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Title:
The pulsating hot subdwarf Balloon 090100001: results of the 2005 multisite campaign
Authors:
Baran, A.; Oreiro, R.; Pigulski, A.; Pérez Hernández, F.; Ulla, A.; Reed, M. D.; Rodríguez-López, C.; Moskalik, P.; Kim, S.-L.; Chen, W.-P.; Crowe, R.; Siwak, M.; Armendarez, L.; Binder, P. M.; Choo, K.-J.; Dye, A.; Eggen, J. R.; Garrido, R.; González Pérez, J. M.; Harms, S. L.; Huang, F.-Y.; Kozieł, D.; Lee, H.-T.; MacDonald, J.; Fox Machado, L.; Monserrat, T.; Stevick, J.; Stewart, S.; Terry, D.; Zhou, A.-Y.; Zoła, S.
Affiliation:
AA(Cracow Pedagogical University, ul. Podchorażych 2, 30 - 084 Kraków, Poland; Toruń Centre for Astronomy, ul. Gagarina 11, Toruń, Poland), AB(Instituto de Astrofísica de Canarias, 38200 La Laguna, Spain; Institute of Astronomy, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium), AC(Instytut Astronomiczny Uniwersytetu Wrocławskiego, ul. Kopernika 11, 51 - 622 Wrocław, Poland), AD(Instituto de Astrofísica de Canarias, 38200 La Laguna, Spain; Dpto. Astrofísica, Universidad de La Laguna, 38200 La Laguna, Spain), AE(Dpto. Física Aplicada, Universidade de Vigo, 36200 Vigo, Spain), AF(Department of Physics, Astronomy and Materials Science, Missouri State University, 901 S. National, Springfield, MO 65897, USA), AG(Dpto. Física Aplicada, Universidade de Vigo, 36200 Vigo, Spain; Instituto de Astrofísica de Andalucía - C.S.I.C., Camino Bajo de Huétor 50, E - 18008 Granada, Spain; Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 Av. Edouard Belin, Toulouse 31400, France), AH(Copernicus Astronomical Centre, ul. Bartycka 18, 00-716 Warsaw, Poland), AI(Korea Astronomy and Space Science Institute, Daejeon 305-348, South Korea), AJ(Institute of Astronomy, National Central University, Jhongli 32054, Taiwan), AK(Department of Physics and Astronomy, University of Hawaii - Hilo, 200 West Kawili Street, Hilo, Hawaii 96720-4091, USA), AL(Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland), AM(Department of Physics and Astronomy, University of Hawaii - Hilo, 200 West Kawili Street, Hilo, Hawaii 96720-4091, USA), AN(Department of Physics and Astronomy, University of Hawaii - Hilo, 200 West Kawili Street, Hilo, Hawaii 96720-4091, USA), AO(Korea Astronomy and Space Science Institute, Daejeon 305-348, South Korea), AP(Department of Physics and Astronomy, University of Hawaii - Hilo, 200 West Kawili Street, Hilo, Hawaii 96720-4091, USA), AQ(Department of Physics, Astronomy and Materials Science, Missouri State University, 901 S. National, Springfield, MO 65897, USA), AR(Instituto de Astrofísica de Andalucía - C.S.I.C., Camino Bajo de Huétor 50, E - 18008 Granada, Spain), AS(Instituto de Astrofísica de Canarias, 38200 La Laguna, Spain; GRANTECAN S.A. (CALP), 38712 Brena Baja, La Palma, Spain), AT(Department of Physics, Astronomy and Materials Science, Missouri State University, 901 S. National, Springfield, MO 65897, USA), AU(Institute of Astronomy, National Central University, Jhongli 32054, Taiwan), AV(Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland), AW(Institute of Astronomy, National Central University, Jhongli 32054, Taiwan), AX(Departmentof Physics and Astronomy, University of Delaware, Newark, DE19716, USA), AY(Instituto de Astrofísica de Canarias, 38200 La Laguna, Spain; Observatorio Astronómico Nacional, Instituto de Astronomía, Universidad Nacional Autónoma de México, A.P. 877 Ensenada, BC 22860, Mexico), AZ(Instituto de Astrofísica de Canarias, 38200 La Laguna, Spain), BA(Department of Physics and Astronomy, University of Hawaii - Hilo, 200 West Kawili Street, Hilo, Hawaii 96720-4091, USA), BB(Department of Physics and Astronomy, University of Hawaii - Hilo, 200 West Kawili Street, Hilo, Hawaii 96720-4091, USA), BC(Department of Physics and Astronomy, University of Hawaii - Hilo, 200 West Kawili Street, Hilo, Hawaii 96720-4091, USA), BD(Department of Physics, Astronomy and Materials Science, Missouri State University, 901 S. National, Springfield, MO 65897, USA; National Astronomical Observatories of the Chinese Academy of Sciences, Beijing 100012, P.R. China), BE(Cracow Pedagogical University, ul. Podchorażych 2, 30 - 084 Kraków, Poland; Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 392, Issue 3, pp. 1092-1105. (MNRAS Homepage)
Publication Date:
01/2009
Origin:
MNRAS
MNRAS Keywords:
stars: individual: Balloon090100001 , stars: oscillations , subdwarf
DOI:
10.1111/j.1365-2966.2008.14024.x
Bibliographic Code:
2009MNRAS.392.1092B

Abstract

We present the results of a multisite photometric campaign on the pulsating B-type hot subdwarf star Balloon090100001 (Bal09). The star is one of the two known hybrid hot subdwarfs with both long- and short-period oscillations, theoretically attributed to g and p modes. The campaign involved eight telescopes with three obtaining UBVR data, four B-band data and one Strömgren uvby photometry. The campaign covered 48 nights, providing a temporal resolution of 0.36μHz with a detection threshold of about 0.2mmag in B-filter data.

Bal09 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. Most of the 24 g-mode frequencies are between 0.1 and 0.4mHz. Of the remaining 73, presumably p modes, 72 group into four distinct regions near 2.8, 3.8, 4.7 and 5.5mHz. The density of frequencies requires that some modes must have degrees l larger than 2. The modes in the 2.8 mHz region have the largest amplitudes. The strongest mode (f1) is most likely radial, while the remaining ones in this region form two nearly symmetric multiplets: a triplet and quintuplet, attributed to rotationally split l = 1 and 2 modes, respectively. We find clear increases of splitting in both multiplets between the 2004 and 2005 observing campaigns, amounting to ~15 per cent on average. The observed splittings imply that the rotational rate in Bal09 depends on stellar latitude and is the fastest on the equator. We also speculate on the possible reasons for the changes of splitting. The only plausible explanation we find is torsional oscillation. This hypothesis, however, needs to be verified in the future by detailed modelling. In this context, it is very important to monitor the splittings on a longer time-scale as their behaviour may help to explain this interesting phenomenon.

The amplitudes of almost all terms detected in both 2004 and 2005 were found to vary. This is evident even during one season; for example, amplitudes of modes f8 and fC were found to change by a factor of 2-3 within about 50 d during 2005.

We use a small grid of models to constrain the main mode (f1), which most likely represents the radial fundamental pulsation. The groups of p-mode frequencies appear to lie in the vicinity of the 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.

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