Institute of Astronomy and Astrophysics

Taipei, Taiwan

Institute of Astronomy and Astrophysics

Taipei, Taiwan
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News Article | June 30, 2017
Site: phys.org

Figure 1: Jet, disk, and disk atmosphere in the HH 212 protostellar system. (a) A composite image for the HH 212 jet in different molecules, combining the images from the Very Large Telescope (McCaughrean et al. 2002) and ALMA (Lee et al. 2015). Orange image shows the dusty envelope+disk mapped with ALMA.  (b) A zoom-in to the central dusty disk. The asterisk marks the position of the protostar. A size scale of our solar system is shown in the lower right corner for comparison. (c) Atmosphere of the accretion disk detected with ALMA. In the disk atmosphere, green is for deuterated methanol, blue for methanethiol, and red for formamide. Credit: ALMA (ESO/NAOJ/NRAO)/Lee et al. An international research team, led by Chin-Fei Lee of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA, Taiwan), has used the Atacama Large Millimeter/submillimeter Array (ALMA) to detect complex organic molecules for the first time in the atmosphere of an accretion disk around a very young protostar. These molecules play a crucial role in producing the rich organic chemistry needed for life. The discovery suggests that the building blocks of life are produced in such disks at the very beginning of star formation and that they are available to be incorporated into planets that form in the disk subsequently. It could help us understand how life came to be on Earth. "It is so exciting to discover complex organic molecules on an accretion disk around a baby star," says Chin-Fei Lee at ASIAA. "When such molecules were first found in the protoplanetary disk around a star in a later phase of star formation, we wondered if they could have formed earlier. Now, using ALMA's unprecedented combination of spatial resolution and sensitivity, we not only detect them on a younger accretion disk, but also determine their location. These molecules are the building blocks of life, and they are already there in the disk atmosphere around the baby star in the earliest phase of star formation." Herbig-Haro (HH) 212 is a nearby protostellar system in Orion at a distance of about 1,300 light-years. The central protostar is very young, with an estimated age of only 40,000 years—about 1/100,000th the age of our sun—and a mass of only 0.2 solar mass. It drives a powerful bipolar jet and thus must accrete material efficiently. Indeed, an accretion disk is seen feeding the protostar. The disk is nearly edge-on and has a radius of about 60 astronomical units (AU), or 60 times the average Earth-sun distance. Interestingly, it shows a prominent equatorial dark lane sandwiched between two brighter features, looking like a "space hamburger." The research team's ALMA observations have clearly detected an atmosphere of complex organic molecules above and below the disk. These include methanol (CH OH), deuterated methanol (CH DOH), methanethiol (CH SH), and formamide (NH2CHO). These molecules have been proposed to be the precursors for producing biomolecules such as amino acids and sugars. "They are likely formed on icy grains in the disk and then released into the gas phase because of heating from stellar radiation or some other means, such as shocks," says co-author Zhi-Yun Li of the University of Virginia. The team's observations open up an exciting possibility of detecting complex organic molecules in disks around other baby stars through high-resolution and high-sensitivity imaging with ALMA, which provides strong constraints on theories of prebiotic chemistry in star and planet formation. In addition, the observations open up the possibility of detecting more complex organic molecules and biomolecules that could shed light on the origin of life. Explore further: Methanol detected for first time around young star More information: Chin-Fei Lee et al. Formation and Atmosphere of Complex Organic Molecules of the HH 212 Protostellar Disk, The Astrophysical Journal (2017). DOI: 10.3847/1538-4357/aa7757


News Article | June 12, 2017
Site: phys.org

The massive protostar is surrounded by a disk of gas and dust. The outflow is launched from the surface of the outer disk. Credit: ALMA (ESO/NAOJ/NRAO) Stars form from gas and dust floating in interstellar space. But, astronomers do not yet fully understand how it is possible to form the massive stars seen in space. One key issue is gas rotation. The parent cloud rotates slowly in the initial stage and the rotation becomes faster as the cloud shrinks due to self-gravity. Stars formed in such a process should have very rapid rotation, but this is not the case. The stars observed in the Universe rotate more slowly. How is the rotational momentum dissipated? One possible scenario involves that the gas emanating from baby stars. If the gas outflow rotates, it can carry rotational momentum away from the system. Astronomers have tried to detect the rotation of the outflow to test this scenario and understand its launching mechanism. In a few cases signatures of rotation have been found, but it has been difficult to resolve clearly, especially around massive baby stars. The team of astronomers led by Tomoya Hirota, an assistant professor at the National Astronomical Observatory of Japan (NAOJ) and SOKENDAI (the Graduate University for Advanced Studies) observed a massive baby star called Orion KL Source I in the famous Orion Nebula, located 1,400 light-years away from the Earth. The Orion Nebula is the closest massive-star forming region to Earth. Thanks to its close vicinity and ALMA's advanced capabilities, the team was able to reveal the nature of the outflow from Source I. "We have clearly imaged the rotation of the outflow," said Hirota, the lead author of the research paper published in the journal Nature Astronomy. "In addition, the result gives us important insight into the launching mechanism of the outflow." The new ALMA observations beautifully illustrate the rotation of the outflow. The outflow rotates in the same direction as the gas disk surrounding the star. This strongly supports the idea that the outflow plays an important role in dissipating the rotational energy. Furthermore, ALMA clearly shows that the outflow is launched not from the vicinity of the baby star itself, but rather from the outer edge of the disk. This morphology agrees well with the "magnetocentrifugal disk wind model." In this model, gas in the rotating disk moves outward due to the centrifugal force and then moves upward along the magnetic field lines to form outflows. Although previous observations with ALMA have found supporting evidence around a low-mass protostar, there was little compelling evidence around massive protostars because most of the massive-star forming regions are rather distant and difficult to investigate in detail. "In addition to high sensitivity and fidelity, high resolution submillimeter-wave observation is essential to our study, which ALMA made possible for the first time. Submillimeter waves are a unique diagnostic tool for the dense innermost region of the outflow, and at that exact place we detected the rotation," explained Hirota. "ALMA's resolution will become even higher in the future. We would like to observe other objects to improve our understanding of the launching mechanism of outflows and the formation scenario of massive stars with the assistance of theoretical research." ALMA also imaged rotation of a gas jet from a low-mass protostar. Please read the press release "Baby Star Spits a "Spinning Jet" As It Munches -Down on a "Space Hamburger"" from the Academia Sinica Institute of Astronomy and Astrophysics, Taiwan. Explore further: ALMA returns to boomerang nebula: Companion star provides chilling power of 'coldest object in the universe' More information: Tomoya Hirota et al, Disk-driven rotating bipolar outflow in Orion Source I, Nature Astronomy (2017). DOI: 10.1038/s41550-017-0146


News Article | June 12, 2017
Site: www.eurekalert.org

Stars form from gas and dust floating in interstellar space. But, astronomers do not yet fully understand how it is possible to form the massive stars seen in space. One key issue is gas rotation. The parent cloud rotates slowly in the initial stage and the rotation becomes faster as the cloud shrinks due to self-gravity. Stars formed in such a process should have very rapid rotation, but this is not the case. The stars observed in the Universe rotate more slowly. How is the rotational momentum dissipated? One possible scenario involves that the gas emanating from baby stars. If the gas outflow rotates, it can carry rotational momentum away from the system. Astronomers have tried to detect the rotation of the outflow to test this scenario and understand its launching mechanism. In a few cases signatures of rotation have been found, but it has been difficult to resolve clearly, especially around massive baby stars. The team of astronomers led by Tomoya Hirota, an assistant professor at the National Astronomical Observatory of Japan (NAOJ) and SOKENDAI (the Graduate University for Advanced Studies) observed a massive baby star called Orion KL Source I in the famous Orion Nebula, located 1,400 light-years away from the Earth. The Orion Nebula is the closest massive-star forming region to Earth. Thanks to its close vicinity and ALMA's advanced capabilities, the team was able to reveal the nature of the outflow from Source I. "We have clearly imaged the rotation of the outflow," said Hirota, the lead author of the research paper published in the journal Nature Astronomy. "In addition, the result gives us important insight into the launching mechanism of the outflow." The new ALMA observations beautifully illustrate the rotation of the outflow. The outflow rotates in the same direction as the gas disk surrounding the star. This strongly supports the idea that the outflow plays an important role in dissipating the rotational energy. Furthermore, ALMA clearly shows that the outflow is launched not from the vicinity of the baby star itself, but rather from the outer edge of the disk. This morphology agrees well with the "magnetocentrifugal disk wind model." In this model, gas in the rotating disk moves outward due to the centrifugal force and then moves upward along the magnetic field lines to form outflows. Although previous observations with ALMA have found supporting evidence around a low-mass protostar, there was little compelling evidence around massive protostars because most of the massive-star forming regions are rather distant and difficult to investigate in detail. "In addition to high sensitivity and fidelity, high resolution submillimeter-wave observation is essential to our study, which ALMA made possible for the first time. Submillimeter waves are a unique diagnostic tool for the dense innermost region of the outflow, and at that exact place we detected the rotation," explained Hirota. "ALMA's resolution will become even higher in the future. We would like to observe other objects to improve our understanding of the launching mechanism of outflows and the formation scenario of massive stars with the assistance of theoretical research." ALMA also imaged rotation of a gas jet from a low-mass protostar. Please read the press release "Baby Star Spits a "Spinning Jet" As It Munches -Down on a "Space Hamburger"" from the Academia Sinica Institute of Astronomy and Astrophysics, Taiwan. These observation results were published as Hirota et al. "Disk-Driven Rotating Bipolar Outflow in Orion Source I" in Nature Astronomy on June 12, 2017. Tomoya Hirota (National Astronomical Observatory of Japan / SOKENDAI), Masahiro Machida (Kyushu University), Yuko Matsushita (Kyushu University), Kazuhito Motogi (Yamaguchi University / NAOJ), Naoko Matsumoto (Yamaguchi University / NAOJ), Mi Kyoung Kim (Korean Astronomy and Space Science Institute), Ross A. Burns (Joint Institute for VLBI ERIC), Mareki Honma (NAOJ/SOKENDAI) The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI). ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.


News Article | June 15, 2017
Site: physicsworld.com

China has launched the country's first dedicated X-ray telescope to study the radiation produced by black holes and neutron stars as well as detect gamma-ray bursts. The 1bn RMB Hard X-ray Modulation Telescope (HXMT) was launched today at 11:00 local time from the Jiuquan Satellite Launch Center in north-western China's Gobi Desert. It will now be put into low-Earth orbit with an altitude of around 550 km. First proposed in 1994 and approved in March 2011, HXMT has been developed jointly by the China Academy of Space Technology, Tsinghua University and the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences. The 2700 kg probe carries three instruments that will detect X-rays between 1–250 keV. The high-energy X-ray instrument has a total collecting area of 5000 cm2 and will work between 20–250 keV. The instrument will also be able to detect gamma-ray bursts at energies 3 MeV. "We expect to monitor about 200 gamma-ray bursts every year," says IHEP physicist Shuangnan Zhang, who is the principal investigator of the satellite. The medium-energy X-ray instrument will operate between 5–30 keV while the low-energy X-ray instrument will work from 1–15 keV. Due to atmospheric absorption, cosmic X-rays can only be seen from space with black holes and neutron stars being the two main sources in the universe. By combining sky-survey data with single-point observations, HXMT will develop a high-precision, hard X-ray sky map, looking for new sources and studying in greater detail the temporal properties of known sources to significantly improve our knowledge of the X-ray sky. Dedicated X-ray astronomy began in 1970 with the launch of NASA's Uhuru and since then there have been more than 50 missions, including NASA's Chandra and the European Space Agency's XMM-Newton. But instead of using focusing optics to identify X-ray sources, HXMT adopts a unique "modulation" technique to detect X-rays. "We use collimators to filter the incoming light so that only radiation travelling in a specific direction is allowed through," says Zhang. By swinging the detector in various directions, astronomers can then reconstruct a specific source and eventually render a map of the entire X-ray sky. Paolo Giommi, an astronomer at the Italian Space Agency, says that the modulation technique is "clever" because it does not require "complex and costly X-ray mirrors". That view is backed by astronomer Jonathan Grindlay from Harvard University, who says that the strength of the modulation technique is its simplicity. "With HXMT's broad energy band coverage for wide-field imaging, it should obtain better spectral energy distribution than what is now being done with [NASA's Monitor of All-sky X-ray Image]," he adds. However, HXMT's sensitivity will be limited by using this approach, so Zhang's team is planning to carry out joint observations with other missions such as NASA's NuSTAR, which was launched in 2012. Data from the HXMT will also be shared with international collaborators including those at the University of Tübingen's Institute of Astronomy and Astrophysics. Zhang and colleagues from Tübingen are also working on the proposal for a successor to HXMT – the enhanced X-ray Timing and Polarimetry mission. If approved, it will be an international project involving more than 20 nations and led by China with a launch date as early as 2024/2025.


News Article | November 21, 2016
Site: www.eurekalert.org

An international team led by researchers from Tohoku University has found an extremely faint dwarf satellite galaxy of the Milky Way. The team's discovery is part of the ongoing Subaru Strategic Survey using Hyper Suprime-Cam. The satellite, named Virgo I, lies in the direction of the constellation Virgo. At the absolute magnitude of -0.8 in the optical waveband, it may well be the faintest satellite galaxy yet found. Its discovery suggests the presence of a large number of yet-undetected dwarf satellites in the halo of the Milky Way and provides important insights into galaxy formation through hierarchical assembly of dark matter. Currently, some 50 satellite galaxies to the Milky Way have been identified. About 40 of them are faint and diffuse and belong to the category of so-called "dwarf spheroidal galaxies". Many recently discovered dwarf galaxies, especially those seen in systematic photometric surveys such as the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) are very faint with absolute luminosity in the optical waveband below -8 magnitude. These are so-called "ultra-faint dwarf galaxies". However, previous searches made use of telescopes with a diameter of 2.5 to 4 meters, so only satellites relatively close to the Sun or those with higher magnitudes were identified. Those that are more distant or faint ones in the halo of the Milky Way are yet to be detected. The combination of the large aperture of 8.2-meter Subaru Telescope and the large field-of-view Hyper Suprime-Cam (HSC) instrument is very powerful in this study. It enables an efficient search for very faint dwarf satellites over large areas of the sky. The first step in searching out a new dwarf galaxy is to identify an over density of stars in the sky, using photometric data. Next is to assess that the over dense appearance is not due to line-of-sight or accidental juxtapositions of unrelated dense fields, but is really a stellar system. The standard method for doing this is to look for a characteristic distribution of stars in the color-magnitude diagram (comparable to the Hertzsprung-Russell diagram). Stars in a general field shows no particular patterns in this diagram. Daisuke Homma, a graduate student at Tohoku University, found Virgo I under the guidance of his advisor, Masashi Chiba, and their international collaborators. "We have carefully examined the early data of the Subaru Strategic Survey with HSC and found an apparent over density of stars in Virgo with very high statistical significance, showing a characteristic pattern of an ancient stellar system in the color-magnitude diagram," he said. "Surprisingly, this is one of the faintest satellites, with absolute magnitude of -0.8 in the optical waveband. This is indeed a galaxy, because it is spatially extended with a radius of 124 light years - systematically larger than a globular cluster with comparable luminosity." The faintest dwarf satellites identified so far was Segue I, discovered by SDSS (-1.5 mag) and Cetus II in DES (0.0 mag). Cetus II is yet to be confirmed, as it is too compact as a galaxy. Virgo I may ultimately turn out to be the faintest one ever discovered. It lies at a distance of 280,000 light years from the Sun, and such a remote galaxy with faint brightness has not been identified in previous surveys. It is beyond the reach of SDSS, which has previously surveyed the same area in the direction of the constellation Virgo. According to Chiba, the leader of this search project, the discovery has profound implications. "This discovery implies hundreds of faint dwarf satellites waiting to be discovered in the halo of the Milky Way," he said. "How many satellites are indeed there and what properties they have, will give us an important clue of understanding how the Milky Way formed and how dark matter contributed to it." Formation of galaxies like the Milky Way is thought to proceed through the hierarchical assembly of dark matter, forming dark halos, and through the subsequent infall of gas and star formation affected by gravity. Standard models of galaxy formation in the context of the so-called cold dark matter (CDM) theory predict the presence of hundreds of small dark halos orbiting in a Milky Way-sized dark halo and a comparable number of luminous satellite companions. However, only tens of satellites have ever been identified. This falls well short of a theoretical predicted number, which is part of the so-called "missing satellite problem". Astronomers may need to consider other types of dark matter than CDM or to invoke baryonic physics suppressing galaxy formation to explain the shortfall in the number of satellites. Another possibility is that they have seen only a fraction of all the satellites associated with the Milky Way due to various observational biases. The issue remains unsolved. One of the motivations for the Subaru Strategic Survey using HSC is to do increase observations in the search for Milky Way satellites. The early data from this survey is what led to the discovery of Virgo I. This program will continue to explore much wider areas of the sky and is expected to find more satellites like Virgo I. These tiny companions to be discovered in the near future may tell us much more about history of the Milky Way's formation. Daisuke Homma (Tohoku University, Japan), Masashi Chiba (Tohoku University, Japan), Sakurako Okamoto (Shanghai Astronomical Observatory, China), Yutaka Komiyama (National Astronomical Observatory of Japan (NAOJ), Japan), Masayuki Tanaka (NAOJ, Japan), Mikito Tanaka (Tohoku University, Japan), Miho N. Ishigaki (Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), University of Tokyo, Japan), Masayuki Akiyama (Tohoku University, Japan), Nobuo Arimoto (Subaru Telescope, NAOJ, USA), Jose A, Garmilla (Princeton University, USA), Robert H. Lupton (Princeton University, USA), Michael A. Strauss (Princeton University, USA), Hisanori Furusawa (NAOJ, Japan), Satoshi Miyazaki (NAOJ, Japan), Hitoshi Murayama (Kavli IPMU, WPI, University of Tokyo, Japan), Atsushi J. Nishizawa (Nagoya University, Japan), Masahiro Takada (Kavli IPMU, WPI, University of Tokyo, Japan), Tomonori Usuda (NAOJ, Japan), Shiang-Yu Wang (Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan) Leo II: An Old Dwarf Galaxy with Juvenescent Heart http://subarutelescope.


Basilakos S.,Academy of Athens | Basilakos S.,University of Barcelona | Plionis M.,Institute of Astronomy and Astrophysics | Plionis M.,National Institute of Astrophysics, Optics and Electronics | And 2 more authors.
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

We derive an exact analytical solution for the redshift evolution of linear and scale-independent bias, by solving a second-order differential equation based on linear perturbation theory. This bias evolution model is applicable to all different types of dark energy and modified gravity models. We propose that the combination of the current bias evolution model with data on the bias of extragalactic mass tracers could provide an efficient way to discriminate between geometrical dark energy models and dark energy models that adhere to general relativity. © 2011 American Physical Society.


Rao R.,Institute of Astronomy and Astrophysics | Girart J.M.,Institute Of Ciencies Of Lespai Csic Ieec | Lai S.-P.,National Tsing Hua University | Lai S.-P.,Academia Sinica, Taiwan | Marrone D.P.,University of Arizona
Astrophysical Journal Letters | Year: 2014

We present subarcsecond resolution polarimetric observations of the 878 μm thermal dust continuum emission obtained with the Submillimeter Array toward the IRAS 16293-2422 protostellar binary system. We report the detection of linearly polarized dust emission arising from the circumstellar disk associated with the IRAS 16293-2422 B protostar. The fractional polarization of ≃ 1.4% is only slightly lower than that expected from theoretical calculations in such disks. The magnetic field structure on the plane of the sky derived from the dust polarization suggests a complex magnetic field geometry in the disk, possibly associated with a rotating disk that is wrapping the field lines as expected from the simulations. The polarization around IRAS 16293-2422 A at subarcsecond angular resolution is only marginally detected. © 2014. The American Astronomical Society. All rights reserved..


Girart J.M.,Institute Of Ciencies Of Lespai | Estalella R.,University of Barcelona | Palau A.,Institute Of Ciencies Of Lespai | Torrelles J.M.,Institute Of Ciencies Of Lespai | And 2 more authors.
Astrophysical Journal Letters | Year: 2014

We present CO 3-2, SiO 8-7, C34S 7-6, and 878 μm dust continuum subarcsecond angular resolution observations with the Submillimeter Array (SMA) toward the IRAS 16293-2422 (I16293) multiple low-mass protostellar system. The C34S emission traces the 878 μm dust continuum well, and in addition clearly shows a smooth velocity gradient along the major axis of component I16293A. CO shows emission at moderate high velocities arising from two bipolar outflows, which appear to be perpendicular with respect to each other. The high sensitivity and higher angular resolution of these observations allows us to pinpoint well the origin of these two outflows at the center of component I16293A. Interestingly, the most compact outflow appears to point toward I16293B. Our data show that the previously reported monopolar blueshifted CO outflow associated with component I16293B seems to be part of the compact outflow arising from component I16293A. In addition, the SiO emission is also tracing this compact outflow: on the one hand, the SiO emission appears to have a jet-like morphology along the southern redshifted lobe; on the other hand, the SiO emission associated with the blueshifted northern lobe traces a well-defined arc on the border of component I16293B facing I16293A. The blueshifted CO lobe of the compact outflow splits into two lobes around the position of this SiO arc. All these results lead us to propose that the compact outflow from component I16293A is impacting on the circumstellar gas around component I16293B, possibly being diverged as a consequence of the interaction. © 2014. The American Astronomical Society. All rights reserved.


Alves F.O.,Institute Of Ciencies Of Lespai Ieec Csic | Girart J.M.,Institute Of Ciencies Of Lespai Ieec Csic | Lai S.-P.,National Tsing Hua University | Rao R.,Institute of Astronomy and Astrophysics | Zhang Q.,Harvard - Smithsonian Center for Astrophysics
Astrophysical Journal | Year: 2011

We used the Submillimeter Array to observe the thermal polarized dust emission from the protostellar source NGC 2024 FIR 5. The polarized emission outlines a partial hourglass morphology for the plane-of-sky component of the core magnetic field. Our data are consistent with previous BIMA maps, and the overall magnetic field geometries obtained with both instruments are similar.We resolve the main core into two components, FIR 5A and FIR 5B. A possible explanation for the asymmetrical field lies in depolarization effects due to the lack of internal heating from the FIR 5B source, which may be in a prestellar evolutionary state. The field strength was estimated to be 2.2 mG, in agreement with previous BIMA data. We discuss the influence of a nearby Hii region over the field lines at scales of ∼0.01 pc. Although the hot component is probably compressing the molecular gas where the dust core is embedded, it is unlikely that the radiation pressure exceeds the magnetic tension. Finally, a complex outflow morphology is observed in CO (3→2) maps. Unlike previous maps, several features associated with dust condensations other than FIR 5 are detected. © 2011 The American Astronomical Society. All rights reserved.


Frau P.,CSIC - Institute of Materials Science | Girart J.M.,Institute Of Ciencies Of Lespai | Zhang Q.,Harvard - Smithsonian Center for Astrophysics | Rao R.,Institute of Astronomy and Astrophysics
Astronomy and Astrophysics | Year: 2014

Context. NGC 7538 IRS 1-3 is a high-mass star-forming cluster with several detected dust cores, infrared sources, (ultra)compact HII regions, molecular outflows, and masers. In such a complex environment, interactions and feedback among the embedded objects are expected to play a major role in the evolution of the region. Aims. We study the dust, kinematic, and polarimetric properties of the NGC 7538 IRS 1-3 region to investigate the role of the different forces in the formation and evolution of high-mass star-forming clusters. Methods. We performed SMA high angular resolution observations at 880 μm with the compact configuration. We developed the RATPACKS code to generate synthetic velocity cubes from models of choice to be compared to the observational data. To quantify the stability against gravitational collapse we developed the "mass balance" analysis that accounts for all the energetics on core scales. Results. We detect 14 dust cores from 3.5 M to 37 M arranged in two larger scale structures: a central bar and a filamentary spiral arm. The spiral arm presents large-scale velocity gradients in H13CO+ 4-3 and C17O 3-2, and magnetic field segments aligned well to the dust main axis. The velocity gradient is reproduced well by a spiral arm expanding at 9 km s-1 with respect to the central core MM1, which is known to power a large precessing outflow. The energy of the outflow is comparable to the spiral-arm kinetic energy, which dominates gravitational and magnetic energies. In addition, the dynamical ages of the outflow and spiral arm are comparable. On core scales, those embedded in the central bar seem to be unstable against gravitational collapse and prone to forming high-mass stars, while those in the spiral arm have lower masses that seem to be supported by non-thermal motions and magnetic fields. Conclusions. The NGC 7538 IRS 1-3 cluster seems to be dominated by protostellar feedback. The dusty spiral arm appears to be formed in a snowplow fashion owing to the outflow from the MM1 core. We speculate that the external pressure from the redshifted lobe of the outflow could trigger star formation in the spiral arm cores. This scenario would form a small cluster with a few central high-mass stars, surrounded by a number of low-mass stars formed through protostellar feedback. © ESO, 2014.

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