Weighing the cusp at the Galactic Centre

As stars close to the galactic centre have short orbital periods it has been possible to trace large fractions of their orbits in the recent years. Previously the data of the orbit of the star S2 have been fitted with Keplerian orbits corresponding to a massive black hole (MBH) with a mass of M_BH = 3–4 × 10⁶M_⊙ implying an insignificant cusp mass. However, it has also been shown that the central black hole resides in a ∼1″ diameter stellar cluster of a priori unknown mass. In a spherical potential which is neither Keplerian nor harmonic, orbits will precess resulting in inclined rosetta shaped trajectories on the sky. In this case, the assumption of non‐Keplerian orbits is a more physical approach. It is also the only approach through which cusp mass information can be obtained via stellar dynamics of the cusp members. This paper presents the first exemplary modelling efforts in this direction. Using positional and radial data of star S2, we find that there could exist an unobserved extended mass component of several 10⁵M_⊙ forming a so‐called ‘cusp’ centered on the black hole position. Considering only the fraction of the cusp mass Mequation/tex2gif-inf-4.gif within the apo‐center of the S2 orbit we find as an upper limit that Mequation/tex2gif-inf-6.gif/(M_BH + Mequation/tex2gif-inf-9.gif) ≤ 0.05. A large extended cusp mass, if present, is unlikely to be composed of sub‐solar mass constituents, but could be explained rather well by a cluster of high M/L stellar remnants, which we find to form a stable configuration. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A solar super‐flare as cause for the 14C variation in AD 774/5?

We present further considerations regarding the strong ¹⁴C variation in AD 774/5. For its cause, either a solar super‐flare or a short gamma‐ray burst were suggested. We show that all kinds of stellar or neutron star flares would be too weak for the observed energy input at Earth in AD 774/5. Even though Maehara et al. (2012) present two super‐flares with ∼10³⁵ erg of presumably solar‐type stars, we would like to caution: These two stars are poorly studied and may well be close binaries, and/or having a M‐type dwarf companion, and/or may be much younger and/or much more magnetic than the Sun – in any such case, they might not be true solar analog stars. From the frequency of large stellar flares averaged over all stellar activity phases (maybe obtained only during grand activity maxima), one can derive (a limit of) the probability for a large solar flare at a random time of normal activity: We find the probability for one flare within 3000 years to be possibly as low as 0.3 to 0.008 considering the full 1σ error range. Given the energy estimate in Miyake et al. (2012) for the AD 774/5 event, it would need to be ∼2000 stronger than the Carrington event as solar super‐flare. If the AD 774/5 event as solar flare would be beamed (to an angle of only ∼24°), 100 times lower energy would be needed. A new AD 774/5 energy estimate by Usoskin et al. (2013) with a different carbon cycle model, yielding 4 ot 6 time lower ¹⁴C production, predicts 4–6 times less energy. If both reductions are applied, the AD 774/5 event would need to be only ∼4 times stronger than the Carrington event in 1859 (if both had similar spectra). However, neither ¹⁴C nor ¹⁰Be peaks were found around AD 1859. Hence, the AD 774/5 event (as solar flare) either was not beamed that strongly, and/or it would have been much more than 4‐6 times stronger than Carrington, and/or the lower energy estimate (Usoskin et al. 2013) is not correct, and/or such solar flares cannot form (enough) ¹⁴C and ¹⁰Be. The 1956 solar energetic particle event was followed by a small decrease in directly observed cosmic rays. We conclude that large solar super‐flares remain very unlikely as the cause for the ¹⁴C increase in AD 774/5. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The applications of the MHD Alfvén wave oscillation model for kHz quasi‐periodic oscillations

In this paper, we improve the previous work on the MHD Alfvén wave oscillation model for the neutron star (NS) kHz quasi‐periodic oscillations (QPOs), and compare the model with the updated twin kHz QPO data. For the 17 NS X‐ray sources with the simultaneously detected twin kHz QPO frequencies, the stellar mass M and radius R constraints are given by means of the derived parameter A in the model, which is associated with the averaged mass density of the star as 〈ρ 〉 = 3M /(4πR³) ≃ 2.4 × 10¹⁴ (A /0.7)² g/cm³, and we also compare the M ‐R constraints with the stellar equations of state. Moreover, we also discuss the theoretical maximum kHz QPO frequency and maximum twin peak separation, and some expectations on SAX J1808.4–3658 are mentioned, such as its highest kHz QPO frequency ∼ 870 Hz, which is about 1.4–1.5 times less than those of the other known kHz QPO sources. The estimated magnetic fields for both Z sources (about Eddington accretion rate ) and Atoll sources (∼ 1% ) are approximately ∼10⁹ G and ∼10⁸ G, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Implications of Non-cylindrical Flux Ropes for Magnetic Cloud Reconstruction Techniques and the Interpretation of Double Flux Rope Events

Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections (ICMEs) which exhibit signatures consistent with a magnetic flux rope structure. Techniques for reconstructing flux rope orientation from single-point in situ observations typically assume the flux rope is locally cylindrical, e.g., minimum variance analysis (MVA) and force-free flux rope (FFFR) fitting. In this study, we outline a non-cylindrical magnetic flux rope model, in which the flux rope radius and axial curvature can both vary along the length of the axis. This model is not necessarily intended to represent the global structure of MCs, but it can be used to quantify the error in MC reconstruction resulting from the cylindrical approximation. When the local flux rope axis is approximately perpendicular to the heliocentric radial direction, which is also the effective spacecraft trajectory through a magnetic cloud, the error in using cylindrical reconstruction methods is relatively small (≈ 10^∘). However, as the local axis orientation becomes increasingly aligned with the radial direction, the spacecraft trajectory may pass close to the axis at two separate locations. This results in a magnetic field time series which deviates significantly from encounters with a force-free flux rope, and consequently the error in the axis orientation derived from cylindrical reconstructions can be as much as 90^∘. Such two-axis encounters can result in an apparent ‘double flux rope’ signature in the magnetic field time series, sometimes observed in spacecraft data. Analysing each axis encounter independently produces reasonably accurate axis orientations with MVA, but larger errors with FFFR fitting.     

Expanding shells in low and high density environments

The gravitational instability of expanding shells evolving in a homogeneous and static medium is discussed. In the low density environment (n = 1 cm^-3), the fragmentation starts in shells with diameters of a few 100 pc and fragment masses are in the range of 5 × 10³ - 10⁶ M _⊙. In the high density environment (n = 10⁵ - 10⁷ cm^-3), shells fragment at diameters of pc producing clumps of stellar masses. The mass spectrum in both environments is approximated by a power law dN/dm ∼ m ^-2.3. This is close to the slope of the stellar IMF. To reproduce the observed mass spectrum of clouds (the spectral index close to ∼ -2.0) we have to assume, that the cloud formation time is independent of the cloud size, similarly to the Jeans unstable medium. This revised version was published online in August 2006 with corrections to the Cover Date.     

On stellar encounters and their effect on cometary orbits in the Oort cloud

We systematically investigate the encounters between the Sun and neighbouring stars and their effects on cometary orbits in the Oort cloud, including the intrinsic one with the star Gl 710 (HIP 89 825), with some implications to stellar and cometary dynamics. Our approach is principally based on the combination of a Keplerian‐rectilinear model of stellar passages and the Hipparcos Catalogue (ESA 1997). Beyond the parameters of encounter, we pay particular attention to the observational errors in parallaxes and stellar velocities, and their propagation in time. Moreover, as a special case of this problem, we consider the collision probability of a star passing very closely to the Sun, taking also into account the mutual gravitational attraction between the stars. In the part dealing with the influence of stellar encounters on the orbital elements of Oort cloud comets, we derive new simple formulae calculating the changes in the cometary orbital elements, expressed as functions of the Jeans impulse formula. These expressions are then applied to calculate numerical values of the element changes caused by close encounters of neighbouring stars with some model comets in the Oort cloud. Moreover, the general condition for an ejection of comets from the cloud effected by a single encounter is derived and discussed.     

Titanium isotopic compositions of well‐characterized silicon carbide grains from Orgueil (CI): Implications for s‐process nucleosynthesis

Abstract— We have measured the titanium isotopic compositions of 23 silicon carbide grains from the Orgueil (CI) carbonaceous chondrites for which isotopic compositions of silicon, carbon, and nitrogen and aluminum‐magnesium systematics had been measured previously. Using the 16 most‐precise measurements, we estimate the relative contributions of stellar nucleosynthesis during the asymptotic giant branch (AGB) phase and the initial compositions of the parent stars to the compositions of the grains. To do this, we compare our data to the results of several published stellar models that employ different values for some important parameters. Our analysis confirms that s‐process synthesis during the AGB phase only slightly modified the titanium compositions in the envelopes of the stars where mainstream silicon carbide grains formed, as it did for silicon. Our analysis suggests that the parent stars of the >1 μm silicon carbide grains that we measured were generally somewhat more massive than the Sun (2–3 M_⊙) and had metallicities similar to or slightly higher than solar. Here we differ slightly from results of previous studies, which indicated masses at the lower end of the range 1.5–3 M_⊙ and metallicities near solar. We also conclude that models using a standard ¹³C pocket, which produces a good match for the main component of s‐process elements in the solar system, overestimate the contribution of the ¹³C pocket to s‐process nucleosynthesis of titanium found in silicon carbide grains. Although previous studies have suggested that the solar system has a significantly different titanium isotopic composition than the parent stars of silicon carbide grains, we find no compelling evidence that the Sun falls off of the array defined by those stars. We also conclude that the Sun does lie on the low‐metallicity end of the silicon and titanium arrays defined by mainstream silicon carbide grains.     

Short‐lived radioactivity in the early solar system: The Super‐AGB star hypothesis

Abstract– The composition of the most primitive solar system condensates, such as calcium‐aluminum‐rich inclusions (CAIs) and micron‐sized corundum grains, show that short‐lived radionuclides (SLR), e.g., ²⁶Al, were present in the early solar system. Their abundances require a local or stellar origin, which, however, is far from being understood. We present for the first time the abundances of several SLR up to ⁶⁰Fe predicted from stars with initial mass in the range approximately 7–11 M_⊙. These stars evolve through core H, He, and C burning. After core C burning they go through a “Super”‐asymptotic giant branch (Super‐AGB) phase, with the H and He shells activated alternately, episodic thermal pulses in the He shell, a very hot temperature at the base of the convective envelope (approximately 10⁸ K), and strong stellar winds driving the H‐rich envelope into the surrounding interstellar medium. The final remnants of the evolution of Super‐AGB stars are mostly O–Ne white dwarfs. Our Super‐AGB models produce ²⁶Al/²⁷Al yield ratios approximately 0.02–0.26. These models can account for the canonical value of the ²⁶Al/²⁷Al ratio using dilutions with the solar nebula of the order of 1 part of Super‐AGB mass per several 10² to several 10³ of solar nebula mass, resulting in associated changes in the O‐isotope composition in the range Δ¹⁷O from 3 to 20‰. This is in agreement with observations of the O isotopic ratios in primitive solar system condensates, which do not carry the signature of a stellar polluter. The radionuclides ⁴¹Ca and ⁶⁰Fe are produced by neutron captures in Super‐AGB stars and their meteoritic abundances are also matched by some of our models, depending on the nuclear and stellar physics uncertainties as well as the meteoritic experimental data. We also expect and are currently investigating Super‐AGB production of SLR heavier than iron, such as ¹⁰⁷Pd.     

Tomography of stellar non‐radial pulsations

The stellar surface imaging technique is used for studying stellar non‐radial pulsations on the basis of inversions of time series of variable line profiles without making assumptions on the specific shape of the pulsations. The inversion results in an image of the stellar surface in which sectoral and tesseral modes can be distinguished in many cases and the pulsational degree and the azimuthal order can be determined. The capability of the technique is studied with simulated data. Then, the surface imaging technique is applied to high‐resolution spectra of the rapidly rotating Beta Cep‐type star ω¹ Sco, which shows strong line‐profile variations. Stellar surface imaging is concluded to be a useful technique for pulsation‐mode identification. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hα emission objects in Circinus

We present a new survey for Hα emission objects in the Circinus cloud complex and introduce an efficient photometric method for detecting Hα emission via observations in a narrow‐band filter. The observed flux is compared to a blackbody fit of the continuum. Our search strategy reveals 20 stars with strong Hα emission (EW > 10 Å), eight of them being new detections. All Hα stars display infrared excess corroborating their youth. On the other hand, the region contains a number of infrared excess objects that do not show Hα emission. Our results support the picture that accretion – as witnessed by Hα emission – is a highly variable phenomenon. Therefore, Hα surveys can only trace the temporarily active objects. In contrast, infrared excess is a more robust tracer that reveals most of the population of young stellar objects in a star forming region. Our analysis of the general stellar content of the region yields a reliable distance of 450 pc for the Circinus cloud. Moreover, we find a ratio of total‐to‐selective extinction of R^V = 2.8 suggesting that smaller‐than‐normal dust grains may be present. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effects of disk building on the distributions of refractory materials in the solar nebula

Abstract– Refractory materials, such as calcium‐aluminum‐rich inclusions (CAIs) and crystalline silicates, are widely found in chondritic meteorites as well as comets, taken as evidence for large‐scale mixing in the solar nebula. Most models for mixing in the solar nebula begin with a well‐formed protoplanetary disk. Here, we relax this assumption by modeling the formation and evolution of the solar nebula during and after the period when it accreted material from its parent molecular cloud. We consider how disk building impacts the long‐term evolution of the disk and the implications for grain transport and mixing within it. Our model shows that materials that formed before infall was complete could be preserved in primitive bodies, especially those that accreted in the outer disk. This potentially explains the discovery of refractory objects with low initial ²⁶Al/²⁷Al ratios in comets. Our model also shows that the highest fraction of refractory materials in meteorites formed around the time that infall stopped. Thus, we suggest that the calcium‐aluminum‐rich inclusions in chondrites would be dominated by the population that formed during the transition from class I to class II stage of young stellar objects. This helps us to understand the meaning of t = 0 in solar system chronology. Moreover, our model offers a possible explanation for the existence of isotopic variations observed among refractory materials—that the anomalous materials formed before the collapse of the parent molecular cloud was complete.     

Calculated atmospheric color refraction and observed stellar positions

Astrometric observations at different zenith distances have been performed in Dresden in an area centered atNGC 6791 where there are some stars with reliable color information (widely dispersed spectral types in the MK systemand color indices B_T – V_T) as well as with accurate positions from Tycho‐2 catalog. The results are used to estimate how significant improvements in stellar positions may be when accurate corrections for color refraction are taken into account. We have treated two cases for refraction calculations: (1) a photometric case for color indices and (2) a spectral case for spectral types and luminosity classes. To calculate refraction we use Stone's modified computer code (Malyuto & Meinel 2000). To treat the photometric case we have calculated the synthetic color indices for the spectral energy distributions of Sviderskiene (1988). The positional improvements due to including color refraction corrections are significant and slightly larger in the spectral case. An improvement of about 15% is reached at a zenith distance of 65°. Our basic conclusion is that color refraction should be taken into account for obtaining accurate stellar positions from ground based observations at larger zenith distances. Reliable refraction corrections may be calculated from spectral and/or photometric data.     

Neutrinooszillationen in Neutronensternmaterie

According to the suggestion of T. J. Mazurek (1979) neutrino oscillations occuring during the dynamic stellar collapse (M ≥ 10M) could be result in a transfer of leptonic zero-point energy to baryons. Then the adiabatic index increases above γ ≥ 4/3, and such an increase is necessary to reverse the collapse. From the theory of neutrino oscillations of B. Pontekorvo (1967) we derive the oszillation length L of neutrinos in vacuum and the characteristic oscillation lengh L^* of neutrinos taking into consideration the refraction index n_e of neutron star matter. The comparison of both oscillation lenghts shows that for electron densities, characteristically of neutron star matter, the oscillation lenght L is considerable larger than the oscillation lenght L^*. Therefore neutrino oscillations cannot influence the scenario for neutrino emission of the neutron star.     

Applications to stellar and galactic dynamics

Computation and a wealth of new observational techniques have reinvigorated dynamical studies of galaxies and star clusters. These objects are examples of the gravitationaln-body problem withn in the range from a few hundred to 10¹¹. Relaxation effects dominate at the low end and are completely negligible at the high end. The gravitationaln-body problem is chaotic, and the principal challenge in doing physics where that problem is involved (whether computationally or with analytic theory) is to ensure that chaos has not vitiated the results. Enforcing a Liouville theorem accomplishes this with collision-free large-n problems, but equivalent recipes are not in common use for smallern. We describe some important insights and discoveries that have come from computation in stellar dynamics, discuss chaos briefly, and indicate the way the physics that comes up in different astronomical contexts is addressed in numerical methods currently in use. Graphics is a vital part of any computational approach. The long range prospects are very promising for continued high scientific productivity in stellar dynamics.     

On the intercomponent emission in close binary systems

An empirical relationship is discovered for RS CVn type close binary systems between their absolute luminosity, L(MgII), of the ultraviolet magnesium doublet 2800 MgII, and the intercomponent distancea of the system. It has the following form: L(MgII) a ⁿ(Figure 1). It is shown that for the overwhelming majority of binary systemsn = 1 (Figure 4). This correlation presents itself as a direct confirmation of the intercomponent origin of the observed emission, particularly, in the magnesium doublet in close binary systems. The basic relationship of intercomponent emission is derived in the form: L(MgII) = 1.0 × 10³² a ergs s^–1. At the same time, accidental statistical divergences from this correlation are possible on both sides: asn > 1 as welln < 1 (Figure 4). The correlationn = 1 determines also the character, - i.e. cylindric for a stream - of the transfer of gaseous matter from one component of the system to the other, and in the general gas dynamics of the intercomponent medium.The existence of a new category of stellar atmosphere, - which we callroundchrom, is predicted, representing the common chromosphere of a superclose binary system, surrounding or blending both components of the system (Figure 3). The boundaries between the three most important divisions of magnesium doublet emission - chromosphere of single stars, roundchrom of superclose binary systems and intercomponent space - are established for RS CVn type systems. Finally, a number of new problems, both observational and theoretical, are brought forward.     

Bursts of gravitational radiation from active galactic nuclei and globular clusters

The characteristics of gravitational bursts from active galactic nuclei, and globular clusters are obtained for three astrophysical situations:(i) scattering of stars by massive black holes residued at the centers of galaxies and globular clusters; (ii) the close encounters of stars in the nuclear regions of these objects; (iii) scattering of stars by black holes of stellar mass containing in the stellar population of galactic nuclei and clusters. The most effective source of gravitational bursts appears to be a scattering of stars by the massive central black holes which produces the bursts with dimensionless amplitudeh10^–19–10^–21 and frequencies from 10^–1 to 10^–5 Hz. The characteristics obtained correspond to the possiblities of a future gravitational-wave experiment with use of laser Doppler tracking of interplanetary spacecrafts.     

Constraining the 13C neutron source in AGB stars through isotopic analysis of trace elements in presolar SiC

Abstract— Analyses of the isotopic compositions of multiple elements (Mo, Zr, and Ba) in individual mainstream presolar SiC grains were done by resonant ionization mass spectrometry (RIMS). While most heavy element compositions were consistent with model predictions for the slow neutron capture process (s‐process) in low‐mass (1.5–3 M_⊙) asymptotic giant branch stars of solar metallicity when viewed on single‐element three‐isotope plots, grains with compositions deviating from model predictions were identified on multi‐element plots. These grains have compositions that cannot result from any neutron capture process but can be explained by contamination in some elements with solar system material. Previous work in which only one heavy element per grain was examined has been unable to identify contaminated grains. The multi‐element analyses of this study detected contaminated grains which were subsequently eliminated from consideration. The uncontaminated grains form a data set with a greatly reduced spread on the three‐isotope plots of each element measured, corresponding to a smaller range of ¹³C pocket efficiencies in parent AGB stars. Furthermore, due to this reduced spread, the nature of the stellar starting material, previously interpreted as having solar isotopic composition, is uncertain. The constraint on ¹³C pocket efficiencies in parent stars of these grains may help uncover the mechanism responsible for formation of ¹³C, the primary neutron source for s‐process nucleosynthesis in low‐mass stars.     

High‐mass resolution molecular imaging of organic compounds on the surface of Murchison meteorite

High‐resolution mass spectrometry (HRMS) imaging by desorption electrospray ionization (DESI) coupled with Orbitrap MS using methanol (MeOH) spray was performed on a fragment of the Murchison (CM2) meteorite in this study. Homologues of C_nH_2n–1N₂⁺ (n = 7–9) and C_nH_2nNO⁺ (n = 9–14) were detected on the sample surface by the imaging. A high‐performance liquid chromatography (HPLC)/HRMS analysis of MeOH extracts from the sample surface after DESI/HRMS imaging indicated that the C_nH_2n–1N₂⁺ homologues corresponds to alkylimidazole, and that a few isomers of the C_nH_2nNO⁺ homologues present in the sample. The alkylimidazoles and C_nH_2nNO⁺ homologues displayed different spatial distributions on the surface of the Murchison fragment, indicating chromatographic separation effects during aqueous alteration. Moreover, the distribution pattern of compounds is also different among homologues. This is probably also resulting from the separation of isomers by similar chromatographic effects, or different synthetic pathways. Alkylimidazoles and the C_nH_2nNO⁺ homologues are mainly distributed in the matrix region of the Murchison by mineralogical observations, which is consistent with previous reports. Altered minerals (e.g., Fe‐oxide, Fe‐sulfide, and carbonates) occurred in this region. However, no clear relationship was found between these minerals and the organic compounds detected by DESI/HRMS imaging. Although this result might be due to scale differences between the spatial resolution of DESI/HRMS imaging and the grain size in the matrix of the Murchison, our results would indicate that alkylimidazoles and the C_nH_2nNO⁺ homologues in the Murchison fragment were mainly synthesized by different processes from hydrothermal alteration on the parent body.     

Algorithms for Stellar Perturbation Computations on Oort Cloud Comets

We investigate different approximate methods of computing the perturbations on the orbits of Oort cloud comets caused by passing stars, by checking them against an accurate numerical integration using Everhart’s RA15 code. The scenario under study is the one relevant for long-term simulations of the cloud’s response to a predefined set of stellar passages. Our sample of stellar encounters simulates those experienced by the Solar System currently, but extrapolated over a time of 10¹⁰ years. We measure the errors of perihelion distance perturbations for high-eccentricity orbits introduced by several estimators – including the classical impulse approximation and Dybczyński’s (1994, Celest. Mech. Dynam. Astron. 58, 1330–1338) method – and we study how they depend on the encounter parameters (approach distance and relative velocity). We introduce a sequential variant of Dybczyński’s approach, cutting the encounter into several steps whereby the heliocentric motion of the comet is taken into account. For the scenario at hand this is found to offer an efficient means to obtain accurate results for practically any domain of the parameter space.