The standard model of star formation applied to massive stars: accretion disks and envelopes in molecular lines

We address the question of whether the formation of high-mass stars is similar to or differs from that of solar-mass stars through new molecular line observations and modeling of the accretion flow around the massive protostar IRAS20126+4104. We combine new observations of NH3(1,1) and (2,2) made at the Very Large Array, new observations of CHCN(13-12) made at the Submillimeter Array, previous VLA observations of NH(3,3), NH(4,4), and previous Plateau de Bure observations of C34S(2-1), C34S(5-4), and CHCN(12-11) to obtain a data set of molecular lines covering 15 to 419 K in excitation energy. We compare these observations against simulated molecular line spectra predicted from a model for high-mass star formation based on a scaled-up version of the standard disk-envelope paradigm developed for accretion flows around low-mass stars. We find that in accord with the standard paradigm, the observations require both a warm, dense, rapidly-rotating disk and a cold, diffuse infalling envelope. This study suggests that accretion processes around 10 M stars are similar to those of solar mass stars.Comment: Accepted MNRA

Modeling molecular hyperfine line emission

In this paper we discuss two approximate methods previously suggested for modeling hyperfine spectral line emission for molecules whose collisional transitions rates between hyperfine levels are unknown. Hyperfine structure is seen in the rotational spectra of many commonly observed molecules such as HCN, HNC, NH3, N2H+, and C17O. The intensities of these spectral lines can be modeled by numerical techniques such as Lambda-iteration that alternately solve the equations of statistical equilibrium and the equation of radiative transfer. However, these calculations require knowledge of both the radiative and collisional rates for all transitions. For most commonly observed radio frequency spectral lines, only the net collisional rates between rotational levels are known. For such cases, two approximate methods have been suggested. The first method, hyperfine statistical equilibrium (HSE), distributes the hyperfine level populations according to their statistical weight, but allows the population of the rotational states to depart from local thermodynamic equilibrium (LTE). The second method, the proportional method approximates the collision rates between the hyperfine levels as fractions of the net rotational rate apportioned according to the statistical degeneracy of the final hyperfine levels. The second method is able to model non-LTE hyperfine emission. We compare simulations of N2H+ hyperfine lines made with approximate and more exact rates and find that satisfactory results are obtained.Comment: 34 pages. Pages 22-34 are data tables. For ApJ

Chemistry and Radiative Transfer of Water in Cold, Dense Clouds

The Herschel Space Observatory's recent detections of water vapor in the cold, dense cloud L1544 allow a direct comparison between observations and chemical models for oxygen species in conditions just before star formation. We explain a chemical model for gas phase water, simplified for the limited number of reactions or processes that are active in extreme cold ($<$ 15 K). In this model, water is removed from the gas phase by freezing onto grains and by photodissociation. Water is formed as ice on the surface of dust grains from O and OH and released into the gas phase by photodesorption. The reactions are fast enough with respect to the slow dynamical evolution of L1544 that the gas phase water is in equilibrium for the local conditions thoughout the cloud. We explain the paradoxical radiative transfer of the H$_2$O ($1_{10}-1_{01}$) line. Despite discouragingly high optical depth caused by the large Einstein A coefficient, the subcritical excitation in the cold, rarefied H$_2$ causes the line brightness to scale linearly with column density. Thus the water line can provide information on the chemical and dynamical processes in the darkest region in the center of a cold, dense cloud. The inverse P-Cygni profile of the observed water line generally indicates a contracting cloud. This profile is reproduced with a dynamical model of slow contraction from unstable quasi-static hydrodynamic equilibrium (an unstable Bonnor-Ebert sphere).Comment: submitted to MNRA

The Evolution of Cloud Cores and the Formation of Stars

For a number of starless cores, self-absorbed molecular line and column density observations have implied the presence of large-amplitude oscillations. We examine the consequences of these oscillations on the evolution of the cores and the interpretation of their observations. We find that the pulsation energy helps support the cores and that the dissipation of this energy can lead toward instability and star formation. In this picture, the core lifetimes are limited by the pulsation decay timescales, dominated by non-linear mode-mode coupling, and on the order of ~few x 10^5--10^6 yr. Notably, this is similar to what is required to explain the relatively low rate of conversion of cores into stars. For cores with large-amplitude oscillations, dust continuum observations may appear asymmetric or irregular. As a consequence, some of the cores that would be classified as supercritical may be dynamically stable when oscillations are taken into account. Thus, our investigation motivates a simple hydrodynamic picture, capable of reproducing many of the features of the progenitors of stars without the inclusion of additional physical processes, such as large-scale magnetic fields.Comment: 12 pages, 7 figures, submitted to Ap