An Algol type binary with a δ Scuti component: RZ Cassiopeiae revisited

We present new BV photometry and spectroscopic observations of RZ Cassiopeiae. The light and radial velocity curves were formed by the new observations which have been analyzed simultaneously by using theWilson‐Dewinney code. The non‐synchronous rotational velocity v ₁ sin i = 76 ± 6 km s^–1, deduced for the primary component from the new spectroscopic observations, was also incorporated in the analysis. A time‐series analysis of the residual light curves revealed the multi‐periodic pulsations of the primary component of RZ Cas. The main peak in the frequency spectrum was observed at about 64.197 c d^–1 in both B and V bands. The pulsational constant was calculated to be 0.0116 days. This value corresponds to high overtones (n ∼ 6) of non‐radial mode oscillations.We find significant changes in the pulsational amplitude of the primary component from year to year. The peak‐to‐peak pulsational amplitude of the main frequency displays a decrease from 0.^m013 in 2000 to 0.^m002 in 2001 and thereafter we have found an increase again in the amplitude to 0.^m01 in the year 2002. We propose the mass transfer from the cool secondary to the pulsating primary as a possible explanation for such remarkable changes in the pulsational behavior of the primary component. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Observations of structure in the stellar wind of HD 152408

We report on the presence of substantial, low-velocity stellar wind structure in the extreme O supergiant HD 152408, based on optical spectroscopic time-series observations. Systematic variations in the form of migrating optical depth enhancements occur in the absorption trough of the He I 5876 P Cygni profile. These variations start deep in the stellar wind, slowly accelerate bluewards to 0.5v over 1–2 days, and recur at intervals of about 1 day. Sympathetic variations are apparent in the Balmer emission lines. The observations provide constraints on the stability of the low-velocity stellar wind regime, and indicate the presence of large-amplitude perturbations at great depths in the outflow.     

Search for HH Objects and Emission Stars in Star Formation Regions. III. PMS Stars in NGC 7129

Slitless spectroscopy is used to discover 22 emission stars in the central and northwest regions of the cluster located in the NGC 7129 nebula. 16 of them are found for the first time. This sample is essentially complete up to V 20.0. The emission stars are distributed nonuniformly over the field of the cluster and are concentrated toward its center. Photometry in the V, R, and I bands is conducted on more than a hundred stars in the cluster. This yields an average absorption coefficient A _V=1.7±0.27 for this region. Based on their positions in color diagrams for the optical and near IR ranges, most of the emission stars can be regarded as T Tau objects.     

Testing the 'clump' model of SiO maser emission

Building on the detection of the J =7–6 SiO maser emission in both the v =1 and v =2 vibrational states towards the symbiotic Mira R Aquarii, we have used the James Clerk Maxwell Telescope to study the changes in the SiO maser features from R Aqr over a stellar pulsational period. The observations, complemented by contemporaneous data taken at 86 GHz, represent a test of the popular thermal-instability clump models of SiO masers. The 'clump' model of SiO maser emission considers the SiO masers to be discrete emitting regions which differ from their surroundings in the values of one or more physical variables (SiO abundance, for example). We find that our observational data are consistent with a clump model in which the appearance of maser emission in the J =7–6 transitions coincides with an outward-moving shock impinging on the inner edge of the maser zone.     

X-ray emission from stellar coronae

Summary Stars of nearly all spectral types and luminosity classes are surrounded by tenuous high-temperature (T10⁶-10⁷K) coronae, which emit most of their radiation in the soft X-ray part of the spectrum. This paper reviews our present observational knowledge and theoretical understanding of stellar coronae, as has emerged from the extensive observations carried out with theEinstein and EXOSAT Observatories. We argue that different physical mechanisms are likely to be responsible for coronal emission in different parts of the HR diagram and we discuss the principal scenarios that have been proposed to account for the data. We show that in spite of the enormous progress made during the past decade, our understanding of stellar coronal emission remains incomplete and largely phenomenological. We outline major unsolved problems to be addressed by future space missions.     

Statistical results from a jcmt survey of MM and sub-MM emission in herbig ae/be stars

We have detected 1.1 mm continuum emission from 24 of 53 Herbig Ae/Be stars surveyed with the JCMT. Survival analysis shows that 1.1 mm luminosity is correlated with bolometric luminosity and with IRAS 25µm luminosity. For those stars that were also detected at 0.45 or 0.8 mm we find a typical flux dependence of the form S _#x03BD; ³, which is steeper than that of most classical T Tauri stars.     

ASCA observations of colliding stellar winds in gamma velorum

We present new high spectral resolution X-ray observations of the colliding wind binary Vel taken with the ASCA satellite. We find two spectral components, one of which is post-shock emission from the colliding winds. Spectral variability is also seen, consistent with current notions of colliding wind phenomena.     

X-ray spectroscopy of stellar coronae

Summary From the early discovery in 1948 of X-rays from the Solar corona, X-ray spectroscopy has proven to be an invaluable tool in studying hot astrophysical and laboratory plasmas. Because the emission line spectra and continua from optically thin plasmas are fairly well known, high-resolution X-ray spectroscopy has its most obvious application in the measurement of optically thin sources such as the coronae of stars. In particular X-ray observations with theEINSTEIN observatory have demonstrated that soft X-ray emitting coronae are a common feature among stars on the cool side of the Hertzsprung-Russell diagram, with the probable exception of single very cool giant and supergiant stars and A-type dwarfs. Observations with the spectrometers aboardEINSTEIN andEXOSAT have shown that data of even modest spectral resolution (/ = 10–100) permit the identification of coronal material at different temperatures whose existence may relate to a range of possible magnetic loop structures in the hot outer atmospheres of these stars. The higher spectral resolution of the next generation of spectrometers aboard NASA'sAXAF and ESA'sXMM will allow to fully resolve the coronal temperature structure and to enable velocity diagnostics and the determination of coronal densities, from which the loop geometry (i.e. surface filling factors and loop lengths) can be derived. In this paper various diagnostic techniques are reviewed and the spectral results fromEINSTEIN andEXOSAT are discussed. A number of spectral simulations forAXAF andXMM, especially high-resolution iron K-shell, L-shell, and2s-2p spectra in the wavelength regions around 1.9 Å, 10 Å, and 100 Å, respectively, are shown to demonstrate the capabilities for temperature, density, and velocity diagnostics. Finally, iron K-shell spectra are simulated for various types of detectors such as microcalorimeter, Nb-junction, and CCD.     

Radial velocity study of the roAp star α Circinus

We present results of 548 high-dispersion spectra of the roAp star α Circinus over a five-night period. The pulsational radial velocities measured from the rare-earth elements, Nd iii , Hα and Hβ are easily measured and occur at the photometric period. The amplitude is largest in Nd iii and Hα, is lower in the rare earths and Hβ, and cannot be measured in other metal lines. This behaviour can be understood in terms of an increase of pulsational amplitude with height in the atmosphere coupled with abundance stratification. The radial velocities show a significant variation at the rotational period of 4.463 d and a marginally significant periodicity at a frequency of 8.16 cycle d⁻¹. The latter may be a very low-amplitude δ Scuti pulsation.     

Pulsational mode and detection of gravitational wave source

Based on Chandrasekhar's and Lebovitz's treatment of the roto-vibrational modes of a homogeneous compressible neutron star, we calculate damping rates per unit eccentricity of toroidal mode and pulsational mode by gravitational radiation. It is found that the damping rate of the pulsational mode which becomes the quadrupole mode (emits gravitational wave) has a minimum at the eccentricity e = 0.72 for = 4/3, and 8/5 whereas both the pulsational mode with = 5/3 and the toroidal mode have no minima, i.e., the pseudo-radial mode can last many years longer than the toroidal mode. We suggest to measure the damping rate of pulsational mode only for the detection of a strong gravity wave source.Submitted for presentation in the 6th Asian Pacific Regional Meeting on Astronomy, I.A.U., to be held on 16–20 August 1993 at IUCAA, Pune, India.     

The stellar populations within theρ Oph cloud cores

We have obtained infrared colors and limiting magnitudes from 1.25–4.8µm for a sample of 26 of the cm continuum radio sources located in the core of the Oph molecular cloud. Their colors demonstrate that the majority of the sources appear to be heavily reddened objects surrounded by circumstellar accretion disks. In these cases the radio emission most likely diagnoses accretion driven energetic outflow phenomena: either ionized winds or possibly synchrotron emission from shocked gas associated with stellar jets.     

Interferometric observations of HCN and CN towards carbon stars

Using the IRAM interferometer we have observed four carbon stars (U Cam, CIT6, Y CVn, IRC+40540) in the HCN(J=1 0) and CN(N=1 0) lines. Here we present some results for CIT6 and U Cam.     

Abundance analysis of the γ Doradus–δ Scuti hybrid metallic line (Am) star HD 8801

Low frequency oscillation, typical for γ Doradus g‐mode type stellar core sensitive pulsation, as well as higher frequency δ Scuti type pulsation typical for p ‐modes, sensitive to the envelope, make HD 8801 a remarkable hybrid pulsator with the potential to probe a stellar structure over a wide range of radius. In addition HD 8801 is a rare pulsating metallic line (Am) star. We determined the astro‐physical fundamental parameters to locate HD 8801 in the H‐R diagram. We analyzed the element abundances, paying close attention to the errors involved, and confirm the nature of HD 8801 as a metallic line (Am) star. We also determined an upper limit on the magnetic field strength. Our abundance analysis is based on classical techniques, but uses for the final step a model atmosphere calculated with the abundances determined by us. We also discuss spectropolarimetric observations obtained for HD 8801. This object is remarkable in several respects. It is a nonmagnetic metallic line (Am) star, pulsating simultaneously in p‐ and g‐modes, but also shows oscillations with periods in between these two domains, whose excitation requires explanation. Overall, the pulsational incidence in unevolved classical Am stars is believed to be quite low; HD 8801 does not conform to this picture. Finally, about 75 % of Am stars are located in short‐period binaries, but there is no evidence that HD 8801 has a companion. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low‐mass visual binaries in the solar neighbourhood: The case of HD 141272

We search for stellar and substellar companions of young nearby stars to investigate stellar multiplicity and formation of stellar and substellar companions. We detect common proper‐motion companions of stars via multi‐epoch imaging. Their companionship is finally confirmed with photometry and spectroscopy. Here we report the discovery of a new co‐moving (13 σ) stellar companion ∼17.8 arcsec (350AU in projected separation) north of the nearby star HD141272 (21 pc).With EMMI/NTT optical spectroscopy we determined the spectral type of the companion to be M3±0.5V. The derived spectral type as well as the near infrared photometry of the companion are both fully consistent with a M_⊙ dwarf located at the distance of HD141272 (21 pc). Furthermore the photometry data rules out the pre‐main sequence status, since the system is consistent with the ZAMS of the Pleiades. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High-luminosity single carbon stars in stellar and galactic evolution

Summary In the solar neighborhood, approximately half of all intermediate mass main sequence stars with initially between 1 and about 5 Mbecome carbon stars with luminosities near 10⁴ L for typically less than 10⁶ years. These high luminosity carbon stars lose mass at rates nearly always in excess of 10^–7 M yr^–1 and sometimes in excess of 10^–5 M yr^–1. Locally, close to half of the mass returned into the interstellar medium by intermediate mass stars before they become white dwarfs is during the carbon star phase. A much greater fraction of lower metallicity stars become carbon-rich before they evolve into planetary nebulae than do higher metallicity stars; therefore, carbon stars are much more importan t in the outer than in the inner Galaxy.     

Confirmation of the oblique pulsator model for the rapidly oscillating Ap star HR 3831

We have detected pulsational radial velocity variations in the rapidly oscillating Ap star HR 3831 which are amplitude- and phase-modulated in the same manner as the photometric variations. In particular, the radial velocities show the same 180° phase reversal at magnetic quadrature as the photometric variations. This confirms the oblique pulsator model, and rules out the spotted pulsator model for these stars.     

Rapid differential rotation in intermediate mass stars

ZAMS models with internal differential rotation are computed for 6.5 M_⊙ and 10 M_⊙ models with the 2D evolution code ROTORC. Two applications of these models, their pulsational frequencies and deduced locations in the HR diagram, are discussed in order to show how the internal differential rotation could be investigated. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Millimeter polarimetry of star-forming regions: magnetic field structure around low-mass stars

We have measured polarization of the 1.1 mm and 0.8 mm continuum emission for 3 pre-T Tauri stars and 2 T Tauri stars. Positive detections were made for NGC 1333 IRAS 4 and IRAS 16293-2422, while L1551 IRS 5 and HL Tau were only marginally detected. For GG Tau we measured a 2 upper limit of 3%. The polarization is interpreted in terms of thermal emission by magnetically aligned dust grains in circumstellar disks or envelopes. We have found a definite geometrical relation between the polarization and other circumstellar structure.     

Accurate masses and radii of normal stars

Summary Binary stars are the main source of fundamental data on stellar masses and radii (M, R). Considerable progress has been made in recent years in the quality and quantity of such data, and stellar masses and radii of high accuracy have led to a number of qualitatively new and interesting results on the properties and evolution of normal stars. This paper reviews the current status of fundamentalM andR determinations which (i) have errors 2%, the limit for non-trivial results in many applications, and (ii) can be presumed valid for single stars. These two conditions limit the discussion to data fromdetached, doublelined eclipsing binary systems.After a brief discussion (Sect. 2) of the main tests for accuracy and consistency which must be met for observational data to be included in the sample, data for 45 binary systems (90 single stars) are presented in Sect. 3 (Table 1 and Figs. 2–5). Spectral types are O8-M1 on the main sequence, with only two stars clearly in the red-giant region. From the review by Popper (1980), data for only 6 systems survive unchanged in the present list, while improved data are given for 18 systems; 21 systems are new additions. Broadband colours, effective temperatures, and luminosities are also given, but are scale-dependent and considerably less reliably determined thanM andR.The observed ranges inM andR for a given colour far exceed the observational errors, primarily due to evolutionary effects within the main sequence. For this reason, single-parameter relations used to predictM andR for single stars are limited to an accuracy of some ±15% inM and ±50% inR, basically independent of the number and accuracy of the data used to establish the relations. Two-parameter calibrations are discussed (Sect. 4) which can eventually reduce these errors to & 5% in bothM andR. At this level, abundance effects become significant and presumably account for the residual scatter.Comparison of the data with stellar evolution models is the topic of Sect. 5. Characteristic features of the data which are crucial in such work are emphasized, rather than attempts to prove the validity of any particular set of models. Already fromM andR alone, some significant constraints can be derived (Fig. 4). When bothM, R, andT _e are known, the initial helium abundanceY can be estimated if the metal-abundance parameter Z is assumed or determined. Studies in which binaries with accurate values ofM, R, and Z are fit by models calculated for the precise observed masses, and withY and mixing length constrained to solar values, provide the most stringent tests of the models. Probing further model refinements such as convective overshooting requires full use of the potential of the data. For example, models may yield general main-sequence limits which are consistent with the observations, but still be unable to fit any single system to the precision of the data. Conditions for critical, informative tests are discussed. Tidal effects in binaries are briefly discussed in Sect. 6. As tidal forces are extremely sensitive to the dimensions and internal structure of the stars, the present sample is well suited for such studies. Recent success in matching computed and observed apsidal-motion parameters for early-type binaries is mentioned. Finally, main priorities for future work are outlined.