J. T. Gilker

Publications (3)5.53 Total impact

• Article: GALEX J201337.6+092801: The Lowest Gravity Subdwarf B Pulsator

  R. H. Østensen, P. I. Pápics, R. Oreiro, M. D. Reed, A. C. Quint, J. T. Gilker, L. L. Hicks, A. S. Baran, L. Fox Machado, T. A. Ottosen, and J. H. Telting
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  ABSTRACT: We present the recent discovery of a new subdwarf B variable (sdBV), with an exceptionally low surface gravity. Our spectroscopy places it at T eff = 32,100 ± 1000, log g = 5.15 ± 0.20, and log(N He/N H) = -2.8 ± 0.2. With a magnitude of B = 12.0, it is the second brightest V361 Hya star ever found. Photometry from three different observatories reveals a temporal spectrum with 11 clearly detected periods in the range 376-566 s, and at least five more close to our detection limit. These periods are unusually long for the V361 Hya class of short-period sdBV pulsators, but not unreasonable for p- and g-modes close to the radial fundamental, given its low surface gravity. Of the ~50 short-period sdB pulsators known to date, only a single one has been found to have comparable spectroscopic parameters to GALEX J201337.6+092801(J20136+0928, for short). This is the enigmatic high-amplitude pulsator V338 Ser, and we conclude that J20136+0928 is the second example of this rare subclass of sdB pulsators located well above the canonical extreme horizontal branch in the H-R diagram.
  The Astrophysical Journal Letters 03/2011; 731(1):L13. · 5.53 Impact Factor
• Source

  Available from: Andrzej Slawomir Baran

  Article: RAT J0455+1305: A rare hybrid pulsating subdwarf B star

  A. S. Baran, J. T. Gilker, L. Fox Machado, M. D. Reed, S. D. Kawaler
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  ABSTRACT: We present results on the second-faintest pulsating subdwarf B (sdB) star known, RAT J0455+1305, derived from photometric data obtained in 2009. It shows both short and long periods oscillations, theoretically assigned as pressure and gravity modes. We identify six short-period frequencies (with one being a combination) and six long-period frequencies. This star is the fourth hybrid sdB star discovered so far which makes it of special interest as each type of mode probes a different part of the star. This star is similar to the sdB hybrid pulsator Balloon 090100001 in that it exhibits short-period mode groupings, which can be used to identify pulsation parameters and constrain theoretical models.
  03/2011;
• Source

  Available from: Andrzej Slawomir Baran

  Article: J08069+1527: A newly discovered high amplitude, hybrid subdwarf B pulsator

  A. S. Baran, J. T. Gilker, M. D. Reed, R. H. Ostensen, J. H. Telting, K. Smolders, L. Hicks, R. Oreiro
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  ABSTRACT: We present our discovery of a new hybrid pulsating subdwarf B star, J08069+1527. The effective temperature and surface gravity of 28,500$\pm$400\,K and 5.37$\pm$0.04\,dex, respectively, place this object inside the instability strip and also among other pulsating hot subdwarfs of a hybrid nature, right next to another fascinating star: Balloon\,090100001. From this proximity, we anticipated this star could pulsate in both high and low frequency modes. Indeed, our analysis of photometric data confirmed our prediction. We detected two peaks in the high frequency region and two other peaks at low frequencies. In addition, the amplitude of the dominant mode is very high and comparable to the dominant peaks in other hybrid subdwarf B stars. Since this star is bright, we performed time-series low resolution spectroscopy. Despite a low signal-to-noise (S/N) ratio, we were able to detect the main peak from these data. All our results strongly indicate that J08069+1527 is a high amplitude pulsating hot subdwarf B star of hybrid nature. By analogy to the other pulsating sdB star, we judge that the dominant mode we detected here has radial nature. Future stellar modeling should provide us with quite good constrains as p- and g-modes presented in this star are driven in different parts of its interior.
  03/2011;