arXiv subjects

Alexei Kniazev

Publications and source records attributed to Alexei Kniazev.

At least 19 recordsLinked to original sources

arXiv2026

Studies of Two Post Common Envelope Binary Stars

New time series photometry of the short period ($P<0.2$~d) binary stars ZTF~J032906.36+070408.3 (ZTF~0329) and ATO~J090.7238+00.5996 (ATL~0602) is presented. The primary (hotter) stars in both systems are white dwarfs, while the cooler stars are M dwarfs. Archival photometry and new and archival spectra are used to derive the temperatures of the components in the two binaries. Both are single-lined binaries. Velocity curves are fitted to radial velocity measurements of the red dwarfs and used to place lower limits on the masses of the white dwarfs. New and archival time series photometry show evolution in the shapes of the light curves of both stars. Spectra of ZTF~0329 show an excess of radiation in the $R_C$ band which is ascribed to electron cyclotron radiation, suggesting that the star is a low accretion rate polar. Satisfactory model fits to the multi-filter light curves of ATL~0602 require the presence of cool spots on the red component of the binary. The latter star exhibits a variety of velocities associated with Balmer and Ca~II emission lines within individual spectra.

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arXiv2026

Long-slit spectroscopy of the bipolar planetary nebula ESO\,428$-$05

The results of detailed long-slit spectroscopy of the bipolar planetary nebula \ESO\ are presented. The observations were obtained with the facility spectrograph RSS of the SALT telescope. The total measured extent of the nebula reaches $\approx$55$^{\prime\prime}$, and the kinematics along the slit directly confirms its bipolar structure. The inclination of the nebula to the line of sight is estimated as $i\approx 45^{\circ}$, and the kinematic ages of two morphological structures are measured: the expanding ring and the polar outflow in the direction perpendicular to the ring. The age of the former is $\approx$1.5$\times$10$^4$~yr and that of the latter is $\approx$(3--4)$\times$10$^4$~yr, i.e. the polar outflow is 2--3 times older than the central ring. The ratio of these ages does not depend on the adopted distance. The weighted mean heliocentric velocity of \ESO\ is V$_{\rm hel}$=74.7$\pm$1.6~\kms. Chemical abundances are obtained for 19 regions along the major axis of the nebula, over an extent of 40$^{\prime\prime}$, i.e. for different morphological parts of \ESO. The abundances were derived with the T$_{\rm e}$--method, where the electron temperature was calculated directly using the measured faint auroral lines [\ion{O}{iii}]~$\lambda$4363~\AA\ and [\ion{N}{ii}]~$\lambda$5755~\AA. The abundances of O, N, Ne, S, Ar, Cl and He were determined. Despite a significant stratification, the measured abundances along the slit can be considered nearly constant.

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arXiv2025

The Peekaboo galaxy: new SALT spectroscopy and implications of archive HST data

The dwarf galaxy Peekaboo was recently identified as a Local Volume (LV) gas-rich and 'eXtremely Metal-Poor' (XMP) dIrr. Its gas metallicity is Z~Zsun/50, with +/-1$\sigma$ uncertainty range of [Zsun/72-Zsun/35]). Its the "Tip of Red Giant Stars" (TRGB)-based distance is of 6.8$\pm$0.7 Mpc. HST data for its individual stars reveal that its older RGB stars comprise a smaller part, while the majority of visible stars have ages of less than one to a few Gyr. Thus, Peekaboo dwarf can be considered as the nearest record-low-Z dwarf. As such, the galaxy deserves a deeper multi-method study, including properties of both, young massive stars and the fainter older population, and its ionised gas and the dominant baryonic component of HI gas. We use the direct (Te) method for the east HII region, in which the [OIII]4363A line is well detected, to estimate its parameter 12+log(O/H). Since in the west HII region the line [OIII]4363A is not detected, its O/H is estimated via the empirical "strong-line" method. The resulting value of O/H is very close to that in the east HII region. The new spectroscopy of Peekaboo dwarf allows us to improve substantially the accuracy of its direct O/H estimate, which appears of 12+log(O/H) = 6.99$\pm$0.06. The new data reveal that emission lines in the E region consist of two components with the velocity difference of ~65 km/s. The fainter, approaching, component can be related to a fast-moving WR star thrown from a cluster or a binary system. Using the HST $V$ magnitudes and colour $V-I$, we identify tentative O-type and very hot candidate WO stars, which are likely the ionising stars of the studied HII regions. With the new optical spectra, the Peekaboo galaxy is confirmed as the lowest-metallicity dwarf in the Local Volume and the valuable object for indeep multi-method studies.

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arXiv2025

A 70 pc-Diameter Nova Super-remnant Surrounding the Recurrent Nova RS Ophiuchi

Recurrent novae undergo thermonuclear-powered eruptions separated by less than 100 years, enabled by subgiant or red giant donors transferring hydrogen-rich matter at very high rates onto their massive white dwarf companions. The most-rapidly moving parts of envelopes ejected in successive recurrent nova events are predicted to overtake and collide with the slowest ejecta of the previous eruption, leading to the buildup of vast (~ 10 - 100 parsec) super-remnants surrounding all recurrent novae; but only three examples are currently known. We report deep narrowband imaging and spectroscopy which has revealed a ~ 70-parsec-diameter shell surrounding the frequently recurring nova RS Ophiuchi. We estimate the super-remnant mass to be ~ 20 - 200 solar masses, expanding at a few tens of km/s, with an age of order 50-100 kyr. Its extremely low surface brightness and large angular size help explain the hitherto surprising absence of nova super-remnants. Our results support the prediction that ALL recurrent novae are surrounded by similar extended structures.

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arXiv2025

Study of extragalactic HII regions with echelle spectroscopy: The A2 region in the irregular galaxy IC4662

I present the results of echelle spectroscopy of a bright HII region in the irregular galaxy IC4662 and their comparison with results from long-slit spectroscopy of the same region. All observations were obtained with the standard spectrographs of the SALT telescope: (1) low and medium spectral resolution spectrograph RSS (R~800) and (2) echelle spectrograph HRS (R=16000-17000). In both types of data the intensities of most of the emission lines were measured and abundances of oxygen and N, Ne, S, Ar, Cl and Fe were determined as well as physical parameters of the \ion{H}{ii} region. The chemical abundances were obtained from both types of data with the Te-method. Abundances calculated from both types of data agree to within the cited uncertainties. The analysis of the echelle data revealed three distinct kinematic subsystems within the studied \ion{H}{ii} region: a narrow component (NC, $\sigma ~ 12$~km/s), a broad component (BC, $\sigma ~ 40$~km/s), and a very broad component (VBC, $\sigma ~ 60-110$~km/s, detected only in the brightest emission lines). The velocity dispersion dependence on the ionisation potential of elements showed no correlation for the NC, indicating a well-mixed turbulent medium, while the BC exhibited pronounced stratification, characteristic of an expanding shell. Based on a detailed analysis of the kinematics and chemical composition, it was concluded that the BC is associated with the region surrounding a Wolf-Rayet star of spectral type WN7-8. The stellar wind from this WR star interacts with a shell ejected during an earlier evolutionary stage (either as a red supergiant or a luminous blue variable, LBV), which is enriched in nitrogen. These findings highlight the importance of high spectral resolution for detecting small-scale (~25 pc) chemical inhomogeneities and for understanding the feedback mechanisms of massive stars in low-metallicity environments.

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arXiv2023

Introducing the Condor Array Telescope: IV. A possible nova super-remnant surrounding the putative recurrent nova KT Eridani

Just 10 recurrent novae (RNe) - which erupt repeatedly on timescales shorter than one century - are known in our Galaxy. The most extreme RN known (located in the Andromeda galaxy), M31N 2008-12a, undergoes a nova eruption every year, and is surrounded by a vast nova "super-remnant", 134 pc in extent. Simulations predict that all RNe should be surrounded by similar vast shells, but previous searches have failed to detect them. KT Eri has recently been suggested to be a RN, and we have used the Condor Array Telescope to image its environs through multiple narrowband filters. We report the existence of a large ($\sim$ 50 pc diameter), H$\,\alpha$-bright shell centered on KT Eri, exactly as predicted. This strongly supports the claim that KT Eri is the 11th Galactic recurrent nova, and only the second nova known to be surrounded by a super-remnant. SALT spectra of the super-remnant demonstrate that its velocity width is consistent with that of M31-2008-12a.

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arXiv2023

Searching For Wide Binary Stars with Non-coeval Components in the Southern Sky

We have completed our observational program to search for wide binary systems with non-coeval components in the southern sky and report our results here. The final set of four systems was spectroscopically investigated in this paper. No binary systems with components of different ages were found among them. Taking into account our previous studies, we estimate the fraction of such binaries (i.e., binaries formed, presumably, by capture) not higher than 0.06~\%. The study will be continued on the northern sky.

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arXiv2023

SALT spectroscopy of the HMXB associated with the LMC supernova remnant MCSNR\,J0513$-$6724

We report the results of optical \'echelle spectroscopy with the Southern African Large Telescope (SALT) of the mass donor star BSDL\,923 in the neutron star (NS) high-mass X-ray binary XMMU\,J051342.6$-$672412 associated with the LMC supernova remnant (SNR) MCSNR\,J0513$-$6724. We found that BSDL\,923 is a B0.7\,III star with double peaked emission lines originating in a circumbinary disk-like structure. This classification and the presence of double-peaked emission lines imply that BSDL\,923 is a Be star. Modelling with the stellar atmosphere code {\sc fastwind} was used to derive the effective temperature $T_{\rm eff}=27\pm1$\,kK, surface gravity $\log g=3.22\pm0.10$, projected rotational velocity $v\sin i\approx100\pm45 \, \kms$, colour excess $E(B-V)=0.53\pm0.05$\,mag, and luminosity $\log(L_*/\lsun)=5.46\pm0.10$ of BSDL\,923, as well as to show that the surface of this star is polluted with $\alpha$-elements (O, Mg and Si) from the supernova ejecta. We found also that the NS is orbiting BSDL\,923 in an eccentric ($e=0.158\pm0.061$) orbit with the orbital period of $1.280\pm0.006$\,d and the semi-major axis of $17\pm3 \, \rsun$, and the radius of BSDL\,923 is $25\pm5 \,\rsun$. We speculate that the NS is embedded in the atmosphere of BSDL\,923 either because it was kicked at birth towards this star or because of inflation of BSDL\,923 caused by the energy input from the supernova blast wave. Using long-slit spectroscopy with SALT, we searched for possible signs of the SNR shell in the 2D spectrum, but did not find them. This lack of detection is consistent with the young age ($\approx4^{+2} _{-1}$\,kyr) of MCSNR\,J0513$-$6724, implying that it is still in the adiabatic (non-radiative) phase.

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arXiv2023

Probing the History of the Galaxy Assembly of the Counter-rotating Disk Galaxy PGC 66551

Stellar counter-rotation in disk galaxies directly relates to the complex phenomenon of the disk mass assembly believed to be driven by external processes, such as accretion and mergers. The detailed study of such systems makes it possible to reveal the source of external accretion and establish the details of this process. In this paper, we investigate the galaxy PGC 66551 (MaNGA ID 1-179561) which hosts two large-scale counter-rotating (CR) stellar disks identified in the Sloan Digital Sky Survey MaNGA data and then confirmed using deep follow-up spectroscopy with the 10m Southern African Large Telescope. We measured the properties of ionized gas and stellar populations of both CR disks in PGC 66551. We found that the CR disk is compact, contains young stars with subsolar metallicity, and has a stellar mass of $5\times10^{9}$ M$_\odot$ which amounts to $\approx$20% of the galaxy's total. Surprisingly, the main 8 Gyr old disk has a significantly lower metallicity of -0.8 dex than other CR galaxies. We developed a simple analytic model of the history of the metal enrichment, which we applied to PGC 66551 and constrained the parameters of the galactic outflow wind, and estimated the metallicity of the infalling gas that formed the CR disk to be -0.9 ... -0.5 dex. Our interpretation prefers a merger with a gas-rich satellite over cold accretion from a cosmic filament as a source of gas, which then formed the CR disk in PGC 66551.

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arXiv2022

Wide binary stars with non-coeval components

We have estimated masses of components of visual binaries from their spectral classification. We have selected pairs, where the less massive component looks more evolved. Spectral observations of some of such pairs were made, and at least one pair, HD~156331, was confirmed to have components of different age. Since mass exchange is excluded in wide binaries, it means that HD~156331 can be formed by the capture.

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arXiv2022

How long can LBVs sleep? A long-term photometric vaiability and spectral study of the Galactic candidate luminous blue variable MN112

Luminous Blue Variables (LBVs) are massive stars that show strong spectral and photometric variability. The question of what evolutionary stages they represent and what exactly drives their instability is still open, and thus is it important to understand whether LBVs without significant ongoing activity exist, and for how long such dormant LBVs may ``sleep''. In this article we investigate the long-term variability properties of the LBV candidate MN112, by combining its optical and infrared spectral data covering 12 years with photometric data covering nearly a century acquired by both modern time-domain sky surveys and historical photographic plates. We analyze the spectra, derive physical properties of the star by modelling its atmosphere and use a new distance estimate from Gaia data release 3 (DR3) to determine the position of MN112 both inside the Galaxy and in the Hertzsprung--Russell diagram. Distance estimation increased in almost two times in comparison with Gaia DR2. Due to that MN112 moved in upper part of the diagram and according to our modeling it lies on evolutionary track for star with initial mass $M_*=70 M_\odot$ near Humphreys--Davidson limit. Given the absence of any significant variability we conclude that the star is a dormant LBV that has been inactive for at least a century now.

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arXiv2021

Star formation in outer rings of S0 galaxies. IV. NGC 254 -- a double-ringed S0 with gas counter-rotation

Although S0 galaxies are usually considered "red and dead", they often demonstrate star formation organized into ring structures. We aim to clarify the nature of this phenomenon and how it differs from star formation in spiral galaxies. We investigated the nearby, moderate-luminosity S0 galaxy NGC 254 using long-slit spectroscopy taken with the South African Large Telescope and publicly available imaging data. Applying a full spectral fitting, we analyzed gaseous and stellar kinematics as well as ionized gas excitation and metallicity and stellar population properties resolved by radius. An advanced approach of simultaneously fitting spectra and photometric data allowed us to quantify the fraction of hidden counter-rotating stars in this galaxy. We find that the ionized gas is counter-rotating with respect to the stars throughout NGC 254 disk, indicating an external origin of the gas. We argue the gas-rich galaxy merger from retrograde orbit as a main source of counter-rotating material. The star formation fed by this counter-rotating gas occurs within two rings: an outer ring at R=55 - 70 arcsec and an inner ring at R=18 arcsec. The star formation rate is weak, 0.02 solar mass per year in total, and the gas metallicity is slightly subsolar. We estimated that the accretion of the gas occurred about 1 Gyr ago, and about 1% of all stars have formed in situ from counter-rotating gas.

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arXiv2020

Long-period eclipsing binaries: towards the true mass-luminosity relation. II. Absolute parameters of the NN Del system

We present results of our study of the long-period eclipsing binary star NN Delphini (hereafter NN Del). The results are based on spectral data obtained with the HRS echelle spectrograph of the Southern African Large Telescope (SALT). Our constructed velocity curve is based on 19 spectra obtained between 2017 and 2019 years and covers all phases of the binary's orbit. The orbital period, P=99.252 days, was determined from our spectral data and coincides with the period determined in previous studies, as well as the system eccentricity of $e=0.517$. Calculated velocity amplitudes of both components allow us to determine the masses of both system components M_1 = 1.320 M_sun and M_2 = 1.433 M_sun with the accuracy about of one percent (0.8% and 1.1%), respectively. Luminosities of both components are presented as L_1 = 4.164 L_sun and L_2 = 6.221 L_sun, and the effective temperatures of both components were directly evaluated (T_eff = 6545~K and T_eff = 6190~K) together with the metallicity of the system [Fe/H] = -0.19 dex and its color excess E(B-V)=0.026~mag. Comparison with evolutionary tracks shows that the system age is 2.25+/-0.19 Gyr, and both components are on the main sequence and have not yet passed the turn point. Spectral type is F5V for the hotter component and F8V for another one.

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arXiv2020

Direct evidence for shock-powered optical emission in a nova

Classical novae are thermonuclear explosions that occur on the surfaces of white dwarf stars in interacting binary systems (Bode & Evans 2008). It has long been thought that the luminosity of classical novae is powered by continued nuclear burning on the surface of the white dwarf after the initial runaway (Gallaher & Starrfield 1978). However, recent observations of GeV $γ$-rays from classical novae have hinted that shocks internal to the nova ejecta may dominate the nova emission. Shocks have also been suggested to power the luminosity of events as diverse as stellar mergers (Metzger & Pejcha 2017), supernovae (Moriya et al. 2018), and tidal disruption events (Roth et al. 2016), but observational confirmation has been lacking. Here we report simultaneous space-based optical and $γ$-ray observations of the 2018 nova V906 Carinae (ASASSN-18fv), revealing a remarkable series of distinct correlated flares in both bands. The optical and $γ$-ray flares occur simultaneously, implying a common origin in shocks. During the flares, the nova luminosity doubles, implying that the bulk of the luminosity is shock-powered. Furthermore, we detect concurrent but weak X-ray emission from deeply embedded shocks, confirming that the shock power does not appear in the X-ray band and supporting its emergence at longer wavelengths. Our data, spanning the spectrum from radio to $γ$-ray, provide direct evidence that shocks can power substantial luminosity in classical novae and other optical transients.

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arXiv2018

Milky Way metallicity gradient from Gaia DR2 F/1O double-mode Cepheids

The ratio of the first overtone (1O) / fundamental (F) periods of mixed-mode Cepheids that pulsate simultaneously in these two modes (F/1O) is metallicity-dependent. It can therefore be used to characterize the systems that host such variable stars. We want to take advantage of the F/1O double-mode Cepheids listed in the Gaia DR2 catalogue to derive the metallicity gradient in the Milky Way disk. The metallicity is derived from the ratio of the first overtone and fundamental periods provided by Gaia DR2 while the Gaia DR2 parallaxes are used to determine the Galactocentric distances of the stars. From a visual inspection of the light curves, it turns out that a large fraction (77%) of the Galactic F/1O double-mode Cepheids in Gaia DR2 are spurious detections. Gaia DR2 provides 3 new bona fide F/1O Cepheids. Combining them with the currently known F/1O Cepheids and using the Gaia DR2 parallaxes for the entire sample, we can derive the metallicity gradient in the Milky Way disk. We find a slope of -0.045$\pm$0.007 dex/kpc using a bootstrap method, and of -0.040$\pm$0.002 dex/kpc using a total least squares method. These results are in good agreement with previous determinations of the [Fe/H] gradient in the disk based on canonical Cepheids. The period ratio of F/1O Cepheids allows for a reliable determination of the metallicity gradient in the Milky Way, and in turn, in other systems that would be difficult to reach via classical spectroscopic methods.

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arXiv2017

The orbit of the gamma-ray binary 1FGL J1018.6-5856

Gamma-ray binaries are a small subclass of the high mass X-ray binary population which exhibit emission across the whole electromagnetic spectrum. We present radial velocities of 1FGL J1018.6$-$5856 based on observations obtained with the Southern African Large Telescope (SALT). We combine our measurements with those published in the literature to get a broad phase coverage. The mass function obtained supports a neutron star compact object, although a black hole mass is possible for very low inclination angles. The improved phase coverage allows constraints to be placed on the orbital eccentricity (e = 0.31 $\pm$ 0.16), which agrees with estimates from high-energy data.

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arXiv2017

High Resolution Spectroscopic Abundances of Red Giant Branch Stars in NGC 6681

We obtain high resolution spectra of nine red giant branch stars in NGC 6681 and perform the first detailed abundance analysis of stars in this cluster. We confirm cluster membership for these stars based on consistent radial velocities of 214.5$\pm$3.7 km/s and find a mean [Fe/H] = -1.63$\pm$0.07 dex and [α/Fe] = 0.42$\pm$0.11 dex. Additionally, we confirm the existence of a Na-O anti-correlation in NGC 6681 and identify two populations of stars with unique abundance trends. With the use of HST photometry from Sarajedini et al. (2007) and Piotto et al. (2015) we are able to identify these two populations as discrete sequences in the cluster CMD. Although we cannot confirm the nature of the polluter stars responsible for the abundance differences in these populations, these results do help put constraints on possible polluter candidates.

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arXiv2015

The empty ring galaxy ESO 474 - G040

We describe observations of the apparently empty ring galaxy ESO 474-G040 obtained with the Southern African Large Telescope (SALT). The observations, consisting of imaging, long-slit spectroscopy and Fabry-Perot mapping of the H-alpha line, allow determining the ring kinematics as well as estimating the metallicity of the ring and the stellar population composition in its various parts. We propose that the object could best be understood as being the result of a past merger of disk galaxies, which formed a gas ring that subsequently disrupted via the bead instability and is presently forming stars.

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