We present subkiloparsec-scale mapping of the 870 m ALMA continuum emission in six luminous (LIR ∼ 5 × 1012 L ) submillimeter galaxies (SMGs) from the ALESS survey of the Extended Chandra Deep Field ...South. Our high-fidelity 0 07-resolution imaging (∼500 pc) reveals robust evidence for structures with deconvolved sizes of 0.5-1 kpc embedded within (dominant) exponential dust disks. The large-scale morphologies of the structures within some of the galaxies show clear curvature and/or clump-like structures bracketing elongated nuclear emission, suggestive of bars, star-forming rings, and spiral arms. In this interpretation, the ratio of the "ring" and "bar" radii (1.9 0.3) agrees with that measured for such features in local galaxies. These potential spiral/ring/bar structures would be consistent with the idea of tidal disturbances, with their detailed properties implying flat inner rotation curves and Toomre-unstable disks (Q < 1). The inferred one-dimensional velocity dispersions ( r 70-160 km s−1) are marginally consistent with the limits implied if the sizes of the largest structures are comparable to the Jeans length. We create maps of the star formation rate density ( SFR) on ∼500 pc scales and show that the SMGs are able to sustain a given (galaxy-averaged) SFR over much larger physical scales than local (ultra)luminous infrared galaxies. However, on 500 pc scales, they do not exceed the Eddington limit set by radiation pressure on dust. If confirmed by kinematics, the potential presence of nonaxisymmetric structures would provide a means for net angular momentum loss and efficient star formation, helping to explain the very high star formation rates measured in SMGs.
ABSTRACT The spiral arms of the Milky Way are being accurately located for the first time via trigonometric parallaxes of massive star-forming regions with the Bar and Spiral Structure Legacy Survey, ...using the Very Long Baseline Array and the European VLBI Network, and with the Japanese VLBI Exploration of Radio Astrometry project. Here we describe a computer program that leverages these results to significantly improve the accuracy and reliability of distance estimates to other sources that are known to follow spiral structure. Using a Bayesian approach, sources are assigned to arms based on their (l, b, v) coordinates with respect to arm signatures seen in CO and H i surveys. A source's kinematic distance, displacement from the plane, and proximity to individual parallax sources are also considered in generating a full distance probability density function. Using this program to estimate distances to large numbers of star-forming regions, we generate a realistic visualization of the Milky Way's spiral structure as seen from the northern hemisphere.
We compile and analyze approximately 200 trigonometric parallaxes and proper motions of molecular masers associated with very young high-mass stars. Most of the measurements come from the BeSSeL ...Survey using the VLBA and the Japanese VERA project. These measurements strongly suggest that the Milky Way is a four-arm spiral, with some extra arm segments and spurs. Fitting log-periodic spirals to the locations of the masers, allowing for "kinks" in the spirals and using well-established arm tangencies in the fourth Galactic quadrant, allows us to significantly expand our view of the structure of the Milky Way. We present an updated model for its spiral structure and incorporate it into our previously published parallax-based distance-estimation program for sources associated with spiral arms. Modeling the three-dimensional space motions yields estimates of the distance to the Galactic center, , the circular rotation speed at the Sun's position, km s−1, and the nature of the rotation curve. Our data strongly constrain the full circular velocity of the Sun, km s−1, and its angular velocity, km s−1 kpc-1. Transforming the measured space motions to a Galactocentric frame which rotates with the Galaxy, we find non-circular velocity components typically 10 km s−1. However, near the Galactic bar and in a portion of the Perseus arm we find significantly larger non-circular motions. Young high-mass stars within 7 kpc of the Galactic center have a scale height of only 19 pc, and thus are well suited to define the Galactic plane. We find that the orientation of the plane is consistent with the IAU-defined plane to within 0 1, and that the Sun is offset toward the north Galactic pole by pc. Accounting for this offset places the central supermassive black hole, Sgr A*, in the midplane of the Galaxy. The measured motions perpendicular to the plane of the Galaxy limit precession of the plane to 4 km s−1 at the radius of the Sun. Using our improved Galactic parameters, we predict the Hulse-Taylor binary pulsar to be at a distance of 6.54 0.24 kpc, assuming its orbital decay from gravitational radiation follows general relativity.
Aims.Our current knowledge of high-mass star formation is mainly based on follow-up studies of bright sources found by IRAS, and is thus biased against its earliest phases, inconspicuous at infrared ...wavelengths. We therefore started searching, in an unbiased way and in the closest high-mass star-forming complexes, for the high-mass analogs of low-mass pre-stellar cores and class 0 protostars. Methods.We have made an extensive 1.2 mm continuum mosaicing study of the Cygnus X molecular cloud complex using the MAMBO cameras at the IRAM 30 m telescope. The $\sim 3^{\circ^2}$ imaged areas cover all the high-column density (AV ≥ 15 mag) clouds of this nearby (~1.7 kpc) cloud complex actively forming OB stars. We then compared our millimeter maps with mid-infrared images, and have made SiO(2-1) follow-up observations of the best candidate progenitors of high-mass stars. Results.Our complete study of Cygnus X with ~0.09 pc resolution provides, for the first time, an unbiased census of massive young stellar objects. We discover 129 massive dense cores (FWHM size ~0.1 pc, $M_{1.2~mm}$ = 4-950 $\mbox{$M_\odot$}$, volume-averaged density ~105 cm-3), among which ~42 are probable precursors of high-mass stars. A large fraction of the Cygnus X dense cores (2/3 of the sample) remain undetected by the MSX satellite, regardless of the mass range considered. Among the most massive (≥40 $\mbox{$M_\odot$}$) cores, infrared-quiet objects are driving powerful outflows traced by SiO emission. Our study qualifies 17 cores as good candidates for hosting massive infrared-quiet protostars, while up to 25 cores potentially host high-luminosity infrared protostars. We fail to discover the high-mass analogs of pre-stellar dense cores (~0.1 pc, > 104 cm-3) in Cygnus X, but find several massive starless clumps (~ 0.8 pc, 7 $\times$ 103 cm-3) that might be gravitationally bound. Conclusions.Since our sample is derived from a single molecular complex and covers every embedded phase of high-mass star formation, it gives the first statistical estimates of their lifetime. In contrast to what is found for low-mass class 0 and class I phases, the infrared-quiet protostellar phase of high-mass stars may last as long as their better-known high-luminosity infrared phase. The statistical lifetimes of high-mass protostars and pre-stellar cores (~ 3 $\times$ 104 yr and < 103 yr) in Cygnus X are one and two order(s) of magnitude smaller, respectively, than what is found in nearby, low-mass star-forming regions. We therefore propose that high-mass pre-stellar and protostellar cores are in a highly dynamic state, as expected in a molecular cloud where turbulent processes dominate.
By matching infrared-selected, massive young stellar objects (MYSOs) and compact H ii regions in the Red MSX Source survey to massive clumps found in the submillimetre ATLASGAL (APEX Telescope Large ...Area Survey of the Galaxy) survey, we have identified ∼1000 embedded young massive stars between 280° < ℓ < 350° and 10° < ℓ < 60° with | b | < 1
$_{.}^{\circ}$
5. Combined with an existing sample of radio-selected methanol masers and compact H ii regions, the result is a catalogue of ∼1700 massive stars embedded within ∼1300 clumps located across the inner Galaxy, containing three observationally distinct subsamples, methanol-maser, MYSO and H ii-region associations, covering the most important tracers of massive star formation, thought to represent key stages of evolution. We find that massive star formation is strongly correlated with the regions of highest column density in spherical, centrally condensed clumps. We find no significant differences between the three samples in clump structure or the relative location of the embedded stars, which suggests that the structure of a clump is set before the onset of star formation, and changes little as the embedded object evolves towards the main sequence. There is a strong linear correlation between clump mass and bolometric luminosity, with the most massive stars forming in the most massive clumps. We find that the MYSO and H ii-region subsamples are likely to cover a similar range of evolutionary stages and that the majority are near the end of their main accretion phase. We find few infrared-bright MYSOs associated with the most massive clumps, probably due to very short pre-main-sequence lifetimes in the most luminous sources.
ABSTRACT We present high-resolution (0 16) 870 m Atacama Large Millimeter/submillimeter Array (ALMA) imaging of 16 luminous ( ) submillimeter galaxies (SMGs) from the ALESS survey of the Extended ...Chandra Deep Field South. This dust imaging traces the dust-obscured star formation in these galaxies on ∼1.3 kpc scales. The emission has a median effective radius of Re = 0 24 0 02, corresponding to a typical physical size of 1.8 0.2 kpc. We derive a median Sérsic index of n = 0.9 0.2, implying that the dust emission is remarkably disk-like at the current resolution and sensitivity. We use different weighting schemes with the visibilities to search for clumps on 0 12 (∼1.0 kpc) scales, but we find no significant evidence for clumping in the majority of cases. Indeed, we demonstrate using simulations that the observed morphologies are generally consistent with smooth exponential disks, suggesting that caution should be exercised when identifying candidate clumps in even moderate signal-to-noise ratio interferometric data. We compare our maps to comparable-resolution Hubble Space Telescope -band images, finding that the stellar morphologies appear significantly more extended and disturbed, and suggesting that major mergers may be responsible for driving the formation of the compact dust disks we observe. The stark contrast between the obscured and unobscured morphologies may also have implications for SED fitting routines that assume the dust is co-located with the optical/near-IR continuum emission. Finally, we discuss the potential of the current bursts of star formation to transform the observed galaxy sizes and light profiles, showing that the descendants of these SMGs are expected to have stellar masses, effective radii, and gas surface densities consistent with the most compact massive ( 1-2 × 1011 ) early-type galaxies observed locally.
Context. As the number of complex organic molecules (COMs) detected in the interstellar medium increases, it becomes even more important to place meaningful constraints on the origins and formation ...pathways of such chemical species. The molecular cloud Sagittarius B2(N) is host to several hot molecular cores in the early stage of star formation, where a great variety of COMs are detected in the gas phase. Given its exposure to the extreme conditions of the Galactic center (GC) region, Sgr B2(N) is one of the best targets to study the impact of environmental conditions on the production of COMs. Aims. Our main goal is to characterize the physico-chemical evolution of Sgr B2(N)’s sources in order to explain their chemical differences and constrain their environmental conditions. Methods. The chemical composition of Sgr B2(N)’s hot cores, N2, N3, N4, and N5 is derived by modeling their 3 mm emission spectra extracted from the Exploring Molecular Complexity with ALMA (EMoCA) imaging spectral line survey performed with the Atacama Large Millimeter/submillimeter Array (ALMA). We derived the density distribution in the envelope of the sources based on the masses computed from the ALMA dust continuum emission maps. We used the radiative transfer code RADMC-3D to compute temperature profiles and inferred the current luminosity of the sources based on the COM rotational temperatures derived from population diagrams. We used published results of 3D radiation-magnetohydrodynamical (RMHD) simulations of high-mass star formation to estimate the time evolution of the source properties. We employed the astrochemical code MAGICKAL to compute time-dependent chemical abundances in the sources and to investigate how physical properties and environmental conditions influence the production of COMs. Results. The analysis of the abundances of 11 COMs detected toward Sgr B2(N2-N5) reveals that N3 and N5 share a similar chemical composition while N2 differs significantly from the other sources. We estimate the current luminosities of N2, N3, N4, and N5 to be 2.6 × 105 L⊙, 4.5 × 104 L⊙, 3.9 × 105 L⊙, and 2.8 × 105 L⊙, respectively. We find that astrochemical models with a cosmic-ray ionization rate of 7 × 10−16 s−1 best reproduce the abundances with respect to methanol of ten COMs observed toward Sgr B2(N2-N5). We also show that COMs still form efficiently on dust grains with minimum dust temperatures in the prestellar phase as high as 15 K, but that minimum temperatures higher than 25 K are excluded. Conclusions. The chemical evolution of Sgr B2(N2-N5) strongly depends on their physical history. A more realistic description of the hot cores’ physical evolution requires a more rigorous treatment with RMHD simulations tailored to each hot core.
Complex stellar winds from evolved stars
Stars less than eight times the mass of the Sun end their lives as planetary nebulae, structures of ionized gas thrown off by the star and heated by the ...exposed stellar core. Planetary nebulae are often bipolar in shape or contain complex morphological features such as rings or spirals. Decin
et al.
observed the stellar winds of 14 stars during their asymptotic giant branch (AGB) phase of stellar evolution, which immediately precedes the planetary nebula phase. They found morphologies in the AGB winds similar to planetary nebulae and demonstrated that they are produced by the influence of a binary companion on the AGB wind.
Science
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Complex morphologies in the winds of asymptotic giant branch stars and planetary nebulae are produced by binary interactions.
Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe.
Context. The discovery of amino acids in meteorites fallen to Earth and the detection of glycine, the simplest of them, in samples returned from a comet to Earth strongly suggest that the chemistry ...of the interstellar medium is capable of producing such complex organic molecules and that they may be widespread in our Galaxy. Aims. Our goal is to investigate the degree of chemical complexity that can be reached in the interstellar medium, in particular in dense star-forming regions. Methods. We performed an unbiased, spectral line survey toward Sgr B2(N) and (M), two regions where high-mass stars are formed, with the IRAM 30 m telescope in the 3 mm atmospheric transmission window. Partial surveys at 2 and 1.3 mm were performed in parallel. The spectra were analyzed with a simple radiative transfer model that assumes local thermodynamic equilibrium but takes optical depth effects into account. Results. About 3675 and 945 spectral lines with a peak signal-to-noise ratio higher than 4 are detected at 3 mm toward Sgr B2(N) and (M), i.e. about 102 and 26 lines per GHz, respectively. This represents an increase by about a factor of two over previous surveys of Sgr B2. About 70% and 47% of the lines detected toward Sgr B2(N) and (M) are identified and assigned to 56 and 46 distinct molecules as well as to 66 and 54 less abundant isotopologues of these molecules, respectively. In addition, we report the detection of transitions from 59 and 24 catalog entries corresponding to vibrationally or torsionally excited states of some of these molecules, respectively, up to a vibration energy of 1400 cm-1 (2000 K). Excitation temperatures and column densities were derived for each species but should be used with caution. The rotation temperatures of the detected complex molecules typically range from ~50 to 200 K. Among the detected molecules, aminoacetonitrile, n-propyl cyanide, and ethyl formate were reported for the first time in space based on this survey, as were five rare isotopologues of vinyl cyanide, cyanoacetylene, and hydrogen cyanide. We also report the detection of transitions from within twelve new vibrationally or torsionally excited states of known molecules. Absorption features produced by diffuse clouds along the line of sight are detected in transitions with low rotation quantum numbers of many simple molecules and are modeled with ~30–40 velocity components with typical linewidths of ~3–5 km s-1. Conclusions. Although the large number of unidentified lines may still allow future identification of new molecules, we expect most of these lines to belong to vibrationally or torsionally excited states or to rare isotopologues of known molecules for which spectroscopic predictions are currently missing. Significant progress in extending the inventory of complex organic molecules in Sgr B2(N) and deriving tighter constraints on their location, origin, and abundance is expected in the near future thanks to an ongoing spectral line survey at 3 mm with ALMA in its cycles 0 and 1. The present single-dish survey will serve as a solid basis for the line identification and analysis of such an interferometric survey.
Trigonometric Parallax of W51 Main/South Sato, M; Reid, M. J; Brunthaler, A ...
Astrophysical journal/The Astrophysical journal,
09/2010, Letnik:
720, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We report measurement of the trigonometric parallax of W51 Main/South using the Very Long Baseline Array. We measure a value of 0.185 {+-} 0.010 mas corresponding to a distance of 5.41{sup +0.31}{sub ...-0.28} kpc. W51 Main/South is a well-known massive star-forming region near the tangent point of the Sagittarius spiral arm of the Milky Way. Our distance to W51 yields an estimate of the distance to the Galactic center of R{sub 0} = 8.3 {+-} 0.46 (statistical) {+-}1.0 (systematic) kpc by simple geometry. Combining the parallax and proper motion measurements for W51, we obtained the full-space motion of this massive star-forming region. We find W51 is in a nearly circular orbit about the Galactic center. The H{sub 2}O masers used for our parallax measurements trace four powerful bipolar outflows within a 0.4 pc size region, some of which are associated with dusty molecular hot cores and/or hyper- or ultra-compact H II regions.
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