A. H. Cerqueira

Emission lines from rotating proto-stellar jets with variable velocity profiles - I. Three-dimensional numerical simulation of the non-magnetic case

Using the Yguazu-a three-dimensional hydrodynamic code, we have computed a set of numerical simulations of heavy, supersonic, radiatively cooling jets including var iabilities in both the ejection direction (precession) and the jet velocity (intermittence). In order to investigate the effects of jet rotation on the shape of the line profiles, we also i ntroduce an initial toroidal rotation velocity profile, in agreement with some r ecent observational evidence found in jets from T Tauri stars which seems to support the presence of a rotation velocity pattern inside the jet beam, near the jet production region. Since th e Yguazu- a code includes an atomic/ionic network, we are able to compute the emission coeffi cients for several emission lines, and we generate line profiles for the H �, (O I)�6300, (S II)�6716 and (N II)�6548 lines. Using initial parameters that are suitable for the DG Tau microjet, we show that the computed radial velocity shift for the medium-velocity component of the line profile as a function of distance from the jet axis is strikingly simi lar for rotating and non-rotating jet models. These findings lead us to put forward some caveats on the interpretation of the observed radial velocity distribution from a few outflows from yo ung stellar objects, and we claim that these data should not be di rectly used as a doubtless confirmation of the magnetocentri fugal wind acceleration models.

Gemini Multi-Object Spectrograph Integral Field Unit Spectroscopy of the 167-317 (LV2) Proplyd in Orion*

We present high spatial resolution spectroscopic observations of the proplyd 167-317 (LV2) near the Trapezium cluster in the Orion Nebula, obtained during the system verification run of the Gemini Multi-Object Spectrograph (GMOS) Integral Field Unit (IFU) at the Gemini South Observatory. We have detected 38 forbidden and permitted emission lines associated with the proplyd and its redshifted jet. We have been able to detect three velocity components in the profiles of some of these lines: a peak with a 28–33 km s-1 systemic velocity that is associated with the photoevaporated proplyd flow, a highly redshifted component associated with a previously reported jet (which has receding velocities of about 80–120 km s-1 with respect to the systemic velocity and which is spatially distributed to the southeast of the proplyd), and a less obvious, approaching structure that may possibly be associated with a faint counterjet with a systemic velocity of -75 ± 15 km s-1. We find evidence that the redshifted jet has a variable velocity, with slow fluctuations as a function of the distance from the proplyd. We present several background-subtracted, spatially distributed emission-line maps, and we use this information to obtain the dynamical characteristics over the observed field. Using a simple model and extinction-corrected Hα fluxes, we estimate the mass-loss rate for both the proplyd photoevaporated flow and the redshifted microjet, obtaining proplyd = (6.2 ± 0.6) × 10-7 M⊙ yr-1 and jet = (2.0 ± 0.7) × 10-8 M⊙ yr-1, respectively.We present high spatial resolution spectroscopic observations of the proplyd 167-317 (LV2) near the Trapezium cluster in the Orion nebula, obtained during the System Verification run of the Gemini Multi Object Spectrograph (GMOS) Integral Field Unit (IFU) at the Gemini South Observatory. We have detected 38 forbidden and permitted emission lines associated with the proplyd and its redshifted jet. We have been able to detect three velocity components in the profiles of some of these lines: a peak with a 28-33 km/s systemic velocity that is associated with the photoevaporated proplyd flow, a highly redshifted component associated with a previously reported jet (which has receding velocities of about 80-120 km/s with respect to the systemic velocity and is spatially distributed to the southeast of the proplyd) and a less obvious, approaching structure, which may possibly be associated with a faint counter-jet with systemic velocity of (-75 +/- 15) km/s. We find evidences that the redshifted jet has a variable velocity, with slow fluctuations as a function of the distance from the proplyd. We present several background subtracted, spatially distributed emission line maps and we use this information to obtain the dynamical characteristics over the observed field. Using a simple model and with the extinction corrected Halpha fluxes, we estimate the mass loss rate for both the proplyd photoevaporated flow and the redshifted microjet, obtaining (6.2 +/- 0.6) x 10^{-7} M_sun/year and (2.0 +/- 0.7) x 10^{-8} M_sun/year, respectively.

3D numerical simulations of photodissociated and photoionized disks

Aims. In this work we study the influence of the UV radiation field of a massive star on the evolution of a disklike mass of gas and dust around a nearby star. This system has similarities with the proplyds seen in Orion. Methods. We study disks with different inclinations and distances from the source, performing fully 3D numerical simulations. We use the YGUAZU-A adaptative grid code modified to account for EUV/FUV fluxes and non-spherical mass distributions. We treat H and C photoionization in order to reproduce the ionization fronts and photodissociation regions observed in proplyds. We also incorporate a wind from the ionizing source, in order to investigate the formation of the bow shock observed in several proplyds. We examine density and Hα maps, as well as the mass loss rates in the photoevaporated winds. Results. Our results show that a photoevaporated wind propagates from the disk surface and becomes ionized after an ionization front (IF) seen as a bright peak in the Hα maps. We follow the development of an HI region inside the photoevaporated wind which corresponds to a photodissociated region (PDR) for most of our models, except those without a FUV flux. For disks that are at a distance from the source d ≥ 0.1 pc, the PDR is thick and the IF is detached from the disk surface. In contrast, for disks that are closer to the source, the PDR is thin and not resolved in our simulations. The IF then coincides with the first grid points of the disk that are facing the ionizing photon source. In both cases, the photoevaporated wind shocks (after the IF) with the wind that comes from the ionizing source, and this interaction region is bright in Hα. Conclusions. Our 3D models produce two emission features: a hemispherically shaped structure (associated with the IF) and a detached bow shock where both winds collide. A photodissociated region develops in all of the models exposed to the FUV flux. More importantly, disks with different inclinations with respect to the ionizating source have relatively similar photodissociation regions. If the disk axis is not aligned with the direction of the ionizing photon flux, the IF displays moderate side-to-side asymmetries, in qualitative agreement with images of proplyds, which also show such asymmetries. The mass loss rates are ∼ 10 -7 M o yr -1 for face-on disks, and 5 x 10 -8 M ⊙ yr -1 for inclined disks at distances from 0.1 to 0.2 pc from the ionizing photon sources.

On the mean field dynamo with Hall effect

Context. MHD turbulence with Hall effect. Aims. Study how Hall effect modifies the quenching process of the electromotive force (e.m.f.) in Mean Field Dynamo (MFD) theories. Methods. We write down the evolution equations for the e.m.f. and for the large and small scale magnetic helicity, treat Hall effect as a perturbation and integrate the resulting equations, assuming boundary conditions such that the total divergencies vanish. Results. For force-free large scale magnetic fields, Hall effect acts by coupling the small scale velocity and magnetic fields. For the range of parameters considered, the overall effect is a stronger quenching of the e.m.f. than in standard MHD and a damping of the inverse cascade of magnetic helicity. Conclusions. In astrophysical environments characterized by the parameters considered here, Hall effect would produce an earlier quenching of the e.m.f. and consequently a weaker large scale magnetic field.

The photoevaporation of a neutral structure by an EUV+FUV radiation field

The EUV photoionizing radiation and FUV dissociating radiation from newly born stars photoevaporate their parental neutral cloud, leading to the formation of dense clumps that could eventually form additional stars. We study the effects of including a photodissociating FUV flux in models of the fragmentation of a photoevaporating, self-gravitating molecular cloud. We compute 3D simulations of the interaction of an inhomogeneous, neutral, self-gravitating cloud with external EUV and FUV radiation fields, and calculate the number of collapsing clumps and their mass. We find that the presence of an outer photodissociation region has an important effect on the formation of dense structures due to the expansion of an HII region. In particular, including a FUV field leads to the earlier formation of a larger number of dense clumps, which might lead to the formation of more stars.

GMOS-IFU Observations of LV2

We present GMOS-IFU spectroscopic observations of the proplyd 167-317 (LV2) near the Trapezium cluster in the Orion nebula, obtained at the Gemini South Observatory. The proplyd was observed in optical wavelengths ranging from ≃ 5515 Ato ≃ 7630A. The ionized photo-evaporated flow was detected together with a microjet and possibly the blue-shifted jet that appear in most of the detected emission lines as multiple peaks at different velocities. Typically, there is a low velocity peak at ∼ 30 km s-1, a high velocity red-shifted peak at ∼ 100 km s-1 and a very faint blue-shifted peak at ∼ -60 km s-1. The observations also allowed us to determine spatially the emitting regions. The red-shifted jet is located to the SE of the central emission. We find evidences that the red-shifted jet has a variable velocity, which slowly drops with increasing distance from the proplyd

Radial velocity asymmetries from jets with variable velocity profiles

We have computed a set of 3‐D numerical simulations of radiatively cooling jets including variabilities in both the ejection direction (precession) and the jet velocity (intermittence), using the Yguazu‐a code. In order to investigate the effects of jet rotation on the shape of the line profiles, we also introduce an initial toroidal rotation velocity profile. Since the Yguazu‐a code includes an atomic/ionic network, we are able to compute the emission coefficients for several emission lines, and we generate line profiles for the Hα, [O I]λ6300, [S II]λ6716 and [N II]λ6548 lines. Using initial parameters that are suitable for the DG Tau microjet, we show that the computed radial velocity shift for the medium‐velocity component of the line profile as a function of distance from the jet axis is strikingly similar for rotating and non‐rotating jet models.

Adaptive grid simulations of ionized flows

This paper presents a discussion of our “yguazu‐a” code, which integrates systems of differential equations (generally, the gasdynamic or MHD equations together with and atomic/chemical network) on a hierarchical, binary adaptive grid. Examples of recent calculations are shown, and a general discussion of the capabilities of the code and the published results is presented.

Self‐Consistent Mean Field Dynamo with Hall Effect

We investigate the influence of the Hall effect on the mean field dynamo action. As a starting point, we consider the theory developed by E. Blackman and G. Field, where the generation of a mean magnetic field is treated consistently by considering an evolution equation for the magnetic helicity and for the stochastic electromotive force, instead of prescribing its form as it is usually done. We found that the presence of the Hall effect, namely, the fact that the magnetic field is frozen in the electron flux and not in the bulk flux, introduces substantial changes in these evolution equations. We also present results of the numerical integration of the corresponding equations and compare them with previous results obtained without Hall effect. Our main result is that the dynamo effect also operates through an inverse cascade of magnetic helicity as in the standard dynamo theory, but now this inverse cascade is due to the electron fluid flux instead of the bulk fluid. We also find that the Hall effect can...