S. -L. Kim

Publications (73)53.54 Total impact

• Article: Asteroseismology of the ZZ Ceti star HS 0507+0434B

  J. -N. Fu, N. Dolez, G. Vauclair, L. Fox Machado, S. -L. Kim, C. Li, L. Chen, M. Alvarez, J. Su, S. Charpinet, M. Chevreton, R. Michel, X. H. Yang, Y. Li, Y. P. Zhang, L. Molnar, E. Plachy
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  ABSTRACT: The pulsating DA white dwarfs (ZZ Ceti stars) are $g$-mode non-radial pulsators. Asteroseismology provides strong constraints on their global parameters and internal structure. Since all the DA white dwarfs falling in the ZZ Ceti instability strip do pulsate, the internal structure derived from asteroseismology brings knowledge for the DA white dwarfs as a whole group. HS 0507+0434B is one of the ZZ Ceti stars which lies approximately in the middle of the instability strip for which we have undertaken a detailed asteroseismological study. We carried out multisite observation campaigns in 2007 and from December 2009 to January 2010. In total, 206 hours of photometric time-series have been collected. They have been analysed by means of Fourier analysis and simultaneous multi-frequency sine-wave fitting. In total, 39 frequency values are resolved including 6 triplets and a number of linear combinations. We identify the triplets as $\ell$=1 $g$-modes split by rotation. We derived the period spacing, the rotational splitting and the rotation rate. From the comparison of the observed periods with the theoretical periods of a series of models we estimate the fundamental parameters of the star: its total mass M$_{*}$/M$_{\odot}$ = 0.675, its luminosity L/L$_{\odot}$=3.5$\times 10^{-3}$, and its hydrogen mass fraction M$_{H}$/M$_{*}$= 10$^{-8.5}$.
  11/2012;
• Article: Empirical Determination of Convection Parameters in White Dwarfs I : Whole Earth Telescope Observations of EC14012-1446

  J. L. Provencal, M. H. Montgomery, A. Kanaan, S. E. Thompson, J. Dalessio, H. L. Shipman, D. Childers, J. C. Clemens, R. Rosen, P. Henrique, [......], A. Kniazev, R. Silvotti, S. Galleti, T. Nagel, G. Vauclair, N. Dolez, J. R. Fremy, J. Perez, J. M. Almenara, L. Fraga
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  ABSTRACT: We report on analysis of 308.3 hrs of high speed photometry targeting the pulsating DA white dwarf EC14012-1446. The data were acquired with the Whole Earth Telescope (WET) during the 2008 international observing run XCOV26. The Fourier transform of the light curve contains 19 independent frequencies and numerous combination frequencies. The dominant peaks are 1633.907, 1887.404, and 2504.897 microHz. Our analysis of the combination amplitudes reveals that the parent frequencies are consistent with modes of spherical degree l=1. The combination amplitudes also provide m identifications for the largest amplitude parent frequencies. Our seismology analysis, which includes 2004--2007 archival data, confirms these identifications, provides constraints on additional frequencies, and finds an average period spacing of 41 s. Building on this foundation, we present nonlinear fits to high signal-to-noise light curves from the SOAR 4.1m, McDonald 2.1m, and KPNO 2m telescopes. The fits indicate a time-averaged convective response timescale of 99.4 +/- 17 s, a temperature exponent 85 +/- 6.2 and an inclination angle of 32.9 +/- 3.2 degrees. We present our current empirical map of the convective response timescale across the DA instability strip.
  04/2012;
• Source

  Available from: Andrzej Pigulski

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

  M. Jerzykiewicz, G. Kopacki, A. Pigulski, Z. Kołaczkowski, S. -L. Kim
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  ABSTRACT: 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 Gamma Doradus-type variable is a foreground star. The eight fainter ones, including a Gamma Cassiopeiae-type variable, two Lambda 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-I)_C 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 get log (age/yr) = 7.70, V_0 - M_V = 12.10 mag. For 103 brightest stars in our field we obtained the alpha index, a measure of the equivalent widths of the Halpha line. We find Halpha emission in five stars, including the Gamma Cas-type variable and the two Lambda Eri-type variables.
  10/2011;
• Source

  Available from: Patricia Lampens

  Article: Multi-site, multi-year monitoring of the oscillating Algol-type eclipsing binary CT Her

  P. Lampens, A. Strigachev, S. -L. Kim, E Rodriguez, M. J. Lopez-Gonzalez, J. Vidal-Sainz, D. Mkrtichian, J. -R. Koo, Y. B. Kang, P. Van Cauteren, P. Wils, Z. Kraicheva, D Dimitrov, J. Southworth, E. Garcia Melendo, J. M. Gomez Forellad
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  ABSTRACT: We present the results of a multi-site photometric campaign carried out in 2004-2008 for the Algol-type eclipsing binary system CT Her, the primary component of which shows Delta Scuti-type oscillations. Our data consist of differential light curves collected in the filters B and V which have been analysed using the method of Wilson-Devinney (PHOEBE). After identification of an adequate binary model and removal of the best-matching light curve solution, we performed a Fourier analysis of the residual B and V light curves to investigate the pulsational behaviour. We confirm the presence of rapid pulsations with a main period of 27.2 min. Up to eight significant frequencies with semi-amplitudes in the range 3 to 1 mmag were detected, all of which surprisingly lie in the frequency range 43.5-53.5 c\d. This result is independent from the choice of the primary's effective temperature (8200 or 8700 K) since the light curve models for the binary are very similar in both cases. This is yet another case of a complex frequency spectrum observed for an accreting Delta Scuti-type star (after Y Cam). In addition, we demonstrate that the amplitudes of several of these pulsation frequencies show evidence of variability on time scales as short as 1-2 years, perhaps even less. Moreover, our analysis takes into account some recently acquired spectra, from which we obtained the corresponding radial velocities for the years 2007-2009. Investigation of the O-C diagram shows that further monitoring of the epochs of eclipse minima of CT Her will cast a new light on the evolution of its orbital period.
  05/2011;
• Source

  Available from: Barry Smalley

  Article: Kepler photometry of KIC 10661783: a binary star with total eclipses and delta Scuti pulsations

  John Southworth, W. Zima, C. Aerts, H. Bruntt, H. Lehmann, S. -L. Kim, D. W. Kurtz, K. Pavlovski, A. Prsa, B. Smalley, R. L. Gilliland, J. Christensen-Dalsgaard, S. D. Kawaler, H. Kjeldsen, M. T. Cote, P. Tenenbaum, J. D. Twicken
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  ABSTRACT: We present Kepler satellite photometry of KIC 10661783, a short-period binary star system which shows total eclipses and multi-periodic delta Scuti pulsations. A frequency analysis of the eclipse-subtracted light curve reveals at least 68 frequencies of which 55 or more can be attributed to pulsation modes. The main limitation on this analysis is the frequency resolution within the 27-day short-cadence light curve. Most of the variability signal lies in the frequency range 18 to 31 c/d, with amplitudes between 0.1 and 4 mmag. One harmonic term (2.f) and a few combination frequencies (f_i+f_j) have been detected. From a plot of the residuals versus orbital phase we assign the pulsations to the primary star in the system. The pulsations were removed from the short-cadence data and the light curve was modelled using the Wilson-Devinney code. We are unable to get a perfect fit due to the residual effects of pulsations and also to the treatment of reflection and reprocessing in the light curve model. A model where the secondary star fills its Roche lobe is favoured, which means that KIC 10661783 can be classified as an oEA system. Further photometric and spectroscopic observations will allow the masses and radii of the two stars to be measured to high precision and hundreds of delta Scuti pulsation frequencies to be resolved. This could lead to unique constraints on theoretical models of delta Scuti stars, if the evolutionary history of KIC 10661783 can be accounted for.
  02/2011;
• Source

  Available from: Joseph Ryan Eggen

  Article: Towards ensemble asteroseismology of the young open clusters χ Persei and NGC 6910

  S. Saesen, A. Pigulski, F. Carrier, G. Michalska, C. Aerts, J. De Ridder, M. Briquet, G. Handler, Z. Kołaczkowski, B. Acke, [......], V. Van Helshoecht, E. Vanhollebeke, H. Van Winckel, T. Verhoelst, M. Vučković, C. Waelkens, G.W. Wolf, K. Yakut, C. Zhang, W. Zima
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  ABSTRACT: As a result of the variability survey in χ Persei and NGC 6910, the number of β Cep stars that are members of these two open clusters is increased to twenty stars, nine in NGC 6910 and eleven in χ Persei. We compare pulsational properties, in particular the frequency spectra, of β Cep stars in both clusters and explain the differences in terms of the global parameters of the clusters. We also indicate that the more complicated pattern of the variability among B-type stars in χ Persei is very likely caused by higher rotational velocities of stars in this cluster. We conclude that the sample of pulsating stars in the two open clusters constitutes a very good starting point for the ensemble asteroseismology of β Cep-type stars and maybe also for other B-type pulsators (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
  Astronomische Nachrichten 11/2010; 331(9‐10):1080 - 1083. · 1.01 Impact Factor
• Article: δ Sct‐type pulsations in eclipsing binary systems: Y Cam

  E. Rodríguez, J. M. García, V. Costa, P. Lampens, P. van Cauteren, D. E. Mkrtichian, E. C. Olson, P. J. Amado, J. Daszyńska-Daszkiewicz, V. Turcu, [......], D. Díaz-Fraile, M. A. Wood, E. Hintz, A. Pop, D. Moldovan, P. B. Etzel, V. Casanova, A. Sota, F. J. Aceituno, D.-J. Lee
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  ABSTRACT: We present the results of a three-continent multisite photometric campaign carried out on the Algol-type eclipsing binary system Y Cam, in which the primary component is a multiperiodic δ Sct-type pulsator. The observations consist of 86 nights and more than 450 h of useful data collected mainly during the Northern winter 2002–2003. This means that this is the most extensive time series for such kind of systems obtained so far. These observations were collected mostly in the Johnson V filter, but they also include, for the first time, nearly complete binary light curves in simultaneous Strömgren uvby filters together with a few Crawford Hβ data obtained around the orbital phase of the first quadrature. A detailed photometric analysis is presented for both binarity and pulsation. The results indicate a semidetached system with the secondary filling its Roche lobe. No significant contribution from a third body is found. The residuals from the computed binary solution were then used to investigate the pulsational content of the primary component. The frequency analysis of the out-of-primary-eclipse data leads to a set of eight significant and independent pulsational peaks in a well-defined region of the frequency domain. This means that this is the largest set of excited modes discovered so far in the pulsating component of such kind of systems. The possibility of aliasing problems during the present run or short-term time-scale amplitude variations in some of them was investigated with null results. Indeed the results indicate that f1 and f3 form a frequency doublet with a beat period of Pbeat= 17.065 d. Our results confirm the frequencies already detected by earlier authors and show the presence of some additional significant peaks. The observed amplitudes during the present run are also consistent with those derived from older data sets. We perform a preliminary mode identification for most of the frequencies on the basis of the collected multicolour photometry, the observed frequency spacings and the mode visibility in eclipsing binaries.
  Monthly Notices of the Royal Astronomical Society 11/2010; 408(4):2149 - 2162. · 4.90 Impact Factor
• Source

  Available from: Greg Stachowski

  Article: Whole Earth Telescope Observations of the subdwarf B star KPD 1930+2752: A rich, short period pulsator in a close binary

  M. D. Reed, S. L. Harms, S. Poindexter, A. -Y. Zhou, J. R. Eggen, M. A. Morris, A. C. Quint, S. McDaniel, A. Baran, N. Dolez, [......], Z. Csubry, D. Zsuffa, R. Silvotti, S. Marinoni, I. Bruni, G. Vauclair, M. Chevreton, J. M. Matthews, C. Cameron, H. Pablo
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  ABSTRACT: KPD 1930+2752 is a short-period pulsating subdwarf B (sdB) star. It is also an ellipsoidal variable with a known binary period just over two hours. The companion is most likely a white dwarf and the total mass of the system is close to the Chandresakhar limit. In this paper we report the results of Whole Earth Telescope (WET) photometric observations during 2003 and a smaller multisite campaign from 2002. From 355 hours of WET data, we detect 68 pulsation frequencies and suggest an additional 13 frequencies within a crowded and complex temporal spectrum between 3065 and 6343 $\mu$Hz (periods between 326 and 157 s). We examine pulsation properties including phase and amplitude stability in an attempt to understand the nature of the pulsation mechanism. We examine a stochastic mechanism by comparing amplitude variations with simulated stochastic data. We also use the binary nature of KPD 1930+2752 for identifying pulsation modes via multiplet structure and a tidally-induced pulsation geometry. Our results indicate a complicated pulsation structure that includes short-period ($\approx 16$ h) amplitude variability, rotationally split modes, tidally-induced modes, and some pulsations which are geometrically limited on the sdB star. Comment: 30 pages, 17 figures, 11 tables. Accepted for publication by MNRAS
  11/2010;
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  Available from: Maja Vuckovic

  Article: Towards ensemble asteroseismology of the young open clusters Chi Persei and NGC 6910

  S. Saesen, A. Pigulski, F. Carrier, G. Michalska, C. Aerts, J. De Ridder, M. Briquet, G. Handler, Z. Kolaczkowski, B. Acke, [......], V. Van Helshoecht, E. Vanhollebeke, H. Van Winckel, T. Verhoelst, M. Vuckovic, C. Waelkens, G. W. Wolf, K. Yakut, C Zhang, W. Zima
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  ABSTRACT: As a result of the variability survey in Chi Persei and NGC6910, the number of Beta Cep stars that are members of these two open clusters is increased to twenty stars, nine in NGC6910 and eleven in Chi Persei. We compare pulsational properties, in particular the frequency spectra, of Beta Cep stars in both clusters and explain the differences in terms of the global parameters of the clusters. We also indicate that the more complicated pattern of the variability among B type stars in Chi Persei is very likely caused by higher rotational velocities of stars in this cluster. We conclude that the sample of pulsating stars in the two open clusters constitutes a very good starting point for the ensemble asteroseismology of Beta Cep-type stars and maybe also for other B-type pulsators. Comment: 4 pages, Astronomische Nachrichten, HELAS IV Conference, Arecife, Lanzarote, Feb 2010, submitted
  04/2010;
• Source

  Available from: Johannes Puschnig

  Article: Photometric multi-site campaign on the open cluster NGC 884 I. Detection of the variable stars

  S. Saesen, F. Carrier, A. Pigulski, C. Aerts, G. Handler, A. Narwid, J. N. Fu, C Zhang, X. J. Jiang, J. Vanautgaerden, [......], P. Degroote, P. Beck, J. Puschnig, L. Schmitzberger, G. A. Gelven, B. Steininger, J. Blommaert, R. Drummond, M. Briquet, J. Debosscher
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  ABSTRACT: CONTEXT: Recent progress in the seismic interpretation of field beta Cep stars has resulted in improvements of the physics in the stellar structure and evolution models of massive stars. Further asteroseismic constraints can be obtained from studying ensembles of stars in a young open cluster, which all have similar age, distance and chemical composition. AIMS: To improve our comprehension of the beta Cep stars, we studied the young open cluster NGC 884 to discover new B-type pulsators, besides the two known beta Cep stars, and other variable stars. METHODS: An extensive multi-site campaign was set up to gather accurate CCD photometry time series in four filters (U, B, V, I) of a field of NGC884. Fifteen different instruments collected almost 77500 CCD images in 1286 hours. The images were calibrated and reduced to transform the CCD frames into interpretable differential light curves. Various variability indicators and frequency analyses were applied to detect variable stars in the field. Absolute photometry was taken to deduce some general cluster and stellar properties. RESULTS: We achieved an accuracy for the brightest stars of 5.7 mmag in V, 6.9 mmag in B, 5.0 mmag in I and 5.3 mmag in U. The noise level in the amplitude spectra is 50 micromag in the V band. Our campaign confirms the previously known pulsators, and we report more than one hundred new multi- and mono-periodic B-, A- and F-type stars. Their interpretation in terms of classical instability domains is not straightforward, pointing to imperfections in theoretical instability computations. In addition, we have discovered six new eclipsing binaries and four candidates as well as other irregular variable stars in the observed field. Comment: Accepted for publication in Astronomy and Astrophysics, 21 pages, 14 figures, 4 tables. The full appendix is available at http://www.ster.kuleuven.be/~sophies/Appendix.pdf (74 MB, 169 pages, 343 figures, 1 table)
  01/2010;
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  Available from: ArXiv

  Article: RACE-OC Project: Rotation and variability in the open cluster M11 (NGC6705)

  S. Messina, P. Parihar, J. -R. Koo, S. -L. Kim, S. -C. Rey, C. -U. Lee
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  ABSTRACT: Rotation and magnetic activity are intimately linked in main-sequence stars of G or later spectral types. The presence and level of magnetic activity depend on stellar rotation, and rotation itself is strongly influenced by strength and topology of the magnetic fields. Open clusters represent especially useful targets to investigate the rotation/activity/age connection. The open cluster M11 has been studied as a part of the RACE-OC project (Rotation and ACtivity Evolution in Open Clusters), which is aimed at exploring the evolution of rotation and magnetic activity in the late-type members of open clusters with different ages. Photometric observations of the open cluster M11 were carried out in June 2004 using LOAO 1m telescope. The rotation periods of the cluster members are determined by Fourier analysis of photometric data time series. We further investigated the relations between the surface activity, characterized by the light curve amplitude, and rotation. We have discovered a total of 75 periodic variables in the M11 FoV, of which 38 are candidate cluster members. Specifically, among cluster members we discovered 6 early-type, 2 eclipsing binaries and 30 bona-fide single periodic late-type variables. Considering the rotation periods of 16 G-type members of the almost coeval 200-Myr M34 cluster, we could determine the rotation period distribution from a more numerous sample of 46 single G stars at an age of about 200-230 Myr and determine a median rotation period P=4.8d. A comparison with the younger M35 cluster (~150 Myr) and with the older M37 cluster (~550 Myr) shows that G stars rotate slower than younger M35 stars and faster than older M37 stars. The measured variation of the median rotation period is consistent with the scenario of rotational braking of main-sequence spotted stars as they age. Comment: Accepted by Astronomy and Astrophysics on Dec 15, 2009
  12/2009;
• Article: RACE-OC project: M11 (NGC6705) (Messina+, 2010)

  S Messina, P Padmakar, J ~R Koo, S -L Kim, S -C Rey, C -U Lee
  VizieR Online Data Catalog. 11/2009; 351:39029.
• Article: Whole Earth Telescope Observations of the Pulsating Subdwarf B Star PG 0014+067

  M. Vučković, S. D. Kawaler, S. O'Toole, Z. Csubry, A. Baran, S. Zola, P. Moskalik, E. W. Klumpe, R. Riddle, M. S. O'Brien, [......], G. Vauclair, R. Oreiro, R. Østensen, A. Bronowska, B. G. Castanheira, S. O. Kepler, L. Fraga, H. L. Shipman, J. L. Provencal, and D. Childers
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  ABSTRACT: PG 0014+067 is one of the most promising pulsating subdwarf B stars for seismic analysis, as it has a rich pulsation spectrum. The richness of its pulsations, however, poses a fundamental challenge to understanding the pulsations of these stars, as the mode density is too complex to be explained only with radial and nonradial low-degree (l < 3) p-modes without rotational splittings. One proposed solution, suggested by Brassard et al. in 2001 for the case of PG 0014+067 in particular, assigns some modes with high degree (l = 3). On the other hand, theoretical models of sdB stars suggest that they may retain rapidly rotating cores, and so the high mode density may result from the presence of a few rotationally split triplet (l = 1) and quintuplet (l = 2) modes, along with radial (l = 0) p-modes. To examine alternative theoretical models for these stars, we need better frequency resolution and denser longitude coverage. Therefore, we observed this star with the Whole Earth Telescope for two weeks in 2004 October. In this paper we report the results of Whole Earth Telescope observations of the pulsating subdwarf B star PG 0014+067. We find that the frequencies seen in PG 0014+067 do not appear to fit any theoretical model currently available; however, we find a simple empirical relation that is able to match all of the well-determined frequencies in this star.
  The Astrophysical Journal 12/2008; 646(2):1230. · 6.02 Impact Factor
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  Available from: David Mkrtichian

  Article: Frequency analysis of the δ Scuti type pulsations in the semi-detachedeclipsing binary CT Her

  P. Lampens, A. Strigachev, S.-L. Kim, E. Rodriguez, M. J. Lopez-Gonzales, J. Vidal-Sainz, D. Mkrtichian, D. Litvinenko, P. van Cauteren, P. Wils
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  ABSTRACT: We present the latest results of a multisite photometric campaign carried out in 2004-2007 on the Algol-type eclipsing binary system CT Her, the primary component of which is a δ Scuti- type pulsator with a main pulsation period of only 27 min. CT Her belongs to the new class of oscillating Algol-type binary systems. We collected enough data in two passbands to perform a modelling of the light curves using PHOEBE and detected up to 7 significant pulsation frequencies in the frequency range between 45-53 d-1 in the B-residual data (independent of the adopted solution for the binary model). The remaining standard deviation of the ca. 7500 B-residuals spread over 4 years is 5.0 mmag.
  Communications in Asteroseismology 11/2008; 157:328-329.
• Source

  Available from: Greg Stachowski

  Article: 2006 Whole Earth Telescope Observations of GD358: A New Look at the Prototype DBV

  J. L. Provencal, M. H. Montgomery, A. Kanaan, H. L. Shipman, D. Childers, A. Baran, S. O. Kepler, M. Reed, A. Zhou, J. Eggen, [......], A. Hoffman, E. Reiff, R. Kowalski, G. Vauclair, S. Charpinet, M. Chevreton, J. -E. Solheim, E. Pakstiene, L. Fraga, J. Dalessio
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  ABSTRACT: We report on the analysis of 436.1 hrs of nearly continuous high-speed photometry on the pulsating DB white dwarf GD358 acquired with the Whole Earth Telescope (WET) during the 2006 international observing run, designated XCOV25. The Fourier transform (FT) of the light curve contains power between 1000 to 4000 microHz, with the dominant peak at 1234 microHz. We find 27 independent frequencies distributed in 10 modes, as well as numerous combination frequencies. Our discussion focuses on a new asteroseismological analysis of GD358, incorporating the 2006 data set and drawing on 24 years of archival observations. Our results reveal that, while the general frequency locations of the identified modes are consistent throughout the years, the multiplet structure is complex and cannot be interpreted simply as l=1 modes in the limit of slow rotation. The high k multiplets exhibit significant variability in structure, amplitude and frequency. Any identification of the m components for the high k multiplets is highly suspect. The k=9 and 8 modes typically do show triplet structure more consistent with theoretical expectations. The frequencies and amplitudes exhibit some variability, but much less than the high k modes. Analysis of the k=9 and 8 multiplet splittings from 1990 to 2008 reveal a long-term change in multiplet splittings coinciding with the 1996 "sforzando" event, where GD358 dramatically altered its pulsation characteristics on a timescale of hours. We explore potential implications, including the possible connections between convection and/or magnetic fields and pulsations. We suggest future investigations, including theoretical investigations of the relationship between magnetic fields, pulsation, growth rates, and convection.
  11/2008;
• Article: γ Doradus variable stars in the Pleiades cluster: results from a photometric multiste campaign

  S Martín-Ruiz, E Rodríguez, A Grigahcène, J C Suárez, A Moya, V Costa, A Rolland, P J Amado, E G Hintz, S L Kim, S Oh, A V Kusakin, A Y Zhou, A Arellano, R Peniche, J H Peña, J P Sareyan
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  ABSTRACT: The variability of the two γ Doradus star belong to Pleiades cluster, HD 22702 and HD 23585, have been confirmed by using new photometric measurements collected during a multisite campaign in 1998. Respect to previous observing runs, the frequency analysis shows new peaks close the 3 cd-1 for both stars. With the aim of performing a modal identification, the method based on amplitudes ratios and phase shifts with non-adiabatic time dependent convection (TDC) has been applied. The physical parameters obtained from the photometry put the star HD 23585 out the blue observational edge of the γ Dor region in the HR diagram. This behaviour together with the high value of vsini have not allowed us to perform an identification of the excited modes. Respect to the the star HD 22702, a preliminary study of stability provides results consistent with a l = 2 identification for the found oscillation frequencies except for f3, where a l = 1 mode is also probable. Also for this star, the TDC treatment has not been able to discrimante the found modes. Therefore, in addition to new vsini measurements, it is fundamental to carry out an exhaustive modelling by considering the pertinent corrections in the rotation.
  Journal of Physics Conference Series 10/2008; 118(1):012049.
• Source

  Available from: Michal Siwak

  Article: The pulsating hot subdwarf Balloon 090100001: results of the 2005 multisite campaign

  A. Baran, R. Oreiro, A. Pigulski, F. Perez Hernandez, A. Ulla, M. D. Reed, C. Rodriguez-Lopez, P. Moskalik, S. -L. Kim, W P Chen, [......], D. Koziel, H. T. Lee, J. MacDonald, L. Fox Machado, T. Monserrat, J. Stevick, S. Stewart, D. Terry, A. -Y. Zhou, S. Zola
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  ABSTRACT: We present the results of a multisite photometric campaign on the pulsating sdB star Balloon 090100001. The star is one of the two known hybrid hot subdwarfs with both long- and short-period oscillations. The campaign involved eight telescopes with three obtaining UBVR data, four B-band data, and one Stromgren uvby photometry. The campaign covered 48 nights, providing a temporal resolution of 0.36microHz with a detection threshold of about 0.2mmag in B-filter data. Balloon 090100001 has the richest pulsation spectrum of any known pulsating subdwarf B star and our analysis detected 114 frequencies including 97 independent and 17 combination ones. The strongest mode (f_1) in the 2.8mHz region is most likely radial while the remaining ones in this region form two nearly symmetric multiplets: a triplet and quintuplet, attributed to rotationally split \ell=1 and 2 modes, respectively. We find clear increases of splitting in both multiplets between the 2004 and 2005 observing campaigns, amounting to 15% on average. The observed splittings imply that the rotational rate in Bal09 depends on stellar latitude and is the fastest on the equator. We use a small grid of models to constrain the main mode (f_1), which most likely represents the radial fundamental pulsation. The groups of p-mode frequencies appear to lie in the vicinity of consecutive radial overtones, up to the third one. Despite the large number of g-mode frequencies observed, we failed to identify them, most likely because of the disruption of asymptotic behaviour by mode trapping. The observed frequencies were not, however, fully exploited in terms of seismic analysis which should be done in the future with a larger grid of reliable evolutionary models of hot subdwarfs. Comment: accepted for publication in MNRAS
  Monthly Notices of the Royal Astronomical Society 10/2008; · 4.90 Impact Factor
• Article: Photometric multi-site campaign on massive B stars in the open cluster $chi$ Persei (NGC 884)

  S Saesen, A Pigulski, F Carrier, J DeRidder, C Aerts, G Handler, A Narwid, J ~N Fu, C Zhang, X ~J Jiang, [......], E Vanhollebeke, G Raskin, Y Blom, J ~R Eggen, P ~G Beck, J Puschnig, L Schmitt, G ~A Gelven, B Steiniger, R Drummond
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  ABSTRACT: In 2005 a photometric observation campaign started on the open cluster $chi$ Persei, involving 13 telescopes spread over the whole northern hemisphere. After two years we gathered almost 1200 hours of data. We present here preliminary results on the variability search, especially from the 60-cm telescope in Bialków (Poland), which show seven confirmed $beta$ Cephei stars, four candidate B-type pulsators and other interesting variable stars.
  Journal of Physics Conference Series 10/2008; 118(1):012071.
• Article: V994 Herculis: the multiple system with a quadruple‐lined spectrum and a double eclipsing feature

  C.-U. Lee, S.-L. Kim, J. W. Lee, C.-H. Kim, Y.-B. Jeon, H.-I. Kim, J.-N. Yoon, A. Humphrey
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  ABSTRACT: We report the discovery of a multiple system with a quadruple-lined spectrum and a double eclipsing feature. Our photometric and high-resolution spectroscopic data show that V994 Herculis (V994 Her; ADS 11373 AB) is composed of two pairs of double-lined eclipsing binaries, which we designate as A and B. System A consists of a B8V+A0V binary with an orbital period of 2.083 264 d and system B of a A2V+A4V binary with 1.420 033 d. Our light curves show that both of them have a detached binary configuration. We derive masses and radii of four components (Aa, Ab, Ba and Bb) from the synthetic analyses of light curves and radial velocity curves. The masses of systems A and B are MAa= 2.83 ± 0.20 M⊙, MAb= 2.30 ± 0.16 M⊙, MBa= 1.87 ± 0.12 M⊙ and MBb= 1.86 ± 0.12 M⊙, with radii RAa= 2.15 ± 0.05 R⊙, RAb= 1.71 ± 0.04 R⊙, RBa= 1.59 ± 0.08 R⊙ and RBb= 1.50 ± 0.08 R⊙, respectively. These masses and radii are well consistent with the empirical relation for double-lined eclipsing binaries.
  Monthly Notices of the Royal Astronomical Society 08/2008; 389(4):1630 - 1636. · 4.90 Impact Factor
• Article: Rate of change of the pulsation periods in the PG 1159 star PG 0122+200

  G. Vauclair, J.-N. Fu, J.-E. Solheim, S.-L. Kim, M. Chevreton, N. Dolez, L. Chen, M. A. Wood, I. M. Silver
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  ABSTRACT: The pre-white dwarf pulsators of PG 1159 type, or GW Virginis variable stars, are in a phase of rapid evolution towards the white dwarf cooling sequence. The rate of change of their nonradial g-mode frequencies can be measured on a reasonably short time scale. From a theoretical point of view, it was expected that one could derive the rate of cooling of the stellar core from such measurements. At the cool end of the GW Virginis instability strip, it is predicted that the neutrinos flux dominates the cooling. PG 0122+200 which defines the red edge of the instability strip is in principle a good candidate to check this prediction. It has been followed-up through multisite photometric campaigns for about fifteen years. We report here the first determination of the rate of change of its 7 largest amplitude frequencies. We find that the amplitudes of the frequency variations are one to two orders of magnitude larger than predicted by theoretical models based on the assumption that these variations are uniquely caused by cooling. The time scales of the variations are much shorter than the ones expected from a neutrino dominated core cooling. These results point to the existence of other mechanisms responsible for the frequency variability. We discuss the role of nonlinearities as one possible mechanism.
  Proceedings of the International Astronomical Union 03/2008; 4:157 - 162.