Recent Observations of PMS δ Scuti Stars

We present time series observations of the intermediate mass Pre-Main Sequence star H254 belonging to the young star cluster IC 348 and of the Herbig Ae star V351 Ori.Both these stars present light variation on short time scale (a few hours) typical of the Scuti pulsation. The new data are briefly described together with the plan for future observational campaigns on PMS Scuti stars.     

Gravity Modes in Delta Scuti Stars

This paper examines the observational evidence for the detectionof gravity modes in Scuti stars, which are p-mode pulsators.Low-order gravity modes have also been found in at least one star (FG Vir).Some reports of gravity modes may be due to systematic errors in theabsolute magnitude calibrations for slowly rotating stars. Furthermore,many detected low frequencies are not high-order gravity modes, but linear combinations,f _i -f _j , of the main pulsation modes. Other low frequencies are caused bya close binary companion leading to tidal deformation as well as tidallyexcited gravity modes.     

Nonlinearity of Nonradial Modes in Evolved Stars

We show that in evolved stars, even at relatively low surfaceamplitudes, nonradial modes become strongly nonlinear in thehydrogen shell source, where the Brunt-Väisälä frequency hasits absolute maximum. The measure of nonlinearity is the productof horizontal displacement times the radial wavenumber, . It becomes large already in evolved -Scuti stars.This nonlinearity presents a major problem for interpretations ofamplitude modulation in RR Lyrae stars in terms of nonradial modeexcitation.     

STARE Results on a Single Field: Tens of New Pulsating Stars

We present preliminary results on variable stars of aSTARE's three month observational run centered at the Cygnus constellation.A total amount of aprox. 14000 stars with 9^2 FOV, have been analyzed to obtainlightcurves for each of these stars.The data spans for 90 nights.In this single field, we detect more that 40 stars withpulsation modes between 5 and 40 c/d, the vast mayoritypreviously unknown to be variables.     

Present Observational Status of the Intermediate Mass Stars: δ Sct Stars, γ Dor Stars and roAp Stars

The present observational status of the Sct stars, Dor stars and roAp stars is discussed. The Sct stars are the most intensively observed of the three groups, but it has become clear that there are severe problems in extracting asteroseismic information from them. Dozens of frequencies are observed, but hundreds of frequencies are predicted from the models; unique matches of observation and theory still elude us. The Sct stars are observationally complex – some recent `best case' campaigns are discussed. It is possible that substantial observational advances for Sct stars may need to await upcoming satellite missions. New Dor stars are beingdiscovered frequently, and new behaviour is being found for them. They constitutean observationally young field. Their pulsational frequency range is being expanded, their position in the HR diagram is becoming better known (but is yet to be fully constrained), and the possibility exists of hybrid Dor – Sct stars that have greatasteroseismic promise, although it is clear such stars are rare, if they do exist. It has been observationally challenging to extract more than a fewfrequencies for any Dor star so far. Exciting spectroscopic discoveries of new behaviour in roAp stars promise unprecedented information about the structure of the peculiar atmospheres ofthose stars – pulsation amplitude and phase in 3D, magnetic field structurein 3D, abundance stratification in 3D, realistic T- for the most peculiarstars – as well as entirely new information about the interaction of pulsation,rotation and magnetic fields. Recent theoretical work has led to new understandingof the previously inexplicable frequency spacing of HR 1217 with new Whole Earth Telescope observations supporting this theory. An `improved oblique pulsator model' has been developed in which the pulsationaxis is not the magnetic axis; this model has passed several observationaltests and new ones are being devised to examine it further.     

Period analysis of theδ Scuti star HD93044

HD93044 was observed electrophotometricaly on April 21, May 1, 2 and 4, 1991 at Xinglong Station of Beijing Astronomical Observatory. Combining the data with Li Zhiping's data together and a period analysis was completed using a program which consists of a combination of Fourier transforms of prewhitened data and the multifrequency least squares of brightness residuals (LSR). Three pulsation frequencies (11.90809, 16.79553, 22.44827 cycles per day) with visual amplitudes between 0.0056 and 0.0203 mag were found. The solution fits the observations to +0.0071 mag which is equal to the mean square deviation of observations. The first frequency (11.90809 cycles per day) must be the right value of the fundamental frequency of HD93044.     

Subarcsec 10μm imaging of bipolar flow sources

We have started a program of high-resolution (0.4/pixel) 10m imaging of bipolar outflow sources using the 10m camera CAMIRAS. We present recent results obtained at the Canada France Hawaii Telescope which reveal extended emission or IR companions in several luminous objects. The extended emission we detected probably arises from transiently heated very small grains, while the newly discovered companions could contribute significantly to the outflow activity and extended far-IR emission usually attributed to the main optical source.     

Field azimuth disambiguation using ambiguity-free currents

Using the ambiguity-free vertical current defined by Semel and Skumanich (1995) we derive a minimum-current azimuth disambiguation for the observed magnetic field in the active region NOAA 7201. A comparison of such a minimum-current azimuth resolution with those from other extant methods indicates that the resulting resolution, even though found to be limited by noise, is a useful first approximation. A comparison of our minimum current distribution with the currents we derive from an extant disambiguation (Lites et al., 1995) indicates the presence of current discontinuities in the form of linear features near the magnetic neutral line of the associated -spot.The National Center for Atmospheric Research is funded by the National Science Foundation     

Magnetic field measurements on four yellow supergiants. I

Multiyear high precision measurements of the longitudinal component of the magnetic field (B_e) of four supergiants are reported: Aqr (G0 Ib), Aqr (G2 Ib), Gem (G8 Ib), and Peg (K2 Ib). The best measurement accuracy, =0.8 G, was achieved for Peg. A Monte Carlo method was used to test the reliability of the derived measurement errors. The differences between the observational errors and the calculated Monte Carlo errors were 3.2%. For Aqr and Aqr no statistically significant value of the magnetic field was recorded when averaged over a night. For eGem the following overnight average values of the magnetic field were recorded on five nights: 11.1±2.7 G, 9.8±2.5 G, –10.5±3.0 G, 38.1±7.4 G, and 5.3±1.5 G. For Peg the magnetic field recorded over two nights was –5.3±0.9 G and – 2.7±0.8 G.Translated from Astrofizika, Vol. 48, No. 1, pp. 15–28 (February 2005).     

The Mel’nikov Subharmonic Function and the Non-existence of Analytic Integrals in Non-autonomous Systems

We consider a one-dimensional non-degenerate Hamiltonian system perturbed by a periodically time dependent non-Hamiltonian vector field and show that the non-vanishing of the Melnikov subharmonic function is strongly related to the non-existence of an analytic integral in the perturbed system.     

Rosat X-ray light curves of early-type stars

We have investigated ROSAT X-ray data of OB stars to search for evidence of time variability in the X-ray emission from early-type stars. As an example for such studies we present a detailed variability analysis for our two program stars Ori and Ori which have been multiply observed with ROSAT. The long-term analysis of both stars now covers a time range of 2.5 years and includes six pointed PSPC observations, an additional pointed HRI observation of Ori and the ROSAT all-sky survey data of both stars. Over a long time range the X-ray light curves of Ori and Ori show no evidence for variability. In the case of Ori we detected a moderate increase in X-ray count rate during a period of 2 days which can be explained as a strong shock propagating in the wind of an O-type star.     

Sub-arcsec 10 μm imaging of β pictoris and other star disk candidates

We have used the ESO 10 m camera, TIMMI, to image with a very high angular resolution (PFoV: 0.3; FWHM:0.9) several main-sequence star disk candidates. Information on the -Pictoris dust disk has been obtained in a region largely inaccessible to previous observations: 0–80 AU, with a resolution of about 5 AU after deconvolution. Another promising target for 10 m imaging, 51 Ophiuchi, appears point-like.based on data collected at the European Southern Observatory (ESO), La Silla, Chile     

p-mode Oscillations of α Cen A

Models of Cen A & B have been computed using the masses determinedby Pourbaix et al. (2002) and the data derived from the spectroscopicanalysis of Neuforge and Magain (1997). The seismological dataobtained by Bouchy and Carrier (2001, 2002) do help improveour knowledge of the evolutionary status of the system. All the constraintsare satisfied with a model which gives an age of about 6 Gyr for the binary.     

Low energy γ-ray events in imaging Čerenkov telescope arrays

One of the major goals in VHE--ray astronomy is to open the energy range below 100 GeV with earthbound detectors. This paper demonstrates a new method for analyzing erenkov light of a shower in a erenkov telescope array. This method is successful for showers in this low energy regime where previous techniques (e.g. alpha analysis) are not applicable. A Monte Carlo simulation is applied to a system of 19 Whipple type [3, Cawley 1990] Imaging Atmospheric erenkov telescopes (IAT), each built as a 10 m diameter reflector and equipped with a 109 photomultiplier tube camera. The energy threshold for a single detector of this type is given [5, Kerrick et al. 1995] as 250 GeV. Analysis of simulated coincident events of the system for those events not having enough light to apply a standard imaging analysis [4, Hillas 1985], leads to a considerably lower threshold of 85 GeV. With a new analysis method of these events it is shown that it should be possible to distinguish between -ray induced and proton induced showers. The improvement of sensitivity (Q = figure of merit) of this analysis method is found to be Q=2.9.     

A Friedmann-Lemaître Universe Model with a Positive Cosmological Constant

The possibility of using a Friedmann-Lemaître model with a non-zero cosmological constant is investigated on the basis of analyses of the RATAN-600 and 87GB radio surveys and optical constraints. A zero deceleration parameter and critical matter density are features of the model. The model is in agreement with current observational data, including estimates of the age of the Universe, Hubble constant measurements, and the - z relation. A value for the cosmological constant is determined and its physical nature discussed.     

A deeply embedded companion to LKHα198

The region around the Herbig Ae/Be star LkH198 was imaged at 10m with the CAMIRAS camera at the Canada-France-Hawaii Telescope and the Nordic Optical Telescope. We discovered a deeply embedded companion 6 north of LkH198, which may be responsible for most of the far-infrared emission in the region and for driving the molecular flow. LkH198 appears point-like at our resolution (FWHM 1.3), excluding an extended envelope of transiently heated small grains as the dominant origin of the mid-infrared excess in that star.     

Comparison between a radiative wind model for be stars and high angular resolution data from the GI2T interferometer

I present theoretical line profiles and intensity maps from an axi-symmetric radiative wind model from a rapidly rotating Be star. The introduction of a viscosity parameter in the latitude-dependent hydrodynamic code enables us to consider the effects of the viscous force in the azimuthal component of momentum equations (Araújo et al. 1994). Both velocity field and density law derived from the hydrodynamic equations have been used for solving the statistical equilibrium equations. By adopting the Sobolev approximation, we could easily obtain a good estimate of both electronic density and hydrogen level populations throughout the envelope. The numerical calculation was performed for parameters characterisic of the Be star Cassiopeiae.     

Follow-up observations of β-pic-like stars

Following the discovery, by IRAS, of the dust disc around Vega and three other main sequence stars, searches have been made for other candidates. The-Pic-like candidates have 12µm excesses and 100µm fluxes (unlike the Vega-like candidates), so they can be further investigated using ground-based techniques. Data are presented here, comprising 10µm spectroscopy and sub-mm observations, for several candidates from the Walker & Wolstencroft list, showing that the stars have silicate dust, and optically thick dust discs even at 1300µm.