Numerical Simulations of Radiatively-Driven Dusty Winds

[abridged] Radiation pressure on dust grains may be an important mechanism in driving winds in a wide variety of astrophysical systems. However, the efficiency of the coupling between the radiation field and the dusty gas is poorly understood in environments characterized by high optical depths. We present a series of idealized numerical experiments, performed with the radiation-hydrodynamic code ORION, in which we study the dynamics of such winds and quantify their properties. We find that, after wind acceleration begins, radiation Rayleigh-Taylor instability forces the gas into a configuration that reduces the rate of momentum transfer from the radiation field to the gas by a factor ~ 10 - 100 compared to an estimate based on the optical depth at the base of the atmosphere; instead, the rate of momentum transfer from a driving radiation field of luminosity L to the gas is roughly L/c multiplied by one plus half the optical depth evaluated using the photospheric temperature, which is far smaller than the optical depth one would obtain using the interior temperature. When we apply our results to conditions appropriate to ULIRGs and star clusters, we find that the asymptotic wind momentum flux from such objects should not significantly exceed that carried by the direct radiation field, L/c. This result constrains the expected mass loss rates from systems that exceed the Eddington limit to be of order the so-called "single-scattering" limit, and not significantly higher. We present an approximate fitting formula for the rate of momentum transfer from radiation to dusty gas through which it passes, which is suitable for implementation in sub-grid models of galaxy formation. Finally, we provide a first map of the column density distribution of gas in a radiatively-driven wind as a function of velocity, and velocity dispersion.Comment: 19 pages, 17 figures, MNRAS in press; some additional discussion compared to previous version, no changes in conclusion

Achieving the Potential of Health Care Performance Measures: Timely Analysis of Immediate Health Policy issues

The United States is on the cusp of a new era, with greater demand for performance information, greater data availability, and a greater willingness to integrate performance information into public policy. This era has immense promise to deliver a learning health care system that encourages collaborative improvements in systems-based care, improves accountability, helps consumers make important choices, and improves quality at an acceptable cost. However, to curtail the possibility of unintended adverse consequences, it is important that we invest in developing sound measures, understand quality measures' strengths and limitations, study the science of quality measurement, and reduce inaccurate inferences about provider performance

Direct Numerical Simulation of Radiation Pressure-Driven Turbulence and Winds in Star Clusters and Galactic Disks

[abridged] The pressure exerted by the radiation of young stars may be an important feedback mechanism in forming star clusters and the disks of starburst galaxies. However, there is great uncertainty in how efficiently radiation couples to matter in these high optical depth environments. In particular, it is unclear what levels of turbulence the radiation can produce, and whether the infrared radiation trapped by the dust opacity can give rise to heavily mass-loaded winds. In this paper we report a series of two-dimensional flux-limited diffusion radiation-hydrodynamics calculations performed with the code ORION in which we drive strong radiation fluxes through columns of dusty matter confined by gravity. We consider both systems where the radiation flux is sub-Eddington throughout the gas column, and where it is super-Eddington at the midplane but sub-Eddington in the atmosphere. In the latter, we find that the radiation-matter interaction gives rise to radiation-driven Rayleigh-Taylor instability, which drives supersonic turbulence at a level sufficient to fully explain the turbulence seen in Galactic protocluster gas clouds, and to make a non-trivial contribution to the turbulence observed in starburst galaxy disks. However, the instability also produces a channel structure in which the radiation-matter interaction is reduced because the radiation field is not fully trapped. For astrophysical parameters relevant to forming star clusters and starburst galaxies, we find that this effect reduces the net momentum deposition rate in the dusty gas by a factor of ~2-6 compared to simple analytic estimates, and that in steady state the Eddington ratio reaches unity and there are no strong winds. We provide an approximation formula, appropriate for implementation in analytic models and non-radiative simulations, for the force exerted by the infrared radiation field in this regime.Comment: 20 pages, 11 figures, emulateapj format, accepted to ApJ. This version has extra discussion, but the results are unchanged. For movies of simulation results, see http://www.ucolick.org/~krumholz/downloads.htm

A Preliminary Ecological Study of Areas to be Impounded in the Salt River Basin of Kentucky

This report covers work that is an extension of Project No. A-019-KY. A series of 25 sampling stations was established in the mainstream and tributaries of the Salt River that extend from the source of the stream in Boyle County to a few miles below the site of Taylorsville Darn in Spencer County. Sampling for water chemistry and biota was carried out semimonthly. Data on temperature, oxygen, depth, and discharge, along with analyses for cations (Ca, Mg, Fe, Mn) and anions (PO4, NO3, NO2, CO3, HCO3) have been accumulated and analyzed. Bottom fauna, fishes, and plants have been sampled at each station and relative abundance and species composition of the biota have been made. Physical and chemical data, along with flora and fauna taken from the stream present the characteristics of a relatively healthy ecosystem. Water temperatures reflect air temperatures closely and dissolved oxygen values are near saturation. Turbidity increased with runoff, the stream flow increasing rapidly during rainy periods and falling to a minimum during dry periods. Total alkalinities ranged from 135 to 210 mg/1 as CaCO3 with ranges in pH from 6.3 to 8.2. Nitrate nitrogen ranged from 2.0 to 11.3 mg/1 and orthophosphate from 0.25 to 2.78 mg/1. Iron and manganese ranged from 0.07 to 0.46 and 0.09 to 0.39 mg/1, respectively. A total of 74 species of algae referable to 35 families were collected and identified. Green algae (Chlorophyta) were represented by 38 species, reflecting the contention that the Salt River is a relatively clean, oligosaprobic stream. More than 200 species of vascular plants referable to 50 families have been collected from the riparian vegetation. Bottom fauna includes 98 species of insects representing 8 orders and 42 families. Prominent among these are the 23 species of chironomids that have been identified to date, The most common crustaceans are Orconectes rusticus and Lirceus lineatus along with several species of Gammarus. Molluscs include gastropods, fingernail clams, and unionids. More than 50 species of fishes have been collected and will provide data for a preliminary report to be published in the open literature. Plans include a further inventory of the fish population and continued study of the physiochemical and biological aspects of the stream ecosystem, The study of economic aspects of the area will continue at an accelerated pace

Gamma-Ray Bursts and the Cosmic Star Formation Rate

We have tested several models of GRB luminosity and redshift distribution functions for compatibility with the BATSE 4B number versus peak flux relation. Our results disagree with recent claims that current GRB observations can be used to strongly constrain the cosmic star formation history. Instead, we find that relaxing the assumption that GRBs are standard candles renders a very broad range of models consistent with the BATSE number-flux relation. We explicitly construct two sample distributions, one tracing the star formation history and one with a constant comoving density. We show that both distributions are compatible with the observed fluxes and redshifts of the bursts GRB970508, GRB971214, and GRB980703, and we discuss the measurements required to distinguish the two models.Comment: 12 pages, 2 postscript figures, uses AAS LaTex macros v4.0. To be published in Astrophysical Journal Letters, accepted August 20, 1998. Revised for publicatio

The Maximum Flux of Star-Forming Galaxies

The importance of radiation pressure feedback in galaxy formation has been extensively debated over the last decade. The regime of greatest uncertainty is in the most actively star-forming galaxies, where large dust columns can potentially produce a dust-reprocessed infrared radiation field with enough pressure to drive turbulence or eject material. Here we derive the conditions under which a self-gravitating, mixed gas-star disc can remain hydrostatic despite trapped radiation pressure. Consistently taking into account the self-gravity of the medium, the star- and dust-to-gas ratios, and the effects of turbulent motions not driven by radiation, we show that galaxies can achieve a maximum Eddington-limited star formation rate per unit area $\dot{\Sigma}_{\rm *,crit} \sim 10^3 M_{\odot}$ pc$^{-2}$ Myr$^{-1}$, corresponding to a critical flux of $F_{\rm *,crit} \sim 10^{13} L_{\odot}$ kpc$^{-2}$ similar to previous estimates; higher fluxes eject mass in bulk, halting further star formation. Conversely, we show that in galaxies below this limit, our one-dimensional models imply simple vertical hydrostatic equilibrium and that radiation pressure is ineffective at driving turbulence or ejecting matter. Because the vast majority of star-forming galaxies lie below the maximum limit for typical dust-to-gas ratios, we conclude that infrared radiation pressure is likely unimportant for all but the most extreme systems on galaxy-wide scales. Thus, while radiation pressure does not explain the Kennicutt-Schmidt relation, it does impose an upper truncation on it. Our predicted truncation is in good agreement with the highest observed gas and star formation rate surface densities found both locally and at high redshift.Comment: Version accepted for publication in MNRAS. 12 pages, 8 figures. New appendix on photon tirin

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 surface density (Sigma_SFR) and the mass surface density of gas (Sigma_gas) in NGC 5194. VIRUS-P integral field spectroscopy is used to measure H-alpha, H-beta [NII]6548,6584, and [SII]6717,6731 emission line fluxes for 735 regions ~170 pc in diameter, and derive extinction corrected Sigma_SFRs. HI 21cm and CO maps are used to measure the HI and H_2 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, and is free of the systematics involved in performing linear correlations over incomplete data in logarithmic space. We use the [SII]/H-alpha ratio to separate the H-alpha flux contribution from the diffuse ionized gas (DIG). After removing the DIG, 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~2 Gyr, in good agreement with Bigiel et al. (2008). The HI density shows no correlation with the SFR, and the total gas SFL closely follows the molecular gas SFL. We assess the validity of corrections applied in narrow-band H-alpha measurements to overcome issues related to continuum subtraction, underlying photospheric absorption, and contamination by the [NII] doublet. The disagreement with the super-linear molecular SFL measured by Kennicutt et al. (2007) is due to differences in the fitting method. Our results support a low and close to constant star formation efficiency (SFE = 1/tau) in the molecular ISM. The data shows excellent agreement with the model of Krumholz et al (2009). The large intrinsic scatter may imply the existence of other important parameters setting the SFR.Comment: 23 pages, 19 figures, 14 pages of tables. Accepted for publication in ApJ. (Figure 16 has been corrected from the first submitted version.

Morphology of Hydrodynamic Winds: A Study of Planetary Winds in Stellar Environments

Bathed in intense ionizing radiation, close-in gaseous planets undergo hydrodynamic atmospheric escape, which ejects the upper extent of their atmospheres into the interplanetary medium. Ultraviolet detections of escaping gas around transiting planets corroborate such a framework. Exposed to the stellar environment, the outflow is shaped by its interaction with the stellar wind and by the planet's orbit. We model these effects using Athena to perform 3-D radiative-hydrodynamic simulations of tidally-locked hydrogen atmospheres receiving large amounts of ionizing extreme-ultraviolet flux in various stellar environments for the low-magnetic-field case. Through a step-by-step exploration of orbital and stellar wind effects on the planetary outflow, we find three structurally distinct stellar wind regimes: weak, intermediate, and strong. We perform synthetic Lyman-$\alpha$ observations and find unique observational signatures for each regime. A weak stellar wind$\textrm{---}$which cannot confine the planetary outflow, leading to a torus of material around the star$\textrm{---}$has a pre-transit, red-shifted dayside arm and a slightly redward-skewed spectrum during transit. The intermediate regime truncates the dayside outflow at large distances from the planet and causes periodic disruptions of the outflow, producing observational signatures that mimic a double transit. The first of these dips is blue-shifted and precedes the optical transit. Finally, strong stellar winds completely confine the outflow into a cometary tail and accelerate the outflow outwards, producing large blue-shifted signals post-transit. Across all three regimes, large signals occur far outside of transit, offering motivation to continue ultraviolet observations outside of direct transit.Comment: 33 pages, 21 figures (7 of which have embedded movies viewable with Adobe Acrobat Pro), Submitted to Ap