arXiv subjects

Gesesew R. Habtie

Publications and source records attributed to Gesesew R. Habtie.

5 recordsLinked to original sources

arXiv2026

Optical Spectroscopy and Temporal Evolution of the Nova V1405 Cas

This paper presents the findings from our study of nova V1405~Cas over the first 1051 days after the outburst. The study includes an analysis of the photometric light curve evolution, along with a detailed spectroscopic evolution. Photometric analysis shows that the nova is a very slow nova, with a decline timescale of $t_2 \approx 165$ days. The mass of the white dwarf is calculated as $ M_{\rm WD} \sim 0.7M_{\odot} $. The secondary star is a low-mass main-sequence star, with a mass of $ M_{\rm sec} \approx 0.43M_{\odot} $. Spectral observations show initial dominance by Balmer emission lines accompanied by prominent P Cygni profiles during the first +339 days. These features disappeared in later epochs, being replaced by high-ionization coronal lines, indicating that the nova had transitioned to the coronal phases by day +371. To investigate the physical conditions of the ejecta and the central source, we conducted photoionization modeling using \textsc{cloudy}. Our model reveals a gradual increase in the temperature and luminosity of the system, and suggests that the ejecta were primarily composed of He, N, Fe, Ne, and Ca, with noticeable temporal variations in their relative abundances. The estimated mean ejected mass is approximately $1.10~\times~10^{-4}M_{\odot}$, which is relatively high and suggests a low-mass white dwarf. Our optical spectroscopic and photometric analyses, combined with detailed photoionization modeling, indicate that the white dwarf in Nova V1405 Cas is unlikely to be of the ONeMg type.

astro-ph.SR↗

arXiv2025

Multiple outflows and delayed ejections revealed by early imaging of novae

Novae are thermonuclear eruptions on accreting white dwarfs in interacting binaries. Although most of the accreted envelope is expelled, the mechanism -- impulsive ejection, multiple outflows or prolonged winds, or a common-envelope interaction -- remains uncertain. GeV $γ$-ray detections from $>20$ Galactic novae establish these eruptions as nearby laboratories for shock physics and particle acceleration, underscoring the need to determine how novae eject their envelopes. Here we report on near-infrared interferometry, supported with multiwavelength observations, of two $γ$-ray detected novae. The images of the very fast 2021 nova V1674~Her, taken just 2--3 days after discovery, reveal the presence of two perpendicular outflows. The interaction between these outflows likely drives the observed $γ$-ray emission. Conversely, the images of the very slow 2021 nova V1405~Cas suggest a delay in the ejection of the bulk of the accreted envelope of more than 50 days after the start of eruption, as the nova slowly rises to visible peak and during which the envelope engulfed the system in a common envelope phase. These unprecedented images offer direct observational evidence that the mechanisms driving mass ejection from the surfaces of accreting white dwarfs are not as simple as previously thought, revealing multiple outflows and delayed ejections.

astro-ph.HE↗

arXiv2025

Spectroscopic study of the Quiescent Stages in between the 2006 and 2021 outbursts of RS Ophiuchi

This paper presents a comprehensive spectroscopic analysis of the quiescent stage of the recurrent nova RS Ophiuchi between its 2006 and 2021 outbursts. The spectra shows prominent low-ionization emission features, including hydrogen, helium, iron emissions, and TiO absorption features. The \ion{H}{$α$} and \ion{H}{$β$} lines showed double-peaked emission profiles, indicating that both originate from the accretion disc. The central peaks of the \ion{H}{$α$} and \ion{H}{$β$} emission profiles exhibited subtle shifts towards the blue or red side, attributed to orbital motion and fluctuations in the accretion rate. Using the double-peak features observed in the \ion{H}{$α$} and \ion{H}{$β$} lines, we have estimated the accretion disc size to be $ R_{AD} = 3.10 \pm 0.04 \times 10^{12} \, \text{cm} $. The \textsc{cloudy} photoionization code is employed to model the quiescent phase spectra, allowing us to study the evolution of various physical parameters such as temperature, luminosity, hydrogen density, elemental abundances, accreted mass, and accretion rate. The central ionizing sources exhibit temperatures in the range of $1.05 - 1.80~\times 10^4$ K and luminosities between $0.10 - 7.94~\times 10^{30}$ \ergs. The mean accretion rate, calculated from the model, is $\sim$ $1.25 \times 10^{-8} M_{\odot}$ yr$^{-1}$. The model results reveal that the accretion rate rose substantially in the later phase. The accreted mass in the 16 months, preceding the 2021 outburst exceeds 47\% of the critical mass, and more than 88\% of the critical mass was accreted in the last three years.

astro-ph.SR↗

arXiv2023

Study of the fastest classical nova, V1674 Her: Photoionization and Morpho-kinemetic model analysis

We present the results of the investigation of the nova V1674 Her (2021), recognised as the swiftest classical nova, with $t_2 \sim 0.90$ days. The distance to the nova is estimated to be 4.97 kpc. The mass and radius of the WD are calculated to be $\sim~1.36~M_\odot$ and $\sim 0.15~R_\oplus$, respectively. Over the course of one month following the outburst, V1674 Her traversed distinct phases -- pre-maxima, early decline, nebular, and coronal -- displaying a remarkably swift transformation. The nebular lines emerged on day 10.00, making it the classical nova with the earliest observed commencement to date. We modelled the observed optical spectrum using the photoionization code \textsc{cloudy}. From the best-fitting model we deduced different physical and chemical parameters associated withe the system. The temperature and luminosity of the central ionizing sources are found in the range of $1.99 - 2.34~\times 10^5$ K and $1.26 - 3.16~ \times 10^{38}$ \ergs, respectively. Elements such as He, O, N, and Ne are found to be overabundant compared to solar abundance in both the nebular and coronal phases. According to the model, Fe II abundance diminishes while Ne abundance increases, potentially elucidating the rare hybrid transition between Fe and He/N nova classes. The ejected mass across all epochs spanned from $3.42 - 7.04~ \times 10^{-5}~M_\odot$. Morpho-kinematic modelling utilising \textsc{shape} revealed that the nova V1674 Her possesses a bipolar structure with an equatorial ring at the centre and an inclination angle of i = 67$\pm$ 1.5$^{\circ}$.

astro-ph.SR↗

arXiv2022

Study of 2021 outburst of the recurrent nova RS Ophiuchi: Photoionization and morpho-kinematic modelling

We present the evolution of the optical spectra of the 2021 outburst of RS Ophiuchi (RS Oph) over about a month after the outburst. The spectral evolution is similar to the previous outbursts. Early spectra show prominent P Cygni profiles of hydrogen Balmer, \ion{Fe}{ii}, and \ion{He}{i} lines. The emission lines were very broad during the initial days, which later became narrower and sharper as the nova evolved. This is interpreted as the expanding shocked material into the winds of the red giant companion. We find that the nova ejecta expanded freely for $\sim 4$ days, and afterward, the shock velocity decreased monotonically with time as $v\propto t^{-0.6}$. The physical and chemical parameters associated with the system are derived using the photoionization code \textsc{cloudy}. The best-fit \textsc{cloudy} model shows the presence of a hot central white dwarf source with a roughly constant luminosity of $\sim$1.00 $\times$ 10$^{37}$ erg s$^{-1}$. The best-fit photoionization models yield absolute abundance values by number, relative to solar of He/H $\sim 1.4 - 1.9$, N/H = $70 - 95$, O/H = $0.60 - 2.60$, and Fe/H $\sim 1.0 - 1.9$ for the ejecta during the first month after the outburst. Nitrogen is found to be heavily overabundant in the ejecta. The ejected hydrogen shell mass of the system is estimated to be in the range of $3.54 - 3.83 \times 10^{-6} M_{\odot}$. The 3D morpho-kinematic modelling shows a bipolar morphology and an inclination angle of $i=30^{\circ}$ for the RS Oph binary system.

astro-ph.SR↗