Photoevaporation of Circumstellar Disks Due to External Far-Ultraviolet Radiation in Stellar Aggregates

By Fred C. Adams, D. Hollenbach, Gregory Laughlin, U. Gorti. 2004

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When stars form within small groups (with N* ≈ 100-500 members), their circumstellar disks are exposed to relatively little extreme-ultraviolet (EUV; hν > 13.6 eV) radiation but a great deal of far-ultraviolet (FUV; 6 eV < hν < 13.6 eV) radiation (~103 times the local interstellar FUV field) from the most massive stars in the group. This paper calculates the mass-loss rates and evaporation timescales for circumstellar disks exposed to external FUV radiation. Previous work treated large disks and/or intense radiation fields in which the disk radius rd exceeds the critical radius rg where the sound speed in the FUV heated surface layer exceeds the escape speed; it has often been assumed that photoevaporation occurs for rd > rg and is negligible for rd < rg. Since rg ≳ 100 AU for FUV heating, this would imply little mass loss from the planet-forming regions of a disk. In this paper we focus on systems in which photoevaporation is suppressed because rd < rg and show that significant mass loss still takes place as long as rd/rg ≳ 0.1-0.2. Some of the gas extends beyond the disk edge (or above the disk surface) to larger distances where the temperature is higher, the escape speed is lower, and an outflow develops. The resulting evaporation rate is a sensitive function of the central stellar mass and disk radius, which determine the escape speed, and the external FUV flux, which determines the temperature structure of the surface layers and outflowing gas. Disks around red dwarfs, low-mass stars with M* ≲ 0.5 M☉, are evaporated and shrink to disk radii rd ≲ 15 AU on short timescales t ≲ 10 Myr when exposed to moderate FUV fields with G0 = 3000 (where G0 = 1.7 for the local interstellar FUV field). The disks around solar-type stars are more durable. For intense FUV radiation fields with G0 = 30,000, however, even these disks shrink to rd ≲ 15 AU on timescales t ~ 10 Myr. Such fields exist within about 0.7 pc of the center of a cluster with N* ≈ 4000 stars. If our solar system formed in the presence of such strong FUV radiation fields, this mechanism could explain why Neptune and Uranus in our solar system are gas-poor, whereas Jupiter and Saturn are relatively gas-rich. This mechanism for photoevaporation can also limit the production of Kuiper Belt objects and can suppress giant planet formation in sufficiently large clusters, such as the Hyades, especially for disks associated with low-mass stars.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/ae3ca1a9e9d3941449ae7527cb40763a48440c93

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Citations in the corpus

Theory of Star Formation (2007, background): “Pressure gradients enable flows to emerge down to (0.1− 0.2)rg (Begelman, McKee, & Shields, 1983; Font et al., 2004; Adams et al., 2004).”
TIME EVOLUTION OF VISCOUS CIRCUMSTELLAR DISKS DUE TO PHOTOEVAPORATION BY FAR-ULTRAVIOLET, EXTREME-ULTRAVIOLET, AND X-RAY RADIATION FROM THE CENTRAL STAR (2009, methodology): “We note that the flows induced by FUV/X-rays at these regions of the disk are predominantly subsonic (Adams et al. 2004).”
Protoplanetary Disks and Their Evolution (2011, background): “The corresponding thermal velocity, ∼ 3 km s−1, implies that gas is bound to the central star for radii <∼ 100AU (Adams et al. 2004).”
On the fraction of star formation occurring in bound stellar clusters (2012, background): “It has been known for some time that circumstellar discs and planet formation are affected by external photoevaporation (Scally & Clarke 2001; Adams et al. 2004) and dynamical encounters with other stars (Olczak, Pfalzner & Spurzem 2006; Dukes & Krumholz 2011; de Juan Ovelar et al. 2012; Parker &…”
Planet Formation around Stars of Various Masses: The Snow Line and the Frequency of Giant Planets (2007, background): “Although the expected distance for external photoevaporation in “typical” clusters reaches closer to the central star for low-mass stars (Adams et al. 2004), it lies outside the core-forming region for our standard case of Mcore = 10 M⊕, tcore = 10 6 yr, and δ = 1.5.”
Characterization of exoplanets from their formation - II. The planetary mass-radius relationship (2012, background): “We have also neglected the findings of Adams et al. (2004) that significant mass loss due to external FUV radiation might also happen well inside Rg,I and leave this for improvements in further work.”
Photoevaporation of protoplanetary discs - II. Evolutionary models and observable properties (2006, background): “Adams et al. (2004) have shown that external FUV radiation can produce a significant disc wind, which is dominated by flow from the outer disc edge.”
An ALMA Survey of Protoplanetary Disks in the σ Orionis Cluster (2017, background): “At larger separations of ∼0.03–0.3 pc, where less energetic FUV emission is expected to dominate (Adams et al. 2004), they found a range of disk masses representative of typical low-mass star-forming regions, indicating that the lower mass-loss rates in FUVdominated regions can preserve disk masses…”
PHOTOEVAPORATION OF CIRCUMSTELLAR DISKS BY FAR-ULTRAVIOLET, EXTREME-ULTRAVIOLET AND X-RAY RADIATION FROM THE CENTRAL STAR (2008, background): “In order for matter to escape the disk, the gas temperature at the base of the flow has to necessarily satisfy the criterion r rcr with rcr ∼ 0.1GM∗ / c2s to 0.2GM∗ / c2s (see Equation (5); also Liffman 2003; Adams et al. 2004).”
A Spitzer View of Protoplanetary Disks in the γ Velorum Cluster (2008, background): “In clusters with high mass stars, disk photoevaporation due to the strong ultraviolet radiation field can be important, and is directly observed in the Orion Nebula Cluster (Richling & Yorke 2000; Adams et al. 2004; Hollenbach & Adams 2004; Throop & Bally 2005; Clarke 2007; Megeath et al. 2007).”
A Decreased Probability of Habitable Planet Formation around Low-Mass Stars (2007, background): “There are observational (Endl et al. 2006; Butler et al. 2006) and theoretical (Ward & Hahn 1995; Adams et al. 2004; Laughlin et al. 2004, Ida & Lin 2005) reasons to think that the fraction of M stars with Jovian planets is significantly smaller than for solar-mass stars.”
The planet nine hypothesis (2019, background): “More importantly, these clusters also produce intense FUV radiation fields that evaporate circumstellar disks, removing all of the material beyond ∼ 30 − 40 AU over a time scale of 10 Myr (Adams et al., 2004).”
Extrasolar planet population synthesis I: Method, formation tracks and mass-distance distribution (2009, background): “One should for example, when external UV sources are considered, take into account the stellar environment as O or B stars in rich clusters can greatly enhance the far-ultraviolet flux (Adams et al. 2004; Armitage 2000).”
Early Evolution of Stellar Groups and Clusters: Environmental Effects on Forming Planetary Systems (2005, background): “In Figure 9, the vertical lines at G0 ¼ 300, 3000, and 30,000 are values for which the photoevaporation of circumstellar disks due to FUV radiation has been calculated in detail (Adams et al. 2004).”
Globular clusters as the relics of regular star formation in ‘normal’ high-redshift galaxies (2015, background): “…discs to survive for 5–10Myr (the disc lifetime problem), which may be longer than discs survive in nearby star-forming regions (Haisch, Lada & Lada 2001; Bell et al. 2013), especially at the extreme stellar densities of GCs (Adams et al. 2004; de Juan Ovelar et al. 2012; Rosotti et al. 2014).”
The Core Accretion Model Predicts Few Jovian-Mass Planets Orbiting Red Dwarfs (2004, background): “Most stars form in groups and clusters, where external radiation from other nearby stars can efficiently drive mass loss from disks around M stars (Adams et al. 2004).”
X-Ray-Irradiated Protoplanetary Disk Atmospheres. I. Predicted Emission-Line Spectrum and Photoevaporation (2008, background): “A dramatic illustration of the latter is the apparent photoevaporation of Orion “proplyds” in the vicinity of OB stars (e.g. Johnstone et al. 1998, Adams et al., 2004; Fatuzzo & Adams, 2008).”