Stellar egg hunt with ALMA
Tracing evolution from embryo to baby star

Date:
August 7, 2020
Source:
National Institutes of Natural Sciences
Summary:
Astronomers using the Atacama Large Millimeter/submillimeter Array
(ALMA) took a census of stellar eggs in the constellation Taurus
and revealed their evolution state. This census helps researchers
understand how and when a stellar embryo transforms to a baby star
deep inside a gaseous egg. In addition, the team found a bipolar
outflow, a pair of gas streams, that could be telltale evidence
of a truly newborn star.



FULL STORY ========================================================================== Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA)
took a census of stellar eggs in the constellation Taurus and revealed
their evolution state. This census helps researchers understand how and
when a stellar embryo transforms to a baby star deep inside a gaseous
egg. In addition, the team found a bipolar outflow, a pair of gas streams,
that could be telltale evidence of a truly newborn star.


========================================================================== Stars are formed by gravitational contraction of gaseous clouds. The
densest parts of the clouds, called molecular cloud cores, are the very
sites of star formation and mainly located along the Milky Way. The
Taurus Molecular Cloud is one of the active star-forming regions and
many telescopes have been pointed at the cloud. Previous observations
show that some cores are actually stellar eggs before the birth of stars,
but others already have infant stars inside.

A research team led by Kazuki Tokuda, an astronomer at Osaka Prefecture University and the National Astronomical Observatory of Japan (NAOJ),
utilized the power of ALMA to investigate the inner structure of the
stellar eggs. They observed 32 starless cores and nine cores with baby protostars. They detected radio waves from all of the nine cores with
stars, but only 12 out of 32 starless cores showed a signal. The team
concluded that these 12 eggs have developed internal structures, which
shows they are more evolved than the 20 quite cores.

"Generally speaking, radio interferometers using many antennas, like
ALMA, are not good at observing featureless objects like stellar eggs,"
says Tokuda. "But in our observations, we purposely used only the 7-m
antennas of ALMA. This compact array enables us to see objects with smooth structure, and we got information about the internal structure of the
stellar eggs, just as we intended." Increasing the spacing between the antennas improves the resolution of a radio interferometer, but makes
it difficult to detect extended objects. On the other hand, a compact
array has lower resolution but allows us to see extended objects. This
is why the team used ALMA's compact array of 7-m antennas, as known as
the Morita Array, not the extended array of 12-m antennas.

They found that there is a difference between the two groups in the gas
density at the center of the dense cores. Once the density of the center
of a dense core exceeds a certain threshold, about one million hydrogen molecules per cubic centimeter, self-gravity leads the egg to transform
into a star.

A census is also useful for finding a rare object. The team noticed that
there is a weak but clear bipolar gas stream in one stellar egg. The
size of the stream is rather small, and no infrared source has been
identified in the dense core. These characteristics match well with the theoretical predictions of a "first hydrostatic core," a short-lived
object formed just before the birth of a baby star. "Several candidates
for the first hydrostatic cores have been identified in other regions," explains Kakeru Fujishiro, a member of the research team. "This is
the first identification in the Taurus region. It is a good target for
future extensive observation." Kengo Tachihara, an associate professor
at Nagoya University mentions the role of Japanese researchers in this
study. "Japanese astronomers have studied the baby stars and stellar eggs
in Taurus using the Nagoya 4-m radio telescope and Nobeyama 45-m radio telescope since the 1990s. And, ALMA's 7-m array was also developed by
Japan. The present result is part of the culmination of these efforts."
"We have succeeded in illustrating the growth history of stellar eggs up
to their birth, and now we have established the method for the research," summarizes Tokuda. "This is an important step to obtain a comprehensive understanding of star formation."

========================================================================== Story Source: Materials provided by
National_Institutes_of_Natural_Sciences. Note: Content may be edited
for style and length.


========================================================================== Journal References:
1. Kazuki Tokuda, Kakeru Fujishiro, Kengo Tachihara, Tatsuyuki
Takashima,
Yasuo Fukui, Sarolta Zahorecz, Kazuya Saigo, Tomoaki Matsumoto,
Kengo Tomida, Masahiro N. Machida, Shu-ichiro Inutsuka, Philippe
Andre', Akiko Kawamura, Toshikazu Onishi. FRagmentation and
Evolution of Dense Cores Judged by ALMA (FREJA). I. Overview:
Inner ~1000 au Structures of Prestellar/Protostellar Cores
in Taurus. The Astrophysical Journal, 2020; 899 (1): 10 DOI:
10.3847/1538-4357/ab9ca7
2. Kakeru Fujishiro, Kazuki Tokuda, Kengo Tachihara, Tatsuyuki
Takashima,
Yasuo Fukui, Sarolta Zahorecz, Kazuya Saigo, Tomoaki Matsumoto,
Kengo Tomida, Masahiro N. Machida, Shu-ichiro Inutsuka, Philippe
Andre', Akiko Kawamura, Toshikazu Onishi. A Low-velocity Bipolar
Outflow from a Deeply Embedded Object in Taurus Revealed by the
Atacama Compact Array. The Astrophysical Journal Letters, 2020
[abstract] ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200807093733.htm

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