High-frequency modes in solar-like stars

p-mode oscillations in solar-like stars are excited by the outer convection zone in these stars and reflected close to the surface. The p modes are trapped inside an acoustic cavity, but the modes only stay trapped up to a given frequency [known as the acoustic cut-off frequency  (ν_ac)  ] as modes with larger frequencies are generally not reflected at the surface. This means that modes with frequency larger than the acoustic cut-off frequency must be travelling waves. The high-frequency modes may provide information about the physics in the outer layers of the stars and the excitation source and are therefore highly interesting as it is the estimation of these two phenomena that cause some of the largest uncertainties when calculating stellar oscillations.
High-frequency modes have been detected in the Sun, in β Hydri and in α Cen A and α Cen B by smoothing the so-called echelle diagram and the large frequency separation as a function of frequency has been estimated. The large frequency separation has been compared with a simple model of the acoustic cavity which suggests that the reflectivity of the photosphere is larger at high frequency than predicted by standard models of the solar atmosphere and that the depth of the excitation source is larger than what has been estimated by other models and might depend on the order n and degree l of the modes.     

Seismic diagnostics of solar-like oscillating stars

High precision and long-lasting Kepler data enabled us to estimate stellar properties with asteroseismology as an accurate tool. We performed asteroseismic analysis on six solar-like stars observed by the Kepler mission: KIC 6064910, KIC 6766513, KIC 7107778, KIC 10079226, KIC 10147635 and KIC 12069127. The extraction of seismic information includes two parts. First, we obtained two global asteroseismic parameters, mean large separation ?_ν and frequency of maximum power ν_(max),with autocorrelation function and collapsed autocorrelation function. Second, we extracted individual oscillation modes ν_(nl) with low-l degree using a least-squares fit. Stellar grid models were built with Yale Rotating Stellar Evolution Code(YREC) to analyze stellar properties. They covered the range of M = 0.8 ~ 1.8 M_⊙with a step of 0.02 M_⊙ and [Fe/H] =-0.3 ~ 0.4 dex with a step of 0.1 dex.We used a Bayesian approach to estimate stellar fundamental parameters of the six stars, under the constraints of asteroseismic parameters(?_ν, ν_(max)) and non-asteroseismic parameters(Teff, [Fe/H]). We discover that the six targets include five sub-giant stars with 1.2 ~1.5 M_⊙ and one main-sequence star with 1.08 M_⊙, and with ages in the range of 3 ~5 Gyr.     

Nonradiative activity across the H-R diagram: Which types of stars are solar-like?

Major advances in our understanding of nonradiatively heated outer atmospheric layers (coronae, transition regions, and chromospheres) and other solar-like activity in stars has occurred in the past few years primarily as a result of ultraviolet spectroscopy from IUE, X-ray imaging from the Einstein Observatory, microwave detections by the VLA, and new optical observing techniques. I critically review the observational evidence and comment upon the trends with spectral type, gravity, age, and rotational velocity that are now becoming apparent. I define a solar-like star as one which has a turbulent magnetic field sufficiently strong to control the dynamics and energetics in its outer atmospheric regions. The best indicator of a solar-like star is the direct measurement of a strong, variable magnetic field and such data are now becoming available, but good indirect indicators include photometric variability on a rotational time scale indicating dark starspots and nonthermal microwave emission. X-rays and ultraviolet emission lines produced by plasma hotter than 10⁴ K imply nonradiative heating processes that are likely magnetic in character, except for the hot stars where the heating is likely by shocks in the wind resulting from radiative instabilities. I conclude that dwarf stars of spectral type G-M and rapidly rotating subgiants and giants of spectral type F-K in spectroscopic binary systems are definitely solar-like. Dwarf stars of spectral type A7-F7 are almost certainly solar-like, and T Tauri and other pre-Main-Sequence stars are probably solar-like. Slowly rotating single giants of spectral type F to early K are also probably solar-like, and the helium-strong hottest Bp stars are interesting candidates for being solar-like. The O and B stars exhibit some aspects of activity but probably have weak fields and are not solar-like. Finally, the A dwarfs and the cool giants and supergiants show no evidence of being solar-like.Staff Member, Quantum Physics Division, National Bureau of Standards.     

Hipparcos observations of pre-main-sequence stars

We present first results of Hipparcos observations of nearby low-mass pre-main-sequence (PMS) stars (T Tauri and Herbig Ae/Be stars). The data obtained by Hipparcos allow us to derive weighted mean parallaxes for three major nearby star-forming regions (SFRs), Lupus, Chamaeleon I and Taurus–Auriga. Furthermore, results on the isolated objects AB Dor and TW Hya are presented. Finally, we discuss the evolutionary status of Herbig Ae/Be (HAEBE) stars on the basis of Hipparcos results.     

Infrared observations of symbiotic stars

Near infrared measurements in the J, H and K bands have been carried out for a number of symbiotic stars with the 1.5 m telescope at the Rothney Astrophysical Observatory (RAO). A comparison with the earlier observations shows that the S-type symbiotic stars do not have any significant variation in the infrared flux over the past five years. However a small variation ∼ 0.3 magnitude in the infrared flux has been detected for CH Cygni. The observations of HM Sagittae show large decreases in the infrared flux compared to the previous measurements. The variability in the infrared fluxes of both these objects could be attributed to a variation in the temperature due to the cooling of the dust shell. The variability observed for V1016 Cygni is found consistent with the previous measurements.     

Spectrophotometric observations of variable stars

We present the results of our visual and near-infrared spectrophotometric observations for 77 variable stars obtained during 1971–1991 in Chile, Armenia, and Bolivia. The quasi-monochromatic, extraatmospheric fluxes from the stars are given in absolute energy units (W m^?2 m^?1) at all wavelengths of the spectral range at 2.5-nm intervals.     

250 GHz observations of Be stars

New 250 GHz flux density measurements with the 30 m telescope at Pico Veleta are presented for 3 Be stars. We suggest that the radio spectral indexis typically close to 1.4 for these stars. We present in a discusssion of own and literature data some evidence that in few cases slow variations with a time scale of about a year occur. Both evidence is compatible with the idea that there is no stellar activity type radio emission of Be stars but a may be slightly modulated quiescent radiation of the outer parts of selfsimilar circumstellar disks. An immediate improvement of existing models is not possible.     

Fresip: A mission to determine the character and frequency of extra-solar planets around solar-like stars

FRESIP (FRequency of Earth-Sized Inner Planets) is a mission designed to detect and characterize Earth-sizes planets around solar-like stars. The sizes of the planets are determined from the decrease in light from a star that occurs during planetary transits, while the orbital period is determined from the repeatability of the transits. Measurements of these parameters can be compared to theories that predict the spacing of planets, their distribution of size with orbital distance, and the variation of these quantities with stellar type and multiplicity. Because thousands of stars must be continually monitored to detect the transits, much information on the stars can be obtained on their rotation rates and activity cycles. Observations of p-mode oscillations also provide information on their age and composition. These goals are accomplished by continuously and simultaneously monitoring 500 solar-like stars for evidence of brightness changes caused by Earth-sized or larger planetary transits. To obtain the high precision needed to find planets as small as the Earth and Venus around solar-like stars, a wide field of view Schmidt telescope with an array of CCD detectors at its focal plane must be located outside of the Earth's at mosphere. SMM (Solar Maximum Mission) observations of the low-level variability of the Sun (1:100,000) on the time scales of a transit (4 to 16 hours), and our laboratory measurements of the photometric precision of charge-coupled devices (1:100,000) show that the detection of planets as small as the Earth is practical. The probability for detecting transits is quite favorable for planets in inner orbits. If other planetary systems are similar to our own, then approximately 1% of those systems will show transits resulting in the discovery of 50 planetary systems in or near the habitable zone of solar-like stars.     

Sio observations of mass-losing AGB stars

We have observed the first overtone rotation-vibration absorption bands of SiO near = 4µm for a sample of 23 oxygen-rich Mira and Semiregular variables using the ESO NTT and IRSPEC. We discuss the strength of the SiO absorption in terms of the near infrared, IRAS and pulsational properties of the stars. Especially among the Miras there are big differences in the strength of the SiO bands between individual objects, which are probably due to pulsational variability.This work is supported by the Austrian Science Fund Project P9638-AST.     

Rosat observations of pleiades flare stars

The X-ray data of a sample of 104 flare stars (FSs) in the Pleiades cluster region obtained by Stauffer et al. [1] on the basis of deep ROSAT PSPC observations are analyzed. If we divide the X-ray emission detected in late-type stars of the Pleiades cluster into FSs and non-FSs, we find that X-ray luminosities of stars of both groups can be considered as coming from the same parent population. Moreover, in order to classify stars in a sample of 23 late-type Pleiades stars of unknown nature discriminant analysis in a four-dimensional parameter space (log (L_x, log (L_x/L_bol), and ROSAT hardness ratios HR1 and HR2) has been used. It can be shown that the majority of these stars (16) are very likely FSs rather than non-FSs. Published in Astrofizika, Vol. 40, No. 4, pp. 545–557, October-December, 1997.     

Polarimetric observations of pulsating variable stars

Results are presented for polarimetric observations of 17 red giants and supergiants, of which nine are long-period Mira variables, five are semiregular variables (SR), and three are slowly fluctuating variables (Lb). Light polarization is detected for eight stars, seven of them for the first time.Translated fromAstrofizika, Vol. 39, No. 3, pp. 385–391, July–September, 1996.     

First VLTI observations of Mira stars

We present first observations of Mira stars obtained at the Very Large Telescope Interferometer (VLTI) at 2 microns, equipped with the VINCI instrument, using different baselines with both the test siderostats and the 8.2 m unit telescopes. These observations, collected in the course of the VLTI commissining program, have targeted so far about sixty cool giant stars. In this talk, we list and describe the measurements obtained for 14 Mira stars, many of them representing first-time determinations of the visibility. In particular, we devote special attention to a study of Mira itself, for which very accurate determinations of the visibility at several baseline lenghts and orientations could be obtained. We find that a two-component model is more consistent with the data than a single stellar disc. Further observations are needed for a better understanding of this source. The VLTI will constitute an ideal instrument for such studies in the future. In particular it will permit detailed investigations of southern AGB stars, such as accurate measurements of surface structureparameters (diameters, diameter variations, asymmetries, center-to-limbvariations, special features like hot spots) and of circumstellar envelopes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interferometric observations of rapidly rotating stars

Optical interferometry provides us with a unique opportunity to improve our understanding of stellar structure and evolution. Through direct observation of rotationally distorted photospheres at sub-milliarcsecond scales, we are now able to characterize latitude dependencies of stellar radius, temperature structure, and even energy transport. These detailed new views of stars are leading to revised thinking in a broad array of associated topics, such as spectroscopy, stellar evolution, and exoplanet detection. As newly advanced techniques and instrumentation mature, this topic in astronomy is poised to greatly expand in depth and influence.     

R-mode instability of strange stars and observations of neutron stars in LMXBs

Using a realistic equation of state(EOS) of strange quark matter, namely,the modified bag model, and considering the constraints on the parameters of EOS by the observational mass limit of neutron stars, we investigate the r-mode instability window of strange stars, and find the same result as in the brief study of Haskell,Degenaar and Ho in 2012 that these instability windows are not consistent with the spin frequency and temperature observations of neutron stars in low mass X-ray binaries.     

Physics of solar-like oscillations

The physics of solar and stellar oscillations determines their observable properties: frequencies, amplitudes, lifetimes, line asymmetries and phase relations. In the solar case these quantities have been measured, often with high precision, and much has been learned about the properties of the solar interior, and the properties of the oscillations. With recent advances in observational techniques, such seismic investigations are now being extended to solar-like oscillations in distant stars. I provide a brief overview of the basic properties of stellar oscillations, and of the information about stellar properties that may be inferred from them, concentrating mostly on the low-degree modes for which information may be expected for distant stars. In addition, I consider the current state of investigations of solar-like oscillations in other stars, and the prospects for an improved understanding of the physics of such oscillations.     

Rosat and EUVE observations of B stars

Recent observations of the X-ray and EUV emission of non-supergiant B stars are summarized. As compared with O stars, the X-rays of most of the near-main-sequence B stars are soft, and the stars show a departure from theL _x = 10^?7 L _bol relation. Using line driven wind models to provide an estimate of the density distribution, it is concluded that a major fraction of the wind emission measure is hot, whereas in shocked wind theory less than 10 percent of the wind emission measure should be hot. The X-ray observations suggest that all of the B stars are X-ray emitters with a basal X-ray luminosity of about 10^?8.5 L _bol . A hard component dominates the X-ray emission of τ Sco, and possible causes are discussed. For the Be stars, the X-ray emission is that which is expected from a normal B-star wind coming from the poles, as in the Wind Compressed Disk (WCD) model of Be stars. None of the stars, including theβ Cep stars, show noticeable variability in their X-rays.EUVE observations of∈ CMa B2 II, find it to be the brightest object in the EUV sky at 500 to 700 Å. It shows a Lyman continuum flux that is a factor of 30 higher than line blanketed model atmospheres. The continuum is seen on both sides of the He I 504 Å edge, and the discrepancy with model atmospheres is even greater shortward of 504 Å. TheEUVE spectra show emission lines both from high stages of ionization ( Feix to Fexvi) and from low stages (Heii and Oiii). The Heii Lymanα results from recombination following X-ray photoionization in the wind, and the Oiii resonance line is found to be present because of the Bowen fluorescence mechanism. Thus, there is and interesting coupling between the wind production by the EUV photospheric emission, the production of X-ray and line EUV emission by winds, and the production of fluorescence by recombination in the wind; all of these processes are now observable in B stars.