4-8 October 2015
Hans Harnack Haus
Europe/Berlin timezone
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Contribution Invited Talk

Hans Harnack Haus -
FROM CLOUDS TO DENSE CORES 1

New insights on prestellar cores: deeper and stronger

Speakers

  • Dr. Charlotte VASTEL

Primary authors

Content

Within the ASAI (Astrochemical Surveys At Iram) Large Program, we conducted an unbiased spectral exploration of a carefully selected sample of template sources, which cover the full formation process of solar-type stars to understand the chemical evolution of the matter during the long process that brought it from prestellar cores and protostars to protoplanetary disks, and ultimately to the bodies of the Solar System. One of the targeted source is the prototypical prestellar core L1544 on the verge of gravitational collapse (see Caselli et al. 2012 and references within), located in the Taurus molecular cloud complex at d $\sim$ 140 pc. The observations were performed using fast Fourier transform spectrometers with a spectral resolution of 50 kHz covering a frequency range between 81 and 110 GHz. The sensitivity of the receivers allowed to push the limits and reach an rms between 3 (lower frequency) and 7 mK (higher frequency). The high sensitivity obtained for those observations led to the surprising detections of many Complex Organic Molecules (COMs) as well as tricarbon monoxide C$_3$O (Vastel et al. 2014). This prestellar core is characterized by a central high-density (2 $\times$ 10$^6$ cm$^{-3}$), low-temperature ($\sim$ 7 K), and high CO depletion, accompanied by a large degree of molecular deuteration. Its physical and dynamical structure has recently been reconstructed by Caselli et al. (2012) and Keto et al. (2014) using numerous existing observations toward L1544. Among them, we emphasize the recent detection of water vapor by the Herschel Space Observatory, the first water detection in a prestellar core, which provided key information for reconstructing the physical and chemical structure of L1544. I will present the observations of deuterated water in this core (Quénard et al., 2015). Based on a multi-line analysis (Vastel et al. 2014) of the methanol lines, we could establish that the detected COMs originate in an outer ring, where UV photons photo-evaporate methanol, whose presence in the gas phase triggers a cold gas-phase chemistry. This is the first evidence that the COMs' chemistry may be driven by the non-thermal desorption of simple ice components, namely, hydrogenated species like methanol and ethene, and not necessarily by other processes such as reactive desorption in which he exothermicity of surface chemical reactions cause the species to be desorbed after their formation (Vasyunin & Herbst 2013). I will also report the detection of the cyanomethyl radical for the first time in a prototypical prestellar core (Vastel et al., 2015). A complex structure for which we suspected the presence of the hyperfine transitions of the ortho (see Figure 1) and para forms, is observed due to the high spectral resolution of $\sim$ 0.1 km/s. We performed computation of all transition frequencies and line intensities for all transitions including satellite hyperfine components for the ortho and para forms of CH$_2$CN at the frequencies observed by ASAI. This is first detection of the fine and hyperfine structure of the ortho and para forms of cyanomethyl radical at $\sim$ 101 GHz, resolved in this cold dense core. Given the key role that the presence of COMs in prestellar cores has in understanding the general mechanisms of their formation, it is of paramount importance (1) to have a census that is as complete as possible of the COMs present in prestellar cores, (2) to better characterize where the COMs' emission comes from in these cold objects, and, as a consequence, (3) to have a better determination of their abundance, at present obtained by dividing the measured species' column density by the total H$_2$ column density of the core.