Abstract
We report on an Atacama Large Millimeter/submillimeter Array study of the Class I or II intermediate-mass protostar DK Cha in the Chamaeleon II region. The
12
CO(
J
= 2–1) images have an ...angular resolution of ∼1″ (∼250 au) and show high-velocity blueshifted (≳70 km s
−1
) and redshifted (≳50 km s
−1
) emissions, which have 3000 au scale crescent-shaped structures around the protostellar disk traced in the 1.3 mm continuum. Because the high-velocity components of the CO emission are associated with the protostar, we concluded that the emission traces the pole-on outflow. The blueshifted outflow lobe has a clear layered velocity gradient with a higher-velocity component located on the inner side of the crescent shape, which can be explained by a model of an outflow with a higher velocity in the inner radii. Based on the directly driven outflow scenario, we estimated the driving radii from the observed outflow velocities and found that the driving region extends over 2 orders of magnitude. The
13
CO emission traces a complex envelope structure with arc-like substructures with lengths of ∼1000 au. We identified the arc-like structures as streamers because they appear to be connected to a rotating infalling envelope. DK Cha is useful for understanding characteristics that are visible by looking at nearly face-on configurations of young protostellar systems, providing an alternative perspective for studying the star formation process.
Abstract
We present the detection of a secondary outflow associated with a Class I source, Ser-emb 15, in the Serpens Molecular Cloud. We reveal two pairs of molecular outflows consisting of three ...lobes, that is, primary and secondary outflows, using Atacama Large Millimeter/submillimeter Array
12
CO and SiO line observations at a resolution of ∼318 au. The secondary outflow is elongated approximately perpendicular to the axis of the primary outflow in the plane of the sky. We also identify two compact structures, Sources A and B, within an extended structure associated with Ser-emb 15 in the 1.3 mm continuum emission at a resolution of ∼40 au. The projected sizes of Sources A and B are 137 au and 60 au, respectively. Assuming a dust temperature of 20 K, we estimate the dust mass to be 2.4 × 10
−3
M
⊙
for Source A and 3.3 × 10
−4
M
⊙
for Source B. C
18
O line data imply rotational motion around the extended structure, but we cannot resolve rotational motion in Source A and/or B because the angular and frequency resolutions are insufficient. Therefore, we cannot conclude whether Ser-emb 15 is a single or binary system. Thus, either Source A or Source B could drive the secondary outflow. We discuss two scenarios that might explain the driving mechanism of the primary and secondary outflows: the Ser-emb 15 system is (1) a binary system composed of Sources A and B, or (2) a single-star system composed of Source A alone. In either case, the system could be a suitable target for investigating the disk and/or binary formation processes in complicated environments. Detecting these outflows should contribute to understanding complex star-forming environments, which may be common in the star formation processes.
We have conducted ALMA CO isotopes and 1.3 mm continuum observations toward filamentary molecular clouds of the N159W-South region in the Large Magellanic Cloud with an angular resolution of ∼0 25 ...(∼0.07 pc). Although the previous lower-resolution (∼1″) ALMA observations revealed that there is a high-mass protostellar object at an intersection of two line-shaped filaments in 13CO with the length scale of ∼10 pc, the spatially resolved observations, in particular, toward the highest column density part traced by the 1.3 mm continuum emission, the N159W-South clump, show complicated hub-filamentary structures. We also discovered that there are multiple protostellar sources with bipolar outflows along the massive filament. The redshifted/blueshifted components of the 13CO emission around the massive filaments/protostars have complementary distributions, which is considered to be possible evidence for a cloud-cloud collision. We propose a new scenario in which the supersonically colliding gas flow triggers the formation of both the massive filament and protostars. This is a modification of the earlier scenario of cloud-cloud collision, by Fukui et al., that postulated the two filamentary clouds occur prior to the high-mass star formation. A recent theoretical study of the shock compression in colliding molecular flows by Inoue et al. demonstrates that the formation of filaments with hub structure is a usual outcome of the collision, lending support for the present scenario. The theory argues that the filaments are formed as dense parts in a shock compressed sheet-like layer, which resembles "an umbrella with pokes."
We have performed survey-type observations in 1 mm continuum and molecular lines toward dense cores (32 prestellar + 7 protostellar) with an average density of 105 cm−3 in the Taurus molecular clouds ...using the Atacama Large Millimeter/submillimeter Array-Atacama Compact Array (ALMA-ACA) stand-alone mode with an angular resolution of 6 5 (∼900 au). The primary purpose of this study is to investigate the innermost part of dense cores with view to understanding the initial condition of star formation. In the protostellar cores, contributions from protostellar disks dominate the observed continuum flux with a range of 35%-90%, except for the very low-luminosity object. For the prestellar cores, we have successfully confirmed continuum emission from dense gas with a density of 3 × 105 cm−3 toward approximately one-third of the targets. Thanks to the lower spatial frequency coverage with the ACA 7 m array, the detection rate is significantly higher than that of the previous surveys, which have zero or one continuum-detected sources among a large number of starless samples using the ALMA Main Array. The statistical counting method tells us that the lifetime of prestellar cores until protostar formation therein approaches the freefall time as the density increases. Among the prestellar cores, at least two targets have possible internal substructures, which are detected in continuum emission with the size scale of ∼1000 au if we consider the molecular line (C18O and N2D+) distributions. These results suggest that small-scale fragmentation/coalescence processes occur in a region smaller than 0.1 pc, which may determine the final core mass associated with individual protostar formation before starting the dynamical collapse of the core with a central density of ∼(0.3-1) × 106 cm−3.
We present ALMA observations of CO isotopes and 1.3 mm continuum emission toward the N159E-Papillon Nebula in the Large Magellanic Cloud (LMC). The spatial resolution is 0 25-0 28 (0.06-0.07 pc), ...which is a factor of 3 higher than previous ALMA observations in this region. The high resolution allowed us to resolve highly filamentary CO distributions with typical widths of ∼0.1 pc (full width half maximum) and line masses of a few 100 M pc−1. The filaments (more than ten in number) show an outstanding hub-filament structure emanating from the nebular center toward the north. We identified for the first time two massive protostellar outflows of ∼104 yr dynamical age along one of the most massive filaments. The observations also revealed several pillar-like CO features around the Nebula. The H ii region and the pillars have a complementary spatial distribution and the column density of the pillars is an order of magnitude higher than that of the pillars in the Eagle nebula (M16) in the Galaxy, suggesting an early stage of pillar formation with an age younger than ∼105 yr. We suggest that a cloud-cloud collision triggered the formation of the filaments and protostar within the last ∼2 Myr. It is possible that the collision is more recent, as part of the kpc-scale H i flows come from the tidal interaction resulting from the close encounter between the LMC and SMC ∼200 Myr ago as suggested for R136 by Fukui et al.
We present maps of the dust properties in the Small and Large Magellanic Clouds (SMC, LMC) from fitting Spitzer and Herschel observations with the Draine & Li dust model. We derive the abundance of ...the small carbonaceous grain (or polycyclic aromatic hydrocarbon; PAH) component. The global PAH fraction ( , the fraction of the dust mass in the form of PAHs) is smaller in the SMC ( %) than in the LMC ( %). We measure the PAH fraction in different gas phases (H ii regions, ionized gas outside of H ii regions, molecular gas, and diffuse neutral gas). H ii regions appear as distinctive holes in the spatial distribution of the PAH fraction. In both galaxies, the PAH fraction in the diffuse neutral medium is higher than in the ionized gas, but similar to the molecular gas. Even at equal radiation field intensity, the PAH fraction is lower in the ionized gas than in the diffuse neutral gas. We investigate the PAH life-cycle as a function of metallicity between the two galaxies. The PAH fraction in the diffuse neutral medium of the LMC is similar to that of the Milky Way (∼4.6%), while it is significantly lower in the SMC. Plausible explanations for the higher PAH fraction in the diffuse neutral medium of the LMC compared to the SMC include: more effective PAH production by fragmentation of large grains at higher metallicity, and/or the growth of PAHs in molecular gas.
We studied star formation activities in the molecular clouds in the Large Magellanic Cloud. We have utilized the second catalog of 272 molecular clouds obtained by NANTEN to compare the cloud ...distribution with signatures of massive star formation including stellar clusters, and optical and radio H II regions. We find that the molecular clouds are classified into three types according to the activities of massive star formation: Type I shows no signature of massive star formation; Type II is associated with relatively small H II region(s); and Type III with both H II region(s) and young stellar cluster(s). The radio continuum sources were used to confirm that Type I giant molecular clouds (GMCs) do not host optically hidden H II regions. These signatures of massive star formation show a good spatial correlation with the molecular clouds in the sense that they are located within ~100 pc of the molecular clouds. Among possible ideas to explain the GMC types, we favor that the types indicate an evolutionary sequence; i.e., the youngest phase is Type I, followed by Type II, and the last phase is Type III, where the most active star formation takes place leading to cloud dispersal. The number of the three types of GMCs should be proportional to the timescale of each evolutionary stage if a steady state of massive star and cluster formation is a good approximation. By adopting the timescale of the youngest stellar clusters, 10 Myr, we roughly estimate the timescales of Types I, II, and III to be 6 Myr, 13 Myr, and 7 Myr, respectively, corresponding to a lifetime of 20-30 Myr for the GMCs with a mass above the completeness limit, 5 X 104 M.
The spatial variations of the gas-to-dust ratio (GDR) provide constraints on the chemical evolution and life-cycle of dust in galaxies. We examine the relation between dust and gas at 10-50 pc ...resolution in the Large and Small Magellanic Clouds (LMC and SMC) based on Herschel far-infrared (FIR), H I 21 cm, CO, and H alpha observations. We investigate the range of CO-to-Hsub 2 conversion factor to best account for all the molecular gas in the beam of the observations, and find upper limits on XCO to be 6 x 10sup 20 cmsup -2 Ksup -1 kmsup -1 s in the LMC (Z = 0.5Z) at 15 pc resolution, and 4 x 1021 cmsup -2 Ksup -1 kmsup -1 s in the SMC (Z = 0.2Z) at 45 pc resolution. Our analysis demonstrates that obtaining robust ISM masses remains a non-trivial endeavor even in the local Universe using state-of-the-art maps of thermal dust emission.
ABSTRACT
We report millimeter/submillimeter continuum and molecular line observations of the Galactic super star cluster RCW 38, obtained from the Atacama Large Millimeter/Submillimeter Array with a ...minimum angular resolution of ${0{^{\prime \prime }_{.}}17}\times {0{^{\prime \prime }_{.}}15}$ (≃ 289 au × 255 au). The C18O image reveals many massive condensations embedded within filamentary structures extending along the northwest–southeast direction in the center of cluster. The condensations have sizes of 0.01–0.02 pc, H2 column densities of 1023–1024 cm−2, and H2 masses of 10–130 M⊙. In addition, the 233 GHz continuum image reveals two dense, small millimeter-sources with radii of 460 and 200 au (Source A and Source B). Source A is embedded within the most massive C18O condensation, whereas no counterpart is seen for Source B. The masses of Source A and Source B are estimated as 13 and 3 M⊙ at the optically thin limit, respectively. The C18O emission shows a velocity gradient of 2 km s−1 at the central 2000 au of Source A, which could be interpreted as a Keplerian rotation with a central mass of a few M⊙ or infall motion of gas. Further, the ALMA 12CO data reveal that Source A and Source B are associated with molecular outflows exhibiting maximum velocities of ∼30–70 km s−1. The outflows have short dynamical timescales of <1000 yr and high-mass outflow rates of ∼10−4–10−3 M⊙ yr−1. These observational signatures suggest an early evolutionary phase of the massive star formation in Source A and Source B.
We report on continuous wave lasing characteristics of GaN vertical cavity surface emitting lasers (VCSELs). The VCSEL operates at room temperature under current injection by using highly reflective ...distributed Bragg reflectors (DBRs) made up of transparent {\rm ZrO}_{2} and {\rm SiO}_{2} film stacks. Together with high reflectivity and the wide stop band of the DBR, the long cavity of 6 \mu{\rm m} allows multimode lasing oscillation with narrow mode spacing of 2.9 nm. In addition, a short cavity structure of 2 \mu{\rm m} is fabricated and shows quasi-single mode operation. The spacing of the lasing modes shows a clear dependence of the actual cavity lengths with fairly good agreement to theory taking account of the refractive index dispersion.