Fluorescent Ly-alpha emission from the high-redshift intergalactic medium

We combine a high-resolution hydro-simulation of the LambdaCDM cosmology with two radiative transfer schemes (for continuum and line radiation) to predict the properties, spectra and spatial distribution of fluorescent Ly-alpha emission at z~3. We focus on line radiation produced by recombinations in the dense intergalactic medium ionized by UV photons. In particular, we consider both a uniform background and the case where gas clouds are illuminated by a nearby quasar. We find that the emission from optically thick regions is substantially less than predicted from the widely used static, plane-parallel model. The effects induced by a realistic velocity field and by the complex geometric structure of the emitting regions are discussed in detail. We make predictions for the expected brightness and size distributions of the fluorescent sources.Our results account for recent null detections and can be used to plan new observational campaigns both in the field (to measure the intensity of the diffuse UV background) and in the proximity of bright quasars (to understand the origin of high colum-density absorbers).Comment: 15 pages, 12 figures, accepted for publication in Ap

The Spin Temperature of Warm Interstellar H I

Collisional excitation of the 21cm HI hyperfine transition is not strong enough to thermalize it in warm neutral (``intercloud'') interstellar gas, which we show by simultaneously solving the equations of ionization and collisional equilibrium under typical conditions. Coupling of the 21cm excitation temperature and local gas motions may be established by the Ly-alpha radiation field, but only if strong Galactic Ly-alpha radiation permeates the gas in question. The Ly-alpha radiation tends to impart to the gas its own characteristic temperature, which is determined by the range of gas motions that occur on the spatial scale of the Ly-alpha scattering. In general, the calculation of H I spin temperatures is a more difficult and interesting problem than might have been expected, as is any interpretation of H I spin temperature measurements.Comment: 11 pages, 8 figures, accepted for A&

The Low- and Intermediate-Mass Stellar Population in the Small Magellanic Cloud: The Central Stars of Planetary Nebulae

We present a study on the central stars (CSs) of Planetary Nebulae (PNe) observed in the Small Magellanic Cloud (SMC) with the Space Telescope Imaging Spectrograph instrument on-board the HST. The stellar magnitudes have been measured using broad-band photometry, and Zanstra analysis of the nebulae provided the stellar temperatures. From the location of the CSs on the HR diagram, and by comparing the observed CSs with current models of stellar evolution, we infer the CSs masses. We examine closely the possibility of light contamination in the bandpass from an unrecognized stellar companion, and we establish strong constraints on the existence and nature of any binary companion. We find an average mass of 0.63 Msun, which is similar to the mass obtained for a sample of CSs in the LMC (0.65 Msun). However, the SMC and LMC CS mass distributions differ slightly, the SMC sample lacking an intermediate-mass stellar population (0.65 to 0.75 Msun). We discuss the significance and possible reasons for the difference between the two mass distributions. In particular, we consider the differences in the star formation history between the clouds and the mass-loss rate dependence on metallicity.Comment: 30 pages, 6 figures, 5 tables. To be published in ApJ (October 20

Possible Detection of Lyman-alpha Fluorescence from a Damped Lyman Alpha system at Redshift z=2.8

We have detected Lyman-alpha emission from a damped Lyman-alpha system (DLA) that lies near the bright quasar HS1549+1919. The DLA has the same redshift as HS1549+1919 and was discovered in the spectrum of a faint QSO that lies 49" away (380 proper kpc). The emission line's luminosity, double-peaked profile, and small spatial separation from the DLA suggest that it may be fluorescent Lyman-alpha emission from gas that is absorbing the nearby QSO's radiation. If this is the case, our observations show that the DLA has a size of at least 1.5" and that the QSO's luminosity one million years ago was similar to its luminosity today. A survey for similar systems within 1' of bright QSOs would put interesting limits on the mean quasar lifetime.Comment: 6.1 pages including 4 figures; accepted for publication in the Ap

Modeling of Photoionized Plasmas

In this paper I review the motivation and current status of modeling of plasmas exposed to strong radiation fields, as it applies to the study of cosmic X-ray sources. This includes some of the astrophysical issues which can be addressed, the ingredients for the models, the current computational tools, the limitations imposed by currently available atomic data, and the validity of some of the standard assumptions. I will also discuss ideas for the future: challenges associated with future missions, opportunities presented by improved computers, and goals for atomic data collection.Comment: 17 pages, 8 figures, to appear in the proceedings of Xray2010, Utrecht, the Netherlands, March 15-17 201

Three-dimensional chemically homogeneous and bi-abundance photoionization models of the "super-metal-rich" planetary nebula NGC 6153

Deep spectroscopy of the planetary nebula (PN) NGC\,6153 shows that its heavy element abundances derived from optical recombination lines (ORLs) are ten times higher than those derived from collisionally excited lines (CELs), and points to the existence of H-deficient inclusions embedded in the diffuse nebula. In this study, we have constructed chemically homogeneous and bi-abundance three-dimensional photoionization models, using the Monte Carlo photoionization code {\sc mocassin}. We attempt to reproduce the multi-waveband spectroscopic and imaging observations of NGC\,6153, and investigate the nature and origin of the postulated H-deficient inclusions, as well as their impacts on the empirical nebular analyses assuming a uniform chemical composition. Our results show that chemically homogeneous models yield small electron temperature fluctuations and fail to reproduce the strengths of ORLs from C, N, O and Ne ions. In contrast, bi-abundance models incorporating a small amount of metal-rich inclusions ($\sim 1.3$ per cent of the total nebular mass) are able to match all the observations within the measurement uncertainties. The metal-rich clumps, cooled down to a very low temperature ($\sim 800$~K) by ionic infrared fine-structure lines, dominate the emission of heavy element ORLs, but contribute almost nil to the emission of most CELs. We find that the abundances of C, N, O and Ne derived empirically from CELs, assuming a uniform chemical composition, are about 30 per cent lower than the corresponding average values of the whole nebula, including the contribution from the H-deficient inclusions. Ironically, in the presence of H-deficient inclusions, the traditional standard analysis of the optical helium recombination lines, assuming a chemically homogeneous nebula, overestimates the helium abundance by 40 per cent.Comment: 19 pages, 18 figures, accepted for publication in MNRA

On-disk coronal rain

Small and elongated, cool and dense blob-like structures are being reported with high resolution telescopes in physically different regions throughout the solar atmosphere. Their detection and the understanding of their formation, morphology and thermodynamical characteristics can provide important information on their hosting environment, especially concerning the magnetic field, whose understanding constitutes a major problem in solar physics. An example of such blobs is coronal rain, a phenomenon of thermal non- equilibrium observed in active region loops, which consists of cool and dense chromospheric blobs falling along loop-like paths from coronal heights. So far, only off-limb coronal rain has been observed and few reports on the phenomenon exist. In the present work, several datasets of on-disk H{\alpha} observations with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar Telescope (SST) are analyzed. A special family of on-disk blobs is selected for each dataset and a statistical analysis is carried out on their dynamics, morphology and temperatures. All characteristics present distributions which are very similar to reported coronal rain statistics. We discuss possible interpretations considering other similar blob-like structures reported so far and show that a coronal rain interpretation is the most likely one. Their chromospheric nature and the projection effects (which eliminate all direct possibility of height estimation) on one side, and their small sizes, fast dynamics, and especially, their faint character (offering low contrast with the background intensity) on the other side, are found as the main causes for the absence until now of the detection of this on-disk coronal rain counterpart.Comment: 18 pages, 10 figures. Accepted for Solar Physic

Temperature of the central stars of planetary nebulae and the effect of the nebular optical depth

The effect of the nebula optical depth on the determination of the temperature (T*) of the central stars in planetary nebulae is discussed. Based on photoionization models for planetary nebulae with different optical depths, we show, quantitatively, that the details of the distribution of the H and He II Zanstra temperatures are mainly explained by an optical depth effect; in particular, that the discrepancy is larger for low stellar temperatures. The results also show that for high stellar temperatures the He II Zanstra temperature underestimates the stellar temperature, even for high optical depths. The stellar temperature, as well as the optical depth, can be obtained from a Zanstra temperature ratio (ZR) plot ZR = Tz(He II)/Tz(H) versus Tz(He II). The effects of departures from a blackbody spectrum, as well as of the He abundance in the nebulae, are also discussed. For nebulae of very low optical depth and/or high stellar temperature the distribution ZR versus Tz(He II) only provides lower limits for T*. In order to obtain better values for the optical depth and T*, we propose the use of the line intensity ratio He II/He I versus Tz(He II) diagram.Comment: 24 pages, 4 figures; to appear in ApJ, Nov 200