J. Franco

On the formation and expansion of H II regions

The evolution of H II regions in spherical clouds with small, constant-density cores and power-law density distributions r exp -w outside the core is described analytically. It is found that there is a critical exponent above which the cloud becomes completely ionized. Its value in the formation phase depends on the initial conditions, but it has a well-defined value w(crit) = 3/2 during the expansion phase. For w less than w(crit), the radius of the H II region grows at a given rate, while neutral mass accumulates in the interphase between the ionization and shock fronts. For w = w(crit), the fronts move together without mass accumulation. Cases with w greater than w(crit) lead to the champagne phase: once the cloud is fully ionized, the expansion becomes supersonic. For self-gravitating disks without magnetic fields, the main features include a new variable-size stage. The initial shape of the H II region has a critical point beyond which the disk becomes completely ionized. 40 refs.

Photoionized and Photodissociated Regions around Main-Sequence Stars

Within a molecular cloud, the strong ultraviolet radiation field produced by newly formed stars dissociates and ionizes the surrounding molecular gas. The radiative flux depends on the effective temperature and metallicity of the star. Using the most recent line-blanketed atmosphere models from Kurucz, we obtain the rates of ionizing and dissociating photons from stars with effective temperatures of 7.5 × 103 to 5 × 104 K, and for metallicities between 0.01 times solar and solar. With a radiative transfer model, we then compute the basic structures and sizes of the photoionized and photodissociated regions produced by stars embedded in a molecular gas with uniform densities. Absorption of the UV flux by dust decreases the mass of H II and H I produced within the cloud, and its effects are taken into account in our model. We also discuss the constraints imposed by photodissociated regions on the number of intermediate- and high-mass stars that can form in molecular clouds.

Magnetically Driven Winds from Post-Asymptotic Giant Branch Stars: Solutions for High-Speed Winds and Extreme Collimation

This paper explores the effects of post-asymptotic giant branch (AGB) winds driven solely by magnetic pressure from the stellar surface. It is found that winds can reach high speeds under this assumption and lead to the formation of highly collimated proto-planetary nebulae. Bipolar knotty jets with periodic features and constant velocity are well reproduced by the models. Several wind models with terminal velocities from a few tens of km s-1 up to 103 km s-1 are calculated, yielding outflows with linear momenta in the range 1036-1040 g cm s-1, and kinetic energies in the range 1042-1047 ergs. These results are in accord with recent observations of proto-planetary nebulae that have pointed out serious energy and momentum deficits if radiation pressure is considered as the only driver for these outflows. Our models strengthen the notion that the large mass loss rates of post-AGB stars, together with the short transition times from the late AGB to the planetary nebula stage, could be directly linked with the generation of strong magnetic fields during this transition stage.

On the massive star-forming capacity of molecular clouds

Assuming that photoionization is the self-limiting process for continued star formation, we estimate the maximum number of massive (OB) stars that can form within a molecular cloud. The most efficient cloud destruction mechanism in the early stages of H II region evolution is the evaporation of the cloud by stars located near the cloud boundary. The maximum number of OB stars is of order 1 per 10(exp 4) solar mass of average molecular gas, or 10 per 10(exp 4) solar mass of dense molecular gas. The resulting star-forming efficiencies within cloud complexes range from 2% to 16% depending on both the location of the stars in the cloud and the details of the initial mass function, with an overall value of about 5% for average molecular gas.

Evolution of dust grains through a hot gaseous halo

The evolution of bare spherical dust grains in the halo of spiral galaxies is analyzed. Two different grain compounds, graphites and astronomical silicates, are considered. The detailed mass and luminosity distributions for the Milky Way and NGC 3198 (considered as representative of the Sb and Sc types, respectively) are used to evaluate the range of possible evolutionary tracks. Aside from radiative and gravitational forces, the effects of drag and sputtering from a gaseous halo are included. A simple isothermal and hydrostatic density structure, with temperatures in the range 3 × 10 5 -10 6 K, has been used for this gaseous halo

The Collisions of HVCs with a Magnetized Gaseous Galactic Disk

Resumen en: We discuss 2-D MHD numerical simulations for the interaction of high-velocit y clouds with a magnetized Galactic disk. The initial magnetic field is orie...

Molecular clouds in galaxies with different Z - Fragmentation of diffuse clouds driven by opacity

Molecular clouds are formed from diffuse interstellar clouds when the external ultraviolet radiation field is prevented from penetrating into the cloud. The opacity is provided mainly by dust grains and the required column density to the cloud center is N 5×1020(Zsun;/Z) cm-2. This high-opacity criterion could have a significant impact on the radial trends observed in spiral galaxies, and on the distinctions between spiral and dwarf irregular galaxies.

The Effects of Dust on Compact and Ultracompact H II Regions

We calculate numerical models of dusty H II regions using the Cloudy photoionization code with a grain size distribution. Dust sublimation causes a depletion of grain sizes and types within the ionized region, with large graphite grains being able to exist closer to the star than smaller graphite grains or silicate grains. We investigate the time-dependent hydrodynamic expansion of dusty H II regions and find that the fraction of ionizing photons absorbed by dust decreases with time. Furthermore, dusty H II regions stall earlier and at smaller radii than their dust-free counterparts. Comparison is made between our models and observable parameters, such as the electron density. We find that the electron density in dusty ionized regions estimated from radio continuum observations is likely to be an overestimate, and we quantify the discrepancy. Finally, we calculate the infrared emission from dusty H II regions and their surrounding circumnebular dust shells using the DUSTY code. We find that the far-infrared emission depends strongly on the parameters assumed for the circumnebular dust shell.

The Density Structure of Highly Compact H II Regions

We report the density structure of the ultracompact (UC) H ii regions G35.2021.74, G9.6210.19-E, and G75.7810.34-H O. The density profiles are derived from radio continuum emission at wavelengths from 6 to 2 0.3 cm. In the case of G35.2021.74, a cometary UC H ii region with a core and a tail, the spectrum of the core varies as , implying that the density structure is . The emission from the tail has a flatter spectrum, 0.6 22 S / n n / r n e indicating that the density gradient is also negative but shallower. For the case of G9.62 10.19, which is an H ii region complex with several components, the spectrum of the region designated component E is , cor0.95 S / n n responding to . The steepest spectral index, , is for the super UC H ii region G75.7810.34-H O; 22.5 1.4 n / rS / n e n 2 its density stratification may be as steep as . The actual density gradient may be smaller, owing to an 24 n / r e exponential (rather than a power-law) density distribution or to the effects of finite spatial extent. The contribution from dust emission and some of the possible implications of these density distributions are briefly discussed. Stellar groups form in the dense cores of molecular clouds. The structure of these clouds has been unveiled with optical, infrared, and radio observations over the last three decades, and a great effort has been made to understand the connections between cloud properties and star-forming activity. In particular, knowledge of the gas density structure is required to determine fundamental properties, such as the mass and stability of star-forming cores. Extinction studies can be used to derive the density profiles of clouds, but given the large opacities involved, they can only probe the outermost gas layers. Tracers occurring at radio frequencies, on the other hand, can penetrate deeper into the cloud and can reveal the density stratification of dark clouds and cloud cores. The information obtained from extinction and molecular-line studies shows that molecular clouds are centrally condensed. For instance, moderate-density (10 3 ‐10 5 cm ) envelopes sur23 rounding higher density (10 7 cm ) cores suggest the existence 23

Toroidal Magnetic Fields and the Evolution of Wind-driven Nebulae

Recent analytical work on magnetic shaping of wind-driven nebulae suggests that diverging stellar winds with frozen-in toroidal magnetic fields (occurring naturally when the wind originates from a rotating source) may evolve into collimated outflows. We report numerical simulations which show that the MHD collimation of stellar winds is indeed possible provided that the field carried by the wind is sufficiently strong. Our collimation mechanism operates in the shocked wind region of the nebula, leading to the formation of an astrophysical plasma gun recently invoked to accelerate jets associated with accretion disks.