Study finds HD 179821 less massive than previously thought

HD 179821 is not a supergiant.

Study finds HD 179821 less massive than previously thought
Image: Brain Campbell, Flickr

A new study suggests that a post-asymptotic giant branch (post-AGB) star known as HD 179821 ends up being essentially less massive than recently thought. According to the data gathered from ESA’s Gaia satellite, HD 179821 is not a supergiant, instead, it is a solar-mass star.

This yellow solar-mass star in the constellation of Aquila, surrounded by a detached dust shell. It is a semi-regular variable and either a moderate-mass post-AGB star or distant massive yellow hypergiant among the largest known stars.

Even if conducting several spectroscopic observations, the precise distance to this star remains a subject of debate. Some studies suggest that it is located about 19,500 light years from the Earth whereas some studies suggest that cite a much closer distance of some 12,700 light years.

A team of astronomers led by Mudumba Parthasarathy of the Indian Institute of Astrophysics in Bangalore used data to obtain the parallax of HD 179821, which allowed them to make new distance calculations. The data suggest that the parallax of HD 179821 was found to be approximately 0.31 mas.

Employing an inference procedure developed by other researchers in 2018, astronomers calculated that this value corresponds to a distance of about 9,600 light years. Gaia parallax also allowed them to find that the star is located some 850 light years below the Galactic plane.

Furthermore, the derived distance and other parameters provided by previous studies, such as spectral type and observed V magnitude, were essential for finding the absolute magnitude and bolometric luminosity of HD 179821. These values were estimated to be –5.7 and 4.3 respectively.

According to the paper, the derived properties exclude the possibility that HD 179821 is a supergiant star with a mass between 19 and 30 solar masses, which was suggested by observations conducted in recent years.

Astronomers noted, “The derived MV = -5.7 clearly establishes that HD 179821 is not a 30 or 19 Msun post-red supergiant. (…) The bolometric magnitude clearly excludes the possibility, defended by many authors, that it is a very massive (30 to 19 Msun) post-red supergiant star.”

“HD 179821 is a post-AGB star of mass in the range of about 0.8 solar masses, an effective temperature of 5,660 K, and a radial velocity of approximately 81.8 km/s.”

The finding is presented in a paper published January 28 on arXiv.org.