arXiv subjects

V. V. Kovtyukh

Publications and source records attributed to V. V. Kovtyukh.

At least 37 records · Page 2Linked to original sources

arXiv2012

Accurate luminosities from the oxygen 7771-4 A triplet and the fundamental parameters of F-G supergiants

The oxygen 7771-4 A triplet is a good indicator of luminosity in A-G supergiants. However, its strength also depends on other atmospheric parameters. In this study, we present the luminosity calibrations, where, for the first time, the effects of the effective temperature, microturbulent velocity, surface gravity, and the abundance have been disentangled. The calibrations are derived on the basis of a dataset of high-dispersion spectra of 60 yellow supergiants with highly reliable luminosities and accurate atmospheric parameters. This allowed to bring the uncertainty of the triplet-based absolute magnitudes down to 0.26 mag. The calibrations are suitable for spectral types F0-K0 and luminosity classes I-II, covering absolute magnitudes Mv from -1.0 to -10 mag.

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arXiv2011

Galactic restrictions on iron production by various Types of supernovae

We propose a statistical method for decomposition of contributions to iron production from various sources: supernovae Type II and the subpopulations of supernovae Type Ia -- prompt (their progenitors are short-lived stars of ages less then $\sim$100 Myr) and tardy (whose progenitors are long-lived stars of ages $>$100 Myr). To do that, we develop a theory of oxygen and iron synthesis which takes into account the influence of spiral arms on amount of the above elements synthesized by both the supernovae Type II and prompt supernovae Ia. We solve this task without of any preliminary suppositions about the ratio among the portions of iron synthesized by the above sources. The relative portion of iron synthesized by tardy supernovae Ia for the life-time of the Galaxy is $\sim$35 per cent (in the present ISM this portion is $\sim$50 per cent). Correspondingly, the total portion of iron supplied to the disc by supernovae Type II and prompt supernovae Ia is $\sim$65 per cent (in the present ISM this portion is $\sim$50 per cent). The above result slightly depends on the adopted mass of oxygen and iron synthesized during one explosion of supernovae and the shape (bimodal or smooth) of the so-called Delay Time Distribution function. The portions of iron mass distributed between the short-lived supernovae are usually as follows: depending on the ejected masses of oxygen or iron during one supernovae Type II event the relative portion of iron, supplied to the Galactic disc for its age, varies in the range 12 - 32 per cent (in the present ISM 9-25 per cent); the portion supplied by prompt supernovae Ia to the Galactic disc is 33 - 53 per cent (in ISM 26 - 42 per cent).

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arXiv2011

The Distribution of the Elements in the Galactic Disk II. Azimuthal and Radial Variation in Abundances from Cepheids

This paper reports on the spectroscopic investigation of 101 Cepheids in the Carina region. These Cepheids extend previous samples by about 35% in number and increase the amount of the galactic disk coverage especially in the direction of l \approx 270°. The new Cepheids do not add much information to the radial gradient, but provide a substantial increase in azimuthal coverage. We find no azimuthal dependence in abundance over an 80° angle from the galactic center in an annulus of 1 kpc depth centered on the Sun. A simple linear fit to the Cepheid data yields a gradient d[Fe/H]/dRG = -0.055 \pm 0.003 dex/kpc which is somewhat shallower than found from our previous, smaller Cepheid sample.

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arXiv2010

KP Cyg: an Unusual Metal-rich RR Lyr Type Star of Long Period

We present the results of a detailed spectroscopic study of the long period ($P=0.856$ days) RR Lyrae star, KP Cyg. We derived abundances of many chemical elements including the light species, iron-group elements and elements of the s-processes. Most RR Lyrae stars with periods longer than 0.7 days are metal-deficient objects. Surprisingly, our results show that KP Cyg is very metal rich ([Fe/H] $= +0.18\pm 0.23$). By comparison with a number of short period ($P=1\sim 6$ days), metal-rich CWB stars, we suggest that KP Cyg may be a very short period CWB star (BL Her star) rather than an RR Lyrae star. As seen in some CWB stars, KP Cyg shows strong excesses of carbon and nitrogen in its atmosphere. This indicates that the surface of KP Cyg has been polluted by material that has undergone helium burning (to enhance carbon) and proton capture (to transform carbon into nitrogen). We also note that UY CrB, whose period is 0.929 days, also shows an enhancement of C and N, and that two carbon cepheids of short period, V553 Cen and RT TrA, show similar excesses of carbon and nitrogen.

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arXiv2009

Enhancing Our Knowledge of Northern Cepheids through Photometric Monitoring

A selection of known and newly-discovered northern hemisphere Cepheids and related objects are being monitored regularly through CCD observations at the automated Abbey Ridge Observatory, near Halifax, and photoelectric photometry from the Saint Mary's University Burke-Gaffney Observatory. Included is Polaris, which is displaying unusual fluctuations in its growing light amplitude, and a short-period, double-mode Cepheid, HDE 344787, with an amplitude smaller than that of Polaris, along with a selection of other classical Cepheids in need of additional observations. The observations are being used to establish basic parameters for the Cepheids, for application to the Galactic calibration of the Cepheid period-luminosity relation as well as studies of Galactic structure.

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arXiv2009

The Cepheid Impostor HD 18391 and its Anonymous Parent Cluster

New and existing photometry for the G0 Ia supergiant HD 18391 is analyzed in order to confirm the nature of the variablity previously detected in the star, which lies off the hot edge of the Cepheid instability strip. Small-amplitude variability at a level of ΔV = 0.016+-0.002 is indicated, with a period of P=123.04+-0.06 d. A weaker second signal may be present at P=177.84+-0.18 with ΔV = 0.007+-0.002, likely corresponding to fundamental mode pulsation if the primary signal represents overtone pulsation (123.04/177.84 = 0.69). The star, with a spectroscopic reddening of E(B-V) = 1.02, is associated with heavily-reddened B-type stars in its immediate vicinity that appear to be outlying members of an anonymous young cluster centered ~10 arcmin to the west and 1661+-73 pc distant. The cluster has nuclear and coronal radii of r_n=3.5 arcmin and R_c=14 arcmin, respectively, while the parameters for HD 18391 derived from membership in the cluster with its outlying B stars are consistent with those implied by its Cepheid-like pulsation, provided that it follows the semi-period-luminosity relation expected of such objects. Its inferred luminosity as a cluster member is M_V=-7.76+-0.10, its age (9+-1)x10^6 years, and its evolutionary mass ~19 M_{\sun}. HD 18391 is not a classical Cepheid, yet it follows the Cepheid period-luminosity relation closely, much like another Cepheid impostor, V810 Cen.

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arXiv2009

Carbon-rich RR Lyr type stars

We have derived CNO abundances in 12 RR Lyrae stars. Four stars show [C/Fe] near 0.0 and two stars show [C/Fe] = 0.52 and 0.65. Red giant branch stars, which are known to be the predecessors of RR Lyrae stars, generally show a deficiency of carbon due to proton captures during their evolution from the main sequence up the giant branch. We suggest that the enhancement of carbon is due to production during the helium flash combined with mixing to the surface by vigorous convection induced by the flash itself.

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arXiv2008

Reddenings of FGK supergiants and classical Cepheids from spectroscopic data

Accurate and homogeneous atmospheric parameters (Teff, log (g), Vt, [Fe/H]) are derived for 74 FGK non-variable supergiants from high-resolution, high signal-to-noise ratio, echelle spectra. Extremely high precision for the inferred effective temperatures (10-40 K) is achieved by using the line-depth ratio method. The new data are combined with atmospheric values for 164 classical Cepheids, observed at 675 different pulsation phases, taken from our previously published studies. The derived values are correlated with unreddened B-V colours compiled from the literature for the investigated stars in order to obtain an empirical relationship of the form: (B-V)o = 57.984 - 10.3587(log Teff)^2 + 1.67572(log Teff)^3 - 3.356(log (g)) + 0.0321(Vt) + 0.2615[Fe/H] + 0.8833((log (g))(log Teff)). The expression is used to estimate colour excesses E(B-V) for individual supergiants and classical Cepheids, with a precision of +-0.05 mag. for supergiants and Cepheids with n=1-2 spectra, reaching +-0.025 mag. for Cepheids with n>2 spectra, matching uncertainties for the most sophisticated photometric techniques. The reddening scale is also a close match to the system of space reddenings for Cepheids. The application range is for spectral types F0--K0 and luminosity classes I and II.

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arXiv2007

Vertical distribution of Galactic disk stars IV - AMR and AVR from clump giants

We present the parameters of 891 stars, mostly clump giants, including atmospheric parameters, distances, absolute magnitudes, spatial velocities, galactic orbits and ages. One part of this sample consists of local giants, within 100 pc, with atmospheric parameters either estimated from our spectroscopic observations at high resolution and high signal-to-noise ratio, or retrieved from the literature. The other part of the sample includes 523 distant stars, which we have estimated atmospheric parameters from high resolution but low signal-to-noise Echelle spectra. This new sample is kinematically unbiased, with well-defined boundaries in magnitude and colours. We revisit the basic properties of the Galactic thin disk as traced by clump giants. We find the metallicity distribution to be different from that of dwarfs, with less metal-rich stars. We find evidence for a vertical metallicity gradient of -0.31 dex/kpc and for a transition at 4-5 Gyr in both the metallicity and velocities. The age - metallicity relation (AMR), which exhibits a very low dispersion, increases smoothly from 10 to 4 Gyr, with a steeper increase for younger stars. The age-velocity relation (AVR) is characterized by the saturation of the V and W dispersions at 5 Gyr, and continuous heating in U.

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arXiv2006

Elemental abundances in the atmosphere of clump giants

The aim of this paper is to provide the fundamental parameters and abundances for a large sample of local clump giants with a high accuracy. The selection of clump stars for the sample group was made applying a color - absolute magnitude window to nearby Hipparcos stars. The abundances of carbon and nitrogen were obtained from molecular synthetic spectrum, the Mg and Na abundances were derived using the non-LTE approximation. The "classical" models of stellar evolution without atomic diffusion and rotation-induced mixing were employed. The atmospheric parameters (Teff, log g, [Fe/H], Vt) and Li, C, N, O, Na, Mg, Si, Ca and Ni abundances in 177 clump giants of the Galactic disc were determined. The underabundance of carbon, overabundance of nitrogen and "normal" abundance of oxygen were detected. A small sodium overabundance was found. A possibility of a selection of the clump giants based on their chemical composition and the evolutionary tracks was explored. The theoretical predictions based on the classical stellar evolution models are in good agreement with the observed surface variations of the carbon and nitrogen just after the first dredge-up episode. The giants show the same behavior of the dependencies of O, Mg, Ca, Si (alpha-elements) and Ni (iron-peak element) abundances vs. [Fe/H] as dwarfs do. This allows one to use such abundance ratios to study the chemical and dynamical evolution of the Galaxy.

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arXiv2005

Vertical distribution of Galactic disk stars: III. The Galactic disk surface mass density from red clump giants

We used red clump stars to measure the surface mass density of the Galactic disk in the solar neighbourhood. High resolution spectra of red clump stars towards the NGP have been obtained with the ELODIE spectrograph at OHP for Tycho-2 selected stars, and nearby Hipparcos counterparts were also observed. We determined their distances, velocities, and metallicities to measure the gravitational force law perpendicular to the Galactic plane. As in most previous studies, we applied one-parameter models of the vertical gravitational potential. We obtained a disk surface mass density within 1.1kpc of the Galactic plane, Sigma_{1.1kpc}=64+/-5 Msun_pc^{-2}, with an excellent formal accuracy, however we found that such one-parameter models can underestimate the real uncertainties. Applying two-parameter models, we derived more realistic estimates of the total surface mass density within 800pc from the Galactic plane, Sigma_{0.8kpc}=57-66 Msun pc^{-2}, and within 1.1kpc, Sigma{1.1kpc}=57-79 Msun pc^{-2}. This can be compared to literature estimates of \sim40 Msun pc^{-2} in stars and to 13 Msun pc^{-2} in the less accurately measured ISM contribution. We conclude that there is no evidence of large amounts of dark matter in the disk and, furthermore, that the dark matter halo is round or not vey much flattened. A by-product of this study is the determination of the half period of oscillation by the Sun through the Galactic plane, 42+/-2Myr, which cannot be related to the possible period of large terrestrial impact craters \sim 33-37Myr.

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arXiv2004

A new Bohm-Vitense gap in the temperature range 5560 to 5610K in the Main Sequence

Highly precise temperatures (sigma = 10-15 K) have been determined from line depth ratios for a set of 248 F-K field dwarfs of about solar metallicity (-0.5 < [Fe/H] < +0.4), based on high resolution (R=42000), high S/N echelle spectra. A new gap has been discovered in the distribution of stars on the Main Sequence in the temperature range 5560 to 5610 K. This gap coincides with a jump in the microturbulent velocity Vt and the well-known Li depression near 5600 K in field dwarfs and open clusters. As the principal cause of the observed discontinuities in stellar properties we propose the penetration of the convective zone into the inner layers of stars slightly less massive than the Sun and related to it, a change in the temperature gradient.

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arXiv2004

On the correlation of elemental abundances with kinematics among galactic disk stars

We have performed the detailed analysis of 174 high-resolution spectra of FGK dwarfs obtained with the ELODIE echelle spectrograph at the Observatoire de Haute-Provence. Abundances of Fe, Si and Ni have been determined from equivalent widths under LTE approximation, whereas abundances of Mg have been determined under NLTE approximation using equivalent widths of 4 lines and profiles of 5 lines. Spatial velocities with an accuracy better than 1 km/s, as well as orbits, have been computed for all stars. They have been used to define 2 subsamples kinematically representative of the thin disk and the thick disk in order to highlight their respective properties. A transition occurs at [Fe/H]=-0.3. Stars more metal-rich than this value have a flat distribution with Zmax<1 kpc and sigma_W<20 km/s, and a narrow distribution of [alpha/Fe]. There exist stars in this metallicity regime which cannot belong to the thin disk because of their excentric orbits, neither to the thick disk because of their low scale height. Several thin disk stars are identified down to [Fe/H]=-0.80. Their Mg enrichment is lower than thick disk stars with the same metallicity. We confirm from a larger sample the results of Feltzing et al (2003) and Bensby et al (2003) showing a decrease of [alpha/Fe] with [Fe/H] in the thick disk interpreted as the signature of the SNIa which have progressively enriched the ISM with iron. However our data suggest that the star formation in the thick disk stopped when the enrichment was [Fe/H]=-0.30, [Mg/Fe]=+0.20, [Si/Fe]=+0.17. A vertical gradient in [alpha/Fe] may exist in the thick disk but should be confirmed with a larger sample. Finally we have identified 2 new candidates of the HR1614 moving group.

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arXiv2003

High precision effective temperatures for 181 F--K dwarfs from line-depth ratios

Line depth ratios measured on high resolution (R=42000), high S/N echelle spectra are used for the determination of precise effective temperatures of 181 F,G,K Main Sequence stars with about solar metallicity (-0.5 < [Fe/H] < +0.5). A set of 105 relations is obtained which rely Teff on ratios of the strengths of lines with high and low excitation potentials, calibrated against previously published precise (1%) temperature estimates. The application range of the calibrations is 4000--6150 K (F8V--K7V). The internal error of a single calibration is less than 100 K, while the combination of all calibrations for a spectrum of S/N=100 reduces uncertainty to only 5-10 K, and for S/N=200 or higher, to better than 5 K. The zero point of the temperature scale is directly defined from reflection spectra of the Sun with an uncertainty about 1 K. The application of this method to investigation of the planet host stars properties is discussed.

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arXiv2003

Line profile variations in classical Cepheids. Evidence for non-radial pulsations?

We have investigated line profiles in a large sample of Cepheid spectra, and found four stars that show unusual (for Cepheids) line profile structure (bumps or/and asymmetries). The profiles can be phase dependent but the behavior persists over many cycles. The asymmetries are unlikely to be due to the spectroscopic binarity of these stars or the specific velocity field in their atmospheres caused by shock waves. As a preliminary hypothesis, we suggest that the observed features on the line profiles in the spectra of X Sgr, V1334 Cyg, EV Sct and BG Cru can be caused by the non-radial oscillations. It is possible that these non-radial oscillations are connected to resonances between the radial modes (3.2, 7.5 or 10.0 days).

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arXiv2002

Abundances of Cu and Zn in metal-poor stars: clues for Galaxy evolution

We present new observations of copper and zinc abundances in 90 metal-poor stars, belonging to the metallicity range -3< [Fe/H] < -0.5. The present study is based on high resolution spectroscopic measurements collected at the Haute Provence Observatoire (R= 42000, S/N > 100). The trend of Cu and Zn abundances as a function of the metallicity [Fe/H] is discussed and compared to that of other heavy elements beyond iron. We also estimate spatial velocities and galactic orbital parameters for our target stars in order to disentangle the population of disk stars from that of halo stars using kinematic criteria. In the absence of a firm a priori knowledge of the nucleosynthesis mechanisms controlling Cu and Zn production, and of the relative stellar sites, we derive constraints on these last from the trend of the observed ratios [Cu/Fe] and [Zn/Fe] throughout the history of the Galaxy, as well as from a few well established properties of basic nucleosynthesis processes in stars. We thus confirm that the production of Cu and Zn requires a number of different sources (neutron captures in massive stars, s-processing in low and intermediate mass stars, explosive nucleosynthesis in various supernova types). We also attempt a ranking of the relative roles played by different production mechanisms, and verify these hints through a simple estimate of the galactic enrichment in Cu and Zn. In agreement with suggestions presented earlier, we find evidence that Type Ia Supernovae must play a relevant role, especially for the production of Cu.

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arXiv2002

Using Cepheids to determine the galactic abundance gradient. III.First results for the outer disc

As a continuation of our previous work, which concerned the radial abundance distribution in the galactic disc over the distances 4-10 kpc this paper presents the first results on the metallicicty in the outer disc (RG > 10 kpc). Based on high-resolution spectra obtained for 19 distant Cepheids we sampled galactocentric distances from 10 to 12 kpc. Combined with the results of our previous work on the inner and middle parts of the galactic disc, the present data enable one to study the structure of the radial abundance distribution over a large baseline. In particular, we find indications of a discontinuity in the radial abundance distribution for iron as well as a number of the other elements. The discontinuity is seen at a galactocentric distance RG = 10 kpc. This finding supports the results reported earlier by Twarog et al. (1997).

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arXiv2001

Using Cepheids to determine the galactic abundance gradient I. The solar neighbourhood

A number of studies of abundance gradients in the galactic disk have been performed in recent years. The results obtained are rather disparate: from no detectable gradient to a rather significant slope of about -0.1 dex kpc -1. The present study concerns the abundance gradient based on the spectroscopic analysis of a sample of classical Cepheids. These stars enable one to obtain reliable abundances of a variety of chemical elements. Additionally, they have well determined distances which allow an accurate determination of abundance distributions in the galactic disc. Using 236 high resolution spectra of 77 galactic Cepheids, the radial elemental distribution in the galactic disc between galactocentric distances in the range 6-11 kpc has been investigated. Gradients for 25 chemical elements (from carbon to gadolinium) are derived...

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