Scientists sketch aged star system using over a century of observations


Date:
March 12, 2021
Source:
NASA/Goddard Space Flight Center
Summary:
Astronomers have painted their best picture yet of an RV Tauri
variable, a rare type of stellar binary where two stars - one
approaching the end of its life - orbit within a sprawling disk
of dust. Their 130-year dataset spans the widest range of light
yet collected for one of these systems, from radio to X-rays.



FULL STORY ========================================================================== Astronomers have painted their best picture yet of an RV Tauri variable,
a rare type of stellar binary where two stars -- one approaching the end
of its life - - orbit within a sprawling disk of dust. Their 130-year
dataset spans the widest range of light yet collected for one of these
systems, from radio to X- rays.


========================================================================== "There are only about 300 known RV Tauri variables in the Milky Way
galaxy," said Laura Vega, a recent doctoral recipient at Vanderbilt
University in Nashville, Tennessee. "We focused our study on the second brightest, named U Monocerotis, which is now the first of these systems
from which X-rays have been detected." A paper describing the findings,
led by Vega, was published in The Astrophysical Journal.

The system, called U Mon for short, lies around 3,600 light-years away
in the constellation Monoceros. Its two stars circle each other about
every six and a half years on an orbit tipped about 75 degrees from
our perspective.

The primary star, an elderly yellow supergiant, has around twice the
Sun's mass but has billowed to 100 times the Sun's size. A tug of war
between pressure and temperature in its atmosphere causes it to regularly expand and contract, and these pulsations create predictable brightness
changes with alternating deep and shallow dips in light -- a hallmark
of RV Tauri systems. Scientists know less about the companion star,
but they think it's of similar mass and much younger than the primary.

The cool disk around both stars is composed of gas and dust ejected
by the primary star as it evolved. Using radio observations from the Submillimeter Array on Maunakea, Hawai'i, Vega's team estimated that the
disk is around 51 billion miles (82 billion kilometers) across. The binary orbits inside a central gap that the scientists think is comparable to
the distance between the two stars at their maximum separation, when
they're about 540 million miles (870 million kilometers) apart.



==========================================================================
When the stars are farthest from each other, they're roughly aligned
with our line of sight. The disk partially obscures the primary and
creates another predictable fluctuation in the system's light. Vega and
her colleagues think this is when one or both stars interact with the
disk's inner edge, siphoning off streams of gas and dust. They suggest
that the companion star funnels the gas into its own disk, which
heats up and generates an X-ray-emitting outflow of gas. This model
could explain X-rays detected in 2016 by the European Space Agency's
XMM-Newton satellite.

"The XMM observations make U Mon the first RV Tauri variable detected
in X- rays," said Kim Weaver, the XMM U.S. project scientist and an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt,
Maryland. "It's exciting to see ground- and space-based multiwavelength measurements come together to give us new insights into a long-studied
system." In their analysis of U Mon, Vega's team also incorporated 130
years of visible light observations.

The earliest available measurement of the system, collected on Dec. 25,
1888, came from the archives of the American Association of Variable Star Observers (AAVSO), an international network of amateur and professional astronomers headquartered in Cambridge, Massachusetts. AAVSO provided additional historical measurements ranging from the mid-1940s to the
present.

The researchers also used archived images cataloged by the Digital
Access to a Sky Century @ Harvard (DASCH), a program at the Harvard
College Observatory in Cambridge dedicated to digitizing astronomical
images from glass photographic plates made by ground-based telescopes
between the 1880s and 1990s.



==========================================================================
U Mon's light varies both because the primary star pulsates and because
the disk partially obscures it every 6.5 years or so. The combined
AAVSO and DASCH data allowed Vega and her colleagues to spot an even
longer cycle, where the system's brightness rises and falls about every
60 years. They think a warp or clump in the disk, located about as far
from the binary as Neptune is from the Sun, causes this extra variation
as it orbits.

Vega completed her analysis of the U Mon system as a NASA Harriett
G. Jenkins Predoctoral Fellow, a program funded by the NASA Office of
STEM Engagement's Minority University Research and Education Project.

"For her doctoral dissertation, Laura used this historical dataset
to detect a characteristic that would otherwise appear only once
in an astronomer's career," said co-author Rodolfo Montez Jr., an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian,
also in Cambridge. "It's a testament to how our knowledge of the
universe builds over time." Co-author Keivan Stassun, an expert in
star formation and Vega's doctoral advisor at Vanderbilt, notes that
this evolved system has many features and behaviors in common with
newly formed binaries. Both are embedded in disks of gas and dust,
pull material from those disks, and produce outflows of gas. And in
both cases, the disks can form warps or clumps. In young binaries,
those might signal the beginnings of planet formation.

"We still have questions about the feature in U Mon's disk,
which may be answered by future radio observations," Stassun
said. "But otherwise, many of the same characteristics are there. It's fascinating how closely these two binary life stages mirror each other." ========================================================================== Story Source: Materials provided by
NASA/Goddard_Space_Flight_Center. Original written by Jeanette
Kazmierczak. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Laura D. Vega, Keivan G. Stassun, Rodolfo Montez, Tomasz
Kamiński,
Laurence Sabin, Eric M. Schlegel, Wouter H. T. Vlemmings, Joel H.

Kastner, Sofia Ramstedt, Patricia T. Boyd. Multiwavelength
Observations of the RV Tauri Variable System U Monocerotis:
Long-term Variability Phenomena That Can Be Explained by Binary
Interactions with a Circumbinary Disk. The Astrophysical Journal,
2021; 909 (2): 138 DOI: 10.3847/1538-4357/abe302 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/03/210312155446.htm

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