Pablo del Mazo Sevillano, new doctor

On January 25th 2021, Pablo del Mazo Sevillano has defended in the Autonomous University of Madrid his PhD work entitled “Superficie de energía potencial y dinámica para la reacción H2CO + OH” (Potential Energy Surface and Dynamics for H2CO + OH reaction) supervised by professors Octavio Roncero Villa (IFF-CSIC) and Alfredo Aguado Gómez (UAM). Congratulations!

In this text, he explains the content of his tesis:

“The mechanism by which complex organic molecules (COMs) are found in the dense molecular clouds in the interstellar medium (ISM) has been assumed to proceed through their formation on ices. Later, as these ices evolve to hotter regions, the COMs would be released to the gas phase. Nevertheless, this complex organic molecules have been found in cold regions, shielded from UV-radiation, so it is mandatory to rethink this process.

One possible explanation comes from the results shown by CRESU experiment. In particular, it has been found that reactions between COMs and OH exhibit a huge increase in their kinetic rate constants as temperature decreases, what is surprising for reactions that usually present a barrier, hence should have an Arrhenius-like behavior. This opens the possibility of gas phase reactivity that has been previously dismissed. Unfortunately, there is no current experimental setup that can reproduce the conditions found in the ISM, in particular, the low pressure, so it is not possible to calculate the zero-pressure kinetic rate constants to be used in the astrochemical models. It is here that we can take some advantage from the computational simulations, since it is much easier to reproduce this low pressure conditions. In this PhD work we have focused on the simulation of the reactive process between H2CO + OH to form HCO + H2O and HCOOH + H, to evaluate the zero-pressure kinetic rate constants and provide a dynamical explanation of its behavior with temperature.

The dynamical simulation of a reactive process usually involves two steps:

First we must obtain the potential energy surface (PES) of the process. The PES is a function that encodes the energy for every nuclear configuration the system can reach during the simulation. It is from this function that the classical forces the system experience emerge, so a correct description of this function is crucial. In this work we have explored and propose different methodologies to express this function, with special interest in the use of artificial neural networks (ANN). ANNs are functions that present huge flexibility along with a short evaluation time, what makes them an excellent alternative to describe a PES.

The second step is the dynamical simulation itself. Once the interactions between particles are known (the PES) it is possible simulate collisions between the reactive partners and determine whether they react or not. From this, the kinetic rate constant can be calculated. In this work we have employed two methods: a quassiclassical  trajectory  method (including the zero-point energy of the reactants only at the beginning of dynamical calculations) and a Ring-polymer Molecular dynamics (RPMD) method (based on path integral and including quantum effects through the use of replicas or beads), to include quantum effects important at the low temperatures considered,  such as zero-point energy and tunneling. We found two key aspects that explain the increase of the kinetic rate constant for HCO + H2O formation as temperature lowers. On the one hand, due to the permanent dipole moment present in both reactants there is a very efficient capture mechanism at long distances. That way, reactants that are far away feel each other and start approaching until they finally collide. On the other hand, during this approach and finally collision, translational energy is transferred to rotation and vibration. This means that once the energy transfer is produced, it is not so easy for the system to redissociate back to reactants and the system gets trap for long times in a reactive complex. During this time the system has plenty of time to explore the configuration space, increasing the probability that reaction may take place. This is the mayor reaction mechanism for temperatures below 200 K, as has been found both experimentally and theoretically. Above this temperature, the direct mechanism becomes more important, leading to the well-known Arrhenius behavior”.

Representation of a PES (blue surface) over which a trajectory is being propagated (red line). Molecular geometries along the trajectory are shown.

José Cernicharo, awarded with the “Miguel Catalán” 2020 prize

José Cernicharo, Researcher of the IFF-CSIC and Head of our group, has been awarded the “Miguel Catalán” 2020 Prize for his scientific career within the research awards of the Community of Madrid, “for the quality of his research, his recognized leadership at national and international level in the area of Molecular Astrophysics and for his contributions and technological innovations”.

The Community of Madrid convenes annually the Research Awards “Miguel Catalán” and “Julián Marías” to the scientific career, in order to recognize scientific activity, as well as the scientific and humanistic values developed by researchers who throughout their professional career have been in some way linked to the Community of Madrid, and the Research Awards of the Community of Madrid “Miguel Catalán” and “Julián Marías” to researchers of less than forty years , in order to recognize the quality and excellence of scientific and research work developed at the beginning of his research career. They are convened annually in the two Areas, Sciences and Humanities.

The ceremony, which will take place tomorrow, November 20 at 10:30, can be followed online through the following links:

  1. Madrid Community Canal
  2. Twitter of the Community of Madrid
  3. Complutense University Of Madrid Canal
  4. Youtube from the Complutense University of Madrid

Funambulist stars

Why do we study chemical equilibrium in red giants?

You may have often read that “we are stardust.” It is a rather accurate expression, especially if we think that most of the elements that make us up (this scarce 5% of the baryonic matter of the universe) emerged from the core of a star and from a whole process of death and destruction. But what do we call stardust?

Medium-sized stars (between one and eight solar masses) go through several phases throughout their lives. The longest phase is the so-called  “main sequence”, in which time is spent transforming hydrogen into helium. When hydrogen from the nucleus (where this activity occurs in principle) is depleted, the star begins to use other elements as “fuels”. This results in the star starting to “swell”, becoming a red giant and releasing its outer layers to the interstellar medium. This occurs in one of the final stages of its life, the AGB phase (from  Asymptotic Giant Branch).

A multitude of chemical phenomena take place in the atmospheres of these evolved stars, enriched by successive material dredged-up. In principle, when we talk about the birth of dust grains, we mean precisely those atmospheres of evolved stars (also supernova explosions, which are much more massive stars, but this is another story).

The layers of material released by the AGB star and forming its atmosphere are composed of a huge amount of gas molecules and a small proportion of dust grains. We are interested to know how these grains of dust are formed, from what basic elements and under what physical conditions.

To understand this whole process it is important to know the chemical equilibrium in these atmospheres, that is, the state in which, although the chemical activity continues, the chemical composition remains stable. If we know the chemical equilibrium, we will know what atoms and molecules are at the origin of the formation of dust grains (which is what we are interested in). In addition, we will have the theoretical scenario with the sequence of the different types of grains that are expected to appear as matter flows and cools from the AGB star into the interstellar medium.

In our case, in the atmospheres of AGB stars, we know that the gas is at high temperature and pressure, which is very fortunate as these are the conditions under which it is valid to use chemical equilibrium. This would be very different if we talked about, for example, the interstellar medium, where temperatures and pressures are very small and the chemical composition is given by chemical kinetics. In that case, as Marcelino Agúndez (from our group Astromol in the Institute of Fundamental Physics, IFF-CSIC) states, “it would be necessary to know the constant kinetics of a large number of reactions, in those environments everything is rather more uncertain. The interest of applying chemical equilibrium to the atmospheres of AGB stars is that, in them, temperatures and pressures are high, ergo we can use chemical equilibrium and the chemical composition can be calculated in a simple way. This has been known since the 1970s.”.

And here we get into a paper recently published by a team of researchers from the Spanish Council for Scientific Research (CSIC), focused on studying the atmospheres of AGB stars (as we said before, medium-sized stars in the final stages of their  lives).

The idea was to update all the information available so, first of all, the team compiled and updated a large dataset of thermochemical properties for 919 gaseous and 185 condensed species involving 34 elements. The chemical composition in AGB star atmospheres was calculated thanks to a recently developed code. All available information obtained from astronomical observations was also updated to, as a last step, compare the predictions of the calculations with the observations. All with the idea of reviewing what we knew so far, what our successes and mistakes were, and future prospects about molecules, molecular aggregates, and solid condensates in the atmospheres of red giant stars.

Funambulist stars, what the equilibrium tells us

This study not only tells us which aggregates could act as “bricks” of dust grains construction (and  what may be the most likely gas precursors of these grains) but even throws information on what we do not find:  potentially detectable molecules that have not yet been observed have been identified, making them good candidates for detection with observatories such as ALMA.

Finally, there are also things that don’t fit: in some cases, theoretical analysis and data don’t match. In fact, there are discrepancies of several orders of magnitude for some molecules which are observed with abundances several orders of magnitude above the expectations from chemical equilibrium. This means that, in some areas of the stellar atmosphere, there are no chemical equilibrium conditions due to unidentified phenomena that could be related to ultraviolet photons or hydrodynamic processes.

What’s new thanks to the lab

To do this type of studies it is necessary to know the thermodynamic properties of each molecular species. In the case of the most common molecules, this data is known, but for the rarest ones it is necessary to calculate or measure them.

This study has incorporated, for the first time, the properties of a large number of molecular aggregates of titanium and carbon analyzed within the Nanocosmos (ERC) project in the laboratories of the Institute of Materials Science of Madrid (ICMM-CSIC). In carbon-rich AGB stars, forecasts say the first condensates would be carbon itself, titanium carbide (TiC) and silicon carbide (SiC). (In O-rich atmospheres, the first condensate expected is Al2OR3). And it was necessary to have data on these under-studied species in order to compare them with observations.

In short, this work tells us about our mistakes and successes, about the path on which these atoms and molecules pass, about the origins of dust grains and where they are expected to appear, about the agglomeration that initiates the formation of the first solid materials from a gas of atoms and small molecules.

We are interested in how its composition can influence what happens much later, when the birth cycle of a new star begins again with the condensation of those dust grains resulting in nuclear reactions. We will continue to build knowledge on this data, relegating the wrong ones and analyzing the new information, because that’s how science works.

More information:

Scientific paper:Chemical equilibrium in AGB atmospheres: Successes, failures and prospects for small molecules, clusters, and condensates“, M. Agúndez  (IFF-CSIC), J. I. Martínez  (ICMM-CSIC), P. L. de Andres (ICMM-CSIC), J. Cernicharo  (IFF-CSIC) and J. A. Martín-Gago (ICMM-CSIC).