MAGNETIC FIELD EFFECTS NEAR THE LAUNCHING REGION OF ASTROPHYSICAL JETS

One of the fundamental properties of astrophysical magnetic fields is their ability to change topology through reconnection and in doing so, to release magnetic energy, sometimes violently. In this work, we review recent results on the role of magnetic reconnection and associated heating and particle acceleration in jet/accretion disk systems, namely young stellar objects (YSOs), microquasars, and active galactic nuclei (AGNs).

Cross-entropy optimizer: a new tool to study precession in astrophysical jets

Evidence of jet precession in many galactic and extragalactic sources has been reported in the literature. Much of this evidence is based on studies of the kinematics of the jet knots, which depends on the correct identification of the components to determine their respective proper motions and position angles on the plane of the sky. Identification problems related to fitting procedures, as well as observations poorly sampled in time, may influence the follow-up of the components in time, which consequently might contribute to a misinterpretation of the data. In order to deal with these limitations, we introduce a very powerful statistical tool to analyse jet precession: the cross-entropy method for continuous multi-extremal optimization. Only based on the raw data of the jet components (right ascension and declination offsets from the core), the cross-entropy method searches for the precession model parameters that better represent the data. In this work we present a large number of tests to validate this technique, using synthetic precessing jets built from a given set of precession parameters. With the aim of recovering these parameters, we applied the cross-entropy method to our precession model, varying exhaustively the quantities associated with the method. Our results have shown that even in the most challenging tests, the cross-entropy method was able to find the correct parameters within a 1 per cent level. Even for a non-precessing jet, our optimization method could point out successfully the lack of precession.

Climatic simulations of the eastern Andes low-level jet and its dependency on convective parameterizations

The South American low level jet (SALLJ) of the Eastern Andes is investigated with Regional Climate Model version 3 (RegCM3) simulations during the 2002-2003 austral summer using two convective parameterizations (Grell and Emanuel). The simulated SALLJ is compared with the special observations of SALLJEX (SALLJ Experiment). Both the Grell and Emanuel schemes adequately simulate the low level flow over South America. However, there are some intensity differences. Due to the larger (smaller) convective activity, the Emanuel (Grell) scheme simulates more intense (weaker) low level wind than analysis in the tropics and subtropics. The objectives criteria of Sugahara (SJ) and Bonner (BJ) were used for LLJ identification. When applied to the observations, both criteria suggest a larger frequency of the SALLJ in Santa Cruz, followed by Mariscal, Trinidad and Asuncin. In Mariscal and Asuncin, the diurnal cycle indicates that SJ occurs mainly at 12 UTCs (morning), while the BJ criterion presents the SALLJ as more homogenously distributed. The concentration into two of the four-times-a-day observations does not allow conclusions about the diurnal cycle in Santa Cruz and Trinidad. The simulated wind profiles result in a lower than observed frequency of SALLJ using both the SJ and BJ criteria, with fewer events obtained with the BJ. Due to the stronger simulated winds, the Emanuel scheme produces an equal or greater relative frequency of SALLJ than the Grell scheme. However, the Grell scheme using the SJ criterion simulates the SALLJ diurnal cycle closer to the observed one. Although some discrepancies between observed and simulated mean vertical profiles of the horizontal wind are noted, there is large agreement between the composites of the vertical structure of the SALLJ, especially when the SJ criterion is used with the Grell scheme. On an intraseasonal scale, a larger southward displacement of SALLJ in February and December when compared with January has been noted. The Grell and Emanuel schemes simulated this observed oscillation in the low-level flow. However, the spatial pattern and intensity of rainfall and circulation anomalies simulated by the Grell scheme are closer to the analyses than those obtained with the Emanuel scheme.

MODELING VERY LONG BASELINE INTERFEROMETRIC IMAGES WITH THE CROSS-ENTROPY GLOBAL OPTIMIZATION TECHNIQUE

We present a new technique for obtaining model fittings to very long baseline interferometric images of astrophysical jets. The method minimizes a performance function proportional to the sum of the squared difference between the model and observed images. The model image is constructed by summing N(s) elliptical Gaussian sources characterized by six parameters: two-dimensional peak position, peak intensity, eccentricity, amplitude, and orientation angle of the major axis. We present results for the fitting of two main benchmark jets: the first constructed from three individual Gaussian sources, the second formed by five Gaussian sources. Both jets were analyzed by our cross-entropy technique in finite and infinite signal-to-noise regimes, the background noise chosen to mimic that found in interferometric radio maps. Those images were constructed to simulate most of the conditions encountered in interferometric images of active galactic nuclei. We show that the cross-entropy technique is capable of recovering the parameters of the sources with a similar accuracy to that obtained from the very traditional Astronomical Image Processing System Package task IMFIT when the image is relatively simple (e. g., few components). For more complex interferometric maps, our method displays superior performance in recovering the parameters of the jet components. Our methodology is also able to show quantitatively the number of individual components present in an image. An additional application of the cross-entropy technique to a real image of a BL Lac object is shown and discussed. Our results indicate that our cross-entropy model-fitting technique must be used in situations involving the analysis of complex emission regions having more than three sources, even though it is substantially slower than current model-fitting tasks (at least 10,000 times slower for a single processor, depending on the number of sources to be optimized). As in the case of any model fitting performed in the image plane, caution is required in analyzing images constructed from a poorly sampled (u, v) plane.

Mapping low- and high-density clouds in astrophysical nebulae by imaging forbidden line emission

Emission line ratios have been essential for determining physical parameters such as gas temperature and density in astrophysical gaseous nebulae. With the advent of panoramic spectroscopic devices, images of regions with emission lines related to these physical parameters can, in principle, also be produced. We show that, with observations from modern instruments, it is possible to transform images taken from density-sensitive forbidden lines into images of emission from high- and low-density clouds by applying a transformation matrix. In order to achieve this, images of the pairs of density-sensitive lines as well as the adjacent continuum have to be observed and combined. We have computed the critical densities for a series of pairs of lines in the infrared, optical, ultraviolet and X-rays bands, and calculated the pair line intensity ratios in the high- and low-density limit using a four- and five-level atom approximation. In order to illustrate the method, we applied it to Gemini Multi-Object Spectrograph (GMOS) Integral Field Unit (GMOS-IFU) data of two galactic nuclei. We conclude that this method provides new information of astrophysical interest, especially for mapping low- and high-density clouds; for this reason, we call it `the ld/hd imaging method`.

High-frequency extensions of magnetorotational instability in astrophysical plasmas

High-frequency extensions of magnetorotational instability driven by the Velikhov effect beyond the standard magnetohydrodynamic (MHD) regime are studied. The existence of the well-known Hall regime and a new electron inertia regime is demonstrated. The electron inertia regime is realized for a lesser plasma magnetization of rotating plasma than that in the Hall regime. It includes the subregime of nonmagnetized electrons. It is shown that, in contrast to the standard MHD regime and the Hall regime, magnetorotational instability in this subregime can be driven only at positive values of dln Omega/dlnr, where Omega is the plasma rotation frequency and r is the radial coordinate. The permittivity of rotating plasma beyond the standard MHD regime, including both the Hall regime and the electron inertia regime, is calculated.

Radiative capture of nucleons at astrophysical energies with single-particle states

Radiative capture of nucleons at energies of astrophysical interest is one of the most important processes for nucleosynthesis. The nucleon capture can occur either by a compound nucleus reaction or by a direct process. The compound reaction cross sections are usually very small, especially for light nuclei. The direct capture proceeds either via the formation of a single-particle resonance or a non-resonant capture process. In this work we calculate radiative capture cross sections and astrophysical S-factors for nuclei in the mass region A < 20 using single-particle states. We carefully discuss the parameter fitting procedure adopted in the simplified two-body treatment of the capture process. Then we produce a detailed list of cases for which the model works well. Useful quantities, such as spectroscopic factors and asymptotic normalization coefficients, are obtained and compared to published data. (C) 2010 Elsevier Inc. All rights reserved.

Astrophysical S factors for fusion reactions involving C, O, Ne, and Mg isotopes

Using the Sao Paulo potential and the barrier penetration formalism we have calculated the astrophysical factor S(E) for 946 fusion reactions involving stable and neutron-rich isotopes of C, O, Ne, and Mg for center-of-mass energies E varying from 2 to approximate to 18-30 MeV (covering the range below and above the Coulomb barrier). We have parameterized the energy dependence, S(E), by an accurate universal 9-parameter analytic expression and present tables of fit parameters for all the reactions. We also discuss the reduced 3-parameter version of our fit which is highly accurate at energies below the Coulomb barrier, and outline the procedure for calculating the reaction rates. The results can be easily converted to thermonuclear or pycnonuclear reaction rates to simulate various nuclear burning phenomena, in particular, stellar burning at high temperatures and nucleosynthesis in high density environments. (C) 2010 Elsevier Inc. All rights reserved

Indirect study of (11)B(p,alpha(0))(8)Be and (10)B(p,alpha)(7)Be reactions at astrophysical energies by means of the Trojan Horse Method: recent results

Nuclear (p,alpha) reactions destroying the so-called ""light-elements"" lithium, beryllium and boron have been largely studied in the past mainly because their role in understanding some astrophysical phenomena, i.e. mixing-phenomena occurring in young F-G stars [1]. Such mechanisms transport the surface material down to the region close to the nuclear destruction zone, where typical temperatures of the order of similar to 10(6) K are reached. The corresponding Gamow energy E(0)=1.22 (Z(x)(2)Z(X)(2)T(6)(2))(1/3) [2] is about similar to 10 keV if one considers the ""boron-case"" and replaces in the previous formula Z(x) = 1, Z(X) = 5 and T(6) = 5. Direct measurements of the two (11)B(p,alpha(0))(8)Be and (10)B(p,alpha)(7)Be reactions in correspondence of this energy region are difficult to perform mainly because the combined effects of Coulomb barrier penetrability and electron screening [3]. The indirect method of the Trojan Horse (THM) [4-6] allows one to extract the two-body reaction cross section of interest for astrophysics without the extrapolation-procedures. Due to the THM formalism, the extracted indirect data have to be normalized to the available direct ones at higher energies thus implying that the method is a complementary tool in solving some still open questions for both nuclear and astrophysical issues [7-12].

Indirect Study of the (2)H(d,p)(3)H and (2)H(d,n)(3)He Reactions at Astrophysical Energies via the Trojan Horse Method

The (2)H(d,p)(3)H and (2)H(d,n)(3)He reactions have been indirectly studied by means of the Trojan Horse Method applied to the quasi-free (2)H((3)He, p(3)H)(1)H (2)H((3)He, n(3)He)(1)H reaction at 18 MeV of beam energy. This is the first experiment where the spectator (here (1)H) has been detected in coincidence with the charged participant, avoiding the limitations of standard neutron detectors. The d - d relative energy has been measured from 1.5 MeV down to 2 keV, at center of mass angles from 40A degrees to 170A degrees. Indirect angular distributions are compared with the direct behaviour in the overlapping regions.