A Comparative Study of the Parker Instability under Three Models of the Galactic Gravity

To examine how non-uniform nature of the Galactic gravity might affect length and time scales of the Parker instability, we took three models of gravity, uniform, linear and realistic ones. To make comparisons of the three gravity models on a common basis, we first fixed the ratio of magnetic pressure to gas pressure at $\alpha$ = 0.25, that of cosmic-ray pressure at $\beta$ = 0.4, and the rms velocity of interstellar clouds at $a_s$ = 6.4 km s$^{-1}$, and then adjusted parameters of the gravity models in such a way that the resulting density scale heights for the three models may all have the same value of 160 pc. Performing linear stability analyses onto equilibrium states under the three models with the typical ISM conditions, we calculate the maximum growth rate and corresponding length scale for each of the gravity models. Under the uniform gravity the Parker instability has the growth time of 1.2$\times10^{8}$ years and the length scale of 1.6 kpc for symmetric mode. Under the realistic gravity it grows in 1.8$\times10^{7}$ years for both symmetric and antisymmetric modes, and develops density condensations at intervals of 400 pc for the symmetric mode and 200 pc for the antisymmetric one. A simple change of the gravity model has thus reduced the growth time by almost an order of magnitude and its length scale by factors of four to eight. These results suggest that an onset of the Parker instability in the ISM may not necessarily be confined to the regions of high $\alpha$ and $\beta$.Comment: Accepted for publication in ApJ, using aaspp4.sty, 18 text pages with 9 figure

The most massive progenitors of neutron stars: CXO J164710.2-455216

The evolution leading to the formation of a neutron star in the very young Westerlund 1 star cluster is investigated. The turnoff mass has been estimated to be 35 Msun, indicating a cluster age ~ 3-5 Myr. The brightest X-ray source in the cluster, CXO J164710.2-455216, is a slowly spinning (10 s) single neutron star and potentially a magnetar. Since this source was argued to be a member of the cluster, the neutron star progenitor must have been very massive (M_zams > 40 Msun) as noted by Muno et al. (2006). Since such massive stars are generally believed to form black holes (rather than neutron stars), the existence of this object poses a challenge for understanding massive star evolution. We point out while single star progenitors below M_zams < 20 Msun form neutron stars, binary evolution completely changes the progenitor mass range. In particular, we demonstrate that mass loss in Roche lobe overflow enables stars as massive as 50-80 Msun, under favorable conditions, to form neutron stars. If the very high observed binary fraction of massive stars in Westerlund 1 (> 70 percent) is considered, it is natural that CXO J164710.2-455216 was formed in a binary which was disrupted in a supernova explosion such that it is now found as a single neutron star. Hence, the existence of a neutron star in a given stellar population does not necessarily place stringent constraints on progenitor mass when binary interactions are considered. It is concluded that the existence of a neutron star in Westerlund 1 cluster is fully consistent with the generally accepted framework of stellar evolution.Comment: 5 pages of text and 4 figures (submitted to Astrophysical Journal

Are the jets accelerated from the disk coronas in some active galactic nuclei?

We use a sample of radio-loud active galactic nuclei (AGNs) with estimated central black hole masses to explore their jet formation mechanisms. The jet power of AGNs is estimated from their extended radio luminosity. It is found that the jets in several AGNs of this sample are too powerful to be extracted from the standard thin accretion disks or rapidly spinning black holes surrounded by standard thin disks. If the advection dominated accretion flows (ADAFs) are present in these AGNs, their bright optical continuum luminosity cannot be produced by pure-ADAFs due to their low accretion rates and low radiation efficiency, unless the ADAFs transit to standard thin disks at some radii $R_{\rm tr}$. If this is the case, we find that the dimensionless accretion rates as high as 0.05 and transition from ADAFs to standard thin disks at rather small radii around 20GM/c^2 are required to explain their bright optical continuum emission. We propose that the disk-corona structure is present at least in some AGNs in this sample. The plasmas in the corona are very hot, and the pressure scale-height of the corona H\sim R. Powerful jets with Q_jet \sim L_bol (bolometric luminosity) can form by the large-scale magnetic fields created by dynamo processes in the disk corona of some AGNs. The maximal jet power extractable from the corona Q_jet^max\le 0.6L_c (L_c is the corona luminosity) is expected by this jet formation scenario. The statistic results on the sample of AGNs are consistent with the predictions of this scenario. Finally, the possibility that the jet is driven from a super-Keplerian rotating hot layer located between the corona and the cold disk is discussed. We find that, in principle, this layer can also produce a powerful jet with Q_jet\sim L_bol.Comment: 9 pages, accepted for publication in Ap

Zeeman tomography of magnetic white dwarfs. II, The quadrupole-dominated magnetic field of HE 1045-0908

We report time-resolved optical flux and circular polarization spectroscopy of the magnetic DA white dwarf HE 1045−0908 obtained with FORS1 at the ESO VLT. Considering published results, we estimate a likely rotational period of P rot 2.7 h, but cannot exclude values as high as about 9 h. Our detailed Zeeman tomographic analysis reveals a field structure which is dominated by a quadrupole and contains additional dipole and octupole contributions, and which does not depend strongly on the assumed value of the period. A good fit to the Zeeman flux and polarization spectra is obtained if all field components are centred and inclinations of their magnetic axes with respect to each other are allowed for. The fit can be slightly improved if an offset from the centre of the star is included. The prevailing surface field strength is 16 MG, but values between 10 and ∼ 75 MG do occur. We derive an effective photospheric temperature of HE 1045−0908 of T eff = 10 000 ± 1000 K. The tomographic code makes use of an extensive database of pre-computed Zeeman spectra (Paper I)

Interacting Binaries with Eccentric Orbits. Secular Orbital Evolution Due To Conservative Mass Transfer

We investigate the secular evolution of the orbital semi-major axis and eccentricity due to mass transfer in eccentric binaries, assuming conservation of total system mass and orbital angular momentum. Assuming a delta function mass transfer rate centered at periastron, we find rates of secular change of the orbital semi-major axis and eccentricity which are linearly proportional to the magnitude of the mass transfer rate at periastron. The rates can be positive as well as negative, so that the semi-major axis and eccentricity can increase as well as decrease in time. Adopting a delta-function mass-transfer rate of 10^{-9} M_\sun {\rm yr}^{-1} at periastron yields orbital evolution timescales ranging from a few Myr to a Hubble time or more, depending on the binary mass ratio and orbital eccentricity. Comparison with orbital evolution timescales due to dissipative tides furthermore shows that tides cannot, in all cases, circularize the orbit rapidly enough to justify the often adopted assumption of instantaneous circularization at the onset of mass transfer. The formalism presented can be incorporated in binary evolution and population synthesis codes to create a self-consistent treatment of mass transfer in eccentric binaries.Comment: 16 pages, 8 figures, Accepted by The Astrophysical Journa

The Binarity of Eta Carinae and its Similarity to Related Astrophysical Objects

I examine some aspects of the interaction between the massive star Eta Carinae and its companion, in particular during the eclipse-like event, known as the spectroscopic event or the shell event. The spectroscopic event is thought to occur when near periastron passages the stellar companion induces much higher mass loss rate from the primary star, and/or enters into a much denser environment around the primary star. I find that enhanced mass loss rate during periastron passages, if it occurs, might explain the high eccentricity of the system. However, there is not yet a good model to explain the presumed enhanced mass loss rate during periastron passages. In the region where the winds from the two stars collide, a dense slow flow is formed, such that large dust grains may be formed. Unlike the case during the 19th century Great Eruption, the companion does not accrete mass during most of its orbital motion. However, near periastron passages short accretion episodes may occur, which may lead to pulsed ejection of two jets by the companion. The companion may ionize a non-negligible region in its surrounding, resembling the situation in symbiotic systems. I discuss the relation of some of these processes to other astrophysical objects, by that incorporating Eta Car to a large class of astrophysical bipolar nebulae.Comment: Updated version. ApJ, in pres

Three-dimensional simulations of the Parker instability in a uniformly rotating disk

We investigate the effects of rotation on the evolution of the Parker instability by carrying out three-dimensional numerical simulations with an isothermal magnetohydrodynamic code. These simulations extend our previous work on the nonlinear evolution of the Parker instability by J. Kim and coworkers. The initial equilibrium system is composed of exponentially stratified gas and a field (along the azimuthal direction) in a uniform gravity (along the downward vertical direction). The computational box, placed at the solar neighborhood, is set to rotate uniformly around the Galactic center with a constant angular speed. The instability has been initialized by random velocity perturbations. In the linear stage, the evolution is not much different from that without rotation, and the mixed (undular + interchange) mode regulates the system. The interchange mode induces alternating dense and rarefied regions with small radial wavelengths, while the undular mode bends the magnetic field lines in the plane of the azimuthal and vertical directions. In the nonlinear stage, flow motion overall becomes chaotic, as in the case without rotation. However, as the gas in higher positions slides down along field lines forming supersonic flows, the Coriolis force becomes important. As oppositely directed flows fall into valleys along both sides of the magnetic field lines, they experience the Coriolis force toward opposite directions, which twists the magnetic field lines there. Hence, we suggest that the Coriolis force plays a role in randomizing the magnetic field. The three-dimensional density structure formed by the instability is still sheetlike with the short dimension along the radial direction, as in the case without rotation. However, the long dimension is now slightly tilted with respect to the mean field direction. The shape of high-density regions is a bit rounder. The maximum enhancement factor of the vertical column density relative to its initial value is about 1.5, which is smaller than that in the case without rotation. We conclude that uniform rotation does not change our point of view that the Parker instability alone is not a viable mechanism for the formation of giant molecular cloudsopen252

A Search for Nitrogen-Enhanced Metal-Poor Stars

Theoretical models of very metal-poor intermediate-mass Asymptotic Giant Branch (AGB) stars predict a large overabundance of primary nitrogen. The very metal-poor, carbon-enhanced, s-process-rich stars, which are thought to be the polluted companions of now-extinct AGB stars, provide direct tests of the predictions of these models. Recent studies of the carbon and nitrogen abundances in metal-poor stars have focused on the most carbon-rich stars, leading to a potential selection bias against stars that have been polluted by AGB stars that produced large amounts of nitrogen, and hence have small [C/N] ratios. We call these stars Nitrogen-Enhanced Metal-Poor (NEMP) stars, and define them as having [N/Fe] > +0.5 and [C/N] < -0.5. In this paper, we report on the [C/N] abundances of a sample of 21 carbon-enhanced stars, all but three of which have [C/Fe] < +2.0. If NEMP stars were made as easily as Carbon-Enhanced Metal-Poor (CEMP) stars, then we expected to find between two and seven NEMP stars. Instead, we found no NEMP stars in our sample. Therefore, this observational bias is not an important contributor to the apparent dearth of N-rich stars. Our [C/N] values are in the same range as values reported previously in the literature (-0.5 to +2.0), and all stars are in disagreement with the predicted [C/N] ratios for both low-mass and high-mass AGB stars. We suggest that the decrease in [C/N] from the low-mass AGB models is due to enhanced extra-mixing, while the lack of NEMP stars may be caused by unfavorable mass ratios in binaries or the difficulty of mass transfer in binary systems with large mass ratios.Comment: 14 pages, 7 figures, to be published in Ap

The two-hour orbit of a binary millisecond X-ray pulsar

Typical radio pulsars are magnetized neutron stars that are born rapidly rotating and slow down as they age on time scales of 10 to 100 million years. However, millisecond radio pulsars spin very rapidly even though many are billions of years old. The most compelling explanation is that they have been "spun up" by the transfer of angular momentum during accretion of material from a companion star in so-called low-mass X-ray binary systems, LMXBs. (LMXBs consist of a neutron star or black hole accreting from a companion less than one solar mass.) The recent detection of coherent X-ray pulsations with a millisecond period from a suspected LMXB system appears to confirm this link. Here we report observations showing that the orbital period of this binary system is two hours, which establishes it as an LMXB. We also find an apparent modulation of the X-ray flux at the orbital period (at the two per cent level), with a broad minimum when the pulsar is behind this low-mass companion star. This system seems closely related to the "black widow" millisecond radio pulsars, which are evaporating their companions through irradiation. It may appear as an eclipsing radio pulsar during periods of X-ray quiescence.Comment: 4 pages with 1 figure. Style files included. Fig. 2 deleted and text revised. To appear in Nature. Press embargo until 18:00 GMT on 1998 July 2

X-ray Emission from Young Stellar Objects in the \epsilon Chamaeleontis Group: the Herbig Ae Star HD 104237 and Associated Low-Mass Stars

We present Chandra-HETGS observations of the Herbig Ae star HD 104237 and the associated young stars comprising lower mass stars, in the 0.15-1.75\msol mass range, in their pre-main sequence phase. The brightest X-ray source in the association is the central system harboring the Herbig Ae primary, and a K3 companion. Its X-ray variability indicates modulation possibly on time scales of the rotation period of the Herbig Ae star, and this would imply that the primary significantly contributes to the overall emission. The spectrum of the Herbig Ae+K3 system shows a soft component significantly more pronounced than in other K-type young stars. This soft emission is reminiscent of the unusually soft spectra observed for the single Herbig Ae stars HD 163296 and AB Aur, and therefore we tentatively attribute it to the Herbig Ae of the binary system. The HETGS spectrum shows strong emission lines corresponding to a wide range of plasma temperatures. The He-like triplet of MgXI and NeIX suggest the presence of plasma at densities of about $10^{12}$ cm$^{-3}$, possibly indicating accretion related X-ray production mechanism. The analysis of the zero-order spectra of the other sources indicates X-ray emission characteristics typical of pre-main sequence stars of similar spectral type, with the exception of the T Tauri HD104237-D, whose extremely soft emission is very similar to the emission of the classical T Tauri star TW Hya, and suggests X-ray production by shocked accreting plasma.Comment: accepted for publication on the Astrophysical Journa