A survey of 10-μm silicate emission from dust around young sun-like stars

We obtained low resolution (R=100) mid-infrared (8–13 μm wavelengths) spectra of 8 nearby young main sequence stars with the Keck 1 telescope and Long-Wavelength Spectrometer (LWS) to search for 10 μm silicate (Si–O stretch) emission from circumstellar dust. No stars exhibited readily apparent emission: Spectra were then analyzed by least-squares fitting of a template based on a spectrum of Comet Hale-Bopp. Using this technique, we were able to constrain the level of silicate emission to a threshold 10 times below what was previously possible from space. We found one star, HD 17925, with a spectrum statistically different from its calibrator and consistent with a silicate emission peak of 7% of the photosphere at a wavelength of 10 μm. Excess emission at 60 μm from this star has already been reported.     

Theβ index in cool stars

If we follow recent work and in order to extend theuvby photometric calibrations to spectral types later than G0, we present an attempt to use the combineduvby and systems for stars in the range G5-K7 and luminosity classes V to III.The behaviour in the MK-, (b–y)– and cl– diagrams of the 200 stars, only good spectroscopic data being considered, suggests the usefulness of the index as an independent parameter for late-type Main-Sequence stars, following in a natural way the general trend defined by Crawford for F- and G-type stars.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain     

Searching for r-process-enhanced stars in the LAMOST surveyⅠ:the method

The abundance patterns of r-process-enhanced stars contain key information required to constrain the astrophysical site(s)of r-process nucleosynthesis,and to deepen our understanding of the chemical evolution of our Galaxy.To expand the sample of known r-process-enhanced stars,we have developed a method to search for candidates in the LAMOST medium-resolution(R7500)spectroscopic survey by matching the observed spectra to synthetic templates around the EuⅡline at 6645.1 A.We obtain a sample of 13 metal-poor(-2.35<[Fe/H]<-0.91)candidates from 12209 unique stars with 32774 mediumresolution spectra.These candidates will be further studied by high-resolution follow-up observations in the near future.We describe some extensions of this effort to include larger samples of stars,in particular at lower metallicity,using the strength of the BaⅡline at 6496.9 A.     

UVbYβ electrophotometry of the RR Lyrae stars

The behaviour of the spectral metallicity parameters S of the RR Lyrae stars in the course of light change is considered. Electrophotometry results of 9 stars of the given class inUVbY are presented. Analysis of the data obtained shows that the variables for which variation of S with the light phase was established do not show any real variations of metallicity indexm ₁.     

Solar-type stars in Cepheus OB 2: Hα emission stars and IRAS point sources

A survey for emission line stars was carried out in 1980 with the Schmidt telescope of Konkoly Observatory in the region of IC 1396 (Kun, 1986a, hereafter referred to as Paper I). This work was aimed at a preliminary mapping of the probable medium mass members of the association Cepheus OB 2. The stars of the region have been followed since then with the same telescope in order to obtain a more complete view on the stellar content of this giant star-forming region. The common variability of the H emission strength makes the sense of repeating such surveys. The new survey resulted in the discovery of 65 further emission stars. Equatorial coordinates and finding charts are given for them.PhotographicBVRI photometry was carried out for all known emission stars of the field. The colourmagnitude and two-colour diagrams derived from theB, V, R, andI magnitudes show that these stars are probably F-G-type (1.5M <M<3M ), partly pre-Main-Sequence members of Cep OB 2, whereas a few of them may be distant red giants.The youngest objects of the region can be found among the IRAS point sources. Their properties are discussed on the basis of IRAS data alone.     

Analysis of the X-ray emission from OB stars Ⅲ: low-resolution spectra of OB stars

This paper is the third in our series of papers devoted to the investigation of X-ray emission from OB stars.In our two previous papers,we study the high-resolution X-ray spectra of 32 O stars and 25 B stars to investigate the correlations between the properties of X-ray emission and stellar parameters.We checked if the X-ray hardness and post-shock plasma temperature grow with increasing stellar magnetic field,mass loss rate and terminal wind velocity.Our previous analysis of high-resolution spectra showed that the correlations are weak or even absent.In the present paper,we analyzed low-resolution X-ray spectra,using model-independent X-ray hardness values for checking the above mentioned dependencies.We establish that X-ray luminosities L_X weakly depend on the stellar magnetic field.At the same time,Lx ∝ M~(0.5) and L_X ∝ E_(kin)~(0.5),where M is the mass loss rate and E_(kin) is the kinetic energy of the wind.The X-ray luminosities decrease with growing magnetic confinement parameter η.We also argue that there is an additional(probably non-thermal) component contributed to the stellar X-ray emission.     

Hβ luminosities of planetary nebula central stars

The determination of the luminosities of planetary nebula central stars from H nebular fluxes is investigated. A correlation is obtained with the luminosities derived from independent stellar parameters. An average scaling factor is determined for H luminosities of optically thick nebulae, as well as correlations of this parameter with the Zanstra He II and H I temperatures.     

Hα line emission and infrared excess in Be stars

The H observations of a selected sample of bright Be stars are presented. The available infrared observations at K band (2.2 m) of these stars have been used to find the infrared excess emission. The analysis of the combined data show thatL _H, the luminosity of the H emission line, is proportional toL _IR, the luminosity of the infrared excess emission. The linear correlation betweenL _IR andL _H shows that both the infrared excess and the H line originate in a common region. It is also detected that the infrared excess emission is produced throughout the whole envelope whereas the H is emitted in some defined region of the circumstellar (CS) envelope.     

Hα emission stars in the region of IC 1396

155 H emission stars not catalogued earlier have been detected with the 60/90/180 cm Schmidt telescope of Konkoly Observatory in a field of 19.5 square degrees centred on theHii region IC 1396.UBVR photohraphic magnitudes were obtained for most of them. Equatorial coordinates for the epoch 1950.0 as well as identification charts are given. Space distribution of the emission stars and their connection with the association Cepheus OB 2 are briefly discussed.     

Enhanced α-elements and He-burning Population II stars

Spectroscopic analysis of Population II stars indicates that oxygen and other -elements appear to be overabundant with respect to iron (Sneden, 1985; Grattonet al., 1986; Gustafsson, 1987; Gratton, 1987; Gratton and Ortolani, 1989). By use of the Los Alamos Tapes Library, new opacities for Population II stars have been computed, including oxygen, neon, magnesium, and calcium enhanced by a factor five with respect to the solar ratios mixture. On this basis, a grid of evolutionary sequences of He-burning models has been computed for low-mass stars covering the He-burning phase from the Zero-Age Horizontal Branch (ZAHB) up to the onset of thermal pulses in an advanced phase of He-shell burning, for the following assumption on the stellar parameters: helium abundance in the envelopeY=0.20 and 0.25; intial chemical composition [Fe/H]=–2.3, and [Fe/H]=–1.3; initial helium-core massM _c=0.48 and 0.50 and total mass values fromM=0.56 up toM=0.76M . This choice of parameters was suggested by the opportunity of covering the range of values expected in galactic globular cluster HB stars, when an original abundance of heliumY=0.23 is assumed, as adopted throughout the recent literature. At this conference we present just a short overview of the main results while the large set of computed tracks and a detailed discussion on the topic will be presented elsewhere (Bencivenniet al., 1989).     

WR stars with the Ovi λλ3811, 3834 emission doublet

The catalog of the classical WR stars which have the emission doublet Ovi 3811, 3834 in their optical spectra (the catalog of the WR-Ovi stars) and the results of the spectroscopic investigations of the WR-Ovi stars HD 16523, HD 17638, and HD 192103 are presented. Rapid spectral variability of the emission doublet Ovi 3811, 3834 in the spectra of WR-Ovi stars HD 16523, HD 17638, and of the emission band 3680-3780 Å in the spectra of the WR-Ovi star HD 16523 is observed. It is shown that spectral sub-types of the stars HD 16523 and HD 17638 as estimated from different criteria are uncertain. We argue that the WR-Ovi stars HD 16523 and HD 17638, the optical spectra of which display emission doublet Ovi 3811, 3834, may be considered as WO5 stars. The sub-type WO5 is proposed for the first time. Classification criteria of the WO5 sub-type are represented. The possible contribution of the ions Heii to the emission at 3811 Å and 3834 Å is investigated. Thez-distributions of WR-Ovi stars and WR stars with the probable relativistic companions are found to be similar.     

Rapid variations of Hα emission strength of eight Be stars

Rapid observations of H profiles of eight Be stars have been obtained and it is found that rapid (in a time-scale of 4 to 6 minutes) and irregular variations of total emission strength of H line are present in these stars. It has been suggested that these variations may be due to the material circulation in the envelope of Be stars.     

Empirical P-L-C relation for δ scuti stars

Empirical period-luminosity-colour (P-L-C) relations are obtained for the four lowest modes corresponding to radial pulsations. Agreement with predicted values indicates that, in general, both Strömgren photometric calibration and pulsation theory work well for these stars.Paper presented at the 11 th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

UV survey of δ Scuti stars: Final results and open problems

The results obtained from our IUE observations of some Scuti stars and in particular 71 Tau and 69 Tau are reviewed. 71 Tau is the second brightest X-ray source of the Hyades cluster; on the contrary 69 Tau, very similar to 71 Tau in the optical range, does not show X emissions. The complex and in somewhat controversial scenario obtained from this survey is also discussed. In particular we consider the X-ray emission detected in some Scuti with respect to the chromospheric activity level as measured through the MgII line and to other stellar parameters.     

Non-thermal emission in Wolf–Rayet stars: are massive companions required?

We examine the radio spectral indices of 23 Wolf–Rayet (WR) stars to identify the nature of their radio emission. We identify nine systems as non-thermal emitters. In seven of these systems the non-thermal emission dominates the radio spectrum, while in the remaining two it is of comparable strength to the thermal, stellar wind emission, giving 'composite' spectra. Among these nine systems, seven have known spectroscopic or visual binary companions. The companions are all massive O or early B-type stars, strongly supporting a connection between the appearance of non-thermal emission in WR stars and the presence of a massive companion. In three of these binaries, the origin of non-thermal emission in a wind-collision region between the stars has been well established in earlier work. The binary systems that exhibit only thermal emission are all short‐period systems where a wind-collision zone is deep within the opaque region of the stellar wind of the WR star. To detect non-thermal emission in these systems requires optically thin lines of sight to the wind-collision region.     

Searches for HH-objects and emission stars in star-formation regions. VIII. Stars with Hα emission in the vicinity of the nebula GM 2-41

43 stellar objects with Hα emission are discovered by slitless spectroscopy in a 14’ × 14' area near the nebula GM 2-41 located in the HII region DR 15 at the southern edge of the Cyg OB2 association. Emission is detected for the first time for 30 of these objects. Based on VI and JHK photometric data, the overwhelming majority of these objects are classified as young stellar objects in spectral classes F7-M3 with ages of ~1 Myr. The stars are distributed nonuniformly and form two groups which have the same coordinates as the UCHII regions G0.79 + 0.3 and G79.2 + 0.4.     

Luminosity–colour relations for red clump stars

We calibrated the absolute magnitudes M _V , M _J , $M_{K_{s}}$ and M _g of red clump stars in terms of colours. M _V and M _g are strongly dependent on colour, while the dependence of M _J and $M_{K_{s}}$ on colour is rather weak. The calibration of the absolute magnitudes M _V and $M_{K_{s}}$ is tested on 101 RC stars in the field SA 141. The Galactic model parameters estimated with this sample are in good agreement with earlier studies.     

New Hα stars. NGC 6910 region. II.

A search for Hα objects in four fields with an overall area of about 0.14 square degrees in the region of NGC 6910 (the core of the known Cyg OB9 association) has led to the discovery of 64 emission stars, of which 49 are newly detected. The high surface density is indicative of a very rich T association or, possibly, of the projection of several T associations onto one another in this direction. The latter possibility is also indicated by the range of the R photometric stellar magnitudes of the emission stars, 9^m.68-19^m.42. A comparison with earlier observations shows that for many of the stars, the emission line intensity is variable. A P Cyg Hα line profile is recorded for the star V 1515 Cyg with an emission component equivalent width that greatly exceeds the equivalent absorption width.     

Strömgren and Hβ photometry of OB stars in Serpens

Strömgren and H photometry of 25 stars in the field of Serpens are presented. The excesses, the absolute magnitude and the temperatures of the stars are obtained. The masses and the ages are also estimated.