arXiv subjects

A. F. J. Moffat

Publications and source records attributed to A. F. J. Moffat.

At least 127 records · Page 7Linked to original sources

arXiv2008

MOST detects SPBe pulsations in HD 127756 & HD 217543: Asteroseismic rotation rates independent of vsini

The MOST (Microvariability and Oscillations of Stars) satellite has discovered SPBe (Slowly Pulsating Be) oscillations in the stars HD 127756 (B1/B2 Vne) and HD 217543 (B3 Vpe). For HD 127756, 30 significant frequencies are identified from 31 days of nearly continuous photometry; for HD 217543, up to 40 significant frequencies from 26 days of data. In both cases, the oscillations fall into three distinct frequency ranges, consistent with models of the stars. The variations are caused by nonradial g-modes (and possibly r-modes) distorted by rapid rotation and excited by the opacity mechanism near the iron opacity bump. A comparison of pulsation models and observed frequency groups yields a rotation frequency for each star, independently of vsini. The rotation rates of these stars, as well as those of the SPBe stars previously discovered by MOST, HD 163868 and $β$ CMi, are all close to their critical values.

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arXiv2008

MOST photometry of the roAp star 10 Aql

Context: We present 31.2 days of nearly continuous MOST photometry of the roAp star 10Aql. Aims:The goal was to provide an unambiguous frequency identification for this little studied star, as well as to discuss the detected frequencies in the context of magnetic models and analyze the influence of the magnetic field on the pulsation. Methods: Using traditional Fourier analysis techniques on three independent data reductions, intrinsic frequencies for the star are identified. Theoretical non-adiabatic axisymmetric modes influenced by a magnetic field having polar field strengths Bp = 0-5kG were computed to compare the observations to theory. Results: The high-precision data allow us to identify three definite intrinsic pulsation frequencies and two other candidate frequencies with low S/N. Considering the observed spacings, only one (50.95microHz) is consistent with the main sequence nature of roAp stars. The comparison with theoretical models yields a best fit for a 1.95Msun model having solar metallicity, suppressed envelope convection, and homogenous helium abundance. Furthermore, our analysis confirms the suspected slow rotation of the star and sets new lower limits to the rotation period (Prot>1 month) and inclination (i>30\pm10deg.). Conclusions:The observed frequency spectrum is not rich enough to unambiguously identify a model. On the other hand, the models hardly represent roAp stars in detail due to the approximations needed to describe the interactions of the magnetic field with stellar structure and pulsation. Consequently, errors in the model frequencies needed for the fitting procedure can only be estimated. Nevertheless, it is encouraging that models which suppress convection and include solar metallicity, in agreement with current concepts of roAp stars, fit the observations best.

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arXiv2008

MOST photometry and modeling of the rapidly oscillating (roAp) star gamma Equ

Aims. Despite photometry and spectroscopy of its oscillations obtained over the past 25 years, the pulsation frequency spectrum of the rapidly oscillating Ap (roAp) star gamma Equ has remained poorly understood. Better time-series photometry, combined with recent advances to incorporate interior magnetic field geometry into pulsational models, enable us to perform improved asteroseismology of this roAp star. Methods. We obtained 19 days of continuous high-precision photometry of gamma Equ with the MOST (Microvariability & Oscillations of STars) satellite. The data were reduced with two different reduction techniques and significant frequencies were identified. Those frequencies were fitted by interpolating a grid of pulsation models that include dipole magnetic fields of various polar strengths. Results. We identify 7 frequencies in gamma Equ that we associate with 5 high-overtone p-modes and 1st and 2nd harmonics of the dominant p-mode. One of the modes and both harmonics are new discoveries for this star. Our best model solution (1.8 M_sun, log T_eff ~ 3.882; polar field strength ~8.1 kG) leads to unique mode identifications for these frequencies (ell = 0, 1, 2 and 4). This is the first purely asteroseismic fit to a grid of magnetic models. We measure amplitude and phase modulation of the primary frequency due to beating with a closely spaced frequency which had never been resolved. This casts doubts on theories that such modulation - unrelated to the rotation of the star - is due to a stochastic excitation mechanism.

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arXiv2007

New insights into the nature of the SMC WR/LBV binary HD 5980

We present the results of optical wavelength observations of the unusual SMC eclipsing binary system HD 5980 obtained in 1999 and 2004--2005. Radial velocity curves for the erupting LBV/WR object (star A) and its close WR-like companion (star B) are obtained by deblending the variable emission-line profiles of N IV and N V lines under the simplistic assumption that these lines originate primarily in the winds of star A and star B. The derived masses M_A=58--79 Mo and M_B=51--67 Mo, are more consistent with the stars' location near the top of the HRD than previous estimates. The presence of a wind-wind interaction region is inferred from the orbital phase-dependent behavior of He I P Cygni absorption components. The emission-line intensities continued with the declining trend previously seen in UV spectra. The behavior of the photospheric absorption lines is consistent with the results of Schweickhardt (2002) who concludes that the third object in the combined spectrum, star C, is also a binary system with P(starC)~96.5 days, e=0.83. The data used in this paper will be made publicly available for further analysis.

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arXiv2007

Nonradial p-modes in the G9.5 giant epsilon Oph? Pulsation model fits to MOST photometry

The G9.5 giant eps Oph shows evidence of radial p-mode pulsations in both radial velocity and luminosity. We re-examine the observed frequencies in the photometry and radial velocities and find a best model fit to 18 of the 21 most significant photometric frequencies. The observed frequencies are matched to both radial and nonradial modes in the best model fit. The small scatter of the frequencies about the model predicted frequencies indicate that the average lifetimes of the modes could be as long as 10-20d. The best fit model itself, constrained only by the observed frequencies, lies within 1 sigma of eps Oph's position in the HR-diagram and the interferometrically determined radius.

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arXiv2007

HD 114839 - An Am star showing both delta Scuti and gamma Dor pulsations discovered through MOST photometry

Using MOST (Microvariability and Oscillations of STars) satellite guide star photometry, we have discovered a metallic A star showing hybrid p- and g-mode pulsations. HD 114839 was observed nearly continuously for 10 days in March, 2005. We identify frequencies in three groups: the first centered near 2 cycles/day, in the gamma Dor pulsation range, and two others near 8 and 20, both in the delta Scuti range. This is only the fourth known such hybrid pulsator, including another MOST discovery (Rowe et al. 2006, this issue).

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arXiv2007

Reduction of time-resolved space-based CCD photometry developed for MOST Fabry Imaging data

The MOST (Microvariability & Oscillations of STars) satellite obtains ultraprecise photometry from space with high sampling rates and duty cycles. Astronomical photometry or imaging missions in low Earth orbits, like MOST, are especially sensitive to scattered light from Earthshine, and all these missions have a common need to extract target information from voluminous data cubes. They consist of upwards of hundreds of thousands of two-dimensional CCD frames (or sub-rasters) containing from hundreds to millions of pixels each, where the target information, superposed on background and instrumental effects, is contained only in a subset of pixels (Fabry Images, defocussed images, mini-spectra). We describe a novel reduction technique for such data cubes: resolving linear correlations of target and background pixel intensities. This stepwise multiple linear regression removes only those target variations which are also detected in the background. The advantage of regression analysis versus background subtraction is the appropriate scaling, taking into account that the amount of contamination may differ from pixel to pixel. The multivariate solution for all pairs of target/background pixels is minimally invasive of the raw photometry while being very effective in reducing contamination due to, e.g., stray light. The technique is tested and demonstrated with both simulated oscillation signals and real MOST photometry.

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arXiv2007

Spectroscopy of SMC Wolf-Rayet Stars Suggests that Wind-Clumping does not Depend on Ambient Metallicity

The mass-loss rates of hot, massive, luminous stars are considered a decisive parameter in shaping the evolutionary tracks of such stars and influencing the interstellar medium on galactic scales. The small-scale structures (clumps) omnipresent in such winds may reduce empirical estimates of mass-loss rates by an evolutionarily significant factor of >=3. So far, there has been no direct observational evidence that wind-clumping may persist at the same level in environments with a low ambient metallicity, where the wind-driving opacity is reduced. Here we report the results of time-resolved spectroscopy of three presumably single Population I Wolf-Rayet stars in the Small Magellanic Cloud, where the ambient metallicity is ~1/5 Z_Sun.We detect numerous small-scale emission peaks moving outwards in the accelerating parts of the stellar winds.The general properties of the moving features, such as their velocity dispersions,emissivities and average accelerations, closely match the corresponding characteristics of small-scale inhomogeneities in the winds of Galactic Wolf-Rayet stars.

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arXiv2006

Early-type stars in the core of the young open cluster Westerlund2

Aims. The properties of the early-type stars in the core of the Westerlund2 cluster are examined in order to establish a link between the cluster and the very massive Wolf-Rayet binary WR20a as well as the HII complex RCW49. Methods. Photometric monitoring as well as spectroscopic observations of Westerlund2 are used to search for light variability and to establish the spectral types of the early-type stars in the cluster core. Results. The first light curves of the eclipsing binary WR20a in B and V filters are analysed and a distance of 8kpc is inferred. Three additional eclipsing binaries, which are probable late O or early B-type cluster members, are discovered, but none of the known early O-type stars in the cluster displays significant photometric variability above 1% at the 1-sigma level. The twelve brightest O-type stars are found to have spectral types between O3 and O6.5, significantly earlier than previously thought. Conclusions. The distance of the early-type stars in Westerlund2 is established to be in excellent agreement with the distance of WR20a, indicating that WR20a actually belongs to the cluster. Our best estimate of the cluster distance thus amounts to 8.0pm1.4kpc. Despite the earlier spectral types, the currently known population of early-type stars in Westerlund2 does not provide enough ionizing photons to account for the radio emission of the RCW49 complex. This suggests that there might still exist a number of embedded early O-stars in RCW49.

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arXiv2006

MOST Photometry of the roAp Star HD 134214

We present 10.27$ \rm{hrs}$ of photometry of the roAp star HD 134214 obtained by the MOST satellite. The star is shown to be monoperiodic and oscillating at a frequency of 2948.97$ \pm 0.55 μ\rm{Hz}$. This is consistent with earlier ground based photometric campaigns (e.g. Kreidl et al. 1994). We do not detect any of the additional frequencies identified in the recent spectroscopic study by Kurtz et al. (2006) down to an amplitude limit of 0.36$ \rm{mmag}$ ($2σ$ significance limit).

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arXiv2006

Discovery of hybrid $γ$ Dor and $δ$ Sct pulsations in BD+18 4914 through MOST spacebased photometry

We present a total of 57 days of contiguous, high-cadence photometry (14 days in 2004 and 43 in 2005) of the star BD+18 4914 obtained with the MOST satellite. We detect 16 frequencies down to a signal-to-noise of 3.6 (amplitude \~ 0.5 mmag). Six of these are less than 3 cycles/day, and the other ten are between 7 and 16 cycles/day. We intrepret the low frequencies as g-mode $γ$ Doradus-type pulsations and the others as $δ$ Scuti-type p-modes, making BD+18 4914 one of the few known hybrid pulsators of its class. If the g-mode pulsations are high-overtone non-radial modes with identical low degree l, we can assign a unique mode classification of n={12, 20, 21, 22, 31, 38} based on the frequency ratio method.

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arXiv2006

MOST Detects g-Modes in the Late-Type be Star beta CMi (B8Ve)

The Microvariability and Oscillations of stars (MOST) satellite has detected low-amplitude light variations ($Δm\sim$1 mmag) in the Be star $β$ CMi (B8Ve). The observations lasted 41 days and the variations have typical periods $\sim 0.3$ days. We demonstrate that the dominant frequencies are consistent with prograde high-order g-modes of $m=-1$ excited by the Fe-bump of opacity in an intermediate-mass ($\approx 3.5 M_\odot$) star with a nearly critical rotation period of 0.38 days. This is the first detection of nonradial g-mode pulsations in a Be star later than B6 leading to the possibility that pulsations are excited in all classical Be stars.

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arXiv2006

Are peculiar Wolf-Rayet Stars of type WN8 Thorne-Zytkow Objects?

Most population I Wolf-Rayet (WR) stars are the He-rich descendants of the most massive stars (M_i = 25 - 100 M_sun). Evidence has been accumulating over the years that among all pop I WR stars, those of the relatively cool, N-rich subtype "WN8" are among the most peculiar: 1. They tend to be runaways, with large space velocity and/or avoid clusters. 2. Unlike their equally luminous WN6,7 cousins, only a very small number of WN8 stars are known to belong to a close binary with an OB companion. 3. They are the systematically most highly stochastically variable among all (single) WR stars. Taken together, these suggest that many WN8 stars may originally have been in close binaries (like half of all stars), in which the original primary exploded as a supernova, leaving behind a very close binary containing a massive star with a neutron star/black hole companion (like Cyg X-3). When the massive remaining star evolved in turn, it engulfed and eventually swallowed the compact companion, leading to the presently puffed-up, variable WN8 star. Such stars could fall in the realm of the exotic Thorne-Zytkow objects.

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arXiv2006

Most Detects G- and P-Modes in the B Supergiant HD 163899 (B2Ib/II)

The {\it Microvariability and Oscillations of Stars (MOST)} satellite observed the B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star and detected 48 frequencies $\la$ 2.8 c d$^{-1}$ with amplitudes of a few milli-magnitudes (mmag) and less. The frequency range embraces g- and p-mode pulsations. It was generally thought that no g-modes are excited in less luminous B supergiants because strong radiative damping is expected in the core. Our theoretical models, however, show that such g-modes are excited in massive post-main-sequence stars, in accordance with these observations. The nonradial pulsations excited in models between $20M_\odot$ at $\log T_{\rm eff} \approx 4.41$ and $15M_\odot$ at $\log T_{\rm eff} \approx 4.36$ are roughly consistent with the observed frequency range. Excitation by the Fe-bump in opacity is possible because g-modes can be partially reflected at a convective zone associated with the hydrogen-burning shell, which significantly reduces radiative damping in the core. The {\it MOST} light curve of HD 163899 shows that such a reflection of g-modes actually occurs, and reveals the existence of a previously unrecognized type of variable, slowly pulsating B supergiants (SPBsg) distinct from $α$ Cyg variables. Such g-modes have great potential for asteroseismology.

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arXiv2006

Discovery of the new slowly pulsating B star HD 163830 (B5II/III) from MOST spacebased photometry

We report the discovery of a new slowly pulsating B (SPB) star, with the largest number of detected frequencies to date by more than a factor of three, based on 37 days of MOST (Microvariability & Oscillations of STars) satellite guide star photometry. The star HD 163830 (V = 9.3, B5II/III) varies in twenty detected frequencies in the range $0.035 - 1.06$ d$^{-1}$ ($0.4 - 12.3 ~ μ$Hz) with amplitudes from 0.7 to 7.6 mmag ($S/N$ from {4 to 43}). Eighteen of these frequencies are consistent with low-degree, high-order nonradial g-modes {of seismic models of an evolved 4.5$M_{\odot}$ star. We are unable to identify one unique model due to lack of mode identifications. The lowest two frequencies may be associated with the rotation of HD 163830, but firm proof of this must await future spectroscopic data.

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arXiv2006

An Upper Limit on the Albedo of HD 209458b: Direct Imaging Photometry with the MOST Satellite

We present space-based photometry of the transiting exoplanetary system HD 209458 obtained with the MOST (Microvariablity and Oscillations of STars) satellite, spanning 14 days and covering 4 transits and 4 secondary eclipses. The HD 209458 photometry was obtained in MOST's lower-precision Direct Imaging mode, which is used for targets in the brightness range $6.5 < V < 13$. We describe the photometric reduction techniques for this mode of observing, in particular the corrections for stray Earthshine. We do not detect the secondary eclipse in the MOST data, to a limit in depth of 0.053 mmag (1 σ). We set a 1 σupper limit on the planet-star flux ratio of 4.88 x 10^-5 corresponding to a geometric albedo upper limit in the MOST bandpass (400 to 700 nm) of 0.25. The corresponding numbers at the 3 σlevel are 1.34 x 10^-4 and 0.68 respectively. HD 209458b is half as bright as Jupiter in the MOST bandpass. This low geometric albedo value is an important constraint for theoretical models of the HD209458b atmosphere, in particular ruling out the presence of reflective clouds. A second MOST campaign on HD 209458 is expected to be sensitive to an exoplanet albedo as low as 0.13 (1 sigma), if the star does not become more intrinsically variable in the meantime.

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arXiv2006

Origin of Structures in Wolf-Rayet Winds: FUSE Observations of WR 135

We report the detection with FUSE of strong, highly blue-shifted absorption features appearing in the absorption troughs of practically all major P Cygni profiles in the presumably single Wolf-Rayet star WR135. These features also appear in the shock-sensitive OVI 1032/38 Ådoublet, coincident both in time and velocity space with the rest of the lower-ionization species. Choosing between two alternative interpretations: large-scale, coherent structures vs. localized, random shocks, we favor the latter. The absolute value of velocity, as well as velocity dispersion in the shocked region, the density of the shocked gas, and the time scales of the observed variability allow us to relate the observed shocks to the incidence of numerous over-dense clumps (blobs) in the wind of a hot, massive star.

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arXiv2006

delta Ceti is not monoperiodic: seismic modeling of a beta Cephei star from MOST spacebased photometry

The beta Cephei star delta Ceti was considered one of the few monoperiodic variables in the class. Despite (or perhaps because of) its apparently simple oscillation spectrum, it has been challenging and controversial to identify this star's pulsation mode and constrain its physical parameters seismically. Broadband time-resolved photometry of delta Ceti spanning 18.7 days with a duty cycle of about 65% obtained by the MOST (Microvariability & Oscillations of STars) satellite -- the first scientific observations ever obtained by MOST -- reveals that the star is actually multiperiodic. Besides the well-known dominant frequency of f1 = 6.205886/d, we have discovered in the MOST data its first harmonic 2f1 and three other frequencies (f2 = 3.737/d, f3 = 3.673/d and f4 = 0.318/d), all detected with S/N > 4. In retrospect, f2 was also present in archival spectral line profile data but at lower S/N. We present seismic models whose modes match exactly the frequencies f1 and f2. Only one model falls within the common part of the error boxes of the star's observed surface gravity and effective temperature from photometry and spectroscopy. In this model, f1 is the radial (l = 0) first overtone and f2 is the g2 (l = 2, m = 0) mode. This model has a mass of 10.2+/-0.2 Msun and an age of 17.9+/-0.3 million years, making delta Ceti an evolved beta Cephei star. If f2 and f3 are rotationally split components of the same g2 mode, then the star's equatorial rotation velocity is either 27.6 km/s or half this value. Given its vsini of about 1 km/s, this implies we are seeing delta Ceti nearly pole-on.

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