MHD Models of Axisymmetric Protostellar Jets

We present the results of a series of axisymmetric time-dependent magnetohydrodynamic (MHD) simulations of the propagation of cooling, overdense jets, motivated by the properties of outflows associated with young stellar objects. A variety of initial field strengths and configurations are explored for both steady and time-variable (pulsed) jets. Even apparently weak magnetic fields with strengths B < 60 micro-G in the pre-shocked jet beam can have a significant effect on the dynamics, for example by altering the density, width, and fragmentation of thin shells formed by cooling gas. A linear analysis predicts that axisymmetric pinch modes of the MHD Kelvin-Helmholtz instability should grow only slowly for the highly supermagnetosonic jets studied here; we find no evidence for them in our simulations. Some of our models appear unstable to current-driven pinch modes, however the resulting pressure and density variations induced in the jet beam are not large, making this mechanism an unlikely source of emission knots in the jet beam. In the case of pulsed jets, radial hoop stresses confine shocked jet material in the pulses to the axis, resulting in a higher density in the pulses in comparison to purely hydrodynamic models.Comment: 28 pages, 16 figures, accepted by Ap.

Magnetic Field Effects on the Head Structure of Protostellar Jets

We present the results of 3-D SPMHD numerical simulations of supermagnetosonic, overdense, radiatively cooling jets. Two initial magnetic configurations are considered: (i) a helical and (ii) a longitudinal field. We find that magnetic fields have important effects on the dynamics and structure of radiative cooling jets, especially at the head. The presence of a helical field suppresses the formation of the clumpy structure which is found to develop at the head of purely hydrodynamical jets. On the other hand, a cooling jet embedded in a longitudinal magnetic field retains clumpy morphology at its head. This fragmented structure resembles the knotty pattern commonly observed in HH objects behind the bow shocks of HH jets. This suggests that a strong (equipartition) helical magnetic field configuration is ruled out at the jet head. Therefore, if strong magnetic fields are present, they are probably predominantly longitudinal in those regions. In both magnetic configurations, we find that the confining pressure of the cocoon is able to excite short-wavelength MHD K-H pinch modes that drive low-amplitude internal shocks along the beam. These shocks are not strong however, and it likely that they could only play a secondary role in the formation of the bright knots observed in HH jets.Comment: 14 pages, 2 Gif figures, uses aasms4.sty. Also available on the web page http://www.iagusp.usp.br/preprints/preprint.html. To appear in The Astrophysical Journal Letter

Magnetic Field Effects on the Structure and Evolution of Overdense Radiatively Cooling Jets

We investigate the effect of magnetic fields on the propagation dynamics and morphology of overdense, radiatively cooling, supermagnetosonic jets, with the help of fully three-dimensional SPMHD simulations. Evaluated for a set of parameters which are mainly suitable for protostellar jets (with density ratios between the jet and the ambient medium 3-10, and ambient Mach number ~ 24), these simulations are also compared with baseline non-magnetic and adiabatic calculations. We find that, after amplification by compression and re-orientation in nonparallel shocks at the working surface, the magnetic field that is carried backward with the shocked gas into the cocoon improves the jet collimation relative to the purely hydrodynamic (HD) systems. Low-amplitude, approximately equally spaced internal shocks (which are absent in the HD systems) are produced by MHD K-H reflection pinch modes. The longitudinal field geometry also excites non-axisymmetric helical modes which cause some beam wiggling. The strength and amount of these modes are, however, reduced (by ~ twice) in the presence of radiative cooling relative to the adiabatic cases. Besides, a large density ratio between the jet and the ambient medium also reduces, in general, the number of the internal shocks. As a consequence, the weakness of the induced internal shocks makes it doubtful that the magnetic pinches could produce by themselves the bright knots observed in the overdense, radiatively cooling protostellar jets.Comment: To appear in ApJ; 36 pages + 16 (gif) figures. PostScript files of figures are available at http://www.iagusp.usp.br/preprints/preprint.htm

PSR 0943+10: a bare strange star?

Recent work by Rankin & Deshpande strongly suggests that there exist strong ``micro-storms'' rotating around the magnetic axis of the 1.1s pulsar PSR 0943+10. Such a feature hints that most probably the large-voltage vacuum gap proposed by Ruderman & Sutherland (RS) does exist in the pulsar polar cap. However, there are severe arguments against the formation of the RS-type gap in pulsars, since the binding energies of both the Fe ions and the electrons in a neutron star's surface layer is too small to prevent thermionic ejection of the particles from the surface. Here we propose that PSR 0943+10 (probably also most of the other ``drifting'' pulsars) might be bare strange stars rather than normal neutron stars, in which the ``binding energy'' at the surface is merely infinity either for the case of ``pulsar'' or ``anti-pulsar''. It is further proposed that identifying a drifting pulsar as an anti-pulsar is the key criterion to distinguish strange stars from neutron stars.Comment: 4 pages, no figures, LaTeX, accepted 1999 July 9 by ApJ Letter

Three-dimensional MHD simulations of Radiatively cooling, Pulsed Jets

(Abridged) We here investigate, by means of fully 3-D Smoothed Particle Magnetohydrodynamic numerical simulations, the effects of magnetic fields on overdense, radiatively cooling, pulsed jets, using different initial magnetic field topologies and strengths ($B \simeq 260 \mu$G-0). The relative differences that have been previously detected in 2-D simulations involving distinct magnetic field configurations are diminished in the 3-D flows. While the presence of toroidal magnetic components can modify the morphology close to the jet head inhibiting its fragmentation in the early jet evolution, as previously reported in the literature, the impact of the pulsed-induced internal knots causes the appearance of a complex morphology at the jet head (as required by the observations of H-H jets) even in the MHD jet models with toroidal components. The detailed structure and emission properties of the internal working surfaces can be also significantly altered by the presence of magnetic fields. The increase of the magnetic field strength improves the jet collimation, and amplifies the density (by factors up to 1.4, and 4) and the H\alpha$intensity (by factors up to 4, and 5) behind the knots of jets with helical field and$\beta \simeq 1-0.1$(respectively), relative to a non magnetic jet. As a consequence, the corresponding$I_{[SII]}}/I_{H}\alpha}$ratio (which is frequently used to determine the excitation level of HH objects) can be decreased in the MHD models with toroidal components relative to non-magnetic calculations. We also find that the helical mode of the K-H instability can be triggered in MHD models with helical magnetic fields, causing some jet wiggling. No evidence for the formation of the nose cones is found in the 3-D flows, nor even in the$\beta \simeq 0.1$ case.Comment: 31 pages, 10 figures (see higher resolution figures in: http://www.iagusp.usp.br/~dalpino/mhd-jets/apj0301.tar.gz), ApJ in pres

Vacuum gaps in pulsars and PSR J2144-3933

In this paper we revisit the radio pulsar death line problem within the framework of curvature radiation and/or inverse compton scattering induced vacuum gap model above neutron star polar caps. Our special interest is in the recently detected pulsar PSR J2144-3933 with extremal period 8.5 seconds, which lies far beyond conventional death lines. We argue, that formation of vacuum gaps requires a complicated multipolar surface magnetic field, with a strenght $B_s$ much higher than the surface dipolar component $B_d$, and radii of curvature ${\cal R}$ much smaller than the neutron star radius $R=10^6$ cm. Such a multipolar surface field is also consistent with death lines including the extremal pulsar PSR J2144-3933. Since vacuum gap models produce sparks, our paper naturally supports the spark related models of subpulse drift phenomenon as well as to the spark associated models of coherent pulsar radio emission.Comment: 19 pages, 1 postscript figure, Latex, uses aastex.st

Atomic calculation for the atmospheres of strongly-magnetized neutron stars

Complete modeling of radiative transfer in neutron star atmospheres is in progress, taking into account the anisotropy induced by magnetic fields, non-ideal effects and general relativity. As part of our modeling, we present a novel atomic calculation method producing an extensive atomic data set including energy values and oscillator strengths in the so-called Landau regime ($B > 4.7\times10^9Z^2$ G). Conventional atmosphere models for B=0 are not applicable to typical field strengths of cooling neutron stars ($B \sim10^{12}-10^{13}$ G), since an atom no longer keeps its spherical shape. The elemental composition and the configuration of the magnetic field in the atmosphere are presently unknown, so that atomic data must be produced for ground and excited states of several ions as a function of magnetic field. To accomplish this efficiently, we minimized the iterations in the Hartree equation and treated exchange terms and higher Landau states by perturbation methods. This method has the effect of reducing the computation time significantly. Inclusion of higher Landau states gives us much more accurate data for inner orbitals unlike other methods based on the adiabatic approximation. While existing atomic data in the Landau regime are available only for low $Z$ atoms, our method can be used in elements up to Fe with sufficient accuracy to be of use for spectroscopic missions such as {\it Chandra}, {\it XMM-Newton} and next-generation X-ray telescopes.Comment: 19 pages, AASTeX, 4 figures, accepted for publication in Ap

Surgical Outcomes in Benign Gynecologic Surgery Patients during the COVID-19 Pandemic (SOCOVID study)

Study Objective To determine the incidence of perioperative coronavirus disease (COVID-19) in women undergoing benign gynecologic surgery and to evaluate perioperative complication rates in patients with active, previous, or no previous severe acute respiratory syndrome coronavirus 2 infection. Design A multicenter prospective cohort study. Setting Ten institutions in the United States. Patients Patients aged >18 years who underwent benign gynecologic surgery from July 1, 2020, to December 31, 2020, were included. All patients were followed up from the time of surgery to 10 weeks postoperatively. Those with intrauterine pregnancy or known gynecologic malignancy were excluded. Interventions Benign gynecologic surgery. Measurements and Main Results The primary outcome was the incidence of perioperative COVID-19 infections, which was stratified as (1) previous COVID-19 infection, (2) preoperative COVID-19 infection, and (3) postoperative COVID-19 infection. Secondary outcomes included adverse events and mortality after surgery and predictors for postoperative COVID-19 infection. If surgery was delayed because of the COVID-19 pandemic, the reason for postponement and any subsequent adverse event was recorded. Of 3423 patients included for final analysis, 189 (5.5%) postponed their gynecologic surgery during the pandemic. Forty-three patients (1.3% of total cases) had a history of COVID-19. The majority (182, 96.3%) had no sequelae attributed to surgical postponement. After hospital discharge to 10 weeks postoperatively, 39 patients (1.1%) became infected with severe acute respiratory syndrome coronavirus 2. The mean duration of time between hospital discharge and the follow-up positive COVID-19 test was 22.1 ± 12.3 days (range, 4–50 days). Eleven (31.4% of postoperative COVID-19 infections, 0.3% of total cases) of the newly diagnosed COVID-19 infections occurred within 14 days of hospital discharge. On multivariable logistic regression, living in the Southwest (adjusted odds ratio, 6.8) and single-unit increase in age-adjusted Charlson comorbidity index (adjusted odds ratio, 1.2) increased the odds of postoperative COVID-19 infection. Perioperative complications were not significantly higher in patients with a history of positive COVID-19 than those without a history of COVID-19, although the mean duration of time between previous COVID-19 diagnosis and surgery was 97 days (14 weeks). Conclusion In this large multicenter prospective cohort study of benign gynecologic surgeries, only 1.1% of patients developed a postoperative COVID-19 infection, with 0.3% of infection in the immediate 14 days after surgery. The incidence of postoperative complications was not different in those with and without previous COVID-19 infections

Data from: Processing of simple and complex acoustic signals in a tonotopically organized ear

Processing of complex signals in the hearing organ remains poorly understood. This paper aims to contribute to this topic by presenting investigations on the mechanical and neuronal response of the hearing organ of the tropical bushcricket species Mecopoda elongata to simple pure tone signals as well as to the conspecific song as a complex acoustic signal. The high-frequency hearing organ of bushcrickets, the crista acustica (CA), is tonotopically tuned to frequencies between about 4 and 70 kHz. Laser Doppler vibrometer measurements revealed a strong and dominant low-frequency-induced motion of the CA when stimulated with either pure tone or complex stimuli. Consequently, the high-frequency distal area of the CA is more strongly deflected by low-frequency-induced waves than by high-frequency-induced waves. This low-frequency dominance will have strong effects on the processing of complex signals. Therefore, we additionally studied the neuronal response of the CA to native and frequency-manipulated chirps. Again, we found a dominant influence of low-frequency components within the conspecific song, indicating that the mechanical vibration pattern highly determines the neuronal response of the sensory cells. Thus, we conclude that the encoding of communication signals is modulated by ear mechanics