Perseus Arm Research Articles

PSRs J0248+6021 and J2240+5832: young pulsars in the northern Galactic plane

Pulsars PSR J0248+6021 (rotation period P=217 ms and spin-down power Edot = 2.13E35 erg/s) and PSR J2240+5832 (P=140 ms, Edot = 2.12E35 erg/s) were discovered in 1997 with the Nancay radio telescope during a northern Galactic plane survey, using the Navy-Berkeley Pulsar Processor (NBPP) filter bank. GeV gamma-ray pulsations from both were discovered using the Fermi Large Area Telescope. Twelve years of radio and polarization data allow detailed investigations. The two pulsars resemble each other both in radio and in gamma-ray data. Both are rare in having a single gamma-ray pulse offset far from the radio peak. The high dispersion measure for PSR J0248+6021 (DM = 370 pc cm^-3) is most likely due to its being within the dense, giant HII region W5 in the Perseus arm at a distance of 2 kpc, not beyond the edge of the Galaxy as obtained from models of average electron distributions. Its high transverse velocity and the low magnetic field along the line-of-sight favor this small distance. Neither gamma-ray, X-ray, nor optical data yield evidence for a pulsar wind nebula surrounding PSR J0248+6021. The gamma-ray luminosity for PSR J0248+6021 is L_ gamma = (1.4 \pm 0.3)\times 10^34 erg/s. For PSR J2240+5832, we find either L_gamma = (7.9 \pm 5.2) \times 10^34 erg/s if the pulsar is in the Outer arm, or L_gamma = (2.2 \pm 1.7) \times 10^34 erg/s for the Perseus arm. These luminosities are consistent with an L_gamma ~ sqrt(Edot) rule. Comparison of the gamma-ray pulse profiles with model predictions, including the constraints obtained from radio polarization data, favor emission in the far magnetosphere. These two pulsars differ mainly in their inclination angles and acceleration gap widths, which in turn explains the observed differences in the gamma-ray peak widths.

PHYSICAL PROPERTIES AND GALACTIC DISTRIBUTION OF MOLECULAR CLOUDS IDENTIFIED IN THE GALACTIC RING SURVEY

We derive the physical properties of 580 molecular clouds based on their 12CO and 13CO line emission detected in the University of Massachusetts-Stony Brook (UMSB) and Galactic Ring surveys. We provide a range of values of the physical properties of molecular clouds, and find a power-law correlation between their radii and masses, suggesting that the fractal dimension of the ISM is around 2.36. This relation, M = (228 +/- 18) R^{2.36+/-0.04}, allows us to derive masses for an additional 170 GRS molecular clouds not covered by the UMSB survey. We derive the Galactic surface mass density of molecular gas and examine its spatial variations throughout the Galaxy. We find that the azimuthally averaged Galactic surface density of molecular gas peaks between Galactocentric radii of 4 and 5 kpc. Although the Perseus arm is not detected in molecular gas, the Galactic surface density of molecular gas is enhanced along the positions of the Scutum-Crux and Sagittarius arms. This may indicate that molecular clouds form in spiral arms and are disrupted in the inter-arm space. Last, we find that the CO excitation temperature of molecular clouds decreases away from the Galactic center, suggesting a possible decline in the star formation rate with Galactocentric radius. There is a marginally significant enhancement in the CO excitation temperature of molecular clouds at a Galactocentric radius of about 6 kpc, which in the longitude range of the GRS corresponds to the Sagittarius arm. This temperature increase could be associated with massive star formation in the Sagittarius spiral arm.

COBEAND THE GALACTIC INTERSTELLAR MEDIUM: GEOMETRY OF THE SPIRAL ARMS FROM FIR COOLING LINES

We present a new model for the spiral structure of the Milky Way based upon the essentially all-sky intensity maps of the [C II] 158 {mu}m and [N II] 205 {mu}m lines of the interstellar medium (ISM) obtained by the FIRAS instrument of the Cosmic Background Explorer (COBE), with ancillary data from the Balloon-borne Infrared Carbon Explorer, and Infrared Space Observatory. These lines are important coolants of the ISM and strong tracers of the spiral structure. The model provides the volume emissivity of these species as a function of position within the Galaxy. Two-, three-, and four-arm models are examined, using a number of spiral functional forms. Two-arm models are found to be inconsistent with the COBE/FIRAS data. A three-arm model can be constructed that reproduces the [C II] and [N II] intensity profiles along the Galactic plane. This model, however, is discounted by historical observations of the Perseus and Cygnus ('Outer') arms. A four-arm model, with arms defined by logarithmic spiral forms, reproduce the observations extremely well. Models of the Milky Way's spiral geometry proposed from {approx}1980 to the present are examined in light of the COBE data and compared with the model presented herein. The preponderance of themore » evidence supports the existence of four well-defined logarithmic spiral arms in the gaseous component of the ISM. We note that essentially all two-arm models proposed since the mid-1980s are based upon observations of older evolved stars. We address the question of why studies based upon observations of stellar densities yield two-arm models while models based upon observations of more traditional tracers of spiral arms, i.e., enhanced gas and dust densities, star formation, and young stellar populations, yield four-arm models.« less

The Red MSX Source survey: distribution and properties of a sample of massive young stars

The Red MSX Source (RMS) survey has identified a large sample of massive young stellar objects (MYSOs) and ultra compact (UC) HII regions from a sample of ~2000 MSX and 2MASS colour selected sources. Using a recent catalogue of molecular clouds derived from the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (GRS), and by applying a Galactic scaleheight cut off of 120 pc, we solve the distance ambiguity for RMS sources located within 18\degr < |l| > 54\degr. These two steps yield kinematic distances to 291 sources out of a possible 326 located within the GRS longitude range. Combining distances and integrated fluxes derived from spectral energy distributions, we estimate luminosities to these sources and find that > 90% are indicative of the presence of a massive star. We find the completeness limit of our sample is ~10^4 Lsun, which corresponds to a zero age main sequence (ZAMS) star with a mass of ~12 Msun. Selecting only these sources, we construct a complete sample of 196 sources. Comparing the properties of the sample of young massive stars with the general population, we find the RMS-clouds are generally larger, more massive, and more turbulent. We examine the distribution of this sub-sample with respect to the location of the spiral arms and the Galactic bar and find them to be spatially correlated. We identify three significant peaks in the source surface density at Galactocentric radii of approximately 4, 6 and 8 kpc, which correspond to the proposed positions of the Scutum, Sagittarius and Perseus spiral arms, respectively. Fitting a scale height to the data we obtain an average value of ~29+-0.5 pc, which agrees well with other reported values in the literature, however, we note a dependence of the scale height on galactocentric radius with it increases from 30 pc to 45 pc between 2.5 and 8.5 kpc.

PHOTOSPHERIC RADIUS EXPANSION DURING MAGNETAR BURSTS

On August 24th 2008 the new magnetar SGR 0501+4516 (discovered by SWIFT) emitted a bright burst with a pronounced double-peak structure in hard X-rays, reminiscent of the double-peak temporal structure seen in some bright thermonuclear bursts on accreting neutron stars. In the latter case this is due to Photospheric Radius Expansion (PRE): when the flux reaches the Eddington limit, the photosphere expands and cools so that emission becomes softer and drops temporarily out of the X-ray band, re-appearing as the photosphere settles back down. We consider the factors necessary to generate double-peaked PRE events, and show that such a mechanism could plausibly operate in magnetar bursts, despite the vastly different emission process. Identification of the magnetic Eddington limit in a magnetar would constrain magnetic field and distance and could, in principle, enable a measurement of gravitational redshift. It would also locate the emitting region at the neutron star surface, constraining the burst trigger mechanism. Conclusive confirmation of PRE events will require more detailed radiative models for bursts. However for SGR 0501+4516 the predicted critical flux (using the magnetic field strength inferred from timing and the distance suggested by its probable location in the Perseus arm of our Galaxy) is consistent with that observed in the August 24th burst.

Characterization of 15 overlooked Ruprecht clusters with ages within 400 Myr and 3 Gyr

We derive fundamental, structural, and photometric parameters of 15 overlooked Ruprecht (hereafter Ru) star clusters by means of 2MASS photometry and field-star decontamination. Ru\,1, 10, 23, 26, 27, 34, 35, 37, 41, 54, 60, 63, 66, and 152 are located in the third Galactic quadrant, while Ru\,174 is in the first. With the constraints imposed by the field-decontaminated colour-magnitude diagrams (CMDs) and stellar radial density profiles (RDPs), we derive ages in the range 400\,Myr --- 1\,Gyr, except for the older Ru\,37, with $\sim3$\,Gyr. Distances from the Sun are within $\rm1.5\la\ds(kpc)\la8.0$. The RDPs are well-defined and can be described by a King-like profile for most of the radial range, except for Ru\,23, 27, 41, 63, and 174, which present a conspicuous stellar density excess in the central region. The clusters dwell between (or close to) the Perseus and Sagittarius-Carina arms. We derive evidence in favour of cluster size increasing with distance to the Galactic plane ($\zgc$), which is consistent with a low frequency of tidal stress associated with high-$|\zgc|$ regions. The clusters are rather faint even in the near-infrared, with apparent integrated \jj\ magnitudes within $6.4\la m_J\la9.8$, while their absolute magnitudes are $-6.6\la M_J\la-2.6$. Extrapolation of the relation between $M_V$ and $M_J$, derived for globular clusters, suggests that they are low-luminosity optical clusters, with $-5\la M_V\la-1$.

A synthetic 21-cm Galactic Plane Survey of a smoothed particle hydrodynamics galaxy simulation

We have created synthetic neutral hydrogen (HI) Galactic Plane Survey data cubes covering 90 degrees < l < 180 degrees, using a model spiral galaxy from SPH simulations and the radiative transfer code TORUS. The density, temperature and other physical parameters are fed from the SPH simulation into TORUS, where the HI emissivity and opacity are calculated before the 21-cm line emission profile is determined. Our main focus is the observation of Outer Galaxy `Perseus Arm' HI, with a view to tracing atomic gas as it encounters shock motions as it enters a spiral arm interface, an early step in the formation of molecular clouds. The observation of HI self-absorption features at these shock sites (in both real observations and our synthetic data) allows us to investigate further the connection between cold atomic gas and the onset of molecular cloud formation.

Distance to the SNR CTB109/AXP 1E 2259+586 by Hiabsorption and self-absorption

AbstractWe suggest a revised distance to the supernova remnant G109.1−1.0 (CTB109) and its associated anomalous X-ray pulsar (AXP)1E2259+586 by analysing 21-cm Hi-line and 12CO-line spectra of CTB109, Hii region Sh152 and the adjacent molecular cloud complex. CTB109 has been established to be interacting with a large molecular cloud (recession velocity at v=−55kms−1). The highest radial velocities of absorption features towards CTB109 (−56kms−1) and Sh152 (−65kms−1) are larger than the recombination line velocity (−50kms−1) of Sh152 demonstrating the velocity reversal within the Perseus Arm. The molecular cloud has cold Hi column density large enough to produce Hi self-absorption (HISA) and Hi narrow self-absorption (HINSA) if it was at the near side of the velocity reversal. Absence of both HISA and HINSA indicates that the cloud is at the far side of the velocity reversal within the Perseus Arm, so we obtain a distance for CTB109 of 4 ± 0.8kpc. The new distance still leads to a normal explosion energy for CTB 109/AXP 1E 2259+586.

H i 21 cm opacity fluctuations power spectra towards Cassiopeia A

Abstract The angular power spectrum of Hi 21 cm opacity fluctuations is a useful statistic for quantifying the observed opacity fluctuations as well as for comparing these with theoretical models. We present here the Hi 21cm opacity fluctuation power spectrum towards the supernova remnant Cas A from interferometric data with spacial resolution of 5arcsec and spectral resolution of 0.4 km s−1. The power spectrum has been estimated using a simple but robust visibility-based technique. We find that the power spectrum is well fitted by a power law Pτ(U) =Uα with a power-law index of α∼−2.86 ± 0.10 (3σ error) over the scales of 0.07–2.3 pc for the gas in the Perseus spiral arm and 0.002–0.07 pc (480–15730 au) for that in the local arm. This estimated power-law index is consistent with earlier observational results based on both Hi emission over larger scales and absorption studies over a similar range of scales. We do not detect any statistically significant change in the power-law index with the velocity width of the frequency channels. This constrains the power-law index of the velocity structure function to be β= 0.2 ± 0.6 (3σ error).

VERA Observations of H$_2$O Maser Sources in Three Massive Star-Forming Regions and Galactic Rotation Measurements

We present the results of VLBI observations of three H$_2$O maser sources in massive star-forming regions: IRAS 06058$+$2138, IRAS 19213$+$1723, and AFGL 2789. We determined the annual parallaxes and proper motions for these sources by a phase-referencing method. The estimated distances to IRAS 06058$+$2138 and AFGL 2789 are 1.76$\pm$0.11 kpc and 3.07$\pm$0.30 kpc, which are located in the Perseus spiral arm. The distance to IRAS 19213$+$1723 is 3.98$\pm$0.57 kpc, placing it in the Carina–Sagittarius arm. With estimated 3-dimensional velocity components, we have shown that the galactic rotation curve is flat in the range of 6.4 kpc $\le R \le$ 13 kpc as combining with S 269, which was observed with VERA. We have also found that the overall rotation curve of the Galaxy do not depend on $\Theta_0$. The maser sources also show deviations from flat rotation curves by a few kilometers per second. We discuss possible reasons for these peculiar motions.

NGC 2309: A Relatively Young Open Cluster Projected onto a Random Stellar Concentration

ABSTRACTWe have obtained CCD UBVIKC photometry down to V ∼ 22.0 for the open cluster NGC 2309 and its surrounding field. Twenty three probable cluster members have been identified for the first time on the basis of sound photometric criteria. Because NGC 2309 is a relatively poor cluster projected onto a rich star field showing density fluctuations, the frequently used membership criteria based on stellar density profiles or proper motion measurements would not be enough to assess the star membership status if employed independently. We estimated a cluster radius of 7.9′ and a radius at half the maximum of the cluster density profile of 1.7′. Based on the best fits of isochrones computed by the Geneva group to the cluster color–magnitude and color–color diagrams, previously shifted by the cluster reddening (E(B - V) = 0.32) and distance (d = 2.5 kpc), we derived the cluster age (250 Myr) and metallicity ([Fe/H] ≃ 0.0). If an 8.5 kpc distance from the Sun to the center of the Galaxy is assumed, then NGC 2309 is found to be located just in front of the Perseus spiral arm.

DISCOVERY OF A NEW SOFT GAMMA REPEATER: SGR J0418 + 5729

On 2009 June 5, the Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-ray Space Telescope triggered on two short, and relatively dim bursts with spectral properties similar to Soft Gamma Repeater (SGR) bursts. Independent localizations of the bursts by triangulation with the Konus-RF and with the Swift satellite, confirmed their origin from the same, previously unknown, source. The subsequent discovery of X-ray pulsations with the Rossi X-ray Timing Explorer (RXTE), confirmed the magnetar nature of the new source, SGR J0418+5729. We describe here the Fermi/GBM observations, the discovery and the localization of this new SGR, and our infrared and Chandra X-ray observations. We also present a detailed temporal and spectral study of the two GBM bursts. SGR J0418+5729 is the second source discovered in the same region of the sky in the last year, the other one being SGR J0501+4516. Both sources lie in the direction of the galactic anti-center and presumably at the nearby distance of ~2 kpc (assuming they reside in the Perseus arm of our galaxy). The near-threshold GBM detection of bursts from SGR J0418+5729 suggests that there may be more such dim SGRs throughout our galaxy, possibly exceeding the population of bright SGRs. Finally, using sample statistics, we conclude that the implications of the new SGR discovery on the number of observable active magnetars in our galaxy at any given time is <10, in agreement with our earlier estimates.

FERMIOBSERVATIONS OF CASSIOPEIA AND CEPHEUS: DIFFUSE GAMMA-RAY EMISSION IN THE OUTER GALAXY

We present the analysis of the interstellar γ-ray emission measured by the Fermi Large Area Telescope toward a region in the second Galactic quadrant at 100° ⩽ l ⩽ 145° and −15° ⩽ b ⩽ +30°. This region encompasses the prominent Gould Belt clouds of Cassiopeia, Cepheus, and the Polaris flare, as well as atomic and molecular complexes at larger distances, like that associated with NGC 7538 in the Perseus arm. The good kinematic separation in velocity between the local, Perseus, and outer arms, and the presence of massive complexes in each of them, make this region well suited to probe cosmic rays (CRs) and the interstellar medium beyond the solar circle. The γ-ray emissivity spectrum of the gas in the Gould Belt is consistent with expectations based on the locally measured CR spectra. The γ-ray emissivity decreases from the Gould Belt to the Perseus arm, but the measured gradient is flatter than expectations for CR sources peaking in the inner Galaxy as suggested by pulsars. The XCO = N(H2)/WCO conversion factor is found to increase from (0.87 ± 0.05) × 1020 cm−2 (K km s−1)−1 in the Gould Belt to (1.9 ± 0.2) × 1020 cm−2 (K km s−1)−1 in the Perseus arm. We derive masses for the molecular clouds under study. Dark gas, not properly traced by radio and microwave surveys, is detected in the Gould Belt through a correlated excess of dust and γ-ray emission: its mass amounts to ∼50% of the CO-traced mass.

Boron abundances in diffuse interstellar clouds

AbstractWe present a comprehensive survey of B abundances in diffuse interstellar clouds from HST/STIS observations along 56 Galactic sight lines. Our sample is the result of a complete search of archival STIS data for the B II λ1362 resonance line, with each detection confirmed by the presence of absorption from other dominant ions at the same velocity. The data probe a range of astrophysical environments including both high-density regions of massive star formation as well as low-density paths through the Galactic halo, allowing us to clearly define the trend of B depletion onto interstellar grains as a function of gas density. Many extended sight lines exhibit complex absorption profiles that trace both local gas and gas associated with either the Sagittarius-Carina or Perseus spiral arm. Our analysis indicates a higher B/O ratio in the inner Sagittarius-Carina spiral arm than in the vicinity of the Sun, which may suggest that B production in the current epoch is dominated by a secondary process. The average gas-phase B abundance in the warm diffuse ISM [log ϵ(B) = 2.38±0.10] is consistent with the abundances determined for a variety of Galactic disk stars, but is depleted by 60% relative to the solar system value. Our survey also reveals sight lines with enhanced B abundances that potentially trace recent production of 11B either by cosmic-ray or neutrino-induced spallation. Such sight lines will be key to discerning the relative importance of the two production routes for 11B synthesis.

The open cluster Berkeley 53

AbstractWe present a photometric study of the neglected open cluster Berkeley 53. We derived its fundamental parameters, such as the age, the interstellar reddening, and the distance from the Sun, based on BV photometry combined with near‐infrared JHKS data. The structure and the mass function of the cluster were also studied and the total number of members and the total mass were estimated. The cluster was found to be a rich and massive stellar system, located in the Perseus Arm of the Milky Way, 3.1 ± 0.1 kpc from the Sun. Its age exceeds 1 Gy but it seems tobe very young in the context of its dynamical evolution. The analysis of the two‐color diagrams and color‐magnitude diagrams indicates that the cluster is significantly reddened. However, both methods resulted in different values of E (B – V), i.e. 1.21 ± 0.04 and 1.52 ± 0.01, respectively. This discrepancy suggests the presence of an abnormal interstellar extinction law toward the cluster (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

IC 361, a distant intermediate-age cluster in Camelopardalis

AbstractWe have undertaken CCD photometry in the eight-color Vilnius + I system for the open cluster IC 361 (l = 147.5°, b = 5.7°) located in the constellation of Camelopardalis. Based on multicolor data, estimates of distance moduli, foreground reddening and metallicity have been obtained for individual stars measured in the field of the cluster. This allowed us to eliminate most of the field stars and to identify the probable cluster members, from which we derived the true distance modulus (m − M)0 = 12.7 mag and metallicity [Fe/H] = −0.3 dex. The interstellar extinction is found to be nonuniform across the field, with values of AV ranging from 1.9 to 2.6 mag. A comparison of the color–magnitude diagrams with Padova isochrones yields (m − M)0 = 12.6 mag, or a cluster distance of 3.3 kpc, and an age of 1.0 Gyr. Therefore, IC 361 appears to be a mildly metal-deficient cluster of intermediate age, located as far as, or just beyond, the Perseus spiral arm.

TRIGONOMETRIC PARALLAXES OF MASSIVE STAR-FORMING REGIONS. VI. GALACTIC STRUCTURE, FUNDAMENTAL PARAMETERS, AND NONCIRCULAR MOTIONS

We are using the Very Long Baseline Array and the Japanese VLBI Exploration of Radio Astronomy project to measure trigonometric parallaxes and proper motions of masers found in high-mass star-forming regions across the Milky Way. Early results from 18 sources locate several spiral arms. The Perseus spiral arm has a pitch angle of 16 degrees +/- 3 degrees, which favors four rather than two spiral arms for the Galaxy. Combining positions, distances, proper motions, and radial velocities yields complete three-dimensional kinematic information. We find that star-forming regions on average are orbiting the Galaxy approximate to 15 km s(-1) slower than expected for circular orbits. By fitting the measurements to a model of the Galaxy, we estimate the distance to the Galactic center R(0) = 8.4 +/- 0.6 kpc and a circular rotation speed Theta(0) = 254 +/- 16 km s(-1). The ratio Theta(0)/R(0) can be determined to higher accuracy than either parameter individually, and we find it to be 30.3 +/- 0.9 km s(-1) kpc(-1), in good agreement with the angular rotation rate determined from the proper motion of Sgr A*. The data favor a rotation curve for the Galaxy that is nearly flat or slightly rising with Galactocentric distance. Kinematic distances are generally too large, sometimes by factors greater than 2; they can be brought into better agreement with the trigonometric parallaxes by increasing Theta(0)/R(0) from the IAU recommended value of 25.9 km s(-1) kpc(-1) to a value near 30 km s(-1) kpc(-1). We offer a revised prescription for calculating kinematic distances and their uncertainties, as well as a new approach for defining Galactic coordinates. Finally, our estimates of Theta(0) and Theta(0)/R(0), when coupled with direct estimates of R(0), provide evidence that the rotation curve of the Milky Way is similar to that of the Andromeda galaxy, suggesting that the dark matter halos of these two dominant Local Group galaxy are comparably massive.

KINEMATIC DISTANCES TO MOLECULAR CLOUDS IDENTIFIED IN THE GALACTIC RING SURVEY

Kinematic distances to 750 molecular clouds identified in the 13CO J = 1 → 0 Boston University–Five College Radio Astronomy Observatory Galactic Ring Survey (GRS) are derived assuming the Clemens rotation curve of the Galaxy. The kinematic distance ambiguity is resolved by examining the presence of H i self-absorption toward the 13CO emission peak of each cloud using the Very Large Array Galactic Plane Survey. We also identify 21 cm continuum sources embedded in the GRS clouds in order to use absorption features in the H i 21 cm continuum to distinguish between near and far kinematic distances. The Galactic distribution of GRS clouds is consistent with a four-arm model of the Milky Way. The locations of the Scutum-Crux and Perseus arms traced by GRS clouds match star-count data from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire star-count data. We conclude that molecular clouds must form in spiral arms and be short-lived (lifetimes < 107 yr) in order to explain the absence of massive, 13CO bright molecular clouds in the interarm space.

THE IONIZED STRUCTURE OF THE LOCAL SPUR

The distribution of the H166α radio recombination line emission at the end of the first quadrant is obtained and compared with the distribution of H I and other young objects. The data analyzed lie in the Galactic longitude range between l = 60° and l = 91° and was observed with single dish antenna at 1.4 GHz. The extended low-density warm ionized medium (ELDWIM) is used to trace the Local Spur (LS), which is strongly correlated with neutral hydrogen and low-brightness H II regions. The Galactic distribution of gas in the (X, Y, z) Galactic coordinates yields the location of ionized hydrogen in an arclike structure at an almost constant distance of 8 kpc from the galactic center. The ELDWIM and the low-brightness H II region define the LS; it is located between r 0.5 and 5 kpc from the Sun. We found sources of ionized gas at larger distances; they are probably located in the Perseus arm. The structure of the LS does not seem to join the Perseus arm at farther distances, a fact gleaned from the H I emission. The LS is very thick between 2 and 5 kpc, where the ionized gas spreads from z –150 to +250 pc. The z distribution of the ionized gas appears tilted with respect to the Galactic plane, which starts at 3 kpc from the Sun and runs toward higher galactocentric radii on the Perseus arm.

Unified Rotation Curve of the Galaxy — Decomposition into de Vaucouleurs Bulge, Disk, Dark Halo, and the 9-kpc Rotation Dip —

We present a unified rotation curve of the Galaxy re-constructed from existing data by re-calculating the distances and velocities for a set of Galactic constants, $R_0$$=$ 8 kpc and $V_0$$=$ 200 km s$^{-1}$. We decomposed it into a bulge with de Vaucouleurs-law profile of half-mass scale radius 0.5 kpc and mass 1.8 $\times$ 10$^{10}M_\odot$, an exponential disk of scale radius 3.5 kpc and 6.5 $\times$ 10$^{10}M_\odot$, and an isothermal dark halo of terminal velocity 200 km s$^{-1}$. A $r^{1/4}$-law fit was obtained for the first time for the Milky Way’s rotation curve. After fitting by these fundamental structures, two local minima, or dips, of the rotation velocity are prominent at radii 3 and 9 kpc. The 3-kpc dip is consistent with the observed bar. It is alternatively explained by a massive ring with the density maximum at a radius of 4 kpc. The 9-kpc dip is clearly exhibited as the most peculiar feature in the Galactic rotation curve. We explain it by a massive ring of amplitude as large as 0.3 to 0.4-times the disk density with the density peak at a radius of 11 kpc. This great ring may be related to the Perseus arm, while no peculiar feature of HI-gas is associated.