Complex organic molecules in organic-poor massive young stellar objects

Abstract

Context. Massive young stellar objects (MYSOs) with hot cores are classic sources of complex organic molecules. The origins of these molecules in such sources, as well as the small-and large-scale differentiation between nitrogen-and oxygen-bearing complex species, are poorly understood. Aims. We aim to use complex molecule abundances toward a chemically less explored class of MYSOs with weak hot organic emission lines to constrain the impact of hot molecular cores and initial ice conditions on the chemical composition toward MYSOs. Methods. We use the IRAM 30 m and the Submillimeter Array to search for complex organic molecules over 8−16 GHz in the 1 mm atmospheric window toward three MYSOs with known ice abundances, but without luminous molecular hot cores. Results. Complex molecules are detected toward all three sources at comparable abundances with respect to CH 3 OH to classical hot core sources. The relative importance of CH 3 CHO, CH 3 CCH, CH 3 OCH 3 , CH 3 CN, and HNCO differ between the organic-poor MYSOs and hot cores, however. Furthermore, the N-bearing molecules are generally concentrated toward the source centers, while most O-and C-bearing molecules are present both in the center and in the colder envelope. Gas-phase HNCO/CH 3 OH ratios are tentatively correlated with the ratios of NH 3 ice over CH 3 OH ice in the same lines of sight, which is consistent with new gas-grain model predictions. Conclusions. Hot cores are not required to form complex organic molecules, and source temperature and initial ice composition both seem to affect complex organic distributions toward MYSOs. To quantify the relative impact of temperature and initial conditions requires, however, a larger spatially resolved survey of MYSOs with ice detections.

Complex Organic Molecules toward Embedded Low-mass Protostars∗

“…However, for molecules with similar dipole moments, rotational temperatures should be proportional to kinetic temperatures; for this reason we can compare CH 3 CHO and CH 3 CN with CH 3 OH to put qualitative constraints on their emission regions. As for previous observations of low-mass (Öberg et al 2014) and high-mass (Bisschop et al 2007;Fayolle et al 2015) protostars, we find a relatively low rotational temperature for CH 3 CHO, suggesting that it is mainly emitting from the cold outer envelope. Likewise, we find a higher rotational temperature for CH 3 CN, suggesting that it emits from closer in.…”

“…The MYSOs in Fayolle et al (2015) have large envelopes with ice detections and lack a central hot core, and should therefore represent a comparable evolutionary stage to our LYSO sample. LYSOs have slightly higher abundances relative to MYSOs of the cold molecules CH 3 CHO and HNCO, and comparable abundances for the warm molecules CH 3 OCH 3 and CH 3 CN.…”

“…We assume that all molecules within the beam can be described by a single temperature (e.g. Bisschop et al 2007;Öberg et al 2014;Fayolle et al 2015). For detected molecules, upper limits are calculated and included in the rotational diagrams for all listed transitions with an equal or greater transition strength and an upper level energy at most 15K greater than the detected lines.…”

“…Blake et al 1987;Fayolle et al 2015;Bottinelli et al 2004bBottinelli et al , 2007Arce et al 2008;Öberg et al 2010;Bacmann et al 2012;Cernicharo et al 2012). COMs formed at these early stages of star formation can become incorporated into protoplanetary disks (Visser et al 2009(Visser et al , 2011 and further into planetesimals and planets, seeding nascent planets with complex organic material.…”

ATLASGAL-selected massive clumps in the inner Galaxy

“…For temperatures lower than this value, the possibility remains that methyl acetylene is mostly locked in grains. The extended nature of the emitting region and the absence of a strong jump in abundance for CH 3 CCH in hot gas is confirmed by interferometric observations of a small sample of high-mass star-forming regions in Öberg et al (2013) and Fayolle et al (2015). These works find that only a minor fraction (10 − 15%) of the single-dish line flux is recovered by interferometric data on scales of 4 , showing that most of the emission is originated in the envelope, also for high-excitation lines.…”