arXiv subjects

A. T. Bajkova

Publications and source records attributed to A. T. Bajkova.

At least 55 records · Page 3Linked to original sources

arXiv2019

Kinematics of Hot Subdwarfs from the Gaia DR2 Catalogue

We have studied the kinematic properties of the candidates for hot subdwarfs (HSDs) selected by Geier et al. from the Gaia DR2 catalogue. We have used a total of 12 515 stars with relative trigonometric parallax errors less than 30\%. The HSDs are shown to have different kinematics, depending on their positions on the celestial sphere. For example, the sample of low-latitude $(|b|<20^\circ)$ HSDs rotates around the Galactic center with a linear velocity $V_0=221\pm5$ km s$^{-1}$. This suggests that they belong to the Galactic thin disk. At the same time, they lag behind the local standard of rest by $ΔV_\odot\sim16$ km s$^{-1}$ due to the asymmetric drift. The high-latitude ($|b|\geq20^\circ$) HSDs rotate with a considerably lower velocity, $V_0=168\pm6$ km s$^{-1}$. Their lagging behind the local standard of rest is already $ΔV_\odot\sim40$ km s$^{-1}$. Based on the entire sample of 12 515 HSDs, we have found a positive rotation around the $x$ axis significantly differing from zero with an angular velocity $ω_1=1.36\pm0.24$ km s$^{-1}$ kpc$^{-1}$. We have studied the samples of HSDs that are complete within $r<1.5$ kpc. Based on them, we have traced the evolution of the parameters of the residual velocity ellipsoid as a function of both latitude $|b|$ and coordinate $|z|.$ The following vertical disk scale heights have been found: $h=180\pm6$ and $290\pm10$ pc from the low- and high-latitude HSDs, respectively. A new estimate of the local stellar density $Σ_{out}=53\pm4~M_{\odot}$/kpc$^2$ kpc$^{-2}$ has been obtained for $z_{out}=0.56$ kpc from the high-latitude HSDs.

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arXiv2019

Features of the Residual Velocity Ellipsoid of Hot Subdwarfs from the Gaia DR2 Catalog

The evolution of the parameters of the residual velocity ellipsoid of hot subdwarfs (HSDs) with their position relative to the Galactic plane is traced, using the HSDs selected by Geier et al. from the Gaia DR2 catalog. Parameters of the Galactic rotation are determined for two $|z|$ zones. These are used to estimate the gradient of the circular rotation velocity, $V_0,$ versus $|z|,$ found to be $ΔV_0/Δ|z|=-71\pm7$ km s$^{-1}$ kpc$^{-1}$. The size of the residual velocity ellipsoid is $(σ_1,σ_2,σ_3)=(36.1,27.6,22.8)\pm(0.4,0.8,0.6)$ km/s for HSDs at $|z|<0.5$ kpc and $(σ_1,σ_2,σ_3)=(56.9,55.8,39.7)\pm(0.9,1.1,0.8)$ km/s for HSDs at $|z|\geq0.5$ kpc. When forming the HSD residual velocities, the Galactic rotation was taken into account using individual approaches for each $z$ zone. Parameters of the residual velocity ellipsoids for HSDs located in four plane-parallel layers are also determined. The size of the ellipsoid increases with $z,$ and the inclination of the first axis relative to the Galactic plane also increases. This inclination is close to zero in zones close to the Galactic plane, $z\sim\pm0.2$ kpc, and rises to $\mp12\pm4^\circ$ for $z\sim\pm0.9$ kpc.

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arXiv2019

Search for the evolutionary relationship between Galactic globular and open clusters using data from the Gaia DR2 Catalog

Passing through the Galactic disk, a massive object such as a globular cluster, can trigger star formation process leading to the birth of open clusters. Here, we analyze such possible evolutionary relationship between globular and open clusters. To search for the closest rapprochement between objects we computed backwards the orbits of 150 Galactic globular and 232 open clusters (younger than 100 Myr) with proper motions, derived from the Gaia DR2 Catalog. The orbits were computed using the recently modified three-component (disk, bulge and halo) axisymmetric Navarro-Frenk-White potential, which was complemented by non-axisymmetric bar and spiral density wave potentials. We obtained a new estimate for the frequency of impacts of globular clusters about the Galactic disk, which is equal to 4 events for 1 million years. In the framework of the considered scenario, we highlight the following nine pairs of globular and open clusters, with rapprochement within 1 kpc at the time of the intersection the Galactic disk by a globular cluster for the latest 100 Myr: NGC 104 - Turner 3, NGC 104 - NGC 6396, NGC 104 - Ruprecht 127, NGC 5139 - Trumpler 17, NGC 5139 - NGC 6520, NGC 6341 - NGC 6613, NGC 6838 - NGC 6520, NGC 7078 - NGC 7063, NGC 6760 - Ruprecht 127.

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arXiv2019

Galactic Rotation Based on OB Stars from the Gaia DR2 Catalogue

We have studied a sample containing about 6000 OB stars with proper motions and trigonometric parallaxes from the Gaia DR2 catalogue. The following parameters of the angular velocity of Galactic rotation have been found: $Ω_0=29.70\pm0.11$ km s$^{-1}$ kpc$^{-1}$, $Ω'_0=-4.035\pm0.031$ km s$^{-1}$ kpc$^{-2}$, and $Ω''_0= 0.620\pm0.014$ km s$^{-1}$ kpc$^{-3}$. The circular rotation velocity of the solar neighborhood around the Galactic center is $V_0=238\pm5$ km s$^{-1}$ for the adopted Galactocentric distance of the Sun $R_0=8.0\pm0.15$ kpc. The amplitudes of the tangential and radial velocity perturbations produced by the spiral density wave are $f_θ=4.4\pm1.4$ km s$^{-1}$ and $f_R=5.1\pm1.2$ km s$^{-1}$, respectively; the perturbation wavelengths are $λ_θ=1.9\pm0.5$ kpc and $λ_R=2.1\pm0.5$ kpc for the adopted four-armed spiral pattern. The Sun's phase in the spiral density wave is $χ_\odot=-178^\circ\pm12^\circ$.

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arXiv2019

Kinematic Properties of Open Star Clusters with Data from the Gaia DR2 Catalogue

We consider open star clusters (OSCs) with the proper motions, parallaxes, and line-of-sight velocities calculated by various authors from Gaia DR2 data. The distance scale factor has been found by analyzing the separate solutions of the basic kinematic equations to be $p=1.00\pm0.04.$ It shows that the distances calculated using the parallaxes from the Gaia DR2 catalogue do not need any correction factor. We have investigated the solutions obtained from various OSC samples differing in both age and accuracy of their parallax and line-of-sight velocity measurements. The solution obtained from a sample of 930 OSCs satisfying the constraints on the age logt<9 and the relative trigonometric parallax error <30% is recognized to be the best one, with 384 OSCs in this sample having the mean line-of-sight velocities calculated from at least three probable members of the corresponding clusters. As a result of the simultaneous solution of all the basic kinematic equations, we have found the following kinematic parameters from this sample: $(U,V,W)_\odot=(8.53,11.22,7.83)\pm(0.38,0.46,0.32)$ km s$^{-1}$, $Ω_0=28.71\pm0.22$ km s$^{-1}$ kpc$^{-1}$, $Ω'_0=-4.100\pm0.058$ km s$^{-1}$ kpc$^{-2}$, and $Ω''_0= 0.736\pm0.033$ km s$^{-1}$ kpc$^{-3}$. The linear rotation velocity at the adopted at the adopted solar distance $R_0=8\pm0.15$ kpc is $V_0=229.7\pm4.6$ km s$^{-1}$. An analysis of the proper motions for these 930 OSCs has shown that, apart from the rotation around the Galactic z axis, there is rotation of the entire sample around the x axis with an angular velocity of $0.48\pm0.15$ km s$^{-1}$ kpc$^{-1}$ differing significantly from zero.

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arXiv2019

Kinematics of the Galaxy from a Sample of Young Open Star Clusters with Data from the Gaia DR2 Catalogue

We have selected a sample of 326 young $(\log t<8)$ open star clusters with the proper motions and distances calculated by various authors from Gaia DR2 data. The mean values of their line-of-sight velocities have also been taken from various publications. As a result of our kinematic analysis, we have found the following parameters of the angular velocity of Galactic rotation: $Ω_0=29.34\pm0.31$ km s$^{-1}$ kpc$^{-1}$, $Ω'_0=-4.012\pm0.074$ km s$^{-1}$ kpc$^{-2}$, and $Ω''_0=0.779\pm0.062$ km s$^{-1}$ kpc$^{-3}$. The circular rotation velocity of the solar neighborhood around the Galactic center is $V_0=235\pm5$ km s$^{-1}$ for the adopted Galactocentric distance of the Sun $R_0=8.0\pm0.15$ kpc. The amplitudes of the tangential and radial velocity perturbations produced by the spiral density wave are $f_θ=3.8\pm1.2$ km s$^{-1}$ and $f_R=4.7\pm1.0$ km s$^{-1}$, respectively; the perturbation wavelengths are $λ_θ=2.3\pm0.5$ kpc and $λ_R=2.2\pm0.5$ kpc for the adopted four-armed spiral pattern. The Sun's phase in the spiral density wave is close to $-120\pm10^\circ.$

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arXiv2019

Search for possibly evolutionary linked globular and open clusters

Based on a large sample of 133 Galactic globular clusters we obtained a new estimate of the frequency of globular-cluster impacts onto the Galactic plane, which we found to be equal to three events per 1 Myr. Our computations involving new kinematical data do not support the well-known hypothesis about the possible origin of the open cluster Stephenson~2 as a result of the massive globular cluster $ω$ Cen crossing the Galactic disk. Our results also do not support the well-known hypothesis that the globular cluster NGC6397 could trigger the formation of the open cluster NGC6231. We found for the first time six globular clusters, which could have triggered the formation of an open cluster when crossing the Galactic plane. These are the globular clusters NGC104, NGC2808, NGC6362, NGC6540, NGC6749, and NGC6752. For each of these clusters we identify one or several open clusters, which were possibly born via such scenario. In our opinion, of greatest interest are the pairs NGC104-Ruprecht 129, and NGC6362-Pismis 11.

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arXiv2018

Kinematics of the Galaxy from OB Stars with Data from the Gaia DR2 Catalogue

We have selected and analyzed a sample of OB stars with known line-of-sight velocities determined through ground-based observations and with trigonometric parallaxes and proper motions from the Gaia DR2 catalogue. Some of the stars in our sample have distance estimates made from calcium lines. A direct comparison with the trigonometric distance scale has shown that the calcium distance scale should be reduced by 13\%. The following parameters of the Galactic rotation curve have been determined from 495 OB stars with relative parallax errors less than 30\%: $(U,V,W)_\odot=(8.16,11.19,8.55)\pm(0.48,0.56,0.48)$ km s$^{-1}$, $Ω_0=28.92\pm0.39$ km s$^{-1}$ kpc$^{-1}$, $Ω^{'}_0=-4.087\pm0.083$ km s$^{-1}$ kpc$^{-2}$ and $Ω^{''}_0=0.703\pm0.067$ km s$^{-1}$ kpc$^{-3}$, where the circular velocity of the local standard of rest is $V_0=231\pm5$ km s$^{-1}$ (for the adopted $R_0=8.0\pm0.15$ kpc). The parameters of the Galactic spiral density wave have been found from the series of radial, $V_R,$ residual tangential, $ΔV_{circ}$, and vertical, $W,$ velocities of OB stars by applying a periodogram analysis. The amplitudes of the radial, tangential, and vertical velocity perturbations are $f_R=7.1\pm0.3$ km s$^{-1}$, $f_θ=6.5\pm0.4$ km s$^{-1}$, and $f_W=4.8\pm0.8$ km s$^{-1}$, respectively; the perturbation wavelengths are $ λ_R=3.3\pm0.1$ kpc, $λ_θ=2.3\pm0.2$ kpc, and $ λ_W=2.6\pm0.5$ kpc; and the Sun's radial phase in the spiral density wave is $ (χ_\odot)_R=-135\pm5^\circ$, $(χ_\odot)_θ=-123\pm8^\circ$, and $ (χ_\odot)_W=-132\pm21^\circ$ for the adopted four-armed spiral pattern.

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arXiv2018

A Search for a Globular Cluster whose Passage through the Galactic Disk Could Induce the Formation of the Gould Belt

The distribution of sites where globular clusters have crossed the Galactic disk during the last 100 million years has been analyzed using the most recent kinematic data for 133 globular clusters (GCs). Three GCs (NGC 6341, NGC 7078, and $ω$ Cen) whose distances between the positions where they crossed the Galactic disk and trajectories of the Gould Belt are less than 20\% of their heliocentric distances at the crossing time (82, 98, and 96 million years ago, respectively) have been identified. For each of the clusters, this was their next to last, rather than their last, crossing of the Galactic disk. The passage of any one of these three GCs through the disk could potentially have initiated the formation of the Gould Belt.

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arXiv2018

OB Stars and Cepheids From the Gaia TGAS Catalogue: Test of their Distances and Proper Motions

We consider young distant stars from the Gaia TGAS catalog. These are 250 classical Cepheids and 244 OB stars located at distances up to 4 kpc from the Sun. These stars are used to determine the Galactic rotation parameters using both trigonometric parallaxes and proper motions of the TGAS stars. In this case the considered stars have relative parallax errors less than 200%. Following the well-known statistical approach, we assume that the kinematic parameters found from the line-of-sight velocities $V_r$ are less dependent on errors of distances than the found from the velocity components $V_l$. From values of the first derivative of the Galactic rotation angular velocity $Ω{'}_0$, found from the analysis of velocities $V_r$ and $V_l$ separately, the scale factor of distances is determined. We found that from the sample of Cepheids the scale of distances of the TGAS should be reduced by 3%, and from the sample of OB stars, on the contrary, the scale should be increased by 9%.

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arXiv2018

Search for Stellar Streams Based on Data from the RAVE5 and Gaia TGAS Catalogues

We have analyzed the space velocities of stars with the proper motions and trigonometric parallaxes from the Gaia TGAS catalogue in combination with the line-of-sight velocities from the RAVE5 catalogue. In the (V,\sqrt{U^2+2V^2}) velocity plane we have identified three clumps, BB17-1, BB17-2, and BB17-3, in the region of large velocities (V<-150 km/s). The stars of the BB17-1 and BB17-2 clumps are associated with the kinematic groups VelHel-6 and VelHel-7 detected previously by Helmi et al. We give the greatest attention to the BB17-3 clump. The latter is shown to be most closely linked with the debris of the globular cluster ωCen. In the BB17-3 clump we have identified 28 stars with a low velocity dispersion with respect to the center of their distribution. All these stars have very close individual age estimates: \log t\approx 10. The distribution of metallicity indices in this sample is typical for the stars of the globular cluster ωCen. In our opinion, the BB17-3 clump can be described as a homogeneous stream in the debris of the cluster ωCen.

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arXiv2018

Testing the distance scale of the Gaia TGAS catalogue by kinematic method

We have studied the simultaneous and separate solutions of the basic kinematic equations obtained using the stellar velocities calculated on the basis of data from the Gaia TGAS and RAVE5 catalogues. By comparing the values of Omega_0' found by separately analyzing only the line-of-sight velocities of stars and only their proper motions, we have determined the distance scale correction factor p to be close to unity, 0.97+/-0.04. Based on the proper motions of stars from the Gaia TGAS catalogue with relative trigonometric parallax errors less than 10% (they are at a mean distance of 226 pc), we have found the components of the group velocity vector for the sample stars relative to the Sun (U,V,W)o=(9.28,20.35,7.36)+/-(0.05,0.07,0.05) km s^{-1}, the angular velocity of Galactic rotation Omega0=27.24+/-0.30 km s^{-1} kpc^{-1}, and its first derivative Omega_0'=-3.77+/-0.06 km s^{-1} kpc^{-2}; here, the circular rotation velocity of the Sun around the Galactic center is V_0=218+/-6 km s^{-1} kpc (for the adopted distance R_0=8.0+/-0.2 kpc), while the Oort constants are A=15.07+/-0.25 km s^{-1} kpc^{-1} and B=-12.17+/-0.39 km s^{-1} kpc^{-1}, p=0.98+/-0.08. The kinematics of Gaia TGAS stars with parallax errors more than 10% has been studied by invoking the distances from a paper by Astraatmadja and Bailer-Jones that were corrected for the Lutz-Kelker bias. We show that the second derivative of the angular velocity of Galactic rotation Omega_0"=0.864+/-0.021 km s^{-1} kpc^{-3} is well determined from stars at a mean distance of 537 pc. On the whole, we have found that the distances of stars from the Gaia TGAS catalogue calculated using their trigonometric parallaxes do not require any additional correction factor.

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arXiv2017

Galactic Kinematics Derived from Data in the RAVE5, UCAC4, PPMXL, and Gaia TGAS Catalogs

The spatial velocities of the stars with high-precision positions, proper motions and parallaxes from the Gaia TGAS catalog and line-of-sight velocities from the RAVE5 catalog are considered. From the sample of 92395 stars with the age estimates we have obtained the following kinematic parameters: (U,V,W)=(9.42,20.34,7.21)+/-0.12,0.10,0.09) km/s, Ω=26.29+/-0.39 km/s/kpc and Ω'=-3.89+/-0.08 km/s/kpc^2, where V0=210+/-6 km/s (for adopted R0=8.0+/-0.2 kpc), and the Oort constants A=15.57+/-0.31 km/s/kpc and B=-10.72+/-0.50 km/s/kpc. It is shown that the parameters Ωand Ω' are stable to the star age. A comparative analysis of the Bottlinger model parameters obtained separately from the RAVE5 catalog line-of-sight velocities and the Gaia TGAS, UCAC4 and PPMXL catalogs proper motion has been made. It is shown that these parameters are in good agreement with each other when derived from the proper motions of both the terrestrial catalogs and catalog Gaia TGAS. At the same time, it was established that the values of the Bottlinger model parameters obtained from the line-of-sight velocities can differ from the corresponding parameters obtained from the proper motions. The reduction of the line-of-sight velocities from the RAVE5 catalog is proposed for eliminating these differences.

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arXiv2017

Galactic Orbits of Selected Companions of the Milky Way

High-accuracy absolute proper motions, radial velocities, and distances have now been measured for a number of dwarf-galaxy companions of the Milky Way, making it possible to study their 3D dynamics. Galactic orbits for 11 such galaxies (Fornax, Sagittarius, Ursa Minor, LMC, SMC, Sculptor, Sextans, Carina, Draco, Leo I, Leo II) have been derived using two previously refined models for the Galactic potential with the Navarro-Frenk-White and Allen-Santill'an expressions for the potential of the dark-matter halo, and two different masses for the Galaxy within 200 kpc - 0.75x10^12 Mo and 1.45x10^12 Mo. The character of the orbits of most of these galaxies indicates that they are tightly gravitationally bound to the Milky Way, even with the lower-mass model for the gravitational potential. One exception is the most distant galaxy in the list, Leo I, whose orbit demonstrates that it is only weakly gravitationally bound, even using the higher-mass model of the gravitational potential.

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arXiv2017

Searching for Stars Closely Encountering with the Solar System Based on Data from the Gaia DR1 and RAVE5 Catalogues

We have searched for the stars that either encountered in the past or will encounter in the future with the Solar system closer than 2 pc. For this purpose, we took more than 216 000 stars with the measured proper motions and trigonometric parallaxes from the Gaia DR1 catalogue and their radial velocities from the RAVE5 catalogue. We have found several stars for which encounters closer than 1 pc are possible. The star GJ 710, for which the minimum distance is d_m=0.063+/-0.044 pc at time t_m=1385+/-52 thousand years, is the record-holder among them. Two more stars, TYC 8088-631-1 and TYC 6528-980-1, whose encounter parameters, however, are estimated with large errors, are of interest.

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arXiv2017

Vertical Distribution and Kinematics of Protoplanetary Nebulae in the Galaxy

The catalogue of protoplanetary nebulae by Vickers et al. has been supplemented with the line-of-sight velocities and proper motions of their central stars from the literature. Based on an exponential density distribution, we have estimated the vertical scale height from objects with an age less than 3 Gyr belonging to the Galactic thin disk (luminosities higher than 5000 Lo) to be h=146+/-15 pc, while from a sample of older objects (luminosities lower than 5000 Lo) it is h=568+/-42 pc. We have produced a list of 147 nebulae in which there are only the line-of-sight velocities for 55 nebulae, only the proper motions for 25 nebulae, and both line-of-sight velocities and proper motions for 67 nebulae. Based on this kinematic sample, we have estimated the Galactic rotation parameters and the residual velocity dispersions of protoplanetary nebulae as a function of their age. We have established that there is a good correlation between the kinematic properties of nebulae and their separation in luminosity proposed by Vickers. Most of the nebulae are shown to be involved in the Galactic rotation, with the circular rotation velocity at the solar distance being V_0=227+/-23 km/s. The following principal semiaxes of the residual velocity dispersion ellipsoid have been found: (sigma1, sigma2, sigma3) = (47, 41, 29) km/s from a sample of young protoplanetary nebulae (with luminosities higher than 5000 Lo), (sigma1, sigma2, sigma3) = (50, 38, 28) km/s from a sample of older protoplanetary nebulae (with luminosities of 4000 Lo or 3500 Lo), and (sigma1, sigma_2, sigma3) = (91, 49, 36) km/s from a sample of halo nebulae (with luminosities of 1700 Lo).

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arXiv2017

Galactic Orbits of Globular Clusters in the Region of the Galactic Bulge

Galactic orbits have been constructed over long time intervals for ten globular clusters located near the Galactic center. A model with an axially symmetric gravitational potential for the Galaxy was initially applied, after which a non-axially symmetric potential corresponding to the central bar was added. Variations in the trajectories of all these globular clusters in the XY plane due to the influence of the bar were detected. These were greatest for the cluster Terzan 4 in the meridional (RZ) plane. The globular clusters Terzan 1, Terzan 2, Terzan 4, Terzan 9, NGC 6522, and NGC 6558 always remained within the Galactic bulge, no farther than 4 kpc from the Galactic center.

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arXiv2017

Vertical Distribution and Kinematics of Planetary Nebulae in the Milky Way

Based on published data, we have produced a sample of planetary nebulae (PNe) that is complete within 2 kpc of the Sun. We have estimated the total number of PNe in the Galaxy from this sample to be 17000+/-3000 and determined the vertical scale height of the thin disk based on an exponential density distribution to be 197+/-10 pc. The next sample includes PNe from the Stanghellini-Haywood catalog with minor additions. For this purpose, we have used about 200 PNe with Peimbert's types I, II, and III. In this case, we have obtained a considerably higher value of the vertical scale height that increases noticeably with sample radius. We have experimentally found that it is necessary to reduce the distance scale of this catalog approximately by 20%. Then, for example, for PNe with heliocentric distances less than 4 kpc the vertical scale height is 256+/-12 kpc. A kinematic analysis has confirmed the necessity of such a reduction of the distance scale.

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