The tip of the iceberg: the frequency content of the δ Sct star HD 50844 from CoRoT space photometry (original) (raw)
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HD 50844: a new look at δ Scuti stars from CoRoT space photometry
Astronomy and Astrophysics, 2009
Context. The ground-based campaigns on δ Sct stars revealed richer and richer frequency spectra of these opacity-driven pulsators, thanks to continuous improvements in their exploitation. It has also been suggested that the detection of a wealth of very low amplitude modes was only a matter of signal-to-noise ratio. Access to this treasure, impossible from the ground, is one of the scientific aims of the space mission CoRoT. Aims. This work presents the results obtained on HD 50844, the only δ Sct star observed in the CoRoT Initial Run (57.6 d). The aim of these CoRoT observations was to investigate and characterize for the first time the pulsational behaviour of a δ Sct star, when observed at a level of precision and with a much better duty cycle than from the ground. Methods. The 140,016 datapoints were analysed using independent approaches (SigSpec software and different iterative sine-wave fittings) and several checks performed (splitting of the timeseries in different subsets, investigation of the residual light curves and spectra). A level of 10 −5 mag was reached in the amplitude spectra of the CoRoT timeseries. The space monitoring was complemented by ground-based high-resolution spectroscopy, which allowed the mode identification of 30 terms. Results. The frequency analysis of the CoRoT timeseries revealed hundreds of terms in the frequency range 0-30 d −1 . All the crosschecks confirmed this new result. The initial guess that δ Sct stars have a very rich frequency content is confirmed. The spectroscopic mode identification gives theoretical support since very high-degree modes (up to ℓ=14) are identified. We also prove that cancellation effects are not sufficient in removing the flux variations associated to these modes at the noise level of the CoRoT measurements. The ground-based observations indicate that HD 50844 is an evolved star that is slightly underabundant in heavy elements, located on the Terminal Age Main Sequence. Probably due to this unfavourable evolutionary status, no clear regular distribution is observed in the frequency set. The predominant term ( f 1 =6.92 d −1 ) has been identified as the fundamental radial mode combining ground-based photometric and spectroscopic data. Conclusions. The CoRoT scientific programme contains other δ Sct stars, with different evolutionary statuses. The very rich and dense frequency spectrum discovered in the light curve of HD 50844 is the starting point for future investigations. 2 Poretti et al.: HD 50844: the new look of δ Sct stars from CoRoT space photometry ,
Astronomy & Astrophysics, 2011
The detection of small-amplitude nonradial modes in high-amplitude Delta Sct (HADS) variables has been very elusive until at least five of them were detected in the light curve of V974 Oph obtained from ground-based observations. The combination of radial and nonradial modes has a high asteroseismic potential, thanks to the strong constraints we can put in the modelling. The continuous monitoring of ASAS 192647-0030.0=CoRoT 101155310 (P=0.1258 d, V=13.4) ensured from space by the CoRoT (Convection, Rotation and planetary Transits) mission constitutes a unique opportunity to exploit such potential. The 22270 CoRoT measurements were performed in the chromatic mode. They span 152 d and cover 1208 consecutive cycles. After the correction for one jump and the long-term drift, the level of the noise turned out to be 29 micromag. The phase shifts and amplitude ratios of the coloured CoRoT data, the HARPS spectra, and the period-luminosity relation were used to determine a self-consistent physical model. In turn, it allowed us to model the oscillation spectrum, also giving feedback on the internal structure of the star. In addition to the fundamental radial mode f1=7.949 c/d with harmonics up to 10f1, we detected 12 independent terms. Linear combinations were also found and the light curve was solved by means of 61 frequencies (smallest amplitude 0.10 mmag). The newest result is the detection of a periodic modulation of the f1 mode (triplets at +/-0.193 c/d centred on f1 and 2f1), discussed as a rotational effect or as an extension of the Blazhko effect to HADS stars. The physical model suggests that CoRoT 101155310 is an evolved star, with a slight subsolar metallic abundance, close to the terminal age main sequence. All the 12 additional terms are identified with mixed modes in the predicted overstable region.
CoRoT sounds the stars: p-mode parameters of Sun-like oscillations on HD 49933
Astronomy and Astrophysics, 2008
Context. The first asteroseismology results from CoRoT are presented, on a star showing Sun-like oscillations. We have analyzed a 60 day lightcurve of high-quality photometric data collected by CoRoT on the F5 V star HD 49933. The data reveal a rich spectrum of overtones of low-degree p modes. Aims. Our aim was to extract robust estimates of the key parameters of the p modes observed in the power spectrum of the lightcurve. Methods. Estimation of the mode parameters was performed using maximum likelihood estimation of the power spectrum. A global fitting strategy was adopted whereby 15 mode orders of the mode spectrum (45 modes) were fitted simultaneously. Results. The parameter estimates that we list include mode frequencies, peak linewidths, mode amplitudes, and a mean rotational frequency splitting. We find that the average large frequency (overtone) spacing derived from the fitted mode frequencies is 85.9 ± 0.15 μHz. The frequency of maximum amplitude of the radial modes is at 1760 μHz, where the observed rms mode amplitude is 3.75 ± 0.23 ppm. The mean rotational splitting of the non-radial modes appears to be in the range ≈2.7 μHz to ≈3.4 μHz. The angle of inclination offered by the star, as determined by fits to the amplitude ratios of the modes, appears to be in the range ≈50 degrees to ≈62 degrees.
CoRoT 102749568: mode identification in aδScuti star based on regular spacings
Astronomy & Astrophysics, 2013
Context. The high accuracy of space data increased the number of the periodicities determined for pulsating variable stars, but the mode identification is still a critical point in the non-asymptotic regime. Aims. We use regularities in frequency spacings for identifying the pulsation modes of the recently discovered δ Sct star ID 102749568. Methods. In addition to analysing CoRoT light curves (15 252 datapoints spanning 131 days), we obtained and analysed both spectroscopic and extended multi-colour photometric data. We applied standard tools (MUFRAN, Period04, SigSpec, and FAMIAS) for time-series analysis. Results. A satisfactory light-curve fit was obtaining by means of 52 independent modes and 15 combination terms. The frequency spacing revealed distinct peaks around large (25.55-31.43 μHz), intermediate (9.80, 7.66 μHz), and low (2.35 μHz) separations. We directly identified 9 modes, and the l and n values of other three modes were extrapolated. The combined application of spectroscopy, multi-colour photometry, and modelling yielded the precise physical parameters and confirmed the observational mode identification. The large separation constrained the log g and related quantities. The dominant mode is the radial first overtone.
Pulsation spectrum ofδScuti stars: the binary HD 50870 as seen with CoRoT and HARPS
Astronomy & Astrophysics, 2012
Aims. We present the results obtained with the CoRoT satellite for HD 50870, a δ Sct star which was observed for 114.4 d. The aim of these observations was to evaluate the results obtained for HD 50844, the first δ Sct star monitored with CoRoT, on a longer time baseline. Methods. The 307,570 CoRoT datapoints were analysed with different techniques. The photometric observations were complemented over 15 nights of high-resolution spectroscopy with HARPS on a baseline of 25 d. These spectra were analysed to study the line profile variations and to derive the stellar physical parameters. Some uvby photometric observations were also obtained to better characterize the pulsation modes. Results. HD 50870 proved to be a low-amplitude, long-period spectroscopic binary system seen almost pole-on (i 21 •). The brighter component, which also has the higher rotational velocity (v sin i = 37.5 km s −1), is a δ Sct-type variable with a full light amplitude variation of about 0.04 mag. There is a dominant axisymmetric mode (17.16 d −1). Moreover, there are two groups of frequencies (about 19) in the intervals 6−9 and 13−18 d −1 , with amplitudes ranging from a few mmag to 0.3 mmag. After the detection of about 250 terms (corresponding to an amplitude of about 0.045 mmag) a flat plateau appears in the power spectrum in the low-frequency region up to about 35 d −1. We were able to detect this plateau only thanks to the short cadence sampling of the CoRoT measurements (32 s). The density distribution vs. frequency of the detected frequencies seems to rule out the possibility that this plateau is the result of a process with a continuum power spectrum. The spacings of the strongest modes suggest a quasi-periodic pattern. We failed to find a satisfactory seismic model that simultaneously matches the frequency range, the position in the HR diagram, and the quasiperiodic pattern interpreted as a large separation. Nineteen modes were detected spectroscopically from the line profile variations and associated to the photometric ones. Tentative , m values have been attributed to the modes detected spectroscopically. Prograde as well as retrograde modes are present with values up to 9. There are no traces of variability induced by solar-like oscillations.
Astronomy & Astrophysics, 2013
Context. The internal structure of pre-main-sequence (PMS) stars is poorly constrained at present. This could change significantly through high-quality asteroseismological observations of a sample of such stars. Aims. We concentrate on an asteroseismological study of HD 261711, a rather hot δ Scuti-type pulsating member of the young open cluster NGC 2264 located at the blue border of the instability region. HD 261711 was discovered to be a PMS δ Scuti star using the time series photometry obtained by the MOST satellite in 2006. Methods. High-precision, time-series photometry of HD 261711 was obtained by the MOST and CoRoT satellites in four separate new observing runs that are put into context with the star's fundamental atmospheric parameters obtained from spectroscopy. Frequency Analysis was performed using Period04. The spectral analysis was performed using equivalent widths and spectral synthesis. Results. With the new MOST data set from 2011/12 and the two CoRoT light curves from 2008 and 2011/12, the δ Scuti variability was confirmed and regular groups of frequencies were discovered. The two pulsation frequencies identified in the data from the first MOST observing run in 2006 are confirmed and 23 new δ Scuti-type frequencies were discovered using the CoRoT data. Weighted average frequencies for each group were determined and are related to l = 0 and l = 1 p-modes. Evidence for amplitude modulation of the frequencies in two groups is seen. The effective temperature (T eff ) was derived to be 8600 ± 200 K, log g is 4.1 ± 0.2, and the projected rotational velocity (υ sin i) is 53 ± 1 km s −1 . Using our T eff value and the radius of 1.8 ± 0.5 R derived from spectral energy distribution (SED) fitting, we get a luminosity log L/L of 1.20 ± 0.14 which agrees well to the seismologically determined values of 1.65 R and, hence, a log L/L of 1.13. The radial velocity of 14 ± 2 km s −1 we derived for HD 261711, confirms the star's membership to NGC 2264. Conclusions. Our asteroseismic models suggest that HD 261711 is a δ Scuti-type star close to the zero-age main sequence (ZAMS) with a mass of 1.8 to 1.9 M . With an age of about 10 million years derived from asteroseismology, the star is either a young ZAMS star or a late PMS star just before the onset of hydrogen-core burning. The observed splittings about the l = 0 and 1 parent modes may be an artifact of the Fourier derived spectrum of frequencies with varying amplitudes.
The solar-like CoRoT target HD 170987: Spectroscopic and seismic observations
2010
Context. The CoRoT mission is in its third year of observation and the data from the second long run in the galactic centre direction are being analysed. The solar-like oscillating stars that have been observed up to now have given some interesting results, specially concerning the amplitudes that are lower than predicted. We present here the results from the analysis of the star HD 170987. Aims. The goal of this research work is to characterise the global parameters of HD 170987. We look for global seismic parameters such as the mean large separation, maximum amplitude of the modes, and surface rotation because the signal-to-noise ratio in the observations do not allow us to measure individual modes. We also want to retrieve the stellar parameters of the star and its chemical composition. Methods. We have studied the chemical composition of the star using ground-based observations performed with the NARVAL spectrograph. We have used several methods to calculate the global parameters from the acoustic oscillations based on CoRoT data. The light curve of the star has been interpolated using inpainting algorithms to reduce the effect of data gaps. Results. We find power excess related to p modes in the range [400 -1200] µHz with a mean large separation of 55.2 ± 0.8 µHz with a probability above 95 % that increases to 55.9 ± 0.2 µHz in a higher frequency range [500 -1250] µHz and a rejection level of 1 %. A hint of the variation of this quantity with frequency is also found. The rotation period of the star is estimated to be around 4.3 days with an inclination axis of i = 50 • +20 −13 . We measure a bolometric amplitude per radial mode in a range [2.4 -2.9] ppm around 1000 µHz. Finally, using a grid of models, we estimate the stellar mass, M = 1.43 ± 0.05 M , the radius, R = 1.96 ± 0.046 R , and the age ∼2.4 Gyr.
Pulsational content and abundance analysis of some δ Scuti stars observed by CoRoT
Astronomische Nachrichten, 2010
Several δ Sct stars were photometrically monitored with the satellite CoRoT and observed in high-resolution spectroscopy from ground. We present here the preliminary analysis of the abundance analysis of several potential CoRoT targets. Moreover, new insights are given about the problem of the huge number of detected frequencies by comparing δ Sct stars with normal A-stars.
Asteroseismic analysis of the CoRoT δ Scuti star HD 174936
Astronomy & Astrophysics, 2009
We present an analysis of the δ-Scuti star HD 174936 (ID 7613) observed by CoRoT during the first short run SRc01 (27 days). A total number of 422 frequencies were extracted from the light curve using standard prewhitening techniques. This number of frequencies was obtained by considering a spectral significance limit of sig = 10 using the software package SigSpec. Our analysis of the oscillation frequency spectrum reveals a spacing periodicity around 52 μHz. Although modes considered here are not in the asymptotic regime, a comparison with stellar models confirms that this signature may stem from a quasi-periodic pattern similar to the so-called large separation in solar-like stars.
2009
Context. We present spectroscopic ground-based observations of the early Be star HD 49330 obtained simultaneously with the CoRoT-LRA1 run just before the burst observed in the CoRoT data. Aims. Ground-based spectroscopic observations of the early Be star HD 49330 obtained during the precursor phase and just before the start of an outburst allow us to disantangle stellar and circumstellar contributions and identify modes of stellar pulsations in this rapidly rotating star. Methods. Time series analysis (TSA) is performed on photospheric line profiles of He i and Si iii by means of the least squares method. Results. We find two main frequencies f 1 = 11.86 c d−1 and f 2 = 16.89 c d−1 which can be associated with high order p-mode pulsations. We also detect a frequency f 3 = 1.51 c d−1 which can be associated with a low order g-mode. Moreover we show that the stellar line profile variability changed over the spectroscopic run. These results are in agreement with the results of the CoRo...