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The disk atmosphere is the gas above and below the midplane dust layer, which bisects the disk atmosphere and is the location where planets form. Disks have three classifications. Protoplanetary disks have complete disk atmospheres, transition disks have partial disk atmospheres, and dusty disks have no disk atmospheres. Disk atmospheres are dissipated quickly by large stars but my persist for long periods around smaller stars. With the ALMA telescope array, scientists study the progression of disk evolution around many protostars and stars.
The ALMA telescope (Figure 4‑17) has a detection range in the millimeter (10-3 m) to submillimeter (10-4 m) range, which includes the range of radiation given off by cold disks. It is the most expensive land-based telescope system in history with a cost of $1.4 billion.
Figure 4‑17. Alma telescope array (above) and artist rendering of array (below). Credit: ALMA (ESO/NAOJ/NRAO) and ESO - http://www.eso.org/public/images/alma-chajnantor-scene1/
The ALMA array is able to detect disks at high resolution with its 66 twelve-meter diameter antennas and 12 seven-meter diameter antennas that can be driven around and positioned at different points on the plateau. All 66 antennae are synchronized to observe the same object with different beams (interferometry) with a massive computer system with over 134 million processors, performing up to 17 quadrillion operations per second. The diameter of the entire array is the effective diameter of the telescope.
This section focuses on three disks observed by ALMA. The TW Hydrae star is a relatively old and small star surrounded by a transition disk with an inner dust section and an outer disk atmosphere sections. HL Tauri is a relatively small and young star that is surrounded by a protoplanetary disk. V883 Orionis is a young but large star that has completely removed the disk atmosphere and is surrounded by a dust disk.
Figure 4‑18. TW Hydrae transition disk. Credit: ALMA (ESO/NAOJ/NRAO).
The TW Hydrae transition disk (Figure 4‑18) encircles a star that is approximately 8 million years old. The TW Hydrae star is 175 light years from Earth and is a K6 spectral class star, which means it is smaller than the sun and emits an orange color. The star has not reached the main sequence phase and has a surface temperature of 4,000 K. The radius of TW Hydrae is 111% of the sun, and the luminosity is 28% of the sun. The disk surrounding TW Hydrae is a transition disk. The region within 1 AU of the star has no gas or dust. The range from 1 to 4 AU only has dust. The gas has been blown away by the star. There is a likely planet at 22 AU distance from the star, which is represented by the first gap in Figure 4‑18, the distance of Uranus from the sun.
Figure 4‑19. Region within 4 AU of TW Hydrae. “This ALMA image of the young nearby star TW Hydrae has a resolution of 1 AU (Astronomical Unit, the distance from the Earth to the Sun in the Solar System).” Credit: ALMA (ESO/NAOJ/NRAO).
Figure 4‑19 represents the region within 4 AU of the star. Each of the brighter sections in Figure 4‑19 represents a pixel. Examination of the pixel dimensions indicates that the limit of the ALMA resolution is 1 AU for the TW Hydrae disk, which is 0.0002 LY. Based on the distance of the star (~200 AU) and resolution (~0.0002 LY), the telescope resolution is one ten millionth of the distance to the star. This high resolution is due to the fact that the telescopes are widely spaced on the plane.
Figure 4‑20. ALMA image with green representing carbon monoxide beginning 30 AU from the TW Hydrae star. Credit: ALMA. (ESO/NAOJ/NRAO)
The gas section of the disk begins at 4 AU and reaches the full disk thickness relatively quickly and then has a relatively flat shape extending 200 AU from the star. Scientists detect relatively large grains of dust (mm size) in this gaseous section of the disk out to a distance of 60 AU from the star. There is a very high fraction of water vapor in the disk between 4 AU and 30 AU. Scientists also detect water vapor in the outer part of the disk. There is a carbon monoxide snowline approximately 30 AU from the star, which means that there is carbon monoxide ice and vapor outside of 30 AU (Figure 4‑20). Water ice is also in the outer part of the disk. The majority of ice particles in the disk are up to millimeter-sized grains and are within 50 AU of the star. There is also methyl alcohol (methanol) in the disk (Figure 4‑21).
Figure 4‑21. Artist's representation of TW Hydrae disk and gaseous methanol molecules in disk atmosphere. Credit: ALMA ESO/NAOJ/NRAO/M. Kommesar.[1]
Figure 4‑22. HL Tauri protoplanetary disk with gaps were planets are probably forming. Credit: ALMA (ESO/NAOJ/NRAO).
The HL Tauri protoplanetary disk (Figure 4‑22) surrounds a young (100,000 years old) K9 star (smaller than sun and larger than TW Hydrae). The disk radius is 80 AU, and the disk mass is 0.13 solar masses. The disk is optically thick, which means it is a protoplanetary disk with a thick disk atmosphere. One interesting feature of this disk is that planets are forming after only 100,000 years, sooner than scientists had expected.
V883 Orionis is 30% more massive than the sun and 400 times more luminous. It is a young unstable star with flash heating events. Although it is a young star, the disk surrounding it is a dust disk with no disk atmosphere due to the high radiation from the star burning away the disk atmosphere. It has an icy outer disk with frozen water ice and an inner dust disk. Scientists refer to the transition zone between the hot and cold regions as the snow line (Figure 4‑23). It is the same concept as the snow line on a mountain, above which there is snow because of cold temperatures. Because of temperature change from inner to outer disks, the hot inner sections might be observed at mm wavelengths and outer cooler sections might be observed at micron (long) wavelengths.
Figure 4‑23. Water snowline (artist’s rendition on the left) around protostar V883 Orionis observed with ALMA telescope. Credit: ALMA (ESO/NAOJ/NRAO).
Figure 4‑24. The fraction of stars with protoplanetary disks vs. time after formation. Credit: Mamajek.[2] Used here per CC BY-SA 3.0
Based on observation and theory, most protostars have protoplanetary disks for the first one to three million years after formation, and most disks dissipate by ten million years after protostar formation (Figure 4‑24).
References
References
[1] Walsh, Catherine, Ryan A. Loomis, Karin I. Öberg, Mihkel Kama, Merel L. R. van 't Hoff, Tom J. Millar, et al. First detection of gas-phase methanol in a protoplanetary disk. 2016 May 13. The American Astronomical Society. The Astrophysical Journal Letters, Volume 823, Number 1
[2] E. Mamajek (2009) in EXOPLANETS AND DISKS: THEIR FORMATION AND DIVERSITY: Proceedings of the International Conference. AIP Conference Proceedings, Volume 1158, pp. 3-10."Initial Conditions of Planet Formation: Lifetimes of Primordial Disks"
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