Pre-stellar Core L1544 Research Articles

Simulated observations of young gravitationally unstable protoplanetary discs

The formation and earliest stages of protoplanetary discs remain poorly constrained by observations. ALMA will soon revolutionise this field. Therefore, it is important to provide predictions which will be valuable for the interpretation of future high sensitivity and high angular resolution observations. Here we present simulated ALMA observations based on radiative transfer modelling of a relatively massive (0.39 M_solar) self-gravitating disc embedded in a 10 M_solar dense core, with structure similar to the pre-stellar core L1544. We focus on simple species and conclude that C17O 3-2, HCO+ 3-2, OCS 26-25 and H2CO 404-303 lines can be used to probe the disc structure and kinematics at all scales.

FIRST DETECTION OF WATER VAPOR IN A PRE-STELLAR CORE

Water is a crucial molecule in molecular astrophysics as it controls much of the gas/grain chemistry, including the formation and evolution of more complex organic molecules in ices. Pre-stellar cores provide the original reservoir of material from which future planetary systems are built, but few observational constraints exist on the formation of water and its partitioning between gas and ice in the densest cores. Thanks to the high sensitivity of the Herschel Space Observatory, we report on the first detection of water vapor at high spectral resolution toward a dense cloud on the verge of star formation, the pre-stellar core L1544. The line shows an inverse P-Cygni profile, characteristic of gravitational contraction. To reproduce the observations, water vapor has to be present in the cold and dense central few thousand AU of L1544, where species heavier than Helium are expected to freeze-out onto dust grains, and the ortho:para H2 ratio has to be around 1:1 or larger. The observed amount of water vapor within the core (about 1.5x10^{-6} Msun) can be maintained by Far-UV photons locally produced by the impact of galactic cosmic rays with H2 molecules. Such FUV photons irradiate the icy mantles, liberating water wapor in the core center. Our Herschel data, combined with radiative transfer and chemical/dynamical models, shed light on the interplay between gas and solids in dense interstellar clouds and provide the first measurement of the water vapor abundance profile across the parent cloud of a future solar-type star and its potential planetary system.

The depletion of NO in pre-protostellar cores

Understanding the depletion of heavy elements is a fundamental step towards determining the structure of pre-protostellar cores just prior to collapse. We study the dependence of the NO abundance on position in the pre-protostellar cores L1544 and L183. We observed the 150 GHz and 250~GHz transitions of NO and the 93 GHz transitions of \NTHP \ towards L1544 and L183 using the IRAM 30 m telescope. We compare the variation of the NO column density with position in these objects with the H column density derived from dust emission measurements. We find that NO behaves differently from \NTHP \ and appears to be partially depleted in the high density core of L1544. Other oxygen-containing compounds are also likely to be partially depleted in dense-core nuclei. The principal conclusions are that: the prestellar core L1544 is likely to be 'carbon-rich'; the nitrogen chemistry did not reach equilibrium prior to gravitational collapse, and nitrogen is initially (at densities of the order of $10^4$~cm$^{-3}$) mainly in atomic form; the grain sticking probabilities of atomic C, N and, probably, O are significantly smaller than unity.

The evolutionary state of the southern dense core Chamaeleon-MMS1

Aims.Our goal is to set constraints on the evolutionary state of the dense core Cha-MMS1 in the Chamaeleon I molecular cloud. Methods: .We analyze molecular line observations carried out with the new submillimeter telescope APEX. We look for outflow signatures around the dense core and probe its chemical structure, which we compare to predictions of models of gas-phase chemistry. We also use the public database of the Spitzer Space Telescope (SST) to compare Cha-MMS1 with the two Class 0 protostars IRAM 04191 and L1521F, which are at the same distance. Results: .We measure a large deuterium fractionation for N2H+ (11 ± 3 %), intermediate between the prestellar core L1544 and the very young Class 0 protostar L1521F. It is larger than for HCO+ (2.5 ± 0.9 %), which is probably the result of depletion removing HCO+ from the high-density inner region. Our CO(3-2) map reveals the presence of a bipolar outflow driven by the Class I protostar Ced 110 IRS 4 but we do not find evidence for an outflow powered by Cha-MMS1. We also report the detection of Cha-MMS1 at 24, 70 and 160 μm by the instrument MIPS of the SST, at a level nearly an order of magnitude lower than IRAM 04191 and L1521F. Conclusions: .Cha-MMS1 appears to have already formed a compact object, either the first hydrostatic core at the very end of the prestellar phase, or an extremely young protostar that has not yet powered any outflow, at the very beginning of the Class 0 accretion phase.

The Distribution of Ortho–H2D+(11,0–11,1) in L1544: Tracing the Deuteration Factory in Prestellar Cores

Prestellar cores are unique laboratories for studying the chemical and physical conditions preceding star formation. We observed the prestellar core L1544 in the fundamental transition of ortho-H_(2)D^(+)(1_(1,0)-1_(1,1)) at different positions over 100 and found a strong correlation between its abundance and the CO depletion factor. We also present a tentative detection of the fundamental transition of para-D_(2)H^+ (1_(1,0)-1_(0,1)) at the dust emission peak. Maps in N_(2)H^+, N_(2)D^+, HCO^+, and DCO^+ are used and interpreted with the aid of a spherically symmetric chemical model that predicts the column densities and abundances of these species as a function of radius. The correlation between the observed deuterium fractionation of H^(+)_3, N_(2)H^+, and HCO^+ and the observed integrated CO depletion factor across the core can be reproduced by this chemical model. In addition, a simpler model is used to study the H_(2)D^+ ortho-to-para ratio. We conclude that, in order to reproduce the observed ortho-H_(2)D^+ observations, the grain radius should be larger than 0.3 μm.

Observations of L1521F: A highly evolved starless core

We observed the pre-stellar core L1521F in dust emission at 1.2 mm and in two transitions each of N2H + ,N 2D + ,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. N2H + and N2D + 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(H2) ∼ 10 6 cm −3 and a CO depletion factor, integrated along the line of sight, fD ≡ 9.5 × 10 −5 /xobs(CO) ∼ 15 in the central 20 �� , similar to the pre-stellar core L1544. However, the N(N2D + )/N(N2H + ) 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 N2H + is slightly (factor of ∼2 in fractional abundance) depleted in the central 3000 AU. The N2H + and N2D + 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.

Abundant H$\mathsf{_{2}}$D$\mathsf{^{+}}$ in the pre–stellar core L1544

We have detected the 372 GHz line of ortho-H2D + towards the pre-stellar core L1544. The strongest emission (Tmb1 K) occurs at the peak of the millimeter continuum emission, while measurements at oset positions indicate that H 2D + is confined within 20 00 , where CO is highly depleted. The derived H2D + abundance of10 9 is comparable with previous estimates of the electron abundance in the core, which suggests that H2D + is the main molecular ion in the central 20 00 (2800 AU) of L1544. This confirms the expectations that H 2D + is dramatically enhanced in gas depleted of molecules other than H2 .T he measured abundance even exceeds the present model predictions by about a factor ten. One possibility is that all CNO-bearing neutral species, including atomic oxygen, are almost completely (>98%) frozen within a radius of2800 AU.

A coupled, dynamical and chemical model for the prestellar core L1544: Comparison of modeled and observed C18O, HCO+, and CS emission spectra

We consider radiative transfer in C18O, HCO+, and CS molecular lines in a spherically symmetrical, coupled, dynamical and chemical model of a prestellar core whose evolution is determined by ambipolar diffusion. Theoretical and observed line profiles are compared for the well-studied core L1544, which may be a collapsing protostellar cloud. We study the relationship between the line shapes and model parameters. The structure of the envelope and kinematic parameters of the cloud are the most important factors determining the shape of the lines. Varying the input model parameters for the radiative transfer—the kinetic temperature and microturbulent velocity—within the limits imposed by observations does not result in any substantial variations of the line profiles. The comparison between the model and observed spectra indicates that L1544 displays a flattened structure, and is viewed at an oblique angle. A two-dimensional model is needed to reproduce this structure.

An Empirical Model for Protostellar Collapse

We propose a new analytic model for the initial conditions of protostellar collapse in relatively isolated regions of star formation. The model is nonmagnetic and is based on a Plummer-like radial density profile as its initial condition. It fits: the observed density profiles of prestellar cores and class 0 protostars; recent observations in prestellar cores of roughly constant contraction velocities over a wide range of radii; and the lifetimes and accretion rates derived for class 0 and class I protostars. However, the model is very simple, having in effect only two free parameters, so it should provide a useful framework for interpreting observations of prestellar cores and protostars and for calculations of radiation transport and time-dependent chemistry. As an example, we model the prestellar core L1544.