arXiv subjects

A. K. Dambis

Publications and source records attributed to A. K. Dambis.

At least 19 recordsLinked to original sources

arXiv2026

Kinematics of the System of Young Open Clusters based on Gaia DR3 Data

We use a kinematical model including circular rotation of the Milky-Way disk incorporating effects produced by a spiral density wave with constant radial and vertical components of the velocity dispersion tensor and adopted solar Galactocentric distance R_0=8.277kpc (inferred by GRAVITY collaboration in 2022) to derive the velocity field parameters for a sample of 2384 open clusters from the Hunt and Reffert (2024) catalog having ages no greater than 100 Myr almost all of which reside at Galactoaxial distances between 5 and 14 kpc. We estimate the velocity-field parameters using maximum-likelyhood method and the affine-invariant variant of the Markov chain Monte-Carlo method proposed by Goodman and Weare (2010) with both methods yieilding almost identical parameter values. The inferred rotation curve, on the whole, is consistent with the results based on our kinematical analysis of a sample of Galactic masers. We find the linear rotation velocity at the solar distance to be V=242.3+/-1.1~km/s; it reaches its maximum V~244 km/s at Galactoaxial distance of about R_g~7 kpc followed by a slow and smooth decline with V~227 km/s at R_g=14 kpc. The inferred radial and vertical components of the velocity dispersion tensor are (sigma U_0, sigma W_0)~(10.63+/-0.15, 4.56+/-0.15) km/s. We find the pitch angle and the phase of the Sun for a four-armed spiral pattern to be i~-11.7+/-0.3 degrees and chi_0~138+/-5 degrees, respectively, and the amplitudes of radial and tangential perturbations, f_R=-3.0+/-0.4 and f_{theta}=-3.1+/-0.4 km/s, respectively.

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arXiv2023

Periodic changes in the morphology of the Galactic resonance rings

We study the periodic enhancement of either trailing or leading segments of the resonance elliptical rings in the dynamical model of the Galaxy which reproduces distributions of observed velocities derived from Gaia DR3 (EDR3) data along the Galactocentric distance. The model disc forms a nuclear ring, an inner combined ring and outer resonance rings R1 and R2. The backbone of the inner combined ring is banana-type orbits around the Lagrange equilibrium points L4 and L5. Orbits associated with the unstable equilibrium points L1 and L2 also support the inner ring. We have found the changes of the morphology of the inner ring with a period of P=0.57+/-0.02 Gyr, which is close to the period of revolution along the long-period orbits around the points L4 and L5. A possible explanation of these morphological changes is the formation of an overdensity which then begins circulating along the closed contour. In the region of the Outer Lindblad Resonance (OLR), we have found the changes of the morphology of the outer rings with a period of P=2.0+/-0.1 Gyr. Probably, the morphological changes of the outer rings are due to the orbits trapped by the OLR. These orbits exhibit librations of the direction of orbital elongation with respect to the minor axis of the bar as well as the long-term variations in the stellar angular momentum, energy, average radius of the orbit, and eccentricity. Among many librating orbits, we discovered orbits with the libration period of P=1.91+/-0.01 Gyr, which may cause the morphological changes of the outer rings.

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arXiv2021

Influence of the Galactic bar on the kinematics of the disc stars with Gaia EDR3 data

A model of the Galaxy with the outer ring R1R2 can explain the observed distribution of the radial, VR, and azimuthal, VT, velocity components along the Galactocentric distance, R, derived from the Gaia EDR3 data. We selected stars from the Gaia EDR3 catalogue with reliable parallaxes, proper motions and line-of-sight velocities lying near the Galactic plane, |z|<200 pc, and in the sector of the Galactocentic angles |theta|<15 degrees and calculated the median velocities VR and VT in small bins along the distance R. The distribution of observed velocities appears to have some specific features: the radial velocity VR demonstrates a smooth fall from +5 km s-1 at the distance of R=R0-1.5 kpc to -3 km s-1 at R=R0+1.0 kpc while the azimuthal velocity VT shows a sharp drop by 7 km s-1 in the distance interval R0<R<R0+1.0 kpc, where R0 is the solar Galactocentric distance. We build a model of the Galaxy including bulge, bar, disc and halo components, which reproduces the observed specific features of the velocity distribution in the Galactocentric distance interval |R-R0|< 1.5 kpc. The best agreement corresponds to the time 1.8+/-0.5 Gyr after the start of the simulation. A model of the Galaxy with the bar rotating at the angular velocity of Omega_b=55+/-3 km s-1 kpc-1, which sets the OLR of the bar at the distance of R0-0.5+/-0.4 kpc, provides the best agreement between the model and observed velocities. The position angle of the bar, theta_b, corresponding to the best agreement between the model and observed velocities is theta_b=45+/-15 degrees.

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arXiv2021

Kinematics and Multi Band Period-Luminosity-Metallicity Relation of RR Lyrae Stars via Statistical Parallax

This paper presents results from photometric and statistical-parallax analysis of a sample of 850 field RR Lyrae (RRL) variables. The photometric and spectroscopic data for sample RRLs are obtained from (1) our new spectroscopic observations (for 448 RRLs) carried out with the Southern African Large Telescope (SALT); (2) our photometric observations using the 1.0-m telescope of the South African Astronomical Observatory (SAAO), and (3) literature. These are combined with accurate proper motion data from the second release of \textit{Gaia} mission (DR2). This study primarily determines the velocity distribution of solar neighborhood RRLs, and it also calibrates the zero points of the RRLs visual V-band luminosity-metallicity (LZ or $M_V-$[\text{Fe/H}]) relation and their period-luminosity-metallicity (PLZ) relations in the \textit{WISE} $W_1-$ and \textit{2MASS} $Ks-$band. The calibrated PLZ and LZ relations are used to estimate the Galactic Center distance and the distance modulus of the Large Magellanic Cloud (LMC), which are found to be 7.99$\pm$0.49\,kpc and 18.46$\pm$0.09 \,mag, respectively. All our results are in excellent agreement with available literature based on statistical parallax analysis, but are considerably more accurate and precise. Moreover, the zero-points of our calibrated PLZ and LZ relations are quite consistent with current results found by other techniques and yield the LMC distance modulus that is within 0.04\,mag of the current most precise estimate.

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arXiv2020

Calibrating the BHB Star Distance Scale and the Halo Kinematic Distance to the Galactic Centre

We report the first determination of the distance to the Galactic centre based on the kinematics of halo objects. We apply the statistical-parallax technique to the sample of ~2500 Blue Horizontal Branch (BHB) stars compiled by Xue et al. (2011) to simultaneously constrain the correction factor to the photometric distances of BHB stars as reported by those authors and the distance to the Galactic centre to find R0=8.2+/-0.6 kpc. We also find that the average velocity of our BHB star sample in the direction of Galactic rotation, V0=-240+/-4 km/s, is greater by about 20 km/s in absolute value than the corresponding velocity for halo RR Lyrae type stars (V0=-222+/-4 km/s) in the Galactocentric distance interval from 6 to 18 kpc, whereas the total (sigma V) and radial (sigma r) velocity dispersion of the of the BHB sample are smaller by about 40-45 km/s than the corresponding parameters of the velocity dispersion ellipsoid of halo RR Lyrae type variables. The velocity dispersion tensor of halo BHB stars proved to be markedly less anisotropic than the corresponding tensor for RR Lyrae type variables: the corresponding anisotropy parameter values are equal to beta(BHB)=0.51+/-0.02 and beta(RR)=0.71+/-0.03, respectively.

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arXiv2020

Contribution of binary stars to the velocity dispersion inside OB associations with Gaia DR2 data

We estimated the contribution of binary systems to the velocity dispersion inside OB-associations derived from Gaia DR2 proper motions. The maximum contribution to the velocity dispersion is given by the systems with the period of revolution of P=5.9 yr whose components shift by a distance of about the diameter of the system during the base-line time of Gaia DR2 observations. We employed two methods to study the motion of the photocenter of the binary system: the first one uses the total displacement between the initial and final visibility periods and the second one is based on solving a system of n equations defining the displacements at the times t_n. The first and second methods yield very similar sigma_bn values of 0.90 and 0.87 km s-1, respectively. Taking into account the fact that orbits are elliptical slightly decreases the inferred sigma_bn. We estimated the eccentricity-averaged sigma_bn value to be sigma_bn=0.81 km s-1 assuming that the orbital eccentricities of massive binary systems are distributed uniformly in the [0, 0.9] interval. The choice of the exponent gamma in the power-law distribution, p_q ~ q^gamma, of the component-mass ratios q=M_2/M_1 of binary systems appears to have little effect on sigma_bn. A change of gamma from 0 (flat distribution) to -2.0 (preponderance of systems with low-mass components) changes sigma_bn from 0.90 to 1.07 km s-1.

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arXiv2020

Distance scale for high-luminosity stars in OB associations and in field with Gaia DR2. Spurious systematic motions

We calculated the median parallaxes for 47 OB associations including at least 10 stars with known Gaia DR2 parallaxes. A comparison between trigonometric and photometric parallaxes of OB associations reveals a zero-point offset of delta_pi=-0.11 +\- 0.04 mas indicating that Gaia DR2 parallaxes are, on average, underestimated and the distances derived from them are overestimated. The correction of delta_pi=-0.11 mas is consistent with the estimate that Arenou et al. (2018) obtained for bright stars. An analysis of parallaxes of OB associations and high-luminosity field stars confirms our previous conclusion (Dambis et al. 2001) that the distance scale for OB stars established by Blaha and Humphreys (1989) must be reduced by 10--20%. Spurious systematic motions of 10--20 km s-1 at the distances of 2--3 kpc from the Sun are found to arise from the use of the uncorrected Gaia DR2 parallaxes.

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arXiv2020

Internal motions in OB-associations with Gaia DR2

We study the motions inside 28 OB-associations with the use of Gaia DR2 proper motions. The average velocity dispersion calculated for 28 OB-associations including more than 20 stars with Gaia DR2 proper motion is sigma_v =4.5 km s-1. The median virial and stellar masses of OB-associations are M_vir=8.9 x 10^5 and M_st=8.1 x 10^3 Ms, respectively. The median star-formation efficiency in parent giant molecular clouds appears to be epsilon=1.2 per cent. Gaia DR2 proper motions confirm the expansion in the Per OB1, Car OB1 and Sgr OB1 associations found earlier with Gaia DR1 data. We also detect the expansion in Gem OB1, Ori OB1 and Sco OB1 associations which became possible for the first time now when analyzed with Gaia DR2 proper motions. The analysis of the distribution of OB-stars in the Per OB1 association shows the presence of a shell-like structure with the radius of 40 pc. Probably, the expansion of the Per OB1 association started with the velocity greater than the present-day expansion velocity equal to 5.0 +\- 1.7 km s-1.

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arXiv2018

A comprehensive study of 94 open clusters based on the data from IPHAS, GAIA DR2, and other sky surveys

We determine the color excesses, photometric distances, ages, astrometric parallaxes and proper motions for 94 open clusters in the northern part of the Milky Way. We estimate the color excesses and photometric distances based on the data from IPHAS photometric survey of the northern Galactic plane using individual total-to-selective extinction ratios R_r=A_r/E_(r-i) for each cluster computed via the color-difference method based on IPHAS r, i, and H_alpha-band, 2MASS J, H, and K_s-band, WISE W1-band, and Pan-STARRS i, z, and y-band data. The inferred R_r values vary significantly from cluster to cluster spanning the R_r=3.1--5.2 interval with a mean and standard deviation equal to =3.99 and sigma R_r=0.34, respectively. We identified cluster members using (1) absolute proper motions determined from individual-epoch positions of stars retrieved from IPHAS, 2MASS, URAT1, ALLWISE, UCAC5, and Gaia DR1 catalogs and positions of stars on individual Palomar Sky Survey plates reconstructed based on the data provided in USNO-B1.0 catalog and (2) absolute proper motions provided in Gaia DR2 catalog, and computed the average Gaia DR2 trigonometric parallaxes and proper motions of the clusters. The mean formal error of the inferred astrometric parallaxes of clusters is of about 7microarcseconds, however, a comparison of astrometric and photometric parallaxes of our cluster sample implies that Gaia DR2 parallaxes are, on the average, systematically underestimated by 45 +/-9 microarcseconds. This result agrees with estimates obtained by other authors using other objects. At the same time, we find our photometric distance scale to be correct within the quoted errors (the inferred correction factor is equal to unity to within a standard error of 0.025).

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arXiv2018

Globular clusters: absolute proper motions and Galactic orbits

We cross-match objects from several different astronomical catalogs to determine the absolute proper proper motions of stars within the 30-arcmin radius fields of 115 Milky-Way globular clusters with the accuracy of 1--2~mas/yr. The proper motions are based on positional data recovered from the USNO-B1, 2MASS, URAT1, ALLWISE, UCAC5, and GAIA DR1 surveys with up to 10 positions spanning an epoch difference of up to $\sim$~65~years, and reduced to GAIA DR1 TGAS frame using UCAC5 as the reference catalog. Cluster members are photometrically identified by selecting horizontal- and red-giant branch stars on color-magnitude diagrams, and the mean absolute proper motions of the clusters with a typical formal error of $\sim$~0.4~mas/yr are computed by averaging the proper motions of selected members. The inferred absolute proper motions of clusters are combined with available radial-velocity data and heliocentric distance estimates to compute the cluster orbits in terms of the Galactic potential models based on Miyamoto and Nagai disk, Hernquist spheroid, and modified isothermal dark-matter halo (axisymmetric model without a bar) and the same model + rotating Ferre's bar (non-axisymmetric). Five distant clusters have higher-than-escape velocities, most likely due to large errors of computed transversal velocities, whereas the computed orbits of all other clusters remain bound to the Galaxy. Unlike previously published results, we find the bar to affect substantially the orbits of most of the clusters, even those at large Galactocentric distances, bringing appreciable chaotization, especially in the portions of the orbits close to the Galactic center, and stretching out the orbits of some of the thick-disk clusters.

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arXiv2017

Kinematics of OB-associations in Gaia epoch

We use stellar proper motions from the TGAS catalog to study the kinematics of OB-associations. The TGAS proper motions of OB-associations generally agree well with the Hipparcos proper motions. The parameters of the Galactic rotation curve obtained with TGAS and Hipparcos proper motions agree within the errors. The average one-dimensional velocity dispersion inside 18 OB-associations with more than 10 TGAS stars is sigma_v=3.9 km s-1, which is considerably smaller, by a factor of 0.4, than the velocity dispersions derived from Hipparcos data. The effective contribution from orbital motions of binary OB-stars into the velocity dispersion sigma_v inside OB-associations is sigma_b=1.2 km s-1. The median virial and stellar masses of OB-associations are equal to 7.1 10^5 and 9.0 10^3 Ms, respectively. Thus OB-associations must be unbound objects provided they do not include a lot of dense gas. The median star-formation efficiency is epsilon=2.1 percent. Nearly one third of stars of OB-associations must lie outside their tidal radius. We found that the Per OB1 and Car OB1 associations are expanding with the expansion started in a small region of 11--27 pc 7--10 Myr ago. The average expansion velocity is 6.3 km s-1.

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arXiv2016

Galactic masers: kinematics, spiral structure and the disk dynamic state

We applied the currently most comprehensive version of the statistical-parallax technique to derive kinematical parameters of the maser sample with 136 sources. Our kinematic model comprises the overall rotation of the Galactic disk and the spiral density-wave effects. We take into account the variation of radial velocity dispersion with Galactocentric distance. The best description of the velocity field is provided by the model with constant radial and vertical velocity dispersions, $(σU0, σW0) \approx (9.4 \pm 0.9~, 5.9 \pm 0.8)~ km/s$. We compute flat Galactic rotation curve over the Galactocentric distance interval from 3 to 15 kpc and find the local circular rotation velocity to be $ V_0 \approx (235-238)$~ km/s $\pm 7$~ km/s. We also determine the parameters of the four-armed spiral pattern (pitch angle $i \approx (-10.4 \pm 0.3)^\circ$ and the phase of the Sun $χ_0 \approx (125 \pm 10) ^\circ$). The radial and tangential spiral perturbations are about $f_R \approx (-6.9 \pm 1.4)$~km/s, $f_Θ\approx (+2.8 \pm 1.0$) ~km/s. The kinematic data yield a solar Galactocentric distance of $R_0 \approx (8.24 \pm 0.12)~kpc$. Based on rotation curve parameters and the asymmetric drift we Infer the exponential disk scale $H_D \approx (2.7 \pm 0.2)$ ~kpc under assumption of marginal stability of the intermediate-age disk, and finally we estimate the minimum local surface disk density, $Σ(R_0) > (26 \pm 3) ~ M_\odot pc^{-2}$.

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arXiv2016

Evidence of the Galactic outer ring R1R2' from young open clusters and OB-associations

The distribution of young open clusters in the Galactic plane within 3 kpc from the Sun suggests the existence of the outer ring R1R2' in the Galaxy. The optimum value of the solar position angle with respect to the major axis of the bar, theta_b, providing the best agreement between the distribution of open clusters and model particles is theta_b=35 +/- 10 degrees. The kinematical features obtained for young open clusters and OB-associations with negative Galactocentric radial velocity VR indicate the solar location near the descending segment of the outer ring R2.

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arXiv2015

Classical Cepheids and the spiral structure of the Milky Way

We use the currently most complete collection of reliable Cepheid positions (565 stars) out to ~5 kpc based mostly on our photometric data to outline the spiral pattern of our Galaxy. We find the pitch-angle to be equal to 9--10 degrees with the most accurate estimate (i=9.5 +/-0.1 degrees) obtained assuming that the spiral pattern has a four-armed structure, and the solar phase angle in the spiral pattern to be chi_0 = 121+/-3 degrees. The pattern speed is found to be Omega_P=25.2+/-0.5km/s/kpc based on a comparison of the positions of the spiral arms delineated by Cepheids and maser sources and the age difference between these objects.

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arXiv2014

Classical Cepheids in the Galactic outer ring R1R2'

The kinematics and distribution of classical Cepheids within ~3 kpc from the Sun suggest the existence of the outer ring R1R2' in the Galaxy. The optimum value of the solar position angle with respect to the major axis of the bar, theta_b, providing the best agreement between the distribution of Cepheids and model particles is theta_b=37 +/- 13 degrees. The kinematical features obtained for Cepheids with negative Galactocentric radial velocity VR are consistent with the solar location near the descending segment of the outer ring R2. The sharp rise of extinction toward of the Galactic center can be explained by the presence of the outer ring R1 near the Sun.

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arXiv2014

Mid-infrared PL relations for Globular Cluster RR Lyrae

The period - metallicity - WISE W1- and W2-band luminosity relations are derived for RR Lyrae stars based on WISE epoch photometry for 360 and 275 stars in 15 and 9 Galactic globular clusters, respectively. Our final relations have the form = gamma(W1) - (2.381 +/- 0.097) log PF + (0.096 +/- 0.021)[Fe/H] and = gamma(W2)-(2.269 +/- 0.127)log PF + (0.108 +/- 0.021)[Fe/H], where [Fe/H] values are on the scale of Carretta et al. (2009). We obtained two appreciably discrepant estimates for the zero points gamma(W1) and gamma(W2) of both relations: one based on a statistical-parallax analysis -- gamma(W1) = -0.829 +/- 0.093 and gamma(W2)=-0.776 +/- 0.093 and another, significantly brighter one, based on HST FGS trigonometric parallaxes -- gamma(W1, HST) =-1.150 +/- 0.077 and gamma(W2, HST) =-1.105 +/- 0.077. The period-metallicity-luminosity relations in the two bands yield highly consistent distance moduli for the calibrator clusters and the distance moduli computed using the W1- and W2-band relations with the HST zero points agree well with those computed by \citet{sollima} based on their derived period-metallicity-K-band luminosity relation whose zero point is tied to the HST trigonometric parallax of RR Lyrae itself (Delta DM0 = +0.04 and +0.06, respectively, with a scatter of only 0.06).

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arXiv2013

RR Lyrae variables: visual and infrared luminosities, intrinsic colours, and kinematics

We use UCAC4 proper motions and WISE W1-band apparent magnitudes intensity-mean for almost 400 field RR Lyrae variables to determine the parameters of the velocity distribution of Galactic RR Lyrae population and constrain the zero points of the metallicity- relation and those of the period-metallicity- -band and period-metallicity- -band luminosity relations via statistical parallax. We find the mean velocities of the halo- and thick-disc RR Lyrae populations in the solar neighbourhood to be (U0(Halo), V0(Halo), W0(Halo)) = (-7 +/- 9, -214 +/- 10, -10 +/- 6) km/s and (U0(Disc), V0(Disc), W0(Disc)) =(-13 +/- 7, -37 +/- 6, -17 +/- 4) km/s, respectively, and the corresponding components of the velocity-dispersion ellipsoids, (sigma VR(Halo), sigma Vphi(Halo), sigma Vtheta(Halo)) = (153 +/- 9, 101 +/- 6, 96 +/- 5) km/s and (sigma VR(Disc), sigma Vphi(Disc), sigma Vtheta(Disc)) = (46 +/- 7, 37 +/- 5, 27 +/- 4) km/s, respectively. The fraction of thick-disc stars is estimated at 0.22 +/- 0.03. The corrected IR period-metallicity-luminosity relations are = -0.769 +0.088 [Fe/H]- 2.33 mathoprm log PF and = -0.825 + 0.088 [Fe/H] -2.33 mathoprm log PF, and the optical metallicity-luminosity relation, [Fe/H]- , is = +1.094 + 0.232 [Fe/H], with a standard error of +/- 0.089, implying an LMC distance modulus of 18.32 +/- 0.09, a solar Galactocentric distance of 7.73 +/- 0.36 kpc, and the M31 and M33 distance moduli of DM(M31) = 24.24 +/- 0.09 (D = 705 +/- 30 kpc) and DM(M33) = 24.36 +/- 0.09 (D = 745 +/- 31 kpc), respectively. Extragalactic distances calibrated with our RR Lyrae star luminosity scale imply a Hubble constant of ~80 km/s/Mpc. Our results suggest marginal prograde rotation for the population of halo RR Lyraes in the Milky Way.

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arXiv2010

Estimating the Kinematic Parameters and the Distance-Scale Zero Point for the Thin-Disk, Thick-Disk, and Halo Population Tracers via 3D Velocity Data

We use the method of statistical parallax to constrain the distance-scale zero points and analyze the kinematics of extensive samples of Galactic classical Cepheids, RR Lyrae type variables, and blue horizontal branch stars, which serve as standard candles/kinematic tracers of various Galactic populations. We obtain three consistent estimates for the local circular velocity based on the mean velocities of halo RR Lyrae variables, BHB stars, and Galactic rotation curve inferred from Cepheid data with an average value of 210+/-6 km/s, which is close to the average circular velocity in the 5-40 kpc interval of Galactocentric distances inferred from BHB star data (195+/-5 km/s), thereby providing further supporting evidence for the practically flat shape of the Galactic rotation curve beyond ~5 kpc from the center. The inferred distance-scale corrections imply a solar Galactocentric distance of 7.7+/-0.4 kpc, an LMC distance modulus of 18.42+/-0.06, and a Hubble constant of 73-85 km/s/Mpc.

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