[en] We report subarcsecond images of the high-mass star-forming region Onsala 1 (ON 1) made with the Submillimeter Array (SMA) at 0.85 mm and the Very Large Array (VLA) at 1.3 cm and 3.6 cm. ON 1 is one of the smallest ultracompact H II regions in the Galaxy and exhibits various star formation signposts. With our VLA and SMA observations, two new centimeter-wave sources and five submillimeter dust sources, respectively, within a field of ∼5'' (corresponding to a linear scale of 0.05 pc) are identified, indicating the multiplicity at the center of the ON 1 region. The dust and gas masses of these submillimeter sources are in the range of 0.8 to 6.4 M _sun. Among the five submillimeter dust sources, SMA2, with a dust and gas mass of 2.6 M _sun, demonstrates several star formation signatures, and hence likely represents an intermediate-mass (or even high-mass) star-forming core. Due to the low star formation efficiency of ∼10%, we suggest that star formation in the ON 1 region will continue. For example, SMA4 and SMA5 are not associated with any star formation signatures and likely mark star formation cores at very early evolutionary stages.

[en] The structure of the coronal magnetic field prior to eruptive processes and the conditions for the onset of eruption are important issues that can be addressed through studying the magnetohydrodynamic (MHD) stability and evolution of nonlinear force-free field (NLFFF) models. This paper uses data-constrained NLFFF models of a solar active region (AR) that erupted on 2010 April 8 as initial conditions in MHD simulations. These models, constructed with the techniques of flux rope insertion and magnetofrictional relaxation (MFR), include a stable, an approximately marginally stable, and an unstable configuration. The simulations confirm previous related results of MFR runs, particularly that stable flux rope equilibria represent key features of the observed pre-eruption coronal structure very well, and that there is a limiting value of the axial flux in the rope for the existence of stable NLFFF equilibria. The specific limiting value is located within a tighter range, due to the sharper discrimination between stability and instability by the MHD description. The MHD treatment of the eruptive configuration yields a very good agreement with a number of observed features, like the strongly inclined initial rise path and the close temporal association between the coronal mass ejection and the onset of flare reconnection. Minor differences occur in the velocity of flare ribbon expansion and in the further evolution of the inclination; these can be eliminated through refined simulations. We suggest that the slingshot effect of horizontally bent flux in the source region of eruptions can contribute significantly to the inclination of the rise direction. Finally, we demonstrate that the onset criterion, formulated in terms of a threshold value for the axial flux in the rope, corresponds very well to the threshold of the torus instability in the considered AR.

[en] Twelve new kinds of Al-Cu-Mg-Zn alloys with different components were designed on the basis of optimizing main alloying elements, including Zn, Mg and Cu to resolve hot crack problems of Al-Cu-Mg-Zn series alloys during liquid forging. Hot cracking susceptibility (HCS) was adopted to characterize hot cracking tendency, evaluated using constrained rod casting in a steel mould. The influence of those three main alloying elements on the hot cracking tendency and the mechanical properties of new alloys were researched. The results show that most of those new alloys have better hot crack tendency performances than 2024 alloy and 7075 alloy. The 8th alloy, Al-5Cu-4.5Mg-2.5Zn, shows best mechanical properties after liquid forging, which hot crack tendency index is the lowest, only reaching 40. The tensile strength is 327 MPa and the elongation to fracture is 2.7%, which Brinell hardness reaches 107 N/mm². (paper)

[en] We have observed the J = 3 - 2 transition of N₂H⁺ and N₂D⁺ to investigate the trend of deuterium fractionation with evolutionary stage in three selected regions in the infrared dark cloud (IRDC) G28.34+0.06 with the Submillimeter Telescope and the Submillimeter Array. A comprehensible enhancement of roughly 3 orders of magnitude in deuterium fractionation over the local interstellar D/H ratio is observed in all sources. In particular, our sample of massive star-forming cores in G28.34+0.06 shows a moderate decreasing trend over a factor of 3 in the N(N₂D⁺)/N(N₂H⁺) ratio with evolutionary stage, a behavior resembling that previously found in low-mass protostellar cores. This suggests a possible extension for the use of the N(N₂D⁺)/N(N₂H⁺) ratio as an evolutionary tracer to high-mass protostellar candidates. In the most evolved core, MM1, the N₂H⁺(3-2) emission appears to avoid the warm region traced by dust continuum emission and emission of ¹³CO sublimated from grain mantles, indicating an instant release of gas-phase CO. The majority of the N₂H⁺ and N₂D⁺ emission is associated with extended structures larger than 8'' (∼0.2 pc).

[en] In this paper, we report our multiwavelength observations of the C4.2 circular-ribbon flare in active region (AR) 12434 on 2015 October 16. The short-lived flare was associated with positive magnetic polarities and a negative polarity inside, as revealed by the photospheric line-of-sight magnetograms. Such a magnetic pattern is strongly indicative of a magnetic null point and spine-fan configuration in the corona. The flare was triggered by the eruption of a mini-filament residing in the AR, which produced the inner flare ribbon (IFR) and the southern part of a closed circular flare ribbon (CFR). When the eruptive filament reached the null point, it triggered null point magnetic reconnection with the ambient open field and generated the bright CFR and a blowout jet. Raster observations of the Interface Region Imaging Spectrograph show plasma upflow at speeds of 35–120 km s⁻¹ in the Fe xxi λ1354.09 line ($\mathrm{l}\mathrm{o}\mathrm{g}T\approx <!--\; ???\; -->7.05$) and downflow at speeds of 10–60 km s⁻¹ in the Si iv λ1393.77 line ($\mathrm{l}\mathrm{o}\mathrm{g}T\approx <!--\; ???\; -->4.8$) at certain locations of the CFR and IFR during the impulsive phase of the flare, indicating explosive chromospheric evaporation. Coincidence of the single hard X-ray source at 12–25 keV with the IFR and calculation based on the thick-target model suggest that the explosive evaporation was most probably driven by nonthermal electrons.

[en] In this paper, we report our multiwavelength observations of the large-amplitude longitudinal oscillations of a filament observed on 2015 May 3. Located next to active region 12335, the sigmoidal filament was observed by the ground-based H α telescopes from the Global Oscillation Network Group and by the Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory . The filament oscillations were most probably triggered by the magnetic reconnection in the filament channel, which is characterized by the bidirectional flows, brightenings in EUV and soft X-ray, and magnetic cancellation in the photosphere. The directions of oscillations have angles of 4°–36° with respect to the filament axis. The whole filament did not oscillate in phase as a rigid body. Meanwhile, the oscillation periods (3100–4400 s) have a spatial dependence, implying that the curvature radii ( R ) of the magnetic dips are different at different positions. The values of R are estimated to be 69.4–133.9 Mm, and the minimum transverse magnetic field of the dips is estimated to be 15 G. The amplitudes of S5-S8 grew with time, while the amplitudes of S9-S14 damped with time. The oscillation amplitudes range from a few to ten Mm, and the maximum velocity can reach 30 km s⁻¹. Interestingly, the filament experienced mass drainage southward at a speed of ∼27 km s⁻¹. The oscillations continued after the mass drainage and lasted for more than 11 hr. After the mass drainage, the oscillation phases did not change much. The periods of S5-S8 decreased, while the periods of S9-S14 increased. The amplitudes of S5-S8 damped with time, while the amplitudes of S9-S14 grew. Most of the damping (growing) ratios are between −9 and 14. We offer a schematic cartoon to explain the complex behaviors of oscillations by introducing thread-thread interaction.

[en] For the first time, using the Submillimeter Array, H₂CS emission was observed to show a typical bipolar outflow morphology toward DR21(OH). The emission consisted of a strong concentration toward a hot molecular core (MM1a) and a symmetrically extended feature aligned roughly in the north-south direction. H₂CS is one of the hot core species, and its extended emission is expected to result from the interaction between the outflow of MM1a and the dense ambient gas. We derive H₂CS column densities of ∼3 x 10¹⁵ cm^-2 and ∼1 x 10¹⁵ cm^-2 toward MM1a and the centers of the extended emission, respectively. Its fractional abundance, f (H₂CS) ∼ 10^-9 relative to the total H₂ abundance, is comparable to those in other star-forming regions, suggesting that H₂CS may be present in a region closer to the center than previously thought. Our results suggest that the observed H₂CS emission arises via direct evaporation or sputtering of the solid H₂CS on the grain surface, and H₂CS may be one of the major sulfur-bearing species residing in the ice grain mantles in a solid form, at least in the case of DR21(OH).

[en] We present sensitive high angular resolution (∼1'') millimeter continuum and line observations from the massive star-forming region DR21(OH) located in the Cygnus X molecular cloud. Within the well-known dusty MM1-2 molecular cores, we report the detection of a new cluster of about 10 compact continuum millimeter sources with masses between 5 and 24 M_☉, and sizes of a few thousands of astronomical units. These objects are likely to be large dusty envelopes surrounding massive protostars, some of them most probably driving several of the outflows that emanate from this region. Additionally, we report the detection of strong millimeter emission of formaldehyde (H₂CO) and methanol (CH₃OH) near 218 GHz as well as compact emission from the typical outflow tracers carbon monoxide and silicon monoxide (CO and SiO) toward this massive star-forming region. The H₂CO and CH₃OH emission is luminous (∼10^–4 L_☉), well resolved, and found along the collimated methanol maser outflow first identified at centimeter wavelengths and in the sources SMA6 and SMA7. Our observations suggest that this maser outflow might be energized by a millimeter source called SMA4 located in the MM2 dusty core. The CO and SiO emission traces some other collimated outflows that emanate from MM1-2 cores, and are not related with the low-velocity maser outflow.

[en] The massive star-forming core MM1 of W75N was observed using the Submillimeter Array with ∼1'' and 2'' spatial resolutions at 217 and 347 GHz, respectively. From the 217 GHz continuum we found that the MM1 core consists of two sources, separated by about 1'': MM1a (∼0.6 M_sun) and MM1b (∼1.4 M_sun), located near the radio continuum sources VLA 2/VLA 3 and VLA 1, respectively. Within MM1b, two gas clumps were found to be expanding away from VLA 1 at about ±3 km s^-1, as a result of the most recent star formation activity in the region. Observed molecular lines show emission peaks at two positions, MM1a and MM1b: sulfur-bearing species have emission peaks toward MM1a, but methanol and saturated species at MM1b. We identified high-temperature (∼200 K) gas toward MM1a and the hot core in MM1b. This segregation may result from the evolution of the massive star-forming core. In the very early phase of star formation, the hot core is seen through the evaporation of dust ice-mantle species. As the mantle species are consumed via evaporation the high-temperature gas species (such as the sulfur-bearing molecules) become bright. The SiO molecule is unique in having an emission peak exactly at the VLA 2 position, probably tracing a shock powered by VLA 2. The observed sulfur-bearing species show similar abundances both in MM1a and MM1b, whereas the methanol and saturated species show significant abundance enhancement toward MM1b, by about an order of magnitude, compared to MM1a.

[en] The properties of tidally induced arms provide a means to study molecular cloud formation and the subsequent star formation under environmental conditions which, in principle, are different from quasi-stationary spiral arms. We report the properties of a newly discovered molecular gas arm of likely tidal origin at the south of NGC 4039 and the overlap region in the Antennae galaxies, with a resolution of 1.''68 × 0.''85, using the Atacama Large Millimeter/submillimeter Array science verification CO(2-1) data. The arm extends 3.4 kpc (34'') and is characterized by widths of ∼<200 pc (2'') and velocity widths of typically ΔV ≅ 10-20 km s^–1. About 10 clumps are strung out along this structure, most of them unresolved, with average surface densities of Σ_gas ≅ 10-100 M_☉ pc^–2, and masses of (1-8)×10⁶ M_☉. These structures resemble the morphology of beads on a string, with an almost equidistant separation between the beads of about 350 pc, which may represent a characteristic separation scale for giant molecular associations. We find that the star formation efficiency at a resolution of 6''(600 pc) is in general a factor of 10 higher than in disk galaxies and other tidal arms and bridges. This arm is linked, based on the distribution and kinematics, to the base of the western spiral arm of NGC 4039, but its morphology is different to that predicted by high-resolution simulations of the Antennae galaxies.