Formation and Evolution of Planetary Systems: Upper Limits to the Gas Mass in Disks around Sun‐like Stars

Abstract

We have carried out a sensitive search for gas emission lines at infrared and millimeter wavelengths for a sample of 15 young sun-like stars selected from our dust disk survey with the Spitzer Space Telescope. We have used mid-infrared lines to trace the warm (300-100 K) gas in the inner disk and millimeter transitions of 12 CO to probe the cold (∼20 K) outer disk. We report no gas line detections from our sample. Line flux upper limits are first converted to warm and cold gas mass limits using simple approximations allowing a direct comparison with values from the literature. We also present results from more sophisticated models following Gorti and Hollenbach (2004) which confirm and extend our simple analysis. These models show that the [S i] line at 25.23 µm can set constraining limits on the gas surface density at the disk inner radius and traces disk regions up to a few AU. We find that none of the 15 systems have more than 0.04 M J of gas within a few AU from the disk inner radius for disk radii from 1 AU up to ∼40 AU. These gas mass upper limits even in the 8 systems younger than ∼30 Myr suggest that most of the gas is dispersed early. The gas mass upper limits in the 10-40 AU region, that is mainly traced by our CO data, are < 2 M ⊕ . If these systems are analogs of the Solar System, either they have already formed Uranus-and Neptune-like planets or they will not form them beyond 100 Myr. Finally, the gas surface density upper limits at 1 AU are smaller than 0.01% of the minimum mass solar nebula for most of the sources. If terrestrial planets form frequently and their orbits are circularized by gas, then circularization occurs early.

Resolved Co Gas Interior to the Dust Rings of the Hd 141569 Disk

“…Typical protoplanetary disks have gas masses of ∼10 −4 -10 −2 M e Williams et al 2013;Carpenter et al 2014;Mann et al 2015) while we infer M gas =3.8-+ -10 2.8 15 5 M e in the HD 141569 system. Conversely, Pascucci et al (2006), using mid-infrared spectroscopy and single disk mm CO spectroscopy, put upper limits on the gas in the outer radii of debris disks of a few earth masses, below what we derive in the HD 141569 system. Our gas mass is similar to that derived using CO emission from cold gas in the disks around 49 Ceti, with M gas = 3.9±0.9×10 −5 M e (Hughes et al 2008a), HD 21997, with M gas = 8-20×10 −5 M e (Kóspál et al 2013), and HD 131835, with M gas = 1.3±0.6×10 −5 M e (Moór et al 2015), all examples of massive CO disks.…”

“…The CO(1-0) flux implies a CO mass of M CO =1.05±0.06×10 −8 M e , assuming a temperature derived from the line ratio, which is similar to 49 Ceti, HD 21997, and HD 131835 (Hughes et al 2008a;Kóspál et al 2013;Moór et al 2015). This corresponds to M gas = 1.05±0.06×10 −4 M e = 0.11± 0.01 M jup , assuming CO/H 2 = 10 −4 , which is not enough to create a Jupitersized gas giant, but is larger than typically found among debris disks (Pascucci et al 2006). This mass falls within the range infered by Zuckerman et al (1995) from single-dish CO observations ( Table 2).…”

“…Most observations focus on this dust emission (Wyatt 2008;Krivov 2010;Matthews et al 2014, and references therein) but the presence of gas can substantially influence the observed dust structures (Takeuchi & Artymowicz 2001;Lyra & Kuchner 2013). While surveys have found that most debris disks have a negligible gas mass (Pascucci et al 2006), some systems possess significant amounts of gas in addition to their debris-like dust distributions. Radio surveys revealed that HD 21997, a 30 Myr old A star, is surrounded by a broad disk of CO with a mass of M CO ∼5×10 −2 M ⊕ (Kóspál et al 2013), while 49 Ceti also has a large ring of CO emission, with a mass estimated at M CO ∼10 −3 M ⊕ (Hughes et al 2008a).…”

c2dSpitzerIRS spectra of embedded low-mass young stars: gas-phase emission lines

“…Najita et al 2003;Blake & Boogert 2004). Such disks show various atomic transitions as well, most prominently the [Ne ii] 12.8 μm line (Lahuis et al 2007b;Pascucci et al 2006;Espaillat et al 2007; see also models by Meijerink et al 2008). An interesting question is therefore whether young disks in the embedded phase studied here have similar spectra as the more evolved disks, or whether they show additional features, for example due to the accretion shock of material falling on to the disk (Pontoppidan et al 2002;Watson et al 2007).…”

Disk Dispersal: Theoretical Understanding and Observational Constraints

“…The earliest study was the seminal CO survey of ∼ 10 disks by Zuckerman et al (1995) which set a loose constraint on the gas disk dispersal time (at large ∼ 100AU radii) of ∼ 10 Myr. The FEPS legacy survey on the Spitzer Science Telescope, based on non-detections of H 2 , set a dispersal timescale of the order of about 5 − 30 Myr at radii 1 − 40 AU (Pascucci et al 2006). A more recent [OI]63µm survey using the Herschel Space Observatory (GASPS program) derived a similar timescale for the dispersal of gas (∼ 5 − 200AU).…”