Structure and Composition of the Disk Around V4046 Sgr

The structure and composition of the protoplanetary disk around V4046 Sgr has been stud-ied extensively via (radio) mm-wave molecular line spectroscopy and interferometric imag-ing (Kastner et al., 2008; Rodriguez et al., 2010; ¨Oberg et al., 2011). These observations

are used to determine the chemistry of the gas and dust components of the disk, detect the existence and extent of the disk, and ultimately ascertain if the conditions within the disk are conducive for giant planet formation.

Kastner et al. (2008) detected rotational transitions of ¹²CO, ¹³CO, HCN, CN, and HCO⁺ in the mm spectrum of V4046 Sgr with the 30 m telescope of the Institut de Radio Astronomie Millimetrique (IRAM). At the time, these detections established V4046 Sgr as only the fourth known nearby (D < 100 pc) pre-main sequence star with a molecular disk (the others are TW Hya, 49 Cet, and HD 141569; Zuckerman et al., 1995), and the only such nearby disk orbiting a binary system. The ¹²CO and¹³CO (J = 2→1 rotational transition) line profiles are double peaked, which allowed for a measurement of the radial velocity of the material and thus, assuming the disk is Keplerian, an estimate for the size of the disk. Line profile fitting of the ¹²CO line resulted in a projected rotational velocity of

∼1.5 km s⁻¹for the disk, which corresponds to an outer disk radius of ∼ 250 AU, assuming the total mass of the binary is 1.8 M(Kastner et al., 2008). Using the disk dust tempera-ture estimated from the submm SED, Kastner et al. (2008) estimated the lower limit of the dust mass in the disk to be 6×10⁻⁵M (∼20 M⊕).

Rodriguez et al. (2010) imaged V4046 Sgr with the SMA and confirmed that the CO detected by Kastner et al. (2008) arises from a rotating circumbinary disk. The SMA maps show a disk which extends 5⁰⁰in radius or ∼370 AU at a distance of 73 pc, thereby refining the results of Kastner et al. (2008). The changes in position and morphology of the CO with radial velocity are as expected for a Keplerian disk viewed at an intermediate inclination (i∼ 35^◦Beckwith & Sargent, 1993; Rodriguez et al., 2010).

Rodriguez et al. (2010) also modeled the CO line maps as a Keplerian disk in hydro-static equilibrium and the best fit model suggests a combined mass for the binary of 1.8 M

and a disk inclination of 33^◦. Rosenfeld et al. (2012a) repeat the modeling of Rodriguez et al. (2010) while including more recent sub-compact and very extended SMA data of

12CO in their analysis, and further refined the Rodriguez et al. (2010) results, finding a combined mass of 1.75 ^+0.09_−0.06 M and a disk inclination of 33^◦.5^+0.7_−1.4. Previous work by Quast et al. (2000) showed that the inclination of the orbit of the central binary was 35^◦.

Thus, the binary and disk are inclined at roughly the same angle, with respect to our line of sight, suggesting that the binary and disk formed in this configuration (Monin et al., 2007).

Since CO is often used as a tracer for molecular hydrogen gas, the CO line intensities can be used to estimate the gas mass of the disk. Rodriguez et al. (2010) used ¹²CO and

13CO line intensities to determine a gas mass for the V4046 Sgr disk (assuming¹³CO is optically thin) of ∼100 M⊕. Assuming a dust temperature of 37 K and a dust opacity of 1.15 cm²g⁻¹, the dust mass is estimated to be 40 M⊕, double that estimated by Kastner et al.

(2008); the difference likely due to different assumed opacities. Regardless, the apparent gas to dust mass ratio is much smaller than the typically assumed ratio of 100 for molecular clouds and young circumstellar disks. This indicates that either the gas is heavily depleted in this system that the CO is optically thick and not representative of the total gas mass of the disk. Since the disk extends to large radii and is thus very cold, much of the CO in the disk may be in the form of ice mantles on dust grains rather than gas (Rodriguez et al., 2010).

SMA¹²CO and¹³CO line and 1.3 mm continuum data (Rodriguez et al., 2010; Rosen-feld et al., 2012a) were also used by RosenRosen-feld et al. (2013) to study the distribution of large (∼mm-sized) and small (∼µm-sized) dust grains in the V4046 Sgr disk. Rosenfeld et al.

(2013) developed a radiative transfer model that reproduces the CO and 1.3 mm continuum emission, as well as optical to far-infrared photometry. This model suggests that the disk around V4046 Sgr has three-components: 1) a “gap” at R.29 AU that is depleted of large dust grains and contains only small amounts of µm-sized particles close to the central stars, 2) a narrow concentration or ring of mm-sized dust centered at a radius of ∼37 AU, and 3) an extended halo of CO gas and small dust particles. The model also suggests a total gas+dust mass of ∼0.094 M and a gas to dust ratio of ∼20. Although the latter is larger than the Rodriguez et al. (2010) estimate, it is nevertheless still somewhat smaller than the gas to dust ratio of 100 calculated for the interstellar medium (Bohlin et al., 1978), and typically assumed for circumstellar disks. The inner (R.29 AU) region of the disk is best probed at near- to far-infrared wavelengths, and is the focus of Chapters 2 and 4 of this dissertation.

Oberg et al. (2011) also used SMA observations of V4046 Sgr and 11 other young¨ stars with disks to study the structure and chemical composition of circumstellar disks.

They detected transitions of CN, HCN, H₂CO, N₂H⁺, DCO⁺, HCO⁺, and CO towards V4046 Sgr and most of the other sources in their sample. There is little correlation in the strength of the ¹²CO 2-1 line and the dust continuum emission, which is due to the fact that the dust is optically thin and the CO emission is optically thick. It does not suggest a lack of correlation between gas and dust mass. These maps also show that the HCO⁺ emission tends to be concentrated at higher velocities, whereas the CN emission extends to larger radii. Models from Aikawa et al. (2002) attribute this steep decline of HCO⁺to CO freeze out at low temperatures, which would also help explain the apparent low gas to dust ratio inferred by Rodriguez et al. (2010). Position velocity maps of the molecules detected by ¨Oberg et al. (2011) show that the H₂CO and N₂H⁺ emission peak at a slightly larger position offset than other molecules and thus appear more abundant in the outer portions of the disk ( ¨Oberg et al., 2011). These molecules require CO freeze-out to become abundant, furthering the argument that CO must be locked up on dust grains at large radii. All of these data suggests that CO is not an optimal tracer for the mass of gas in the protoplanetary disk around V4046 Sgr, as much of it has frozen onto dust grains at radii >10 AU.

Table 1.1 summarizes the key properties of the V4046 Sgr AB star-disk system as de-termined by the studies mentioned above and adopted in this thesis. Infrared observations of V4046 Sgr are crucial for understanding the structure and gas content of the inner disk, a region that has not been well studied in the literature so far. This dissertation explores V4046 Sgr at near- to far-infrared wavelengths, which will fill in the gap (pun intended) and reveal the molecular content and dynamics of the inner regions of this protoplanetary disk.