Enlightening the Chemistry of Infalling Envelopes and Accretion Disks Around Sun-Like Protostars: The ALMA FAUST Project

The huge variety of planetary systems discovered in recent decades likely depends on the early history of their formation. In this contribution, we introduce the FAUST Large Program which focuses specifically on the early history of solar-like protostars and their chemical diversity at scales of ∼ 50 au, where planets are expected to form. In particular, the goal of the project is to reveal and quantify the variety of chemical composition of the envelope/disk system at scales of 50 au in a sample of Class 0 and I protostars representative of the chemical diversity observed at larger scales. For each source, we propose a set of molecules able to (1) disentangle the components of the 50–2000 au envelope/disk system, (2) characterize the organic complexity in each of them, (3) probe their ionization structure, and (4) measure their molecular deuteration. The output will be a homogeneous database of thousands of images from different lines and species, i.e., an unprecedented source survey of the chemical diversity of solar-like protostars. FAUST will provide the community with a legacy dataset that will be a milestone for astrochemistry and star formation studies.

Limits to Ionization-parameter Mapping as a Diagnostic of Hii Region Optical Depth

We employ ionization-parameter mapping (IPM) to infer the optical depth of H ii regions in the northern half of M33. We construct [O iii]λ5007/[O ii]λ3727 and [O iii]λ5007/[S ii]λ6724 ratio maps from narrowband images continuum-subtracted in this way, from which we classify the H ii regions by optical depth to ionizing radiation, based on their ionization structure. This method works relatively well in the low-metallicity regime, 12 + log ( O / H ) ≤ 8.4 , where [O iii]λ λ4959, 5007 is strong. However, at higher metallicities, the method breaks down due to the strong dependence of the [O iii]λ λ4959, 5007 emission lines on the nebular temperature. Thus, although O++ may be present in metal-rich H ii regions, these commonly used emission lines do not serve as a useful indicator of its presence, and hence the O ionization state. In addition, IPM as a diagnostic of optical depth is limited by spatial resolution. We also report a region of highly excited [O iii] extending over an area ∼1 kpc across and [O iii]λ5007 luminosity of 4.9 ± 1.5 × 1038 erg s−1, which is several times higher than the ionizing budget of any potential sources in this portion of the galaxy. Finally, this work introduces a new method for continuum subtraction of narrowband images based on the dispersion of pixels around the mode of the diffuse-light flux distribution. In addition to M33, we demonstrate the method on C iii]λ1909 imaging of Haro 11, ESO 338-IG004, and Mrk 71.

Tracing the Ionization Structure of the Shocked Filaments of NGC 6240

We study the ionization and excitation structure of the interstellar medium in the late-stage gas-rich galaxy merger NGC 6240 using a suite of emission-line maps at ∼25 pc resolution from the Hubble Space Telescope, Keck/NIRC2 with Adaptive Optics, and the Atacama Large Millimeter/submillimeter Array (ALMA). NGC 6240 hosts a superwind driven by intense star formation and/or one or both of two active nuclei; the outflows produce bubbles and filaments seen in shock tracers from warm molecular gas (H2 2.12 μm) to optical ionized gas ([O iii], [N ii], [S ii], and [O i]) and hot plasma (Fe XXV). In the most distinct bubble, we see a clear shock front traced by high [O iii]/Hβ and [O iii]/[O i]. Cool molecular gas (CO(2−1)) is only present near the base of the bubble, toward the nuclei launching the outflow. We interpret the lack of molecular gas outside the bubble to mean that the shock front is not responsible for dissociating molecular gas, and conclude that the molecular clouds are partly shielded and either entrained briefly in the outflow, or left undisturbed while the hot wind flows around them. Elsewhere in the galaxy, shock-excited H2 extends at least ∼4 kpc from the nuclei, tracing molecular gas even warmer than that between the nuclei, where the two galaxies’ interstellar media are colliding. A ridgeline of high [O iii]/Hβ emission along the eastern arm aligns with the southern nucleus’ stellar disk minor axis; optical integral field spectroscopy from WiFeS suggests this highly ionized gas is centered at systemic velocity and likely photoionized by direct line of sight to the southern active galactic nucleus.

The COS Absorption Survey of Baryon Harbors: Unveiling the Physical Conditions of Circumgalactic Gas through Multiphase Bayesian Ionization Modeling

Quasar absorption systems encode a wealth of information about the abundances, ionization structure, and physical conditions in intergalactic and circumgalactic media. Simple (often single-phase) photoionization models are frequently used to decode such data. Using five discrete absorbers from the COS Absorption Survey of Baryon Harbors (CASBaH) that exhibit a wide range of detected ions (e.g., Mg ii, S ii – S vi, O ii – O vi, Ne viii), we show several examples where single-phase ionization models cannot reproduce the full set of measured column densities. To explore models that can self-consistently explain the measurements and kinematic alignment of disparate ions, we develop a Bayesian multiphase ionization modeling framework that characterizes discrete phases by their unique physical conditions and also investigates variations in the shape of the UV flux field, metallicity, and relative abundances. Our models require at least two (but favor three) distinct ionization phases ranging from T ≈ 104 K photoionized gas to warm-hot phases at T ≲ 105.8 K. For some ions, an apparently single absorption “component” includes contributions from more than one phase, and up to 30% of the H i is not from the lowest ionization phase. If we assume that all of the phases are photoionized, we cannot find solutions in thermal pressure equilibrium. By introducing hotter, collisionally ionized phases, however, we can achieve balanced pressures. The best models indicate moderate metallicities, often with sub-solar N/α, and, in two cases, ionizing flux fields that are softer and brighter than the fiducial Haardt & Madau UV background model.

Two new nova shells associated with V4362 Sagittarii and DO Aquilae

ABSTRACT A classical nova is an eruption on the surface of a white dwarf in an accreting binary system. The material ejected from the white dwarf surface generally forms an axisymmetric shell. The shaping mechanisms of nova shells are probes of the processes that take place at energy scales between planetary nebulae and supernova remnants. We report on the discovery of nova shells surrounding the post-nova systems V4362 Sagittarii (1994) and more limited observations of DO Aquilae (1925). Distance measurements of $0.5\substack{+1.4 \\ -0.2}$ kpc for V4362 Sgr and 6.7 ± 3.5 kpc for DO Aql are found based on the expansion parallax method. The growth rates are measured to be 0.07 arcsec yr−1 for DO Aql and 0.32 arcsec yr−1 for V4362 Sgr. A preliminary investigation into the ionization structure of the nova shell associated with V4362 Sgr is presented. The observed ionization structure of nova shells depends strongly on their morphology and the orientation of the central component towards the observer. X-ray, IR, and UV observations as well as optical integral field unit spectroscopy are required to better understand these interesting objects.

Morphology and ionization characteristics of planetary nebulae PB 1 and PC 19

ABSTRACT We present results of our study of two planetary nebulae (PNe), PB1 and PC 19. We use the optical spectra of these two PNe observed at 2 m Himalayan Chandra Telescope and also archival and literature data for the study. We use the morphokinematic code shape to construct 3D morphologies of the PNe and the photoionization code cloudy to model the observed spectra. The 3D model of PB 1 consists of an elongated shell surrounded by a bipolar halo and that of PC 19 consists of an open lobed bipolar structure and a spiral filamentary pair. We analyse the ionization structure of the PNe by deriving several plasma parameters and by photoionization modelling. We estimate the elemental abundances of the elements, He, C, N, O, Ne, S, Ar, and Cl, from our analysis. We find He, C, and N abundances to be significantly higher in case of PB 1. We estimate different physical parameters of the central stars, namely effective temperature, luminosity, and gravity, and of the nebula, namely hydrogen density profiles, radii, etc., from photoionization modelling. We estimate distances to the PNe as ∼4.3 kpc for PB 1 and as ∼5.6 kpc for PC 19 by fitting the photoionization models to absolute observed fluxes. Progenitor masses are estimated from theoretical evolutionary trajectories and are found to be ∼1.67 and ∼2.38 M⊙ for PB 1 and PC 19, respectively.

Probing inhomogeneity in the helium ionizing UV background

ABSTRACT We present an analysis combining the simultaneous measurement of intergalactic absorption by hydrogen (${\rm {H\,\small {I}}}$), helium (${\rm {He\,\small {II}}}$), and oxygen (${\rm {O\,\small {VI}}}$) in UV and optical quasar spectra. The combination of the ${\rm {H\,\small {I}}}$ and ${\rm {He\,\small {II}}}$ Lyman-alpha forests through η (the ratio of column densities of singly ionized helium to neutral hydrogen) is thought to be sensitive to large-scale inhomogeneities in the extragalactic UV background. We test this assertion by measuring associated five-times-ionized oxygen (${\rm {O\,\small {VI}}}$) absorption, which is also sensitive to the UV background. We apply the pixel optical depth technique to ${\rm {O\,\small {VI}}}$ absorption in high and low η samples filtered on various scales. This filtering scale is intended to represent the scale of any coherent oxygen excess/deficit. We find a 2σ detection of an ${\rm {O\,\small {VI}}}$ opacity excess in the low η sample on scales of ∼10 cMpc for HE 2347-4342 at $\bar{z}\approx 2.6$, consistent with a large-scale excess in hard UV photons. However, for HS 1700 + 6416 at $\bar{z}\approx 2.5$ we find that the measured ${\rm {O\,\small {VI}}}$ absorption is not sensitive to differences in η. HS 1700 + 6416 also shows a relative absence of ${\rm {O\,\small {VI}}}$ overall, which is 6σ inconsistent with that of HE 2347-4342. This implies UV background inhomogeneities on ≳200 cMpc scales, hard UV regions having internal ionization structure on ∼10 cMpc scales, and soft UV regions showing no such structure. Furthermore, we perform the pixel optical depth search for oxygen on the ${\rm {He\,\small {II}}}$ Gunn-Peterson trough of HE 2347-4342 and find results consistent with post-${\rm {He\,\small {II}}}$-reionization conditions.

Numerical models for the diffuse ionized gas in galaxies

Context. The diffuse ionized gas (DIG) constitutes the largest fraction of the total ionized interstellar matter in star-forming galaxies, but it is still unclear whether the ionization is driven predominantly by the ionizing radiation of hot massive stars, as in H II regions, or whether additional sources of ionization have to be considered. Key to understanding the ionization mechanisms in the DIG is the line emission by the ionized gas. Aims. We systematically explore a plausible subset of the parameter space involving effective temperatures and metallicities of the ionizing sources, the effects of the hardening of their radiation by surrounding “leaky” H II regions with different escape fractions, as well as different scenarios for the clumpiness of the DIG, and compute the resulting line strength ratios for a number of diagnostic optical emission lines. Methods. For the ionizing fluxes we computed a grid of stellar spectral energy distributions (SEDs) from detailed, fully non-LTE model atmospheres that include the effects of stellar winds and line blocking and blanketing. To calculate the ionization and temperature structure in the interstellar gas we used spherically symmetric photoionization models and state-of-the-art three-dimensional (3D) non-LTE radiative transfer simulations, considering hydrogen, helium, and the most abundant metals. We first applied these methods to classical H II regions around hot stars, using the model SEDs at different metallicities and effective temperatures as ionizing fluxes, and compute the SEDs of the escaping radiation for different escape fractions of hydrogen-ionizing photons. In a second step, we studied the effects of the escaping radiation on the more dilute and extended DIG. Using 3D models simulating a section of a galactic spiral arm, we computed the ionization structure in the DIG for different scenarios for the inhomogeneity of the gas, assuming ionization by a stellar population SED based on plausible parameters. Results. We provide quantitative predictions of how the line ratios from H II regions and the DIG vary as a function of metallicity Z, stellar effective temperature Teff, and escape fraction fesc from the H II region. The range of predicted line ratios reinforces the hypothesis that the DIG is ionized by (filtered) radiation from hot stars. At one-tenth solar metallicity, radiation hardening is mostly due to hydrogen and helium, whereas at solar metallicity absorption by metals plays a significant role. The effects of hardening are seen primarily in the increase in the emission line ratios of the most important cooling lines of the gas, [N II]∕Hβ and [O II]∕Hβ at lower Teff, and [O III]∕Hβ at higher Teff. For low Teff nearly the entire He I-ionizing radiation is absorbed in the H II regions, thereby preventing the formation of high ionization stages such as O III in the DIG. The ionization structure of the DIG depends strongly on both the clumping factor fcl = 〈nH2〉/〈nH2〉 and the large-scale distribution of the gas. In our simulations about 10% of the ionizing radiation produced by hot massive stars in a spiral arm is sufficient to ionize the DIG up to a height of approximately 1 kpc above the galactic plane for a clumping factor close to the observed value of fcl ~ 5. Even small changes in simulation parameters such as the clumping factor can lead to considerable variation in the ionized volume. Both for a more homogeneous gas and a very inhomogeneous gas containing both dense clumps and channels with low gas density, the ionized region in the dilute gas above the galactic plane can cease to be radiation-bounded, allowing the ionizing radiation to leak into the intergalactic medium. Comparison of observed and predicted line ratios indicates that the DIG is typically ionized with a softer SED than predicted by the chosen stellar population synthesis model.