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Unveiling Unusual Nitrogen-rich Massive Star in Metal-poor Dwarf Galaxy

Russian version

    Using the 6-m SAO RAS telescope, we discovered an interesting star in the NGC 4068 galaxy. Massive stars have significant impact on the gaseous environment of galaxies by ejecting into it a huge amount of energy in terms of radiation, stellar wind, and shock waves in a short time. They are primarily responsible for enriching the Universe with heavier elements. Therefore, vision of the evolution of galaxies and the Universe as a whole is closely related to understanding the processes that occur with massive stars. At the same time, their key parameters (the mass, wind velocity, and lifetime), and hence, the impact on the interstellar medium, largely depend on the abundance of heavier elements in them.
    While studying the kinematics of the ionized gas of the dwarf galaxy NGC 4068 with the 6-m SAO RAS telescope, a compact nebula with relatively high internal gas velocities was found. Subsequent spectroscopic studies carried out with both the 6-m SAO RAS telescope and the 2.5-m telescope of the Caucasian Mountain Observatory of SAI MSU have shown that there is a star at the center of the nebula enriching the surrounding nebula with nitrogen, so its relative abundance [N/O] is almost the same as that of the Sun; although, in the gas surrounding the galaxy itself, the abundance of heavier elements is only 1/10 of that of the Sun. Based on the observations, numerical models of the expanding stellar atmosphere were constructed. They have shown that it is a case of a massive star at a short evolutionary stage (a blue supergiant or a Wolf-Rayet star), when a powerful stellar wind actively affects the environment. The total bolometric luminosity of the star is (1.5-1.7)·106 solar luminosity, the supposed mass is about 80 solar masses. The radius of the surrounding expanding nebula is smaller than 15 pc, and its kinematic age is smaller than 500,000 years.
Fig.1. Observations of the NGC 4068 galaxy with the SCORPIO-2 instrument at the 6-m SAO RAS telescope.
On the left, there is the image of the galaxy in the H-alpha line; the inset shows the neighborhood of the unique massive star according to the Hubble telescope data, the star itself is in the center of the white circle.
On the right, there is the spectrum of the star and the surrounding nebula, the best model description of this spectrum is shown in red.

The work was carried out within the framework of the grant from the Ministry of Science and Higher Education of the Russian Federation № 075-15-2022-262 (13.MNPMU.21.0003).

Yarovova A.D., Egorov O.V., Moiseev A.V., Maryeva O.V., “Unveiling the nitrogen-rich massive star in the metal-poor galaxy NGC 4068”, 2022, MNRAS, 518, 2256

Contact person - A.V. Moiseev, DSc, Senior Researcher in the Laboratory of Spectroscopy and Photometry of Extragalactic Objects.