Mean Ionization Fraction Research Articles

Beorn: a fast and flexible framework to simulate the epoch of reionization and cosmic dawn

ABSTRACT In this study, we introduce beorn (Bubbles during the Epoch of Reionization Numerical Simulator), a publicly available python code that generates three-dimensional maps of the 21-cm signal from the cosmic dawn and the epoch of reionization. Built upon N-body simulation outputs, beorn populates haloes with stars and galaxies based on a flexible source model. It then computes the evolution of Ly α coupling, temperature, and ionization profiles as a function of source properties, and paints these profiles around each source on to a three-dimensional grid. The code consistently deals with the overlap of ionized bubbles by redistributing photons around the bubble boundaries, thereby ensuring photon conservation. It accounts for the redshifting of photons and the source look-back effect for the temperature and Ly α coupling profiles that extend far into the intergalactic medium to scales of order 100 cMpc. We provide a detailed description of the code and compare it to results from the literature. After validation, we run three different benchmark models based on a cosmological N-body simulation. All three models agree with current observations from ultravoilet luminosity functions and estimates of the mean ionization fraction. Due to different assumptions regarding the small-mass stellar-to-halo relation, the X-ray flux emission, and the ionizing photon escape fraction, the models produce unique signatures ranging from a cold reionization with deep absorption trough to an emission-dominated 21-cm signal, broadly encompassing the current uncertainties at cosmic dawn. The code beorn is publicly available at https://github.com/cosmic-reionization/BEoRN.

Jet from the enigmatic high-latitude star BP Psc and evolutionary status of its driving source

ABSTRACT BP Psc is an active late-type (sp:G9) star with unclear evolutionary status lying at high-galactic latitude b = −57°. It is also the source of the well collimated bipolar jet. We present results of the proper motion and radial velocity study of BP Psc outflow based on the archival Hα imaging with the GMOS camera at 8.1-m Gemini-North telescope as well as recent imaging and long-slit spectroscopy with the SCORPIO multimode focal reducer at 6-m BTA telescope of SAO RAS. The 3D kinematics of the jet revealed the full spatial velocity up to ∼140 km s−1 and allows us to estimate the distance to BP Psc system as D = 135 ± 40 pc. This distance leads to an estimation of the central source luminosity L* ≈ 1.2 L⊙, indicating that it is the ≈1.3M⊙ T Tauri star with an age t ≲ 7 Myrs. We measured the electron density of order Ne ∼ 102 cm−3 and mean ionization fraction f ≈ 0.04 within the jet knots and estimated upper limit of the mass-loss rate in NE lobe as $\dot{M}_{\rm out}\approx 1.2\cdot 10^{-8}{\rm M}_{\odot }\,{\rm yr}^{-1}$. The physical characteristics of the outflow are typical for the low-excitation YSO jets and consistent with the magnetocentrifugal mechanism of its launching and collimation. Prominent wiggling pattern revealed in Hα images allowed us to suppose the existence of a secondary substellar companion in a non-coplanar orbit and estimate its most plausible mass as Mp ≈ 30MJup. We conclude that BP Psc is one of the closest to the Sun young jet-driving systems and its origin is possibly related to the episode of star formation triggered by expanding supershells in second Galactic quadrant.

Effect of intergalactic medium on the observability of Lyα emitters during cosmic reionization

We perform a systematic study of how the inhomogeneities in the Inter-Galactic Medium (IGM) affect the observability of Lyman Alpha Emitters (LAEs) around the Epoch of Reionization. We focus on the IGM close to the galaxies as the detailed ionization distribution and velocity fields of this region could significantly influence the scattering of Ly-alpha photons off neutral H atoms as they traverse the IGM after escaping from the galaxy. We simulate the surface brightness (SB) maps and spectra of more than 100 LAEs at z=7.7 as seen by an observer at z=0. To achieve this, we extract the source properties of galaxies and their surrounding IGM from cosmological simulations of box sizes 5-30 Mpc/h and follow the coupled radiative transfer of ionizing and Ly-alpha radiation through the IGM using CRASH-alpha. We find that the simulated SB profiles are extended and their detailed structure is affected by inhomogeneities in the IGM, especially at high neutral fractions. The detectability of LAEs and the fraction of the flux observed depend heavily on the shape of the SB profile and the SB threshold (SB_th) of the observational campaign. Only ultradeep observations (e.g. SB_th ~ 10^-23 ergs/s/cm^2/arcsec^2) would be able to obtain the true underlying mass-luminosity relation and luminosity functions of LAEs. The details of our results depend on whether Ly-alpha photons are significantly shifted in the galaxy to longer wavelengths, the mean ionization fraction in the IGM and the clustering of ionizing sources. These effects can lead to an easier escape of Ly-alpha photons with less scattering in the IGM and a concentrated SB profile similar to the one of a point source. Finally, we show that the SB profiles are steeper at high ionization fraction for the same LAE sample which can potentially be observed from the stacked profile of a large number of LAEs.

INTENSITY MAPPING OF THE [C II] FINE STRUCTURE LINE DURING THE EPOCH OF REIONIZATION

The atomic CII fine-structure line is one of the brightest lines in a typical star-forming galaxy spectrum with a luminosity ~ 0.1% to 1% of the bolometric luminosity. It is potentially a reliable tracer of the dense gas distribution at high redshifts and could provide an additional probe to the era of reionization. By taking into account of the spontaneous, stimulated and collisional emission of the CII line, we calculate the spin temperature and the mean intensity as a function of the redshift. When averaged over a cosmologically large volume, we find that the CII emission from ionized carbon in individual galaxies is larger than the signal generated by carbon in the intergalactic medium (IGM). Assuming that the CII luminosity is proportional to the carbon mass in dark matter halos, we also compute the power spectrum of the CII line intensity at various redshifts. In order to avoid the contamination from CO rotational lines at low redshift when targeting a CII survey at high redshifts, we propose the cross-correlation of CII and 21-cm line emission from high redshifts. To explore the detectability of the CII signal from reionization, we also evaluate the expected errors on the CII power spectrum and CII-21 cm cross power spectrum based on the design of the future milimeter surveys. We note that the CII-21 cm cross power spectrum contains interesting features that captures physics during reionization, including the ionized bubble sizes and the mean ionization fraction, which are challenging to measure from 21-cm data alone. We propose an instrumental concept for the reionization CII experiment targeting the frequency range of $\sim$ 200 to 300 GHz with 1, 3 and 10 meter apertures and a bolometric spectrometer array with 64 independent spectral pixels with about 20,000 bolometers.

Time-dependent MHD shocks and line emission: the case of the DG Tau jet

The line emission from a growing number of Herbig-Haro jets can be observed and resolved at angular distances smaller than a few arcseconds from the central source. The interpretation of this emission is problematic, since the simplest model of a cooling jet cannot sustain it. It has been suggested that what one actually observes are shocked regions with a filling factor of $\sim 1%$. In this framework, up to now, comparisons with observations have been based on stationary shock models. Here we introduce for the first time the self-consistent dynamics of such shocks and we show that considering their properties at different times, i.e. locations, we can reproduce observational data of the DG Tau microjet. In particular, we can interpret the spatial behavior of the [SII]6716/6731 and [NII]/[OI]6583/6300 line intensity ratios adopting a set of physical parameters that yield values of mass loss rates and magnetic fields consistent with previous estimates. We also obtain the values of the mean ionization fraction and electron density along the jet, compare these values with the ones derived from observations using the sulfur doublet to constrain the electron density (e.g. Bacciotti et al. 1995).

Constraints on the Ionization Balance of Hot‐Star Winds fromFUSEObservations of O Stars in the Large Magellanic Cloud

We present Far Ultraviolet Spectroscopic Explorer (FUSE) spectra for 25 O stars in the Large Magellanic Cloud. We analyze wind profiles for the resonance lines from C III, N III, S IV, P V, S VI, and O VI in the FUSE range using a Sobolev with exact integration method. In addition, the available data from either IUE or the Hubble Space Telescope for the resonance lines of Si IV, C IV, and N V are also modeled. Because several of the FUSE wind lines are unsaturated, the analysis provides meaningful optical depths (or equivalently, mass-loss rate times ionization fractions, q) as a function of normalized velocity, w = v/v∞. Ratios of q (which are independent of ) determine the behavior of the relative ionization as a function of w. The results demonstrate that, with the exception of O VI in all stars and S VI in the later stars, the ionization in the winds shifts toward lower ionization stages at higher w (contrary to the expectations of the nebular approximation). This result implies that the dominant production mechanism for O VI and S VI in the late O stars differs from the other ions. Using the Vink et al. relationship between stellar parameters and mass-loss rate, we convert the measurements into mean ionization fractions for each ion, qi. Because the derived ion fractions never exceed unity, we conclude that the derived values of are not too small. However, q(P V), which is expected to be the dominant stage of ionization in some of these winds, is never greater than 0.20. This implies that either the calculated values of are too large, the assumed abundance of phosphorus is too large, or the winds are strongly clumped. The implications of each possibility are discussed. Correlations between the mean ion fractions and physical parameters such as Teff, v∞, and the mean wind density, ρ, are examined. Two clear relationships emerge. First, as expected, the mean ionization fraction of the lower ions (C III, N III, Si IV, S IV) decreases with increasing Teff. Second, the mean ion fraction of S VI in the latest stars and O VI in all stars increases with increasing v∞. This reaffirms the notion, first introduced by Cassinelli & Olson, that O VI is produced nonradiatively. Finally, we discuss specific characteristics of three stars, BI 272, BI 208, and Sk -67°166. For BI 272, the ionic species present in its wind suggest it is much hotter than its available (uncertain) spectral type of O7: II-III:. In the case of BI 208, our inability to fit its observed profiles suggests that its wind is not spherically symmetric. For Sk -67°166, quantitative measurements of its line strengths confirm the suggestion by Walborn et al. that it is a nitrogen-rich O star.

An analysis of HST UV spectra of Cyg OB2 stars

As a first step of a vigorous program to investigate the Wind Momentum - Luminosity Relationship (WLR) of Galactic O-stars by analyzing stars belonging to the same cluster we present UV HST observations of six supergiants and one giant in the Cyg OB2 association. Terminal and turbulent wind velocities, velocity laws and metal ion column densities are derived and mean ionization fractions are estimated. Turbulent velocities are mostly in the range 10-14% of $v_\infty$. The terminal velocities agree well with the average $v_\infty$ vs. spectral class relationship compiled by Kudritzki & Puls ([CITE]). We compare the observed $v_\infty$ vs. escape velocity (depending on the diagnostics of the stellar mass) correlation with the predictions of the radiatively driven wind theory and find better agreement with the spectroscopic masses rather than with the evolutionary ones. The β velocity field exponents are in the range 0.7-0.8, without any trend towards larger values. We show that for a single luminosity class there is a tight relationship between T eff and $ $ (the mean density at the point in the stellar wind, where half the wind terminal velocity is reached). In consequence, the ionization fractions show the same trend with both, T eff and $ $: we find that Nv increases with T eff , Siiv decreases and Civ does not clearly correlate. As a byproduct, we also derive interestellar Hi column densities towards Cyg OB2, which turn out to be quite large. For one object (Cyg OB2$\#$2) we find inconsistencies making the association membership questionable.

The Ionization in the Winds of O Stars and the Determination of Mass‐Loss Rates from Ultraviolet Lines

Empirical ionization fractions of C IV ,N V ,S iIV, and empirical ionization plus excitation fractions of C III* and N IV* in the winds of 34 O stars and one B star have been derived. We combine the mass-loss rates derived from radio measurements and Ha with the line —tting of ultraviolet resonance lines and subordinate lines using the Sobolev plus exact integration (SEI) method. The dependence of the empiri- cal ionization fractions, SqT, on the stellar eUective temperature and on the mean wind density is dis- cussed. This sets constraints for the models of ionization in the winds of hot stars. The ionization and excitation fractions can be expressed in terms of an empirical radiation temperature. This radiation tem- perature scales with and we derive empirical relations for as a function of The radiation T eff , T rad T eff . temperatures are on the order of 0.5¨0.9 with signi—cant diUerences between the ions. The derived T eff , relations between the ionization fractions and the stellar parameters have an uncertainty of 0.2 dex for Si IV ,N V, and C III*, and about 0.26 dex for N IV*. For C IV, we can only derive an expression for the mean ionization fraction in the wind if the mass-loss rate is small, yr~1, because the C IV M 0 10~6 M _ lines are usually saturated for higher mass-loss rates. The resulting expressions for can be used to T rad derive the mass-loss rates from studies of ultraviolet P Cygni pro—les in the range of stellar parameters studied here: 30,000 K, and ( 7.5 K ( T eff ( 50,500 5.2 ( log L /L * ( 6.4, M _ ( log M0 ( (4.6 M _ yr~1. An accuracy of about a factor of 2 or better can be reached, depending on the lines that are used and the accuracy of the line —ts and the stellar parameters. The Si IV lines give the most reliable mass- loss rates, because the abundance is about the same for all O stars, the lines saturate only for high mass-loss rates, the doublet lines only partly overlap, and the mass-loss rate is proportional to the square root of the column density. The radiation temperature of N V shows a surprisingly strict relation with with a scatter of only The mass-loss rate cannot be derived from the N V T eff , *T rad /T eff 0.01. lines, because the column density of the N V ions in the wind is independent of A consistency check M0 . and a test of the method for the stars HD 14749 and HD 190429 show that the mass-loss rate derived from the UV lines with the ionization fractions of this paper agree very well with the mass-loss rate derived from new radio —ux measurements. Subject headings: stars: atmospheresstars: early-typestars: mass lossultraviolet: stars

Interstellar and photospheric opacity from EUV spectroscopy of DA white dwarfs

We present a detailed analysis of the extreme-ultraviolet (EUV) spectra of 13 hydrogen-rich DA white dwarfs, observed by the Extreme Ultraviolet Explorer (EUVE) satellite, paying attention to the possible sources of absorbing material along the lines of sight both in the local interstellar medium (ISM) and in the photospheres of the stars themselves. The range of interstellar column densities seen are consistent with our previous understanding of the local distribution of material. Absorption from interstellar Heii is found in the direction of five stars, allowing us to measure directly the He ionization fraction and estimate, indirectly, that of H. The weighted mean ionization fractions along these lines of sight are 0.27+/-0.04 and 0.35+/-0.1 respectively. Where Heii is directly detected, the observed ionization fractions are not correlated with direction or with the volume/column density of material along the line of sight. Furthermore, the limits on the amount of Heii established in all other directions completely encompass the range of observed values. Indeed, all the data can be consistent with more or less constant He and H ionization fractions throughout the local ISM. It is clear that there is little photospheric opacity, from either He or heavier elements, in the majority of the stars we have studied. This poses further difficulties in explaining the observed division of white dwarfs into H- and He-rich groups, the temperature gap in the He-rich sequence and the detailed spectral evolution of the H-rich DA white dwarfs as they cool. A striking observational result is that our spectroscopic evidence indicates that radiative levitation effects are only important at temperatures above 50000K, rather than the 40000K suggested by broad-band photometry. There is clearly an urgent need for further theoretical work on the mechanisms that determine the photospheric composition of white dwarf stars.

Disks, Winds, and Veiling Curtains: Dissecting the Ultraviolet Spectrum of the Dwarf Nova Z Camelopardalis in Outburst

We present a far-ultraviolet spectrum of the dwarf nova Z Cam near the peak of a normal outburst as observed with the Hopkins Ultraviolet Telescope (HUT) on the Astro-2 mission. The continuum shape and luminosity are almost identical to an Astro-1 HUT spectrum of the same object in a similar state obtained about 4 years or 50 outburst episodes earlier. This suggests that, following the onset of an outburst, the system quickly reaches a (quasi-) steady state that is insensitive to the interoutburst history. A variance analysis of the Astro-2 data reveals no evidence for spectral variability on a timescale of minutes. The rms amplitude of any intrinsic fluctuations is <5% of the flux in both continuum and lines. Z Cam's continuum can be described moderately well in terms of an optically thick, steady state accretion disk with acc 3 × 1017 g s-1 if the disk is assumed to radiate as an ensemble of stellar atmospheres. This type of model reproduces the turnover in the data at about 1050 A, but the predicted spectrum is somewhat too blue at longer wavelengths, causing it to underpredict the flux longward of about 1500 A. This discrepancy appears to be resistant to all potential remedies we have tested, which include differential limb-darkening, reddening, and white dwarf, boundary layer, or hot spot spectral components. This suggests either that our modeling of the standard accretion disk picture is too simplistic—the effects of radial interactions and disk irradiation, for example, have been ignored—or that the standard picture itself may require modification. Blackbody disk models actually match the data better at longer HUT wavelengths, but the redder color of these models is a direct consequence of the neglect of all radiative transfer effects. The same neglect prevents blackbody models from reproducing the turnover in the spectrum and causes them to overpredict the accretion rate. We use a Monte Carlo line profile synthesis code to model five of the high-ionization lines in Z Cam's spectrum in terms of a simple, kinematic description of a rotating, biconical accretion disk wind. Adopting the picture of such an outflow that has recently been proposed for another cataclysmic variable, UX UMa, we find that acceptable fits to the data can be obtained. The relative mean ionization fractions we derive for the ionic species included in our wind modeling appear to be consistent with photoionization by a radiation field with T ~ 1.2 × 105 K. This temperature is within the range that has recently been inferred for Z Cam's soft X-ray component from ROSAT data and similar to the boundary layer temperature that has been derived on the basis of Extreme-Ultraviolet Explorer (EUVE) observations for the dwarf nova U Gem in outburst. An important feature of our adopted outflow model is the existence of a vertically extended, dense, slow-moving transition region between the disk photosphere and the fast-moving wind. Using a static LTE slab to crudely model this region, we find many of the absorption features in Z Cam's line spectrum that we have not modeled with our Monte Carlo code. The physical conditions expected in the extended disk atmosphere—ne ~ 1012 cm-3, NH ~ 1022 cm-2, and T few × 104 K—are similar to those in the Fe II curtain that has been found to veil the white dwarf in the dwarf nova OY Car in quiescence.

The Interstellar and Circumstellar Environment of White Dwarfs

AbstractWe have studied the EUV spectra of 13 DA white dwarfs, observed by the EUVE satellite, paying attention to the possible sources of absorbing material along the lines-of-sight in both the local interstellar medium and in the photospheres of the stars themselves. The range of interstellar column densities seen are consistent with our previous understanding of the local distribution of material. Absorption from interstellar He II is found in the direction of five stars, allowing us to measure directly the He ionization fraction and estimate, indirectly, that of H. The weighted mean ionization fractions along these lines-of-sight are 0.27 ± 0.04 and 0.35 ± 0.1 respectively. Where He II is directly detected, the observed ionization fractions are not correlated with direction or with the volume/column density of material along the line-of-sight. Furthermore, the limits on the amount of He II established in all other directions completely encompass the range of observed values. Indeed, all the data can be consistent with more or less constant He and H ionization fractions throughout the local ISM. However, observation of very hot DA stars, indicating higher He II columns, might contradict this picture if the material is not photospheric or circumstellar.