Mystery of Extreme Helium Stars Solved

The origins and evolution of hydrogen-deficient stars are still a mystery. Their chemical characteristics, along with a lack of hydrogen, set them apart from the others. Severe fluorine enrichment in the range of 800–8000 is one of the distinguishing characteristics of a class of hydrogen-deficient stars, primarily the RCBs (R Coronae Borealis stars) and cool EHes (Extreme Helium stars), which binds them together. 

This link between hot EHes and colder EHes, depending on fluorine abundance, is unknown. To try to establish an evolutionary relationship, early estimates of fluorine abundances in hot EHes are provided and debated in comparison to their cooler counterparts.

Indian Institute of Astronomical Observatory: 

The Indian Institute of Astrophysics (IIA) is a world-class research centre dedicated to astronomy, astrophysics, and associated disciplines. The institute’s roots may be traced back to a Madras observatory founded in 1786 that began formally operating as the Madras Observatory in Nungambakkam in 1792. The observatory was relocated to Kodaikanal in 1899.

The new Indian Astronomical Observatory (IAO) at Hanle, in southeastern Ladakh, has improved nighttime astronomy facilities. In 2001, a 2-meter Himalayan Chandra Telescope was installed here. At the Hanle site, a seven-unit High Altitude Gamma Ray (HAGAR) telescope is in service. A large library, a computer centre, physics laboratories, and electronics, photonics, and mechanical laboratories serve the Bengaluru campus.

Extreme Helium Stars (EHes): 

“Extreme helium stars” are among the oddest creatures in the cosmic zoo. Instead of the abundant hydrogen that makes up the bulk of normal stars like the sun, these unusual and perplexing stars appear to be predominantly made up of helium. An extreme helium star, or EHe, is a low-mass supergiant that is nearly devoid of hydrogen, the universe’s most abundant chemical element. 

So far, 21 of them have been discovered in our galaxy. The origins and evolution of these hydrogen-deficient objects remain a mystery. Because no known circumstances exist for the formation of hydrogen-free stars from molecular clouds, they are thought to be the result of the merger of helium-core and carbon-oxygen core white dwarfs.

Himalayan Chandra Telescope: 

The Indian Astronomical Observatory’s Himalayan Chandra Telescope (HCT) is currently one of the most heavily subscribed telescopes in India. Since May 2003, the telescope has been used for scientific observations by both national and international astronomy communities. It is situated at a height of 4,500 meters on Mt Saraswati. The Near-IR Imager, Himalayan Faint Object Spectrograph, and Optical CCD Imager are the three main science equipment on the telescope. The telescope is controlled remotely using the INSAT-3B satellite link, which allows it to operate in sub-zero temperatures. In a year, it created 1000 GB of data.

Detection of Fluorine in EHes: 

1. The main creation of these stars, according to this research, is the merger of a carbon-oxygen (CO) and helium (He) white dwarf. Fluorine enrichment is particularly high in singly ionized fluorine (F II). This is a factor of a hundred to ten thousand times greater than typical stars.
2. Based on their fluorine abundance, the researchers looked into the connections between hot EHes and colder EHes and discovered an evolutionary link.
3. The Hanle Echelle Spectrograph (HESP), which is mounted on the 2-m Himalayan Chandra Telescope at the Indian Institute of Astronomical Observatory (IAO) in Hanle, Ladakh and is operated remotely by IIA, was used to obtain high-resolution EHs, along with data from McDonald Observatory in the United States.
4. By comparing the results, the study concludes that fluorine is abundantly created during the merging of a He-CO WD, resulting in carbon-rich EHes, whereas the He-He-WD merger produces deficient carbon EHes with no fluorine.

Significance of the study conducted: 

• The origins and evolution of these hydrogen-deficient objects remain a mystery. The observed chemical composition of these stars does not match that predicted for low mass evolved stars, posing a serious challenge to the well-accepted stellar evolution theory. 
• Fluorine is extremely important in determining the true evolutionary sequence of these hydrogen-deficient structures.
• Based on their fluorine abundance, the scientists investigated the association between hot EHes (effective temperature 14000K) and cooler EHes, and found it in the former, demonstrating an evolutionary connection across a wide range of effective temperatures.
• The discovery of increased fluorine abundances in the atmospheres of hot EHes answers a ten-year-old question regarding how they develop. It firmly establishes hot EHes in an evolutionary sequence alongside cold EHes and other hydrogen-deficient stars, focusing on an evolutionary scenario involving the merger of two double degenerate white dwarfs (WDs).

Hydrogen-Deficient Stars: 

Within the larger category of hydrogen-deficient stars, extreme helium stars are a subset. For example, R Coronae Borealis variables, population I Wolf–Rayet stars, helium-rich spectral class O or B stars, AM CVn stars, white dwarfs of spectral type WC, and transition stars like PG 1159.

Conclusion: 

The research clarified a long-standing question about the genesis of the hot EHes. It involves the merger of two double degenerate white dwarfs and sets hot EHes in an evolutionary sequence with cool EHes (WDs). The study also demonstrates Indian scientists’ ability to conduct advanced astrophysics research.