Perseus Arm Research Articles

$$\varvec{U\!BV\!I}$$ CCD photometry of Berkeley 55 open cluster

Fundamental astrophysical parameters have been derived for Berkeley 55 (Be 55) open cluster based on the UBVI charge-coupled device (CCD) photometric data observed using a AZT-22 1.5 m telescope at Maidanak Astronomical Observatory (MAO) in Uzbekistan. The mean reddening is obtained as \(E(B-V)=1.77\pm 0.10\,\mathrm {mag}\) from early-type members. The zero-age main sequence fitting in the \(Q_{V\lambda }\) – \(Q'\) diagrams indicates the distance modulus, \((V_{0}-M_{V})=12.4\pm 0.20\,\mathrm {mag}\,(d=3.02\pm 0.28\,\mathrm {kpc}\)). This photometric distance is consistent with the distances of Gaia EDR3 (\(d = 3.09 \pm 0.16\,\mathrm {kpc}\)) and period–luminosity relation (\(d =\) 2.78 ± 0.32 kpc) of its Cepheid S5 within the uncertainties. This distance also locates the cluster near the Perseus spiral arm. The Geneva isochrone fittings to the Hertzsprung–Russell diagram and observational colour–magnitude diagrams derive turn-off age, 85 ± 13 Myr, by taking care of five red supergiants/bright giants. The possible inconsistencies in the locations of the bright giants with the rotating/non-rotating isochrones may be due to both the age spread of stars in young open clusters and the diversity in rotational velocities.

1I/‘Oumuamua as an N2 Ice Fragment of an exo‐Pluto Surface: I. Size and Compositional Constraints

AbstractThe origin of the interstellar object 1I/‘Oumuamua has defied explanation. We perform calculations of the non‐gravitational acceleration that would be experienced by bodies composed of a range of different ices and demonstrate that a body composed of N2 ice would satisfy the available constraints on the non‐gravitational acceleration, size, and albedo, and lack of detectable emission of CO or CO2 or dust. We find that ‘Oumuamua was small, with dimensions 45 m × 44 m × 7.5 m at the time of observation at 1.42 au from the Sun, with a high albedo of 0.64. This albedo is consistent with the N2 surfaces of bodies like Pluto and Triton. We estimate ‘Oumuamua was ejected about 0.4–0.5 Gyr ago from a young stellar system, possibly in the Perseus arm. Objects like ‘Oumuamua may directly probe the surface compositions of a hitherto‐unobserved type of exoplanet: “exo‐plutos”. In a companion paper (Desch & Jackson, 2021) we demonstrate that dynamical instabilities like the one experienced by the Kuiper belt, in other stellar systems, plausibly could generate and eject large numbers of N2 ice fragments. ‘Oumuamua may be the first sample of an exoplanet brought to us.

1I/‘Oumuamua as an N2 Ice Fragment of an Exo‐Pluto Surface II: Generation of N2 Ice Fragments and the Origin of ‘Oumuamua

AbstractThe origin of the interstellar object 1I/‘Oumuamua has defied explanation. In a companion paper (Jackson & Desch, 2021), we show that a body of N2 ice with axes 45 m × 44 m × 7.5 m at the time of observation would be consistent with its albedo, nongravitational acceleration, and lack of observed CO or CO2 or dust. Here we demonstrate that impacts on the surfaces of Pluto‐like Kuiper belt objects (KBOs) would have generated and ejected ∼1014 collisional fragments—roughly half of them H2O ice fragments and half of them N2 ice fragments—due to the dynamical instability that depleted the primordial Kuiper belt. We show consistency between these numbers and the frequency with which we would observe interstellar objects like 1I/‘Oumuamua, and more comet‐like objects like 2I/Borisov, if other stellar systems eject such objects with efficiency like that of the Sun; we infer that differentiated KBOs and dynamical instabilities that eject impact‐generated fragments may be near‐universal among extrasolar systems. Galactic cosmic rays would erode such fragments over 4.5 Gyr, so that fragments are a small fraction (∼0.1%) of long‐period Oort comets, but C/2016 R2 may be an example. We estimate ‘Oumuamua was ejected about 0.4–0.5 Gyr ago, from a young (∼108 yr) stellar system, which we speculate was in the Perseus arm. Objects like ‘Oumuamua may directly probe the surface compositions of a hitherto‐unobserved type of exoplanet: “exo‐Plutos.” ‘Oumuamua may be the first sample of an exoplanet brought to us.

Dynamics of the Spiral-Arm Corotation and Its Observable Footprints in the Solar Neighborhood

This article discusses the effects of the spiral-arm corotation on the stellar dynamics in the Solar Neighborhood (SN). All our results presented here rely on: (1) observational evidence that the Sun lies near the corotation circle, where stars rotate with the same angular velocity as the spiral-arm pattern; the corotation circle establishes domains of the corotation resonance (CR) in the Galactic disk; (2) dynamical constraints that put the spiral-arm potential as the dominant perturbation in the SN, comparing with the effects of the central bar in the SN; (3) a long-lived nature of the spiral structure, promoting a state of dynamical relaxing and phase-mixing of the stellar orbits in response to the spiral perturbation. With an analytical model for the Galactic potential, composed of an axisymmetric background deduced from the observed rotation curve, and perturbed by a four-armed spiral pattern, numerical simulations of stellar orbits are performed to delineate the domains of regular and chaotic motions shaped by the resonances. Such studies show that stars can be trapped inside the stable zones of the spiral CR, and this orbital trapping mechanism could explain the dynamical origin of the Local arm of the Milky Way (MW). The spiral CR and the near high-order epicyclic resonances influence the velocity distribution in the SN, creating the observable structures such as moving groups and their radially extended counterpart known as diagonal ridges. The Sun and most of the SN stars evolve inside a stable zone of the spiral CR, never crossing the main spiral-arm structure, but oscillating in the region between the Sagittarius-Carina and Perseus arms. This orbital behavior of the Sun brings insights to our understanding of questions concerning the solar system evolution, the Earth environment changes, and the preservation of life on Earth.

Molecular Clouds in the Second Quadrant of the Milky Way Midplane from l = 104.°75 to l = 119.°75 and b = −5.°25 to b = 5.°25

Abstract In this work, we study the properties of molecular clouds in the second quadrant of the Milky Way Midplane, from l = 104.°75 to l = 119.°75, and b = −5.°25 to b = 5.°25, using the 12CO, 13CO, and C18O J = 1 − 0 emission line data from the Milky Way Imaging Scroll Painting project. We identify 857 and 300 clouds in the 12CO and 13CO spectral cubes, respectively, using the DENDROGRAM + SCIMES algorithms. The distances of the molecular clouds are estimated, and physical properties such as the mass, size, and surface densities of the clouds are tabulated. The molecular clouds in the Perseus Arm are about 30−50 times more massive, and 4−6 times larger than the clouds in the Local Arm. This result, however, is likely to be biased by distance selection effects. The surface densities of the clouds are enhanced in the Perseus Arm, with an average value of ∼100 M ⊙ pc−2. Here. we select the 40 most extended (>0.35 arcdeg2) molecular clouds from the 12CO catalog to build the H2 column density probability distribution function (N-PDF). Some 78% of the N-PDFs of the selected molecular clouds are well fitted with log-normal functions with only small deviations at high densities, corresponding to star-forming regions with scales of ∼1–5 pc in the Local Arm, and ∼5–10 pc in the Perseus Arm. About 18% of the selected molecular clouds have power-law N-PDFs at high densities. In these molecular clouds, the majority of the regions fitted with the power law correspond to molecular clumps at sizes of ∼1 pc, or filaments at widths of ∼1 pc.

The Alma catalogue of OB stars – II. A cross-match with Gaia DR2 and an updated map of the solar neighbourhood

ABSTRACT We cross-match the Alma catalogue of OB stars with Gaia DR2 astrometry and photometry as a first step towards producing a clean sample of massive stars in the solar neighbourhood with a high degree of completeness. We analyse the resulting colour–absolute magnitude diagram to divide our sample into categories and compare extinction estimates from two sources, finding problems with both of them. The distances obtained with three different priors are found to have few differences among them, indicating that Gaia DR2 distances are robust. An analysis of the 3D distribution of massive stars in the solar neighbourhood is presented. We show that a kinematically distinct structure we dub the Cepheus spur extends from the Orion–Cygnus spiral arm towards the Perseus arm and is located above the Galactic mid-plane, likely being related to the recently discovered Radcliffe wave. We propose that this corrugation pattern in the Galactic disc may be responsible for the recent enhanced star formation at its crests and troughs. We also discuss our plans to extend this work in the immediate future.

First Phase Space Portrait of a Hierarchical Stellar Structure in the Milky Way

Abstract We present the first detailed observational picture of a possible ongoing massive cluster hierarchical assembly in the Galactic disk as revealed by the analysis of the stellar full phase space (3D positions and kinematics and spectro-photometric properties) of an extended area (6° diameter) surrounding the well-known h and χ Persei double stellar cluster in the Perseus Arm. Gaia-EDR3 shows that the area is populated by seven comoving clusters, three of which were previously unknown, and by an extended and quite massive (M ∼ 105 M ⊙) halo. All stars and clusters define a complex structure with evidence of possible mutual interactions in the form of intra-cluster overdensities and/or bridges. They share the same chemical abundances (half-solar metallicity) and age (t ∼ 20 Myr) within a small confidence interval and the stellar density distribution of the surrounding diffuse stellar halo resembles that of a cluster-like stellar system. The combination of these pieces of evidence suggests that stars distributed within a few degrees from h and χ Persei are part of a common, substructured stellar complex that we named LISCA I. Comparison with results obtained through direct N-body simulations suggest that LISCA I may be at an intermediate stage of an ongoing cluster assembly that can eventually evolve in a relatively massive (a few times 105 M ⊙) stellar system. We argue that such a cluster formation mechanism may be quite efficient in the Milky Way and disk-like galaxies and, as a consequence, it has a relevant impact on our understanding of cluster formation efficiency as a function of the environment and redshift.

Trigonometric Parallaxes of Four Star-forming Regions in the Distant Inner Galaxy

Abstract We have measured trigonometric parallaxes for four H2O masers associated with distant massive young stars in the inner regions of the Galaxy using the Very Long Baseline Array as part of the BeSSeL Survey. G026.50 + 0.28 is located at the near end of the Galactic bar, perhaps at the origin of the Norma spiral arm. G020.77−0.05 is in the Galactic Center region and is likely associated with a far-side extension of the Scutum arm. G019.60−0.23 and G020.08−0.13 are likely associated and lie well past the Galactic Center. These sources appear to be in the Sagittarius spiral arm, but an association with the Perseus arm cannot be ruled out.

The historical record of massive star formation in Cygnus

Context. The Cygnus region, which dominates the local spiral arm of the Galaxy, is one of the nearest complexes of massive star formation, extending over several hundred parsecs. Its massive stellar content, regions of ongoing star formation, and molecular gas have been studied in detail at virtually all wavelengths. However, little is known of the history of the region beyond the past 10 Myr. Aims. We use the correlations between age, mass and luminosity of red supergiants to explore the history of star formation in Cygnus previous to the formation of the present-day associations. The brightness and spectroscopic characteristics of red supergiants make it easy to identify them and build up a virtually complete sample of such stars at the distance of the Cygnus region, thus providing a record of massive star formation extending several tens of Myr into the past, a period inaccessible through the O and early B stars observable at present. Methods. We have made a selection based on the 2MASS colors of a sample of bright, red stars in an area of 84 square degrees covering the whole present extension of the Cygnus association in the Local Arm. We have obtained spectroscopy in the red visible range allowing an accurate, homogeneous spectral classification as well as a reliable separation between supergiants and other cool stars. Our data are complemented with Gaia Data Release 2 astrometric data. Results. We have identified 29 red supergiants in the area, 17 of which had not been previously classified as supergiants. Twenty-four of the 29 most likely belong to the Cygnus region and four of the remaining to the Perseus arm. We have used their derived luminosities and masses to infer the star formation history of the region. Intense massive star formation activity is found to have started approximately 15 Myr ago, and we find evidence for two other episodes, one taking place between 20 and 30 Myr ago and another one having ended approximately 40 Myr ago. There are small but significant differences between the kinematic properties of red supergiants younger or older then 20 Myr, hinting that stars of the older group were formed outside the precursor of the present Cygnus complex, possibly in the Sagittarius-Carina arm.

Untangling the Galaxy. II. Structure within 3 kpc

Abstract We present the results of the hierarchical clustering analysis of the Gaia DR2 data to search for clusters, comoving groups, and other stellar structures. The current paper builds on the sample from the previous work, extending it in distance from 1 to 3 kpc and increasing the number of identified structures up to 8292. To aid in the analysis of the population properties, we developed a neural network called Auriga to robustly estimate the age, extinction, and distance of a stellar group based on the input photometry and parallaxes of the individual members. We apply Auriga to derive the properties of not only the structures found in this paper, but also previously identified open clusters. Through this work, we examine the temporal structure of the spiral arms. Specifically, we find that the Sagittarius Arm has moved by >500 pc in the last 100 Myr and the Perseus Arm has been experiencing a relative lull in star formation activity over the last 25 Myr. We confirm the findings of the previous paper on the transient nature of the spiral arms, with the timescale of transition of a few 100 Myr. Finally, we find a peculiar ∼1 Gyr old stream of stars that appears to be heliocentric. Its origin is unclear.

Detailed Studies of IPHAS sources. II. Sab 19, a true planetary nebula and its mimic crossing the Perseus Arm.

Abstract The INT Photometric Hα Survey (IPHAS) has provided us with a number of new-emission line sources, among which planetary nebulae (PNe) constitute an important fraction. Here we present a detailed analysis of the IPHAS nebula Sab 19 (IPHASX J055242.8+262116) based on radio, infrared, and optical images and intermediate- and high-dispersion longslit spectra. Sab 19 consists of a roundish 0.10 pc in radius double-shell nebula surrounded by a much larger 2.8 pc in radius external shell with a prominent H-shaped filament. We confirm the nature of the main nebula as a PN whose sub-solar N/O ratio abundances, low ionized mass, peculiar radial velocity, and low-mass central star allow us to catalog it as a type III PN. Apparently, the progenitor star of Sab 19 became a PN when crossing the Perseus Arm during a brief visit of a few Myr. The higher N/O ratio and velocity shift ≃40 km s−1 of the external shell with respect to the main nebula and its large ionized mass suggest that it is not truly associated with Sab 19, but it is rather dominated by a Strömgren zone in the interstellar medium ionized by the PN central star.

Evolution of the OH and H2O Maser Emission in the Active Star-Forming Region IRAS 05358+3543 (S231)

We present the results of our observations of the star-forming region located in the Perseus arm and associated with the Sharpless nebula S231 in the lines of H2O at a wavelength of 1.35 cm and OH at a wavelength of 18 cm obtained with the Pushchino 22-m radio telescope (Russia) and the Nancay Large Radio Telescope (France), respectively. We provide a catalog of H2O maser spectra in the period from March 2003 to March 2020. The variability of the integrated flux that we have calculated based on the monitoring by Felli et al. (1987–2007) and our monitoring (2003–2020) has a quasi-cyclic pattern with a cycle duration from 4.3 to 7.7 yr. Flares of maser emission of both single and several features occurred in each activity cycle. The mean rise and decay times of the flare emission were $$0.30 \pm 0.02$$ and $$0.35 \pm 0.02$$ yr, respectively. The observed radial velocity drift and jumps of the H2O emission features can be a consequence of the impact of shocks emerging as the protostar activity rises on them. The complex pattern of variability shows that the H2O masing medium is fragmented and small-scale turbulent motions of matter can occur in it. We have observed the OH maser emission in the 1665 and 1667 MHz main lines and the 1720 MHz satellite line. A large number of OH emission features and their variability have been detected. We have found one Zeeman pair at ‒9.2 km/s with a small splitting, with the longitudinal magnetic field in the period 2008–2020 having decreased monotonically from 0.24 to 0.10 mG. For the three most intense features we have calculated the degrees of linear polarization and position angles. A correlation between the H2O and OH maser activities has been found.

Sixteen overlooked open clusters in the fourth Galactic quadrant

Aims. This paper has two main objectives: (1) To determine the intrinsic properties of 16 faint and mostly unstudied open clusters in the poorly known sector of the Galaxy at 270° −300° to probe the Milky Way structure in future investigations. (2) To address previously reported systematics in Gaia DR2 parallaxes by comparing the cluster distances derived from photometry with those derived from parallaxes. Methods. Deep UBVI photometry of 16 open clusters was carried out. Observations were reduced and analyzed in an automatic way using the ASteCA package to obtain individual distances, reddening, masses, ages, and metallicities. Photometric distances were compared to those obtained from a Bayesian analysis of Gaia DR2 parallaxes. Results. Ten out of the sixteen clusters are true or highly probable open clusters. Two of them are quite young and follow the trace of the Carina Arm and the already detected warp. The remaining clusters are placed in the interarm zone between the Perseus and Carina Arms, as expected for older objects. We found that the cluster van den Berg-Hagen 85 is 7.5 × 109 yr old, which means that it is one of the oldest open clusters detected in our Galaxy so far. The relationship of these ten clusters with the Galaxy structure in the solar neighborhood is discussed. The comparison of distances from photometry and parallaxes data in turn reveals a variable level of disagreement. Conclusions. Various zero-point corrections for Gaia DR2 parallax data recently reported were considered for a comparison between photometry- and parallax-based distances. The results tend to improve with some of these corrections. Photometric distance analysis suggests an average correction of ∼+0.026 mas (to be added to the parallaxes). The correction may have a more intricate dependence on distance, but addressing this level of detail will require a larger cluster sample.

Searching for Molecular Outflows with Support Vector Machines: The Dark Cloud Complex in Cygnus

Abstract We present a survey of molecular outflows across the dark cloud complex in the Cygnus region, based on a 46.75 deg2 field of CO isotopologue data from the Milky Way Imaging Scroll Painting survey. A supervised machine-learning algorithm, the support vector machine, is introduced to accelerate our visual assessment of outflow features in the data cube of 12CO and 13CO J = 1−0 emission. A total of 130 outflow candidates are identified, 77 of which show bipolar structures and 118 are new detections. Spatially, these outflows are located inside dense molecular clouds, and some of them are found in clusters or in elongated linear structures tracing the underlying gas filament morphology. Along the line of sight, 97, 31, and 2 candidates reside in the Local, Perseus, and Outer Arms, respectively. Young stellar objects as outflow drivers are found near most outflows, while 36 candidates show no associated source. The clusters of outflows that we detect are inhomogeneous in their properties; nevertheless, we show that the outflows cannot inject turbulent energy on cloud scales. Instead, at best, they are restricted to affecting the so-called “clump” and “core” scales, and only on short (∼0.3 Myr) estimated timescales. Combined with outflow samples in the literature, our work shows a tight outflow mass–size correlation.

A comprehensive study of open clusters Czernik 14, Haffner 14, Haffner 17 and King 10 using multicolour photometry and Gaia DR2 astrometry

ABSTRACT This paper presents an investigation into the four open clusters Czernik 14, Haffner 14, Haffner 17 and King 10, located near the Perseus arm of the Milky Way, using the Gaia DR2, 2MASS, WISE, APASS and Pan-STARRS1 data sets. We find normal interstellar extinction in 12 photometric bands for these clusters. Using Gaia DR2 proper motion data, the numbers of likely cluster members are found to be 225, 353, 350 and 395 for Czernik 14, Haffner 14, Haffner 17 and King 10, respectively. Radii are determined as 3.5, 3.7, 6.2 and 5.7 arcmin for Czernik 14, Haffner 14, Haffner 17 and King 10, respectively. Mean proper motions in RA and Dec. are estimated as (−0.42 ± 0.02, −0.38 ± 0.01), (−1.82 ± 0.009, 1.73 ± 0.008), (−1.17 ± 0.007, 1.88 ± 0.006) and (−2.75 ± 0.008, −2.04 ± 0.006) mas yr−1 for Czernik 14, Haffner 14, Haffner 17 and King 10, respectively. The comparison of observed colour–magnitude diagrams with solar metallicity isochrones leads to ages of 570 ± 60, 320 ± 35, 90 ± 10 and 45 ± 5 Myr for these clusters. The distances of 2.9 ± 0.1, 4.8 ± 0.4, 3.6 ± 0.1 and 3.8 ± 0.1 kpc determined using parallax are similar to the values derived by the isochrone-fitting method. Mass function slopes are found to be in good agreement with the Salpeter value. The total masses are derived as 348, 595, 763 and 1088 M⊙ for Czernik 14, Haffner 14, Haffner 17 and King 10, respectively. Evidence for the existence of a mass-segregation effect is observed in each cluster. Using the Galactic potential model, Galactic orbits are derived for the clusters. It is found that all four clusters follow a circular path around the Galactic Centre.

Interarm islands in the Milky Way – the one near the Cygnus spiral arm

ABSTRACT This study extends to the structure of the Galaxy. Our main goal is to focus on the first spiral arm beyond the Perseus arm, often called the Cygnus arm or the ‘Outer Norma’ arm, by appraising the distributions of the masers near the Cygnus arm. The method is to employ masers whose trigonometric distances were measured with accuracy. The maser data come from published literature – see column 8 in Table 1 here, having been obtained via the existing networks (US VLBA, the Japanese VERA, the European VLBI, and the Australian LBA). The new results for Cygnus are split in two groups: those located near a recent CO-fitted global model spiral arm and those congregating within an ‘interarm island’ located halfway between the Perseus arm and the Cygnus arm. Next, we compare this island with other similar interarm objects near other spiral arms. Thus, we delineate an interarm island (6 × 2 kpc) located between the two long spiral arms (Cygnus and Perseus arms); this is reminiscent of the small ‘Local Orion arm’ (4 × 2 kpc) found earlier between the Perseus and Sagittarius arms and of the old ‘Loop’ (2 × 0.5 kpc) found earlier between the Sagittarius and Scutum arms. Various arm models are compared, based on observational data (masers, H II regions, H I gas, young stars, CO 1–0 gas).

New insights in giant molecular cloud hosting the S147/S153 complex: signatures of interacting clouds

Abstract In order to understand the formation of massive OB stars, we report a multi-wavelength observational study of a giant molecular cloud hosting the S147/S153 complex (size ${\sim}90\:\:\mbox{pc} \times 50\:$pc). The selected complex is located in the Perseus arm, and contains at least five H ii regions (S147, S148, S149, S152, and S153) powered by massive OB stars with dynamical ages ${\sim}0.2$–$0.6\:$Myr. The Canadian Galactic Plane Survey $^{12}$CO line data (beam size ${\sim}{100{^{\prime \prime}_{.}}4}$) trace the complex in a velocity range of [$-59$, $-43]\:$km$\:$s$^{-1}$, and also reveal the presence of two molecular cloud components around $-54$ and $-49\:$km$\:$s$^{-1}$ in the direction of the complex. Signatures of the interaction/collision between these extended cloud components are investigated through their spatial and velocity connections. These outcomes suggest the collision of these molecular cloud components about $1.6\:$Myr ago. Based on the observed overlapping zones of the two clouds, the collision axis appears to be parallel to the line-of-sight. Deep near-infrared photometric analysis of point-like sources shows the distribution of infrared-excess sources in the direction of the overlapping zones of the molecular cloud components, where all the H ii regions are also spatially located. All elements put together, the birth of massive OB stars and embedded infrared-excess sources seems to be triggered by two colliding molecular clouds in the selected site. High-resolution observations of the dense gas tracer will be required to further confirm the proposed scenario.

Open Cluster IC 1369 and Its Vicinity: Multicolor Photometry and Gaia DR2 Astrometry

Abstract The open cluster IC 1369, located in Cygnus close to the galactic equator at the 89.°6 longitude, and its vicinity are investigated by applying two-dimensional photometric classification of stars down to V = 19 mag measured in the Vilnius seven-color photometric system. Employing the Gaia DR2 coordinates, parallaxes, and proper motions, 143 cluster members are found having cluster membership probabilities ≥0.75. The hottest stars of the evolved main sequence are of spectral class B9. Distances to the cluster, determined by two methods based on the Gaia parallaxes, are 3280 and 3370 pc with an uncertainty of 400–500 pc. Employing the physical HR diagram with the coordinates versus and the Padova isochrones, the cluster age is found to be between 300 and 350 Myr. Among the cluster members we identified six red giants of spectral classes G5–G8 and luminosities II–III, 17 Am stars, one Ap star, and one B5 blue straggler. The extinction shows a steep rise up to ∼2.0 mag at 300–700 pc, caused by the extensions of the dust cloud LDN 970 and the Great Cygnus Rift, and up to ∼2.5 mag at the edge of the Perseus arm at 3.0 kpc. The extinction is almost constant between 0.9 and 2.5 kpc from the Sun. The average extinction of the cluster members is 2.54 mag. A systematic error of color excesses found in the Argonaut calculator results is discussed.

The HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and H I overview

Context.The Galactic plane has been observed extensively by a large number of Galactic plane surveys from infrared to radio wavelengths at an angular resolution below 40′′. However, a 21 cm line and continuum survey with comparable spatial resolution is lacking.Aims.The first half of THOR data (l= 14.0°−37.9°, andl= 47.1°−51.2°, |b|≤ 1.25°) has been published in our data release 1 paper. With this data release 2 paper, we publish all the remaining spectral line data and Stokes I continuum data with high angular resolution (10′′–40′′), including a new H Idataset for the whole THOR survey region (l= 14.0−67.4° and |b|≤ 1.25°). As we published the results of OH lines and continuum emission elsewhere, we concentrate on the H Ianalysis in this paper.Methods.With theKarl G. JanskyVery Large Array (VLA) in C-configuration, we observed a large portion of the first Galactic quadrant, achieving an angular resolution of ≤40′′. AtLBand, the WIDAR correlator at the VLA was set to cover the 21 cm H Iline, four OH transitions, a series of Hnαradio recombination lines (RRLs;n= 151 to 186), and eight 128 MHz-wide continuum spectral windows, simultaneously.Results.We publish all OH and RRL data from the C-configuration observations, and a new H Idataset combining VLA C+D+GBT (VLA D-configuration and GBT data are from the VLA Galactic Plane Survey) for the whole survey. The H Iemission shows clear filamentary substructures at negative velocities with low velocity crowding. The emission at positive velocities is more smeared-out, likely due to higher spatial and velocity crowding of structures at the positive velocities. Compared to the spiral arm model of the Milky Way, the atomic gas follows the Sagittarius and Perseus Arm well, but with significant material in the inter-arm regions. With the C-configuration-only H I+continuum data, we produce an H Ioptical depth map of the THOR areal coverage from 228 absorption spectra with the nearest-neighbor method. With thisτmap, we corrected the H Iemission for optical depth, and the derived column density is 38% higher than the column density with optically thin assumption. The total H Imass with optical depth correction in the survey region is 4.7 × 108 M⊙, 31% more than the mass derived assuming the emission is optically thin. If we applied this 31% correction to the whole Milky Way, the total atomic gas mass would be 9.4–10.5 × 109 M⊙. Comparing the H Iwith existing CO data, we find a significant increase in the atomic-to-molecular gas ratio from the spiral arms to the inter-arm regions.Conclusions.The high-sensitivity and resolution THOR H Idataset provides an important new window on the physical and kinematic properties of gas in the inner Galaxy. Although the optical depth we derive is a lower limit, our study shows that the optical depth correction issignificant for H Icolumn density and mass estimation. Together with the OH, RRL and continuum emission from the THOR survey, these new H Idata provide the basis for high-angular-resolution studies of the interstellar medium in different phases.

A large catalogue of molecular clouds with accurate distances within 4 kpc of the Galactic disc

ABSTRACT We present a large, homogeneous catalogue of molecular clouds within 4 kpc from the Sun at low Galactic latitudes (|b| < 10°) with unprecedented accurate distance determinations. Based on the 3D dust reddening map and estimates of colour excesses and distances of over 32 million stars presented in Chen et al., we have identified 567 dust/molecular clouds with a hierarchical structure identification method and obtained their distance estimates by a dust model fitting algorithm. The typical distance uncertainty is less than 5 per cent. As far as we know, this is the first large catalogue of molecular clouds in the Galactic plane with distances derived in a direct manner. The clouds are seen to lie along the Sagittarius, Local and Perseus Arms. In addition to the known structures, we propose the existence of a possible spur, with a pitch angle of about 34°, connecting the Local and the Sagittarius Arms in the fourth quadrant. We have also derived the physical properties of those molecular clouds. The distribution of cloud properties in different parameter spaces agrees grossly with the previous results. Our cloud sample is an ideal starting point to study the concentration of dust and gas in the solar vicinity and their star formation activities.