A Census of the Circumstellar Disk Populations in the Sco-Cen Complex*

I have used mid-infrared (IR) photometry from the Wide-field Infrared Survey Explorer (WISE) to perform a census of circumstellar disks among ∼10,000 candidate members of the Sco-Cen complex that were recently identified with data from the Gaia mission. IR excesses are detected for more than 1200 of the WISE counterparts that are within the commonly adopted boundary for Sco-Cen, ∼400 of which are newly reported in this work. The richest population in Sco-Cen, UCL/LCC, contains the largest available sample of disks (>500) for any population near its age (∼20 Myr). UCL/LCC also provides the tightest statistical constraints to date on the disk fractions of low-mass stars for any single age beyond that of Upper Sco (∼11 Myr). For Upper Sco and UCL/LCC, I have measured the disk fractions as a function of spectral type. The disk fraction in Upper Sco is higher at later spectral types, which is consistent with the results for previous samples of candidate members. In UCL/LCC, that trend has become more pronounced; the disk fractions in UCL/LCC are lower than those in Upper Sco by factors of ∼10, 5.7, and 2.5 at B7–K5.5, K6–M3.5, and M3.75–M6, respectively. The data in UCL/LCC also demonstrate that the disk fraction for low-mass stars remains nonnegligible at an age of 20 Myr (0.09 ± 0.01). Finally, I find no significant differences in the ages of disk-bearing and diskless low-mass stars in Upper Sco and UCL/LCC based on their positions in color–magnitude diagrams.

Influence of the scattering particle shape on the SiO2 silicate feature

Silicate dust particles are part of many astronomical objects such as comets and circumstellar disks. In a spectrum, silicates exhibit a number of characteristic silicate emission features. To study these features, Mie’s theory is usually used. This theory assumes that the scattering object is an ideal sphere. In this work, we investigated the contribution of non-spherical quartz particles (SiO2) to these features. We studied the influence of the deviation from sphericity on the 10-micron silicate feature of quartz. It is shown that the deviation from sphericity has a significant effect on both the scattered light intensity and the scattering factor Qsca, and this effect increases with increasing scattering particle size. The main peculiarities of the 10-micron silicate feature have been studied for both prolate and oblate spheroids.

An Unbiased Mineral Compositional Analysis Technique for Circumstellar Disks

A circumstellar disk that surrounds a star is composed of gas, dust, and rocky objects that are in orbit around it. Around infant stars, this disk can act as a source of material that can be used to form planetesimals, which can then accrete more material and form into planets. Studying the mineral composition of these disks can provide insight into the processes that created our solar system. The purpose of this paper is to analyze the mineral composition of these disks by using a newly created python package, Min-CaLM. This package determines the relative mineral abundance within a disk by using a linear regression technique called non-negative least square minimization. The circumstellar disks that are capable of undergoing compositional analysis must have a spectrum with both a detectable mid-infrared excess and prominent silicate features. From our sample, there are only eight debris disks that qualify to be candidates for the Min-CaLM program. The mineral compositions calculated by Min-CaLM are then compared to the Tholen asteroid classification scheme. HD 23514, HD 105234, HD 15407A, BD+20 307, HD 69830, and HD 172555 are found to have a compositions similar to that expected for C-type asteroids, TYC 9410-532-1 resembles the composition of S-type asteroids, and HD 100546 resembles D-type asteroids. Min-CaLM also calculates the mineral compositions of the comets Tempel 1 and Hale-Bopp, and they are used as a comparison between the material in our early solar system and the debris disk compositions. KEYWORDS: Debris disk; Mineral; Composition; Analysis; Asteroid; Circumstellar; Spectroscopy; Python

Growth of Magnetorotational Instability in Circumstellar Disks around Class 0 Protostars

We investigate the possibility of the growth of magnetorotational instability (MRI) in disks around Class 0 protostars. We construct a disk model and calculate the chemical reactions of neutral and charged atoms, molecules and dust grains to derive the abundance of each species and the ionization degree of the disk. Then, we estimate the diffusion coefficients of non-ideal magnetohydrodynamics effects such as ohmic dissipation, ambipolar diffusion and the Hall effect. Finally, we evaluate the linear growth rate of MRI in each area of the disk. We investigate the effect of changes in the strength and direction of the magnetic field in our disk model and we adopt four different dust models to investigate the effect of dust size distribution on the diffusion coefficients. Our results indicate that an MRI active region possibly exists with a weak magnetic field in a region far from the protostar where the Hall effect plays a role in the growth of MRI. On the other hand, in all models the disk is stable against MRI in the region within <20 au from the protostar on the equatorial plane. Since the size of the disks in the early stage of star formation is limited to ≲ 10–20 au, it is difficult to develop MRI-driven turbulence in such disks.

10-micron emission feature of nonspherical olivine particles

Many astronomical objects, from comets to circumstellar disks, contain silicate particles. The spectrum of silicates has a set of emission features, the most characteristic of which is in the spectral range of about 10 microns, and is called a 10-micron emission feature. The 10-micron emission feature is often studied under the assumption that the scattering particles are spherical. In this work, we investigated the contribution of nonspherical particles (elongated and oblate spheroids) of olivine to the 10-micron emission feature. It is shown that enough large non-spherical particles (whose size more than 20 microns) cause a noticeable increasing of scattering cross section Csca in comparison with spheres. Thus, the observed 10-micron emission feature of large particles can be explained by the non-sphericity of the scattering particles.

Activity evolution for F, G, K, and M dwarfs and the importance for planetary atmospheres

<p>The magnetic activity of stars is a crucially important factor influencing planet formation processes and the subsequent evolution of planetary atmospheres. Understanding how stellar activity evolves for stars with different masses is crucially important for understanding the effects of stellar winds and radiation at X-ray and ultraviolet wavelengths on the erosion of circumstellar disks and planetary atmospheres. I will present a new and comprehensive description of the rotational evolution of stars and the resulting evolution of X-ray and ultraviolet emission for F, G, K, and M dwarfs. I will demonstrate the importance of the star's initial rotation rate on the subsequent activity evolution and clarify common misunderstandings regarding the dependence on stellar mass, including the common belief that M dwarfs are more XUV active than G dwarfs. I will show why these results are important for the evolution of planetary atmospheres.</p>