… 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?
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 . 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!)?
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 .
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  (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…  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 .
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.
 This one in particular has a brightness of L ~ 5 × 105L⊙ and an estimated initial mass of Minit ~ 50M⊙.
 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.
 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.
 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.
 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.
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).