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Complex Organic Molecules in Star-Forming Regions of the Magellanic Clouds

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Complex Organic Molecules in Star-Forming Regions of the Magellanic Clouds Complex Organic Molecules in Star-Forming Regions of the Magellanic Clouds Marta Sewiło,∗,y,y Steven B. Charnley,{ Peter Schilke,x Vianney Taquet,k Joana M. Oliveira,? Takashi Shimonishi,#,@ Eva Wirström,4 Remy Indebetouw,r,yy Jacob L. Ward,zz Jacco Th. van Loon,? Jennifer Wiseman,{{ Sarolta Zahorecz,xx,kk Toshikazu Onishi,xx Akiko Kawamura,?? C.-H. Rosie Chen,## Yasuo Fukui,@@ and Roya Hamedani Golshanx yCRESST II and Exoplanets and Stellar Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA zDepartment of Astronomy, University of Maryland, College Park, MD 20742, USA {Astrochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA xI. Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937, Köln, Germany kINAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy ?Lennard-Jones Laboratories, Keele University, ST5 5BG, UK #Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Aramakiazaaoba 6-3, Aoba-ku, Sendai, Miyagi, 980-8578, Japan @Astronomical Institute, Tohoku University, Aramakiazaaoba 6-3, Aoba-ku, Sendai, Miyagi, 980-8578, Japan 4Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992, Onsala, Sweden rDepartment of Astronomy, University of Virginia, PO Box 400325, Charlottesville, VA 22904, USA yyNational Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA zzAstronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg Germany {{NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA xxDepartment of Physical Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan arXiv:1909.06843v1 [astro-ph.GA] 15 Sep 2019 kkChile Observatory, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan ??National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan ##Max-Planck-Institut fu?r Radioastronomie, Auf dem Hügel 69 D-53121 Bonn, Germany @@Nagoya University, School of Science, Furo-cho, Chikusa-ku, Nagoya, JP 464-8602 E-mail: [email protected] 1 Abstract Introduction The Large and Small Magellanic Clouds (LMC An important issue for astrochemistry is to un- and SMC), gas-rich dwarf companions of the derstand how the organic chemistry in low- Milky Way, are the nearest laboratories for de- metallicity environments (i.e., with low abun- tailed studies on the formation and survival dances of elements heavier than hydrogen or of complex organic molecules (COMs) under helium), relevant for star formation at ear- metal poor conditions. To date, only methanol, lier epochs of cosmic evolution, differs from methyl formate, and dimethyl ether have been that in the Galaxy. Large aromatic organic detected in these galaxies – all three toward molecules such as polycyclic aromatic hydro- two hot cores in the N113 star-forming region carbons (PAHs) are detected in high-redshift in the LMC, the only extragalactic sources ex- galaxies, but the formation efficiency of smaller hibiting complex hot core chemistry. We de- complex organic molecules has been unknown. scribe a small and diverse sample of the LMC A quantitative determination of the organic and SMC sources associated with COMs or hot chemistry in high-redshift galaxies with differ- core chemistry, and compare the observations ent star formation histories, and lower abun- to theoretical model predictions. Theoretical dances of the important biogenic elements (C, models accounting for the physical conditions O, N, S, P), can shed light on the inventory and metallicity of hot molecular cores in the of complex organic molecules available to plan- Magellanic Clouds have been able to broadly etary systems and their potential for harbor- account for the existing observations, but fail ing life (e.g.1). The nearest laboratories for to reproduce the dimethyl ether abundance by detailed studies of star formation under metal more than an order of magnitude. We discuss poor conditions are the Large and Small Mag- future prospects for research in the field of com- ellanic Clouds (LMC and SMC). plex chemistry in the low-metallicity environ- The LMC and SMC are gas-rich dwarf com- ment. The detection of COMs in the Magellanic panions of the Milky Way located at a distance Clouds has important implications for astrobi- of (50.0 ± 1.1) kpc3 and (62.1 ± 2.0) kpc4, re- ology. The metallicity of the Magellanic Clouds spectively. They are the nearest star-forming is similar to galaxies in the earlier epochs of galaxies with metallicities Z (the mass frac- the Universe, thus the presence of COMs in the tions of all the chemical elements other than LMC and SMC indicates that a similar prebi- hydrogen and helium) lower than that in the so- 5 otic chemistry leading to the emergence of life, lar neighborhood (Z = 0.0134) : ZLMC∼0.3– 6,7 as it happened on Earth, is possible in low- 0.5 Z and ZSMC∼0.2 Z . Apart from the metallicity systems in the earlier Universe. lower elemental abundances of gaseous C, O, and N atoms (e.g.,8), low metallicity leads to less shielding (due to the lower dust abundance; 9,10 LMC e.g., ), greater penetration of UV photons SMC into the interstellar medium, and consequently warmer dust grains (e.g.,11,12). The interstel- lar ultraviolet radiation field in the LMC and CH 3OH SMC is 10–100 higher than typical Galactic val- ues (e.g.,13). Gamma-ray observations indicate CH OCH 3 3 that the cosmic-ray density in the LMC and CH 3OCHO Spitzer SMC is, respectively, ∼25% and ∼15% of that 14,15 Accepted for publication in the ACS Earth and measured in the solar neighborhood ( ). All Space Chemistry journal on August 27, 2019. these low metallicity effects may have direct The article is part of the “Complex Organic consequences on the formation efficiency and Molecules (COMs) in Star-Forming Regions” survival of COMs, although their relative im- Special Issue. portance remains unclear. 2 LMC -66.0 -68.0 N144 N113 N159 -70.0 250 µm 160 µm 100 µm (RA, Dec) J2000 90.0 80.0 70.0 SMC N78 -72.0 250 µm 160 µm 100 µm -73.0 -74.0 (RA, Dec) J2000 24.0 20.0 16.0 12.0 Figure 1: Three-color composite mosaics of the LMC (top) and SMC (bottom), combining the Herschel/HERITAGE 250 µm (red), 160 µm (green), and 100 µm (blue) images.2 Star-forming regions harboring sources with the detection of COMs (N 113 and N 159 in the LMC and N 78 in the SMC; see Table 2) are indicated with yellow arrows and labeled. The location of the star- forming region N 144 hosting ST 11, a hot core without COMs, is indicated in white. North is up and east to the left. 3 The range of metallicities observed in the most luminous CO clouds from this survey were Magellanic Clouds is similar to galaxies at observed with 4500 resolution using the Australia redshift z ∼ 1:5 − 2, i.e. at the peak Telescope National Facility (ATNF) Mopra 22- of the star formation in the Universe (be- m Telescope (the MAGMA survey;27). tween ∼2.8 and ∼3.5 billion years after the The molecular gas in the Magellanic Clouds Big Bang; e.g.,16), making them ideal tem- was also investigated using the CO data from plates for studying star formation and com- the ESO SEST (Swedish/ESO Submillimeter plex chemistry in low-metallicity systems in Telescope) Key-Program (CO J = 1 − 0 and an earlier Universe where direct measurements 2 − 1 lines with 5000 and 2500 half-power beam of resolved stellar populations are not possi- width, HPBW;28) toward 92 and 42 positions ble. Only very recently have gaseous com- in the LMC and SMC, respectively, mostly co- plex organic molecules (methanol: CH3OH, incident with IRAS sources; the results were methyl formate: HCOOCH3, dimethyl ether: reported in multiple papers, including detailed 17–19 CH3OCH3) been detected in the LMC and studies of major star-forming giant molecu- in the SMC20, using the Atacama Large Mil- lar clouds (GMCs) and less active cloud com- limeter/submillimeter Array (ALMA). These plexes in the LMC and small molecular cloud observations showed that interstellar complex complexes in the SMC (see29 and references organic molecules (COMs) can form in the low- therein). Many other CO studies of individ- metallicity environments, probably in the hot ual star-forming regions with SEST in both the molecular cores and corinos associated with LMC and SMC followed (e.g., LMC regions massive and low- to intermediate-mass proto- N159W, N113, N44BC, and N214DE30). stars, respectively. COMs had previously been The first unbiased spectral line survey of an detected outside the Milky Way, but in galaxies individual star-forming region in the LMC was with metallicities comparable to or higher than performed by31 using SEST and centered on the solar (21 and references therein). N 159W molecular peak in N 159 – the bright- In this article, we summarize the current state est NANTEN GMC with H ii regions. The of knowledge concerning the organic chemistry observations covered a 215–245 GHz frequency in the Magellanic Clouds. range in several bands and selected bright lines at lower frequencies (∼100 GHz; HPBW∼2300 at 220 GHz and 5000 at 100 GHz). No COM Molecular Line Inventories in transitions covered by these observations (e.g., Star-Forming Regions in the propyne: CH3CCH J = 5 − 4 and J = 6 − 5 31 and CH3OCH3 J = 6−5 lines) were detected. Magellanic Clouds found that fractional abundances with respect 13 to H2 for molecules detected in N 159W ( CO, The proximity of the LMC/SMC enables both C18O, CS, SO, HCO+, HCN, HNC, CN, and detailed studies of individual star-forming re- H2CO – formaldehyde) are typically a factor of gions and statistical studies of molecular clouds 10 lower than those observed in the Galaxy.
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