Mapping the prestellar core Ophiuchus D (L1696A) in ammonia

Abstract

Context. The gas kinetic temperature in the centres of starless, high-density cores is predicted to fall as low as 5−6 K. The temperature gradient, which affects the dynamics and chemistry of these objects, should be discernible with radio interferometers reaching a spatial resolution of 1000 AU or better. Aims. The aim of this study was to determine the kinetic temperature distribution in the low-mass prestellar core Oph D where previous observations suggest a very low central temperature.Methods. The densest part of the Oph D core was mapped in the NH 3 (1, 1) and (2, 2) inversion lines using the Very Large Array (VLA). The physical quantities were derived from the observed spectra by fitting the hyperfine structure of the lines, and subsequently the temperature distribution of Oph D was calculated using the standard rotational temperature techniques. A physical model of the cores was constructed, and the simulated spectra after radiative transfer calculations with a 3D Monte Carlo code were compared with the observed spectra. Temperature, density, and ammonia abundance of the core model were tuned until a satisfactory match with the observation was obtained.Results. The high resolution of the interferometric data reveals that the southern part of Oph D comprises of two small cores in consistence with the 1.3 mm dust continuum map. The gas kinetic temperatures, as derived from ammonia towards the centres of the southern and northern core are 7.4 and 8.9 K, respectively. These values represent line-of-sight averages using the LTE assumption. A model using modified Bonnor-Ebert spheres, in which the temperature decreases to 6.1 K and 8.9 K in the centres of southern and northern core, matched the observed values satisfactorily. The southern core, which has more steep temperature gradient, has central density of n c = 4 × 10 6 cm −3 , and the data suggests depletion of ammonia within 700 AU from the centre. The northern core, which is almost isothermal, seems to be less dense. The radial velocity gradients in these cores are almost opposite in direction, which may be an indication that turbulent fragmentation has a role in the formation of these cores. The observed masses of the cores are only ∼0.2 M . Their potential collapse could lead to formation of brown dwarfs or low-mass stars.

Complex cyanides as chemical clocks in hot cores

“…While we did not include molecular gas in our initial abundances, the chemistry quickly converts the free atoms into stable molecules ( Figure 3). These gas-phase abundances are within an order of magnitude of reported abundances in starless and prestellar cores (Ruoskanen et al 2011;Koumpia et al 2016;Vastel et al 2016).…”

Triggering Collapse of the Presolar Dense Cloud Core and Injecting Short-Lived Radioisotopes With a Shock Wave. Ii. Varied Shock Wave and Cloud Core Parameters

“…The standard target clouds were Bonnor-Ebert spheres of molecular hydrogen gas with initial central densities of 1.24 × 10 −18 g cm −3 , radii of 0.058 pc, and masses of 2.2 M ⊙ , centered at R = 0 and Z = 0.13 pc. Bonnor-Ebert-like density profiles have been inferred for dense molecular cloud cores, such as Barnard 68 (Burkert & Alves 2009) and L1696A (Ruoskanen et al 2011). In the Perseus molecular cloud, starless cores appear to have radial density profiles that are shallower than that of the singular isothermal sphere, where ρ ∝ r −2 (Schnee et al 2010), and hence are roughly consistent with a Bonnor-Ebert-like initial profile.…”

Why does ammonia not freeze out in the centre of pre-stellar cores?

“…These results are not in agreement with observations. Ammonia depletion has been observed toward pre-stellar cores, but it seems to occur only at very high ⋆ E-mail: osipila@mpe.mpg.de (column) densities (Friesen et al 2009;Ruoskanen et al 2011;Chitsazzadeh et al 2014). To add to the conundrum, Crapsi et al (2007) derived an ammonia abundance profile in L1544, a well-studied pre-stellar core in Taurus, that increases toward the dust peak and shows no signs of depletion in the center of the core despite the high gas density (n(H 2 ) > 10 6 cm −3 ).…”