Kpc Arm Research Articles

CO Emission Delineating the Interface between the Milky Way Nuclear Wind Cavity and the Gaseous Disk

Based on the MWISP survey, we study high-z CO emission toward the tangent points, in which the distances of the molecular clouds (MCs) are well determined. In the region of l = 12°–26° and ∣b∣ ≲ 5.°1, a total of 321 MCs with ∣z∣ ≳ 110 pc are identified, of which nearly 30 extreme high-z MCs (EHMCs at ∣z∣ ≳ 260 pc) are concentrated in a narrow region of R GC ∼ 2.6–3.1 kpc. The EHMC concentrations, together with other high-z MCs at R GC ≲ 2.3–2.6 kpc, constitute molecular crater-wall structures surrounding the edges of the H i voids that are physically associated with the Fermi bubbles. Intriguingly, some large high-z MCs, which lie in the crater walls above and below the Galactic plane, show cometary structures with the head toward the plane, favoring the scenario that the entrained molecular gas moves with the multiphase flows from the plane to the high-z regions. We suggest that the Milky Way nuclear wind has a significant impact on the Galactic gaseous disk. The powerful nuclear wind at ∼3–6 Myr ago is likely responsible for the observational features: (1) the enhanced CO gas lying in the edges of the H i voids, (2) the deficiency of atomic and molecular gas within R GC ≲ 3 kpc, (3) the possible connection between the EHMC concentrations and the 3 kpc arm, and (4) the elongated high-z MCs with the tail pointing away from the Galactic plane.

Gas Dynamics in the Galaxy: Total Mass Distribution and the Bar Pattern Speed

Abstract Gas morphology and kinematics in the Milky Way contain key information for understanding the formation and evolution of our Galaxy. We present hydrodynamical simulations based on realistic barred Milky Way potentials constrained by recent observations. Our model can reproduce most features in the observed longitude–velocity diagram, including the Central Molecular Zone, the Near and Far 3 kpc arms, the Molecular Ring, and the spiral arm tangents. It can also explain the noncircular motions of masers from the recent BeSSeL2 survey. The central gas kinematics are consistent with a mass of 6.9 × 108 M ⊙ in the Nuclear Stellar Disk. Our model predicts the formation of an elliptical gaseous ring surrounding the bar, which is composed of the 3 kpc arms, the Norma arm, and the bar-spiral interfaces. This ring is similar to those “inner” rings in some Milky Way analogs with a boxy/peanut-shaped bulge (e.g., NGC 4565 and NGC 5746). The kinematics of gas near the solar neighborhood are governed by the Local arm. The bar pattern speed constrained by our gas model is 37.5–40 km s−1 kpc−1, corresponding to a corotation radius of R CR = 6.0–6.4 kpc. The rotation curve of our model rises gently within the central ∼ 5 kpc, significantly less steep than those predicted by some recent zoom-in cosmological simulations.

A New Transition Wolf–Rayet WN/C Star in the Milky Way

Abstract We report the discovery of a new transition type Wolf–Rayet (W-R) WN/C star in the Galaxy. According to its coordinates (R.A., decl.)J2000 = 18 h 51 m 39 .ˢ 7 , −05°34′51.″1, and the distance (7.11 − 1.22 + 1.56 kpc away from Earth) inferred from the second Gaia data release, it is found that W-R 121-16 is located in the Far 3 kpc Arm, and it is 3.75 kpc away from the Galactic Center. The optical spectra obtained by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope and the 2.16 m telescope, both located at the Xinglong Observatory in China, indicate that this is a W-R star of the transitional WN7o/WC subtype. A current stellar mass of about 7.1 − 1.1 + 1.7 M ☉, a mass-loss rate of M ̇ = 10 − 4.97 − 0.20 + 0.16 M ☉ yr − 1 , a bolometric luminosity of log L/L ☉ = 4.88 − 0.15 + 0.17 , and a stellar temperature of T * = 47 − 5 + 9 kK are derived by fitting the observed spectrum with a specific Potsdam W-R model. The magnitude in V-band varies between 13.95 and 14.14 mag, while no period is found. Based on the optical spectra, the time domain data, and the indices of the astrometric solution of the Gaia data, W-R 121-16 is likely a transitional WN/C single star rather than a WN+WC binary.

FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45 m telescope (FUGIN). V. Dense gas mass fraction of molecular gas in the Galactic plane

Abstract Recent observations of the nearby Galactic molecular clouds indicate that the dense gas in molecular clouds has quasi-universal properties on star formation, and observational studies of extra-galaxies have shown a galactic-scale correlation between the star formation rate (SFR) and the surface density of molecular gas. To reach a comprehensive understanding of both properties, it is important to quantify the fractional mass of dense gas in molecular clouds, fDG. In particular, for the Milky Way (MW) there are no previous studies resolving fDG disk over a scale of several kpc. In this study, fDG was measured over 5 kpc in the first quadrant of the MW, based on the CO J = 1–0 data in l = 10°–50° obtained as part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) project. The total molecular mass was measured using 12CO, and the dense gas mass was estimated using C18O. The fractional masses, including fDG, in the region within ±30% of the distances to the tangential points of the Galactic rotation (e.g., the Galactic Bar, Far-3 kpc Arm, Norma Arm, Scutum Arm, Sagittarius Arm, and inter-arm regions) were measured. As a result, an averaged fDG of $2.9^{+2.6}_{-2.6}$% was obtained for the entirety of the target region. This low value suggests that dense gas formation is the primary factor in inefficient star formation in galaxies. It was also found that fDG shows large variations depending on the structures in the MW disk. In the Galactic arms, fDG was estimated to be ∼4%–5%, while in the bar and inter-arm regions it was as small as ∼0.1%–0.4%. These results indicate that the formation/destruction processes of the dense gas and their timescales are different for different regions in the MW, leading to differences in Star formation efficiencies.

Small-scale physical and chemical structure of diffuse and translucent molecular clouds along the line of sight to Sgr B2

Context. The diffuse and translucent molecular clouds traced in absorption along the line of sight to strong background sources have so far been investigated mainly in the spectral domain because of limited angular resolution or small sizes of the background sources. Aims. We aim to resolve and investigate the spatial structure of molecular clouds traced by several molecules detected in absorption along the line of sight to Sgr B2(N). Methods. We have used spectral line data from the EMoCA survey performed with the Atacama Large Millimeter/submillimeter Array (ALMA), taking advantage of its high sensitivity and angular resolution. The velocity structure across the field of view is investigated by automatically fitting synthetic spectra to the detected absorption features, which allows us to decompose them into individual clouds located in the Galactic centre (GC) region and in spiral arms along the line of sight. We compute opacity maps for all detected molecules. We investigated the spatial and kinematical structure of the individual clouds with statistical methods and perform a principal component analysis to search for correlations between the detected molecules. To investigate the nature of the molecular clouds along the line of sight to Sgr B2, we also used archival Mopra data. Results. We identify, on the basis of c-C3H2, 15 main velocity components along the line of sight to Sgr B2(N) and several components associated with the envelope of Sgr B2 itself. The c-C3H2 column densities reveal two categories of clouds. Clouds in Category I (3 kpc arm, 4 kpc arm, and some GC clouds) have smaller c-C3H2 column densities, smaller linewidths, and smaller widths of their column density PDFs than clouds in Category II (Scutum arm, Sgr arm, and other GC clouds). We derive opacity maps for the following molecules: c-C3H2, H13CO+, 13CO, HNC and its isotopologue HN13C, HC15N, CS and its isotopologues C34S and 13CS, SiO, SO, and CH3OH. These maps reveal that most molecules trace relatively homogeneous structures that are more extended than the field of view defined by the background continuum emission (about 15′′, that is 0.08–0.6 pc depending on the distance). SO and SiO show more complex structures with smaller clumps of size ~5–8′′. Our analysis suggests that the driving of the turbulence is mainly solenoidal in the investigated clouds. Conclusions. On the basis of HCO+, we conclude that most line-of-sight clouds towards Sgr B2 are translucent, including all clouds where complex organic molecules were recently detected. We also conclude that CCH and CH are good probes of H2 in both diffuse and translucent clouds, while HCO+ and c-C3H2 in translucent clouds depart from the correlations with H2 found in diffuse clouds.

ALMA observations of molecular absorption in four directions toward the Galactic bulge

Context. Alma Cycle 3 observations serendipitously showed strong absorption from diffuse molecular gas in the Galactic bulge at − 200 km s−1 < v < −140 km s−1 toward the compact extragalactic continuum source J1744-3116 at (l, b) = −2.13°, − 1.00°. Aims. We aimed to test whether molecular gas in the bulge could also be detected toward the three other, sufficiently strong mm-wave continuum sources seen toward the bulge at |b| < 3°. Methods. We took absorption profiles of HCO+ (1−0), HCN(1−0), C2H (1−0), CS(2−1) and H13CO+ (1−0) in ALMA Cycle 4 toward J1713-3418, J1717-3341, J1733-3722 and J1744-3116. Results. Strong molecular absorption from disk gas at |v|≲30 km s−1 was detected in all directions, and absorption from the 3 kpc arm was newly detected toward J1717 and J1744. However, only the sightline toward J1744 is dominated by molecular gas overall and no other sightlines showed molecular absorption from gas deep inside the bulge. No molecular absorption was detected toward J1717 where H I emission from the bulge was previously known. As observed in HCO+, HCN, C2H and CS, the bulge gas toward J1744 at v < −135 km s−1 has chemistry and kinematics like that seen near the Sun and in the Milky Way disk generally. We measured isotopologic ratios N(HCO+)/N(H13CO+) > 51 (3σ) for the bulge gas toward J1744 and 58 ± 9 and 64 ± 4 for the disk gas toward J1717 and J1744, respectively, all well above the value of 20−25 typical of the central molecular zone. Conclusions. The kinematics and chemistry of the bulge gas observed toward J1744 more nearly resemble that of gas in the Milky Way disk than in the central molecular zone.

On the many ‘3-kiloparsec arms’—shocked wave and nuclear rotation

Many features near the Galactic Center have been called 3-kiloparsec arms. We reached a point of having too many divergent data, making it difficult to be constrained by a single physical model. Their differing characteristics suggest different physical and dynamical objects. Radial velocity data on the so-called 3-kpc arms do not coincide with radial velocities of major spiral arms near 3kpc, nor near 2 kpc, nor near 4 kpc from the Galactic Center (Fig. 1 and 2). Different 3-kpc arm features may require different models: turbulence around a shock in a Galactic density wave between 2 and 4 kpc from the Galactic Center (Table 1), or nuclear rotation between 0 and 2 kpc from the Galactic Center region (Table 2), or a putative radial expansion between 0 and 4 kpc from the Galactic Center. Despite their naming as Near 3-kiloparsec arms or Far 3-kiloparsec arms, these features are not major arms. Those 3-kpc arm features nearer the Galactic Center (within 13o of Galactic longitude) may be different than those farther out (Table 2). Here we show that the plethora of observed '3-kpc arm' features can be separated in two: those with Galactic longitude of 13 degrees or more away from the Galactic Center (Table 1 - some of which are possibly associated with the observed major spiral arms), and those within 13 degrees from the Galactic Center (Table 2 - some of which are possibly associated with the observed central bars; Fig.1 and Fig.2).

CLOUD STRUCTURE OF GALACTIC OB CLUSTER-FORMING REGIONS FROM COMBINING GROUND- AND SPACE-BASED BOLOMETRIC OBSERVATIONS

ABSTRACT We have developed an iterative procedure to systematically combine the millimeter and submillimeter images of OB cluster-forming molecular clouds, which were taken by ground-based (CSO, JCMT, APEX, and IRAM-30 m) and space telescopes (Herschel and Planck). For the seven luminous ( L ⊙) Galactic OB cluster-forming molecular clouds selected for our analyses, namely W49A, W43-Main, W43-South, W33, G10.6-0.4, G10.2-0.3, and G10.3-0.1, we have performed single-component, modified blackbody fits to each pixel of the combined (sub)millimeter images, and the Herschel PACS and SPIRE images at shorter wavelengths. The ∼10″ resolution dust column density and temperature maps of these sources revealed dramatically different morphologies, indicating very different modes of OB cluster-formation, or parent molecular cloud structures in different evolutionary stages. The molecular clouds W49A, W33, and G10.6-0.4 show centrally concentrated massive molecular clumps that are connected with approximately radially orientated molecular gas filaments. The W43-Main and W43-South molecular cloud complexes, which are located at the intersection of the Galactic near 3 kpc (or Scutum) arm and the Galactic bar, show a widely scattered distribution of dense molecular clumps/cores over the observed ∼10 pc spatial scale. The relatively evolved sources G10.2-0.3 and G10.3-0.1 appear to be affected by stellar feedback, and show a complicated cloud morphology embedded with abundant dense molecular clumps/cores. We find that with the high angular resolution we achieved, our visual classification of cloud morphology can be linked to the systematically derived statistical quantities (i.e., the enclosed mass profile, the column density probability distribution function (N-PDF), the two-point correlation function of column density, and the probability distribution function of clump/core separations). In particular, the massive molecular gas clumps located at the center of G10.6-0.4 and W49A, which contribute to a considerable fraction of their overall cloud masses, may be special OB cluster-forming environments as a direct consequence of global cloud collapse. These centralized massive molecular gas clumps also uniquely occupy much higher column densities than what is determined by the overall fit of power-law N-PDF. We have made efforts to archive the derived statistical quantities of individual target sources, to permit comparisons with theoretical frameworks, numerical simulations, and other observations in the future.

GAS DYNAMICS IN THE MILKY WAY: A LOW PATTERN SPEED MODEL

ABSTRACT We present gas flow models for the Milky Way based on high-resolution grid-based hydrodynamical simulations. The basic galactic potential we use is from an N-body model constrained by the density of red clump giants in the Galactic bulge. We augment this potential with a nuclear bulge, two pairs of spiral arms, and additional mass at the bar end to represent the long bar component. With this combined model we can reproduce many features in the observed ( ) diagram with a bar pattern speed of and a spiral pattern speed of . The shape and kinematics of the nuclear ring, Bania’s Clump 2, the Connecting arm, the Near and Far 3 kpc arms, the Molecular Ring, and the spiral arm tangent points in our simulations are comparable to those in the observations. Our results imply that a low pattern speed model for the bar in our Milky Way reproduces the observations for a suitable Galactic potential. Our best model gives a better match to the ( ) diagram than previous high pattern speed hydrodynamical simulations.

THE START OF THE SAGITTARIUS SPIRAL ARM (SAGITTARIUS ORIGIN) AND THE START OF THE NORMA SPIRAL ARM (NORMA ORIGIN): MODEL-COMPUTED AND OBSERVED ARM TANGENTS AT GALACTIC LONGITUDES −20° < l < +23°

ABSTRACT Here, we fitted a four-arm spiral model to the more accurate data on global arm pitch angle and arm longitude tangents to get the start of each spiral arm near the galactic nucleus. We find that the tangent to the “start of the Sagittarius” spiral arm (arm middle) is at l = −17° ± 0.°5, while the tangent to the “start of the Norma” spiral arm (arm middle) is at l = +20° ± 0.°5. Earlier, we published a compilation of observations and analysis of the tangent to each spiral arm tracer from longitudes +23° to +340°; in this paper we cover the arm tracers in the remaining longitudes +340 (=−20°) to +23°. Our model arm tangents are confirmed through the recent observed masers data (at the arm's inner edge). Observed arm tracers in the inner Galaxy show an offset from the mid arm; this was also found elsewhere in the Milky Way disk. In addition, we collated the observed tangents to the so-called 3 kpc arm features; statistically they are found to be near l = −18° ± 2° and near l = +21° ± 2°, after excluding misidentified spiral arms. We find that the model-computed arm tangents in the inner Galaxy are spatially coincident with the mean longitude of the observed tangents to the 3 kpc arm features (same galactic longitudes, within the errors). These spatial similarities may be suggestive of a contiguous space.

A long-period Cepheid variable in the starburst cluster VdBH222

Galactic starburst clusters play a twin role in astrophysics, serving as laboratories for the study of stellar physics and also delineating the structure and recent star formation history of the Milky Way. In order to exploit these opportunities we have undertaken a multi-epoch spectroscopic survey of the red supergiant dominated young massive clusters thought to be present at both near and far ends of the Galactic Bar. Significant spectroscopic variability suggestive of radial pulsations was found for the yellow supergiant VdBH 222 #505. Follow-up photometric investigations revealed modulation with a period of ~23.325d; both timescale and pulsational profile are consistent with a Cepheid classification. As a consequence #505 may be recognised as one of the longest period Galactic cluster Cepheids identified to date and hence of considerable use in constraining the bright end of the period/luminosity relation at solar metallicities. In conjunction with extant photometry we infer a distance of ~6kpc for VdBH222 and an age of ~20Myr. This results in a moderate reduction in both integrated cluster mass (~2x10^4Msun) and the initial stellar masses of the evolved cluster members (~10Msun). As such, VdBH222 becomes an excellent test-bed for studying the properties of some of the lowest mass stars observed to undergo type-II supernovae. Moreover, the distance is in tension with a location of VdBH 222 at the far end of the Galactic Bar. Instead a birthsite in the near 3kpc arm is suggested; providing compelling evidence of extensive recent star formation in a region of the inner Milky Way which has hitherto been thought to be devoid of such activity.

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 HIPASS survey of the Galactic plane in radio recombination lines

We present a Radio Recombination Line (RRL) survey of the Galactic Plane from the HI Parkes All-sky Survey and associated Zone of Avoidance survey, which mapped the region l=196degr -- 0degr --52degr and |b| < 5degr at 1.4 GHz and 14.4 arcmin resolution. We combine three RRLs, H168$\alpha$, H167$\alpha$, and H166$\alpha$ to derive fully sampled maps of the diffuse ionized emission along the inner Galactic plane. The velocity information, at a resolution of 20 km/s, allows us to study the spatial distribution of the ionized gas and compare it with that of the molecular gas, as traced by CO. The longitude-velocity diagram shows that the RRL emission is mostly associated with CO gas from the molecular ring and is concentrated within the inner 30degr of longitude. A map of the free-free emission in this region of the Galaxy is derived from the line-integrated RRL emission, assuming an electron temperature gradient with Galactocentric radius of $496\pm100$ K/kpc. Based on the thermal continuum map we extracted a catalogue of 317 compact (<15 arcmin) sources, with flux densities, sizes and velocities. We report the first RRL observations of the southern ionized lobe in the Galactic centre. The line profiles and velocities suggest that this degree-scale structure is in rotation. We also present new evidence of diffuse ionized gas in the 3-kpc arm. Helium and carbon RRLs are detected in this survey. The He line is mostly observed towards HII regions, whereas the C line is also detected further away from the source of ionization. These data represent the first observations of diffuse C RRLs in the Galactic plane at a frequency of 1.4 GHz.

DISCOVERY OF A PAIR OF CLASSICAL CEPHEIDS IN AN INVISIBLE CLUSTER BEYOND THE GALACTIC BULGE

We report the discovery of a pair of extremely reddened classical Cepheid variable stars located in the Galactic plane behind the bulge, using near-infrared time-series photometry from the VVV Survey. This is the first time that such objects have ever been found in the opposite side of the Galactic plane. The Cepheids have almost identical periods, apparent brightnesses and colors. From the near-infrared Leavitt law, we determine their distances with ~1.5% precision and ~8% accuracy. We find that they have a same total extinction of A(V)~32 mag, and are located at the same heliocentric distance of <d>=11.4+/-0.9 kpc, and less than 1 pc from the true Galactic plane. Their similar periods indicate that the Cepheids are also coeval, with an age of ~48+/-3 Myr, according to theoretical models. They are separated by an angular distance of only 18.3", corresponding to a projected separation of ~1 pc. Their position coincides with the expected location of the Far 3 kpc Arm behind the bulge. Such a tight pair of similar classical Cepheids indicates the presence of an underlying young open cluster, that is both hidden behind heavy extinction and disguised by the dense stellar field of the bulge. All our attempts to directly detect this "invisible cluster" have failed, and deeper observations are needed.

A COMPLETE ATLAS OF H I ABSORPTION TOWARD H II REGIONS IN THE SOUTHERN GALACTIC PLANE SURVEY (SGPS I)

We present a complete catalog of H I emission and absorption spectrum pairs, toward H II regions, detectable within the boundaries of the Southern Galactic Plane Survey (SGPS I), a total of 252 regions. The catalog is presented in graphical, numerical and summary formats. We demonstrate an application of this new dataset through an investigation of the locus of the Near 3kpc Arm.

SNR G349.7+0.2: A γ-RAY SOURCE IN THE FAR 3 kpc ARM OF THE GALACTIC CENTER

We analyze the HI absorption profile for TeV Supernova Remnant (SNR) G349.7+0.2 based on updated knowledge of the inner Galaxy's structure. We significantly revise its kinematic distance from the previous ~ 22 kpc to ~11.5 kpc, indicating it is in the far 3 kpc arm of the Galactic center. We give a revised age of ~ 1800 year for G349.7+0.2 which has a low explosion energy of ~ 2.5 x 10^50 ergs. This removes G349.7+0.2 from the set of brightest SNRs in radio, X-ray to gamma-ray wavebands and helps understand gamma-ray emission originating from this remnant better. However, one needs to use caution for old kinematic distances of Galactic objects (e.g. SNRs, Pulsars and HII regions) in the range of -12 degree =< l =< 12 degree and having distance estimates of >= 5.5 kpc.

TRIGONOMETRIC PARALLAXES TO STAR-FORMING REGIONS WITHIN 4 kpc OF THE GALACTIC CENTER

We report four trigonometric parallaxes for high-mass star-forming regions within 4 kpc of the Galactic center. These measurements were made with the VLBA as part of the BeSSeL Survey. By associating these sources kinematically with large-scale features in CO and HI longitude-velocity diagrams, we begin to outline some major features of the inner Milky Way: the Connecting arm, the near and far 3 kpc arms, and the Norma arm. The Connecting arm in the first Galactic quadrant lies closer to the Galactic center than the far 3 kpc arm and is offset by the long-bar's major axis near its leading edge, supporting the presence of an inner Lindblad resonance. Assuming the 3 kpc arms are a continuous physical structure, the relative Galactocentric distance of its near and far sides suggests highly elliptical streamlines of gas around the bar(s) and a bar corotation radius, r_CR > 3.6 kpc. At a Galactic longitude near 10{\deg} and a heliocentric distance of about 5 kpc, the near 3 kpc arm and the Norma arm intersect on a face-on view of our Galaxy, while passing at different Galactic latitudes. We provide an accurate distance measurement to the W31 star-forming complex of 4.95 (+0.5;-0.43) kpc from the Sun, which associates it with a bright CO feature belonging to the near 3 kpc arm.

H I ABSORPTION TOWARD H II REGIONS AT SMALL GALACTIC LONGITUDES

We make a comprehensive study of HI absorption toward HII regions located within Galactic longitudes less than 10 degrees. Structures in the extreme inner Galaxy are traced using the longitude-velocity space distribution of this absorption. We find significant HI absorption associated with the Near and Far 3kpc Arms, the Connecting Arm, Banias Clump 1 and the H I Tilted Disk. We also constrain the line of sight distances to HII regions, by using HI absorption spectra together with the HII region velocities measured by radio recombination lines.

Hydroxyl, water, ammonia, carbon monoxide, and neutral carbon towards the Sagittarius A complex

Aims. The Sagittarius A complex in the Galactic centre comprises an ensemble of molecular clouds of different species with a variety of geometrical and kinematic properties. This work aims to study molecular abundances, morphology, and kinematics by comparing hydroxyl, water, carbon monoxide, ammonia, and atomic carbon and some of their isotopologues, in the +50 km s-1 cloud, the circumnuclear disk (CND), the +20 km s-1 cloud, the expanding molecular ring and the line-of-sight spiral arm features, including the Local/Sgr arm, the −30 km s-1 arm, and the 3-kpc arm. Methods. We observed the +50 km s-1 cloud, the CND and the +20 km s-1 cloud, and other selected positions at the Galactic centre with the VLA, and the Odin satellite. The VLA was used to map the 1665 and 1667 MHz OH lambda doublet main lines of the (2Π3/2) state, and the Odin satellite was used to map the 557 GHz H2O (110 − 101) line as well as to observe the 548 GHz H218O (110-101) line, the 572 GHz NH3 (10 − 00) line, the 576 GHz CO J = 5 − 4 line and the 492 GHz C i (3P1–3P0) line. Furthermore, the SEST was used to map a region of the Sgr A complex in the 220 GHz C18O J = 2−1 line. Results. Strong OH absorption, H2O emission and absorption lines were seen at all observed positions, and the H218O line was detected in absorption towards the +50 km s-1 cloud, the CND, the +20 km s-1cloud, the expanding molecular ring, and the 3-kpc arm. Strong CO J = 5−4, C18O J = 2−1, and neutral carbon C i emissions were seen towards the +50 and +20 km s-1 clouds. NH3 was only detected in weak absorption originating in the line-of-sight spiral arm features. The abundances of OH and H2O in the +50 and +20 km s-1 clouds reflect the different physical environments in the clouds, where shocks and star formation prevail in the +50 km s-1 cloud and giving rise to a higher rate of H2O production there than in the +20 km s-1 cloud. In the CND, cloud collisions and shocks are frequent, and the CND is also subject to intense UV-radiation emanating from the supermassive black hole and the central star cluster. The CND is rich in H2O and OH, and these abundances are considerably higher than in the +50 and +20 km s-1 clouds. We compare our estimated abundances of OH, H2O, and NH3 with similar and differing results for some other sources available in the literature. As compared to the quiescent cloud values of a few × 10-9, or lower, the H2O abundance is markedly enhanced in the front sides of the Sgr A molecular cloud cores, (2−7) × 10-8, as observed in absorption, and highest in the CND. A similar abundance enhancement is seen in OH. The likely explanation is PDR chemistry including grain surface reactions, and perhaps also the influence of shocks. In the redward high-velocity line wings of the +50 and +20 km s-1 clouds and the CND, the H2O abundances are estimated to be (1−6) × 10-6 or higher, i.e., similar to the water abundances in outflows of the Orion KL and DR21 molecular clouds, which are said to be caused by the combined action of shock desorption from icy grain mantles and high-temperature, gas-phase shock chemistry.

Photometric distances to young stars in the inner Galactic disk

We present results of the first extensive and deep CCD photometric survey carried out in the field of the open cluster Trumpler 27, an object in the constellation of Sagittarius not far from the Galaxy center. We look for young stars clumps that might plausibly be associated with spiral structure. Wide-field UBVI photometry combined with infrared information allows us to reconstruct the distribution in reddening and distance of young stars in the field using the Color-Color and Color-Magnitude diagrams. The analysis of our data, combined too with extensive spectroscopy taken from literature shows that the real entity of Trumpler 27 as an open cluster is far from being firmly stated. In fact, instead of finding a relatively compact group of stars confined to a small distance range, we found that stars associated to Trumpler 27 are, indeed, a superposition of early type stars seen along the line of sight extending over several kiloparsecs beyond even the center of the galaxy. We demonstrate that at each distance range it becomes possible to generate a color-magnitude diagram resembling that of an open cluster. This way, our analysis indicates that what was considered an open cluster characterized by a significant age spread is a stellar continuum that reaches its maximum number of stars at approximately 3.5 kpc from the Sun, the distance of the Scutum-Crux arm approximately. At the same time, and after analyzing the way early type stars distribute with distance, we found some of these stellar groups may be linked, within the distance errors, with other inner spiral arms of our galaxy, including the Near 3 kpc arm at approximately 5 kpc from the Sun. However, very young stars by themselves do not seem to trace strongly the inner spiral arms since they are distributed evenly across several kiloparsecs toward the center of the Galaxy.