Chemical tracers of episodic accretion in low-mass protostars

Abstract

Aims. Accretion rates in low-mass protostars can be highly variable in time. Each accretion burst is accompanied by a temporary increase in luminosity, heating up the circumstellar envelope and altering the chemical composition of the gas and dust. This paper aims to study such chemical effects and discusses the feasibility of using molecular spectroscopy as a tracer of episodic accretion rates and timescales. Methods. We simulate a strong accretion burst in a diverse sample of 25 spherical envelope models by increasing the luminosity to 100 times the observed value. Using a comprehensive gas-grain network, we follow the chemical evolution during the burst and for up to 10 5 yr after the system returns to quiescence. The resulting abundance profiles are fed into a line radiative transfer code to simulate rotational spectra of C 18 O, HCO + , H 13 CO + , and N 2 H + at a series of time steps. We compare these spectra to observations taken from the literature and to previously unpublished data of HCO + and N 2 H + 6−5 from the Herschel Space Observatory. Results. The bursts are strong enough to evaporate CO throughout the envelope, which in turn enhances the abundance of HCO + and reduces that of N 2 H + . After the burst, it takes 10 3 -10 4 yr for CO to refreeze and for HCO + and N 2 H + to return to normal. The H 2 O snowline expands outwards by a factor of ∼10 during the burst; afterwards, it contracts again on a timescale of 10 2 -10 3 yr. The chemical effects of the burst remain visible in the rotational spectra for as long as 10 5 yr after the burst has ended, highlighting the importance of considering luminosity variations when analyzing molecular line observations in protostars. The spherical models are currently not accurate enough to derive robust timescales from single-dish observations. As follow-up work, we suggest that the models be calibrated against spatially resolved observations in order to identify the best tracers to be used for statistically significant source samples.

Setting the volatile composition of (exo)planet-building material

“…The second scenario is the case where the material coming from the dark cloud experiences heating events from the protostar (i.e., accretion bursts or regular stellar irradiation). These heating events are assumed to alter the chemistry in disks significantly (see, e.g., Visser et al 2015). In the extreme case, the chemistry is reset, meaning that the molecules are assumed to be dissociated into atoms out to R = 30 AU, which can then reform molecules and solids in a condensation sequence, as traditionally assumed for the inner solar nebula (e.g.…”

Protostellar accretion traced with chemistry

“…Once the burst ends, the envelope cools rapidly (Johnstone et al 2013), whereas the time scale for the molecules to refreeze back onto the dust grains is in the range 10 3 yr to 10 5 yr for typical envelope densities ranging from 10 7 cm −3 to 10 5 cm −3 (Rodgers & Charnley 2003). Observations of this out-of-equilibrium state, which manifests itself by stronger line intensities extending over larger areas of spatially resolved emission, may be used as a method to detect past accretion bursts (Lee 2007;Visser & Bergin 2012;Vorobyov et al 2013;Visser et al 2015). It is also possible to constrain the accretion history of the protostar by looking at absorption bands of interstellar ices.…”

How chemistry influences cloud structure, star formation, and the IMF

“…In regions such as cold dense clumps and cores with dust temperatures below 20 K, CO molecules will be locked up in ice. These CO molecules cannot thermally evaporate back into the gas phase and the freeze-out timescale is sufficiently short ( 10 4 yr, Visser, Bergin & Jørgensen 2015) to deplete all molecules within a cloud lifetime (10 6 yr). Other molecules, like water and methanol, await a similar fate.…”