Tracing the evolutionary stage of Bok globules: CCS and NH₃

Abstract

Aims. We investigate a previously proposed correlation between the chemical properties and the physical evolutionary stage of isolated low-mass star-forming regions. The N NH 3 /N CCS abundance ratio has been proposed to be a potentially useful indicator of the evolutionary stage of cloud cores, and we study its applicability for isolated Bok globules. Methods. We searched for CCS(2 1 -1 0 ) emission in 42 Bok globules both with and without signs of current star formation. A set of NH 3 measurements was compiled from measurements available in the literature and from our own observations. The abundance ratio of both molecules is discussed with respect to the evolutionary stage of the objects and in the context of chemical models.Results. We determine the N NH 3 /N CCS ratio for 18 Bok globules and find that it is moderately high and roughly similar across all evolutionary stages from starless and prestellar cores towards internally heated cores harboring protostars of Class 0, Class I, or later. We do not find any Bok globules with extremely high CCS abundances analogous to carbon-chain producing regions in dark cloud cores. The observed range of N NH 3 /N CCS implies that all of the observed Bok globules are in a relatively evolved chemical state.

Physical Conditions of the Earliest Phases of Massive Star Formation: Single-Dish and Interferometric Observations of Ammonia and CCS in Infrared Dark Clouds

“…They found that the [CCS]/[N 2 H + ] ratios of even 24 µm dark cores were typically less than 1 and concluded that these cores were more chemically evolved than low-mass starless cores where the ratio is 2.6-3.2. Marka et al (2012), however, found no variation in the column density ratio, N (CCS)/N (NH 3 ), with evolutionary state of Bok globules, the low-mass dark cloud analogs to IRDCs. All of these studies targeted dense cores but were performed with single-dish observations.…”

An Ammonia Spectral Map of the L1495-B218 Filaments in the Taurus Molecular Cloud. II. CCS and HC7N Chemistry and Three Modes of Star Formation in the Filaments

“…Benson et al (1998) found no clear trend of [CCS]/[N 2 H + ] as a function of the evolutionary stage of the 20 dense cores with both CCS and N 2 H + emission in their survey of 60 dense cores. Marka et al (2012) also showed that the isolated Bok globules in their survey do not have a clear anti-correlation of the N(CCS)/N(NH 3 ) ratio with evolutionary state moving from starless cores to protostellar cores. Observations toward infrared dark clouds (IRDCs) in NH 3 and CCS using the GBT and the VLA do not show any evidence of a trend in N(CCS)/N(NH 3 ) from starless to protostellar clumps but rather have highly scattered values (Dirienzo et al 2015).…”

“…CCS is a carbon-chain molecule which is frequently found in dense cores in nearby, low-mass star-forming clouds (e.g., Suzuki et al 1992;Wolkovitch et al 1997;Rathborne et al 2008;Roy et al 2011;Marka et al 2012;Tatematsu et al 2014a). Yet, what physical and chemical properties of star-forming clouds can be probed by CCS and how CCS traces the chemical evolution during star formation are still debated.…”

Characterising the physical and chemical properties of a young Class 0 protostellar core embedded in the Orion B9 filament

“…14 Compared to the aforementioned reference studies, the ammonia abundance is SMM3 appears to be elevated by a factor of about two or more, although differences in the assumptions of dust properties should be borne in mind. The chemical modelling of the Class 0 sources performed by Marka et al (2012), which included reactions taking place on dust grain surfaces, predicted that an NH 3 abundance exceeds ∼ 10 −8 after 10 5 yr of evolution (see also Hily-Blant et al 2010 for a comparable result). This compares well with the fragmentation timescale in SMM3 estimated above.…”

“…Their sources might represent the sites of triggered star formation, and could therefore resemble the case of SMM3 -a core that might have initially formed as a result of external feedback. More recently, Marka et al (2012) found that the average NH 3 abundance in their sample of globules hosting Class 0 protostars is 3 × 10 −8 with respect to H 2 . 14 Compared to the aforementioned reference studies, the ammonia abundance is SMM3 appears to be elevated by a factor of about two or more, although differences in the assumptions of dust properties should be borne in mind.…”