Sagittarius Arm Research Articles

Three-dimensional interstellar dust reddening maps of the Galactic plane

We present new three-dimensional (3D) interstellar dust reddening maps of the Galactic plane in three colours, E(G-Ks), E(Bp-Rp) and E(H-Ks). The maps have a spatial angular resolution of 6 arcmin and covers over 7000 deg$^2$ of the Galactic plane for Galactic longitude 0 deg $<$ $l$ $<$ 360 deg and latitude $|b|$ $<$ $10$ deg. The maps are constructed from robust parallax estimates from the Gaia Data Release 2 (Gaia DR2) combined with the high-quality optical photometry from the Gaia DR2 and the infrared photometry from the 2MASS and WISE surveys. We estimate the colour excesses, E(G-Ks), E(Bp-Rp) and E(H-Ks), of over 56 million stars with the machine learning algorithm Random Forest regression, using a training data set constructed from the large-scale spectroscopic surveys LAMOST, SEGUE and APOGEE. The results reveal the large-scale dust distribution in the Galactic disk, showing a number of features consistent with the earlier studies. The Galactic dust disk is clearly warped and show complex structures possibly spatially associated with the Sagittarius, Local and Perseus arms. We also provide the empirical extinction coefficients for the Gaia photometry that can be used to convert the colour excesses presented here to the line-of-sight extinction values in the Gaia photometric bands.

The spiral potential of the Milky Way

Context. The location of young sources in the Galaxy suggests a four-armed spiral structure, whereas tangential points of spiral arms observed in the integrated light at infrared and radio wavelengths indicate that only two arms are massive. Aims. Variable extinction in the Galactic plane and high light-to-mass ratios of young sources make it difficult to judge the total mass associated with the arms outlined by such tracers. The current objective is to estimate the mass associated with the Sagittarius arm by means of the kinematics of the stars across it. Methods. Spectra of 1726 candidate B- and A-type stars within 3◦ of the Galactic center (GC) were obtained with the FLAMES instrument at the VLT with a resolution of ≈6000 in the spectral range of 396–457 nm. Radial velocities were derived by least-squares fits of the spectra to synthetic ones. The final sample was limited to 1507 stars with either Gaia DR2 parallaxes or main-sequence B-type stars having reliable spectroscopic distances. Results. The solar peculiar motion in the direction of the GC relative to the local standard of rest (LSR) was estimated to U⊙ = 10.7 ± 1.3kms−1. The variation in the median radial velocity relative to the LSR as a function of distance from the sun shows a gradual increase from slightly negative values near the sun to almost 5 km s−1 at a distance of around 4 kpc. A sinusoidal function with an amplitude of 3.4 ± 1.3kms−1 and a maximum at 4.0 ± 0.6 kpc inside the sun is the best fit to the data. A positive median radial velocity relative to the LSR around 1.8 kpc, the expected distance to the Sagittarius arm, can be excluded at a 99% level of confidence. A marginal peak detected at this distance may be associated with stellar streams in the star-forming regions, but it is too narrow to be associated with a major arm feature. Conclusions. A comparison with test-particle simulations in a fixed galactic potential with an imposed spiral pattern shows the best agreement with a two-armed spiral potential having the Scutum–Crux arm as the next major inner arm. A relative radial forcing dFr ≈ 1.5% and a pattern speed in the range of 20–30 km s−1 kpc−1 yield the best fit. The lack of a positive velocity perturbation in the region around the Sagittarius arm excludes it from being a major arm. Thus, the main spiral potential of the Galaxy is two-armed, while the Sagittarius arm is an inter-arm feature with only a small mass perturbation associated with it.

A comparison of the local spiral structure from Gaia DR2 and VLBI maser parallaxes

Context. The Gaia mission has released the second data set (Gaia DR2), which contains parallaxes and proper motions for a large number of massive, young stars. Aims. We investigate the spiral structure in the solar neighborhood revealed by Gaia DR2 and compare it with that depicted by VLBI maser parallaxes. Methods. We examined three samples with different constraints on parallax uncertainty and distance errors and stellar spectral types: (1) all OB stars with parallax errors of less than 10%; (2) only O-type stars with 0.1 mas errors imposed and with parallax distance errors of less than 0.2 kpc; and (3) only O-type stars with 0.05 mas errors imposed and with parallax distance errors of less than 0.3 kpc. Results. In spite of the significant distance uncertainties for stars in DR2 beyond 1.4 kpc, the spiral structure in the solar neighborhood demonstrated by Gaia agrees well with that illustrated by VLBI maser results. The O-type stars available from DR2 extend the spiral arm models determined from VLBI maser parallaxes into the fourth Galactic quadrant, and suggest the existence of a new spur between the Local and Sagittarius arms.

Formation of the young compact cluster GM 24 triggered by a cloud–cloud collision

Abstract High-mass star formation is an important step which controls galactic evolution. GM 24 is a heavily obscured star cluster including a single O9 star with more than ∼100 lower-mass stars within a 0.3 pc radius toward (l, b) ∼ (${350{{^{\circ}_{.}}}5}$, ${0{{^{\circ}_{.}}}96}$), close to the Galactic mini-starburst NGC 6334. We found two velocity components associated with the cluster by new observations of 12CO J =2–1 emission, whereas the cloud was previously considered to be single. We found that the distribution of the two components of 5 $\rm {km}\:$s−1 separation shows complementary distribution; the two fit well with each other if a relative displacement of 3 pc is applied along the Galactic plane. A position–velocity diagram of the GM 24 cloud is explained by a model based on numerical simulations of two colliding clouds, where an intermediate velocity component created by the collision is taken into account. We estimate the collision time scale to be ∼Myr in projection of a relative motion tilted to the line of sight by 45°. The results lend further support for cloud–cloud collision as an important mechanism of high-mass star formation in the Carina–Sagittarius Arm.

The Carina Nebula and Gum 31 molecular complex – II. The distribution of the atomic gas revealed in unprecedented detail

We report high spatial resolution observations of the H i 21cm line in the Carina Nebula and the Gum 31 region obtained with the Australia Telescope Compact Array. The observations covered ∼12 |$\deg ^2$| centred on l = 287|$_{.}^{\circ}$|5, b = −1°, achieving an angular resolution of ∼35 arcsec. The H i map revealed complex filamentary structures across a wide range of velocities. Several ‘bubbles’ are clearly identified in the Carina Nebula complex, produced by the impact of the massive star clusters located in this region. An H i absorption profile obtained towards the strong extragalactic radio source PMN J1032−5917 showed the distribution of the cold component of the atomic gas along the Galactic disc, with the Sagittarius–Carina and Perseus spiral arms clearly distinguishable. Preliminary calculations of the optical depth and spin temperatures of the cold atomic gas show that the H i line is opaque (τ ≳ 2) at several velocities in the Sagittarius–Carina spiral arm. The spin temperature is ∼100 K in the regions with the highest optical depth, although this value might be lower for the saturated components. The atomic mass budget of Gum 31 is ∼35 per cent of the total gas mass. H i self-absorption features have molecular counterparts and good spatial correlation with the regions of cold dust as traced by the infrared maps. We suggest that in Gum 31 regions of cold temperature and high density are where the atomic to molecular gas-phase transition is likely to be occurring.

The Spiral Arm Segments of the Galaxy within 3 kpc from the Sun: A Statistical Approach

Abstract As can be reasonably expected, upcoming large-scale APOGEE, GAIA, GALAH, LAMOST, and WEAVE stellar spectroscopic surveys will yield rather noisy Galactic distributions of stars. In view of the possibility of employing these surveys, our aim is to present a statistical method to extract information about the spiral structure of the Galaxy from currently available data, and to demonstrate the effectiveness of this method. The model differs from previous works studying how objects are distributed in space in its calculation of the statistical significance of the hypothesis that some of the objects are actually concentrated in a spiral. A statistical analysis of the distribution of cold dust clumps within molecular clouds, H ii regions, Cepheid stars, and open clusters in the nearby Galactic disk within 3 kpc from the Sun is carried out. As an application of the method, we obtain distances between the Sun and the centers of the neighboring Sagittarius arm segment, the Orion arm segment in which the Sun is located, and the Perseus arm segment. Pitch angles of the logarithmic spiral segments and their widths are also estimated. The hypothesis that the collected objects accidentally form spirals is refuted with almost 100% statistical confidence. We show that these four independent distributions of young objects lead to essentially the same results. We also demonstrate that our newly deduced values of the mean distances and pitch angles for the segments are not too far from those found recently by Reid et al. using VLBI-based trigonometric parallaxes of massive star-forming regions.

Is the Galactic Spiral Potential 2- or 4-arms?

AbstractYoung objects (e.g. OB-associations and HII regions) in the Galaxy outline a 4-armed spiral structure whereas the tangent points of arms observed in the near-infrared indicate a 2-armed pattern. The more important issue is whether the spiral potential in the Galaxy is 2- or 4-armed i.e. if all arms traced by young objects also have a significant mass perturbation associated to them. This can be tested by studying the mean radial velocity of a well defined stellar population across the spiral arms and thereby estimating the surface density change.The current paper presents a preliminary analysis of the radial velocities of a sample of 736 early-type stars toward the Galactic center observed with FLAMES/VLT. A comparison with N-body models in a fixed spiral potential with 2 or 4 arms suggests that no significant mass is associated to the Sagittarius arm. The data are consistent with 2-armed models with pattern speeds in the range of 15-30 km s−1 kpc−1 and relative radial forces of less than 4%.

The nearby spiral density‐wave structure of the Galaxy: Line‐of‐sight and longitudinal velocities of 61 masers

This paper is the eighth in a series presenting the results of the dynamical analysis of velocity fields of Galactic objects on the assumption that the system is subject to moderately unstable, low‐amplitude, tightly wound, and rigidly rotating Lin–Shu density waves. The data of 61 masers associated with star‐forming regions having distances d&lt;4 kpc from the Sun and h&lt;200 pc from the plane are collected from the work by Reid et al. Both line‐of‐sight vlos and transverse along the Galactic longitude vlvelocities of these objects are analyzed. It is shown that there may coexist at least two radial scales of irregularity of a velocity field with different wavelengths λ≈2 and λ≈3.5 kpc and amplitudes of the perturbed gravitational potential for all values of the number of spiral arms considered, m = 1, 2, 3, and 4. To be consistent with the current ideas as to the spiral structure of the Galaxy, we argue respectively that the Sun is located at the inner edge of the Cygnus–Orion arm (very close to the arm center), between the nearby inner Carina–Sagittarius and outer Perseus arms (closer to the inner one). Estimates obtained from an analysis of two independent velocities vlos and vl are fairly consistent. As for us, the latter establishes further evidence to support the Lin–Shu wave proposal.

Three-dimensional structure of the magnetic field in the disk of the Milky Way

We present Rotation Measures (RM) of the diffuse Galactic synchrotron emission from the Canadian Galactic Plane Survey (CGPS) and compare them to RMs of extragalactic sources in order to study the large-scale reversal in the Galactic magnetic field (GMF). Using Stokes Q, U and I measurements of the Galactic disk collected with the Synthesis Telescope at the Dominion Radio Astrophysical Observatory, we calculate RMs over an extended region of the sky, focusing on the low longitude range of the CGPS (l=52deg to l=72deg). We note the similarity in the structures traced by the compact sources and the extended emission and highlight the presence of a gradient in the RM map across an approximately diagonal line, which we identify with the well-known field reversal of the Sagittarius-Carina arm. We suggest that the orientation of this reversal is a geometric effect resulting from our location within a GMF structure arising from current sheets that are not perpendicular to the Galactic plane, as is required for a strictly radial field reversal, but that have at least some component parallel to the disk. Examples of models that fit this description are the three-dimensional dynamo-based model of Gressel et al. (2013) and a Galactic scale Parker spiral (Akasofu & Hakamada 1982), although the latter may be problematic in terms of Galactic dynamics. We emphasize the importance of constructing three-dimensional models of the GMF to account for structures like the diagonal RM gradient observed in this dataset.

The distance to the pulsar B1818-04 and the distribution of turbulent interstellar plasma toward B0833-45, B1818-04, and B1933+16

The dependence of the scatter broadening of extragalactic sources on the dispersion measures of distant pulsars observed along nearby lines of sight and the dependence of broadening of pulsar pulses on the scatter broadening observed for the pulsars themselves and for extragalactic sources observed along nearby lines of sight are constructed and analyzed. These dependences can be used to study turbulent plasma in the Galaxy. The effective scattering layer in the direction toward the pulsar B1933+16 is located in the Sagittarius arm at a distance of ≈3.4 kpc from the observer, and has an extent of ≈0.55 kpc. The scatter broadening and pulse broadening of B0833-45 are due to the turbulent medium in the shell of the Gum Nebula. The distance from the pulsar to the center of the scattering layer is≈43 pc. Data on scattering of the radiation of the pulsar B1818-04 and of the extragalactic source J1821-0502, together with data on the distribution of OB stars in the direction toward this pulsar, are used to show that the distance to the pulsar is ≈0.6 kpc; an H II region around the O7V star HD 171198, located 0.42 kpc from the Sun, is responsible for the scattering of this pulsar’s radiation.

Ionized gas clouds near the Sagittarius Arm tangent

Radio recombination lines (RRLs) are the best tracers of ionized gas. Simultaneous observations of multi-transitions of RRLs can significantly improve survey sensitivity. We conducted pilot RRL observations near the Sagittarius Arm tangent by using the 65-m Shanghai Tian Ma Radio Telescope (TMRT) equipped with broadband feeds and a digital backend. Six hydrogen RRLs (H96 H101α) at C band (6289 MHz–7319 MHz) were observed simultaneously toward a sky area of 2° × 1.2° by using on-the-fly mapping mode. These transitions were then stacked together for detection of ionized gas. Star forming complexes G48.6+0.1 and G49.5−0.3 were detected in the integrated intensity map. We found agreements between our measured centroid velocities and previous results for the 21 known HII regions in the mapped area. For more than 80 cataloged HII region candidates without previous RRL measurements, we obtained new RRL spectra at 30 targeted positions. In addition, we detected 25 new discrete RRL sources with spectral S/N σ, and they were not listed in the catalogs of previously known HII regions. The distances for 44 out of these 55 new RRL sources were estimated.

Warm Ionized Medium throughout the Sagittarius–Carina Arm

Abstract Wisconsin H-Alpha Mapper observations of Hα and [S ii] λ6716 emission are used to trace the vertical distribution and physical conditions of the warm ionized medium along the Sagittarius–Carina arm. CO emission, tracing cold molecular gas in the plane of the Galaxy, is used as a guide to isolate Hα and [S ii] λ6716 emission along individual spiral arms. Exponential scale heights of electron density squared (or emission measure) are determined using Hα emission above (below) the midplane to be along the near Sagittarius arm, along the near Carina arm, and along the far Carina arm. The emission measure scale height tends to increase as a function of Galactocentric radius along the Sagittarius–Carina arm for . Physical conditions of the ionized gas are analyzed using the [S ii]/Hα line ratio, which more closely traces I Hα than height above the plane, z, suggesting a stronger relationship with the in situ electron density. We interpret this result as further evidence for the majority of the observed diffuse emission originating from in situ ionized gas as opposed to scattered light from classical H ii regions in the plane.

The deep OB star population in Carina from the VST Photometric Hα Survey (VPHAS+)

Massive OB stars are critical to the ecology of galaxies, and yet our knowledge of OB stars in the Milky Way, fainter than $V \sim 12$, remains patchy. Data from the VST Photometric H$\alpha$ Survey (VPHAS+) permit the construction of the first deep catalogues of blue excess-selected OB stars, without neglecting the stellar field. A total of 14900 candidates with 2MASS cross-matches are blue-selected from a 42 square-degree region in the Galactic Plane, capturing the Carina Arm over the Galactic longitude range $282^{\circ} \lesssim \ell \lesssim 293^{\circ}$. Spectral energy distribution fitting is performed on these candidates' combined VPHAS+ $u,g,r,i$ and 2MASS $J,H,K$ magnitudes. This delivers: effective temperature constraints, statistically separating O from early-B stars; high-quality extinction parameters, $A_0$ and $R_V$ (random errors typically $< 0.1$). The high-confidence O-B2 candidates number 5915 and a further 5170 fit to later B spectral type. Spectroscopy of 276 of the former confirms 97% of them. The fraction of emission line stars among all candidate B stars is 7--8% . Greyer ($R_V > 3.5$) extinction laws are ubiquitous in the region, over the distance range 2.5--3 kpc to $\sim$10~kpc. Near prominent massive clusters, $R_V$ tends to rise, with particularly large and chaotic excursions to $R_V \sim 5$ seen in the Carina Nebula. The data reveal a hitherto unnoticed association of 108 O-B2 stars around the O5If$+$ star LSS 2063 ($\ell = 289.77^{\circ}$, $b = -1.22^{\circ}$). Treating the OB star scale-height as a constant within the thin disk, we find an orderly mean relation between extinction ($A_0$) and distance in the Galactic longitude range, $287.6^{\circ} < \ell < 293.5^{\circ}$, and infer the subtle onset of thin-disk warping. A halo around NGC 3603, roughly a degree in diameter, of $\sim$500 O-B2 stars with $4 < A_0 (\rm{mag}) < 7$ is noted.

The local spiral structure of the Milky Way.

The nature of the spiral structure of the Milky Way has long been debated. Only in the last decade have astronomers been able to accurately measure distances to a substantial number of high-mass star-forming regions, the classic tracers of spiral structure in galaxies. We report distance measurements at radio wavelengths using the Very Long Baseline Array for eight regions of massive star formation near the Local spiral arm of the Milky Way. Combined with previous measurements, these observations reveal that the Local Arm is larger than previously thought, and both its pitch angle and star formation rate are comparable to those of the Galaxy's major spiral arms, such as Sagittarius and Perseus. Toward the constellation Cygnus, sources in the Local Arm extend for a great distance along our line of sight and roughly along the solar orbit. Because of this orientation, these sources cluster both on the sky and in velocity to form the complex and long enigmatic Cygnus X region. We also identify a spur that branches between the Local and Sagittarius spiral arms.

KOSMA 12CO(2–1) and (3–2) observations toward Infrared Dark Clouds

Infrared Dark Clouds (IRDCs) are a valuable sample to study the initial conditions and the very early stages of the evolution of massive stars. Using the KOSMA telescope, we performed simultaneous single point 12CO(2–1) and (3–2) observations toward 117 IRDCs with a detection rate of 74 % for 12CO(2–1) and 58 % for 12CO(3–2). The non-detected sources are found not to represent real dark clouds. The distances of the IRDCs have been calculated using a Bayesian distance estimator indicating that the sources are mainly concentrated in the Outer, Perseus, Local, Sagittarius and Scutum arms. The excitation temperature and the filling factor have been solved simultaneously for the two emission lines. The average excitation temperature is about 10 K. The filling factors are used to correct the integrated intensity ratio of 12CO(3–2) to (2–1) resulting in a typical value of the ratio of $0.69\pm0.47$ for the majority of sources. A small number of sources exhibit enhanced values for the ratio because of the presence of a nearby HII region. The ratios do not show significant differences for sources with embedded far-infrared sources.

The global chemical properties of high-mass star forming clumps at different evolutionary stages

A total of 197 relatively isolated high-mass star-forming clumps were selected from the Millimeter Astronomy Legacy Team 90 GHz (MALT90) survey data and their global chemical evolution investigated using four molecular lines, \(\mbox{N}_{2}\mbox{H}^{+}\ \mbox{(1--0)}\), \(\mbox{HCO}^{+}\ \mbox{(1--0)}\), HCN (1–0), and HNC (1–0). The results suggest that the global averaged integrated intensity ratios \(I(\mbox{HCO}^{+})/I(\mbox{HNC})\), \(I(\mbox{HCN})/I(\mbox{HNC})\), \(I(\mbox{N}_{2}\mbox{H}^{+})\mbox{/}I(\mbox{HCO}^{+})\), and \(I(\mbox{N}_{2}\mbox{H}^{+})\mbox{/} I(\mbox{HCN})\) are promising tracers for evolution of high-mass star-forming clumps. The global averaged column densities and abundances of \(\mbox{N}_{2}\mbox{H}^{+}\), \(\mbox{HCO}^{+}\), \(\mbox{HCN}\), and \(\mbox{HNC}\) increase as clumps evolve. The global averaged abundance ratios \(X(\mbox{HCN})\mbox{/}X(\mbox{HNC})\) could be used to trace evolution of high-mass star forming clumps, \(X(\mbox{HCO}^{+})\mbox{/}X(\mbox{HNC})\) is more suitable for distinguishing high-mass star-forming clumps in prestellar (stage A) from those in protostellar (stage B) and HII/PDR region (stage C). These results suggest that the global averaged integrated intensity ratios between HCN (1–0), HNC (1–0), \(\mbox{HCO}^{+}\ \mbox{(1--0)}\) and \(\mbox{N}_{2}\mbox{H}^{+}\ \mbox{(1--0)}\) are more suitable for tracing the evolution of high-mass star forming clumps. We also studied the chemical properties of the target high-mass star-forming clumps in each spiral arm of the Galaxy, and got results very different from those above. This is probably due to the relatively small sample in each spiral arm. For high-mass star-forming clumps in Sagittarius arm and Norma-Outer arm, comparing two groups located on one arm with different Galactocentric distances, the clumps near the Galactic Center appear to be younger than those far from the Galactic center, which may be due to more dense gas concentrated near the Galactic Center, and hence more massive stars being formed there.

The electron density in clouds of turbulent interstellar plasma

The dependence of the emission measure on the dispersion measure due to the Galactic background has been derived for 120 directions in the Galaxy. This analysis has yielded the mean electron density, effective thickness of the electron layer, and the volume filling factor of the clouds of ionized gas along the line of sight. The pulsar J1745−2900, which lies in a direction close to the direction toward the center of the Galaxy, is located at least 100 pc closer to the observer than the source Sgr A* along the line of sight. The scatter-broadened angular size of J1745−2900 is determined by the turbulent medium in the Sagittarius Arm.

A catalog of early-type stars toward the Galactic center

It is still unclear whether the Sagittarius spiral arm is a major spiral arm in the Galaxy or whether it just outlines a region of enhanced star formation because of the local compression of gas. The best way to separate these scenarios out is to study the kinematics across the arm to determine the velocity perturbation it induces. A survey of early-type stars in the direction of the Galactic center is performed covering an area of 100 square degrees with the aim of identifying candidates for a radial velocity study. Objective prism plates were obtained with the 4deg prism on the ESO Schmidt telescope using IIaO, 4415, and IIIaJ emulsions. The plates were digitized and more than 100k spectra were extracted down to a limiting magnitude of B = 15m. The spectra were cross-correlated with a template with Balmer lines, which yielded a candidate list of 12675 early-type stars. Magnitudes and equivalent widths of strong lines were calculated from the spectra, which allowed us to estimate the individual extinctions and distances for 11075 stars. The survey identified 9571 candidate stars with a spectral type earlier than A1 and B < 14.5m out to distances of more than 2kpc, which is beyond the Sagittarius arm. This is indicated by the increase of absorption in the plane at distances larger than 0.5kpc.

Offset between stellar spiral arms and gas arms of the Milky Way

Spiral arms shown by different components may not be spatially coincident, which can constrain formation mechanisms of spiral structure in a galaxy. We reassess the spiral arm tangency directions in the Milky Way through identifying the bump features in the longitude plots of survey data for infrared stars, radio recombination lines (RRLs), star formation sites, CO, high density regions in clouds, and HI. The bump peaks are taken as indications for arm tangencies, which are close to the real density peaks near the spiral arm tangency point but often have $\sim$ 1$^\circ$ offset to the interior of spiral arms. The arm tangencies identified from the longitudes plots for RRLs, HII regions, methanol masers, CO, high density gas regions, and HI gas appear nearly the same Galactic longitude, and therefore there is no obvious offset for spiral arms traced by different gas components. However, we find obvious displacements of 1.3$^\circ-$ 5.8$^\circ$ between gaseous bump peaks from the directions of the maximum density of old stars near the tangencies of the Scutum-Centaurus Arm, the northern part of the Near 3 kpc Arm, and maybe also the Sagittarius Arm. The offsets between the density peaks of gas and old stars for spiral arms are comparable with the arm widths, which is consistent with expectations for quasi-stationary density wave in our Galaxy.

The Lin–Shu type density wave structure of our Galaxy: line-of-sight velocities of 396 HII regions

In this fifth paper in a series, we check again the Lin–Shu idea of small-amplitude density waves in our Galaxy. The updated catalogue of Galactic objects of Hou and Han (Hou and Han in Astron Astrophys 569:125–146, 2014), available in the literature, is used. The line-of-sight velocity field of 396 HII regions with known trigonometric or photometric distances and their uncertainties within 4 kpc from the Sun is investigated in terms of a wave perturbation. Our previous study is enlarged by examining the secondary minima of the residual sum of squares of the predicted and measured velocities of a sample of objects in a considerable region surrounding the Sun. Both the geometrical and physical parameters of the wave pattern are evaluated. Here we show, for the first time, that the local Cygnus–Orion spiral arm, where the Sun is located, does form a part of the major density wave structure of the system under the question but is not a spur originating at the inner Carina–Sagittarius arm or at the outer Perseus arm.