Time-series Spectroscopy Research Articles

A new method for the spectroscopic identification of stellar non-radial pulsation modes

Aims. We present the Fourier parameter fit method, a new method for spectroscopically identifying stellar radial and non-radial pulsation modes based on the high-resolution time-series spectroscopy of absorption-line profiles. In contrast to previous methods this one permits a quantification of the statistical significance of the computed solutions. The application of genetic algorithms in seeking solutions makes it possible to search through a large parameter space. Methods. The mode identification is carried out by minimizing Χ 2 , using the observed amplitude and phase across the line profile and their modeled counterparts. Computations of the theoretical line profiles are based on a stellar displacement field, which is described as superposition of spherical harmonics and that includes the first order effects of the Coriolis force. Results. We made numerical tests of the method on a grid of different mono- and multi-mode models for 0 ≤ l ≤ 4 in order to explore its capabilities and limitations. Our results show that whereas the azimuthal order m can be unambiguously identified for low-order modes, the error of l is in the range of ±1. The value of m can be determined with higher precision than with other spectroscopic mode identification methods. Improved values for the inclination can be obtained from the analysis of non-axisymmetric pulsation modes. The new method is ideally suited to intermediatley rotating 6 Scuti andβ Cephei stars.

On Coordinating Time Series Spectroscopy with the WET

Die Serie Communications in Asteroseismology (CoAst) stellt eine international gefragte Plattform für wissenschaftliche Publikationen der neuesten Forschungsergebnisse auf dem Gebiet der Asteroseismologie dar. Forschungsgegenstand der Asteroseismologie sind pulsierende Sterne. Neben den halbjährlich erscheinenden regulären Ausgaben mit Beiträgen aus der laufenden Forschung werden auch Sonderausgaben für Konferenzberichte bzw. Programmhandbücher herausgegeben. Der Sammelband Vol. 154 beinhaltet die Konferenzbeiträge des in Delaware 2007 abgehaltenen Delaware Asteroseismic Research Center and Whole Earth Telescope Workshops. Im Rahmen dieses Arbeitstreffens wurden die neuesten theoretischen und beobachtungstechnischen Forschungsergebnisse auf dem Gebiet der Weißen Zwerge präsentiert. Contents: H. L. SHIPMAN: The Whole Earth Telescope Network's August 2008 Workshop S. J. KLEINMAN: Do You Need the WET: A Modest Proposal A. BISCHOFF-KIM: Preliminary Asteroseismology of EC20058-5234 and Limits on Plasmon Neutrinos J. L. PROVENCAL, H. L. SHIPMAN, and the WET TEAM: An Update on XCOV25: GD358 M. H. MONTGOMERY: What We Can Learn from the Light Curves of GD 358 and PG 1351+489 S. E. THOMPSON: On Coordinating Time Series Spectroscopy with the WET M. PAPARÓ, E. PLACHY, L. MOLNÁR, P. I. PÁPICS, Zs. BOGNÁR, N. SZTANKÓ, Gy. KEREKES, A. MÁR & E. BOKOR: Preliminary Results in White Dwarf Research by a New Group H. O. VATS & K. S. BALIYAN: Recent observations of WD1524 at MIRO and participation in WET observing program J. DALESSIO, J. L. PROVENCAL & A. KANAAN: Introducing SPA, "The Stellar Photometry Assistant"

Discovery of very low amplitude 9-minute multiperiodic pulsations in the magnetic Ap star HD 75445

We present our discovery of pulsational radial-velocity variations in the cool Ap star HD75445, an object spectroscopically similar to the bright, rapidly-oscillating Ap (roAp) star gamma Equ. Based on high-resolution time-series spectroscopy obtained with the HARPS spectrometer at the European Southern Observatory 3.6-m telescope, we detected oscillations in Nd II and Nd III lines with a period close to 9 min and amplitudes of 20-30 m s(-1). Substantial variation in the pulsational amplitude during our 3.8 h observing run reveals the presence of at least three excited non-radial modes. The detection of extremely low amplitude pulsations in HD75445 indicates that the roAp excitation mechanism produces variability in the radial velocity amplitude of between a few tens ms(-1) and several km s(-1). This supports the idea that many, if not all, cool Ap stars occupying the roAp instability strip may harbour non-radial pulsations, which currently remain undetected due to their small photometric and radial-velocity amplitudes.

Time-series spectroscopy and photometry of the pulsating subdwarf B star PG 1219+534 (KY UMa)

Aims. We present observations and analysis of time-series spectroscopy and photometry of the pulsating subdwarf B star PG 1219+534 (KY UMa). Subdwarf B stars are blue horizontal branch stars that have shed most of their hydrogen envelopes. Pulsating subdwarf B stars allow a probe into this interesting phase of evolution. Methods. Low-resolution spectra were obtained at the Nordic Optical Telescope and Kitt Peak National Observatory, and photometric observations were obtained at MDM and Baker observatories in 2006. We extracted radial velocity and equivalent width variations from several Balmer and He I lines in individual spectra. The pulsation frequencies were separated via phase binning to detect lineprofile variations in Balmer and helium lines, which were subsequently matched to atmospheric models to infer effective temperature and gravity changes throughout the pulsation cycle. Results. From the photometry we recovered the four previously observed frequencies and detected a new fifth frequency. From the spectra we directly measured radial velocity and equivalent width variations for the four main frequencies and from atmospheric models. We successfully inferred temperature and gravity changes for these four frequencies. We compared amplitude ratios and phase differences of these quantities and searched for outliers that could be identified as high-degree modes. These are the first such measurements for a “normal” amplitude pulsating subdwarf B star, indicating that spectroscopic studies can benefit the majority of pulsating subdwarf B stars.

Towards the understanding of radial velocity pulsation in roAp stars

High-resolution spectroscopic time series of rapidly oscillating Ap stars show evidence for a co-existence of standing and running waves in their atmospheric layers. With the purpose of understanding these observations we have carried out a theoretical analysis of the pulsations in the outermost layers of these stars, starting from the simplest possible model that still retains all important physical ingredients. In our analysis we considered an isothermal atmosphere in a plane-parallel approximation. Moreover we assumed that in the region considered the magnetic pressure is much larger than the gas pressure and, consequently, that the magnetoacoustic wave has decoupled into its acoustic and magnetic components. Using the analytical solutions for the velocity components appropriate to this model we estimate the velocity component parallel to the line of sight averaged over the visible stellar disk. Fitting the latter to a function of the form Acos(σt+phase), with σ the dimensionless oscillation frequency and t the dimensionless time, we derive the amplitude A and the phase for our model as function of height in the atmosphere.

Spectroscopic observations for asteroseismology

Spectroscopic observations of stellar oscillations provide an indispensable input for asteroseismology. High-resolution and high signal-to-noise ratio time-series spectroscopy can reveal radial velocity variations as well as profile variations in stellar absorption lines which are caused by non-radial pulsations. Oscillations in solar-like stars have been detected in radial velocity measurements obtained with stabilized spectrographs. For hotter stars on the main sequence, besides for the purpose of abundance analyses, spectroscopy is mainly acquired to carry out mode identification, the assignment of the spherical degree l, and the azimuthal order m, to the observed pulsation frequencies. It has been demonstrated for several stars on the main sequence hotter than the sun that only combined spectroscopic and photometric time-series measurements can lead to successful seismic modeling. In this article, I will focus on the application of spectroscopic mode identification on pulsating stars and discuss some recent results.

Deciphering the pulsations of G 29-38 with optical time series spectroscopy

We present optical time series spectroscopy of the pulsating white dwarf star G 29-38 taken at the Very Large Telescope (VLT). By measuring the variations in brightness, Doppler shift and line shape of each spectrum, we explore the physics of pulsation and measure the spherical degree (l) of each stellar pulsation mode. We measure the physical motion of the g modes correlated with the brightness variations for three of the eight pulsation modes in this data set. The varying line shape reveals the spherical degree of the pulsations, an important quantity for properly modelling the interior of the star with asteroseismology. Performing fits to the Hβ, Hy and Hδ lines, we quantify the changing shape of the line and compare them to models and previous time series spectroscopy of G 29-38. These VLT data confirm several l identifications and add four new values, including an additional l = 2 and a possible l = 4. In total, from both sets of spectroscopy of G 29-38, eleven modes now have known spherical degrees.

CLOUDS search for variability in brown dwarf atmospheres

Context: L-type ultra-cool dwarfs and brown dwarfs have cloudy atmospheres that could host weather-like phenomena. The detection of photometric or spectral variability would provide insight into unresolved atmospheric heterogeneities, such as holes in a global cloud deck. Aims: It has been proposed that growth of heterogeneities in the global cloud deck may account for the L- to T-type transition as brown dwarf photospheres evolve from cloudy to clear conditions. Such a mechanism is compatible with variability. We searched for variability in the spectra of five L6 to T6 brown dwarfs in order to test this hypothesis. Methods: We obtained spectroscopic time series using VLT/ISAAC, over 0.99-1.13um, and IRTF/SpeX for two of our targets, in J, H and K bands. We search for statistically variable lines and correlation between those. Results: High spectral-frequency variations are seen in some objects, but these detections are marginal and need to be confirmed. We find no evidence for large amplitude variations in spectral morphology and we place firm upper limits of 2 to 3% on broad-band variability, on the time scale of a few hours. The T2 transition brown dwarf SDSS J1254-0122 shows numerous variable features, but a secure variability diagnosis would require further observations. Conclusions: Assuming that any variability arises from the rotation of patterns of large-scale clear and cloudy regions across the surface, we find that the typical physical scale of cloud cover disruption should be smaller than 5-8% of the disk area for four of our targets. The possible variations seen in SDSS J1254-0122 are not strong enough to allow us to confirm the cloud breaking hypothesis.

Weighted Target Testing (WTT) — An improved principle for modeling reaction kinetics

This paper presents Weighted Target Testing (WTT) as a hard modeling principle to resolve the reaction kinetics from online spectroscopic time series (two-way data). While classical Target Testing (TT) offers several advantages as being able to deal with non-linearity in the data, the lack of appropriate weights that reflect the structure of the data may be a major reason why TT estimations fail in certain cases. To address this weakness of TT we propose to use the weights as available in the singular values of the data and show the resulting impact on the cost function. The performance of WTT compared to TT is assessed using simulated data and experimental data taken from the literature. The interplay between the order of the singular values and the ordering of the columns of the concentration matrix is thoroughly discussed. When dealing with complicated reaction models, multi-component systems, and noisy data WTT outperforms the classical principle.

A NEW, BRIGHT, SHORT-PERIOD, EMISSION LINE BINARY IN OPHIUCHUS

The 11th magnitude star LS IV-08°3 has been classified previously as an OB star in the Luminous Stars survey, or alternatively as a hot subdwarf. It is actually a binary star. We present spectroscopy, spectroscopic orbital elements, and time-series photometry from observations made at the Kitt Peak National Observatory 2.1 m, Steward Observatory 2.3 m, MDM Observatory 1.3 m and 2.4 m, Hobby-Eberly 9.2 m, and Michigan State University 0.6 m telescopes. The star exhibits emission of varying strength in the cores of H and He I absorption lines. Emission is also present at 4686 A (He II) and near 4640/4650 A (N III/C III). Time-series spectroscopy collected from 2005 July to 2007 June shows coherent, periodic radial velocity variations of the Hα line, which we interpret as orbital motion with a period of 0.1952894(10) days. High-resolution spectra show that there are two emission components, one broad and one narrow, moving in antiphase, as might arise from an accretion disk and the irradiated face of the mass donor star. Less coherent, low-amplitude photometric variability is also present on a timescale similar to the orbital period. Diffuse interstellar bands indicate considerable reddening, which however is consistent with a distance of ~100-200 pc. The star is the likely counterpart of a weak ROSAT X-ray source, whose properties are consistent with accretion in a cataclysmic variable (CV) binary system. We classify LS IV-08°3 as a new member of the UX UMa subclass of CV stars.

The discovery of high-amplitude, 10.9-minute oscillations in the cool magnetic Ap star HD 115226

We present the discovery of pulsational variations in the cool magnetic Ap star HD 115226 – the first high-amplitude rapidly oscillating Ap (roAp) star discovered with time-series spectroscopy. Using high-resolution spectra obtained with the HARPS instrument at the European Southern Observatory 3.6-m telescope, we detect radial velocity variations with a period of 10.86 min in Pr iii ,N diii, Dy iii lines, and in the narrow cores of hydrogen lines. Pulsational amplitudes exceed 1 km s −1 in individual lines of Nd iii .T he presence of running waves in the stellar atmosphere is inferred from a phase shift between the radial velocity maxima of rare-earth and hydrogen lines. Our abundance analysis demonstrates that HD 115226 exhibits a typical roAp spectroscopic signature, notably ionization anomaly of Pr, Nd, and Dy. We discuss the discovery of pulsations in HD 115226 in the context of recent spectroscopic studies of roAp stars and point to the existence of a correlation between spectroscopic pulsational amplitude and the stellar rotation rate.

Deciphering the pulsations of G 29-38 with optical time series spectroscopy

Deciphering the pulsations of G 29-38 with optical time series spectroscopy

The pre-cataclysmic variable, LTT 560

Aims. System parameters of the object LTT560 are determined in order to clarify its nature and evolutionary status. Methods. We apply time-series photometry to reveal orbital modulations of the light curve, time-series spectroscopy to measure radial velocities of features from both the primary and the secondary star, and flux-calibrated spectroscopy to derive temperatures of both components. Results. We find that LTT 560 is composed of a low temperature (T ∼ 7500 K) DA white dwarf as the primary and an M5.5±1 mainsequence star as the secondary component. The current orbital period is Porb = 3.54(07) h.We derive a mass ratio Msec/Mwd = 0.36(03) and estimate the distance to d = 25–40 pc. Long-term variation of the orbital light curve and an additional Hα emission component on the white dwarf indicate activity in the system, probably in the form of flaring and/or accretion events.

High-resolution spectroscopy of theδScuti star 44 Tauri: photospheric element abundances and mode identification

Aims. We carried out a detailed spectroscopic study of the δ Sct star 44 Tau to investigate its element abundances and stellar parameters as well as to provide an identification of its non-radial oscillation modes. Methods. We acquired high-resolution high-signal-to-noise ratio time-series spectroscopy of 44 Tau from two observing sites in Europe during a time span of 30 days. We determined atmospheric element abundances and stellar properties using spectroscopic methods. Due to the low value of v sini the mode identification was carried out by applying the moment method. Results. A very low value of the projected rotational velocity of v sini = 2 ± 1k m s −1 was determined. We detected spectroscopically 12 independent frequencies between 6.8 and 11.6 d −1 and four combination frequencies by analyzing the line moments as well as the pixel-intensities across the profiles of carefully selected Fe absorption lines. The mode identification applying the moment method yielded six axisymmetric pulsation modes. Two frequencies are low-degree prograde pulsation modes. A rather low value of log g = 3.6 was determined implying an evolved evolutionary status of 44 Tau. The derived atmospheric element abundances are close to solar. Conclusions. Our analysis revealed that 44 Tau is an exceptionally slowly rotating δ Scuti star pulsating with a rich spectrum of axisymmetric pulsation modes. This simplifies the seismic modeling since rotational perturbations of higher order can be neglected.

Observations of pulsations in roAp stars

Inhalt (u.a.): M. BREGER: Delta Scuti stars: Observational aspects; M.S. Cunha: Theory of rapidly oscillating Ap stars; M. SACHKOV et al.: Vertical structure of pulsations in roAp stars; G. HOUDEK: Theoretical asteroseismology of solar-like oscillations; B.L. POPIELSKI: Core modes as a seismic probe of mixing beyond the convective core; M. GODART: Temperature gradients in the core overshooting region; W.A. DZIEMBOWSKI – J. DASZYŃSKA-DASZKIEWICZ – A.A. PAMYATNYKH: Interpretation of the Be star HD 163868 oscillation spectrum based on the MOST observations; D. KILKENNY: Pulsating Hot Subdwarfs – An Observational Review; M.H. MONTGOMERY: Mapping Convection using Pulsating White Dwarf Stars; M. ROTH: The Network Activities in HELAS; I. KUDZEJ et al.: A New Slovak Observatory 500 km from Vienna; W.W. WEISS: Microsatellites; G. KOPACKI – S. FRANDSEN: RR Lyrae stars in M4

Vertical structure of pulsations in roAp stars

Inhalt (u.a.): M. BREGER: Delta Scuti stars: Observational aspects; M.S. Cunha: Theory of rapidly oscillating Ap stars; M. SACHKOV et al.: Vertical structure of pulsations in roAp stars; G. HOUDEK: Theoretical asteroseismology of solar-like oscillations; B.L. POPIELSKI: Core modes as a seismic probe of mixing beyond the convective core; M. GODART: Temperature gradients in the core overshooting region; W.A. DZIEMBOWSKI – J. DASZYŃSKA-DASZKIEWICZ – A.A. PAMYATNYKH: Interpretation of the Be star HD 163868 oscillation spectrum based on the MOST observations; D. KILKENNY: Pulsating Hot Subdwarfs – An Observational Review; M.H. MONTGOMERY: Mapping Convection using Pulsating White Dwarf Stars; M. ROTH: The Network Activities in HELAS; I. KUDZEJ et al.: A New Slovak Observatory 500 km from Vienna; W.W. WEISS: Microsatellites; G. KOPACKI – S. FRANDSEN: RR Lyrae stars in M4

An Observational Study of Tidal Synchronization in Solar‐Type Binary Stars in the Open Clusters M35 and M34

We present rotation periods for the solar-type primary stars in 13 close (a ≲ 5 AU) single-lined spectroscopic binaries with known orbital periods (P) and eccentricities (e). All binaries are members of the open clusters M35 (NGC 2168; ~150 Myr) and M34 (NGC 1039; ~250 Myr). The binary orbital parameters and the rotation periods of the primary stars were determined from time-series spectroscopy and time-series photometry, respectively. Knowledge of the ages, orbital periods, and eccentricities of these binaries combined with the rotation periods and masses of their primary stars makes them particularly interesting systems for studying the rates of tidal circularization and synchronization. Our sample of 13 binaries includes six with orbital periods shortward of 13 days (a ≲ 0.12 AU). The stars in these binaries orbit sufficiently close that their spins and orbits have evolved toward synchronization and circularization due to tidal interactions. We investigate the degree of tidal synchronization in each binary by comparing the angular rotation velocity of the primary stars (Ω⋆) to the angular velocity expected if the primary star was synchronized (e = 0) or pseudosynchronized (e > 0) with the orbital motion (Ωps). Of the six closest binaries, two with circular orbits are not synchronized, one being subsynchronous and one being supersynchronous, and the primary stars in two binaries with eccentric orbits are rotating more slowly than pseudosynchronism. The remaining two binaries have reached the equilibrium state of both a circularized orbit and synchronized rotation. As a set, the six binaries present a challenging case study for tidal evolution theory, which in particular does not predict subsynchronous rotation in such close systems.

The superimposed photospheric and stellar wind variability of the O-type supergiant $\mathsf{\alpha}$ Camelopardalis

Aims. This study seeks to provide empirical constraints on the different physical components that can yield temporal variability in predominantly or partially wind-formed optical lines of luminous OB stars, and thus potentially affect the reliable determination of fundamental parameters, including mass-loss rates via clumped winds. Methods. Using time-series spectroscopy from epochs spread over ∼4 years, we present a case study of the O9.5 supergiant α Cam. We demonstrate that the HeI λ5876 (2 3 P 0 –3 3 D) line is an important diagnostic for photospheric and wind variability in this star. The actions of large radial velocity shifts (up to ∼30 km s −1 ) in the photospheric absorption lines can also affect the morphology of the Hα line profile, which is commonly used for measuring mass-loss rates in massive stars. Results. We identify a 0.36-day period in subtle absorption profile changes in HeI λ5876, which likely reveals the photospheric structure, perhaps due to low-order non-radial pulsations. This signal persists over ∼2 months, but it is not present 2 years later (November 2004); it is also not seen in the stellar wind components of the line profiles. Using a pure Hα line-synthesis code we interpret maximum changes in the redward and peak emission of α Cam in terms of mass-loss rate differences in the range ∼5.1 × 10 −6

A new method for the spectroscopic identification of stellar non-radial pulsation modes

We present the Fourier parameter fit method, a new method for spectroscopically identifying stellar radial and non-radial pulsation modes based on the high-resolution time-series spectroscopy of absorption-line profiles. In contrast to previous methods this one permits a quantification of the statistical significance of the computed solutions. The application of genetic algorithms in seeking solutions makes it possible to search through a large parameter space. The mode identification is carried out by minimizing chi-square, using the observed amplitude and phase across the line profile and their modeled counterparts. Computations of the theoretical line profiles are based on a stellar displacement field, which is described as superposition of spherical harmonics and that includes the first order effects of the Coriolis force. We made numerical tests of the method on a grid of different mono- and multi-mode models for 0 <= l <= 4 in order to explore its capabilities and limitations. Our results show that whereas the azimuthal order m can be unambiguously identified for low-order modes, the error of l is in the range of pm 1. The value of m can be determined with higher precision than with other spectroscopic mode identification methods. Improved values for the inclination can be obtained from the analysis of non-axisymmetric pulsation modes. The new method is ideally suited to intermediatley rotating Delta Scuti and Beta Cephei stars.

Multi-periodic photospheric pulsations and connected wind structures in HD 64760

We report on the results of an extended optical spectroscopic monitoring campaign on the early-type B supergiant HD64760 (B0.5Ib) designed to probe the deep-seated origin of spatial wind structure in massive stars. This new study is based on high-resolution echelle spectra obtained with the Feros instrument at ESO La Silla. 279 spectra were collected over 10 nights between February 14 and 24, 2003. From the period analysis of the line-profile variability of the photospheric lines we identify three closely spaced periods around 4.810 h and a splitting of +/- 3%. The velocity-phase diagrams of the line-profile variations for the distinct periods reveal characteristic prograde non-radial pulsation patterns of high order corresponding to pulsation modes with l and m in the range 6-10. A detailed modeling of the multi-periodic non-radial pulsations with the Bruce and Kylie pulsation-model codes (Townsend 1997b, MNRAS, 284, 839) favors either three modes with l = -m and l = 8, 6, 8 or m = - 6 and l = 8, 6, 10 with the second case maintaining the closely spaced periods in the co-rotating frame. The pulsation models predict photometric variations of 0.012-0.020 mag consistent with the non-detection of any of the spectroscopic periods by photometry. The three pulsation modes have periods clearly shorter than the characteristic pulsation time scale and show small horizontal velocity fields and hence are identified as p-modes. The beating of the three pulsation modes leads to a retrograde beat pattern with two regions of constructive interference diametrically opposite on the stellar surface and a beat period of 162.8h (6.8 days). This beat pattern is directly observed in the spectroscopic time series of the photospheric lines. The wind-sensitive lines display features of enhanced emission, which appear to follow the maxima of the photospheric beat pattern. The pulsation models predict for the two regions normalized flux amplitudes of A = +0.33, -0.28, sufficiently large to raise spiral co-rotating interaction regions (Cranmer & Owocki 1996, ApJ, 462, 469). We further investigate the observed Ha wind-profile variations with a simple rotating wind model with wind-density modulations to simulate the effect of possible streak lines originating from the localized surface spots created by the NRP beat pattern. It is found that such a simple scenario can explain the time scales and some but not all characteristics of the observed Ha line-profile variations.