Solar Galactocentric Distance Research Articles

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

ABSTRACT 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 Galactocentric angles |θ| < 15° 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 Ωb = 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, θb, corresponding to the best agreement between the model and observed velocities is θb = 45 ± 15°.

A New Estimate of the Best Value for the Solar Galactocentric Distance

Using data from the literature, we made a list of individual estimates of the solar Galactocentric distance, which were performed after 2017 by different methods. These values have not yet been used to calculate the best value of mean $R_0$. For the sample containing 21 estimates, based on the standard approach, we found the weighted mean ${\overline R_0}=8.14$ kpc with the dispersion $\sigma=0.16$ kpc, and using the median statistics, we obtained the estimate $R_0=8.15\pm0.11$ kpc. For practical use, the value $R_0=8.1\pm0.1$ kpc can be recommended.

Constraining the Solar Galactic Reflex Velocity using Gaia Observations of the Sagittarius Stream

Abstract Because of its particular orientation around the Galaxy—i.e., in a plane nearly perpendicular to the Galactic plane and containing both the Sun and Galactic center—the Sagittarius (Sgr) stream provides a powerful means by which to measure the solar reflex velocity, and thereby infer the velocity of the Local Standard of Rest (LSR), in a way that is independent of assumptions about the solar Galactocentric distance. Moreover, the solar reflex velocity with respect to the stream is projected almost entirely into the proper motion component of Sgr stream stars perpendicular to the Sgr plane, which makes the inferred velocity relatively immune to most Sgr model assumptions. Using Gaia Data Release 2 proper motions of ∼2000 stars identified to be Sgr stream candidates in concert with the Law & Majewski Sgr N-body models (which provide a good match to the Gaia observations), we constrain the solar reflex velocity induced by its orbital motion around the Galaxy to be Θ⊙ = 253 ± 6 km s−1. Assuming a solar peculiar motion in the direction of orbital rotation of 12 km s−1, and an LSR velocity of 12 km s−1 with respect to the local circular speed, the implied circular speed of the Milky Way at the solar circle is 229 ± 6 km s−1.

An Extremely Low Mid-infrared Extinction Law toward the Galactic Center and 4% Distance Precision to 55 Classical Cepheids

Abstract Distances and extinction values are usually degenerate. To refine the distance to the general Galactic Center region, a carefully determined extinction law (taking into account the prevailing systematic errors) is urgently needed. We collected data for 55 classical Cepheids projected toward the Galactic Center region to derive the near- to mid-infrared extinction law using three different approaches. The relative extinction values obtained are , , , and . We also calculated the corresponding systematic errors. Compared with previous work, we report an extremely low and steep mid-infrared extinction law. Using a seven-passband “optimal distance” method, we improve the mean distance precision to our sample of 55 Cepheids to 4%. Based on four confirmed Galactic Center Cepheids, a solar Galactocentric distance of R 0 = 8.10 ± 0.19 ± 0.22 kpc is determined, featuring an uncertainty that is close to the limiting distance accuracy (2.8%) for Galactic Center Cepheids.

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, $(\sigma U0, \sigma W0) \approx (9.4 \pm 0.9~, 5.9 \pm 0.8)~ km/s$. 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 $\chi_0 \approx (125 \pm 10) ^\circ$). The radial and tangential spiral perturbations are about $f_R \approx (-6.9 \pm 1.4)$~km/s, $f_\Theta \approx (+2.8 \pm 1.0$) ~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, $\Sigma (R_0) > (26 \pm 3) ~ M_\odot pc^{-2}$.

RR Lyrae star distance scale and kinematics from inner bulge to 50 kpc

We use the currently most complete sample of ∼ 3500 type ab RR Lyraes in our Galaxy with available radial-velocity and [Fe/H] measurements to perform a statisticalparallax analysis for a subsample of ∼ 600 type ab RR Lyraes located within 5 kpc from the Sun to refine the parameters of optical and WISE W1-band period-metallicityluminosity relations and adjust our preliminary distances. The new zero point implies the rescaled estimates for the solar Galactocentric distance (RG = 7.99 ± 0.37 kpc) and the LMC distance modulus (DMLMC = 18.39 ±0.09). We use the kinematic data for the entire sample to explore the dependence of the halo and thick-disk RR Lyrae velocity ellipsoids on Galactocentric distance from the inner bulge out to R ∼ 50 kpc.

CLUSTERING OF LOCAL GROUP DISTANCES: PUBLICATION BIAS OR CORRELATED MEASUREMENTS? IV. THE GALACTIC CENTER

ABSTRACT Aiming at deriving a statistically well-justified Galactic Center distance, R 0, and reducing any occurrence of publication bias, we compiled the most comprehensive and most complete database of Galactic Center distances available to date, containing 273 new or revised R 0 estimates published since records began in 1918 October until 2016 June. We separate our R 0 compilation into direct and indirect distance measurements. The latter include a large body of estimates that rely on centroid determinations for a range of tracer populations, as well as measurements based on kinematic observations of objects at the solar circle, combined with a mass and/or rotational model of the Milky Way. Careful assessment of the Galactic Center distances resulting from orbital modeling and statistical parallax measurements in the Galactic nucleus yields our final Galactic Center distance recommendation of . The centroid-based distances are in good agreement with this recommendation. Neither the direct measurements nor the post-1990 centroid-based distance determinations suggest that publication bias may be important. The kinematics-based distance estimates are affected by significantly larger uncertainties, but they can be used to constrain the Galaxy’s rotation velocity at the solar galactocentric distance, . Our results imply that the International-Astronomical-Union-recommended Galactic Center distance ( ) needs a downward adjustment, while its recommendation ( km s−1) requires a substantial upward revision.

Determination of the Solar Galactocentric Distance from the Kinematics of Masers

Abstract We determine the parameters of Galactic rotation and the solar galactocentric distance R 0 by simultaneously solving Bottlinger’s kinematic equations using data for masers with known line-of-sight velocities and highly accurate VLBI trigonometric parallaxes and proper motions. Our sample includes 93 masers spanning the range of galactocentric distances R from 3 to 15 kpc. The inferred parameters are Ω0 = 29.7 ± 0.5 km s−1 kpc−1, Ω′0 = −4.20 ± 0.11 km s−1 kpc−2, Ω″0 = 0.730 ± 0.029 km s−1 kpc−3, and R 0 = 8.03 ± 0.12 kpc, implying a circular rotation velocity of V 0 = 238 ± 6 km s−1 at the solar distance R 0.

On the Possibility of Determining the Distance to the Galactic Center from the Geometry of Spiral Arm Segments

AbstractA new approach to determining the solar galactocentric distance,

Analysis of galactic rotation from masers based on a nonlinear oort model

Based on data on Galactic masers with measured trigonometric parallaxes, we have tested a nonlinear model of Galactic rotation using generalized Oort formulas. This model is shown to yield pretty good results up to heliocentric distances of 3--4 kpc. The main feature of the method is the possibility of estimating the solar Galactocentric distance Ro. This distance has been found by analyzing approximately 60 masers to be Ro=8.3+/-0.4 kpc. Our study of the three-dimensional kinematics of more than 100 masers based on the Ogorodnikov--Milne model has shown that significant nonlinearities are present only in the xy plane (rotation around the Galactic z axis) due to the peculiarities of the Galactic rotation curve. No significant linear dependences have been found in the xz and yz planes. We show the presence of a wave in the velocities w as a function of coordinate x or distance R with a wavelength of 3 kpc and an amplitude of 10 km/s. The wave is particularly prominent in the Local and Perseus arms.

Determination of galactic rotation parameters and the solar galactocentric distance R 0 from 73 masers

We have determined the Galactic rotation parameters and the solar Galactocentric distance R 0 by simultaneously solving Bottlinger’s kinematic equations using data on masers with known line-of-sight velocities and highly accurate trigonometric parallaxes and proper motions measured by VLBI. Our sample includes 73 masers spanning the range of Galactocentric distances from 3 to 14 kpc. The solutions found are Ω0 = 28.86 ± 0.45 km s−1 kpc−1, Ω′0 = −3.96 ± 0.09 km s−1 kpc−2, Ω″0 = 0.790 ± 0.027 km s−1 kpc−3, and R 0 = 8.3 ± 0.2 kpc. In this case, the linear rotation velocity at the solar distance R 0 is V = 241 ± 7 km s−1. Note that we have obtained the R 0 estimate, which is of greatest interest, from masers for the first time; it is in good agreement with the most recent estimates and even surpasses them in accuracy.

Flare in the Galactic stellar outer disc detected in SDSS-SEGUE data

Aims. We explore the outer Galactic disc up to a Galactocentric distance of 30 kpc to derive its parameters and measure the magnitude of its flare. Methods. We obtained the 3D density of stars of type F8V-G5V with a colour selection from extinction-corrected photometric data of the Sloan Digital Sky Survey - Sloan Extension for Galactic Understanding and Exploration (SDSS-SEGUE) over 1,400 deg^2 in off-plane low Galactic latitude regions and fitted it to a model of flared thin+thick disc. Results. The best-fit parameters are a thin-disc scale length of 2.0 kpc, a thin-disc scale height at solar Galactocentric distance of 0.24 kpc, a thick-disc scale length of 2.5 kpc, and a thick-disc scale height at solar Galactocentric distance of 0.71 kpc. We derive a flaring in both discs that causes the scale height of the average disc to be multiplied with respect to the solar neighbourhood value by a factor of 3.3^{+2.2}_{-1.6} at R=15 kpc and by a factor of 12^{+20}_{-7} at R=25 kpc. Conclusions. The flare is quite prominent at large R and its presence explains the apparent depletion of in-plane stars that are often confused with a cut-off at R>15 kpc. Indeed, our Galactic disc does not present a truncation or abrupt fall-off there, but the stars are spread in off-plane regions, even at z of several kpc for R>20 kpc. Moreover, the smoothness of the observed stellar distribution also suggests that there is a continuous structure and not a combination of a Galactic disc plus some other substructure or extragalactic component: the hypothesis to interpret the Monoceros ring in terms of a tidal stream of a putative accreted dwarf galaxy is not only unnecessary because the observed flare explains the overdensity in the Monoceros ring observed in SDSS fields, but it appears to be inappropriate.

CCD Washington photometry of four poorly studied open clusters in the two inner quadrants of the galactic plane

Complementing our Washington photometric studies on Galactic open clusters (OCs), we now focus on four poorly studied OCs located in the first and fourth Galactic quadrants, namely BH84, NGC5381, BH211 and Czernik37. We have obtained CCD photometry in the Washington system C and T1 passbands down to T1∼8.5mag for these four clusters. Their positions and sizes were determined using the stellar density radial profiles. We derived reddening, distance, age and metallicity of the clusters from extracted (C-T1,T1) color-magnitude diagrams (CMDs), using theoretical isochrones computed for the Washington system. There are no previous photometric data in the optical band for BH84, NGC5381 and BH211. The CMDs of the observed clusters show relatively well defined main sequences, except for Czernik37, wherein significant differential reddening seems to be present. The red giant clump is clearly seen only in BH211. For this cluster, we estimated the age in (1000-200+260)Myr, assuming a metallicity of Z=0.019. BH84 was found to be much older than it was previously believed, while NGC5381 happened to be much younger than previously reported. The heliocentric distances to these clusters are found to range between 1.4 and 3.4kpc. BH84 appears to be located at the solar galactocentric distance, while NGC5381, BH211 and Czernik37 are situated inside the solar ring.

Estimation of the solar galactocentric distance and galactic rotation velocity from near-solar-circle objects

We have tested the method of determining the solar Galactocentric distance R_0 and Galactic rotation velocity $V_0$ modified by Sofue et al. using near-solar-circle objects. The motion of objects relative to the local standard of rest has been properly taken into account. We show that when such young objects as star-forming regions or Cepheids are analyzed, allowance for the perturbations produced by the Galactic spiral density wave improves the statistical significance of the estimates. The estimate of R_0=7.25\pm0.32 kpc has been obtained from 19 star-forming regions. The following estimates have been obtained from a sample of 14 Cepheids (with pulsation periods P>5^d): R_0=7.66\pm0.36 kpc and V_0=267\pm 17 km s^{-1}. We consider the influence of the adopted Oort constant A and the character of stellar proper motions (Hipparcos or UCAC4). The following estimates have been obtained from a sample of 18 Cepheids with stellar proper motions from the UCAC4 catalog: R_0=7.64\pm0.32 kpc and V_0=217\pm 11 km s^{-1}.

ON THE VERTICAL EQUILIBRIUM OF THE LOCAL GALACTIC DISK AND THE SEARCH FOR DISK DARK MATTER

Estimates of the dynamical surface mass density at the solar Galactocentric distance are commonly derived assuming that the disk is in vertical equilibrium with the Galactic potential. This assumption has recently been called into question, based on the claim that the ratio between the kinetic and the gravitational energy in such solutions is a factor of 3 larger than required if Virial equilibrium is to hold. Here we show that this ratio between energies was overerestimated and that the disk solutions are likely to be in Virial equilibrium after all. We additionally demonstrate, using one-dimensional numerical simulations, that the disks are indeed in equilibrium. Hence, given the uncertainties, we find no reason to cast doubt on the steady-state solutions which are traditionally used to measure the matter density of the disk.

NO EVIDENCE FOR A DARK MATTER DISK WITHIN 4 kpc FROM THE GALACTIC PLANE

We estimated the dynamical surface mass density (Sigma) at the solar Galactocentric distance between 2 and 4 kpc from the Galactic plane, as inferred from the observed kinematics of the thick disk. We find Sigma(z=2 kpc)=57.6+-5.8 Mo pc^-2, and it shows only a tiny increase in the z-range considered by our investigation. We compared our results with the expectations for the visible mass, adopting the most recent estimates in the literature for contributions of the Galactic stellar disk and interstellar medium, and proposed models of the dark matter distribution. Our results match the expectation for the visible mass alone, never differing from it by more than 0.8 $Mo pc^-2 at any z, and thus we find little evidence for any dark component. We assume that the dark halo could be undetectable with our method, but the dark disk, recently proposed as a natural expectation of the LambdaCDM models, should be detected. Given the good agreement with the visible mass alone, models including a dark disk are less likely, but within errors its existence cannot be excluded. In any case, these results put constraints on its properties: thinner models (scale height lower than 4 kpc) reconcile better with our results and, for any scale height, the lower-density models are preferred. We believe that successfully predicting the stellar thick disk properties and a dark disk in agreement with our observations could be a challenging theoretical task.

Evolution of the velocity ellipsoids in the thin disk of the Galaxy and the radial migration of stars

Data from the revised Geneva-Copenhagen catalog are used to study the influence of radial migration of stars on the age dependences of parameters of the velocity ellipsoids for nearby stars in the thin disk of the Galaxy, assuming that the mean radii of the stellar orbits remain constant. It is demonstrated that precisely the radial migration of stars, together with the negative metallicity gradient in the thin disk, are responsible for the observed negative correlation between the metallicities and angular momenta of nearby stars, while the angular momenta of stars that were born at the same Galactocentric distances do not depend on either age or metallicity. The velocity components of the Sun relative to the Local Standard of Rest derived using data for stars born at the solar Galactocentric distance are (U ⊙, V ⊙, W ⊙) LSR = (5.1 ± 0.4, 7.9 ± 0.5, 7.7 ± 0.2) km/s. The two coordinates of the apex of the solar motion remain equal to 〈l ⊙〉 = 70° ± 7° and 〈b ⊙〉 = 41° ± 2°, within the errors. The indices for the power-law age dependences of them ajor, middle, and minor semi-axes become 0.26±0.04, 0.32±0.03, and 0.07±0.03, respectively. As a result, with age, the velocity ellipsoid for thin-disk stars born at the solar Galactocentric distance increases only in the plane of the disk, remaining virtually constant in the perpendicular direction. Its shape remains far from equilibrium, and the direction of the major axis does not change with age: the ellipsoid vertex deviation remains constant and equal to zero within the errors (〈L〉 = 0.7° ± 0.6°, 〈B〉 = 1.9° ± 1.1°). Such a small increase in the velocity dispersion perpendicular to the Galactic plane with age can probably be explained by “heating” of the stellar system purely by spiral density waves, without a contribution from giant molecular clouds.

Regularities in the distribution of star/gas complexes in the spiral arms of our galaxy and M31

The fragmentation of gaseous spiral arms in the outer Galaxy into superclouds has been studied using recently published data on the HI distribution in the Galactic disk. Regular chains of superclouds have been found or confirmed in the Cygnus (Outer) and Carina arms, with the spacings between the superclouds being concentrated near 0.1 and 0.2 of the solar Galactocentric distance. The star complexes in the northwestern arm of the galaxy M31 are spaced, on average, 1.2 kpc apart, with the most distinct chain of complexes being located in the arm region where Beck et al. (1989) detected a strong and wavy (along the arm) magnetic field. Its wavelength turns out to be related to the spacing between the complexes. In this arm, the HII regions lie inside the star complexes, which, in turn, are located inside the gas-dust lane. In contrast, the southwestern arm of M31 is split into a gas-dust lane and a dense stellar arm, which is not fragmented into star complexes. Here, the HII regions are located along the boundary between the gas-dust and stellar components of the arm; other evidence for the presence of a spiral shock wave triggering star formation is also observed, which is probably attributable to the large pitch angle of this segment of the southwestern arm. It may be suggested that the shock wave rapidly leads to star formation everywhere in this arm, while in the northwestern arm, where the shock wave is absent, star formation begins in the superclouds formed along the arm by the magneto-gravitational instability. This is how the chains of star complexes in the northwestern arm of M31 and, obviously, the chains of superclouds in the Carina and Cygnus arms of our Galaxy have been formed. The detection of a regularmagnetic field in the corresponding segments of these arms can be predicted.

The kinematics and zero-point of the log P-〈MK〉 relation for Galactic RR Lyrae variables via statistical parallax

We use accurate absolute proper motions and Two-Micron All-Sky Survey K s -band apparent magnitudes for 364 Galactic RR Lyrae variables to determine the kinematical parameters of the Galactic RR Lyrae population and constrain the zero-point of the K s -band period-luminosity relation for these stars via statistical parallax. We find the mean velocities of the halo- and thick-disc RR Lyrae populations in the solar neighbourhood to be [U o (Halo), V 0 (Halo), W 0 (Halo)] = (―12 ± 10, ―217 ± 9, ―6 ± 6) km s ―1 and [U 0 (Disc), V 0 (Disc), W 0 (Disc)] = (―15 ± 7, ―44 ± 7, ―25 ± 5) km s ―1 , respectively, and the corresponding components of the velocity-dispersion ellipsoids, [σ V R (Halo), σ V θ (Halo), σ W(Halo)] = (167 ± 9, 86 ± 6, 78 ± 5) km s ―1 and [σ V R (Disc), σ V θ (Disc), σ W(Disc)] = (55 ± 7, 44 ± 6, 30 ± 4) km s ―1 , respectively. The fraction of thick-disc stars is estimated at 0.25 ± 0.03. The corrected infrared period-luminosity relation is = ― 2.33 log P F ― 0.818 ± 0.081, implying a Large Magellanic Cloud (LMC) distance modulus of 18.27 ± 0.08 and a solar Galactocentric distance of 7.58 ± 0.40 kpc. Our results suggest no or slightly prograde rotation for the population of halo RR Lyraes in the Milky Way.

The transition between the thick and thin Galactic disks

AbstractWe study the transition between the thick and thick disks using solar neighbourhood data, focusing in particular on the status of local metal-poor thin disk stars ([Fe/H]<−0.3 dex, [α/Fe]<0.1 dex). The orbital properties of these stars, which are responsible for the hiatus in metallicity between the two disks, suggest that they most likely originate from the outer Galactic thin disk. It implies that the transition between the two stellar populations at a solar galactocentric distance must have occurred at a metallicity of about −0.3 dex. Transition stars at this metallicity are in fact present in local samples and fill the gap in α-element between the thick and thin disks. These results imply that, at least from the local data, there is a clear evolutionary link between the thick and thin disks.