Figure 1. An artist's concept of the distribution of the ambient gas around IRAS 16547-4247. The central high-density gas cloud is thought to contain multiple high-density protostars. Two outflows of gas spurt from the central part in the vertical and horizontal directions respectively while pushing the ambient gas away, which makes a balloon-like structure. A pair of narrow jets is the one that was found in past observations.
Credit: ALMA(ESO/NAOJ/NRAO). Click to Enlarge (2.83MB / JPEG)
Figure 2. A mesh 3D model of gas distribution. The orange-colored, peanut-hull-like structure at the center represents the high-density gas cloud observed with ALMA; the blue-colored, big rugby-ball-like structure stretching out in the vertical direction represents the big outflow observed in past observations; and the lime-green-colored and purple-colored structures represent the outflows discovered with ALMA.
Credit: ALMA(ESO/NAOJ/NRAO). Click to Enlarge (2.45MB / JPEG)
Figure 3. Figure 1+Figure 2 (4.02MB / JPEG)
Credit: ALMA(ESO/NAOJ/NRAO)
Credit: ALMA(ESO/NAOJ/NRAO)
Overview
A research group led by Aya Higuchi, a researcher at Ibaraki
University, conducted observations of the massive-star forming region
IRAS 16547-4247 with the Atacama Large Millimeter/submillimeter Array
(ALMA). The observation results shows the presence of multiple, or at
least two, gas outflows from a protostar, indicating the possible
existence of two new-born stars in this region. Also, the radio
observation results of molecular line emission of methanol revealed in
vivid detail an hourglass structure created by gas outflows spreading
outward while thrusting the ambient gas cloud away. It is the first time
that such an hourglass structure was found in observations of methanol
in high-mass star forming regions. Detailed observations of high-mass
stars have been considered difficult so far because high-mass stars form
in a complex environment with multiple protostars in clusters, and
their forming regions are located farther away from the Earth compared
to those of low-mass stars. However, high angular resolution
observations with ALMA opened a new window to understand their formation
environment in further details.
Research Background
All stars that twinkle in the night sky vary in their masses. While
some stars have masses smaller than 1/10 of solar masses, others have
masses larger than 100 solar masses. How such a wide variety of stars
are born and what factors make the difference in their masses; these are
the most fundamental and most enigmatic astronomical questions which
have yet to be answered. To solve these mysteries, it is essential to
make detailed observations of various stars of different masses during
formation.
The formation process of high-mass stars which have masses larger
than 10 times solar mass still has much to be explored. Detailed
observations of high-mass stars at an early stage of formation are
difficult because the number of high-mass stars is smaller than that of
one-solar-mass stars and the evolution process of high-mass stars is
faster than low-mass stars (*1). Another adverse condition in the study
of high-mass stars is the distance from the Earth; while the forming
regions of low-mass stars are about 500 light years away from the Earth,
those of high-mass stars are farther and even the closest one in the
Orion Nebula is about 1500 light years away. Since it is thought that
high-mass stars are born in clusters far away from the Earth, it is
impossible to understand their formation process in detail without high
angular resolution observations. In this regard, ALMA is the most
desirable telescope for this purpose as being capable of observing gas
and dust which will be ingredients of stars at high sensitivity and high
resolution.
Observations with ALMA
The research team led by Aya Higuchi at Ibaraki University made
observations of the luminous infrared source IRAS 16547-4247 in the
direction of the Scorpion. IRAS 16547-4247 is an object emitting strong
radiation with about 60 times solar luminosity and being surrounded by
high-density molecular cloud with a mass of 1300 times solar mass in a
distance of 9500 light years away from the Earth. Past radio
observations of molecular carbon monoxide (CO) in this region revealed a
pair of outflows which was thought to be emitted from a young star, and
some other radio sources have been found in addition to a bright object
at the center. "Even though many of the astronomers assumed that this
would be a fertile high-mass star forming region, we couldn't probe the
kinematics of gas around high-mass protostars at the level of resolution
provided by existing telescopes," Higuchi said.
To study the structure and kinematics of gas around IRAS 16547-4247,
the research group observed molecular line emission of dust, CO, and
methanol (CH3OH). From the observation results of dust, it was first
found that the center of the region contains two high-density compact
gas clouds with masses 10 to 20 times solar mass. It is thought that
these gas clouds are surrounding a newly forming high-mass star like a
cocoon
And the observation results of CO indicates that the outflows which
looked like a blurred object extending in the north-south direction was
actually two pairs of outflows aligned with the north-south and
east-west direction respectively. Since the angular resolution provided
by ALMA was 36 times higher than that applied to the past CO
observations, the observation results clearly revealed the details of
complex structure and kinematics of gas. As it is assumed that one
protostar is able to produce only a pair of outflows, these results
suggests that multiple stars are being formed simultaneously in this
region.
On top of these, the research group discovered that methanol molecule
is spreading from the center of IRAS1654-4247 in the form of hourglass
structure. CH3OH is normally produced on the surface of dust, but when
the temperature increases by some process, it will be released from the
dust surface and turn into gas which emits radio waves. Since the
hourglass structure made by the distribution of CH3OH traces the contour
of the observed CO outflow, CH3OH is assumed to have been produced by
the interaction with the ambient gas which was pushed away by the
outflow from the protostar, resulting in the increase of temperature and
consequent transition into gas. This kind of hourglass structure has
often been found around low-mass protostars, but it was the first time
that the distribution of CH3OH with this structure was found in a
high-mass-star forming region.
Furthermore, past observation results
indicates the presence of a maser source (*2) emitting extremely strong
radio waves on the extended line of the CO outflow. Although it was
unknown what is responsible for the maser source in this object, the
observation results this time suggests that the maser source is excited
by the shock influence between a high-velocity outflow and the ambient
gas.
"We conducted radio observations of carbon monoxide and methanol to
explore the details of the distribution and kinematics of gas in the
region where high-mass stars are forming in clusters," Higuchi said. "A
typical example of a high-mass star forming region is the Orion Nebula,
but ALMA enabled us to see the complex formation environment of star
clusters which is even 7 times farther away than the Orion Nebula with
the highest imaging resolution ever achieved. ALMA will become
indispensable for the future research on the high-mass star forming
region."
(*1) The formation of high-mass stars completes over the course of a hundred thousand years, which is approximately one tenth of the formation period of low-mass stars
(*2) Maser is a phenomenon that emits strong electromagnetic radiation of a coherent wavelength. Laser which is used in our daily life is also strong radiation produced on the same principle applied to maser. Since maser is produced when atoms are excited to a high energy state, the presence of a maser source suggests the possibility of a physical state which is different from that of common interstellar cloud.
Note
(*1) The formation of high-mass stars completes over the course of a hundred thousand years, which is approximately one tenth of the formation period of low-mass stars
(*2) Maser is a phenomenon that emits strong electromagnetic radiation of a coherent wavelength. Laser which is used in our daily life is also strong radiation produced on the same principle applied to maser. Since maser is produced when atoms are excited to a high energy state, the presence of a maser source suggests the possibility of a physical state which is different from that of common interstellar cloud.
Paper and Research Team
These observation results were published as Higuchi et al. "IRAS 16547-4247: A New Candidate of a Protocluster Unveiled with ALMA" in the astronomical journal Astrophysical Journal Letters, issued in January 2015.
This research was conducted by:
- Aya Higuchi (Ibaraki University)
- Kazuya Saigo (National Astronomical Observatory of Japan)
- James Chibueze (National Astronomical Observatory of Japan/University of Nigeria)
- Patricio Sanhueza (National Astronomical Observatory of Japan)
- Shigehisa Takakuwa (Academia Sinica Institute of Astronomy and Astrophysics)
- Guido Garay (University of Chile)
This research is supported by Grant-in-Aid for Scientific Research on
Innovative Areas "New Frontiers of Extrasolar Planets: Exploring
Terrestrial Planets". Guide Garay is supported by CONICYT project
PFB-06.
ALMA array from the air
Credit: Clem & Adri Bacri-Normier (wingsforscience.com)/ESO
Credit: Clem & Adri Bacri-Normier (wingsforscience.com)/ESO
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.