arXiv subjects

G. Handler

Publications and source records attributed to G. Handler.

At least 127 records · Page 7Linked to original sources

arXiv2005

Detection of 75+ pulsation frequencies in the Delta Scuti star FG Vir

Extensive photometric multisite campaigns of the Delta Scuti variable FG Vir are presented. For the years 2003 and 2004, 926 hours of photometry at the millimag precision level were obtained. The combinations with earlier campaigns lead to excellent frequency resolution and high signal/noise. A multifrequency analysis yields 79 frequencies. This represents a new record for this type of star. The modes discovered earlier were confirmed. Pulsation occurs over a wide frequency band from 5.7 to 44.3 c/d with amplitudes of 0.2 mmag or larger. Within this wide band the frequencies are not distributed at random, but tend to cluster in groups. A similar feature is seen in the power spectrum of the residuals after 79 frequencies are prewhitened. This indicates that many additional modes are excited. The interpretation is supported by a histogram of the photometric amplitudes, which shows an increase of modes with small amplitudes. The old question of the 'missing modes' may be answered now: the large number of detected frequencies as well as the large number of additional frequencies suggested by the power spectrum of the residuals con rms the theoretical prediction of a large number of excited modes. FG Vir shows a number of frequency combinations of the dominant mode at 12.7162 c/d (m = 0) with other modes of relatively high photometric amplitudes. The amplitudes of the frequency sums are higher than those of the differences. A second mode (20.2878 c/d) also shows combinations. This mode of azimuthal order m = -1 is coupled with two other modes of m = +1.

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arXiv2005

Asteroseismology of the Beta Cephei star Nu Eridani: photometric observations and pulsational frequency analysis

We undertook a multisite photometric campaign for the Beta Cephei star Nu Eridani. More than 600 hours of differential photoelectric uvyV photometry were obtained with 11 telescopes during 148 clear nights. The frequency analysis of our measurements shows that the variability of Nu Eri can be decomposed into 23 sinusoidal components, eight of which correspond to independent pulsation frequencies between 5 - 8 c/d. Some of these are arranged in multiplets, which suggests rotational m-mode splitting of nonradial pulsation modes as the cause. If so, the rotation period of the star must be between 30 - 60 d. One of the signals in the light curves of Nu Eri has a very low frequency of 0.432 c/d. It can be a high-order combination frequency or, more likely, an independent pulsation mode. In the latter case Nu Eri would be both a Beta Cephei star and a slowly pulsating B (SPB) star. The photometric amplitudes of the individual pulsation modes of Nu Eri appear to have increased by about 20 per cent over the last 40 years. So do the amplitudes of the dominant combination frequencies of the star. Among the latter, we only could identify sum frequencies with certainty, not difference frequencies, which suggests that neither light-curve distortion in its simplest form nor resonant mode coupling are their single cause. One of our comparison stars, Mu Eridani, turned out to be variable with a dominant time scale of 1.62 d. We believe that it is either an SPB star just leaving its instability strip or that its variations are of rotational origin.

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arXiv2004

The Delta Scuti star FG Vir. V. The 2002 photometric multisite campaign

A high-accuracy multisite campaign was carried out from 2002 January to May with a photometric coverage of 398 hours at five observatories. The concentration on a few selected sites gives better consistency and accuracy than collecting smaller amounts from a larger number of sites. 23 frequencies were detected with a high statistical significance. 6 of these are new. The 17 frequencies found in common with the 1992-1995 data are the modes with highest amplitudes. This indicates that the pulsation spectrum of FG Vir is relatively stable over the ten-year period. Two frequencies have variable amplitudes and phases from year to year as well as during 2002. These were both found to be double modes with close frequencies. For the mode at 12.15 c/d this leads to an apparent modulation with a time scale of about 129d. The close frequencies at 12.15 c/d are composed of a radial and a nonradial mode, suggesting a similarity with the Blazhko Effect seen in RR Lyrae stars.

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arXiv2004

Asteroseismology of the Beta Cephei star Nu Eridani: Interpretation and applications of the oscillation spectrum

The oscillation spectrum of Nu Eri is the richest known for any variable of the Beta Cephei type. We interpret the spectrum in terms of normal mode excitation and construct seismic models of the star. The frequency data combined with data on mean colours sets the upper limit on the extent of overshooting from the convective core. We use data on rotational splitting of two dipole (l=1) modes (g_1 and p_1) to infer properties of the internal rotation rate. Adopting a plausible hypothesis of nearly uniform rotation in the envelope and increasing rotation rate in the mu-gradient zone, we find that the mean rotation rate in this zone is about three times faster than in the envelope. In our standard model only the modes in the middle part of the oscillation spectrum are unstable. To account for excitation of a possible high-order g-mode at nu = 0.43 c/d as well as p-modes at nu > 6 c/d we have to invoke an overabundance of Fe in the driving zone.

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arXiv2003

2MASSJ0516288+260738: Discovery of the first eclipsing late K + Brown dwarf system?

We report the discovery of a new eclipsing system less than one arcminute south of the pulsating DB white dwarf KUV 05134+2605. The object could be identified with the point source 2MASSJ0516288+260738 published by the Two Micron All Sky Survey. We present and discuss the first light curves as well as some additional colour and spectral information. The eclipse period of the system is 1.29d, and, assuming this to be identical to the orbital period, the best light curve solution yields a mass ratio of m2/m1=0.11, a radius ratio of r2/r1~1 and an inclination of 74 deg. The spectral anaylsis results in a Teff=4200K for the primary. On this basis, we suggest that the new system probably consists of a late K + Brown dwarf (which would imply a system considerably younger than ~0.01 Gyr to have r2/r1~1), and outline possible future observations.

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arXiv2003

Preparing the COROT space mission: incidence and characterisation of pulsation in the Lower Instability Strip

By pursuing the goal to find new variables in the COROT field-of-view we characterised a sample of stars located in the lower part of the instability strip. Our sample is composed of stars belonging to the disk population in the solar neighbourhood. We found that 23% of the stars display multiperiodic light variability up to few mmag of amplitude. uvbyBeta photometry fixed most of the variables in the middle of the instability strip and high-resolution spectroscopy established that they have vsin i>100 km/s. The comparison with delta Sct stars in the whole Galaxy shows slightly different features, i.e., most delta Sct stars have a 0.05-mag redder (b-y)_0 index and lower vsin i values. Additional investigation in the open cluster NGC 6633 confirms the same incidence of variability, i.e., around 20%. The wide variety of pulsational behaviours of delta Sct stars (including unusual objects such as a variable beyond the blue edge or a rapidly rotating high-amplitude pulsator) makes them very powerful asteroseismic tools to be used by COROT. Being quite common among bright stars, delta Sct stars are suitable targets for optical observations from space.

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arXiv2003

Amplitude and frequency variability of the pulsating DB white dwarf stars KUV 05134+2605 and PG 1654+160 observed with the Whole Earth Telescope

We have acquired new time series photometry of the two pulsating DB white dwarf stars KUV 05134+2605 and PG 1654+160 with the Whole Earth Telescope. Additional single-site photometry is also presented. We use all these data plus all available archival measurements to study the temporal behaviour of the pulsational amplitudes and frequencies of these stars for the first time. We demonstrate that both KUV 05134+2605 and PG 1654+160 pulsate in many modes whose amplitudes are variable in time; some frequency variability of PG 1654+160 is also indicated. Beating of multiple pulsation modes cannot explain our observations; the amplitude variability must therefore be intrinsic. We cannot find stable modes to be used for determinations of the evolutionary period changes of the stars. Some of the modes of PG 1654+160 appear at the same periods whenever detected. The mean spacing of these periods (about 40s) suggests that they are probably caused by non-radial gravity-mode pulsations of spherical degree l=1. If so, PG 1654+160 has a mass around 0.6 Msun. The time scales of the amplitude variability of both stars (down to two weeks) are consistent with theoretical predictions of resonant mode coupling, a conclusion which might however be affected by the temporal distribution of our data.

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arXiv2003

Asteroseismological studies of three Beta Cephei stars: IL Vel, V433 Car and KZ Mus

We have acquired between 127 and 150 h of time-resolved multicolour photometry for each of the three Beta Cephei stars IL Vel, V433 Car and KZ Mus over a time span of four months from two observatories. All three objects are multiperiodic with at least three modes of pulsation. Mode identification from the relative colour amplitudes is performed. We obtain unambiguous results for the two highest-amplitude modes of IL Vel (both are l=1) and the three strongest modes of KZ Mus (l=2,0 and 1), but none for V433 Car. Spectroscopy shows the latter star to be a fast rotator (v sin i = 240 km/s), whereas the other two have moderate v sin i (65 and 47 km/s, respectively). We performed model calculations with the Warsaw-New Jersey stellar evolution and pulsation code. We find that IL Vel is an object of about 12 Msun in the second half of its main sequence evolutionary track. Its two dipole modes are most likely rotationally split components of the mode originating as p1 on the ZAMS; one of these modes is m=0. V433 Car is suggested to be an unevolved 13 Msun star just entering the Beta Cephei instability strip. KZ Mus seems less massive (about 12.7 Msun) and somewhat more evolved, and its radial mode is probably the fundamental one. In this case its quadrupole mode would be the one originating as g1, and its dipole mode would be p1. It is suggested that mode identification of slowly rotating Beta Cephei stars based on photometric colour amplitudes is reliable; we estimate that a relative accuracy of 3% in the amplitudes is sufficient for unambiguous identifications. Due to the good agreement of our theoretical and observational results we conclude that the prospects for asteroseismology of multiperiodic slowly rotating Beta Cephei star are good.

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arXiv2002

The ^12C(alpha,gamma)^16O Nuclear Reaction Rate from Asteroseismology of the DBV White Dwarf CBS 114

We have identified seven independent pulsation modes in the helium-atmosphere variable (DBV) white dwarf star CBS 114, based on 65 hours of time-resolved CCD photometry from the 0.75-m telescope at SAAO. We interpret these pulsations as non-radial g-modes with the same spherical degree l=1, as suggested by the mean period spacing of 37.1+/-0.7 seconds. We use a genetic-algorithm-based fitting method to find the globally optimal model parameters, including the stellar mass (M_* = 0.73 M_sun), the effective temperature (T_eff = 21,000 K), the mass of the atmospheric helium layer (log[M_He/M_*] = -6.66), and the central oxygen mass fraction (X_O = 0.61). The latter value implies a rate for the ^12C(alpha,gamma)^16O reaction near S_300 = 180 keV b, consistent with laboratory measurements.

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arXiv2002

On the Period-Luminosity-Colour-Metallicity relation and the pulsational characteristics of lambda Bootis type stars

The group of lambda Bootis type stars comprises late B- to early F-type, Population I objects which are basically metal weak, in particular the Fe group elements, but with the clear exception of C, N, O and S. The present work is a continuation of the studies by Paunzen et al. (1998, A&A, 335, 533), who presented first results on the pulsational characteristics of the lambda Bootis stars. Since then, we have observed 22 additional objects; we found eight new pulsators and confirmed another one. Furthermore, new spectroscopic data (Paunzen, 2001, A&A, 373, 633) allowed us to sort out misidentified candidates and to add true members to the group. From 67 members of this group, only two are not photometrically investigated yet which makes our analysis highly representative. We have compared our results on the pulsational behaviour of the lambda Bootis stars with those of a sample of delta Scuti type objects. We find that at least 70% of all lambda Bootis type stars inside the classical instability strip pulsate, and they do so with high overtone modes (Q < 0.020d). Only a few stars, if any, pulsate in the fundamental mode. Compared to the delta Scuti stars, the cool and hot borders of the instability strip of the lambda Bootis stars are shifted by about 25mmag, towards smaller (b-y)0. Using published abundances and the metallicity sensitive indices of the Geneva 7-colour and Stroemgren uvbybeta systems, we have derived [Z] values which describe the surface abundance of the heavier elements for the group members. We find that the Period-Luminosity-Colour relation for the group of lambda Bootis stars is within the errors identical with that of the normal delta Scuti stars. No clear evidence for a statistically significant metallicity term was detected.

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arXiv2002

The unusual pulsation spectrum of the cool ZZ Ceti star HS 0507+0434B

We present the analysis of one week of single-site high-speed CCD photometric observations of the cool ZZ Ceti star HS 0507+0434B. Ten independent frequencies are detected in the star's light variations: one singlet and three nearly-equally spaced triplets. We argue that these triplets are due to rotationally split modes of spherical degree l=1. This is the first detection of consistent multiplet structure in the amplitude spectrum of a cool ZZ Ceti star and it allows us to determine the star's rotation period: 1.70 +/- 0.11 d. We report exactly equal frequency, not period, spacings between the detected mode groups. In addition, certain pairs of modes from the four principal groups have frequency ratios which are very close to 3:4 or 4:5; while these ratios are nearly exact (within one part in 10^4), they still lie outside the computed error bars. We speculate that these relationships between different frequencies could be caused by resonances. One of the three triplets may not be constant in amplitude and/or frequency. We compare our frequency solution for the combination frequencies (of which we detected 38) to Wu's (1998, 2001) model thereof. We obtain consistent results when trying to infer the star's convective thermal time and the inclination angle of its rotational axis. Theoretical combination-frequency amplitude spectra also resemble those of the observations well, and direct theoretical predictions of the observed second-order light-curve distortions were also reasonably successful assuming the three triplets are due to l=1 modes. Attempts to reproduce the observed combination frequencies adopting all possible l=2 identifications for the triplets did not provide similarly consistent results, supporting their identification with l=1.

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arXiv2002

The asteroseismological potential of the pulsating DB white dwarf stars CBS 114 and PG 1456+103

We have acquired 65 h of single-site time-resolved CCD photometry of the pulsating DB white dwarf star CBS 114 and 62 h of two-site high-speed CCD photometry of another DBV, PG 1456+103. The pulsation spectrum of PG 1456+103 is complicated and variable on time scales of about one week and could only partly be deciphered with our measurements. The modes of CBS 114 are more stable in time and we were able to arrive at a frequency solution somewhat affected by aliasing, but still satisfactory, involving seven independent modes and two combination frequencies. These frequencies also explain the discovery data of the star, taken 13 years earlier. We find a mean period spacing of 37.1 +/- 0.7 s significant at the 98% level between the independent modes of CBS 114 and argue that they are due to nonradial g-mode pulsations of spherical degree l=1. We performed a global search for asteroseismological models of CBS 114 using a genetic algorithm, and we examined the susceptibility of the results to the uncertainties of the observational frequency determinations and mode identifications (we could not provide m values). The families of possible solutions are identified correctly even without knowledge of m. Our optimal model suggests Teff = 21,000 K and M_* = 0.730 M_sun as well as log(M_He/M_*) = -6.66, X_O = 0.61. This measurement of the central oxygen mass fraction implies a rate for the ^12C(alpha,gamma)^16O nuclear reaction near S_300=180 keV b, consistent with laboratory measurements.

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arXiv2002

Pulsation of the Delta Scuti star Theta2 Tau: New multisite photometry and modelling of instability

The results of a multisite photometric campaign of Theta2 Tau are reported. This binary system consists of evolved and main-sequence A stars inside the instability strip. The 12th Delta Scuti Network campaign included 152 hours of high-precision photometry obtained at four observatories. This leads to the derivation of 11 frequencies of pulsation in the 10.8 to 14.6 c/d range. These frequencies confirm the results from previous Earth-based (1982-1986) as well as satellite (2000) photometry,although amplitude variability on a time scale of several years is present. We show that at least two high frequencies (26.18 and 26.73 c/d are also present in Theta 2 Tau. Arguments are given that these high frequencies originate in the main-sequence companion and are not combination frequencies, fi+fj, from the primary. Models for both the primary and the secondary components were checked for the instability against radial and nonradial oscillations. All hot models of the primary with Teff > 8000K are stable in the observed frequency range. The best fit between the theoretical and observed frequency ranges is achieved by models with Teff approximately equal to 7800K (or slightly higher), in agreement with photometric calibrations. The instability range spans two or three radial orders in the range p4 to p6 for radial modes. Post-main-sequence models (with or without overshooting) are preferable for the primary, but main-sequence models with overshooting cannot be excluded. For the less luminous secondary component the instability range is wider and spans 5 to 7 radial orders from p2 to p8. The observed frequencies lie around radial modes p5 to p6. The main uncertainties of these results are caused by a simple treatment of the convective flux in the hydrogen ionization zone.

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arXiv2002

Discovery and analysis of p-mode and g-mode oscillations in the A-type primary of the eccentric binary HD 209295

We have discovered both intermediate-order gravity mode and low-order pressure mode pulsation in the same star, HD 209295. It is therefore both a Gamma Doradus and a Delta Scuti star, which makes it the first pulsating star to be a member of two classes. The star is a single-lined spectroscopic binary with an orbital period of 3.10575 d and an eccentricity of 0.352. Weak pulsational signals are found in both the radial velocity and line-profile variations, allowing us to show that the two highest-amplitude Gamma Doradus pulsation modes are consistent with l=1 and |m|=1. In our 280 h of BVI multi-site photometry we detected ten frequencies in the light variations, one in the Delta Scuti regime and nine in the Gamma Doradus domain. Five of the Gamma Doradus frequencies are exact integer multiples of the orbital frequency. This observation leads us to suspect they are tidally excited. Results of theoretical modeling (stability analysis, tidal excitation) were consistent with the observations. We could not detect the secondary component of the system in infrared photometry, suggesting that it may not be a main-sequence star. Archival data of HD 209295 show a strong ultraviolet excess, the origin of which is not known. The orbit of the primary is consistent with a secondary mass of M > 1.04 Msun indicative of a neutron star or a white dwarf companion.

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arXiv2002

On the relationship between the Delta Scuti and Gamma Doradus pulsators

We searched for Delta Scuti-type pulsations amongst known and candidate Gamma Doradus stars. We acquired 270 h of observations and monitored a total of 26 stars. One target, HD 209295, turned out to be a member of both classes of pulsating star. We classify six of our targets as new bona fide Gamma Doradus stars, nine more as good Gamma Doradus candidates, and three as ellipsoidal variables. The remainder comprise one Delta Scuti star and five unexplained variables. We revise the blue border of the Gamma Doradus phenomenon towards cooler temperatures. This new blue edge extends from a temperature of about 7550 K on the ZAMS to 7400 K one magnitude above it. Five bona fide Gamma Doradus stars we observed are located inside the Delta Scuti instability strip, but none of them exhibited observable Delta Scuti pulsations. We therefore suggest that Gamma Doradus stars are less likely to be Delta Scuti pulsators compared to other normal stars in the same region of the lower instability strip. In addition, we show that there is a clear separation between the pulsation constants Q of Delta Scuti and Gamma Doradus stars. The Gamma Doradus stars known to date all have Q>0.23 d.

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arXiv2001

Asteroseismology and forced oscillations of HD 209295, the first member of two classes of pulsating star

We report the discovery of both intermediate-order gravity mode and low-order pressure mode pulsation in the same star, HD 209295. It is therefore both a gamma Doradus and a delta Scuti star, which makes it the first confirmed member of two classes of pulsating star. This object is located in a close binary system with an unknown, but likely degenerate companion in an eccentric orbit, and some of the gamma Doradus pulsation frequencies are exact integer multiples of the orbital frequency. We suggest that these pulsations are tidally excited. HD 209295 may be the progenitor of an intermediate-mass X-Ray binary.

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arXiv2000

The Discovery of an Outburst and Pulsed X-ray Flux from SMC X-2 from RXTE Observations

Rossi X-ray Timing Explorer All Sky Monitor observations of SMC X-2 show that the source experienced an outburst in January to April 2000 reaching a peak luminosity of greater than ~10^38 ergs s^-1. RXTE Proportional Counter Array observations during this outburst reveal the presence of pulsations with a 2.37s period. However, optical photometry of the optical counterpart showed the source to be still significantly fainter than it was more than half a year after the outburst in the 1970s when SMC X-2 was discovered.

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arXiv2000

The observation of Delta Sct and Gamma Dor stars by using MONS satellite

The MONS space experiment will be able to verify some of the most important topics in the asteroseismologic studies of Delta Sct and Gamma Dor stars. We discuss how the $γ$ Doradus can be considered possible "bridges" between the opacity driven overstable pulsation and the stochastically excited solar like oscillation. For this reason the possible inclusion of a Gamma Dor representative in the MONS main target list is presented and discussed. To study Delta Sct stars, an appropriate frequency resolution, comparable to or better than what is currently achieved by ground-based observations, is necessary and hence two runs are recommended. To avoid over-scheduling of the Main Camera, a possible combination of observations from the Main Camera and from the Star Trackers is proposed. As a result, key-objects could be adequately monitored, affording the possibility to enter into details of amplitude variations. Simultaneously, the same results could be obtained on two Gamma Dor stars.

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