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Register a new userC+ Emission from the Magellanic Clouds II. [CII] maps of star-forming regions LMC-N 11, SMC-N 66, and several others
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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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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.
The Magellanic Clouds (MCs) offer an outstanding variety of young stellar associations, in which large samples of low-mass stars (with masses less than 1 solar mass) currently in the act of formation can be resolved and explored sufficiently with the Hubble Space Telescope. These pre-main sequence (PMS) stars provide a unique snapshot of the star formation process, as it is being recorded for the last 20 Myr, and they give important information on the low-mass Initial Mass Function (IMF) of their host environments. We present the latest results from observations with the Advanced Camera for Surveys (ACS) of such star-forming regions in the MCs, and discuss the importance of Hubble}for a comprehensive collection of substantial information on the most recent low-mass star formation and the low-mass IMF in the MCs.
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.
We have selected 43 southern massive star-forming regions to study the spatial distribution of HNCO 4$_{04}$-3$_{03}$, SiO 2-1 and HC$_{3}$N 10-9 line emission and to investigate their spatial association with the dust emission. The morphology of HNCO 4$_{04}$-3$_{03}$ and HC$_{3}$N 10-9 agrees well with the dust emission. HC$_{3}$N 10-9 tends to originate from more compact regions than HNCO 4$_{04}$-3$_{03}$ and SiO 2-1. We divided our sources into three groups: those in the Central Molecular Zone (CMZ), those associated with bubbles (Bubble), and the remaining sources, which are termed normal star forming regions (NMSFR). These three groups, subdivided into three different categories with respect to line widths, integrated intensities, and column densities, hint at the presence of different physical and chemical processes. We find that the dust temperature $T_{rm d}$, and the abundance ratios of $N_{rm HNCO}/N_{rm SiO}$ and $N_{rm HNCO}/N_{rm HC3N}$ show a decreasing trend towards the central dense regions of CMZ sources, while $N_{rm HC3N}/N_{rm SiO}$ moves into the opposite direction. Moreover, a better agreement is found between $T_{rm d}$ and $N_{rm HC3N}/N_{rm SiO}$ in Bubble and NMSFR category sources. Both outflow and inflow activities have been found in eight of the sixteen bubble and NMSFR sources. The low outflow detection rate indicates that in these sources the SiO 2-1 line wing emission is either below our sensitivity limit or that the bulk of the SiO emission may be produced by the expansion of an H{sc,ii} region or supernova remnant, which has pushed molecular gas away forming a shock and yielding SiO.
We report the first detection of C$^{15}$N in diffuse molecular gas from a detailed examination of CN absorption lines in archival VLT/UVES spectra of stars probing local diffuse clouds. Absorption from the C$^{15}$N isotopologue is confidently detected (at $gtrsim4sigma$) in three out of the four directions studied and appears as a very weak feature between the main $^{12}$CN and $^{13}$CN absorption components. Column densities for each CN isotopologue are determined through profile fitting, after accounting for weak additional line-of-sight components of $^{12}$CN, which are seen in the absorption profiles of CH and CH$^+$ as well. The weighted mean value of C$^{14}$N/C$^{15}$N for the three sight lines with detections of C$^{15}$N is $274pm18$. Since the diffuse molecular clouds toward our target stars have relatively high gas kinetic temperatures and relatively low visual extinctions, their C$^{14}$N/C$^{15}$N ratios should not be affected by chemical fractionation. The mean C$^{14}$N/C$^{15}$N ratio that we obtain should therefore be representative of the ambient $^{14}$N/$^{15}$N ratio in the local interstellar medium. Indeed, our mean value agrees well with that derived from millimeter-wave observations of CN, HCN, and HNC in local molecular clouds.
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