arXiv subjects

S. Georgiev

Publications and source records attributed to S. Georgiev.

4 recordsLinked to original sources

arXiv2023

A long-term study of the magnetic field and activity in the M giant RZ Ari

We present a detailed long-term study of the single M6 III giant RZ~Ari to obtain direct and simultaneous measurements of the magnetic field, activity indicators, and radial velocity in order to infer the origin of its activity. We study its magnetic activity in the context of stellar evolution, and for this purpose, we also refined its evolutionary status and Li abundance. In general, for the M giants, little is known about the properties of the magnetic activity and its causes. RZ~Ari posses the strongest surface magnetic field of the known Zeeman-detected M giants and is bright enough to allow a deep study of its surface magnetic structure. The results are expected to shed light on the activity mechanism in these stars.

astro-ph.SR↗

arXiv2023

The height of convective plumes in the red supergiant $μ$ Cep

Aims. We seek to understand convection in red supergiants and the mechanisms that trigger the mass loss from cool evolved stars. Methods. Linear spectropolarimetry of the atomic lines of the spectrum of $μ$ Cep reveals information well outside the wavelength range expected from previous models. This is interpreted as structures in expansion that are visible in the front hemisphere and sometimes also in the back hemisphere. We model the plasma distribution together with its associated velocities through an inversion algorithm to fit the observed linear polarization. Results. We find that supposing the existence of plasma beyond the limb rising high enough to be visible above it can explain the observed linear polarization signatures as well as their evolution in time. From this we are able to infer the geometric heights of the convective plumes and establish that this hot plasma rises to at least 1.1 R*. Conclusions. $μ$ Cep appears to be in an active phase in which plasma rises often above 1.1 R* . We generalize this result to all red supergiants in a similarly evolved stage, which at certain epochs may easily send plasma to greater heights, as $μ$ Cep appears to be doing at present. Plasma rising to such heights can easily escape the stellar gravity.

astro-ph.SR↗

arXiv2022

Three-dimensional imaging of convective cells in the photosphere of Betelgeuse

Understanding convection in red supergiants and the mechanisms that trigger the mass loss from these evolved stars are the general goals of most observations of Betelgeuse and its inner circumstellar environment. Linear spectropolarimetry of the atomic lines of the spectrum of Betelgeuse reveals information about the three-dimensional (3D) distribution of brightness in its atmosphere. We model the distribution of plasma and its velocities and use inversion algorithms to fit the observed linear polarization. We obtain the first 3D images of the photosphere of Betelgeuse. Within the limits of the used approximations, we recover vertical convective flows and measure the velocity of the rising plasma at different heights in the photosphere. In several cases, we find this velocity to be constant with height, indicating the presence of forces other than gravity acting on the plasma and counteracting it. In some cases, these forces are sufficient to maintain plasma rising at 60\,\kms to heights where this velocity is comparable to the escape velocity. Forces are present in the photosphere of Betelgeuse that allow plasma to reach velocities close to the escape velocity. These mechanisms may suffice to trigger mass loss and sustain the observed large stellar winds of these evolved stars.

astro-ph.SR↗

arXiv2019

High-resolution optical spectroscopy of Nova V392 Per

Here we analyze high-resolution spectra of the Nova V392 Per obtained during the 2018 outburst. The H$α$ and H$β$ emission lines show a triple-peak structure with radial velocities of about -2000 km/s, -250 km/s and 1900 km/s respectively. The near infrared spectrum is dominated by the narrow and single-peaked Paschen lines of hydrogen and the OI $λ$8446 and OI $λ$7773 emission lines. Using DIBs and the KI line, we estimate the interstellar excess towards V392 Per. Based on AAVSO and ASAS-SN photometry data, we calculate that the t$_2$ and t$_3$ decline times are $\sim$ 3 d and $\sim$ 11 d respectively, which classifies V392 Per as a very fast nova. We also briefly discuss the similarity between V392 Per and other very fast novae and the possible future evolution of the system in terms of the hibernation model.

astro-ph.SR↗