We have carried out a multiwavelength study of the plane of our Galaxy in order to establish a star-forming-complex catalogue which is as complete as possible. Features observed include H, H109, CO, the radio continuum and absorption lines. For each complex we have determined the position, the systemic velocity, the kinematic distance and, when possible, the stellar distance and the corresponding uncertainties. All of these parameters were determined as homogeneously as possible, in particular all the stellar distances have been (re)calculated with the same calibration and the kinematic distances with the same mean Galactic rotation curve. Through the complexes with stellar distance determination, a rotation curve has been fitted. It is in good agreement with the one of Brand & Blitz (1993). We also investigated the residual velocities relative to the circular rotation model. We find that departures exist over large areas of the arms, with dierent values from one arm to another. From our data and in good agreement with previous studies, the Galactic warp is observed. It does not seem correlated with the departures from circular rotation. Finally, as segment-like features are noted from the complexes' distribution, we tried to find if they are indicative of a larger underlying structure. Then, we attempted to interpret the complexes' distribution in terms of spiral structure by fitting models with two, three and four logarithmic spiral arms. The four-arm model seems more appropriate to represent the grand design of our Galaxy. In this model the Norma arm and the external arm appear as being the two extremities of a single arm called the Norma-Cygnus arm. The new data and fitted model confirm the four-segment model of Georgelin & Georgelin (1976), clarifying the arms' design and extension and doubling their known length.

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Star-forming complexes and the spiral structure of our Galaxy

The Oort constants describe the local spatial variations of the stellar streaming field. The classic way for their determination employs their effect on stellar proper motions. We discuss various problems arising in this procedure. A large, hitherto apparently overlooked, source of potential systematic error arises from longitudinal variations of the mean stellar parallax, caused by intrinsic density inhomogeneities and interstellar extinction. Together with the reflex of the solar motion, these variations by mode mixing create contributions to the longitudinal proper motions (l) that are indistinguishable from the Oort constants at 20% of their amplitude. Fortunately, we can correct for this mode mixing using the latitudinal proper motions μb(l). We use about 106 stars from the ACT/Tycho-2 catalogs brighter than V ≈ 11 with median proper-motion error of ≈3 mas yr-1, taking every precaution to avoid or correct for the various sources of systematic error, significant deviations from expectations based on a smooth axisymmetric equilibrium, in particular nonzero C for old red giant stars. We also find variations of the Oort constants with the mean color, which correlate nicely with the asymmetric drift of the subsample considered. In addition, these correlations are different in nature than those expected for an axisymmetric galaxy. The most reliable tracers for the "true" Oort constants are red giants, which are old enough to be in equilibrium and distant enough to be unaffected by possible local anomalies. For these stars we find, after correction for mode mixing and the axisymmetric asymmetric-drift effects, A ≈ 16, B ≈ -17, A - B ≈ 33, and C ≈ -10 km s-1 kpc-1 with internal errors of about 1-2 and external error of perhaps the same order. These values are consistent with our knowledge of the Milky Way (flat rotation curve and Ω ≡ A - B ≈ 28 ± 2), based on observations made with the ESA Hipparcos astrometry satellite.

https://iopscience.iop.org/article/10.1086/379278/pdf

The Oort Constants Measured from Proper Motions

The Gaia mission is designed as a Galaxy explorer, and will measure simultaneously, in a survey mode, the five or six phase-space parameters of all stars brighter than 20th magnitude, as well as providing a description of their astrophysical characteristics. These measurements are obtained by combining an astrometric instrument with micro-arcsecond capabilities, a photometric system giving the magnitudes and colours in 15 bands and a medium-resolution spectrograph named the Radial Velocity Spectrometer (RVS). The latter instrument will produce spectra in the 848- to 874-nm wavelength range, with a resolving power R = 11500, from which radial velocities, rotational velocities, atmospheric parameters and abundances can be derived. A companion paper has presented the characteristics of the RVS and its performance. The present paper details the outstanding scientific impact of this important part of the Gaia satellite on some key open questions in present-day astrophysics. The unbiased and simultaneous acquisition of multi-epoch radial velocities and individual abundances of key elements in parallel with the astrometric parameters is essential for the determination of the dynamical state and formation history of our Galaxy. Moreover, for stars brighter than V similar or equal to 15, the resolving power of the RVS will give information about most of the effects that influence the position of a star in the Hertzsprung-Russell diagram, placing unprecedented constraints on the age, internal structure and evolution of stars of all types. Finally, the RVS multi-epoch observations are ideally suited to the identification, classification and characterization of the many types of double, multiple and variable stars.

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Spectroscopic Survey of the Galaxy with Gaia II. The expected science yield from the Radial Velocity Spectrometer

Analyzing Hipparcos proper motions of 1352 O–B5 stars other than the Gould belt stars, which are representative of the young disk population, we have found a clear stellar warping motion that is a systematic rotation +3.8 ± 1.1 km s-1 kpc-1 of stars about the axis pointing to the Galactic center in the sense of increasing the inclination of the H I warp, and a remarkably large Galactic rotation of (V0)O–B5 = 268.7 ± 11.9 km s-1, compared with the IAU recommendation (V0 = 220 ± 20 km s-1), given the Galactocentric distance of the Sun R0 = 8.5 kpc. We have carried out a similar analysis for 170 Hipparcos Cepheids as well and obtained a solution that apparently shows neither the rotation nor the shear, other than the Oort differential rotation. The Cepheids are purely rotating around the Galactic center with the velocity (V0)Cep = 243.3 ± 12.0 km s-1, again larger than the IAU recommendation. However, the solution for the Cepheids, which are considered the same young disk population as the O–B5 stars, seems to be different from that for the O–B5 stars. In order to find the above systematic stellar motion as generally as possible, we apply the Ogorodnikov-Milne model to the Hipparcos proper motions, and solve for nine kinematic parameters: three components of solar motion, three components of vorticity (rotation), and three components of strain velocity (shear). This paper discusses the systematic difference in the proper-motion systems between the ground-based catalogs and the Hipparcos Catalogue, in order to examine one of the main causes of the large difference between the present (270 km s-1) and previous (220 km s-1) Galactic rotations.

Galactic Interior Motions Derived from Hipparcos Proper Motions. I. Young Disk Population

We present a new model of the four Galilean satellites Io, Europa, Ganymede and Callisto, able to deliver accurate ephemerides over a very long time span (several centuries). In the first paper (Lainey et al. 2004, A&A, 420, 1171) we gave the equations of the dynamical model. Here we present the fit of this model to the observations, covering more than one century starting from 1891. Our ephemerides, based on this first fit called L1, are available on the web page of the IMCCE at the URL http://www.imcce.fr/ephemeride_eng.html.

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New accurate ephemerides for the Galilean satellites of Jupiter - II. Fitting the observations

The systematic stellar velocity field of about 30 000 K-M giants in the heliocentric distance interval 0.5-1.0 kpc is examined, on the basis of the three-dimensional Ogorodnikov-Milne model for the overall pattern analysis of the proper motions. The basic material of this investigation consists of the proper motions provided by the astrometric catalogue, ACRS Part 1, and of the spectroscopic and photometric data given by SKYMAP.

Is the vorticity vector of the galaxy perpendicular to the galactic plane? I. Precessional correction and equinoctial motion correction to the FK5 system

A warping motion inherent in the galactic warp is detected, based on the proper motions of about 2000 young generation stars which consist of O-B5 stars, supergiants and bright giants of any spectral type in the heliocentric distance interval 0.5 to 3.0 kpc. To examine the warping motion, the overall pattern analysis of the proper motions of these stars is performed on the basis of the three-dimensional Ogorodnikov-Milne model, instead of the classical Oort-Lindblad model. It is found that the kinematics of these stars is quite different from that of K-M giants which, as has been demonstrated in the previous work (Paper I), show only the familiar plane-parallel galactic rotation described by the Oort constants A and B

Is the vorticity vector of the Galaxy perpendicular to the Galactic plane? II - Kinematics of the Galactic warp

This paper derives midtimes and relative satellite positions from nearly 200 light curves of mutual eclipses and occultations of the Galilean satellites that occurred in 1985. It is shown that, at least for observations of the highest quality, the standard errors on the midtimes are smaller than 1 s and close to 0.01 arcsec on the positional offsets in right ascension and declination. 12 refs.

An Analysis of the 1985 Observations of Mutual Phenomena of the Galilean Satellites

Commission 4 was among the first set of commissions formed within the IAU at its founding in 1919. (Commissions were originally called “Standing Committees.”) During its 96 years of service to the IAU and astronomical community in general, the commission has been fortunate to have been led by many distinguished scientists — see the list of presidents below.

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Mitsuru Soma

Papers

Is the vorticity vector of the galaxy perpendicular to the galactic plane? I. Precessional correction and equinoctial motion correction to the FK5 system

Is the vorticity vector of the Galaxy perpendicular to the Galactic plane? II - Kinematics of the Galactic warp

An Analysis of the 1985 Observations of Mutual Phenomena of the Galilean Satellites

Historical reflections on the work of commission 4

Ten Years of the Tokyo Photoelectric Meridian Circle (Tokyo PMC) : An activity report for the years from 1982 to 1993