Probing CO and N₂ Snow Surfaces in Protoplanetary Disks with N₂H⁺ Emission

Abstract

Snowlines of major volatiles regulate the gas and solid C/N/O ratios in the planet-forming midplanes of protoplanetary disks. Snow surfaces are the 2D extensions of snowlines in the outer disk regions, where radiative heating results in a decreasing temperature with disk height. CO and N 2 are two of the most abundant carriers of C, N and O. N 2 H + can be used to probe the snow surfaces of both molecules, because it is destroyed by CO and formed from N 2 . Here we present Atacama Large Millimeter/submillimeter Array (ALMA) observations of N 2 H + at ∼0. 2-0. 4 resolution in the disks around LkCa 15, GM Aur, DM Tau, V4046 Sgr, AS 209, and IM Lup. We find two distinctive emission morphologies: N 2 H + is either present in a bright, narrow ring surrounded by extended tenuous emission, or in a broad ring. These emission patterns can be explained by two different kinds of vertical temperature structures. Bright, narrow N 2 H + rings are expected in disks with a thick Vertically Isothermal Region above the Midplane (VIRaM) layer (LkCa 15, GM Aur, DM Tau) where the N 2 H + emission peaks between the CO and N 2 snowlines. Broad N 2 H + rings come from disks with a thin VIRaM layer (V4046 Sgr, AS 209, IM Lup). We use a simple model to extract the first sets of CO and N 2 snowline pairs and corresponding freeze-out temperatures towards the disks with a thick VIRaM layer. The results reveal a range of N 2 and CO snowline radii towards stars of similar spectral type, demonstrating the need for empirically determined snowlines in disks.

Temperature Structures of Embedded Disks: Young Disks in Taurus Are Warm

“…These young disks are thus warmer than T Tauri disks, where the snowline is typically at a few tens of au, as can be seen in Figure 11. We only include class II disks for which a CO snowline location has been reported based on molecular line observations, either 13 C 18 O (for TW Hya; Zhang et al 2017) or N 2 H + (Qi et al 2019). There is no clear trend between CO snowline location and bolometric luminosity for either Class, but the Class I disks have CO snow lines at larger radii compared to Class II disks with similar bolometric luminosities.…”

“…Molecular line observations require more telescope time than continuum observations; hence, studies of the chemical structure generally target individual disks or small samples of bright disks (e.g., Dutrey et al 1997;Thi et al 2004;Öberg et al 2010;Cleeves et al 2015;Huang et al 2017). The picture that is emerging for the global composition of Class II disks around solar analogs is that they have a large cold outer region (T20 K) where CO is frozen out in the disk midplanes (e.g., Aikawa et al 2002;Mathews et al 2013;Qi et al 2013bQi et al , 2015Qi et al , 2019Dutrey et al 2017).…”

Unusually high CO abundance of the first active interstellar comet

“…Currently, the outer edge of the Solar System (as defined by a drop-off in the Kuiper Belt density), is close to 50 au 36 . In other planetary systems, the CO iceline has been identified at similar or greater distances from the star 34,37,38 , with a radius depending on the luminosity of the young star. Observations of disks during the initial stages of planetary assembly find that their outer radii extend 5-200 au 39 .…”

Quantum-mechanical Rotational and Vibrational Signatures of Astrophyscially relevant Gas-Phase Stereo-isomeric Species of Leucine

“…For example, in the typical case of N 2 , its protonated form N 2 H + was successfully rationalized in the ISM while utilizing the quantum-mechanically computed data, thereby, acting as a tracer for locating the N 2 . 67 In a similar manner, the charged isomeric species (EQ1 R1 , EQ1 R3a , EQ2 R3 ) being investigated in the present work, which in fact have a high dipole moment, can act as tracer for the detection of corresponding conformers of Leucine. These conformers have low dipole moment, therefore, exhibit only weak rotational and vibrational transitions.…”