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تسجيل مستخدم جديدEmbedded protostellar disks around (sub-)solar stars. II. Disk masses, sizes, densities, temperatures and the planet formation perspective
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We present basic properties of protostellar disks in the embedded phase of star formation (EPSF), which is difficult to probe observationally using available observational facilities. We use numerical hydrodynamics simulations of cloud core collapse and focus on disks formed around stars in the 0.03-1.0 Msun mass range. Our obtained disk masses scale near-linearly with the stellar mass. The mean and median disk masses in the Class 0 and I phases (M_{d,C0}^{mean}=0.12 Msun, M_{d,C0}^{mdn}=0.09 Msun and M_{d,CI}^{mean}=0.18 Msun, M_{d,CI}^{mdn}=0.15 Msun, respectively) are greater than those inferred from observations by (at least) a factor of 2--3. We demonstrate that this disagreement may (in part) be caused by the optically thick inner regions of protostellar disks, which do not contribute to millimeter dust flux. We find that disk masses and surface densities start to systematically exceed that of the minimum mass solar nebular for objects with stellar mass as low as M_st=0.05-0.1 Msun. Concurrently, disk radii start to grow beyond 100 AU, making gravitational fragmentation in the disk outer regions possible. Large disk masses, surface densities, and sizes suggest that giant planets may start forming as early as in the EPSF, either by means of core accretion (inner disk regions) or direct gravitational instability (outer disk regions), thus breaking a longstanding stereotype that the planet formation process begins in the Class II phase.
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Eduard I. Vorobyov The Institute for Computational Astrophysics
, n Saint Marys University
, Halifax
2010
We perform a comparative numerical hydrodynamics study of embedded protostellar disks formed as a result of the gravitational collapse of cloud cores of distinct mass (M_cl=0.2--1.7 M_sun) and ratio of rotational to gravitational energy (beta=0.0028-
-0.023). An increase in M_cl and/or beta leads to the formation of protostellar disks that are more susceptible to gravitational instability. Disk fragmentation occurs in most models but its effect is often limited to the very early stage, with the fragments being either dispersed or driven onto the forming star during tens of orbital periods. Only cloud cores with high enough M_cl or beta may eventually form wide-separation binary/multiple systems with low mass ratios and brown dwarf or sub-solar mass companions. It is feasible that such systems may eventually break up, giving birth to rogue brown dwarfs. Protostellar disks of {it equal} age formed from cloud cores of greater mass (but equal beta) are generally denser, hotter, larger, and more massive. On the other hand, protostellar disks formed from cloud cores of higher beta (but equal M_cl) are generally thinner and colder but larger and more massive. In all models, the difference between the irradiation temperature and midplane temperature triangle T is small, except for the innermost regions of young disks, dense fragments, and disks outer edge where triangle T is negative and may reach a factor of two or even more. Gravitationally unstable, embedded disks show radial pulsations, the amplitude of which increases along the line of increasing M_cl and beta but tends to diminish as the envelope clears. We find that single stars with a disk-to-star mass ratio of order unity can be formed only from high-beta cloud cores, but such massive disks are unstable and quickly fragment into binary/multiple systems.
We used chromospheric activity to determine the ages of 2,820 field stars.. We searched these stars for excess emission at 22 um with the Wide-Field Infrared Survey Explorer. Such excess emission is indicative of a dusty debris disk around a star. We
investigated how disk incidence trends with various stellar parameters, and how these parameters evolve with time. We found 22 um excesses around 98 stars (a detection rate of 3.5%). Seventy-four of these 98 excess sources are presented here for the first time. We also measured the abundance of lithium in 8 dusty stars in order to test our stellar age estimates.
Since giant planets scatter planetesimals within a few tidal radii of their orbits, the locations of existing planetesimal belts indicate regions where giant planet formation failed in bygone protostellar disks. Infrared observations of circumstellar
dust produced by colliding planetesimals are therefore powerful probes of the formation histories of known planets. Here we present new Spitzer IRS spectrophotometry of 111 Solar-type stars, including 105 planet hosts. Our observations reveal 11 debris disks, including two previously undetected debris disks orbiting HD 108874 and HD 130322. Combining our 32 micron spectrophotometry with previously published MIPS photometry, we find that the majority of debris disks around planet hosts have temperatures in the range 60 < T < 100 K. Assuming a dust temperature T = 70 K, which is representative of the nine debris disks detected by both IRS and MIPS, we find that debris rings surrounding Sunlike stars orbit between 15 and 240 AU, depending on the mean particle size. Our observations imply that the planets detected by radial-velocity searches formed within 240 AU of their parent stars. If any of the debris disks studied here have mostly large, blackbody emitting grains, their companion giant planets must have formed in a narrow region between the ice line and 15 AU.
The proper motions of the three stars ejected from Orions OMC1 cloud core are combined with the requirement that their center of mass is gravitationally bound to OMC1 to show that radio source I (Src I) is likely to have a mass around 15 Solar masses
consistent with recent measurements. Src I, the star with the smallest proper motion, is suspected to be either an AU-scale binary or a protostellar merger remnant produced by a dynamic interaction ~550 years ago. Near-infrared 2.2 um images spanning ~21 years confirm the ~55 km/s motion of `source x (Src x) away from the site of stellar ejection and point of origin of the explosive OMC1 protostellar outflow. The radial velocities and masses of the Becklin-Neugebauer (BN) object and Src I constrain the radial velocity of Src x to be V_{LSR} = -28 +/-10 km/s . Several high proper-motion radio sources near BN, including Zapata 11 ([ZRK2004] 11) and a diffuse source near IRc 23, may trace a slow bipolar outflow from BN. The massive disk around Src I is likely the surviving portion of a disk that existed prior to the stellar ejection. Though highly perturbed, shocked, and re-oriented by the N-body interaction, enough time has elapsed to allow the disk to relax with its spin axis roughly orthogonal to the proper motion.
The evolution of protoplanetary disks is dominated by the conservation of angular momentum, where the accretion of material onto the central star is driven by viscous expansion of the outer disk or by disk winds extracting angular momentum without ch
anging the disk size. Studying the time evolution of disk sizes allows us therefore to distinguish between viscous stresses or disk winds as the main mechanism of disk evolution. Observationally, estimates of the disk gaseous outer radius are based on the extent of the CO rotational emission, which, during the evolution, is also affected by the changing physical and chemical conditions in the disk. We use physical-chemical DALI models to study how the extent of the CO emission changes with time in a viscously expanding disk and investigate to what degree this observable gas outer radius is a suitable tracer of viscous spreading and whether current observations are consistent with viscous evolution. We find that the gas outer radius (R_co) measured from our models matches the expectations of a viscously spreading disk: R_co increases with time and for a given time R_co is larger for a disk with a higher viscosity alpha_visc. However, in the extreme case where the disk mass is low (less than 10^-4 Msun) and alpha_visc is high (larger than 10^-2), R_co will instead decrease with time as a result of CO photodissociation in the outer disk. For most disk ages R_co is up to 12x larger than the characteristic size R_c of the disk, and R_co/R_c is largest for the most massive disk. As a result of this difference, a simple conversion of R_co to alpha_visc will overestimate the true alpha_visc of the disk by up to an order of magnitude. We find that most observed gas outer radii in Lupus can be explained using a viscously evolving disk that starts out small (R_c = 10 AU) and has a low viscosity (alpha_visc = 10^-4 - 10^-3).
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