Pulsations in the late-type Be star HD 50 209 detected by CoRoT (original) (raw)

A photometric study of Be stars located in the seismology fields of COROT

Astronomy and Astrophysics, 2007

Context. In preparation for the COROT mission, an exhaustive photometric study of Be stars located in the seismology fields of the COROT mission has been performed. The very precise and long-time-spanned photometric observations gathered by the COROT satellite will give important clues of the origin of the Be phenomenon. Aims. The aim of this work is to find short-period variable Be stars located in the seismology fields of COROT and to study and characterise their pulsational properties. Methods. Light curves obtained at the Observatorio de Sierra Nevada together with data from Hipparcos and ASAS-3 of a total of 84 Be stars have been analysed in order to search for short-term variations. We have applied standard Fourier techniques and non-linear least-square fitting to the time series. Results. We have found 7 multiperiodic, 21 mono-periodic and 26 non-variable Be stars. Short-term variability has been detected in 74% of early-type Be stars and in 31% of mid-to late-type Be stars. We have shown that non-radial pulsations are most frequent among Be stars than in slow-rotating B stars of the same spectral range.

Asteroseismology and mass loss in be stars: study with corot

2010

describes the basics of the Fourier analysis and the rudiments of the time series analysis. In this Chapter, we introduce the pasper code for the frequency analysis of photometric light curves. At the early begin of this Ph.D. thesis, the CoRoT satellite was still on ground getting ready for the launch. At this time, we were performing and improving our pasper code and we needed some variable stars to check the performance of our methods. In this context, we perform a search for short-period B and Be star variables in the low metallicity environment of the Magellanic Clouds. This study constitutes the Part I of this Ph.D. thesis. This Part has a double goal: i) to test the frequency analysis codes; and ii) to detect observationally β Cephei and SPB-like B-type pulsators in low metallicity environments, actually not predicted by the pulsational theory and models. This Part is organized in four Chapters: Chapter 4 depicts the scientific context for the search of B-type pulsators in the Magellanic Clouds. An overview of the MACHO survey and the description of the B and Be studied samples are given in Chapter 5. Finally, Chapter 6 and Chapter 7 describe the results and discussion for the SMC and LMC studies, respectively. Part II is devoted to the study of Be stars with the CoRoT space mission. This Part is also organized in four Chapters: Chapter 8 depicts a complete review on the CoRoT mission, describing the spacecraft, the different observing programmes and the mission design. Chapter 9 presents the CoRoT Be Team, a collaboration for the study of Be stars using the CoRoT data. We describe some general remarks about the instrumental effects present in the CoRoT light curves and information on the frequency analysis of the CoRoT data. In Chapter 10 we describe the results on the analysis of three Be stars from the CoRoT exoplanet field. Finally, in Chapter 11 we present the results on the frequency analysis of the late Be star HD 50 209, observed in the seismology field of the CoRoT satellite. The analysis of this Be star has revealed up to sixty frequencies, grouped in six different and separated sets, attributed to g-mode pulsations. Finally, in Chapter 12 we resume the main conclusions of the whole project, including prospects and future work to be done. An addendum with all the published results derived from this project has been added in Chapter 13. P. D. Diago with = 1 or = 2 is sometimes called a dipole mode, respectively quadrupole mode. 1.2.3 Historical background The early history of studies on stellar pulsation was concisely described in the introduction of the famous textbook The Pulsation Theory of Variable Stars (Rosseland 1949). It is interesting to see that the theory of non-radial pulsation developed by Kelvin (1863) preceded the theory of radial pulsation developed by Ritter (1879). However, the Cepheids have been the chief concern of pulsation theory, which was founded by Eddington as summarised in his book The Internal Constitution of the Stars (Eddington 1926). In spite of the remarkable progress in the development of the theory of radial pulsation, the theoretical study of nonradial pulsation remained largely within academic circles until recently. But the work of Pekeris and Cowling should be mentioned. Pekeris (1938) obtained the exact analytic solution for adiabatic non-radial oscillations in the homogeneous compressible model. Cowling (1941) extended the study for the polytrope model. For a description of these and other studies, readers can refer to the comprehensive article by Ledoux and Walraven (1958). Ledoux developed the study of non-radial pulsations in 1951 (see Ledoux 1951). He suggested that non-radial oscillations could explain the double periodicity and the large temporal variations in the broadening of spectral lines observed in β Canis Majoris (a prototype of β Cephei type variable stars). Osaki (1971) examined Ledoux's theory by calculating line profiles for a star undergoing non-radial oscillations and compared the result with observations available at that time. He also suggested (Osaki 1974) a possible mechanism for the origin and maintenance of β Cephei pulsation based on non-radial oscillation. The discovery of the five-minute Solar oscillations by Leighton et al. (1962) was also epoch-making. A number of interesting theories had been proposed to explain this phenomenon (see Stein and Leibacher 1974). Some fifteen years later, Deubner (1975) succeeded in resolving observed oscillations into discrete modes 12 Asteroseismology in the so-called diagnostic diagram. A comparison between his observation and theoretical eigenfrequencies of non-radial modes calculated by Ando and Osaki (1975) has established that the solar five-minute oscillations are global non-radial p-modes of the Sun with high spherical harmonic degree (= 200 − 1 000). Furthermore, low-degree (= 0 − 5) and intermediate-degree (= 1 − 200) have also been detected. Since the late 1960s and early 1970s, pulsations and oscillation-related phenomena have been observed in many stars that were regarded as non-pulsating stars before. They include white dwarfs, hot subdwarfs, Ap stars, γ Dor stars, and early type O and B stars. It is now believed that non-radial oscillations are responsible for variability observed in these stars in most cases. Along with these observational developments, much progress has been made in the theoretical side of non-radial oscillations theory. Since the middle of the 1970s, full equations of linear adiabatic and non-adiabatic non-radial oscillations have been solved numerically for realistic stellar models with the help of computers. The introduction of the so-called propagation diagram, the phase diagram and the concept of "wave trapping" (Scuflaire 1974; Unno 1975; Osaki 1975; Shibahashi and Osaki 1976) have greatly improved the understanding on non-radial oscillation in stars. A number of theoretical problems remain to be studied. Among them are non-linear problems, including the mixing of matter due to finite oscillations, oscillations in the presence of a strong magnetic field or rotation, and energy and momentum transport by waves. Both theory and observation are still in progress, so further developments can be expected in the near future. 1.2.4 The effect of rotation From Eqs. 1.1 and 1.4 we can derive that for modes with m = 0 the exponentials in the two equations combine to give a time dependence that goes as e −i (2πνt−mϕ). This phase factor in the time dependence means that the m = 0 modes are travelling waves. Conventionally, we assign positive values of m to modes that propagate in the same direction as the stellar rotation (pro-grade modes), and 14 http://kepler.arc.nasa.gov/.

Photometric variability of the Be star CoRoT-ID 102761769

2010

Context. Classical Be stars are rapid rotators of spectral type late O to early A and luminosity class V-III, wich exhibit Balmer emission lines and often a near infrared excess originating in an equatorially concentrated circumstellar envelope, both produced by sporadic mass ejection episodes. The causes of the abnormal mass loss (the so-called Be phenomenon) are as yet unknown.

Low-amplitude variations detected by CoRoT in the B8IIIe star HD 175869

2009

Context. The origin of the short-term variability in Be stars remains a matter of controversy. Pulsations and rotational modulation are the components of the favored hypothesis. Aims. We present our analysis of CoRoT data of the B8IIIe star HD 175869 observed during the first short run in the center direction (SRC1). Methods. We review both the instrumental effects visible in the CoRoT light curve and the analysis methods used by the CoRoT Be team. We applied these methods to the CoRoT light curve of the star HD 175869. A search for line-profile variations in the spectroscopic data was also performed. We also searched for a magnetic field, by applying the LSD technique to spectropolarimetric data. Results. The light curve exhibits low-amplitude variations of the order of 300 μmag with a double wave shape. A frequency within the range determined for the rotational frequency and 6 of its harmonics are detected. The main frequency and its first harmonic exhibit amplitude variations of a few days. Other significant frequencies of low-amplitude from 25 to a few μmag are also found. The analysis of line profiles from ground-based spectroscopic data does not detect any variation. In addition, no Zeeman signature was found. Conclusions. Inhomogeneities caused by stellar activity in or just above the photosphere are proposed to produce the photometric variability detected by CoRoT in the Be star HD 175869. The hypothesis that non-radial pulsations are the origin of these variations cannot be excluded.

Fundamental parameters of Be stars located in the seismology fields of COROT

Astronomy & Astrophysics, 2006

In preparation for the COROT space mission, we determined the fundamental parameters (spectral type, temperature, gravity, vsini) of the Be stars observable by COROT in its seismology fields (64 Be stars). We applied a careful and detailed modeling of the stellar spectra, taking into account the veiling caused by the envelope, as well as the gravitational darkening and stellar flattening due to rapid rotation. Evolutionary tracks for fast rotators were used to derive stellar masses and ages. The derived parameters will be used to select Be stars as secondary targets (i.e. observed for 5 consecutive months) and short-run targets of the COROT mission. Furthermore, we note that the main part of our stellar sample is falling in the second half of the main sequence life time, and that in most cases the luminosity class of Be stars is inaccurate in characterizing their evolutionary status.

Short-term variations in Be stars observed by the CoRoT and Kepler space missions

Proceedings of the International Astronomical Union, 2010

Thecorotandkeplerspace missions are collecting very high-precision long-duration photometric data of many Be stars, allowing us to better understand the origin of their short-term variability and the link between these variations and the Be phenomenon. In this paper, we present a brief summary of the results obtained in the analysis of several Be stars observed withcorotin terms of pulsations. In addition, we show that variations of the Be star HD 175869 can be explained as two active regions separated by 150 degrees or as unstable pulsating modes in a star with an extensive mixing in radiative layers corresponding to a core overshooting of 0.35Hp. A preliminary study of the photometric and spectroscopic variability seen in the B1.5IVe star HD 51193 is performed. Currently thekeplersatellite is observing the only confirmed Be star in its field of view, namely KIC 6954726. From low-resolution spectra we derived a spectral type of B2.5Ve for this star and we studied the long-term vari...

The pulsations of the B5IVe star HD 181231 observed with CoRoT and ground-based spectroscopy

Astronomy & Astrophysics, 2009

Context. HD 181231 is a B5IVe star, which has been observed with the CoRoT satellite during ∼5 consecutive months and simultaneously from the ground in spectroscopy and spectropolarimetry. Aims. By analysing these data, we aim to detect and characterize as many pulsation frequencies as possible, to search for the presence of beating effects possibly at the origin of the Be phenomenon. Our results will also provide a basis for seismic modelling. Methods. The fundamental parameters of the star are determined from spectral fitting and from the study of the circumstellar emission. The CoRoT photometric data and ground-based spectroscopy are analysed using several Fourier techniques: Clean-ng, Pasper, and Tisaft, as well as a time-frequency technique. A search for a magnetic field is performed by applying the LSD technique to the spectropolarimetric data. Results. We find that HD 181231 is a B5IVe star seen with an inclination of ∼45 degrees. No magnetic field is detected in its photosphere. We detect at least 10 independent significant frequencies of variations among the 54 detected frequencies, interpreted in terms of non-radial pulsation modes and rotation. Two longer-term variations are also detected: one at ∼14 days resulting from a beating effect between the two main frequencies of short-term variations, the other at ∼116 days due either to a beating of frequencies or to a zonal pulsation mode. Conclusions. Our analysis of the CoRoT light curve and ground-based spectroscopic data of HD 181231 has led to the determination of the fundamental and pulsational parameters of the star, including beating effects. This will allow a precise seismic modelling of this star.

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.

CoRoT’s view on variable B8/9 stars: spots versus pulsations

Astronomy & Astrophysics, 2011

Context. There exist few variability studies of stars in the region in the Hertzsprung-Russell diagram between the A and B-star pulsational instability strips. With the aid of the high precision continuous measurements of the CoRoT space satellite, low amplitudes are more easily detected, making a study of this neglected region worthwhile. Aims. We collected a small sample of B stars observed by CoRoT to determine the origin of the different types of variability observed. Methods. We combine literature photometry and spectroscopy to measure the fundamental parameters of the stars in the sample, and compare asteroseismic modelling of the light curves with (differentially rotating) spotted star models. Results. We found strong evidence for the existence of spots and differential rotation in HD 174648, and formulated hypotheses for their origin. We show that the distinction between pulsations and rotational modulation is difficult to make solely based on the light curve, especially in slowly rotating stars.

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.