Figure 2: The central part of the OB cluster-forming region G33.92+0.11, observed
by ALMA. Left: Dust continuum image taken at 1.3 mm. Right: False color
image showing the integrated emission of three molecules: CH3CN in yellow, 13CS in green, and DCN in magenta, respectively. The CH3CN emission mainly traces the hot molecular cores, which harbor massive stars. The 13CS
emission traces warm dense gas and shocks. The DCN emission appears to
follow the bulk of dense gas traced by the dust continuum emission.
Credit: ALMA(ESO/NAOJ/NRAO), H. B. Liu et al. | Download Image
A Taiwanese research team used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe a large molecular gas clump [1] named G33.92+0.11, where a cluster with massive stars is forming. The
excellent imaging power of ALMA allowed to reveal with unprecedented
detail, the fine structure of the molecular gas at the center of the
region, where two surprisingly large molecular gas arms, with sizes of ~
3.2 light years [2],
appear to be spiraling around two massive molecular cores. These
results showed that the large molecular arms are indeed the cradles of
dense cores, which are current or future birthplaces of massive stars.
This is a crucial step forward in the understanding of how mass
distributes to form both massive cores and massive stars.
How the gravitationally bound stellar clusters, for example, the
young massive clusters (YMCs) and globular clusters (GCs) come to the
existence, remains a fundamental problem in astrophysics. To form such
complex systems, it is required that massive amounts of gas can be
converted with little losses, into stars, before they start to disperse
the gas by the action of their winds —the so called stellar feedback—,
and such process is far from trivial. Current models propose that in
order to quench the action of stellar feedback, the global collapse of
the parent molecular cloud has to be very rapid.
However, this global collapse of giant [3]
molecular clouds (GMC) represents an observational challenge for
astronomers, because they cannot measure distances along their line of
sight (data is projected in two dimensions) and because it is near
impossible to measure gas velocities in the transverse directions.
Nevertheless, the amplified effects of the initial rotation (angular
momentum) of the clouds may translate into the formation of massive
molecular clumps that are supported by centrifugal forces at the center
of the collapsing GMC.
The identification of rotating structures at scales larger than the
cores, may serve as evidence of such an outcome of global collapse.
Also, because the massive molecular clumps are the densest regions in a
collapsing GMC, they are likely the sites where the most massive stars
of stellar clusters can form. To resolve the details of the morphology
and kinematics of these systems will be key to understand how mass
distributes in the sites of star cluster formation, such that it can
form both massive and not massive stars.
A research team led by Hauyu Baobab Liu at the Academia Sinica
Institute of Astronomy and Astrophysics (ASIAA) observed with ALMA the
luminous OB cluster-forming region G33.92+0.11, located at a distance of
about 23.000 lightyears. This source is at a beginning phase of forming
an OB association, which has a contained luminosity of 250 thousand
times the luminosity of the Sun. Most of this light is provided by a few
embedded massive stars. The research team used the archival Herschel
350 μm, which were combined with another 350 μm image from the Caltech
Submillimeter Observatory (CSO) with a higher angular resolution.
"The Herschel Space Telescope archive images provided a high quality map of the 350 μm thermal emission of the external dusty gas structures around G33.92+0.11. We completed the missing small-scale pixels of this map with data from the Caltech Submillimeter Observatory. The final map revealed two molecular arms twisted in opposite directions, north and south of the cluster, converging at the central molecular clumps, indicating that perhaps the gas is being transported toward the central cluster along these spiral arms from distances as large as 20 light years," says co-author Román-Zúñiga, from the Astronomy Institute of the Universidad Nacional Autónoma de México.
The unprecedented high angular resolution and high imaging fidelity
of ALMA allowed the astronomers to reveal in G33.92+0.11 A two centrally
located massive molecular cores (~100-300 solar masses), connected by
several spiraling dense molecular gas arms. This kind of morphology
resembles the previous ALMA images of molecular gas arms surrounding the
low-mass protostellar binary L1551 NE [4],
however, but linearly scaled-up by a factor between 100 and 1000
(Figure 1). In addition, the observed gas arms in G33.92+0.11 A appear
to be fragmenting, which results in the formation of multiple satellite
cores orbiting the central two highest mass cores. Comparing the
simultaneously observed molecular gas tracers including CH3CN, 13CS,
and DCN shows that the gas excitation conditions in these molecular
arms and cores far from being uniform across the system (Figure 2). For
instance, the two highest mass cores at the center already harbor
massive stars and present bright CH3CN emission. The
molecular arms embedded with satellite cores in the north may be
relatively cool, indicated by the good correlation between the DCN line
and the 1.3 mm dust continuum emission. Finally the molecular arms
connecting the central massive molecular cores from the west may contain
gas that is shocked to a higher temperature or are subject to stellar
heating and show stronger 13CS emission.
This team propose that the central ~1 pc scale region of G33.92+0.11 A
is a flattened, massive molecular clump that is currently accreting
material, which is being fed by the exterior gas filaments, and is
marginally supported by centrifugal forces. At all spatial scales, the
regions of higher density, that contain larger amounts of mass, form at
the center of the system. Accretion may be prohibited by the angular
momentum, but might be alleviated by fragmentation. The authors further
propose that in the dense eccentric accretion flows, the formation of
spiraling arm-like structures may be essential to the process. The
subsequent fragmentation of the dense molecular arms may lead to the
formation of the second generation high-mass stars.
"Gas structures similar to spiral
arms should be common in many systems at many different scales, as long
as they are unstable to gravity and have non-negligible rotation. The
superb images made with ALMA are starting to show this,"says co-author Galván-Madrid.
Notes
[1] In our nomenclature, massive molecular clumps refer to dense molecular gas structures with sizes of ∼0.5-1 pc, massive molecular cores refer to the ⟨ 0.1 pc size overdensities embedded within a clump, and condensations refer to the distinct molecular substructures within a core. Fragmentation refers to the dynamical process that produces or enhances the formation of multiple objects.
[2] 1 parsec (pc) ~ 3.2 light years ~ 3.086×1016 meters.
[3] The typical spatial scales of stellar cluster-forming molecular clouds are 101-2 pc.
[4] More in the press release Dec 04, 2014: Astronomers Identify Gas Spirals as a Nursery of Twin Stars through ALMA Observation
[5] For details, please see Dale, J. E., Ngoumou, J., Ercolano, B., Bonnell, I. A., 2014, MNRAS, 442, 694
More information
These observational results were published in the Astrophysical Journal (ApJ, 804, 37) by Liu et al. as "ALMA resolves the spiraling accretion flow in the luminous OB cluster forming region G33.92+0.11".
This research was conducted by Hauyu Baobab Liu (Academia Sinica Institute of Astronomy and Astrophysics); Roberto Galván-Madrid (Centro de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México); Izaskun Jiménez-Serra (Department of Physics and Astronomy, University College London and European Southern Observatory, Garching Germany); Carlos Román-Zúñiga (Instituto de Astronomía, Universidad Nacional Autónoma de México); Qizhou Zhang (Harvard-Smithsonian Center for Astrophysics); Zhiyun Li (Department of Astronomy, University of Virginia); Huei-Ru Chen (Institute of Astronomy and Department of Physics, National Tsing Hua University).
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organization 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.
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