MOST found evidence for solar-type oscillations in the K2 giant star HD20884 (original) (raw)

Solar-like oscillations in HD 181420: data analysis of 156 days of CoRoT data

Astronomy and Astrophysics, 2009

Context. The estimate of solar-like oscillation properties, such as their frequencies, amplitudes and lifetimes, is challenging because of their low amplitudes and will benefit from long and uninterrupted observing runs. The space telescope CoRoT allows us to obtain high-performance photometric data over a long and quasi continuous period. Among its main targets are stars for which we expect solar-like oscillations. Aims. HD 181420, an F2 main sequence star, has been observed by CoRoT during its first long run covering about 156 days. With this unprecedently high-quality set of data, our aim is to derive the p-mode parameters that can be used to probe the stellar interior. Methods. The CoRoT data obtained on HD 181420 is analysed using a classical Fourier approach for the search for the p mode signature. The p-mode parameters are then derived using global fitting of the power spectrum by a Lorentzian model, as used widely in the solar case. Results. From the p-mode frequencies, the mean value of the large spacing is estimated to be 75 μHz. The p-mode amplitudes are slightly less than 4 ppm with a line width of about 8 μHz at the maximum of the p modes. The inclination angle is estimated to be around 45 • . The large mode line-width combined with the observed mode spacing make it difficult to identify the = 2 modes and to estimate the rotational splitting. We explore two scenarios for the identification of the modes.

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.

HS 0702+6043: a star showing both short-period p-mode and long-period g-mode oscillations

Astronomy & Astrophysics, 2006

Context. The hot subdwarf B star HS 0702+6043 is known as a large-amplitude, short-period p-mode pulsator of the EC 14026 type. Its atmospheric parameters place it at the common boundary between the empirical instability regions of the EC 14026 variables and the typically cooler long-period g-mode pulsators of the PG 1716 kind. Aims. We analyse and interpret the photometric variability of HS 0702+6043 in order to explore its asteroseismological potential. Methods. We report on rapid wide band CCD photometric observations to follow up on and confirm the serendipitous discovery of multiperiodic long-period luminosity variations with typical time scales of ∼1 h in HS 0702+6043, in addition to the two previously known pulsations at 363 s and 383 s. In particular, we isolate a relatively low-amplitude (∼4 mmag), long-period (3538±130 s) light variation. Results. We argue that the most likely origin for this luminosity variation is the presence of an excited g-mode pulsation. If confirmed, HS 0702+6043 would constitute a rare addition to the very select class of pulsating stars showing simultaneously parts of their pressure and gravity mode pulsation spectra. The asteroseismological potential of such stars is immense, and HS 0702+6043 thus becomes a target of choice for future investigations. While our discovery appears consistent with the location of HS 0702+6043 at the common boundary between the two families of pulsating sdB stars, it does challenge theory's current description of stability and driving mechanisms in pulsating B subdwarfs.

Discovery of the ‘missing’ mode in HR 1217 by the Whole Earth Telescope

Monthly Notices of the Royal Astronomical Society, 2002

HR 1217 is a prototypical rapidly oscillating Ap star that has presented a test to the theory of nonradial stellar pulsation. Prior observations showed a clear pattern of five modes with alternating frequency spacings of 33.3 µHz and 34.6 µHz, with a sixth mode at a problematic spacing of 50.0 µHz (which equals 1.5 × 33.3µHz) to the high-frequency side. Asymptotic pulsation theory allowed for a frequency spacing of 34 µHz, but hipparcos observations rule out such a spacing. Theoretical calculations of magnetoacoustic modes in Ap stars by Cunha predicted that there should be a previously undetected mode 34 µHz higher than the main group, with a smaller spacing between it and the highest one. In this Letter, we present preliminary results from a multi-site photometric campaign on the rapidly oscillating Ap star HR 1217 using the "Whole Earth Telescope". While a complete analysis of the data will appear in a later paper, one outstanding result from this run is the discovery of a newly detected frequency in the pulsation spectrum of this star, at the frequency predicted by Cunha .

Detection of solar-like oscillations in the red giant star ε Ophiuchi by MOST spacebased photometry

Astronomy and Astrophysics, 2007

Context. Solar-like oscillations have been discovered in a few red giants, including Oph, through spectroscopy. Acoustic modes around 60 µHz were clearly seen in this star, but daily aliasing of the groundbased data made it impossible to unambiguously isolate the p-mode frequencies in the eigenspectrum, and hence the correct value of the large spacing, to asteroseismically constrain the mass of this pulsating star. Aims. We obtained about 28 days of contiguous high-precision photometry of Oph in May-June 2005 with the MOST (Microvariability & Oscillations of STars) satellite. The thorough time sampling removes the ambiguity of the frequency identifications based on the groundbased discovery data. Methods. We identify equidistant peaks in the Fourier spectrum of the MOST photometry in the range where the p-modes were seen spectroscopically. Those peaks are searched by autocorrelation of the power spectrum to estimate the value of the large separation in the p-mode eigenspectrum. Having isolated the oscillation modes, we determine their mode parameters (frequency, amplitude and line width) by fitting the distribution of peaks to Lorentzian profiles. Results. The clear series of equidistant peaks in the power spectrum, with amplitudes from about 30 to 130 ppm, are consistent with radial modes spaced by a mean value of (5.3 ± 0.1) µHz. This large separation matches one of the two possibilities allowed by the groundbased observations thus constraining the stellar models to a much greater extent than previously possible. The line widths and Lorentzian fits indicate a rather short average mode lifetime: (2.7

The temporal spectrum of the sdB pulsating star HS?2201+2610 at 2?ms resolution

Astronomy and Astrophysics, 2002

In this article we present the results of more than 180 hours of time-series photometry on the low gravity (log g = 5.4, T eff = 29 300 K, log He/H = −3.0 by number) sdB pulsating star HS 2201+2610, obtained between September 2000 and August 2001. The temporal spectrum is resolved and shows 5 close frequencies: three main signals at 2860.94, 2824.10 and 2880.69 µHz, with amplitudes of about 1%, 0.5% and 0.1% respectively, are detected from single run observations; two further peaks with very low amplitude (<0.07%) at 2738.01 and 2921.82 µHz are confirmed by phase analysis on several independent runs. Due to the small number of detected frequencies, it is not possible to obtain a univocal identification of the excited modes and perform a detailed seismological analysis of the star. No clear signatures of rotational splitting are seen. Nevertheless, the observed period spectrum is well inside the excited period window obtained from pulsation calculations with nonadiabatic models having effective temperature and surface gravity close to the spectroscopic estimates. Due to its relatively simple temporal spectrum, HS 2201+2610 is a very good candidate for trying to measure the secular variation of the pulsation periods in time. With this purpose a long-term monitoring of the star was started. The results of the first 11 months show amplitude variations up to ∼20% on timescales of months, which are probably real, and allow us to measure the pulsation frequencies with an unprecedented 0.02 µHz resolution.

Short time-scale frequency and amplitude variations in the pulsations of an roAp star: HD 217522

Monthly Notices of the Royal Astronomical Society, 2014

Photometric observations of HD 217522 in 1981 revealed only one pulsation frequency ν 1 = 1.21529 mHz. Subsequent observations in 1989 showed the presence of an additional frequency ν 2 = 2.0174 mHz. New observations in 2008 confirm the presence of the mode with ν 2 = 2.0174 mHz. Examination of the 1989 data shows amplitude modulation over a time scale of the order of a day, much shorter than what has been observed in other roAp stars. High spectral and time resolution data obtained using the VLT in 2008 confirm the presence of ν 2 and short term modulations in the radial velocity amplitudes of rare earth elements. This suggests growth and decay times shorter than a day, more typical of solar-like oscillations. The driving mechanism of roAp stars and the Sun are different, and the growth and decay seen in the Sun are due to stochastic nature of the driving mechanism. The driving mechanism in roAp stars usually leads to mode stability on a longer timescale than in the Sun. We interpret the reported change in ν 1 between the 1982 and 1989 data as part of the general frequency variability observed in this star on many time scales.

Photometric and spectroscopic variability of the B5IIIe star HD 171219

Astronomy and Astrophysics, 2017

We analyzed the star HD 171219, one of the relatively bright Be stars observed in the seismo field of the CoRoT satellite, in order to determine its physical and pulsation characteristics. Classical Be stars are main-sequence objects of mainly B-type, whose spectra show, or had shown at some epoch, Balmer lines in emission and an infrared excess. Both characteristics are attributed to an equatorially concentrated circumstellar disk fed by non-periodic mass-loss episodes (outbursts). Be stars often show nonradial pulsation gravity modes and, as more recently discovered, stochastically excited oscillations. Applying the CLEANEST algorithm to the high-cadence and highly photometrically precise measurements of the HD 171219 light curve led us to perform an unprecedented detailed analysis of its nonradial pulsations. Tens of frequencies have been detected in the object compatible with nonradial g-modes. Additional high-resolution ground-based spectroscopic observations were obtained at La Silla (HARPS) and Haute Provence (SOPHIE) observatories during the month preceding CoRoT observations. Additional information was obtained from low-resolution spectra from the BeSS database. From spectral line fitting we determined physical parameters of the star, which is seen equator-on (i = 90 •). We also found in the ground data the same frequencies as in CoRoT data. Additionally, we analyzed the circumstellar activity through the traditional method of violet to red emission Hα line variation. A quintuplet was identified at approximately 1.113 c d −1 (12.88 µHz) with a separation of 0.017 c d −1 that can be attributed to a pulsation degree ℓ ∼ 2. The light curve shows six small-to medium-scale outbursts during the CoRoT observations. The intensity of the main frequencies varies after each outburst, suggesting a possible correlation between the nonradial pulsations regime and the feeding of the envelope.

Seismic analysis of HD 43587Aa, a solar-like oscillator in a multiple system

Astronomy & Astrophysics, 2014

Context. The object HD 43587Aa is a G0V star observed during the 145-day LRa03 run of the COnvection, ROtation and planetary Transits space mission (CoRoT), for which complementary High Accuracy Radial velocity Planet Searcher (HARPS) spectra with S/N > 300 were also obtained. Its visual magnitude is 5.71, and its effective temperature is close to 5950 K. It has a known companion in a highly eccentric orbit and is also coupled with two more distant companions. Aims. We undertake a preliminary investigation of the internal structure of HD 43587Aa. Methods. We carried out a seismic analysis of the star, using maximum likelihood estimators and Markov chain Monte Carlo methods. Results. We established the first table of the eigenmode frequencies, widths, and heights for HD 43587Aa. The star appears to have a mass and a radius slightly larger than the Sun, and is slightly older (5.6 Gyr). Two scenarios are suggested for the geometry of the star: either its inclination angle is very low, or the rotation velocity of the star is very low. Conclusions. A more detailed study of the rotation and of the magnetic and chromospheric activity for this star is needed, and will be the subject of a further study. New high resolution spectrometric observations should be performed for at least several months in duration.