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Register a new userThe abundances of O, N, S, Cl, Ne, Ar, and Fe in H II regions of the Magellanic Clouds
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Gisela Dom\\'inguez-Guzm\\'an
Publication date
2017
fields
Physics
and research's language is
English
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We use very deep spectra obtained with the Ultraviolet-Visual Echelle Spectrograph in the Very Large Telescope in order to determine the physical conditions, the chemical abundances and the iron depletion factors of four H II regions of the Large Magellanic Cloud and four H II regions of the Small Magellanic Cloud. The spectral range covered is 3100-10400 $mathring{A}$ with a resolution of $Deltalambda sim lambda / 8800$. We measure the intensity of up to 200 emission lines in each object. Electron temperature and electron density are determined using different line intensity ratios. The ionic and total abundances are derived using collisionally excited lines for O, N, S, Cl, Ne, Ar, and Fe. The uncertainties are calculated using Monte Carlo simulations. This is the largest available set of high quality spectra for H II regions in the Magellanic Clouds. Thus, we can derive chemical abundances and depletion factors and constrain their variations across each galaxy with better accuracy than previous studies. In particular, we find that the amount of Fe depleted on to dust grains in the H II regions of the Magellanic Clouds is similar to that found in Galactic H II regions.
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Dynamical expansion of H II regions around star clusters plays a key role in dispersing the surrounding dense gas and therefore in limiting the efficiency of star formation in molecular clouds. We use a semi-analytic method and numerical simulations to explore expansion of spherical dusty H II regions and surrounding neutral shells and the resulting cloud disruption. Our model for shell expansion adopts the static solutions of Draine (2011) for dusty H II regions and considers the contact outward forces on the shell due to radiation and thermal pressures as well as the inward gravity from the central star and the shell itself. We show that the internal structure we adopt and the shell evolution from the semi-analytic approach are in good agreement with the results of numerical simulations. Strong radiation pressure in the interior controls the shell expansion indirectly by enhancing the density and pressure at the ionization front. We calculate the minimum star formation efficiency $epsilon_{min}$ required for cloud disruption as a function of the clouds total mass and mean surface density. Within the adopted spherical geometry, we find that typical giant molecular clouds in normal disk galaxies have $epsilon_{min} lesssim 10$%, with comparable gas and radiation pressure effects on shell expansion. Massive cluster-forming clumps require a significantly higher efficiency of $epsilon_{min} gtrsim 50$% for disruption, produced mainly by radiation-driven expansion. The disruption time is typically of the order of a free-fall timescale, suggesting that the cloud disruption occurs rapidly once a sufficiently luminous H II region is formed. We also discuss limitations of the spherical idealization.
The Large and Small Magellanic Clouds (LMC and SMC), gas-rich dwarf companions of the Milky Way, are the nearest laboratories for detailed studies on the formation and survival of complex organic molecules (COMs) under metal poor conditions. To date, only methanol, methyl formate, and dimethyl ether have been detected in these galaxies - all three toward two hot cores in the N113 star-forming region in the LMC, the only extragalactic sources exhibiting complex hot core chemistry. We describe a small and diverse sample of the LMC and SMC sources associated with COMs or hot core chemistry, and compare the observations to theoretical model predictions. Theoretical models accounting for the physical conditions and metallicity of hot molecular cores in the Magellanic Clouds have been able to broadly account for the existing observations, but fail to reproduce the dimethyl ether abundance by more than an order of magnitude. We discuss future prospects for research in the field of complex chemistry in the low-metallicity environment. The detection of COMs in the Magellanic Clouds has important implications for astrobiology. The metallicity of the Magellanic Clouds is similar to galaxies in the earlier epochs of the Universe, thus the presence of COMs in the LMC and SMC indicates that a similar prebiotic chemistry leading to the emergence of life, as it happened on Earth, is possible in low-metallicity systems in the earlier Universe.
We have compared Monte Carlo photoionization models of H II regions with a uniform density distribution with models with the same central stars and chemical compositions but with 3-D hierarchical clumps. We compare the abundances of He, N, O, Ne, and S obtained from emission line strengths and [O III] and [N II] temperatures to those in our models. We consider stellar temperatures in the range 37.5 -- 45kK and ionizing luminosities from 10^{48} to 10^{51} photons/s. Clumped models have different ionic abundances than uniform. For hot stars, He^0/He^+ is 2 -- 3%, much larger than with uniform models. This amount of He I is independent of metallicity and so impacts the determination of the primordial abundance of He. The total abundances of O, Ne, and S obtained by the usual methods of analysis, using T([OIII) for high stages of ionization and T([NII]) for low, are about as accurate for clumped models as for uniform and within about 20% of the true values. If T([OIII]) is used for analyzing all ions, the derived (O/H) is 40 to 60% too large for cool stars but is good for hot stars. Uniform models have similar errors, so the clumping does not change the accuracy of abundance analysis. The physical causes of the ionic abundance errors are present in real nebulae. In clumped models, helium ionizing radiation from zones of high ionization (low He^0 and low UV opacity) can penetrate nearby regions near the edge of the ionized zone. This effect allows He^0 to absorb more stellar photons than in uniform or radially symmetrical geometries. In turn, these absorptions compete with O+, etc., for those energetic stellar photons.
We report on a direct method to measure the internuclear potential energy curve of diatomic systems. A COLTRIMS reaction microscope was used to measure the squares of the vibrational wave functions of H$_{2}$, He$_{2}$, Ne$_{2}$, and Ar$_{2}$. The Schrodinger equation relates the curvature of the wave function to the potential V(R) and therefore offers a simple but elegant way to extract the shape of the potential.
We study the 158 micron [CII] fine-structure line emission from star-forming regions as a function of metallicity. We have measured and mapped the [CII] emission from the very bright HII region complexes N 11 in the LMC and N 66 in the SMC, as well as the SMC HII regions N 25, N 27, N 83/N 84, and N 88, with the FIFI instrument on the Kuiper Airborne Observatory. In both the LMC and SMC, the ratio of the [CII] line to the CO line and to the far-infrared continuum emission is much higher than seen almost anywhere else, including Milky Way star-forming regions and whole galaxies. In the low metallicity, low dust-abundance environment of the LMC and the SMC, UV mean free path lengths are much greater than those in the higher-metallicity Milky Way. The increased photoelectric heating efficiencies cause significantly greater relative [CII] line emission strengths. At the same time, similar decreases in PAH abundances have the opposite effect, by diminishing photoelectric heating rates. Consequently, in low-metallicity environments the relative [CII] strengths are high but exhibit little further dependence on actual metallicity. Relative [CII] strengths are slightly higher in the LMC than in the SMC, which has both lower dust and lower PAH abundances.
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