The big yellow void…

… and the loss of mass of the yellow hypergiant star IRC+10420

Why is there a seemingly empty area of the Hertzprung-Russell diagram? And why is it called a “yellow void”? And, while we are ai it… what is the Hertzprung-Russell or H-R diagram?

Image1: IRC+10420. Yellow hypergiant star IRC+10420 surrounded by ejected material. Credit: Roberta M Humphreys.

First of all, welcome to a report that is going to be totally yellow (but not yellow press, we leave that to IRC+10216).

I’m going to take it for a fact that you have no idea about astronomy. (If you already know what the H-R diagram is, you can go to the next heading: ‘The “yellow empty” area’).

The Hertzsprung-Russell diagram (yes, I know you can search for it on Wikipedia, but I’m going to explain it to you anyway) is a way to visually illustrate a set of star-related data, so that we can understand its distribution according to certain parameters. There are many types of diagrams, but the one that Hertzprung and Russell invented independently makes us see the distribution of the stars according to their brightness and temperature. It’s like taking all the stars in the Milky Way – imagine a bunch of colored marbles – and put them in a box distributed according to those parameters. We would see them classified into a single image, giving us an idea of how many of them are in each class.

Well, there’s supposed to be something yellow in one part of that box… And there isn’t.

The “yellow empty” area

In the H-R diagram there are two regions that have very few stars: the ‘Hertzsprung Gap’ and the Yellow Void.  In the first, it is believed that the problem is that stars have not yet been observed at that stage because it is a rapid stage in the life of a solar-type star; in the case of yellow void, it is believed that there should be yellow hypergiants, but there is none.

Yellow hypergiant stars are a type of evolved massive star that have extreme initial masses and very high luminosities [1]. In fact, they are supposed to end up exploding as supernovae after going through several phases in which they lose a lot of mass.

The thing is these stars are very unstable. When, by the evolution of their characteristics (specifically, by the changes in their effective temperature), they are about to enter the ‘yellow void’ area of the diagram, they “bounce” and go back to an area where they appear as red… But how on Earth! What’s wrong with yellow hypergiants? Why don’t they step into that empty area (there is so many space!)?

Image 2: The yellow void. Illustration of the hypergiant star HR 8752 through the yellow void. The diagram shows the surface temperature of the star observed in the last 100 years. It increased from ~5000 to ~8000 degrees between 1985 and 2005, while the radius decreased from 750 to 400 times the radius of the Sun. Credit: A. Lobel ROB.


There are two types of yellow hypergiant stars. The first are stars that are starting to age after finishing their main sequence, moving to the red supergiant phase (i.e., they do not yet have an envelope created by matter that they release into the environment when they start “dying”).

But our yellow hypergiants are of the second type, evolved stars with envelopes and large mass loss that go from the yellow supergiant phase to the WR (Wolf-Rayet) star phase. Afterwards, they will move to the blue luminous variable star stage, then to hydrogen-poor WR star and eventually explode like supernovae (to see it very clearly, go to  “Stellar Evolution”  of @molasaber).

IRC+10420 is a prototypical yellow hypergiant (located in the Aquila Constellation) that has already passed the red supergiant phase (in which they can lose up to half of their initial mass) and has evolved to higher temperatures in the H-R diagram [2].

The yellow void stage results in a series of episodes of mass ejection that occurs in the form of bursts. As a result, the star is surrounded by dust, so perhaps what is happening is that we cannot measure one of the parameters of the H-R diagram correctly because the effective temperature of the star, that is, the temperature of its detectable surface… can’t be detected!

Our yellow hypergiant is hidden behind the dust, so we see her as a reddish star. But the actual effective temperature continues to rise and a pseudo-photosphere that keeps them at the low temperature limits of the yellow void in the HR diagram is formed around the yellow hypergiant stars. As the ejected material is diluted in the outer area, we induce that at the end they will appear just beyond the high temperature limit of the yellow void. That’s why they look like bouncing off the diagram!

Therefore, the evolution of yellow hypergiants remains hidden until they become what has been called slash stars [3]  (renamed by myself as oldyoung stars) and eventually enter the Wolf-Rayet phase.

The rich chemistry of this yellow hypergiant star

There is something important to keep in mind: these stars are aging, they are losing mass at an extremely high rate, they are ejecting matter to the medium at an incredible speed… [4] And all that matter is the one that feeds back the interstellar medium. Here we get to the heart of the matter: what we are interested in is knowing more about the chemical properties of circumstellar matter ejected by the most massive evolved stars. And studying their behavior helps us better understand the processes that occur before supernova explosions and determine when different species of molecules are formed.

Using the IRAM30m telescope, a team of astronomers, leaded by Quintana-Lacaci (CSIC), did an IRC+10420 probe confirming that the chemistry of this object is especially rich: they detected 22 molecular species in the circumstellar envelope of this object [5].

Although it is predicted that the mass ejections are huge in these objects, only three yellow hypergiants, IRC+10420, AFGL 2343 and IRAS 17163-3907, have shown molecular emission.

The expulsion of this material may also be explained in a similar way to that of ejections that take place in low-mass AGB stars, the small counterpart of massive stars, which also age by ejecting matter but do not end up bursting as supernovae. In this case, mass ejection is driven by radiation pressure in dust grains.

But this is not all: the ejections prior to the yellow hypergiant phase… are no longer part of the envelope around them. For Quintana-Lacaci “All the molecular material we observe had to be expelled only during the yellow hypergiant phase. Any gas expelled during the previous phase of red giant star would have been rapidly diluted in the interstellar medium and would have been photodissociated by the ultraviolet radiation of the interstellar medium”.

Which means that there may be no stars in the “yellow void” of the diagram because they hide behind the dust they have recently ejected, while the dust they ejected in previous episodes has already become part of that interstellar medium composed of dust grains (1%) and gas (the remaining 99%).

Behold, we have a possible answer to that yellow void. Along the way, we seem to have learned things about huge stars who like to play hide-and-seek, hiding behind layers of dust and leaving an “empty space” in our knowledge. A void we strive to fill.


[1] This one in particular has a brightness of L ~ 5 × 105L⊙ and an estimated initial mass of Minit ~ 50M⊙.

[2] In particular, the spectral type of IRC+10420 has changed from F8Ia (with an effective temperature of 6300K) to A5Ia (8300K) in just 20 years.

[3] Slash stars are massive, hot stars that have typical characteristics of both old stars (in this case Wolf-Rayet) and young stars (type O). Slash comes from the “/” symbol that is used to separate young star characteristics from old star characteristics (as happens, for example, in Ofpe/WN9 stars). I mean, they’re  oldyoung stars.

[4] In particular, for the IRC+10420 yellow hypergiant, they detected a separate circumstellar envelope with an extension of 5×1017cm expanding at speeds of ~37 km/s. There are two episodes of strong mass ejection responsible of the formation of this circumstellar envelope, which occurred within 1,200 years and reached a mass loss rate of 3 ×10−4M⊙ yr−1.

[5] The team has conducted a survey of IRC+10420 at wavelengths of 1 and 3 mm, identifying 106 molecular emission lines from 22 molecular species: CO,  13CO, CN, H13CN, HCN, SiO, 29SiO, SO, SiS, HCO+,CN, HNC, HN13C and CS.

More information:

Paper “A λ 3 mm and 1 mm line survey toward the yellow hypergiant IRC +10420: N-rich chemistry and IR flux variations”, by G. Quintana-Lacaci (ICMM-CSIC), M. Agúndez (ICMM-CSIC), J. Cernicharo (ICMM-CSIC), V. Bujarrabal (OAN-IGN), C. Sánchez Contreras (CAB/INTA-CSIC), A. Castro-Carrizo (IRAM France), and  J. Alcolea (OAN-IGN).


Image1: IRC+10420. Yellow hypergiant star IRC+10420 surrounded by ejected material. Credit: Roberta M Humphreys.

Image 2: The yellow void. Illustration of the hypergiant star HR 8752 through the yellow void. The diagram shows the surface temperature of the star observed in the last 100 years. It increased from ~5000 to ~8000 degrees between 1985 and 2005, while the radius decreased from 750 to 400 times the radius of the Sun. Credit: A. Lobel ROB.

Originally published in Spanish on the Naukas website: “El gran vacío amarillo… (2017/10/30).

Baby, baby, baby, light my way

Yes, may be this reminds you the lyrics of “Ultraviolet”, a song from U2 that fits perfectly with our topic. Let’s sing some astrochemistry.

Image 1: Orion Nebula.

The world of science often says that space is hostile, that it is very difficult to find complex molecules (even if there are). The fact is that one of the culprits of breaking bonds between atoms and leaving everything broken is ultraviolet radiation from stars, and in this case, the distant ultraviolet. But also, like Ying and yang, it may be responsible for liven up certain organic molecules.

Astrochemistry looks for those molecules and study their endurance and how they react. In this particular work, scientists have studied the molecular gas in space that is being strongly irradiated by ultraviolet rays. The team, led by Sara Cuadrado (ICMM-CSIC), has performed (the words in bold are explained below) a complete spectral survey of lines in the millimeter range using the IRAM30m telescope. This spectral survey (a kind of thorough review of that entire range of the electromagnetic spectrum) has been carried out at the edge of the photodissociation region of the Orion Bar, which is being irradiated by a very intense field of distant ultraviolet radiation.

But let us take it one step at a time.

Maybe you already know the Orion Bar from other previous reports: it is located within the well-known Orion Nebula, about 1,300 light years from Earth, and is the closest massive stars formation region.

The energetic far ultraviolet reaches this area of the Orion Bar. This ultraviolet range of light comes from young and massive stars that are forming  nearby (the stars known by the name of the Trapezium) and emit much of their energy in this range of the electromagnetic spectrum. The far ultraviolet is responsible, in this case, for the photodissociation of the molecules.

The photodissociation region is the one in which ultraviolet light is dissociating, that is, separating the atoms of the molecules (although, at the same time, new bonds between atoms may be forming, creating new molecules). The photodissociation region of the Orion Bar is very special because, being “close”, we can study it in detail.

The millimeter range is the range of the electromagnetic spectrum that allows us to study cold areas of the cosmos. For an astronomer a “cold” environment is one in which the temperature does not allow us to observe it because it emits poorly and it is very difficult to detect objects. Infrared and millimeter help us “see” those cold objects. 

Finally, the lines we are talking about are like fingerprints of chemical species. We detect them in space and they are reflected in lines like the ones I show you:

Image 2: Orion Bar Spectrum.

Well, despite the fact that the Orion Bar is a hostile environment where it was only expected to  find very simple molecules, the observations show spectra with many lines (the team has detected more than 850!), of which about 250 correspond to complex organic molecules and related precursors  [1]: methanol, formaldehyde,  formic acid  (the ants one), acetaldehyde, etc.

What does this mean?

La zona de la Barra de Orión sufre el castigo constante de la radiación ultravioleta emitida por estrellas masivas jóvenes del entorno. Por eso se pensaba que no podía haber complejidad química. Pero la hay. Y, aunque se desconocen los procesos por los cuales se forman estas especies descubiertas en el borde de la Barra, se han planteado varios escenarios que explicarían cómo se forman las moléculas orgánicas complejas halladas:

The Orion Bar area suffers the constant punishment of ultraviolet radiation emitted by young massive stars in the environment. That’s why it was thought that there could be no chemical complexity. But there is. And, although the processes by which these species discovered at the edge of the Bar are formed are unknown, several scenarios have been proposed that would explain the formation of the complex organic molecules found:

The first scenario would take into account new chemical reactions that only occur in the hottest gas and that have not yet been included in current theoretical chemistry models that try to reproduce the processes that occur in the interstellar medium.

In the second, complex organic molecules would be produced on the hot surfaces of the nearly bare grains (without ice sheets [2]).

And, in the third scenario, the dynamics of the photodissociation regions (something like currents of motion within the cloud) would cause complex organic molecules or their precursors, which have formed in the icy mantles of dust grains inside the molecular cloud, to sublimate and reach the edge of the Bar.

In short: the presence of complex organic molecules in the interstellar medium is more ubiquitous than initially expected. It includes environments as adverse as gas in the process of colliding at high speeds and, now, gas strongly illuminated by distant ultraviolet radiation. The formation of complex organic molecules reflects the complicated interaction between the chemical processes that occur in the gas phase and on the surface of the dust grains, leaving us with the question of what do all these molecules do in the Bar?

For Sara Cuadrado, “The formation routes of these species are not entirely clear and may not even be the same in different environments. More theoretical studies and laboratory experiments are needed to investigate the different chemical processes that take place on the surface of grains. The next step is, thanks to the new and increasingly powerful telescopes, to study regions similar to the Orion Bar to learn more about the different mechanisms taking place in these chemically surprising regions.”

Going back to the title of this report, you will understand that we sing the chorus of the “Ultraviolet” song from U2, “Baby, baby, baby, light (photodissociate, photoionize and photodesorb) my way”.


[1] H2CO, CH3OH, HCO, H2CCO, CH3CHO, H2CS, HCOOH, CH3CN, CH2NH, HNCO, H213 CO, and HC3N (in decreasing order of abundance). The inferred column densities are in the range 1011— 1013 cm-2. The work also provides the upper limit of abundance for some organic molecules that have not been detected in spectral scanning, but are present in other star formation regions: HDCO, CH3O, CH3NC, CH3CCH, CH3OCH3, HCOOCH3, CH3CH2OH, CH3CH2CN, and CH2CHCN.

[2] The desorption of complex organic molecules from the icy mantles that coat the dust grains by the action of UV radiation is one of the main mechanisms of formation of these species in the interstellar medium, a mechanism known as photodesorption. But this process does not occur on the illuminated and warmer edge of the Orion Bar, as the dust grains are no longer coated by ice.

More information:

This work has been published in the paper Complex organic molecules in strongly UV-irradiated gas”, by S. Cuadrado (Molecular Astrophysics Group, Institute of Materials Science of Madrid –CSIC, Spain); J. R. Goicoechea (Molecular Astrophysics Group, ICMM-CSIC, Spain); J. Cernicharo (Molecular Astrophysics Group, ICMM-CSIC, Spain); A. Fuente (Observatorio Astronómico Nacional – IGN, Spain); J. Pety (Institute of Millimeter Radio Astronomy (IRAM); LERMA, Paris Observatory, CNRS/PSL Research University, France); and B. Third (Molecular Astrophysics Group, ICMM-CSIC, Spain).


Image 1: Orion Nebula: The Orion Nebula, an immense stellar nursery about 1,500 light-years away. This stunning false-color view has been based on infrared data obtained with the Spitzer Space Telescope.  Credits: NASA/JPL-Caltech

Image 2: Orion Bar spectrum: Part of the spectral survey in the Orion Bar photodissociation region obtained with the IRAM-30m radio telescope. Credit: Sara Cuadrado.

Originally published in Spanish on the Naukas website: “¿Qué tiene el ultravioleta que a todas horas…? (2017/10/23).

Three new doctors

The Molecular Astrophysics Group has three new doctors, who defended their thesis in the last months. Luis Velilla, Alicia López and Sara Cuadrado: here we present a resume of their work. Congratulations!

Luis Velilla:

“Molecular complexity in envelopes of evolved stars: detailed study of the molecular emission of the objects IKTau, OH231.8+4.2, and IRC+10216”

Circumstellar envelopes of evolved stars are the main contributors to the enrichment of the interstellar medium, and are excellent laboratories to study the molecular complexity and the chemical evolution of the Universe. In this thesis, we present our study of the molecular emission in the millimeter wavelength range with the IRAM-30m telescope, Herschel-HIFI, and ALMA, of three circumstellar envelopes around the evolved stars IKTau, OH231.8+4.2, and IRC+10º216.

The main results obtained show that the chemistry of oxygen-rich objects is not as poor as it was previously thought. In particular, the chemistry of OH231.8+4.2 has been probably altered by high-speed shocks caused by the interaction between the slow AGB wind and fast (few 100 km· s−1) highly collimated bipolar winds. We also present the first sub-arcsecond resolution observations obtained with ALMA, for species such as SiO, SiS, or SiC2 towards IRC+10º216. This work will serve as a reference for future studies of the molecular emission in circumstellar envelopes of evolved stars, particularly for the oxygen rich envelopes.

Thesis defense: 09/06/2017

Thesis directors: Carmen Sánchez Contreras, José Cernicharo.

Alicia López:

 “Organic molecules chemistry in massive stars formation regions”

“Radioastronomy needs information from the laboratory for the spectral characterization and identification of abundant molecules in the Orion-KL molecular cloud. The temperature of this high-mass star forming region causes many of the low-lying vibrational states of these molecules to be excited so that, in addition to lines from rare isotopologues, we have to identify lines arising from vibrationally excited states, thanks to the availability of laboratory measurements in the millimeter and submillimeter domains. This work has permitted to characterize the spectrum of this prototypical hot core and will be of great importance to detect and identify molecular lines using ALMA in other high-mass star forming regions.”

Thesis defense: 14/09/2017

Thesis directors: José Cernicharo, Belén Tercero.

Sara Cuadrado:

“Molecular content in the Orion Bar photodissociation region”

“In this PhD thesis, a detailed study of the molecular emission of the Orion Bar photodissociation region (PDR) has been presented. The Orion Bar is the prototypical warm PDR with a far-UV (FUV) radiation field of a few 104 times the mean interstellar field. Owing to its proximity (~414 pc) and nearly edge-on orientation, the Orion Bar offers the opportunity to determine the chemical content, spatial stratification of different species, and chemical formation-destruction routes in strongly FUV-illuminated gas.

We carried out a millimetre line survey of the irradiated edge of the Orion Bar PDR using the IRAM-30m telescope, and complemented it with ~7′′ resolution maps at 0.8 mm, in order to study the chemistry prevailing in molecular gas that is directly exposed to strong FUV fields. Despite being a very harsh environment, our observations show a relatively rich molecular line spectra, with hundreds of lines arising from hydrocarbons and complex organic molecules (Cuadrado et al. 2015, 2017). We have also reported the first interstellar detection of the less stable conformer of formic acid, cis-HCOOH (Cuadrado et al. 2016). In addition, we have used ALMA to observe a small field-of-view with a high angular resolution (~1′′) where the transition from atomic to molecular gas takes place, in the context of investigating the structure and dynamics of FUV-irradiated molecular gas. The images (in the rotationally excited emission of CO, HCO+, SH+, HOC+, SO+, and SO) reveal a pattern of high-density substructures, photo-ablative gas flows and instabilities at the edge of the molecular cloud (Goicoechea et al. 2016, 2017).

Thesis defense: 15/09/2017

Thesis directors: Javier R. Goicoechea, José Cernicharo.

Acknowledgements: AYA2009-07304, AYA2012-32032, CSD2009-00038, and ERC-610256 (Nanocosmos).