Browsing by Subject "stars: variables: general"
Now showing 1 - 14 of 14
- Results Per Page
- Sort Options
Item The Chemical Compositions Of RR Lyrae Type C Variable Stars(2014-02) Govea, Jose; Gomez, Thomas; Preston, George W.; Sneden, Christopher; Govea, Jose; Gomez, Thomas; Sneden, ChristopherWe present a detailed chemical abundance study of eight RR Lyrae variable stars of subclass c (RRc). The target RRc stars chosen for study exhibit "Blazhko-effect" period and amplitude modulations to their pulsational cycles. Data for this study were gathered with the echelle spectrograph of the 100 inch du Pont telescope at Las Campanas Observatory. Spectra were obtained throughout each star's pulsation cycle. Atmospheric parameters-effective temperature, surface gravity, microturbulent velocity, and metallicity-were derived at multiple phase points. We found metallicities and element abundance ratios to be constant within observational uncertainties over the pulsational cycles of all stars. Moreover, the alpha-element and Fe-group abundance ratios with respect to iron are consistent with other horizontal-branch members (RRab, blue and red non-variables). Finally, we have used the [Fe/H] values of these eight RRc stars to anchor the metallicity estimates of a large-sample RRc snapshot spectroscopic study being conducted with the same telescope and instrument combination employed here.Item Discovery of an Ultramassive Pulsating White Dwarf(2013-07) Hermes, J. J.; Kepler, S. O.; Castanheira, Barbara G.; Gianninas, A.; Winget, D. E.; Montgomery, M. H.; Brown, Warren R.; Harrold, Samuel T.; Hermes, J. J.; Castanheira, Barbara G.; Winget, D. E.; Montgomery, M. H.; Harrold, Samuel T.We announce the discovery of the most massive pulsating hydrogen-atmosphere white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a 12,030 +/- 210 K WD with a log g = 9.08 +/- 0.06, which corresponds to a mass of 1.20 +/- 0.03M(circle dot). Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core. Such a massive WD should also be significantly crystallized at this temperature. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425 to 595 s and amplitudes up to 0.7%, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode pulsations driven by a hydrogen partial ionization zone. Successfully unraveling both the total mass and core composition of GD 518 provides a unique opportunity to investigate intermediate-mass stellar evolution, and can possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in turn constrains Type Ia supernovae progenitor systems.Item Evidence For Temperature Change And Oblique Pulsation From Light Curve Fits Of The Pulsating White Dwarf GD 358(2010-06) Montgomery, Michael H.; Provencal, J. L.; Kanaan, A.; Mukadam, Anjum S.; Thompson, Susan E.; Dalessio, J.; Shipman, H. L.; Winget, D. E.; Kepler, S. O.; Koester, Detlev; Montgomery, Michael H.Convective driving, the mechanism originally proposed by Brickhill for pulsating white dwarf stars, has gained general acceptance as the generic linear instability mechanism in DAV and dbV white dwarfs. This physical mechanism naturally leads to a nonlinear formulation, reproducing the observed light curves of many pulsating white dwarfs. This numerical model can also provide information on the average depth of a star's convection zone and the inclination angle of its pulsation axis. In this paper, we give two sets of results of nonlinear light curve fits to data on the dbV GD 358. Our first fit is based on data gathered in 2006 by the Whole Earth Telescope; this data set was multiperiodic containing at least 12 individual modes. Our second fit utilizes data obtained in 1996, when GD 358 underwent a dramatic change in excited frequencies accompanied by a rapid increase in fractional amplitude; during this event it was essentially monoperiodic. We argue that GD 358's convection zone was much thinner in 1996 than in 2006, and we interpret this as a result of a short-lived increase in its surface temperature. In addition, we find strong evidence of oblique pulsation using two sets of evenly split triplets in the 2006 data. This marks the first time that oblique pulsation has been identified in a variable white dwarf star.Item GD 358: The Case For Oblique Pulsation And Temperature Change(2010-08) Montgomery, M. H.; Montgomery, M. H.We provide multiple lines of evidence that GD 358's convection zone was much thinner in 1996 than in 2006 and we interpret this as a result of a short-lived increase in its surface temperature. In addition, we find strong evidence of oblique pulsation using two sets of evenly split triplets in the 2006 data. This marks the first time that oblique pulsation has been identified in a variable white dwarf star as well as the first time that a temperature change has been observed in a single white dwarf.Item The Hot R Coronae Borealis Star DY Centauri is a Binary(2012-11) Rao, N. Kameswara; Lambert, David L.; Garcia-Hernandez, D. A.; Jeffery, C. Simon; Woolf, Vincent M.; McArthur, Barbara; Rao, N. Kameswara; Lambert, David L.; McArthur, BarbaraThe remarkable hot R Coronae Borealis (RCB) star DY Cen is revealed to be the first and only binary system to be found among the RCB stars and their likely relatives, including the extreme helium stars and the hydrogen-deficient carbon stars. Radial velocity determinations from 1982 to 2010 have shown that DY Cen is a single-lined spectroscopic binary in an eccentric orbit with a period of 39.67 days. It is also one of the hottest and most H-rich member of the class of RCB stars. The system may have evolved from a common envelope to its current form.Item Kepler Mission Design, Realized Photometric Performance, and Early Science(2010-04) Koch, David G.; Borucki, William J.; Basri, Gibor; Batalha, Natalie M.; Brown, Timothy M.; Caldwell, Douglas; Christensen-Dalsgaard, Jorgen; Cochran, William D.; DeVore, Edna; Dunham, Edward W.; Gautier, Thomas N., III; Geary, John C.; Gilliland, Ronald L.; Gould, Alan; Jenkins, Jon; Kondo, Yoji; Latham, David W.; Lissauer, Jack J.; Marcy, Geoffrey; Monet, David; Sasselov, Dimitar; Boss, Alan; Brownlee, Donald; Caldwell, John; Dupree, Andrea K.; Howell, Steve B.; Kjeldsen, Hans; Meibom, Soren; Morrison, David; Owen, Tobias; Reitsema, Harold; Tarter, Jill; Bryson, Stephen T.; Dotson, Jessie L.; Gazis, Paul; Haas, Michael R.; Kolodziejczak, Jeffrey; Rowe, Jason F.; Van Cleve, Jeffrey E.; Allen, Christopher; Chandrasekaran, Hema; Clarke, Bruce D.; Li, Jie; Quintana, Elisa V.; Tenenbaum, Peter; Twicken, Joseph D.; Wu, Hayley; Cochran, William D.The Kepler Mission, launched on 2009 March 6, was designed with the explicit capability to detect Earth-size planets in the habitable zone of solar-like stars using the transit photometry method. Results from just 43 days of data along with ground-based follow-up observations have identified five new transiting planets with measurements of their masses, radii, and orbital periods. Many aspects of stellar astrophysics also benefit from the unique, precise, extended, and nearly continuous data set for a large number and variety of stars. Early results for classical variables and eclipsing stars show great promise. To fully understand the methodology, processes, and eventually the results from the mission, we present the underlying rationale that ultimately led to the flight and ground system designs used to achieve the exquisite photometric performance. As an example of the initial photometric results, we present variability measurements that can be used to distinguish dwarf stars from red giants.Item KOI-54: The Kepler Discovery of Tidally Excited Pulsations and Brightenings in a Highly Eccentric Binary(2011-11) Welsh, William F.; Orosz, Jerome A.; Aerts, Conny; Brown, Timothy M.; Brugamyer, Erik; Cochran, William D.; Gilliland, Ronald L.; Guzik, Joyce Ann; Kurtz, D. W.; Latham, David W.; Marcy, Geoffrey W.; Quinn, Samuel N.; Zima, Wolfgang; Allen, Christopher; Batalha, Natalie M.; Bryson, Steve; Buchhave, Lars A.; Caldwell, Douglas A.; Gautier, Thomas N., III; Howell, Steve B.; Kinemuchi, K.; Ibrahim, Khadeejah A.; Isaacson, Howard; Jenkins, Jon M.; Prsa, Andrej; Still, Martin; Street, Rachel; Wohler, Bill; Koch, David G.; Borucki, William J.; Cochran, William D.Kepler observations of the star HD 187091 (KIC 8112039, hereafter KOI-54) revealed a remarkable light curve exhibiting sharp periodic brightening events every 41.8 days with a superimposed set of oscillations forming a beating pattern in phase with the brightenings. Spectroscopic observations revealed that this is a binary star with a highly eccentric orbit, e = 0.83. We are able to match the Kepler light curve and radial velocities with a nearly face-on (i = 5 degrees.5) binary star model in which the brightening events are caused by tidal distortion and irradiation of nearly identical A stars during their close periastron passage. The two dominant oscillations in the light curve, responsible for the beating pattern, have frequencies that are the 91st and 90th harmonic of the orbital frequency. The power spectrum of the light curve, after removing the binary star brightening component, reveals a large number of pulsations, 30 of which have a signal-to-noise ratio greater than or similar to 7. Nearly all of these pulsations have frequencies that are either integer multiples of the orbital frequency or are tidally split multiples of the orbital frequency. This pattern of frequencies unambiguously establishes the pulsations as resonances between the dynamic tides at periastron and the free oscillation modes of one or both of the stars. KOI-54 is only the fourth star to show such a phenomenon and is by far the richest in terms of excited modes.Item Limits Of Perturbative Nonlinear Light Curve Analyses: The Case Of G117-B15A(2010-08) Montgomery, M. H.; Hermes, J. J.; Winget, D. E.; Montgomery, M. H.; Hermes, J. J.; Winget, D. E.This poster examines the nonlinear combination frequencies found in the DAV G117-B15A. Using multiple epochs of observations we show that the ratio of the amplitude of the combination frequencies to those of the parent frequencies is not constant in time but rather varies on a time scale of years. This variation is difficult to explain in any model in which the modeled nonlinearities are second order, such as that of Brassard et al. [2], which accounts for these nonlinearities via the conversion from temperature variations to flux variations. In contrast, due to its extreme sensitivity, the convective light curve model can accomodate these variations with only modest variations in parameters such as T(eff).Item Measuring The Evolutionary Rate Of Cooling Of ZZ Ceti(2013-07) Mukadam, Anjum S.; Bischoff-Kim, Agnes; Fraser, Oliver; Corsico, A. H.; Montgomery, Michael H.; Kepler, S. O.; Romero, A. D.; Winget, D. E.; Hermes, J. J.; Riecken, T. S.; Kronberg, M. E.; Winget, K. I.; Falcon, Ross E.; Chandler, Dean W.; Kuehne, J. W.; Sullivan, Denis J.; Reaves, D.; von Hippel, Ted; Mullally, Fergal; Shipman, H.; Thompson, Susan E.; Silvestri, N. M.; Hynes, Robert I.; Winget, D. E.; Hermes, J. J.; Winget, K. I.; Falcon, Ross E.; Reaves, D.We have finally measured the evolutionary rate of cooling of the pulsating hydrogen atmosphere (DA) white dwarf ZZ Ceti (Ross 548), as reflected by the drift rate of the 213.13260694 s period. Using 41 yr of time-series photometry from 1970 November to 2012 January, we determine the rate of change of this period with time to be dP/dt = (5.2 +/- 1.4) x 10(-15) s s(-1) employing the O - C method and (5.45 +/- 0.79) x 10(-15) s s(-1) using a direct nonlinear least squares fit to the entire lightcurve. We adopt the dP/dt obtained from the nonlinear least squares program as our final determination, but augment the corresponding uncertainty to a more realistic value, ultimately arriving at the measurement of dP/dt = (5.5 +/- 1.0) x 10(-15) s s(-1). After correcting for proper motion, the evolutionary rate of cooling of ZZ Ceti is computed to be (3.3 +/- 1.1) x 10(-15) s s(-1). This value is consistent within uncertainties with the measurement of (4.19 +/- 0.73) x 10(-15) s s(-1) for another similar pulsating DA white dwarf, G 117-B15A. Measuring the cooling rate of ZZ Ceti helps us refine our stellar structure and evolutionary models, as cooling depends mainly on the core composition and stellar mass. Calibrating white dwarf cooling curves with this measurement will reduce the theoretical uncertainties involved in white dwarf cosmochronometry. Should the 213.13 s period be trapped in the hydrogen envelope, then our determination of its drift rate compared to the expected evolutionary rate suggests an additional source of stellar cooling. Attributing the excess cooling to the emission of axions imposes a constraint on the mass of the hypothetical axion particle.Item A New Timescale For Period Change In The Pulsating DA White Dwarf WD 0111+0018(2013-03) Hermes, J. J.; Montgomery, Michael H.; Mullally, Fergal; Winget, D. E.; Bischoff-Kim, Agnes; Hermes, J. J.; Mullally, Fergal; Winget, D. E.We report the most rapid rate of period change measured to date for a pulsating DA (hydrogen atmosphere) white dwarf (WD), observed in the 292.9 s mode of WD0111+0018. The observed period change, faster than 10(-12) s s(-1), exceeds by more than two orders of magnitude the expected rate from cooling alone for this class of slow and simply evolving pulsating WDs. This result indicates the presence of an additional timescale for period evolution in these pulsating objects. We also measure the rates of period change of nonlinear combination frequencies and show that they share the evolutionary characteristics of their parent modes, confirming that these combination frequencies are not independent modes but rather artifacts of some nonlinear distortion in the outer layers of the star.Item Precision Asteroseismology Of The Pulsating White Dwarf Gd 1212 Using A Two-Wheel-Controlled Kepler Spacecraft(2014-07) Hermes, J. J.; Charpinet, S.; Barclay, Thomas; Pakstiene, E.; Mullally, Fergal; Kawaler, Steven D.; Bloemen, S.; Castanheira, Barbara G.; Winget, D. E.; Montgomery, Michael H.; Van Grootel, V.; Huber, Daniel; Still, Martin; Howell, Steve B.; Caldwell, Douglas A.; Haas, Michael R.; Bryson, Stephen T.; Winget, D. E.; Montgomery, Michael H.We present a preliminary analysis of the cool pulsating white dwarf (WD) GD 1212, enabled by more than 11.5 days of space-based photometry obtained during an engineering test of the two-reaction-wheel-controlled Kepler spacecraft. We detect at least 19 independent pulsation modes, ranging from 828.2-1220.8 s, and at least 17 nonlinear combination frequencies of those independent pulsations. Our longest uninterrupted light curve, 9.0 days in length, evidences coherent difference frequencies at periods inaccessible from the ground, up to 14.5 hr, the longest-period signals ever detected in a pulsating WD. These results mark some of the first science to come from a two-wheel-controlled Kepler spacecraft, proving the capability for unprecedented discoveries afforded by extending Kepler observations to the ecliptic.Item Radius Constraints From High-Speed Photometry Of 20 Low-Mass White Dwarf Binaries(2014-09) Hermes, J. J.; Brown, Warren R.; Kilic, Mukremin; Gianninas, A.; Chote, Paul; Sullivan, Denis J.; Winget, D. E.; Bell, Keaton J.; Falcon, Ross E.; Winget, K. I.; Mason, Paul A.; Harrold, Samuel T.; Montgomery, Michael H.; Hermes, J. J.; Winget, K. I.; Harrold, Samuel T.; Montgomery, Michael H.We carry out high-speed photometry on 20 of the shortest-period, detached white dwarf binaries known and discover systems with eclipses, ellipsoidal variations (due to tidal deformations of the visible white dwarf), and Doppler beaming. All of the binaries contain low-mass white dwarfs with orbital periods of less than four hr. Our observations identify the first eight tidally distorted white dwarfs, four of which are reported for the first time here. We use these observations to place empirical constraints on the mass-radius relationship for extremely low-mass (<= 0.30 M-circle dot) white dwarfs. We also detect Doppler beaming in several of these binaries, which confirms their high-amplitude radial-velocity variability. All of these systems are strong sources of gravitational radiation, and long-term monitoring of those that display ellipsoidal variations can be used to detect spin-up of the tidal bulge due to orbital decay.Item SDSS J184037.78+642312.3: the First Pulsating Extremely Low Mass White Dwarf(2012-05) Hermes, J. J.; Montgomery, M. H.; Winget, D. E.; Brown, Warren R.; Kilic, Mukremin; Kenyon, Scott J.; Hermes, J. J.; Montgomery, M. H.; Winget, D. E.We report the discovery of the first pulsating extremely low mass (ELM) white dwarf (WD), SDSS J184037.78+642312.3 (hereafter J1840). This DA (hydrogen-atmosphere) WD is by far the coolest and the lowest-mass pulsating WD, with T-eff = 9100 +/- 170 K and log g = 6.22 +/- 0.06, which corresponds to a mass of similar to 0.17 M-circle dot. This low-mass pulsating WD greatly extends the DAV (or ZZ Ceti) instability strip, effectively bridging the log g gap between WDs and main-sequence stars. We detect high-amplitude variability in J1840 on timescales exceeding 4000 s, with a non-sinusoidal pulse shape. Our observations also suggest that the variability is multi-periodic. The star is in a 4.6 hr binary with another compact object, most likely another WD. Future, more extensive time-series photometry of this ELM WD offers the first opportunity to probe the interior of a low-mass, presumably He-core WD using the tools of asteroseismology.Item A Second Case of Outbursts in A Pulsating White Dwarf Observed By Kepler(2015-09) Hermes, J. J.; Montgomery, M. H.; Bell, Keaton J.; Chote, P.; Gansicke, B. T.; Kawaler, Steven D.; Clemens, J. Christopher; Dunlap, B. H.; Winget, D. E.; Armstrong, D. J.; Montgomery, M. H.; Bell, Keaton J.; Winget, D. E.We present observations of a new phenomenon in pulsating white dwarf stars: large-amplitude outbursts at timescales much longer than the pulsation periods. The cool (T-eff = 11,060 K), hydrogen-atmosphere pulsating white dwarf PG 1149+057 was observed nearly continuously for more than 78.8 day by the extended Kepler mission in K2 Campaign 1. The target showed 10 outburst events, recurring roughly every 8 day and lasting roughly 15 hr, with maximum flux excursions up to 45% in the Kepler bandpass. We demonstrate that the outbursts affect the pulsations and therefore must come from the white dwarf. Additionally, we argue that these events are not magnetic reconnection flares, and are most likely connected to the stellar pulsations and the relatively deep surface convection zone. PG 1149+057 is now the second cool pulsating white dwarf to show this outburst phenomenon, after the first variable white dwarf observed in the Kepler mission, KIC 4552982. Both stars have the same effective temperature, within the uncertainties, and are among the coolest known pulsating white dwarfs of typical mass. These outbursts provide fresh observational insight into the red edge of the DAV instability strip and the eventual cessation of pulsations in cool white dwarfs.