Publications (32)71.49 Total impact
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Article: A spectroscopic analysis of the chemically peculiar star HD207561
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ABSTRACT: In this paper we present a high-resolution spectroscopic analysis of the chemically peculiar star HD207561. During a survey programme to search for new roAp stars in the Northern hemisphere, Joshi et al. (2006) observed significant photometric variability on two consecutive nights in the year 2000. The amplitude spectra of the light curves obtained on these two nights showed oscillations with a frequency of 2.79 mHz [P~6-min]. However, subsequent follow-up observations could not confirm any rapid variability. In order to determine the spectroscopic nature of HD207561, high-resolution spectroscopic and spectro-polarimetric observations were carried out. A reasonable fit of the calculated Hbeta line profile to the observed one yields the effective temperature (Teff) and surface gravity (log g) as 7300 K and 3.7 dex, respectively. The derived projected rotational velocity (vsin i) for HD207561 is 74 km/sec indicative of a relatively fast rotator. The position of HD207561 in the H-R diagram implies that this is slightly evolved from the main-sequence and located well within the delta-Scuti instability strip. The abundance analysis indicates the star has slight under-abundances of Ca and Sc and mild over-abundances of iron-peak elements. The spectro-polarimetric study of HD207561 shows that the effective magnetic field is within the observational error of 100 gauss (G). The spectroscopic analysis revealed that the star has most of the characteristics similar to an Am star, rather than an Ap star, and that it lies in the delta-Scuti instability strip; hence roAp pulsations are not expected in HD207561, but low-overtone modes might be excited.05/2012; -
Article: δ Sct pulsation in magnetic Ap stars: the discovery of δ Sct pulsations in HD 218994AB and measurement of the magnetic fields of HD 218994A and HD 21190★
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ABSTRACT: We have discovered that at least one component of the visual binary star HD 218994AB is a δ Sct star. The primary star of this system is a known rapidly oscillating Ap (roAp) star; therefore, it is possible that it is both an roAp star and a δ Sct star – a hybrid. There are no confirmed such hybrid roAp–δ Sct stars, and they are predicted theoretically not to exist. More probably, HD 218994B is the δ Sct star, as it is 0.54 mag fainter than the primary and also lies in the δ Sct instability strip. It then offers the potential to determine Teff and L for the primary which appears to be one of the more evolved roAp stars and an important test the theoretically calculated roAp star instability strip. In this case, the system also is important since the stars are a co-eval pair of very similar mass, yet one is a magnetic roAp star and the other is probably a non-magnetic δ Sct star with a normal spectrum. We detect a significant longitudinal magnetic field strength of 440 ± 23 G in the roAp star HD 218994A. We also present the first measurement of the longitudinal magnetic field strength for the δ Sct star HD 21190, the most-evolved Ap star known. We obtain a formally significant value of 47 ± 13 G. If confirmed, this will make HD 21190 the first magnetic δ Sct star known.Monthly Notices of the Royal Astronomical Society 04/2008; 386(3):1750 - 1755. · 4.90 Impact Factor -
Article: The pulsation modes of the pre-white dwarf PG 1159-035
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ABSTRACT: PG 1159-035, a pre-white dwarf with T_eff=140,000 K, is the prototype of both two classes: the PG1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. In this work, we used all available WET photometric data from 1983, 1985, 1989, 1993 and 2002 to identify the pulsation periods and identified 76 additional pulsation modes, increasing to 198 the number of known pulsation modes in PG 1159-035, the largest number of modes detected in any star besides the Sun. From the period spacing we estimated a mass M = 0.59 +/- 0.02 solar masses for PG 1159-035, with the uncertainty dominated by the models, not the observation. Deviations in the regular period spacing suggest that some of the pulsation modes are trapped, even though the star is a pre-white dwarf and the gravitational settling is ongoing. The position of the transition zone that causes the mode trapping was calculated at r_c = 0.83 +/- 0.05 stellar radius. From the multiplet splitting, we calculated the rotational period P_rot = 1.3920 +/- 0.0008 days and an upper limit for the magnetic field, B < 2000 G. The total power of the pulsation modes at the stellar surface changed less than 30% for l=1 modes and less than 50% for l=2 modes. We find no evidence of linear combinations between the 198 pulsation mode frequencies. PG 1159-035 models have not significative convection zones, supporting the hypothesis that nonlinearity arises in the convection zones in cooler pulsating white dwarf stars.12/2007; -
Article: The Nainital-Cape Survey: contributions to asteroseismology of CP stars
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ABSTRACT: We present a progress report on the Nainital-Cape Survey. Pulsations of the δ Scuti type have been discovered in the chemically peculiar A-type stars HD 13038, HD 13079, HD 98851, HD 102480, HD 113878 and HD 118660. HD 12098 has been discovered to be a roAp star. We have also detected evidence for roAp-like 6.1-minute oscillations in the Am star HD 207561.Communications in Asteroseismology 05/2007; 150:79. -
Article: The rapidly oscillating Ap star HD 99563 and its distorted dipole pulsation mode
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ABSTRACT: We undertook a time-series photometric multi-site campaign for the rapidly oscillating Ap star HD 99563 and also acquired mean light observations over two seasons. The pulsations of the star, that show flatter light maxima than minima, can be described with a frequency quintuplet centred on 1557.653 microHertz and some first harmonics of these. The amplitude of the pulsation is modulated with the rotation period of the star that we determine with 2.91179 +/- 0.00007 d from the analysis of the stellar pulsation spectrum and of the mean light data. We break the distorted oscillation mode up into its pure spherical harmonic components and find it is dominated by the l=1 pulsation, and also has a notable l=3 contribution, with weak l=0 and 2 components. The geometrical configuration of the star allows one to see both pulsation poles for about the same amount of time; HD 99563 is only the fourth roAp star for which both pulsation poles are seen and only the third where the distortion of the pulsation modes was modelled. We point out that HD 99563 is very similar to the well-studied roAp star HR 3831. Finally, we note that the visual companion of HD 99563 is located in the Delta Scuti instability strip and may thus show pulsation. We show that if the companion was physical, the roAp star would be a 2.03 solar mass object, seen at a rotational inclination of 44 degrees, which then predicts a magnetic obliquity of 86.4 degrees. Comment: 10 pages, 6 figures, accepted for publication by MNRAS11/2005; -
Article: A Whole Earth Telescope campaign on the pulsating subdwarf B binary system PG 1336−018 (NY Vir)
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ABSTRACT: We present results from a multisite (‘Whole Earth Telescope’) photometric campaign on PG 1336−018, the close eclipsing binary system containing a pulsating subdwarf B (sdB) star. The main part of the campaign (1999 April) resulted in ∼172 h of observations, representing a coverage of about 47 per cent, and additional data were obtained outside the core campaign. Periodogram analysis shows that the light variations are dominated by three frequencies near 5757, 5585 and 5369 μHz (∼174, 179 and 186 s, respectively), although many frequencies are present, particularly in the range 5000–6000 μHz (∼200–170 s). We identify, with some confidence, 28 frequencies down to a semi-amplitude of 0.0005 in fractional intensity (equivalent to about 0.5 mmag). It is clear that the pulsation frequencies of PG 1336−018 have changed substantially since the 1996 discovery observations were made, and that amplitude changes occur, at least in the dominant three frequencies, on relatively short time-scales (of the order of a day). On the assumption that the pulsating star is phase-locked in the binary system, we have searched for rotational splitting of frequencies near the orbital and half of the orbital period, but the results are confused by aliasing at those frequencies (due to the data gaps caused by the eclipses). A preliminary model qualitatively matches the distribution of frequencies in PG 1336−018, with some good individual correspondences, but cannot be considered adequate because geometric cancellation should hide some of the modes which are apparently detected. Analysis of the pulsations during eclipse recovers three of the strongest modes, but the limited eclipse data – which can, at best, be only about 9 per cent of the total – do not allow mode identification at this stage. Simulations indicate that an overall coverage of about 80 per cent would be required for this to be viable. An attempt was made to determine phase shifts in the pulsation frequencies as a way of directly measuring the size of the binary orbit, but the uncertainties in the method are comparable to the light travel time across the orbit (probably less than a second).Monthly Notices of the Royal Astronomical Society 10/2003; 345(3):834 - 846. · 4.90 Impact Factor -
Article: Discovery of unusual pulsations in the cool, evolved Am stars HD 98851 and HD 102480
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ABSTRACT: The chemically peculiar (CP) stars HD 98851 and HD 102480 have been discovered to be unusual pulsators during the ‘Naini Tal–Cape Survey’ programme to search for pulsational variability in CP stars. Time series photometric and spectroscopic observations of these newly discovered stars are reported here. Fourier analyses of the time series photometry reveal that HD 98851 is pulsating mainly with frequencies 0.208 and 0.103 mHz, and HD 102480 is pulsating with frequencies 0.107, 0.156 and 0.198 mHz. The frequency identifications are all subject to 1 d−1 cycle count ambiguities. We have matched the observed low-resolution spectra of HD 98851 and HD 102480 in the range 3500–7400 Å with theoretical synthetic spectra using Kurucz models with solar metallicity and a micro-turbulent velocity of 2 km s−1. These yield Teff= 7000 ± 250 K, log g= 3.5 ± 0.5 for HD 98851 and Teff= 6750 ± 250 K, log g= 3.0 ± 0.5 for HD 102480. We determined the equivalent H-line spectral class of these stars to be F1 IV and F3 III/IV, respectively. A comparison of the location of HD 98851 and HD 102480 in the HR diagram with theoretical stellar evolutionary tracks indicates that both stars are about 1-Gyr-old, 2-M⊙ stars that lie towards the red edge of the δ Sct instability strip. From comparison between the observed and calculated physical parameters, we conclude that HD 98851 and HD 102480 are cool, evolved Am pulsators. The light curves of these pulsating stars have alternating high and low amplitudes, nearly harmonic (or subharmonic) period ratios, high pulsational overtones and Am spectral types. This is unusual for both Am and δ Sct pulsators, making these stars interesting objects for further observational and theoretical studies.Monthly Notices of the Royal Astronomical Society 08/2003; 344(2):431 - 438. · 4.90 Impact Factor -
Article: Delta Scuti Network observations of XX Pyx: detection of 22 pulsation modes and of short‐term amplitude and frequency variations
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ABSTRACT: We report multisite observations devoted to the main-sequence δ Scuti star XX Pyx, conducted as the 17th run of the Delta Scuti Network. Over 125 nights a total of 550 h of usable time-series photometric B- and V-filter data were acquired involving both photoelectric and CCD measurements at eight observatories spread around the world, which represents the most extensive single time-series for any pulsating star other than the Sun obtained so far.We describe our observations and reduction methods, and present the frequency analysis of our new data. First, we detect six new pulsation and five new combination frequencies in the star's light curves. We also discover evidence for amplitude and/or frequency variations of some of the modes during the observations. These can occur on time-scales as short as 20 d and show quite diverse behaviour. To take them into account in the frequency analysis, a so-called non-linear frequency analysis method was developed, allowing us to quantify the temporal variability of the modes and to compensate for it. Following that we continue the frequency search and we also incorporate published multisite observations. In this way, we reveal three more pulsation and two more combination frequencies. In the end, we report a total of 30 significant frequencies – 22 of which correspond to independent pulsation modes. This is the largest number of independent modes ever detected in the light curves of a δ Scuti star.The frequencies of the modes show preferred separations as already suggested by previous work on this star; they are also arranged in clear patterns. These results lead to a refinement of the stellar mean density and to a new constraint on the rotation rate of XX Pyx (νrot=1.1±0.3 d−1). However, our attempts to identify the modes by pattern recognition failed. Moreover, mode identification from multicolour photometry failed as well because the high pulsation frequencies make this method unfavourable. The diverse behaviour of the amplitude and frequency variations of some of the modes leaves resonances as the only presently known possibility for their explanation.Monthly Notices of the Royal Astronomical Society 08/2003; 318(2):511 - 525. · 4.90 Impact Factor -
Article: IRAS 08544-4431 : a new post-AGB star in a binary system surrounded by a dusty disc
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ABSTRACT: We present an analysis of our extensive data-set on IRAS 08544-4431. It is the first object we discuss of our newly defined sample of stars, selected for their position in the 'RV Tauri' box in the IRAS [12]-[25], [25]-[60] two-color diagram. Moreover, our selection criteria included an observed excess in the L-band, indicative of a dusty disc. The SED of IRAS 08544-4431 shows a broad IR excess starting already at H. Our optical photometric data reveal some evidence for deep and] shallow minima in the light curve and a pulsation time-scale of around 100 days with a small amplitude (peak-to-peak amplitude in V = 0.17 mag). Our CORALIE radial velocity measurements show that IRAS 08544-4431 is a binary system with a period of 499 +/- 3 days and a mass function of 0.02 M_sun. Moreover, IRAS 08544-4431 is detected in both the CO (2-1) and (1-0) mm-wave emission lines. The triangular shape of the weak CO profile confirms that part of the circumstellar material is not freely expanding but resides probably in a dusty circumbinary disc. Our chemical abundance analysis of a high resolution spectrum of high S/N reveals that a depletion process has modified the photospheric abundances to a moderate extent ([Zn/Fe]=+0.4). All these findings confirm that the F-type IRAS 08544-4431 is another good example of a binary Post-AGB star surrounded by a dusty disc. The H-alpha P-Cygni profile shows ongoing mass-loss with a very high outflow velocity, the origin of which is not understood. The strength and velocity of the H-alpha-absorption are modulated with the orbital motion; the maxima of both quantities (+/- 400 km/s, 5 Angstrom, respectively) occur at superior conjunction. Comment: 14 pages, 13 figures06/2003; -
Article: The Everchanging Pulsating White Dwarf GD358
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ABSTRACT: We report 323 hours of nearly uninterrupted time series photometric observations of the DBV star GD 358 acquired with the Whole Earth Telescope (WET) during May 23rd to June 8th, 2000. We acquired more than 232 000 independent measurements. We also report on 48 hours of time-series photometric observations in Aug 1996. We detected the non-radial g-modes consistent with degree l=1 and radial order 8 to 20 and their linear combinations up to 6th order.We also detect, for the first time, a high amplitude l=2 mode, with a period of 796s. In the 2000 WET data, the largest amplitude modes are similar to those detected with the WET observations of 1990 and 1994, but the highest combination order previously detected was 4th order. At one point during the 1996 observations, most of the pulsation energy was transferred into the radial order k=8 mode, which displayed a sinusoidal pulse shape in spite of the large amplitude. The multiplet structure of the individual modes changes from year to year, and during the 2000 observations only the k=9 mode displays clear normal triplet structure. Even though the pulsation amplitudes change on timescales of days and years, the eigenfrequencies remain essentially the same, showing the stellar structure is not changing on any dynamical timescale. Comment: 34 pages, 14 figures, WET data, accepted to A&A01/2003; -
Article: The WSO, a world-class observatory for the ultraviolet
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ABSTRACT: The World Space Observatory is an unconventional space project proceeding via distributed studies. The present design, verified for feasibilty, consists of a 1.7-meter telescope operating at the second Largangian point of the Earth-Sun system. The focal plane instruments consist of three UV spectrometers covering the spectral band from Lyman alpha to the atmospheric cutoff with R~55,000 and offering long-slit capability over the same band with R~1,000. In addition, a number of UV and optical imagers view adjacent fields to that sampled by the spectrometers. Their performance compares well with that of HST/ACS and the spectral capabilities of WSO rival those of HST/COS. Comment: 11 pages, including 7 figures and 5 tables To be published in Proc SPIE, vol 485409/2002; -
Article: WSO/UV: World Space Observatory/Ultraviolet
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ABSTRACT: We summarize the capabilities of the World Space Observatory (UV) Project (WSO/UV). An example of the importance of this project (with a planned launch date of 2007/8) for the study of Classical Novae is given.08/2002; -
Article: 29 frequencies for the $delta$ Scuti variable BI CMi: the 1997-2000 multisite campaigns
mnras. 01/2002; 329:531-542. -
Article: 29 frequencies for the δ Scuti variable BI CMi: the 1997–2000 multisite campaigns
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ABSTRACT: A multisite campaign of BI CMi was carried out with excellent frequency resolution and high photometric accuracy from 1997 to 2000, including two long observing seasons. 29 pulsation frequencies could be extracted from the 1024 h (177 nights) of photometry used. The detected frequencies include 20 pulsation modes in the main pulsation frequency range from 4.8 to 13.0 cycle d−1 (55 to 150 μHz), eight linear combinations of these frequencies, and a very low frequency at 1.66 cycle d−1. Since the value of the low frequency at 1.66 cycle d−1 cannot be identified with a linear combination of other frequencies, g-mode pulsation is suspected, but rotational modulation of abundance spots cannot be ruled out. BI CMi, which is situated near the cool edge of the classical instability strip, may be both a δ Scuti and a γ Doradus star. Another outstanding property of BI CMi is the presence of a number of close frequency pairs in the power spectrum with separations as small as 0.01 cycle d−1.A rotational velocity of was determined from a high-dispersion spectrum. From phase differences, the dominant modes can be identified with ℓ values from 0 to 2. The spectral type and evolutionary status of BI CMi are examined.Monthly Notices of the Royal Astronomical Society 12/2001; 329(3):531 - 542. · 4.90 Impact Factor -
Article: HD 12098 - A new norhtern hemisphere roAp star
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ABSTRACT: We present the analysis of 65 hours of high speed photometric observations of HD 12098 taken from State Observatory, Naini Tal and Gurushikhar Observatory, Mt.Abu on sixteen nights spanning from November 1999 to November 2000. HD 12098 is the first rapidly oscillating Ap star discovered from the `Naini Tal-Cape survey for northern hemisphere roAp stars'. It is the 32nd in the complete list. HD 12098 exhibits one predominant mode of oscillation at nu_1 = 2.1738 mHz. The second-most significant frequency in our data is at nu_2 = 2.1641 mHz with a 1 cycle/day alias ambiguity. We argue that nu_2 is a rotational sidelobe of nu_1, rather than an independent pulsation mode. Evidence for the presence of two other frequencies at 2.1807 and 2.3056 mHz is also presented.09/2001; -
Article: HD 12098 a new far-northern roAp star
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ABSTRACT: The rapidly oscillating Ap (roAp) stars are cool, magnetic, chemically peculiar stars which pulsate in non-radial p-modes in the period range 4-16 min and have Johnson B amplitudes less than 8 mmag. "The NainiTal-Cape survey" to search for and study new roAp stars in the northern hemisphere was initiated in 1998 in collaboration between ISAC & UPSO from India and SAAO & UCT from South Africa. HD12098 is the first roAp star discovered in this survey and also the first far northern hemisphere roAp star. During the initial observations the star showed modulation in the pulsation amplitude indicating the multi-periodicity of pulsations. The multi-periodicity may be either due to the excitation of different modes or due to the rotation of the star. In order to resolve these frequencies HD12098 was observed extensively in October 2000. The preliminary results of these observations are presented here.Bulletin of the Astronomical Society of India 08/2001; 29:305-307. · 2.72 Impact Factor -
Article: Discovery of delta Scuti Pulsations in HD 98851
Information Bulletin on Variable Stars. 04/2000; 4900:1. -
Article: A search for rotational photometric variability in the pulsating Ap star HD 119027
Observatory -Didcot- 05/1998; 118:153-158. · 0.48 Impact Factor -
Article: Pulsation of the lambda Bootis stars HD 111786 and HD 142994
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ABSTRACT: The detection of possible nonradial pulsation modes among some lambda Bootis stars offers the prospect of using asteroseismology to determine their masses and ages, thereby testing competing theories for the origin of their chemical peculiarities. As a step toward this goal, we conducted multi-site photometric campaigns spanning two weeks each for two lambda Bootis stars already known to show oscillations with periods from 0.75 to 4 hr: HD 111786} \def\2{HD 142994 and \2. Comparison of the observed eigenfrequency spectrum with model eigenmode spectra can constrain the age of the star. Since two of the main proposed mechanisms for the lambda Bootis phenomenon (mass loss coupled with diffusion, and accretion) predict widely different stellar ages, such an analysis could distinguish between these options. Frequency analysis of campaign data yielded four frequencies for each program star. The best matches of our observed eigenfrequencies to Main Sequence stellar models (which include the effects of rotation on the stellar structure and the pulsation modes) suggest that both stars are too far from the Zero-Age Main Sequence, to be consistent with the widely favoured accretion scenario. However, we caution that the results are preliminary and suggest future observing strategies to confirm/deny this finding. Based on observations obtained at CTIO, SAAO and with the Hipparcos satelliteAstronomy and Astrophysics 01/1998; 330:605-611. · 4.59 Impact Factor -
Article: The p-mode spectrum of γ Equ (HR 8097).
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ABSTRACT: The authors present new high-speed photometric observations of the rapidly oscillating Ap (roAp) star γ Equ acquired on 26 nights during 1992. A frequency analysis of these observations reveals the presence of four p modes with frequencies of 1339, 1366, 1397 and 1427 μHz. These frequencies are all roughly equally spaced by about 30 kHz, which the authors interpret as the spacing of alternating even and odd l modes for a slightly evolved A-type star. This interpretation leads to an asteroseismological luminosity estimate for γ Equ of MV(Δν) = 1.8, which is in good agreement with the parallax luminosity MV(π) = 1.9.Monthly Notices of the Royal Astronomical Society 08/1996; 282:243-251. · 4.90 Impact Factor
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1998–2008
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South African Astronomical Observatory
Cape Town, Province of the Western Cape, South Africa
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