Molecular Ions in L1544. I. Kinematics

Spezzano

et al. 2014

A&A

Self Cite

103

152

Context. High methanol (CH 3 OH) deuteration has been revealed in Class 0 protostars with the detection of singly, doubly, and even triply D-substituted forms. Methanol is believed to form during the pre-collapse phase via gas-grain chemistry and then eventually injected into the gas when the heating produced by the newly formed protostar sublimates the grain mantles. The molecular deuterium fraction of the warm gas is thus a relic of the cold pre-stellar era and provides hints of the past history of the protostars. Aims. Pre-stellar cores represent the preceding stages in the process of star formation. We aim at measuring methanol deuteration in L1544, a prototypical dense and cold core on the verge of gravitational collapse. The aim is to probe the deuterium fractionation process while the "frozen" molecular reservoir is accumulated onto dust grains. Methods. Using the IRAM 30 m telescope, we mapped the methanol emission in the pre-stellar core L1544 and observed singly deuterated methanol (CH 2 DOH and CH 3 OD) towards the dust peak of L1544. Non-LTE radiative transfer modelling was performed on three CH 3 OH emissions lines at 96.7 GHz, using a Bonnor-Ebert sphere as a model for the source. We have also assumed a centrally decreasing abundance profile to take the molecule freeze-out in the inner core into account. The column density of CH 2 DOH was derived assuming LTE excitation and optically thin emission. Results. The CH 3 OH emission has a highly asymmetric morphology, resembling a non-uniform ring surrounding the dust peak, where CO is mainly frozen onto dust grains. The observations provide an accurate measure of methanol deuteration in the cold pre-stellar gas. The derived abundance ratio is [CH 2 DOH]/[CH 3 OH] = 0.10 ± 0.03, which is significantly smaller than the ones found in lowmass Class 0 protostars and smaller than the deuterium fraction measured in other molecules towards L1544. The singly-deuterated form CH 3 OD was not detected at 3σ sensitivity of 7 mK km s −1 , yielding a lower limit of [CH 2 DOH]/[CH 3 OD] ≥ 10, consistent with previous measurements towards Class 0 protostars. Conclusions. The low deuterium fractionation observed in L1544 and the morphology of the CH 3 OH emission suggest that we are mainly tracing the outer parts of the core, where CO just started to freeze-out onto dust grains.

Section: Singly Deuterated Methanol (Ch 2 Doh)supporting

confidence: 80%

Deuterated methanol in the pre-stellar core L1544

Bizzocchi

Spezzano

et al. 2014

A&A

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103

152

“…3 is that in L1521F there is evidence for an "infall signature" although less marked than in L1544, where it is attributed to extended infall onto the core (Williams et al 1999) or to central infall in a depleted core nucleus (Caselli et al 2002a). In fact, we see evidence for Table 2.…”

Section: Integrated Intensity Maps and Continuum Emissionmentioning

confidence: 98%

“…6.472 ± 0.001 0.299 ± 0.001 17.9 ± 0.1 5.859 ± 0.016 4.94 ± 0.03 N 2 H + (3−2) 6.388 ± 0.010 a 0.290 ± 0.030 1.4 ± 1.0 0.692 ± 0.075 5.1 ± 3.7 N 2 D + (2−1) 6.505 ± 0.004 0.268 ± 0.010 2.2 ± 0.4 0.978 ± 0.026 4.6 ± 0.9 N 2 D + (3−2) 6.507 ± 0.006 0.222 ± 0.024 1.2 ± 0.9 0.332 ± 0.029 4.1 ± 3.2 NOTE: a The N 2 H + (3−2) frequency is as in Caselli et al (2002a), whereas for the other lines we used the updated values determined by Dore et al (2004). b The integration includes all hyperfines.…”

Section: Integrated Intensity Maps and Continuum Emissionmentioning

confidence: 99%

Observations of L1521F: A highly evolved starless core

Crapsi

Walmsley

et al. 2004

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161

Abstract. We observed the pre-stellar core L1521F in dust emission at 1.2 mm and in two transitions each of N 2 H + , N 2 D + , C 18 O and C 17 O in order to increase the sample of well studied centrally concentrated and chemically evolved starless cores, likely on the verge of star formation, and to determine the initial conditions for low-mass star formation in the Taurus Molecular Cloud. The dust observation allows us to infer the density structure of the core and together with measurements of CO isotopomers gives us the CO depletion. N 2 H + and N 2 D + lines are good tracers of the dust continuum and thus they give kinematic information on the core nucleus. We derived in this object a molecular hydrogen number density n(H 2 ) ∼ 10 6 cm −3 and a CO depletion factor, integrated along the line of sight, f D ≡ 9.5 × 10 −5 /x obs (CO) ∼ 15 in the central 20 , similar to the pre-stellar core L1544. However, the N(N 2 D + )/N(N 2 H + ) column density ratio is ∼0.1, a factor of about 2 lower than that found in L1544. The observed relation between the deuterium fractionation and the integrated CO depletion factor across the core can be reproduced by chemical models if N 2 H + is slightly (factor of ∼2 in fractional abundance) depleted in the central 3000 AU. The N 2 H + and N 2 D + linewidths in the core center are ∼0.3 km s −1 , significantly larger than in other more quiescent Taurus starless cores but similar to those observed in the center of L1544. The kinematical behaviour of L1521F is more complex than seen in L1544, and a model of contraction due to ambipolar diffusion is only marginally consistent with the present data. Other velocity fields, perhaps produced by accretion of the surrounding material onto the core and/or unresolved substructure, are present. Both chemical and kinematical analyses suggest that L1521F is less evolved than L1544, but, in analogy with L1544, it is approaching the "critical" state.

“…Thus, the N 2 D + /N 2 H + abundance ratio and the N 2 D + fractional abundance increase towards the core center, so that N 2 D + is particularly suitable to investigate the kinematics as well as the physical and chemical conditions of the highly CO-depleted core nuclei. In fact, N 2 D + observations have recently allowed an accurate analysis of the velocity field and kinematics across the core nucleus of L1544, a well-known starless core (Caselli et al 2002a). N 2 D + lines can be used to determine the line of sight component of the velocity of ions in the densest portions of cloud cores.…”

Section: Introductionmentioning

confidence: 99%

“…As far as higher J transitions are concerned, Sastry et al (1981) observed unresolved rotational lines up to the J = 6 ← 5 one in the positive column of a DC glow discharge, while some hyperfine structure for the J = 2 → 1 and 3 → 2 transitions has been detected by several radioastronomers (Gerin et al 2001;Turner 2001;Caselli et al 2002a).…”

Section: Introductionmentioning

confidence: 99%

Laboratory and radio-astronomical spectroscopy of the hyperfine structure of N₂D$\mathsf{^+}$

Dore

Beninati

et al. 2004

A&A

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Abstract. We present the first laboratory measurements of the hyperfine structure of the J = 1 ← 0 rotational transition of N 2 D + , a good tracer of the dense regions of molecular cloud cores, and the spectra of unresolved high J transitions recorded in the 308−463 GHz region. Together with a high sensitivity radio-astronomical spectrum of the N 2 D + J = 1 → 0 rotational transition in a quiescent cloud core, we determined with high precision the frequencies of the seven hyperfine components and the molecular spectroscopic constants, allowing us to make predictions on the N 2 D + frequencies of higher J transitions occurring in the submillimeter-wave region.