The Spiral Structure in the Solar Neighborhood

The spiral structure in the solar neighborhood is an important issue in astronomy. In the past few years, there has been significant progress in observation. The distances for a large number of good spiral tracers, i.e., giant molecular clouds, high-mass star-formation region masers, HII regions, O-type stars, and young open clusters, have been accurately estimated, making it possible to depict the detailed properties of nearby spiral arms. In this work, we first give an overview about the research status for the Galaxy’s spiral structure based on different types of tracers. Then the objects with distance uncertainties better than 15% and <0.5 kpc are collected and combined together to depict the spiral structure in the solar neighborhood. Five segments related with the Perseus, Local, Sagittarius-Carina, Scutum-Centaurus, and Norma arms are traced. With the large dataset, the parameters of the nearby arm segments are fitted and updated. Besides the dominant spiral arms, some substructures probably related to arm spurs or feathers are also noticed and discussed.

Monoceros OB4: a new association in Gaia DR2

ABSTRACT We use Gaia DR2 data to survey the classic Monoceros OB1 region and look for the existence of a dispersed young population, co-moving with the cloud complex. An analysis of the distribution of proper motions reveals a 20–30 Myr association of young stars, about 300–400 pc away from the far side of the Mon OB1 complex, along the same general line of sight. We characterize the new association, Monoceros OB4, and estimate it contains between 1400 and 2500 stars, assuming a standard initial mass function, putting it on par in size with NGC 2264. We find from the internal proper motions that Mon OB4 is unbound and expanding. Our results seem to unveil a larger and more complex Monoceros star formation region, suggesting an elongated arrangement that seems to be at least 300 × 60 pc.

Estimation of physical conditions in star formation region S255IR-SMA1

The physical conditions were estimated from the methanol radio lines observed at 0.8 mm with IRAM30m in the star-forming region S255IR-SMA1. In the approximation of a large velocity gradient (LVG), the values of the gas kinetic temperature (170 K), the number density of molecular hydrogen (3×106 cm−3), the specific column density of methanol (2×1012 cm−3s), the relative abundance of methanol (10−7) and the filling factor (10 %). The parameter values are typical for hot dense cores. It is shown that the S255IR-SMA1 object is significantly inhomogeneous within the diagram (7.5 ).

Physical parameters of molecular clumps in the S254—S258 star formation region

This research is focusing on studying of physical parameters in the S254—S258 star-formation complex. The following parameters were derived: column densities, clump masses, and highdensity tracers optical depth, including CS and HCO+ molecules. Different gas and dust tracers maps were used, including CO molecular lines, Bolocam 1.1 mm continuum emission and near-infrared extinction map. We also used the available CS and HCO+ data, which cover part of the S254-S258 region to make conclusions regarding the distribution of highdensity gas. The resulted values of physical parameters obtained through different tracers are slightly different. The discussion about the possible reason for the difference is provided.

Ammonia observations towards the Aquila Rift cloud complex

We surveyed the Aquila Rift complex including the Serpens South and W 40 regions in the NH3 (1,1) and (2,2) transitions making use of the Nanshan 26-m telescope. Our observations cover an area of ~ 1.5° × 2.2° (11.4 pc × 16.7 pc). The kinetic temperatures of the dense gas in the Aquila Rift complex obtained from NH3 (2,2)/(1,1) ratios range from 8.9 to 35.0 K with an average of 15.3 ± 6.1 K (errors are standard deviations of the mean). Low gas temperatures are associated with Serpens South ranging from 8.9 to 16.8 K with an average of 12.3 ± 1.7 K, while dense gas in the W 40 region shows higher temperatures ranging from 17.7 to 35.0 K with an average of 25.1 ± 4.9 K. A comparison of kinetic temperatures derived from para-NH3 (2,2)/(1,1) against HiGal dust temperatures indicates that the gas and dust temperatures are in agreement in the low-mass-star formation region of Serpens South. In the high-mass-star formation region W 40, the measured gas kinetic temperatures are higher than those of the dust. The turbulent component of the velocity dispersion of NH3 (1,1) is found to be positively correlated with the gas kinetic temperature, which indicates that the dense gas may be heated by dissipation of turbulent energy. For the fractional total-NH3 (para+ortho) abundance obtained by a comparison with Herschel infrared continuum data representing dust emission, we find values from 0.1 ×10−8 to 2.1 ×10−7 with an average of 6.9 (±4.5) × 10−8. Serpens South also shows a fractional total-NH3 (para+ortho) abundance ranging from 0.2 ×10−8 to 2.1 ×10−7 with an average of 8.6 (±3.8) × 10−8. In W 40, values are lower, between 0.1 and 4.3 ×10−8 with an average of 1.6 (±1.4) × 10−8. Weak velocity gradients demonstrate that the rotational energy is a negligible fraction of the gravitational energy. In W 40, gas and dust temperatures are not strongly dependent on the projected distance to the recently formed massive stars. Overall, the morphology of the mapped region is ring-like, with strong emission at lower and weak emission at higher Galactic longitudes. However, the presence of a physical connection between the two parts remains questionable.

From clump to disc scales in W3 IRS4

Context. High-mass star formation typically takes place in a crowded environment, with a higher likelihood of young forming stars affecting and being affected by their surroundings and neighbours, as well as links between different physical scales affecting the outcome. However, observational studies are often focused on either clump or disc scales exclusively. Aims. We explore the physical and chemical links between clump and disc scales in the high-mass star formation region W3 IRS4, a region that contains a number of different evolutionary phases in the high-mass star formation process, as a case-study for what can be achieved as part of the IRAM NOrthern Extended Millimeter Array (NOEMA) large programme named CORE: “Fragmentation and disc formation in high-mass star formation”. Methods. We present 1.4 mm continuum and molecular line observations with the IRAM NOEMA interferometer and 30 m telescope, which together probe spatial scales from ~0.3−20′′ (600−40 000 AU or 0.003−0.2 pc at 2 kpc, the distance to W3). As part of our analysis, we used XCLASS to constrain the temperature, column density, velocity, and line-width of the molecular emission lines. Results. The W3 IRS4 region includes a cold filament and cold cores, a massive young stellar object (MYSO) embedded in a hot core, and a more evolved ultra-compact (UC)H II region, with some degree of interaction between all components of the region that affects their evolution. A large velocity gradient is seen in the filament, suggesting infall of material towards the hot core at a rate of 10−3−10−4 M⊙ yr−1, while the swept up gas ring in the photodissociation region around the UCH II region may be squeezing the hot core from the other side. There are no clear indications of a disc around the MYSO down to the resolution of the observations (600 AU). A total of 21 molecules are detected, with the abundances and abundance ratios indicating that many molecules were formed in the ice mantles of dust grains at cooler temperatures, below the freeze-out temperature of CO (≲35 K). This contrasts with the current bulk temperature of ~50 K, which was obtained from H2CO. Conclusions. CORE observations allow us to comprehensively link the different structures in the W3 IRS4 region for the first time. Our results argue that the dynamics and environment around the MYSO W3 IRS4 have a significant impact on its evolution. This context would be missing if only high resolution or continuum observations were available.

Detection of a high-redshift molecular outflow in a primeval hyperstarburst galaxy

We report the discovery of a high-redshift, massive molecular outflow in the starburst galaxy SPT 0346-52 (z = 5.656) via the detected absorption of high-excitation water transitions (H2O 42,3 − 41,4 and H2O 33,0 − 32,1) with the Atacama Large Millimeter/submillimeter Array (ALMA). The host galaxy is one of the most powerful starburst galaxies at high redshift (star formation rate; SFR ∼3600 M⊙ year−1), with an extremely compact (∼320 pc) star formation region and a SFR surface density (ΣSFR ∼ 5500 M⊙ year−1 kpc−2) five times higher than “maximum” (i.e. Eddington-limited) starbursts, implying a highly transient phase. The estimated outflow rate is ∼500 M⊙ year−1, which is much lower than the SFR, implying that in this extreme starburst the outflow capabilities saturate and the outflow is no longer capable of regulating star formation, resulting in a runaway process in which star formation will use up all available gas in less than 30 Myr. Finally, while previous kinematic investigations of this source revealed possible evidence for an ongoing major merger, the coincidence of the hyper-compact starburst and high-excitation water absorption indicates that this is a single starburst galaxy surrounded by a disc.

Mapping the location of terrestrial impacts and extinctions onto the spiral arm structure of the Milky Way

High-density regions within the spiral arms are expected to have profound effects on passing stars. Understanding of the potential effects on the Earth and our Solar System is dependent on a robust model of arm passage dynamics. Using a novel combination of data, we derive a model of the timings of the Solar System through the spiral arms and the relationship to arm tracers such as methanol masers. This reveals that asteroid/comet impacts are significantly clustered near the spiral arms and within specific locations of an average arm structure. The end-Permian and end-Cretaceous extinctions emerge as being located within a small star-formation region in two different arms. The start of the Solar System, greater than 4.5 Ga, occurs in the same region in a third arm. The model complements geo-chemical data in determining the relative importance of extra-Solar events in the diversification and extinction of life on Earth.