Dense Protostellar Envelopes Research Articles

Deep observations of O2toward a low-mass protostar withHerschel-HIFI

According to traditional gas-phase chemical models, O2 should be abundant in molecular clouds, but until recently, attempts to detect interstellar O2 line emission with ground- and space-based observatories have failed. Following the multi-line detections of O2 with low abundances in the Orion and rho Oph A molecular clouds with Herschel, it is important to investigate other environments, and we here quantify the O2 abundance near a solar-mass protostar. Observations of O2, at 487 GHz toward a deeply embedded low-mass Class 0 protostar, NGC 1333-IRAS 4A, are presented, using the HIFI instrument on the Herschel Space Observatory. Complementary data of the chemically related NO and CO molecules are obtained as well. The high spectral resolution data are analysed using radiative transfer models to infer column densities and abundances, and are tested directly against full gas-grain chemical models. The deep HIFI spectrum fails to show O2 at the velocity of the dense protostellar envelope, implying one of the lowest abundance upper limits of O2/H2 at <6x10^-9 (3 sigma). However, a tentative (4.5 sigma) detection of O2 is seen at the velocity of the surrounding NGC 1333 molecular cloud, shifted by 1 km/s relative to the protostar. For the protostellar envelope, pure gas-phase models and gas-grain chemical models require a long pre-collapse phase (~0.7-1x10^6 years), during which atomic and molecular oxygen are frozen out onto dust grains and fully converted to H2O, to avoid overproduction of O2 in the dense envelope. The same model also reproduces the limits on the chemically related NO molecule. The tentative detection of O2 in the surrounding cloud is consistent with a low-density PDR model with small changes in reaction rates. The low O2 abundance in the collapsing envelope around a low-mass protostar suggests that the gas and ice entering protoplanetary disks is very poor in O2.

Herschel-HIFI observations of H2O in high-mass star-forming regions: first results

This paper reviews the first results of observations of H2O line emission with Herschel-HIFI towards high-mass star-forming regions, obtained within the WISH guaranteed time program. The data reveal three kinds of gas-phase H2O: `cloud water' in cold tenuous foreground clouds, `envelope water' in dense protostellar envelopes, and `outflow water' in protostellar outflows. The low H2O abundance (1e-10 -- 1e-9) in foreground clouds and protostellar envelopes is due to rapid photodissociation and freeze-out on dust grains, respectively. The outflows show higher H2O abundances (1e-7 -- 1e-6) due to grain mantle evaporation and (probably) neutral-neutral reactions.

Cosmic-ray ionization of molecular clouds

Low-energy cosmic rays are a fundamental source of ionization for molecular clouds, influencing their chemical, thermal and dynamical evolution. The purpose of this work is to explore the possibility that a low-energy component of cosmic-rays, not directly measurable from the Earth, can account for the discrepancy between the ionization rate measured in diffuse and dense interstellar clouds. We collect the most recent experimental and theoretical data on the cross sections for the production of H2+ and He+ by electron and proton impact, and we discuss the available constraints on the cosmic-ray fluxes in the local interstellar medium. Starting from different extrapolations at low energies of the demodulated cosmic-ray proton and electron spectra, we compute the propagated spectra in molecular clouds in the continuous slowing-down approximation taking into account all the relevant energy loss processes. The theoretical value of the cosmic-ray ionization rate as a function of the column density of traversed matter is in agreement with the observational data only if either the flux of cosmic-ray electrons or of protons increases at low energies. The most successful models are characterized by a significant (or even dominant) contribution of the electron component to the ionization rate, in agreement with previous suggestions. However, the large spread of cosmic-ray ionization rates inferred from chemical models of molecular cloud cores remains to be explained. Available data combined with simple propagation models support the existence of a low-energy component (below about 100 MeV) of cosmic-ray electrons or protons responsible for the ionization of molecular cloud cores and dense protostellar envelopes.

Who is eating the outflow? High-angular resolution study of an intermediate-mass protostar in L1206

Up to now only a few intermediate-mass molecular outflows have been studied with enough high-angular resolution. The aim of this work is to study in detail the intermediate-mass YSO IRAS 22272+6358A, which is embedded in L1206, and its molecular outflow, in order to investigate the interaction of the outflow with the dense protostellar material, and to compare their properties with those of lower mas counterparts. We carried out OVRO observations of the 2.7 mm continuum emission, CO(1-0), C18O(1-0), and HC3N(12-11) in order to map with high-angular resolution the core of L1206, and to derive the properties of the dust emission, the molecular outflow and the dense protostellar envelope. The 2.7 mm continuum emission has been resolved into four sources, labeled OVRO~1, 2, 3, and 4. The intermediate-mass Class~0/I object OVRO 2, with a mass traced by the dust emission of 14.2 Msun, is the source associated with IRAS 22272+6358A. The CO(1-0) observations have revealed a very collimated outflow driven by OVRO 2, at a PA ~140 degr, that has a very weak southeastern red lobe and a much stronger northwestern blue lobe. Photodissociation toward the red lobe produced by the ionization front coming from the bright-rimmed diffuse HII region could be responsible of the morphology of the outflow. The spatial correlation between the outflow and the elongated dense protostellar material traced by HC3N(12-11) suggests an interaction between the molecular outflow and the protostellar envelope. Shocks produced by the molecular outflow, and possibly by the shock front preceding the ionization front could account for the southern enhancement of HC3N. The properties of the intermediate-mass protostar OVRO 2 and the molecular outflow are consistent with those of lower mass counterparts.

Millimeter Interferometric Polarization Imaging of the Young Stellar Object NGC 1333/IRAS 4A

We present a 3.4 mm polarization image of the dust emission associated with the young stellar object NGC 1333/IRAS 4A made with the Owens Valley Millimeter Array with 5'' resolution. The integrated linear polarization of the dust continuum is 4% of the total intensity. The polarization is produced by magnetically aligned dust grains and arises from very dense gas (n > 108 cm-3), which indicates that the dust alignment process remains viable in the dense protostellar envelope. The magnetic field directions inferred from our observations are aligned with features seen in the high-velocity outflow emanating from IRAS 4A. The magnetic field directions are not aligned with the field on much larger scales as measured by optical and infrared selective extinction, which suggests significant field structure in the cloud core. The peak of the polarized emission is offset from the total intensity peak, which perhaps indicates considerable unresolved structure in the magnetic field.