Prevalence of Complex Organic Molecules in Starless and Prestellar Cores within the Taurus Molecular Cloud

Abstract

The detection of complex organic molecules (COMs) toward dense, collapsing prestellar cores has sparked interest in the fields of astrochemistry and astrobiology, yet the mechanisms for COM formation are still debated. It was originally believed that COMs initially form in ices which are then irradiated by UV radiation from the surrounding interstellar radiation field as well as forming protostars and subsequently photodesorbed into the gas-phase. However, starless and prestellar cores do not have internal protostars to heat-up and sublimate the ices. Alternative models using chemical energy have been developed to explain the desorption of COMs, yet in order to test these models robust measurements of COM abundances are needed toward representative samples of cores. We've conducted a large-sample survey of 31 starless and prestellar cores in the Taurus Molecular Cloud, detecting methanol (CH 3 OH) in 100% of the cores targeted and acetaldehyde (CH 3 CHO) in 70%. At least two transition lines of each molecule were measured, allowing us to place tight constraints on excitation temperature, column density and abundance. Additional mapping of methanol revealed extended emission, detected down to A V as low as ∼ 3 mag. We find complex organic molecules are detectable in the gas-phase and are being formed early, at least hundreds of thousands of years prior to star and planet formation. The precursor molecule, CH 3 OH, may be chemically linked to the more complex CH 3 CHO, however higher spatial resolution maps are needed to further test chemical models.

A survey of CH2DOH towards starless and pre-stellar cores in the Taurus molecular cloud

“…The deuterium fraction, f D = N(CH 2 DOH)/N(CH 3 OH), of methanol in the cores is calculated by deriving the column density of CH 2 DOH and dividing by the total column density (adding A + E symmetry states) of CH 3 OH previously published by Scibelli & Shirley (2020). CH 3 OH column densities (see Table 1) were determined using the one dimensional non-LTE RADEX code (van der Tak et al 2007).…”

“…CH 3 OH column densities (see Table 1) were determined using the one dimensional non-LTE RADEX code (van der Tak et al 2007). Three transitions of CH 3 OH were modeled assuming the core was characterized by a homogeneous average density derived from Herschel continuum observations (Palmeirim et al 2013;Marsh et al 2016) and an isothermal gas kinetic temperature derived from NH 3 observations (Seo et al 2015) with no large scale velocity motions (see Scibelli & Shirley (2020) for details). Since there are no published collision rates for CH 2 DOH, we cannot use RADEX to estimate column densities for the deuterated species.…”

“…Since a reliable single value for the CH 2 DOH T ex could not be calculated from the two observed transitions, the column densities of CH 2 DOH were calculated for a range of possible excitation temperatures using only the a-type transition. These column densities were divided by the total CH 3 OH column densities (adding A + E symmetry states) measured for the cores (Scibelli & Shirley 2020) to obtain values of f D . A plot of f D vs T ex of CH 2 DOH for each detected source shows that the resultant curves increase for T ex < 4 K, and climb again for T ex > 8 K (Fig.…”

“…In the past decade, observations have shown that complex organic molecules (molecules with more than 5 atoms containing C, H, and either N or O; Herbst & van Dishoeck 2009) are detectable towards prestellar cores (e.g., Bacmann et al 2012, Jiménez-Serra et al 2016, Soma et al 2018, Scibelli & Shirley 2020. A recent survey of 31 starless and prestellar cores in the L1495-B218 regions of Taurus showed methanol (100 percent detection rate) and acetaldehyde (70 percent detection rate) to be extremely common in starless and prestellar cores (Scibelli & Shirley 2020). Given the ★ E-mail: heha2455@email.arizona.edu (HEA) Senior Honors Thesis at The University of Arizona † E-mail: yshirley@email.arizona.edu (YLS) typical phase lifetimes of starless cores with densities of 10 5 cm −3 (André et al 2014), these results indicate complex organic molecule formation is occurring at least hundreds of thousands of years prior to the formation of a protostar.…”

“…This survey targeted low-mass (0.31-1.22 M ) starless and prestellar dense cores in the Barnard 10 (B10) region of the Taurus Molecular Cloud, which were identified in the NH 3 mapping survey of Seo et al (2015). The twelve starless and prestellar cores, Seo06-Seo17, contain the highest average abundance of methanol among the L1495-B218 regions (Scibelli & Shirley 2020). The B10 region has no embedded protostars (no Class 0/I/II protostars) (Rebull et al 2010) and is considered to be a less-evolved, quiescent region free from stellar and protostellar activity (Hacar et al 2013;Seo et al 2015).…”