From NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory) (US) via AAS NOVA : “Accretion in Action in an Angled Disk”

From NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory) (US)

via

AASNOVA

AAS NOVA

12 May 2021
Susanna Kohler

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Artist’s impression of a young star surrounded by an accreting circumstellar disk. [NASA/JPL-Caltech]

How does material move through an accretion disk to the young star at its center? Surprising detections from a fortuitously angled disk have now provided new insights.

Driving Inflow

When stars are born from the collapse of a dense molecular cloud, they spend their early stages surrounded by circumstellar disks: disks of gas and dust that we understand to be accreting onto the young stars at their centers.

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Herschel infrared view of the Taurus Molecular Cloud complex, the home of GV Tau. Credit: R. Hurt [European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU)/Herschel/National Aeronautics and Space Administration (US)/Jet Propulsion Laboratory-California Institute of Technology(US)]

How do we know that the disk matter is flowing onto the stars? Evidence for accretion comes from the high-energy light emitted when inflowing material strikes the surface of young stars, producing accretion shocks. But, though these observations provide evidence that accretion is occurring, they don’t tell us much about the mechanisms that drive these flows within the disk.

For material to move inwards within a disk, it must first lose angular momentum — but where does that momentum go? What processes remove or redistribute it? In a new study led by Joan Najita (National Science Foundation (US)’s NOIRLab [National Optical-Infrared Astronomy Research Laboratory] (US)), a team of scientists presents high-resolution observations of an unusual disk — one that happens to be angled in such a way as to help us answer these questions.

Lucky Alignment

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Schematics representing the likely observing geometry of GV Tau N; the observer is on the right. Top: The line of sight to the disk continuum (orange) passes through the warm molecular atmosphere at larger radii (pink), producing absorption. Bottom: View of the molecular gas velocities. The combination of rotation (blue arrows) and inflow (green arrows) produces net redshifted (red arrows) absorption velocities. [Adapted from Najita et al. 2021]

Najita and collaborators used the TEXES spectrograph on the Gemini North 8-m telescope to conduct mid-infrared observations of GV Tau N, a young star surrounded by a nearly edge-on circumstellar disk. The authors’ observations revealed rare molecular absorption lines, a result of the nearly edge-on inclination of the disk.

The unique viewing angle for GV Tau N means that our sightline passes through the disk atmosphere in the inner few au of the disk — the region where planet formation is thought to occur. The molecules in this gas absorb some of the continuum light emitted by the interior disk, leaving signatures in the spectrum that provide valuable insight into the composition and motions of the gas at the surface of the inner disk.

Caught in the Act

Najita and collaborators found evidence for a variety of molecular species in the disk: acetylene (C2H2), hydrogen cyanide (HCN), water (H20), and even ammonia (NH3), which has never before been detected in an inner accretion disk. But the especially interesting result is that these molecules’ absorption lines are redshifted, lying at longer wavelengths than expected if the gas were moving in a stable circular orbit.

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Spectrum showing various ammonia absorption lines from GV Tau N. [Adapted from Najita et al. 2021]

This redshift is an indication that the gas observed is flowing rapidly (about 1 au per year) inward along the disk surface — direct evidence for accretion in action. The authors show that their observations match expected mass accretion rates for active T Tauri stars: roughly a few to a few tens of Earth masses per year. The observations fit neatly with a disk accretion model in which angular momentum is redistributed within the disk, causing surface gas to flow in and accrete while the midplane of the disk spreads outward.

GV Tau N is a lucky break — its orientation allowed us to make these unique measurements. But it’s surely not alone! With more observations of systems like GV Tau N, we’ll be able to further deepen our understanding of disk accretion.
Citation

“High-resolution Mid-infrared Spectroscopy of GV Tau N: Surface Accretion and Detection of NH3 in a Young Protoplanetary Disk,” Joan R. Najita et al 2021 ApJ 908 171.
https://iopscience.iop.org/article/10.3847/1538-4357/abcfc6

See the full article here .


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AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
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Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

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What is NOIRLab?

NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory) (US), the US center for ground-based optical-infrared astronomy, operates the international Gemini Observatory (US) (a facility of National Science Foundation (US), NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and Korea Astronomy and Space Science Institute [한국천문연구원] (KR)), NOAO Kitt Peak National Observatory(US) (KPNO), Cerro Tololo Inter-American Observatory(CL) (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (in cooperation with DOE’s SLAC National Accelerator Laboratory (US)). It is managed by the Association of Universities for Research in Astronomy (AURA) (US) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawaiʻi, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O’odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.

National Science Foundation(US) NOIRLab (US) NOAO (US) Kitt Peak National Observatory (US) on Kitt Peak of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono O’odham Nation, 88 kilometers (55 mi) west-southwest of Tucson, Arizona, Altitude 2,096 m (6,877 ft). annotated.

NOIRLab(US)NOAO Cerro Tololo Inter-American Observatory(CL) approximately 80 km to the East of La Serena, Chile, at an altitude of 2200 meters.

The NOAO-Community Science and Data Center(US)

The NSF NOIRLab Vera C. Rubin Observatory. It is managed by the Association of Universities for Research in Astronomy(US) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawaiʻi, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O’odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.

NSF (US) NOIRLab (US) NOAO (US) Vera C. Rubin Observatory [LSST] Telescope currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing NSF (US) NOIRLab (US) NOAO (US) Gemini South Telescope and NSF (US) NOIRLab (US) NOAO (US) Southern Astrophysical Research Telescope.