Perseus Arm Research Articles

Abstract Within the molecular clouds of the Camelopardis OB1 (Cam OB1) association exists a region previously noted as one subgroup. However, bulk clustering from Gaia astrometry has recently shown three distinctive kinematically coherent groups, all found in a similar location in the sky (137 ⪅ l ⪅ 145 and −2 ⪅ b ⪅ 5) and at a similar distance (∼1 kpc). In this work, we derive from first principles the three proposed clusters in this region, refine the membership list and cluster ages, and, for the first time, examine the 3D structure, motion, and origin of the clusters. Using clustering of Gaia data in 3D position + 2D velocity space, supplemented by available fifth-generation Sloan Digital Sky Survey radial velocities, we find clusters of ages 10, 15.8, and 20 Myr with members numbering 140, 469, and 184, respectively. All three clusters overlap currently in 3D space. Tracing their previous locations, based on present-day motions, shows that each cluster originated in its own distinct region and exhibited no influence on each other’s formation. Two of the clusters trace their origin to different areas within the Cam OB1 association, with the oldest cluster tracing its origins to the near edge of the Perseus Arm, in the direction of the Perseus OB1 or Cassiopeia OB6 associations. Overall, this work illustrates how different stellar groups, even those originating in a different spiral arm, can visit and pass through each other as they travel through the Galaxy.

Abstract The spiral structure of the Milky Way remains under debate. Y. Xu et al. proposed, for the first time, a multiple-arm morphology for our Galaxy, with two spiral arms in the inner disk bifurcating outward alongside additional independent arms in the outer disk. This model aligns with the statistical properties of spiral arms seen in many extragalactic spiral galaxies and differs from the traditional standard model primarily in the presence of bifurcation points. This study aims to search for observational evidence of spiral arm bifurcations in the Milky Way and constrain its structure. Using the Herschel Infrared Galactic Plane Survey and the APEX Telescope Large Area Survey of the Galaxy clump catalogs, we first reassess the spiral arm tangent points, then analyze the statistical distributions of Galactic longitude, latitude, and velocity to identify signatures of bifurcations and constrain their potential locations. Our results suggest that the Perseus and Sagittarius arms may merge near Galactic longitude ℓ ∼ 15° in the first quadrant. Symmetrically, in the fourth quadrant, the sources are distributed along the Galactic longitude with a concentration at ℓ ∼ 333°, where the Norma and Centaurus arms are likely to converge.

Neutral hydrogen (HI) is fundamental for tracing spiral structure in galaxies, including the Milky Way, through its 21-cm emission line, revealing features otherwise obscured by dust and gas. However, much of the Milky Way remains poorly resolved due to the Solar System’s embedded location within the galactic disk. Here, we present the first direct comparison between zircon oxygen isotope kurtosis, a terrestrial deep time record, and spatial variations in HI density along the Solar System’s galactic orbit. Significant correlations between Earth’s magmatic zircon oxygen isotope kurtosis and HI density in spiral arms suggest astrophysical influences on Earth’s crustal evolution. Peaks in HI density near the Scutum-Centaurus and Perseus spiral arms are consistent with the hypothesis that periodic disruptions of the Oort cloud during galactic arm crossings increased impact rates on Earth. These periods of elevated impact flux may have contributed substantial thermal energy, leading to more variable magmatic petrogenesis. This variability is recorded by zircon oxygen isotopes, a proxy sensitive to the depth of crustal melting and the degree of interaction with surface-derived water. These results provide new insights into the interplay between galactic-scale processes and Earth’s geological history, arguing for consideration of astrophysical drivers of planetary evolution.

Abstract The Milky Way’s disk–halo interface mediates energy and mass exchange between the interstellar thin disk and the halo. In the first detailed study of the Perseus arm’s disk–halo interface, we combine Hubble Space Telescope/Space Telescope Imaging Spectrograph and Cosmic Origins Spectrograph absorption spectra toward six stars and 23 active galactic nuclei (AGNs) projected behind a narrow section (95° < l < 145°, −46° < b < 0°). This provides a unique data set that bridges the disk and its extended vertical structure in these directions. We measure S ii, Si iv, and C iv absorption, along with H i 21 cm emission, at heights −70 pc to −3.3 kpc from the midplane. The arm’s southern vertical structure shows distinct height-dependent behaviors: H i and S ii column densities sharply decline with height up to 1.5 kpc, then continue declining at a much shallower rate at greater heights. In contrast, high ion (Si IV and C IV) column densities remain relatively constant throughout the entire height range. In the disk–halo interface, where warm neutral medium dominates, S ii and the high ions show similar kinematics, and we find a remarkably uniform C iv/Si iv ratio (〈N C IV/N Si IV〉 = 2.5 ± 0.5) within −0.9 to −3.25 kpc. Both the kinematic correspondence and high-ion ratio are consistent with the high ions probing turbulent mixing layers at the interfaces between warm/cool and hot gas phases. The AGN sight lines reveal minimal circumgalactic medium contribution in the low-velocity gas at ∣v LSR∣ < 100 km s−1. The extraplanar absorbing gas may trace material ejected from previous Galactic fountain activity.

Abstract Massive young clusters with rich populations of high-mass stars are ideal laboratories to explore their evolutionary paths. Despite being the most prominent cluster in the Perseus-arm Cas OB8 association, NGC 663 remains comparatively little studied. We present a comprehensive investigation of its properties, integrating astrometric, photometric and spectroscopic data for the cluster and its surroundings, including accurate spectral classification for over 150 members. Gaia astrometry indicates over 300 B-type members, possibly rendering NGC 663 the most massive cluster in the Perseus arm, with initial mass likely exceeding 104 M⊙. This large population makes NGC 663 an excellent laboratory for studying massive star evolution. Spectral analysis of the earliest members reveals approximately solar metallicity and a turn-off mass of ≈8.5 M⊙, consistent with the photometric age of 23 Ma. We identify five spectroscopic blue stragglers, including the Be/X-ray binary RX J0146.9+6121. We outline its evolutionary history and compare its properties with other Be stars. Although the cluster contains many Be stars, their relative fraction is not particularly high. Intriguingly, four of the six blue supergiant members appear to have significantly higher masses than the brightest giants near the Hertzsprung gap. These observations suggest that most mid-B supergiants may form via mergers, unless stars of 10–12 M⊙ born as primaries in binaries rarely undergo supernova explosions. Similarly, if Be stars form through the binary channel, then either most are produced through case A evolution or supernovae are uncommon among primaries in this mass range.

We provide a purely dynamical global map of the non-axisymmetric structure of the Milky Way disk. For this, we exploited the information contained within the in-plane motions of disk stars from Gaia DR3 to adjust a model of the Galactic potential, including a detailed parametric form for the bar and spiral arms. We explored the parameter space of the non-axisymmetric components with the backward integration method, first adjusting the bar model to selected peaks of the stellar velocity distribution in the solar neighborhood, and then adjusting the amplitude, phase, pitch angle, and pattern speed of spiral arms to the median radial velocity as a function of position within the disk. We checked a posteriori that our solution also qualitatively reproduces various other features of the global non-axisymmetric phase-space distribution, including most moving groups and phase-space ridges, despite those not being primarily used in the adjustment. This fiducial model has a bar with a pattern speed of 37 km s−1 kpc−1 and two spiral modes that are twoarmed and three-armed, respectively. The two-armed spiral mode has a ~25% local contrast surface density and a low pattern speed of 13.1 km s−1 kpc−1, and matches the location of the Crux-Scutum, Local, and Outer arm segments. The three-armed spiral mode has a ~9% local contrast density, a slightly higher pattern speed of 16.4 km s−1 kpc−1, and matches the location of the Carina-Sagittarius and Perseus arm segments. The Galactic bar, with a higher pattern speed than both spiral modes, has recently disconnected from those two arms. The fiducial non-axisymmetric potential presented in this paper, reproducing most non-axisymmetric signatures detected in the stellar kinematics of the Milky Way disk, can henceforth be used to confidently integrate orbits within the Galactic plane.

ABSTRACT Mapping the Milky Way spiral arms in the vertical direction remains a challenging task that has received little attention. Taking advantage of recent results that link the position of the Galactic spiral arms to metal-rich regions in the disc, we analyse a sample of young giant stars from Gaia DR3 and use their metallicity distribution to produce a 3D metallicity excess map. The map shows signatures of the spiral arms, whose vertical height vary across the Galactic disc, reaching up to 400 ${\rm \, pc}$ in amplitude and exhibiting vertical asymmetries with respect to the mid-plane. Specifically, the Perseus arm displays a high vertical asymmetry consistent with the Galactic warp. Moreover, we find evidence of a metal-rich stellar structure that undulates vertically, nearly in phase with the arrangement of star-forming regions named the Radcliffe Wave. This new structure is larger and extends beyond the Radcliffe Wave, reaching vertical amplitudes of $\sim$270 ${\rm \, pc}$ and extending for at least 4 ${\rm \, kpc}$ in length. We confirm that for at least half of its length this Extended Radcliffe Wave is the inner edge of the Local Arm. The finding of a metal-rich stellar counterpart of the Radcliffe Wave shows that mapping the three-dimensional metallicity distribution of young stellar populations reveals key information about the structures and chemical enrichment in the Galactic disc.

As a relatively young and bright population and the archetype of standard candles, classical Cepheids are an ideal population on which to trace the non-axisymmetric structure in the young stellar disk to large distances. We used the new distances derived in Paper I based on mid-IR WISE photometry for a selected sample of 2857 dynamically young Cepheids to trace the spiral arms of the Milky Way. The Perseus and Sagittarius-Carina arms are clearly evident in the third and fourth Galactic quadrants, while the Local and Scutum arms are much weaker, and extinction severely limits our view of the latter innermost spiral arm. Pitch angles were derived for each arm over various ranges of Galactic azimuth, each covering at least 90° in azimuth. Our method of detecting spiral arms and deriving pitch angles does not rely on pre-assigning sources to specific arms. While the spiral structure in the first and second quadrant is not obvious in part because of extinction effects, it is not inconsistent with the structure seen in the third and fourth quadrants. In summary, the Cepheids allow us to map spiral structure in the third and fourth Galactic quadrants where currently few masers have astrometric parallaxes, significantly extending our understanding of the Milky Way at large scales.

Abstract Galactic neutral hydrogen (H i) data drawn from several large-scale surveys have been used to study 32 small clouds found between Galactic longitudes 70° and 140° and latitudes −20° and −45°. This places them below the Perseus spiral arm in the second quadrant of Galactic longitude. The velocities of the sample range from −30 to −57 km s−1, and their z-heights are between −1.5 and −3.7 kpc. The mapped clouds are localized brightness enhancements along the length of interstellar H i filaments. This H i may have been propelled into the Galactic halo by the action of many supernovae and stellar winds originating in the abundant star-forming regions in this longitude region of the Perseus spiral arm. Adopting this model allows the distance to each cloud to be derived. Their emission profiles are characterized by narrow lines ≈5 km s−1 wide. Application of the virial theorem for the ensemble of features indicates that an average magnetic field of 1.0 ± 0.4 μG can balance internal pressure.

Context. Cas OB5 is an OB association located at a distance of 2.5–3 kpc that intercepts the Perseus spiral arm. It carries a moderate amount of reddening (AV ~ 2–3 mag) and contains several well-known open clusters within its boundaries, such as King 12, NGC 7788, and NGC 7790. The availability of modern clustering algorithms, together with Gaia DR3 kinematics and complementary spectroscopic data, makes it a suitable site for studies of Galactic structure. Aims. We seek to quantify the spatial scale of star formation in the spiral arms, using Cas OB5 as a pilot target before extending our study to more distant and extinguished regions of the Galaxy. Methods. We selected 129 695 candidate OBA stars in a 6x8 deg2 region around Cas OB5. We applied a spectral energy distribution (SED) fitting process to this sample to derive the physical parameters. Through this process, we found 56 379 OBA stars, which we then clustered using HDBSCAN. Results. We identified 17 open clusters inside this area, four of which appear to form a coherent structure that we identify as Cas OB5. Nevertheless, our findings suggest that these clusters belong to two different age groups despite sharing a similar position and kinematics. Spectroscopic observations confirm the youth of NGC 7788 (10–15 Myr) compared to NGC 7790 (110 ± 15 Myr). Conclusions. We have determined a spatial scale for star formation of a few tens of pc to a few hundreds of pc, comparing the clustered to the diffuse population of Cas OB5 across this part of the Perseus arm. A spectroscopic analysis was required to complement the clustering algorithm, so that we could separate younger OCs (tracers of the spiral arm) from older ones. These results highlight the need to combine these techniques to fully disentangle the Milky Way structure.

Abstract We conducted an ammonia survey targeting 217 sources from the BGPS v1.0 using the Nanshan 26 m radio telescope, focusing on the $\rm NH_3$(1,1) and (2,2) lines, selecting sources based on the criteria that their 1.1 mm fluxes are greater than 5 Jy and that they are observable with the Nanshan 26 m radio telescope at the Xinjiang Astronomical Observatory. We successfully detected 188 (86.6%) of our sources. These sources span a galactic longitude range of 1.2○ to 192.6○ and reside in the local, Perseus, Saggitarius-Carina, Scutum-Centaurus and Norma spiral arms and between them. Among them, 185 sources exhibit NH3 (1,1) and (2,2) inversion lines, all showing well-defined structural characteristics. Total $\rm NH_3$ column densities vary from 0.12× 1015 to 15× 1015cm−2, with para-$\rm NH_3$ fractional abundances averaging 1.8 × 10−7. This study concludes that 82% of our observed sources exhibit star formation activity, evidenced by 22 GHz water maser emission, and 52% are identified as High-Mass Star Formation Regions (HMSFRs) through 6.7 GHz class II methanol maser detections. Turbulence, reflected in non-thermal velocity dispersion, correlates positively with kinetic temperature, influencing cloud evolution and star formation. These results emphasize the role of turbulence in star formation processes and maser emission as their indicator. We also find a weak negative trend between ammonia column density and galactocentric distance with the 2σ significance, potentially linked to Galactic isotope ratio and gas density gradients.

ABSTRACT The external environments surrounding molecular clouds vary widely across galaxies such as the Milky Way, and statistical samples of clouds are required to understand them. We present the Perseus Arm Molecular Survey (PAMS), a James Clerk Maxwell Telescope (JCMT) survey combining new and archival data of molecular-cloud complexes in the outer Perseus spiral arm in $^{12}$CO, $^{13}$CO, and C$^{18}$O (J = 3–2). With a survey area of $\sim$8 deg$^2$, PAMS covers well-known complexes such as W3, W5, and NGC 7538 with two fields at $\ell \approx 110^{\circ }$ and $\ell \approx 135^{\circ }$. PAMS has an effective resolution of 17 arcsec, and rms sensitivity of $T_\mathrm{mb}= 0.7$–1.0 K in 0.3 km s$^{-1}$ channels. Here we present a first look at the data, and compare the PAMS regions in the Outer Galaxy with Inner Galaxy regions from the CO Heterodyne Inner Milky Way Plane Survey (CHIMPS). By comparing the various CO data with maps of H$_2$ column density from Herschel, we calculate representative values for the CO-to-H$_2$ column-density X-factors, which are $X_\mathrm{^{12}CO\, (3-2)}$$\, =4.0\times 10^{20}$ and $X_\mathrm{^{13}CO\, (3-2)}$$\, =4.0\times 10^{21}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$ with a factor of 1.5 uncertainty. We find that the emission profiles, size–linewidth, and mass–radius relationships of $^{13}$CO-traced structures are similar between the Inner and Outer Galaxy. Although PAMS sources are slightly more massive than their Inner Galaxy counterparts for a given size scale, the discrepancy can be accounted for by the Galactic gradient in gas-to-dust mass ratio, uncertainties in the X-factors, and selection biases. We have made the PAMS data publicly available, complementing other CO surveys targeting different regions of the Galaxy in different isotopologues and transitions.

Abstract Using data from the Large Area Multi-Object fiber Spectroscopic Telescope Medium-Resolution Spectroscopic Survey of Nebulae, we create a sample of 17,821 diffuse ionized gas spectra in the anticenter region of the Milky Way by excluding fibers in the directions of H ii regions and supernova remnants. We then analyze the radial and vertical distributions of three line ratios ([N ii]/Hα, [S ii]/Hα, and [S ii]/[N ii]), as well as the oxygen abundance. [N ii]/Hα and [S ii]/Hα do not exhibit a consistent, monotonic decrease with increasing Galactocentric distance (R gal). Instead, they show enhancement within the interarm region, positioned between the Local Arm and the Perseus Arm. [S ii]/[N ii] has a radial gradient of 0.1415 ± 0.0646 kpc−1 for the inner disk (8.34 < R gal < 9.65 kpc) and remains nearly flat for the outer disk (R gal > 9.65 kpc). In the vertical direction, [N ii]/Hα, [S ii]/Hα, and [S ii]/[N ii] increase with increasing Galactic disk height (∣z∣) in both southern and northern disks. Based on the N2S2Hα method, which combines [S ii]/[N ii] and [N ii]/Hα, we estimate the oxygen abundance. The oxygen abundance exhibits a consistent radial gradient with R gal, featuring a slope of −0.0559 ± 0.0209 dex kpc−1 for the inner disk and a similar slope of −0.0429 ± 0.0599 dex kpc−1 for the outer disk. A single linear fitting to the entire disk yields a slope of −0.0317 ± 0.0124 dex kpc−1. In the vertical direction, the oxygen abundance decreases with increasing ∣z∣ in both southern and northern disks.

ABSTRACT Star clusters provide unique advantages for investigating Galactic spiral arms, particularly due to their precise ages, positions, and kinematic properties, which are further enhanced by ongoing updates from the astrometric data. In this study, we employ the latest extensive catalogue of open clusters from Gaia DR3 to examine the positional deviations of clusters belonging to different age groups. Additionally, we employ dynamical simulations to probe the evolutionary behaviour of spiral arm positions. Our analysis reveals an absence of a theoretical age pattern in the spiral arms traced by open clusters, and the pattern speeds of the spiral arms are consistent with the rotation curve. Both of these results do not align with the predictions of quasi-stationary density wave theory, suggesting a more dynamic or transient arm scenario for the Milky Way. From this perspective, combined with vertex deviation estimates, it appears that the Local arm is in a state of growth. In contrast, the Sagittarius-Carina arm and the Perseus arm exhibit opposing trends. Consequently, we speculate that the Galactic stellar disc does not exhibit a grand-design spiral pattern with a fixed pattern speed, but rather manifests as a multi-armed structure with arms that continuously emerge and dissipate.

Abstract We report detection and analysis of the largest ever low-frequency sample of Crab giant pulses (GPs) detected in frequency band 200–231.25 MHz. In total, about $\sim$ 95 000 GPs were detected, which, to our knowledge is the largest low-frequency sample of Crab GPs presented in the literature. The observations were performed between 2024-12-14 and 2025-03-31 with the Engineering Development Array 2, a prototype station of the low-frequency Square Kilometre Array telescope. The fluence distribution of GPs in the entire sample is very well characterised with a single power law N(F) $\propto$ F $^\alpha$ , where $\alpha =-3.17\pm0.02$ for all GPs, and $\alpha_{MP} =$ $-3.13\pm0.02$ and $\alpha_{IP} =-3.59\pm0.06$ for GPs at the phases of the main pulse and low-frequency interpulse, respectively. We do not observe flattening of the fluence distribution at the higher fluences. Although the index of the power law fluence distribution remained approximately constant over the observing period, the normalisation of the distribution was strongly anti-correlated (coefficient $\approx -0.9$ ) with the scatter broadening time. The timescale ( $\sim$ weeks) of these variations indicates that intrinsic GP emission was modulated by the refractive scintillation as the signals propagated through the Crab Nebula and ISM. As a result, the measured fluence distribution was augmented for lower ( $\tau \approx$ 2 ms) and diminished for higher ( $\tau \approx$ 5 ms) scatter broadening time $\tau$ causing the GP detection rate to vary between 3 000 and 100 per hour, respectively (the correlation coefficient $\approx -0.9$ ). Furthermore, for the first time at low frequencies we observe indications of positive correlation (correlation coefficient $\approx$ 0.7) between the scatter broadening time ( $\tau$ ) and dispersion measure. Our modelling favours the screen size $\sim10^{-5}$ pc with mean electron density $\sim 400\textit{e}^{-}$ cm $^{-3}$ located within 100 pc from the pulsar (Crab Nebula or Perseus arm of the Milky Way galaxy). The observed frequency scaling of the scattering broadening time $\beta \approx -3.6\pm0.1$ (where $\tau \propto \nu^{\beta}$ ) is in agreement with the previous measurements. The observed maximum spectral luminosities $\sim 10^{25}$ erg/Hz/s approach those of the weakest pulses from some repeating fast radio bursts (FRBs). However, the distribution of pulse arrival times is consistent with a purely random Poisson process, and we do not find evidence of clustering. Overall, our results agree with the current views that GPs from extra-galactic Crab-like pulsars can be responsible for some very weak repeating FRBs, but cannot explain the entire FRB population. Finally, these results demonstrate an enormous transient science potential of individual SKA-Low stations, which can be unlocked by milli-second all-sky imaging.

Abstract Based on 32,162 molecular clouds from the Milky Way Imaging Scroll Painting project, we obtain new face-on molecular gas maps of the northern outer Galaxy. The total molecular gas surface density map reveals three segments of spirals, extending 16–43 kpc in length. The Perseus and Outer arms stand out prominently, appearing as quasi-continuous structures along most of their length. At the Galactic outskirts, about 1306 clouds connect the two segments of the new spiral arm discovered by T. M. Dame & P. Thaddeus (2011) in the first quadrant and Y. Sun et al. in the second quadrant, possibly extending the arm into the outer third quadrant. Logarithmic spirals can be fitted to the CO arm segments with pitch angles ranging from 4∘ to 12∘. These CO arms extend beyond previous CO studies and the optical radius, reaching a Galactic radius of about 22 kpc, comparable to the H i radial range.

We develop a new method for studying the Galactic magnetic field along the spiral arms using pulsar Faraday rotation measures (RMs). Our new technique accounts for the dot-product nature of Faraday rotation and also splits the associated path integral into segments corresponding to particular zones along the line of sight. We apply this geometrically corrected, arm-by-arm technique to the low-latitude portion of a recently published set of Arecibo Faraday RMs for 313 pulsars, along with previously obtained RMs in the same regions. We find disparities >1σ between the magnitude of the field above and below the plane in the Local Arm, Sagittarius Arm, Sagittarius-to-Scutum Interarm, Scutum Arm, and Perseus Arm. We find evidence for a single field reversal near the Local Arm–Sagittarius Arm boundary. Interestingly, our results suggest that this field reversal is dependent on latitude, occurring inside the Sagittarius Arm at negative Galactic latitudes and at the Local Arm–Sagittarius Arm boundary at positive Galactic latitudes. We discuss all of our results in the context of different models and other observational Galactic magnetic field analyses.

Aims. Our objective is to investigate the distribution of dust and associated large-scale structures of the Galaxy using optical linear polarization measurements of various open clusters located at different distances in the Galactic anticenter direction. Methods. We present R-band linear polarization observations of stars toward five open clusters: Kronberger 1, Berkeley 69, Berkeley 71, Berkeley 19, and King 8 in the anticenter direction. The polarization observations were carried out using the ARIES (Aryabhatta Research Institute of Observational Sciences) IMaging POLarimeter mounted on the 104 cm Sampurnanand telescope of ARIES, Nainital, making it the first study to target the polarization observations toward distant clusters (~6 kpc). We combined the observed polarization data with the distance information from the Gaia space telescope to infer the dust distribution along the line of sight. Results. The variation in the degree of polarization and extinction with distance reveals multiple dust layers in each cluster direction. In addition, common foreground-dust layers detected toward different cluster directions highlight global features such as spiral arms. Our results show that the dust clouds at 2 kpc toward Berkeley 69 and Berkeley 71 coincide with the Perseus arm, while the dust layer at ~4 kpc toward the distant clusters Berkeley 19 and King 8 indicates the presence of the Outer arm. The large-scale dust distribution obtained by combining our polarization results with previous polarization studies of nearby open clusters suggests that the anticenter direction is characterized by a low-extinction homogeneous dust distribution with a somewhat uniform orientation of the plane-of-sky component of the magnetic field along the line of sight. Conclusions. Our study demonstrates that polarization is useful as a tool for studying the large-scale dust distribution and structural features where kinematic distance methods are inadequate and cannot provide accurate distances to the dust clouds. The global dust distribution in the anticenter direction shows signatures of the intervening spiral arms.

Context. The Villafranca project is combining Gaia data with ground-based surveys to analyze Galactic stellar groups (clusters, associations, or parts thereof) with OB stars. Aims. We want to analyze the poorly studied cluster Stock 18 within the Villafranca project, as it is a very young stellar cluster with a symmetrical and compact H II region around it, Sh 2-170, so it is likely to provide insights into the structure and dynamics of such objects at an early stage of their evolution. Methods. We used Gaia astrometry, photometry, spectrophotometry, and variability data as well as ground-based spectroscopy and imaging to determine the characteristics of Stock 18. We used these data to analyze its core, massive star population, extinction, distance, membership, internal dynamics, density profile, IMF, stellar variability, and Galactic location. Results. Stock 18 is a very young (∼1.0 Ma) cluster located at a distance of 2.91 ± 0.10 kpc and is dominated by the GLS 13 370 system, whose primary (Aa) is an O9 V star. We propose that Stock 18 was in a very compact state (∼0.1 pc) about 1.0 Ma ago and that most massive stars were ejected at that time without significantly affecting the less massive stars as a result of multi-body dynamical interactions. Different age estimates also point toward an age close to 1.0 Ma, indicating that the dynamical interactions took place very shortly after massive star formation. Well-defined expanding stellar clusters have been observed before, but none are as young as this one. If we include all of the stars, the initial mass function is top heavy, but if we discard the ejected ones, it becomes nearly canonical. Therefore, this is another example (in addition to the previous one we found – the Bermuda cluster) of (a) a very young cluster with an already evolved present day mass function (b) that has significantly contributed to the future population of free-floating compact objects. If confirmed in more clusters, the number of such compact objects may be higher in the Milky Way than previously thought. Stock 18 has a variable extinction with an average value of R5495 higher than the canonical one of 3.1. We have discovered a new visual component (Ab) in the GLS 13 370 system. The cluster is above our Galactic mid-plane, likely as a result of the Galactic warp, and it has a distinct motion with respect to its surrounding old population, which is possibly an influence of the Perseus spiral arm.

ABSTRACT Studying the nature of spiral arms is essential for understanding the formation of the intricate disc structure of the Milky Way. The European Space Agency’s Gaia mission has provided revolutionary observational data that have uncovered detailed kinematical features of stars in the Milky Way. However, so far the nature of spiral arms continues to remain a mystery. Here, we present that the stellar kinematics traced by the classical Cepheids around the Perseus and Outer spiral arms in the Milky Way show strikingly different kinematical properties from each other: the radial and azimuthal velocities of Cepheids show positive and negative correlations in the Perseus and Outer arms, respectively. We also found that the dynamic spiral arms commonly seen in an N-body/hydrodynamic simulation of a Milky Way-like galaxy can naturally explain the observed kinematic trends. Furthermore, a comparison with such a simulation suggests that the Perseus arm is being disrupted, while the Outer arm is growing. Our findings suggest that two neighbouring spiral arms in distinct evolutionary phases – growing and disrupting phases – coexist in the Milky Way.