Astronomers have observed a red giant star in unprecedented detail as it endures its final death throes, revealing an unusual feature. The star known as V Hydrae (or V Hya for short) ejected six different material rings, according to a form accepted for publication in the Astrophysical Journal. The exact mechanism of the formation of these mysterious “smoke rings” is not yet understood. Still, the observation could potentially shake current models for this particular late stage in stellar evolution and shed further light on stellar evolution fate of our own sun.
“V Hydrae was caught shedding its atmosphere—ultimately most of its mass—which is something most late-stage red giants do.” said co-author Mark Morris, an astronomer at UCLA. “However, this is the first and only time that a series of expanding rings has been seen around a star in its death throes – a series of expanding ‘smoke rings’ that we have calculated are blown out every few hundred years. “
Red Giants are one of the final stages of stellar evolution. Once a star’s core stops converting hydrogen into helium through nuclear fusion, gravity begins to compress the star and raise its internal temperature. This process ignites a shell of hydrogen that burns around an inert core. Eventually, core compression and heating causes the star to expand significantly, reaching diameters of between 62 million and 620 million miles (100 million to 1 billion kilometers). Surface temperatures are relatively cool by stellar standards: just 4,000 to 5,800 degrees F (2,200 to 3,200 degrees C). So these stars take on an orange-red appearance, hence the nickname red giant.
Eventually, the helium in a red giant’s core will run out and the core will shrink again. The star then becomes one Asymptotic giant branch (AGB) Stern (the last giant red stage). The internal structure of an AGB star consists of a central core of carbon and oxygen, a shell where fusion converts helium to carbon, and another shell where hydrogen is converted to helium. These stars typically produce dramatic pulses of increased brightness every 100 to 1,000 days. In addition, intense surface winds form a gaseous cloud known as the circumstellar envelope around the star.
These intense stellar winds will eventually expel the atmosphere and stellar envelope, and the star will become a white dwarf in a planetary nebula. The faster a AGB star loses its mass, the closer it is to this final transition. Our Sun will eventually become a red giant in about 5 billion years, eventually evolving into an AGB before finally evolving into a planetary nebula with a white dwarf star at its center.
This is the process as astronomers have understood it for years. However, V Hya’s unusual characteristics make her reconsider. Located 1,300 light-years away in the Hydra constellation, V Hya is a carbon-rich star, meaning its atmosphere contains more carbon than oxygen. It has a high loss rate for its mass, so astronomers suspect it’s probably in the process of shedding its atmosphere to become a planetary nebula.
This AGB star is also fascinating because large plasma outbursts occur about every eight years and a sharp drop in brightness occurs about every 17 years. These events point to the presence of a barely visible companion star. (The dips in brightness could be caused by a cloud associated with this second star passing in front of V Hya.)
This latest study combines data from the Hubble Space Telescope with observations from the Atacama Large Millimeter/submillimeter Array (ALMA), which includes infrared, optical, and ultraviolet data, to capture V Hya’s death spasms across multiple wavelengths. The star is distant and surrounded by dense dust, but ALMA’s higher resolution capability revealed its rings and outflows in great detail.
The timing was also random. “V Hya is in the brief but critical transitional phase that dying stars go through at the end of their lives,” said co-author Raghvendra Sahai, an astronomer at NASA’s Jet Propulsion Laboratory. “This is the phase when they lose the most mass. It’s likely that this stage doesn’t last very long, so catching it in the act is difficult. We were lucky with V Hya and were able to visualize all the different activities in and around this star to better understand how dying stars lose mass at the end of their lives.”
Sahai and his co-authors found that the star is losing its atmosphere by blowing out a series of smoke rings that have expanded outward over the past 2,100 years to form a dusty, disk-like region around V Hya. The team synchronized this structure DUDE (disk is dynamically expanded).
Their observations also showed bursts of high-velocity gas being ejected from the star in opposite directions perpendicular to the smoke rings, forming two hourglass-shaped structures. These structures are rapidly expanding at more than half a million miles per hour (240 km/s). “The discovery that this process can involve the ejection of gas rings simultaneously with the production of intermittent high-velocity jets of material brings a new and intriguing fold to our understanding of how stars end their lives.” said Morris.
All this suggests that the star is undergoing a particularly rapid evolution, contrary to the current model. “Our study dramatically shows that the traditional model of how AGB stars die – from the mass ejection of fuel by a slow, relatively steady spherical wind over 100,000 years or more – is at best incomplete, or at worst incorrect.” said Sahai. “It is very likely that a close stellar or substellar companion played a significant role in their deaths. In the case of V Hya, the combination of a nearby and a hypothetical distant companion is responsible, at least to some degree, for the presence of its six rings and the high-velocity outflows that cause the star’s miraculous death.”
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