Radiation pressure feedback in the formation of massive stars

We investigate the radiation pressure feedback in the formation of massive stars in 1, 2, and 3D radiation hydrodynamics simulations of the collapse of massive pre-stellar cores. In contrast to previous research, we consider frequency dependent stellar radiation feedback, resolve the dust sublimation front in the vicinity of the forming star down to 1.27 AU, compute the evolution for several 10^5 yrs covering the whole accretion phase of the forming star, and perform a comprehensive survey of the parameter space. The most fundamental result is that the formation of a massive accretion disk in slowly rotating cores preserves a high anisotropy in the radiation field. The thermal radiation escapes through the optically thin atmosphere, effectively diminishing the radiation pressure feedback onto the accretion flow. Gravitational torques in the self-gravitating disk drive a sufficiently high accretion rate to overcome the residual radiation pressure. Simultaneously, the radiation pressure launches an outflow in the bipolar direction, which grows in angle with time and releases a substantial fraction of the initial core mass from the star-disk system. Summarized, for an initial core mass of 60, 120, 240, and 480 Msol these mechanisms allow the star to grow up to 28.2, 56.5, 92.6, and at least 137.2 Msol respectively.Comment: 5 pages, 3 figures, Proceedings of the 39th Liege International Astrophysical Colloquium: The multi-wavelength view of Hot, Massive Star

Simulating the Formation of Massive Protostars: I. Radiative Feedback and Accretion Disks

We present radiation hydrodynamic simulations of collapsing protostellar cores with initial masses of 30, 100, and 200 M$_{\odot}$. We follow their gravitational collapse and the formation of a massive protostar and protostellar accretion disk. We employ a new hybrid radiative feedback method blending raytracing techniques with flux-limited diffusion for a more accurate treatment of the temperature and radiative force. In each case, the disk that forms becomes Toomre-unstable and develops spiral arms. This occurs between 0.35 and 0.55 freefall times and is accompanied by an increase in the accretion rate by a factor of 2-10. Although the disk becomes unstable, no other stars are formed. In the case of our 100 and 200 M$_{\odot}$ simulation, the star becomes highly super-Eddington and begins to drive bipolar outflow cavities that expand outwards. These radiatively-driven bubbles appear stable, and appear to be channeling gas back onto the protostellar accretion disk. Accretion proceeds strongly through the disk. After 81.4 kyr of evolution, our 30 M$_{\odot}$ simulation shows a star with a mass of 5.48 M$_{\odot}$ and a disk of mass 3.3 M$_{\odot}$, while our 100 M$_{\odot}$ simulation forms a 28.8 M$_{\odot}$ mass star with a 15.8 M$_{\odot}$ disk over the course of 41.6 kyr, and our 200 M$_{\odot}$ simulation forms a 43.7 M$_{\odot}$ star with an 18 M$_{\odot}$ disk in 21.9 kyr. In the absence of magnetic fields or other forms of feedback, the masses of the stars in our simulation do not appear limited by their own luminosities.Comment: 24 pages, 14 figures. Accepted to The Astrophysical Journa

A general hybrid radiation transport scheme for star formation simulations on an adaptive grid

Radiation feedback plays a crucial role in the process of star formation. In order to simulate the thermodynamic evolution of disks, filaments, and the molecular gas surrounding clusters of young stars, we require an efficient and accurate method for solving the radiation transfer problem. We describe the implementation of a hybrid radiation transport scheme in the adaptive grid-based FLASH general magnetohydrodynamics code. The hybrid scheme splits the radiative transport problem into a raytracing step and a diffusion step. The raytracer captures the first absorption event, as stars irradiate their environments, while the evolution of the diffuse component of the radiation field is handled by a flux-limited diffusion (FLD) solver. We demonstrate the accuracy of our method through a variety of benchmark tests including the irradiation of a static disk, subcritical and supercritical radiative shocks, and thermal energy equilibration. We also demonstrate the capability of our method for casting shadows and calculating gas and dust temperatures in the presence of multiple stellar sources. Our method enables radiation-hydrodynamic studies of young stellar objects, protostellar disks, and clustered star formation in magnetized, filamentary environments.Comment: 16 pages, 15 figures, accepted to Ap

CCS and NH_3 Emission Associated with Low-Mass Young Stellar Objects

In this work we present a sensitive and systematic single-dish survey of CCS emission (complemented with ammonia observations) at 1 cm, toward a sample of low- and intermediate-mass young star-forming regions known to harbor water maser emission, made with NASA's 70 m antenna at Robledo de Chavela, Spain. Out of the 40 star-forming regions surveyed in the CCS (2_(1)-1_(0)) line, only six low-mass sources show CCS emission: one transitional object between the prestellar and protostellar Class 0 phase (GF9-2), three Class 0 protostars (L1448-IRS3, L1448C, and B1-IRS), a Class I source (L1251A), and a young T Tauri star (NGC 2071 North). Since CCS is considered an "early-time" (≲10^5 yr) molecule, we explain these results by either proposing a revision of the classification of the age of NGC 2071 North and L1251A, or suggesting the possibility that the particular physical conditions and processes of each source affect the destruction/production of the CCS. No statistically significant relationship was found between the presence of CCS and parameters of the molecular outflows and their driving sources. Nevertheless, we found a significant relationship between the detectability of CCS and the ammonia peak intensity (higher in regions with CCS), but not with its integrated intensity. This tendency may suggest that the narrower ammonia line widths in the less turbulent medium associated with younger cores may compensate for the differences in ammonia peak intensity, rendering differences in integrated intensity negligible. From the CCS detection rate we derive a lifetime of this molecule of ≃(0.7-3) × 10^4 yr in low-mass star-forming regions

Three-dimensional simulation of massive star formation in the disk accretion scenario

The most massive stars can form via standard disk accretion - despite of the radiation pressure generated - due to the fact that the massive accretion disk yields a strong anisotropy in the radiation field, releasing most of the radiation pressure perpendicular to the disk accretion flow. Here, we analyze the self-gravity of the forming circumstellar disk as the potential major driver of the angular momentum transport in such massive disks responsible for the high accretion rates needed for the formation of massive stars. For this purpose, we perform self-gravity radiation hydrodynamics simulations of the collapse of massive pre-stellar cores. The formation and evolution of the resulting circumstellar disk is investigated in 1.) axially symmetric simulations using an alpha-shear-viscosity prescription and 2.) a three-dimensional simulation, in which the angular momentum transport is provided self-consistently by developing gravitational torques in the self-gravitating accretion disk. The simulation series of different strength of the alpha-viscosity shows that the accretion history of the forming star is mostly independent of the alpha-viscosity-parameter. The accretion history of the three-dimensional run driven by self-gravity is more time-dependent than the viscous disk evolution in axial symmetry. The mean accretion rate, i.e. the stellar mass growth, is nearly identical to the alpha-viscosity models. We conclude that the development of gravitational torques in self-gravitating disks around forming massive stars provides a self-consistent mechanism to efficiently transport the angular momentum to outer disk radii. Also the formation of the most massive stars can therefore be understood in the standard accretion disk scenario.Comment: accepted for publication at Ap

High-Resolution Observations in B1-IRS: ammonia, CCS and water masers

We present a study of the structure and dynamics of the star forming region B1-IRS (IRAS 03301+3057) using the properties of different molecules at high angular resolution (~4''). We have used VLA observations of NH3, CCS, and H2O masers at 1 cm. CCS emission shows three clumps around the central source, with a velocity gradient from red to blueshifted velocities towards the protostar, probably due to the interaction with outflowing material. Water maser emission is elongated in the same direction as a reflection nebula detected at 2micron by 2MASS, with the maser spots located in a structure of some hundreds of AU from the central source, possibly tracing a jet. We propose a new outflow model to explain all our observations, consisting of a molecular outflow near the plane of the sky. Ammonia emission is extended and anticorrelated with CCS. We have detected for the first time this anticorrelation at small scales (1400 AU) in a star forming region.Comment: 6 pages, 3 figures. To appear in the Proceedings of the 2004 European Workshop: "Dense Molecular Gas around Protostars and in Galactic Nuclei", Eds. Y.Hagiwara, W.A.Baan, H.J.van Langevelde, 2004, a special issue of ApSS, Kluwe

Localized Control of Curie Temperature in Perovskite Oxide Film by Capping-layer- induced Octahedral Distortion

With reduced dimensionality, it is often easier to modify the properties of ultra-thin films than their bulk counterparts. Strain engineering, usually achieved by choosing appropriate substrates, has been proven effective in controlling the properties of perovskite oxide films. An emerging alternative route for developing new multifunctional perovskite is by modification of the oxygen octahedral structure. Here we report the control of structural oxygen octahedral rotation in ultra-thin perovskite SrRuO3 films by the deposition of a SrTiO3 capping layer, which can be lithographically patterned to achieve local control. Using a scanning Sagnac magnetic microscope, we show increase in the Curie temperature of SrRuO3 due to the suppression octahedral rotations revealed by the synchrotron x-ray diffraction. This capping-layer-based technique may open new possibilities for developing functional oxide materials.Comment: Main-text 5 pages, SI 6 pages. To appear in Physical Review Letter

Statistical Studies of Giant Pulse Emission from the Crab Pulsar

We have observed the Crab pulsar with the Deep Space Network (DSN) Goldstone 70 m antenna at 1664 MHz during three observing epochs for a total of 4 hours. Our data analysis has detected more than 2500 giant pulses, with flux densities ranging from 0.1 kJy to 150 kJy and pulse widths from 125 ns (limited by our bandwidth) to as long as 100 microseconds, with median power amplitudes and widths of 1 kJy and 2 microseconds respectively. The most energetic pulses in our sample have energy fluxes of approximately 100 kJy-microsecond. We have used this large sample to investigate a number of giant-pulse emission properties in the Crab pulsar, including correlations among pulse flux density, width, energy flux, phase and time of arrival. We present a consistent accounting of the probability distributions and threshold cuts in order to reduce pulse-width biases. The excellent sensitivity obtained has allowed us to probe further into the population of giant pulses. We find that a significant portion, no less than 50%, of the overall pulsed energy flux at our observing frequency is emitted in the form of giant pulses.Comment: 19 pages, 17 figures; to be published in Astrophysical Journa

DSN co-observing operations to support space VLBI missions

Reliable radio astronomy support of space very long baseline interferometry (VLBI) missions by ground radio telescopes is mandatory in order to achieve a high scientific return from the missions. The 70 m DSN antennas along with other ground radio telescopes will perform as the ground segment of the earth-space interferometer. Improvements of radio astronomy VLBI operations at the DSN to achieve higher reliability, efficiency, flexibility, and lower operations costs is a major goal in preparing for radio astronomy support of SVLBI. To help realize this goal, a remote control and monitoring mode for radio astronomy operations at the DSN has been developed