A&A 624, L5 (2019) Astronomy https://doi.org/10.1051/0004-6361/201935267 & c L. Pagani et al. 2019 Astrophysics LETTER TO THE EDITOR The complexity of Orion: an ALMA view III. The explosion impact? L. Pagani1, E. Bergin2, P. F. Goldsmith3, G. Melnick4, R. Snell5, and C. Favre6 1 LERMA & UMR8112 du CNRS, Observatoire de Paris, PSL University, Sorbonne Universités, CNRS, 75014 Paris, France e-mail: [email protected] 2 Department of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Ave, Ann Arbor, MI 48109, USA 3 JPL, Pasadena, CA, USA 4 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA 5 Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA 6 Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France Received 14 February 2019 / Accepted 16 March 2019 ABSTRACT The chemistry of complex organic molecules in interstellar dark clouds is still highly uncertain in part because of the lack of con- straining observations. Orion is the closest massive star-forming region, and observations making use of ALMA allow us to separate the emission regions of various complex organic molecules (COMs) in both velocity and space. Orion also benefits from an excep- tional situation, in that it is the site of a powerful explosive event that occurred ∼550 years ago. We show that the closely surrounding Kleinmann-Low region has clearly been influenced by this explosion; some molecular species have been pushed away from the dens- est parts while others have remained in close proximity. This dynamical segregation reveals the time dependence of the chemistry and, therefore allows us to better constrain the formation sequence of COMs and other species, including deuterated molecules. Key words. astrochemistry – ISM: kinematics and dynamics – ISM: clouds – evolution – ISM: molecules – ISM: individual objects: Orion KL 1. Introduction results including the detection of several species not previously seen in Orion (n– and i–propyl cyanide, C H CN, . ) as well Though Orion is a well-studied region and has been explored 3 7 as several vibrationally excited levels of cyanoacetylene (HC3N) with a wide variety of instruments, including the NOEMA and of its 13C isotopologues. A companion paper (Favre et al. (former Plateau de Bure) Interferometer, the Berkeley-Illinois- 2017) presents the first detection of gGg0 ethylene glycol (gGg0 Millimeter-Array (BIMA), the Combined Array for Research 1 (CH2OH)2) and of acetic acid (CH3COOH ) in Orion. in Millimeter-wave Astronomy (CARMA), and the Submillime- One remarkable feature present in the central region of Orion ter Array (SMA), the arrival of the Atacama Large Millimeter is an explosive event that occurred 550 ± 25 years ago (J. Bally, Array (ALMA) holds the promise of new discoveries thanks to priv. comm.) and was revealed by the three runaway stars BN, n, its higher angular resolution and sensitivity. The ALMA instru- and I (Gómez et al. 2005; Rodríguez et al. 2005, 2017), and by the ment provides high-velocity resolution while maintaining a high CO and H2 fingers (e.g., Allen & Burton 1993; Zapata et al. 2009; dynamic range that enables the detection of previously unseen Nissen et al. 2012; Youngblood et al. 2016; Bally et al. 2017). structures, leading to a better understanding of the source struc- However, Luhman et al.(2017) showed that the object n is no ture and evolution. longer a runaway member because its real proper motion is much Based on the search for the lowest observable rotational tran- 16 18 lower than previously estimated; but conversely another object, sition of O O at 234 GHz, we performed deep observations named x, is moving away at high speed from the same explo- of this source during Cycle 2 with 37–39 antennas, surveying sion center. J. Bally (priv. comm.) confirms the fast proper motion 16 GHz in ALMA band 6, and improving the sensitivity by a of x and the absence of movement of n from ground-based H ∼ 2 factor 5 compared to the Cycle 0 Science Verification (SV0) images 14 years apart. X is further out having passed our 20% observations for the frequencies in common (Pagani et al. 2017, beam coupling mark (see Fig. 1 of Paper I), and therefore does hereafter Paper I). Paper I presents a more detailed history of the not appear in the figures presented in this Letter. Zapata et al. recent work on Orion. In Paper I, we present the data and first (2011) and Orozco-Aguilera et al.(2017) in their follow-up work ? This paper makes use of the following ALMA data: with ALMA proposed that the hot core (HC) is externally ADS/JAO.ALMA#2013.1.00533.S. ALMA is a partnership of heated despite its high temperature, and that the heating source ESO (representing its member states), NSF (USA) and NINS (Japan), could be the nearby explosion. Similarly, Blake et al.(1987), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The 1 Though not explicitly announced nor discussed in their paper, acetic Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. acid was already identified in Cernicharo et al.(2016). L5, page 1 of 10 Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A&A 624, L5 (2019) Wang et al.(2011), and Favre et al.(2011) advocated that the elongations that can be traced back to the 550-year-old explo- Compact Ridge is also externally heated, although the heating sion center as suggested by the yellow arrows (Fig.1). Not only source should not be the same since we presented evidence in do the lowest contours form convex structures oriented away Paper I that the Compact Ridge has not yet been affected by the from the explosion center but the peak emission is always dis- impact of the explosion. A possibility could be the outflow from placed outward with respect to the local continuum peaks. The source I hitting the Compact Ridge (Liu et al. 2002). In Paper I, we offset is especially strong southeastward in the 9 and 10 km s−1 also presented evidence for an interaction between the explosive channel maps where the maximum emission from MF6 and the event and the main components of the Orion KL region including EGP is clearly displaced (sources are labeled in Fig. 1 of Paper I the HC, several infrared (IR) components (Rieke et al. 1973), and and in Fig. A.1). There is also a northwest elongation past MF5 −1 methyl formate (CH3OCHO; hereafter MF) peaks (Favre et al. between 7 and −8 km s . This feature is also clearly seen in 2011). We showed that the IRc6/MF5 and IRc20/MF4 sources, OCS and D2CO, and in other species such as NO but at a much west of the explosion center, display emission lines of various lower intensity level (∼0.15 Jy beam−1 for NO). Similarly, the species having only red wings, while sources on the east and south emission is neatly stretching away from the IRc7 spot at veloc- sides display emission lines having only blue wings. We also con- ities 3–6 km s−1. The elongated emission north of the HC east firmed that excited emission lines are found preferentially sur- from the explosion center, typically at 0 to 3 km s−1, is seen rounding the explosion center and that complex organic molecules (Fig. B.1) in all species that are emitting strongly enough in that (COMs) rich in oxygen (O-COMs) do not occupy the same vol- velocity range. umes as CN rich COMs (CN-COMs). We identified the ethylene Spatial velocity cuts along the two most prominent northeast glycol peak (EGP) to be coincident with a hollow sphere of mate- and southwest features of the H2CO maps reveal essentially con- rial, which we interpreted to have originated from the impact of stant velocity (<1 km s−1 shift) over their ∼5–1000 (0.01–0.02 pc) a “bullet” launched from the explosion center (Favre et al. 2017; extent. The dynamical time of these features cannot be inferred Wright & Plambeck 2017). We also proposed that the Compact from their line-of-sight velocity since these movements are close Ridge (MF1) is sufficiently far away from the rest of the KL region to the plane of the sky with relative velocities of only one to a −1 00 to have not yet been perturbed by the explosion, the evidence being few km s . Conversely, the longest H2CO elongation, ∼10 can the absence of asymmetric emission line wings and the narrow- be attained in 550 years if the gas is moving at ∼40 km s−1. This ness of the lines themselves (∼1 km s−1). In this Letter, we study is much lower than the maximum 12CO clump velocities noticed further the interaction of the explosion blowout with the surround- in various data sets including our own, on the order of 140 km s−1 ing gas and dense sources. (Zapata et al. 2009; Bally et al. 2017). It is remarkable that a number of species do not show these var- ious elongations (except that at ∼2 km s−1) and remain close to the 2. Observations dust emission peaks. This is the case of the H2CNH shown (Fig.2) but also of, for example, CH OCH , CH OCHO, NH CHO, and The observations have been described in detail in Paper I, and we 3 3 3 2 NH2D.