Inside the Cats's Paw Nebula, as seen in an infrared image from NASA's Spitzer Space Telescope (left), ALMA discovered that an infant star is undergoing an intense growth spurt, shining nearly 100 brighter than before and reshaping its stellar nursery (right). Credit: ALMA (ESO/NAOJ/NRAO), T. Hunter; C. Brogan, B. Saxton (NRAO/AUI/NSF); GLIMPSE, NASA/JPL-Caltech
ALMA image of the glowing dust inside NGC 6334I, a protocluster containing an infant star that is undergoing an intense growth spurt, likely triggered by an avalanche of gas falling onto its surface. Credit: ALMA (ESO/NAOJ/NRAO); C. Brogan, B. Saxton (NRAO/AUI/NSF)
Comparing observations by two different millimeter-wavelength telescopes, ALMA and the SMA, astronomers noted a massive outburst in a star-forming cloud. Because the ALMA images are more sensitive and show finer detail, it was possible to use them to simulate what the SMA could have seen in 2015 and 2016. By subtracting the earlier SMA images from the simulated images, astronomers could see that a significant change had taken place
in MM1 while the other three millimeter sources (MM2, MM3, and MM4) are unchanged. Credit: ALMA (ESO/NAOJ/NRAO); SMA, Harvard/Smithsonian CfA
A massive protostar, deeply nestled in its dust-filled stellar nursery, recently roared to life, shining nearly 100 times brighter than before. This outburst, apparently triggered by an avalanche of star-forming gas crashing onto the surface of the star, supports the theory that young stars can undergo intense growth spurts that reshape their surroundings.
Astronomers made this discovery by comparing
new observations from the Atacama Large Millimeter/submillimeter Array
(ALMA) in Chile with earlier observations from the Submillimeter Array
(SMA) in Hawaii.
"We were amazingly fortunate to detect this
spectacular transformation of a young, massive star,” said Todd Hunter,
an astronomer at the National Radio Astronomy Observatory (NRAO) in
Charlottesville, Va., and lead author on a paper published in the Astrophysical Journal Letters.
"By studying a dense star-forming cloud with both ALMA and the SMA, we
could see that something dramatic had taken place, completely changing a
stellar nursery over a surprisingly short period of time."
In
2008, before the era of ALMA, Hunter and his colleagues used the SMA to
observe a small but active portion of the Cat's Paw Nebula (also known
as NGC 6334), a star-forming complex located about 5,500 light-years
from Earth in the direction of the southern constellation Scorpius. This
nebula is similar in many respects to its more northern cousin, the
Orion Nebula, which is also brimming with young stars, star clusters,
and dense cores of gas that are on the verge of becoming stars. The
Cat's Paw Nebula, however, is forming stars at a faster rate.
The
initial SMA observations of this portion of the nebula, dubbed NGC
6334I, revealed what appeared to be a typical protocluster: a dense
cloud of dust and gas harboring several still-growing stars.
Young
stars form in these tightly packed regions when pockets of gas become
so dense that they begin to collapse under their own gravity. Over time,
disks of dust and gas form around these nascent stars and funnel
material onto their surfaces helping them grow.
This process,
however, may not be entirely slow and steady. Astronomers now believe
that young stars can also experience spectacular growth spurts, periods
when they rapidly acquire mass by gorging on star-forming gas.
The
new ALMA observations of this region, taken in 2015 and 2016, reveal
that dramatic changes occurred toward a portion of the protocluster
called NGC 6334I-MM1 after the original SMA observations. This region is
now about four times brighter at millimeter wavelengths, meaning that
the central protostar is nearly 100 times more luminous than before.
The
astronomers speculate that leading up to this outburst, an uncommonly
large clump of material was drawn into the star's accretion disk,
creating a logjam of dust and gas. Once enough material accumulated, the
logjam burst, releasing an avalanche of gas onto the growing star.
This
extreme accretion event greatly increased the star’s luminosity,
heating its surrounding dust. It’s this hot, glowing dust that the
astronomers observed with ALMA. Though similar events have been observed
in infrared light, this is the first time that such an event has been
identified at millimeter wavelengths.
To ensure that the observed
changes were not the result of differences in the telescopes or simply a
data-processing error, Hunter and his colleagues used the ALMA data as a
model to accurately simulate what the SMA -- with its more modest
capabilities -- would have seen if it conducted similar observations in
2015 and 2016. By digitally subtracting the actual 2008 SMA images from
the simulated images, the astronomers confirmed that there was indeed a
significant and consistent change to one member of the protocluster.
"Once
we made sure we were comparing the two sets of observations on an even
playing field, we knew that we were witnessing a very special time in
the growth of a star," said Crystal Brogan, also with the NRAO and
co-author on the paper.
Further confirmation of this event came
from complementary data taken by the Hartebeesthoek Radio Astronomy
Observatory in South Africa. This single-dish observatory was monitoring
the radio signals from masers in the same region. Masers are the
naturally occurring cosmic radio equivalent of lasers. They are powered
by a variety of energetic processes, including outbursts from rapidly
growing stars.
The data from the Hartebeesthoek observatory
reveal an abrupt and dramatic spike in maser emission from this region
in early 2015, only a few months before the first ALMA observation. Such
a spike is precisely what astronomers would expect to see if there were
a protostar undergoing a major growth spurt.
"These observations
add evidence to the theory that star formation is punctuated by a
sequence of dynamic events that build up a star, rather than a smooth
continuous growth," concluded Hunter. "It also tells us that it is
important to monitor young stars at radio and millimeter wavelengths,
because these wavelengths allow us to peer into the youngest, most
deeply embedded star-forming regions. Catching such events at the
earliest stage may reveal new phenomena of the star-formation process."
The
National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.
# # #
The Atacama Large
Millimeter/submillimeter Array (ALMA), an international astronomy
facility, is a partnership of 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.
This
research is presented in a paper titled "An extraordinary outburst in
the massive protostellar system NGC6334I-MM1: Quadrupling of the
millimeter continuum," by T.R. Hunter et al., published in the
Astrophysical Journal Letters [https://doi.org/10.3847/2041-8213/aa5d0e].
Contact:
Charles Blue
434-296-0314;
cblue@nrao.edu
