16124844
Sp i t zer Mid-Infrared Spectroscopy of Ices toward Extincted Background Stars
A powerful way to directly observe the solid-state inventory of dense molecular clouds is by infrared spectroscopy of background stars. We present Spitzer IRS 5-20 μm spectra of ices toward stars behind the Serpens and Taurus molecular clouds, probing visual extinctions of 10-34 mag. These data provide the first complete inventory of solid-state material in dense clouds before star formation begins. The spectra show prominent 6.0 and 6.85 μm bands. In contrast to some young stellar objects (YSOs), most (~75%) of the 6.0 μm band is explained by the bending mode of pure H2O ice. In realistic mixtures this number increases to 85%, because the peak strength of the H2O bending mode is very sensitive to the molecular environment. The strength of the 6.85 μm band is comparable to what is observed toward YSOs. Thus, the production of the carrier of this band does not depend on the energetic input of a nearby source. The spectra show large abundances of CO and CO2 (20%-40% with respect to H2O ice). Compared with YSOs, the band profile of the 15 μm CO2 bending mode lacks the signatures of crystallization, confirming the cold, pristine nature of these lines of sight. After the dominant species are removed, there are residuals that suggest the presence of minor species such as HCOOH and possibly NH3. Clearly, models of star formation should begin with dust models already coated with a fairly complex mixture of ices.
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
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.
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.
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
15147744
Majorana neutrino masses can save one-family technicolour models
We make non-perturbative estimates of the electroweak radiative correction parameter S in dynamical symmetry breaking models with Majorana neutrino masses. The Majorana masses are treated as perturbations to a non-local chiral model of the strong interactions. We argue that parameter ranges exist that would allow realistic values of S and T in one-family technicolour models.
15575800
Flavor universal dynamical electroweak symmetry breaking
The top condensate seesaw mechanism of Dobrescu and Hill allows electroweak symmetry to be broken while deferring the problem of flavor to an electroweak singlet, massive sector. We provide an extended version of the singlet sector that naturally accommodates realistic masses for all the standard model fermions, which play an equal role in breaking electroweak symmetry. The models result in a relatively light composite Higgs sector with masses typically in the range of (400{endash}700) GeV. In more complete models the dynamics will presumably be driven by a broken gauged family or flavor symmetry group. As an example of the higher scale dynamics a fully dynamical model of the quark sector with a GIM mechanism is presented, based on an earlier top condensation model of King using broken family gauge symmetry interactions (that model was itself based on a technicolor model of Georgi). The crucial extra ingredient is a reinterpretation of the condensates that form when several gauge groups become strong close to the same scale. A related technicolor model of Randall which naturally includes the leptons too may also be adapted to this scenario. We discuss the low energy constraints on the massive gauge bosons and scalars of these modelsmore » as well as their phenomenology at the TeV scale. thinsp thinsp {copyright} {ital 1999} {ital The American Physical Society}« less
119208225
The baryonic phase in holographic descriptions of the QCD phase diagram
A bstractWe study holographic models of the QCD temperature-chemical potential phase diagram based on the D3/D7 system with chiral symmetry breaking. The baryonic phase may be included through linked D5-D7 systems. In a previous analysis of a model with a running gauge coupling a baryonic phase was shown to exist to arbitrarily large chemical potential. Here we explore this phase in a more generic phenomenological setting with a step function dilaton profile. The change in dilaton generates a linear confining qq potential and opposes the screening effect of temperature. We show that the persistence of the baryonic phase depends on the step size and that QCD-like phase diagrams can be described. The baryonic phase’s existence is qualitatively linked to the existence of confinement in Wilson loop computations in the background.
17498179
Diagrammatic analysis of duality in supersymmetric gauge theories
We introduce a diagrammatic notation for supersymmetric gauge theories. The notation is a tool for exploring duality and helps to present the field content of more complicated models in a simple visual way. We introduce the notation with a few examples from the literature. The power of the formalism allows us to study models with gauge group [SU(N)]{sup k} and their duals. Among these are models which, contrary to a naive analysis, possess no conformal phase. {copyright} {ital 1997} {ital The American Physical Society}
8365482
Non-equilibrium physics at a holographic chiral phase transition
The D3/D7 system holographically describes an $ \mathcal{N} = 2 $ gauge theory which spontaneously breaks a chiral symmetry by the formation of a quark condensate in the presence of a magnetic field. At finite temperature it displays a first order phase transition. We study out of equilibrium dynamics associated with this transition by placing probe D7 branes in a geometry describing a boost-invariant expanding or contracting plasma. We use an adiabatic approximation to track the evolution of the quark condensate in a heated system and reproduce the phase structure expected from equilibrium dynamics. We then study solutions of the full partial differential equation that describes the evolution of out of equilibrium configurations to provide a complete description of the phase transition including describing aspects of bubble formation.