arXiv subjects

A. Y. Kniazev

Publications and source records attributed to A. Y. Kniazev.

At least 37 records · Page 2Linked to original sources

arXiv2018

Two circumstellar nebulae discovered with the Wide-field Infrared Survey Explorer and their massive central stars

We report the discovery of two mid-infrared nebulae in the northern hemisphere with the Wide-field Infrared Survey Explorer and the results of optical spectroscopy of their central stars, BD+60 2668 (composed of two components, separated from each other by \approx 3 arcsec) and ALS 19653, with the Calar Alto 3.5-m telescope and the Southern African Large Telescope (SALT), respectively. We classify the components of BD+60 2668 as stars of spectral types B0.5 II and B1.5 III. ALS 19653 is indicated in the SIMBAD data base as a planetary nebula, while our observations show that it is a massive B0.5 Ib star, possibly in a binary system. Using the stellar atmosphere code FASTWIND, we derived fundamental parameters of the three stars as well as their surface element abundances, implying that all of them are either on the main sequence or only recently left it. This provides further evidence that massive stars can produce circumstellar nebulae while they are still relatively unevolved. We also report the detection of optical counterparts to the mid-infrared nebulae and a second, more extended optical nebula around ALS 19653, and present the results of SALT spectroscopy of both nebulae associated with this star. The possible origin of the nebulae is discussed.

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arXiv2018

CPD-64 2731: a massive spun-up and rejuvenated high-velocity runaway star

We report the results of our study of the high-velocity (\approx160 km/s) runaway O star CPD-64 2731 and its associated horseshoe-shaped nebula discovered with the Wide-field Infrared Survey Explorer. Spectroscopic observations with the Southern African Large Telescope and spectral analysis indicate that CPD-64 2731 is a fast-rotating main-sequence O5.5 star with enhanced surface nitrogen abundance. We derive a projected rotational velocity of \approx300 km/s which is extremely high for this spectral type. Its kinematic age of \approx6 Myr, assuming it was born near the Galactic plane, exceeds its age derived from single star models by a factor of two. These properties suggest that CPD-64 2731 is a rejuvenated and spun-up binary product. The geometry of the nebula and the almost central location of the star within it argue against a pure bow shock interpretation for the nebula. Instead, we suggest that the binary interaction happened recently, thereby creating the nebula, with a cavity blown by the current fast stellar wind. This inference is supported by our results of 2D numerical hydrodynamic modelling.

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arXiv2018

Study of galaxies in the Eridanus void. Sample and oxygen abundances

We present a sample of 66 galaxies belonging to the equatorial part (Dec.= -7$^o$, +7$^o$) of the large so called Eridanus void (after Fairall 1998). The void galaxies are selected as to be separated from the luminous galaxies ($M_{\rm B} < M_{\rm B}^{*} +1$), delineating the void, by more than 2 Mpc. Our main goal is to study systematically the evolutionary parameters of the void sample (metallicity and gas content) and to compare the void galaxy properties with their counterparts residing in denser environments. Besides the general galaxy parameters, compiled mainly from the literature, we present the results of dedicated observations to measure the oxygen abundance O/H in HII-regions of 23 void galaxies obtained with the 11-m SALT telescope (SAAO) and the 6-m BTA telescope (SAO), as well as the O/H estimates derived from the analysis of the SDSS DR12 spectra for 3 objects. We compiled all available data on O/H in 36 these void galaxies, including those for 11 galaxies available in the literature (for one object both SDSS and SALT spectra were used), and analyze this data in relation to galaxy luminosity ($\log$(O/H) versus $M_{\rm B}$). Comparing them with the control sample of similar type galaxies from the Local Volume, we find clear evidence for a substantially lower average metallicity of the Eridanus void galaxies. This result matches well the conclusions of our recent similar study for galaxies in the Lynx-Cancer void.

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arXiv2018

Line identification and photometric history of the hot post-AGB star Hen 3-1013 (IRAS 14331-6435)

We present a study of the high-resolution optical spectrum for the hot post-asymptotic giant branch (post-AGB) star, Hen 3-1013, identified as the optical counterpart of the infrared sourceIRAS 14331-6435. For the first time the detailed identifications of the observed absorption and emission features in the wavelength range 3700-9000 Å is carried out. Absorption lines of HI, HeI, CI, NI, OI, NeI, CII, NII, OII, SiII, SII, ArII, FeII, MnII, CrII, TiII, CoII, NiII, SIII, FeIII and SIV were detected. From the absorption lines, we derived heliocentric radial velocities of $V_r=-29.6\pm0.4$ km/s. We have identified emission permitted lines of OI, NI, FeII, MgII, SiII and AlII. The forbidden lines of [NI], [FeII], [CrII] and [NiII] have been identified also. Analysis of [NiII] lines in the gaseous shell gives an estimate for the electron density $N_e\sim10^7$ cm$^{-3}$ and the expansion velocity of the nebula $V_{exp}=12$ km/s. The mean radial velocity as measured from emission features of the envelope is $V_r=-36.0\pm0.4$ km/s. The Balmer lines H$α$, H$β$ and H$γ$ show P Cyg behaviour which indicate ongoing post-AGB mass-loss. Based on ASAS and ASAS-SN data, we have detected rapid photometric variability in Hen 3-1013 with an amplitude up to 0.2 mag in the V band. The star's low-resolution spectrum underwent no significant changes from 1994 to 2012. Based on archival data, we have traced the photometric history of the star over more than 100 years. No significant changes in the star brightness have been found.

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arXiv2017

Modelling interstellar structures around Vela X-1

We report the discovery of filamentary structures stretched behind the bow-shock-producing high-mass X-ray binary Vela X-1 using the SuperCOSMOS H-alpha Survey and present the results of optical spectroscopy of the bow shock carried out with the Southern African Large Telescope (SALT). The geometry of the detected structures suggests that Vela X-1 has encountered a wedge-like layer of enhanced density on its way and that the shocked material of the layer partially outlines a wake downstream of Vela X-1. To substantiate this suggestion, we carried out 3D magnetohydrodynamic simulations of interaction between Vela X-1 and the layer for three limiting cases. Namely, we run simulations in which (i) the stellar wind and the interstellar medium (ISM) were treated as pure hydrodynamic flows, (ii) a homogeneous magnetic field was added to the ISM, while the stellar wind was assumed to be unmagnetized, and (iii) the stellar wind was assumed to possess a helical magnetic field, while there was no magnetic field in the ISM. We found that although the first two simulations can provide a rough agreement with the observations, only the third one allowed us to reproduce not only the wake behind Vela X-1, but also the general geometry of the bow shock ahead of it.

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arXiv2017

Optical spectroscopy of the blue supergiant Sk-69 279 and its circumstellar shell with SALT

We report the results of optical spectroscopy of the blue supergiant Sk-69 279 and its circular shell in the Large Magellanic Cloud (LMC) with the Southern African Large Telescope (SALT). We classify Sk-69 279 as an O9.2 Iaf star and analyse its spectrum by using the stellar atmosphere code CMFGEN, obtaining a stellar temperature of \approx 30 kK, a luminosity of log(L_*/Lsun)=5.54, a mass-loss rate of log(\dot{M}/(Msun/yr))=-5.26, and a wind velocity of 800 km/s. We found also that Sk-69 279 possesses an extended atmosphere with an effective temperature of \approx 24 kK and that its surface helium and nitrogen abundances are enhanced, respectively, by factors of \approx 2 and 20--30. This suggests that either Sk-69 279 was initially a (single) fast-rotating (\ga 400 km/s) star, which only recently evolved off the main sequence, or that it is a product of close binary evolution. The long-slit spectroscopy of the shell around Sk-69 279 revealed that its nitrogen abundance is enhanced by the same factor as the stellar atmosphere, which implies that the shell is composed mostly of the CNO processed material lost by the star. Our findings support previous propositions that some massive stars can produce compact circumstellar shells and, presumably, appear as luminous blue variables while they are still on the main sequence or have only recently left it.

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arXiv2017

Nuclei of dwarf spheroidal galaxies KKs3 and ESO269-66 and their counterparts in our Galaxy

We present the analysis of medium-resolution spectra obtained at the Southern African Large Telescope (SALT) for nuclear globular clusters (GCs) in two dwarf spheroidal galaxies (dSphs). The galaxies have similar star formation histories, but they are situated in completely different environments. ESO269-66 is a close neighbour of the giant S0 NGC5128. KKs3 is one of the few truly isolated dSphs within 10 Mpc. We estimate the helium abundance $Y=0.3$, $\rm age=12.6\pm1$ Gyr, $[Fe/H]=-1.5,-1.55\pm0.2$ dex, and abundances of C, N, Mg, Ca, Ti, and Cr for the nuclei of ESO269-66 and KKs3. Our surface photometry results using HST images yield the half-light radius of the cluster in KKs3, $\rm r_h=4.8\pm0.2$ pc. We demonstrate the similarities of medium-resolution spectra, ages, chemical compositions, and structure for GCs in ESO269-66 and KKs3 and for several massive Galactic GCs with $[Fe/H]\sim-1.6$ dex. All Galactic GCs posses Extended Blue Horizontal Branches and multiple stellar populations. Five of the selected Galactic objects are iron-complex GCs. Our results indicate that the sample GCs observed now in different environments had similar conditions of their formation $\sim$1 Gyr after the Big Bang.

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arXiv2017

On utmost multiplicity of hierarchical stellar systems

According to theoretical considerations, multiplicity of hierarchical stellar systems can reach, depending on masses and orbital parameters, several hundred, while observational data confirm existence of at most septuple (seven-component) systems. In this study, we cross-match very high multiplicity (six and more components) stellar systems in modern catalogues of visual double and multiple stars, to find candidates to hierarchical systems among them. After cross-matching with catalogues of closer binaries (eclipsing, spectroscopic, etc.), some of their components were found to be binary/multiple themselves, which increases the system's degree of multiplicity. Optical pairs, known from literature or filtered by the authors, are flagged and excluded from the statistics. We have compiled a list of potentially very high multiplicity hierarchical systems that contains 10~objects. Their multiplicity does not exceed 12, and we discuss a number of ways to explain the lack of extremely high multiplicity systems.

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arXiv2016

IRAS 18153-1651: an H II region with a possible wind bubble blown by a young main-sequence B star

We report the results of spectroscopic observations and numerical modelling of the H II region IRAS 18153-1651. Our study was motivated by the discovery of an optical arc and two main-sequence stars of spectral type B1 and B3 near the centre of IRAS 18153-1651. We interpret the arc as the edge of the wind bubble (blown by the B1 star), whose brightness is enhanced by the interaction with a photoevaporation flow from a nearby molecular cloud. This interpretation implies that we deal with a unique case of a young massive star (the most massive member of a recently formed low-mass star cluster) caught just tens of thousands of years after its stellar wind has begun to blow a bubble into the surrounding dense medium. Our two-dimensional, radiation-hydrodynamics simulations of the wind bubble and the H II region around the B1 star provide a reasonable match to observations, both in terms of morphology and absolute brightness of the optical and mid-infrared emission, and verify the young age of IRAS 18153-1651. Taken together our results strongly suggest that we have revealed the first example of a wind bubble blown by a main-sequence B star.

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arXiv2016

SALT spectroscopy of evolved massive stars

Long-slit spectroscopy with the Southern African Large Telescope (SALT) of central stars of mid-infrared nebulae detected with the Spitzer Space Telescope and Wide-field Infrared Survey Explorer (WISE) led to the discovery of numerous candidate luminous blue variables (cLBVs) and other rare evolved massive stars. With the recent advent of the SALT fibre-fed high-resolution echelle spectrograph (HRS), a new perspective for the study of these interesting objects is appeared. Using the HRS we obtained spectra of a dozen newly identified massive stars. Some results on the recently identified cLBV Hen 3-729 are presented.

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arXiv2016

Central stars of mid-infrared nebulae discovered with Spitzer and WISE

Searches for compact mid-IR nebulae with the Spitzer Space Telescope and the Wide-field Infrared Survey Explorer (WISE), accompanied by spectroscopic observations of central stars of these nebulae led to the discovery of many dozens of massive stars at different evolutionary stages, of which the most numerous are candidate luminous blue variables (LBVs). In this paper, we give a census of candidate and confirmed Galactic LBVs revealed with Spitzer and WISE, and present some new results of spectroscopic observations of central stars of mid-IR nebulae.

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arXiv2016

High proper motion objects towards the inner Milky Way: characterisation of newly identified nearby stars from the VISTA Variables in the Via Lactea Survey

The census of the Solar neighbourhood is still incomplete, as demonstrated by recent discoveries of many objects within 5-10 pc from the Sun. The area around the mid-plane and bulge of the Milky Way presents the most difficulties in searches for such nearby objects, and is therefore deficient in the known population. This is largely due to high stellar densities encountered. Spectroscopic, photometric and kinematic characterization of these objects allows better understand the local mass function, the binary fraction, and provides new interesting targets for more detailed studies. We report the spectroscopic follow-up and characterisation of 12 bright high PM objects, identified from the VISTA Variables in Via Lactea survey (VVV). We used the 1.9-m telescope of the South African Astronomical Observatory (SAAO) for low-resolution optical spectroscopy and spectral classification, and the MPG/ESP 2.2m telescope Fiber-fed Extended Range Optical Spectrograph (FEROS) high-resolution optical spectroscopy to obtain the radial and space velocities for three of them. Six of our objects have co-moving companions. We derived optical spectral types and photometric distances, and classified all of them as K and M dwarfs within 27-264 pc of the Sun. Finally, we found that one of the sources, VVV J141421.23-602326.1 (a co-moving companion of VVV J141420.55-602337.1), appears to be a rare massive white dwarf that maybe close to the ZZ Ceti instability strip. Many of the objects in our list are interesting targets for exoplanet searches.

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arXiv2016

MN48: a new Galactic bona fide luminous blue variable revealed by Spitzer and SALT

In this paper, we report the results of spectroscopic and photometric observations of the candidate evolved massive star MN48 disclosed via detection of a mid-infrared circular shell around it with the Spitzer Space Telescope. Follow-up optical spectroscopy of MN48 with the Southern African Large Telescope (SALT) carried out in 2011--2015 revealed significant changes in the spectrum of this star, which are typical of luminous blue variables (LBVs). The LBV status of MN48 was further supported by photometric monitoring which shows that in 2009--2011 this star has brightened by approx 0.9 and 1 mag in the V and I_c bands, respectively, then faded by approx 1.1 and 1.6 mag during the next four years, and apparently started to brighten again recently. The detected changes in the spectrum and brightness of MN48 make this star the 18th known Galactic bona fide LBV and increase the percentage of LBVs associated with circumstellar nebulae to more than 70 per cent. We discuss the possible birth place of MN48 and suggest that this star might have been ejected either from a putative star cluster embedded in the HII region IRAS 16455-4531 or the young massive star cluster Westerlund 1.

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arXiv2015

[O III]$λ5007$ and X-ray Properties of a Complete Sample of Hard X-ray Selected AGNs in the Local Universe

We study the correlation between the [O III]$λ5007$ and X-ray luminosities of local Active Galactic Nuclei (AGNs), using a complete, hard X-ray ($>10$ keV) selected sample in the Swift/BAT 9-month catalog. From our optical spectroscopic observations at the South African Astronomical Observatory and the literature, a catalog of [O III]$λ5007$ line flux for all 103 AGNs at Galactic latitudes of $|b|>15^\circ$ is complied. Significant correlations with intrinsic X-ray luminosity ($L_{\rm X}$) are found both for observed ($L_{\rm [O~III]}$) and extinction-corrected ($L_{\rm [O~III]}^{\rm cor}$) luminosities, separately for X-ray unabsorbed and absorbed AGNs. We obtain the regression form of $L_{\rm [O~III]}$ $\propto L_{\rm 2-10\; keV}^{1.18\pm0.07}$ and $L_{\rm [O~III]}^{\rm cor}$ $\propto L_{\rm 2-10\; keV}^{1.16\pm0.09}$ from the whole sample. The absorbed AGNs with low ($<$0.5\%) scattering fractions in soft X-rays show on average smaller $L_{\rm [O~III]}/L_{\rm X}$ and $L_{\rm [O~III]}^{\rm cor}/L_{\rm X}$ ratios than the other absorbed AGNs, while those in edge-on host galaxies do not. These results suggest that a significant fraction of this population are buried in tori with small opening angles. By using these $L_{\rm [O~III]}$ vs. $L_{\rm X}$ correlations, the X-ray luminosity function of local AGNs (including Compton thick AGNs) in a standard population synthesis model gives much better agreement with the [O III]$λ5007$ luminosity function derived from the Sloan Digital Sky Survey than previously reported. This confirms that hard X-ray observations are a very powerful tool to find AGNs with high completeness.

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arXiv2015

Discovery of a new bona fide luminous blue variable in Norma

We report the results of optical spectroscopy of the candidate evolved massive star MN44 revealed via detection of a circular shell with the Spitzer Space Telescope. First spectra taken in 2009 May--June showed the Balmer lines in emission as well as numerous emission lines of iron, which is typical of luminous blue variables (LBVs) near the visual maximum. New observations carried out in 2015 May--September detected significant changes in the spectrum, indicating that the star became hotter. We found that these changes are accompanied by significant brightness variability of MN44. In particular, the I_c-band brightness decreased by \approx 1.6 mag during the last six years and after reaching its minimum in 2015 June has started to increase. Using archival data, we also found that the I_c-band brightness increased by \approx 3 mag in \approx 30 yr preceding our observations. MN44 therefore represents the seventeenth known example of the Galactic bona fide LBVs. We detected a nitrogen-rich knot to the northwest of the star, which might represent an interstellar cloudlet interacting with the circumstellar shell. We discuss a possible association between MN44 and the INTEGRAL transient source of hard X-ray emission IGR J16327-4940, implying that MN44 might be either a colliding-wind binary or a high-mass X-ray binary.

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arXiv2015

The blue supergiant MN18 and its bipolar circumstellar nebula

We report the results of spectrophotometric observations of the massive star MN18 revealed via discovery of a bipolar nebula around it with the Spitzer Space Telescope. Using the optical spectrum obtained with the Southern African Large Telescope, we classify this star as B1 Ia. The evolved status of MN18 is supported by the detection of nitrogen overabundance in the nebula, which implies that it is composed of processed material ejected by the star. We analysed the spectrum of MN18 by using the code CMFGEN, obtaining a stellar effective temperature of \approx 21 kK. The star is highly reddened, E(B-V)\approx 2 mag. Adopting an absolute visual magnitude of M_V=-6.8\pm0.5 (typical of B1 supergiants), MN18 has a luminosity of log L/Lsun \approx 5.42\pm0.30, a mass-loss rate of \approx (2.8-4.5)\times10^{-7} Msun/yr, and resides at a distance of \approx 5.6^{+1.5} _{-1.2} kpc. We discuss the origin of the nebula around MN18 and compare it with similar nebulae produced by other blue supergiants in the Galaxy (Sher 25, HD 168625, [SBW2007] 1) and the Large Magellanic Cloud (Sk-69 202). The nitrogen abundances in these nebulae imply that blue supergiants can produce them from the main sequence stage up to the pre-supernova stage. We also present a K-band spectrum of the candidate luminous blue variable MN56 (encircled by a ring-like nebula) and report the discovery of an OB star at \approx 17 arcsec from MN18. The possible membership of MN18 and the OB star of the star cluster Lynga 3 is discussed.

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arXiv2015

Oxygen abundance distributions in six late-type galaxies based on SALT spectra of HII regions

Spectra of 34 H II regions in the late-type galaxies NGC1087, NGC2967, NGC3023, NGC4030, NGC4123, and NGC4517A were observed with the South African Large Telescope (SALT). In all 34 H II regions, oxygen abundances were determined through the "counterpart" method (C method). Additionally, in two H II regions in which the auroral lines were detected oxygen abundances were measured through the classic Te method. We also estimated the abundances in our H II regions using the O3N2 and N2 calibrations and compared those with the C-based abundances. With these data we examined the radial abundance distributions in the disks of our target galaxies. We derived surface-brightness profiles and other characteristics of the disks (the surface brightness at the disk center and the disk scale length) in three photometric bands for each galaxy using publicly available photometric imaging data. The radial distributions of the oxygen abundances predicted by the relation between abundance and disk surface brightness in the W1 band obtained for spiral galaxies in our previous study are close to the radial distributions of the oxygen abundances determined from the analysis of the emission line spectra for four galaxies where this relation is applicable. Hence, when the surface-brightness profile of a late-type galaxy is known, this parametric relation can be used to estimate the likely present-day oxygen abundance in its disk.

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arXiv2015

The stellar populations in the low luminosity, early-type galaxy NGC59

Low luminosity galaxies may be the building blocks of more luminous systems. Southern African Large Telescope (SALT) observations of the low luminosity, early-type galaxy NGC59 are obtained and analysed. These data are used to measure the stellar population parameters in the centre and off-centre regions of this galaxy, in order to uncover its likely star formation history. We find evidence of older stars, in addition to young stars in the emission line regions. The metallicity of the stellar population is constrained to be [Z/H] ~ -1.1 to -1.6, which is extremely low, even for this low luminosity galaxy, since it is not classed as a dwarf spheroidal galaxy. The measured [alpha/Fe] ratio is sub-solar, which indicates an extended star formation history in NGC59. If such objects formed the building blocks of more massive, early-type galaxies, then they must have been gaseous mergers, rather than dry mergers, in order to increase the metals to observed levels in luminous, early-type galaxies.

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