The death of stars is the main phenomenon responsible for the enrichment of the interstellar medium, both in terms of the number of elements and in terms of their chemical complexity, and one of the objectives of astrochemistry is to understand how chemical processes that we know on Earth take place in space.
The difference in environmental conditions, however, could make us think that it is impossible for some events that do take place on our planet to happen in space. Nothing further from the truth: despite being hostile environments (in most cases) in the universe there are processes of chemical complexity that are often unexpected.
This is the case of IK Tau, an evolved solar-type star that is in one of the last stages of its life, the AGB or Asymptotic Giant Branch phase. At this stage, the star inflates, increasing its size hundreds of times, and creates an envelope that will gradually release its material into the middle.
For the first time, an international research team, led by Luis Velilla, from the Institute of Materials Science of Madrid (CSIC), has made an extensive survey of the envelope of this oxygen-rich AGB star.
And why is this study interesting? Normally, evolved stars are either rich in oxygen or rich in carbon, and the latter have always been considered more complex and chemically active. Let’s say stars with oxygen-rich envelopes were considered more “boring”, chemically speaking. In fact, the models on which the studies of these two types of evolved stars are based take these factors into account, confirming a more active character in carbon-rich envelopes.
But, given the results of this work, perhaps the approach that has prevailed so far will have to be changed. We will have to start looking at the “oxygenated” stars with different eyes.
After several observation campaigns carried out between 2009 and 2013 with the IRAM-30m telescope, located in Sierra Nevada (Granada, Spain), the data have revealed the presence of approximately 350 spectral lines emitted by numerous species carrying elements such as H-, O-, C-, N-, S-, Si- and P (approximately 20 molecular species). In addition, for the first time in this source, lines of HCO+,NS, NO, and H2CO, as well as several isotopologues of previously identified molecules have been detected .
Several lines of high excitation of SO2 have also been detected, which is interesting, since the models do not predict that, under the conditions of temperature and density of the inner region of the envelope, SO2 can form so abundantly.
This would prove that oxygen-rich molecular envelopes have a much more complex chemical activity than previously believed.
A reservoir of data
One of the important aspects of this work, apart from the discoveries of molecular species not expected in these environments, is the large amount of data it provides to continue studying this type of object, since it has detected numerous lines of molecules previously identified in IK Tau, allowing a detailed study of molecular abundances and excitation temperatures. That is, if before there were simple detections, now there is enough data to make larger and deeper studies.
On the other hand, this work will not only serve as a reference for the studies of the chemical evolution of solar-type stars (in particular, of stars 4,000 million years older than our Sun), but also makes it necessary to rethink the current chemical models when reproducing the formation of species in the inner regions of oxygen-rich AGB stars.
Studying these envelopes helps us understand how the elements react to form, first, simple molecules composed of two atoms (such as NS) and, later, molecules such as formaldehyde (H2CO), keys to the subsequent formation of complex organic molecules.
As always when we talk about astrochemistry, our search is associated with the great unknown that we have not yet managed to clear: the chemical origin of life.
 C18O, Si17O, Si18O, 29SiS, 30SiS, Si34S, H13CN, 13CS, C34S, H234S, 34SO, y 34SO2.
IK Tau is an evolved star surrounded by an envelope composed of dust and gas. This envelope blocks most of the visible light emitted by the star. In this false color image, from the Sloan Digital Sky Survey, we see the object in an intense red color.
This work has been published in the paper “The millimeter IRAM-30m line survey toward IK Tau“, and its authors are Luis Velilla Prieto (Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC; Centro de Astrobiología, CAB/INTA-CSIC, Spain); Carmen Sánchez Contreras (CAB/INTA-CSIC, Spain); José Cernicharo (ICMM-CSIC, CSIC, Spain); Marcelino Agúndez (ICMM-CSIC, Spain); Guillermo Quintana-Lacaci (ICMM-CSIC, Spain); Valentín Bujarrabal (National Astronomical Observatory, OAN-IGN, Spain); Javier Alcolea (OAN-IGN, Spain); C. Balança (LERMA, Paris Observatory, Sorbonne University, France); F. Herpin (University of Bordeaux, LAB; CNRS, LAB, France); K.M. Menten (Max-Planck Institute for Radio Astronomy, Germany); and F. Wyrowski (Max-Planck Institute for Radio Astronomy, Germany).
Originally published in Spanish on the CulturaCientífica website: “IK Tau, una muerte oxigenada” (2017/02/13).