The β Cephei instability strip

We carried out a series of linear stability analyses of the radial and low-degree non-radial p modes for stellar models with initial masses of     . The stellar models were computed by using convective overshoot distance     , 0.25 and 0.40  H _P. Our numerical results show that the β Cephei instability strip forms a horn-shaped region pointing upwards near the main sequence on the Hertzsprung–Russell diagram (HRD). The lower part of the instability strip for the radial modes join the zero-age main-sequence (ZAMS) at     , while the top of the instability strip extends up to     . The instability strip for the non-radial modes is even wider. The overall instability strip is dominated by the radial and non-radial fundamental modes. The first overtone (the radial-order index     is also pulsationally unstable. We have shown that the β Cephei stability is almost independent of the overshoot parameter d _over used for the stellar models, while it depends critically on the metal abundance. With decreasing metal abundance, the instability region shrinks and eventually disappears for     .     

Catalogue of averaged stellar effective magnetic fields – II. Re-discussion of chemically peculiar A and B stars

This paper presents a catalogue and the method of determining averaged quadratic effective magnetic fields  〈 B _e〉  for 1212 main-sequence and giant stars, and 11 white dwarf stars. The catalogue includes stars that are members of several open clusters. We have compiled measurements of the longitudinal magnetic field for those stars, which were scattered in the existing literature. A new parameter, magnetization (MA), has been defined, and we present values of MA for stars of various spectral classes. Our sample includes a subset of 610 chemically peculiar early-type stars. We confirm the conclusion of our previous study that the number distribution of all chemically peculiar stars versus the averaged magnetic field strength is described by a decreasing exponential function. Relations of this type also hold for stars of all the analysed subclasses of chemical peculiarity. Magnetization tends to correlate with the effective temperature only at high MA, for He-weak and He-rich stars.     

The Solar Ultraviolet Spectrum Estimated Using the Mg <Emphasis Type="SmallCaps">ii</Emphasis> Index and Ca <Emphasis Type="SmallCaps">ii</Emphasis> K Disk Activity

As part of a program to estimate the solar spectrum back to the early twentieth century, we have generated fits to UV spectral irradiance measurements from 1 – 410 nm. The longer wavelength spectra (150 – 410 nm) were fit as a function of two solar activity proxies, the Mg ii core-to-wing ratio, or Mg ii index, and the total Ca ii K disk activity derived from ground based observations. Irradiance spectra at shorter wavelengths (1 – 150 nm) where used to generate fits to the Mg ii core-to-wing ratio alone. Two sets of spectra were used in these fitting procedures. The fits at longer wavelengths (150 to 410 nm) were derived from the high-resolution spectra taken by the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) on the Upper Atmospheric Research Satellite (UARS). Spectra measured by the Solar EUV Experiment (SEE) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite were used for the fits at wavelengths from 1 to 150 nm. To generate fits between solar irradiance and solar proxies, this study uses the above irradiance data, the NOAA composite Mg ii index, and daily Ca ii K disk activity determined from images measured by Big Bear Solar Observatory (BBSO). In addition to the fitting coefficients between irradiance and solar proxies, other results from this study include an estimated relationship between the fraction of the disk with enhanced Ca ii K activity and the Mg ii index, an upper bound of the average solar UV spectral irradiance during periods where the solar disk contains only regions of the quiet Sun, as was believed to be present during the Maunder Minimum, as well as results indicating that slightly more than 60% of the total solar irradiance (TSI) variability occurs between 150 and 400 nm.     

X‐shooting Herbig Ae/Be stars: Accretion probed by near‐infrared He I emission

The Herbig Ae/Be stars are intermediate mass pre‐main sequence stars that bridge the gap between the low mass T Tauri stars and the Massive Young Stellar Objects. In this mass range, the acting star forming mechanism switches from magnetically controlled accretion to an as yet unknown mechanism, but which is likely to be direct disk accretion onto the star. We observed a large sample of Herbig Ae/Be stars with X‐shooter to address this issue from a multi‐wavelength perspective. It is the largest such study to date, not only because of the number of objects involved, but also because of the large wavelength coverage from the blue to the near‐infrared. This allows many accretion diagnostics to be studied simultaneously. By correlating the various properties with mass, temperature and age, we aim to determine where and whether the magnetically controlled mass accretion mechanism halts and the proposed direct disk accretion takes over. Here, we will give an overview of the background, present some observations and discuss our initial results. We will introduce a new accretion diagnostic for the research of Herbig Ae/Be stars, the HeI 1.083 μm line (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Robust Scalings in Compressible Flux Pile-Up Reconnection

Flux pile-up magnetic reconnection is traditionally considered only for incompressible plasmas. The question addressed in this paper is whether the pile-up scalings with resistivity are robust when plasma compressibility is taken into account. A simple analytical argument makes it possible to understand why the transition from a highly compressible limit to the incompressible one is difficult to discern in typical simulations spanning a few decades in resistivity. From a practical standpoint, however, flux pile-up reconnection in a compressible plasma can lead to anomalous electric resistivity in the current sheet and flare-like energy release of magnetic energy in the solar corona.     

The solar dynamo in the light of the distribution of various sunspot magnetic classes over butterfly diagram

We present the data concerning the distribution of various sunspot magnetic classes over the solar butterfly diagram and discuss how this data can inform solar dynamo models. We use the statistics of sunspots that violate the Hale polarity law to estimate the ratio of the fluctuating and mean components of the toroidal magnetic field inside the solar convective zone. An analysis of the spatial distribution of bipolar, unipolar and complex sunspot groups in the context of simple dynamo models results in the conclusion that the mean toroidal field is relatively simple and maintains its shape during the course of the solar cycle (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Predictions of Energy and Helicity in Four Major Eruptive Solar Flares

In order to better understand the solar genesis of interplanetary magnetic clouds (MCs), we model the magnetic and topological properties of four large eruptive solar flares and relate them to observations. We use the three-dimensional Minimum Current Corona model (Longcope, 1996, Solar Phys. 169, 91) and observations of pre-flare photospheric magnetic field and flare ribbons to derive values of reconnected magnetic flux, flare energy, flux rope helicity, and orientation of the flux-rope poloidal field. We compare model predictions of those quantities to flare and MC observations, and within the estimated uncertainties of the methods used find the following: The predicted model reconnection fluxes are equal to or lower than the reconnection fluxes inferred from the observed ribbon motions. Both observed and model reconnection fluxes match the MC poloidal fluxes. The predicted flux-rope helicities match the MC helicities. The predicted free energies lie between the observed energies and the estimated total flare luminosities. The direction of the leading edge of the MC’s poloidal field is aligned with the poloidal field of the flux rope in the AR rather than the global dipole field. These findings compel us to believe that magnetic clouds associated with these four solar flares are formed by low-corona magnetic reconnection during the eruption, rather than eruption of pre-existing structures in the corona or formation in the upper corona with participation of the global magnetic field. We also note that since all four flares occurred in active regions without significant pre-flare flux emergence and cancelation, the energy and helicity that we find are stored by shearing and rotating motions, which are sufficient to account for the observed radiative flare energy and MC helicity.     

Spectroscopic Study of a Large Sample of Galaxies Discovered in the Second Byurakan Survey Fields

The Second Byurakan Survey (SBS) is a well known combined survey, which uses the presence of UV-excess radiation in the continuum, or the presence of emission-lines in the spectra for the identification of active and star-forming galaxies. This paper reports on a comparative study of 77 galaxies identified with UV-excess, and 34 galaxies identified via emission-line techniques in the fields of the SBS. The spectroscopic parameters used for the comparison are the [OII]3727/H and [OIII]5007/H emission-lines ratios, the equivalent widths of [OII]3727, [OIII]5007 and H emission-lines, and the C _[OII]-C _H index. Spectroscopic parameters as well as new redshifts were determined from the spectra obtained with the 6m telescope of the Special Astrophysical Observatory (Russia). The main results are: 1) Galaxies discovered via UV-excess technique are preferably more active. 2) Galaxies discovered via emission-line technique are preferably high-excitation low-luminosity star-forming galaxies. 3) UV-excess galaxies with faintest UV-excess radiation are likely candidates to be LINER or Sy2 type objects.     

A model of impulsive loop flares revisited

A critical examination of the components of the recent impulsive loop flare model of Takakura is made. It is found that his analysis of the stability of the electron distribution resulting from anomalous heat conduction is in error and electron plasma waves would not be excited. Rather, in the regions where the electron/proton temperature ratioT _e/T _i 10, electrostatic ion-cyclotron waves would be excited and in the regions whereT _e 10, ion-acoustic waves would be excited. Ratios ofT _e/T _i 10 occur only in the late time development behind the conduction fronts. Since the anomalous resistivity due to electrostatic ion-cyclotron waves is fortuitously about 70% of the one used by Takakura, the general development will follow closely the one calculated by him. Because the anomalous resistivity due to ion-acoustic waves is about 95 times the one used by Takakura, the development in the parts of the loop whereT _e/T _i 10 for late times would be altered considerably.Also Guest Worker at NOAA Space Environment Laboratory, Boulder, Colorado, U.S.A.