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1

Does the A-type Metallic-Line Star IW Persei Have Non-Uniform Chemical Anomaly on the Surface?

Takeda Y.

Acta Astronomica

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2023

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Vol. 73, No. 1

35--55

EN

IW Per, a single-lined spectroscopic binary with a short period of 0.92 d, is known to be a A-type metallic-line (Am) star showing anomalous line strengths of specific elements. It was previously reported that its equivalent widths of CaII 3934, SrII 4215, and ScII 4320 lines (important key lines characterizing the Am anomaly) show cyclic variations in accordance with the rotation phase, implying that the chemical peculiarities on the surface are not uniform but of rather patchy distribution. However, no attempts of reconfirmation have been done so far. In order to check the validity of this finding, ten high-dispersion spectra of IW Per covering different phases were analyzed for these lines by using the spectrum-fitting technique to determine the abundances of Ca, Sr, and Sc and the corresponding equivalent widths. It turned out, however, that no firm evidence of such phase-dependent line-strength variations could be found, suggesting that significant chemical inhomogeneity on the surface of IW Per is unlikely to exist, at least during the period of our observations (2010 December). Meanwhile, the abundances of O, Si, Ca, Ba, and Fe resulting from the 6130-6180 Å region corroborate that IW Per is a distinct Am star (though the degree of peculiarity differs from element to element) despite that its rotational velocity (≈100 km/s) is near to the limit of Am phenomenon.

2

OGLE-LMC-ECL-09937: The Most Massive Algol-Type Binary System with a Mass Measurement Accurate to 2%

Skowron D. M., Kourniotis M., Prieto J. L., Castro N., Bonanos A. Z., Pieńkowski D.

Acta Astronomica

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2017

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Vol. 67, No. 4

329--344

EN

This paper presents a detailed analysis of the light and radial velocity curves of the semi-detached eclipsing binary system OGLE-LMC-ECL-09937. The system is composed of a hot, massive and luminous primary star of a late-O spectral type, and a more evolved, but less massive and luminous secondary, implying an Algol-type system that underwent a mass transfer episode. We derive masses of 21.04±0.34 M☉ and 7.61±0.09 M☉ and radii of 9.93±0.06 R☉ and 9.18±0.04 R☉, for the primary and the secondary component, respectively, which make it the most massive known Algol-type system with masses and radii of the components measured with <2% accuracy. Consequently, the parameters of OGLE-LMC-ECL-09937 provide an important contribution to the sparsely populated high-mass end of the stellar mass distribution, and an interesting object for stellar evolution studies, being a possible progenitor of a binary system composed of two neutron stars.

3

Variability Survey in the Open Cluster Stock 14 and the Surrounding Fields

Drobek D., Pigulski A., Shobbrook R. R., Narwid A.

Acta Astronomica

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2013

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Vol. 63, No. 3

339--368

EN

We present the results of a photometric variability survey in the young open cluster Stock 14 and the surrounding fields. In total, we detected 103 variable stars of which 88 are new discoveries. We confirm short-period, low-amplitude light variations in two eclipsing members of the cluster, HD 101838 and HD 101794. In addition, we find two new β Cep stars of which one, HD 101993, is also a member. The sample of pulsating cluster members is supplemented by one multimode slowly pulsating B-type star and several single-mode candidates of this type. The other pulsating stars in our sample are mostly field stars. In particular, we found fourteen δ Sct stars including one γ Dor/δ Sct hybrid pulsator. From our UBV photometry we derived new parameters of Stock 14: the mean reddening E(B-V)=0.21±0.02 mag, the true distance modulus, 11.90±0.05 mag, and the age, 20±10 Myr. Finally, we use the new photometry to analyze changes of the 6.322-d orbital period of the bright eclipsing binary and the member of the cluster, V346 Cen. In addition to the known apsidal motion, we find that another effect, possibly light-time effect in a hierarchical system of a very long orbital period, affects these changes. The updated value of the period of apsidal motion for this system amounts to 306±4 yr. The open cluster Stock 14 was found to be a fairly good candidate for successful ensemble asteroseismology.

4

A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. VIII. NGC 6834

Jerzykiewicz M., Kopacki G., Pigulski A., Kołaczkowski Z., Kim S.-L.

Acta Astronomica

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2011

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Vol. 61, No. 3

247--273

EN

We present results of a CCD variability search in the field of the young open cluster NGC 6834. We discover 15 stars to be variable in light. The brightest, a multiperiodic γ Doradus-type variable is a foreground star. The eight fainter ones, including a γ Cassiopeiae-type variable, two λ Eridani-type variables, an ellipsoidal variable, an EB-type eclipsing binary, and three variable stars we could not classify, all have E(B-V) within proper range, thus fulfilling the necessary condition to be members. One of the three unclassified variables may be a non-member on account of its large angular distance from the center of the cluster. Four of the six faintest variable stars, which include two eclipsing binaries and two very red stars showing year-to-year variations, are certain non-members. One of the remaining two faintest variable stars, an EA-type eclipsing binary may be a member, while the faintest one, a W Ursae Majoris-type variable, is probably a non-member. For 6937 stars we provide the V magnitudes and V-IC color indices on the standard system. Because of nonuniform reddening over the cluster's face, a direct comparison of these data with theoretical isochrones is not possible. We therefore obtain E(B-V) from available UBV photometry, determine the lower and upper bound of E(B-V) for NGC 6834, and then fit properly reddened Padova isochrones to the data. Assuming HDE 332843, an early-F supergiant, to be a member we obtain log(age/yr)=7.70, V0-MV=12.10 mag. For 103 brightest stars in our field we obtained the α index, a measure of the equivalent widths of the Hα line. We find Hα emission in five stars, including the γ Cas-type variable and the two λ Eri-type variables.

5

New β Cephei and SPB Stars Discovered in Hipparcos Photometry

Molenda-Żakowicz J., Połubek G.

Acta Astronomica

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2005

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Vol. 55, No. 4

375--388

EN

We discuss 34 stars for which we detected new frequencies in Hipparcos Hp magnitudes. 13 of these stars are variables discovered in this paper. For 20 stars, we derive log Teff and log g from Strömgren or Geneva photometry. We classify one new β Cep star, HIP 88352, two new β Cep suspects, HIP 54753 and 88123, four new SPBs, HIP 1030, 39206, 46192 and 111147, and two SPB suspects, HIP 75787 and 98778. We find the last star to be triply-periodic and we show that the frequencies detected in Hp magnitudes are present in the photoelectric observations of Hill et al., (1976). Finally, we discover the hottest variable of the SPB type, namely, HIP 1030.

6

Empirical Absolute Magnitudes, Luminosities and Effective Temperatures of SPB Variables and the Problem of Variability Classification of Monoperiodic Stars

Molenda-Żakowicz J., Połubek G.

Acta Astronomica

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2004

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Vol. 54, No. 3

281--297

EN

We derive semi-empirical log L/Lsolar for 27 stars classified as SPB on the basis of Hipparcos photometry and we plot these stars on the log Teff - log L/Lsolar diagram. We confirm pulsations of HIP 63210 and HIP 108348 and show that luminosities and masses derived from photometry are of limited use for asteroseismology. For HIP 69174 and 77227, two SB2 systems with an SPB primary, we compute the age of the systems, the orbital inclination - i, the large semi-axis - a, and the masses, radii, log Teff, log g and log L/Lsolar of the components. We discover five new multiperiodic stars classified in the literature as SPB, namely, HIP 5161, 20963, 26243, 26464 and 44996. One of these stars, HIP 26243, shows periods on the time-scales of days and hours. Finally, we discuss classification of monoperiodic SPBs and show that photometry combined with evolutionary models can be helpful in preselecting tentative pulsators.

7

A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. VI. NGC 6910

Kołaczkowski Z., Pigulski A., Kopacki G., Michalska G.

Acta Astronomica

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2004

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Vol. 54, No. 1

33--55

EN

We present results of variability search in the field of the young open cluster NGC 6910. We found four &beta Cep-type stars in the cluster, which makes it rather exceptional among the northern clusters observed in our project. Two of these &beta Cep stars show three modes with amplitudes above the detection level, in the remaining two we detect single modes. We discuss the possibility that the large number of &beta Cep stars in NGC 6910 is due to higher metallicity of the cluster. In addition, we found four other variables, one ellipsoidal or eclipsing binary and three that show irregular variations. The latter three stars have Hα in emission. We also provide VIC photometry for 139 stars down to V=16.6 mag and Hα photometry for 73 stars. Using this photometry we estimate the cluster age to be equal to 6±2 Myr, and the distance modulus, 11.0±0.3 mag. The reddening is high and variable across the cluster. In terms of the E(V-IC) color-excess it amounts to 1.25-1.7 mag, corresponding to E(B-V) between 1.0 mag and 1.4 mag. The average reddening in the central field is E(V-IC)=1.47±0.04 mag.

8

A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. V. NGC 2169

Jerzykiewicz M., Kopacki G., Molenda-Żakowicz J., Kołaczkowski Z.

Acta Astronomica

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2003

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Vol. 56, No. 2

151--165

EN

We present results of a search for variable stars in the field of the young open cluster NGC 2169. The General Catalogue of Variable Stars (GCVS, http://www.sai.msu.su/groups/cluster/gcvs/) lists four variable stars in the field we observed, viz., two β Cep stars, V 916 and V 917 Ori, an α2 CVn variable, V 1356 Ori, and an RRc variable, V 1154 Ori. We find V 916 and V 1154 Ori to be constant in light. We confirm the variability of V 917 Ori, but not the period given in GCVS. For the chemically peculiar A0 V Si star V 1356 Ori we definitely establish the period of 1.565 d, thus settling the uncertainty persisting in the literature since the star was discovered to be variable. In addition, we find two other stars to be variable in light. Both show irregular variations. For V 917 Ori, one of the two GCVS &beta Cep variables, we determine a period of 0.267 d (frequency 3.7477 d-1). However, prewhitening with this period leaves a significant amount of the star's light-variation unaccounted for. Since the star shows emission at Hα, we hypothesize that the unaccounted for variation is caused by an erratic, Be-type activity. As to the periodic term, we consider three hypotheses: (1) β Cep-type pulsation, (2) rotational modulation of the λ Eri type, and (3) ellipsoidal variation due to distorted primary component in a close binary system. After deriving the star's effective temperature from Strömgren indices and the luminosity from the distance modulus of the cluster, we show that while the third hypothesis is untenable, the first two should be retained. However, neither is entirely satisfactory. For a number of stars we provide the V magnitudes. For 14 brightest stars in our field we also obtain the photometric α-index, a measure of the Hα equivalent width. From the α index, we detect mild emission at Hα in two stars, V 917 Ori and NGC 2169-8.

9

A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. IV. NGC 663

Pigulski A., Kopacki G., Kołaczkowski Z.

Acta Astronomica

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2001

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Vol. 51, No. 2

159--174

EN

We present results of the variability search in the field of the young open cluster NGC 663. In addition to the one β Cep-type variable known in this cluster, we found another one. It is a mono-periodic pulsator changing brightness with a period of 0.27640 d. In total, 19 new variables were discovered and the variability of 5 other ones was confirmed. Out of all 24 variables in the observed field, 21 are probable cluster members. One SPB candidate and three eclipsing or ellipsoidal variables could be classified. Moreover, ten out of fourteen Be stars we observed vary in brightness. Only one of them shows periodic variations of the λ Eri-type, while the remaining ones exhibit irregular changes with the range up to 0.4 mag in the IC band. We also provide new VIC photometry of 477 stars in the field of the cluster and check the consistence of the present photometry with cluster parameters derived earlier. The average cluster E(V-IC) color excess amounts to about 0.92 mag.

10

Erratum: A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. III. NGC 6823

Pigulski A., Kołaczkowski Z., Kopacki G.

Acta Astronomica

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2000

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Vol. 50, No. 2

278

EN

In the paper ”A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. III. NGC 6823” (Acta Astron., 50, p. 113) the entries in the last column of Table 2 (times of maximum light) are too large by 1000 days. The correct Table 2 follows:

11

A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. III. NGC 6823

Pigulski A., Kołaczkowski Z., Kopacki G.

Acta Astronomica

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2000

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Vol. 50, No. 1

113--132

EN

We present the results of variability search in the field of a very young open cluster NGC 6823. Two δ Sct stars, still at the pre-main sequence (PMS) stage of evolution, were found. These objects could be used for future testing of the evolutionary period changes in this class of variable stars. In addition, we found 13 other variables including a bright cluster eclipsing binary and an SPB candidate. A few other variables could be PMS stars of the UX Ori type. The Hα photometry, which we made for 69 brightest stars, revealed only one object with strong emission, discovered recently by the photographic methods. Two other stars announced to have Hα emission, do not show any evidence for its presence. For a number of stars in the search field, we also provide the BV(RI)C photometry. We explain how the distribution of absorbing matter along the line of sight results in the unusual morphology of the cluster color-magnitude (CM) diagram. The dereddened CM diagram is used in the calculation of the extinction map for the observed field, in which the E(R-I)C color excess varies from 0.54 to 0.72 mag, with the average value equal to 0.62 mag. It is also shown that all cluster stars with spectral types later than A0 are PMS objects. We use these stars to estimate the cluster age: 3±1 Myr. Using the cluster CM diagram, we compare and discuss the position of the two discovered δ Sct stars with respect to the theoretical instability strip for PMS stars of this type.