17992435
Evolution of Chemistry and Molecular Line Profiles during Protostellar Collapse
Understanding the chemical evolution in star-forming cores is a necessary precondition to correctly assessing physical conditions when using molecular emission. We follow the evolution of chemistry and molecular line profiles through the entire star formation process, including a self-consistent treatment of dynamics, dust continuum radiative transfer, gas energetics, chemistry, molecular excitation, and line radiative transfer. In particular, the chemical code follows a gas parcel as it falls toward the center, passing through regimes of density, dust temperature, and gas temperature that are changing because of both the motion of the parcel and the evolving luminosity of the central source. We combine a sequence of Bonnor-Ebert spheres and the inside-out collapse model to describe dynamics from the pre-protostellar stage to later stages. The overall structures of abundance profiles show complex behavior that can be understood as interactions between freezeout and evaporation of molecules. We find that the presence or absence of gas-phase CO has a tremendous effect on the less abundant species. In addition, the ambient radiation field and the grain properties have important effects on the chemical evolution, and the variations in abundance have strong effects on the predicted emission-line profiles. Multitransition and multiposition observations are necessary to constrain the parameters and interpret observations correctly in terms of physical conditions. Good spatial and spectral resolution is also important in distinguishing evolutionary stages.
119073347
Fragmentation and Evolution of Molecular Clouds. I. Algorithm and First Results
We present a series of simulations of the fragmentation of a molecular cloud, leading to the formation of a cluster of protostellar cores. We use Gaussian initial conditions with a power spectrum P(k) ∝ k-2, assume an isothermal equation of state, and neglect turbulence and magnetic fields. The purpose of these simulations is to address a specific numerical problem called artificial fragmentation, which plagues simulations of cloud fragmentation. We argue that this is a serious problem, and that the only reasonable and practical way to address it within the smoothed particle hydrodynamics (SPH) algorithm is to use a technique called particle splitting. We performed three simulations, with Ngen = 0, 1, and 2 levels of particle splitting. All simulations start up with 643 SPH particles, but their effective resolutions correspond to 643, 1283, and 2563 particles, respectively. The third simulation properly resolves the Jeans mass throughout the entire system, at all times, thus preventing artificial fragmentation. The high resolution of our simulations results in the formation of a large number of protostellar cores, nearly 3000 for the largest simulation. The final mass distribution of cores is lognormal, and the distribution shifts down in mass as the resolution improves. The width of the distribution is about 1.5 (e.g., a factor of 30 in the mass), and the low-mass edge of the distribution corresponds to the lowest core mass that the code can resolve. This result differs from previous claims of a relationship between the mean of the distribution and the initial Jeans mass.
118722765
CO2 ICE TOWARD LOW-LUMINOSITY EMBEDDED PROTOSTARS: EVIDENCE FOR EPISODIC MASS ACCRETION VIA CHEMICAL HISTORY
We present Spitzer IRS spectroscopy of CO{sub 2} ice bending mode spectra at 15.2 {mu}m toward 19 young stellar objects (YSOs) with luminosity lower than 1 L{sub Sun} (3 with luminosity lower than 0.1 L{sub Sun }). Ice on dust grain surfaces can encode the history of heating because pure CO{sub 2} ice forms only at elevated temperature, T > 20 K, and thus around protostars of higher luminosity. Current internal luminosities of YSOs with L < 1L{sub Sun} do not provide the conditions needed to produce pure CO{sub 2} ice at radii where typical envelopes begin. The presence of detectable amounts of pure CO{sub 2} ice would signify a higher past luminosity. Many of the spectra require a contribution from a pure, crystalline CO{sub 2} component, traced by the presence of a characteristic band splitting in the 15.2 {mu}m bending mode. About half of the sources (9 out of 19) in the low-luminosity sample have evidence for pure CO{sub 2} ice, and 6 of these have significant double-peaked features, which are very strong evidence of pure CO{sub 2} ice. The presence of the pure CO{sub 2} ice component indicates that the dust temperature, and hence luminosity of the centralmore » star/accretion disk system, must have been higher in the past. An episodic accretion scenario, in which mixed CO-CO{sub 2} ice is converted to pure CO{sub 2} ice during each high-luminosity phase, explains the presence of pure CO{sub 2} ice, the total amount of CO{sub 2} ice, and the observed residual C{sup 18}O gas.« less
118401675
The Diskionary: A Glossary of Terms Commonly Used for Disks and Related Objects, First Edition
Based on a panel discussion at the meeting "New Light on Young Stars: Spitzer's View of Circumstellar Disks", we provide some definitions of common usage of terms describing disks and related objects.
5949463
THE SPATIALLY RESOLVED STAR FORMATION LAW FROM INTEGRAL FIELD SPECTROSCOPY: VIRUS-P OBSERVATIONS OF NGC 5194
We investigate the relation between the star formation rate (SFR) surface density (SIGMA{sub SFR}) and the mass surface density of gas (SIGMA{sub gas}) in NGC 5194 (a.k.a. M51a, Whirlpool Galaxy). Visible Integral field Replicable Unit Spectrograph Prototype (VIRUS-P) integral field spectroscopy of the central 4.1 x 4.1 kpc{sup 2} of the galaxy is used to measure Halpha, Hbeta, [O III]lambda5007, [N II]lambdalambda6548,6584, and [S II]lambdalambda6717,6731 emission line fluxes for 735 regions approx170 pc in diameter. We use the Balmer decrement to calculate nebular dust extinctions, and correct the observed fluxes in order to accurately measure SIGMA{sub SFR} in each region. Archival H I 21 cm and CO maps with spatial resolution similar to that of VIRUS-P are used to measure the atomic and molecular gas surface density for each region. We present a new method for fitting the star formation law (SFL), which includes the intrinsic scatter in the relation as a free parameter, allows the inclusion of non-detections in both SIGMA{sub gas} and SIGMA{sub SFR}, and is free of the systematics involved in performing linear regressions over incomplete data in logarithmic space. After rejecting regions whose nebular spectrum is affected by the central active galactic nucleus in NGC 5194,more » we use the [S II]/Halpha ratio to separate spectroscopically the contribution from the diffuse ionized gas (DIG) in the galaxy, which has a different temperature and ionization state from those of H II regions in the disk. The DIG only accounts for 11% of the total Halpha luminosity integrated over the whole central region, but on local scales it can account for up to a 100% of the Halpha emission, especially in the inter-arm regions. After removing the DIG contribution from the Halpha fluxes, we measure a slope N = 0.82 +- 0.05, and an intrinsic scatter epsilon = 0.43 +- 0.02 dex for the molecular gas SFL. We also measure a typical depletion timescale tau=SIGMA{sub H{sub i+H{sub 2}}}/SIGMA{sub SFR}approx2 Gyr, in good agreement with recent measurements by Bigiel et al. The atomic gas density shows no correlation with the SFR, and the total gas SFL in the sampled density range closely follows the molecular gas SFL. Integral field spectroscopy allows a much cleaner measurement of Halpha emission line fluxes than narrow-band imaging, since it is free of the systematics introduced by continuum subtraction, underlying photospheric absorption, and contamination by the [N II] doublet. We assess the validity of different corrections usually applied in narrow-band measurements to overcome these issues and find that while systematics are introduced by these corrections, they are only dominant in the low surface brightness regime. The disagreement with the previous measurement of a super-linear molecular SFL by Kennicutt et al. is most likely due to differences in the fitting method. Our results support the recent evidence for a low, and close to constant, star formation efficiency (SFE =tau{sup -1}) in the molecular component of the interstellar medium. The data show an excellent agreement with the recently proposed model of the SFL by Krumholz et al. The large intrinsic scatter observed may imply the existence of other parameters, beyond the availability of gas, which are important in setting the SFR.« less
119498034
Submillimeter Dust Continuum Studies of Low and High Mass Star Formation
Studying the physical environments of low mass and high mass cores using dust continuum emission provides important observational constraints on theoretical models of star formation. The motivation and procedure for modeling dust continuum emission is reviewed and the results of recent surveys towards low mass and high mass star forming regions are compared.
117922628
Minimal walking technicolour, the top mass and precision electroweak measurements
We consider a minimal technicolour theory with two techniflavours in the adjoint representation of an SU(2) technicolour gauge group which has been argued to feature walking dynamics. We show how to naturally embed this theory in an extended technicolour model capable of generating the top quark mass. We investigate the precision constraints and conclude that such models, in the light of the most recent precision data fit, are not ruled out.
11037353
Lattice tests of supersymmetric Yang-Mills theory?
Supersymmetric Yang Mills theory is directly accessible to lattice simulations using current methodology, and can provide a non-trivial check of recent exact results in SQCD. In order to tune the lattice simulation to the supersymmetric point it is neccessary to understand the behavior of the theory with a small supersymmetry breaking gaugino mass. We introduce a soft breaking gaugino mass in a controlled fashion using a spurion analysis. We compute the gluino condensate, vacuum energy and bound-state masses as a function of the gaugino mass, providing more readily accessible predictions which still test the supersymmetric results. Finally we discuss diagnostics for obtaining the bare lattice parameters that correspond to the supersymmetric continuum limit.
118470786
Chemical potential in the gravity dual of a 2 + 1 dimensional system
We study probe D5 branes in D3 brane AdS5 and AdS5-Schwarzschild backgrounds as a prototype dual description of strongly coupled 2+1 dimensional quasiparticles. We introduce a chemical potential through the U(1)R symmetry group, U(1) baryon number, and a U(1) of isospin in the multiflavor case. We find the appropriate D5 embeddings in each case—the embeddings do not exhibit the spontaneous symmetry breaking that would be needed for a superconductor. The isospin chemical potential does induce the condensation of charged meson states.
118397983
E, B, μ, T phase structure of the D3/D7 holographic dual
The large N$ \mathcal{N} = 4 $ gauge theory with quenched $ \mathcal{N} = 2 $ quark matter displays chiral symmetry breaking in the presence of a magnetic field. We previously studied the temperature and chemical potential phase structure of this theory in the grand canonical ensemble-here we, in addition, include the effect of an electric field which acts to counter chiral symmetry breaking by dissociating mesons. We compute using the gravity dual based on the D3/probe-D7 brane system. The theory displays two transition at one of which chiral symmetry is restored. At the other transition density switches on, the mesons of the theory become unstable and a current forms, making it a conductor-insulator transition. Through the temperature, electric field, chemical potential volume (at fixed magnetic field parallel to the electric field) these transitions can coincide or separate at critical points, and be first order or second order. We map out this full phase structure which provides varied computable examples relevant to strongly coupled gauge theories and potentially condensed matter systems.
15006058
An Effective field theory approach to color superconductivity at high quark density
Abstract We investigate the Fermi Liquid theory of high density QCD. Using the renormalization group (RG), we determine the behavior of effective fermion interactions near the Fermi surface. At sufficiently high densities the matching between the Fermi Liquid theory and QCD can be accomplished in perturbation theory and a reliable calculation of Cooper pair formation performed, modulo the existence of screening masses for the gluon. The presence of a chemical potential leads to different RG flows for sub-components of the effective interactions which would ordinarily be linked by Lorentz invariance. We also study instanton-induced operators and find that near the Fermi surface they are likely to be subdominant relative to operators induced by perturbative gluon exchange. We discuss the implications of our results for the phase structure of QCD at finite baryon density and temperature.