Found 16 talks width keyword HII regions

We present some results on a project based on high-resolution UVES@VLT spectroscopy and HST imaging of photoionized Herbig-Haro (HH) objects in the Orion Nebula. We study physical conditions, chemical abundances and other properties such as proper motions and the origin of the driving jets. Our study will include at least 9 HH objets, of which we will focus on HH529II, HH529III and HH204 in this talk. Our data allow us to separate the spectrum of the outflows from the main nebular emission, studying each object with an unprecedented detail. The HHs are located at different distances from the main ionization source of the Orion Nebula, with different ionization/physical conditions and flow velocities. In all objects, the electronic density (ne) is substantially higher than in the surrounding Orion Nebula, while the electronic temperature (Te) is maintained under photoionization equilibrium for the most abundant ion stages. In HH204 we observe a Te([OIII]) gradient due to the contribution of [OIII] emission from the cooling layer behind the bow shock, which is also detected in the HST imaging. The ionization degree of the gas in the different HH objects is very different, allowing us to determine the chemical composition of the Orion Nebula under both ionization conditions, avoiding the use of ionization correction factors (ICFs) for many elements. HH204 shows an abundance discrepancy  -the difference between abundances derived from recombination and collisionally excited lines- thas is actually zero. We find direct evidence of dust destruction in the bow shock in all objects. This increases the gaseous abundances of Fe, Ni and Cr with respect to the Orion Nebula ones. We show that a failure to resolve the different kinematic components -as in a low spectral resolution spectrum- can lead to significant error in the determination of chemical abundances -40% underestimate of O in the case of HH204-, mainly due to incorrect estimation of the electron density.

Molecular hydrogen (H2) is a fundamental component of galaxies, being the most abundant element in molecular clouds, where stars form, and an important source of radiative cooling at low temperature. With the advent of the ALMA telescope, a large amount of data about the distribution of H2 in galaxies has become available. However, the large majority of numerical simulations on galactic and cosmological scales still lacks the ability to directly follow the formation and dissociation of H2, and must rely on pre-calibrated sub-grid models to compare the results with observations. I will present a new model to self-consistently track the evolution of H2, including gas and dust shielding, H2 self-shielding, star formation (SF), supernova feedback, and extragalactic and local stellar radiation. I will discuss the results of a suite of hydrodynamic simulations of an isolated gas-rich galaxy at z=3, showing that the model can naturally reproduce the observed correlation between SF and H2 surface densities, without assuming any a priori dependence of SF on the H2 abundance. I will also present a study of the kinematics and dynamics of molecular gas in high-redshift quasars (z=6), where we investigate whether a central accreting black hole (BH) can significantly affect the H2 distribution in the host galaxy and generate molecular outflows.

From the structure of PHL 293B and the physical properties of its ionizing cluster and based on results of hydrodynamic models, we point at the various events required to explain in detail the emission and absorption components seen in its optical spectrum. We ascribe the narrow and well centered emission lines, showing the low metallicity of the galaxy, to an HII region that spans through the main body of the galaxy. The broad emission line components are due to two off-centered supernova remnants evolving within the ionizing cluster volume and the absorption line profiles are due to a stationary cluster wind able to recombine at a close distance from the cluster surface as originally suggested by Silich et al. 2004. Our numerical models and analytical estimates confirm the ionized and neutral column density values and the inferred X-ray emission derived from the observations.

There is an increasing multiplicity of proposed methodologies to derive chemical abundances in HII regions from the measurement of the relative fluxes of their optical emission lines. Particularly there is a known discrepancy between the prediction of some widely used grids of photoionization models and the results of the direct analysis of the spectra from their integrated physical properties (i.e. density, temperature). In this seminar, I will introduce HII-CHI-mistry, a Chi square approximation to compare observations with results of a large grid of models calculated using CLOUDY and varying the oxygen abundance, the nitrogen-to-oxygen ratio and the ionization parameter, covereing all possible conditions observed so far in massive complexes of star-formation. Including N/O as an additional variable allows the correct interpretation of the [NII] 6584 emission lines, widely used to derive abundances both in the Local and the Early Universe in the infra-red part of the spectrum.

The use of this method leads to a derivation of both Z and N/O totally consistent with the results from the direct method when emission line ratios sensitive to the temperature are available (e.g. [OIII] 50007/4363). On the contrary, when these ratios are not available, what is the most common situation in metal-rich/distant objects, it is necessary to assume empirical constraints to the space of parameters covered by the model-grid to arrive to solutions in the whole range of metallicity. Among the applications of this methods it is a consistent study of the metallicity in a wide range of potential variations (e.g. gradients of Z in disc galaxies, mass-metallicity relation, etc ...)

The direct accretion of pristine gas streams is predicted to be the main mode of galaxy disk growth in the early universe (cold-flows). We (think we) have discovered this physical process at work in the local Universe. The finding is one of the outcomes of our in-depth study of local extremely metal poor (XMP) galaxies. I will explain the main observational properties of XMPs, in particular, their tendency to have cometary or tadpole morphology, with a bright peripheral clump (the head) on a faint tail. Tadpole galaxies are rare in the nearby universe but turn out to be very common at high redshift, where they are usually interpreted as disk galaxies in early stages of assembling. We have found the heads to be giant HII regions displaced with respect to the rotation center, with the galaxy metallicity being smallest at the head and larger elsewhere. The resulting chemical abundance gradient is opposite to the one observed in local spirals, and suggests a recent gas accretion episode onto the head. Thus, local XMP galaxies seem to be primitive disks, with their star formation sustained by accretion of external metal poor gas. I will argue how the same mechanism may be driving the star formation in many other local galaxies. Ongoing observational projects to confirm these findings and conjectures will be briefly mentioned.

The fate of ionizing radiation from massive stars has fundamental consequences on scales ranging from the physics of circumstellar disks to the ionization state of the entire universe. On galactic scales, the radiative feedback from massive stars is a major driver for the energetics and phase balance of the interstellar medium in star-forming galaxies. While even starburst galaxies appear to be largely optically thick in the Lyman continuum, ionization-parameter mapping shows that significant populations of HII regions within galaxies are optically thin, powering the diffuse, warm ionized medium. I will discuss our multi-faceted work to clarify our understanding of radiative feedback in star-forming galaxies from the Magellanic Clouds to starbursts.

Morphologies of star-forming galaxies at z>1 are typically irregular containing a handful of dominant bright regions. Recent observational evidence suggest that many of these galaxies are governed by disc-like rotation. Using Halpha galaxy kinematics from OSIRIS+LGSAO we find that within z~1 turbulent discs star-forming regions have average sizes of 1.5 kpc and average Jeans masses of 4.2x10^9 \Msun, in total accounting for 20-30% of the stellar mass of the discs. These findings lend observational support to models that predict larger star-forming regions will form as a result of higher disc velocity dispersions driven-up by cosmological gas accretion.  As a consequence of the changes in global environment, it may be predicted that star-forming regions at high redshift should not resemble star-forming regions locally. Yet despite the increased sizes and dispersions, high-z star-forming regions and HII regions are found to follow tight scaling relations over the range z=0-2 for Halpha size, velocity dispersion, luminosity and mass when comparing >2000 HII regions locally and 30 regions at z>1.  While the turbulence of discs may have important implications for the size and luminosity of regions which form within them, the same processes likely govern their formation from high redshift to the current epoch. We are now able to test this conclusion with first results from a new sample of z=0.1-0.2 highly star-forming turbulent galaxies from the Sloan Digital Sky Survey.

The spectral analysis of HII regions allows one to determine the chemical composition of the ionized gas phase of the interstellar medium (ISM) from the solar neighborhood to the high-redshift galaxies. Therefore, it stands as an essential tool for our knowledge of the chemical evolution of the Universe. However, it turns out that chemical abundances of heavy-element ions determined from the bright collisionally excited lines (CELs) are systematically lower than the abundances derived from the faint recombination lines (RLs) emitted by the same ions. Today, this controversial issue is known as abundance discrepancy problem and it is far from negligible. In the analysis of Galactic and extragalactic HII regions the O2+/H+ ratio calculated from the OII RLs is between 0.10 and 0.35 dex higher than that obtained from the [OIII] CELs. In this talk, we will face this problem in the benchmark object of the solar vicinity, the Orion Nebula. Due to its high surface brightness and proximity, the Orion Nebula is an ideal lab, which allows us to study in detail the possible role of its rich and well-resolved internal structure (such as Herbig-Haro objects, protoplanetary disks or bars) on the abundance discrepancy.

We have selected the Galactic HII region M43, a close-by apparently spherical nebula ionized by a single star (HD37061, B0.5V) to investigate several topics of recent interest in the field of HII regions and massive stars. We perform a combined, comprehensive study
of the nebula and its ionizing star by using as many observational constraints as possible. For this study we collected a set of high-quality observations, including the optical spectrum of HD3706, along with nebular optical imaging and long-slit spatially resolved spectroscopy. On the one hand, we have carried out a quantitative spectroscopic analysis of the ionizing star from which we have determined the stellar parameters of HD37061 and the total number of ionizing photons emitted by the star; on the other hand, we have done a
empirical analysis of the nebular images and spectroscopy from which we have find observational evidence of scattered light from the Huygens region (the brightest part of the Orion nebula) in the M43 region. We show the importance of an adequate correction of this scattered light in both the imagery and spectroscopic observations of M43 in accurately determining the total nebular Halpha luminosity, the nebular physical
conditions. and chemical abundances. We have computed total abundances for three of the analyzed elements (O, S, and N), directly from
observable ions (no ionization correction factors are needed). The comparison of these abundances with those derived from the spectrum of the Orion nebula indicates the importance of the atomic data and, specially in the case of M42, the considered ionization correction factors.

In this talk I consider two questions. First, I investigate the formation of molecular clouds from diffuse interstellar gas. It has been argued that the midplane pressure controls the fraction of molecular hydrogen present, and thus the star formation rate. Alternatively, I and others have suggested that the gravitational instability of the disk controls both. I present numerical results demonstrating that the observed correlations between midplane pressure, molecular hydrogen fraction, and star formation rate can be explained within the gravitational instability picture. Second, I discuss how ionization affects the formation of massive stars. Although most distinctive observables of massive stars can be traced back to their ionizing radiation, it does not appear to have a strong effect on their actual formation. Rather, I present simulations suggesting that stars only ionize large volumes after their accretion has already been throttled by gravitational fragmentation in the accretion flow. At the same time these models can explain many aspects of the observations of ultracompact H II regions.