A tale of hydrogen-deficient stars
Anirban Bhowmick
‘Bhaiyya! Do hydrogen-deficient stars really exist?’ asked Rony with amazement.
‘Yes! Not only do they exist, but they also have different types’ explained Ani.
R: ‘Unbelievable! I always thought stars shine due to nuclear fusion of hydrogen and hence they are hydrogen-rich!’
A: ‘It’s not just you, even the astronomers were not ready to accept that stars can be hydrogen-deficient till 1935.’
R: ‘Oh! How did it change then?’
A: ‘It all started in 1891 when a brilliant Scottish woman scientist named Williamna Fleming found that hydrogen lines are very faint in the spectrum of a binary star Upsilon Sgr. A few years later in 1906, another scientist named Lundendorff discovered the complete absence of hydrogen in the atmosphere of another star called R-Coronae Borealis (RCrB). Finally, around 1935-1940, after many deliberations, astronomers agreed that these two stars are indeed hydrogen-deficient.’
R: ‘Ok. So are there more stars like these? What is the cause of hydrogen-deficiency?
A: ‘Wait! I will answer all of your queries. Let me take you on an exciting journey about hydrogen-deficient stars and my little personal investigation on them.’
R: ‘Sure!’
A: ‘Hydrogen deficient stars are extremely rare and very little was known about them even 40 years back. But with the arrival of modern telescopes with dedicated surveys, many more were identified with proper classification based on their spectra, mass and temperature. They are broadly classified into two based on their mass – the massive hydrogen-deficient (hdef) stars and low-mass hydrogen-deficient stars. Some types of massive hdef-stars are the Wolf-Rayet stars, hydrogen-deficient binaries like Upsilon Sgr and hydrogen-deficient supernovae. However, in this story, I will tell you about the low-mass hdef-stars.’
A: ‘The low-mass hdef-stars are again divided into three principal categories based on their temperature, surface composition and variability. A few examples are the peculiar variables R-Coronae Borealis stars (RCBs), of which RCrB is the prototype, the non-variable hydrogen-deficient carbon stars (HdCs) and hot extreme helium Stars (EHes).’
R: ‘Variable stars? How do you understand a star is variable?
A: ‘Astronomers observe the brightness of a star at regular intervals. If they find that the brightness is varying with time in a pattern, it’s called a variable star. They plot this brightness variation with time, which is called a lightcurve.’
R: ‘Wow! So there are variable as well as non-variable stars which are hydrogen-deficient?’
A: ‘Yes! In fact, one of the most peculiar and exciting groups of variable stars are the RcrB and their counterparts. RCrB like stars can suddenly become very faint within a matter of a few days, almost disappearing from naked eyesight, slowly recovering after months and sometimes years.’
R: ‘That’s interesting! Tell me more about them.’
A: ‘RCrBs are hydrogen-deficient supergiants having surface temperature 5000K to 12000K. Similar to RCrBs, there is a cooler group of non-variable hydrogen-deficient stars known as hydrogen-deficient carbon stars or HdCs. They have temperatures of 4000K to 6000K and share a similar chemical composition with RCrBs. The hotter cousins of the previous two groups of stars are known as Extreme Helium Stars or EHes, which are so-called due to the presence of high amounts of neutral helium lines in their spectrum. They are hotter (8000K to 30000K), non-variable, and helium is enhanced with respect to hydrogen by more than 1000 times when compared to normal stars!’
R: ‘So there are only three types of low-mass hdef stars? Any chance of finding a new group?’
Astronomers observe the brightness of a star at regular intervals. If they find that the brightness is varying with time in a pattern, it’s called a variable star.
A: ‘Yes, I was about to say that. Very recently, one more type of star similar to the above group was observed. They are known as DYPersei variables which may be the coolest member of the group (2000K to 4000K) yet not much is known about them. They also show an abrupt drop in their brightness like RcrBs, but their rise and drop in brightness is symmetric.’
R: ‘Wow! You said these stars are rare. Is that due to observational limitations? Isn’t it easier to detect RCBs by seeing their spectacular light curves?’
A: ‘Brilliant! In fact, most of the RCB stars are discovered by amateur astronomers using lightcurves. Brightness variations are much easier to monitor than detailed spectrum analysis, which is time-consuming. That’s why the number of detected RCB stars are around 500 in our galaxy whereas the two other non-variables HdCs and EHes are around 5 and 23 only!’
R: ‘So less! Compared to 200-400 billions of stars in our galaxy, the numbers are basically insignificant!’
A: ‘Yes, since these stars are extremely rare in nature, the exact reasons for their formation and evolution are merely speculative.’
R: ‘So how do you predict their origins? By seeing some extraordinary characteristics common only to them, like hydrogen-deficiency?’
A: ‘Excellent, yes! The only way we can connect these stars and predict their evolution is to look at their chemical composition, the common peculiarities they share and compare with those of normal stars.’
A: ‘In the year 2007, Clayton and his team found that the cooler ones, the HdCs and cool RCBs (< 6000K) show considerable enhancement of 18O with respect to 16O in comparison to normal stars. While for normal stars 16O/18O > 200, for these hydrogen deficient stars this ratio is almost unity. They also found that the 12C/13C ratios for these stars are 5-20 times higher than those of normal stars. It’s worth mentioning that a few RCBs showed normal values of 12C/13C and 16O/18O. Similarly, Pandey in 2006 found that the warm RCBs (> 8000K) and cool EHes show remarkable overabundance in neutral fluorine (F), almost 800-8000 times higher than normal stars.’
A: ‘Now, to explain the reasons for these anomalies, astronomers tried to simulate the conditions required to create such abundances. Based on these simulations and the observed surface compositions of hydrogen-deficient stars, two principal formation scenarios stood the test of time and rigorous observational analyses – The Double Degenerate (DD) white dwarf merger scenario and Final helium shell Flash (FF) scenario. The DD scenario supports low 16O/18O, high 12C/ 13C and high fluorine abundances, explaining most of the observed cases, whereas the FF scenario that doesn’t predict these anomalies explains the rest of the observations.’
R: ‘White dwarf merger! Aren’t white dwarfs dead stars? How does the merger of two faint white dwarfs result in such brightly shining supergiants? Also, what about the second case?’
There are two principal formation scenarios for hydrogen-deficient stars – The Double Degenerate (DD) white dwarf merger and the Final helium shell Flash (FF).
A: ‘In a DD scenario, two white dwarfs, mostly a low-mass Helium white dwarf and a massive CO white dwarf, co-evolves in a binary system, and their orbits slowly decay. Finally, the massive white dwarf swallows the low-mass one. The ingested helium-rich material from the He white dwarf forms an envelope around the massive one. The temperature and pressure are raised to such levels that helium fusion starts again, and the star expands to supergiant sizes.’
A: ‘Now about the second case: the stars formed in such a situation are also called Born-Again stars. In general, when a star is about to become a white dwarf, hydrogen burns only in a thin outer layer of the star, depositing helium below before slowly turning off. In a few rare cases, the mass of the deposited helium reaches a critical amount known as critical mass, and even with a little disturbance, violently ignites, ingesting all the remaining hydrogen and giving the appearance of a helium-burning supergiant star. As if from the brink of death these stars are born again like a phoenix, hence the name Born-Again stars!’
A: ‘However, the lifetimes of born-again stars are very short, and they evolve very fast from bright supergiants to faint white dwarfs. This evolution can be observed in human timelines (50-60 years), which is extraordinary since they have actual evolutionary timelines of millions of years!’
R: ‘Whoa! Both these scenarios involve the evolution of stars near the white dwarf sequence! So these stars must be super old!’
A: ‘Exactly, that’s why they are so exotic. On top of that, most of the single/binary white dwarfs may not give rise to such enigmatic objects. Based on the probability of occurrence of the above scenarios, it is predicted that there may be around 2000 hdef stars in our galaxy.’
R: ‘So do you look for similar signatures in newly discovered hydrogen-deficient objects?’
A: ‘Yes. We try to determine the abundances of newly discovered hydrogen-deficient objects to connect them with the observed ones. For example, as I mentioned before the cooler stars show enhanced 18O. 18O was detected through molecular lines of 12C18O in the infrared wavelength region of 2 microns. As the star gets hotter, molecular lines disappear. So we don’t see 18O in hot RCBs or EHes. We instead look for a common anomaly in them, which in this case was found to be overabundance of fluorine. It suggests that maybe cool HdCs are connected with hot EHes through RCBs. Proceeding like this, we look for common connections between different groups of these stars.’
R: ‘So what about DYPers? Is there any similar connection for them?’
A: ‘Good question. Most of the sparsely available literature on DYPersei type stars are on the DYPersei prototype, suggesting it as a cooler RCB based on their high 12C/13C ratio. But as a group, the results were mostly unavailable. For the first time, in our paper Bhowmick et al. 2018, we explored the connections of DYPersei variables with cool RCBs, HdCs and normal stars based on the observed 16O/18O and 12C/13C ratios determined from the strengths of 12C16O, 13C16O and 12C18O bands in the 2 microns infrared region of their spectrum. We used the instrument TIRSPEC, mounted on the 2 meter Himalayan Chandra Telescope (HCT) at Indian Astronomical Observatory IAO, operated by IIA at Hanle, Ladakh to observe such stars. Surprisingly, we found most of the DYPersei variables are just like HdCs and RCBs with very low 16O/18O and high 12C/13C values. This opened up a new venue for research, and with increasing discoveries of more DYPersei objects, will help us in classifying these stars. If indeed these DYPersei are cooler cousins of RCBs, then they are related to both HdCs and EHes, which is an exciting result in the research of low-mass hydrogen-deficient stars.
R: ‘Wow! Searching for so few stars in this gigantic galaxy is like searching for a needle in a haystack!’
A: ‘Yes. But when the needle is finally found, it stitches together all the loose threads about the existence of enigmatic objects in our universe, which may someday lead to answers about the origin and existence of all the objects in the universe, including us!’
About the author
Anirban Bhowmick is an SRF at IIA and works on Hydrogen-deficient star systems.
