Solar Galactocentric Distance Research Articles

Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: II. The Molecular Gas Disk

Abstract We created a three-dimensional distribution map of molecular gas throughout the Milky Way galaxy using the latest ${{12\atop} \mathrm{CO}}$ ($J = 1 \hbox{--} 0$) survey data cube and the rotation curve based on the kinematic distance. The radial distribution of the molecular gas shows a central peak and a second peak around $0.5 \,R_0$ ($R_0$: the solar Galactocentric distance). The thickness of the molecular disk slightly increases from 48 pc to 160 pc with the Galactocentric distance within a radius range of 0–11 kpc. We were able to trace the Outer, the Perseus, the Sagittarius–Carina, the Scutum–Crux, and the Norma arms as logarithmic spiral arms with pitch angles of $11 {{}^{\circ }} \hbox{--} 15^\circ$. Considering that the pitch angles of the spiral arms are within this range, the Norma and the Outer arms seem to be identified as the same spiral arm. We could also trace a midplane displacement, whose amplitude is nearly constant inside a 10 kpc radius and increases beyond this radius. The ridges of the midplane displacement form the leading spiral arms.

Galactic parameters derived from open-cluster data

The components U 0 and V 0 of the solar motion and the Oort constant A 0 are determined using the data of a homogeneous open-cluster catalog with corrected distance moduli. The results are based on a sample of 146 open clusters with known radial velocities located in the Galactic plane (b<7°) within 4 kpc of the Sun. The solar Galactocentric distance R 0 is determined using two kinematic methods. The following results are obtained: A 0=17.0±0.9 km/s kpc, U 0=10.5±1.0 km/s, V 0=11.5±1.1 km/s, R 0=8.3±0.3 pc.

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

AbstractThe kinematical parameters of the local field RR Lyrae population and the zero point of the log P –〈MK〉 relation for these variables are inferred by applying the statistical parallax (maximum-likelihood) technique to a sample of 379 RR Lyrae stars with known periods, radial-velocities, metallicities, K-band photometry, and absolute proper motions on the ICRS system. Hipparcos, Tycho-2, SPM, UCAC1 and NPM1 were used as the sources of proper motions. The K-band magnitudes were adopted from the 2MASS All-Sky Data Release. The parameters of the velocity distribution are found to be (U0 = −13 ± 9, V0 = −41 ± 7, W0 = −19±4) kms−1, (σU = 66±9, σV = 47±7, σW = 23±5) km s−1 and (U0 = −11±10, V0 = −224±8, W0 = −8±6) km s−1 (σU = 158±9, σV = 95±5, σW = 88±5) km s−1 for the thick-disk (the purest disk sample, 56 stars) and halo (the purest halo sample, 264 stars) objects, respectively. The zero point of the infrared PL relation of Jones et al. (1992 – based on the results obtained using the Baade-Wesselink method) is confirmed: we find 〈MK〉 = −2.33⋅log PF−0.89±0.09, which is only brighter than found using the Baade-Wesselink method (Jones et al. 1992). A conversion of the resulting log P –〈MK〉 relation to V-band luminosities yields the metallicity-luminosity relation 〈MV〉 = +1.12 + 0.18 ⋅ [Fe/H] ± 0.10. Our results imply a solar Galactocentric distance of R0 = 7.8±0.4 kpc and an LMC distance modulus of DMLMC = 18.17 ±0.10 (cluster RR Lyraes) or DMLMC = 18.10±0.10 (field RR Lyraes), thereby favoring the so-called short distance scale.

On the oxygen abundance in our Galaxy

The compilation of published spectra of Galactic HII regions with available diagnostic [OIII]4363 line has been carried out. Our list contains 71 individual measurements of 13 HII regions in the range of galactocentric distances from 6.6 to 14.8 kpc. The oxygen abundances in all the HII regions were recomputed in the same way, using the classic Te - method. The oxygen abundance at the solar galactocentric distance traced by those HII regions is in agreement with the oxygen abundance in the interstellar medium in the solar vicinity derived with high precision from the interstellar absorption lines towards stars. The derived radial oxygen abundance distribution was compared with that for HII regions from the Shaver et al. (1983) sample which is the basis of many models for the chemical evolution of our Galaxy. It was found that the original Shaver et al.'s oxygen abundances are overestimated by 0.2-0.3 dex. Oxygen abundances in HII regions from the Shaver et al. sample have been redetermined with the recently suggested P - method. The radial distribution of oxygen abundances from the Shaver et al. sample redetermined with the P - method is in agreement with our radial distribution of (O/H)_Te abundances.

The log P - $\langle M_{K}\rangle$ Relation of Galactic Field RR Lyrae Variables

The kinematical parameters of the local field RR Lyrae population and the zero-point of the log relation for these variables are inferred by applying the statistical parallax (maximum-likeli-hood) technique to a sample of 182 RR Lyraes with known periods, radial-velocities, metallicities, K-band photometry, and absolute proper motions on the ICRS system. Hipparcos, Tycho-2, SPM, UCAC, NPM1, and the Four-Million Star Catalog (Volchkov et al. 1992) were used as the sources of proper motions; the proper motions of the last two catalogs are reduced to the Hipparcos (ICRS) system (Dambis & Rastorguev 2001). The K-band magnitudes were adopted from the list of Fernley et al. (1998) and supplemented by the data of the 2MASS Second Incremental Data Release. The parameters of the velocity distribution are found to be: (, , km s-1; (, , ) km s-1: and (, , ) km s-1; (, , ) km s-1 for the thick-disk (41 stars) and halo (141 stars) objects, respectively. The zero-point of the infrared PL relation of Jones et al. (1992) (based on the results obtained using the Baade-Wesselink method) is confirmed: we find compared to as inferred by Jones et al. (1992). A conversion of the resulting log relation to V-band luminosities yields the metallicity-luminosity relation Fe/H. Our results imply a solar Galactocentric distance of kpc and an LMC distance modulus of (cluster RR Lyraes) or (field RR Lyraes), thereby favoring the so-called short distance scale.

Dynamics of the Galactic Bulge Using Planetary Nebulae

Evidence for a bar at the center of the Milky Way triggered a renewed enthusiasm for dynamical modeling of the Galactic bar-bulge. Our goal is to compare the kinematics of a sample of tracers, planetary nebulae, widely distributed over the bulge with the corresponding kinematics for a range of models of the inner Galaxy. Three of these models are N-body barred systems arising from the instabilities of a stellar disk (Sellwood, Fux, and Kalnajs), and one is a Schwarzschild system constructed to represent the three-dimensional distribution of the COBE/DIRBE near-IR light and then evolved as an N-body system for a few dynamical times (Zhao). For the comparison of our data with the models, we use a new technique developed by Saha. The procedure finds the parameters of each model, i.e., the solar galactocentric distance R0 in model units, the orientation angle , the velocity scale (in km s-1 per model unit), and the solar tangential velocity that best fit the data.

Oxygen and helium abundances in Galactic H II regions -- II. Abundance gradients

The absolute integrated fluxes that we obtained (Paper I) for 34 H ii regions, with Galactocentric distances RG in the 6.6–17.7 kpc range, in the emission lines [O ii] λλ3726 and 3729, [O iii] λλ4363 and 5007, He iλ5876, Hα and Hβ, are analysed to derive the extinctions, the electron densities and temperatures, the ionic abundances O+/H+, O++/H+ and He+/H+ and the O/H abundances. The electron temperature has been derived from the [O iii] λ4363/λ5007 ratio in six H ii regions with RG between 6.6 and 14.8 kpc. These new measurements of Te, which are in good agreement with those from radio recombination lines, widen the RG range for which reliable Te measurements exist. Combining our new Te([O iii]) results with the radio values, we obtain the following temperature relationship: TeK]=(372±38)RG+4260±350. The O+/H+ and O++/H+ abundances have been obtained assuming a two-temperature H ii region model. Our O/H relationship for 5 kpc<RG<15 kpc is 12+log O/H= (−3.95±0.49)×10−2RG+(8.82±0.05). The slope is lower, by a factor of 2, than that previously obtained by Shaver et al. No significant flattening of this relation is obtained out to 15 kpc. At the solar Galactocentric distance, 12+log(O/H)=8.48, in good agreement with the O/H abundance measured in the local interstellar medium by Meyer et al., and a factor of 2 lower than the solar abundance. Our helium observations show that even a region ionized by a star earlier than O6.5 may contain a significant amount of neutral helium. We confirm the high ΔY/Δ(O/H) value (≥180) measured in the Galaxy by Peimbert et al.

On non-baryonic dark objects in galactic haloes

Taking into account that a hierarchical pattern of clustering appears in cold dark matter scenarios, the existence of cold dark matter substructure in galactic haloes is explored. For this purpose, well-known dynamical constraints are imposed on non-baryonic objects, at solar Galactocentric distance. In this paper, aggregates of axions, aggregates of fermions and clusters of these aggregates have been considered, and ranges for their masses and radii have been estimated. Non-baryonic objects with radii and masses different from those allowed must have been disrupted by dynamical effects, so that their components must be orbiting freely within the Galactic halo. Upper limits for the ratio between the mass (per volume unit) that remains bound to dark objects and the cold dark matter total mass (per volume unit) in the Galactic halo have also been estimated.

A determination of the thick disk chemical abundance distribution: Implications for galaxy evolution

We present a determination of the thick disk iron abundance distribution obtained from an in situ sample of F/G stars. These stars are faint, $15 \simlt V \simlt 18$, selected on the basis of color, being a subset of the larger survey of Gilmore and Wyse designed to determine the properties of the stellar populations several kiloparsecs from the Sun. The fields studied in the present paper probe the iron abundance distribution of the stellar populations of the Galaxy at 500--3000pc above the plane, at the solar Galactocentric distance.

On local stellar age distributions and the history of the stellar birthrate in the galactic disk

AbstractThe fraction of nearby stars born at different galactocentric radii is roughly estimated using the epicyclic approximation of stellar orbits in the galactic disk. Especially, the question of a possible burst origin of features in the local stellar age distribution is discussed. If the “bump” at about 4 Gyr in the frequency distribution of stellar isochrone ages (MEUSINGER 1991) is explained by star formation events then they have been locally at the solar galactocentric distance or globally in the whole disk.

Problems in determining the surface density of the Galactic disk

A new method is presented for determining the local surface density of the Galactic disk from distance and velocity measurements of stars toward the Galactic poles. The procedure is fully three-dimensional, approximating the Galactic potential by a potential of Staeckel form and using the analytic third integral to treat the tilt and the change of shape of the velocity ellipsoid consistently. Applying the procedure to artificial data superficially resembling the K dwarf sample of Kuijken and Gilmore (1988, 1989), it is shown that the current best estimates of local disk surface density are uncertain by at least 30 percent. Of this, about 25 percent is due to the size of the velocity sample, about 15 percent comes from uncertainties in the rotation curve and the solar galactocentric distance, and about 10 percent from ignorance of the shape of the velocity distribution above z = 1 kpc, the errors adding in quadrature. Increasing the sample size by a factor of 3 will reduce the error to 20 percent. To achieve 10 percent accuracy, observations will be needed along other lines of sight to constrain the shape of the velocity ellipsoid.