known as a Pumpkin Nebula or Rotten Egg Nebula. Why that name? Cover your nose…
Dying stars are real molecule factories, as they throw into the interstellar medium part of the material that forms them, creating a kind of cloud of gas and dust around them called “circumstellar envelope”. Using the IRAM 30m radio telescope, a team of researchers has discovered new species with nitrogen (N) in OH231.8+4.2, an oxygen-rich circumstellar envelope also known as the Pumpkin Nebula or Rotten Egg Nebula.
For about 40 years, circumstellar chemistry has been a fertile source for new molecular discoveries and for the development of physical and chemical models. Circumstellar envelopes around evolved stars (at the stage of the Asymptotic Giant Branch or AGBs) are formed as a result of the intense mass loss process suffered by these objects and are composed mainly of molecular gas and dust, which makes them the areas of space with one of the most complex chemical environments.
These circumstellar envelopes are classified according to the relative abundances of carbon and oxygen, being rich in one element or another, which will determine what kind of chemistry will predominate in each of these environments.
In the case of oxygen-rich circumstellar envelopes, carbon plays the role of “limiting reagent” and is almost entirely contained in carbon monoxide (CO), which is a very abundant and stable species, while the remaining oxygen is free to react with other atoms, forming additional oxygen-carrying molecules.
For this reason, apart from CO, oxygen-rich circumstellar envelopes are relatively poor in carbon-carrying molecular species, while carbon-rich envelopes show low abundances of oxygen-carrying species. To date, most observation efforts to detect new circumstellar molecules had focused on carbon-rich sources, as they are believed to have a more complex chemistry than their oxygen-rich counterparts (in fact, the most studied object of this type is the evolved star CW Leonis, in whose envelope some 80 molecules have been discovered).
However, recent work suggests that oxygen-rich envelopes may be chemically more diverse than originally thought. For example, some unexpected chemical compounds (such as HNC, HCO+, CS, CN, etc.) have been identified in a number of oxygen-rich late-type stars, including the Rotten Egg Nebula, studied in this work: using data obtained with the IRAM 30m telescope, in a survey conducted at millimeter wavelengths, the molecular species HNCO, HNCS, HC3N and NO have been detected .
This work encloses many “first times”: this is the first time HNCO and HNCS have been detected in any type of circumstellar envelope; it is the first time HC3N has been detected in an oxygen-rich envelope; finally, the finding of NO (nitrogen monoxide) represents the first detection in an envelope around an evolved low/intermediate mass star (stars with a mass between one and eight times the mass of the Sun).
The importance of these detections lies not only in their discovery, but also in that they also give us clues to the chemical processes that could form them: the study of this finding suggests that shock processes could be the cause of their formation. But first, let’s get to know our protagonist a little more.
OH231.8+4.2, the Pumpkin or Rotten Egg Nebula
OH231.8+4.2  is a well-known bipolar nebula that is also known as the Rotten Egg Nebula (by the way, it could also be called the pestilent nebula… as you know, it’s not the first time we’ve talked about stinky gases or other compounds with a characteristic bad smell. In this case, our nebula has so many sulphurous compounds that the thing must be very bad up there).
The evolutionary stage of this nebula is unclear due to its multiple and unusual properties. It is believed to be a precursor to planetary nebula detected, probably in a short-lived transition phase. The central star is QX Pup , and is obscured in the visible range by gas and dust.
The late evolution of this object may have been complex, as it has a binary companion star indirectly identified from the spectral analysis of light reflected by the nebula.
A companion star could influence and alter a circumstellar envelope in both physical (for example, due to the influence of its gravitational field) and chemical development (for example, if it is very hot, it could emit ultraviolet-like light capable of dissociating some of the molecules).
Most nebular material is found in the form of cold and massive molecular gas and dust. This gas is found in a very elongated and lumpy structure formed by two main components: a central core and a highly collimated bipolar jet.
Both the presence of bipolar jets and the presence of shocks are common characteristics to objects that have left the AGB phase and are evolving into the planetary pre-nebula phase.
However, this nebula still preserves characteristics related to the spherical star envelope that are still in the AGB phase. In fact, the central star, QX Pup, is classified as AGB. As we said at the beginning, it is not clear to us at what stage of her life the star is at, and it is likely to be in a moment of transition: we have caught this star “in fraganti”.
But that is not the only topic of debate: it is still discussed what the process that gave rise and shape to this nebula as we see it today has been. It is believed that, at first, a spherical envelope was created around the AGB star (which occurs in most stars of this type). However, a mechanism of unknown origin created bipolar jets of collimated and strongly accelerated matter . The shock of these jets with the AGB envelope, which previously occupied that place in the pole area, would push that material away from the star, creating that form of pumpkin or hourglass.
The origin of these differences and the acceleration itself  is likely to be a consequence of the presence of this potential star companion, a plausible scenario that has been proposed to explain the shape and acceleration of bipolar planetary pre-nebulae and planetary nebulae.
The importance of shocks
We let you in standby, yearning for more, by overtaking you of the importance of shocks in this research work. After studying and comparing the observations and results of the models, it was inferred that the amounts found of these molecular species in this nebula (remember, HNCO, HNCS, HC3N and NO) could not be a product of UV photon-induced chemistry or cosmic ray-induced chemistry, and that other processes, such as shocks, played an important role in their formation.
It is very likely that the molecules located in the shock zone, which is located between the jet and the spherical envelope, dissociate. Material may even have been extracted from the dust grains. Then, after the shock, the collided material would cool over time, thus allowing new molecules to form again.
In short, this survey in the millimeter range has led the team to obtain very detailed information about the overall physical-chemical structure of this envelope. OH231.8+4.2 could be the best example of an environment around an evolved star that has suffered shocks and therefore a unique environment for the study of chemical processes induced by these shocks.
Rotten eggs, violent impacts, smelly gases… our oxygenated nebula seemed like a dull environment. But it is full of action.
Paper: “New N-bearing species towards OH231.8+4.2: HNCO, HNCS, HC3N and NO”. Authors: L. Velilla Prieto (Molecular Astrophysics Group, ICMM-CSIC; Astrobiology Center, CAB/INTA-CSIC, Spain); C. Sánchez Contreras (CAB/INTA-CSIC, Spain); J. Cernicharo (Molecular Astrophysics Group, ICMM-CSIC; CAB/INTA-CSIC, Spain); M. Agúndez (Molecular Astrophysics Group, ICMM-CSIC; CAB/INTA-CSIC, Spain; Laboratoire d’Astrophysique de Bordeaux, LAB/Université de Bordeaux, France); G. Quintana-Lacaci (Molecular Astrophysics Group, ICMM-CSIC; CAB/INTA-CSIC, Spain); J. Alcolea (National Astronomical Observatory, OAN-IGN, Spain); V. Bujarrabal (OAN-IGN, Spain); F. Herpin (LAB/Université de Bordeaux, France); K. M. Menten (Max-Planck Institute for Radio Astronomy, Germany); and F. Wyrowski (Max-Planck Institute for Radio Astronomy, Germany).
 In addition to these molecular species, this survey in the millimeter range has detected hundreds of molecular transitions, discovering more than 30 new species (including different isotopologues) and expanding the sequence of rotational transitions detected for many other species at this source.
 Discovered by Turner (1971), the OH231.8+4.2 bipolar nebula surrounds an OH/IR source: OH/IR objects -seen in the infrared- are evolved bright stellar objects with an envelope featuring an eminent OH maser emission.
 This star is classified as M9-10 III and has a Mira-type variability consistent with an evolved AGB star.
 With speeds of up to ∼400 km s−1.
 It appears that the acceleration of the lobes could have taken place about ∼800 years ago in less than ∼150 years and it is believed that the low-speed central core is probably the fossil vestige of the AGB’s circumstellar envelope.
Image 1: Messier 46 (Pumpkin Nebula or Rotten Egg Nebula). Credits: Valentín Bujarrabal (OAN, National Astronomical Observatory, IGN, Spain), WFPC2, HST, ESA, NASA.
Image 2: Area of the sky where Messier 46 (the Pumpkin Nebula or Rotten Egg Nebula) is located. Credits: ESA; Valentín Bujarrabal (National Astronomical Observatory, IGN, Spain) and Digitized Sky Survey.
Originally published in Spanish on the Naukas website: Sorpresas en la nebulosa del Huevo podrido (2015/04/07).